chapter 9, Effect of Various Parameters on Image Quality

Excerpt

9.1 Introduction

In the previous chapter, various metrics were analyzed for their suitability for the description of image quality. In these metrics, use was made of the MTF of the imaging system and the contrast sensitivity of the eye. From these metrics, the SQRI appeared to have the best relation with perceived image quality. The SQRI was originally developed for the evaluation of the effect of resolution on perceived image quality (Barten, 1987), but later, it appeared that it could also be used for the description of the effect of many other parameters on image quality (Barten, 1989a, 1990a). In this chapter, the effect of a number of parameters on image quality will be treated. They will be analyzed with the aid of the SQRI and will largely be compared with published measurements. For the contrast sensitivity of the eye, Eq. (3.26) in Chapter 3 will be used with the there given typical values of the constants.

Beyond the MTF of the imaging system, the contrast sensitivity of the eye plays an important role in the perception of image quality. Generally, some parameters influence the MTF, other parameters influence contrast sensitivity, and still other parameters influence both MTF and contrast sensitivity. In the SQRI the following parameters can be taken into account: Resolution is taken into account in the MTF. Addressability or addressable resolution is taken into account in the upper limit of the integration. Addressability is determined by the number of lines or the number of pixels with which the image is reproduced. Luminance, defined here as the average luminance of the image, influences contrast sensitivity and is therefore taken into account in the contrast sensitivity function. Contrast, defined by the size of the luminance variations divided by the average luminance, is taken into account in the MTF, because contrast variations cause a multiplication of the MTF with a certain factor. Gamma (see section 9.7) is also taken into account in the MTF, as it influences image quality in a similar way. Viewing distance is both taken into account in the contrast sensitivity function, because of its effect on field size, and in the MTF, because of its effect on angular spatial frequency.