Q: Why has the luminous paint on my watch stopped glowing?
Luminous paint makes it easier to read the time in the dark. However, some older watches no longer glow. This is because they use self-luminous tritium. Tritium is a radioactive substance that emits beta rays as it decays, eventually ceasing to glow. This period lasts for approximately 10 years. Watches that use tritium have a "T" mark at the 6 o'clock position on the dial. While tritium is a fairly safe substance, Japan, with its strict standards, requires that the total amount of beta rays emitted be kept below 25 microcuries (= 925 kilobecquerels). When this type of tritium is used, the "T25" mark is displayed below the 6 o'clock position on the dial.
On the other hand, the luminous paints currently in use are not self-luminous, but rather phosphorescent paints that accumulate light from outside and then emit light. The majority of these are Nemoto Specialty Chemical's N-Luminova. These phosphorescent paints are primarily aluminum-based. As a result, they do not decay like tritium and are resistant to heat and water. Furthermore, the color of the paint applied does not change with age. However, since they emit accumulated light, the brightness will gradually decrease over time. In this case, it is a good idea to expose the item to plenty of light beforehand.
In the past, Luminova was only available in green, but with advances in technology, it is now also available in white, red, and black. Some Luminova watches have the letter "L" on the dial.
