On 18 Mar 99, Bruce Horn <email@example.com> wrote:
> 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
> 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
> 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 (firstname.lastname@example.org)
> Medical Physicist
Thanks for beating me to the punch, Bruce. Saves some work on my part
:.) I will add a few comments though...
There is a difference between Snellen acuity, being able to identify an
object, and being able to detect the presence of an object. There is a
term called minimal peerceptible acuity. It refers to the dection of
fine objects, such as dots or line, against a plain background. The
objects may be bright on a dark background, drak on a bright
background, or of low contrast (i.e. of nearly the same luminance as
their background). A long black line (eg Phillystran) subtending 0.5
seconds of arc is visible against a white background. The image of the
line is diffracted and blurred over many cones, instead of being imaged
as a geometric shadow of approx 1/60 of the cone diameter. What makes
the line perceptible is that the energy on the column of cones under
the diffracted image is about 1% less than the background energy. This
is evidently the detectable difference threshold for brightness for the
given condx. Thinner lines would produce less than 1% redcution in
luminance on the retina and would not be detected.
Consequently, detecting black lines on bright backgrounds and detecting
a fine white line on a black background differ. The latter type of
target will be detected regardless of how thin it is, provided
sufficenet illumination reaches the retina.
Reference - Duane's Clinical Ophthalmology, vol 1, Ch 33.
Fred, as you can see, the issue is not black and white (pun intended).
There are many variables to consider, usch as color of the guy,
lighting condx, etc. Does it matter that the guy wires are not visible,
if the tower is still visible?
73 Barry Kutner, M.D. (ophthalmologist)
Barry Kutner, W2UP Internet: email@example.com
Newtown, PA Frankford Radio Club
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