Film-Tech Forum ARCHIVE: Xenon color temp question

This is topic Xenon color temp question in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Jeff Taylor (Member # 443) on 01-23-2002, 09:47 AM:

Does anyone know if there is any change in the color temp of xenons when you drop back a bit on amperge (thanks to Film-Tech I never go below 80%)? I haven't noticed anything significant--certainly not like incandescent or halogens--but would be curious to know as some prints with less density do look too bright at full amperage.

Posted by Pat Moore (Member # 380) on 01-23-2002, 10:28 AM:

Jeff;

The testing I've done previously shows no change in color temperature due to amperage changes. I suspect there is a color temp change over the life of the lamp, particularly if the quartz envelope darkens, but I don't claim to have any good data on that.

Pat

Posted by Jeff Taylor (Member # 443) on 01-23-2002, 10:44 AM:

Thanks, Pat. That confirms what I suspected. My screening room setup is really a little too bright much of the time with the alternative to lower amperage being defocusing the lamp, which I wanted to avoid if possible. Appreciate the info!

Posted by John Pytlak (Member # 331) on 01-23-2002, 12:58 PM:

If you have too much screen luminance, you can reduce light level by reducing current, defocusing the lamp, or perhaps using a three-blade shutter. Reducing current slightly (but staying within the operational current range specified for the lamp) shouldn't significantly change color temperature. Going too low may cause premature blackening of the envelope (running too cool) or arc instability. Defocusing will tend to improve light uniformity, but may shift color slightly. Backing off focus too far might produce a "doughnut" light distribution, where the screen edges are brighter than the center. A three blade shutter will halve the light level compared to an equivalent two blade shutter, but will greatly reduce flicker perception. Increasing the shutter blade angle is also an option to reduce light. If you have light "to burn", I favor consideration of the three blade shutter.

Don't forget that SMPTE 196M allows theatres to be as high as 22 footlamberts. Some feel that pictures actually look better at higher luminance, but the aim is 16 footlamberts:
http://www.kodak.com/US/en/motion/newsletters/reel/spring98/pointers.shtml

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Frank Angel (Member # 248) on 01-25-2002, 04:12 AM:

A fellow projectionist had the same problem using 70mm in a small auditorium. He felt the 70mm image was just too bright, but not the 35mm image. Refocusing the bulb and/or dropping the amperage -- these would affect his 35mm image which he didn't want to do. His solution was to use "black wrap" -- a kind of black aluminum foil that they use in stage lighting -- to reduce the light. He cut out a circle in the center of a piece of this foil, slightly smaller than the back element of his 70mm lens and then centered the cut-out at the rear of the lens barrel (obviouly avoiding any contact to the lens element). This reduced the light output. He claimed there was no vinetting problems and the light reduction was uniform -- no center hotspots. I am not sure if this would be true for all focal lenghts and for every optical system, but it sure is a cheap way to go and worth checking it out. He said he had to cut a few sizes to find the just the right diameter cut-out.

Also, if all else fails, Schneider does makes neutral density filters for cameras that, in theory, should work perfectly well with projection lenses. Only question here is, how much heat can they stand. You may not be able to mount them directly to the front of the lens with out damage from radient heat. But you probably could get them in the large formats (I think they make them in 4x4in square glass plates) and mount them a further distance in front of the lens. But these are quite a bit more expensive than black-wrap!

Posted by Jeff Taylor (Member # 443) on 01-25-2002, 08:06 AM:

Thanks for the suggestions, guys. The one point I should have added is that while I have too much light for some flat prints, especially ones that are not really dense, I can use all of it for scope. For that reason I wanted to avoid any "permanent" fixes and keep the ability to put all my lumens on the screen when I need them. The black wrap or neutral density filters sound worth a little experimentation.

Posted by John Pytlak (Member # 331) on 01-25-2002, 10:43 AM:

Yes, aperture "stops" in a lens can be used to cut down on light level and increase depth of focus. But the optimum positioning of the "stop" will vary with the lens design, and may be internal to the lens. Check with the lens manufacturer for their advice for your particular lens.