An image is defined as the optical counterpart of a physical object produced when light rays pass through a lens or are reflected from a mirror (see also Television signal processing and systems, p. 367). An image space is thus an abstract concept formed by a transformation from an object space in the real world. However, images can be classified in a wider sense, dependent upon the wavelength of the radiation that is used to produce them:

(1) images obtained from radiation with wavelengths longer than light such as radar mappings;

(2) optical or visible images, including those obtained from ultraviolet or infra-red radiation (infra-red images are often described as thermic);

(3) images produced by very short wavelength penetrating radiation such as X-rays.