Laser-Induced Fluorescence (LIF)
Definition
Fluorescence excited by a monochromatic laser source, providing higher photon density than a broadband ALS; used for fingerprints on difficult substrates and for time-gated detection.
- Excitation source
- Monochromatic laser
- Advantage over ALS
- Higher photon density
- Application
- Fingerprints on difficult substrates
- Extended capability
- Time-gated fluorescence detection
Common questions
Why does higher photon density matter for fingerprint detection?+
A more intense, focused excitation produces brighter fluorescence emission from treated or naturally fluorescent latent print residue, improving contrast against difficult backgrounds such as multicoloured or highly patterned surfaces where a broadband ALS struggles.
What does time-gated detection add that continuous LIF does not?+
Time-gating captures fluorescence only after background reflection and short-lived fluorescence have decayed, isolating the longer-lived emission from certain print treatments or substrates and suppressing interfering background fluorescence.
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...
- Infrared Luminescence (IRL)
- A photophysical process where visible-light absorption produces NIR emission; exploited in questioned-document examination to differentiate and sequence ink strokes with different luminescent...
- 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...