Film-Tech Forum ARCHIVE: Could this STRONG manual override SMPTE guideline ?

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Posted by Kamakshipalya Dhananjay (Member # 1391) on 08-13-2002, 11:08 AM:

We have a genuine problem resolving what wattage XENON bulb to use for our screen which has a height of 21.5 and width is 51.5.

Applying the formula of 5 watts per square foot- we get :

1110 Feet and therefore, 1110 * 5 =5550 Wattage.

Also applying, 21.5 * 21.5 * 12, we still find to use a 5500 Watt Xenon bulb.

What really put us in doubt is a STRONG XENON Manual which says that

Super 80 LAMPHOUSE - S803000/SW - Wattage 3000 - is suitable for screens upto 50' Wide

S804200/SW - Wattage 4200 - is suitable for screens upto 60' Wide

S807000/SW - Wattage 7000 - is suitable for much larger screens.

So, if STRONG were to advise that a 3 KW XENON bulb is enough to illuminate a 50' Wide screen, should it be weighed above other formulas or authorities ?

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Posted by Leo Enticknap (Member # 534) on 08-13-2002, 11:25 AM:

I don't know where the formula of 5 watts per square foot comes from, but in arriving at the 16 foot lamberts of illumination which the SMPTE spec requires, then the bulb's output is not the only relevant factor - the amount of light absorbed by the lens, the amount of ambient light in the auditorium during the show (from the emergency lighting) and the efficiency of the projector's shutter being the most obvious things which spring to mind...

Posted by Randy Stankey (Member # 64) on 08-13-2002, 11:54 AM:

In the places I used to work they used 4200 W to light the screens up to around 50 ft. wide. Of course, the console was a CFS which was a real light hog. If they had used Christie, or some other brand of consoles, I bet they could get away with a 3000 W. In the largest stadium theatres with 60+ foot screens they used 6000 W lamps in a Christie console. No problem getting light there!

When you're on the fence between deciding on whether or not to use a larger size lamp, personally, I'd go for the larger one. It's easier to "reign in" a larger lamp/console than it is to "pump up" a smaller one. The caveat being that you don't want to use a lamp that's grossly over powered lest you risk overheating the projector or burning the film.

Most projectors, if they are set up properly, should be able to handle a lamp in the 4000-5000 watt range without any problems. IF you wanted to be extra safe you could fit it with a water cooled trap, although it's not 100% necessary.

Posted by John Pytlak (Member # 331) on 08-13-2002, 12:08 PM:

That 5 watt per square foot guideline is for a MATTE screen surface having a gain of about 1.0. A properly curved GAIN screen requires less light from the projector, since it reflects more light to the audience. Really large screens need to have a curved gain surface to meet the 12-22 footlambert range allowed by standard SMPTE 196M, since 7000 watts is the largest practical xenon lamp used for 35mm.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Pat Moore (Member # 380) on 08-13-2002, 01:30 PM:

Hi;

I have to point out that there are all kinds of disclaimers along with that Strong recommended wattage. First, it assumes a Scope image, a gain screen, fast high quality lenses in what was then "standard" focal lengths, and a newer xenon lamp within its warranted life and at its nominal light output. That chart goes back to when I started in the business and has pretty much remained unchanged.

Advertising is one thing and the real world is another. When I set up a theatre specification I tend to be rather conservative. I want a bright image when the lamp has 800 hours on it, flat, scope or otherwise.

While a 3kw might provide adequate light under the prescribed conditions, it does not properly account for a Flat image, a matte screen, an old xenon lamp, short focal length lenses, etc. I have nothing against the 5 watts per square foot estimate, I think that's generally comfortable. The exception is top or top & bottom movable masking that makes the flat image larger than the scope picture.

Randy -- I'm with you generally, but I do disagree on the benefits of water-cooling. I think keeping the ambient temperature of the film trap area cooler is a great benefit, helping to minimize film flex as it reacts to the light that pours through it. I also sense a phenomenon that the metal pads get a bit "sticky" the hotter they get, and water-cooling helps that problem. I'd water-cool anything 4000W and above, and an insertion-type heat filter would also be in my equipment mix above 3000W.

Pat

Posted by Steve Guttag (Member # 268) on 08-13-2002, 06:00 PM:

Once upon a time...

The Strong figures were based on a f/1.7 lens on Pearlescent screen. Remember, every lamphouse manufacturer likes to boast theirs lights the most with the least...if Strong were to use more stringent requirements than other manufacturers, one would mistakenly think the Strong lamphouses put out less light.

In your case, however, I would recommend about 6000 watts. Once you get to around 4000 watts, you are going to want a heat filter, that will knock off some of your light...so use 13 as your multiplier rather than 12.

Now, if you curve your screen (properly) and install a 1.5-1.7 gain screen...you wattage can drop a bit to say 5000 watts or maybe a tad less. Dropping your wattage will save you on lamps, electricity and focus aggrivation.

Steve

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"Old projectionists never die, they just changeover!"

Posted by John Walsh (Member # 168) on 08-13-2002, 06:27 PM:

Many years ago, someone wrote to "Road and Track" magazine, saying they had installed several of those devices to increase your car's gas mileage. The writer installed one that claimed to increase your mileage by 40%, another that would increase it by 30%, and two different devices each increasing it by another 20%. The writer noted that, with all these devices he should not only be saving gas, he should actually be producing it.

Everyone is on the right track, but I think Steve is totally right about what manufaacturers say in their literature. I'm not saying manufacturers' are lying, just that they are sometimes overly optimistic.

Posted by Pat Moore (Member # 380) on 08-13-2002, 07:29 PM:

Optimism's a good thing -- right? Advertising's pretty much the same with all the manufacturers.

Anyway, I'm on board with Steve. I tend to think a well setup 5kw system will meet the needs, IF you have a gain screen that's properly curved and good lenses.

But I have to note that in my India travels, I never did see a good gain screen installed, and I saw a few theatres around the country. Most of what I looked at closely were "local made". The newer mattes actually looked okay in terms of surface quality, but perforations weren't too hot. Iwould bet this theatre in Bangalore is a flat, matte white surface. Hope I'm wrong -- a good 50ft curved screen would really look nice.

Pat