Witness the beauty of underwater fluorescence on your next night dive with the Bigblue Fluorodiving Yellow Barrier Filter for masks. It fits over most standard dive, scuba, and snorkeling masks/goggles, and is shaped to accommodate their nose pockets. This filter must be used in conjunction with a dark blue or UV light, which stimulates fluorophores in the proteins of certain coral, fish, and other organisms--exciting them into emitting a fluorescent glow. The filter blocks the excitation light and allows you to see this fluorescence, which is invisible to the naked eye. It also increases the intensity of the colors emitted by the sea life.