English: Two " black light " ultraviolet fluorescent lamps. The top one is an F15T8/BLB 18 inch 15 watt black light tube in a standard plugin fluorescent fixture. This is often used for special lighting effects, such as illuminating fluorescent posters. The bottom one is an F8T5/BLB 11 inch 8 watt tube in a portable black light, with a solid state ballast, run by eight AA size batteries. It is sold as a pet urine detector, to make urine stains on carpet and furniture visible by its fluorescence. Both radiate "long-wave" or UVA ultraviolet radiation , with a wavelength in the range 350 to 370 nm . The purple glow of the tubes is not the ultraviolet light itself, which is invisible, but visible purple light from mercury's 404 nm spectral line which leaks through the dark blue filter coating on the glass tube. This purple color is not quite accurately represented in this photo, due to differences between the color response of the camera sensor and the human eye.