The performance of the bandpass photon detector for inverse photoemission spectroscopy has been significantly improved. The detector basically consists of a simple combination of a SrF2 entrance window and a commercial Cu–BeO photomultiplier. RbCl (KBr) evaporation onto the SrF2 window realizes a FWHM of 0.51 eV (0.58 eV), at the sacrifice of sensitivity. On the other hand, KCl evaporation onto the first dynode of the photomultiplier provides good characteristics for the photon detector not only with respect to the FWHM but also in terms of the sensitivity. The new detector has a FWHM of 0.47 eV with a maximum response at a photon energy of 9.4 eV. The sensitivity is improved by about one order of magnitude.