Choice of Gas Type

Gases used for filling incandescent lamps must be chemically inert, i.e. they should not react with the hot filament (with the exception of halogen lamps). This means that they also need to be extremely pure, as very small amounts of some substances can have disastrous effects. For example, a single drop of water in a gas cylinder capable of filling half a million GLS lamps would cause every one of those lamps to blacken and fail early! The following table shows the gases that are inert to tungsten and make suitable fillings:

The molecular weight is the weight of one atom or molecule of the gas. The heavier atoms are more effective in reducing filament evaporation, making the lamp last longer. It can be seen that they also feature lower thermal conductivity, reducing gas losses so the filament will run hotter and produce light more efficiently.

Consequently for incandescent lamps it would appear that xenon would be the best choice of fill gas. But this is only present in very small quantities in the atmosphere, which makes it expensive. Krypton is about ten times cheaper but still too scarce, so we generally use argon which is roughly 500 times cheaper. However the first gas-filled lamps had to use nitrogen, because at the turn of the century there were no argon distillation factories in existence. When argon lamps were eventually introduced, it was found that its lower electrical ionisation potential resulted in problems of arcing between the lead wires. This was overcome by the admixture of a small amount of nitrogen, and an 85:15 argon:nitrogen mixture is common in European lamps. Lower voltage American lamps can get away with a 95:5 gas and thus are slightly more efficient. The fill pressure is about 0.8 atmospheres which normally rises to just above atmospheric pressure when the lamp is in operation.

By the 1930s the price of krypton had reduced considerably as new distillation plants were opened, and in 1936 Tungsram of Hungary put the first krypton-filled GLS lamp on the market. To reduce the required volume of this expensive gas, a new glass bulb shape was designed. The result was a lamp having a mushroom shaped envelope, which is still produced today. The Krypton Mushroom lamps usually offer a combination of about 5% more light and 50% longer life than ordinary argon-filled bulbs. Alternatively an energy-saving range of lamps producing the same light but with 10% lower wattage is available. It is also standard practice to employ krypton in high efficiency miniature lamps, because the benefits of krypton are much more pronounced for lower voltage types. Miners' cap lamps are an important application, since a smaller battery can provide the same light level. Even xenon is sometimes found in today's miniature halogen capsules where a very high efficacy or long life is called for, especially in automotive and projection lamps.

The only other gas which has been commercially employed in incandescent lamps is hydrogen. It is characterised by a very high thermal conductivity making it inefficient for general lighting, but it has been employed in special signal lamps because it cools the filament more rapidly between pulses. The faster nigrescence time permits higher speed signalling by light pulses. The table below shows the main applications of the available lamp filling gases: