Abstract: The cutting horizon for mining machines, such as coal ploughs and roll loaders, in particular the position of the coal-rock interlayer, can be determined with the aid of light signals of selected wavelengths at reflection layers, at least one sensor head guiding iding dragging along the floor and having at least one optical waveguide bundle constructed as measured-value pickup and guided in a passage sealed with a crystal window being arranged on the mining machine and a transmitted and receving station being arranged on the machine body. For clear identification of the carbon-rock interlayer the passage (6) receiving the optical waveguide bundles (5,5') extends in the exit of the sensor head (3) at an angle of 30.degree. and the lower surface (13) of the crystal window (7) extends parallel to the floor (2).