34 - Image quality criteria

Summary

Introduction

Image quality criteria are used to assess how faithfully an optical system can image an object. The aberrations discussed in the previous chapter directly do not give this information, except for the distortion aberration expressed as a fraction or percentage difference in magnification. As will be seen in this chapter, aberration values can be used to calculate some but not all of these image quality criteria. For example, aberrations by themselves do not take into account veiling glare although some of the image quality criteria, using aberrations as input, do take diffraction effects into account. In fact, the quality of the image depends upon the following three factors:

1. (a) Veiling glare, which is due to unwanted reflections or scatter from surfaces

2. (b) Monochromatic and chromatic aberrations, discussed in the previous chapter

3. (c) Diffraction, discussed in Chapter 26

Because aberration and diffraction levels depend upon the size of the pupil and the aberrations also depend upon the position of the image point in the field, the form and magnitude of the image quality criteria depend upon pupil size and field position. Thus to assess fully the quality of a system, the image quality criteria should be evaluated for various pupil sizes and at several field positions.

Many image quality criteria are multi-dimensional functions and as a result are not always easy to use as criteria for comparing two similar systems. Therefore most image quality criteria have a reduced and sometimes approximate equivalent one dimensional form. That is, the image quality can be described by a single number on some suitable scale. Examples will be given at appropriate sections in this chapter.