« Luminous Hands and Indices »
Luminous hands and indices, made possible by Super-LumiNova, for example, are a feature many watch enthusiasts value for practical as well as aesthetic reasons. On the one hand, a watch with luminous numerals or indices is useful because it lets you read the time effortlessly in the dark or in poor lighting. On the other hand, there is a unique appeal to the glowing pattern that lume-coated indices and hands create in the dark. We take a look at the historical development, the materials, and the physics involved.
What Is Luminous Paint and What Is It Made Of?
The term luminous paint – or luminous compound – refers to various types of coatings that can emit more visible light than is falling on them from outside at that moment. This sets them apart from reflectors, which merely bounce back incoming light. The ability to emit more visible light than is received is also known as luminescence. This already explains the name Luminova or Super-LumiNova. Luminescence is what distinguishes so-called luminous pigments from other types of color pigments. To create luminous paint, a sufficient quantity of luminous pigments must be added to the paint mixture. The glow effect can be based on different physical principles.
Daylight fluorescent paints convert incoming UV light, which is invisible to the human eye, into visible light. This is the case, for example, with neon colors used as signal colors on safety vests or on police and emergency vehicles. Phosphorescent paints, by contrast, store incoming light in the form of so-called excitation energy and then release it again after a certain delay. Practical applications include emergency exit markings and various glow-in-the-dark decorations, trinkets, and novelty items. The third group of luminous paints is radioactive luminous paint.
With these, the energy needed for the glow is produced either directly or indirectly by the decay of a radioactive substance. This is combined with a fluorescent substance that it excites. In the past, the radioactive substance used in luminous paints was mainly radium salt, which has since been largely replaced by tritium compounds or promethium salt.
Luminous Paint on Watches and the Radium Girls Scandal
After Marie and Pierre Curie discovered radium in December 1898, luminous paints based on this material were developed in the years that followed. They quickly found widespread use in military technology and in various civilian industries, especially the watch industry. In some cases, self-luminous dials were made by coating them with luminous compound; in others, the compound was applied not to the dial but to the hands and indices.
Both methods make it possible to read the time even in total darkness without any other light source.
Initially, however, the risks associated with radioactive radiation were underestimated or simply unknown. In the early years, radium and radium-containing substances were even sold on a large scale as health-promoting products. The health hazards posed by the radiation were not examined more closely and brought to wider public attention until the 1920s, in the wake of the scandal surrounding the so-called Radium Girls. The women referred to in the media and literature as the Radium Girls were factory workers in the United States whose job had been to apply radioactive luminous paint to watch dials or to make luminous hands, and who had suffered radium poisoning as a result.
In hindsight, a particularly fatal mistake turned out to be the habit of licking the brush used to apply the paint. The women had been advised to do this by their supervisors in order to bring the bristles back to a fine point so that numerals, lettering, and fine lines could be rendered precisely. Some of the women had also painted their fingernails or even their faces and teeth with it to playfully surprise people in their private lives. You can get a rough idea of the scale of the problem when you consider that in 1920, for example, three major American manufacturers produced a total of four million watches whose dials featured radium-based luminous numerals.
Only after certain illnesses such as anemia, bone fractures, cancers of the tongue and jaw area, and necrosis of the jaw had become strikingly frequent among the women did the problem attract attention, prompting a more detailed investigation of the connections and the introduction of appropriate protective measures.
For many of those affected, however, it was already too late, and at several sites there were numerous deaths attributable to sustained and repeated radium contamination. Fortunately, this has nothing to do with the modern luminous paints Luminova or Super-LumiNova.
Watch Lume Today
Anyone who wears a watch with luminous numerals made today by Rolex, Omega, Tag Heuer, or another renowned luxury watchmaker, however, need not worry about possible radium poisoning. Today, only alpha emitters and low-energy beta emitters are used. The direct radiation from the radioactive substances used has a short range of only a few centimeters and cannot penetrate either the watch crystal or the metal case.
Caution is advisable at most with an older watch with luminous numerals or a lume-coated dial. This applies especially if the luminous compound is already beginning to flake off or if the watch is worn constantly against the body. With the availability of weakly radioactive isotopes such as promethium or tritium from the late 1950s onward, the era of radium luminous hands and dials quickly drew to a close. Tritium luminous hands and indices in a modern wristwatch no longer pose a health risk.
In addition, non-radioactive photoluminescent materials have become increasingly established in recent years and are now used by most watch manufacturers. Particularly noteworthy here are Luminova and its further development, Super-LumiNova.
Luminova, Super-LumiNova & Co.
In 1993, the Japanese manufacturer Nemoto & Co., Ltd., launched inorganic phosphorescent pigments under the name Luminova; based on alkaline earth aluminates, they had been developed by Nemoto itself. Unlike the luminous pigments used previously, they are completely free of radioactivity. They are characterized by good durability and long-lasting luminosity, and they are available in various colors. Luminous hands and indices coated with Luminova can thus glow yellow-green, blue-green, or blue-violet.
Luminova was further developed specifically for use in the watch industry and modified so that the substance glows even more brightly. This product has been marketed under the name Super-LumiNova since 2000 and is now standard among many renowned watch manufacturers. Today, Super-LumiNova no longer has to be imported from Japan, as RC Tritec, Ltd., has been producing it as a 100% Swiss Made product since 2007 under a license from Nemoto. Some manufacturers, however, also use their own luminous compounds, such as Chromalight, which has been used increasingly in Rolex Deepsea watches since 2008 and is distinguished by a characteristic light blue glow.
The rapid spread of Super-LumiNova is mainly due to the fact that this material offers roughly one hundred times the luminance of the non-radioactive phosphorescent pigments previously available. Incidentally, the color of the luminous compound visible in daylight can differ considerably from the hue of the glow visible in the dark. Even a watch with luminous numerals and hands that look matte and dark gray in daylight – as with the models of the Hamilton Khaki Field Black series, for example – can display the time in bright blue-green at night.
