An apparatus for measuring optical properties of transparent materials with a first illumination device which illuminates the material to be investigated along a pre-set illumination path with a pre-set radiation, with a radiation recording space which records radiation passed on by the material to be investigated. The radiation recording space is arranged so that radiation emitted by the first illumination device first strikes the material and then at least for a time an inner wall of the radiation recording space. A radiation detector device is arranged to record radiation reflected and/or scattered essentially only from the inner wall. A second illumination device suitable for emitting modulated radiation also illuminates the inner wall.