Photometric system

In astronomy, a Photometric system is a set of well-defined passbands (of 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.

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:
 * broadband (passbands wider than 30 nm (the most widely used is Johnson-Morgan UBV system)),
 * intermediate band (passbands widths between 10 and 30 nm),
 * narrow band (passbands widths less than 10 nm).

Photometric Letters
The letters designate a region of a wavelength of light. Majority of the letters span from near-ultraviolet (NUV) to visible and majority of the near-infrared (NIR).

Note, indigo and cyan are not standard colors. Orange and yellow fall under visual bands, while violet and purple are under the blue bands.

Used Filters
The filters currently being used by other telescopes or organizations.

Units of measurements:
 * Å = Ångström
 * nm = nanometer
 * μm = micron