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Hiding power (HP) may be qualitatively defined as the property of a paint that enables it to obliterate beyond recognition any background over which it may be spread. Quantitatively it may be expressed as the square feet of background that may be obliterated by a gallon of paint. The values necessarily depend upon the manner and conditions of conducting the test. It is usual to apply the paint over a background of black and white squares although areas of different reflectances, such as black and gray or gray and white, have been used. It is generally agreed that complete hiding has been reached when the paint applied over the black background has a reflectance 0.98 of that applied in equal thickness over the white background. Thus, HP becomes a function of the contrast ratio (CR) of the background and of the thickness of the paint required to reduce the contrast difference to 0.02. The value 0.02 is based on the Weber-Fechner law which states that differences of less than 2 percent for moderate illumination are imperceptible to the average eye.