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» Film-Tech Forum   » Operations   » Digital Cinema Forum   » Luminance Uniformity Falloff (Page 1)

 
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Author Topic: Luminance Uniformity Falloff
Lucas Iaccarino
Film Handler

Posts: 34
From: capital federal, buenos aires, argentina
Registered: Feb 2015


 - posted 06-02-2017 06:20 PM      Profile for Lucas Iaccarino   Email Lucas Iaccarino   Send New Private Message       Edit/Delete Post 
Hello everybody.
I have a big question. Today I went to make a Screening test at a Local Cinema, and the luminance falloff was huge. I have never seen something like this, with the edges so clear. It´s look like a Vignette.
The center is 10 fL but the edges are 1.8 fL. Clearly far away from the 75% SMPTE Standards.

Do you say it's all about Lamp hours and Alignment?

Thank you very much

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Monte L Fullmer
Film God

Posts: 8297
From: Nampa, Idaho, USA
Registered: Nov 2004


 - posted 06-02-2017 07:29 PM      Profile for Monte L Fullmer   Email Monte L Fullmer   Send New Private Message       Edit/Delete Post 
Definitely have a hot spot.
Using a silver screen and 3D, per chance?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12089
From: Annapolis, MD
Registered: Dec 1999


 - posted 06-03-2017 06:39 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
The higher the gain of the screen, the higher the fall off on light. I've said it before and I'll say it again, if your screen gain is above 1.3, you will NEVER achieve SMPTE specs on light uniformity. Curving the screen will help/eliminate the hot spot in the lateral direction though it will still fall off top and bottom (but not as bad since image aspect ratios are wider than tall).

It would be very odd if it was the lamp or its alignment. If it was film, then that could be a possibility based on the lamp focus (fore/aft adjustment) or the reflector working distance but in a digital projector it would be pretty difficult to get a uniform dispersion AND have that sort of falloff. You can get uneven light still and you can have sharp fall offs (squared off corners/sides) due to alignment but not what you are describing.

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Lucas Iaccarino
Film Handler

Posts: 34
From: capital federal, buenos aires, argentina
Registered: Feb 2015


 - posted 06-03-2017 01:46 PM      Profile for Lucas Iaccarino   Email Lucas Iaccarino   Send New Private Message       Edit/Delete Post 
So, you say it's all about Screen Gain?

The Screen is 2.1 Silver Screen. It's very odd to me that I have already seen a lot of Silver Screens and NEVER seen a vignette so clear. It's very evident.

I will get a little more information. Thanks!

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Marco Giustini
Film God

Posts: 2536
From: Reading, UK
Registered: Nov 2007


 - posted 06-04-2017 04:00 AM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
Silver screen, short throw, flat frame? Very normal. I have measured those numbers myself. In my case it was 14fL on centre, 1.4fL on corners.

Walk left to right and vice/versa and you'll see the huge hotspot following you.

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Lucas Iaccarino
Film Handler

Posts: 34
From: capital federal, buenos aires, argentina
Registered: Feb 2015


 - posted 06-05-2017 07:53 AM      Profile for Lucas Iaccarino   Email Lucas Iaccarino   Send New Private Message       Edit/Delete Post 
Silver screen, 21m Throw, Scope format

You say it´s normal? Maybe it´s the projection Angle....

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Bajsic Bojan
Expert Film Handler

Posts: 188
From: Ljubljana, Si, Eu
Registered: Aug 2008


 - posted 06-05-2017 10:29 AM      Profile for Bajsic Bojan   Email Bajsic Bojan   Send New Private Message       Edit/Delete Post 
what is the width of the screen? and do you know the exact material / manufacturer of the screen?

if you look at any gain/viewing angle graph for silver screens with "2.1 gain", most will have 50% gain factor at about 20-25degrees, 20% gain at about 60degrees viewing angle, so if you're looking from the first rows especially and there is no curve on the screen, you'll be in for those kinds of numbers, unless its some extra rare specialty screen not normaly used in commercial cinemas.

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Tom Bert
Film Handler

Posts: 83
From: Belgium
Registered: Apr 2010


 - posted 07-24-2017 09:04 AM      Profile for Tom Bert   Email Tom Bert   Send New Private Message       Edit/Delete Post 
If it's screen gain, than you should measure different fall-off in different positions. From the center of the room: screen center > screen sides. From the right hand side: right > center > left. ...

