NONDESTRUCTIVE TESTING .------___N W "-J
HANDBOO
Volume 9
Visual
Testing
Editor
Patrick O. Moore
Technical Editors
Michael W. Allgaier
Robert E. Cameron
American Society for Nondestructive Testing
NONDESTRUCTIVE TESTING Third Edition
HANDBOOK
Volume 9
Visual Testing
Editor
Patrick O. Moore
Technical Editors
Michael W. Allgaier
Robert E. Cameron
® American Society for Nondestructive Testing
FOUNDED 1941
Copyright © 2010
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Library of Congress Cataloging-in-Publication Data
Visual testing / editor, Patrick O. Moore -- 3rd ed.
p. cm. -- (Nondestructive testing handbook ; v. 9)
Rev. ed. of: Visual and optical testing, 1993
Includes bibliographical references and index.
ISBN 978-1-57117-186-3 (alk. paper)
1. Nondestructive Testing. 2. Engineering inspection. 3. Optical
measurements. I. Moore, Patrick O., II. American Society for Nondestructive
Testing. III. Visual and optical testing.
TA417.2.V57 2010
620.1'127--dc22
2010018505
Errata
Errata if available for this printing may be obtained from ASNT’s Web site, <www.asnt.org>.
first printing 05/10
Published by the American Society for Nondestructive Testing
PRINTED IN THE UNITED STATES OF AMERICA
President’s Foreword
ASNT exists to create a safer world by
promoting the profession and
technologies of nondestructive testing.
The dedicated efforts of the Technical
and Education Council continue to
advance NDT technology through their
tireless efforts in creating new NDT
education and resource materials. Their
important achievements are a testimonial
to the efforts of these dedicated
volunteers.
One of the best ways to promote NDT
technology is to update and maintain our
handbooks as science and technology
advances. The NDT Handbook series is one
of ASNT’s premier products. It is
recognized both nationally and
internationally as a valuable study and
reference resource for NDT.
Visual Testing, Volume 9 of the third
edition, is the result of the dedicated
efforts of volunteers and ASNT staff to
update the handbook and align with
today’s technological advancements.
Vision is an integral part of everyday
life. It is not surprising that visual testing
is usually the initial examination
performed on components, parts and
structures.
As the demand for inspectors continues
to increase, there will be a significant
demand to keep materials current and
develop new NDT technology handbooks.
As technology continues to advance,
ASNT will continue to keep its library of
resources current and useful as an
essential resource to the NDT community.
The opportunities for the NDT
professional are endless. Involvement on
the Technical and Education Committee
is an excellent way to give back to this
proud profession. I encourage each ASNT
member to become involved and give
back to the profession of NDT. I guarantee
that you will get more than you give.
Joel W. Whitaker
ASNT President, 2009-2010
Visual Testing iii
Foreword
Aims of a Handbook take great pains to ensure that their
documents are definitive in wording and
The volume you are holding in your hand technical accuracy. People writing
is the ninth in the third edition of the contracts or procedures should consult
Nondestructive Testing Handbook. In the the actual standards when appropriate.
beginning of each volume, ASNT has
stated the purposes and nature of the Those who design qualifying
NDT Handbook series. examinations or study material for them
draw on ASNT handbooks as a quick and
Handbooks exist in many disciplines of convenient way of approximating the
science and technology, and certain body of knowledge. Committees and
features set them apart from other individuals who write or anticipate
reference works. A handbook should questions are selective in what they draw
ideally provide the basic knowledge from any source. The parts of a handbook
necessary for an understanding of the that give scientific background, for
technology, including both scientific instance, may have little bearing on a
principles and means of application. The practical examination except to provide
third edition of the NDT Handbook the physical foundation to assist handling
provides this knowledge through method of more challenging tasks. Other parts of
specific volumes. a handbook are specific to a certain
industry. This handbook provides a
The typical reader may be assumed to collection of perspectives on its subject to
have completed a few years of college broaden its value and convenience to the
toward a degree in engineering or science nondestructive testing community.
and has the background of an elementary
physics or mechanics course. Additionally, The present volume is a worthy
this volume allows for computer based addition to the third edition. The editors,
media that enhance all levels of education technical editors, ASNT staff, many
and training. contributors and reviewers worked
together to bring the project to
Standards, specifications, completion. For their scholarship and
recommended practices and inspection dedication, I thank them all.
procedures are discussed for instructional
purposes, but at a level of generalization Richard H. Bossi
that is illustrative rather than Handbook Development Director
comprehensive. Standards writing bodies
iv Visual Testing
Preface
The first visual testing report is found accessible. As the light source progressed
written in the book of Genesis, “He saw from a candle to a light bulb, to a fiber
that it was good.” optic cable, to an illumination bundle,
the limiting factor was the lens optic
Visual testing is the test that precedes system and eventually the fiber optic
every other test. For years, a certification system.
in magnetic particle testing or liquid
penetrant testing would suffice to be the The main content difference of this
equivalent of a visual testing edition of the visual volume of the NDT
qualification. Handbook is the significant addition of the
topic of indirect (or remote) visual testing.
The inspector had to “look” at the Coupling the recent advances in remote
object, part, component or system before visual test techniques with modern image
performing any other nondestructive recording capabilities makes the recording
testing (NDT) to “see” if the surface was and transferring of visual images a major
suitable for further testing. advance in recording, transferring and
retaining visual data of a test object. This
Its primary role as first test makes it the technology is a major advantage over
most important of all the methods of other NDT methods.
nondestructive testing. For years, how to
look at something defined visual testing. Visual testing allows direct
What the inspector is looking at entails a interpretation of test results without
broad spectrum of applications. This is encoding, decoding, extrapolating and
probably why visual testing was evaluating data from other NDT methods.
formalized so late in industry — codified To assess the condition of the test object,
by the nuclear industry, in the 1980s, and what the inspectors see is what they get.
appearing last in the sequence of NDT Visual is the most directly useful test
Handbook volumes, in 1993. method to assess the condition of an
object.
Its main limitation is that the test
surface must be accessible. Direct visual Michael W. Allgaier
testing has always addressed direct line of Robert E. Cameron
sight from the eyeball to the test surface. Technical Editors
With the help of a candle and a mirror,
otherwise inaccessible surfaces became
Visual Testing v
Editor’s Preface
Early in 1986, Robert McMaster sat up in One of the intriguing things about VT
his hospital bed and handed me a piece of is that very few publications have been
paper from a technical committee dedicated to it as nondestructive testing,
member. On the paper was scratched an distinct from fields such as astronomy or
outline for the book you are now reading. medical endoscopy. By 1990, there were
two books on VT, one on borescopes and
This book on visual testing (VT) began one mainly on direct viewing.
with Robert McMaster. McMaster was
ASNT’s president from 1952 to 1953. He The next step was taken by Michael
compiled and in 1959 published the first Allgaier and ASNT’s VT Committee.
edition of the NDT Handbook. That Allgaier collected available material, and
edition was a milestone in the history of in 1993 ASNT published it as Volume 8 in
nondestructive testing (NDT). the second edition of the NDT Handbook.
That volume defined the method.
McMaster is revered in ASNT because Henceforth, VT was to include both direct
of two major visions that he imparted. and indirect techniques. It would be
First, he believed that NDT had a mission, scientifically grounded in the physics of
an important role among applied sciences light. Its study would include basic
such as engineering: NDT’s purpose was to optometry, since the eye is the primary
improve the quality of products and sensor. VT’s representation in standards
services, to preserve not just the quality of for industries such as energy and
life but to preserve life itself through petroleum would be duly noted.
public safety. He often compared
nondestructive inspectors to physicians, Before that book, the method would
saving lives. Without NDT, airplanes crash usually go unnoticed: inspectors would
and buildings fall and boilers explode. not even realize that their visual
inspection was actually nondestructive
Second, McMaster wanted to ground testing. After that book, the foundation
NDT solidly as a material science. He had was laid for an ASNT method — with
studied under Enrico Fermi and Robert trainers, qualifying examinations and a
Millikan at CalTech. McMaster believed in literature for study.
the nobility of science, that it improved
our lives through understanding natural The present volume builds on the
laws and applying that understanding. success of that 1993 volume. Information
has been added on digital capabilities that
His first edition of the NDT Handbook inspectors use routinely. The coverage of
was monumental, 54 sections in two indirect techniques (sometimes called
volumes. There were fifteen sections for remote inspection) has been updated to
radiographic testing and two for visual reflect current technology for cameras and
testing. That the visual method was measurement. The discussions of
represented at all is remarkable, and optometry and physics are updated. The
reflects McMaster’s scientific bent and the chapter on metals is completely revised
conviction that NDT should be with an eye for practicality. The material
represented in every band in the on direct techniques is presented in one
electromagnetic spectrum, even the visible chapter. References are updated
radiation we call light. But on that winter throughout. The entire book has been
afternoon in 1986, an exasperated revised to be clearly organized and
McMaster pointed to the brief outline: functionally complete.
“It’s just a list of different kinds of
borescopes! Just borescopes!” McMaster’s stay in the hospital in the
winter of 1986 was one of several that
The challenge for the writer of that would end with his death in July. I like to
outline, as for McMaster in 1959 and for think that, if he had lived to see it, he
others since, is precisely how the method would have celebrated this book and VT’s
is to be defined. For some, it was defined place as an NDT method.
by its instruments, mainly the industrial
endoscopes called borescopes. Others Dozens of contributors and reviewers
believed, wrongly, that the term visual freely shared their expertise; in particular
denoted viewing unmediated by lenses Technical Editors Michael Allgaier and
and that another word, optical, was Robert Cameron provided leadership and
needed to include instruments such as encouragement. On ASNT staff, Senior
borescopes. For McMaster, however, as for Manager of Publications Timothy Jones
every volume of the third edition of the provided essential administrative support.
NDT Handbook, the word visual carved out My colleague, Technical Publications
a niche in the electromagnetic spectrum Supervisor Hollis Humphries, proofed the
somewhere between infrared and X-rays entire book and supervised all its graphics.
(both of which, by the way, are also A hearty thanks to them all.
mediated through optics). Still, as late as
the 1980s, some people assumed that the Patrick Moore
term visual testing meant only “vision NDT Handbook Editor
acuity examination.”
vi Visual Testing
Acknowledgments
All contributors are also reviewers but are Contributors
listed once, as contributors.
Michael W. Allgaier, Mistras
Handbook Development David R. Atkins, Packer Engineering
Committee David R. Bajula, Acuren Inspection
Bruce L. Bates
Richard H. Bossi, Boeing Aerospace Thomas D. Britton, General Electric
Michael W. Allgaier, Mistras
David R. Bajula, Acuren Inspection Sensing and Inspection Technologies
Albert S. Birks, Naval Surface Warfare Brian P. Buske, General Electric Sensing
Center and Inspection Technologies
Lisa Brasche, Iowa State University Donald R. Christina, Boeing Company
James E. Cox, Zetec, Incorporated John C. Duke, Jr., Virginia Polytechnic
David L. Culbertson, El Paso Corporation
James L. Doyle, Jr., NorthWest Research Institute and State University
Mohamed El-Gomati, University of York,
Associates
Nat Y. Faransso, KBR United Kingdom
Gerard K. Hacker, Teledyne Brown Nat Y. Faransso, KBR
Gregory W. Good, Ohio State University,
Engineering
Harb S. Hayre, Ceie Specs College of Optometry
Eric v.K. Hill, Embry-Riddle Aeronautical Doron Kishoni, Business Solutions USA,
University Canada
James W. Houf, American Society for Douglas G. Krauss, Huddleston Technical
Nondestructive Testing Services, Redstone Arsenal
Frank A. Iddings William J. Lang, Lenox Instrument
Morteza K. Jafari, Fugro South
Timothy E. Jones, American Society for Company
Trevor Liddell, General Electric Sensing
Nondestructive Testing
John K. Keve, DynCorp Tri-Cities Services and Inspection Technologies
Doron Kishoni, Business Solutions USA, Zheng Liu, Research Officer, National
Canada Research Council Canada
Xavier P.V. Maldague, University Laval Joseph L. Mackin, Team Industrial Services
George A. Matzkanin, Texas Research Stephen L. Meiley, Champion
Institute International
Ronnie K. Miller, Mistras Richard T. Nademus, Exelon Corporation
Scott D. Miller Yoshihiro Ohno, National Institute of
Mani Mina, Technology Resource Group
David G. Moore, Sandia National Standards and Technology
Donald Parrish, Southern Company
Laboratories
Patrick O. Moore, American Society for Services
David A. Pasquazzi, David Pasquazzi and
Nondestructive Testing
Stanislav I. Rokhlin, Ohio State University Associates
Frank J. Sattler Stanislav I. Rokhlin, Ohio State University
Fred Seppi, Williams International Donald J. Roth, National Aeronautics and
Kermit A. Skeie
Roderic K. Stanley, NDE Information Space Administration, Glenn Research
Center
Consultants Gregory C. Sayler, MD Helicopters
Stuart A. Tison, Millipore Corporation Roderic K. Stanley, NDE Information
Noel A. Tracy, Universal Technology Consultants
Marvin W. Trimm, Savannah River
Corporation National Laboratory
Satish S. Udpa, Michigan State University Hiroyuki Ukida, University of Tokushima,
Mark F.A. Warchol, Alcoa Japan
Glenn A. Washer, University of Missouri Michael A. Urzendowski, Valero Energy
Robert W. Warke, LeTourneau University
— Columbia
George C. Wheeler Reviewers
Gary L. Workman, University of Alabama,
Steven E. Anderson, Canam Steel
Huntsville Jerry D. Beasley, Omaha Public Power
Kenneth Becker, Sigma Transducers
James J. Bogner, GPR Testing and
Inspection
Visual Testing vii
Richard H. Bossi, Boeing Research and Walter R. Matulewicz, Tinker Air Force
Technology Base
Lisa Brasche, Iowa State University Charles H. Mazel, BlueLine NDT
Robert H. Bushnell Eugene A. Mechtly, University of Illinois
James R. Cahill, General Electric Sensing
at Urbana-Champaign
and Inspection Technologies John W. Miller
Robert E. Cameron Scott D. Miller
Eugene J. Chemma, Arcelor Mittal Steel Van B. Nakagawara, Federal Aviation
David Clark, LightDancer Interactive
Authority, Civil Aerospace Medical
Technologies Institute
Christopher I. Collins, Olympus Industrial David K. Park, Olympus Industrial
America
Systems Europa Bruce A. Pellegrino, General Electric
Jackson R. Crissey, Jr., Plant Performance Sensing and Inspection Technologies,
Everest RVI
Services William C. Plumstead, Sr., PQT Services
Claude D. Davis, Unified Testing Services Frank J. Sattler
Edward R. Generazio, NASA Langley David Sentelle, American Society for
Nondestructive Testing
Research Center Robert E. Stevens, United Airlines
Lawrence O. Goldberg, Seatest Mark F.A. Warchol, Alcoa
Jack K. Harper, Babcock and Wilcox, Oak Stanley L. Weatherly, Boeing Company
Ridge
James W. Houf, American Society for
Nondestructive Testing
Charles P. Longo, American Society for
Nondestructive Testing
viii Visual Testing
CONTENTS
Chapter 1. Introduction to Chapter 7. Machine Vision for Visual
Visual Testing . . . . . . . . . . . . . . . . 1 Testing . . . . . . . . . . . . . . . . . . . 157
Part 1. Nondestructive Testing . . . . . 2 Part 1. System Architecture of
Part 2. Management of Visual Machine Vision
System . . . . . . . . . . . . . . 158
Testing . . . . . . . . . . . . . . . 13
Part 3. History of Visual Part 2. Algorithms and
Software . . . . . . . . . . . . . 164
Testing . . . . . . . . . . . . . . . 24
Part 4. Measurement Units for References . . . . . . . . . . . . . . . . . . 177
Visual Testing . . . . . . . . . 34 Chapter 8. Visual Testing of
References . . . . . . . . . . . . . . . . . . . 37 Metals . . . . . . . . . . . . . . . . . . . 179
Chapter 2. Light . . . . . . . . . . . . . . . . . . 41 Part 1. Metal Processing . . . . . . . . 180
Part 1. Physics of Light . . . . . . . . . 42 Part 2. Visual Testing of Cast
Part 2. Refraction and Color . . . . . 45
Part 3. Photometry . . . . . . . . . . . . . 51 Ingots . . . . . . . . . . . . . . . 182
References . . . . . . . . . . . . . . . . . . . 58 Part 3. Visual Testing of Forgings
Chapter 3. Vision Acuity for and Rolled Metal . . . . . . 185
Nondestructive Testing . . . . . . . 61 Part 4. Visual Testing of
Part 1. Vision . . . . . . . . . . . . . . . . . 62 Welds . . . . . . . . . . . . . . . 191
Part 2. Vision Acuity . . . . . . . . . . . 66 Part 5. Discontinuities from
Part 3. Vision Testing . . . . . . . . . . . 71
References . . . . . . . . . . . . . . . . . . . 79 Processes Other than
Welding . . . . . . . . . . . . . 197
Chapter 4. Visual Test Imaging . . . . . . . 83 Part 6. Service Induced
Part 1. Photography in Visual Discontinuities . . . . . . . . 200
Testing . . . . . . . . . . . . . . . 84 References . . . . . . . . . . . . . . . . . . 210
Part 2. Digital Processing and
Archiving for Visual Chapter 9. Chemical and Petroleum
Testing . . . . . . . . . . . . . . . 95 Applications of Visual
Part 3. Video . . . . . . . . . . . . . . . . . 100 Testing . . . . . . . . . . . . . . . . . . . 211
References . . . . . . . . . . . . . . . . . . 108
Part 1. Chemical and Petroleum
Chapter 5. Direct Visual Testing . . . . . 111 Industry . . . . . . . . . . . . . 212
Part 1. Circumstances of
Viewing . . . . . . . . . . . . . . 112 Part 2. Visual Acceptance Criteria
Part 2. Illumination . . . . . . . . . . . 116 for Welds . . . . . . . . . . . . 215
Part 3. Magnification . . . . . . . . . . 121
Part 4. Surface Characteristics . . . 127 Part 3. Petroleum Tubular
Part 5. Dimensional Specifications . . . . . . . . . 220
Measurement . . . . . . . . . 130
References . . . . . . . . . . . . . . . . . . 134 Part 4. Visual Testing of Pipe
Threads . . . . . . . . . . . . . . 223
Chapter 6. Indirect Visual Testing . . . 135
Part 1. Introduction to Indirect References . . . . . . . . . . . . . . . . . . 229
Visual Testing . . . . . . . . . 136
Part 2. Borescopy . . . . . . . . . . . . . 141 Chapter 10. Electric Power
Part 3. Camera Based Applications of Visual
Measurement . . . . . . . . . 148 Testing . . . . . . . . . . . . . . . . . . . 233
References . . . . . . . . . . . . . . . . . . 155
Part 1. Visual Testing of
Welds . . . . . . . . . . . . . . . 234
Part 2. Visual Testing of Various
Components . . . . . . . . . . 250
References . . . . . . . . . . . . . . . . . . 264
Visual Testing ix
Chapter 11. Aerospace Applications Chapter 13. Visual Testing
of Visual Testing . . . . . . . . . . . . 265 Glossary . . . . . . . . . . . . . . . . . . . 303
Part 1. Visual Testing of Aircraft Definitions . . . . . . . . . . . . . . . . . 304
Structure . . . . . . . . . . . . . 266 References . . . . . . . . . . . . . . . . . . 322
Part 2. Visual Testing of Jet Index . . . . . . . . . . . . . . . . . . . . . . . . . . 323
Engines . . . . . . . . . . . . . . 274
Figure Sources . . . . . . . . . . . . . . . . . . . 330
Part 3. Visual Testing of
Composite Materials . . . 278
References . . . . . . . . . . . . . . . . . . 283
Chapter 12. Techniques Allied to
Visual Testing . . . . . . . . . . . . . . 285
Part 1. Indications Not from
Visual Testing . . . . . . . . . 286
Part 2. Replication . . . . . . . . . . . . 291
Part 3. Etching . . . . . . . . . . . . . . . 297
References . . . . . . . . . . . . . . . . . . 302
x Visual Testing
1
CHAPTER
Introduction to
Visual Testing
Mohamed El-Gomati, University of York, Heslington,
North Yorkshire, United Kingdom (Part 3)
William J. Lang, Lenox Instrument Company, Trevose,
Pennsylvania (Part 3)
Marvin W. Trimm, Savannah River National Laboratory,
Aiken, South Carolina (Part 2)
PART 1. Nondestructive Testing
Scope of Nondestructive pressure testing is a form of proof testing
Testing that sometimes destroys the test object.
