emission wavelength. Some of the colors that have been manufactured in addition to the common phosphors are green, red, blue, yellow, purple, orange, and white. The GTLSs used in watches give off a small amount of light: Not enough to be seen in daylight, but visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. Being an unstable isotope with a half-life of 12.32 years, the rate of beta emissions decreases by half in that period. Additionally, phosphor degradation will cause the brightness of a tritium tube to drop by more than half in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10, 15, or 20 year useful life expectancies. The difference between the signs is how much tritium the manufacturer installs. The light produced by GTLSs varies in color and size. Green usually appears as the brightest color with a brightness as high as 2 cd/m2 and red appears the least bright. For comparison, most consumer desktop liquid crystal displays have luminances of 200 to 300 cd/m2. Sizes range from tiny tubes small enough to fit on the hand of a watch to ones the size of a pencil. Large tubes (5 mm diameter and up to 100 mm long) are usually only found in green, and can surprisingly be not as bright as the standard 22.5 mm × 3 mm sized tritium, this is due to the lower concentration and high cost of tritium; this smaller size is usually the brightest and is used mainly in keychains available commercially. Uses These light sources are most often seen as "permanent" illumination for the hands of wristwatches intended for diving, nighttime, or combat use. They are also used in glowing novelty keychains and in self-illuminated exit signs. They are favored by the military for applications where a power source may not be available, such as for instrument dials in aircraft, compasses, and sights for weapons. In the case of solid tritium light sources, the tritium replaces some of the hydrogen atoms in the paint, which also contains a phosphor such as zinc sulfide. Tritium lights or beta lights were formerly used in fishing lures. Some flashlights have slots for tritium vials