Terry Grant, of Levin, asks :-

My wristwatch had a luminous dial but with time it faded so much as to be unreadable. However when I was in the bush one night I had cause to shine my flashlight on the dial, and when I returned to the tent my watch dial was luminous again. It did not last for long though. What caused it to be re-charged?

Roger Reeves, a phosphor physicist at the University of Canterbury, responded.

The concept of luminous watch dials is old but has several tragic stories. A popular method in the early part of the twentieth century for the luminous marks was to use radioactive radium atoms embedded in a paint-like substance. The high energy particles emitted from the radium nuclei during their radioactive decay, struck luminous phosphor atoms in the dried paint and imparted some of their energy. The absorbed energy was released as light making the dial visible in the dark.

The dial marks remained luminous for many years because the half-life of the longest-lived isotope (Radium) is 1600 years. However, eventually the damage from the radiation causes the paint to degrade and the effect becomes lost.

The tragic stories around this product relates to the factory workers employed to paint the dials. They became known as the ”Radium Girls” as a result of the litigation they invoked as a result of the severe health effects they suffered from the process. The radioactive paint was applied with a brush – the mostly women employees licked their brushes to better make a fine point to apply the small marks. Thus significant radiation material became ingested and many became sick and died of radiation poisoning.

Because of the risk of unnecessary radiation exposure the use of radioactivity in luminous dials was discontinued in the mid-twentieth century and replaced by a process where the phosphor is “activated” by ultraviolet (UV) light. The UV from everyday sunlight is absorbed by the phosphor to create excited electrons in what are called traps in the phosphor. Over a period of time these electrons relax from the traps back to their unexcited states and in the process release energy in the form of light we can see with our eyes – visible light.

The appearance of the dial marks in the dark is the result of the very long storage times these excited electrons can have – perhaps several hours in fact after the initial UV excitation. But as with the case of the radioactivity luminous, the ultraviolet light degrades the phosphor and eventually the effect becomes weaker as the number of traps decreases.

In this example the more intense light from the flashlight – as opposed to sunlight – excited some electrons, but because of the phosphor degradation the long lived excited states no longer exist and the luminescence faded away more quickly. Incidentally, I suspect that the flashlight used here might have been one of the new white LED based design. They have much more blue light than the previous old-fashioned filament bulb technology and thus more power in the UV to re-charge the phosphor.