In meteorological usage, contrast
C is defined by
where
L0 is the
luminance of a
target object and
Ls is that of its immediate visual surroundings.
Both luminances may include
airlight or reflected
glare, and
Ls is always assumed to be greater than zero. Because
detection outdoors often depends on luminance rather than chromaticity differences,
C is usually calculated from spectrally integrated luminances. Contrast for a perfectly black target is -1 but, in principle,
C has no upper limit for a self-luminous target. If
is less than a variable threshold contrast, the target is visually indistinguishable from its surroundings.