The History of Luminous Paint in Wristwatches - The Light and Dark of Luminous Technology
1. The Origin of Luminescence - The Age of Radium
■ Application to clocks (1910s onwards)
Radium + zinc sulfide - Semi-permanently glowing - Explosive popularity in military watches
During World War I, visibility was vital for soldiers who often operated at night. It is often said that the religious revolution led to the spread of wristwatches around the world, but the classic story of wristwatches is that they were born in war and then made their way to peace.
First, the radium luminous material on the watch spreads all at once. ( Older watches that use radium (mainly from the 1910s to the 1950s) often do not have any distinctive markings or inscriptions, so the only way to identify them is by measuring their radiation levels or by looking for a reaction that "does not glow (or rapidly loses its light)" when exposed to a black light. The "T" at the 6 o'clock position, also known as the Swiss Tritium mark, indicates the later tritium (1960s to 1990s), and is therefore distinguished from radium.
・Production to be discontinued
Due to the dangers of radium, tritium (³H) was adopted as the next generation. A female worker who was working on painting dials in the United States was forced to lick the tips of the brushes, which caused her to develop osteonecrosis of the jaw and cancer. This became a social issue, and the dangers of radiation became widely known, leading to a temporary halt in production.
2. Transition to tritium (1960s onwards)
Features
- Emits only beta rays (weak penetrating power)
Half-life: Approximately 12.3 years. Light intensity gradually decreases.
Dialing example:
T SWISS T (also known as Swiss Tritium)
T < 25
・Even if the tritium luminous paint reaches the end of its life and stops glowing, the watch itself can still be used. However, nighttime visibility will be impaired. Repainting or a replacement of the dial may be necessary. Furthermore, the amount of radiation is considered to be so small that it has no effect on the human body. Although the risk of external exposure was low, internal ingestion remained dangerous.
Many luxury watches from the 1970s to the 1990s used tritium. Due to the dangers of radium, tritium (³H) was adopted as the next generation. The luminous paint (self-luminous) in watches using tritium typically lasts for around 10 to 12 years. Because it has a half-life of around 12 years, after this period, its luminous ability significantly decreases and it no longer glows. Older models manufactured up until the late 1990s, in particular, barely glow at all today. In the antique market, "tritium tarnish (cream)" increases vintage value.
・Production to be discontinued again
Currently, its use is restricted in many countries. It was such an essential feature for watches that some watches even had the "Wiss Made" label (with the "T" disappearing). However, it is true that it uses radioactive materials, just like radium. It was discontinued because safety could not be ensured during the manufacturing process.
3. The 1990s: Revolution - The birth of Luminova
It has become a safe technology used in most modern watches. Now, let's take a closer look at why watches glow. Now that it's finally safe to use, let's take a closer look at why watches glow.
Unlike previous radioactive materials, Luminova is not a "paint" but rather a "powder."
Luminova is basically a strontium aluminate-based phosphorescent (long-lasting) pigment, and the long-lasting glow characteristic is created by dopants such as europium (Eu) and dysprosium (Dy).
In other words, the main body is a "glowing powder" that only becomes "paint (binder + solvent + powder)" when it is placed on the watch.
Luminova is basically a strontium aluminate-based phosphorescent (long-lasting) pigment, and the long-lasting glow characteristic is created by dopants such as europium (Eu) and dysprosium (Dy).
(Not strictly radioactive: important)
In modern phosphorescent pigments, electrons inside the pigment are excited by light, trapped in defect levels (traps), and then released as light as they return over time. Essentially, it's not a "battery that stores light," but rather a "trap release within a solid." These two characteristics (initial brightness = "shallow traps" emit all at once, long afterglow = "deep traps" emit gradually) are difficult to achieve at the same time, and their properties change depending on the formulation and particle size (which is why the same color number can look different).
The color numbering system is a feature of Luminova, which means it can produce a variety of colors, which was a real revolution in the watch industry.
The structural characteristics of Luminova vary depending on the watch factory, but it is divided into four basic structures.
1. Base (primer/white base)
2. Luminous layer (LumiNova powder + transparent binder)
3. Color adjustment layer (pigments and dyes) Aged, light green, ivory, etc.
4. Top coat (gloss, abrasion resistance, moisture resistance)
Why does the brightness differ even with the same "BGW9"?
*1BGW9 is a high-performance luminous paint (luminous paint) under the "Super-LumiNova®" brand manufactured by RC TRITEC of Switzerland. It is widely used on watch dials, hands, and bezels, and is characterized by its white glow during the day and blue glow in the dark.
The color number (C3 / BGW9 / C1, etc.) indicates the "luminous color, daylight color, and relative brightness tendency," but the luminous effect of the watch varies greatly depending on the manufacturing process. For example, it is commonly said that BGW9 has a whitish daylight color with a blue glow, but the actual product will differ depending on the "paint."
Why are the colors different? Why are the brightnesses different even with the same "BGW9 *1"? Factors that cause the difference (from a field perspective):
Film thickness: The thicker the film, the brighter it will be and the longer it will last (but there will be unevenness).
