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Ian L Bailey; The Visual Acuities and Their Clinical Measurement. Journal of Vision 2016;16(4):37-38. doi: 10.1167/16.4.36.
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There is a multitude of options available for the clinical measurement of visual acuity. Variations in the characteristics of the tests and their administration can affect the scores of visual acuity. And the test task influences acuity scores can be very dependent on the nature of any ocular disorders.
Acuity scores are affected by whether the optotypes are Landolt rings, tumblingE's various families of letters, numbers, pictures or symbols. Reading acuity tests might include flowing text or unrelated words.. Other acuity tests use gratings, checkerboards or vanishing optotypes. Optotypes might be presented singly, in rows, in chart format and sometimes flanking bars are used. The visual acuity score is invariably a measure of angle dependent on the viewing distance and the size of a specific feature of the test target, but it is not always clear what that specific feature should be.
There is no one “true” visual acuity. All presentations of visual acuity results should include a full description of the test and procedures. The score of visual acuity on one test cannot reliably predict the score from another. For example, in many persons with abnormal macular function, the ability to identify visual acuity targets can be very dependent on the congestion of detail within the test task.
Computer driven displays enable more variations in optypes, formats and testing protocols, but there are associated limits imposed by pixellation and screen sizes.
Accordingly, we can expect to see more variations in visual acuity testing procedures, changes to simpler tests when visual acuity is poorer, and new special tests to quantify visual efficiency and the effects of task congestion, luminance and contrast.
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