TheUBV photometric system (fromUltraviolet, Blue, Visual), also called theJohnson system (orJohnson-Morgan system), is aphotometric system usually employed forclassifying stars according to their colors. It was the first standardized photometric system. Theapparent magnitudes of stars in the system are often used to determine thecolor indices B−V and U−B, the difference between the B and V magnitudes and the U and B magnitudes respectively.^[1] The system is defined using a set of coloroptical filters in combination with anRMA 1P21photomultiplier tube.^[2]

The choice of colors on the blue end of the spectrum was assisted by the bias that photographic film has for those colors. It was introduced in the 1950s byAmericanastronomersHarold Lester Johnson andWilliam Wilson Morgan. A 13 in (330 mm) telescope andthe 82 in (2,100 mm) telescope atMcDonald Observatory were used to define the system.^[1][3] The filters that Johnson and Morgan used were Corning 9 863 for U and 3 384 for V. The B filter used a combination of Corning 5 030 and Schott GG 13.^[4]

The filters are selected so that the mean wavelengths of response functions (at which magnitudes are measured to mean precision) are364 nm for U,442 nm for B,540 nm for V. Zero-points were calibrated in theB−V (B minus V) and U−B (U minus B)color indices selecting suchA0 main sequence stars which are not affected byinterstellar reddening.^[1] These stars correspond with a meaneffective temperature (T_eff (K)) of between 9727 and 9790 Kelvin, the latter being stars with class A0^[a]V (V meaning five)^[b].

The system has a key limit drawback. The short wavelength cutoff that is the shortest limit of the U filter is set by any given terrestrial atmosphere rather than the filter itself; thus, it (and observed magnitudes) varies chiefly with altitude and atmospheric water (humidity plus condensation into clouds).^[5] However, many measurements have been made in this system, including thousands of the bright stars.^[6]

The Johnson-Kron-Cousins UBVRI photometric system is a common extension of Johnson's original system that provides redder passbands.^[7][8]

• Strömgren photometric system
• Photometric System

• Photometric systems
• American inventions
• Stellar astronomy

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