At the surface, numerically evaluating imaging systems with monochromatic light is a simple extension of two-dimensional discrete convolution, as discussed in Sec. 3.1. This is because the response of light to an imaging system, whether the light is coherent or incoherent, can be modeled as a linear system. Determining the impulse response of an imaging system is more complicated, particularly when the system does not perfectly focus the image. This happens because of aberrations present in the imaging system. In this chapter, aberrations are treated first. Then, we show how aberrations affect the impulse response of imaging systems. Finally, the chapter finishes with a discussion of imaging system performance.
The light from an extended object can be treated as a continuum of point sources. Each point source emits rays in all directions as shown in Fig. 5.1. In geometric optics, the rays from a given object point that pass all the way through an ideal imaging system are focused to another point. Each point of the object emits (or reflects) an optical field which becomes a diverging spherical wave at the entrance pupil of the imaging system. To focus the light to a point in the image plane, the imaging system must apply a spherical phase delay to convert a diverging spherical wavefront into a converging spherical wavefront. Aberrations are deviations from the spherical phase delay that cause the rays from a given object point to misfocus and form a finite-sized spot. When the image is viewed as a whole, the aberration manifests itself as blur. Light from different object points can experience different aberrations in the image plane depending on their distance from the optical axis. However, for the purposes of this book, we are not concerned with these field-angle-dependent aberrations but assume that they are constant.