In today's on-demand society we are used to being able to read the time from our watches when and wherever we want.  When wristwatches first became popular during and after World War I, they were novel enough in themselves, but it wouldn't take long before owners wanted the ability to be able to read the time at night too.

Fortunately, there was radium, a ready-made solution familiar to clock makers and pocket watch makers.

No longer the wonder stuff of its first discovery, when its energetic nature was thought to confer health benefits and led to it being incorporated into products as diverse as toothpaste and hair creams, radium's adverse health effects were beginning to be understood. But it lent itself readily to being used to make dial markings glow intensely enough to be read clearly in the dark.

It's a common misconception that the glow of the luminous dial is from the radium itself. In sufficient concentration, it does in fact glow blue, which is what helped Marie and Pierre Curie discover the element in pitchblende. However, in our wristwatch application, there is insufficient radium for it to luminesce itself (typically in the order of about 25 to perhaps 300 micrograms per dial). Instead its radioactive properties come into play.

Radium is a very strong emitter of alpha particles, which can be used to excite a phosphorescent material into emitting visible light in a process called radioluminescence. Watches with radium lume are also referred to as "self-luminescent" as they glow all by themselves without any trigger being necessary.

For dials, small amounts of a radium salt were mixed with zinc sulfide (ZnS) and a bonding agent and applied to the dial. The ZnS could be mixed with various other compounds to amend the color of the lume, which could mask some of the changes of the radium salt itself as it aged from white through yellow to ultimately a darker brown.

The isotope of radium normally used was radium-226, which has a half-life (the time it takes for the element to decrease by half, thanks to radioactive decay into other elements) of about 1,600 years. That means that it would take sixteen centuries for the radium to be half as effective. It also means that the radium is active long beyond the lifetime of the watch. In fact, since the ZnS phosphor was under constant attack from the radium's radioactivity, the limiting factor for the glow of watches of this vintage is never the radium itself, but rather the lume material.

Little risk
Despite its strength, radium would typically have posed little risk to the watch wearer. The same cannot be said of the people involved in producing the watches unless stringent precautions were taken. The major risk to human life from radium compounds comes from inhalation and ingestion– and this is just what the dial painters were exposing themselves to.

The novelty of the glowing material led some of them to paint their nails with it, or run it through their hair. But their jobs required them to paint very fine lines with the radium mix, so they used to "point" their brushes by licking the ends to make a fine tip.

Unfortunately, this meant some of the material was swallowed and potentially absorbed into bone, leading to painful, disfiguring conditions and possible death. This came to light in the infamous "Radium girls" case in the US in the 1920s, with the court finding against their employer, the United States Radium Corporation, and labor laws relating to occupational health being introduced or revised as a result.

More stringent procedures practically eliminated diseases related to radium ingestion, but the industry and public were now much more aware of the risks and the search for alternatives to radium gained momentum. The amount of radium was gradually reduced, to the extent that a wristwatch from 1960 would only be about 1/100th as active as a pocket watch from 1910. Radium use in wristwatches has been banned in the U.S. since 1968 by the National Council on Radiation Protection & Measurements (NCRP), with the rest of the watchmaking world following suit.

The search for a safer replacement for radium didn't stray too far. The benefits of self-luminescence were clear and so the alternatives considered tended to be radioactive too. The frontrunners were promethium-247 and tritium (a form of hydrogen), both low energy beta emitters.

The impact of radiation on biological tissue is measured by "relative biological effectiveness" (RBE) and expressed as a radiation weighting figure. The beta particles emitted by promethium and tritium have a weighting of 1, comparable to having a medical x-ray. Radium decay however, emitting alpha particles, has a weighting of 20, as high as the scale goes.

Despite the differing strength, tritium lume works exactly as radium does, with radioactive decay triggering typically zinc sulfide. One big difference is in their half-lives. As opposed to radium's 1600+ years, tritium only has a half-life of just over twelve, meaning the lume is only half as effective after twelve years, then half as much again after twenty-four years, and so on. The lume material is not attacked radioactively as aggressively as with radium, but the reduced half-life means 1960s watches rarely remain illuminated.

