Let it shine, shine, shine!

How watches can be read in the dark

by Marcus Hanke

 

Ever since people had the technological know-how to measure time, there were some of the chronically sleepless type, who wanted to know what time it is, even when it was too dark to see the dials and hands. In the medieval cities, the nightshift guards sang out the hour, and in the more noble households, technically more refined innovations were introduced: clocks with a moving strip of paper, onto which numbers were painted. Before going to bed, people would light a candle and place it into the clock’s case, so that it would illuminate the paper strip from behind, clearly showing the hour numbers. Since it is never a very clever idea to combine candle and paper and go to bed then, the alternative was a more solid, opaque strip made from leather or metal, from which the numbers were cut out. The candlelight would then project the current hour through the cutout onto the wall; a very smart idea, that, amusingly, is currently seeing a renaissance with modern alarm clocks that have a small laser optics, projecting the time onto the ceiling or wall.

 

Another way to tell time at night of course were striking clocks, and finally repeaters, which are able to indicate the time down to the individual minute, exclusively by acoustic chiming. However, this is a technically complicated path, and only works well, when the environment is mostly silent.

 

Before the 20th century, wars were normally fought during daytime only, starting at sunrise, or maybe dawn, with the battles normally ending when it became too dark to tell between friend and foe. This changed, and it thus became crucial, at least for the officers, to tell the exact time even in complete darkness. Since wars are either very loud, or life depends on staying very silent, acoustic time signals, like those from repeaters and sonneries, barely make sense in the field, aside from their mechanical complexity, that barely withstands the inevitable stress during battle.

 

The solution was simple: at least the hands, but ideally also the hour markers had to be illuminated somehow. The ability of some agents to emit light for some time, after having been subjected to energy, the so-called luminescence, is known since a long time already. White phosphor was used as a night light since the late 17th century. However, phosphor is a chemoluminescent agent, since its luminosity is the result of a rather violent reaction of the highly toxic white phosphor with oxygen. When the reaction is finished, also would be the shine. Therefore, a different material had to be found for watch dials.

 

What is luminescence?

 

The basic principle behind luminescence is the fascinating simplicity of our universe: What goes in, must come out - or in this case: what comes out, has to go in before. It is impossible to produce energy, it is only possible to transform it from one state to another. Thus the movement of a steam ship (=kinetic energy) is not “produced” by its engine, but in the engine the caloric energy stored in the water is transformed into the movement of a piston, or a turbine. However, it is clearly not enough to tell the water: “make the ship move!” You have to invest further energy to heat the water, until it vaporises, and the steam, that needs more space than liquid water, violently is pushing the piston.

 

The same is valid for luminous hands and markers of watches: their glow in the dark is an emission of energy, and this has to come from somewhere before. Consequently, we have to take into account not only the luminescent material itself, but also the energy source. In our task to make watch dials shine at night, we are confronted with the necessity of searching not one, but two entirely different materials, that only together can do what we want it to do.

 

Luminescent materials

 

There are many materials emitting light, but for us only “cold light” is relevant, that means light that is not the result of a combustion or heating process. This is limiting the choice already, but not enough. Also, the duration of the light emission is crucial. You certainly know those elements often used on textiles that glow as soon as they are subjected to ultraviolet or “black” light. These are frequently used in discotheques and night clubs. The use the UV light as energy input, but stop shining as soon as the light source is switched off. This short-time luminescence is called “fluorescence”, and would be useless for watches, since it depends on constant energy input.

 

What we need is “phosphorescence”, that shines for longer than a few nano-seconds.

 

How does phosphorescence work?

 

I am not an expert in elementary physics, thus you cannot expect a complete treatise on the process within the elementary particles. What I may point out instead is how fascinating this process is, since it is “impossible”. Normally, the “charging” with energy is done by lifting the material’s electrons on a higher energy level. From there, they want to return to their former, stable level. They start to move quicker, and consequently the charged molecules collide with others, thus losing energy, that is normally emitted as heat. Sometimes, though, not all energy is lost as heat, but also by emission of a single photon the elementary particle of light.

 

However, the electrons return to their stable level immediately after the energy supply has ceased, which results in the fluorescence mentioned above. Under certain conditions, though, electrons can be “parked” with their enriched energy (so-called intersystem crossing), and then slowly emit low-level energy, until they have reached their original level again. This can last for several hours.

