In astronomy, a photometric system is a set of well-defined passbands (or filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.
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The first known standardized photometric system is the Johnson-Morgan or UBV photometric system (1953). At present, there are more than 200 photometric systems.
Photometric systems are usually characterized according to the widths of their passbands:
Photometric letters
Each letter designates a particular section of the electromagnetic spectrum; most of these sections fall within the region spanning the near-ultraviolet (NUV), the visible and the majority of the near-infrared (NIR).
Indigo and cyan are not standard colors. Orange, yellow, and green fall under visual bands, while violet and purple are under the blue bands. The letters are not standards, but are recognized by common agreement among astronomers and astrophysicists.
Combinations of these letters are frequently used; for example the combination JHK has been used more or less as a synonym of "near-infrared", and appears in the title of many papers.
Filters used
The filters currently being used by other telescopes or organizations.
Units of measurements: