NIR (Near-Infrared)
Definition
The spectral region from approximately 700 to 1100 nm; used in reflectance and transmittance imaging of documents, obliterated text, soot patterns under blood, and ink differentiation.
- Wavelength range
- Approximately 700 to 1100 nm
- Applications
- Reflectance/transmittance document imaging, obliterated text, soot-under-blood, ink differentiation
- Detection basis
- Material-dependent infrared absorption invisible to the naked eye
Common questions
Why can NIR imaging reveal text that has been obliterated with ink or correction fluid?+
Different inks and covering materials absorb and reflect infrared light differently even when they appear visually identical or opaque in the visible spectrum, so the underlying writing can become visible as a contrast difference under NIR illumination that the naked eye cannot perceive.
How does NIR help examine soot patterns underneath blood?+
Because blood strongly absorbs certain visible wavelengths but is more transparent in parts of the near-infrared range, imaging in that band can reveal soot or gunshot-residue patterns deposited on skin or fabric before the blood obscured them visually.
Is NIR imaging destructive to the evidence?+
No, it is a non-contact optical technique that captures the reflected or transmitted infrared light without altering the document or exhibit, which is why it is typically performed before any chemical or destructive testing.
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...
- 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...
- 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...