Particle size: Large particles = bright but prone to graininess / Small particles = smooth but prone to darkening Binder transparency: Yellowing makes both daytime and luminous colors cloudy Coloring (aging): The stronger the coloring, the more likely it is that "the color is beautiful in the daytime but is a loss at night" Reflection of the base: The experience changes depending on whether or not there is a white base
Key Features of the BGW9 Appearance: In bright light, it appears pure white, giving a modern and clean impression.
Luminous color: Glows a vibrant light blue in the dark.
Brightness: It has approximately 95% of the brightness of Super-LumiNova, second only to C3 (yellowish green), which boasts the highest brightness.
Visibility: It has excellent luminous properties for long periods of time in dark places, and is particularly used in watches that require high practicality, such as diver's watches.
3-1. Luminova's "charging conditions" and "decay curve" - How long will the luminescence last?
In theory, LumiNova (Super-LumiNova) is phosphorescent, so the "rise time" will change depending on the conditions of exposure to light (wavelength, illuminance, time), and it will then always darken according to a decay curve.
1) Charging conditions: What is the difference between sunlight, LED, and UV?
A. Sunlight (strongest. Outdoors is a special case)
Sunlight provides both strong illumination and a wide wavelength range (ultraviolet to visible). Luminova can be excited by either sunlight or artificial light, meaning it has a wide wavelength tolerance.
Even a few minutes of direct sunlight or being near a window will cause the dial to "quickly approach maximum brightness." However, it should be noted that in the long term, factors such as ultraviolet rays, heat, and humidity may contribute to deterioration of the entire dial (although this will not last forever).
B. LED lighting (indoor "normal charging")
The spectrum of LEDs varies depending on the product (strong blue peak, weak red, etc.).
The explanation goes something like this: "The luminous material is charged by the UV component of the light source, and the amount of UV varies depending on the light source. LEDs may have less UV." If you only use LEDs, it's easy for the phenomenon to occur where "even if you think you've fully charged it, it won't last."
If you're happy with just the natural glow you get from everyday life, then LEDs will suffice, but if your goal is night photography or visibility in the dark, then UV, as discussed below, will work.
C. UV light (easy to reach peak in a short time)
Luminova is known to be well excited by UV light, and RC Tritec's measurement process also introduces an example of charging it with UV light for a certain period of time and measuring the afterglow.
3-2.So how can you make Luminova last longer?
If you want a strong glow, UV is the most effective!
However, prolonged exposure to UV light at close range can be damaging to materials, printing, and resins, so for watches, a short exposure time and an appropriate distance is more practical (don't overdo it).
Decay curve: Luminova "falls off at first, then holds on"
Roughly speaking, the decay is as follows: Immediately after the light is turned off, for a few minutes: The light drops sharply (the initial brightness fades).
After that: Gradually declines (the key is the persistence of the afterglow).
This "sudden drop → slow drop" is measured as a typical "afterglow decay curve" of a phosphorescent material (following the intensity as a time-dependent curve immediately after UV irradiation is stopped).
The curve changes depending on the "grade" (there are also grades of luminous material).
Super-LumiNova is categorized so that the initial brightness does not vary significantly depending on the grade, but the rate at which it decays differs (i.e., how long it lasts in the dark).
RC Tritec also boasts improved afterglow performance in its higher grades (e.g., X2).
Color and tinting change the curve
There is also an explanation to the effect that the more strongly colored the type, the faster the light intensity will decrease.
(In other words, there tends to be a trade-off between a beautiful aged color and the strongest luminescence.)
- The most realistic luminous watch charging
Simple protocol (for comparison)
Condition A: 10 minutes under indoor LED light. Condition B: 5 minutes under natural light by a window. Condition C: 30-60 seconds under UV light (weak to medium).
Immediately after lights out
10 minutes later
Photograph or visually evaluate at three points after 60 minutes (try this experiment with your own watch when you have a lot of free time).
Thank you for listening to this tale of luminous watches, a nerdy subject.
The rate at which the glow will decrease varies depending on the light source used by the watch, in the following order: sunlight > LED > UV light.
In the store, you can experience the glow and duration of the luminous effect of the actual watch by shining LED lights or black light (ultraviolet light) on it.
Even though they are all called "luminous clocks," there are types that glow brightly, types that glow softly and for a long time, types that glow green, types that glow blue, etc.
Each watch has its own unique way of illuminating. How it appears the moment you move from a bright place to a dark place. How it remains after 10 minutes and 30 minutes. You can see the differences that aren't apparent in photographs with your own eyes. Luminous light isn't just a function; it's an important element that reflects the watch's usage and design philosophy. Experience the beauty that emerges in the dark. While vintage watches have become more secure, it could be said that we're now in an era where wristwatches can be enjoyed even more. New watch brands have also recently emerged that allow you to tell the time using luminous light.
Comptoine, an authorized watch and jewelry dealer
Comptoine, an authorized watch and jewelry dealer, is an authorized watch and jewelry specialty store located in the Hatsudai/Yoyogi area.
We carry a wide range of products, from independent brands to mechanical watches and jewelry.
We also provide after-sales services such as battery replacement and strap adjustment.
Please feel free to ask us for a comparison experience of luminous materials.
CONTÉVANOU
Comptoine, an authorized watch and jewelry dealer
Nishi-sando Terrace E1, 28-7 Yoyogi 4-chome, Shibuya-ku, Tokyo 151-0053
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