Film-Tech Forum ARCHIVE: Measuring screen luminance (without destroying the lens)

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Topic: Measuring screen luminance (without destroying the lens)

Michael Barry
Jedi Master Film Handler

Posts: 584
From: Sydney, NSW, Australia
Registered: Nov 1999

posted 11-21-2000 09:28 AM

It seems to be widely accepted that projecting raw light from a xenon lamphouse straight through a lens without any film in the gate can damage a lens. At the same time, SMPTE specifies measuring screen luminance using this 'open gate' method.

Is there a way to do this safely without jeopardising projection lenses in any way?

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 11-21-2000 10:08 AM

Get a helper in the booth to open the douser only when you are in the auditorium ready to take readings. You don't need to take readings all at once. Do them one at a time and allow the lens to cool for a few seconds between each reading.

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 11-21-2000 10:27 AM

If you are using a matte-white screen, I think it would be OK to take readings from the booth where you could operats the dowser.

We usually did 15 seconds "on" then 15 seconds "off."

A guy I know used to test lenses using the SMPTE 35PA. (This was before it was made on tougher polyester stock.) He and I hooked up a wired remote control switch that would operate the zipper shutter. It would dange out the port window to the auditourium. That way, he could go right up to the screen and see the focus/resoultion charts, but if the film broke, he could hit the remote to shut the zipper and protect the lens.

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Rory Burke
Expert Film Handler

Posts: 181
From: Burbank, CA, USA
Registered: Jun 2000

posted 11-21-2000 05:23 PM

This brings about many qustions I guess. Yes it is proper ettiquette to keep the dowser closed when projecting white lite thru the screen while checking Luminance as long as possible...but how many times has a service technician entered a theater booth to find a projector that just finished running manually sitting there blasting white lite for God knows how long while the projectionist has been, the entire, time downstairs hitting on the candy girls. Ultimately these typical lenses ie. ISco and Schneider are made of steel(metaphorically speaking) and can handle much abuse ultimately. I am sure schneider will at least back that up. Also it is important to note that while you are checking light Luminance levels that the motor is also on as well. I know it seems silly to point on the basics but this Low paying Industry has much ineptness running amuk and much doesn't surprise me anymore nowadays.

Rory

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Dave Bird
Jedi Master Film Handler

Posts: 777
From: Perth, Ontario, Canada
Registered: Jun 2000

posted 11-21-2000 06:36 PM

I know a drive-in where they regularly fired pure light onto the screen during intermission so kids could make hand-puppets from in front of the ground-level ports. Don't know if they used an old lens or not. And I didn't see it happen this year, though I suspect that's more as a result of rude kids than anything else.

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 11-21-2000 08:31 PM

In every industry, there are always people who will not follow proper procedures, and get away with it.

I suspect the reason many theaters get away with projecting white light is the lamp is not bright enough- so the picture is dark, too.

"Burning" the lens is the thing I've heard will happen if the light is left on too long. I'm not exactly sure what that means; I guess it means the bluing will discolor. I've never seen it happen.

What I have seen is that the back element lens (closest to the film) crack. I've only seen two lenses that this happend to; both had a vertical crack about in the center of the rear element. We actually ran like that for a week with one of them. Since the crack was at a right angle to the light, there was very little distortion visible on screen.

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Greg Mueller
Phenomenal Film Handler

Posts: 1687
From: Port Gamble, WA
Registered: Jun 99

posted 11-21-2000 09:09 PM

I've seen older lenses that had cemented elements, get bubbles in the "glue" from being cooked with to much light/heat. Most modern lenses are "air spaced" now. I suppose you could "burn" the anti-reflective coatings etc. and probably crack the elements

------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut

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David Kilderry
Master Film Handler

Posts: 355
From: Melbourne Australia
Registered: Sep 1999

posted 11-22-2000 04:38 AM

It comes down to the amount of heat you are putting through the lens. If you have a 2,000 watt lamp then even 45 seconds is fine once every few months for an alignment and measure. On the other hand, if you are running 7,000 watts like we just installed in one of our new screens, you don't want to be going over 20 secs too often. The heat not only effects the balsam but also the elements and blooming. Once you have "cooked a lens, there is no going back.

I always have a projectionist open the dowser for only the amount of time I need to take my readings, measuring from the booth is to be used as a rough guide only.

David

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 11-22-2000 08:36 AM

All new quality lens are airgapped and don't use balsam for cementing elements
I use the rule 30sec of light 1 min to cool

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Michael Barry
Jedi Master Film Handler

Posts: 584
From: Sydney, NSW, Australia
Registered: Nov 1999

posted 11-22-2000 09:55 AM

David K: What kind of heat filtration or air/water cooling system are you using for the 7K? What kind of lamphouse and lenses and what is the screen size? Did you measure the screen luminance?

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David Kilderry
Master Film Handler

Posts: 355
From: Melbourne Australia
Registered: Sep 1999

posted 11-24-2000 04:30 AM

Michael,

The projectors are Kinoton FP50 (our spec) the lamphouse uses the usual Kinoton glass heat filter with a water cooled gate. The water circulates through a water bin via a submersible pump. Head above gate is coll at all times. We run 5K in Kinoton FP50 machines without water cooling and have no problems. Most other brands can not handle the heat with these lamps. Screen width is 17 meters light is between 18 and 22 footlamberts. Lenses are brang new Schneider. As a matter of interest the smallest lamps in the complex are 4K!

The complex is located in Glenorchy Tasmania and opened a few weeks ago. It will be joined in 3 weeks by another complex at Eastlands in Tasmania also with 7K on the big screen.

You would be shocked at how low the light is on most big screens run by the other two majors in this country. Many of the big screens at one chain that are branded with a catchy title have less than 10 footlamberts!

David

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John Pytlak
Film God

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

posted 11-27-2000 07:05 AM

When running "open gate", there is no film in the gate to absorb some of the energy. Even film that has a very light image (e.g., white sky or snow scene) transmits only 50 percent of the energy, and typical scenes only let about 10 percent of the energy go through the lens. So, when running "open gate" to measure screen luminance, there is a risk of damage to the lens, especially with lamps larger than 2000 watts. Old lenses with cemented elements are the most likely to be damaged by excessive heat, but even modern lenses could be damaged with very large lamps. Gordon's suggestion of 30 seconds of light, and at least 60 seconds to cool between exposures is a good one.

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

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Aaron Sisemore
Flaming Ribs beat Reeses Peanut Butter Cups any day!

Posts: 3061
From: Rockwall TX USA
Registered: Sep 1999

posted 11-27-2000 08:34 PM

I have had the pleasure of having to replace several fried lenses (all of them 50's B&Ls and 50's-60's Kollmorgens) and thge typical cause is the lamp manual override being left on (CFS consoles/ORC/PerkinElmer bulbs, natch) and the film wrapping and going white screen and the dowser failing to close, etc etc... The damage is always the same: a dark brown burned spot in the center of one or more of the lens elements. in some situations the rear element actually cracked! John is correct, however that most if not all newer lenses made with air-spacing won;t discolor from overheating, but may still crack...

Aaron

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