| Definition: | quotient of the real difference in luminance between the target and its background, ΔL, and the luminance difference needed between a target of a certain angular size and its background for the target to be just visible, ΔLth, that is at the threshold FVL = ΔL / ΔLth Note 1 to entry: The visibility level indicates how much the contrast of the target is above threshold contrast. Note 2 to entry: The visibility level has unit one.
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