Are glow-in-the-dark watches a radiation hazard?

14 September 2008

Question

I was just wondering, is there any radiation that would come from a glow-in-the-dark watch that would be harmful to the wearer?

Answer

Philip Clark, University of Edinburgh, Experimental Particle Physics Group:

It depends very much on the type of dial that you're considering. By far the most common watch that you come across that's glow in the dark is called a phosphorescent watch. Essentially the watch is coated in a paint which absorbs light and then re-emits it. These watches are completely harmless. The second type of watch is called a tritium watch. The modern way to do this is you have the same phosphorescent paint but this time it's mixed with small tubes filled with tritium. Tritium is radioactive and emits beta particles. These have the same effect of exciting the phosphorescent paint. This time tritium has got a half-life of 12 years. The beta particles that are emitted are not very energetic so if anything they couldn't even penetrate the outermost skin layer. The third watch I'd like to mention is a radium watch. They have very much the same design but this time instead of tritium they're mixed with radium. The half-life is sixteen hundred years. However, they may not seem to be as radioactive because the phosphor in the paint gets eaten up by the radium. I've got a small demonstration here so I've got an old watch that I'm going to hold a Geiger counter to. If I turn the Geiger counter on you'll hear it clicking:

That's when I hold it slightly close to the watch. If I hold it really close to the watch:

And the take it away from the watch then the background count - you can hear the occasional count just now is much lower than if you hold it closer to the watch. These watches are extremely radioactive. However they're still not too harmful unless you were to break the watch and inhale it or somehow ingest the watch.

Comments

I am.planning to purchase casio watch and i have heard that it glows.....any idea qhat substance does companies like casio use for glow factor

HI. I just got a sports watch by PASNEW (Lapgo) that says it has an "electronic fluorescent lamp" in it. Furthermore, they don't want you to over-press the light button, as it will weaken over time. Question 1: Is this the same technology as in all "electroluminescent" products (ie. Casio) or is it different? I'm just confused about the word "fluorescent"; perhaps it's just a word tranlation choice... Got any thoughts? Also...why does the light "weaken over time"? Just curious. Thanks for any info!

The nomenclature is a bit confusing! The word "phosphorescent" used in these contexts often makes you think of the element "phosphorus", but its a misleading term. There is unlikely to be much phosphorus in a "phosphor"; these are coatings made from, usually, mixtures of chemicals that absorb and emit light at certain frequencies. Painted onto a screen or a lamp coating, they can generate certain ranges of colours; the phosphor on the inside of a strip light, for instane, converts short wavelength ultraviolet into visible "white" light. The word probably comes from the observation that elemental white phosphorus can spontaneously ignite and burn with a white glow.

Fluorescence is another way of making things glow. Jellyfish do it by pumping energy into a protein called GFP. Fireflies use an enzyme called luciferase and a substrate called luciferin to do it. Your watch does it by using electricity from an on-board battery to excite - most likely - a semiconductor, like an LED to produce light.

Good info. Okay next: do companies still use radium to effect a light reaction? I think the article said some are still made...how do I know? I'd rather have my son wear ultra-safe materials on his wrist if possible. So what are the chances a company (Chinese in this case) still uses radium or any other element I'd rather avoid? Is there a way to find out? Thx!

Probably not. For a start, radium is universally acknowledged to be a risky substance to work with, even in places like China, where health and safety is often an after thought. There are strict controls on the production, movement and use of radioactive materials, so sourcing radium for an application like watch paint is likely to be preclusively tricky.

Tritium is used (brands like Rolex use it), because it's regarded as safer; the half life is - unfortunately - shorter, meaning that the watch face needs a refresh every decade or so...

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