Specialised Photography: UV, IR and Close-Up at the Crime Scene

Visible light occupies a narrow band of the electromagnetic spectrum from about 400 to 700 nm. Forensic photography spreads either side of that band into UV and IR.

Each band needs different gear: different illumination, different lens coatings (modern lenses are usually IR-blocked, which is a problem for IR work), and different filters in the optical path.

UV photography divides into two distinct techniques. They are routinely confused because the slogan "UV reveals body fluids" hides which of the two techniques is being used.

Reflected UV

UV illuminates the scene. UV reflected from the surface is captured. Visible light is blocked at the lens by a UV-pass filter (typically a Wratten 18A or Hoya U-340). The sensor captures only the UV reflection. Standard digital sensors are sensitive enough into the near-UV for this to work with care.

Reveals:

• Bruises that have not yet developed visibly under skin
• Bite-mark patterns on skin
• Some body fluids on light surfaces
• Altered documents where the original and overwrite reflect UV differently

UV fluorescence

UV illuminates the scene. A UV-blocking filter (a yellow or amber pass) is placed in front of the lens so only the visible fluorescence emitted by the excited substance reaches the sensor. The UV light itself is excluded.

Reveals:

• Semen on dark fabrics (the classic case)
• Saliva traces
• Certain narcotics
• Fluorescent fingerprint dusts

Near-infrared (700-1100 nm) is the band most useful for forensic photography. It's not thermal IR (that's a separate, much-longer-wavelength domain used in surveillance). Near-IR penetrates a few millimetres into surfaces and reveals what's underneath.

Standard IR forensic uses:

• Ink overwrites. Two different inks may look identical under visible light but absorb IR differently. Document examiners use this to reveal altered cheques, post-dated entries, or rewritten signatures.
• Soot and char. Burnt documents and burnt fabric retain enough surface contrast under IR to read text the visible image lost.
• Obscured tattoos. Tattoo pigment sits a few millimetres under skin. IR reveals tattoos through scarring, certain skin conditions, or post-mortem decomposition that obscures them visibly.
• Camouflaged blood on dark fabric. Dried blood absorbs IR strongly; many dark fabrics reflect IR. The contrast reveals stains that visible imaging loses.

The gear specifics:

• The camera body must be IR-sensitive. Most modern DSLRs include an IR-blocking filter (the "hot mirror") in front of the sensor. Dedicated forensic cameras have this removed, producing a "full-spectrum" body.
• The illumination source emits in the IR band. IR-converted continuous lamps or modified flashes are the standard kit.
• The lens has an IR-pass filter in front (Wratten 87, 88A, 89B depending on cutoff). The filter blocks visible light and admits only the desired IR band.

An alternate light source is a tunable narrow-band lamp covering UV through visible into IR. The major brands (Foster + Freeman Crime-lite, SPEX Forensics CrimeScope and Mini-CrimeScope) have been standard in well-equipped Indian SOCO and FSL kits. The ALS replaces several single-purpose lamps with a single device.

The workflow:

• Sweep the scene with white light first, documenting what's visible.
• Switch to UV (typically 365 nm) and re-sweep, photographing fluorescence with the appropriate yellow-pass filter on the lens.
• Step through visible bands (450 nm, 515 nm, 530 nm) with matched goggles and barrier filters, photographing any new fluorescence.
• Switch to IR (typically 780 nm illumination) with an IR-pass filter, photographing IR-distinct features.
• Document each band's findings separately in the photo log; the same physical evidence can produce different findings under different bands.

The ALS workflow is what makes specialised photography practical at scene. A static UV lamp and a separate IR lamp would each require their own setup and tear-down; an ALS does the whole sweep with filter changes only.

Macro photography for forensic work captures evidence at 1:1 reproduction ratio or closer. The classic uses are toolmarks, fingerprint detail, fibre identification at scene, ammunition base markings, and bite-mark documentation.

A few technique points:

• Lens choice is secondary to lighting. A good ring flash or twin-flash on a mediocre macro lens beats a great macro lens with the wrong lighting. Forensic macro is a lighting problem first.
• Working distance matters. Longer focal-length macros (105mm, 180mm) give more working distance, which is useful when you need to fit a ring flash, an evidence scale, and a polariser into the gap between the lens and the subject.
• Focus stacking is increasingly used. Multiple exposures at slightly different focus points are stacked in post-processing for an image with deep depth of field at high magnification. The stacking is disclosed in the photo log.
• Polarising filters defeat reflections. Wet evidence, glass, plastic, and chrome surfaces all reflect light into the lens. A polariser oriented correctly suppresses reflections and reveals surface detail underneath.
• Side-lighting reveals impression evidence. Toolmarks, footwear impressions, fabric weaves, and indented writing all show better under raking light (light coming in at a low angle to the surface) than under flat front-lighting.

The macro work feeds the same documentation pipeline laid out in Forensic Photography. Macros are usually the close-up element of the three-shot rule, with overview and mid-range shots in standard visible light.