The physics of infrared luminescence21 photography is similar to ultraviolet fluorescence photography although the results are different in a practical sense. In both techniques, the painting is irradiated (excited) with electromagnetic radiation resulting in emission of radiation of longer wavelength. In ultraviolet fluorescence photography (see below), the exciting radiation is ultraviolet and the emitted radiation is visible light. In the case of infrared luminescence photography, the exciting radiation is blue-green visible light and the emitted radiation is in the infrared region of the spectrum and is recorded on black-and-white infrared film. Infrared luminescence photography is applicable when thick layers of varnish are present.