In this paper, factors controlling the radiance incident on a remote-sensing device and the interaction of the sensor with upwelling radiance are considered. We also discuss systematic and random radiance variations across an image: while the former may be corrected for (to an extent) the latter may not, so that it is necessary to be aware of the impact of random variations on target identification, discrimination and quantification. Both the optical reflective and the thermal-infrared parts of the spectrum are considered. Equations are presented to concisely illustrate the processes involved