The UBV photometric system, also called the Johnson system (or Johnson-Morgan system), is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan. A 13" telescope and the 82" telescope at McDonald Observatory were used to define the system.
The filters are selected so that the mean wavelengths of response functions are 364 nm for U, 442 nm for B, 540 nm for V. The zero point of the B−V and U−B color indices were defined such as to be about zero for A0 main sequence stars not affected by interstellar reddening.
The UBV system has some disadvantages. The short wavelength cutoff of the U filter is defined mainly by the terrestrial atmosphere rather than the filter itself; thus, it (and observed magnitudes) can vary with altitude and atmospheric conditions. However, a large number of measurements have been made in this system, including many of the bright stars.
Extensions
The Johnson-Cousins UBVRI photometric system is a common extension of Johnson's original system that provides redder passbands.