# Are xenon short arc lamps dimmable?



## Artlav (Mar 2, 2015)

I've been making a driver for a Xenon short arc lamp.
The lamp is rated at 150W, to be powered by DC.






I seem to remember reading a few years ago that running such bulbs at less than their rated power can damage them, but can't quite google the source any more or remember the details.

So, the question is - would an arc lamp like that be damaged by running at significantly less than it's rated current?
I.e. at 100W or 75W?
It does not stay lit below 50% rated power.

The dimming is done by reducing the current, not by rapid switching/PWM.


----------



## FRITZHID (Mar 2, 2015)

I'm not 100% but I believe xenon SA lamps don't really have an issue with running lower than recommended amperage.
Mercury SA on the other hand could very likely have issues with under driving.
Biggest issues with SA failures is repeated striking (wears electrodes), unclean power and adequate cooling. Overdriving isn't recommended either.


----------



## BVH (Mar 2, 2015)

The Xenon lamps made by A.R.C. are good for 10% plus or minus power levels with no significant affects. I'd guess you could do a 20% long-term reduction with no ill effects, but it's a guess. IIRC, one of the issues with Mercury Xenon is that if not run at proper temps/power levels, the electrode materials as they wear/evaporate away, cling to the inside of the arc chamber and start to darken the quartz.


----------



## FRITZHID (Mar 2, 2015)

^^^^^ what he said!
He's the guy I go to with SA questions, lol.
Merc SA are far picker then xenon SA in my exp.
I assume that's because metals need to be vaporized and cooler arc temps could impede this.
Overdriving/overstriking I know causes electrode degradation and deposit issues but with pure xenon I don't think underdriving would cause any issues. Mathematically, it should prolong lamp life.


----------



## Artlav (Mar 4, 2015)

FRITZHID said:


> I assume that's because metals need to be vaporized and cooler arc temps could impede this.


Not in a pure Xenon, so that is good.



FRITZHID said:


> Mathematically, it should prolong lamp life.


One thing i noticed is waves, noise in the light itself.
No luck getting it visible on camera, but it's something similar to reflections of light from water surface, only imposed on light given.
What could that be?



FRITZHID said:


> adequate cooling





BVH said:


> one of the issues with Mercury Xenon is that if not run at proper temps/power levels


While we are at it, how much cooling is too much cooling?
Is it ok to blow a fan at the bulb itself?

The scraps of documentation i found for this bulb says "don't blow at the bulb" in one place, and a few lines later specifies "Air cooling speed: 5 m/s".
How does that supposed to work?

I'm making my own enclosure/reflector for it, so "convection cooling" isn't going to work.


----------



## BVH (Mar 4, 2015)

The "noise" you refer to is frequently called "shimmering" and it is completely normal. It is easily seen when the spot is shone on close-up surfaces but pretty much disappears at any significant distance so it is really not an issue for searchlight use. All of my lights exhibit shimmering including the ones using Cermax-type ceramic body lamps.

I have only one Mercury Xenon searchlight - the Locator. It utilizes a small fan inside the glass dome to simply recirculate the completely encapsulated air. There is no fresh air intake and exhaust. The air is not directed at the lamp. On all my large Xenon searchlights, large amounts of cooling air are sent axially over the lamp. Some from Anode to cathode and some vice versa.

Found the engineering piece on short arc lamp shimmering I had seen years ago. It is from the "Cermax Xenon Lamp Engineering Guide":

"The third effect is shimmer. If the light from an elliptical Cermax lamp is allowed to expand and shine on a surface 1 or 2 meters away, there will be a large illuminated area where the light shimmers, similar to light going through air rising from a hot surface. This is caused by variable refraction of light throughout the convectively flowing xenon gas in the lamp. Shimmer does not appear in applications where the light from different parts of the lamp is mixed in either a fiberoptic lightguide or an optical integrator, but usually appears in visual systems where no integrators are present. There is no known way of eliminating this effect in the lamp. Peak-to-peak variations of the light on a very large illuminated surface from a lamp exhibiting shimmer can be 10—20%. An effect related to shimmer is the instability in output of a Cermax® lamp when the lamp is run with the window pointing upward. Running a Cermax lamp in this position is not recommended, except at very low powers, because the hot xenon gas from the arc rises directly to the window and can overheat the window. However, even when the lamp output points within 45 degrees of vertical, the output will become unstable because convection inside the lamp will disturb the arc."


----------



## PolarLi (Mar 4, 2015)

Here is a good document from Osram on their XBO lineup, which pretty much is identical to your lamp: https://assets.sylvania.com/assets/Documents/ENGR_BLTN11.161355cc-1d94-4996-b6cd-a3001fea6f1a.pdf
It describe those 5m/s of axial cooling on page 28,29 and also why you should avoid directing a lot of air on the bulb/arc chamber.


