Luminescence is a technical term that refers to anything that glows in the dark without a specific energy supply. Several animals display luminescence, for example, glow worms and jellyfish (technically, this is bio-luminescence). There are numerous ways that luminescence has been applied to watches and dials over the years. It can be very confusing, so here is a guide to the most common methods used over the years. Let us shine the light!
For the Love of Luminescence
For watch lovers, luminescence seems to be a love-hate relationship. Those who love it tend to call it “lume.” Some want to place as much luminescence material on as many surfaces as possible. Then others go for the completely minimalist approach. It is very personal whether you want your watch to glow in the dark or highlight key points. Daniel & I aim to give everyone a unique watch that they can call their own and truly reflects who they are, so here at SNGLRTY, we have embraced luminescence. There are many options, and some of our efforts – and even my wife agrees – are quite magical.
But how does luminescence work? That is a regular question we receive, so I thought it would be good to review the science to create light out of the dark. Then we can take a walk down the history of luminescence in the watch industry. Before that, we need to go through a few basics to ensure that we all understand what light is! Our eyes are sensitive to electromagnetic waves that travel through our atmosphere and have, for the most part, been emitted by the sun. We see light waves with a wavelength starting around 400 nanometers (one nanometer is 0.000000001 of a meter) up to 700 nanometers. The shorter wavelengths are seen as the violets/blues and the longer wavelengths as reds. This is typically referred to as the visible spectrum. Our eyes are most sensitive to light of 550 nanometers, and it is seen as green.
For the vast majority of the earth’s existence, these photons of light came exclusively from the sun (we shall ignore starlight for these purposes). The first “artificial” light was from fire; this then morphed into controlled fire such as candles and incandescent electric light bulbs. All of these light sources use the same process to create radiant light. First, a substance is heated to a high enough temperature to radiate electromagnetic waves within the visible spectrum.
By contrast, luminescence is the emission of electromagnetic waves within the visible spectrum of light by a substance that is not the result of heating. Sometimes referred to as a form of cold-body radiation, luminescence can result from chemical reactions, subatomic motions, and even stress on a crystal.
How We See In The Dark
It is important to understand how our eyes perceive light too. Especially in the dark, as that is when luminescence is most important for watch lovers. The brightness required for distinguishing a scale in darkness by the human eye that is acclimatized to the dark is 3.2 nano-candela per square millimeter. That is quite a technical definition of brightness but sufficient to say that a 25W compact fluorescent light bulb has a brightness of approximately 130 Candela. Hence, our eyes are very sensitive when acclimated to the dark.
For a watch to be readable in the dark, both the dial and the hands need to have a luminescent material applied to them. The process of coating dials, hands, indexes, and signs of any instrument with a luminescent material is called ‘luminising .’ There are two principal materials used in the luminising process: radio-luminescence or photo-luminescence material.
A radio-luminescence material is defined as a material that emits light when an atomic process occurs. This can be a radioactive material that emits a photon of light when it goes through radioactive decay. Or A particle of radioactive decay bombards a secondary material, the phosphor, that then emits a photon of light. For example, one of the first practical applications of radioactive materials was when radium was mixed with zinc sulfide crystals to make the crystals glow in the dark. The energy from the radioactive decay is transformed into visible light in the zinc sulfide crystals.
There is One Large Drawback
The most obvious issue with radio-luminescence is that it relies on a radioactive process that is extremely dangerous. Thankfully we fully understand the dangers of radioactive materials today. But, unfortunately, it was not until the late 1950s that scientists fully understood the dangers of radioactive decay. The danger comes from the emission of gamma rays or extremely high-energy electromagnetic waves associated with radioactive decay. These gamma rays are hazardous as they pass straight through a watch case and cause damage to any living material they hit. If that were not enough, a byproduct of the decay process is radon gas, a highly radioactive gas. Radon can cause further damage as it decays in the environment and is especially dangerous if inhaled.
All classic watch collectors need to be aware that many watches manufactured before the 1950s still contain radio-luminescent material on the hands and indexes. If you purchase a luminous vintage watch, it is best to check very carefully for radioactivity as the half-life of radium is 1,600 years. This means that it remains a potent radioactive material even after 70 years on a watch. If you are concerned, it is easy to confirm with a Geiger counter, place it over the watch face, and if you get a high reading, you should seek advice on how to dispose of your watch!
A Dangerous Problem Solved – Well, Almost
Once science had identified these very material risks from radio-luminescent materials, there was a lot of effort to identify new luminous compounds that were safer to use. A new compound that phosphors, when radioactive particles bombard it, was developed. It was based on the radioactive decay characteristics of tritium. A beta particle is emitted when tritium decays, and the advantage is that it has significantly less energy than gamma rays. Beta particles can be stopped by a thin sheet of aluminum when used in a watch; radiation cannot leak from the watch case. Another advantage is that tritium does not decay into another radioactive isotope, so it becomes less dangerous over time. With a half-life of 12.5 years, the radioactive strength of a radio-luminescent material based on tritium will reduce to one-sixteenth over a 50 year period. In practical terms, it is unlikely the luminescence will be visible in the dark.
