Night Time Performance
Night performance varies dramatically depending on the 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).
The RAF Seiko Chronograph below has Promethium markings
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 anonther 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 totally disappears after a few hours.
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. Below is a typical military watch from the 1950s, in this case an RAF Omega which at the time like all military watches used radioactive paint.
IWC 1950s Watch With Radioactive Paint
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. Below is a CWC G10 with Tritium paint (see encircled T) which can be obtained from Anchor Surplus (Used) for GBP59.95 or Silvermans / CWC new for GBP89.99
Self-Powered Micro Gas Lights (3H) GTLS
The successor to Tritium paint is the self-powered illumination system often called self luminous or GTLS. 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 for over 290 years and they are covered by procurement specification MIL-W-6374F.
Super-LumiNova was first made around 50 years ago. This is a safe non-radioactive material which is initially much brighter than Tritium or GTLS. The drawback is that it will not come close to the performance of a radioactive substance after an hour or two has gone by. Keep in mind that if the material is activated by artificial light, the brightness is often only 1/10 as bright as when activated by sunlight and this can make charging potentially problematic. Even in bright sunlight it can take an hour to reach a full charge.