Specialised Imaging: UV, IR, Laser and ALS

Ultraviolet reflectance imaging captures the way a surface scatters or absorbs UV radiation rather than visible light. Most biological tissues, white substrates, and many mineral surfaces scatter UV radiation at the surface level without fluorescence, producing a reflected UV image. The diagnostic value comes from the contrast: regions with altered surface chemistry, sub-surface haemorrhage, or chemical deposits scatter UV differently from undisturbed surrounding surfaces.

UV bands and filter combinations. The UV spectrum is divided into UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm). For reflectance imaging on living or recently deceased subjects, UV-A and the near-UV-B range (around 300-360 nm) are the practical bands. The light source emits in this range (a UV-filtered xenon flash, a Spectroline UV-A lamp, or the short-wave output of a filtered forensic light source such as the Polilight or Crime-lite 2L series). A UV-transmitting, visible-blocking filter placed over the camera lens passes only the reflected UV and blocks the visible radiation that would swamp the image. The Kodak Wratten 18A is the classical filter for this purpose; modern equivalents include the Hoya U-330 and the Schott UG-11 glass filter. The camera sensor must be either a modified camera with the UV-blocking hot mirror removed (standard consumer cameras block UV at the sensor) or a purpose-built UV-sensitive scientific camera.

Latent bruise documentation. The Vogeley et al. (2002) protocol, published in Archives of Pediatric and Adolescent Medicine (vol. 156, pp. 265-268), demonstrated that UV-reflectance photography could reveal patterned bruising not visible under standard white light, particularly in child-abuse cases where recent bruising on darker skin tones is difficult to assess grossly. The protocol specifies a 360 nm UV-A source, a Wratten 18A lens filter, and RAW capture with a camera modified to remove the UV-blocking filter. In the UK, paediatric forensic-medical examinations increasingly include UV-reflectance photography as a standard adjunct to visible-light examination, per guidance from the Faculty of Forensic and Legal Medicine (FFLM). The Royal Canadian Mounted Police and FBI Evidence Response Teams include modified UV-capable cameras in their specialist evidence kit. Indian FSL teams in the CFSL Hyderabad and CFSL Chandigarh units have incorporated UV imaging for bite-mark and patterned-injury documentation.

Gunshot residue (GSR) and UV reflectance. Gunshot residue deposits on skin and fabric scatter UV radiation distinctly from the surrounding surface, increasing contrast in UV-reflectance images. The pattern of GSR scatter under UV can help determine firing distance and muzzle-to-skin orientation in a manner complementary to the chemical GSR tests (SEM-EDS for Pb-Sb-Ba particles, covered in the electron microscopy module). UK Home Office research in the 1990s and subsequent ENFSI GSR working group publications document UV photography as a standard pre-sampling step before chemical GSR testing.

Bite-mark documentation under UV. A patterned bite-mark may have a subdermal bruise component that is invisible in visible light but shows contrast under UV reflectance. The ABFO's bite-mark evidence guidelines recommend UV photography as a supplementary technique alongside standard white-light and ALS-fluorescence photography for every bite-mark examination. The documentation value is highest within 24-48 hours of the injury, when the sub-surface haemorrhage is shallowest and most accessible to UV surface scatter.

UV fluorescence imaging exploits the Stokes shift: a molecule absorbs incident UV (or near-UV) radiation and emits fluorescent radiation at a longer (lower-energy) wavelength. The shift from, say, 365 nm excitation to 450 nm emission makes the emission detectable through a filter that blocks the excitation wavelength while passing the emission band.

Filter system: the excitation-barrier pair. The forensic ALS system works with two filters: an excitation filter (on the light source) that restricts the output to a defined wavelength band, and a barrier filter (goggles worn by the examiner, or a filter on the camera lens) that blocks the excitation wavelength while passing the longer-wavelength fluorescent emission. Without the barrier filter, the bright excitation light masks the weak fluorescent signal. The Foster + Freeman Crime-lite 2L series, the Rofin Polilight PL500, and the Spex Mini-Crimescope all use interchangeable bandpass filter sets covering typically 415 nm, 450 nm, 470 nm, 485 nm, 505 nm, and 530 nm excitation bands, each paired with a matched barrier filter.

