UBV photometric system

The UBV photometric system measures stellar brightness in three broad photometric passbands: ultraviolet $U$, blue $B$ and visual $V$. Each measurement is an apparent magnitude defined relative to its calibration. Each of $U-B$ and $B-V$ is a color index: a difference in magnitudes that constrains spectral shape.

The measurement includes the filter, detector response and calibration convention, not merely a nominal wavelength. Effective temperature, composition, surface gravity and interstellar extinction can all affect the colours. Combining both indices with spectral classification therefore provides more information than interpreting one colour as a unique thermometer.