English: Four F40T12/BLB 48 inch 40 watt black light fluorescent tubes in a standard fluorescent ceiling fixture. These emit long-wave (UVA) ultraviolet light at around 365 nm. Large black light fixtures like these are often used in produce storage rooms to attract fruit flies to bug zappers.
The purple glow that black light tubes emit is not the ultraviolet light itself, which is invisible, but the purple 404 nm mercury vapor spectral line which passes through the dark blue filter material on the tube. The color is not rendered completely correctly in this photo; the tube looks darker to the naked eye because of differences in response between the eye and the camera sensor.