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» Film-Tech Forum   » Operations   » Large Format Forum   » Intermittant "Dirty" Spot (Page 1)

 
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Author Topic: Intermittant "Dirty" Spot
Mark Lensenmayer
Phenomenal Film Handler

Posts: 1572
From: Upper Arlington, OH
Registered: Sep 1999


 - posted 05-10-2006 08:22 PM      Profile for Mark Lensenmayer   Email Mark Lensenmayer   Send New Private Message       Edit/Delete Post 
Can anyone explain this?

AMC Easton 30 Imax MPX (I call it "Imax Jr")
4:10pm 5/10/06 4:10 presentation
"V for Vendetta"

Film starts, and I notice what appears to be a dirty spot on the screen, about 6-8' right of center and almost mid-point up-down...right where it is very noticeable.

Then, I notice it is gone...then back again...then gone. So, I started counting. About 20 seconds there, then 20 seconds gone. It did not appear to be a scratch on the film as it never varied in location or size during the entire film. Looked more like a dirty spot in the optical chain. I've never seen anything quite like this.

Usually dirt specks in Imax move at least a little bit...this was absolutely stationary.

Drove me NUTS. Can any Imax folks out there explain this?

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

Posts: 3832
From: Albuquerque, NM
Registered: Feb 2000


 - posted 05-10-2006 08:41 PM      Profile for Paul Mayer   Author's Homepage   Email Paul Mayer   Send New Private Message       Edit/Delete Post 
Probably something sticking to the back side of the field flattener quartz.

If the field flattener is set to auto-cycle, it moves up or down every 20 seconds or so. The film side of the flattener is wiped by brushes as the glass moves, which usually remove anything deposited on the glass by the film as each frame is rolled on to and then off of the glass.

The other side of the flattener doesn't get brushed - sometimes things do end up back there. The backside of the glass is far enough from the film to render small particles out of focus and invisible. But a big enough particle could produce a dark soft-edged spot within the field.

That's my guess anyway.

[Edit: Removed certain evidences of dumbness ^_^;; ]

[ 05-11-2006, 12:51 PM: Message edited by: Paul Mayer ]

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Dan Suomi
Film Handler

Posts: 53
From: Aurora/Oswego, IL
Registered: Jul 2004


 - posted 05-11-2006 09:07 AM      Profile for Dan Suomi   Author's Homepage   Email Dan Suomi   Send New Private Message       Edit/Delete Post 
You are correct Paul.

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Mark Lensenmayer
Phenomenal Film Handler

Posts: 1572
From: Upper Arlington, OH
Registered: Sep 1999


 - posted 05-11-2006 03:16 PM      Profile for Mark Lensenmayer   Email Mark Lensenmayer   Send New Private Message       Edit/Delete Post 
Thank you, Paul and Dan. I thought the Field Flattener was involved, but I wansn't totally sure of how that worked on the MPX.

Just for the record, I spoke with the Projection Manager of the Easton 30 after the film. After I explained the problem, he told me the problem was on the film itself, it lasted much longer than 20 seconds (the film is dark, so you can't see it all the time), and he would tell the operators to "change the pad rollers".

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John Pytlak
Film God

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


 - posted 05-11-2006 03:27 PM      Profile for John Pytlak   Author's Homepage   Email John Pytlak   Send New Private Message       Edit/Delete Post 
As Paul and Dan noted, dirt particles that remain on one portion of the image for about 20 seconds, and then get "swept away" only to come back again, were likely "stuck" on the field flattener and not removed by the wipers. Projectionists sometimes treat the quartz field flattener with anti-stick compounds (e.g., "Rain-X") to reduce the risk a dirt particle will stick.

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Brian Michael Weidemann
Expert cat molester

Posts: 944
From: Costa Mesa, CA United States
Registered: Feb 2004


 - posted 05-12-2006 05:35 AM      Profile for Brian Michael Weidemann   Author's Homepage   Email Brian Michael Weidemann   Send New Private Message       Edit/Delete Post 
The other week I came in for a shift and the show that had been running had a big, blurry, dirty patch that wouldn't wipe away. And, sure enough, it was a static clinging piece of debris on the backside (the non-wiping side) of the field lens. When that happens, you just have to turn off the autowipe and do a quick, manual, double wipe during the dark scenes ... for the whole rest of the show! [thumbsup]

Every now and then, yes, we get that stubborn piece of dirt that SHOULD wipe away, but just plain DOESN'T. (We use an alcohol/Rain-X solution every show.) Same thing, in that situation: creative manual wipes!

quote: Mark Lensenmayer
and he would tell the operators to "change the pad rollers".
You sure he didn't say "PTR rollers"? [Confused]

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Mark Lensenmayer
Phenomenal Film Handler

Posts: 1572
From: Upper Arlington, OH
Registered: Sep 1999


 - posted 05-12-2006 06:41 AM      Profile for Mark Lensenmayer   Email Mark Lensenmayer   Send New Private Message       Edit/Delete Post 
Brian Michael Wiedemann said:
quote: Brian Michael Weidemann
You sure he didn't say "PTR rollers"?
That is possible, but it sure sounded like "pad rollers".

