The present invention regards methods and devices for detecting plaque on a surface in the oral cavity to which a fluorescent agent capable of binding to plaque has been applied, whereby a radiation source emits incident radiation for contacting the surface, reflected light and fluorescent emission resulting from contact of the radiation with the surface is collected by an optical collector and conveyed by an optical pathway in the device, where the optical light signal of the reflected light and fluorescent emission is converted to an electrical signal, and where the electrical signals of the fluorescent emission and the reflected light are then mathematically manipulated to provide a compensated plaque value as a function of the distance from the optical collector and the surface of the oral cavity to which the fluorescent agent has been applied.