English: A Sanax 15W "black light" compact fluorescent bulb. It emits long-wave (UVA) ultraviolet light at around 365 nm, and is used for special lighting effects, such as illuminating fluorescent posters. The purple glow of the bulb is not the ultraviolet light itself, which is invisible, but visible purple light from mercury's 404 nm spectral line which leaks through the filter coating on the bulb, which is designed to stop most visible light and pass the ultraviolet. To the human eye, the light is much more blue, due to differences between the response of the eye and the camera sensor.