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» Film-Tech Forum   » Operations   » Digital Cinema Forum   » Color correcting (Page 1)

 
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Author Topic: Color correcting
Richard May
Jedi Master Film Handler

Posts: 993
From: Floral Park, NY USA
Registered: Aug 2004


 - posted 10-10-2017 04:10 PM      Profile for Richard May   Email Richard May   Send New Private Message       Edit/Delete Post 
When checking red, green, blue, black and white, what values are you looking for? Do you check x,y or X,Y,Z? I was always curious about this?

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Harold Hallikainen
Jedi Master Film Handler

Posts: 593
From: Denver, CO, USA
Registered: Aug 2009


 - posted 10-10-2017 05:33 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
SMPTE st0431-1-2006.pdf calls for xyY values of 0.314, 0.351, 48.0 with Y in cd/m^2 for peak white. 48 cd/m^2 is equivalent to 14.0 fL. Luminance tolerance is 21% for theaters and 7.3% for review rooms. Chromaticity tolerance is .006 for theaters and .002 for review rooms.

The LSS-100P ( https://www.qsc.com/cinema/products/test-and-measurement/usl-lss-100p/ ) measures XYZ in cd/m^2 and then displays what the user wants (typically xyY with Y in either fL or cd/m^2).

Harold

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Richard May
Jedi Master Film Handler

Posts: 993
From: Floral Park, NY USA
Registered: Aug 2004


 - posted 10-10-2017 08:38 PM      Profile for Richard May   Email Richard May   Send New Private Message       Edit/Delete Post 
What values are you looking for in the primary colors?

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Harold Hallikainen
Jedi Master Film Handler

Posts: 593
From: Denver, CO, USA
Registered: Aug 2009


 - posted 10-10-2017 09:44 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
SMPTE does not have a specification on this that I am aware of. Others may have more information.

Harold

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Leo Enticknap
Film God

Posts: 6513
From: Loma Linda, CA
Registered: Jul 2000


 - posted 10-10-2017 10:56 PM      Profile for Leo Enticknap   Author's Homepage   Email Leo Enticknap   Send New Private Message       Edit/Delete Post 
The color calibration procedure (at least on NEC and Barco projectors - I haven't done it on a Christie or Sony) consists of projecting red, green, blue, then white test patterns, and taking a reading from the screen for each one using a photospectrometer. You enter the X and the Y for each color that that the meter gives you into the software. At the end of the process, the software figures out what changes it has to make to each color to get you to 0.314 and 0.351, does it, and then you measure the white again, to confirm that the adjustments have been successful.

This should ideally be done whenever any significant change to the optical system takes place (xenon bulb is replaced, lens is taken out and the back of it cleaned, etc.), but the meters are so expensive that in reality, it's usually only done at planned maintenance service calls.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 593
From: Denver, CO, USA
Registered: Aug 2009


 - posted 10-10-2017 11:06 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Of course, with the previously mentioned LSS, you can do daily automated tests of peak white chromaticity and luminance....

Harold
(designer of the LSS)

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Tom Bert
Film Handler

Posts: 62
From: Belgium
Registered: Apr 2010


 - posted 10-11-2017 06:17 AM      Profile for Tom Bert   Email Tom Bert   Send New Private Message       Edit/Delete Post 
Do note that:
x=X/(X+Y+Z)
y=Y/(X+Y+Z)

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Marcel Birgelen
Film God

Posts: 2136
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012


 - posted 10-11-2017 08:34 AM      Profile for Marcel Birgelen   Email Marcel Birgelen   Send New Private Message       Edit/Delete Post 
Harold, I know it's not your job, but can you name a figure for a current list price for the LSS-100?

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Leo Enticknap
Film God

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From: Loma Linda, CA
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 - posted 10-11-2017 10:27 AM      Profile for Leo Enticknap   Author's Homepage   Email Leo Enticknap   Send New Private Message       Edit/Delete Post 
I've heard rumors (Maybe Tom could confirm?) that Barco is looking in to building a photospectrometer into their projectors, so that color calibration becomes like the CLO system for ensuring that overall luminance stays the same throughout a bulb's service life: a one-time during installation, set-and-forget procedure, and the projector automatically makes adjustments to keep the color calibration on target thereafter.

