What Kind of Invisible Stains Do Black Lights Detect?

A black light produces ultraviolet light, which can make some invisible stains visible to the human eye. The ultraviolet waves react with phosphorus in certain organic substances, illuminating fluorescent molecules.

Petroleum Jelly

Under black light, petroleum jelly has a bright blue glow. People probably aren't normally looking for invisible Vaseline stains, but it can be fun to show kids this effect.

Blood Residue

Residue from blood can show up under black light even when the blood has been washed away. Blood stains under ultraviolet light look about 4 times darker than normal, nearly black.

Urine

Black light illuminates urine stains, which is useful if you're smelling cat or dog urine in your house but can't pinpoint where the odor is coming from. Urine has a bright yellow or yellow-green glow when illuminated by black light.

Saliva and Semen

Semen and saliva stains also glow as a bright yellow or green-yellow color under ultraviolet light.

Expert Insight

Using a black light, employees of the television show "ABC News Primetime" investigated 20 hotels in the United States for cleanliness. Results showed traces of semen or urine in every room they checked, even in expensive five-star hotels.