Method of checking the coating of a metal surface and device for carrying out the same

Apparatus for checking the presence of discontinuities in the coating of a metal surface, which comprises emitting means for emitting electro-magnetic radiation onto the coated surface whose wavelength spectrum includes wavelengths for which the coating is highly absorptive as well as wavelengths for which the coating is only slightly absorptive, first monitoring means for receiving electro-magnetic radiation reflected from said coated metal surface and for producing an output signal whose value increases as the intensity of reflected radiation impinging thereon increases, first filter means for allowing substantially only that reflected radiation which corresponds to the wavelengths for which the coating is highly absorptive to impinge upon said first monitoring means, second monitoring means for receiving electro-magnetic radiation emitted by said emitting means and for producing an output signal whose value increases as the intensity of radiation impinging thereon increases, and second filter means for allowing substantially only radiation emitted by the emitter means which corresponds to the wavelengths for which the coating is highly absorptive to impinge upon said second monitoring means, whereby the difference between the values of said output signals indicates the presence of discontinuities of said coating, and a radiation-opaque mounting plate having one face directed toward said coated surface and an opposite face directed toward said emitting means, said first monitoring means being mounted on said one face and said second monitoring means being mounted on said opposite face.