 

For manufacturers of dials and hands, the goal was to find a paint that would shine very brightly over a very long time. And as you might have recognized now, this is impossible; a bright shine means high energy output, that means the electrons of the luminous material would return to their former low energy level quickly, which again means a quick end of the beautiful shine. Or you get a longtime light emission, but have to live with a very low output energy level. The “flashlight all night long” does not exist, sorry, watch enthusiasts! Before undertaking the difficult quest of searching a compromise that would eventually satisfy all, the short-time luminescence, the fluorescence, offered an interesting aspect: radio-luminescence

 

Initiating luminescence

 

It was found that some luminescent materials could be stimulated by radioactive energy as well. Since the radiation emitted by radioactive materials is more or less constant, it is not necessary to find a luminescent material active for hours after the energy source has been switched off. The idea was to simply pack the radiation source onto the luminescent material, which would thereby be permanently activated.

 

The result was zinc sulfide, paired with radium. This element, discovered by Marie Curie, was thought to be a practical solution. Well, you can imagine that the longtime radiation effect of radium is not healthy at all. However, at first, mostly dials and hands of clocks and (aviation) instruments were treated with that material, and they are normally not in close contact with human tissue. Yet the first victims of contamination with radium were dial painters, who used to lick the tips of their brushes, in order to get a fine and narrow painting tip. In the 1920s the adverse effect of radium on the health of dial painters was scientifically stated for the first time. In spite of that, radium continued to be employed especially in wristwatches made according to military specifications. For the military, the health of their soldiers was an issue only as long as they were serving, and during war, the risk of cancer caused by radioactive materials was not high on the list of things to be considered. Sometimes, specific symbols printed onto the dials were the only warning message, and a famous watch even received its name from the radium used on its dial. The combat divers wearing these watches had a life expectance of less than a year, so who should be worried about cancer in some decades?

 

Luckily for us, times changed, and the the health effects of nuclear energy became an issue for all generations following World War II. This resulted in increased efforts to find less harmful alternatives for radium, which was eventually found: tritium. Tritium is a natural hydrogen isotope and emits weak beta radiation. This can be easily and effectively shielded already by a thin plate of aluminum, so the watch crystal and even more the movement and the caseback are protecting the watch wearer from the tritium radiation, even when it is on his wrist during 24 hours a day, 7 days a week.

 

Watch manufacturers gladly adopted that new technology, and especially watches issued to the various authorities, were boldly signed with symbols showing that no harmful radiation is emitted from the watch: “T”, “3H” - both symbols for tritium, or simply a nuclear radiation warning sign crossed out. On watches made in Switzerland, two tiny “T” prints at both sides of the small “Swiss made” lettering at 6 became common and was used for decades.

 

Yet radio-luminescent paints based on tritium were not without problems: While the contact of the skin with tritium would do no harm, tritium is nonetheless highly toxic when swallowed. Especially for professions involved with the production, the storage or the application of that luminous paint, there is an increased risk of contamination (although no figures were published on the factual cases of such a contamination). Additionally, as every radioactive material, tritium decays, by spending its stored energy. This decay is normally expressed by the halftime figure, that is the time needed for the radiation nergy to decrease by 50%. While radium has a halftime of 1,600 years, tritium is losing half of its energy already after 12.3 years. As a consequence, already six to eight years after the production of the paint, its luminosity is noticeably weaker. Therefore, many of the watches still equipped with tritium-based paint, have already lost much of their nighttime legibility by now.

 

Getting away from radioactivity

 

These problems caused the industry to switch over from radio-luminescence to phosphorescence, to a luminescence that is the result of the material being “charged” by light only. The commonly used zinc sulfide was replaced by strontium oxide aluminate, that was ten times more efficient than the former. A breakthrough in the technology of luminescent paints was achieved already in 1941, by the Japanese company Nemoto. Its patented “LumiNova” paint, based on strontium oxide aluminate, showed all the desired attributes: it is non-toxic, is activated by a wide spectre of light, emits a bright shine for a short time after being charged, but also shows a steady glow for a long time. However, despite its early invention, LumiNova found its way onto the watchdials of the world only in the nineties. By the way, the more commonly term “Super-LumiNova” (SL) is used by Nemoto’s licensed European partner, Tritec in Switzerland. Technically, Super-LumiNova and LumiNova are the same.

