Crime Detection Devices: UV, IR, X-Ray and Detective Dyes

Forensic detection uses four bands of the electromagnetic spectrum, each assigned to distinct evidence types. Device specifications are given in nanometres or angstroms, so wavelength is the practical reference unit.

Shorter wavelengths carry more energy and greater penetration, but also more scatter and greater biological hazard. X-ray and neutron radiography can image through dense metal but require shielding and licensing. UV is energetic enough to drive fluorescence in biological material but won't penetrate more than the top few cell layers, which is why UV is a surface technique. IR is the gentlest of the four; it carries less energy than visible light, doesn't damage samples, and is the workhorse for non-destructive examination.

UV detection splits into two operational sub-bands. Long-wave UV at 365 nm (UV-A) drives fluorescence in most biological body fluids, certain inks, fibres and synthetic adhesives. Short-wave UV at 254 nm (UV-C) is more energetic, gives sharper contrast on some surfaces, and is the band that requires protective eyewear and limited skin exposure. Most field kit ships with both, switchable on the same handheld unit.

What UV reliably reveals:

• Semen. Bright bluish-white fluorescence under 365 nm or 450 nm light through an orange filter. The screening test of choice for sexual-offence scenes; confirmation runs on acid phosphatase or P30 strip and then PCR.
• Saliva, urine, vaginal secretions. Weaker fluorescence than semen, often visible but variable; requires confirmatory chemistry.
• Bruises, especially older ones. Sub-surface haemoglobin breakdown products absorb differently under UV. Bruises invisible to the eye on dark skin become visible under reflected UV imaging, which is the basis of Specialised Photography work.
• Inks and document alterations. Some inks fluoresce under UV; matched colours to the eye sometimes diverge under UV, exposing erasures, overwrites or substituted sheets.
• Fibres. Many synthetic fibres fluoresce; comparison under UV separates fibres that look identical in visible light.
• Detective-dye traps. Treated banknotes transfer dye to the suspect's hands on contact; UV reveals the transfer at the moment of arrest. Covered in detail in Section 5.

What UV doesn't do: read through paint, skin or fabric, or work well in direct sunlight. UV is a darkroom-or-shadow technique. Field practice in India is to drape an opaque cover over the area of interest if direct sunlight can't be avoided.

Infrared detection works on the principle that materials reflect, absorb and transmit IR differently from how they handle visible light. The most useful IR window in forensic work sits between 700 and 1000 nm (near IR), which silicon CMOS sensors capture if the manufacturer's IR-cut filter is removed. Many Indian state FSL labs run an IR-converted DSLR alongside a standard one.

What IR reliably reveals:

• Obliterated writing. Ink covered by another ink of the same visible colour often has a different IR-absorption profile. The covering ink becomes transparent in IR; the original writing reads through. Standard for cheque-fraud and tampered-document cases.
• Gunshot residue (GSR) burn patterns. GSR contains carbon and nitrocellulose that absorb strongly in IR. Burn patterns and powder distribution become visible against bloodstained or dark fabric where the eye sees only a uniform stain.
• Bloodstains on dark substrates. Blood absorbs IR strongly; the bloodstain appears nearly black against the substrate's IR reflectance, raising contrast on dark fabrics, dark upholstery and dried wood.
• Latent prints on certain substrates. IR-luminescent powders developed under IR illumination reveal prints on multi-coloured substrates (currency notes, magazines) where visible-light powders blur into the background.
• Tattoo recovery on decomposed skin. Carbon-based tattoo ink reads through superficial skin breakdown when visible light has lost contrast. Forensic odontology and identification teams use this for late-stage decomposition cases.
• Bruise-pattern documentation. Sub-surface tissue contrast in IR is different from UV; the two together give the photographer two independent looks at injury.

What IR doesn't do: distinguish two visually similar inks if their IR absorption is also similar (a known failure mode for some modern gel pens), or read through opaque metal. IR is also less effective in scenes with strong IR-emitting heat sources (cooking surfaces, vehicle engines that were running).

X-ray and neutron radiography are the two penetrating techniques in the forensic kit. They are used when the question is what's inside something opaque, not what's on its surface.

X-ray applications

The standard medical X-ray machine, repurposed for forensic use, handles most casework. Lab-grade setups range from desktop cabinet X-ray units (for small items) to industrial radiography units (for vehicles and large packages).

