Contrast Sensitivity Function

The contrast sensitivity function described here will be used to develop the color image metric described in chapter 8. Contrast sensitivity is sometimes called visual accuity [LaRP97], [FPSG96]. We will use the term contrast sensitivity here, since we have used this terminology throughout chapter 8. Mannos and Sakrison [MaSa74] proposed a model of the human contrast sensitivity function. The contrast sensitivity function tells us how sensitive we are to the various frequencies of visual stimuli. If the frequency of visual stimuli is too high we will not be able to recognize the stimuli pattern any more. Imagine an image consisting of vertical black and white stripes. If the stripes are very thin (i.e. a few thousand per millimeter) we will be unable to see individual stripes. All that we will see is a gray image. If the stripes then become wider and wider, there is a threshold width, from which on we are able to distinguish the stripes. The contrast sensitivity function proposed by Manos and Sakrison is
 
f in equation 2.23 is the spatial frequency of the visual stimuli given in cycles/degree. The function has a peak of value 1 aproximately at f=8.0 cycles/degree, and is meaningless for frequencies above 60 cycles/degree. Figure 2.5 shows the contrast sensitivity function A(f).

  
Figure 2.5: Contrast sensitivity function

The reason why we can not distinguish patterns with high frequncies is the limited number of photoreceptors in our eye. There are several other functions proposed by other authors, but we choose the above function [MaSa74] because it can be simply analitically described. The same function is also used by Rushmeier et al. [RWPSR95] and Gaddipati et al. [GaMY97], which was another motivating factor in using this function.