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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.
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.
illuminated watches
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.
Substitutes
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.
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.
Non-radioactive
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 after it 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.
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.
Source:
http://iwmagazine.com/2012/01/in-the-dark/