1. Concealed objects in body cavities
   Drug couriers swallowing packets, smuggling investigations, suspected internal weapons. Standard procedure in customs and NCB cases. The X-ray is also the documentary record for trial.
2. Internal examination in suspicious deaths
   Pre-autopsy X-ray locates retained projectiles, bone fractures and embedded foreign bodies before the pathologist begins dissection. Saves time and prevents the loss of small fragments during cutting.
3. Concealed objects in luggage and packages
   Standard airport security overlap. CBI and customs use forensic-grade cabinet X-ray for chain-of-custody-grade imaging of seized parcels.
4. Document layer analysis
   Forensic X-ray reveals layered or laminated documents, hidden microdots, and counterfeit currency security features that are below the visible surface.
5. Firearm and explosive device exam
   Pre-disassembly X-ray of a suspect IED images the internal wiring, the initiator, and any anti-tamper features before the bomb squad cuts open the device. Direct safety implication.
6. Skeletal age estimation in unidentified remains
   X-ray of long-bone epiphyses, dental development and pelvic morphology estimates age in unidentified-body cases, working alongside DNA and dental records.

Neutron radiography (NR)

NR uses thermal neutrons rather than X-ray photons. The key difference: neutrons interact strongly with light elements (hydrogen, carbon, boron, lithium) and weakly with heavy elements (most metals). This is the opposite of X-ray sensitivity. NR is the technique you use when the thing you want to see is a plastic or explosive component hidden inside a metal casing, where X-ray would show only the casing and miss the contents.

Realistic Indian uses of NR:

• IED examination. A metal-cased device with a plastic explosive charge reads clearly under NR where X-ray sees only the casing.
• Anti-tamper feature detection. Plastic anti-tamper switches and adhesive layers inside a sealed device.
• Composite-material analysis. Aerospace and defence samples where the question is the distribution of polymer matrix inside a metal-reinforced structure.

NR is rare in routine casework. The facilities are concentrated at DRDO and BARC, with NSG playing a coordination role for IED data and inter-agency response rather than operating dedicated neutron radiography installations. A forensic student should know what NR is, when it's chosen over X-ray, and that hands-on operation is unlikely outside a few national facilities. The detail worth holding is the contrast inversion: heavy elements transparent, light elements opaque.

Detective dyes are fluorescent powders applied to banknotes, file pages or other objects that a corrupt official is expected to touch. The dye is invisible to the eye but transfers readily to skin and fluoresces under UV at 365 nm. When the suspect is apprehended, a UV scan of their hands shows the transfer; the contact is the offence.

The two dyes that have done the bulk of Indian trap-case work for fifty years:

• Phenolphthalein. Colourless on the banknote, turns pink in alkaline solution. The standard CBI dye in trap cases. Hand-wash residues run pink in a sodium-carbonate bath, producing the visible courtroom demonstration that the suspect handled the treated notes.
• Anthracene. Fluoresces a vivid blue-violet under 365 nm UV. Applied dry as a powder; transfers to skin and fluoresces directly without a chemical-bath step. Faster to demonstrate at the scene than phenolphthalein.

How a typical CBI trap runs:

1. Treat the bait
   Banknotes or file pages are dusted with phenolphthalein or anthracene, photographed, serial-noted and sealed in a witness's presence. The treatment is logged as the first step in the chain of custody.
2. Hand-over surveillance
   The complainant approaches the suspect with the treated bait, supervised by CBI/anti-corruption officers from a distance. The transfer is captured on video and audio.
3. Apprehension and hand-wash test
   On the prearranged signal, officers move in. Phenolphthalein cases use a sodium-carbonate hand-wash; the wash water turns pink if the suspect handled the treated notes. Anthracene cases use a 365 nm UV scan.
4. Seizure and documentation
   The hand-wash water (or UV photograph of the hands), the treated bait notes, the suspect's outer clothing and any container holding the notes are seized, separately packaged, sealed and forwarded to the FSL. Each item runs its own chain.
5. FSL confirmation
   Chemistry division confirms the phenolphthalein reaction or the anthracene fluorescence, matches the dye to the bait treatment, and issues the forensic report. The report is filed alongside the panchnama and the video for trial.

The detective-dye method is not a magic bullet. Defence challenges typically attack three points: that the suspect handled the notes innocently (mistaken hand-over, planted), that the hand-wash test was contaminated by the officer's own gloves, and that the chain of custody for the seized notes broke between scene and FSL. Well-prepared trap cases pre-empt all three with strict on-camera protocol, separate seizure packets for officer gloves, and panch-witness signatures at each transition. Sloppy trap cases routinely collapse on cross-examination.

Indian SOCO practice splits crime detection devices into a portable field kit and a lab-grade fixed installation. The field kit gets you 80% of routine hits; the lab handles the edge cases.

The mobile-laboratory programmes covered in CSI Tools, Kits and the Mobile Laboratory carry a meaningful subset of this kit on-scene, which has tightened the field-to-lab escalation loop in Maharashtra, Tamil Nadu and Delhi. Tele-forensics workflows covered in Tele-Forensics and Technology Innovation let an FSL specialist review a UV or IR image in near-real-time and call for additional captures while the SOCO is still on scene, which has cut escalation cycles from days to hours in well-resourced state forces.