Contrast Sensitivity

The ability of an individual to discriminate between two very similar shades of gray.

Various “gratings” are used to assess an individual’s contrast sensitivity. Gratings are an alternating pattern of two colors, as with a barcode. When a grating is below an individual’s contrast sensitivity, they will see a solid color.

The Contrast Sensitivity Function measures an individual’s contrast sensitivity as a function of spatial dimensions, since humans are generally more able to see contrast at ‘medium’ frequency gratings than low or high frequency gratings.

Flicker tests – where the colors are flickered temporally instead of arranged in grating spatially – are also possible to determine contrast sensitivity, but are not standard.

Contrast Sensitivity Function peaking at medium grating frequency (from Wikipedia user Star Whitt-Frousiakis)
Contrast Sensitivity of the Colorblind was a central concept to the discovery of canals on Mars