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Low Vision Assessment and Rehabilitation

Initial Low Vision Assessment

Contrast Sensitivity Testing

Visual acuity is measured with letters of maximum contrast provided by black letters on a white background. However, most visual tasks involve objects with considerably less contrast. Eye disease can impair the relative ability to detect differing levels of contrast. Simply put, a person may easily see a black pen on a white background, but the same pen may be invisible on a dark background.

Contrast sensitivity function (CSF) is most commonly determined by measuring contrast thresholds of alternating light and dark bars (the grating), the luminance (brightness) of which varies. The size of the grating is specified according to spatial frequency by the number of grating pairs per degree of visual angle.

Several test materials are available for contrast sensitivity testing. The Vistech Contrast Sensitivity (CS) chart is simple to administer. From left to right on the chart the contrast of the targets decrease. From top to bottom, the sine wave frequency increases. Pelli-Robson charts are also often used, and are in fact easier and more time-efficient.

Dr. Cole was tested using the Vistech Contrast Sensitivity chart. He was unable to see any of the testing circles with his right eye; the eye was too damaged to assess contrast sensitivity. The left eye showed moderate loss at low frequency gradients and profound high frequency loss. This indicates that at this stage of the disease process he would experience some difficulties with mobility in conditions of poor contrast and/or poor illumination (low frequency loss), and also indicates that he would have considerable difficulty with fine detail (high frequency loss).