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 Encyclopedia - Contrast Sensitivity

What is contrast sensitivity?
How is contrast sensitivity testing performed?
Why is contrast sensitivity testing important?

What is contrast sensitivity?

Contrast sensitivity is the visual ability to see objects that may not be outlined clearly or that do not stand out from their background. The ability to see a shade of gray on a white background or to see white on a light gray background declines with age. Cataracts, diabetic retinopathy and other conditions can also affect contrast sensitivity.

A person with low contrast sensitivity may have such vision difficulties as:

  • Trouble seeing traffic lights or cars at night
  • Not being able to see spots on clothes, counters, or dishes
  • Missing facial gestures
  • Not seeing whether a flame is burning on a stove
  • Needing a great deal of light to read
  • Experiencing tired eyes while watching television.

How is contrast sensitivity testing performed?

Contrast sensitivity tests measure the degree to which this ability has been lost. Unlike the Snellen visual acuity test, which measures the ability to see objects (or letters) of different sizes, a contrast sensitivity test measures two variables, size and contrast. The ability to detect objects of different sizes at lower contrasts is expressed as a contrast sensitivity function (CSF). The test determines the person’s contrast detection threshold, the lowest contrast at which a pattern can be seen.

Typically, the best scores of CSF are recorded for medium-sized objects when their contrast is low. The smallest objects can be detected only when their contrast is very high. Imagine an image comprising vertical black and white stripes. If the stripes are very thin, individual stripes will not be visible. Only a gray image is visible. As the stripes then become wider, there is a threshold width from where it is possible to distinguish the stripes.

The fact that larger objects require higher contrast is explained not by how the eye gathers information but by how the brain processes that information. The brain is relatively insensitive to what neurologists call "low spatial frequencies." Contrast sensitivity readings are presented as a curve, which plots the lowest contrast level at which a person can detect an object of a given size. The higher the contrast sensitivity, the lower the contrast level at which an object can be seen.

Why is contrast sensitivity testing important?

CSF measurements can be important in diabetes and cataract medicine. Cataracts are a clouding of the eye’s natural lens, which causes light to scatter on the retina, reducing image contrast and causing dimness of vision. Many cataract patients have good visual acuity, but they still notice a loss of their visual capability. Contrast sensitivity testing can provide a true "real world" vision of a patient’s functional vision. Tests of contrast sensitivity are normally done before and after cataract surgery, to document the degree of improvement that has been achieved.

Studies in recent years suggest that early signs of diabetic retinopathy can be detected by evaluating contrast sensitivity. A person with diabetes may have a 20/30 score for visual acuity and still be experiencing eye damage that requires treatment to prevent progression. A low CSF score for a patient with good visual acuity is an indication of diabetic retinopathy. So is a difference in readings of contrast sensitivity between the two eyes.

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