Film-Tech Forum ARCHIVE: Screen size vs. wattage

This is topic Screen size vs. wattage in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Dennis Atkinson (Member # 345) on 08-18-2000, 09:51 PM:

What is the standard and the equation to figure out the proper lamp size for a screen?
Before a bulb is bought, there must be a way to get a ballpark number.

Posted by John Walsh (Member # 168) on 08-19-2000, 12:30 AM:

This has been talked about in previous posts, but generally, take the height of the screen, square it, then multipy by 12. That gives suggested wattage.

10 foot high screen

(10 x 10) x 12 = 1200 watts.

If the lens focal you are using is about 60mm or less, you need even more.

Posted by Pat Moore (Member # 380) on 08-19-2000, 06:59 AM:

I've found the (height x height x 12) formula to be a good starting point at times, but there are other considerations. Curved gain screens have different lighting requirements than matte surfaces. Top/Bottom masking situations really skew the Flat/Scope balance (which isn't really balanced in the first place). Focal length and quality of the projection lenses play a big part, and certainly efficiency of the xenon systems have to be figured in.
Sadly, good old economics comes into play as well. How many times do we see a complex with under- (and over-) lit screens so one size xenon lamp can be stocked in the building?
There are other times when I feel compromises are made the wrong way. Designers seem to want the entire front wall filled with screen, despite problems with audience viewing angles, too large-appearing image size, etc. Too often the xenon system is "pushed" to light a screen that really doesn't need that extra foot or two of height or width, so picture quality is unnecessarily compromised. Overlooked is the fact that a better appearing image, both in brightness and in relative size, can be obtained with a slightly smaller screen and not something that's wall-to-wall.

Posted by John Pytlak (Member # 331) on 08-21-2000, 05:51 AM:

Another "rule of thumb" is 5 watts per square foot of "scope" image area for a matte white (gain=1) screen. Using a gain screen can give more efficiency, but the screen needs to be curved to properly distribute the light throughout the auditorium.

Note that by Pat's formula, the largest 35mm screen that can be adequately lit with even a huge 7000 watt lamp is 24 feet high. Theatres installing 30 x 72 foot screens will have dim pictures or burned film.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by Barry Floyd (Member # 385) on 08-21-2000, 08:09 AM:

Using that formula... a 40 ft. high drive-in screen would require almost 20kw ??

Most of the drive-ins in Tennessee I've visited run 4kw max. Does the drive-in world require a totally diiferent set of specs?

Posted by John Walsh (Member # 168) on 08-21-2000, 10:43 AM:

"Does the drive-in world require a totally diiferent set of specs?"

No. Drive ins usually just run with low light. A 40ft high screen would be about 95ft wide; that's pretty big!

Some drive in projectors have "fast pull-down" intermittents (allows the shutter to stay open longer) to get more light. But nothing is going to light that screen. 35mm film just can't take that kind of light (heat.)

I tried to find when in the past we talked about this at length, but the search feature isn't working.

Posted by Rory Burke (Member # 512) on 08-21-2000, 12:16 PM:

Are the above equations set to calculate 16ft/L +/- 2ft/L??? I am assuming they are. I am gonna take those equations and compare them to my theaters and see which one is the closest. Anyone else have any others?

Rory

Posted by John Pytlak (Member # 331) on 08-21-2000, 12:25 PM:

SMPTE Recommended Practice RP 12 "Screen Luminance for Drive-In Theaters" notes that "in an ideal situation...the screen luminance and distribution shall be that specified in ANSI/SMPTE 196M, 16 fL +/- 2 fL (55 cd/m2 +/- 7 cd/m2). But "When maximum compromise must be made...the luminance at the center of the screen, measured from any car position, shall in no case be less than 4.5 fL (15 cd/m2)." RP 12 also allows more non-uniformity on an outdoor screen than for an indoor theatre.

In reality, any screen larger than about 25 x 60 feet will be difficult to light to the SMPTE aim of 16 fL without risk of heat damage to 35mm film. A 70mm print allows spreading the energy of the lamp over a much larger area of film, so huge screens are much easier to light properly with 70mm formats. If you have a 30 or 35 foot high screen, the only way to light it properly is with a 70mm print.

Posted by Pat Moore (Member # 380) on 08-21-2000, 12:28 PM:

John found his SMPTE reference before I did. Let's face it, outdoor screens are a bear to light and if we made anything over 4fL back in the old days (when there were drive-ins), we were pretty happy about it.

Posted by John Pytlak (Member # 331) on 08-21-2000, 12:52 PM:

Rory said: "Are the above equations set to calculate 16ft/L +/- 2ft/L??? I am assuming they are. I am gonna take those equations and compare them to my theaters and see which one is the closest."