Nondestructive testing is a materials A gray area in the definition of
science concerned with many aspects of nondestructive testing is the phrase future
quality and serviceability of materials and usefulness. Some material investigations
structures. The science of nondestructive involve taking a sample of the test object
testing incorporates all the technology for for a test that is inherently destructive. A
process monitoring and for detection and noncritical part of a pressure vessel may
measurement of significant properties, be scraped or shaved to get a sample for
including discontinuities, in items electron microscopy, for example.
ranging from research test objects to Although future usefulness of the vessel is
finished hardware and products in service. not impaired by the loss of material, the
Nondestructive testing examines materials procedure is inherently destructive and
and structures without impairment of the shaving itself — in one sense the true
serviceability and reveals hidden test object — has been removed from
properties and discontinuities. service permanently.
Nondestructive testing is becoming The idea of future usefulness is relevant
increasingly vital in the effective conduct to the quality control practice of
of research, development, design and sampling. Sampling (that is, less than
manufacturing programs. Only with 100 percent testing to draw inferences
appropriate nondestructive testing can the about the unsampled lots) is
benefits of advanced materials science be nondestructive testing if the tested sample
fully realized. The information required is returned to service. If steel bolts are
for appreciating the broad scope of tested to verify their alloy and are then
nondestructive testing is available in returned to service, then the test is
many publications and reports. nondestructive. In contrast, even if
spectroscopy in the chemical testing of
Definition many fluids is inherently nondestructive,
the testing is destructive if the samples are
Nondestructive testing (NDT) has been poured down the drain after testing.
defined as those methods used to test a
part or material or system without Nondestructive testing is not confined
impairing its future usefulness.1 The term to crack detection. Other anomalies
is generally applied to nonmedical include porosity, wall thinning from
investigations of material integrity. corrosion and many sorts of disbonds.
Nondestructive material characterization
Nondestructive testing is used to is a field concerned with properties
investigate specifically the material including material identification and
integrity or properties of a test object. A microstructural characteristics — such as
number of other technologies — for resin curing, case hardening and stress —
instance, radio astronomy, voltage and that directly influence the service life of
current measurement and rheometry the test object.
(flow measurement) — are nondestructive
but are not used specifically to evaluate Methods and Techniques
material properties. Radar and sonar are
classified as nondestructive testing when Nondestructive testing has also been
used to inspect dams, for instance, but defined by listing or classifying the
not when used to chart a river bottom. various techniques.1-3 This approach to
nondestructive testing is practical in that
Nondestructive testing asks “Is there it typically highlights methods in use by
something wrong with this material?” In industry.
contrast, performance and proof tests ask
“Does this component work?” It is not In the Nondestructive Testing Handbook,
considered nondestructive testing when the word method is used for a group of test
an inspector checks a circuit by running techniques that share a form of probing
electric current through it. Hydrostatic energy. The ultrasonic test method, for
example, uses acoustic waves at a
frequency higher than audible sound.
Infrared and thermal testing and
2 Visual Testing
radiographic testing are two test methods satisfaction and maintain the
that use electromagnetic radiation, each manufacturer’s reputation, (5) to aid in
in a defined wavelength range. The word better product design, (6) to control
technique, in contrast, denotes a way of manufacturing processes, (7) to lower
adapting the method to the application. manufacturing costs, (8) to maintain
Through-transmission immersion testing uniform quality levels and (9) to ensure
is a technique of the ultrasonic method, operational readiness.
for example.
These reasons for widespread and
Purposes of profitable nondestructive testing are
Nondestructive Testing sufficient in themselves but parallel
developments have contributed to the
Since the 1920s, the art of testing without technology’s growth and acceptance.
destroying the test object has developed
from a laboratory curiosity to an Increased Demand on Machines
indispensable tool of fabrication,
construction, manufacturing and In the interest of greater performance and
maintenance processes. No longer is reduced cost for materials, the design
visual testing of materials, parts and engineer is often under pressure to reduce
complete products the principal weight. Weight can be saved sometimes
nondestructive test for quality. by substituting aluminum alloys,
Nondestructive tests in great variety are in magnesium alloys or composite materials
worldwide use to detect variations in for steel or iron but such light parts may
structure, minute changes in surface not be the same size or design as those
finish, the presence of cracks or other they replace. The tendency is also to
physical discontinuities, to measure the reduce the size. These pressures on the
thickness of materials and coatings and to designer have subjected parts of all sorts
determine other characteristics of to increased stress levels. Even such
industrial products. Scientists and commonplace objects as sewing
engineers of many countries have machines, sauce pans and luggage are also
contributed greatly to nondestructive test lighter and more heavily loaded than ever
development and applications. before. The stress to be supported is
known as dynamic stress or dynamic
How is nondestructive testing useful? loading, as opposed to static stress. It
Why do thousands of industrial concerns often fluctuates and reverses at low or
buy the test equipment, pay the high frequencies. Frequency of stress
subsequent operating costs of the testing reversals increases with the speeds of
and even reshape manufacturing modern machines, so components tend to
processes to fit the needs and findings of fatigue and fail more rapidly.
nondestructive testing? Modern Another cause of increased stress on
nondestructive tests are used by modern products is a reduction in the
manufacturers (1) to ensure product safety factor. An engineer designs with
integrity and in turn reliability, (2) to certain known loads in mind. On the
avoid failures, prevent accidents and save supposition that materials and
human life (Figs. 1 and 2), (3) to make a workmanship are never perfect, a safety
profit for the user, (4) to ensure customer
FIGURE 2. Boilers operate with high internal steam pressure.
FIGURE 1. Fatigue cracks caused damage to aircraft fuselage, Material discontinuities can lead to sudden, violent failure
causing death of flight attendant and injury to passengers with possible injury to people and damage to property.
(April 1988).
Introduction to Visual Testing 3
factor of 2, 3, 5 or 10 is applied. However, accident. This demand for personal safety
a lower factor is often used that depends has been another strong force in the
on considerations such as cost or weight. development of nondestructive tests.
New demands on machinery have also Rising Costs of Failure
stimulated the development and use of
new materials whose operating Aside from awards to the injured or to
characteristics and performances are not estates of the deceased and aside from
completely known. These new materials costs to the public (because of evacuations
could create greater and potentially occasioned by chemical leaks, for
dangerous problems. For example, an example), there are other factors in the
aircraft part was built from an alloy whose rising costs of mechanical failure.
work hardening, notch resistance and
fatigue life were not well known. After These costs are increasing for many
relatively short periods of service, some of reasons. Some important ones are
the aircraft using these parts suffered (1) greater costs of materials and labor,
disastrous failures. Sufficient and proper (2) greater costs of complex parts,
nondestructive tests could have saved (3) greater costs because of the complexity
many lives. of assemblies, (4) a greater probability that
failure of one part will cause failure of
As technology improves and as service others because of overloads, (5) the
requirements increase, machines are probability that the failure of one part
subjected to greater variations and will damage other parts of high value and
extremes of all kinds of stress, creating an (6) part failure in an integrated automatic
increasing demand for stronger or more production machine, shutting down an
damage tolerant materials. entire high speed production line. In the
past, when production was carried out on
Engineering Demands for Sounder many separate machines, the broken one
Materials could be bypassed until repaired. Today,
one machine is often tied into the
Another justification for nondestructive production cycles of several others. Loss
tests is the designer’s demand for sounder of such production is one of the greatest
materials. As size and weight decrease and losses resulting from part failure.
the factor of safety is lowered, more
emphasis is placed on better raw material Classification of Methods
control and higher quality of materials,
manufacturing processes and The National Materials Advisory Board
workmanship. (NMAB) Ad Hoc Committee on
Nondestructive Evaluation classified
An interesting fact is that a producer of techniques into six major method
raw material or of a finished product categories: visual, penetrating radiation,
sometimes does not improve quality or magnetic-electrical, mechanical vibration,
performance until that improvement is thermal and chemical/electrochemical.3
demanded by the customer. The pressure A modified version of their system is
of the customer is transferred to presented in Table 1.1
implementation of improved design or
manufacturing. Nondestructive testing is Each method can be completely
frequently called on to confirm delivery characterized in terms of five principal
of this new quality level. factors: (1) energy source or medium used
to probe the object (such as X-rays,
Public Demands for Greater Safety ultrasonic waves or thermal radiation),
(2) nature of the signals, image or
The demands and expectations of the signature resulting from interaction with
public for greater safety are widespread. the object (attenuation of X-rays or
Review the record of the courts in reflection of ultrasound, for example),
granting high awards to injured persons. (3) means of detecting or sensing
Consider the outcry for greater resultant signals (photoemulsion,
automobile safety as evidenced by the piezoelectric crystal or inductance coil),
required automotive safety belts and the (4) means of indicating or recording
demand for air bags, blowout proof tires signals (meter deflection, oscilloscope
and antilock braking systems. The trace or radiograph) and (5) basis for
publicly supported activities of the interpreting the results (direct or indirect
National Safety Council, Underwriters indication, qualitative or quantitative and
Laboratories, the Occupational Safety and pertinent dependencies).
Health Administration, the Federal
Aviation Administration and other The objective of each method is to
agencies around the world are only a few provide information about one or more of
of the ways in which this demand for the following material parameters:
safety is expressed. It has been expressed (1) discontinuities and separations (such
directly by passengers who cancel as cracks, voids, inclusions and
reservations following a serious aircraft
4 Visual Testing
delaminations), (2) structure or Classification by Test Object
malstructure (such as crystalline structure,
grain size, segregation and misalignment), Nondestructive test techniques may be
(3) dimensions and metrology (such as classified according to how they detect
thickness, diameter, gap size and indications relative to the surface of a test
discontinuity size), (4) physical and object. Surface methods include liquid
mechanical properties (such as reflectivity, penetrant testing, visual testing and moiré
conductivity, elastic modulus and sonic testing. Surface/near-surface methods
velocity), (5) composition and chemical include tap, holographic, shearographic,
analysis (such as alloy identification, magnetic particle and electromagnetic
impurities and elemental distributions), testing. When surface or near-surface
(6) stress and dynamic response (such as methods are applied during intermediate
residual stress, crack growth, wear and manufacturing, they provide preliminary
vibration), (7) signature analysis (such as assurance that volumetric methods
image content, frequency spectrum and performed on the completed object or
field configuration) and (8) heat sources. component will reveal few rejectable
discontinuities. Volumetric methods
Material characteristics in Table 1 are include radiography, ultrasonic testing
further defined in Table 2 with respect to and acoustic emission testing.
specific objectives and specific attributes Through-boundary techniques include
to be measured, detected and defined. leak testing, some infrared thermographic
techniques, airborne ultrasonic testing
Methods that use electromagnetic and certain techniques of acoustic
radiation (Table 3) can be divided emission testing. Other less easily
according to the segment of the spectrum classified methods are material
each uses as interrogating energy: radar, identification, vibration analysis and
thermography, visual testing and strain gaging.
X-radiography (Fig. 3). Methods using
vibration and ultrasound are in a different No one nondestructive test method is
spectrum: the acoustic. all revealing. In some cases, one method
or technique may be adequate for testing
The limitations of a method include a specific object or component. However,
conditions (such as access, physical in most cases, it takes a series of test
contact and surface preparation) and methods to do a complete nondestructive
requirements to adapt the probe to the test of an object or component. For
test object. Other factors limit the example, if surface cracks must be
detection or characterization of detected and eliminated and if the object
discontinuities or attributes and limit or component is made of ferromagnetic
interpretation of signals or images.
