ALS (Alternate Light Source)
Definition
An adjustable-wavelength high-intensity light source with interchangeable excitation filters, used for scene examination across UV, visible, and NIR bands; examples include the Rofin Polilight and Foster + Freeman Crime-lite.
- Function
- Adjustable-wavelength high-intensity light
- Bands covered
- UV, visible, near-infrared
- Example devices
- Rofin Polilight, Foster + Freeman Crime-lite
- Use case
- Scene screening for fluorescent evidence
Common questions
What kinds of evidence does an ALS commonly reveal?+
Body fluids, fibers, gunshot residue, latent fingerprints treated with fluorescent powders or dyes, and some bruising or bite marks can all fluoresce or contrast under specific ALS wavelengths that are invisible to the naked eye under normal room lighting.
Why do examiners need matching barrier filters or goggles when using an ALS?+
The excitation light itself would overwhelm and mask the faint fluorescence being sought, so a filter tuned to block the excitation wavelength while passing the longer-wavelength emitted light is required to actually see the evidence rather than just the light source's own glare.
Related terms
- 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...
- 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...