UV-a
Definition
The ultraviolet band from 315 to 400 nm; the primary band used for UV-reflectance forensic imaging of bruises, GSR, and bite marks without causing tissue damage.
- Wavelength range
- 315 to 400 nanometers
- Common use
- ALS fingerprint and biological-fluid detection
- Advantage on skin
- Images bruises, gunshot residue, and bite marks without tissue damage
Common questions
What is UV-A and why does forensics use it?+
UV-A is ultraviolet light in the 315 to 400 nanometer band. Forensic examiners use it with alternate-light-source (ALS) systems to find fingerprints and biological fluids like blood and semen on evidence without damaging the surface.
Can UV-A show injuries or marks on skin?+
Yes. UV-A reflectance imaging reveals bruises, gunshot residue (GSR), and bite marks on skin clearly. Because it's in the longer ultraviolet range, it won't harm tissue during examination the way shorter UV wavelengths might.
What does 315-400 nm mean in UV-A?+
These are nanometers, the unit for measuring light wavelengths. UV-A sits at the border between visible light (which starts around 400 nm) and more energetic ultraviolet. This specific band works best for exciting fluorescence in fingerprints and body fluids.
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...
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- 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...
- Ionising Radiation
- Electromagnetic radiation whose photon energy exceeds the ionisation potential of atoms (typically above ~10 eV); includes gamma rays and X-rays. UV below...
- 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...
- Mid-Infrared (Mid-IR)
- The 2500-25000 nm (400-4000 cm^-1) spectral region where functional-group vibrational absorptions serve as chemical fingerprints; the operating region of FTIR spectroscopy.
- 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...
- Photon
- A quantum of electromagnetic radiation; a discrete energy packet with energy E = hν where h is Planck's constant and ν is...
- Photon Energy (eV)
- Photon energy expressed in electron volts; UV photons carry 3-12 eV (chemically reactive), visible 1.8-3.1 eV, mid-IR 0.05-0.5 eV (vibrational excitation only).
Explained in these topics
- Light, Photons and the Forensic EM SpectrumUltraviolet radiation in the 315-400 nm band; the primary excitation range for alternate-light-source (ALS) fingerprint and biological-fluid examination.
- Specialised Imaging: UV, IR, Laser and ALSThe ultraviolet band from 315 to 400 nm; the primary band used for UV-reflectance forensic imaging of bruises, GSR, and bite marks without causing tissue damag...