TABLE 1. Nondestructive test method categories. Test Objectives
Categories
Basic Categories
Mechanical and optical color; cracks; dimensions; film thickness; gaging; reflectivity; strain distribution and magnitude; surface
Penetrating radiation finish; surface flaws; through-cracks
Electromagnetic and electronic
cracks; density and chemistry variations; elemental distribution; foreign objects; inclusions; microporosity;
Sonic and ultrasonic misalignment; missing parts; segregation; service degradation; shrinkage; thickness; voids
Infrared and thermal alloy content; anisotropy; cavities; cold work; local strain, hardness; composition; contamination;
corrosion; cracks; crack depth; crystal structure; electrical conductivities; flakes; heat treatment;
Chemical and analytical hot tears; inclusions; ion concentrations; laps; lattice strain; layer thickness; moisture content;
Auxiliary Categories polarization; seams; segregation; shrinkage; state of cure; tensile strength; thickness; disbonds; voids
Image generation crack initiation and propagation; cracks, voids; damping factor; degree of cure; degree of impregnation;
Signal image analysis degree of sintering; delaminations; density; dimensions; elastic moduli; grain size; inclusions;
mechanical degradation; misalignment; porosity; radiation degradation; structure of composites;
surface stress; tensile, shear and compressive strength; disbonds; wear
anisotropy; bonding; composition; emissivity; heat contours; plating thickness; porosity; reflectivity;
stress; thermal conductivity; thickness; voids; cracks; delaminations; heat treatment; state of cure;
moisture; corrosion
alloy identification; composition; cracks; elemental analysis and distribution; grain size; inclusions;
macrostructure; porosity; segregation; surface anomalies
dimensional variations; dynamic performance; anomaly characterization and definition; anomaly
distribution; anomaly propagation; magnetic field configurations
data selection, processing and display; anomaly mapping, correlation and identification; image
enhancement; separation of multiple variables; signature analysis
Introduction to Visual Testing 5
material, then magnetic particle testing then ultrasonic testing or radiography
would be the appropriate choice. If the would be chosen. The exact technique in
material is aluminum or titanium, then each case depends on the thickness and
the choice would be liquid penetrant or nature of the material and the types of
electromagnetic testing. However, if discontinuities that must be detected.
internal discontinuities are to be detected,
TABLE 2. Objectives of nondestructive test methods.
Objectives Attributes Measured or Detected
Discontinuities and Separations
Surface anomalies roughness, scratches, gouges, crazing, pitting, imbedded foreign material
Surface connected anomalies
Internal anomalies cracks, porosity, pinholes, laps, seams, folds, inclusions
cracks, separations, hot tears, cold shuts, shrinkage, voids, lack of fusion, pores, cavities, delaminations,
disbonds, poor bonds, inclusions, segregations
Structure molecular structure; crystalline structure and/or strain; lattice structure; strain; dislocation; vacancy;
Microstructure deformation
Matrix structure grain structure, size, orientation and phase; sinter and porosity; impregnation; filler and/or reinforcement
distribution; anisotropy; heterogeneity; segregation
Small structural anomalies
Gross structural anomalies leaks (lack of seal or through-holes), poor fit, poor contact, loose parts, loose particles, foreign objects
assembly errors; misalignment; poor spacing or ordering; deformation; malformation; missing parts
Dimensions and Measures linear measurement; separation; gap size; discontinuity size, depth, location and orientation
unevenness; nonuniformity; eccentricity; shape and contour; size and mass variations
Displacement, position film, coating, layer, plating, wall and sheet thickness; density or thickness variations
Dimensional variations
Thickness, density
Physical and Mechanical Properties
Electrical properties resistivity; conductivity; dielectric constant and dissipation factor
Magnetic properties polarization; permeability; ferromagnetism; cohesive force, susceptibility
Thermal properties conductivity; thermal time constant and thermoelectric potential; diffusivity; effusivity; specific heat
Mechanical properties compressive, shear and tensile strength (and moduli); Poisson’s ratio; sonic speed; hardness; temper
Surface properties and embrittlement
color, reflectivity, refraction index, emissivity
Chemical Composition and Analysis
Elemental analysis detection, identification, distribution and/or profile
Impurity concentrations contamination, depletion, doping and diffusants
Metallurgical content variation; alloy identification, verification and sorting
Physiochemical state moisture content; degree of cure; ion concentrations and corrosion; reaction products
Stress and Dynamic Response heat treatment, annealing and cold work effects; stress and strain; fatigue damage and residual life
wear, spalling, erosion, friction effects
Stress, strain, fatigue corrosion, stress corrosion, phase transformation
Mechanical damage radiation damage and high frequency voltage breakdown
Chemical damage crack initiation, crack propagation, plastic deformation, creep, excessive motion, vibration, damping,
Other damage
Dynamic performance timing of events, any anomalous behavior
Signature Analysis potential; intensity; field distribution and pattern
Electromagnetic field
Thermal field isotherms, heat contours, temperatures, heat flow, temperature distribution, heat leaks, hot spots, contrast
Acoustic signature
noise, vibration characteristics, frequency amplitude, harmonic spectrum, harmonic analysis, sonic
Radioactive signature emissions, ultrasonic emissions
Signal or image analysis
distribution and diffusion of isotopes and tracers
image enhancement and quantization; pattern recognition; densitometry; signal classification, separation
and correlation; discontinuity identification, definition (size and shape) and distribution analysis;
discontinuity mapping and display
6 Visual Testing
Nondestructive Testing’s methods. The following section briefly
Value describes major methods and the
applications associated with them.
In manufacturing, nondestructive testing
may be accepted reluctantly because its Visual Testing
contribution to profits may not be
obvious to management. Nondestructive Visual testing is the subject of the present
testing is sometimes thought of only as a volume and of a volume in the previous
cost item and can be curtailed by industry edition.4
downsizing. When a company cuts costs,
two vulnerable areas are quality and Principles. Visual testing (Fig. 4) is the
safety. When bidding contract work, observation of a test object, either directly
companies add profit margin to all cost with the eyes or indirectly using optical
items, including nondestructive testing, so instruments, by an inspector to evaluate
a profit should be made on the the presence of surface anomalies and the
nondestructive testing. The attitude object’s conformance to specification.
toward nondestructive testing is positive Visual testing should be the first
when management understands its value. nondestructive test method applied to an
item. The test procedure is to clear
Nondestructive testing should be used obstructions from the surface, provide
as a control mechanism to ensure that adequate illumination and observe. A
manufacturing processes are within design prerequisite necessary for competent
performance requirements. When used visual testing of an object is knowledge of
properly, nondestructive testing saves the manufacturing processes by which it
money for the manufacturer. Rather than was made, of its service history and of its
costing the manufacturer money, potential failure modes, as well as related
nondestructive testing should add profits industry experience.
to the manufacturing process.
Applications. Visual testing is widely used
Nondestructive Test on a variety of objects to detect surface
Methods discontinuities associated with various
structural failure mechanisms. Even when
To optimize nondestructive testing, it is other nondestructive tests are performed,
necessary first to understand the visual tests often provide a useful
principles and applications of all the supplement. When the eddy current
testing of process tubing is performed, for
example, visual testing is often performed
to verify and more closely examine the
TABLE 3. Nondestructive test methods and corresponding parts of electromagnetic spectrum.
Interrogating Energy Test Method Approximate Approximate
Wavelengths (m) Frequencies (Hz)
X-rays or gamma rays radiography (RT) 10–16 to 10–8 1024 to 1017
Ultraviolet radiation various minor methodsa 10–8 to 10–7 1017 to 1015
Light (visible radiation) visual testing (VT) 4 × 10–7 to 7 × 10–7 1015
Heat or thermal radiation infrared and thermal testing (IR) 10–6 to 10–3 1015 to 1011
Radio waves radar and microwave methods 10–3 to 101 1011 to 107
a. Ultraviolet radiation is used in various methods: (1) viewing of fluorescent indications in liquid penetrant testing and
magnetic particle testing; (2) lasers and optical sensors operating at ultraviolet wavelengths.
FIGURE 3. Electromagnetic spectrum.
Radiation wavelength (nm)
106 105 104 103 102 10 1 10–1 10–2 10–3 10–4 10–5 10–6
Visible X–rays
light
Radio Infrared Ultraviolet Cosmic rays
Gamma rays
10–9 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10–1 1 10 102 103
Photon energy (MeV)
Introduction to Visual Testing 7
surface condition. The following surface as dry particles or as wet particles
discontinuities may be detected by a in a liquid carrier such as water or oil.
simple visual test: surface discontinuities,
cracks, misalignment, warping, corrosion, Applications. The principal industrial uses
wear and physical damage. of magnetic particle testing include final,
receiving and in-process testing; testing
Magnetic Particle Testing for quality control; testing for
maintenance and overhaul in the
Principles. Magnetic particle testing transportation industries; testing for plant
(Fig. 5) is a method of locating surface and machinery maintenance; and testing
and near-surface discontinuities in of large components. Some discontinuities
ferromagnetic materials. It depends on the typically detected are surface
fact that when the test object is discontinuities, seams, cracks and laps.
magnetized, discontinuities that lie in a
direction generally transverse to the Liquid Penetrant Testing
direction of the magnetic field will cause a
magnetic flux leakage field to be formed Principles. Liquid penetrant testing (Fig. 6)
at and above the surface of the test object. reveals discontinuities open to the
The presence of this leakage field and surfaces of solid and nonporous materials.
therefore the presence of the Indications of a wide variety of
discontinuity is detected with fine discontinuity sizes can be found regardless
ferromagnetic particles applied over the of the configuration of the test object and
surface, with some of the particles being regardless of discontinuity orientations.
gathered and held to form an outline of Liquid penetrants seep into various types
the discontinuity. This generally indicates of minute surface openings by capillary
its location, size, shape and extent. action. The cavities of interest can be very
Magnetic particles are applied over a small, often invisible to the unaided eye.
The ability of a given liquid to flow over a
FIGURE 4. Visual test using borescope to surface and enter surface cavities depends
view interior of cylinder. on the following: cleanliness of the
surface, surface tension of the liquid,
configuration of the cavity, contact angle
of the liquid, ability of the liquid to wet
the surface, cleanliness of the cavity and
size of the surface opening of the cavity.
Applications. The principal industrial uses
of liquid penetrant testing include
postfabrication testing, receiving testing,
in-process testing and quality control,
testing for maintenance and overhaul in
the transportation industries, in-plant and
machinery maintenance testing and
testing of large components. The
following are some of the typically
detected discontinuities: surface
discontinuities, seams, cracks, laps,
porosity and leak paths.
FIGURE 6. Liquid penetrant indication of
cracking.
FIGURE 5. Test object demonstrating
magnetic particle method.
8 Visual Testing
Eddy Current Testing wavelength or particulate radiation
(X-rays, gamma rays and neutrons).
Principles. Based on electromagnetic Different portions of an object absorb
induction, eddy current testing is perhaps different amounts of penetrating radiation
the best known of the techniques in the because of differences in density and
electromagnetic test method. Eddy variations in thickness of the test object
current testing is used to identify or or differences in absorption characteristics
differentiate among a wide variety of caused by variation in composition. These
physical, structural and metallurgical variations in the attenuation of the
conditions in electrically conductive penetrating radiation can be monitored
ferromagnetic and nonferromagnetic by detecting the unattenuated radiation
metals and metal test objects. The method that passes through the object.
is based on indirect measurement and on
correlation between the instrument This monitoring may be in different
reading and the structural characteristics forms. The traditional form is through
and serviceability of the test objects. radiation sensitive film. Radioscopic
sensors provide digital images. X-ray
With a basic system, the test object is computed tomography is a
placed within or next to an electric coil in three-dimensional, volumetric
which high frequency alternating current radiographic technique.
is flowing. This excitation current
establishes an electromagnetic field Applications. The principal industrial uses
around the coil. This primary field causes of radiographic testing involve testing of
eddy currents to flow in the test object castings and weldments, particularly
because of electromagnetic induction
(Fig. 7). Inversely, the eddy currents FIGURE 7. Electromagnetic testing:
affected by characteristics (conductivity, (a) representative setup for eddy current
permeability, thickness, discontinuities test; (b) inservice detection of
and geometry) of the test object create a discontinuities.
secondary magnetic field that opposes the
primary field. This interaction affects the (a) Primary Direction of
coil impedance and can be displayed in primary alternating
various ways. electromagnetic current
field
Eddy currents flow in closed loops in
the test object. Their two most important Coil in
characteristics, amplitude and phase, are eddy current
influenced by the arrangement and
characteristics of the instrumentation and probe
test object. For example, during the test of
a tube, the eddy currents flow Induced field
symmetrically in the tube when
discontinuities are not present. However, Induced field
when a crack is present, then the eddy
current flow is impeded and changed in Direction of Conducting
direction, causing significant changes in eddy current test object
the associated electromagnetic field.
Eddy current intensity
Applications. An important industrial use decreases with
of eddy current testing is on heat
exchanger tubing. For example, eddy increasing depth
current testing is often specified for thin
wall tubing in pressurized water reactors, (b)
steam generators, turbine condensers and
air conditioning heat exchangers. Eddy
current testing is also used in aircraft
maintenance. The following are some of
the typical material characteristics that
may affect conductivity and be evaluated
by eddy current testing: cracks, inclusions,
dents and holes; grain size; heat
treatment; coating and material thickness;
composition, conductivity or
permeability; and alloy composition.
Radiographic Testing
Principles. Radiographic testing (Fig. 8) is
based on the test object’s attenuation of
penetrating radiation — either
electromagnetic radiation of very short
Introduction to Visual Testing 9
where there is a critical need to ensure service and acoustic emission testing is
freedom from internal discontinuities. used because it gives valuable additional
Radiographic testing is often specified for information about the expected
thick wall castings and for weldments in performance of the structure under load.
steam power equipment (boiler and Other times, acoustic emission testing is
turbine components and assemblies). The selected for reasons of economy or safety
method can also be used on forgings and and loading is applied specifically for the
mechanical assemblies, although with acoustic emission test.
mechanical assemblies radiographic
testing is usually limited to testing for Applications. Acoustic emission is a
conditions and proper placement of natural phenomenon occurring in the
components. Radiographic testing is used widest range of materials, structures and
to detect inclusions, lack of fusion, cracks, processes. The largest scale events
corrosion, porosity, leak paths, missing or observed with acoustic emission testing
incomplete components and debris. are seismic; the smallest are microscopic
dislocations in stressed metals.
Acoustic Emission Testing
The equipment used is highly sensitive
Principles. Acoustic emissions are stress to any kind of movement in its operating
waves produced by sudden movement in frequency (typically 20 to 1200 kHz). The
stressed materials. The classic sources of equipment can detect not only crack
acoustic emission are crack growth and growth and material deformation but also
plastic deformation. Sudden movement at such processes as solidification, friction,
the source produces a stress wave that impact, flow and phase transformations.
radiates out into the test object and Therefore, acoustic emission testing is also
excites a sensitive piezoelectric sensor. As used for in-process weld monitoring, for
the stress in the material is raised, detecting tool touch and tool wear during
emissions are generated. The signals from automatic machining, for detecting wear
one or more sensors are amplified and and loss of lubrication in rotating
measured to produce data for display and equipment, for detecting loose parts and
interpretation. loose particles, for preservice proof testing
and for detecting and monitoring leaks,
The source of acoustic emission energy cavitation and flow.
is the elastic stress field in the material.
