Infrared Luminescence (IRL)
Definition
A photophysical process where visible-light absorption produces NIR emission; exploited in questioned-document examination to differentiate and sequence ink strokes with different luminescent properties.
- Field
- Questioned document examination
- Detects
- NIR emission from visible-light absorption
- Equipment
- ALS or IR-sensitive video comparator with bandpass filters
- Use
- Differentiates and sequences overlapping ink strokes
Common questions
Why do two chemically different inks show different IRL behaviour?+
Ink dyes and pigments vary in their electronic structure, so some absorb visible light and re-emit it as near-infrared while others do not fluoresce at all under the same illumination. That contrast lets an examiner map ink boundaries that look identical to the naked eye.
Can IRL alone prove two inks are chemically identical?+
No. Matching luminescence behaviour is consistent with a common ink source but is not conclusive on its own. Examiners pair it with thin-layer chromatography or other destructive testing when a definitive chemical match is required.
Related terms
- ALS (Alternate Light Source)
- An adjustable-wavelength high-intensity light source with interchangeable excitation filters, used for scene examination across UV, visible, and NIR bands; examples include the...
- Barrier Filter
- An optical long-pass filter placed over the camera lens and worn as goggles by the examiner during ALS examination; blocks the excitation...
- DFO (1,8-Diazafluorenone)
- A chemical reagent for latent fingerprint development on porous surfaces (paper) that produces yellow-green fluorescence under 505-530 nm ALS excitation; effective on...
- Hot Mirror (IR-Cut Filter)
- The infrared-blocking filter bonded in front of the digital sensor in consumer and professional cameras; must be removed or bypassed for NIR...
- Laser-Induced Fluorescence (LIF)
- Fluorescence excited by a monochromatic laser source, providing higher photon density than a broadband ALS; used for fingerprints on difficult substrates and...
- NIR (Near-Infrared)
- The spectral region from approximately 700 to 1100 nm; used in reflectance and transmittance imaging of documents, obliterated text, soot patterns under...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...
- Time-Gated Detection
- A laser fluorescence technique that delays camera shutter opening by nanoseconds after the laser pulse to allow short-lived background fluorescence to decay...
- UV-a
- The ultraviolet band from 315 to 400 nm; the primary band used for UV-reflectance forensic imaging of bruises, GSR, and bite marks...
- UV Reflectance Imaging
- Photography using UV-A/near-UV-B incident radiation and a UV-pass, visible-blocking lens filter; reveals surface scatter differences in tissue and substrate chemistry not visible...
- Vogeley 2002 Protocol
- A UV-A reflectance photography protocol published in the Journal of Forensic Sciences (2002) demonstrating improved detection of latent bruising on dark skin...