Contrast sensitivity

Jagdish Dukre

What is contrast sensitivity ?

Visual ability to see object that may not be outlined clearly or not stand out from their background.

The ability to see a shade of gray on white background or to see white on light gray background.

Contrast sensitivity

Contrast Def.

The difference in the luminance of target against the background.

contrast

target luminance – background luminance

target luminance + background luminance

Contrast threshold

The minimum resolvable contrast.

It described in logarithmic terms.

Contrast sensitivity function

The manner in which contrast sensitivity changes as function of spatial frequency of the target .

Contrast sensitivity can be tested with sine wave grating using either charts or video gratings or

with opto types of variable contrast such as pelli-robson or regan chart.

How is c.s different than acuity :- CS is unlike acuity , c.s measures tow

variable size & contrast while acuity measures only size.

CS measure the degree to which this ability has been lost , while snellen visual acuity measures the ability to see object of different size.

Snellen chart useful for describing changes in vision caused by spherical blur but unable to detect many types of vision loss are not cased by spherical blur.

Compare:

E

E

Letter Size Spatial Frequency

smallerhigher

Some patient can have normal visual acuity & reduced c.s such as those with

1. Optic neuritis .2. Multiple sclerosis3. Parkinson’s disease4. Papilloedema5. Primary open angel glaucoma6. Diabetic retinopathy 7. Compressive lesions of visual pathways

Benefits of c.s testing

early detection of serious eye diseases, It also helps in more accurate monitoring of ocular conditions

Contrast sensitivity testing complements the traditional 20/20 visual acuity value by measuring more subtle vision losses due to problems with spectacle lenses and contact lenses.

It also offers important information in the fitting of contact lens

Contrast sensitivity testing assists in more precise assessments of patients prior to surgery and post-operatively.

Sinusoidal gratings

The most commonly used stimuli for assessing the contrast sensitivity characteristics of the human visual system are sinusoidal gratings, consisting of an alternating pattern of light and dark bars whose luminance varies sinusoidally in the direction perpendicular to the grating's orientation.

photometer

The Physical Stimulus: Spatial pattern

The appearance of a sinusoidal grating is an alternating series of fuzzy light and dark lines.

The size of the light and dark bars of the grating is specified according to spatial frequency, which is the number of cycles (pairs of light and dark bars) of the grating pattern per degree of visual angle.

Typically, the range of spatial frequencies examined is between 0.5 and 30 cycles per degree.

The contrast of the sinusoidal grating is determined by the luminance of the peaks (Lmax) and troughs (Lmin) of the luminance profile

Contrast sensitivity curve:-

C .S is presented as curve which plots the lowest contrast level a patient can detect for specific size target.

The x-axis curve is for spatial frequency, y-axis is for contrast sensitivity.

Seeing your own CSF

Contrast

Spatial Frequency

Low spatial frequency High

Hig

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c

ontr

ast

Lo

wTHECONTRASTSENSITIVITYFUNCTION

Brightness Perception

What are symptoms of low c.s?

Have problems with night driving, including inability to see traffic lights

May require extra light to read

Their eyes may become tired when they read or watch television

Not being able to see spots on clothes, counters, or dishes

Missing facial gestures

CONTRAST SENSITIVITY TESTING It will be done while you wear your eye

glasses or contact lenses if you require vision correction

Testing done with both eye together to determine ability to see low contrast object

For evaluation of eye disease contrast sensitivity contrast sensitivity is tested on each eye individually .

Contrast sensitivity measured at normal room illumination which about 30-70 footlamberts

Luminance must be kept constant when contrast sensitivity tested because mean luminance has an effect on the shape at normal contrast sensitivity function

Contrast sensitivity

Conditions affects cs Pupil size has effect on contrast sensitivity as it does on

snellen acuity miotic pupils =diffraction reduce contrast sensitivity Dilated pupils leads to optical aberration Contrast sensitivity can also be affected by a distortion or

edema. Lens changes due cataract leads to decrease contrast

sensitivity. Retinal pathology may affect contrast sensitivity. Retinitis pigmentosa. Central serous retinopathy. Macular degeneration. Glaucoma. Retrobulber optic neuritis. Amblyopia.

Pelli-robson letter contrast sensitivity It is simply of quickly measured and provides

a reliable measurement of low spatial frequency contrast sensitivity ( 0.5 – 2 cycles)

Method: Sit the patient 1m from the chart with his or

her correction if needs Occlude one eye or with both eye With both eyes result high with 0.15 log units Asked patient to read lowest letter that can

see

Pelli-robson letter contrast sensitivity

Record the contrast sensitivity score in log units

Results: For patient 20-50 years old monocular

contrast sensitivity should be 1.80 log unit Patient less than 20 years and older so

years =1.65 log units

Most common errors

Not allowing patient at least 20 second for letters to become visible

Not pushing patient to guess Illumination is poor

Cambridge low contrast gratings It is a rapid and simple screening test for

contrast sensitivity. Performed at a distance of 6m. It comprises of 12 pair of plates consisting of

stripes of varying contrast. First one is for demonstration and rest are

for the proper testing and are numbered from 1-10.

The plates are changed sequentially starting from plate 1 till the patient fails to respond.

Cambridge low contrast gratings

Then a new series is begun starting 4 plates

prior to where the patient failed to respond. Four such series are completed and the

score of each series is noted (numbered as per the number of plate read) and added.

The final total value is converted into contrast sensitivity from the provided table.

Functional Acuity Contrast Testing (FACT) FACT charts were developed by Dr. Arthur

Ginsburg. FACT comprises of a chart with sine-wave

gratings of varying contrast as well as varying spatial frequencies.

It is an accurate and comprehensive grating chart that tests functional visual acuity.

This test is performed at a distance of 10 feet.

The chart tests five spatial frequencies (sizes) and nine levels of contrast.

The Contrast varies in a row, decrease from left to right. And the spatial frequencies increase as one move down the

various columns from top to bottom The patient determines the last grating seen for each row (A, B,

C, D and E) and reports the orientation of the grating: right, up or left.

The last correct grating seen for each spatial frequency is plotted on a contrast sensitivity curve.