Alternate Light Source (ALS) and Fingerprint Photography

Alternate Light Source (ALS) and Fingerprint Photography

Why ALS Works

Latent fingerprints are invisible or nearly invisible under ordinary white light. An Alternate Light Source (ALS) exploits the fact that certain substances fluoresce — they absorb energy at one wavelength and re-emit it at a longer, lower-energy wavelength. When a fingerprint has been treated with a fluorescent developer such as cyanoacrylate (CA) followed by Rhodamine 6G, DFO, or a fluorescent powder, the developed ridge detail glows when struck by the correct excitation wavelength. The ALS provides that narrow-band excitation; a barrier filter placed over the camera lens (or worn as goggles) blocks the excitation light and passes only the fluorescent emission, making ridges appear bright against a dark background.

This principle sits squarely within the electromagnetic spectrum. Visible light runs roughly from 400 nm (violet) to 700 nm (red). ALS units produce selectable outputs across that range and into the near-ultraviolet and near-infrared. The key rule: the barrier filter must have a cut-on wavelength above the ALS output wavelength — it blocks excitation light but transmits the longer-wavelength fluorescent emission.

Matching Wavelength to Developer

Different fluorescent developers have different excitation and emission peaks. While your agency's documented protocols are authoritative, the general relationships are well established:

• CA + Rhodamine 6G: Blue-green excitation (approximately 450–530 nm range) works well; an orange barrier filter is used.
• DFO (1,8-Diazafluoren-9-one): Blue-violet excitation (approximately 450–490 nm); a yellow or orange barrier filter.
• Fluorescent powders: Vary by product; consult the manufacturer's data sheet and your lab's standard operating procedure.
• Physical developer: PD produces a silver-gray metallic deposit and does NOT fluoresce. It is imaged under white oblique or raking light, not ALS.

Always consult the SDS and your agency's validated protocol before use. Never assume one wavelength fits all developers.

Safety First

ALS units emit concentrated light that can cause serious eye injury, particularly at short ultraviolet and blue-violet wavelengths.

• Always wear the correct barrier filter goggles for the wavelength in use before activating the ALS. Never look at the beam directly or reflect it toward anyone without eye protection.
• Chemicals such as Rhodamine 6G and DFO are potential irritants and sensitizers. Wear nitrile gloves, eye protection, and a lab coat when handling chemically developed prints. Consult the SDS for each chemical.
• Keep the room dark and controlled. Post a warning outside any blacked-out area so others do not enter unexpectedly.
• Follow your instructor's lockout/shutoff protocol for the ALS unit: power off when not actively imaging.

Camera Setup for ALS Photography

ALS photography in a darkened environment requires deliberate camera settings. Auto-exposure will almost always fail.

ISO: Use the lowest native ISO your DSLR offers (commonly ISO 100 or 200). A high ISO increases noise and obscures fine ridge detail.

Aperture: A mid-range aperture (f/8 to f/16 on a macro lens) maximizes depth of field so that ridge detail across the print surface is sharp.

Shutter speed: Because the room is dark and exposures may be long (several seconds), you must use a tripod and a cable or remote shutter release to eliminate camera shake. Trial exposures are normal; bracket your exposures.

Focus: Use manual focus. Autofocus systems struggle in low light and can hunt or lock onto the wrong plane. Use live view magnified to the ridge level to confirm critical focus before capturing.

White balance: Set a custom white balance or use a fixed preset appropriate to the ALS wavelength output. Auto white balance can shift color unpredictably and reduce ridge contrast.

Barrier filter on lens: Attach or hold the barrier filter over the lens for all ALS exposures. Remove it for white-light comparison shots.

The Court-Admissible Documentation Series

Forensic photography follows a systematic sequence to establish context and ensure the image can be admitted as evidence. For a latent print, the required series is:

1. Overall/Orientation shot — shows the case label, item, and surrounding scene. The case identifier must be legible.

2. Mid-range shot — shows the print's location on the item.

3. Close-up under white light, without scale — captures ridge detail without obstruction.

4. Close-up under white light, with ABFO No. 2 scale — the scale provides size reference and must be in the same plane as the print.

5. Close-up under ALS, without scale — fluorescent ridge detail.

6. Close-up under ALS, with ABFO No. 2 scale — fluorescent detail with size reference.

Never move the scale between shots 4 and 6 if the print has not moved. Record all camera settings, ALS wavelength, and filter used in your case notes. Images are saved to a write-protected or hashed media card as part of chain-of-custody practice.

Connecting to Your Credentials

The IAI CCSI and Latent Print certifications both evaluate your ability to properly document and recover latent prints using validated methods. Employers and courts expect technicians to understand why each step is taken, not just follow a checklist. A well-executed ALS photographic series is one of the most common deliverables you will produce in this trade.