Tritium / GTLS
Watches using Tritium Light Sources which are also referred to as watches with Gaseous Tritium Light Sources (GTLS) or self luminous watches are used by many of the worlds special forces units, including the SAS, SBS, Israeli Defence Forces and US Navy Seal Teams. Watches which empty this technology such as the MWC 2008 Divers Watches are by far the easiest watches in the world to see in either low light or zero light/night conditions. The dials which employ this technology are on average 100 times brighter than any other type of watch dial and are easily readable under any conditions. The tritium light sources require no charging as in the case of Luminova and are guaranteed for 10 years although the life of the light sources can exceed this by a sizable margin.
The MWC and Marathon above are two typical contemporary Tritium watches with GTLS
GTLS, Tritium Vials etc are generic terms describing devices which have the ability to glow continuously without an external power source or the need to be charged by exposure to light. Although we are looking at the application for GTLS in watches this technology is also utilized for target indicators, signs, gun sights and numerous other applications such as cockpit instruments.
Radium and Tritium Paint
Up until the 1960s many manufactures continued to use Radium. It was applied the by hand using a fine paint brush. To apply the Radium exactly to the places where it was supposed to be, workers would tend to lick the tip of the painting brush to get a sharper point, thus being better able to apply more accurately the Radium. This habit of licking the painting brush resulted in numerous cases of workers getting cancer, especially mouth cancer. But as stated, Radium was used up until 1960, where it was finally stopped being used due to serious health risks.
Tritium was used from the 1960s until the late 1990s. Also being radio-active, but more mildly so (beta waves). The half-life mentioned above is of approx. 12,3 years. After that, Tritium (in most cases) will not glow anymore.
There were two degrees of Tritium employed at the time:
a) the dials emitting radio-activity of less than 25 mC. Dials were often marked "SWISS T < 25"
b) the dial emitting radio-activity of less than 7,5 mC. Dials often marked "T SWISS T".
The primary health risk of both Radium and Tritium is that it is radioactive and if it is ingested or if dust particles containing radium are inhaled, it can lead to various bone diseases and forms of cancer. While there is no safe level or radiation exposure, everyone is exposed to a certain amount of background radiation every day. The question is: "are the levels of radiation from the radium dials high enough to worry about?"
Not all the radioactive material remains where the paint is visible. Most watches with genuine tritium dials and hands are now quite old and over time, this paint will become very dusty and it is important not to inhale any of this dust. If the watch needs servicing the crystal should not be removed without a good reason, nor should the watch case be opened up without taking precautions. The dust is due not only to the normal aging of the paint, but also because when radium breaks down, the force of emitting the alpha particle during the radioactive decay causes the daughter atom to recoil. This recoil is strong enough to break the chemical bonds that hold the atoms in place. The resulting daughter atoms will move a little ways away and then settle. These daughter elements, such as radon-222, polonium-218, lead-214, and such, are all radioactive and just as much a problem as the radium or tritium itself.
Starting in the 1920s and especially in WWII radium dials were widely produced for civilian and military watches. The manufacturers would hire girls and young women to paint the dials and they were encouraged to lick the tips of the paint brushes to get a sharp point. Many of these girls had previously painted toys such as lead soldiers and the whole environment in some of these factories was very lax about the radium and tritium dials. The women would sometimes do things like paint their teeth, put it in their hair or on their faces and paint their nails with this material to surprise their boyfriends. Many of these women would later develop serious health problems such as mouth cancers and some even died due to ingesting radium.
If you do a web search on the Radium Girls, you will find lots information about them but the best one in my opinion is here by Bill Kovarik the Radium Girls.
Although this all sounds horrific the fact remains that these women ingested a LOT of this paint, but in reality "only" a few died, and probably not more than 20%-40% had any future health problems. It is not clear how many of these health problems were due to the radiation or due to the heavy metal poisoning because many had previously (and afterwards) painted lead soldiers. The fact remains that there are lots of risky activities in this world, from climbing cliffs or having sex to eating chicken wings (probably a big killer of Canadians!) that many people don't think twice about doing these things. The crazy thing is people seem to panic as soon as you mention radiation, sunbathing and skin cancer or "mad cow disease".
It does seem however that ingesting radium might be considerably less dangerous than breathing it. Ingested radium will either pass through your system or act chemically like calcium and enter your bones. The CDC's Radium webpage says that 80% of ingested radium will pass directly through you and some of the rest will eventually be filtered out by your kidneys. Radium dust in your lungs, on the other hand can stay there permanently and potentially cause lung cancer in the long term.
One particular concern of radium decay is the daughter element radon-222. Radon is a noble element so it won't chemically rebind and it is also a gas so it will float away from the dial. Radon has a half life of about 4 days, which is probably enough for it to diffuse to anywhere in the watch case but probably not long enough to escape before it decays. The daughter elements of radon-222 are radioactive solids, which will settle on the watch parts. So, it seems likely that a watch with a radium dial will cause the entire movement and inside of the case to be covered with a very find dust made of radioactive material.
Now it is true that as time goes on, the decay of the radium will cause it to lose some of its radioactivity. Unfortunately, the half-life of radium-226 is about 1600 years, so these watches will be about as radioactive 200 years from now as they are today. For all practical purposes, these watches will be radioactive forever.
One final comment on the possible health risks of radium is that the dial painting companies weren't exacting in their formulas. It wasn't just radium-226 that was used, but a variety of different radioactive elements. Most of these other elements will decay faster than radium-226, so they may be less dangerous now. Also, depending on how the paint was mixed up, some dials were very hot, while others were not. If you are concerned about a particular watch, you should have it measured. We tested an old Second World War clock dial and the Geiger counter still went crazy!
Tritium Vials or Gaseous Tritium Light Sources (GTSL)
Tritium lighting is made using glass tubes or vials with a phosphor layer in them and radioactive tritium (a hydrogen isotope) gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS).
Physics behind the light
The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons which cause the phosphor layer to fluoresce.
During manufacture, a length of borosilicate glass tube which has had the inside surface coated with a phosphor-containing compound is filled with the radioactive tritium. The tube is then fused with a CO2 laser at the desired length. Borosilicate is preferred because it is a type of glass noted for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electrons due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow. One could use any beta particle-emitting substance, but in practice tritium is preferred because it is not very hazardous.
Various preparations of the phosphorus compound can be used to produce different colours of light. Some of the colours that have been manufactured in addition to the common phosphorus green are red, blue, yellow, purple, and orange.
The types of GTLS used in watches give off a small amount of light—not enough to be seen in daylight, but enough to be visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is often more expensive than the rest of the watch itself, manufacturers try to use as little as possible. Being an unstable isotope with a half-life of about 12.36 years, tritium loses half its brightness in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with and the longer its useful life.
Uses of self-powered lighting
A contemporary Marathon US Military Spec Watch appears below which uses Tritium Vials / Gaseous Tritium Light Source (GTLS). I have owned this model and found it to be an excellent watch and superior to my CWC which uses paint but comparable to several other GTLS models I own which are Traser, MWC, Nite and Luminox.
These light sources are most often seen as "permanent" illumination for the hands and dials of wristwatches intended for diving, night use, or "tactical" applications by the military where illumination is needed but a light source may not be available. Some uses of this sort are analog dials in aircraft, in compasses, and sights for weapons.