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Patrick Watkinson
Film Handler

Posts: 19
From: Ottawa, Ontario, Canada
Registered: Mar 2007


 - posted 03-28-2007 11:40 PM      Profile for Patrick Watkinson     Send New Private Message       Edit/Delete Post 
The MPX has a field flattener that does cycle up and down. Its stepper motor can be set to cycle every 45, 30, 15 seconds or put to manual. The unfortunate part about the design is that the plate cannot be cleaned by hand during a show, half of it is being used to project the picture and the other half just sits there. Opening any door will result in a projector alarm, and the machine should perform a controlled stop. Also, the two sets of wiper bar material do not oscillate or move; it is stationary, therefore it does not always remove the dirt that accumulates on the film (the stuff that the PTR’s fail to pick up. It is very frustrating to say the least… [Frown]

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Sam King
Expert Film Handler

Posts: 118
From: Los Angeles, CA
Registered: Oct 2006


 - posted 03-28-2007 11:57 PM      Profile for Sam King   Email Sam King   Send New Private Message       Edit/Delete Post 
GRRRRR, pet peeve...saying "PTR Rollers" is like saying "ATM Machine"

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Monte L Fullmer
Film God

Posts: 8151
From: Nampa, Idaho, USA
Registered: Nov 2004


 - posted 03-29-2007 12:49 AM      Profile for Monte L Fullmer   Email Monte L Fullmer   Send New Private Message       Edit/Delete Post 
quote: Sam King
saying "PTR Rollers
PTR rollers (quite the mouthfulthere - you like saying Automatic Teller Machines?? instead of saving breath by saying ATM's)...transferring particles from one end to the other...good for rolling up particles of dust off the carpets...

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John Pytlak
Film God

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


 - posted 03-29-2007 12:17 PM      Profile for John Pytlak   Author's Homepage   Email John Pytlak   Send New Private Message       Edit/Delete Post 
As the guy who helped develop PTR film cleaning technology at Kodak, I need to say that they are very effective in many applications. Almost every lab and telecine transfer facility uses them for cleaning printing originals. For IMAX prints, they are also very effective. Remember, IMAX films cannot have any solvent or film treatment residue on them since they come into intimate contact with the field flattener. Likewise, any residue on a printing original could cause issues in processing if it transfers to the raw stock.

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Brian Michael Weidemann
Expert cat molester

Posts: 944
From: Costa Mesa, CA United States
Registered: Feb 2004


 - posted 03-30-2007 08:39 PM      Profile for Brian Michael Weidemann   Author's Homepage   Email Brian Michael Weidemann   Send New Private Message       Edit/Delete Post 
I agree with the pet peeve. It's the redundancy ... PTR Rollers, ATM Machine, PIN Number, HIV Virus: the last letter stands for the word you're saying! [Roll Eyes]

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John Wallington
Film Handler

Posts: 1
From: Frankford, ON, Canada
Registered: Jul 2003


 - posted 08-27-2007 03:35 PM      Profile for John Wallington   Author's Homepage   Email John Wallington   Send New Private Message       Edit/Delete Post 
Bravo Sam King:

I hate the use of the word following the letters of which the last letter stands for. I am not active anymore as a Projectionist :-( I am now a railroad locomotive engineer. We have a device on the railroad called an LCS (local control switch). It drives me nuts when someone calls it an LCS switch!! I thought I was the only person that felt that way.

..........John W.

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Frank Angel
Film God

Posts: 5111
From: Brooklyn NY USA
Registered: Dec 1999


 - posted 09-04-2007 03:36 AM      Profile for Frank Angel   Author's Homepage   Email Frank Angel   Send New Private Message       Edit/Delete Post 
quote: John Wallington
I hate the use of the word following the letters of which the last letter stands for. I am not active anymore as a Projectionist :-( I am now a railroad locomotive engineer. We have a device on the railroad called an LCS (local control switch). It drives me nuts when someone calls it an LCS switch!! I thought I was the only person that felt that way.

But that's what we get for using acronyms for everything instead of real words. The brain needs the language anchor -- if you are talking about a thing,, then you naturally want to use an actual word - the noun - to describe it. Your list of acronyms proves this; most people will put the noun they are talking about after the acronym, and to the ear, it sounds perfectly natural. It's a language thing; people use words, not abbreviations when they are speaking -- the abbreviations only work in text. See how awkward it sounds when you say it out loud: He just found out that he's contracted the HIV. In speech, your mind wants to hear a real word, not an acronym.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 11699
From: Annapolis, MD
Registered: Dec 1999


 - posted 09-04-2007 06:27 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
For me, it is Throw-Distance...you can have projection distance or Throw but Throw-Distance is repetitive.

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