Snellen chart

A Snellen chart (sometimes just called an eye chart) is a chart used by ophthalmologists and opticians to measure visual acuity. It is printed with several lines of block letters. The first line consists of one very large letter. Subsequent rows have increasing numbers of letters that decrease in size. Patients taking the test cover one eye, and read aloud the letters on each row, beginning at the top. The smallest row that can be read accurately indicates the patient's visual acuity in that eye. Charts are available for very young children or illiterate adults that do not require letter recognition. One version uses simple pictures. Others are printed with the capital block letter "E" turned in different orientations. The patient simply indicates which direction each "E" is facing. The "broken ring" chart is similar: rows have circles with different segments missing, and the test-taker describes where each broken piece is located. The last two kinds of charts also reduce the possibility of patient guessing the images. Snellen charts have been the target of some criticism. The fact that the number of letters increases while the size decreases introduces two variables, rather than just one. Several studies indicate that the crowding together of letters makes them inherently more difficult to read. Another issue is that there are fairly large and uneven jumps in acuity level between the rows. To address these concerns, more modern charts have been designed that have the same number of letters on each row, and which use a geometric progression to determine the size of each row of letters. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen (1834-1908), who developed the chart in 1862. Category:Ophthalmology