Without stress, there is no emission. Ultrasonic Testing
Therefore, an acoustic emission test
(Fig. 9) is usually carried out during a Principles. In ultrasonic testing (Fig. 10),
controlled loading of the test object. This beams of acoustic waves at a frequency
can be a proof load before service; a too high to hear are introduced into a
controlled variation of load while the material for the detection of surface and
structure is in service; a fatigue, pressure subsurface discontinuities. These acoustic
or creep test; or a complex loading waves travel through the material with
program. Often, a structure is going to be some energy loss (attenuation) and are
loaded hydrostatically anyway during reflected and refracted at interfaces. The
echoes are then analyzed to define and
FIGURE 8. Representative setup for locate discontinuities.
radiographic testing.
FIGURE 9. Acoustic emission monitoring of floor beam on
Radiation suspension bridge.
source
Test object
Void Sensor
Image plane Discontinuity
images
10 Visual Testing
Applications. Ultrasonic testing is widely pressurized components or into evacuated
used in metals, principally for thickness components. The principles of leak testing
measurement and discontinuity detection. involve the physics of liquids or gases
This method can be used to detect flowing through a barrier where a pressure
internal discontinuities in most differential or capillary action exists.
engineering metals and alloys. Bonds
produced by welding, brazing, soldering Leak testing encompasses procedures
and adhesives can also be ultrasonically that fall into these basic functions: leak
tested. In-line techniques have been location, leakage measurement and
developed for monitoring and classifying leakage monitoring. There are several
materials as acceptable, salvageable or subsidiary methods of leak testing,
scrap and for process control. Also tested entailing tracer gas detection (Fig. 11),
are piping and pressure vessels, nuclear pressure change measurement,
systems, motor vehicles, machinery, observation of bubble formation, acoustic
railroad stock and bridges. emission leak testing and other principles.
Leak Testing Applications. Like other forms of
nondestructive testing, leak testing affects
Principles. Leak testing is concerned with the safety and performance of a product.
the flow of liquids or gases from Reliable leak testing decreases costs by
reducing the number of reworked
FIGURE 10. Classic setups for ultrasonic products, warranty repairs and liability
testing: (a) longitudinal wave technique; claims. The most common reasons for
(b) transverse wave technique. performing a leak test are to prevent the
loss of costly materials or energy, to
(a) prevent contamination of the
environment, to ensure component or
Crack Back system reliability and to prevent an
surface explosion or fire.
Time
Bolt Infrared and Thermal Testing
Principles. Conduction, convection and
radiation are the primary mechanisms of
heat transfer in an object or system.
Electromagnetic radiation is emitted from
all bodies to a degree that depends on
their energy state.
Thermal testing involves the
measurement or mapping of surface
temperatures when heat flows from, to or
through a test object. Temperature
Transducer Crack FIGURE 11. Leakage measurement dynamic leak testing using
Crack vacuum pumping: (a) pressurized system mode for leak
(b) testing of smaller components; (b) pressurized envelope
mode for leak testing of larger volume systems.
(a)
Envelope
Leak detector
System
under test
Source of tracer gas
(b)
Envelope
Entry surface System
Crack under test
Leak detector
Source of tracer gas
Introduction to Visual Testing 11
differentials on a surface, or changes in Other Methods
surface temperature with time, are related
to heat flow patterns and can be used to There are many other methods of
detect discontinuities or to determine the nondestructive testing, including optical
heat transfer characteristics of an object. methods such as holography,
For example, during the operation of an shearography and moiré imaging; material
electrical breaker, a hot spot detected at identification methods such as chemical
an electrical termination may be caused spot testing, spark testing and
by a loose or corroded connection spectroscopy; strain gaging; and acoustic
(Fig. 12). The resistance to electrical flow methods such as vibration analysis and
through the connection produces an tapping.
increase in surface temperature of the
connection. FIGURE 12. Infrared thermography of
automatic transfer switches for emergency
Applications. There are two basic diesel generator. Hot spots appear bright in
categories of infrared and thermal test thermogram (inset).
applications: electrical and mechanical.
The specific applications within these two
categories are numerous.
Electrical applications include
transmission and distribution lines,
transformers, disconnects, switches, fuses,
relays, breakers, motor windings,
capacitor banks, cable trays, bus taps and
other components and subsystems.
Mechanical applications include
insulation (in boilers, furnaces, kilns,
piping, ducts, vessels, refrigerated trucks
and systems, tank cars and elsewhere),
friction in rotating equipment (bearings,
couplings, gears, gearboxes, conveyor
belts, pumps, compressors and other
components) and fluid flow (steam lines;
heat exchangers; tank fluid levels;
exothermic reactions; composite
structures; heating, ventilation and air
conditioning systems; leaks above and
below ground; cooling and heating; tube
blockages; environmental assessment of
thermal discharge; boiler or furnace air
leakage; condenser or turbine system
leakage; pumps; compressors; and other
system applications).
12 Visual Testing
PART 2. Management of Visual Testing
Selection of Visual Testing (5) applicability to irregular shapes,
(6) field mobility, (7) minimal training
Visual testing is an important method in requirements and (8) minimal equipment
the broad field of nondestructive testing. requirements.
Visual testing is used to locate surface
anomalies in most materials and Limitations
subsurface discontinuities in translucent
materials. Visual testing is performed Visual testing requires a line of sight to
either by a direct technique or by a the test surface and lighting adequate to
remote (that is, indirect) technique. One detect and interpret anomalies of interest.
definition of the direct technique is to Visual testing may be limited by
place the eye within 600 mm (24 in.) and component geometry: size, contour,
not less than 30 degrees from the test surface roughness, complexity and
surface. Mirrors may be used to improve discontinuity orientation. Remote visual
the angle of vision, and aids such as equipment may be required to access
magnifying lenses may be used to assist interior surfaces and remote equipment
examinations. The remote, or indirect, providing adequate viewing angles,
technique may include accessories such as sensitivity, resolution and illumination
mirrors, borescopes, video probes or may be costly. For proper interpretation of
cameras to correct for the distance or indications, the inspector needs skill with
angles of view. With a remote (indirect) the technique used, experience using the
technique, resolution must be equivalent visual equipment and knowledge of the
to that of the direct technique. test object.
Visual test equipment is designed to Management of Visual
detect structural characteristics of a part. Testing Programs
These characteristics range from simple
surface discontinuities on flat surfaces to Management of a visual testing program
various fabrication or inservice requires consideration of many items
discontinuities in complex geometries. before it can produce the desired results.
Some basic questions must be answered
As a result, specific applications have before a program can be implemented
been developed using visual testing: effectively.
detecting discontinuities in fabricated
structures such as airframes, piping and 1. Is the program needed?
pressure vessels, ships, bridges, motor 2. Are qualified personnel available?
vehicles and machinery and predicting 3. Are qualified and approved procedures
the impending failure in highly stressed
components exposed to the various in place? Are regulatory requirements
modes of fatigue. in place that mandate program
characteristics?
Advantages 4. What is the magnitude of the program
that will provide desired results?
The visual method is a sensitive means of 5. What provisions must be made for
locating surface anomalies in various personnel safety and for compliance
materials. There is little or no limitation with environmental regulations?
on the size or shape of the part being 6. What is the performance date for a
inspected. Indications provide a graphic program to be fully implemented?
representation of the actual discontinuity. 7. Is there a cost benefit of visual testing?
Precleaning may be necessary if the 8. What are the available resources in
surface cleanliness impairs an adequate material, personnel and money?
view of the test surface, but
discontinuities filled with foreign material Once these questions are answered,
may be detected. The need for precleaning then a recommendation can be made to
will largely depend on the size and type select the type of inspection agency. Three
of discontinuities specified by acceptance primary types of agencies responsible for
criteria. The following are the primary inspection are (1) service companies,
advantages typically associated with visual (2) consultants and (3) in-house programs.
testing: (1) economy, (2) speed,
(3) sensitivity, (4) versatility,
Introduction to Visual Testing 13
Although these are the main agency 7. Who will evaluate the consultant’s
types, some programs may, routinely or as performance (test reports, trending,
needed, require support personnel from a recommendations, root cause analysis
combination of two or more of these and other functions) within the
sources. Before a final decision is made, sponsoring company?
advantages and disadvantages of each
agency type must be considered. 8. Does the consultant possess
qualifications and certifications
Service Companies required by contract and by applicable
regulations?
Once a service company is selected,
responsibilities need to be defined. 9. Does the consultant require site
specific training (confined space entry,
1. Who will identify the components electrical safety, hazardous materials
within the facility to be examined? and others) or clearance to enter and
work in the facility?
2. Will the contract be for time and
materials or have a specific scope of 10. Does the consultant retain any
work? liability for test results?
3. If a time and materials contract is In-House Programs
awarded, who will monitor the time
and materials charged? 1. Who will determine the scope of the
program, such as which techniques
4. If a scope of work is required, who is will be used?
technically qualified to develop and
approve it? 2. What are the regulatory requirements
(codes and standards) associated with
5. What products or documents (test program development and
reports, trending, recommendations, implementation?
root cause analysis and others) will be
provided once the tests are completed? 3. Who will develop a cost benefit
analysis for the program?
6. Who will evaluate and accept the
product (test reports, trending, 4. How much time and what resources
recommendations, root cause analysis are available to establish the program?
and others) within the service
company? 5. What are the qualification
requirements (education, training,
7. Do the service company workers experience and others) for personnel?
possess qualifications and
certifications required by contract and 6. Do program personnel require
by applicable regulations? additional training (safety, confined
space entry or others) or
8. Do the service company workers qualifications?
require site specific training (confined
space entry, electrical safety, hazardous 7. Are subject matter experts required to
materials and others) or clearance to provide technical guidance during
enter and work in the facility? personnel development?
9. Does the service company retain any 8. Are procedures required to perform
liability for test results? work in the facility?
Consultants 9. If procedures are required, who will
develop, review and approve them?
1. Will the contract be for time and
materials or have a specific scope of 10. Who will determine the technical
work? specifications for test equipment?
2. If a scope of work is required, who is Visual Test Procedures
technically qualified to develop and
approve it? The conduct of test operations (in-house
or contracted) should be performed in
3. Who will identify the required accordance with specific instructions from
qualifications of the consultant? an expert. Specific instructions are
typically written as a technical procedure.
4. Is the purpose of the consultant to In many cases, codes and specifications
develop or update a program or is it to will require that a technical procedure be
oversee and evaluate the performance developed for each individual test. In
of an existing program? other cases, the same procedure is used
repeatedly.
5. Will the consultant have oversight
responsibility for tests performed? The procedure can take many forms. A
procedure may comprise general
6. What products or documents instructions that address only major
(trending, recommendations, root aspects of test techniques. Or a procedure
cause analysis and others) are provided may be written as a step-by-step process
once the tests are completed? requiring a supervisor’s or a
qualified/certified worker’s signature after
14 Visual Testing
each step. The following is a typical acceptance criteria and is required by the
format for an industrial procedure. designer, buyer or manufacturer of the
article it covers.
1. The purpose identifies the intent of the
procedure. Specifications are written to eliminate
variables of human operators and system
2. The scope establishes the items, tests designs, to produce an accurate result
and techniques covered and not regardless of who performs the visual test.
covered by the procedure. Specifications must be written with a full
knowledge of (1) visual test techniques,
3. References are specific documents from (2) a technique’s individual sensitivities,
which criteria are extracted or are (3) the test object design, (4) its material
documents satisfied by characteristics and (5) the discontinuities
implementation of the procedure. critical to the test object’s service life. In
most mature manufacturing applications,
4. Definitions are needed for terms and nondestructive tests are considered during
abbreviations that are not common design and such specifications are
knowledge to people who will read the specified on the test object’s original
procedure. drawing.
5. Statements about personnel requirements Visual specifications are produced to
address specific requirements to standardize test results, not to eliminate
perform tasks in accordance with the the initiative of the technician. There is
procedure — issues such as personnel no substitute for an experienced inspector
qualification, certification and access who assumes personal responsibility for
clearance. the quality and accuracy of the test.
6. Calibration requirements and model Testing specifications are working
numbers of qualified equipment must documents that tell how to locate
be specified. discontinuities in a specific test object.
Even well established and successful
7. The test procedure provides a sequential specifications need periodic review and
process to be used to conduct test revision. It is very important that relevant
activities. knowledge of field proven techniques and
advances in inspection technologies be
8. A system performance check is needed incorporated as quickly as possible into
before a test. The check might be daily industry specifications.
or detailed.
Interpretation
9. Acceptance criteria establish component
characteristics that will identify the Interpretation may be complex, especially
items suitable for service (initial use or before a procedure has been established.
continued service). The interpreter must have a knowledge of
the following: (1) the underlying physical
10. Reports (records) document specific test process, (2) techniques and equipment,
techniques, equipment used, (3) details about the test object
personnel, activity, date performed (configuration, material properties,
and test results. fabrication process, potential
discontinuities and anticipated service
11. Attachments may include (if required) conditions) and (4) possible sources of
items such as report forms, instrument false indications that might be mistaken
calibration forms, qualified equipment for meaningful visual indications.
matrix, schedules and others.
After interpretation, acceptance criteria
Once the procedure is written, an and rejection criteria are applied in a
expert in the subject evaluates it. If the phase called evaluation.
procedure meets requirements, the expert
will approve it for use. Some codes and Reliability of Test Results
standards also require the procedure to be
qualified — that is, demonstrated to the When a test is performed, there are four
satisfaction of a representative of a possible outcomes: (1) a rejectable
regulatory body or jurisdictional discontinuity can be found when one is
authority. present, (2) a rejectable discontinuity can
be missed even when one is present, (3) a
Visual Test Specifications4 rejectable discontinuity can be indicated
when none is present and (4) no
A visual test specification must anticipate rejectable discontinuity is found when
issues that arise during testing. A none is present. A reliable testing process
specification is specific to a component or and a qualified inspector should find all
product and may be tailored to comply discontinuities of concern with no
with one or more standards. A discontinuities missed (no errors as in case
specification can require more stringent 2 above) and no false calls (case 3 above).
limits than the standard(s) it was written
to satisfy. In practice, a specification
provides a list of testing parameters that
describes the techniques for locating and
categorizing discontinuities in a specific
test object. A typical specification includes
Introduction to Visual Testing 15
To approach this goal, the probability of Personnel Qualification
finding a rejectable discontinuity must be and Certification
high and the inspector must be both
proficient in the testing process and One of the most critical aspects of the test
motivated to perform with maximum process is the qualification of testing
efficiency. An ineffective inspector may personnel. Nondestructive testing is
accept test objects that contain sometimes referred to as a special process,
discontinuities, with the result of possible special in that it is difficult to determine
inservice part failure. The same inspector the adequacy of a test by merely
may reject parts that do not contain observing the process or the
rejectable discontinuities, with the result documentation it generates. The quality
of unnecessary scrap and repair. Neither of the test largely depends on the skills
scenario is desirable. and knowledge of the inspector.
Visual Test Standards The American Society for
Nondestructive Testing (ASNT) has been a
Traditionally, the purpose of specifications world leader in the qualification and
and standards has been to define the certification of nondestructive testing
requirements that goods or services must personnel since the 1960s. (Qualification
meet. As such, they are intended to be demonstrates that an individual has the
incorporated into contracts so that both required training, experience, knowledge
the buyer and provider have a well and abilities; certification provides written
defined description of what one will testimony that an individual is qualified.)
receive and the other will deliver. By the twenty-first century, the American
Society for Nondestructive Testing had
Standards have undergone a process of instituted three avenues and four major
peer review in industry and can be documents for the qualification and
invoked with the force of law by contract certification of nondestructive testing
or by government regulation. In contrast, personnel.
a specification represents an employer’s
instructions to employees and is specific 1. Recommended Practice
to a contract or workplace. Many a No. SNT-TC-1A, Personnel Qualification
specification originates as a detailed and Certification in Nondestructive
description either as part of a purchaser’s Testing, provides guidelines to
requirements or as part of a vendor’s offer. employers for personnel qualification
Specifications may be incorporated into and certification in nondestructive
standards through the normal review testing. This recommended practice
process. Standards and specifications exist identifies the attributes that should be
in three basic areas: equipment, processes considered when qualifying
and personnel. nondestructive testing personnel. It
requires the employer to develop and
1. Standards for visual equipment implement a written practice, a
include criteria that address surface procedure that details the specific
accessibility, sensitivity, degree of process and any limitation in the
magnification, field of view, depth of qualification and certification of
field, minimum lighting requirements nondestructive testing personnel.6
and other matters.
