Film-Tech Forum ARCHIVE: Incandescent Lamp Life

This is topic Incandescent Lamp Life in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Brian Tristam Williams (Member # 1235) on 01-27-2003, 05:34 AM:

Yes, people, incandescent, not Xenon! First the story, then my question. If you're impatient, skip to the question at the end of this post.

I have just gotten some intelligent lights to work in our premiére cinema. There are four of them, and they all display nice, pretty, colourful company logos on the walls and screen. In order to better see them, we dim the lights a bit in the cinema. The area manager saw this little demonstration, and instructed me to always have them on between shows. I told him that the lamp life of these intelligent lights is, like all lamps, limited, and that, even though I'd like to have them on between each and every show, it might not be cost-effective.

I looked into the cost of the replacements, and their 2000-hour lamp life, and don't really consider it cost-effective. However, I realised that, although we are incurring an extra expense by having them on, we are saving a bit of electricity by having the house lights dimmed for them, and then another question came to mind:

* Do incandescent lamps last longer if you run them at lower (dimmed) levels? If so, is there a formula?

Posted by Stephen Furley (Member # 1277) on 01-27-2003, 06:40 AM:

Brian asked:

quote:

* Do incandescent lamps last longer if you run them at lower (dimmed) levels? If so, is there a formula?

I can't give you a formula, but the life if incandescent lamps is certainly very dependant on current, which, in turn is dependant on the Voltage. For straight tungsten lamps, a few percent reduction in Voltage can double the life. Halogen lamps seem to show a smaller increase in life, but it is still considerable.

Posted by Jon Bartow (Member # 241) on 01-27-2003, 08:26 AM:

Look here:
look at John P's post on the second page
Film-Tech thread
Jonathan

Posted by John Pytlak (Member # 331) on 01-27-2003, 11:11 AM:

Yes, reducing voltage will normally extend the life of an incandescent lamp. In the USA "long life" lamps are often lamps originally designed to run at 130 volts, and used at the nominal 120 volt line voltage.

Here are some links:

http://www.eaoswitch.com/about/lamps.htm

http://home.capecod.net/~pbaum/lamp.htm

Posted by Gordon McLeod (Member # 33) on 01-27-2003, 11:34 AM:

An incandescent lamps life is increased when dimmed but a Quartz Halogen lamp that is usually not the case as the temperature drops the halogen cycle stops and the tungsten boiled from the filament starts to deposit on the quartz wall and blacken it reducing the light output and weakening the filament.
Since light output compared to input voltage follows a S curve a fairly large voltage drop is required at the top end before appreciable light level change occurs and that usually is enough to stop the halogen cycle

Posted by Pete Lawrence (Member # 130) on 01-27-2003, 12:12 PM:

The formula I've used for a number of years on small lamps is as follows:

((lamp rated voltage / applied voltage)^12)X rated lamp life. In words it's rated voltage divided by applied voltage to the twelfth power times the rated life of the lamp.

So a 120 volt lamp run at 100 volts would last about 9 times longer. At 90 volts, about 32 times longer. This came from one of the instrument lamp companies many years ago. It only applies to non-halogen lamps. Not quite the same figures that John shows, but close and there are many factors involved in the different type lamps.

If you run halogens to cold the tungsten doesn't re-deposit on the filament and ends up blackening the envelope. Effectively ending it's life before the filament goes.

Posted by John Pytlak (Member # 331) on 01-27-2003, 12:21 PM:

As a "rule of thumb", don't undervolt tungsten halogen lamps by more than about 80% (e.g., a lamp rated for 120 volts should be run at least 96 volts), or they will prematurely blacken.

Posted by Steve Kraus (Member # 476) on 01-27-2003, 01:54 PM:

Speaking of incandescents, I had been buying the GE Reveal bulbs as I like the whiter light the slight blue tint gives but I've found them to be downright dangerous as in my experience they have a greater than normal propensity of having the glass envelop detach from the screw base. This on top of a greater than usual percentage of bulbs dead out of the box and early failure--earlier than even their short 750 hr life would predict. I had one fail in a ceiling fixture a shortly after putting it in and when I went to re-replace it it the glass dropped almost in my face.

Now you would think General Electric would be interested in my experience but two emails to them went unanswered.

Posted by John Pytlak (Member # 331) on 01-27-2003, 02:01 PM:

General Electric "Reveal" lamps:

http://www.gereveal.com/reveal/media.html

General Electric shatter proof lamps:

http://www.gelighting.com/na/pressroom/pr_saf_t_gard.html

Never the two shall meet? [Wink]

Posted by Steve Kraus (Member # 476) on 01-27-2003, 07:12 PM:

Well, I don't need shatterproof, just something that's not going to drop on a celebrity or anyone else out of the blue. I'm sure that's a headline they don't need and neither do I.

Posted by Brian Tristam Williams (Member # 1235) on 01-29-2003, 02:42 AM:

Thanks guys - very helpful responses. It seems we will be able to afford the intelligent lights after all, since we'll significantly save on the lamp life of the 40-odd PAR38 house lights (downlighters). [I can't help thinking that they should have installed low-voltage downlighters, instead of these 220-volt monsters.]

Posted by Paul G. Thompson (Member # 655) on 01-29-2003, 02:58 AM:

Broadcasting tower marker lights are 116-watt clear incandescent bulbs, the same type stoplights use. They have industrial strength filaments and filament supports. The bulbs are normally rated at 130 volts, with a life expectancy of 8,000 hours.

Posted by Kenneth Wuepper (Member # 1174) on 01-29-2003, 10:04 PM:

Yes Paul
We got lots of those and other long life lamps from Duro Test. It is a tough job replacing a couple of lamps a year when they are 1,200 feet above the ground.

We used Duro Test lamps in the domes of the theatre because it was such a bear to relamp them. Some lamps are rated at 10,000 hours. Expect the cost to be higher but remember that your labor to change the lamps makes up more than the difference.

KEN

P.S. I get airsick looking out the basement window!

Paul, those houselights are the 150 Watt monsters. (EDIT)

[ 01-30-2003, 06:42 AM: Message edited by: Kenneth Wuepper ]

Posted by Paul G. Thompson (Member # 655) on 01-29-2003, 10:11 PM:

Ken, in some areas, Duro-Test sucks! I ordered some 600-watt beacon lamps (the same ones used on airport runways) for our towers, and the quality of the filaments and filament supports were so deplorable, I refused acceptance. [puke]

I ordered GE instead, and they looked like Cadillacs compared to Model "T" Fords. [thumbsup]

Their 116-watt 130 volt lamps seem to be OK, though.

[ 01-30-2003, 12:41 AM: Message edited by: Paul G. Thompson ]

Posted by Barry Floyd (Member # 385) on 01-30-2003, 11:08 AM:

Maybe I mis-understood the question.. Where you questioning the life expectancy of the bulb in the intelligent fixture, or the bulbs in the house lights?

I used 4 Martin Intelligents in my biggest mobile DJ setup for a couple of years, and only had to relace the bulbs twice. Now I have replaced a couple of mirrors and stepper motors, but that was due to one of the guys who worked for me who tossed a roadcase with two lights in it out of the back of the trailer [Mad]

I've found as long as you don't move the fixture until everythings completely cooled off.. you should have no problem.

What type of Intelligents did you get?