Biological fluids. Semen, saliva, urine, and vaginal secretions all contain fluorescent compounds that emit under specific UV-ALS excitation. The operational wavelengths vary: semen fluoresces strongly under 450-470 nm excitation (blue-green emission). Saliva and urine are weaker fluorophores. The standard ALS protocol for biological-fluid screening begins with a sweep of the scene using the ALS at 450 nm with orange barrier goggles, flagging all fluorescent areas for subsequent serological testing. This is standard procedure in sexual assault scene examination in the US (FBI guideline), UK (Home Office Forensic Science guidelines), India (CFSL sexual assault examination protocol), and Australia (AFP crime scene examination procedures). The fluorescence screening result is never itself conclusive for biological fluid identity; it is a location aid that directs the sampling strategy.

Fingerprint fluorescence. Many fluorescent fingerprint powders and cyanoacrylate-fumed fingerprints treated with fluorescent dyes (Rhodamine 6G, RAY dye, Crowle's reagent) fluoresce brightly under specific ALS wavelengths. On multi-coloured or fluorescent substrates where conventional black or white powder would be invisible, ALS-fluorescence examination is the primary detection method. The 532 nm (green) excitation line from an Nd:YAG laser or a filtered ALS system is effective for Rhodamine-treated prints; 405 nm (violet) is used for DFO (1,8-diazafluorenone) treated porous-substrate fingerprints. The ALS provides the broad-spectrum tunable source; the laser provides a higher-intensity monochromatic alternative for substrates requiring greater excitation power.

Infrared (IR) imaging exploits the fact that many materials are transparent or semi-transparent in the near-infrared (NIR, 700-1000 nm) while being opaque in the visible range. Carbon-black-based inks absorb NIR strongly while white paper transmits it, producing high-contrast images of text even on aged or stained substrates. Other materials show the opposite behaviour: surfaces that block visible light (blood, soot, overlaying paint) are semi-transparent in the NIR, allowing the examiner to see through them.

Camera modification for NIR capture. Standard digital cameras have a hot mirror (infrared-blocking filter) bonded in front of the sensor to prevent NIR from reaching the pixels during normal photography. For NIR forensic imaging, either a purpose-modified camera (hot mirror removed, often called a "converted" camera) or a camera with an external NIR-pass filter (Hoya R72, Kodak Wratten 87, or equivalent) is used. The exposure is typically much longer than visible-light photography because sensors are less sensitive in the NIR band after removing the hot mirror correction. A tripod is essential.

Obliterated and altered documents. Carbon-based typewriter ink and many printer inks absorb NIR strongly. If an ink obliteration uses a pigment that absorbs NIR differently from the underlying ink, the underlying text becomes visible through the obliteration in an IR-reflectance image. Ball-point pen inks with different carbon-black concentrations show different NIR absorption, allowing differentiation of pen strokes that are visually identical. This application is a standard first-step examination in questioned-document laboratories worldwide: the ENFSI Document Working Group's Best Practice Manual for Ink Examination, the FBI Laboratory's document examination protocols, and Indian CFSL document examination SOPs all reference IR reflectance as a Tier 1 examination step.

Gunshot soot and blood. When a near-contact or contact gunshot wound bleeds, the soot pattern may be covered by overlying bloodstain. In visible light, the soot pattern is masked. Under NIR reflectance, the blood becomes semi-transparent and the carbon-black soot absorbs strongly, making the soot pattern visible through the blood layer. This technique was documented in forensic pathology literature from the 1990s onward and is applied in both antemortem and postmortem wound examination. The UK's FFLM and RCPath autopsy guidelines reference NIR photography as an adjunct technique for contact and near-contact gunshot wound documentation.

IR transmittance for layered documents. Where IR reflectance shows surface features, IR transmittance (placing the light source behind the document and the camera in front) reveals features through the entire paper thickness: watermarks, chemical alterations that thin the paper, overwriting that penetrates through multiple sheets, and the characteristic paper-fibre patterns of different paper stocks. The Foster + Freeman VSC8000 and the DEXIS document examination system both include transmitted-IR imaging as a standard mode, alongside reflected-UV and reflected-visible modes. The UK's ACPO (Association of Chief Police Officers, now replaced by NPCC) document examination guidelines and the SWGDOC (Scientific Working Group for Forensic Document Examination) best practices both specify IR transmittance as a required examination modality for suspected altered documents.

Laser-induced fluorescence (LIF) uses a monochromatic laser source rather than a broadband ALS to excite fluorescent marks, powders, or treated reagents. The advantage is photon density: a 532 nm 200 mW green laser delivers a highly concentrated beam that can be expanded into a line or sheet with a cylindrical lens, flooding the evidence surface with a far higher photon flux than any filtered broadband source at the same surface area.

