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Issue #218 September 2008
Problem 1—When processing a video signal, what is the process known as “gamma correction,” and what is its purpose?
Answer 1—Gamma correction is the name given to the process that compensates for the nonlinearities of some display technologies, particularly cathode ray tubes (CRTs).
Like any other tube operating near its cut-off region, the current flow, and therefore the brightness of the phosphors, of a CRT has an exponential relationship with respect to the circuit voltage representing intensity. This tends to make dark areas of the image unnaturally dark relative to the bright areas. A typical value for the exponent, called “gamma,” is 2.2.
In order to compensate for this, a precompensating transformation is done on the video signal, using an exponent of 1/2.2 = 0.45. This cancels out the distortion of the CRT, creating a system that is essentially linear from end to end.
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