vision and contrast sensitivity

1. VISUAL ACUITY,
CONTRAST SENSITIVITY,
AMSLER GRID

2. VISUAL ACUITY
• is a measure of keenness of sight
• VA is affected by:
• (a) Optical factors that influence the quality of light reaching the retina
• (b) Physiological factors that determine photoreceptor sensitivity and neural
processing

3. VISUAL ACUITY
• multitude of ways to measure visual function
• The minimum legible threshold (ordinary visual acuity) - the point at which a
patient cannot further distinguish progressively smaller letters or forms from one
another - minimum resolvable
• The minimum visible threshold - minimum brightness of a target at which the
patient can distinguish the target from the background. – minimum visible
• The minimum separable threshold (hyperacuity) - the smallest visual angle formed
by the eye and 2 separate objects at which a patient can discriminate them
individually. --- minimum discriminable
• Vernier acuity - smallest detectable amount of misalignment of 2 line segments

6. FACTORS INFLUENCING VISUAL ACUITY
• Optical factors
• (i) Refractive error
• (ii) Media opacities
• (iii) Pupil size
• • A large pupil (>6 mm) reduces VA by increasing spherical and higher order aberrations
• • A small pupil (<2 mm) reduces VA by increasing diffraction of light
• (iv) Wavelength
• • VA is marginally better for monochromatic light; however, this is most noticeable at
low contrast.
• • This is because chromatic aberration leads to image degradation

7. • Physiologic factors
• (i) Foveal cone density
• • The density of foveal cone packing is a critical determinant of fine visual resolution .
• • Human foveal cones are separated by approximately 30 s of arc; hence, MAR is 1 min
• (ii) Cone to ganglion cell convergence
• • The fovea is characterized by 1:1:1 cone-bipolar-ganglion cell convergence
• (iii) Retinal illumination
• • At low ( scotopic ) luminance levels mediated by rods, VA is reduced; however, it
increases with increasing retinal illumination.
• • Under moderate photopic luminance conditions, VA remains fairly constant

8. CLINICAL MEASUREMENT OF VISUAL
ACUITY
• 1. Snellen chart
• 2. Bailey-Lovie and Early Treatment of Diabetic Retinopathy Study (ETDRS) charts
• 3. Snellen-like tests
• Illiterate E
• Landolt C tests
• 4. Other forms of VA testing (useful in young children)
• Visual evoked reflex
• Optokinetic nystagmus
• Preferential looking

9. SNELLEN VISUAL ACUITY
• letters constructed such that each letter as a whole subtends an angle of 5 minutes
of arc
• each stroke of the letter subtends 1 arcmin
• Letters of different sizes are designated by the distance at which the letter subtends
an angle of 5 arcmin

11. • Though widely accepted, the standard Snellen chart is not perfect
• The letters on different Snellen lines are not related by size in any geometric or
logarithmic sense
• For example, the increase in letter size from the 6/6 line to the 6/9 line differs from
the increase from the 6/9 line to the 6/12 line
• In addition, certain letters (such as C, D, O, and G) are inherently harder to
recognize than others (such as A and J)

12. ETDRS CHART
• each line has the same number of letters, so
crowding is standardized
• decrements in size are uniform: each line is
diminished in size by a factor of 0.1

13. CONTRAST SENSITIVITY
• Contrast is a measure of the lightness or darkness of an object compared to its
background.

14. RELEVANCE OF CONTRAST SENSITIVITY TO
DAILY FUNCTION
• • The object size influences how much contrast is needed to differentiate it from its
background
• Patients with normal visual acuity (VA) may complain of poor vision if CS is reduced
• Snellen visual acuity is approximate 100% contrast
• we are measuring, at approximately 100% contrast, the smallest optotype that the
visual system can resolve

15. • CS is determined by the:
• (a) Stimulus contrast: luminance of the stimulus compared with the background
• (b) Stimulus size: spatial extent of the stimulus against the background

16. CS MEASUREMENT
• Sunisoidal gratings

17. SPATIAL FREQUENCY
• Densely packed lines have high spatial frequency; sparsely packed have low spatial
frequency.
• • Spatial frequency is related to visual acuity
• For example, for a spatial frequency of 30 c/deg, there are 30 alternating black and
white stripes per degree (60 min), and each stripe subtends 1 min of arc.
• • Hence, 30 c/deg is equivalent to Snellen 6/6

18. FACTORS THAT INFLUENCE CONTRAST
SENSITIVITY
• Scotopic vs photopic conditions
• • CS is reduced for scotopic compared to photopic conditions
• • Peak photopic CS occurs at spatial frequency 5 c/deg (6/36); peak scotopic CS
occurs at 1 c/deg
• Retinal eccentricity
• CS is maximal using foveal vision.
• • It reduces with increasing retinal eccentricity
• Mean luminance
• • CS decreases with decreasing mean luminance

19. CLINICAL TESTING OF CONTRAST
SENSITIVITY
• Pelli-Robson chart - - Varying contrast at a fixed spatial frequency corresponding to
peak CS (5c/min)(6/36)
• Functional Acuity Contrast Test - - Varying contrast and several different
frequencies

21. AMSLER GRID
• The Amsler grid evaluates the 20° of the visual field centred on fixation

22. • useful in screening for and monitoring macular disease
• also demonstrate central visual field defects
• Patients with a risk of CNV be provided an Amsler grid for regular use at home

23. CHARTS
• Chart 1 consists of a white grid on a black background,
enclosing 400 smaller 5 mm squares. When
viewed at about one-third of a metre, each small square
subtends an angle of 1°

24. • Chart 2 is similar to chart 1 but has diagonal lines that aid
fixation for patients with a central scotoma

25. • Chart 3 is identical to chart 1 but has red squares
The red-on-black design aims to stimulate long
Wavelength foveal cones
It is used to detect subtle colour scotomas and
desaturation in toxic maculopathy, optic neuropathy
And chiasmal lesions

26. • Chart 4 consists only of random dots and is
used mainly to distinguish scotomas from
metamorphopsia, as there is no form to be
distorted

27. • Chart 5 consists of horizontal lines and is
designed to detect metamorphopsia along
specific meridians
• It is of particular use in the evaluation of
patients describing difficulty reading

28. • Chart 6 is similar to chart 5 but has a white
background and the central lines are closer
together, enabling more detailed evaluation

29. • Chart 7 includes a fine central grid, each square
subtending an angle of a half degree,
and is more sensitive