After a brief introduction to Fourier optics in the previous chapter, we now return to imaging with incoherent light. In Chapter 2 geometrical optics was used to describe image formation and location. Diffraction effects were neglected, and distortions from ideal image quality were attributed to chromatic and spherical aberrations. At this point, another limitation to image quality can be considered, namely, diffraction. Diffraction causes parallel light rays incident on the edges of optical elements such as lenses or mirrors to be deflected away from the focal point, thus degrading image quality. This causes spreading of point images and loss of resolution as described on p. 244 and in Section 7.4 for coherent light. Here we consider primarily incoherent light which is normally used in imaging. Diffraction-limited imaging refers to a situation in which image quality is limited by such diffraction. It is very common, particularly when geometrical optics aberrations are minimized through use of achromatic elements and parabolic rather than spherical surfaces. Moreover, diffraction-limited imaging is a convenient method in which to introduce the use of system concepts such as impulse response and transfer function to characterize image quality. These system concepts are introduced here and emphasized throughout the remainder of this book.