The Science Behind Projector Lamps
High Intensity Discharge
A High Intensity Discharge Projector lamps are technically called High Intensity Discharge (HID) lamps because they produce light from a relatively small intense electrical discharge (compared with the larger discharge in a fluorescent lamp).
A natural phenomenon
We have all seen the technology involved in discharge lighting occur in the natural world. When the high voltage that builds up in storm clouds discharges itself through the air to the ground, the electrical current rushes through the atmosphere, exciting nitrogen atoms along the way. And the result? Lightning.
You may remember from high school chemistry lessons that when ordinary salt is sprinkled on a candle, a bright orange flame is produced. The intense heat excites the sodium atoms to give out this characteristic coloured light. When scientist first saw this, they started to investigate the ability of other metals to produce visible light. Mercury was one of these metals, and it was found suitable for high intensity discharge lighting due to its high vapor pressure and increased emission in the visible light spectrum.
The origin of the metal halide lamp
Projector lamps are further classified into the group "Mercury Halide Lamps", which were invented in 1962 by Dr. Gilbert H. Reiling. All metal halide lamps give a good distribution of the three primary colours so that they produce white light that can be as good as natural daylight in terms of colour rendering.
During their operation, light is generated from the excitation of mercury atoms in an electrical discharge between two electrodes, through argon gas. Halides are used because the metals themselves usually have too low a vapor pressure to participate in the discharge. Halides are vaporized much more readily and since they are much less reactive, corrosion is minimized.
Why so expensive?
People often get a shock when they see the price of a new projector lamp, expecting it to cost a similar amount to a fluorescent lamp. As described above, however, the technologies involved are different. Projector lamps operate at extreme temperatures and pressures, and high-grade quartz is often used to make the bulbs to ensure resistance to these stresses is achieved whilst providing a very high-quality light output. The mercury used inside the bulb is also costly, and in addition, during the manufacture of discharge lamps the bulb may be hand-blown to ensure precision.