Contrast sensitivity (CS) is a very important concept to understand. The loss of CS often results in visual difficulties and dysfunction out of proportion to the patient’s measured visual acuity. For example, patients with nonexudative macular degeneration or diabetic macular edema may have good measured visual acuity of 20/30 or better but report experiencing difficulty when performing routine visual tasks such as reading the newspaper or navigating stairways. Because a conventional Snellen visual acuity is based on high-contrast, achromatic, square-wave stimuli, it does not measure the patient’s ability to perceive the subtleties of light. However, the visual system codes much of what is seen based on contrast rather than spatial resolution, with subtleties of light and dark providing most of the richness of visual perception. For example, when dusk, fog, or smoke reduce contrast, it becomes very difficult for anyone to resolve ordinary objects. Similarly, when patients become unable to perceive contrast under ordinary conditions, for example as a result of many retinal diseases or from media opacities, visual function is adversely impacted.
Testing of contrast sensitivity
Several clinical tests of CS are available. Most relate CS to spatial frequency, which refers to the size of the light–dark cycles. Individuals are typically most sensitive to contrast for objects that have a spatial frequency between 2 and 5 cycles per degree (Fig 3-10), but this sensitivity can change in patients with disease. Some tests use letters or optotypes of varying dimness and size to provide a more clinical context.
The Pelli-Robson test measures contrast sensitivity using a single, large letter size (20/60 optotype), with contrast varying across groups of letters. Patients read the letters, starting with the highest contrast, and continue until they are unable to read 2 or 3 letters in a single group. The subject is assigned a score based on the contrast of the last group in which 2 or 3 letters were correctly read. The Pelli-Robson score is a logarithmic measure of the subject’s contrast sensitivity. Thus, a score of 2 means that the subject could read at least 2 of the 3 letters with a contrast of 1% (contrast sensitivity = 100%, or log10 2). That is, a score of 2.0 indicates normal contrast sensitivity of 100%. A Pelli-Robson contrast sensitivity score of less than 1.5 is consistent with visual impairment, and a score of less than 1.0 represents visual disability (see Fig 3-10).
Figure 3-10 The top 5 of 8 lines of a standard Pelli-Robson contrast sensitivity chart. The top left 3-letter block has a log contrast value of 0.05; there is a log contrast change of 0.15 with each 3-letter block. In the full 8-line chart, the lowest contrast letters have a log value of 2.3; 2.0 represents normal contrast sensitivity.
(Used with permission from Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci. 1988;2(3):187–199.)
Contrast sensitivity testing is discussed further in BCSC Section 3, Clinical Optics, and Section 5, Neuro-Ophthalmology.
Owsley C. Contrast sensitivity. Ophthalmol Clin North Am. 2003;16(2):171–177.
Rubin GS. Visual acuity and contrast sensitivity. In: Ryan SJ, Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P, eds. Retina. 5th ed. Philadelphia: Elsevier/Saunders; 2013:300–306.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.