This is topic Intermittant "Dirty" Spot in forum Large Format Forum at Film-Tech Forum ARCHIVE.


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Posted by Mark Lensenmayer (Member # 134) on 05-10-2006, 08:22 PM:
 
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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Posted by Paul Mayer (Member # 355) on 05-10-2006, 08:41 PM:
 
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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Posted by Dan Suomi (Member # 2585) on 05-11-2006, 09:07 AM:
 
You are correct Paul.
 

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Posted by Mark Lensenmayer (Member # 134) on 05-11-2006, 03:16 PM:
 
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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Posted by John Pytlak (Member # 331) on 05-11-2006, 03:27 PM:
 
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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Posted by Brian Michael Weidemann (Member # 2243) on 05-12-2006, 05:35 AM:
 
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!

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

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and he would tell the operators to "change the pad rollers".

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You sure he didn't say "PTR rollers"?
 

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Posted by Mark Lensenmayer (Member # 134) on 05-12-2006, 06:41 AM:
 
Brian Michael Wiedemann said:

quote: Brian Michael Weidemann

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You sure he didn't say "PTR rollers"?

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That is possible, but it sure sounded like "pad rollers".
 

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Posted by Patrick Watkinson (Member # 4237) on 03-28-2007, 11:40 PM:
 
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…
 

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Posted by Sam King (Member # 4029) on 03-28-2007, 11:57 PM:
 
GRRRRR, pet peeve...saying "PTR Rollers" is like saying "ATM Machine"
 

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Posted by Monte L Fullmer (Member # 2797) on 03-29-2007, 12:49 AM:
 

quote: Sam King

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saying "PTR Rollers

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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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Posted by John Pytlak (Member # 331) on 03-29-2007, 12:17 PM:
 
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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Posted by Brian Michael Weidemann (Member # 2243) on 03-30-2007, 08:39 PM:
 
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!
 

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Posted by John Wallington (Member # 1799) on 08-27-2007, 03:35 PM:
 
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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Posted by Frank Angel (Member # 248) on 09-04-2007, 03:36 AM:
 

quote: John Wallington

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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.

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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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Posted by Steve Guttag (Member # 268) on 09-04-2007, 06:27 AM:
 
For me, it is Throw-Distance...you can have projection distance or Throw but Throw-Distance is repetitive.
 

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Posted by Frank Angel (Member # 248) on 09-04-2007, 07:43 AM:
 
Here's one that even network news anchors get wrong. I hear it over and over. They say EXetera. Which of course anyone who watched THE KING AND I and listened to Yul Brynner knows it's the Latin and it's pronounced exactly like it's spelled ETcetera.
 

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Posted by Martin McCaffery (Member # 37) on 09-04-2007, 11:27 AM:
 
Well, before the grammer police shut us down, my current pet peeve on this topic is WMD's. It is already plural (unless one says a WMD). Oh well, it's like complaining about Attorney Generals
 

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Posted by Tim Rectanus (Member # 3282) on 09-05-2007, 10:19 AM:
 

quote: Frank Angel

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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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