of incidence a of the beam l is relatively small and is between 5 u00b0 and 40 u00b0 preferably 5 u00b0 and the angle of incidence a 39 of the beam l 39 is steeper and is between 60 u00b0 and 85 u00b0 preferably 85 u00b0 if the two light sources l and l 39 are differently modulated then with corresponding demodulation of the signal a single common photoelectric receiver 3 can be employed in this case the signal component originating from the light source 1 is divided in this by that originating from the light source 1 39 however it is also possible to use two light sources 1 1 39 which emit light of differing wavelengths in this case the reflected beam r must be divided on the receiver side and the individual components originating from the two light sources 1 1 39 must be distributed by appropriate filters to two different photoelectric receivers a further illumination variant consists in using two light sources 1 with a small and a light source with a very steep angle of incidence of up to about 90 u00b0 in this case in relation to fig1 b the two light sources 1 are disposed symmetrically on both sides of the reflected beam r and the third light source is situated between the other two this arrangement leads on the one hand to an even better emphasising of periodic structures and on the other hand to the elimination of disturbing influences originating for example from neps and.