Johnson magnitude A magnitude measured in one of Johnson's standard passbands (using a standard filter, i.e. U, B, or V). For accurate photometry the magnitude needs to be determined over well-defined spectral regions as the spectrum of to objects may be quite different. These magnitudes are measured using filters that allows only radiation within specific spectral regions (passbands) to pass through to the detector. These filters have accurately defined transmission curves characterised by a central wavelength and a bandwidth. The UBV system devised by Harold Johnson and William Morgan has been the most important general system until recently. The precise definition requires a reflecting telescope with aluminised mirrors fitted with an RCA 1P21 photomultiplier. The U region corresponds to a region in the violet and ultraviolet, the B region corresponds to typical photographic response and the V region to the visual response region (approximating the eye's response curve). magnitude magnitude