Visual Acuity (VA) Theoretical Optometry
1.The clarity or clearness of the vision, a measure of how well a person sees. The ability to distinguish details and shapes of objects; also called central vision. Visual Acuity 2. Sharpness of vision with respect to ability of the eye to distinguish detail as an object is placed farther away or as it becomes smaller in size.
The word "acuity" comes from the Latin "acuitas" = sharpness. Visual Acuity dependent on The sharpness of the retinal image and The sensitivity of the neural elements of the visual system
1. The angle which an object or detail subtends at the point of observation; usually measured in minutes of arc. Visual Angle 2. The angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size.
Is dependent upon : 1. Size of the object 2. Distance of the object from the eye Visual Angle
Visual Angle
Type of visual acuity Minimum visible Task – detect presence of absence of a target Method – width of a line is varied Best threshold – about 1 second of arc
Minimum discriminable (Hyperacuity , vernier acuity) Task – determine relative location of Type of visual acuity 2 or more visible features with respect to each other Method – vary relative location of 2 features usually 2 lines Best threshold – about 3 second of arc
Minimum discriminable (Hyperacuity , vernier acuity) The eye's resolution is generally considered to be 'normal' if two points that are one minute of an arc (one sixtieth of a degree) apart can be seen as separate. However, the eye has some visual abilities that far out-perform this, such as the detection of an offset between two lines. This ability is termed Vernier Acuity (and the eye's fantastic ability is used in the reading of micrometers). as separate.
Hyperacuity or Vernier acuity
Task – determine the presence of , or distinguish between , more than one Type of visual acuity Minimum resolvable (Minimum separable , ordinary visual acuity) identifying feature in a visible target Method – vary object size or spacing between object components Best threshold – about 30 second of arc
Minimum resolvable Grating acuity Landolt C
Landolt C A Landolt C, also known as a Landolt ring or Landolt broken ring, is an optotype, i.e. a standardized symbol used for testing vision. It was developed by the Swiss developed by the Swiss-born ophthalmologlist Edmund Landolt.
Landolt C The Landolt C consists of a ring that has a gap, thus looking similar to the letter C. The gap can be at various positions (usually left, right, bottom, top and the 45° positions in between) and the task of the tested person is to decide on which side the gap is. The size of the C and the break are reduced until the subject makes a random rate of errors. The minimum angle of the break is judged as the visual acuity. It is generally practiced in the laboratory.
Landolt C The stroke width is 1/5 of the diameter, and the gap width is the same. This is identical to the letter C from a Snellen chart.
a) Detection b) Resolution c) Recognition d) Crowding e) Hyperacuity
Visual Angle Notation Threshold visual angle or Minimum Angle of Resolution (MAR ) The smallest object that the patient can see which subtend 1 minute of arc the MAR = 1´
Decimal Visual Acuity is the decimal expression of the Snellen fraction Or the reciprocal of MAR decimal acuity decreases when increasing the gap size
Percentage acuity Equal decimal VA x 100 Snellen VA Snellen charts are named after the Dutch ophthalmologist Herman snellen who developed the chart in 1862. base on Snellen fraction
Snellen Visual Acuity Can be converted into decimal acuity by treating Snellen VA as a fraction and converting into a decimal
Snellen Visual Acuity
Snellen Visual Acuity
Snellen VA Chart
What is the size of 20/20 letter?
What is the size of 20/20 letter?
What is the size of 20/20 letter?
What is the size of 20/20 letter?
