The equality of the reflected and the transmitted irradiances by a bea
m splitter that consists of a thin absorbing coating (typically a meta
llic film) on a transparent plate is considered. The absorption and th
e phase difference between the reflected and transmitted fields are al
so studied. The lack of reversibility of this beam splitter introduces
an asymmetry that is discussed for a Michelson interferometer. (C) 19
95 Optical Society of America