 

LumiNova is a rather effective paint , that emits light for a prolonged time, after being "activated" only shortly. Activation is made by exposure to daylight or artificial light, which is raising the LumiNova's electrons to a higher energy level. In order to regain their former level, the electrons are slowly emitting energy, that causes the luminescent particles to shine in the dark. The strength of this shine is following a specific curve: very high in the very beginning, after the first significant drop-off, the brightness is reduced gradually, until the material ends to emit light after several hours.

 

Further the effectiveness of the SL material is depending on the thickness of the layer, the fineness (lack of larger grains), the colour of the base material (LumiNova applied on a black base dial is less efficient than on a brightly coloured base), and, finally, the presence of colour pigments. One nice attribute of SL is that it can be combined with various colour pigments, that change the material's appearance in daylight as well when shining in the dark. However, the colour pigments also reduce the material's luminescence effectiveness. The brighter and more saturated the colour, the higher the loss of luminescence, compared with the pure, untinted pigment. Coloured SL on watch dials was pioneered by Audemars Piguet (ROO “End of Days”), and later by smaller brands, such as Temption and Angular Momentum. Today, brightly orange, blue and green SL markers are highly fashionable, and are ubiquitously offered by the industry. It has to be kept in mind, though, that these elements lose their glow a lot faster than the standard, light green/white SL markings. If a long and steady luminescence is important for the buyer, then he should decide against gaudy colours.

 

Another potential issue with SL is that it does not like humidity. Contact with water or even high humidity in the air would result in oxidation of the pigments, which thereby lose a part of their luminescent attributes. Consequently, SL has to be treated with binding and/or protection agents, lacquers mostly. It is important to note their stability regarding UV light, otherwise the SL markings can become grey or brown after extended exposure to ultraviolet light. Note that not the SL particles change their colour, but the binding lacquer, but the result is the same.

 

While the use of LumiNova - or SL - has become absolute standard within the whole watch industry, there are nonetheless massive differences between the luminescent performance of the individual watches. I once conducted a test, comparing Super Luminova with tritium-activated material:

ulyssenardin.watchprosite.com

“Bright as a flashlight, all night long!”

 

Above, we saw that all phosphorescent materials, including SL only permit either a bright afterglow for a short time period, or a faint glow for an extended period of up to twelve hours; or a combination of these two, as in SL: LumiNova pigments reduce their initial afterglow luminance to one seventh within 30 minutes, then the drop curve is becoming more shallow. For diving watches, this is ideal, since a brief “charging” by the diving lamp results in a very bright glow enough for one dive. for everyday use watches, that should stay luminous for a longer time, care with the other factors, such as grain size, thickness of the paint layer, binding agent, base material and colour etc. are more important.

 

But even with all these measures, SL’s afterglow intensity is reduced to 1/40th of its initial value after five hours, so all claims about watch dials and hands glowing bright as flashlights for the whole night are greatly exaggerated. However, those people are no liars, but merely “victims” of their own body’s clever ability to adapt to changing conditions.

 

The miracle of human vision

 

Our optical perception is the result of two different sensors working in our eyes: cones and rods. Cone cells are responsible for daylight vision, they are able to recognize different colours, as well as the direction from where the optical signal is originating. But for this performance, they need a lot of light. For bad light or night vision, our brain is activating other sensors, the rod cells. These are simple, but effective. Already a single photon is enough to trigger a chemo-electrical nerve signal, while cones need 200 photons as minimum. However, rods are unable to give angular and thus directional information, and are limited to black and white vision.

 

The high effectiveness of the rod cells is what compensates the drop in glow intensity on our watch dials and hands. This is the reason why we can still read the watch after six or seven hours of darkness, when non-biological photosensors, such as digital camera sensors are unable to get a signal strong enough to reproduce a picture. Since rods are unable to recognize colours, even tinted SL particles will be perceived as greyish-green by the human brain. Only as long as the SL glow is strong enough to trigger a signal from the cone cells, the different colours of the SL pigments can be seen as such.

 

What about traser?

 

Before closing that brief article, I want to mention a special kind of luminous elements on watch dials and hands: tritium gas tubes, more popular under the designation “traser”. Unlike the zinc sulfide mixed with tritium, tritium in gas state is filled into small transparent tubes, that are inside covered with a radio-luminescent agent. As a gas, tritium is able to activate the luminescent material far better than in solid state, and thus the glow is more intense. However, the problem of tritium’s short halftime of 12.32 years and the reduction of the glow intensity remains.

(c) 2007 Marcus Hanke and PuristSpro

 

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