2. ANSI/ASNT CP-189, Standard for
2. ASTM International and other Qualification and Certification of
organizations publish standards for Nondestructive Testing Personnel,
test techniques. Some other standards resembles SNT-TC-1A but establishes
are for quality assurance procedures specific requirements for the
and are not specific to a test method qualification and certification of
or even to testing in general. Table 4 Level I and II nondestructive testing
lists standards used in visual testing. personnel. For Level III, CP-189
The United States Department of references an examination
Defense has replaced most military administered by the American Society
specifications and standards with for Nondestructive Testing. CP-189 is a
industry consensus specifications and consensus standard as defined by the
standards. A source for nondestructive American National Standards Institute
test standards is the Annual Book of (ANSI). It is recognized as the
ASTM Standards.5 American standard for nondestructive
testing. It is not considered a
3. Qualification and certification of recommended practice; it is a national
testing personnel are discussed below standard.7
with specific reference to
recommendations of ASNT
Recommended Practice
No. SNT-TC-1A.6
16 Visual Testing
TABLE 4. Some standards specifying visual testing. ASTM F 1236, Standard Guide for Visual Inspection of Electrical
Protective Rubber Products (2007).
American Concrete Institute
ASTM F 584, Standard Practice for Visual Inspection of Semiconductor
ACI 201.1R, Guide for Conducting a Visual Inspection of Concrete in Lead-Bonding Wire (2006).
Service (2008).
American Welding Society
American National Standards Institute
AWS B1.11, Guide for the Visual Examination of Welds (2000).
ANSI B3.2, Rolling Element Bearings — Aircraft Engine, Engine
Gearbox, and Accessory Applications — Surface Visual AWS D1.1M, Structural Welding Code — Steel (2008).
Inspection (1999).
AWS D8.1M, Specification for Automotive Weld Quality — Resistance
ANSI/EIA 699, Test Method for the Visual Inspection of Quartz Crystal Spot Welding of Steel (2007).
Resonator Blanks (1997).
AWS D18.2, Guide to Weld Discoloration Levels on Inside of Austenitic
American Petroleum Institute Stainless Steel Tube (1999).
API 5D, Specification for Drill Pipe (2001). AWS G1.6, Specification for the Qualification of Plastics Welding
Inspectors for Hot Gas, Hot Gas Extrusion, and Heated Tool Butt
API 5L, Specification for Line Pipe (2008). Thermoplastic Welds (2006).
API 570, Piping Inspection Code: Inspection, Repair, Alteration, and AWS QC1, Standard for AWS Certification of Welding
Rerating of In-Service Piping Systems (2006). Inspectors (2007).
API 620, Design and Construction of Large, Welded, Low-Pressure Association Connecting Electronics Industries
Storage Tanks (2008).
IPC-OI-645, Standard for Visual Optical Inspection Aids (1993).
API 650, Welded Tanks for Oil Storage (2007).
Compressed Gas Association
API RP-5A5 [ISO 15463-2003], Recommended Practice for Field
Inspection of New Casing, Tubing and Plain End Drill Pipe (2005). CGA C-13, Guidelines for Periodic Visual Inspection and
Requalification of Acetylene Cylinders (2006).
API RP-5L8, Recommended Practice for Field Inspection of New Line
Pipe (1996). CGA C-6, Standards for Visual Inspection of Steel Compressed Gas
Cylinders (2007).
API RP-7G, Recommended Practice for Drill Stem Design and Operating
Limits (2003). CGA C-6.1, Standards for Visual Inspection of High Pressure
Aluminum Compressed Gas Cylinders (2006).
API SPEC 5CT [ISO 11960], Specification for Casing and
Tubing (2006). CGA C-6.2, Guidelines for Visual Inspection and Requalification of
Fiber Reinforced High Pressure Cylinders (2005).
API SPEC 7, Specification for Rotary Drill Stem Elements (2008).
CGA C-6.3, Guidelines for Visual Inspection and Requalification of Low
API STD 1104, Welding of Pipelines and Related Facilities (2005). Pressure Aluminum Compressed Gas Cylinders (1999).
API STD 5T1, Imperfection Terminology (2003). CGA C-6.4, Methods for External Visual Inspection of Natural Gas
Vehicle (NGV) and Hydrogen Vehicle (HV) Fuel Containers and Their
API STD 653, Tank Inspection, Repair, Alteration, and Installations (2007).
Reconstruction (2008).
European Committee for Standardization
ASME International
CEN EN 13508 [DIN 13508] P2, Conditions of Drain and Sewer
ASME Boiler and Pressure Vessel Code: Section I, Rules for Construction Systems Outside Buildings — Part 2: Visual Inspection Coding
of Power Boilers (2007). System (2007).
ASME Boiler and Pressure Vessel Code: Section III, Rules for CEN EN 13018 [BS 13018], Non-Destructive Testing — Visual Testing
Construction of Nuclear Power Plant Components (2007). — General Principles (2007).
ASME Boiler and Pressure Vessel Code: Section IV, Rules for CEN EN 13100-1 [BS 13100-1], Non-Destructive Testing of Welded
Construction of Heating Boilers (2007). Joints of Thermoplastics Semi-Finished Products — Part 1: Visual
Examination (2000).
ASME Boiler and Pressure Vessel Code: Section V, Nondestructive
Examination. Article 9, Visual Examination (2009). CEN EN 3841-201 [BS 3841-201], Circuit Breakers — Test Methods
— Part 201, Visual Inspection (2005).
ASME Boiler and Pressure Vessel Code: Section VI, Recommended Rules
for the Care and Operation of Heating Boilers (2007). Federal Aviation Administration
ASME Boiler and Pressure Vessel Code: Section VII, Recommended FAA AC 43-204, Visual Inspection for Aircraft (1997).
Guidelines for the Care of Power Boilers (2007).
International Electrotechnical Commission
ASME Boiler and Pressure Vessel Code: Section VIII, Rules for
Construction of Pressure Vessels (Divisions 1, 2 and 3) (2007). IEC 60748-23-2, Semiconductor Devices — Integrated Circuits —
PART 23-2: Hybrid Integrated Circuits and Film Structures —
ASME Boiler and Pressure Vessel Code: Section X, Fiber Reinforced Manufacturing Line Certification – Internal Visual Inspection and
Plastic Pressure Vessels (2007). Special Tests (2002).
ASME Boiler and Pressure Vessel Code: Section XI, Rules for Inservice International Organization for Standardization
Inspection of Nuclear Power Plant Components (2007).
ISO 11960 [API SPEC 5CT], Petroleum and Natural Gas Industries —
ASME Boiler and Pressure Vessel Code: Section XII, Rules for Steel Pipes for Use as Casing or Tubing for Wells (2006).
Construction and Continued Service of Transport Tanks (2007).
ISO 17637, Non-Destructive Testing of Welds — Visual Testing of
ASME B 31.1, Power Piping (2007). Fusion-Welded Joints (2003).
ASME B 31.3, Process Piping (2008). ISO 3058, Non-Destructive Testing — Aids to Visual Inspection —
Selection of Low-Power Magnifiers (1998).
ASME B 31.4, Pipeline Transportation Systems for Liquid Hydrocarbons
and Other Liquids (2006). Japanese Institute of Standards
ASME B 31.5, Refrigeration Piping and Heat Transfer JIS H 0613, Non-Ferrous Metals and Metallurgy — Visual Inspection
Components (2006). for Sliced and Lapped Silicon Wafers (1978).
ASME B 31.8, Gas Transmission and Distribution Piping JIS H 0614, Non-Ferrous Metals and Metallurgy — Visual Inspection
Systems (2007). for Silicon Wafers with Specular Surfaces (1996).
ASTM International JIS Z 3090, Visual Testing Method of Fusion-Welded Joints (2005).
ASTM A 802M, Standard Practice for Steel Castings, Surface Manufacturers Standardization Society
Acceptance Standards, Visual Examination (2006).
MSS SP-55, Quality Standard for Steel Castings for Valves, Flanges
ASTM D 2562, Standard Practice for Classifying Visual Defects in Parts and Fittings and Other Piping Components — Visual Method for
Molded from Reinforced Thermosetting Plastics (2008). Evaluation of Surface Irregularities (2006).
ASTM D 2563, Standard Practice for Classifying Visual Defects in South African Bureau of Standards
Glass-Reinforced Plastic Laminate Parts (2008).
SAA AS 3978, Non-Destructive Testing — Visual Inspection of Metal
ASTM D 4385, Standard Practice for Classifying Visual Defects in Products and Components (2003).
Thermosetting Reinforced Plastic Pultruded Products (2008).
SAA AS/NZS 3894.8, Surface Treatment and Coating — Site Testing
ASTM E 1799, Standard Practice for Visual Inspections of Photovoltaic of Protective Coatings — Visual Determination of Gloss (2006).
Modules (1999).
Introduction to Visual Testing 17
3. ANSI/ASNT CP-105, ASNT Standard acceptability of materials or components in
Topical Outlines for Qualification of accordance with the applicable codes,
Nondestructive Testing Personnel, is a standards, specifications and procedures. …
standard that establishes the
minimum topical outline Education, Training, and Experience
requirements for the qualification of Requirements for Initial Qualification …
nondestructive testing (NDT) Candidates for certification in NDT should
personnel. The outlines in this single have sufficient education, training, and
standard are referenced by both experience to ensure qualification in those
SNT-TC-1A and CP-189. CP-105 is a NDT methods in which they are being
consensus standard of the American considered for certification. … Table 6.3.1A
National Standards Institute (ANSI) [see Table 5 in this Nondestructive Testing
and is recognized as an American Handbook chapter, for visual testing] lists
standard for nondestructive testing. It recommended training and experience
is not considered a recommended factors to be considered by the employer in
practice; it is a national standard.8 establishing written practices for initial
qualification of Level I and Level II
4. The ASNT Central Certification individuals. …
Program (ACCP), unlike SNT-TC-1A
and CP-189, is a third party Training Programs … Personnel being
certification process that identifies considered for initial certification should
qualification and certification complete sufficient organized training to
attributes for Level II and Level III become thoroughly familiar with the
nondestructive testing personnel. The principles and practices of the specified
American Society for Nondestructive NDT method related to the level of
Testing certifies that the individual has certification desired and applicable to the
the skills and knowledge for many processes to be used and the products to be
nondestructive test method tested. …
applications. It does not remove the
responsibility for the final Examinations … For Level I and II
determination of personnel personnel, a composite grade should be
qualification from the employer. The determined by simple averaging of the
employer evaluates an individual’s results of the general, specific and practical
skills and knowledge for application of examinations … Examinations
company procedures using designated administered for qualification should result
techniques and equipment identified in a passing composite grade of at least
for specific tests. ACCP is not a 80 percent, with no individual examination
standard or recommended practice; it having a passing grade less than
is a service administered by the 70 percent. …
American Society for Nondestructive
Testing.9 Practical [Examination] (for NDT Level I
and II) … The candidate should
Excerpts from Recommended demonstrate … ability to operate the
Practice No. SNT-TC-1A necessary NDT equipment, record, and
analyze the resultant information to the
To give an idea of the contents of these degree required. ... At least one flawed
documents, the following items are specimen should be tested and the results
excerpted from Recommended Practice of the NDT analyzed by the candidate. …
No. SNT-TC-1A.6 The original text is
arranged in outline format and includes Certification … Certification of all levels of
recommendations that are not specific to NDT personnel is the responsibility of the
visual testing. employer. … Certification of NDT
personnel shall be based on demonstration
Scope … This Recommended Practice has of satisfactory qualification in accordance
been prepared to establish guidelines for with [sections on education, training,
the qualification and certification of NDT experience and examinations] as described
personnel whose specific jobs require in the employer’s written practice. …
appropriate knowledge of the technical Personnel certification records shall be
principles underlying the nondestructive maintained on file by the employer. …
tests they perform, witness, monitor, or
evaluate. … This document provides Recertification … All levels of NDT
guidelines for the establishment of a personnel shall be recertified periodically in
qualification and certification program. … accordance with one of the [following:]
continuing satisfactory technical
Written Practice … The employer shall performance [or reexamination] in those
establish a written practice for the control portions of the examinations … deemed
and administration of NDT personnel necessary by the employer’s NDT Level III.
training, examination, and certification. … … Recommended maximum recertification
The employer’s written practice should intervals are 5 years for all certification
describe the responsibility of each level of levels.
certification for determining the
These recommendations from the 2006
edition of Recommended Practice
No. SNT-TC-1A are cited only to provide
an idea of items that must be considered
in the development of an in-house
nondestructive testing program. Because
the text above is excerpted, those
developing a personnel qualification
18 Visual Testing
program should consult the complete text current testing; infrared thermographic
of SNT-TC-1A and other applicable testing; leak testing (hydraulic pressure
procedures and practices. If an outside tests excluded); magnetic particle testing;
agency is contracted for visual test penetrant testing; radiographic testing;
services, then the contractor must have a strain testing; ultrasonic testing; visual
qualification and certification program to testing (direct unaided visual tests and
satisfy the codes and standards in force. visual tests carried out during the
application of another NDT method are
The minimum number of questions excluded).”
that should be administered in the
written examination for visual test The International Organization for
personnel is as follows: 40 questions in Standardization also publishes a standard
the general examination and 20 questions for something called limited certification.11
in the specific examination. The number Inspectors whose actions are limited
of questions is the same for Level I and sometimes have limited training
Level II personnel. Table 5 shows required requirements. Limited certification would
hours of training for Level I and Level II. not be applicable to visual testing
inspectors in the field, for example, but
Central Certification may be desired for assembly line operators
of remote visual testing equipment to
Another standard that may be a source for detect debris inside fabricated products.
compliance is published by the
International Organization for Safety in Visual Testing12
Standardization (ISO). The work of
preparing international standards is This information is presented solely for
normally carried out through technical educational purposes and should not be
committees of this worldwide federation consulted in place of current safety
of national standards bodies. Each ISO regulations. Note that units of measure
member body interested in a subject for have been converted to this book’s format
which a technical committee has been and are not those commonly used in all
established has the right to be represented industries. Human vision can be disrupted
on that committee. International or destroyed by improper use of any
organizations, governmental and radiation, including light. Consult the
nongovernmental, in liaison with the most recent safety documents and the
International Organization for manufacturer’s literature before working
Standardization, also take part in the near any radiation source.
work.
Need for Safety
Technical Committee ISO/TC 135,
Non-Destructive Testing Subcommittee Developments in optical testing
SC 7, Personnel Qualification, prepared technology have created a need for better
international standard ISO 9712, understanding of the potential health
Non-Destructive Testing — Qualification and hazards caused by high intensity light
Certification of Personnel.10 In its statement sources or by artificial light sources of any
of scope, ISO 9712 states that it “specifies intensity in the work area. The human
the qualification and certification of eye operates optimally in an environment
personnel involved in non-destructive illuminated directly or indirectly by
testing ... in one or more of the following sunlight, with characteristic spectral
methods: acoustic emission testing; eddy distribution and range of intensities that
are very different from those of most
TABLE 5. Recommended training and experience (in artificial sources. The eye can handle only
hours) for visual testing personnel according to a limited range of night vision tasks.
