[TowerTalk] Acuity

[Pete Smith]

At 04:35 PM 3/18/1999 -0800, George Cutsogeorge wrote:
>
>Fred,
>
>The same discussion appears in the Television Engineering
>Handbook edited by Fink.  I quote: "For visual acuity tests of
>the type described, normal vision, corresponding to a Snellen
>20/20 rating, represents an angular discrimination of about 1
>minute."
>
>Your friend has taken the 1 minute number and calculated how far
>away you must be from a 3/16" diameter wire for it to subtend 1
>minute of arc.  The answer is the 53+ feet.

This is analogous to resolution in remote sensing devices, and misleading
for the same reasons, as George points out.  For example, an early Landsat
had resolution (line to line) of 100M, and yet you could easily see the
Lake Pontchartrain causeway on the images, even though the causeway was
only about 25 meters wide.  The explanation lay in the contrast between its
color and the color of the water below.  Its presence changed the average
color of the pixels the causeway passed through enough to provide the
illusion that you were seeing the causeway itself.  Similarly, dark guys
against a bright blue or white sky will shop up well beyond the distance
where they can be resolved as wires, per se.

73,  Pete N4ZR
Loud is good


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