Unless you live in certain parts of the Artic, it will get dark almost every day of your life. During these dark hours, time continues to move on, and conventional watch hands remain unseen.
In order to be able to use your watch when the lights go out, whether in the field or not, finding the proper lume is a must. While some watches and watch manufacturers actually use built-in lights (Think Timex Indiglo) these solutions only work in battery powered quartz models. This is ofter called the "electrical" variety of lighting your watch. If you own a mechanical watch, you are still "left in the dark."
For mechanical, and most analog watches there are essentially two solutions: luminous paint applied to the hands, dial, and bezel, or in some cases, tiny vials of radioactive compounds or "gas".
In regards to luminous paint, there are a number of different companies supplying the paint, and varying levels of quality. On our Atlantis, for example, Smith & Bradley specifies uses high quality Phosphorescent paint. All luminous paints are essentially photo luminescent compound that absorbs light and then gives it off over a period of time. The paint absorbs the light and "stores" the light causing it to illuminate in the dark.
In regards to luminous paint, the quality of the paint, application, and the thickness of application (among other things) all have an effect on the resulting product. Therefore, after "charging" the luminous paint in sunlight or under a synthetic ultra violet light, certain watches will stay illuminated for varying periods of time. All painted hands and dials, however, will only stay lit for a limited amount of time with well-lummed varieties lasting nearly 12 hours and lower-end lummed watches lasting only minutes.
Finally, some watches use radioactive compounds on the watch dial and hands to illuminate the watch. Thesesubstances are called radio-luminescent. These compounds have isotopes selected are typically strong emitters of fast electrons (beta radiation), preferred since this radiation will not penetrate an enclosure. Radioluminescent paints will glow without exposure to light until the radioactive isotope has decayed (or the phosphor degrades), which may be many years. They are therefore sometimes referred to as "self-luminous". Watches that fall into this category can keep their lume ofter times for many years.
Initially, these radio-luminescent compounds were hand painted onto watches. This practice was in use in the early part of the 20th Century until the 1940's. The initial substances used was Radium 226. The factory girls that handled the Radium grew very ill in the 1920's and 1930's and the practice was discontinued.
Now the primary radioactive substance is tritium. In order to prevent a re-occurrence of the “Radium Girls “ of the 1930's, the tritium is stored in small glass vials that are place into the dial.
Smith & Bradley will be introducing tritium into its forthcoming field watch in May of 2013. Keep checking Facebook and www.smithbradleyltd.com for details.