CONTRAST between two areas can be perceived amazingly
well. The Limens for the various tasks are often
Physiological really low, e.g. a few percent, and are further
Aspects decreased by such mechanisms as retinal interactions
and integration.
Stated most generally, the eye's ability to notice
NC EY relative differences of intensity between adjacent
image areas is basically limited by the quantum noise of the effectively
received amount of light per cone or series of cones per O.1 sec,
This statement is not p&rticularly useful, or maybe only in so-far it
reveals that the restrictions of the eye are, in this respect, set by
the statistical nature of light itself. But an important point of
practical value is, that any grainy structure, for instance of the recor-
ding medium, the negative film, severely impairs the abilities of the
contrast discrimination mechanism. This weighs heavier with isolated
points than with long lines or straight edges. Consequently, a sharp but
grainy picture may not be the optimum for small detail recognition = and
the optimum depends on the detail structure as well.
[some facts: the scanning area of the eye for granularity is about
2 mrad, which is a factor 5 larger than the width of the eye's spread
function. This adaptation reduces the perceived graininess, but it
impairs the apparent unsharpness. In the retinal image, two bright
lines need only to be separated by a dark spacing which is only 3%
less bright than the line-images. The saddle in the intensity profile
of two stars has to be lower than 75% of the top-values, in order to
be separated. The Signal to Noise ratio depends on the size of the
details and on the type of granularity.
Colour contrast discrimination forms an additional complex; it adds
a third dimension to almost any visual task. Although much is known
in this field, it is difficult to state it in general terms, except
the basic factor of complementary colours.]
zz Object : Object :
2 bright Lines BD © 2 bright
Z on black background stars
x 37 dip inbetween ! 257 dib
the unsharb Line 1 needed
in wtinal
images at image
the retina suffices intensity profite