The "equations" presented are really only "rules of thumb". As Pat Moore mentioned, there are many factors that affect the "efficiency" of converting watts to light on the screen. These include shutter efficiency, lens f/stop, lamphouse f/stop and match/alignment to the projector optics, port glass transmission, screen gain and curvature, cleanliness of the optics, etc., etc.

As Pat notes, many theatres have screens that use poorly chosen lamp sizes. For example, to reduce lamp inventory, some theatres routinely use only one or two lamp sizes for screens ranging from 10 x 24 feet(which only needs about 1200 watts) to 15 x 36 feet(which needs about 3000 watts) to 20 X 48 feet (which needs about 5000 watts) to 25 x 60 feet (which needs about 7000 watts). Or they try to economize by using a 2000 watt lamp for screens much larger than the 13 x 31 foot size that can be comfortably lit with 2000 watts.

The only sure way to know if you are meeting the SMPTE standard is to regularly MEASURE the screen luminance using a calibrated screen luminance meter. Here are several articles that I wrote on the subject:
http://www.kodak.com/US/en/motion/newsletters/reel/spring98/pointers.shtml http://www.kodak.com/US/en/motion/newsletters/notes/march2000/pytlak.shtml http://www.kodak.com/US/en/motion/newsletters/notes/june2000/pytlak.shtml

Posted by John Pytlak (Member # 331) on 08-21-2000, 01:01 PM:

Pat Moore said: "John found his SMPTE reference before I did."

No, I actually have all the SMPTE Standards, Recommended Practices, and Engineering Guidelines committed to memory!

Actually, committed to my laptop's memory: I purchased the CD-ROM set from SMPTE that has Adobe Acrobat .pdf files of ALL current SMPTE standards. Here is the link:
http://www.smpte.org/stds/index.html

For as little as $250.00, you can be as expert (and fast) as me!

Posted by Antonio Marcheselli (Member # 373) on 08-21-2000, 02:24 PM:

John,

In Italy ARCADIA has the biggest european screen: 98 feet lenght. I've seen some films on that screen but It seems that there were no problem in luminance.
Pls note that ARCADIA is known in Italy as one of the best theater in the world.
The auditorium is THX certified, of course.
Just to say that it seem strange to me...

Bye
Antonio

Posted by Brad Miller (Member # 2) on 08-21-2000, 03:08 PM:

The search is working, but only if you select one specific forum. There is a bug when you do a search of every forum at once that I have not had the time to track down yet.

Posted by John Pytlak (Member # 331) on 08-21-2000, 03:10 PM:

Antonio: If they are really getting 16 footlamberts on a 98-foot wide screen, it would be an engineering breakthrough! I could imagine a 70mm print with a very large lamp (e.g., 10,000 watts) delivering that kind of light, but not with a 35mm print. Please find out what equipment they have, and what the light measurement actually is. I suspect that the light level is actually below standard if you measure it.

Posted by Pat Moore (Member # 380) on 08-21-2000, 03:51 PM:

I'm with you, John. I want to see that 98ft screen with 16fL of illumination, and the equipment doing it.

John, seems like I remember some research done not all that long ago that determined (basically) the bigger the image, the less illumination was required to make the image effective. There was a train of thought saying that very large (appearing) images have so much visual information that it's overwhelming to the brain if it's illuminated at "normal" standards.

Are you aware of anything like this?

I do know when I view a screen from close up (large appearing image) compared to farther away, the close up image sure LOOKS brighter.

Posted by Ben Wales (Member # 92) on 08-21-2000, 04:28 PM:

They are using a 10kw Xenon at a 2,500 seater open air cinema at Expo2000 in Hanover Germany.

Uncertain what projector they are using (Kinoton?), they are only screening 35mm films on a 32m wide screen.

Posted by Tim Sherman (Member # 569) on 08-21-2000, 09:39 PM:

so that means that here at my drive-in i would have to run a 30,000 watt bulb?????? oh my. honestly with that much light you wouldn't see anything on the screen but pure white. my screen size here is 50'x 118' anyone know where one would aquire a 30k watt bulb from. what would the life on a bulb like that be? one night showing? currently on that screen i run a 5,000 watt and it isn't a half bad picture. So honestly what kind of formula is there for drive-in's any ideas anyone

Posted by John Pytlak (Member # 331) on 08-22-2000, 06:35 AM:

Tim:

In practice, a 7000 watt lamp is about as big as can be used for 35mm projection. Larger lamps have larger electrodes, so the plasma of the arc is difficult to efficiently focus on the "postage stamp" sized 35mm frame. And even if you could, the concentration of energy would likely burn the film. For really huge screens, 70mm is needed, since you can then spread out the energy over a much larger area of film.

