Visual acuity

Visual acuity is the most important measurement of the many visual functions. Visual acuity for international reporting purposes is measured as line test acuity. For educational and diagnostic purposes we need to measure visual acuity also with single symbol tests and with the crowded near test and the several different reading acuities (p.17) if the child can read print. In many countries there are no test texts so these have to be created. Local traditions in the assessment of reading difficulty of texts are used, where available, or general rules used in our own languages may be used when composing the first texts at different grade levels. Later the test texts should be written together with linguistic experts for each language so that we employ texts of equal difficulty throughout the test.

Measurement of visual acuity is based on standardised line acuity charts of letters (the American EDTRS chart with Sloan letters or charts with British or Swedish letter charts), numbers (LEA-numbers) or symbols like the LEA-symbols for children who do not know letters or numbers. Only the LEA-symbol vision test system has all three kinds of visual acuity test, single optotypes, line tests with 100% spacing and a more crowded tests to measure the effect of crowding.

Since most visually impaired children in developing countries usually have no exposure to letters and numbers, visual acuities are best measured with symbols that are easy to name. The LEA-Symbols were designed to function in different cultural settings. They have been easily named in numerous countries. However, children in Kenya and Uganda seem to have difficulty in naming the "apple"/ "heart" as "mango" or "tomato" for reasons that at present are not understood. During a project in Ethiopia even teen-age children could not be tested with number charts but all children could be tested with the symbol charts. The EDTRS-letter chart functioned in even fewer cases than the number chart. Measurement of visual acuity is started at near which is a more important distance in learning and communication than greater distances.

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