<unk> s display several rows of optotypes, which are standardized symbols for testing vision. Optotypes are usually letters, numbers, or geometric symbols. Each row of the<unk>  depicts optotypes of a different size. Typically the largest optotypes are in the top row. The optotypes become progressively smaller towards the bottom of the<unk> . The person removes any glasses or contact lenses, and stands or sits a standardized distance from the<unk>  (e.g., 20 feet for the Snellen<unk> . The person is then asked to identify the optotypes on the<unk> , starting with large rows and continuing to smaller rows until the optotypes cannot be reliably identified any more. The row in which the person can reliably identify symbols defines the visual acuity. One<unk>  is tested at a time. Practically, this is accomplished by covering the other<unk>  with a hand, piece of paper, or a small paddle. After testing without glasses or contact lenses, testing is repeated while the person wears them, if applicable. Often, the use of such refractive lenses will correct visual acuity to normal. Refractive error can be corrected using a pinhole occluder. If the visual acuity improves with the use of pinholes, refractive lenses can be utilized to improve visual acuity. Squinting can achieve the same effect as a pinhole occluder. With the Snellen<unk> , the visual acuity is recorded as a fraction with 20 in the numerator (top number) and values ranging from 10 to 600 in the denominator (bottom number). The denominator indicates the distance in feet at which a person with normal vision could stand to correctly identify the same symbols identified by the person tested. For example, a visual acuity of 20/20 is considered normal.
Eye chart
EyeBuyDirect
Snellen chart
Eye bank
Visual field
Solar viewer
Lensmeter
Audio power amplifier