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Mattias Mattsson
Film Handler

Posts: 84
From: Göteborg, Sweden
Registered: May 2007


 - posted 07-25-2017 07:31 PM      Profile for Mattias Mattsson   Email Mattias Mattsson   Send New Private Message       Edit/Delete Post 
With the USL PSA-200 seemingly out of production, what do you all use to measure the luminance fall off? It seems a bit tiresome to manually point your colirometer/luminance meter at nine (or more) different measuring points of the screen.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 722
From: Denver, CO, USA
Registered: Aug 2009


 - posted 07-25-2017 10:56 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Since the PSA used an image sensor, it was a pain to calibrate. Image sensors do not have uniform sensitivity. Lens flare can introduce inaccuracies (sense light outside the desired sense area). Silicon sensors do not have a photopic response, so additional optical filters are required to get the desired optical response so the luminance measurement is accurate. I've always thought it would be interesting to use a stepper motor driven pan/tilt tripod. Mount your favorite spot meter (with USB output) on it. Drive the pan/tilt with software as the computer acquires luminance as the screen is scanned. The software can detect screen edges by discovering the sudden drop in luminance. Not as fast as an image sensor, but I suspect it would cost less and be more accurate.

Harold

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12089
From: Annapolis, MD
Registered: Dec 1999


 - posted 07-26-2017 08:59 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
QSC should bring back the PSA200 (or equivalent). Being able to take 45 luminance readings at once is a VERY valuable tool and not really replicated by a pan/tilt approach single meter. For instance, lets say you are setting a "cold mirror" in a digital projector, you get real time uniformity as you adjust it.

Likewise, if you are adjusting a film system, particularly when matching projectors, you can't really do it from the booth well due to your skewed perspective. With the PSA, you are dead on from the audience perspective.

With respect to screen uniformity, a lot of people make a lot of claims, the PSA can generally prove/disprove such claims, particularly on an installation. Using a single meter that is moved about is fine for documentation and even be better if one is looking for precise numbers that are not subject to the influences discussed by Harold above but it is MUCH more time consuming and for 99% of the people, an unnecessary and non-valued task that few exhibitors will pay for. Sure, the studios will pay for that but they will also pay for a tech to set colors in a room that just had its colors set the screening prior.

Conversely, with the PSA, one can check/set things relatively fast so the improvement in performance versus time spent is easier to justify the effort in the first place.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 722
From: Denver, CO, USA
Registered: Aug 2009


 - posted 07-26-2017 09:45 AM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Getting readings while making adjustments is a very good point! It'd be a pain to wait 10 or 15 minutes for a new set of readings.

Harold

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Lucas Iaccarino
Film Handler

Posts: 34
From: capital federal, buenos aires, argentina
Registered: Feb 2015


 - posted 10-24-2017 03:50 PM      Profile for Lucas Iaccarino   Email Lucas Iaccarino   Send New Private Message       Edit/Delete Post 
Thanks everybody for the answers.
I had been doing some experiments taking multiple measures at different Silvers screens and higher the center measured point, biggest the dropdown at the corners. Making some lamps calibration helped to made a better distribution but still there is a big difference
The main question now is: Which would you consider to be the Center Point to be calibrated at 14 fL? the HotSpot or the point right-next to it? I find the HotSpot at Silver screens is extremedely out of range.

Thanks!

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Marco Giustini
Film God

Posts: 2536
From: Reading, UK
Registered: Nov 2007


 - posted 10-24-2017 04:53 PM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
Personally, I would do an average.

Usually with a Silver/high gain screen you will end up with about 23fL in the hotspot to achieve a 14fL average.

You can't be a purist on such a bad design so let's try and minimise the issue.

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Lucas Iaccarino
Film Handler

Posts: 34
From: capital federal, buenos aires, argentina
Registered: Feb 2015


 - posted 10-24-2017 06:05 PM      Profile for Lucas Iaccarino   Email Lucas Iaccarino   Send New Private Message       Edit/Delete Post 
Hello Marco, thanks for answering both posts.
What do you mean by average? maybe instead of getting 14 fL at center and 3 fL at corners, trying to get 19 at center (still at DCI limits) and around 7 at corners?

When you measured the center, do you point to the HotSpot itself?
THanks!

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