METHOD AND APPARATUS FOR DISTINGUISHING PHOTOLUMINESCENT AND REFLECTING SURFACES IN FORENSIC SCIENCE APPLICATIONS.

摘要

In forensic science it may be necessary to distinguish between two photoluminescent sources which emit radiation at similar, but different, wavelengths, or to distinguish between reflected light from surface regions which reflect light at different characteristic wavelengths. Examples of such sources or surfaces are inks on documents and treated latent fingerprints. The prior art technique for distinguishing such sources or regions involves the systematic use of a large number of narrow band pass interference filters. The present invention uses a single interference filter (17, 14) which is tilted to alter the wavelength of the peak radiation transmitted by the filter. If photoluminescent sources are to be distinguished, the filter may be placed between the sources and a monitor (10) of their emitted radiation, or between an illumination lamp (13) and the sources. An angle of tilt of the filter is selected so that radiation from at least one source is rejected while the intensity of radiation from a wanted source is optimised. If a reflecting surface is to be distinguished, the tiltable filter (14) is positioned between an illumination lamp (13) and the surfaces and an angle of tilt is found so that the illuminating light has a wavelength equal to the characteristic reflecting wavelength of an unwanted surface, so that, to an observer, the unwanted surface merges with the background.