Relations between voltage and lamp life


Active Member
On the very lengthy "Lamp Questions" thread, I noted a quote by Ship:

The following rule of thumb can be derived:
A 5% change in voltage applied to the lamp results in
- halving or doubling the lamp life
- a 15% change in luminous flux
- an 8% change in power
- a 3% change in current
- a 2% change in color temperature

So this means, if I want to get a lot of life out of my lamps, but really aren't concerned about the hottest of light or the brightest, I could trim my dimmers to max out at say 118v at full, and get more hours out of the lamps.

Somewhere I heard that for every volt you deviate from the rated voltage, the life inversely changes by 12%. That means if you run a 120V 1000-hour lamp at 121V, it would be a brighter 880-hour lamp. And if it were run at 119V, it would be a 1120-hour, but less bright lamp.

Also, with regard to the tungsten-halogen reaction, at what level (generally) does the filament get cold enough to were the reaction stops taking place?


Senior Team Emeritus
Premium Member
Depends upon the lamps. If you have 115v lamps, in lowering it to 118v (assuming at the fixture you even have that much voltage) you still would not get even the rated life of the lamp. But yes you understand te concept. in general (see below)

It's possible that what you saw was somewhat correct, but not for each volt - instead each percentage of a volt.
Sometimes a catalog or manual will round up or down in symplifying things or use a slightly different formula. I have 12% noted also in my notes but did not write down it's source. "For every 1% change in supply voltage light output will rise by 3.6% and lamp life will be reduced by 12%. "

Try this:
"The effect of voltage on the light output of a lamp is ±1% voltage over the rated amount stamped on the lamp, gives 3.1/2% more light or Lumens output but decreases the life by 13% and vise a versa.
Do not operate quartz Projection lamps at over 110% of their design voltage as rupture might occur." GE Projection Lamp Catalog, G.E. Lighting #204-01016 (3/93) p.13

A 120v or any voltage over about 64 Volt (Low Voltage) will have sufficient voltage behind it to always have the "Hallogen Effect" taking place. There is no a reaction going on - that would be a bad thing. Don't worry about your halogen effect stopping working at some point.

The following article off the Philips website notes what you are asking about but it is not always evidenced. Not studied or specified is some stage and studio low voltage lamps do not have a problem with dimming upon the lamp, and possibly any low voltage lamps are used in series thus 120v once added up, they will not have a problem with dimming.

"Low Voltage halogen lamps should not be dimmed by more than 10% of their rated voltage since this will result in a reduction in life. Standard tungsten filament lamps (with no halogen filling), can be dimmed to zero volts, resulting in virtually endless life. However if low voltage tungsten halogen lamps are dimmed by more than 10%, the lamp will be operating at too low a temperature and the free halogens in the gas fill, will attack the cooler parts of the tungsten filament i.e. where enters the quartz or glass envelope. The wire at that point will then be eroded and eventually will fail. So if dimmed by 10% or more low voltage tungsten halogen lamps will not have an extended life but are unlikely even to reach their rated life." - Philips Website, Optical p-1


Active Member
We've got 120V lamps all around right now. In our S4's we have Ushio HPL575/120VX lamps, and in the Fresnels we have (Ushio I think) BTL's. I'm considering upgrading these to BTN's once they get old. I might also upgrade a couple of our S4's to HPL750/115VX also.

Since this won't cause any problems with the lamps themselves, I might as well experiment with the max voltage settings in the CEM in the dimmer rack, and see how it looks.

Also, one more question. In your opinion, how important is preheating the lamps? I don't turn off the dimmer rack (the main shutoff is on the roof, and nobody has access except the head custodian and maintenance people), and I'm kind of concerned about leaving the lamps going like that all the time. And I don't want to leave the fan running all the time, if it indeed does in that rack (ETC Sensor SR48 with CEM+ module).


Senior Team Emeritus
Premium Member
I would think Wolf and others will know more about your dimmers and trim setting the balance between cold start and lamp life. Or at least it would be a good debate as to what is the optimum setting.

Given most dimmers have a range of about 80% actual variation when on a 0-FF dimming curve, either you are warming your lamps some which is really good for the filaments in otherwise turning on while cold which is a huge shock to them, or your lamp life is constantly being used up given your dimmers don't get shut off.

Might in my opinion in the end be most cost effective to have a shunt switch or switch relay installed in your theater so as to shut off the dimmers while not in use. If of any help, this is a required feature in many local codes. I have wired up a few relay switches in the past and it when the dimmers are not in the same room were a breaker panel or safety switch was available is a very good idea.

Your lamps already have an expected life at full of 2,000 hours. When used in shows it would tend to be dimmed thus last much longer than that. How often do you have to change these lamps?

The only way to get the above 13% more life would be to decrease the voltage by 12 volts. Your fixtures - given you have 120v of power at them after voltage drop and the dimmers are now running at 108v. (have you metered with a true RMS multi-meter the voltage at the fixture, and/or at least a 75w or more ghost load?)

Given the 120v versions of the HPL lamp is already somewhat dim, and the extended life version is more dim still, dropping the voltage furter would for fixtures with longer throws seriously limit the maximum output and cause huge incandescent like amber shift.

Fine as long as you are used to it, but there is limited punch available especially for the longer range fixtures.

IN any case, if you have 120v at the fixture and trim your dimmer so it's 108v, this would mean about 12v worth of current to pilot light the lamps in keeping them warm, and 12 to 13% more life to them. I would think this not optimum but the maximum balance to shoot for.

At some point about 1.2/3% you will bottom out at right about zero volts and will not be able to save further money without shortening your dimming ratio to an extent it destroys the dimmer curve flexibility. This assuming that you have started at 120v at the fixture. If say 115v, you won't be able to get more than at best the 1% savings in lamp life before you also start to limit the dimming curve of the fixtures. This given you already after about 20% won't really see the light coming out of the fixture.

In other words, if your dimming range is now between say 20v and 108v, that is not much of a range to dim with. Shorten/clip the maximum voltage further and you have very little control over your lights all in saving money.

Doing shows is expensive. At some point you just have to bite the bullet in lamps verses light on stage. This and turning off the dimmers I think is a good idea.

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