Night Time Performance
Night performance varies dramatically depending on the luminous material used. The lowest glow which can be seen by a human eye acclimatized to the dark is 3.2 nCd/mm2 or 3.2 nanocandela per square millimeter.
In order for watches to be readable in the dark a luminescent material must be applied to the dial and hands. There are two main routes to emit the light namely photoluminescence which is created by a substance which stores and then slowly releases the stored light or radioluminescence in the form of tritium paint or gaseous encapsulated tubes which do not need charging and where the light is provided by the luminous material itself. In this case, the use of radioactive material is strictly defined by ISO 3157 Standard which allows only two types of radionucleides: tritium (3H) and promethium (147 Pm). Both of these substances are classified as low yield.
The ISO standards:
A luminous dial with 11 numerals glow with an intensity of 36nCd. Its hands must be at least 10nCd.
The ISO 3157 Standard (tritium (3H) and promethium (147 Pm) ) allows an optional marking for timepieces emitting less than a certain value. The marking may be made on the dial as follows :
1. Tritium : T (sometimes also at the bottom of the dial you will see T 25)
2. Promethium : Pm or sometimes just P (sometimes also at the bottom of the dial you will see Pm 0,5)
The marking "T Swiss made T" means that the watch is Swiss made and contains a specific quantity of tritium which emits less than 227 MBq (7,5 mCi).The indication "Swiss T<25" means that the watch is also Swiss made and contains a specific quantity of tritium which emits less than 925 MBq (25 mCi).
Photoluminescence / Phosphorescence
In the case of photoluminescence, the energy is supplied by electromagnetic radiation (e.g. light). Regardless of whether the material is Luminova, Lumibrite or another substance all photoluminescent materials absorbs light for a significant period of time, then emits the light with a lower frequency than that of the light absorbed originally.
The main disadvantage of photoluminescent materials in watches is that the luminescence diminishes rapidly and sometimes totally disappears after a few hours although in my experience, Breitling, MWC, CWC and Seiko seem to hold up well the only brand where I found the performance is extremely poor was with my Rolex Deepsea, Having said that the Breitling cockpit B50 seems vastly superior to a Breitling emergency I owned previously so I think it's a bit hit and miss. The Breitling top it is still visible at five or 6 am this was never achieved with the Rolex or with the other Breitling model which I had before. Much depends as far as performance is concerned on the amount of paint applied but the one thing which you will always tend to notice with paints such as Luminova and SuperLuminova is that initially they are much brighter than Tritium or GTLS. The drawback is that they will not come close to the performance of a radioactive substance or GTLS after a few hours have gone by.
People often refer to the most common paints which are Luminova and SuperLuminova and often assume SuperLuminova is superior to standard Luminova but this is not the case, the terms Luminova and SuperLuminova are in fact brand names for a photoluminescent material. Unlike tritium, SuperLuminova or Luminova both needs to be charged by light to give an afterglow, thus illuminating the face of a watch in the dark. The history of Luminova is interesting, the paint was developed in Japan at the beginning of WW2 by a Japanese businessman called Kenzo Nemoto it was created for use in military aircraft, submarines and battleships. After the war Nemoto founded Nemoto & Co to continue marketing the product and saw the potential for the watch industry which at the time used mostly radioactive paints such as radium or tritium, of course radioactive paints are a complete no-go area now unless they are encapsulated in GTLS glass vials. Nemoto marketed the paint exclusively themselves to the watch industry until 1993 at which point a licencing agree was signed with a Swiss company and RC Tritec AG was founded as a Swiss based company that still holds the licensing rights to produce and distribute LumiNova under the registered name SuperLuminova. Many watch companies today often use both Luminova and SuperLuminova because some of the colours available from company are not available from the other.
Radioluminescence / Autoluminescence / Radium
Radioluminescence is often also referred to as Self Luminous and can glow without any exposure to light and it is produced by radioactivity. This material in paint form was used for around 90 years and has the advantage that it will perform for years will no deterioration in performance over the course of a night.
Although radioluminescent materials can offer excellent performance they are now banned because of the potential dangers that arise because of the radioactivity, The only exception being encapsulated tritium which is referred to as GTLS and is contained within glass vials. There is a very interesting story which relates to girls who used to paint the dials and would frequently paint their nails and teeth not realising that the radioactive material was potentially lethal the full story can be seen here Radium Girls
Tritium is also a radioactive isotope and it rapidly replaced radium in watches after World War II
Although Tritium is also radioactive the radioactivity is composed entirely of Beta particles that are unable to escape through the glass. The tritium in use today complies with the ISO 3157 and NIHS 97-10, which define the acceptable minimum level which will enable the watch to perform well in the dark. Although Tritium is sometimes still used it is potentially dangerous. The main problem arises during service and repair when dust inside the case can escape or the watchmaker might touch the hands or the dial.
Self-Powered Micro Gas Lights (3H) GTLS
The successor to Tritium paint is the self-powered illumination system often called self luminous or GTLS referred to above. Basically each watch employs tiny self-powered micro gas lights developed by MB Microtech in Switzerland who are still the leaders in this technology. These micro gas lights (borosilicate glass capsules) are fitted to the hands, hour markers, and when necessary, in the case of divers watches to the bezels. The advantage of these tubes is that they need need no external light sources to "charge" the paint as in the gase of paints like Luminova. The US military has used these tubes since the 1990s and they are covered by procurement specification MIL-W-6374F. an example of a typical tritium GTLS watch can be seen at the top of the page on the right.
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