Computer monitor (CRT) gamma measurement and adjustment
The number under the bar with equal brightness top and bottom is the gamma
for your current display setup. The gamma for a properly adjusted system is
1. If the contrast, brightness and gamma are properly set, the figure below
will have nine vertical bars, each a constant brightness half that of the
bar on the left. For an in depth discussion of gamma correction, see
http://www.inforamp.net/~poynton/notes/colour_and_gamma/GammaFAQ.html