When a discharge lamp is cold, it cannot emit all of a sudden the 100% of its power like a normal incandescent lamp, because the temperature and the pressure inside the bulb require a period of heating to catch up the operating levels. According to the type of lamp, such period can last from some seconds to 5 minutes.
For this reason, if the AC power is cut during the lamp’s ignition, the pressure inside the bulb prevents the relighting of the lamp. Before being able to relight the lamp, it is necessary to wait 5 to 10 minutes, to allow the lamp’s cooling off and the crystallization of the metallic salts contained in the bulb. Relighting a lamp still warm can require a period of time still longer than the period needed to relight a completely cold lamp. The greater stress for discharge lamps takes place when the lamp is relit while still warm.
All the discharge lamp bulbs lose strength over time, because of thermal stress and mechanical vibrations. Over time, the lamp loses color, absorbs light and therefore heat. The bulb will continue to weaken till breaking off. Since a discharge lamp contains high-pressured gas, the bulb’s break is inevitably a violent event. For this reason in most cases a discharge lamp bulb is contained inside another bulb, for safety reasons. Although such breaks are normally associated with the lamp’s end of life, they can very rarely happen also when the bulb is new, because of micro-fractures in the glass.
However some precautions can be taken to reduce the risk of lamp’s break:
• avoid using lamps unbranded or of uncertain origin.
• before installing, check if the lamps has micro-fractures in the arc tube or in the external bulb.
• VERY IMPORTANT: replace the lamp before it reaches its end of life. In particular, HMI lamps should not be used beyond half of their life, because they can explode violently at the end of their life.
• avoid moving the projector or letting it fall to the ground when the lamp is hot: the gas within are under pressure and pressure increases as the temperature increases. Therefore letting fall the projector to the ground or shaking it excessively while it is hot could make the lamp explode.
Fingerprints can damage the quartz bulb, especially when it is hot, because the skin fat that remain on the bulb after this has been touched by bare hands, can burn once the lamp has been ignited, rendering the bulb more fragile in the spot where it has been touched, which in turn can lead to breaks. Since the skin fat attracts heat and can cause a weak point on the bulb, we recommend not to touch the bulb with your bare hands, using instead a clean cloth or holding the lamp by its ceramic base. If the bulb has been touched with your fingers, clean it up with an alcohol-soaked cloth. Some lamp models have two bulbs, one inside the other. Only the inner bulb actually contains the electrodes and the gas. The inner bulb is always made of quartz glass, while the outer bulb is made of common glass, and so it can be touched with your fingers. It is necessary to read with attention the producer documentation and warnings: if you are not sure that the outer bulb is made of common glass, it could be made of quartz glass too, so avoid touching it with your fingers.
HMI lamps, containing mercury vapors, generate UV light. You’d better not to watch directly towards the lamp, to prevent damages to the retina.