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Luminescence? Confused? – All Your Questions Answered

Steve Mansfield
Steve Mansfield

Luminescence is a technical term that refers to anything that glows in the dark without a specific energy supply.  A number of 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 just 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.

Luminescence 101

SNGLRTY OHI4 photographed in the dark highlighting the luminescence application on the hour index and the hand
SNGLRTY OHI4 Photographed in the Dark Highlighting the Luminescence Application on the Hour Index and Hour Hand

But how does luminescence work?  That is a regular question we receive so I thought it would be good to review the science that allows us to create light out of 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 make sure 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 then finally incandescent electric light bulbs. All of these light sources use the same process to create the radiant light, a substance is heated to a high enough temperature so that energy is radiated out within the visible spectrum.

By contrast luminescence is the emission of electromagnetic waves that are 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 be the result of 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, so our eyes are very sensitive when acclimated to the dark.

SNGLRTY OHI2 with blue luminescence application on the hour index and nature color on the hour hand
SNGLRTY OHI2 with Blue Luminescence on the Hour Index and Nature on the Hour Hand

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’ and there are two principal materials used in the luminising process, either radio-luminescence or photo-luminescence material.

Radio-Luminescence

A radio-luminescence material is defined as a material that emits light when an atomic process occurs. This is typically the radioactive decay of a radioactive material which either directly emits a photon of light or is mixed with a secondary material that, when bombarded by the byproduct of radioactive decay, emits a photon of light. One of the first practical applications of radioactive materials was when radium was mixed with zinc sulphide crystals to make the crystals glow in the dark. The radiation energy of the radioactive decay process of the radium isotope is transformed into visible light in the zinc sulphide 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 it was not until the late 1950s that the true risk of radioactivity associated with radium was fully understood. The danger comes from the emission of gamma rays or extremely high energy electromagnetic waves, associated with radioactive decay. These gamma rays are particularly dangerous 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 which is 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 there are many watches manufactured prior to the 1950’s that 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 these very material risks from radio-luminescent materials had been identified, there was a lot of effort to identify new luminous compounds that were safer to use. A new compound that phosphors when it is bombarded by radioactive particles was developed, it was based around 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 on the day it was applied 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 are dangerous if handled incorrectly. 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. 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 in certain military applications. Tritium paint that is 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 gas light. 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. As mentioned previously, radio-luminescent technology is only used on very specialized professional watches, for example the US military uses GTLS tubes that are specified in procurement specification MIL-W-46374.

All radio-luminescent material that is 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 of which are classified as low yield radioactive elements.

Photo-luminescence

SNGLRTY OHI4 in a Semi-Light Illustrating the Luminous Dial and Hand
SNGLRTY OHI4 in a Semi-Light Illustrating the Luminous Dial and Hand

A photo-luminescent material is one that absorbs light when it is plentiful and then releases the stored light over a period of time.  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 radio luminescent material there was a lot of interest in developing reliable photo-luminescent materials. Originally phosphorescent zinc sulphide crystals were used but over time many manufacturers have developed proprietary and patented photoluminescent materials that not only have extremely high performance but also offer a huge variety of colours of both the application material but also 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 thus store the energy to be released at a later point of 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.  By manipulating the luminous centers the exact frequency (color) of light that is discharged can be controlled, in this way different luminous emission colors can be created.

Classic watch face with luminous application highlighted
Classic Watch Face with Luminous Application Highlighted

It is interesting to note that immediately the activation process has finished a large number of electrons will fall to ground level.  This results in the initial luminous effect being very strong, 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 photo luminescent 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 radio luminescent material.  Over time further research and development has addressed this issue and now photo luminescence can last well over 12 hours.

Luminescence On Your Watch

In most cases, when you walk in to buy a luminous watch, you have to choose from the selection that has been chosen by the manufacturer.  But what if you could choose for yourself? Choose the color and also the emission color? Well it is possible, join Daniel and I on one of our webinars and we will take you on our journey of discovery that resulted in us creating our own patented watch. The re-engineering of the watch face that was necessary for us to #seetimedifferently means we can offer every one of our owners a unique experience in configuring their very own watch – including luminescence. Want to know more? Come and join us on one of our FREE webinars where we talk about making watches, what to look out for and what to avoid and SNGLRTY.

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