If this is true, this strikes me as a great idea. Building the cost of a $10k measuring device into a $100k capital investment means that a theater is more likely to buy it than as a separate item, the presentation quality boost from which 99.99% of moviegoers in a typical suburban 'plex will not notice or appreciate.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 11569
From: Annapolis, MD
Registered: Dec 1999


 - posted 10-11-2017 11:01 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
LSS-100P is $804.00USD list. Naturally, street prices are substantially lower.

As for primary colors the "native" colors of a xenon projector are typically in the ballpark of: (x,y)

Red (.640, .320)
Green (.265, .690)
Blue (.150, .060)

However MANY things will affect that including the mirrors/reflectors and everything else in the light path including the port, screen and the notch filter (in fact, the notch filter is normally adjusted to minimize color correction of Red or Green or both (it is a yellow notch).

DCI target colors for xenon projectors are (x, y)

Red: (.640, .320)
Green: (.280, .640)
Blue: (.160, .100)
White: (.314, .351)

So with this, you have the three points of the triangle as well as the center (white).

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Tom Bert
Film Handler

Posts: 62
From: Belgium
Registered: Apr 2010


 - posted 10-11-2017 02:00 PM      Profile for Tom Bert   Email Tom Bert   Send New Private Message       Edit/Delete Post 
quote: Leo Enticknap
I've heard rumors (Maybe Tom could confirm?) that Barco is looking in to building a photospectrometer into their projectors, so that color calibration becomes like the CLO system for ensuring that overall luminance stays the same throughout a bulb's service life: a one-time during installation, set-and-forget procedure, and the projector automatically makes adjustments to keep the color calibration on target thereafter.
I can confirm that we currently do NOT have such a feature on our roadmap :-) Wondering where you heard the rumour from.

A really built-IN sensor would yield some practical problems, e.g. the fact that you'd be measuring from behind the porthole.

In the open&agnostic philosophy that we develop products, we are open to interface with 3rd party measurement devices.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 593
From: Denver, CO, USA
Registered: Aug 2009


 - posted 10-11-2017 02:27 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
I like the idea of test equipment being independent of the equipment being measured. That is, I don't think the LSS should be used as a sensor to automatically adjust the projector. With it being an independent instrument, drift in either the LSS or the projector will be caught since it's unlikely they would both drift the same amount in the same direction. If the projector automatically adjusted based on LSS readings, and the LSS readings were incorrect (someone bumped it so it is no longer pointing at the screen), the projector would misadjust itself in attempting to correct the situation. I'd rather just get an alarm that something is wrong. By the way, on DLP projectors, I've found that color is very stable. The amount of each color component emitted for each pixel is determined by pulse width modulating the mirror for that pixel. PWM is very stable and predicatable, so the colors are stable. Any instability would be due to drift in the optics, filters, light source color, etc. You can see some projector stability measurements (and LSS stability measurements) at http://ftp.uslinc.com/ftp/Products/LSS-100P/Documents/Technical/ColorMeasurement/ColorMeasurment%20LSS_160525.pdf .

Harold

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Richard May
Jedi Master Film Handler

Posts: 993
From: Floral Park, NY USA
Registered: Aug 2004


 - posted 10-12-2017 07:43 AM      Profile for Richard May   Email Richard May   Send New Private Message       Edit/Delete Post 
Thanks everyone. Thanks for the x,y coordinates Steve. That's what I was wondering about.

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Lonny Jennings
Film Handler

Posts: 10
From: Boston, MA USA
Registered: Feb 2000


 - posted 10-12-2017 11:01 AM      Profile for Lonny Jennings   Email Lonny Jennings   Send New Private Message       Edit/Delete Post 
Make note, however, that Steve's numbers for the three primary colors are not a "standard". It's just what you nominally would have if the projector is reproducing the whitepoint at the Standard .314, 351. The standard doesn't care about the parts to get there, just the final result.

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Richard May
Jedi Master Film Handler

Posts: 993
From: Floral Park, NY USA
Registered: Aug 2004


 - posted 10-12-2017 01:45 PM      Profile for Richard May   Email Richard May   Send New Private Message       Edit/Delete Post 
I understand. At least I have a general starting point.

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