This chapter continues the work begun in Chapter 1 by discussing several methods used to evaluate image quality. To do this, special objects are placed in the collimator focal plane and viewed by the test optical system: either a point source, or an extended source (such as a resolution target). Of interest is the response of the optical system, i.e., the image, to such sources. For point objects the image is called the point spread function (PSF) or star image. Sometimes you will also hear it called the far-field pattern or impulse response. For an extended source one can think of each point in the "perfect" geometric image as being replaced with the appropriate point spread function. As a result, the amount of detail that can be seen in a resolution target image decreases as the size of the point spread function increases.
The size and shape of the point spread function is controlled by diffraction and aberrations in the lens. If aberrations are negligible, the system is said to be diffraction-limited. The size and shape of the stop determines the size and shape of the point spread function and image quality. This is considered the best one can do. The diffraction limit is a bench mark or standard by which all other imagery is judged. Since most systems have circular stops, the resultant point spread function has a special name. It is called an Airy pattern and is shown in Fig. 2.1. The central core is called the Airy disc.
Most optical systems are not diffraction-limited across their entire field. They suffer from defects called aberration. There are five primary (Seidel) aberrations. In the last chapter we measured two of them: field curvature and distortion. These do not affect point spread function structure but rather its axial and lateral position. The remaining three primary aberrations (spherical aberration, coma, and astigmatism) do change the size and shape of the point spread function.
In refractive systems the aberrations are color dependent due to the dispersive nature of the material. It is common practice to measure the aberration over a restricted spectral bandwidth.The width and center wavelength of the band will depend on the application of the optical system.
There are two primary locations where aberrations can be both described and measured: (a) the image plane; (b) the exit pupil plane. This chapter concentrates on the former. (Chapter 3 will emphasize the latter.) Spherical aberration, coma,and astigmatism will be discussed first. These influence the point spread function and hence image resolution. The second part of this chapter will cover general resolution measurements.
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