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Optical Filters and Light Modifiers for Forensic Imaging in the Visible Spectrum

Wed, 06/15/2011 - 2:36pm
Jason Schneider

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

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

Tiffen Digital HT Filters
Tiffen has been making glass optical filters in the U.S. for over 75 years and they are widely used in professional cinematography. They are manufactured using ColorCore technology, an exclusive lamination process that bonds the filter material permanently in between two layers of optical glass, thus enhancing control over hue and density and assuring uniformity. Until fairly recently the company was reluctant to multi-coat their filters because existing multi-coating was quite fragile and easily abraded, rendering the filter unsuitable for critical work. Tiffen Digital HT filters, announced in 2008, solved these problems with a proprietary Titanium Multi-Coating that is easy to clean, highly resistant to abrasion, and delivers excellent anti-reflection and light transmission characteristics. Since digital CCD and CMOS sensors are much more reflective than film, the combination of high-efficiency multi-coating and the use of high-grade pure Water White optical glass justifies the “Digital” designation. These filters are very well made and have titanium finished mounting rings with milled edges for easy mounting and removal.

Essential Optical Filters for Forensic Imaging
The technical advantages of optical filters are based on the fact that they actually filter the image-forming light passing through the camera lens before it strikes the image sensor or film. Optical polarizers, like the Digital HT Circular Polarizer, actually polarize the light and can thus minimize or eliminate reflections on glass and water (but not metal!) while revealing sub-surface details, such as the contents of a window display or objects under the water. They can also enhance color contrast in brightly lit subjects and you can see the variations by simply turning the front ring of the polarizer as you look through a DSLR viewfinder. You can’t do these things with plug-in filters, which only emulate filter effects. An optical haze filter such as the Digital HT Haze 86 cuts a substantial portion of UV radiation, thereby minimizing haze and bluishness due to atmospheric perspective and providing a clearer view of distant details. Using a graduated or neutral density filter, like the Digital HT Graduated ND .6, lets you attenuate the brightness range of the scene to match the characteristics of the capture medium. This is especially important with digital cameras, which typically don’t have a great deal of overexposure latitude. Finally, optical filters physically protect the front surface of expensive camera lenses from impact in challenging situations.

Fluorescing Body Fluids Using Kodak Wratten Filters Classic Kodak Wratten acetate filters can be used in tandem to fluoresce body fluids such as saliva and semen to make them visible using a conventional DSLR and visible spectrum light sources such as an off camera strobe unit. The outfit consists of a Nikon DSL Rand 105mm f/4 Micro-Nikkor Macrolens with a Wratten 21 Orange filter mounted over the lens and a Nikon Speedlight flash unit with a Wratten 47B blue filter attached over the flash tube. An external power source enables multiple “full-power manual” flashes via a shoe-mounted TTL flash cord.

Suggestions: Always use ISO 400, Auto White Balance (AWB) an aperture between f/5.6 and f/8, a normal sync speed, and a flash setting that provides a full-power exposure on manual setting. The flash should be held 6–8 inches from the subject and held at about a 45-degree oblique angle. The technique was developed a few years ago by Steven O’Brien, a veteran criminalist at the Division for Criminal Investigation, State Crime Lab, Iowa. Steve says it works equally well with film or digital cameras and that the overhead lights can be on or off.

Figure 2: Acetate filters can be used to fluoresce body fluids to make them visible using a conventional DSLR.

Figure 3: The Lowel Blender provides a convenient way to emphazise specific hues or increase their contrast.

Lowel Blender Variable-Color Temperature Light Source
The recently introduced Lowel Blender is an infinitely variable portable light source consisting of a small rectangular reflector head containing a dozen large LEDs, six of them daylight-balanced, the other six tungsten-balanced, arrayed in a compact (about 4 inches wide, 3 inches tall, and 3 inches deep), lightweight (1.2 pounds) fixture with separate variable dimming knobs on the back, coded blue for daylight and yellow for tungsten output. On the bottom of the head is a sturdy polycarbonate and metal bracket that allows fore and aft rotation for basic vertical angle adjustment; provides a standard fitting with a lock screw for mounting the unit on a 5⁄8-inch stand or stud; and includes an accessory slot with a second locking knob for adding a Gel frame, umbrella, etc. There’s an on-off switch, but there’s no need for a fan because the unit’s 56-Watt rated maximum output generates relatively little heat. However, to keep things cool and to maximize LED life there are eight stout metal heat-dissipation fins on the back arrayed in a radial pattern.

