视力表为什么用字母E？

Why do we use the letter E in the visual testing chart?

因为用字母E可以做到最小视角。

Because the letter E can achieve the minimum visual angle.

可能大家以为是因为眼睛的英文单词是EYE，所以用其首字母表示。但这一解释并不科学。事实上，这与我们眼睛看东西时需要用的“视角原理”有关。

Probably we think it is because the English word for it is EYE, so it is indicated by its initial letter E. But this explanation is not scientific. In fact, it is related to the “principle of visual angle” that our eyes depend on to looking at things.

一般情况下，我们的眼睛能分辨两个物体之间的最小距离叫做“最小视角”。当我们检测视力时，往往要站在距离视力表5米远的地方，这时视力表上每两行字母E之间的距离投射在我们眼睛里就刚好是“最小视角”。

In general, the minimum distance between two objects that our eyes can distinguish is called the “minimum visual angle”. When our vision is tested, we often have to stand 5 meters away from the visual testing chart. At this time, the distance between every two lines of letters E on the chart is projected into our eyes just as the “minimum visual angle”.

这个视力表在我国是由卫生部于1984年制定的，并在1990年开始全国推广。

The chart was developed by the Ministry of Health in China in 1984, and began to be popularized nationwide in 1990.

但是并不是所有国家都用E字视力表。

But not all countries use the E visual testing chart.

比如，美国的视力表是由一些拉丁字母组成的。而日本使用的是兰德特环形视力表，因为看上去像字母C，所以也叫C表。我们国家招收飞行员就使用的是这种C表。

For example, the visual testing chart in the US is composed of some Latin letters. And Japanese use the Landolt visual testing chart, which is also called the C chart because the graphics look like the letter C. In China, it’s used to recruit pilots.