Ground-Off Serial Number and a Faded Tattoo
A steel plate from a recovered vehicle has a serial number that has been ground off over an area about 60 mm by 20 mm. Visible grinding marks cross the area. You also have a faded tattoo on the inner forearm of a decedent; the skin is beginning to mummify and the tattoo contrast is low. Your camera is full-spectrum modified. You have a 720 nm IR filter, a UV-pass filter, a hot mirror, a UV torch, incandescent and LED sources, and an electronic flash. The investigator asks you to “make the number appear” and to “bring out the tattoo.” You have not yet consulted the firearms/toolmark examiner or the pathologist.
1. Select a technique for each exhibit and justify it. Be explicit about what photography can and cannot do here.
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For the steel plate, I would not promise that IR or UV will recover the number. The number was stamped into steel and then ground away; the relevant evidence is deformation of the metal grain below the surface, which is usually restored by a firearms/toolmark examiner using chemical etching or magnetic particle methods, not by camera filtration alone. My photographic job is to document the grinding pattern, the plate, and the scene with oblique white light and macro detail, and to coordinate with the examiner before any treatment. IR might help on some coated or painted surfaces, but it is not a reliable see-through method for steel. For the tattoo, IR is the better first attempt: tattoo pigment can show different contrast in the near-IR, and IR is useful on mummified or darkened skin. UV is less promising for deeper pigment and creates safety and exposure problems. I would test IR and visible light, not assume.
2. What focus and exposure problems do you expect with IR and UV, and how do you control them?
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IR focuses at a different plane from visible light because of lens chromatic aberration, so I would use live view focused with the IR filter in place, or the lens IR focus mark if it is accurate, then stop down for margin. The IR filter cuts visible light, so exposures become long or require higher ISO; I would use a tripod, remote release, and a custom white balance on a gray card under IR illumination, then check the histogram because IR highlights clip easily. For UV, I would use a UV-capable lens, a UV-pass filter, and a UV torch, with eye protection and no visible-light leak; focus shift is also an issue, and exposures are long. I would record the filter, light source, exposure, and focus method. I would not judge exposure by the camera's visible-light meter without testing.
3. How do you respond to the investigator's wording and what limitations go in the report?
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I would explain that “make the number appear” is not a photographic instruction I can guarantee. I can document the surface and provide the best controlled images, but recovery of an obliterated serial number is an examiner's restoration process, and any enhancement must not fabricate characters. For the tattoo, I can produce IR and visible-light images that may improve contrast, but I cannot date the tattoo, identify the ink, or claim the image shows more than the pigment pattern. The report should state the equipment, filters, lighting, exposure, and processing; label IR/UV images as non-color-true; retain originals; and note that no chemical or destructive treatment was performed by photography. If the images are inconclusive, I would say so rather than overprocess them.