A beam combiner for combining a plurality of light beams onto an optical path, comprising: a first dichroic element having a dichroic coating that is disposed to transmit light of a first wavelength band along the optical path and to reflect light of a second wavelength band onto the optical path, and a second dichroic element having a dichroic coating that is disposed to transmit the light of the first and second wavelength bands along the optical path and to reflect light of a third wavelength band onto the optical path. The beam combiner further includes a phase difference compensation multilayer thin- film stack that provides at least one reflectance edge transition that lies outside any of the first, second, and third wavelength bands and which provides compensation for an accumulated phase difference for polarization states of the transiting in at least one of the first, second, and third wavelength bands.