# What Makes Luminous watch hands Glow? **URL:** \ **Category:** Factual Questions\ **Created:** [August 14, 2006, 4:49pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692 "2006-08-14T16:49:25Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [August 14, 2006, 4:49pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/1 "2006-08-14T16:49:25Z")
I know they don’t use radium anymore (too dangerous). So what substance is used today? My watcg has luminous hands, but the glow is pretty faint-you can make it out at night, but it is by no means very bright. Are non-radioactive materials used? ---
**Author:** ![Clothahump](https://avatars.discourse-cdn.com/v4/letter/c/51bf81/32.png) [@Clothahump](https://boards.straightdope.com/u/Clothahump)\ **Post date:** [August 14, 2006, 4:57pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/2 "2006-08-14T16:57:55Z")
Tritium. Good explanation [here](http://www.eastman.com/eastmanoutdoors/science/2004/Science101404.pdf) . (PDF) ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 14, 2006, 4:58pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/3 "2006-08-14T16:58:31Z")
There are a few types. Inexpensive watches use phosphorescent materials which “charge” up in bright light then continue to glow for some time in the dark. Others use tritium, a radioactive alpha emitter. These alpha particles are made to excite a fluorescent material which glows; since alpha particles are easily blocked by even a single sheet of paper there is no significant exposure. Another, expensive, option is [Luminox](http://www.luminox.com/technology.html). I’m not sure exactly how it works since the page doesn’t give out many details–proprietary info, I imagine. They call them “self-powered micro gas lights”, whatever that means. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 14, 2006, 5:03pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/4 "2006-08-14T17:03:58Z")
> [@Q.E.D.](#): > > Inexpensive watches use phosphorescent materials which “charge” up in bright light then continue to glow for some time in the dark. Such as luminescent, doped [Calcium sulfide](http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/alkaline-earth-sulfide.shtml) ---
**Author:** ![Bobotheoptimist](https://avatars.discourse-cdn.com/v4/letter/b/919ad9/32.png) [@Bobotheoptimist](https://boards.straightdope.com/u/Bobotheoptimist)\ **Post date:** [August 14, 2006, 5:19pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/5 "2006-08-14T17:19:07Z")
> [@Q.E.D.](#): > > Others use tritium, a radioactive alpha emitter. Another, expensive, option is [Luminox](http://www.luminox.com/technology.html). I’m not sure exactly how it works since the page doesn’t give out many details–proprietary info, I imagine. They call them “self-powered micro gas lights”, whatever that means. Luminox uses tritium (it’s a gas, so “self-powered micro gas lights” is marketing) Luminox is the brightest watch I’ve seen, bright enough to cast shadows in a dark room, where some of the other tritium watches are just barely visible at night. Could be the OP has a S&W, I wasn’t impressed with their glowieness ---
**Author:** ![Placid\_Liver](https://avatars.discourse-cdn.com/v4/letter/p/e99b99/32.png) [@Placid\_Liver](https://boards.straightdope.com/u/Placid_Liver)\ **Post date:** [August 14, 2006, 5:31pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/6 "2006-08-14T17:31:10Z")
> [@Q.E.D.](#): > > There are a few types. Inexpensive watches use phosphorescent materials which “charge” up in bright light then continue to glow for some time in the dark. Others use tritium, a radioactive alpha emitter. These alpha particles are made to excite a fluorescent material which glows; since alpha particles are easily blocked by even a single sheet of paper there is no significant exposure. Another, expensive, option is [Luminox](http://www.luminox.com/technology.html). I’m not sure exactly how it works since the page doesn’t give out many details–proprietary info, I imagine. They call them “self-powered micro gas lights”, whatever that means. So presumably if you wanted to tell phosphorescent from tritium, you could tell it from whether it was a constant glow or a fade proportional to the amount of time since light exposure? ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [August 14, 2006, 5:37pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/7 "2006-08-14T17:37:26Z")
> [@Placid Liver](#): > > So presumably if you wanted to tell phosphorescent from tritium, you could tell it from whether it was a constant glow or a fade proportional to the amount of time since light exposure? Or just read the back of the watch. ---
**Author:** ![Bobotheoptimist](https://avatars.discourse-cdn.com/v4/letter/b/919ad9/32.png) [@Bobotheoptimist](https://boards.straightdope.com/u/Bobotheoptimist)\ **Post date:** [August 14, 2006, 5:46pm UTC](https://boards.straightdope.com/t/what-makes-luminous-watch-hands-glow/368692/8 "2006-08-14T17:46:08Z")
> [@Placid Liver](#): > > So presumably if you wanted to tell phosphorescent from tritium, you could tell it from whether it was a constant glow or a fade proportional to the amount of time since light exposure? That is indeed a way as tritium won’t fade overnight Or the dial may have a “T” on it somewhere (bottom of [this](http://www.luminox.com/catalog/cat/1600/1602.html) dial “T swiss made T”) or an “H3” ([here](http://www.smithandwessonwatches.com/tritium/products/2/enlrg.jpg) )