Recommended Practice No. SNT-TC-1A.6
Evidence has accumulated that
Level I Level II photochemical changes occur in eyes
under the influence of normal daylight
High school graduatea 8h 16 h illumination — short term and long term
Two years of collegeb 4h 8h visual impairment and exacerbation of
Work experience in methodc 70 h retinal disease have been observed and it
Total hours in nondestructive testing 130 h 140 h is important to understand why this
270 h occurs. Periodic fluctuations of visible and
ultraviolet radiation occur with the
a. Or equivalent. regular diurnal light-and-dark cycles and
with the lengthening and shortening of
b. Completion with a passing grade of at least two years of engineering or the cycle as a result of seasonal changes.
science study in a university, college or technical school. These fluctuations are known to affect all
biological systems critically. The majority
c. Minimum work experience in method, per level. Note: For Level II of such light/dark effects is based on
certification, the experience shall consist of time as Level I or equivalent. circadian cycles and controlled by the
If a person is being qualified directly to Level II with no time at Level I,
the required experience shall consist of the sum of the times required for
Level I and Level II and the required training shall consist of the sum of
the hours required for Level I and Level II.
Introduction to Visual Testing 19
pineal system, which can be affected awareness of the hazard from high
directly by the transmission of light to the intensity noncoherent visible sources
pineal gland or indirectly by effects on which may be as great or greater.
the optic nerve pathway. Generally, lasers are used in specialized
environments by technicians familiar
Also of concern are the results of work with the hazards and trained to avoid
that has been done demonstrating that exposure by the use of protective eyewear
light affects immunological reactions in and clothing. Laser standards of
vitro and in vivo by influencing the manufacture and use have been well
antigenicity of molecules, antibody developed and probably have contributed
function and the reactivity of more than anything else to a heightened
lymphocytes. awareness of safe laser operation.
Given the variety of visual tasks and Laser hazard controls are common
illumination that confronts the visual sense procedures designed to (1) restrict
inspector, it is important to consider personnel from entering the beam path
whether failures in performance might be and (2) limit the primary and reflected
a result of excessive exposure to light or beams from occupied areas. Should an
other radiation or even a result of individual be exposed to excessive laser
insufficient light sources. A myth exists light, the probability of damage to the
that 20/20 foveal vision, in the absence of retina is high because of the high energy
color blindness, is all that is necessary for pulse capabilities of some lasers. However,
optimal vision. In fact, there may be the probability of visual impairment is
visual field loss in and beyond the fovea relatively low because of the small area of
centralis for many reasons; the inspector damage on the retina. Once the initial
may have poor stereoscopic vision; visual flash blindness and pain have subsided,
ability may be impaired by glare or the resulting scotomas (damaged
reflection; or actual vision may be affected unresponsive areas) can sometimes be
by medical or psychological conditions. ignored by the accident victim.
Visual Safety Recommendations The tissue surrounding the absorption
site can much more readily conduct away
The American Conference of heat for small image sizes than it can for
Governmental Industrial Hygienists large image sizes. In fact, retinal injury
(ACGIH) has proposed two threshold limit thresholds for less than 0.1 to 10 s
values (TLVs) for noncoherent visible exposure show a high dependence on the
light, one covering damage to the retina image size, 0.01 to 0.1 W·mm–2 for a
by a thermal mechanism and one 1000 µm wide image up to about
covering retinal damage by a 0.01 kW·mm–2 for a 20 µm image. In
photochemical mechanism. Threshold contrast, the sun produces merely a
limit values for visible light, established 160 µm diameter image on the retina.
by the American Conference of
Governmental Industrial Hygienists, are Consensus standards provide guidance
intended only to prevent excessive for the safe use of lasers.15,16
occupational exposure and are limited to
exposure durations of 8 h or less. They are High Luminance Light Sources
not intended to cover photosensitive
individuals.13,14 The normal reaction to a high luminance
light source is to blink and look away
Laser Hazards from the source. The probability of
overexposure to noncoherent light
Loss of vision resulting from retinal burns sources is higher than the probability of
following observation of the sun has been exposure to lasers, yet extended (high
described throughout history. Common luminance) sources are used in a more
technological equivalents to this problem casual and possibly more hazardous way.
are coherent light sources: lasers. In In the nondestructive testing industry,
addition to the development of lasers, extended sources are used as general
further improvement in other high illumination and in many specialized
radiance light sources (a result of smaller, applications. Unfortunately, there are
more efficient reflectors and more comparatively few guidelines for the safe
compact, brighter sources) has presented use of extended sources of visible light.
the potential for chorioretinal injury. It is
thought that chorioretinal burns from Infrared Hazards
artificial sources in industrial situations
have been very much less frequent than Infrared radiation comprises that invisible
similar burns from the sun. radiation beyond the red end of the
visible spectrum up to about 1 mm
Because of the publicity of the health wavelength. Infrared is absorbed by many
hazard caused by exposure to laser substances and its principal biological
radiation, awareness of such hazards is effect is known as hyperthermia, heating
probably much greater than the general that can be lethal to cells. Usually, the
20 Visual Testing
response to intense infrared radiation is with ultraviolet: the source is enclosed
pain and the natural reaction is to move and provided with ultraviolet absorbing
away from the source so that burns do glass or plastic lenses. If injurious effects
not develop. continue to develop, the thickness of the
protective lens is increased.
Ultraviolet Hazards
The photochemical effects of
Before development of the laser, the ultraviolet radiation on the skin and eye
principal hazard in the use of intense are still not completely understood.
light sources was the potential eye and Records of ultraviolet radiation’s relative
skin injury from ultraviolet radiation. spectral effectiveness for eliciting a
Ultraviolet radiation is invisible radiation particular biological effect (referred to by
beyond the violet end of the visible photobiologists as action spectra) are
spectrum with wavelengths down to generally available. Ultraviolet irradiance
about 185 nm. It is strongly absorbed by may be measured at a point of interest
the cornea and the lens of the eye. with a portable radiometer and compared
Ultraviolet radiation at wavelengths with the ultraviolet radiation hazard
shorter than 185 nm is absorbed by air, is criteria.
often called vacuum ultraviolet and is
rarely of concern to the visual inspector. For purposes of determining exposure
Many useful high intensity arc sources levels, it is important to note that most
and some lasers may emit associated, inexpensive, portable radiometers are not
potentially hazardous, levels of ultraviolet equally responsive at all wavelengths
radiation. With appropriate precautions, throughout the ultraviolet spectrum and
such sources can serve very useful visual are usually only calibrated at one
testing functions. wavelength with no guarantees at any
other wavelength. Such radiometers have
Studies have clarified the spectral been designed for a particular application
radiant exposure doses and relative using a particular lamp.
spectral effectiveness of ultraviolet
radiation required to elicit an adverse A common example in the
biological response. These responses nondestructive testing industry is the
include keratoconjunctivitis (known as ultraviolet radiometer used in fluorescent
welder’s flash), possible generation of liquid penetrant and magnetic particle
cataracts and erythema or reddening of applications. These meters are usually
the skin. Longer wavelength ultraviolet calibrated at 365 nm, the predominant
radiation can lead to fluorescence of the ultraviolet output of the filtered 100 W
eye’s lens and ocular media, eyestrain and medium pressure mercury vapor lamp
headache. These conditions lead, in turn, commonly used in the industry. Use of
to low task performance resulting from the meter at any other wavelength in the
the fatigue associated with increased ultraviolet spectrum may lead to
effort. Chronic exposure to ultraviolet significant errors. To minimize problems
radiation accelerates skin aging and in assessing the hazard presented by
possibly increases the risk of developing industrial lighting, it is important to use a
certain forms of skin cancer. radiometer that has been calibrated with
an ultraviolet spectral distribution as close
It should also be mentioned that some as possible to the lamp of interest.
individuals are hypersensitive to
ultraviolet radiation and may develop a If the inspector is concerned about the
reaction following, what would be for the safety of a given situation, ultraviolet
average healthy human, suberythemal absorbing eye protection and facewear is
exposures. However, it is unusual for these readily available from several sources. An
symptoms of exceptional photosensitivity additional benefit of such protection is
to be elicited solely by the limited that it prevents the annoyance of lens
emission spectrum of an industrial light fluorescence and provides the wearer
source. An inspector is typically aware of considerable protection from all
such sensitivity because of earlier ultraviolet radiation. In certain
exposures to sunlight. applications, tinted lenses can also
provide enhanced visibility of the test
In industry, the visual inspector may object.
encounter many sources of visible and
invisible radiation: incandescent lamps, Damage to Retina
compact arc sources (solar simulators),
quartz halogen lamps, metal vapor Although ultraviolet radiation from most
(sodium and mercury) and metal halide of the high intensity visible light sources
discharge lamps, fluorescent lamps and may be the principal concern, the
flash lamps among others. Because of the potential for chorioretinal injury from
high ultraviolet attenuation afforded by visible radiation should not be
many visually transparent materials, an overlooked.
empirical approach is sometimes taken for
the problem of light sources associated It is possible to multiply the spectral
absorption data of the human retina by
the spectral transmission data of the eye’s
Introduction to Visual Testing 21
optical media at all wavelengths to arrive central (foveal) vision, so that the loss of
at an estimate of the relative absorbed this retinal area dramatically reduces
spectral dose in the retina and the visual capabilities. In comparison, the loss
underlying choroid for a given spectral of an area of similar size located in the
radiant exposure of the cornea. In peripheral retina could be subjectively
practice, the evaluation of potential unnoticed.
chorioretinal burn hazards depends on
the maximum luminance and spectral The human retina is normally
distribution of the source; possible retinal subjected to irradiances below
image sizes; the image quality; pupil size; 1 µW·mm–2, except for occasional
spectral scattering and absorption by the momentary exposures to the sun, arc
cornea, aqueous humor, the lens and the lamps, quartz halogen lamps, normal
vitreous humor; and absorption and incandescent lamps, flash lamps and
scattering in the various retinal layers. similar radiant sources. The natural
aversion or pain response to bright lights
Calculation of the permissible normally limits exposure to no more than
luminance from a permissible retinal 0.15 to 0.2 s. In some instances,
illuminance for a source breaks down for individuals can suppress this response
very small retinal image sizes or for very with little difficulty and stare at bright
small hot spots in an extended image sources, as commonly occurs during solar
caused by diffraction of light at the pupil, eclipses.
aberrations introduced by the cornea and
lens and scattering from the cornea and Fortunately, few arc sources are
the rest of the ocular media. Because the sufficiently large and sufficiently bright to
effects of aberration increase with be a retinal burn hazard under normal
increasing pupil size, greater blur and viewing conditions. Only when an arc or
reduced peak retinal illuminance are hot filament is greatly magnified (in an
noticed for larger pupil sizes and for a optical projection system, for example)
given corneal illumination. can hazardous irradiance be imaged on a
sufficiently large area of the retina to
Thermal Factor cause a burn. Visual inspectors do not
normally step into a projected beam at
Visible and near infrared radiation up to close range or view a welding arc with
about 1400 nm (associated with most binoculars or a telescope.
optical sources) is transmitted through the
eye’s ocular media and absorbed in Nearly all conceivable accident
significant doses principally in the retina. situations require a hazardous exposure to
These radiations pass through the neural be delivered within the period of a blink
layers of the retina. A small amount is reflex. If an arc is struck while an
absorbed by the visual pigments in the inspector is located at a very close viewing
rods and cones, to initiate the visual range, it is possible that a retinal burn
response, and the remaining energy is could occur. At lower exposures, an
absorbed in the retinal pigment inspector experiences a short term
epithelium and choroid. The retinal depression in photopic (daylight)
pigment epithelium is optically the most sensitivity and a marked, longer term loss
dense absorbent layer (because of high of scotopic (dark adapted) vision. That is
concentrations of melanin granules) and why it is so important for visual
the greatest temperature changes arise in inspectors in critical fluorescent penetrant
this layer. and magnetic particle test environments
to undergo dark adaptation before
For short (0.1 to 100 s) accidental actually attempting to find
exposures to the sun or artificial radiation discontinuities. Not only does the pupil
sources, the mechanism of injury is have to adapt to the reduced visible level
generally thought to be hyperthermia in a booth but the actual retinal receptors
resulting in protein denaturation and must attain maximum sensitivity. This
enzyme inactivation. Because the large, effect may take half an hour or more,
complex organic molecules absorbing the depending on the preceding state of the
radiant energy have broad spectral eye’s adaptation.
absorption bands, the hazard potential for
chorioretinal injury is not expected to Blue Hazard
depend on the coherence or
monochromaticity of the source. Injury The so-called blue hazard function has
from a laser or a nonlaser radiation source been used with the thermal factor to
should not differ if image size, exposure calculate exposure durations, to avoid
time and wavelength are the same. damaging the retina.
Because different regions of the retina The blue hazard is based on the
play different roles in vision, the demonstration that the retina can be
functional loss of all or part of one of damaged by blue light at intensities that
these regions varies in significance. The do not elevate retinal temperatures
greatest vision acuity exists only for sufficiently to cause a thermal hazard. It
has been found that blue light can
22 Visual Testing
produce 10 to 100 times more retinal protection and other hazard controls have
damage (permanent decrease in spectral been provided on this basis.
sensitivity in this spectral range) than
longer visible wavelengths. Note that Eye protection filters for various
there are some common situations in workers were developed empirically but
which both thermal and blue hazards now are standardized as shades and
may be present. specified for particular applications.
Photosensitizers Other protective techniques include
use of high ambient light levels and
Over the past few decades, a large number specialized filters to further attenuate
of commonly used drugs, food additives, intense spectral lines. Laser eye protection
soaps and cosmetics have been identified is designed to have an adequate optical
as phototoxic or photoallergenic agents density at the laser wavelengths along
even at the longer wavelengths of the with the greatest visual transmission at all
visible spectrum.17 Colored drugs and other wavelengths.
food additives are possible
photosensitizers for organs below the skin Always bear in mind that hazard
because longer wavelength visible criteria must not be considered to
radiations penetrate deeply into the body. represent fine lines between safe and
hazardous exposure conditions. To be
Eye Protection Filters properly applied, interpretation of hazard
criteria must be based on practical
Because continuous visible light sources knowledge of potential exposure
elicit a normal aversion or pain response conditions and the user, whether a
that can protect the eye and skin from professional inspector or a general
injury, visual comfort has often been used consumer. Accuracy of hazard criteria is
as an approximate hazard index and eye limited by biological uncertainties
including diet, genetic photosensitivity
and the large safety factors required to be
built into the recommendations.
Introduction to Visual Testing 23
PART 3. History of Visual Testing
Optics FIGURE 13. Ibn Sahl’s tenth century description of diffraction:
(a) manuscript; (b) simplified enlargement of upper left
Early physicists offered explanations of corner.25
vision and light that have informed later
understanding and made possible the (a)
development of optical devices: sextants,
corrective eyewear, periscopes, telescopes, (b)
microscopes, cameras and borescopes.
These scientists offered mathematical BA E
proofs of optical principles, including
perspective, reflection and refraction. C
1. In perspective, a near object appears D
larger than a distant object of the
same size. Legend
A. Light source.
2. In reflection, light bounces off a B. Point where extension of line CD meets extension of line AE.
surface. If the surface is shiny, the C. Point on illuminated surface.
viewer sees a reversed, or mirror, image D. Point in line of refracted ray of light.
and the shiny surface is called specular, E. Point on surface CE such that AEC forms right angle.
from the Latin speculum, “mirror.”
3. Refraction bends the path of light as it
moves from one medium into another,
for example, from air into water.
Refraction makes it possible for a
convex lens to magnify an image.
With these concepts about the nature of
light were others — for example, that
light travels in a straight line and that it
does not emanate from the viewer’s eye.
The optical principles were not merely
explained but were proven
mathematically. For this reason, the pages
of early optical treatises have diagrams
like those in modern geometry books.
Greeks
The word optics comes from the Greek
word o∆ptikh,√ optike, “sight.” For the
Greeks, optics was part of the study of
geometry. In Greek, the word geometry
literally means “earth measurement.”
Geometry was a practical science, used to
calculate distances and estimate the
height of objects.
