The so-called vision measurement is actually to detect the ability of eyes to see far away (hyperopia chart), which is accurately defined as measuring the smallest image on the retina that can identify its shape. ?
There are many forms of eye chart, the most common one is E, and there are words, numbers, cartoons and so on. For example, pilots test their eyesight with the word C when screening, and cartoons test their eyesight for children (who can't read yet) (let him say what it is).
In the commonly used E-shaped international standard visual acuity chart, the projection of line 1.0 on the retina is exactly 1 degree angle, and the calculation formula of visual acuity is: visual acuity = 1/ degree angle. That is to say, at a distance of 5 meters, the projection of the top line of the eye chart on the retina is 10 degrees, and the visual acuity is 0. 1.
The human eye has many functions, including hyperopia (myopia), myopia (presbyopia), dark vision (night blindness), peripheral vision (visual field defect), and a series of functions such as color vision, spatial contrast and depth vision. (Diseases with corresponding dysfunction are in brackets).
Extended data
An eye chart is a chart used to measure vision. The eye charts used in China are: international standard eye chart, logarithmic eye chart and Landauer eye chart. Functionally, there are myopia chart and hyperopia chart. Eye chart is based on the principle of visual angle. Inspection vision is generally divided into two categories: far vision and near vision. Far vision mostly adopts the international standard visual acuity chart, which consists of 14 lines of "e" with different sizes and different opening directions; Measured from 0. 1- 1.5 (or from 4.0-5.2); Each line is marked, and the operation method line of the examinee's visual acuity chart should be parallel to the line of 1.0, and 5 meters away from the visual acuity chart.
use
The eye chart used in medical examination mainly checks the central vision, that is, the central fovea of macular region, so as to understand the preliminary situation of visual function simply and quickly, which is of great significance to the clinical diagnosis and treatment of ophthalmic diseases.
principle of operation
The eye chart is designed according to the principle of visual angle. The so-called viewing angle is the angle formed by the light emitted by two external points passing through the nodes in the eyes (that is, the angle formed by the intersection of two external objects when they enter the eyes). In general, the human eye can distinguish the smallest distance between two points to form the smallest angle of view, that is, the tiny angle of view. The eye chart aims to effectively detect vision in a unit of one-minute viewing angle.
operational approach
Inspection vision is generally divided into two categories: far vision and near vision. The standard logarithmic visual acuity chart is mainly used in China (the second generation was implemented in May 20 12), which consists of 12 lines of "e" with different sizes and different opening directions; Measured from 0. 1- 1.5 (or from 4.0-5.2); Each line is marked, and the examinee's line of sight should be parallel to the line of 1.0 and 5 meters away from the eye chart. The distance between the eye chart and the examinee must be fixed correctly, and the distance between the examinee and the eye chart should be 5 meters. If the indoor distance is not 5 meters long, a flat mirror should be placed at 2.5 meters to reflect the eye chart. Before the test, cover one eye, use one eye to identify the direction of the "E"-shaped gap from top to bottom until it is unrecognizable, and then record it.
Normal vision should be above 1.0. If candidates can't see at 0. 1, they should move forward to 0. 1 to see, and their vision is "0. 1× distance /5= vision"; If you still can't see 0. 1 within half a meter, candidates can identify indicators and measure manual and light sensitivity. Record vision according to the examination. The "J" myopia eye chart is often used for myopia, and it can also identify the direction of the "E" gap until it is unrecognizable. Myopia vision can be adjusted by itself. Normal myopia can only see the line of 1.0 at 30 cm, and myopia examination is helpful for the diagnosis of ametropia.
Source: Baidu Encyclopedia: Vision Chart