Film-Tech Forum ARCHIVE: Lamphouse alignment

This is topic Lamphouse alignment in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=009580

Posted by Richard May (Member # 2627) on 10-30-2008, 01:31 PM:

After speaking with Strong and searching all related topics, I am out of ideas. We have a Super Highlight II console with a 7K bulb. I have aligned it with the string and with the laser. It all checks out. Working distance is correct (33 1/8"). Left to right brightness is fine. The problem is with up and down. I can't seem to get the light to move down. It is hottest on top. It evenly gets dimmer down the screen. Any ideas?

Posted by Gordon McLeod (Member # 33) on 10-30-2008, 03:16 PM:

What is the gain of the screen and what sort of downward angle is the machine on you may be seeing the hotspot of the gain of the screen

Posted by Richard May (Member # 2627) on 10-30-2008, 03:19 PM:

It is a Silver screen but not sure of the exact gain. It is a 19 degree downward angle. The screen is 70 feet wide. Measurements with the light meter were taken from the booth.

Posted by John Wilson (Member # 269) on 10-30-2008, 04:01 PM:

When you go in the auditorium does the hot spot stay up top or does it 'follow' you wherever you go.

If it follows you, you're not going to get rid of it...it's the silver screen.

Posted by Richard May (Member # 2627) on 10-30-2008, 04:22 PM:

It doesn't seem to move with me.

Posted by Gordon McLeod (Member # 33) on 10-30-2008, 04:45 PM:

A silver screen will exhibit a hot spot usually at the top of the screen when viewed from the booth

Posted by Tim Reed (Member # 137) on 10-30-2008, 05:28 PM:

Oh, yes, that's definitely the silver screen doing that... 19 deg. is pretty steep!

Posted by Frank Angel (Member # 248) on 10-30-2008, 05:40 PM:

If it is the screen -- which does seem likely -- it can be partially remedied by tilting the entire screen frame. You can figure out how much of a tilt by standing in center of the theatre and having someone (a crew is more like it with a 70 footer) and pull the bottom of the screen so it is at a downard angle. You should see the hot spot move toward the center of the screen as the pull the bottom further and further back, basically aiming the light toward the center of the auditorium. It is not going to fix the hotspot problem other than centering it more for a good portion of the center sweet seats. If you can get a fairly even placement of hotspot in the center of the theatre, that about the best you can do. There will still be off-axis seat which will still see a hotspot -- it's just the nature of silver. Don't worry where the hotspot is as seen from the booth....no patrons sit up there. [Big Grin]

Posted by Richard May (Member # 2627) on 10-30-2008, 06:10 PM:

Shouldn't I still be able to have some control over moving the light up and down? Measuring from the booth we get 7.3 on top, 5.8 in the middle, and 4.6 on the bottom.

Posted by John Wilson (Member # 269) on 10-30-2008, 06:15 PM:

Are those measurements are foot lamberts? [Eek!]

When you adjust the lamp (or mirror) vertically, does anything happen?

Posted by John Wilson (Member # 269) on 10-30-2008, 06:17 PM:

Double post

[ 11-02-2008, 02:59 AM: Message edited by: John Wilson ]

Posted by Richard May (Member # 2627) on 10-30-2008, 06:25 PM:

Yes they are in FL. BTW, we are running 70MM with a heat filter. When I adjust the lamp down, the overall level seems to drop a bit. It moves down slightly but not enough to help anything.

Posted by Tim Reed (Member # 137) on 10-30-2008, 06:45 PM:

No. Please. You'll be running around in circles forever trying to balance that out - and it can't be done. Put a black card in front of the porthole and adj the bulb for an even field, then leave it that way. What you get on the screen you are stuck with because it's silvered. Any attempt to compensate will only throw the overall light level down. The screen surface is very directional - therefore, it's not about the rectangle of light you are putting on it, but rather the angle from which the light is falling upon it.

What Frank suggested in tilting the screen back would make it look okay from the booth (and if you have an older theatre it may already be tilted back), but then much of the light would also be reflected in that direction. The closer to vertical this silver screen is, the more light will be bounced to the orchestra. Matte white screens, of course, should be tilted for better focus.

Posted by Richard May (Member # 2627) on 10-30-2008, 06:55 PM:

That would explain a lot Tim. I have been on the phone with Strong all week. I aligned it 3 times with both alignment kits. I also went crazy centering the rear adjustment perfectly, and adjusting the front height of the bulb perfectly to where it not only lined up with the laser but was within Strong's height specs. I went to the extreme of measuring the sides of the bulb to see if all 4 sides were equidistant from the reflector opening. Everything was so centered and aligned and then I hit the screen and was like....oh crap!!! Oh well. I guess that will be the best it's going to be.

Posted by Louis Bornwasser (Member # 3063) on 10-30-2008, 08:42 PM:

The real answer is to engineer a proper matte white (or close) screen. This will solve your problem.

Adjustment of the projection equipment will not solve your problem; it will just confuse the matter. Tim is right.

If the silver screen is what you are stuck with, you will never solve this problem; angling the screen will only move the "best" seat around. Louis

Posted by John Wilson (Member # 269) on 10-31-2008, 01:15 AM:

What are the readings from the auditorium?

