Fluorescence is what happens when one frequency of light falls on a substance, and another frequency of light is returned. Sometimes a viewing filter is needed to exclude the light being used to illuminate the substance so only the fluorescent light can be seen.
After seeing a TV show about finding semen stains in hotel rooms using a blacklight, I started doing some reading about what kind of light works best. I found that in many cases a cheaper blue flashlight combined with an orange viewing filter (orange glasses) is the best combination for showing several fluorescent substances. This works best in a darkened room.
The idea here is that an orange filter will remove all light that isn’t orange (only lets orange through). So the blue light is excluded, except when it causes something to give off extra light (fluoresce). The effect is that you see a mostly very dark orange image, but then a bright greenish looking stain that really pops out.
The TV show only showed that some stains were visible, but there was no way to really identify WHAT the stuff was. In fact, here is a short list of things that will fluoresce under this lighting:
NOTE: Most fluids need to dry before they will fluoresce.
Soap (soap scum, droplets)
Cashews (actually several food residues)
Chewing gum residue
Detergent residue (clothing soap often contains chemicals that make white cloth appear more white under light)
Some insect poisons (ant and roach powders)
Cleaners (like Mr Clean)
Sun screen (some kinds)
Tonic water or club soda (quinine)
Lots of dry powders, certain narcotics
It also will light up insects like scorpions, if you have those around. This is one way to find them in the dark.
From the list, it is clear that just because something glows doesn’t mean it is dirty or worse. Cleaners and soap glow brightly, so remnants of these can be on walls and objects. Conversely, just because it does NOT glow doesn’t mean it is clean from germs or possibly harmful chemicals.
Since I have two cats, and they vomit at least once a week, the blue flashlight and orange goggles are purrfect for finding spots that aren’t obvious. Just because the visible goop was removed doesn’t mean there isn’t a stain. Getting the remnant stains removed can be challenging since some cleaners leave a glowing residue. If the carpet has been cleaned and there are still glowing spots, you did your best. Let it go…
Here are views of soap, soap residue, and urine residue (using blue light and orange filter):
College students have been known to leave hidden messages or elaborate drawings on walls that are invisible under normal light, but show up brightly under black light (ultraviolet or UV).
Checking around a computer (rug, chair, keyboard, mouse, desk, etc) can show splashes of something otherwise invisible. Could be soda, I suppose…
If you delve into forensics more, you will find that they have lights that can change color and other filters for viewing in order to better show certain kinds of substances. It is also a common statement that fluorescence only really helps detect stains, but doesn’t positively identify them as a particular substance (not everything that glows is semen, as is obvious once you look at the list above). Positive identification comes from chemical and DNA testing in a professional lab.
Here is one website that covers some of the various kinds of light and filters, and shows examples of results: