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Chapter 3:
Interferometric Testing of Optical Systems
In the last chapter we made measurements on the image to determined aberration magnitude. In this chapter we will make measurements in the exit pupil plane. The principal means for doing this will be interferometry. We will first consider how aberrations are described in the exit pupil both from a geometrical and mathematical perspective. We then proceed to interferometric measurement techniques, fringe analysis, and finally discuss some operational constraints. The purpose of an optical imaging system is to gather light rays from a point source and redirect them in such a way that the rays converge to a point. This point is called the image. In a perfect optical imaging system the optical path length (OPL) from the object point through the system to the image point is the same for any ray path as illustrated in Fig. 3.1. Let us define the reference optical path length by the optical path of the axial ray between object and exit pupil. All other rays should have this same optical path length. This means that at the exit pupil of a perfect imaging system the rays terminate on a spherical surface centered on the image point. This surface (illustrated in Fig. 3.2) is called a wavefront. Rays are normal to the wavefront.
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