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Rapid Visualization of Biological Fluids at Crime Scenes Using Optical Spectroscopy

NCJ Number
Date Published
March 2011
187 pages
This report presents the findings and methodology of a study whose aim was to demonstrate proof of concept for a rapid and nondestructive tool that uses infrared spectroscopy for visualization of blood at crime scenes.
Current visualization methods for blood are not specific, require dark conditions, and may not be very sensitive. The approach described produced acceptably high signal-to-noise ratios and enabled visualization of blood well below 100x dilutions with visible contrast, while providing some discrimination against substances reported to give false-positive response with other techniques. Besides being rapid, the demonstrated procedure does not expose examiners to chemicals, can be used indoors or outdoors under ambient light, does not smear patterns, and does not dilute stains or alter results by chemical reagents. The project designed a prototype camera using mid-infrared (IR) spectroscopy with a thermal imaging detector that has a spectral response tuned by filters of polymer films. Researchers also devised a lock-in amplifier that constructs the contrast image of the scene pixel-by-pixel basis in real-time using techniques designed to enhance the visualization of blood. An infrared source is used to illuminate a scene with IR light. Light reflected from the scene is used to achieve imaging by chopping the source. Digitally processing each pixel by a lock-in amplifier produces an output that is proportional to contrast between stain/no-stain regions. The infrared camera response is also sensitized to spectral regions where blood components show absorbance using a combinatorial simulation-driven design process that selects chemical filters to maximize discrimination between blood-stained and unstained surfaces. Further data processing methods develop and display scene images. The current instrument is installed on a laboratory optical table. Future studies may involve design of a portable instrument that can be carried to other locations for real-world testing and evaluation. 7 figures and 39 references

Date Published: March 1, 2011