... And If You're Over 70
If you are over 70 and you face Powerpoint1 slides, the following is something most slide showers do not know or do not care about. As we age, the sensitivity to the short-wavelength end of the spectrum decreases relative to the sensitivity to the rest of the spectrum; and in a relatively dimly lit room, fixating [on] the commonly gray-blue slide background (through a small pupil due to senile miosis) in us "oldies" often results in a low retinal illumination in the mesopic range. This in turn means that on a slide that contains, say five curves, each of a different color, seniors may only discover one curve, the black one. The reason, I believe, is a combination of the change in color sensitivity and [low] ... retinal illumination ..., the contrast sensitivity in seniors is mostly very low.