Contrast Sensitivity Testing in Eye Exams

We often undergo standard vision testing by viewing eye charts in high contrast conditions, where black letters or numbers of varying sizes are sharply displayed against well-lit, white backgrounds. But the real world is full of shades of gray in which people hampered by poor contrast sensitivity can have serious problems seeing distinct images under certain conditions.

Even if you have 20/20 vision, you could have poor contrast sensitivity. Contrast sensitivity testing is important because it allows eye doctors to determine how well you function in real-world conditions where objects may "blend" and become indistinguishable in backgrounds with similar coloring.

Contrast sensitivity is crispness of vision, enabling us to see objects that do not stand out from their backgrounds. Contrast sensitivity is often referred to as "functional vision" beyond how well you see details on a standard eye chart.

If you have low contrast sensitivity, you may have problems with night driving, including inability to see traffic lights or spot other cars and pedestrians. People with poor contrast sensitivity also may require extra light to read, and their eyes may become tired when they read or watch television. Poor contrast sensitivity also can cause you to stumble when you fail to see that you need to step down from a curb onto similarly colored pavement.

Low contrast sensitivity also can be a symptom of certain eye conditions or diseases such as cataracts, glaucoma, or diabetic retinopathy. Also, reduced contrast sensitivity sometimes can occur as a usually temporary side effect of laser vision surgery procedures including LASIK and PRK. Lack of contrast sensitivity is not the same as blurred vision, however.

Contrast sensitivity testing gauges your ability to see objects in terms of size and contrast. Small objects, for example, can be seen only when their contrast is very high. Medium-sized and large objects, on the other hand, often can be seen when their contrast is low.

When you undergo a contrast sensitivity test, you may be shown images such as dots and bars with different contrasting backgrounds under varying conditions such as glare. (Photos courtesy of VectorVision, based in Greenville, Ohio)

A person's ability to see objects against low contrasting backgrounds is expressed as a contrast sensitivity function (CSF). Your eyes will be measured to determine the lowest contrast at which certain objects such as bars or dots can be seen, relative to their backgrounds.

Some eye doctors administer the contrast sensitivity test as part of a routine screening. If your eye doctor determines that you need a contrast sensitivity test, it likely will be administered after the visual acuity test that determines your ability to see details such as letters or numbers on an a standard eye chart.

Contrast sensitivity testing likely will be done while you wear your eyeglasses or contact lenses, if you require vision correction. Unlike standard visual acuity testing, both eyes are tested together to determine ability to see low-contrast objects. You will be asked to look at a low-contrast test chart, with images such as bars or dots shown during a sequence of increasingly lower contrasts until you can no longer identify them.

For evaluation of eye disease, contrast sensitivity is tested on each eye individually. Contrast sensitivity is also used to assess visual performance, such as for athletes or contact lens wearers, or after refractive surgery such as LASIK. In this case, unlike conventional acuity testing, both eyes may be tested together to assess how you see in the real world with both eyes working together.
 

Contrast sensitivity readings are shown as a curve that plots the lowest contrast level at which a person can detect certain objects. When you have good contrast sensitivity, you can see objects even when they are displayed against increasingly low-contrast backgrounds.

Many contrast sensitivity tests use bar or dot patterns displayed against backgrounds of varying contrast. These objects are shown at varying intensities (spatial frequencies), making them easier or more difficult to detect. To plot the curve used in test results, your eye doctor determines the highest contrast sensitivity level that you can detect for each spatial frequency used.

Tests can be performed under well-lit and dimly lit conditions to help determine your contrast sensitivity. Outcomes of these contrast sensitivity tests can provide detailed information about how your eyes process images.

Test results may also help your eye doctor determine whether you require specialized lenses or vision correction surgery for vision errors known as higher-order aberrations, or if you have an eye disease.

If you are diagnosed with low contrast sensitivity, your eye doctor may advise you to wear artificial lenses with a yellow filter to help improve contrast. Some eyeglass lens manufacturers also say contrast sensitivity may be improved with lenses designed to offset higher-order aberrations, often called wavefront lenses. In some cases, custom or wavefront LASIK has been shown to reduce higher-order aberrations that can cause low contrast sensitivity.

Certain intraocular lenses (IOLs) used to replace your eye's natural lens following cataract surgery also may be tinted yellow to help improve contrast. Other IOLs also have been designed through wavefront technology to help offset low contrast sensitivity associated with higher-order aberrations.

The U.S. Food and Drug Administration has required contrast sensitivity testing since 1991 for the evaluation of new ophthalmic devices, including excimer lasers used for LASIK and new types of intraocular lenses. So if you volunteer for a clinical trial in which a new type of device or lens is being evaluated, you may be tested for contrast sensitivity under many conditions including daytime, nighttime, and glare.