10. contrast sensitivity

what is contrast

the difference in luminance between the letter and
background

what is contrast threshold

The lowest visible contrast

what is contrast sensitivity

the reciprocal of contrast threshold

what is the formula for contrast

(Lt- Lb)/ Lb

❖ Lt luminance of the target , Lt luminance of the background

❖ However since the development of sine waves as repetitive stimuli used for contrast

sensitivity testing , Michelson Contrast =

❖ (Lmax- Lmin)/ (Lmax + Lmin)

❖ Lmax and Lmin represent the min and max luminance of the grating

what is the most sensitive frequency

you are most sensitive for an intermediate range of spatial

frequencies (around 4-6 cycles/degree

Factors that affect contrast sensitivity

Environmental

• Fog

• Rain

• Illumination (night vs day, etc)

Ocular

• Clarity of ocular media

• Eye disease

Optical

• Defocus

what are the Clinical Measures of Contrast Sensitivity

Vistech

Pelli-Robson

what are the types of contrast sensitivity loss

Type I – Reduced sensitivity to high spatial frequencies only, while low spatial

frequencies remain unaffected

❖ Most commonly occurs in patient is with retinal defocus i.e uncorrected

refractive error

❖ Type II – Contrast Sensitivity loss indicates contrast sensitivity loss across all

spatial frequencies.

❖ Produced by light scattering (e.g. due to cataract ,or corneal opacities

❖ Type III – CS Loss- is reduced sensitivity at low spatial frequencies

❖ Individuals will have normal VA .

what is the pelli robson chart used for

used for determining functional vision loss in patients with low vision and moderate /dense cataract. low spatial frequency loss , patient with Alzheimers

what is the layout of a pelli robson chart

1. letters organised into groups of three (triplets)

2. each triplet has the same contrast

3. the contrast decreases with each triplet

what are the considerations for the pelli robson chart

1. done before dialation

2. patient to sit directly infront of chart

3. wear best distance correction if-necessary add + 0.75

4. RE, then left then both

5. patients should be made to guess even if they cant fully make out letters

6. alllow time for faintest letters to appear

7. dont let give up until he she has guessed 2/3 triplets incorrectly

8. ignore first triplet when scoring with 0.05

9. tell patient that bottom letters are hard for everyone

10. normal 20-50 = 1.80 logless than 20 older than 50 = 1.65 log

what is the bailey lovie chart

1. 5 letters per row

2. equal letter spacing

3. size progression by constant ratio

4. optotype set is the family of ten British Standard (1968) 5×4 non-serifed letters. There are 14 rows with a range of sizes from 38 to 1.9 M- unit

5. distance used depends on chart