Entomologists use black lights, or ultraviolet lights, to sample and study nocturnal insects in an area. The black light attracts night-flying insects, including many moths, beetles, and others. Many insects can see ultraviolet light, which has shorter wavelengths than light visible to the human eye. For this reason, a black light will attract different insects than a regular incandescent light.
If you've ever seen a bug zapper, one of those lights people hang in their backyards to keep mosquitoes at bay, you've observed how UV light attracts a lot of insects. Unfortunately, black lights do not work well to attract biting insects, and bug zappers harm more beneficial insects than pests.
Black light sampling can be done one of two ways. The black light can be suspended in front of a white sheet, giving flying insects a surface on which to land. You can then observe the insects on the sheet, and collect any interesting specimens by hand. A black light trap is constructed by suspending a black light over a bucket or other container, usually with a funnel inside. Insects fly to the light, fall down through the funnel into the bucket, and are then trapped inside the container. Black light traps sometimes contain a killing agent, but can also be used without one to collect live specimens.
When using a black light to collect insects, you should set up your light and sheet or trap just before dusk. Make sure the light faces the area from which you want to attract insects. In other words, if you want to draw insects from a wooded area, position your light between the trees and the sheet. You'll get the greatest diversity of insects if you set up a black light at the intersection of two habitats, such as at the edge of a meadow adjacent to a forest.
Use forceps or an insect aspirator (sometimes called a "pooter") to collect insects from the sheet or trap.