Tritium concerns
Health concerns began to be raised about tritium in the 1960s and regulations around its use and export were tightened. Dials were marked to show tritium was present (typically with 1 or 2 "T"s on civilian watches but also sometimes 3H on military watches, as on Heuer's chronograph supplied to the German Bundeswehr). In 1966, the U.S. Nuclear Regulatory Commission specified an amount of tritium allowed in a watch of 25 millicuries (mCi) and this same maximum was adopted in many other countries.

The markings on tritium and promethium lume watches are governed by ISO 3157; where the watch is simply marked T, it must have less than 7.5 mCi of tritium whereas T<25 as seen on some dive watches like Rolex's Submariner means the tritium is somewhere under the permitted maximum of 25 mCi. Promethium is signified either by a P enclosed within a circle (usually on military issued watches) or the letters Pm or Pm 0,5.

Promethium was relatively rarely used compared to tritium, but both fell out of favor through the decades after the 1970s outside of specific applications where self-luminosity is particularly desirable, as in dive watches.

It became clear that radioactivity was a sensitive solution to making watches glow and was unlikely to be viable longer term, so manufacturers began in the 1960s to search for alternatives.

Chemiluminescence presented itself as a solution, though there were limitations. While it has the potential for self-luminescence like radioactive alternatives, the substances involved are usually consumed in the reaction that produces the glow meaning that the luminescence is not sustainable.

The answer came with photoluminescence. A photoluminescent material absorbs light, typically in the UV spectrum, and releases it again as light over time. For a watch, the material has to be phosphorescent rather than fluorescent, as the latter re-emits light effectively immediately afterit has been absorbed, whereas a phosphorescent material can continue glowing for hours in the right circumstances.

Initially the familiar zinc sulfide was used, but in a photoluminescent application fades undesirably quickly, so alternative compounds were investigated. The answer came with strontium aluminate, typically doped with europium, giving approximately ten times the brightness of ZnS with the glow lasting ten times as long.

This highlights a drawback of photoluminescent lume: it is not self-luminescent but rather has to be "charged" with light in order to glow. A few minutes charge will give minutes of glow, so any use requiring the watch to be readable in the dark for hours typically will require a charging period of several hours too.

In general use this is not an issue, but does explain why tritium continued to be popular in dive watches long after photoluminescent materials became available. Immediately after charging, the photoluminescent watch will typically be brighter than a radioluminescent equivalent but will fade with time after being charged. The luminescence of a tritium watch will remain constant throughout.

Current sources 
Photoluminescent materials currently dominate the market, particularly SuperLuminova from the market leader RC Tritec of Switzerland. However, there are a number of watchmakers such as Ball Watch, Traser and Luminox using tiny glass tubes of gaseous tritium (GTLS or Gaseous Tritium Light Source) from MB-Microtec, also of Switzerland.
Increasingly, the two technologies are being used in combination for the best of both worlds, giving both a constant glow and initial brightness. The major drawback of the GTLS tubes is cost, with the tubes costing in the order of $10 per tube and replacement being advised after twenty-four to thirty-six years.

The future might involve using both GTLS tubes (for specific applications) and photoluminescent pigments for everyday use. The problem there is the two goals, of extra brightness and longer duration of glow, are mutually opposed, and one or the other will suffer. The next breakthrough will be some way of reconciling those two aims in one material.

Mark Moss is a UK-based business architect/analyst who collects Heuer Carreras but has an interest in all aspects of horology. 

Android watches, based in South Florida, is known for unusual and striking designs that cover the spectrum from dress to sport to everyday wear. As tritium has grown in popularity, Wing Liang, Android's owner and chief designer, knew the illumination technology was a perfect fit for his company's watches. With the expansion of import licenses, Liang couldn't resist the opportunity to introduce this still unusual feature to his timepieces. Android's offerings with tritium, like the Divemaster Enforcer T-100 Automatic, offer colors and styles unlike most watches with tubes. This latest offering is a 200-meter diver that will come in a 45mm and 50 mm case with the SII NH35 and NH15 movements. Available with T-100 brightness, Android is another of the several companies to embrace the new, brighter tube configuration with is especially useful in dive watches. The Enforcer retails for $600.

