It acquires information about the object or surface in a set of specific wavebands contained in the energy source used by the sensor. It records the energy coming from the surface in accordance with a geometric model of the sensor. Our knowledge of this geometric model allows us to recreate an image in which the spatial characteristics of the surface are reproduced within a specific level of accuracy. Sensors can be categorised in various ways. By the source of illuminating energy - Passive sensors, such as cameras and most scanners.
Passive sensors are dependent on the characteristics of the independent energy source.
Sensors that depend on sunlight are passive sensors. Active sensors, such as radar, sonar, X-ray imagers, and Electron Microscopes. Active sensors are independent of naturally occurring sources of energy, but they usually cost more to use because of the need to generate their own source of energy. By the waveband characteristics of the energy source - Electromagnetic Energy, such as optical, thermal and radar imagery. Sound, such as sonar, Radiation, such as X-rays and Electron Microscopes.