Imaging is a mapping from some properties of the physical world (object space) into another representation of those properties (image space). The mapping can be carried out by changing the propagation of various types of physical signals. For example, medical ultrasound imaging is the mapping of the acoustic properties of the body tissue into their representation in the transmitted or reflected intensity of the acoustic field. The mapping is carried out by the absorption, scattering, and transmission of the acoustic energy. Optical imaging, the formation of an optical representation separate from the original objects, is a mapping carried out mostly by changing the directions of the electromagneticwaves coming from the objects. Insofar as light can be treated as rays, the spatial mapping from a point in the object space to a point in the image space can be studied geometrically. This field is called the geometrical theory of optical imaging. Situations arise when the wave nature of the light has to be dealt with explicitly. This field is called the physical (or wave) theory of optical imaging. Of course, there are other cases where the quantum nature of the light is the dominant characteristics to be considered.

In this and the next chapter we will study only the basic concepts and processes of optical imaging. The three main subjects to be studied are geometric optics, physical optics, and the radiometry of imaging.