UBV photometric system
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.
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 was defined to be zero for A0 V 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.
- Johnson, H. L.; Morgan, W. W. (1953), Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas, The Astrophysical Journal, vol. 117, pp. 313–352. 
- Iriarte, Braulio, Johnson, Harold L., Mitchell, Richard I., and Wisniewski, Wieslaw K. (1965), Five-Color Photometry of Bright Stars, Sky & Telescope, vol. 30, p. 21
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