UBVRI

UBVRI is a broadband optical photometric system used to measure the brightness of astronomical objects in five standard passbands: U, B, V, R, and I. The system is widely used in stellar astronomy to characterize spectral energy distributions and to derive basic physical parameters such as effective temperature, reddening, and metallicity. The U-band covers near-ultraviolet light, B the blue region, V the visual (green–yellow) region, R the red, and I the near-infrared, with effective wavelengths roughly 365, 445, 551, 658, and 806 nanometers, respectively. The system originated with the Johnson UBV filters introduced in the 1950s and was extended by Cecil G. Cousins in the 1970s with the addition of the R and I filters, forming the widely used Johnson-Cousins UBVRI system.

Calibration in this system typically ties magnitudes to standard stars, especially Landolt standard stars, with Vega commonly used as a zero-point reference for U, B, and V, and small corrections applied for R and I. Observed magnitudes must be corrected for atmospheric extinction and transformed into the standard system using color terms that depend on the instrument and filter set.

Applications of UBVRI include the use of color indices such as U−B, B−V, V−R, and V−I to estimate stellar effective temperature, reddening, and, with models, metallicity. The system remains a fundamental reference for many photometric studies, although modern surveys often include or transform to other systems such as SDSS ugriz.

Variants and notes: The term UBVRI can refer to the Johnson-Cousins photometric system. The exact passbands and zero-points depend on the filter and detector combination, making cross-calibration important when combining datasets.