Visual Acuity Measurement:(what is 20/20 vision?)

20/20 vision is defined as the ability to see, at 20 feet, with the same acuity as an “average” Human can at that distance.

How acurate is that? The standard is that, at 6 meters (~20 feet), the average person can distinguish between objects that are 1.75 mm apart. This corresponds to 0.3 milli-radians or 1 minute of arc.

If you can only distinguish between objects twice that distance apart, then you have “20/40” vision: You can see at 20 feet what the “average” person can at 40 feet. In other words, your maximum resolution is 2 min arc.

Typically, visual acuity is measured by means of “eye-charts”:

A standard “E-type” eye-chart:

The test is to determine which way the open side of each “E” is pointing.

When the chart is viewed from a distance such that, in the smallest row of “E’s”, the dimensions marked below subtend 1 min of arc, then being able to identify the direction of all E’s in that row constituted 20/20 vision.

The larger rows have integer multiples of the smallest’s dimensions, and therefor constitute 20/40, 20/60, 20/80, etc visual acuity.

(The figure as a whole is 5X5 min arc.)

Example Use

If you display this image on your computer monitor at a magnification such that the smallest E’s are 5 mm square (smallest feature is 1 mm); Then viewing it at 3.33 meters (~11 ft) makes the ability to identify the smallest row’s directions constitute 20/20 vision. If you can only see the second to smallest row, that is 20/40, etc.

1 3,333.0003mm mmrad

111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111001111110011111111000000000011111111001111110011111111000000000011111111000000000011111111111111111111111001111110011111111000000000011111111001111110011111111000000000011111111000000000011111111111111111111111001100110011111111001100110011111111001100110011111111111111110011111111001100110011111111111111111111111001100110011111111001100110011111111001100110011111111111111110011111111001100110011111111111111111111111001100110011111111001100110011111111001100110011111111110000000011111111001100110011111111111111111111111001100110011111111001100110011111111001100110011111111110000000011111111001100110011111111111111111111111001100110011111111001100110011111111001100110011111111111111110011111111001100110011111111111111111111111001100110011111111001100110011111111001100110011111111111111110011111111001100110011111111111111111111111000000000011111111001111110011111111000000000011111111000000000011111111001111110011111111111111111111111000000000011111111001111110011111111000000000011111111000000000011111111001111110011111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111000001111111111000001111111111011101111111111011101111111111000001111111111000001111111111111111111111111011111111111111010101111111111010101111111111010101111111111111101111111111010101111111111111111111111111000011111111111010101111111111010101111111111010101111111111100001111111111010101111111111111111111111111011111111111111010101111111111010101111111111010101111111111111101111111111010101111111111111111111111111000001111111111011101111111111000001111111111000001111111111000001111111111011101111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111

You can create a vision chart to use in Zemax’s “Image Simulation” analysis by making a text file of ones and zeros and putting it into the “Images” directory (see “File-Preferences-Folders” to find the right directory). Given the right dimensions, this can be a good way to evaluate the resolution of an optical system designed to be used visually.

This is a fragment of a 105 x 105 text image of the “E-chart”.