Why some saturated colors are darker than others.

Last edited Fri Jun 19, 2015, 01:48 PM - Edit history (2)

Have you ever noticed that blue is darker than red, whereas red is darker than yellow? This gives blue ink the greatest contrast, and yellow the least contrast, against white paper. I've often wondered why this is, and I think I've figured it out.

Color vision involves cones (as opposed to rods). Humans with normal color vision have three types of cones in their eyes. I will refer to them as blue, green, and red cones, although the proper terms are short-wavelength (S), medium-wavelength (M) and long wavelength (L) cones. The following diagram shows the patterns of cones in the foveas of an eye with a normal retina (on the left) and one lacking red cones (on the right). The person with the latter eye suffers from a severe form of red-green colorblindness called protanopia.



The brightness of a color depends on the density of cones that are excited by light of that color. Red and green cones are dense in the normal fovea. Blue cones are very sparse and don't contribute much, if anything, to brightness. Blue light excites mainly the blue cones and has low brightness, i.e., it appears dark. Yellow light excites the red and green cones equally (or approximately so) and has high brightness, since it excites most of the cones. Red light excites mainly the red cones and is intermediate in brightness, since it excites only about half the cones.