As far as drive-ins, SMPTE Recommended Practice RP 12 says that you should try to get 16 footlamberts, but you are allowed to be as low as 4.5 footlamberts. So about 1.5 watts per square foot of scope screen area would be the minimum "rule of thumb" for a drive-in. At 5000 watts, I suspect your "half bad picture" is only half of the SMPTE 4.5 footlambert minimum. It may be heresy, but you would actually improve quality if you didn't try to fill the screen, and used a smaller image, say about 40 x 96 feet, which would at least have a chance of reaching 4.5 footlamberts.

Posted by Dave Bird (Member # 490) on 08-22-2000, 07:47 AM:

We talked about this a few weeks back. All the advice I've gotten says build a smaller screen. I believe Gordon mentioned that when he installed it, the drive-in in Lindsay Ontario was getting 14.5 ftL on both screens there (and it's a great picture, especially for outdoor). Screen 1 was 40ft.
wide with 2K Xenon, Screen 2 was 60ft. wide with 3K, both for 200 +/- cars. Is the indoor goal also around 16 ftL? Screen 1 is a Magnarc conversion, and I think 2 was a Xetron. Picture was fabulous the night I was there, but the owner had ordered new bulbs just the same.

------------------
Dave Bird

Posted by Gordon McLeod (Member # 33) on 08-22-2000, 08:13 AM:

Screen 2 is a xenex2 lamphouse

Posted by John Pytlak (Member # 331) on 08-22-2000, 11:05 AM:

Dave:

Standard SMPTE 196M specifies a screen luminance aim of 16 footlamberts for indoor theatres, with an allowed range of 12 to 22 footlamberts. SMPTE RP 12 sets the same aim for drive-ins, but allows as low as 4.5 footlamberts. It is certainly possible to achieve 16 footlamberts on a 25 x 60 foot screen, but any bigger with 35mm prints risks heat damage to the film and other problems, so the really large screens are probably well below 16 footlamberts.

I agree that getting 14+ footlamberts on a moderate-sized outdoor screen should look "fabulous". But actually MEASURING the light with a screen luminance meter is the only way to be sure what the light level actually is. Some pictures can look okay at 10 footlamberts, but they will look much better at 16 footlamberts.

Posted by Rory Burke (Member # 512) on 08-22-2000, 03:30 PM:

John P: I meant I was going to try and compare all of the "rule of thumb" equations and with my light meter was going to go around all of my screens and compare to see with "rule of thumb" equation was the closest. All of them are THX certified with 16ft/L with the exception of a lamp house here and there that needs a bulb changed out.
Is it possible that one of them coud yeild the best results?

Rory

Posted by John Walsh (Member # 168) on 08-22-2000, 04:14 PM:

Rory: If you would like to make the "rule of thumb" formula more accurate, it might be interesting to make a small survey of the theaters you compare. Recording stuff like the type of screen, (gain or "matte") make and focal length of lenses, throw, screen size, lamphouse type, etc. If you copy that data here, and several other people do the same, then all of us could evaluate it and find a good formula.

I know you may not have time to do all of this. But the reason I suggest it is because there are so many different factors that effect the light. For example, if you measure two different screens, but one has a gain screen and the other doesn't, it will throw off the accuracy of the formula.

THX doesn't get real involved in the "picture" part of theaters. I mean they do make sure the image is not serverly cropped, and it is decently lit, but just because the theaters are THX approved does not mean you are compairing "apples to apples."

Generally speaking, I found that the height x height x 12 formula works to 60mm. Then I would add 5% for each 5mm step less. Even then, we would buy the next bigger lamp, even if the wattage value fell on a standard lamp size.

For top/bottom masking situations, we found that the light could be improved by adjusting the tilt of the projector to the correct angle for scope first. Then, move the framing knob and file a plate for flat. The light is a little better than doing it the other way around. I thought I was so clever when I though of this, but then found a SMPTE article that, although didn't say this directly, implyed it strongly.

I think that without doing a real careful survey, the existing "rule of thumb" is pretty good. But, I would think it kinda fun to work out a better one.

Posted by Pat Moore (Member # 380) on 08-22-2000, 07:53 PM:

A formula as discussed above would be kind of interesting, but there are a lot of variables to get into it.

Start with screen gain, then factor in its curve, and then audience distribution, and masking type, and lenses and film formats and the amount of cropping of each format, and xenon system efficiencies, a bulb degradation factor, and then make an allowance for focus drift...

Might be better to take a basic formula, look at one or two other major variables and then add a little "Kentucky Windage" based on experience.