Posted by Marco Giustini (Member # 4544) on 10-31-2008, 04:21 AM:

My theater has Perlux screens and, despite many are installed in curved frames, I have the "hot spot following me" issue.
I always wonder if it's something that can be fixed with a proper installation of the screens or if it's normal with high gain screens. I understand that it's normal on flat frames.

Yes, you should be able to move the light up and down but if you center the light from the booth moving the lamp, and assuming that what you're seeing is caused by the silver screen, you should then have bottom lighter than top from the auditorium point of view.

Marco

Posted by Steve Guttag (Member # 268) on 10-31-2008, 06:48 AM:

The Perlux comes in a couple of different gains (1.4 and 1.8, I believe).

Curving a screen is not something that one should do just willy-nilly. It requires a curve that takes into acount the dispersion of the screen, the projection point and the seating area. Most screens that are installed today will tend to be too shallow to properly reflect back the light and negate hot-spotting.

A 1.5-1.8 gain screen will typically need a radius about 2/3rds the throw. As the theatre gets shorter/fatter in perspective, the shallower the curve will need to be to cover the entire audience...conversely, long-skinny theatres should have a deeper curve to even out the light better.

The 1 in 20 rule that many seem to use (and that Schneider Optics puts in their lens program)...really only works on matte-white screens or screens with gains about 1.3 or less. It isn't so much for making the light even either. It is more for effect and I find that one can also obtain some better focus in the corners on some lenses.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 10-31-2008, 08:45 AM:

Actually there are three Harkness Perlux "gain Screen surfaces available" 1.4, 1.8, and 2.2. Presently the 2.2 gain screens are only made in England but they will be produced here as well right after the first of the year...

Mark

Posted by Richard May (Member # 2627) on 10-31-2008, 08:59 AM:

Wehave to have the silver screen because we run 3-D. I will take readings from the auditorium tomorrow and let you know.

Posted by Marco Giustini (Member # 4544) on 10-31-2008, 09:21 AM:

Mine are 1.4

Thanks Steve and Mark for your replies.

Marco

Posted by Louis Bornwasser (Member # 3063) on 10-31-2008, 09:48 AM:

If you require silver-screens-for-3D, then you are SCREWED for solving this problem if you have a high angle or a wide auditorium. Suggest you move 3D to a more appropriate venue or have a (smaller) silver screen just for 3D; rolled up or removed when "real" film is projected. Louis

Posted by Steve Guttag (Member # 268) on 10-31-2008, 01:53 PM:

With a Perlux 140...you will find that a radius of 2/3 to 3/4 the throw will make it reflect reasonably evenly. Ideally, you should use some form of ray tracing to calculate the optimal radius.

I suspect that your current radius is much more shallow than that.

Steve

Posted by Marco Giustini (Member # 4544) on 10-31-2008, 02:48 PM:

Thanks Steve

I'll measure it and I'll let you know

Marco

Posted by Richard May (Member # 2627) on 11-01-2008, 12:24 PM:

Well I took all the measurements today from the booth and then from the orchestra. The results were stunning to say the least. The measurements from the orchestra were taken from the back row. Center of the screen was 5.0 from the booth. Center was 18.0 from the orchestra. I did get the same results as far as the light being hotter at the top than the bottom though. I just can't believe there is THAT much of a difference from the booth and the orchestra. It was blinding from down there.

Posted by Tony Bandiera Jr (Member # 2365) on 11-01-2008, 02:22 PM:

ARRRRGGGHHH!!! I can't stand this any longer!!!

QUIT MEASURING FROM THE BOOTH!!!

Unless you have a virtually zero projection angle, and perfectly clear OPTICAL glass in your observation ports, ANY readings from the booth (especially so with a silver screen) are TOTALLY WORTHLESS!!! The light hitting a silver screen is reflected to the audience at an angle roughly matching the angle of projection but at the opposite direction. ( I hope you get what I mean, as it's hard to say it without a picture. [Smile]

)

Your own measurements, when you finally went downstairs, shows that the light is more than the 16fl SMPTE standard.

What you need to do is use a telephone or walkie-talkie, get one person in the booth to adjust the lamp while you take readings IN THE AUDITORIUM, MAIN FLOOR LEVEL. Adjust for 16fl and shift the vertical adjustment for the best eveness top to bottom.

Be sure to only subject the lens to the light for no more than 20-30 seconds at a time, with at least as much time at rest, to avoid cooking the lens.

I strongly reccommend that you go to your local library, and look for two old but excellent books: "Robert A. Mitchell's Manual of Practical Projection" or "Motion Picture Projection" Fourth edition, by James P. Cameron

Both books have excellent discussion of lamphouse optics, silver screens and other projection advice and tips. They are very old, and even though they discuss carbon arcs, the optical theories and principles still apply to modern lamphouses.

Posted by Tim Reed (Member # 137) on 11-02-2008, 12:54 AM:

quote: Richard May
I just can't believe there is THAT much of a difference from the booth and the orchestra.

That's the silver screen doing its thing. They're very directional. You might as well be projecting onto a mirror.