UBV photometric system

[1] The system is defined using a set of color optical filters in combination with an RMA 1P21 photomultiplier tube.

It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.

[4] The filters are selected so that the mean wavelengths of response functions (at which magnitudes are measured to mean precision) are 364 nm for U, 442 nm for B, 540 nm for V. Zero-points were calibrated in the B−V (B minus V) and U−B (U minus B) color indices selecting such A0 main sequence stars which are not affected by interstellar reddening.

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.