The minimum contrast C_thresh at which an observer can just distinguish a target object from its surroundings. Threshold contrast depends on target angular size θ, the surrounding luminance L_s, and the desired probability of detection (usually 50%–99%). For a given L_s, C_thresh 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, C_thresh is usually calculated from spectrally integrated luminances. For alerted observers (i.e., those expecting a target to appear) and daytime luminance levels, C_thresh ranges from ∼0. 005 to 5. 0 or more. Unalerted observers may require C_thresh values that are five or more times greater than those for alerted observers.