Two 13-graduated counter current gray scales are arranged on a gray background (D = 0.75), the gray

　　scales being graduated logarithmically. Related to the densities of the gray scales: gamma = 0.45, related to

　　the transmission values (brightness): gamma = 2.2, that being exactly the reciprocal value of gamma = 0.45.

　　The output signal of a optimally gamma-corrected camera yields two 13-gratuated counter current linear step

　　signals.

　　The contrast range of the gray scales is 200 : 1.