The magnitude of a star measured with an R filter. The filter used has several different definitions. The central wavelength and bandwidth of the R filter in Kron–Cousins RI photometry are 641 and 158 nm; in Johnson photometry they are 710 and 210 nm; in the six-colour system, 714 and 165 nm; and in RGU photometry, 641 and 40 nm. The R most frequently used nowadays is that of the Kron–Cousins system. R is usually measured photoelectrically, except in the RGU system, which is photographic.