There are many forms of vision chart, the most common is e, as well as words, numbers, cartoons and so on. For example, when the pilot is screened, the word C is used to test his eyesight, and the cartoon is used to test the eyesight of the child (before he knows the word) (let him say what it is). What we usually call visual acuity test is actually to test the ability of the eye to see far away (far vision chart). Its accurate definition is to measure the imaging on the smallest retina that can recognize its shape.
The visual acuity chart is designed according to the principle of visual angle. The so-called angle of view is the angle formed by the light emitted from two external points through the nodes in the eye. Under normal circumstances, the human eye can distinguish the minimum distance between two points, and the angle of view formed by it is the minimum angle of view, that is, one point of view. The visual acuity chart is designed in one point of view.
China adopted the E-chart because the Chinese Medical Association recommended the international standard visual acuity chart drawn by Professor Sun Jizhong at the ninth General Assembly of the Chinese Medical Association held in 1952. As the actual national standard, according to the standards of 1-minute visual angle, 5-meter distance and decimal record of the international visual acuity chart, the whole table is finally made with Snellen's e (also known as "illiterate e e"). In 1966, Professor Miao Tianrong proposed a 5-point logarithmic vision chart. After long-term promotion, logarithmic table has been widely used. However, the middle stroke of e visual marker is short, which is easy to identify and guess, and the vision is inaccurate. In 1989, the logarithmic table was determined as the national standard gb11533-1989. In this edition of the national standard, the improved E-Sign proposed by Guo Liangmin is adopted, which is the familiar "E-Sign with equal length of three strokes, and each stroke or gap is one fifth of the square". In contrast, this e is more in line with the design significance of strict viewing angle resolution.
One advantage of table e is that it has a certain grid structure, which is helpful for testing astigmatism. For example, if someone can clearly identify left and right e, but it is difficult to identify up and down e, it can be reasonably suspected that he may have a certain degree of astigmatism. If you use a, F, y, etc., you can also identify the direction of the tip if you don't see it clearly. If the Z and N directions are completely correct, there are only 50% left. If D, O, x, etc. are used, what else can be measured?
So there are not many letters to choose from. The neat square shape on three sides is a major advantage of E. However, e is not a unique choice. It is not impossible to use other letters, such as C and G. The pilot uses chart C for visual acuity test.
Another example is Chinese tabulation. You can use Chinese characters instead of English letters, so there are more choices. It's just that watch e is easy enough to use. It's not necessary.
Snellen chart is used for visual acuity test in the United States, which shows that the visual chart graphics do not have to have both axisymmetric and centrosymmetric characteristics.
Japan uses the ランドルト ring (i.e. table c), and Japan also has some vision charts of hiragana and katakana.
In Japan, because form C is not suitable for children, children's visual acuity chart with animal silhouette as an element will also be used when measuring the visual acuity of infants.
The Soviet Union once used such a table (Golovin – sivtsev table), which was still used by some former Soviet Member States until 2008.
Chinese character (near) vision chart designed by Wenzhou Medical College, the great God of ophthalmology in China