----------



## Artlav (Mar 4, 2015)

PolarLi said:


> Here is a good document from Osram on their XBO lineup, which pretty much is identical to your lamp


Thanks, that is indeed a very useful thing about these lamps.

Hm.
I found this line confusing:
"XBO lamps that do not go out even after the power supply plug has been withdrawn are extremely rare".
Say what?
Is that a joke of some sort?




BVH said:


> The "noise" you refer to is frequently called "shimmering" and it is completely normal.


That is good to know.
It looked weird enough to get me worried that something was melting inside.



BVH said:


> The air is not directed at the lamp. On all my large Xenon searchlights, large amounts of cooling air are sent axially over the lamp. Some from Anode to cathode and some vice versa.


I wish i knew that before i made the housing...
Then again, i didn't consider insufficient cooling before the plexiglas front started softening and bending.


----------



## PolarLi (Mar 5, 2015)

Artlav said:


> Hm.
> I found this line confusing:
> "XBO lamps that do not go out even after the power supply plug has been withdrawn are extremely rare".
> Say what?
> Is that a joke of some sort?



Apparently it is


----------



## more_vampires (Mar 5, 2015)

Lamp doesn't go out when the power is cut?!?!? SIGN ME UP! I'll take 100!


----------



## Echo63 (Mar 5, 2015)

The Maxabeam has the abilty to run the bulb in "low", "normal" and "high" modes - so i would assume slight underdrive or overdrive is ok.

(i have seen the "shimmer" in my MB beam at close range and always wondered what it was, now i know)


----------



## sven_m (Mar 9, 2015)

I believe Xenon arcs are not meant to be dimmed outside their specs. And the specs are relatively tight.

First there is the Osram handout mentioned above ("XBO – theater lamps. Technology and application")
which even repeats
- p. 33 "Undercurrent: This causes the arc to constrict at the cathode tip, resulting in local overheating of the cathode surface and consequently increased vaporization."
- p. 35 "The lower the current, the more constricted the arc root at the cathode; local overheating occurs, causing local partial depletion of emitter material and fissuring of the surface."

Then the specs,
I use the MaxaBeam type now, because it _does_ run being dimmed and because I have some bulbs.
I couldn't find a datasheet for the Ushio 75W types, especially UXL-75PB,
but the sheet for a comparable lamp, the Osram 75W/2, reads 64…86W, which is ±15%.

And third, I don't believe that it is helpful to assume Xenons are dimmable because MaxaBeams do so.
I have no bulb that I bought new which already has been running for a long time.
But I have looked at a bunch of used bulbs. And although I have no knowledge about their history,
they don't look just worn out, but they really look awful.
MaxaBeams usually run at ~45W per default. They can be switched to 85W,
but fall back to 45W after 16 seconds (if not programmed otherwise by the vendor).
They can also be reprogrammed to run on 25W (user programmable) for longer run time of the battery.
And they start at 85W for 3-4 seconds, which is fine.

So I guess they wear out so awfully for one of these reaons
- because of frequently having been switched on/off (don't know, but Xenon arcs are very robust in comparison to MHs, HMIs, UHPs, etc)
- or increased ripple current (don't know yet, but I don't think so)
- or just because they ran at 45W most of the time
- (or they do look so bad after running correctly for quite some time. But I don't think so.)

Here are macro photos of my bulbs, the used ones all came from different hosts, so it's not a faulty ballast
(they are from a much longer article which is in german language though)

Ushio UXL-75PB (bought new, ran only a very few times)




Ushio UXL-75PB (bought new, roughly: ran 30 times, about 2 minutes average, on mid or high)





Ushio UXL-75PB, unknown history





Ushio UXL-75PB, unknown history





original Gen3-bulb ("ARC"), unknown history





original Gen3-bulb ("ARC"), unknown history





Alternatively, they all might have a history of plain bad usage instead, and running on 45W would be no problem.
But currently I just believe: even 45W is too low (let alone 25W).

I have measured such a 75W bulb (run by the MaxaBeam ballast outside the reflector) with a Spyder4 colorimeter,
with the hope to find some variation in color temperature as a hint to the arc temperature at the cathode tip. But unfortunately
I couldn't see anything like a blackbody emitter component (which would be temperature dependent). There's almost no variation,
apparently due to the continous arc discharge spectrum type, let alone all the rest of the arc between the electrodes.


----------



## sven_m (Mar 11, 2015)

sven_m said:


> [...]
> But I have looked at a bunch of used bulbs. And although I have no knowledge about their history,
> they don't look just worn out, but they really look awful.



Oh, and perhaps I should emphasize on this: The very point of a Xenon arc lamp is a cathode tip in best condition. Otherwise the spot brightness decreases _immensely_. The following is a gif animation of these bulbs, in the same host and with the same ballast:


----------