The advantage of tritium is its very low radioactive toxicity. Tritium based luminous compounds were considered a safe alternative to radium, but it is still a radioactive material, and all radioactive material is dangerous if mishandled. The issue arises when tritium luminized watches require maintenance. On removing the case back, the watchmaker can be exposed to the radioactive particles. For this reason, even watches with tritium luminescence must be handled with extreme care as dust particles inside the case can become radioactive over time. These radioactive dust particles can cause damage to any living material and are especially dangerous if inhaled.
Dangerous, But Still In Use
You may have thought that because of all the risks of using a radioactive compound for the luminescence, it would have been completely phased out from the market, but this is not the case. Instead, there remain specific gauges and watches that require a strength of luminescence that decays in years rather than hours, for example, in professional diving watches and certain military applications. Tritium paint, a mixture of tritium and a phosphor, has historically been used, but more recently, there has been a trend towards encapsulated tubes.
The Gaseous Tritium Light Sources (GTLS) is a successor to tritium paint. It is a tube filled with tritium and phosphorus and is, in effect, a tiny self-powered micro gaslight. These tiny tubes are made from borosilicate glass and encapsulate the micro gas lights. A single borosilicate glass tube is fitted to each hand, the hour markers, and, for divers watches, to the bezels. The advantage of these tubes is that there is no need for an external energy source as all the radioactive elements are sealed in the borosilicate glass capsule. However, as mentioned previously, radio-luminescent technology is only used on very specialized professional watches; for example, the US military uses GTLS tubes specified in procurement specification MIL-W-46374.
All radio-luminescent material used in any time measurement instrument must meet ISO 3157:1991 Radio-luminescence for Time Measurement Instruments. This specification permits only two types of radionuclides: tritium (3H) and promethium (147 Pm), both classified as low-yield radioactive elements.
A photo-luminescent material absorbs light when it is plentiful and then releases the stored light over a period of time. Thus, it can be thought of as a light battery. When bright light is incident on the material, it recharges, and then over time, photons of light are discharged at a lower frequency. With the reasonable concerns over the safety of radioluminescent material, there was a lot of interest in developing reliable photo-luminescent materials. Originally phosphorescent zinc sulfide crystals were used. However, over time, many manufacturers developed proprietary and patented photoluminescent materials. These new photoluminescent materials have extremely high performance and offer a huge variety of colors of both the application material and the color of light that the crystals emit.
Charge, Store and Emit – The Photo-luminescent Cycle
There are three distinct phases to the photoluminescent process. First is the charging phase, where photons of light that hit the crystals are absorbed into the crystal structure. When the photon of light hits an area in the crystal that can act as a charging activation point, the electrons in that area are “lifted up” into a higher energy state. Once lifted into the higher energy state, the electrons will migrate to an area of the crystal where they can be most stable and store the energy to be released later in time. The more intense the incident light and the longer the intense light is incident on the crystal during this activation phase, the more electrons are lifted into, the higher energy state and the greater the charge in the crystal. The greater the charge of the crystal, the brighter the luminescent material will appear in the dark.
The electrons in the higher energy state are somewhat unstable, so over time, they return to a lower energy state (sometimes referred to as the “ground state”). In this transition, there will be a release of energy. This energy interacts with the luminous centers and releases a photon of electromagnetic energy (light) at a prescribed color (frequency) which, by necessity, will be at a lower frequency than the photon of energy that charged the crystal. Thus, manufacturers can control the exact frequency (color) of light discharged by manipulating the luminous centers. In this way, manufacturers can create different luminous emission colors.
- It is interesting to note that after the activation process has finished, many electrons will fall to ground level. This results in the initial luminous effect being very strong. However, over time fewer and fewer electrons will fall back to the ground state. As fewer and fewer electrons fall to the ground state, the luminous intensity will diminish. This period is referred to as the half-life (the time for the luminous intensity to drop by 50%). When photoluminescent materials were first introduced, their biggest drawback was that the luminosity would not last a full night period – obviously, this is not a problem for radioluminescent material. Over time further research and development have addressed this issue, and now photoluminescence can last well over 12 hours.
Luminescence On Your Watch
If you are interested in the details of how to select the correct photoluminescent material for your watch, we have the answer for you. Together with Swiss SuperLuminova, we created a series of videos to take you through everything you need to know about this magic material. You can check out the first episode HERE.