Abstract: A method is presented for non-destructive testing of the state of a surface that may have cracks in it by observing waves emitted by a dye applied to the surface and present in the cracks in response to an incident excitation beam of wavelength appropriate to the dye. The incident excitation beam is an ultraviolet light. The beam is made of rectilinear polarized waves. Waves emitted by the dye are observed through a rotatable polarized wave analyzer. The analyzer is rotated first to eliminate from observation the wave due to the residual dye on the surface and thereafter to determine the depths of the cracks.