This chapter describes the sampled imager response (SIR) function. The SIR has two parts: it quantifies the transfer response of a sampled imager, and it quantifies aliasing.

The procedure for finding the SIR function parallels the procedure for finding the transfer response for a nonsampled imager. The SIR function is the Fourier transform of the imager point spread function (psf). As discussed in Chapter 1, the psf of a sampled imager is not shift invariant; it varies depending on sample phase. For this reason, the SIR function is more complicated than the transfer function. The SIR function contains information about both the transfer response of the system and about aliasing.

The SIR function depends on the sensor presample MTF, the sample spacing, and the postsample or display MTF. These imager characteristics are known to the design engineer or system's analyst. The SIR function does not depend on sample values; it characterizes the process by which the samples are taken and displayed.

Since the sampling artifacts produced by an imager depend on the scene being imaged, one might question a mathematical process that quantifies sampling artifacts without including an explicit description of the scene. In that regard, we rely on assumptions identical to those used for nonsampled imagers.

MTF is used to characterize a nonsampled imager. MTF is the Fourier transform of the psf. It describes the blur produced in the image by a point in the scene. The importance of good MTF is not established until the frequency content of the scene is known. The impact or importance of sensor blur depends on scene frequency content.

Nonetheless, MTF has proven to be a good indicator of the overall utility of an imager. The ability of an imager to resolve scene detail is important because of the possibilities it provides. Good MTF is not needed for every task or on every occasion. But the characterization of the imager's response to a point source provides a good indication of the quality of images that are expected in a variety of circumstances.