Argon-ion and Nd:YAG laser lines. The argon-ion laser (488 nm, blue; 514 nm, green) was the first widely deployed laser for fingerprint fluorescence examination in forensic laboratories, used in major forensic institutes from the early 1980s. Its key limitation is size, power consumption, and the need for water cooling, making it a laboratory-only instrument. Frequency-doubled Nd:YAG lasers (532 nm, green) are now more common because they are compact, solid-state, and air-cooled. Semi-conductor diode lasers at 405 nm (violet) are increasingly used for DFO-treated porous substrates and for 1,2-indanedione-zinc prints on paper.

Fingerprints on difficult substrates. The standard latent-fingerprint development workflow uses powder, cyanoacrylate fuming, or chemical reagents (ninhydrin, DFO, physical developer) on the suspect substrate. On multi-coloured, patterned, or fluorescent substrates, any developed fingerprint competes visually with the substrate's own pattern or background fluorescence. Laser excitation, combined with time-gated detection (a technique that delays the camera shutter opening by a few nanoseconds after the laser pulse to let short-lived background fluorescence decay before recording the longer-lived fingerprint fluorescence), can isolate the fingerprint signal. Time-gated laser fluorescence systems are used in major national forensic laboratories, including the UK's DSTL (Defence Science and Technology Laboratory) and the Australian Federal Police's National Centre for Forensic Studies.

Standardisation and documentation. The SWGMAT (Scientific Working Group for Materials Analysis) guidelines on fingerprint examination and the SWGIT Section 15 on digital photomicroscopy both specify that laser examination images must document the laser wavelength, power setting, filter type, exposure settings, and whether time-gating was applied. The ENFSI Fingerprint Working Group's ENFSI-BPM-FIN-01 (Best Practice Manual for Fingerprint Examination) includes LIF examination as a documented procedural step for difficult substrates.

Safety considerations that also constrain the photographic protocol. Laser radiation at any wavelength above a minimum threshold can cause irreversible retinal damage. The laser examination workflow for fingerprints must be conducted with approved laser-safety eyewear matched to the laser line, and photographic documentation using a camera with a lens filter that passes the emission wavelength while blocking the excitation laser line is the only way to record the fluorescent image safely. This filter requirement is not just safety but also quality: direct laser specular reflection would saturate the sensor and destroy the image.

Infrared luminescence (IRL) is a distinct phenomenon from infrared reflectance. In IRL, a material absorbs visible radiation and emits photons at NIR wavelengths, rather than reflecting ambient NIR radiation. Specific classes of ink, particularly certain ball-point and felt-tip pen formulations containing phthalocyanine-based pigments, exhibit this behaviour. IRL allows the forensic document examiner to detect differences between ink strokes that are visually and spectrally similar in reflectance but differ in their luminescent properties.

The ink-sequence problem. When two different inks cross each other on a document, the question of which stroke was deposited first has evidentiary significance: it can reveal whether a signature was written before or after the surrounding text, or whether a correction was made to an authentic document. In transmitted-light photography both inks are visible but their relative sequence is unclear. In IRL examination, one ink luminescent in the NIR and the other non-luminescent can be selectively imaged to reveal the crossing-point sequence. The luminescent ink appears bright in the IRL image regardless of whether it is the upper or lower stroke at the crossing point; geometric analysis of the crossing determines which stroke broke continuity of the other.

Operationalising IRL examination. The procedure requires an excitation source in the visible range (typically 450-550 nm), an NIR-pass filter on the camera (Wratten 87 or 87C, Hoya R72), and a sensitive NIR-capable camera. Exposure times are typically longer than reflectance photography because luminescence emission intensity is lower than reflected emission. The ENFSI Document Working Group's Best Practice Manual for Ink Examination specifies IRL examination as a standard protocol for ink-sequence questions. The FBI's questioned-document examination protocols include IRL as a Tier 1 screening step for stroke-sequence questions.

Limitations. Not all inks exhibit IRL, and the discriminating power of IRL depends on the specific pen formulation. A pair of inks may both be IRL-negative, meaning IRL examination cannot distinguish them. The examiner must document negative IRL results alongside positive ones; a conclusion that "IRL did not differentiate the inks" is a valid examination finding. The RCMP questioned-document laboratory and the UK's DSTL document section both include this negative-result documentation requirement in their examination protocols.

