Contrast sensitivity measures the ability to see details at low contrast levels.
In traffic, the demanding situations are at low contrast levels, for example, seeing in dusk, rain, fog, snow fall, and at night. Contrast sensitivity is the reciprocal of the contrast at threshold, i.e., one divided by the lowest contrast at which forms or lines can be recognized. If a person can see details at very low contrast, his or her contrast sensitivity is high and vice versa.