AS there have been a number of posts asking about luming I thought I'd post a 'Lume 101'
This will be in a couple of parts; this one - The Theory and the following part - The Practical.
The Boring Bit - The Theory
Self-luminous compounds
Self-luminous compounds provide permanent legibility over a long period of time without any prior activation from an external light source.
The energy release of a radioactive isotopes decay is continuously converted into a weak light emission. With tritium for example, traces of a solid, tritiated polymer are coated on zinc sulphide crystals to act as a permanent energy source, only limited by the active life of the element.
The first widely used self-luminous compound used to provide a glow in the dark feature was radium (hence the 'Radiomir' from Panerai). This radioactive isotope was used to excite zinc sulphide crystals to make them glow in the dark.
Once the radioactivity dangers (radium has a half-life of 1600 years) - particularly to the workers applying the compound - were fully realised by the 1950's, new self-luminous compounds using either the radioactive isotope of hydrogen, tritium, or a man-made radioactive element promethium, were developed and totally replaced the use of Radium.
Promethium based lume was known as N-Lume after its developer Nemoto of Japan, but with a half-life of around three years, it's use was as short lived as it's effective life.
Tritium has a half-life of 12 years and this eventually became a major drawback, as it's ability to glow diminished visibly within a few years. Tritium is also used in nuclear fission and so the regulations that came in to control the import, export and storage of radioactive compounds made it's widespread use more and more difficult. Tritium lume is still available from Tritec but under very strict regulations.
This lead to the research that developed the phosphorescent rare earth based compounds, again led by Nemoto.
Phosphorescent compounds
Phosphorescent compounds need activating or 'exciting' by an external energy source for them to glow. The first widely used phosphorescent pigment was Zinc Sulphide in the 1970's. Compared with the current compounds it's not very bright and it only glows for a short time.
Current compounds such as Luminova and SuperLuminova use Alkaline Earth Metal Aluminate compounds often just referred to as Strontium Aluminate, although that's just one of the many variations used.
Depending on the individual product, earth metals can include Strontium, Magnesium, Calcium, and Barium. Silicon and Titanium can also be present. The metal element is typically doped with Europium, the most reactive of the rare earth elements.
These compounds do still have drawbacks. zinc sulphide decays over time in natural light, and the strontium aluminate type products degrade rapidly in the presence of water or moisture. As well as the gradual decay, this also limits the range of binders or mixing medium that can be used. To overcome this, some are available with the individual particles encapsulated in a plastic coating.
A recent development is the use of Crystal Fluoaluminate Compound. These are very stable crystals of oxides and halides of alkaline-earth and aluminum, doped with rare-earth elements. These have the advantage of not being affected by water exposure.
Grain Size
One of the key factors in how well a compound works is the size of the individual particles or grains.
Smaller grains (<10 microns) are much easier to apply and give a smooth paint like finish but are less efficient.
Large grains (75-100 microns) glow like a torch and will hold a charge for up to 36 hrs.
So most manufacturers use varying blends of grain sizes to improve the performance. The larger particles improve the performance, while the smaller particles 'fill the gaps' between them, removing any 'dead space' and creating a smoother surface, so not as susceptible to micro shadowing reducing the effective charged area.
Coming next - The Practical - How to lume a watch.
This will be in a couple of parts; this one - The Theory and the following part - The Practical.
The Boring Bit - The Theory
Self-luminous compounds
Self-luminous compounds provide permanent legibility over a long period of time without any prior activation from an external light source.
The energy release of a radioactive isotopes decay is continuously converted into a weak light emission. With tritium for example, traces of a solid, tritiated polymer are coated on zinc sulphide crystals to act as a permanent energy source, only limited by the active life of the element.
The first widely used self-luminous compound used to provide a glow in the dark feature was radium (hence the 'Radiomir' from Panerai). This radioactive isotope was used to excite zinc sulphide crystals to make them glow in the dark.
Once the radioactivity dangers (radium has a half-life of 1600 years) - particularly to the workers applying the compound - were fully realised by the 1950's, new self-luminous compounds using either the radioactive isotope of hydrogen, tritium, or a man-made radioactive element promethium, were developed and totally replaced the use of Radium.
Promethium based lume was known as N-Lume after its developer Nemoto of Japan, but with a half-life of around three years, it's use was as short lived as it's effective life.
Tritium has a half-life of 12 years and this eventually became a major drawback, as it's ability to glow diminished visibly within a few years. Tritium is also used in nuclear fission and so the regulations that came in to control the import, export and storage of radioactive compounds made it's widespread use more and more difficult. Tritium lume is still available from Tritec but under very strict regulations.
This lead to the research that developed the phosphorescent rare earth based compounds, again led by Nemoto.
Phosphorescent compounds
Phosphorescent compounds need activating or 'exciting' by an external energy source for them to glow. The first widely used phosphorescent pigment was Zinc Sulphide in the 1970's. Compared with the current compounds it's not very bright and it only glows for a short time.
Current compounds such as Luminova and SuperLuminova use Alkaline Earth Metal Aluminate compounds often just referred to as Strontium Aluminate, although that's just one of the many variations used.
Depending on the individual product, earth metals can include Strontium, Magnesium, Calcium, and Barium. Silicon and Titanium can also be present. The metal element is typically doped with Europium, the most reactive of the rare earth elements.
These compounds do still have drawbacks. zinc sulphide decays over time in natural light, and the strontium aluminate type products degrade rapidly in the presence of water or moisture. As well as the gradual decay, this also limits the range of binders or mixing medium that can be used. To overcome this, some are available with the individual particles encapsulated in a plastic coating.
A recent development is the use of Crystal Fluoaluminate Compound. These are very stable crystals of oxides and halides of alkaline-earth and aluminum, doped with rare-earth elements. These have the advantage of not being affected by water exposure.
Grain Size
One of the key factors in how well a compound works is the size of the individual particles or grains.
Smaller grains (<10 microns) are much easier to apply and give a smooth paint like finish but are less efficient.
Large grains (75-100 microns) glow like a torch and will hold a charge for up to 36 hrs.
So most manufacturers use varying blends of grain sizes to improve the performance. The larger particles improve the performance, while the smaller particles 'fill the gaps' between them, removing any 'dead space' and creating a smoother surface, so not as susceptible to micro shadowing reducing the effective charged area.
Coming next - The Practical - How to lume a watch.
