The "gamma" if you will for printed materials is usually defined as the "dot gain", for reflected light one of the key factors is the light adaptation of the ey

The "gamma" if you will for printed materials is usually defined as the "dot gain", for reflected light one of the key factors is the light adaptation of the eye at viewing. The WCAG 2 contrast guidelines are defined using relative luminance "Y", as in CIE XYZ, and not absolute luminance "L", even though they use the letter L somewhat incorrectly (not to mention incorrect math, but that's another story for another day). The 1.4.3 guidelines are not LAB which is more properly L*a*b*, as L* is perceptual lightness, not luminance. Interestingly though, the NEW method for WCAG 3 is APCA, which is using power curves very similar to L* perceptual lightness, but nevertheless is not a ratio. APCA is perceptually uniform, the old method is not, leading to a great deal of misunderstanding, and likely a motivating factor behind "Contrast Rebellion"...