Photometric system

A defined set of wavelength bands, detector responses, and calibration standards used to measure and compare astronomical brightnesses. Its measurements depend on the system’s passbands and calibration.

Photometric passband: A wavelength-dependent transmission profile defining which light a photometric measurement records. Each passband specifies the spectral slice measured by a system.
Spectral energy distribution: The amount of electromagnetic energy emitted or received as a function of wavelength or frequency. A source’s spectrum determines its measured flux in each band.
UBV photometric system: A three-band optical photometric system using ultraviolet, blue, and visual filters. UBV established a widely used framework for stellar colors and magnitudes.
AB magnitude system: A magnitude convention in which zero magnitude corresponds to a constant flux density per unit frequency. It uses a spectral-flux reference rather than Vega’s measured brightness.
Magnitude system: A logarithmic scale expressing the brightness of astronomical sources. Photometric systems report brightness through defined magnitude scales.
Photon-counting detector: A detector that measures incident light by registering individual photons or their accumulated counts. Detector response shapes the effective weighting of wavelengths.
Johnson–Cousins system: A standard optical and near-infrared photometric system with U, B, V, R, and I bands. Its filters extend the classic UBV system toward redder wavelengths.
Vega magnitude system: A magnitude convention that assigns a reference magnitude to Vega or a calibrated Vega-like spectrum. Its band-specific reference differs from the uniform AB flux-density standard.
Zero-point calibration: The procedure that sets the reference level linking measured signal to a standardized magnitude. A zero point anchors instrumental readings to calibrated brightnesses.
Flux density: The received energy or photon rate per unit area, time, and frequency or wavelength interval. Calibrated fluxes underlie brightness measurements in physical units.

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