An overview of Luminous watch dials.
There are a lot of questions about luminous watch dials and hands, what works best, how “Traser” tubes work how Superluminous paint works and how long you should expect a watch to remain legible in the dark.
There are three basic options, if you want to be able to read your watch easily at night:
- Some form of backlight, activated by pushing a button, or constantly active. An example of this is Timex “Indiglo” watches.
- Some form of Tritium material, usually in the form of paint containing radioactive tritium, or small glass tubes, filled with radioactive tritium gas.
- Some form of non-radioactive, luminous, or “Superluminous” paint. This works by a special phosphor-based paint, which absorbs light to “charge”, then glows for a period of time in the dark.
Each of the above options has advantages and disadvantages.
Backlight systems like Timex Indiglo have improved a lot and generally work very well. In an Indiglo watch, a very thin panel uses high voltage to energize phosphor atoms that produce light. The panel itself is extremely simple. As described in the Timex patent, you take a thin glass or plastic layer, coat it with a clear conductor, coat that with a very thin layer of phosphor, coat the phosphor with a thin plastic and then add another electrode. Essentially, what you have is two conductors (a capacitor) with phosphor in between. When you apply 100 to 200 volts AC (alternating current) to the conductors, the phosphor energizes and begins emitting photons.
Creating the high voltage can be a problem in a wristwatch. The watch has only a small 1.5-volt battery. To produce the 100 to 200 volts, the 1:100 transformer is used. By charging the primary coil of the transformer with a transistor that is switching on and off, the secondary coil rises to 150 volts or so. (HowStuffWorks) So this system or similar systems work well, but typically need to be activated by pushing a “light” button, so you need two hands to turn on the light. The Electroluminescent panel also makes the watch a little thicker and adds a level of complication. It also requires some compromise with the dial design, as the dial needs to be semi-transparent for it the panel to light up the whole dial.
Radioactive Radium or Tritium-based luminous: Radium was used in older dials from the 1950’s and 1960’s, but it was fairly unstable and would lose its photoluminescence after a number of years, then decay and release radon (a dangerous carcinogenic gas). More modern systems use Tritium filled tubes, which glow because they contain radioactive gas: the radioactive gas will typically have a half-life of 20-30 years. Thus, the advantage of any tritium dials is that they do not need to be exposed to a bright light to be “charged”: they just glow. This technology has been tested and deemed to be quite safe, as the amounts of radioactive material are generally small. On the other hand, the NRC (Nuclear Regulatory Commission) in the USA will not allow anyone to import tritium-based dials, or any watches that use tritium paint or gas, without a special NRC permit to import and handle radioactive materials. This is a serious issue, and the permit is costly. Watch companies that use tritium and skip the permit can end up paying heavy fines. (e.g. NRC levies fine against watch company).
Of course, the big advantage of tritium systems is that they need no “charging”, so they will always glow and be legible in the dark, for a virtually unlimited number of hours. The disadvantage is that, due to the relatively high cost of the radioactive material (or gas-filled tubes) and the cost of special NRC licenses and special handling, you tend to pay quite a bit more for a watch with a real tritium dial. Or sometimes, the cost of the tritium dial is partly covered by cutting down on other features (case, crystal, band, etc.), when compared to a non-tritium model in the same price range. Of course, many customers simply prefer not to have any kind of radioactive tritium product on their wrist, even if it is just a small amount of tritium and even if it has been declared to be safe.
- Superluminous paints: Luminous paints have come a long way over the last several years. These paints are effective and completely non-radioactive, which is why they are what we choose for most Momentum models. There are many different suppliers of luminous and superluminous paints, but the two best known are SuperLuminova from RC Tritec of Switzerland and Lumi Brite, which is the superluminous paint used by Seiko in Japan. The exact chemical composition of luminous compounds varies from one supplier to another, and is a closely guarded trade secret for each of them, but these luminous paints are based on the same principle: the luminous paint contains a “phosphor”, like Zinc Sulfide, or Calcium Sulfide. This works like a light storage battery, where light charges the battery and afterwards the light is continuously emitted. This activation and subsequent light emission process can be repeated again and again, and the material does not typically suffer any ageing, unless it is made wet or damaged.
Momentum watches use different qualities of luminous, depending on the watch model and target price range. This means that we use a basic grade of superluminous paint on a watch costing $125 but, on a watch costing $500+, we will use a top-grade luminous. This is because the best superluminous material is very expensive and can drive up the price of a watch significantly, so we need to balance performance and value for money.
Momentum does have a few dressier models with little or no luminous on the dials, but most models are designed for legibility in the dark. We typically divide our luminous models into 3 broad categories:
- Basic Grade. A good, basic luminous. Typically, legible for 1-2 hours in the dark, after exposure to a bright light.
- Medium-Grade. A good blend of price and performance. Typically, legible for 2-4+ hours in the dark, after exposure to a bright light.
- Top-Grade. (SuperLuminova or Lumi-Brite) The best luminous. Typically, legible for 5-8+ hours in the dark, after exposure to a bright light.
In knowing what to expect from any luminous watch, there are three additional points to note:
First: the performance of any luminous or superluminous paint is not just a function of the grade of luminous material used, but also depends on the dial-maker or hand-maker and their expertise. For example, a dialmaker that applies several layers of the same high-grade luminous material will offer significantly better luminous performance than a dialmaker who only applies one or two layers of the same quality material. A good dialmaker who applies a white reflective paint under the superluminous material, will achieve better performance than a dialmaker who applies the superluminous directly onto a matte black dial. Even the quality of the varnish used to mix with the luminous material and apply it can make a noticeable difference.
Second: The amount of material applied can make a big difference, so a watch with large, luminous numbers (or indexes) will have more lume material and glow brighter and longer than a dial with fine luminous lines, or small luminous dots, no matter which quality of luminous paint is used.
Third: Non-radioactive luminous watches need to have the lume charged, by exposure to light. The stronger the light, the brighter and longer your watch will “glow” in the dark. If the watch has been under your sleeve for a few hours before you try to read it in the dark, the lume will not be “charged”, and the watch will not be legible in the dark. If your watch has been in direct sunlight, it will be extremely legible.
Finally, it should be noted that a really well-made superluminous dial, using the best material, will glow and remain legible through a long night. If charged in a bright light, good superluminous dials are initially a lot brighter and more legible than dials using tritium tubes, even though the Superlum will fade gradually in the dark and the tritium-based lume will not. For most people, quickly shining a bright light or flashlight on a good superluminous dial that is “fading”, will recharge it almost instantly and make it easily legible again in the dark.
Please contact us at firstname.lastname@example.org if you have any questions not answered above.