The exit sign has a very important role to fill in the event of an emergency. It must guide people to the nearest exit. In order to do this, it must meet certain criteria: first of all, it has to be visible. In situations which may require a quick evacuation it is possible for the power to go out or for the rooms to fill with smoke from a fire. In these cases, the exit signs must still be visible to the naked eye, otherwise it is pretty much useless. Apart from that, other factors which determine the success of a sign is the cost, its durability and its effect on the environment.
The first exit signs, made with an incandescent light bulb used for lighting, were not very good at any of these criteria. They were not very luminous, they were expensive and needed frequent maintenance and they went out in the event of a power failure. Understandably, a lot of changes needed to be made to the exit sign if it were to remain a successful component of the safety code.
Once technology allowed it, the old incandescent lighting exit signs were replaced by LED ones, which were superior in every way. However, these ones still needed electrical power, either from the main power supply of the facility or from a backup battery. They were a step in the right direction, but improvements could still be made. That is how the tritium exit signs were created.
Tritium is a radioactive isotope of hydrogen. However, compared to most other isotopes, it contains very little energy so, even if it is an unstable isotope, it is safe to people. The principle behind tritium exit signs is similar to the one behind old television sets. The tube inside the TV set was also powered by an isotope, only it was much stronger. Tritium does not need all that power since it only needs to light a sign.
In order to work, small amounts of tritium are placed inside a tube which is then coated with a layer of phosphorous on the inside. Since it is unstable, the tritium will decay over time and release electrons called beta particles. When these come in contact with the phosphorous coating they will produce light. This process is completely safe and it also eliminates the need for an electrical power supply or batteries as backups. The sign will remain lit in any circumstance.
Since tritium do not have a lot of energy, it has some limitations on its lighting power. Specifically, signs powered by it will not light up during the day when it is very bright outside. The reaction is still happening, but it is not strong enough to be perceived by the human eye. However, if the power happens to go out, the tritium exit signs will be immediately visible in the midst of the darkness. That reliability is what makes these tritium exit signs the most efficient one available on the market today.