Effect of Burning Position

GLS lamps are designed to be burned in the pendant position, with the cap upwards. When operated horizontally, gas convection currents cause local overheating of the upper portion of filament and this can have an adverse effect on life. In the pendant position, the filament also lies in the single-wire part of the pigtail loop in the supports. When operated in other positions, the filament may fall to the opposite side of the support loop where there are sometimes two parallel wires, and this can cause short-circuiting of filament coils with a subsequent reduction in life - particularly for the C-9 flat-wreath design in Figure I10. Short-circuiting of turns is less likely for the CC-9 pendant wreath design in Figure I1 where support wires are inclined at an angle to the filament.

The traditional pear-shaped glass bulb envelope is the result of many decades of scientific development, to realise a bulb shape which provides the longest lifetime and highest lumen maintenance. The fundamental shape provides adequate cooling of the heated gas without raising the temperature of the glass or cap cement beyond safe working limits. High wattage lamps may incorporate a metal or mica heat shield in the neck area to block the rise of hot gases, and keep the cap temperature within limits.

During operation tungsten is continuously evaporated from the filament and this is carried upwards by the hot gas and deposited on the bulb wall. In the pendant position the tungsten is deposited inside the neck area, where it has very little effect on the light output. Lumen outputs are always quoted for lamps burned in this orientation. For lamps operated cap down or horizontally, the evaporated material will darken the part of the bulb which is vertically above the filament and this can absorb considerable amounts of light. The area of blackening is illustrated in Figure I16.

In recent years there has been a trend to reduce the size of the standard 60mm GLS lamps to smaller 55 or 50mm diameter bulbs. Although this results in a smaller size with reduced packaging and transportation costs, it comes with the penalty of accelerated lumen depreciation because the evaporated tungsten is deposited over a smaller area, and forms a darker light-absorbing coating.

In vacuum type lamps there are no gas convection currents, and the glass bulb will darken over its entire surface, regardless of operating position. It will also blacken much more rapidly because there is no gas filling to reduce the rate of filament evaporation. However vacuum lamps are still intended primarily for pendant use because of the formation of the filament support wires.

Many commercial and domestic lighting fittings are designed to operate lamps upside down or sideways, mainly for compactness or aesthetic reasons. In these applications the slight reduction in lamp life and performance is considered to be acceptable and outweighed by the other advantages. The worst situation is to operate the candle and golfball-size lamps in the horizontal position because they are so small that the light output depreciates more rapidly, and the gas does not have sufficient circulation volume to be fully cooled. The bulb frequently also bears an internal decorative coating, which will release gaseous impurities when overheated. In such applications these lamps may achieve as little as half the rated lifetime.