A system is perceptually uniform if a small perturbation to a component value is approximately equally perceptible across the range of that value. The volume control on your radio is designed to be perceptually uniform: Rotating the knob 10 degrees produces approximately the same perceptual increment in volume anywhere across the range of the control. If the control were physically linear, the logarithmic nature of loudness perception would place all of the perceptual "action" of the control at the bottom of its range. This figure shows the transfer function of a potentiometer with standard audio taper.
The CIE L* system, described on page 88 of A Technical Introduction to Digital Video, assigns a perceptually uniform scale to lightness. Video signals are coded according to perceptual principles, as is explained in Chapter 6, Gamma, of A Technical Introduction to Digital Video.
Excerpted from Chapter 1, Basic Principles, of A Technical Introduction to Digital Video. (New York: John Wiley & Sons, 1996). Page 16. This chapter is available online, in Acrobat PDF format.
See also: Gamma FAQ - Linear and nonlinear coding
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