Have you ever wondered why some watches glow in the dark. Well, this is all thanks to what we refer to as the lume and over many years watchmakers have employed a variety of different materials to make a watch glow – some of which are rather dangerous! Lume is used most in the hands, dial, numerals, and hour markers but isn’t exclusive to these. Lume doesn’t last forever either, the duration of the lume effect depends on the type of chemical used and also the amount used. It can be made up of a single layer or multiple layers which allow the watch to sign brighter and for longer.
It was in the first world war that the demand for a luminescent wristwatch really kicked off. The soldiers of World War One needed to read the time quickly and efficiently and a pocket watch just wouldn’t suffice. Obviously using a flashlight was a sure give-away of location or immediate draw to attention for any soldier on the frontline and this is where watch luminescence really came into its own allowing them to read their watch efficiently and subtly. Since then, the demand for watch luminescence has grown and varied from field watches to dive watches.
In the earlier years when lume technology was consider cutting edge technology and radium was used which is hazardous to both the wearer and the watchmakers who were painting the substance on the dials. However, the risks weren’t well known at the time, and it soon became an appealing job that demanded artistic ability. Applied with a precision-brush workers had very little protection against this radioactive material. Following a lawsuit filed by a group of women dubbed the ‘Radium Girls’ after many of them fell ill and died from the exposure to radium its use was drastically reduced and finally banned in 1968.
This is when we saw the introduction of Tritium which is still sometimes used today and whilst it is still radioactive it was considered a much safer option and was the main option for lume up until the 1990’s until it was banned from use (when painted on) in 1998.
Today’s Watch Lume: Photoluminescence
Today the most common form of watch lume is made up of pigments based on photoluminescent material. This is a non-radioactive material that, similarly to radium, is painted onto the hand and dial and glows in the dark by absorbing and re-emitting light. The main issue with this material is the longevity. The glow generally only lasts for around 7 hours when there is an absence of light. However, saying this that is more than enough for most of us!
Super-Luminova® is an applied fluorescent material that is non-radioactive highly temperature resistant and resistant to many environmental influences. It works similarly to the way a light storage battery would work and stores energy and continuously emits light. The material does not age and will emit light again and again. Swiss Super-Luminova® is available is 8 different colours and these all are interpreted by the human eye differently.
There are 3 different grades of Super-Luminova®. Standard grade was the original and since then Grade A and Grade X1 have been developed. Grade X1 defines a new standard of visibility and is seen on many high-end watches today. The different colour codes for Super-Luminova® are as follows:
C3 – Yellow with the brightest glow
BGW9 – Light bluish white with about 95% glow of C3
C1 – White with about 31% brightness of C3
C5 – Greenish yellow with 89% brightness
C7 – Green with 84% brightness
C9 – Bluish green with about 83% brightness
NTH DevilRay Automatic Dive Watch. Image Credit: WatchGecko
Despite being banned in 1998 Tritium has made a come-back. Tritium was banned from being used as paint due to its radioactive qualities and when painted on there was no protection offered to the watchmaker or wearer. However, Tritium has made a strong come-back in recent years. It is now created in a gas form which is them encapsulated in glass internally coated in a phosphor layer. The tritium gas releases electrons when decaying and this causes the phosphor to glow. Though it is still reactive when safely enclosed in glass it offers little-to-no risk. A handful of watchmakers such as Luminox, Traser, Marathon and UK-based brand NITE Watches are known to use Tritium today and it is also a technical requirement used by the military.
The NITE Alpha. Image Credit: WatchGecko
Most found in battery-powered digital watches electroluminescence produces light due to an electric current passing through the phosphor which coats the crystal of the watch. This is done at a press of a button that releases an electric current triggering the phosphor to react.
Casio SGW-300H-1AV. Image Credit: WatchGecko