Anatomical Bases for VA • VA always mean foveal VA • Fovea centralis – rod-free area, each cone is though to have its own nerve fiber to the visual cortex • ∅ 54 min. of arc • 6/60 (20/200) = 50 min. of arc
Anatomical Bases for VA • Cohen (1975) ⇒ ∅ of cone = 1.5µ ≈ 18 sec. • ≈ 3 rows of cone = a gap of the retinal image of the 6/6 E
Factors responsible for individual variability in VA With refractive error corrected 1. Arrangement of foveal cones 1. Arrangement of foveal cones 2. Optical aberrations of the eye 2. Optical aberrations of the eye 3. Pupil diameter 3. Pupil diameter 4. Clarity of the ocular media 4. Clarity of the ocular media 5. Retinal image magnification 5. Retinal image magnification 6. Ability in interpreting blurred 6. Ability in interpreting blurred images
• Illumination • Contrast Test factors affecting VA • Uncorrected refractive error or defocus • Difficulty of the letter target
Minimum 10 foot-lamberts Illumination visual acuity is greatly affected by the level of background luminance
The ratio of the difference between the maximum and minimal luminance of a test stimulus divided by the sum of the maximum and minimal luminance Contrast
• To express in percentage, x 100 • Highest contrast → dark black letters on a pure white background = 100% Contrast • Lowest contrast → gray letters on an equally gray background = 0% • VA suffers w/ any decrease in contrast below 10%
Contrast
Illumination & Contrast
Uncorrected refractive error
• Letters with more diagonal contours and less horizontal and vertical Difficulty of the letter target contours are more difficult • Common confusions between similar letter
Solutions Difficulty of the letter target –Equal number of Equal number of difficult, easy, and intermediate letters on each line –Use only letters of Use only letters of approximately equal difficulty Example Sloan letters
Sloan letters Sloan letters, designed by Louise Sloan in 1959, are a set of Optotypes currently used to test Visual acuity generally used in Snellen charts and logMAR charts. Z N H R V K D C O S
Subjective Methods of measuring minimum resolvable VA ● Snellen letters ● Landolt Cs ● Thumbling Es ● Detection of a grating or light and dark bars
● Optokinetic nystagmus (OKN) Methods of measuring minimum resolvable VA Objective nystagmus (OKN) ● Visual evoked response (VER) / Visual evoked cortical potential (VECP) ● Preferential looking
Subjective Snellen charts are named after the Dutch ophthalmologist Herman snellen who developed Snellen letters the chart in 1862.
Landolt Cs Subjective A Landolt C, also known as a Landolt ring or Landolt broken ring, is an optotype, i.e. a standardized symbol used for testing vision. It was developed by the Swiss-born ophthalmologist Edmund Landolt. The stroke width is 1/5 of the diameter, and the gap width is the same. This is identical to the letter C from a Snellen chart.
Thumbling Es An E Chart is an ophthalmologist chart used to measure a patient's acuity for distant vision. This chart is useful for patients that are illiterate or too young to read but who can speak. Subjective It contains rows of the letter "E" in various kinds of rotation. The patient is asked to state where the limbs of the E are pointing "up, down, left or right." Depending on how far the patient can read his or her visual acuity is quantified. It works on the same principle as Snellen’s distance vision chart.
Detection of a grating or light and dark bars Subjective
Optokinetic nystagmus (OKN) Objective The optokinetic reflex allows the eye to follow objects in motion when the head remains stationary (e.g. observing individual telephone poles on the side of the road as one travels by them in a car). The reflex develops at about 6 months of age.
Optokinetic nystagmus (OKN) Objective
A visual evoked potential (VEP) is an evoked potential caused by sensory Objective Visual evoked response (VER) / Visual evoked cortical potential (VECP) potential caused by sensory stimulation of a subject's visual field. Commonlyused visual stimuli are flashing lights, or checkerboards on a video screen that flicker between black on white to white on black (invert contrast).
Visual evoked potentials are very useful in detecting blindness in patients that cannot communicate, such as babies or non -human Objective Visual evoked response (VER) / Visual evoked cortical potential (VECP) such as babies or non -human animals. If repeated stimulation of the visual field causes no changes in EEG potentials, then the subject's brain is probably not receiving any signals from his/her eyes. Other applications include the diagnosis of optic neuritis, which causes the signal to be delayed. Visual evoked potentials are also used in the investigation of basic functions of visual perception.