Ball Watch  
Ball watch is a true pioneer in the world of tritium tube watches. Jeff Hess, president of Ball USA, won't hesitate to tell you that there wouldn't be any T-100 without Ball Watch. Back when tritium tubes were still a curiosity or mostly military feature in the U.S., Ball ramped things up by multiplying the amount of tritium in a watch by a factor of four. What could have been a disaster—the US Nuclear Regulatory Commission charged they violated the license—turned into a victory when Hess and his team demonstrated successfully that there was no prohibition to T-100 watches. For some time, Ball was the only company that could bring in watches with this level of tritium illumination. Once MBM had its own license updated, it opened the market to many other brands increasing their tube volume per watch. Ball produces watches filled with tubes and continues to make what might be the brightest watches in the western hemisphere.

Based in Lithuania, Vostok-Europe is the only watch company from that part of the world to incorporate tritium tube technology into its watches. Since their first tube offering came out in 2008 with the Caspian Sea Monster, they have produced dozens of styles and embraced gas tube illumination in a big way. The latest offering, the new N1 Rocket, is a 200-meter diver with T-25 illumination and striking seven-link bracelet. With a 46mm case, the watch, inspired by the world's largest rocket, comes in three dials and on leather as well. Price: $749.

Deep Blue

New York-based Deep Blue has quickly made a name for itself as a maker of serious dive watches for serious divers. Owner Stan Batesh is committed to building watches with exciting form and real world function. From the beginning of the brand, the watches have incorporated tritium tube technology and one of the first to use T-100 after the expanded licenses allowing this level of illumination. "After our initial success with T 100 tritium tubes we decided the next step would be an attempt at the new flat tritium tubes," says owner Stan Batesh. "The Daynight Pro T 100 is our first flat tube tritium watch. The new Daynight Pro retails for $899 with a Swiss ETA 2824 automatic engine, 300 meter water resistant case and 44 mm case.

All Luminox watches are Swiss-made and produced by its partner the Mondaine Watch Company. The company is also an authorized timekeeper for defense contractor Lockheed Martin, builder of the F-16 Fighting Falcon and the F-22 Raptor, among other fighter jets.

"We are recognized as the pioneer in the use of tritium tube technology," Luminox president Barry Cohen says. "One of our major retailers called me once to tell me how a newer brand called on them and, in the course of the conversation explained how they were 'just like Luminox' at least fifteen times," Cohen recalls. "The retailer replied 'why would I want something only like Luminox when we're doing quite well selling the real thing?'"

Most recently Luminox produced a special watch where 100% of the proceeds are going to the Japanese Red Cross in support of the Tsunami victims. The 43-mm watch has a carbon-reinforced polycarbonate case housing a Swiss-made precision quartz movement.

Finding the most eff ective and brightest combination of lume is a passion for Reactor founder and president Jimmy Olmes. His company has experimented with luminous materials from its beginnings in 2003.

"We were the first to seriously investigate how many layers of lume would give optimal glow," Olmes says. "We tried six and eight layers and went all the way up to twelve to learn that anything beyond eight and you reach a point of diminishing returns."

Reactor's patent-pending Never Dark system is a hybrid of the two major illumination technologies. The watches combine both tritium-gas tubes and SuperLuminova. As a result, the watches offer the wearer the advantages of both types of illumination—the initially intense brightness of SuperLuminova and the continuously lit nature of the gas-filled tubes. "We were the first to combine these two materials in one watch," according to Olmes.

As the in-house brand of MB Mictrotec, the makers of the tritium tubes in Switzerland, Traser certainly has the inside edge on all the latest tritium tube technology. As a way showcase the many uses of their tubes and provide military grade watches to both soldiers and the public MBM launched their own brand in the late 1980s. The brand, Traser H3, gained fame and momentum when the US armed forces made them standard equipment during the first Gulf War. Ever since, Traser has remained an authorized watch supplier for the U.S. military and they build watches to those exacting standards.


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