Writing around BCE 300, Euclid, a
Greek, wrote a mathematical treatise that
has dominated geometry for more than
2000 years. He also wrote Optics, a treatise
that described behaviors of light,
including perspective.18
Ptolemy, who lived in Alexandria in
the second century, also touched on
optical principles in his exhaustive
astronomical treatise, called the Almagest,
“great work.”19
After the Roman Empire, Europe
entered a period often called the Dark
24 Visual Testing
Ages, when much ancient learning was burning targets at a distance. Ibn Sahl
lost. Some Greek philosophy survived departed from his predecessors in
because it had been translated into Arabic. studying reflection and refraction of the
Much later, the works of Ptolemy and Sun’s rays. The interest in refraction led
Aristotle were translated from Arabic into him to the study of lenses and their
Latin and so came to European scientists shapes in great detail. In these studies, Ibn
such as Roger Bacon and Johannes Sahl discovered the relationship between
Kepler.20 the incident and refracted rays of light,
the relationship rediscovered by
Medieval Arab Optics Willebrord Snellius some 650 years later
and now referred to as Snell’s law.23-26 In
The Greek era of science was followed by Fig. 13, light from point A enters a new
the Arab scientific Golden Age, from the medium at point C and refracts along the
eighth to the sixteenth century. Nearly all line CD. If the line CD is extended to
of the writing was in Arabic, the scientific point B, the ratio of length AC to length
language before the twelfth century. This BC is the index of refraction.
period began with an intensive period of
translation of Greek books brought to Lens and mirror shapes Ibn Sahl
Baghdad, the imperial and scientific considered were the elliptical, parabolic,
center. hyperbolic and biconvex. Ibn Sahl went
further and designed machines for the
Although early Arab scientists precise drawing of mathematical shapes.
contributed much to other disciplines Ibn Sahl informed the work of another
such as chemistry, biology, medicine and optical physicist, Ibn al-Haytham.
engineering, their enduring legacy was in
mathematics, astronomy and optics. They Ibn al-Haytham
were intrigued by the mechanism of
vision and the function of the eye and Ibn al-Haytham (CE 965-1039), also
brain in processing this information.21,22 known as Alhacen or Alhazen, was born in
Basra, Iraq, and studied in Baghdad
The early Arab scientists were (Fig. 14). In pursuit of knowledge, he
fascinated by what they read in the Greek traveled to Iran and Syria and settled in
books and wanted to understand such Egypt. He wrote more than 90 books and
phenomena, but the respect these Arabs treatises on optics, astronomy,
had for the Greek authorities did not stop mathematics, philosophy, medicine and
them questioning their theories in a new logic.26-29
way, the scientific method known today.
The observation and measurement of data His most important work was a critique
were followed by the formulation and of Ptolemy’s Almagest. Ibn al-Haytham
testing of hypotheses to explain the data. prefaced this critique by stating that his
methods will criticize premises and
Ibn Sahl exercise caution in drawing conclusions,
not to follow authorities blindly. On the
Ibn Sahl (CE circa 940-1000) was an Arab mechanism of vision, he was able to reject
mathematician and physicist. His the two competing Greek theories favored
predecessors and contemporaries by Euclid and Ptolemy. To test these
researched designs of military mirrors for theories in experiments, Ibn al-Haytham
invented the camera obscura (literally the
FIGURE 14. Ibn al-Haytham’s portrait on Iraqi currency, with optics diagram next to him.
Introduction to Visual Testing 25
“dark chamber”), or pinhole camera, the identified it as one of many phenomena
basis of photography. Ibn Haytham wrote where light plays tricks on the brain.
a detailed account of all his experimental
setups and the data he measured. This Ibn al-Haytham’s analysis of his data
book served as the textbook on optics for led him to put forward or question
centuries throughout Europe (Fig. 15).30 models. He was a scientist, using
He dissected the eye and named its parts mathematics to formulate physical
(lens, cornea, retina). He explained for the theories and to conduct careful
first time the imperfection of the eye’s experiments. His writings were
lenses, introducing the concept of transmitted to western Europe in Latin
spherical aberration (Fig. 15c). and founded the technology of optics.
The early Arab interest in the Boiler Inspection,
physiology of the eye together with the 1870-192032,33
mechanism of vision led a later scientist,
Hunayn ibn Ishaq, to write that “it is a The first nondestructive test method was
prerequisite for whoever wants to visual testing, and the term visual testing
understand the function of the eye to be here refers, not to a caveman’s inspection
cognizant of the function of the brain, of his spearhead (although that is indeed
since the process of vision begins and nondestructive testing) but rather to
ends therein” (a translation of the Arabic documented inspection of a product
text in Fig. 16).31 Ibn al-Haytham’s according to a particular procedure or
understanding of the relationship specification designed to recognize
between the eye and the brain enabled material defects. Most specifications for
him to recognize an optical illusion, visual testing ask various quality
where the Moon appears larger on the questions.
horizon than at its zenith. Some have
tried to explain the Moon’s apparent size 1. Are the contracted steps in processing
as diffraction of sunlight through the or fabrication performed completely
atmosphere; some try to explain with and in the correct sequence?
other models. Ibn al-Haytham simply
FIGURE 15. Sixteenth century edition of Ibn al-Haytham’s treatise, in Latin: (a) cover page; (b) caption and engraving on “three
parts of vision, direct, reflected and refracted”; (c) engraved diagram of eye with parts labeled.30
(a) (b) (c)
26 Visual Testing
2. Are the right materials and (Fig. 17). The 1860s saw the introduction
components used throughout? Are of boiler inspection combined with boiler
bolts the right size, for instance? insurance in the United States and the
United Kingdom.32,33
3. Are fasteners and supports spaced and
installed according to specification? Boiler inspection was an early
application of visual testing. Insurance
4. Are protective lubricants, weather inspectors would, of course, look for
strips and coatings applied according corrosion in the inservice boilers they
to specification? insured. Early editions of the ASME Boiler
Code asked the inspector to inspect
5. Are there signs of damage, such as components, that is, to look at them.34 A
wear, corrosion, dents, strain, buckling half century would pass before other
or visible cracking? methods of nondestructive testing would
provide the context needed to make it
These visual checks are, however, not clear that this aspect of the boiler
necessarily nondestructive tests: the inspector’s job was the visual test method
questions except for the last address of nondestructive testing.
fabrication and maintenance quality
rather than material discontinuities. The earliest standards of the American
Society of Mechanical Engineers (ASME),
The introduction of steam power in the although they emphasized proof tests and
nineteenth century led to a rash of boiler destructive tests, say that the boilers must
explosions and to the need for inspection be free of gross surface blemishes and
other signs of poor workmanship. In
FIGURE 16. Thirteenth century manuscript 1915, the first edition of the Boiler Code
page from Hunayn ibn Ishaq, Book of Ten expected the inspector to look at
Treatises on the Eye.22,31 malleable castings to determine that they
were “true to pattern, free from blemishes,
scale or shrinkage cracks. A variation of
1/16 in. per foot [1.6 mm per 0.3 m] shall
be permissible.” The finish of flat bars had
“to be smoothly rolled and free from
slivers, depressions, seams, crop ends,”
and burns. The inspector examined all
parts to be sure that “the finished material
shall be free from injurious defects and
shall have a workmanlike finish.”35
Twenty-first century versions of the Boiler
Code, although briefly, explicitly treat
visual testing as nondestructive testing.36
FIGURE 17. Drawing of steam boiler explosion in nineteenth century.
Introduction to Visual Testing 27
Borescopy37 increased, sectionalized instruments have
been introduced and other special devices
Medical Endoscopy38 have been developed for industrial
applications.
The development of self illuminated
telescopic devices can be traced back to An early inventor and manufacturer
early interest in exploring the interior was a German, Georg Wolf, who
human anatomy without invasive cofounded an optical equipment
procedures.38 The first borescopes were company in 1906.42 He filed patents for
medical endoscopes turned to industrial medical endoscopes in the United States
applications, for an endoscope does not in 1922.43,44 A few months later, a Robert
care what aperture it is interrogating. Wolf filed a patent for a cystoscope.45
Medical endoscopes and industrial When Georg Wolf died in 1938, his son
borescopes share several features: (1) a Richard Wolf continued the family
source of illumination, (2) a means of business, which has continued with his
delivering an image to the viewer’s eye name into the twenty-first century.
and (3) adjustability to view a surface of
interest. Early endoscopes for looking Georg Wolf in 1932 produced a flexible
down the esophagus were called gastroscope, developed by Rudolph
gastroscopes; endoscopes for looking at the Schindler for observing the interior of the
bladder were called cystoscopes. stomach wall.46 The instrument consisted
of a rigid section and a flexible section.
Devices for viewing the interior of Many lenses of small focal distance were
objects are called endoscopes, from the used to allow bending of the instrument
Greek words for “inside view.” Today the to an angle of 34 degrees in several
term endoscope in the United States planes. The tip of the device contained
denotes a medical instrument. Nearly all the objective and the prism, causing the
endoscopes have an integral light source; necessary axial deviation of the bundle of
some incorporate surgical tweezers or rays coming from the illuminated gastric
other devices. Industrial endoscopes are wall. The size of the image depended on
called borescopes because they were the distance of the objective from the
originally used in machined apertures and observed surface. The sharp image could
holes such as gun bores. There are both be magnified or reduced. Later in the
flexible and rigid, fiber optic and direct century, flexible gastroscopes had rubber
light borescopes. tubes over the flexible portion, in
diameters of about 14 mm (0.55 in.) and
In 1806, Philipp Bozzini of Frankfurt 8 mm (0.3 in.).42
announced the invention of his Lichtleiter
(German for “light guide”). Having served Early Patents to Automate Visual
as a surgeon in the Napoleonic wars, Testing
Bozzini envisioned using his device for
medical research. It is considered the first A patent was filed in June 1920 for an
endoscope.39,40 inspection table that would lift rolled
plates so that the underside could be
In 1876, Max Nitze, a urologist, visually tested. The invention was
developed a practical cystoscope to view addressing the steel industry’s need for
the human bladder. A platinum loop in its visual testing.47 In 1922, another
tip furnished a bright light when heated inspection table was patented that made
with galvanic current. Two years later, test objects turn over as they were
Thomas Edison introduced an conveyed past the inspector. This table
incandescent light in the United States. was designed especially for fruits and
Within a short time, scientists in Austria vegetables but could be used, the inventor
made and used a minute electric bulb in said, for anything that should be rotated
Nitze’s cystoscope, even before the electric for inspection on all sides.48
light was in use in America.
In July 1925, Floyd Firestone of the
The early cystoscopes contained simple University of Michigan, Ann Arbor, filed a
lenses; these were soon replaced by patent for automated scanning and flaw
achromatic combinations. In 1900, detection. (This is the same Firestone who
Reinhold Wappler revolutionized the later invented the Supersonic
optical system of the cystoscope and Reflectoscope®, an ultrasonic instrument
produced the first American models. The widely used in the United States in the
forward oblique viewing system was later 1940s.) The optical scanning invention
introduced and has proved very useful in was envisioned for bearing rollers or
both medical and industrial applications. “other articles with surfaces of revolution,
Direct vision and retrospective systems and even to plane surfaces, so long as the
were also first developed for cystoscopy. surface of the article, or as much thereof
as needs inspection, may be moved
Borescopes and related instruments for within the field of view.”49 How could
nondestructive testing have followed the optical inspection have been automated
same basic design used in cystoscopic in the years before computers facilitated
devices. The range of borescope sizes has
28 Visual Testing
decision making? Small areas would application. The patent also provided for
successively be brought into view to a a separate attachment to scour the tube’s
microscope, and a light sensitive cell inside surface before visual testing.
would detect brightness variations below FIGURE 19. Borescopy of tubing: (a) drawing from 1941
a selected threshold and trigger a sorting patent54; (b) photograph of application.
armature. It is not known if this scheme (a)
was ever implemented by industry. A later
design was advanced in 1938 for sheet (b)
metal.50 In the 1980s, microprocessing
made automated vision easier to
implement.51
Industrial Endoscopy: Borescopy
Patents for endoscopes specifically for
industrial applications appeared in the
1920s and 1930s. A patent was filed in
1922 for the inspection of rivets inside
tubing in, for example, a boiler or
airplane. The device resembled a periscope
like those seen in old movies about
submarines, with several differences: (1) it
was portable and small enough to fit
inside tubing; (2) it included light bulbs
for illumination; (3) it provided for
rotation of the objective end while the
eyepiece remained stationary.52
A patent was filed in 1927 literally for a
bore scope — to look inside gun bores
(Fig. 18).53 Another patent to look inside
gun bores was filed on behalf of the Carl
Zeiss company, Jena, Germany, in 1932 in
Germany and in 1933 in the United
States.54
The visual technology for tubing was
represented by a patent filed in 1938; the
invention, which could generically be
called a tube scope, became important for
the inspection of petroleum drill pipe in
the United States.55 A service using the
instrument rather than the instrument
itself was provided to the petroleum
industry. Figure 19 shows the design and
FIGURE 18. Drawing from patent for borescope for gun
barrels.53
Legend 21. Reflector.
22. Guide horn.
1. End of stand of pipe. 23. Barrel.
2. Pipe rack. 24. Joint clamp.
3. Frame. 25. Body of joint sleeve.
4. Strong back. 26. Sleeve split to fit over barrel.
5. Viewing apparatus. 27. Coupling nut.
6. Track. 28. Tapered bores.
7. Trolleys. 29. Tapered or conical outer face
8. Rollers.
9. Chain. of sleeve.
10. Hook. 31. Guide sleeves.
11. A frames. 32. Resilient bow or bar of sleeve.
12. Legs. 33. Loose collar.
13. Saddles. 34. Collar clamped to barrel.
14. Set screw. 35. Tightening nut threaded onto
15. Clamp.
16. Lever with cam head. sleeve.
17. Tightening bolt. 36. Electric lamp cord.
18. Lighting guard. 37. Telescoping means.
19. Electric lamp. 38. Telescoping sections.
20. Crack. 39. Eye piece.
Introduction to Visual Testing 29
Flexible borescopes for industrial use his M.D. from the University of
are more rugged than gastroscopes, Pennsylvania in 1898. While he was
having flexible steel tubes instead of interning at Pennsylvania Hospital,
rubber for the outer tube of the flexible Crampton’s mechanical and engineering
portion. A typical flexible borescope is abilities were recognized and he was
13 mm (0.5 in.) in diameter and has a advised to become an oculist. He returned
1 m (40 in.) working length, with to the university, took a degree in
flexibility in about 500 mm (20 in.) of the ophthalmology and later practiced in
length. Extension sections are available in Philadelphia, Pennsylvania and Princeton,
1, 2 or 3 m (40, 80 or 120 in.) lengths, New Jersey.56
permitting assembly of borescopes up to
10 m (30 ft) in length. In such flexible In 1921, the Westinghouse Company
instruments, the image remains round asked Crampton to make a device that
and sharp when the tube is bent to an could be used to check for discontinuities
angle of about 34 degrees. Beyond that inside the rotor of a steam turbine
limit, the image becomes elliptical but (Fig. 21). Crampton developed the
remains clear until obliterated at about instrument in his Philadelphia shop and
45 degrees of total bending. delivered the prototype within a week —
it was the first borescope produced by his
Crampton company. Crampton continued to supply
custom borescopes for testing inaccessible
After the early medical developments, and often dark areas on power turbines,
certain segments of American industry oil refinery piping, gas mains, soft drink
needed visual testing equipment for tanks and other components. Crampton
special inspection applications. One of the soon was recognized for his ability to
first individuals to help fill this need was design and manufacture borescopes,
George Sumner Crampton. George periscopes and other optical equipment
Crampton (Fig. 20) was born in Rock for specific testing applications.
Island, Illinois, in 1874. He was said to
have set up a small machine shop by the After retiring as emeritus professor of
age of 10 and his first ambition was to ophthalmology at the university,
become an electrical engineer. He chose Crampton continued private practice in
instead to study medicine and received downtown Philadelphia. At the same
time, he worked on borescopes and other
FIGURE 20. George Crampton, developer of borescope. instruments in a small shop he had
established in a remodeled nineteenth
century coach house.
After World War II began, Crampton
devoted much of his energy to the war
effort, filling defense orders for borescopes
(Fig. 22). Crampton practiced medicine
until noon, then went to the nearby
FIGURE 21. Tests of forgings for steam turbine generator
shaft in 1920s.
