[TowerTalk] Acuity

[Bruce Horn]

Visual acuity varies with light intensity. The retina of the eye is
composed of two types of receptors: rods and cones. Cones function well in
bright light and produce color vision, but are relatively blind at low
light levels. Rods function well at low light levels, but are relatively
insensitive to color.

For daytime light conditions, the best visual acuity will occur for objects
in the center of the field of view. For dim light conditions, the best
acuity occurs for objects about 20 degrees off axis (high concentration of
rods).

For discerning a high contrast object from a uniform background in daytime,
the average human could detect an object as small as 0.003 inch at 1 foot
(or 3/16 inch at 62.5 feet). However, for objects of less contrast (maybe
more like a guy line against sky), the average human would be limited to
about 0.01 inch at 1 foot (or 3/16 inch guy line at 18.8 feet). However,
under dim light conditions, visual acuity is reduced by about a factor of
10:  0.03 to 0.1 inch at 1 foot.

I don't think this is the type of calculation where numbers to right of the
decimal place have any meaning. In fact a range of values is probably more
appropriate. For those who like formulas:

D = diameter/acuity where D=distance (ft) at which object will be "invisible"
                          diameter = diameter of guy line (inches)
                          acuity = 0.003 to 0.01 for bright light
                                   0.03 to 0.1 for dim light

Remember, these values are for "average humans" before the aging process
degraded their visual acuity.

73 de Bruce, WA7BNM   (bhorn@hornucopia.com)
Medical Physicist


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