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)