30 Visual Testing
workshop where he visually tested the The original Manhattan Project
bores of 37 mm antiaircraft guns and borescope was later improved with
other weapons.56 During the war, nondarkening optics and a swivel-joint
borescopes were widely used for testing eyepiece that permitted the operator to
warship steam turbines (particularly their work from any angle (this newer
rotating shafts). The United States Army instrument did not require the V trough).
also used borescopes for inspecting the It also was capable of considerable
barrels of tank and antiaircraft weapons bending to snake through the tubes in the
produced in Philadelphia. An even more reactor. Three borescopes were supplied
challenging assignment lay ahead. for this epochal project and they are
believed to be the first optical instruments
The scientists working to develop a to use glass resistant to radioactivity.
successful nuclear chain reaction in the
top secret Manhattan Project asked Aircraft inspection soon became one of
Crampton to provide a borescope for the most important uses of borescope
inspecting tubes near the radioactive pile technology. In 1946, an ultraviolet light
at its guarded location beneath the borescope was developed for fluorescent
stadium seats at the University of testing of the interior of hollow steel
Chicago’s Stagg Field. Crampton devised propeller blades. The 100 W viewing
an aluminum borescope tube 35 mm instrument revealed interior surface
(1.4 in.) in diameter and 10 m (33 ft) discontinuities as glowing green lines.41
long. The device consisted of 2 m (6 ft)
sections of dual tubing joined by bronze Later, in 1958, the entire United States’
couplings which also carried an 8 V B-47 bomber fleet was grounded because
lighting circuit. The inspector standing of metal fatigue cracks resulting from low
directly in front of the bore was subject to level simulated bombing missions. Visual
radioactive emissions from the pile, so testing with borescopes proved to be the
Crampton mounted the borescope outside first step toward resolving the problem.
of a heavy concrete barrier. The operator The program became known as Project
stood at a right angle to the borescope, Milk Bottle, a reference to the bottle
looking through an eyepiece and shaped pin that was a primary connection
revolving the instrument manually. The between the fuselage and wing (Fig. 23).
borescope contained a prism viewing head
and had to be rotated constantly. It was In the late 1950s, a system was
supported in a steel V trough resting on developed for automatic testing of
supports whose height could be varied. helicopter blades. The borescope,
Crampton also mounted a special supported by a long bench, could test the
photographic camera on the eyepiece. blades while the operator viewed results
on a television screen (Fig. 24). The
system was used extensively during the
FIGURE 22. Using a borescope, inspector at a converted FIGURE 23. Inspector uses borescope to check for metal
automobile plant during World War II checks interiors of gun fatigue cracks in B-47 bomber during grounding of bomber
tubes for 90 mm antiaircraft guns. fleet in 1958.
Introduction to Visual Testing 31
Vietnam conflict and helicopter method — there were too many different
manufacturers continue to use borescopes applications on too many test objects to
for such critical tests. permit the use of specific acceptance
criteria. It also was reasoned that visual
In 1962, Crampton sold his borescope testing would occur as a natural result of
business to John Lang of Cheltenham, applying any other nondestructive test
Pennsylvania.56,57 Lang had developed the method.
radiation resistant optics used in the
Manhattan Project borescope, as well as a Expanded Need for Visual
system for keeping it functional in high Certification
temperature environments. Lang also
helped pioneer the use of closed circuit In the early 1970s, the need for certified
television with borescopes for testing the visual inspectors began to increase.
inner surfaces of jet engines and wings, Nuclear power construction was at a peak,
hollow helicopter blades and nuclear visual certification was becoming
reactors. In 1965, the company received a mandatory and nondestructive testing
patent on a borescope whose mirror could was being required. In 1976, the American
be precisely controlled. This borescope Society for Nondestructive Testing began
could zoom to high magnification and considering the need for certified visual
could intensely illuminate the walls of a inspectors. ASNT had become a leading
chamber by means of a quartz force in nondestructive testing and
incandescent lamp containing iodine American industry had accepted its
vapor. Recommended Practice No. SNT-TC-1A as
a guide for certifying other NDT
The basic design of the borescope has inspectors.6
been in use for many decades and it
continues to develop, accommodating In the spring of 1976, ASNT began
advances in video, illumination, robotic surveying industry about their inspection
and computer technologies. needs and their position on visual testing.
Because of the many and varied responses
Certification of Visual to the survey, a society task force was
Inspectors established to analyze the survey data. In
1977, the task force recommended that
The recognition of the visual testing visual inspectors be certified and that
technique and the development of formal visual testing be made a supplement to
procedures for educating and qualifying Recommended Practice No. SNT-TC-1A
visual inspectors were important (1975). At this time, the American
milestones in the history of visual
inspection. Because visual testing can be FIGURE 24. Visual testing of frame of 10 m (32 ft) long
performed without any intervening helicopter blade using 10 m (32 ft) borescope. Inspector
apparatus, it was certainly one of the first could view magnified results on television at bottom left.
forms of nondestructive testing. In its
early industrial applications, visual tests
were used simply to verify compliance to
a drawing or specification. This was
basically a dimensional check. The
soundness of the object was determined
by liquid penetrant, magnetic particle,
radiography or ultrasonic testing.
Following World War II, few inspection
standards included visual testing. By the
early 1960s, visual tests were an accepted
addition to the American Welding
Society’s code books. In NAVSHIPS
250-1500-1, the US Navy included visual
tests with its specifications for other
nondestructive testing techniques for
welds.58
By 1965, there were standards for
testing, and criteria for certifying the
inspector had been established in five test
methods: liquid penetrant, magnetic
particle, eddy current, radiographic and
ultrasonic testing. These five were cited in
ASNT Recommended Practice No.
SNT-TC-1A, introduced in the late 1960s.
The broad use of visual testing hindered
its addition to this group as a specific
32 Visual Testing
Welding Society implemented a program defined the scope and purpose of visual
that, following the United States Navy, testing (dimensional testing was
was the first to certify inspectors whose excluded). In 1984, the Visual Personnel
sole function was visual weld testing. Qualification Committee was formed in
ASNT’s Education and Qualification
During 1978, ASNT subcommittees Council. In 1986, a training outline and a
were formed for the eastern and western recommended reference list was finalized
halves of the United States. These groups and the Board of Directors approved
verified the need for both visual standards incorporation of visual testing into ASNT’s
and trained, qualified and certified Recommended Practice No. SNT-TC-1A.
inspectors. In 1980, a Visual Methods
Committee was formed in ASNT’s
Technical Council and the early meetings
Introduction to Visual Testing 33
PART 4. Measurement Units for Visual Testing
Origin of International meanings of the prefix m (milli) and the
System prefix M (mega) differ by nine orders of
magnitude.
In 1960, the General Conference on
Weights and Measures established the TABLE 6. SI base units.
International System of Units. Le Systéme
International d’Unités (SI) was designed so Quantity Unit Symbol
that a single set of measurement units
could be used by all branches of science, Length meter m
engineering and the general public. Mass kilogram kg
Without SI, the Nondestructive Testing Time second s
Handbook series would contain a Electric current ampere A
confusing mix of obsolete centimeter Temperature kelvin K
gram second (CGS) units, inch pound Amount of substance mole mol
units and the units preferred by certain Luminous intensity candela cd
localities or scientific specialties.
TABLE 7. SI derived units with special names.a
SI is the modern version of the metric
system and ends the division between Quantity Units Symbol Relation
metric units used by scientists and metric
units used by engineers and the public. to Other
Scientists have given up their units based SI Unitsb
on centimeter and gram and engineers
have abandoned the kilogram-force in Capacitance farad F C·V –1
favor of the newton. Electrical engineers Catalytic activity katal kat s –1·mol
have retained the ampere, volt and ohm Conductance siemens S A·V –1
but changed all units related to Energy joule J N·m
magnetism. Frequency (periodic) hertz Hz 1·s –1
Force newton N kg·m·s –2
Table 6 lists the seven SI base units. Inductance henry H Wb·A –1
Table 7 lists derived units with special Illuminance lux lx lm·m –2
names. In SI, the unit of time is the Luminous flux lumen lm cd·sr
second (s) and hour (h) is recognized for Electric charge coulomb C A·s
use with SI. Electric potentialc volt V W·A –1
Electric resistance ohm Ω V·A –1
For more information, the reader is Magnetic flux weber Wb V·s
referred to the information available Magnetic flux density tesla T Wb·m –2
through national standards organizations Plane angle radian rad 1
and specialized information compiled by Power watt W J·s –1
technical organizations.59-61 Pressure (stress) pascal Pa N·m –2
Radiation absorbed dose gray Gy J·kg –1
Multipliers Radiation dose equivalent sievert Sv J·kg –1
Radioactivity becquerel Bq 1·s –1
In science and engineering, very large or Solid angle steradian sr 1
very small numbers with units are Temperature degree celsius °C K
expressed by using the SI multipliers, Timea hour h 3600 s
prefixes of 103 intervals (Table 8). For Volumea liter L dm3
example, a millimeter (mm) is
0.001 meter (m). The volume unit cubic a. Hour and liter are not SI units but are accepted for use with SI.
centimeter (cm3) is (0.01 m)3 or 10–6 m3. b. Number one (1) expresses a dimensionless relationship.
Unit submultiples such as the centimeter, c. Electromotive force.
decimeter, dekameter and hectometer are
avoided in scientific and technical uses of
SI because of their variance from the
convenient 103 or 10–3 intervals that
make equations easy to manipulate.
In SI, the distinction between upper
and lower case letters is meaningful and
should be observed. For example, the
34 Visual Testing
Units for Visual Testing Old units should not be used in science
and engineering; Table 9 gives some
Terms for some quantities have been conversions to SI units. Footcandle (ftc)
replaced. Brightness is now luminance; and phot convert to lux (lx). Stilb (sb),
illumination is illuminance; transmission footlambert and lambert convert to
factor is transmittance. Names of some candela per square meter (cd·m–2).
units have changed: the meter candle is
now lux; the nit is now candela per In visual testing, units express
square meter (cd·m–2). measurements of visible light as part of
the electromagnetic spectrum. Nanometer
TABLE 8. SI prefixes and multipliers. (nm) is used rather than angstrom (Å) for
wavelength. The velocity c of light is
Prefix Symbol Multiplier expressed as a ratio of distance in meters
(m) to time in seconds (s): in a vacuum,
yotta Y 1024 2.997 924 58 × 108 m·s–1.
zetta Z 1021
exa E 1018 Illumination
peta P 1015
tera T 1012 The intensity of visible radiation — that
giga G 109 is, of light — was formerly measured in
mega M 106 footcandles (ftc) and is now expressed in
kilo k 103 lux (lx): 1 ftc = 10 lx. A typical indoor
hectoa h 102 office has illumination of about 400 lx.
dekaa Daylight ranges from 1 to 25 klx; direct
decia da 10 sunlight, several times more.
centia d 10–1
milli c 10–2 Vision requires a source of
micro m 10–3 illumination. The light source is measured
nano μ 10–6 in candela (cd), defined as the luminous
pico n 10–9 intensity in a given direction of a source
femto p 10–12 that emits monochromatic radiation of
atto f 10–15 540 × 1012 hertz (Hz) at a radiant intensity
zepto a 10–18 of 1.46 × 10–3 watt per steradian (W·sr–1).
yocto z 10–21
y 10–24 The luminous flux in a steradian (sr) is
measured in lumens (lm). The
a. Avoid these prefixes (except in dm3 and cm3) for measurement in lumens is the product of
science and engineering. candela and steradian (1 lm = 1 cd·sr). A
light flux of one lumen (1 lm) striking
one square meter (1 m2) on the surface of
the sphere around the source illuminates
it with one lux (1 lx), the unit of
illuminance. If the source itself is scaled to
TABLE 9. Examples of conversions to SI units for visual testing.
Quantity Measurement in Non-SI Unit Multiply by To Get Measurement in SI Unit
Angle minute (min) 2.908 882 × 10–4 radian (rad)
Area
Distance degree (deg) 1.745 329 × 10–2 radian (rad)
Power
Illuminance square inch (in.2) 645 square millimeter (mm2)
Luminance
angstrom (Å) 0.1 nanometer (nm)
Temperature (increment)
Temperature (scale) inch (in.) 25.4 millimeter (mm)
Temperature (scale)
British thermal unit per hour (BTU·h–1) 0.293 watt (W)
footcandle (ftc) 10.76 lux (lx)
phot (ph) 10 000 lux (lx)
candela per square foot (cd·ft–2) 10.76 candela per square meter (cd·m–2)
candela per square inch (cd·in.–2) 1 550 candela per square meter (cd·m–2)
footlambert (ftl) 3.426 candela per square meter (cd·m–2)
lambert 3 183 (= 10 000 ÷ π) candela per square meter (cd·m–2)
nit (nt) 1 candela per square meter (cd·m–2)
stilb (sb) 10 000 candela per square meter (cd·m–2)
degree fahrenheit (°F) 0.556 kelvin (K) or degree celsius (°C)
degree fahrenheit (°F) (°F – 32) ÷ 1.8 degree celsius (°C)
degree fahrenheit (°F) (°F – 32) ÷ 1.8 + 273.15 kelvin (K)
Introduction to Visual Testing 35
one square meter (1 m2) and emits one Although both light and ultraviolet
candela (1 cd), the luminance of the radiation are measured in watts per square
source is 1 cd·m–2. meter, their wavelengths have distinct
ranges. Because ultraviolet radiation is
Quantities and units for photometric invisible, photometric measurement units
measurement of light are discussed in the such as the lumen and lux should never
chapter on light. be applied to ultraviolet radiation.
Optometric Units Ultraviolet radiation is divided into
three ranges: UV-A (320 to 400 nm), UV-B
The diopter is a variable used to express (280 to 320 nm) and UV-C (100 to
the refracting power of curved mirrors, 280 nm). This is analogous to the
lenses and the eye. The diopter is the segmentation of visible light into the
inverse of the distance (in meters) from wavelengths that produce the colors. Blue
the lens (or mirror) to an image of a light, for example, generally has
distant object; that is, the diopter is the wavelengths between 455 and 492 nm.
inverse of the focal distance of the lens or Yellow light is between 577 and 597 nm.
mirror (where that distance is measured in The analogy to visible radiation might
meters). help those first learning to measure
ultraviolet radiation. A certain intensity of
To express retinal illuminance, the yellow light will produce on a surface a
troland (Td) is most often used. It is not a certain illuminance measured in lux. In
true unit of illumination but is the the same way, a certain amount of
product of the target luminance (in ultraviolet radiation will produce an
candela per square meter) and the pupil irradiance on a test surface.
area (in square millimeters).
Ultraviolet irradiance is a time
Ultraviolet Radiation dependent measure of the amount of
energy falling on a prescribed surface area
Ultraviolet radiation is of concern because and is expressed in watts per square meter
some visual inspectors also document the (W·m–2) or (to avoid exponents)
vision acuity and color discrimination of microwatts per square centimeter
personnel who use ultraviolet lamps to (µW·cm–2). One unit of irradiance
perform liquid penetrant and magnetic (1 µW·cm–2) is the power (microwatt)
particle testing. falling on one square centimeter (cm–2) of
surface area. At higher irradiance, the
The term light is widely used for milliwatt per square centimeter
electromagnetic radiation in the visible (mW·cm–2) is sometimes used:
part of the spectrum. The term black light, 1000 µW·cm–2 = 1 mW·cm–2, and
however, should not be used for 1 µW·cm–2 = 10–2 W·m–2.
ultraviolet radiation, because (1) the term
has become ambiguous, denoting More information on the physics and
sometimes the ultraviolet lamp and safe use of ultraviolet radiation can be
sometimes its radiation, (2) the term black found in literature about liquid penetrant
here means merely invisible and not a and magnetic particle testing.
color and (3) ultraviolet radiation is not
light, any more than X-rays are.
36 Visual Testing
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Introduction to Visual Testing 39
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