Humans see a very narrow window of the electromagnetic spectrum. We do not see X-rays, gamma rays or microwaves. We see the colors of the rainbow from red to violet, but there is something beyond red (infrared) and there is something beyond violet (ultraviolet). With electronic help, we can see ultraviolet light (aka black light) and we also have the devices to generate black light--including ultraviolet flashlights.
Finding Certain Rocks
Many minerals are very visible in ultraviolet light. Some even fluoresce in the presence of ultraviolet light. These minerals may appear gray when encountered in a gravel pit but glow brightly with a distinction in the presence of ultraviolet light. The most commercially interesting of these rocks are diamonds. Ultraviolet flashlights are routinely used in diamond mines.
Detecting Counterfeit Bills
UV flashlights can be used to detect counterfeit bills. The United States and many other countries include an invisible fluorescent strip in their currency. These strips only show up when exposed to ultraviolet radiation, such as the beam from an UV flashlight.
Repairmen use UV flashlights to find otherwise invisible leaks in machinery. Injecting a little fluorescent dye into the oil supply, fuel supply or any other liquid supply will make leaks easy to detect if you have an UV flashlight. A repairman might detect an otherwise invisible air-conditioner leak by adding fluorescent dye to the refrigerant and then looking over the air conditioner with an UV flashlight.
Detecting Antique Forgeries
Appraisers use UV flashlights to detect forged antiques. Many paints today contain phosphors that will glow under a black light, while most older paints do not contain phosphors. There is a lot of money in forged antiques, and UV flashlights are an inexpensive way to quickly validate some items--this authentication technique is unknown to some forgers and non-phosphor paint is hard to come by for any forgers.