High Quality Content by WIKIPEDIA articles! 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 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 in such manner, 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.

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