By Gary Heiting, OD
http://www.allaboutvision.com/eye-exam/contrast-sensitivity.htm

Your contrast sensitivity test results can help your eye doctor determine if you have vision errors known as higher-order aberrations or some other problem that could be corrected with eye surgery. Custom or wavefront LASIK can reduce higher-order aberrations and improve contrast sensitivity. The FDA has required contrast sensitivity testing since 1991 for the evaluation of new ophthalmic devices, including excimer lasers used for LASIK!

Contrast sensitivity is a very important measure of visual function, especially in situations of low light, fog or glare, when the contrast between objects and their background often is reduced. Driving at night is an example of an activity that requires good contrast sensitivity for safety. Even if you have 20/20 visual acuity, you can have eye or health conditions that may diminish your contrast sensitivity and make you feel that you are not seeing well.

Symptoms of Reduced Contrast Sensitivity

If you have low contrast sensitivity, you may have problems with night driving, including difficulty seeing pedestrians walking alongside poorly lit streets. Or you might notice that your eyes tire more easily while reading or watching television.
Poor contrast sensitivity also can increase your risk of a fall if you fail to see that you need to step down from a curb onto similarly colored pavement. Low contrast sensitivity can be a symptom of certain eye conditions or diseases such as cataracts, glaucoma or diabetic retinopathy.

Contrast Sensitivity Testing

Contrast sensitivity testing often isn’t included in a routine eye exam. Your eye doctor might perform the test because of a specific visual complaint you have or because he or she suspects you have a condition that is affecting your ability to discern contrast.

Probably the most widely used device to test contrast sensitivity is the Pelli Robson contrast sensitivity chart. The Pelli Robson contrast sensitivity chart tests your ability to detect letters that are gradually less contrasted with the white background as your eyes move down the chart. Like a standard Snellen visual acuity chart, the Pelli Robson chart consists of horizontal lines of capital letters. But instead of the letters getting smaller on each successive line, it is the contrast of the letters (relative to the chart background) that decreases with each line.

Other, more sophisticated devices also may be used to test your contrast sensitivity. These devices often use targets called sine-wave gratings that consist of a number of fuzzy, parallel bars of light and dark. These bars can vary in width (spatial frequency) as well as contrast from target to target, to give a more thorough evaluation of how sensitive your eyes are to differences in contrast.
Some sine-wave grating tests include a bright light source that can be directed toward your eyes during the test to simulate glare situations such as oncoming headlights during night-driving.

If your eye doctor determines that you need a contrast sensitivity test, it likely will be administered after a standard visual acuity test and before your pupils are dilated. In more elaborate contrast sensitivity tests, you may be shown images such as dots and bars with different contrasting backgrounds under varying conditions such as glare. The testing typically is done while you wear your eyeglasses or contact lenses if you require vision correction.

For evaluation of eye disease, contrast sensitivity usually is tested on each eye individually. For other reasons, such as sports vision testing or to evaluate vision after contact lens fitting, LASIK or cataract surgery, the testing might be done with both eyes open.

Contrast Sensitivity Function (CSF)
Detailed contrast sensitivity measurements that include both size (spatial frequency) and contrast are used to plot a person’s contrast sensitivity function (CSF).

Sine-wave grating targets with thicker bars represent low spatial frequencies; targets with thinner bars represent higher spatial frequencies. In this regard, determining a person’s CSF is much like evaluating the sensitivity of his or her hearing, which involves using tones of low and high pitch as well as variations in volume.

Your contrast sensitivity function essentially is a plotting of the curve that defines the lowest contrast level that you can detect for each spatial frequency tested.
Generally, objects with high spatial frequencies (sine-wave gratings with very thin bars) must have significantly higher contrast than objects with lower spatial frequencies (gratings with medium-width bars) to be detected by the human visual system.