Alternate Light Source (ALS)
Definition
A device emitting light at selected wavelengths (typically 415 to 535 nm) used to detect biological fluids that fluoresce or absorb light at those wavelengths. Used with coloured barrier filters. Examination is performed before any physical collection to avoid destroying stains before they are located.
- Primary wavelengths
- 365 nm UV, 415 nm violet, 450 nm blue, 530 nm green, 555 nm
- Key advantage
- Reveals evidence invisible to the naked eye through fluorescence and reflectance contrast
- Evidence types detected
- Biological fluids, gunshot residue, fibers, bruising, latent prints, dye stains, fluorescent powders
Common questions
What exactly is an Alternate Light Source and what does it do at a crime scene?+
An ALS is a tunable lamp that emits narrow-band light across UV, visible and infrared wavelengths. At the scene, it reveals evidence invisible to the naked eye: biological fluids, gunshot residue, fibers, and bruising all fluoresce or show up differently under specific wavelengths. It is paired with matched goggles or barrier filters that block the excitation light and let you see only the fluorescent glow.
Which wavelengths does an ALS typically use and why are they chosen?+
Common ALS wavelengths are 365 nm (UV), 415 nm (violet), 450 nm (blue), 530 nm (green), and 555 nm. Each wavelength excites different evidence materials. Latent print residues and fluorescent dyes respond best to specific colors. The narrow-band design delivers high intensity at exactly the right wavelength instead of wasting energy across a spectrum.
Is the ALS used only for fingerprints or for other evidence types too?+
The ALS is one of the most versatile tools in modern crime scene investigation. It locates biological stains, fibers, gunshot residue, cyanoacrylate-treated fingerprints, and fluorescent powders. It also works post-lab with dye stains applied to latent prints. The same instrument handles all of these with just a wavelength adjustment and the right viewing filter.
Related terms
- Rhodamine 6G
- A fluorescent dye applied as a post-treatment to cyanoacrylate-developed fingerprints; excited at 530 nm (green), it emits at 565-590 nm (yellow-orange), greatly...
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- Crime-Lite
- A compact, battery-powered alternate light source system (Foster+Freeman) designed for scene use by scenes-of-crime officers; the Crime-lite 82S and ML2 are standard...
- 1:1 Macro Photography
- Photography at a reproduction ratio of 1:1 (image sensor size equals subject size); the mandatory standard for all latent fingerprint photography before...
- ABFO #2 Scale
- American Board of Forensic Odontology scale number 2. A right-angle (L-shaped) ruler with three circles at known spacing, used to correct perspective...
- AFIS Workstation
- A dedicated computer system with encoding software and database connectivity used by fingerprint examiners to encode latent minutiae, submit searches against criminal...
- Ardrox
- A commercial fluorescent dye mixture (originally for aircraft-skin crack detection) that fluoresces broadly under 365 nm UV illumination; used as a post-CA...
- Bandpass Filter
- An optical filter transmitting a defined wavelength range (typically 20-40 nm FWHM) while blocking all others; the excitation filter in an ALS...
- 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...
- Basic Yellow 40 (BY40)
- A fluorescent dye (excitation 450-490 nm, emission ~530 nm) applied immediately after CA fuming with a simple aqueous rinse to remove excess;...
- BSA 2023 Section 63
- Bharatiya Sakshya Adhiniyam 2023 Section 63 (the successor to Section 65B of the Indian Evidence Act 1872): the electronic-record certificate that authenticates...
- Cyanoacrylate (CA) Fuming
- The development of latent prints on non-porous surfaces by exposing the item to ethyl or methyl cyanoacrylate vapour in a humidity-controlled environment;...
Explained in these topics
- Biological Evidence Collection ProtocolsA device emitting light at selected wavelengths (typically 415 to 535 nm) used to detect biological fluids that fluoresce or absorb light at those wavelengths....
- Crime Detection Devices: UV, IR, X-Ray and Detective DyesA tunable light unit that emits selectable wavelengths across UV, visible and IR for body-fluid screening, fibre comparison and latent-print enhancement at the...
- Cyanoacrylate Fuming, Alternate Light and Fluorescent StainsAny instrument delivering high-intensity narrowband illumination at a controlled wavelength to excite fluorescence in latent print residue, dye stains, or fluo...
- Forensic Light Sources and Alternate Light ExaminationA lamp that produces narrow-band light at selectable wavelengths, used with a complementary viewing filter to excite fluorescence or enhance reflectance contra...
- Forensic Photography: Types and ApplicationsA tunable narrowband lamp (typically 365 nm UV, 415 nm violet, 450 nm blue, 530 nm green) used with matched goggles or barrier filters to fluoresce biological...
- Lab Equipment and Chain of Custody for Fingerprint CaseworkA forensic illumination instrument that delivers light at selected wavelengths (typically 365, 450, 530, 555 nm) to excite fluorescence in latent print deposit...
- Specialised Photography: UV, IR and Close-Up at the Crime SceneA tunable lamp that emits narrow-band light across UV, visible and IR ranges. The single most useful specialised-photography tool in modern Indian SOCO kits.