Ultraviolet UV flashlights emit a color of light that is useful in the detection of fraudulent documents and money, HVAC leaks, pest control, forensic investigations and more. UV light will cause certain things to fluoresce. Most UV flashlights work best in low light conditions and have about a 1 to 3 foot range. UV output is measured in nm or nanometers, and most UV flashlights will have an output in the range of 365 to 400nm. The higher the number the more visible light.