Measurement of contrast sensitivity has yet to become accepted as a routine test. Although there are numerous reports on new insights that measurement of contrast sensitivity may give regarding the nature of abnormal vision and its use in assisting diagnosis, clinicians have not embraced the new tests which are now available.
Deaf persons using visual sign language are dependent on low contrast information in many communication situations where no special attention is normally given to the contrast between clothing and hands. Lip movements and finger spelling involve low contrast information in motion. Assessment of visual function in the low contrast domain is therefore of particular interest in deaf patients.
Contrast sensitivity tests
Those who have not had experience with contrast sensitivity tests may benefit by a short description of the tests and what these tests measure. Contrast sensitivity refers to the ability to discern small luminance differences present in adjacent surfaces. It can be measured in many different ways.
The technique most often used in research laboratories presents displays of grating patterns generated on a computer screen where mean luminance, exposure time, movement etc. can be controlled. The gratings are either sine wave or square wave gratings. By using gratings of different spatial frequencies (with different numbers of lines per degree of angle of vision) contrast sensitivity may be measured at each spatial frequency, and this allows the determination of what is called contrast sensitivity (CS) function or CS curve or contrast transfer function.
Commercially available grating tests
There are several commercially available printed grating tests and a few tests based on use of a computer. In the assessment of deaf patients Clement Clark's test functions well. It contains a sufficient number of cycles but it is only 2 degrees in diameter at the recommended six metres' testing distance. Clement Clark's test has a demonstration plate at 13% contrast, while the highest test plate is at 5% contrast. Some visually impaired persons do not see anything except the demonstration plate and an occasional patient does not see that either. Since we need grating test values between 13% and 95% contrast it would be of great practical value if a test plate at 25% contrast could be added to this otherwise handy test.
The Low Contrast Grating Acuity test is simple to use. Instead of a fixed spatial frequency it has plates with fixed contrast and by varying the test distance measures the lowest contrast at each spatial frequency used, that is, we can measure grating acuity at different contrast levels. This technique seems to be better than the classical contrast sensitivity measurement in clinical work, the definition of threshold is easier for the patient.
Printed optotype tests
Contrast sensitivity can be measured with low contrast optotypes. Printing of low contrast optotype tests with the required accuracy in contrast rendition is extremely difficult, especially at the lowest contrasts needed. Most commercially available low contrast optotype tests, like the University of Waterloo near vision card and the new Bailey-Lovie charts, have test materials between 95% and 10% contrast. The Pelli-Robson low contrast letter chart was the first printed optotype test at lower contrast levels. It uses one letter size corresponding 0.03 or 20/670 at the test distance of one metre.
Communication during testing of contrast sensitivity
Communication in sign language has not been a problem when measuring contrast sensitivity in a number of deaf persons with Usher's Syndrome, a congenital loss of hearing and later a developing loss of vision due to retinal degeneration. The optotype tests are used the same way as when measuring visual acuity at high contrast. Grating tests require that the patient either shows the direction of lines with his hand, which is easy, or answers on which of the two pages in the Cambridge test the grating is located.