C-type visual acuity chart refers to Lang's ring visual acuity chart, which is mainly used to detect pilots and other professional people who have high vision requirements. Lang's eye diagram adopts a 7.5 mm square ring with a width of 1.5 mm, and the ring has a gap with a width of 1.5 mm, which is C-shaped.
The standard visual acuity was recorded as 1.0 after the decimal point. If the field of view is n, it means that the annular gap visible at 5 meters is a millimeter-wide gap in millimeters square millimeters. Rankine cycle scale is calculated by arithmetic progression, and the growth rates are 0. 1 and 0.2 respectively. . . . . . 2.0, decimal record.
The greater the opening direction, the lower the probability of accurately judging the opening direction of each visual standard, and the higher the inspection accuracy. The eye diagram has only four directions with a probability of 25%, while the eye diagram of C has eight open directions with a probability of 12.5%.
Extended data:
Advantages of C-word table:
When there is no astigmatism or astigmatism has been completely corrected, there is only one or two lines difference between the two visual tables. The visual sensitivity of E-vision workbench is better than that of C-vision workbench. But for patients with astigmatism, if astigmatism is not corrected, with the increase of astigmatism, the difference between the eye charts will be more obvious.
Astigmatism accounts for more than 85% of all ametropia patients. Because the diopters of astigmatism eyes are different in all directions, astigmatism patients have different reactions in all directions when they identify the opening direction.
For example, for myopic astigmatism in the direction of 180 degrees, the aperture in the vertical direction is easy to distinguish, and the aperture in the oblique direction and the aperture in the horizontal direction are difficult to judge. Because the astigmatic eye has a great difference in judging the opening direction of the two visual indexes, the advantages and disadvantages of C and E eye charts in optometry accuracy will be more obvious.
Baidu encyclopedia -c eye map