Many modern HID lamps are rated for universal burning, meaning they can be burned in any position. Some lamps of course are not rated for this, but I have some questions:

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1) Advantages:
Say I had a fixture that burns a lamp base-up. Would there be any benefit at all to using a base-up only lamp instead of a universal burn lamp? Lifespan maybe?

2) Good uses:
Is there any situation at all where a universal-burn lamp would be inferior to a lamp with a specific operating position?

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Thanks!

HID's are sensitive for internal temperature distribution. You want the cold spot to be at exact temperature to get the vapor pressures you need, but also you want the hot spot to be as cold as possible to not stress the materials that much.
There are many tricks to control that, but the general problem with universal burning is the heat rises, but you don't know where will be the "up". So when designing an universal burning lamp, you need to keep way more margins to accomodate the "up" being anywhere, so you can not go that close to the running optimum.
If you are designing a lamp with strictly restricted burning position, you can count on the exact cooling profile and convection currents, so you can reach way more optimal internal conditions for both higher efficacy, better color stability, longer life, better lumen output stability. Or get away with lower cost design, so cheaper product. Even some lamps are open fixture rated only when operated BU, otherwise they need enclosed fixture, because either the failure modes could be different with different orientation, or the internal protection is effective only for operations at the certain positions.

So yes, if you know your burning position is base up, chances are you can find a better performing (according to your criteria) lamp for BU only. But it is just a chance, not guaranteed, it depends what your main criteria are and what the market offerings are.

@Medved: These are usually MH lamps that are sensitive to burning position. HPS and MV lamps aren't sensitive, because in HPS lamp, the arc is wall stabilized, and in MV lamp, the internal pressure is much lower than in MH lamp, and it is an unsaturated vapor lamp.

Indeed, it is mainly related to MH lamps, where the internal thermal margins use to be quite tight.

With MV the problem is with the starting probe and starting reliability: When the probe is on the upper end, there is no way the mercury droplet may settle in such unfortunate way to short out the starting and main probes together, so prevent lamp starting. But normally this is not covered by the formal lamp rating and there is no standard, so no guarantee on which end the starting probe would be, it may vary even along production of the same part number from the same manufacturer.

With HPS the important part is the amalgam reservoir position and its temperature management, but generally the thermal loading of the arctube uses to be rather low for what the material is normally able to handle, so (maybe except the double ended lamps) all the HPS are made as universal burning, so you would not have any indication which position yields to better performance.

In fact it is the fact for any universal burning lamp: Usually you will not get any information which position would lead to best performance, unless it is really part of the formal rating (like open fixture rating valid only for BU).


My point was, once you know your position, it makes sense to not stay with "universal" lamps, but venture into single or limited position range offerings. If anything is guaranteed, it will be mentioned in the datasheet.