In this chapter, the contrast discrimination model given in the previous chapter will be used for the derivation of a measure for the perceived quality of an image. The quality of an image has always been an important aspect in the design of image forming systems. These systems can be cameras and printers for photographic systems, film projectors, display units for television, imaging systems for medical and scientific applications, etc. For the judgment of these systems a quantified measure of image quality is needed. However the design of such a measure is not easy, as the perceived quality of an image depends not only on the physical parameters of the image forming system, like resolution and contrast, but also on the impression of the image received by the eye of the observer. Therefore, the judgment by a panel of observers is often used as a quantitative measure for the obtained image quality. As this method is quite laborious and still rather subjective, investigators have searched for an objective measure for image quality in the form of a mathematical expression that contains a weighted combination of the physical parameters of the image and the psychophysical parameters of the human visual system. Such an expression is called an image quality metric. For such a metric, the MTF of the imaging system is generally used as physical parameter, and the contrast sensitivity function of the human eye as psychophysical parameter. The existing metrics differ from each other in the way these parameters are combined, and in the way the image quality contributions of the different spatial frequency components are taken into account. In this chapter, a survey will be given of the most important of these metrics
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