Dictionary.com defines a black light as “invisible infrared or ultraviolet light.” Under black light, substances not seen by the human eye may be visible.

According to BeginnersGuide.com, there are two main types of black lights—tube and incandescent. A tube black light is a fluorescent light with a special coating, which blocks out certain rays. An infrared black light is just like a light bulb, but has different light filters.

Black lights work with phosphors—substances that produce visible light when exposed to energy—says BeginnersGuide.com. The UV light from a black light makes phosphors noticeable.

Hotels and restaurants use black light inspections for cleaning. Under black light, you can see stains—such as urine, bacteria and mold—that are not otherwise be visible. Cosmetologists and dermatologists use black light to diagnose or treat skin problems.

Black light inspections can identify counterfeit money, determine if documents have been altered and assist in forensic investigations. The Checkmate Group says black lights can even detect art forgeries: “Modern paint will fluoresce or glow under a black light (while) older paints will not. Thus, pictures that have been ‘touched up’ with modern paint will glow.”

From auto mechanics to NASA engineers—whose black light inspections look for “particulate micro-contamination, minute cracks or fluid leaks,” says NASA.gov—black lights identify weaknesses that could put staff at risk.