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Topic: What size Xenon to use????

Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 07-28-2002 04:23 AM

Please excuse my ignorance everyone, but is there a chart or calculation to determine what wattage Xenon is required to light a certain size screen? In this case the screen is 3.35 meters high, and the throw is 15.5 meters. How do I work this out?

BTW , haven't been around lately because .....

I was at one of our other cinemas last weekend replacing a sound system, where I hurt my leg I was on the platform that supports the right stage channel speaker ... (a sturdy construction .. or so I thought) .... when it collapsed with me on it!! Falling 2m or so and been slammed in the legs with snapped timbers isn't my idea of a fun weekend! Luckily the old speaker fell away from me!

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 07-28-2002 06:30 AM

A "rule of thumb" is 5 watts for each square foot of screen area for a matte white (gain=1) screen. Assumptions include a 2-blade (50% efficient) shutter, modern lens design, good alignment, etc.

------------------
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

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Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 07-28-2002 06:47 AM

Thanks John. Do I calculate based on 'C/S' or 'W/S' screen area? Also, the screen looks to have a 'slight sparkle' to it... some sort of coating, but I wouldn't know what type. Any advice on how I wouls 'guesstimate' what the gain of my screen is?

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 07-28-2002 07:08 AM

I believe the scope area should be used (assuming common image height among formats). You probably have a moderate gain "pearlescent" screen, maybe around 1.4 gain. You can measure the gain by comparing the light reflected from the screen to the light reflected from a sheet of white paper of near "100" brightness rating. A gain screen will look brighter "head on", but dimmer when viewed at an angle.

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Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 07-28-2002 07:26 AM

Thanks again John ... I will work on your advice.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 07-28-2002 02:07 PM

Depending on the projector and lenses used, I prefer 12h^2 or 13h^2. Where h = the height of the 1.85:1 image. Thus an older system with a 10 foot tall screen would need 1300 watts. If you have an efficient projector with one of the newer efficient lenses (like the ISCO Ultra Star PLUS lenses) that could be dropped to 1200 watts.

As to gain screens, if they are pearlescent, they are normally in the range of 1.5-1.8. If it is a Stewart brand screen it will be as it is claimed (they offer several gains).

Steve

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"Old projectionists never die, they just changeover!"

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Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 07-28-2002 03:59 PM

Sure would be cool if Hurley or Harkness had a nice computer program where you could plug in the screen size, lens speed and shutter efficency, and it would spit out the bulb wattage needed for each screen type. Or what screen type you would need for an existing lamp size....like Schneider and ISCO have for their lenses. It would be a good marketing tool for them and it would make life easier for the rest of us.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 07-28-2002 10:05 PM

There would be enough unknowns in there to beg the question that if you are that savy, just use your calculator.

Also, it would force Hurley and other common screen manufacturers to have a consistant gain.

Steve

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"Old projectionists never die, they just changeover!"

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 07-29-2002 07:59 AM

The other variable, of course, is lamphouse efficiency. I think the above are assuming current horizontal lamphouse designs.

Pat

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David Rigby
Expert Film Handler

Posts: 134
From: Chorlton, Manchester, UK
Registered: May 2002

posted 07-29-2002 08:50 AM

Speaking of lamphouse efficiency, are vertical lamphouses always a poor second to horizontal ones? Are there any advantages at all of a vertical one?

David

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 07-29-2002 10:38 AM

Steve's formula and my "rule of thumb" give about the same answer:

For a 10 x 23.9 foot screen (239 square feet)

My rule of thumb: 5 x 239 = 1195 watts

Steve's 12h^2 = 12 x 10^2 = 1200 watts

Pat is correct that lamphouse efficiency is assumed to be typical of a modern lamphouse with good optical alignment.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 07-29-2002 10:44 AM

The vertical lamphouse design of the ORC Optimax theoretically was more efficient in reducing infrared radiation, by using a 45-degree heat mirror that reflected visible light and passed the IR to a heatsink. As others have said, alignment was difficult, and the vertical lamp was prone to "candlelight flicker" due to a relatively unstable plasma.

Likewise the vertical lamp design of the old Zeiss Ikon Xenosol III required two mirrors and "honeycomb" condensers, complicating alignment. This was the first xenon lamphouse I ever had to work with.

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David Rigby
Expert Film Handler

Posts: 134
From: Chorlton, Manchester, UK
Registered: May 2002

posted 07-29-2002 11:37 AM

I hadn't thought about the visibility of the arc length causing a shimmering effect, but obviously it could do! How do vertical lamphouses get around this (and if it makes them a more complex engineering task why does/did anyone bother - were vertical lamps the only available type at one time?). Are any modern machines still supplied with vertical lamphouses?

Thanks for the info guys - this is interesting stuff!

David

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 07-29-2002 11:54 AM

The shimmer effect is more noticable on the vertical lamphouses with a 45 degree folding mirror due the reflector collecting the arc image 360 degrees around the bulb of the vertical arc plasma vs the older type ( if properly adjusted ) vertical type with a butterfly type reflector collecting the arc plasma from the side at 120 to 180 degree. The advantage of vertical is longer bulb life but in today's competitive market, bulbs are cheap. Basic rule is horizontal should be 30% more efficient than old style butterfly verticals and the one with the folding mirror "should" equal horizontal iif everything is perfect

Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com

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Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 07-29-2002 01:18 PM

John said: >. Assumptions include a 2-blade (50% efficient) shutter, modern lens design, good alignment, etc.<

But what difference would the age of the lens make if the speed is the same? Wouldn't an old SnapLite with a stated speed of f2 deliver the same light pass-thru as a modern lens that is also rated at f2? In other words, isn't the f rating of a lens a constant?

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