Film-Tech Forum ARCHIVE: One corner of the screen noticably darker

This is topic One corner of the screen noticably darker in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Robert L. Fischer (Member # 2424) on 09-15-2004, 04:38 PM:

In one of our auditoriums, the lower-right corner of the screen is very dark -- so dark that the green band is almost black on trailers. We just changed our xenon bulb a week ago, but it is my understanding that if it is a focus issue, it would be all of the corners. But to be honest, I don't know how to focus a bulb, nor had I ever heard of such a thing until visiting Film-Tech [Roll Eyes]

. The only thing I knew to check was the aperature plate, which is free of dust. I searched through the forums and could only find problems involving all of the corners being dark. So what could be the problem?

Posted by Steve Scott (Member # 630) on 09-15-2004, 04:45 PM:

Sounds like the bulb is not aligned to the center of the screen. Around the focus adjust, you can adjust the horizontal and vertical positioning of the bulb's arc.

Posted by Robert L. Fischer (Member # 2424) on 09-15-2004, 05:00 PM:

Do you happen to know where I can make this adjustment on a XeTron XHM lamphouse?

Posted by Ian Price (Member # 14) on 09-15-2004, 05:04 PM:

First check to see if the lamp is resting on the holder straight. After that, I'm sure one of these fine fellows will guide you about checking the focus.

Posted by Dustin Mitchell (Member # 372) on 09-15-2004, 05:54 PM:

Do you guys have a service tech? If you give him a call I'm sure he could walk you through it. Never worked with that brand of lamphouse myself.

Posted by Dean Kollet (Member # 1789) on 09-15-2004, 06:23 PM:

I don't know about your lamphouse, but I'd ask someone to physically show you if at all possible. It's one of those things (to me anyways) that's very easy when someone shows you. Also, check the manuals section as there are a few XeTron manuals, one of them may be close...

Posted by Michael Schaffer (Member # 1204) on 09-15-2004, 07:48 PM:

In addition to the Xetron manual, you might find it hepful to look into other lamphouse manuals. Obviously the actual controls for the lamp adjustment are different, but they all serve the same purpose, and in some manuals, like the Christie CH series or Strong Highlite, you will also find some illustrations of what the image on screen looks like during various stages of the alignment.
Like Ian said, you should check first if the lamp is sitting on the front support in the proper position. You should also look at the image of the lamp on screen without the lens and check if it actually centered on screen. If not, then the console as a whole is not aimed properly. It will need reaiming and most likely that will also necessitate a new aperture plate. If it gets complicated, it really would be best to call your service technician.
If you find it is aimed properly, maybe it is just a small aligment issue. I would center the horizontal and vertical adjustments first to give you a good starting point. Then look at the image on screen without the lens in. Center the dark spot ( the lamp) in the dark circle (the reflector hole). Remember that what you are looking at is the image coming from the reflector, so everything is reversed. Look at the pictures in the manuals mentioned above, and work your way towards happiness from there.
Remember the alignment should be done with the scope lens and aperture plate. When you check the image with the lens in, don't let the lamp burn on the lens without film for longer then 30 seconds or so. Also remember to switch on the motor, otherwise the aperture area will overheat very quickly and it will start to smell bad...
If you have a lens turret, it is a good idea to take the flat lens out, so you can rotate the turret to compare the results with and without lens in.

Posted by John Pytlak (Member # 331) on 09-15-2004, 10:20 PM:

Since the problem occurred with the installation of a new xenon lamp, I assume the alignment of the lamphouse and working distance to the aperture are unchanged and okay. So it's a matter of lamp focus and position. With horizontal lamps, many use the "bullseye" method of roughing in the horizontal and vertical alignment --- remove the projection lens and you will see an image of the reflector and the shadow of the lamp envelope projected on the screen, looking like a "bullseye" target. You want to adjust for a symmetrical bullseye, and then adjust focus to fill the bullseye with light and no dark areas. Then with the lens in (being careful not to overheat the lens), fine tune the adjustments to get the best uniformity and correct screen luminance (16 footlamberts in the center).

The Strong Super Lume-X manual has a good description of the method.