Metal Halide Lighting
Metal Halide Lamp
Discharge lamp in which the major portion of the light is produced by the radiation from a mixture of a metallic vapor (for example, mercury) and the products of the dissociation of halides (for example, halides of thallium, indium or sodium) - for example: HPI-T lamps.
Metal Halide Lamps:
The Halide Cycle in a Discharge Lamp
Step 1: Metal atoms move from the hot electric arc toward the cooler arc tube wall where the halides are.
Step 2: Near the wall, the temperature and vapor pressure allow the metals and halides to form a stable molecule which will not corrode the arc tube.
Step 3: When the metal halides approach the hot arc, the molecule breaks apart.
Step 4: The halides move away from the arc, while the metals are energized and radiate light.
Sometimes a metal atom will not combine with a halide, but instead migrates through the arc tube. Over time, when enough metal atoms are lost, the lamp will fail.
Metal Halide: The Value of Scientific Training
Edison never considered himself a scientist and cared little for theoretical studies. Trial and error experiments gave him the working knowledge he needed, and if some higher math was called for he had Francis Upton on the payroll. Modern lamp inventors have the knowledge inherited from people like Edison, but they have also inherited complex problems not given to easy solutions. Inventing a lamp today calls for advanced scientific and engineering training, both to define problems and to use the highly specialized equipment needed to find solutions.
As early as 1912 Charles Steinmetz had placed metal halide compounds in mercury lamps hoping to improve the lamps' blue-green color. Iodine, bromine and chlorine are all elements known as "halogens" and react chemically with metals to form salts. The physics of electrical discharges and the chemistry of metal halides turned out to be quite complex, and practical lamps were not made until the late 1950s.
By the 1950s, mercury vapor lamps were common and the subject of much research. In West Germany, Otto Neunhoeffer and Paul Schultz explored the use of halogens to combat electrode evaporation. Bernard Kühl and Horst Krense also tried halogens in a lamp and filed for a patent in August 1960. However, Osram had introduced an improved mercury lamp (designated H-33) without halides in 1959. The H- 33 lasted longer and was more efficient than older designs, and may have tempered commercial willingness to quickly introduce yet another improved mercury lamp.
Metal halide arc tubes
S.I. image #99-4074
At this same time, American physicist Gilbert Reiling was also experimenting with metal halides and mercury lamps. His work at General Electric's Research Laboratory involved a mix of theoretical studies and experimentation. Reiling was able to bring a high level of expertise to bear on the problem. "I had 11 years of college mathematics, from topography to matrices to tensor [states] - everything you could possibly mention in the field of mathematics, and you need that for the physics. I had made some thermodynamic calculations that showed that, with sodium iodide, the iodine was so powerful that sodium would not attack the quartz [envelope]. That's what so many people worried about, that these alkali metals were just going to chew up the envelope, but it turned out that the thermodynamics showed that it wouldn't, and it was that idea that really made this work."
Reiling's experiments with sodium and thallium (see lamps above) were promising enough that in June 1960 he reported to GE, "these lamps appear to have a higher luminous efficiency than the mercury lamp and the possibility for better color rendition." In September the lab's research director C. Guy Suits wrote to GE's Chairman Ralph Cordiner to tell him of the new lamp. Suits reported that, although the lamp produced white light "through a complex mechanism which our scientists are still studying in detail,... it now appears that little change will be required in manufacturing the new lamps other than simply adding a scientifically determined 'pinch' of the optimum compound." GE publicly announced the metal halide lamp in late 1962 and used it at the 1964 World's Fair.
Frequently Asked Questions courtesy of Osram Sylvania.
Mercury and Metal Halide Lamp Operation
What happens when mercury lamps are operated on metal halide
ballasts, and vice versa?
Generally speaking, mercury lamps must be operated on mercury ballasts
and metal halide lamps must be operated on metal halide ballasts. Failure
to do so will compromise performance of the lamp system.
Consequences of operating mercury lamps on metal halide ballasts:
- Higher LPW
- Shorter Lamp Life
- Ballast Incompatibility
Consequences of operating metal halide lamps on mercury ballasts:
- Lower LPW Lumens per watt
- Shorter Lamp Life
- Poor Lumen Maintenance
- Ballast Incompatibility
Some ballast manufacturers no longer make mercury ballasts. They make metal halide ballasts and label them for both mercury and metal halide.
For example, if a ballast is labeled for both 400W mercury and 400W metal halide, it will have H33 and M59 on its label.