The minimum contrast Cthresh
at which an observer can just distinguish a target
object from its surroundings.
Threshold contrast depends on target angular size θ, the surrounding luminance Ls
, and the desired probability of detection
(usually 50%–99%). For a given Ls
has a minimum value at some optimal θ (i.e., the target size that is most easily detected). Threshold contrast also varies among observers and across time for a given observer. Because detection
outdoors primarily depends on luminance rather than chromaticity differences, Cthresh
is usually calculated from spectrally integrated luminances. For alerted observers (i.e., those expecting a target to appear) and daytime luminance levels, Cthresh
ranges from ∼0.005 to 5.0 or more. Unalerted observers may require Cthresh
values that are five or more times greater than those for alerted observers.
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