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BULB DISCUSSION BOARDS => Modern Electric Lighting => Topic started by: Anders Hoveland on May 18, 2015, 05:03:45 pm

Title: Phosphors for Discharge lamps
Post by: Anders Hoveland on May 18, 2015, 05:03:45 pm
Ceramic Metal Halide (CMH) is basically a relatively new type of Metal Halide lamp that operates under high pressure.
Unlike the old Metal Halide lamps, CMH can offer excellent CRI and light quality, I am talking comparable to or even exceeding high CRI LED. The light quality feels very natural, and if one looks at a spectral graph of the light output, it is indeed "full-spectrum".

So I was wondering, could the excellent light quality from CMH be improved by utilizing phosphors? A sort of "hybrid" approach?
CMH bulbs consist of an inner capsule of sintered alumina, that contains the arc, covered by an outer coating of protective glass. Since the inner capsule is transparent to short wave UV, the inside of the outer glass layer could potentially be coated with a layer of phosphor.

I tried to do some research into this and discovered that many old mercury vapor lamps utilized a phosphor coating:

What many of you may not have considered though is that even regular "fluorescent" lamps are really a type of discharge lamp too. If you want to think about it this way, fluorescent tubes are a sort of "hybrid" lighting technology. The 436nm and 405nm blue-violet mercury lines comprise a substantial part of the light spectrum from a typical tri-color fluorescent tube. The only real difference from those mercury lamps is that it is a low pressure glow discharge.

I even found a Metal Halide bulb that uses phosphors to achieve better light quality:
Philips MasterColor? 100 Watt phosphor coated ceramic metal halide lamp with medium base, 4000K 93CRI

Ceramic Metal Halide can achieve 96CRI at 4000K, but at lower color temperatures the CRI is not as high, because these lamps basically add sodium into the discharge tube, creating a yellow-orange line at 589nm.

But I was thinking if phosphors, the light spectrum could be improved. The spectrum of CMH contains a higher ratio of royal blue/violet wavelengths than natural daylight, and as a consequence the cool white lamps tend to be just a little magenta-tinted. But if that violet could be down-converted into cyan, it would make the light quality all the more natural. Phosphors would especially improve the spectrum of warmer color temperature CMH lamps, red could be added instead of yellow-orange.