Biological Evidence Screening: Alternative Light Sources

What are the visual techniques utilized?

daylight, bright or oblique lighting, and alternate light source (ALS)

Oblique Lighting

light directed at a low angle that shows details by creating shadows (good at spotting irregularities, revealing textures, or small objects)

Alternate Light Source (ALS)

The use of specific wavelengths of light to reveal biological materials and other substances that fluoresce or absorb light

What biological materials can ALS be used to detect?

blood, semen, saliva, and urine that are not visible in regular lighting

What is the most common ALS used for biological stains?

1. UV light (<380 nm)
2. Blue light (450-495 nm)
3. IR (>700 nm)

Fluorescence

Occurs when a material absorbs light at one wavelength (excitation) and re-emits it at a longer wavelength (emission)

What is semen fluorescence due to?

It is caused by the non-proteinaceous compound in semen converting into fluorescent products

What wavelengths can semen be viewed under?

450-495 nm of blue light for dried stains through orange goggles

What does blood look like under UV or blue light?

appears dark, but may not be visible if it is on a dark background

What is the best ALS for blood?

IR because it makes the stain darker than the background

Saliva Detection with ALS

produces a weak fluorescence and sometimes appears bluish-white under UV or blue light

Urine Detection with ALS

Produces a faint bluish/yellow fluorescence under UV or blue light (is enhanced when dry due to bacterial growth or crystalline structure)

PoliLight

multi-wavelength portable light source ranging from UV to IR