dor123
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| Why scandium aren't used in CMH lamps? It have very higher efficiency and pressure than rare-earths and may be less corrosive to the ceramic arctube, and with the sodium having wider profile, higher red ratio and CRI can be achieved.
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James
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| Because it is corrosive to the ceramic tube and leads to short life. Scandium lamps are generally better with quartz tubes.
Moreover, scandium has the big drawback of very rapid lumen depreciation due to it dissolving in the tungsten electrodes, which then evaporate faster and blacken the arc tube. Scandium lamps do indeed show higher initial output - but usually after a few hundred hours that advantage has been lost. So that metal was mainly used by the American lampmakers that did not used to openly publish their lumen depreciation curves to the same extent as other countries, and liked to sell based mainly on the higher initial lumen figure.
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dor123
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| I didn't know that scandium is more corrosive to the ceramic arctube than rare-earths.
Also: I thought that scandium causing rapid lumen deprecation only in probe-start MH lamps and not in pulse-start and compact MH lamps that have an elliptical arctubes.
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« Last Edit: March 12, 2025, 10:59:34 AM by dor123 »
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I"m don't speak English well, and rely on online translating to write in this site.
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James
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| In pulse-start metal halide lamps the rate of blackening is of course lower, but that is only because they have much higher argon gas fill pressure than probe-start types. The scandium-induced blackening still happens, but at a much lower rate.
Same principle as in regular vs high xenon pressure HPS lamps - the extra gas pressure slows blackening and improves lumen maintenance, but does not solve the root cause of the problem.
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dor123
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@James: Is only the accelerated arctube blackening in scandium lamps causing the rapid lumen depreciation?
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I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.
I only working with the international date format (dd.mm.yyyy).
I lives in Israel, which is a 220-240V, 50hz country.
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RRK
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| Well, in the addition to what James said, NaSc mixes are low CRI. As ceramic burners are expensive, there is zero commercial point to make an expensive lamp having CRI70. The market is flooded with cheap quartz versions. Color is mediocre, life is mediocre, but so is the price, the last 150W one I got was $0.5 ))) You have multiple of these, too )
One interesting quiestion is why Sylania have chosen a hybrid NaSc + rare earth fill for their low wattage compact PARs. May be James have an insight...
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dor123
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| I thought that increase scandium pressure and temperature, would lead to increased efficiency and color rendering.
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I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.
I only working with the international date format (dd.mm.yyyy).
I lives in Israel, which is a 220-240V, 50hz country.
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dor123
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@James: Some American members said me that they have no problems of rapid lumen depreciation with their pulse-start MH lamps that have Na-Sc of course. How you explain this? And @RRK: The main reason for the increased CRI of CMH lamps versus QMH lamps is the higher sodium iodide pressure. Also, I would expects that more scandium lines would be stronger lead to rich spectra.
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I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.
I only working with the international date format (dd.mm.yyyy).
I lives in Israel, which is a 220-240V, 50hz country.
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James
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| I would not expect anyone to have a problem with the lumen maintenance of a sodium-scandium lamp. After all, they are the most frequently sold quartz metal halide lamp types around the world.
We are talking only in relative terms, that their lumen maintenance is not so good as other chemistries. I can only explain the above views by questioning what measurements might have been made to validate such a claim. The human vision system makes an extremely ineffective light meter - huge deviations in luminous flux can easily look quite similar. Professional lumen maintenance measurements of typical sodium-scandium lamps do not back up such claims.
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