pulsed light sources , such as a flashlamp or laser , remove coatings from substrates via the ablation method . a photodetector circuit , sensing reflected light from the surface being ablated , provides a feedback signal that indicates the reflected color intensity of the surface being ablated . the boundary between the coatings or substrate surfaces is distinguished by a change in color intensity between an upper coating and an undercoating , e . g ., between a topcoat of paint and a primer coat of paint , or between a coating and the substrate surface itself . the color intensity determination thus provides a measure relative to when one coating has been removed and another coating remains . the photodetector circuit is also useful for providing feedback information relative to the quality of a stripped work surface for quality control or other purposes .