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 sensor filters (modern camera sensors are usually IR-blocked by a hot-mirror filter in front of the sensor, 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

Reflected-UV capture needs a sufficiently UV-sensitive camera and transmitting optics, with filtering that rejects unwanted visible and IR radiation. Test the complete system and focus shift.

Reveals:

• Some surface or document features may show useful differential response.
• A response is not a diagnosis of bruising, a person’s identity or a specific fluid.

UV Fluorescence

For induced fluorescence, reject the excitation band and record selected longer-wavelength emission. The appropriate barrier filter depends on the source and target; yellow is not a universal filter.

Reveals:

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

Fluorescence can guide sampling, but detergents and substrates can also fluoresce. Record source, filters, controls and visible-light context; absence of fluorescence does not exclude biological material.

Silicon-camera near-IR imaging often uses a selected band within roughly 700–1100 nm. Contrast and penetration depend on wavelength, material and optics; there is no fixed millimetre penetration depth.

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:

• Use a tested IR-sensitive camera and suitable optics; a full-spectrum conversion is one option, not the only possible system.
• Select illumination and filtering for the intended band and verify unwanted-light rejection.
• Check focus and image geometry under the actual spectral conditions.

An alternate light source provides selected illumination bands; not every device covers UV, visible and IR. Verify output and use appropriate eye and skin protection.

The workflow:

• Record a visible-light comparison and the subject’s original condition.
• Select illumination and matched barrier filtering for the question.
• Use appropriate protection and control samples.
• Record settings and select suitable areas for follow-up testing.

Choose bands through a validated examination plan rather than assume a compulsory UV-to-IR sweep identifies material.

Close-up and macro imaging record small features; report the reproduction scale and resolved detail. Photographs alone do not identify a fibre source or validate bite-mark attribution.

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.