Bandpass Filter
Definition
An optical filter transmitting a defined wavelength range (typically 20-40 nm FWHM) while blocking all others; the excitation filter in an ALS examination.
- Field
- Alternate light source examination
- Typical FWHM
- 20 to 40 nm
- Role in ALS
- Excitation filter
Common questions
Why is a bandpass filter used on the excitation side rather than only relying on the light source itself?+
Many forensic light sources emit a broad range of wavelengths rather than a single clean colour, so the bandpass filter narrows the emitted light to the specific excitation range needed to fluoresce the target evidence type, such as latent prints or biological stains, while blocking wavelengths that would otherwise wash out the fluorescence with unwanted scattered light.
How does a bandpass filter differ from the barrier or viewing filter used with an ALS?+
The bandpass filter sits at the light source and selects the excitation wavelength delivered to the evidence, while the barrier filter is worn or fitted at the viewing side and blocks the reflected excitation light while passing the longer-wavelength fluorescent emission, so the two filters work together but serve opposite ends of the excitation and emission pathway.
Does the choice of bandpass filter change which evidence types become visible?+
Yes, different evidence types fluoresce best under different excitation wavelengths, so an examiner changes the bandpass filter, or selects a different wavelength setting on a tunable forensic light source, to optimise visualisation for the specific evidence being searched for, such as biological fluids versus fibres versus latent prints.
Related terms
- Alternate Light Source (ALS)
- A device emitting light at selected wavelengths (typically 415 to 535 nm) used to detect biological fluids that fluoresce or absorb light...
- 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...
- DFO (1,8-Diazafluoren-9-One)
- A fluorescent ninhydrin analogue used for latent fingerprint development on porous surfaces; excite at 450-470 nm (with secondary absorption extending into the...
- Fluorescent Whitening Agent (FWA)
- An optical brightener added to synthetic fibres and paper during manufacture; fluoresces under UV-A excitation, making treated fibres and paper highly visible...
- Long-Pass (Viewing) Filter
- An optical filter transmitting all wavelengths above a defined cut-on wavelength; worn as goggles by the examiner to block excitation light while...
- Mercury Vapour Lamp
- A gas-discharge lamp producing discrete atomic emission lines, most useful at 253.7 nm (UV-C germicidal) and 365 nm (UV-A); the classic biological-fluid...
- Polilight
- Rofin / Omnichrome xenon-arc-based ALS platform developed in Australia; the reference platform for biological-fluid and fingerprint ALS validation studies in Australasian and...
- 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...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...
- Tungsten-Halogen Lamp
- A broadband (thermal) light source with continuous emission from UV-A through NIR, determined by filament temperature; the first generation of forensic ALS...
- Xenon Arc Lamp
- A high-pressure xenon discharge lamp producing near-continuous white-light emission across 200-2500 nm; used in broadband ALS platforms (Polilight, CrimeScope) with bandpass filters.