Nowadays, knowing the exact time is not a problem. We are surrounded by many different devices that you can always rely on: a wall clock, a computer monitor, a smartphone screen. At the same time, wristwatches are kept apart - they do not need to be taken out of a pocket or bag like a phone, and even more so, their battery will not run out so quickly (if there is one), they are always on the wrist, while a computer or a large interior clock is all do not accompany every moment of our life. Most modern wristwatches will not let you down either day or night due to the presence of some kind of illumination in them.
Luminescent coatings
A wristwatch with luminescent backlighting is a familiar option, when hour markers and hands glow in the dark without a power source. The oldest method used in the watchmaking industry is phosphorescence. A substance that has accumulated energy from an external light source releases it in the form of its own glow relatively slowly.
The simplest example of such a substance is phosphorus, although it is no longer used due to the danger to human health. Modern substances are safe, their advantage is a bright glow in the first minutes after falling into the dark, and the disadvantage is that this glow does not last long. The brightness gradually decreases and after a couple of hours it becomes difficult to see the time in the dark.
Tritium: radioactive and harmless
One of the latest discoveries in this area is tritium illumination, which works on the basis of radioluminescence. This process is caused by the beta decay of tritium, which occurs regardless of external influences and lasts a very long time, tens of years. Tritium is superheavy hydrogen, or rather a radioactive isotope of hydrogen. It was opened back in 1934, but its use in watchmaking began relatively recently, in the first decade of the XNUMXst century.
Until about the 50s, radium-226 was the basis of radioluminescent paints, then they began to replace it with promethium-147, and sometimes krypton-85. Elements of watch dials and other equipment were coated with such paints. Today, the most common is tritium, which emits beta particles.
Protective glass absorbs them almost completely, so it is considered safe for humans. And in general, the energy of the particles is so weak that not only glass, even human skin is capable of repelling them. In addition, to reach a dangerous concentration, the substance must be thousands of times larger than the amount we wear on our wrist.
Principle of operation
Miniature glass flasks are made to use tritium. Their inner surface is covered with a thin layer of a luminous substance. You can choose any color, but traditionally the human eye perceives green best. Gaseous tritium is pumped into the flasks under pressure, after which they are hermetically sealed. In the process of decay, tritium emits electrons, they collide with the coating, and it absorbs their energy and turns it into visible light.
In the space of the dial
Elements of the required size are cut from such flasks, which are installed on hour markers and hands. The decay of tritium continues for tens of years, thanks to which the glow does not stop even for a second. Such illumination does not require external light sources. The watch can lie in the box for a year and shine all the time.
Of course, this is not a perpetual motion machine, and over time, the glow will become weaker, but the figures are impressive. Such backlighting loses about half the brightness within 10-15 years from the date of manufacture and about 75% after 25-30 years. Some manufacturers provide a service of replacing tritium elements in their service centers, for example, the Traser wrist watch company.
