Alternate Light Source (ALS) illumination relies on Stokes shift fluorescence: illuminating evidence with intense narrowband light causes target molecules to absorb high-energy photons and remit them at longer, visible wavelengths.

1. Wavelength Spectrum & Target Matrix

• UV-A (365nm): Visualizes natural fluorescence in untreated semen, saliva, urine, and optical brighteners in trace synthetic fibers without excitation goggles.
• Blue Light (450nm) + Yellow Barrier Filter: Standard crime scene screening wavelength; excites biological stains, latent fingerprints treated with Basic Yellow 40, and bone fragments.
• Green Light (530nm) + Red Barrier Filter: Maximizes contrast for cyanoacrylate prints stained with Rhodamine 6G or Ardrox on reflective metallic surfaces.