Black light is used extensively in non-destructive testing. Fluorescing fluids are applied to metal structures and illuminated with a black light which allows cracks and other weaknesses in the material to be easily detected.
In addition, if a leak is suspected in a refrigerator or an air conditioning system, a UV tracer dye can be injected into the system along with the compressor lubricant oil and refrigerant mixture. The system is then run in order to circulate the dye across the piping and components and then the system is examined with a blacklight lamp. Any evidence of fluorescent dye then pinpoints the leaking part which needs replacement.