In Chapter 2 where the effect of external noise on contrast sensitivity was treated, it was assumed that the noise was white. This means that the spectral density of the noise is constant within the frequency limits of the considered spectrum. Although these conditions are usually met, this is not always the case. Sometimes the spectral density is not constant within the frequency limits of the noise spectrum or is constant only within a limited part of this spectrum. In this chapter, the effect of nonwhite noise on contrast sensitivity will be treated. The disturbance of the observation of a signal by noise with a frequency that is different from that of the signal is called masking. It will be investigated how the formulae for the effect of white noise given in Chapter 2 can be generalized to also become valid for the situation of nonwhite noise. This treatment will, however, be restricted to spatial noise. If temporal noise is also present, it will be assumed that this noise is white.
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