The apparent magnitude a star would have if its energy output could be measured at all wavelengths in the absence of the Earth’s atmosphere. The name arises because bolometers are sensitive to radiation of all wavelengths. In practice, the bolometer is the best detector only for wavelengths longer than 6 μ‎m, and other detectors are used for shorter wavelengths. Between 0.3 and 23 μ‎m the star is compared with a reference source on the ground using only those wavelength windows where the Earth’s atmosphere is relatively transparent. Even within these windows it is important to correct for atmospheric extinction. Between these windows the star’s flux is estimated with the help of a model atmosphere. Outside the range 0.3–23 μ‎m, the flux is measured from satellites.