Vacuum vs Gas Filled Lamps

There are some lamp types for which gas filling may actually reduce performance, or offers no advantage at all. For example in very low wattage lamps, the filament is so fine that even when coiled it is still quite thin and has a large surface area to volume ratio, thus heat losses to the gas will always remain high. It is not possible to coil such thin wires around a larger diameter mandrel and correct this surface area to volume ratio, due to practical constraints in relation to the dimensional stability of such coils. When the mandrel ratio is too great the filament will suffer sag problems.

Consequently an assessment must be made as to whether or not a given lamp design will actually benefit from being gas-filled or not, and in most cases the low wattage versions are produced as vacuum types. The filament is still coiled, but this is done purely to reduce its length and to make the filament easier to mount on the stem assembly. Even coiled-coil vacuum types are in existence for the same reasons. Naturally in vacuum the coiling has no effect whatsoever, since there is no gas flow around the filament and no heat loss to the internal atmosphere.

Because tungsten evaporation is unrestrained in vacuum lamps, the filament must be run at a lower temperature of around 2100°C so that the standard life of 1000 hours can be maintained. Vacuum lamps are thus less efficient than gas-filled types. In practical terms, the changeover point at which it makes sense to switch from gas-filled to vacuum types occurs at about 25 Watts. For miniature lamps, whose filaments are made from thicker wire on account of the low voltage operation, the changeover is at about 3 Watts.

Incandescent striplights are also vacuum types, even in the larger ratings of 150W. This is simply because the filament must extend over the whole length of the luminous tube and therefore cannot be tightly coiled. If such lamps were gas filled the heat losses over such a long filament would be very high. It is clear from the photograph in Figure I13 that the most efficient coils in a gas filling are those which are tightly coiled, and the filaments in striplight lamps are so long that they can never be efficient in a gas atmosphere.

Many single ended tubular lamps also require long filaments, and these must be vacuum types as well. If the filament is long and it is intended for vertical burning, the problem of gas convection currents will return. The hot gases will rise to the upper end of the bulb, overheating one end of the filament and leading to premature failure.

Before coming to a final decision about whether a particular lamp design should be gas-filled or vacuum to deliver optimum performance, consideration must also be given to the application. One of the principal applications of coloured GLS lamps is in festive outdoor illumination, and these are also made with vacuum atmosphere even for wattages that would generally be gas-filled. It has already been seen that the bulb wall temperature is much higher for gas-filled lamps, due to the conduction and convection of heat from the filament to the bulb wall. When coupled with coloured coatings still more heat is absorbed at the bulb wall, and its temperature can become very high. Since these lamps are used outdoors, incident rain or snow will be encountered and it is likely to lead to thermal shock of the bulb envelope with the consequence of glass cracks. Such lamps must therefore be made with vacuum simply in order to keep the glass sufficiently cool.