What can be seen through the barrier filter when using an ALS?

• A. Through barrier filter, the technician can observe the fluorescence of many significant objects - hairs, fibers, semen, saliva, urine.
• B. The color of the surface
• C. Only fingerprints
• D. Only blood

Answer: (A)

When you’re using an alternate light source, the barrier filter is used to block the excitation light and only let the emitted fluorescence be seen. This makes many materials glow, not just appear under normal light. Hair, fibers, and body fluids like semen, saliva, and urine can fluoresce under the right wavelengths, so you can detect and visualize these traces more easily. The barrier filter essentially reveals what is glowing rather than what color the surface is under white light. That broader range of detectable materials is why observing fluorescence of many significant objects is the best answer.