Visual acuity and contrast sensitivity are measures of visual function. Visual acuity is a measure of visual resolution and is affected by optical and physiological factors. It can be measured using letters of decreasing size on charts like the Snellen chart. Contrast sensitivity measures the ability to detect differences in luminance and is more relevant to daily vision tasks. It is assessed using gratings of varying contrast and spatial frequency. The Amsler grid is used to screen for macular diseases by having patients look for visual distortions or gaps within a grid viewed at the fovea.

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