Black light is ultraviolet light, but only a narrow slice of it. The bulbs sold as black lights emit long-wave UV-A, the least energetic part of the ultraviolet spectrum, filtered so almost no visible light escapes. That combination is what makes fluorescent materials glow while the source itself looks dim purple. The Ultraviolet Spectrum Ultraviolet is electromagnetic radiation just past the violet end of visible light, with shorter wavelengths and higher photon energy than the light the eye can see. It divides into three bands: UV-A, 315 to 400 nm. Long-wave, lowest energy. This is the black-light band and the band used for the majority of UV curing and fluorescent inspection. UV-B, 280 to 315 nm. Medium-wave, more energetic, the band responsible for sunburn. UV-C, 100 to 280 nm. Short-wave, highest energy. Nearly all absorbed by the atmosphere and used only in shielded industrial equipment. A black light concentrates its output around 365 to 400 nm and uses a dark filter glass, often called Wood's glass, to block the visible tail. The faint glow you see from the bulb is the small amount of violet that leaks through. Why Things Glow Under Black Light Many materials contain fluorophores: molecules that absorb UV photons and re-emit the energy as visible light. Under a black light, the UV-A is invisible but the re-emitted light is not, so treated papers, certain minerals, optical brighteners in detergents, and fluorescent dyes appear to shine against a dark background. Remove the UV and the glow stops instantly, because fluorescence is not stored energy. This effect is the working principle behind UV inspection of coatings and adhesives. A fluorescent tracer is blended into the material so that a UV-A lamp reveals coverage gaps, thin spots, and defects that are invisible under white light. Where Long-Wave UV-A Is Used Coating and adhesive inspection. A doped conformal coating or bond line fluoresces under 365 nm, making full coverage easy to verify. Leak and crack detection. A fluorescent penetrant wicks into surface flaws and lights up under UV-A. Material sorting and authentication. Minerals, plastics, and security marks on documents and packaging fluoresce differently, allowing rapid separation or verification. Curing. UV-A drives the photoinitiators in many industrial resins; see our guide to selecting a UV lamp for resin curing. Entertainment and display. Stage and venue lighting uses the same band for visual effect. The light guides that carry UV energy in industrial spot systems are built around this band; our explainer on what a light guide does in a UV spot lamp system covers the delivery side. Black Light Sources Fluorescent black-light tubes use a phosphor that emits around 365 nm behind Wood's glass. Inexpensive and wide-area, but low intensity. LED black lights emit a narrow peak, commonly 365, 385, or 395 nm. A true 365 nm LED gives the cleanest fluorescence with the least visible contamination; 395 nm LEDs are cheaper but leak more violet, which washes out faint fluorescence. Mercury-arc lamps produce a strong 365 nm line among other…