Tritium is a colourless gas otherwise known as extra-heavy hydrogen. The name originates from the Greek word ‘tritos’ meaning third, and refers to the three components of the atom (3H). Tritium has been used for decades in all sorts of applications where constant, independent and long-lasting light sources are essential. On the dials of military watches you will often find a red, circular symbol with the notation ‘3H’ that refers to the use of tritium. In ‘civil’ watches, the abbreviation ‘T25’ denotes the same thing. In the past, luminous tritium was applied directly to the dial. Today’s watchmakers are more careful and fill the gas into fine tubes made from borosilicate glass, a highly resilient and ISO-certified glass used in chemical engineering. These Gaseous Tritium Light Sources (or GTLS) are not only exceptionally safe, they also guarantee the watch wearer at least ten years’ constant luminance – without any external energy source.