Some radioactive materials are used to produce light. This is done by bombarding a special material known as a phosphor with the radiation (typically beta radiation) emitted by the radioactive material. Phosphor gets its name from the Greek words for “light” and “to bring.” The phenomenon is called “radioluminescence.”
Radioluminescence can be used to provide a low level light source to allow instruments or signs to be visible at night or for other situations where light is needed for long periods without electricity, such as emergency exit signs.
Paint with radium was the first radioluminescent product. Today, tritium is most commonly used, primarily on wristwatch faces and gun sights. Small tritium lights can be made by sealing tritium and a phosphor layer in small glass tubes. Such a tube is known as a “gaseous tritium light source” (GTLS), or more commonly, a beta light (since the tritium undergoes beta decay).
Tritium is a radioactive isotope with a half-life of about 12 years, which means the glass tube loses half its energy and some of its brightness in that period. So the types of GTLS used in watches generally have a useful life of 10 to 20 years. They give off a small amount of light: not enough to be seen in daylight, but enough to be visible in the dark. The more tritium that is initially placed in the tube, the brighter it is to begin with and the longer its useful life.
The NRC regulates devices that contain small amounts of tritium. Manufacturers and initial distributors of these devices need to have a distribution license issued by the NRC. They also need to have a separate license to possess and use the material. This license can be issued either by the NRC or the state. [There are 37 states that have agreements with us to regulate these types of radioactive materials. They are called Agreement States.] Anyone who initially buys one of these products from someone who has the proper licenses and subsequent owners of the product are exempt from the requirements for an NRC license.
Approval of these types of products would require extremely low risk of radiation exposures to members of the public from normal use, misuse or accidents. The NRC would also need to see the usefulness or benefits of the product. For example, items that could be mishandled, especially by children, will be approved only if they combine an unusual degree of utility and safety. Other countries have different regulatory requirements. That is why some tritium products available for sale internationally are not sold in the U.S.