In a reflectometer for testing beam waveguides, the measurement result is unaffected by undesirable Fresnel reflections which may arise when the light enters the beam waveguide (7) under test. For this purpose, at least one current source (16) is triggered by the presence of Fresnel reflections and emits an output signal. The difference between the output signal and the useful signal is calculated and used to compensate the unwanted signal superimposed on the useful signal. Several Fresnel reflections can be compensated (masking) during one and the same test by repeated triggering of the current source. The reflectometer can also be used to monitor the course of attenuation and to detect imperfections in a beam waveguide under test in the region immediately behind an imperfection giving rise to Fresnel reflections. Application to testing of beam waveguides.