What makes this a particularly useful device for forensic photographers is that it solves one of the basic problems of providing additional light when you’re shooting video or stills in mixed-light situations—especially when shooting, say, a crime scene video on the fly. Instead of just guesstimating, the Blender makes it possible to fine-tune your fill light to match or contrast with the ambient light to get the precise visual effect required. By turning two simple dials you can achieve light outputs ranging from about 2800K to match traditional incandescent lamps, all the way up to around 6000K, roughly equal to daylight on an overcast day. The compact unit is small enough to handhold with the pistol grip included in the kit, and it can run off a camcorder battery (available in configurations to accommodate Sony, Canon, Panasonic, Samsung, and Hitaschi batteries) or powered with the furnished 120V AC adapter. Despite its modest wattage, the unit puts out a surprising amount of light for its size, and it can even be used as a main light for subjects up to around 10 feet away at ISO settings in the 800–1600 range. The kit includes a handle with mounting stud, a nicely made compartmented carrying bag, and three frosted light panels that slide into a groove on the front of the unit to diffuse the light and obviate multiple shadows created by the individual LEDs.

After a two-week field test we conclude that the Lowel Blender provides an efficient and intuitive way of achieving precisely color balanced lighting effects on the fly as well as a convenient way of emphasizing specific hues or increasing their contrast to make them stand out. It’s not a powerhouse that’s intended as the main illumination source for lighting large rooms, but it is a unique product that’s well thought out, very well made, and designed to withstand the rigors of professional use.

Foster + Freeman Crime-Lite 8x4
This exceptionally well made, ultra-sophisticated LED ring light provides 118 modes of illumination, including seven narrow-band (50nm bandwidths at 10% of peak intensity) light sources from violet to red, 98 color mix combinations, variable white light output with ten color temperature settings between 3000K and 10,000K, and ten white light intensity settings at the color temperature of 6500K. It can be mounted on a compatible lab copy stand or on a tripod offered by the manufacturer, can be powered by AC or optional battery power for use at a crime scene, and it incorporates a bellows for safety shielding.

The light source itself is fitted with 32 high-intensity, long-life LEDs configured in a ring pattern to provide the even illumination required in forensic photography, and the unit features an integral control unit for activating lamps individually or in combination. A complete set of nine high-performance Schott glass viewing filters and cross polarizing filters suitable for the visual inspection and photography of fluorescent evidence as well as a storage case are included.

Figure 4: A black-and-white rendering shot through a #25 red filter "removes" the red background on this Snickers wrapper, making the fingerprint easier to see and identify.

Figure 4: A black-and-white rendering shot through a #25 red filter "removes" the red background on this Snickers wrapper, making the fingerprint easier to see and identify.
Figure 4: A black-and-white rendering shot through a #25 red filter "removes" the red background on this Snickers wrapper, making the fingerprint easier to see and identify.

Tiffen Dfx Digital Filter Effects Software
The scientific capabilities of the latest versions of Adobe Photoshop are considered by many forensic photographers to be the gold standard of post-production image control. Photoshop is often used to split out individual R, G, B color channels for image analysis, and many advanced users cite the mechanics of the workflow and multiplicity of possibilities as advantages of this huge and complex system. However, there are other filter emulation and software effect systems that are more intuitive and may even offer useful capabilities that are less cumbersome than Photoshop. A good example is Tiffen Dfx, which provides 113 different filter selections in the Version 2 Standalone and is available as a plug-in for Photoshop or Aperture 2.1.

In its Image Tools & Effects, Dfx provides an EZ Mask tool that lets you easily isolate any specific area of the subject to which you want to apply effects, plus a Color Correction tool that allows you to separately adjust hue, saturation, and brightness, shadows, mid tones, and highlights, as well as gamma, red, green, and blue. You can also apply a black-and-white filter, select filter options, and then dial in the appropriate color channel until the details you want to accentuate are clearly visible. Can you do these things in Photoshop? Yes, but for most users it’s easier and faster in Dfx because it provides an intuitive user interface plus filter parameters that closely match traditional optical filters. Indeed, many CSI units are currently using this software.

Jason Schneider has been writing about hands-on photography, photographic equipment, and photographic technology for over 40 years. He joined the staff of Modern Photography in 1969 and rose to the rank of Editorial Director before becoming Editor-in- Chief of Popular Photography in 1987, a position he held for 16 years. Mr. Schneider is a well respected expert in camera technology and testing and currently writes for a variety of print publications and major Websites in the field.

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