Why use the letter "e" on the chart?

What we usually call vision measurement is actually designed according to the principle of visual angle, and its purpose is to test the hyperopia ability of eyes. Its accurate definition is to measure imaging on the smallest retina whose shape can be recognized. Commonly used "e" eye chart. It is expressed by a formula, that is, V= 1/a indicates that the field of view A is the visual angle.

In general, the human eye can distinguish the minimum distance between two points to form the minimum viewing angle, that is, the projection distance of a viewing angle on the retina is 5 meters, and the 10 line of "e" is only a viewing angle. However, we are familiar with the word "e", which has three strokes of equal length, and each stroke or gap is one-fifth of a square. Compared with this, the word "e" is more in line with the strict design significance of viewing angle resolution.

At the ninth general meeting of the Chinese Medical Association held in 1952, the Chinese Medical Association recommended Professor Sun Jizhong to draw an international standard visual acuity chart, which is what we are seeing now. Since then, this form has been upgraded to the national standard.

So, is the "eye chart" used by China the only one? Not all countries use spreadsheets. Snellen eye chart, which is still used in the United States, is composed of various Latin letters and is the original version of the eye chart. The Landert eye chart adopted in Japan is also often called "C-shape". "Table C" is also the standard adopted by pilots in China.

The "e-list" chosen by China is mainly suitable for people who can't read Latin letters fluently, and also for people whose mother tongue doesn't use the alphabet. In addition, the "alphabet" used in the United States is not suitable for adding a flat mirror (if a flat mirror is used specially, it is usually printed backwards), and the outline of Latin letters increases the possibility of guessing, while the "E-table" does not have these problems.

Many places have also made eye charts specially for young children. In fact, it is not easy for preschool children aged 2-6 to check their eyesight, because even if they recognize the traditional "spreadsheet", it is difficult for children to express it clearly.