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HOLOGRAPHY

Brief Introduction

• The word holography is formed by combining
parts of two Greek words ‘holos’ meaning
“whole” and ‘graphein’ meaning “to write”.
Holography – writing the complete image.

• Holography is a technique that enables a
light field, which is generally the product of a
light source scattered off objects, to be
recorded and later reconstructed in the
absence of the original objects.

• Holography is actually a recording of
interference pattern formed between two
beams of coherent light coming from the
same source.

• In this process both the amplitude and
phase components of light wave are
recorded on a light sensitive medium such
as a photographic plate - Hologram

Principles of Holography

• Holography is a two step process.

• First step – recording of hologram

• Second step – reconstruction, in which the
hologram is transformed into image.

• A hologram is the result of interference
occurring between two waves (an object
beam and reference beam)

• Light scattered off the object and a
coherent background.

• Light reaching the photographic plate
directly.

Recording of the Hologram

• Off axis arrangement of the laser beam is divided into two-
reference beam and object beam

• Each point of the object scatters the incident light and act
as the source of spherical waves.

• Part of the light, scattered by the object, travels towards
the plate.

• In this plate, innumerable spherical waves from the object
combine with the reference beam (direct beam)

• These waves are coherent- from the same laser source.

• Waves interfere and form fringes on the plate.

• The interference fringes are zone plate like
rings and they are super imposed giving rise to
complex pattern of lines.

• The developed negative of these interference
fringe pattern is a hologram.

• The hologram does not contain a distinct image
of the object but carries a record of both the
intensity and the relative phase of the light
wave at each point.

Reconstruction of the Image

• The image can be viewed whenever
required.

• For reconstruction of the image, the
hologram is illuminated by a parallel beam
of laser.

• Most of the light passes straight and the
complex fine fringes act as diffraction
grating.

• The diffracted rays form two images: a
virtual image and a real image.

• The virtual image appears at the location of
object and called as true image.

• The virtual image is only for viewing from
different positions as looking directly at the
real object.

• The real image is formed in front of the
hologram.