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.