Monday, May 22, 2006

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Contrast Sensitivity

Contrast indicates the variation in brightness of an object. When an eye chart uses perfectly black ink on perfectly white paper, 100% contrast is achieved. Acuity charts approximate 100% contrast. Acuity charts are helpful for characterizing central visual acuity. However, they are less helpful for examining visual function away from fixation.

Contrast sensitivity is tested using alternating light and dark bars at varying intensity. The number of light bands per-unit length or per-unit angle is called the spatial frequency. During clinical testing of contrast sensitivity, patients are presented with targets of various spatial frequencies and peak contrasts. The minimum resolvable contrast is the contrast threshold. The reciprocal of the contrast threshold is defined as the contrast sensitivity, and the manner in which contrast sensitivity changes as a function of the spatial frequencies of the target is called the contrast sensitivity function.

Contrast sensitivity can be tested with sine wave gratings presented using either charts or video gratings. Because standard Snellen acuity charts test only the higher spatial frequency (30 cycles per degree), they do not provide an accurate picture of an individual's visual functioning, particularly when the individual has an ocular disease.

Acuity charts provide us with a quantitative assessment of visual functioning while contrast sensitivity charts provide us with a qualitative assessment of visual functioning. Contrast sensitivity testing is similar to current audiological testing which assesses an individual's ability to hear various tones and frequencies.

Contrast sensitivity testing can detect changes in visual function at times when Snellen visual acuity is normal. This can occur when corneal pathology, cataracts, glaucoma and various other ocular diseases are present.



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