The bulbs of conventional incandescent lamps are made of glass. However, these lamps are no longer in use in theatre (except in very old equipment) and tungsten-halogen lamps have taken their place.
Bulbs of tungsten-halogen lamps are smaller in size and are made of quartz glass, for a number of reasons. In order to effectively reduce the vaporization rate of the tungsten, the bulb is filled with gas at high pressures under which regular glass would break. Even if this were not the case and the glass could be fortified, the halogen cycle requires higher bulb temperatures than those of conventional incandescent lamps that would cause undesired vaporization of the gases enclosed in the bulb through the 'soft' glass.
Generally speaking, the optical quality of quartz glass bulbs is considerably lower than that of the blown bulbs used in conventional incandescent lamps.
Quartz
glass allows for bulb temperatures of up to 900°C, as its softening point is far above 1000°C, and pressures of up to 20 bar.
The shape of the glass
envelope
varies in accordance with the shape of the filament and the intended use of the lamp.
Quartz
glass is supplied to tungsten-halogen
lamp
manufacturers in the form of cylindrical tubes, the shape of which is more or less recognizable in the form of the finished bulb, especially in double-ended lamps.
In most single-ended lamps the cylindrical shape is pinched at the bottom where the lead-in wires are connected, and at the exhaust tube at the top. The lower the wattage, the smaller the
lamp
will be, leading to a greater deviation from a cylindrical bulb shape.
The exhaust tube tip-off is the point from which oxygen was removed from the bulb and other gasses inserted. In tungsten-halogen lamps the fill gas add inserted into the
envelope
must be brought to a considerable pressure before the bulb is hermetically sealed.
The first incandescent lamps had an exhaust point or 'belly button' at the top of the bulb, the point at which they were connected to the vacuum pump during manufacture. Later, this exhaust tube tip-off was hidden in the
lamp
base.
In tungsten-halogen lamps, the exhaust tube tip-off is clearly visible on the bulb. In double-ended lamps it is usually in the middle of the tube, in single-ended lamps opposite the pinch.
The pinch is the point at which the leads are hermetically sealed into the quartz or hard glass. The prefabricated filament and internal
lamp
structure is inserted into the bulb, the bottom end of the tube heated to well over 1000°C for quartz or 2200°C for hard glass, and then pinched flat with hard alloy jaws. The outer surface of the pinch is shaped to give it maximum mechanical strength, and to enable a base to be fitted on at a later stage.
