Photometric system


In astronomy, a photometric system is a set of well-defined passbands, 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.
A commonly adopted standardized photometric system is the Johnson-Morgan or UBV photometric system. At present, there are more than 200 photometric systems.
Photometric systems are usually characterized according to the widths of their passbands:
  • broadband
  • intermediate band
  • narrow band

    Photometric letters

Each letter designates a section of light of the electromagnetic spectrum; these cover well the consecutive major groups, near-ultraviolet, visible light, near-infrared and part of mid-infrared. The letters are not standards, but are recognized by common agreement among astronomers and astrophysicists.
The use of U,B,V,R,I bands dates from the 1950s, being single-letter abbreviations.
With the advent of infrared detectors in the next decade, the J to N bands were labelled following on from near-infrared's closest-to-red band, I.
Later the H band was inserted, then Z in the 1990s and finally Y, without changing earlier definitions. Hence, H is out of alphabetical order from its neighbours, while Z,Y are reversed from the alphabetical - higher-wavelength - sub-series which dominates current photometric bands.
Note: colors are only approximate and based on wavelength to sRGB representation.
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:
Note: colors are only approximate and based on wavelength to sRGB representation.