Alternate light source (ALS) examination involves selecting an excitation wavelength, placing the correct barrier filter in front of the camera and the examiner's eyes, and systematically scanning the evidence surface in a darkened environment. The selection logic begins with the evidence type and works backward to the appropriate wavelength, filter combination, and minimum examination time.

Wavelength-evidence matching. The principal forensic applications and their typically effective ALS wavelengths are (based on published ENFSI, SWGIT, and SWGMAT guidelines):

Biological fluids (semen, saliva): 450-470 nm excitation, orange barrier (O-525 or equivalent). Semen is the strongest fluorophore; saliva and urine are weaker and substrate-dependent.

Latent fingerprints (DFO-treated, ninhydrin-zinc): 505-530 nm excitation, yellow or orange barrier.

Fluorescent fibres: 415-450 nm excitation, various barriers depending on fibre type. Certain synthetic fibres (optical brighteners in polyester and nylon) fluoresce strongly under blue-violet ALS.

Bruise documentation: 415-450 nm, or UV-A for reflectance contrast.

Questioned documents (fluorescent inks, alterations): multiple bands from 415 nm through 535 nm, plus near-UV.

Examination protocol across jurisdictions. SWGIT Section 15 specifies that ALS examinations of evidence must document: the ALS model and serial number, the excitation filter used, the barrier filter used, the room conditions (ambient light level), the exposure settings for each photograph, and the result (positive fluorescence, negative, or inconclusive). The UK Home Office's ALS examination protocols for biological-fluid screening in sexual assault investigations require that both positive and negative results be documented, with notes on substrate background fluorescence that might mask a signal. Indian CFSL ALS examination reports follow a parallel documentation structure under the DFSS quality-management framework.

ALS vs laser: when to escalate. An ALS with its bandpass filter set is the first-line examination tool for most forensic applications because it is rapid, portable, and covers a wide evidence area. Laser examination is deployed when the ALS examination returns an equivocal result on a difficult substrate, when substrate background fluorescence is high, or when the evidence type specifically requires the photon density only a laser can provide. The protocol hierarchy (ALS before laser) is specified in SWGIT, ENFSI Fingerprint BPM, and RCMP examination procedures. The full filter architectures and power specifications of individual ALS platforms are covered in the forensic light sources and alternate light topic.

A common failure mode in specialised imaging casework is recording only positive findings. The examiner photographs fluorescent marks and moves on; areas that were examined but returned no fluorescence are left undocumented. If the defence asks whether a specific area was examined, the answer becomes a subjective memory claim rather than a documented record.

Documenting negative examination results. Best practice across SWGIT, ENFSI, and DFSS guidelines requires documenting: the extent of the examination area (a sketch or photograph showing which surfaces were scanned), the ALS parameters applied, and a written statement that no fluorescence was observed in areas outside those documented as positive. In the UK, the College of Policing crime-scene examination guidance and the CPS expert-evidence guidelines both specify that forensic examination reports should state what was examined, what was found, and what was not found. An expert who reports only positive findings without disclosing the full examination scope can face cross-examination challenges in Crown Court about the completeness of the examination. In the US, the National Commission on Forensic Science's 2016 guidance on forensic practitioner competency includes documentation completeness as a fundamental requirement.

Cross-reference to Module 1. The physics of UV absorption, fluorescence emission, the Jablonski diagram, the Stokes shift, and the Kubelka-Munk reflectance model are covered in the light-matter interaction topic in Module 1 of this subject. The ALS platforms (Polilight, Crime-lite, Spectroline), their filter architectures, and the forensic light-source selection logic are covered in the forensic light sources topic in Module 1. This topic does not reproduce that material. The physics of UV absorption, fluorescence emission, the Jablonski diagram, the Stokes shift, and the Kubelka-Munk reflectance model are covered in the light-matter interaction topic in Module 1 of this subject. A reader approaching specialised imaging for the first time will find that Module 1 provides the necessary foundation; this topic connects those principles directly to examination procedure and evidentiary standards.

The chain of custody for ALS/UV/IR images. Specialised imaging photographs are subject to the same chain-of-custody requirements as any other forensic image. Every image must be captured to a write-once medium or immediately written to a forensic media copy with hash verification, documented in the photo log with the ALS parameters, and preserved without lossy post-processing. The integrity standards described in digital imaging evidence and court admissibility apply in full. Any enhancement applied to a specialised imaging photograph (brightness adjustment, contrast enhancement, false-colour mapping) must be documented and applied to a copy of the original, with the unmodified original preserved.