The latest optical filters as well as some tried-and-true classics can be extremely useful in enhancing forensic images and adding to their evidentiary value.

Forensic imaging, the gathering and preservation of photographic evidence essential for investigating or prosecuting a crime, has been around since the days of the wet-plate camera and has been a standard procedure in law enforcement since George Eastman vastly simplified the process by inventing flexible roll film. Indeed, American appellate court cases declaring the admissibility of photographs that are relevant and properly verified go back to 1859! Today most forensic imaging is done using digital cameras, and a fair percentage of forensic images are shot with UV/IR cameras that can reveal trace evidence of fingerprints, tiny blood samples, bite marks, and many other things invisible to the naked eye. Another great advantage of digital capture is that all images shot are tagged with metadata establishing the exact time the picture was taken (assuming the camera was properly calibrated), the exposure data, and (if the camera is equipped with built-in or accessory geo-tagging) fairly precise GPS coordinates of the location where the picture was taken. This is not a substitute for careful record keeping, but it definitely simplifies the organizational process.

Even in today’s brave new world of ultra-sophisticated, high-tech imaging, a significant number of forensic images are still shot in visible light, and that’s why the latest optical filters as well as some tried-and-true classics can be extremely useful in enhancing forensic images and adding to their evidentiary value. Some can reveal faint or unseen details, others can enhance color contrast or highlight specific colors to bring out hidden patterns, and one can even cut through moderate haze and atmospheric perspective to present a clearer view of distant objects. In addition, there are variable color temperature light sources of different degrees of sophistication that can be used to match the color balance of a scene or differentially emphasize certain hues to facilitate forensic analysis. Their ability to deliver a precise visual color balance over a wide range makes them extremely useful while shooting some types of forensic images and videos, and the fact that the RAW or reference image is created at the time of capture without any post-production tweaking minimizes any subsequent legal challenges to their authenticity. Finally, there is new digital filter-emulation software that offers some distinct operational advantages over the widely used scientific version of Adobe Photoshop.

While it is impossible in this short space to provide a detailed step-by-step, how-to guide to using these products in specific forensic applications, the following technical descriptions of each will provide a good basis for understanding their advantages and limitations, and thereby help you to make intelligent, scientific decisions.

Figure 1: Optical polarizers can minimize or eliminate reflections on glass, or water, thus revealing sub-surface details.