A device and method for reliably and accurately detecting impurities in a bulk material comprising two opposing tunnel sections arranged such that a bulk material stream flows between or through the tunnel sections. At least one of the tunnel sections has a lighting means configured for indirectly illuminating the bulk material stream. Furthermore, an optical detector receives the light emitted from the illuminated bulk material. The lighting means and optical detector are configured about the tunnel sections such that the optical radiation optical radiation does not pass directly from the lighting means to the bulk material, nor from the bulk material stream to the optical detector An evaluation apparatus, responsive to measured data from the optical detector, identifies impurities in the bulk material. The invention moreover relates to a method for operating such a device.