Testing the Output of LED Forensic Illuminators

Foster & Freeman CrineLite LED Illuminator

Integrating spheres help in the development of LED-based forensic illuminators

Foster & Freeman Ltd of Evesham, UK manufacture optical instruments used by police and forensic scientists in the laboratory and at the scene of a crime. Traditionally, bulky arc lamp sources have been used for the illumination of latent fingerprints but Mason Vactron is pioneering the use of LED-based sources as more compact and portable alternatives. Using advanced LED technology, the company has created a range of handheld, battery powered, high-intensity forensic light sources for locating evidence at scenes of crimes. The Foster & Freeman Crime-Lite® provides narrow waveband light for fluorescent excitation at wavelengths in the violet, blue, blue-green and green bands for detecting blood stains, gunshot residues, body fluids, and superglue-treated fingerprints.

In the Crime-Lite®, LEDs are clustered to provide high illumination levels, resulting in an output of up to 500mW, in a 50mm diameter, low divergence beam. The radiant flux of this semi-collimated, high power, large-area beam is difficult to measure using bare photodiode detectors, whilst large area calorimeters do not possess sufficient sensitivity for this type of source.

Foster & Freeman partnered with Pro-Lite who developed a radiometer for their LED sources based on a 200mm diameter Labsphere integrating sphere with a 50mm input port. Using this, the power from the Crime-Lite® is measured conveniently, accurately and repeatably. An integrating sphere is a hollow, spherical chamber coated internally with a high reflectance coating that exhibits diffuse reflectance. Spheres are used as directionally-insensitive collectors of light when combined with photodetectors. Light sources that exhibit high powers, large beams, extended source sizes, high divergence or dynamic beam displacement present a challenge to those wishing to determine or track their power in an accurate, repeatable fashion. The integrating sphere provides an elegant solution and offers an increased accuracy alternative to regular optical power meters used with ‘difficult to measure’ sources, such as LEDs, lamps, projectors, optical fibres, laser diodes, VCSELs and laser diode stacks or bars.

Pro-Lite gratefully acknowledges Foster & Freeman for their permission to publish this article. www.fosterfreeman.co.uk