Remote Sensing-->Sensors

The two broadest classes of sensors are Passive (energy leading to radiation received comes from an external source, e.g., the Sun; the MSS is an example) and Active (energy generated from within the sensor system is beamed outward, and the fraction returned is measured; radar is an example). Sensors can be non-imaging (measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance) or imaging (the electrons released are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors (see further down this page for a discussion of detector types); since the radiation is related to specific points in the target, the end result is an image [picture] or a raster display [for example: the parallel horizontal lines on a TV screen]).

Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum. When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to an array of detectors, it is called a spectrometer. One type of spectrometer (used in the laboratory for chemical analysis) passes multiwavelength radiation through a slit onto a dispersing medium which reproduces the slit as lines at various spacings on a film plate (discussed on page I-2a). The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths. Most air/space sensors are spectroradiometers.

Sensors that instantaneously measure radiation coming from the entire scene at once are called framing systems. The eye, a photo camera, and a TV vidicon belong to this group. The size of the scene that is framed is determined by the apertures and optics in the system that define the field of view, or FOV. If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system. Most non-camera sensors operating from moving platforms image the scene by scanning.