Photometric system

In astronomy, a photometric system is a set of well-defined passbands (or optical 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. A commonly adopted 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: broadband (passbands wider than 30 nm, of which the most widely used is Johnson-Morgan UBV system) intermediate band (passbands between 10 and 30 nm wide) narrow band (passbands less than 10 nm wide) Each letter designates a section of light of the electromagnetic spectrum; these cover well the consecutive major groups, near-ultraviolet (NUV), visible light (centered on the V band), near-infrared (NIR) and part of mid-infrared (MIR). 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 (when possible). 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. The filters currently being used by other telescopes or organizations. Units of measurements: Å = Ångström nm = nanometre μm = micrometre Note: colors are only approximate and based on wavelength to sRGB representation (when possible).