Introduction

There are a number of different methods for analysing the image quality of an optical system. One of the simplest is to measure the resolving power, which is a measure of the smallest detail that can be resolved in the image. But even with this simple concept there are several different ways of defining resolving power in terms of the type of detail and test method. Some of the most common are listed below.

1. (1) The detail may be in terms of point objects. In this case the resolving power is a measure of the separation at which two points can just be resolved. This definition is used extensively in optical astronomy since the most common requirement here is to be able to resolve two very close stars.

2. (2) An alternative approach is to measure the resolving power of a periodic target, such as a sinusoidal or square wave pattern. The resolving power is the highest spatial frequency of the periodic pattern that can just be resolved.

In practice, the resolution limit of an optical system is most easily measured using a resolving power chart, which consists of a set of patterns containing detail as described above, but repeated at a number of different levels of detail size.

The Rayleigh criterion

The Rayleigh criterion assumes diffraction limited optics and specifies the resolution limit in terms of the minimum separation of point sources. In a diffraction limited optical system, the image of a point source object is the diffraction limited point spread function discussed in Chapters 26 and 34.