Share thisDigital technology is putting crime labs on the fast track by making infrared and ultraviolet photography easier, quicker, and less expensive then ever before.
The forensic use of infrared and ultraviolet (UVIR) photography to evaluate forged documents and counterfeit currency, gunpowder residues and tissue abrasions, and virtually every other imaginable type of physical evidence is a technique that goes back many decades. It has been used experimentally since the late 19th century and as a routine procedure in cutting-edge crime labs since the first commercial infrared films became available in the early part of the 20th century. However, until quite recently, shooting UVIR was a time-consuming, laborious, and rather expensive process that entailed the use of films withspecial handing requirements and demanding processing procedures.
To quote Chuck McKern, an experienced IR expert, “It’s difficult getting the film, difficult shooting with it, and difficult processing it.” Infrared film must be handled and processed in total darkness, is susceptible to static markings in low humidity, and requires a series of tests using heavy filtration (such as a No. 87, 87C, or 89B filter that blocks all UV radiation and visible light) to determine the best exposure. Focusing is often problematic and must be adjusted by noting the correct visual focusing distance, then resetting it opposite the IR index on the lens to correct for the focusing discrepancy between visible light and IR. Exposures are generally quite long, another major inconvenience.
UV photography using conventional blue-sensitive silver-halide film, light or medium yellow filters, and UV emitters for illumination, is somewhat easier, but optimizing a UV setup still requires a considerable amount of trial and error. In short, while film-based IR and UV photography can and does provide very good results in forensic applications and can record a wide dynamic range, neither can provide instant feedback on exposure, filtration, and other key variables. That single crucial factor severely limits the operational efficiency of film-based IR and UV imaging, and that puts a crimp on both the speed and productivity forensic investigators can achieve. It’s the old “time is money” factor rearing its ugly head, and what’s even more important, having to wait days or weeks to get crucial information makes it that much harder to nail the bad guys!
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Primer on Infrared and Ultraviolet Imaging
Scientists define wavelengths shorter than 400 nanometers as ultraviolet (UV), and wavelengths longer than 700 and up to about 1200 nanometers as infrared (IR). Technically, neither UV nor IR is “light” because humans can’t see it. However, photographs taken under UV and IR illumination can capture and reveal crucial information that is otherwise undetectable by the human eye – literally “colors” we can’t see but that sensors and films sensitive to IR and UV wavelengths are able to record and make visible. This uncanny ability to reveal the unseen is why forensic and law enforcement professionals as well as medical researchers, art historians, and biologists have long used UV and IR photography to uncover crucial observational facts and evidence that would ordinarily elude the keenest human eye. Specific examples of how forensic scientists and investigators use infrared and ultraviolet imaging to solve crimes, determine the causes of death and injury, detect the presence of blood and other substances, and generally reveal unseen truths to catch bad guys could fill a sizeable book. However, herewith a concise list of general categories where UVIR photography has proven to be of immense value:
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The human eye is a remarkable imaging instrument with an impressive dynamic range. However its chromatic sensitivity, which peaks in the green at about 550 nanometers, is limited to the visible spectrum, defined as wavelengths that extend from 400 nanometers (violet) to 700 nanometers (deep red). This is also the realm of ordinary digital and film photography where, with certain exceptions, what you see is what you get. But there are times when ordinary visible-light pictures do not reveal everything a forensic specialist, law enforcement lab technician, scientist, or medical researcher needs to know. That’s where ultraviolet (UV) and infrared (IR) imaging come in.
Black T-Shirt with Blood Stain,captured with the FinePix S3 Pro UVIR using a 700nm filter (Photo courtesy of Julio Sosa, Biomed Studio)
