An extragalactic database V. Conversion of apparent B magnitudes to a standard system

This paper is one of a series dedicated to the construction of homogeneous mean parameters from data available in the Lyon-Meudon Extragalactic Database (LEDA). This paper gives formulae for reducing twenty magnitude systems to the photoelectric B_T_ system of the Third Reference Catalogue, in such a way that the resulting magnitudes are completely free of bias or perturbing effects. We are giving a new interpretation of the mean surface brightness correction term which is used by some authors, and we show that, generally, it has not to be corrected for. Instead, several secondary effects are found.