Film-Tech Forum ARCHIVE: Different light from identical lamphouses

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Posted by Thomas Jonsson (Member # 1902) on 02-06-2006, 03:12 PM:

I run change-over with two Victoria 5 projectors. Both are exactly
identical, with Zenith X 4000 H lamphouses (4 kW horizontal bulbs,
metal reflectors and heat filters). To get the same illumination on
the screen, the left projector runs with 130 amps and the right
with 145 amps. The light from the left projector is a little blue
and from the right a little yellow.

Bulbs has been changed regularly, heat filters and reflector has
been changed once. It´s still the same blue/yellow light, and the
+15 amps on the right lamphouse.

Any idea what´s causing this, and what could be done about it?

Thomas

Posted by Michael Schaffer (Member # 1204) on 02-06-2006, 03:15 PM:

Assuming both have xenon lamps in good condition and the optical alignment has been checked, the most typical problem could simply be that the ampmeters are not calibrated properly. Did you check those current values with a clamp-on ampmeter?
In any case, 145A is a little high for a 4k lamp. That might cause other problems.

Posted by Thomas Jonsson (Member # 1902) on 02-06-2006, 03:35 PM:

The current haven´t been checked with a clamp-meter. You might be
right on that one! I increase the current at the end of the bulbs
life (wich is the case now with roughly 750 hours). I usually run
them some 10 amps lower. I always replace both at the same time.

The optical system has been aligned a couple of times by a tech.
It´s still the blue/yellow light. I don´t think anyone notice it
when running a film, but I still think it´s strange.

Thomas

Posted by Michael Schaffer (Member # 1204) on 02-06-2006, 04:25 PM:

Another factor here is obviosusly the lenses. After checking your ampmeter with a clamp-on, you should swap the lenses. Just for testing, even if lenses of the same type are theoretically all the same, in reality there are variations, so your aperture might appear to be off when you swap the lens. But to test these variations in light output and quality, a little swapping is helpful. Do you have a light meter to read the fL levels? Maybe one of the two reflectors is discolored and/or hazy. But I would check the current first to establish whether you have a variation there, and then proceed with the other steps.

Posted by Brad Miller (Member # 2) on 02-06-2006, 04:49 PM:

I would blame the reflectors. The left one (bluer) has a higher color temperature than the right one. A higher color temperature is generally perceived as brighter.

Posted by Mark Gulbrandsen (Member # 72) on 02-06-2006, 05:45 PM:

I agree with Brad n the mirror thing. It could also be the lenses not being color matched very well. Swap reflectors and lenses and see what follows what in color temp shift.

Mark

Posted by Thomas Jonsson (Member # 1902) on 02-07-2006, 09:18 AM:

The reflectors was replaced less than a year ago, but it´s not
impossible they are different in the same way as the old ones.
I will try your advice on that part next week when I have a
little less shows.

I tried the lens thing, and yes - there was a little difference.
Not much, but noticeable. After that I re-aligned the reflectors,
and achieved almost the same color and brightness on both lamps.
But there is still the blue/yellow color, although a little less
now.

Thanks for your good advice!

Thomas

Posted by Jack Ondracek (Member # 1466) on 02-08-2006, 10:31 PM:

Did I miss whether you're running the same brand of bulb in both lamphouses?

Posted by David Kilderry (Member # 142) on 02-09-2006, 05:36 AM:

Thomas, we had the same problem once with a pair of Bauer U3's. It turned out to be the reflectors. Same brand, they looked identical, but the colour temperature was different and so was the on screen light.

Lamps can also be the reason, especially if they are different brands (or age). I have always found Ushio/Christie on the bluer side compared to Osram.

Another major variable here is the heat filters. Some that have absorbed more heat, due to atomised oil spray or other contaminants on the surface, can change the colour and brilliance of light as it passes through it. Same goes for lenses.

The alignment of the lamphouse is another possibility; the Zeniths could be tricky to align and often varied from unit to unit. Alignment variation could certainly account for less light and the change in colour temperature as you would be focusing a slightly different part of the flame.

One more thing, has this always been a problem with these two projectors? If it has only happened in recent times, what changed?

Good luck with your trouble shooting.

[ 02-09-2006, 06:59 AM: Message edited by: David Kilderry ]

Posted by Thomas Jonsson (Member # 1902) on 02-09-2006, 11:15 AM:

This has been the same for as long as I can remember. I use the
same type of bulb (Osram) and I see to it they both have the same
hour count. Both are always replaced at the same time. Both heat
filters and reflectors was replaced at the same time. Still the
blue/yellow colors.

I will try the advice on the reflectors next week, and see what
happens. It might be a coincident I got a blue one on the left
and a yellow on the right projector once again, when I had them
replaced a year ago.

I have only seen Cinemeccanica projectors with Cinemeccanica
lamphouses on them. Has anybody tried Kinoton or any other make,
and if so - with what result?

Thomas

Posted by Frank Angel (Member # 248) on 02-09-2006, 02:59 PM:

It's a far stretch, but pull out the port glass just on the off chance that at some point in time someone replaced optical glass with plate glass on only one of the windows.

Posted by Thomas Jonsson (Member # 1902) on 02-09-2006, 03:25 PM:

Thanks for the tip, but both port window glasses have the
same classification marks. So I guess that´s not the problem?

Thomas

Posted by John Pytlak (Member # 331) on 02-13-2006, 10:08 AM:

Heat filters / dichroic coatings are the most likely source of noticeable color differences. Lamp focus can shift color, depending upon with part of the xenon plasma is at the focus point of the reflector. Lamp current usually has minimal effect on color.

Posted by Greg Mueller (Member # 17) on 02-13-2006, 11:01 AM:

I noticed the same thing as a kid, when my folks took me to the movies. I asked my mom why sometimes the picture looked more yellow than at other times. She hadn't noticed the difference. The theater was doing change overs (of course) and it was sometime in the 50s

Posted by John Pytlak (Member # 331) on 02-17-2006, 02:29 PM:

quote: Greg Mueller
I noticed the same thing as a kid, when my folks took me to the movies. I asked my mom why sometimes the picture looked more yellow than at other times. She hadn't noticed the difference. The theater was doing change overs (of course) and it was sometime in the 50s

The trim conditions and operating current for a carbon arc can have a significant effect on color. Position of the positive crater and the plasma relative to the focal point of the reflector is a key parameter.

Posted by Greg Mueller (Member # 17) on 02-17-2006, 03:43 PM:

Interesting...
I didn't mention that they were B&W prints which made it even more obvious. And of course (as you mentioned) they were running carbons

Posted by Richard Fowler (Member # 893) on 02-17-2006, 05:18 PM:

I have used Cinemeccanica V-8 with Christie SLC, Xetron, ORC and V-5 on Strong Highlite, Kneisley Xenex II in addition to Cinemeccanica. The obvious one in color ( blue )was the Christie.