Film-Tech Forum ARCHIVE: Luminance Uniformity Falloff

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Posted by Lucas Iaccarino (Member # 8762) on 06-02-2017, 06:20 PM:

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

Posted by Monte L Fullmer (Member # 2797) on 06-02-2017, 07:29 PM:

Definitely have a hot spot.
Using a silver screen and 3D, per chance?

Posted by Steve Guttag (Member # 268) on 06-03-2017, 06:39 AM:

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.

Posted by Lucas Iaccarino (Member # 8762) on 06-03-2017, 01:46 PM:

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!

Posted by Marco Giustini (Member # 4544) on 06-04-2017, 04:00 AM:

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.

Posted by Lucas Iaccarino (Member # 8762) on 06-05-2017, 07:53 AM:

Silver screen, 21m Throw, Scope format

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

Posted by Bajsic Bojan (Member # 4928) on 06-05-2017, 10:29 AM:

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.

Posted by Tom Bert (Member # 5719) on 07-24-2017, 09:04 AM:

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. ...

Posted by Mattias Mattsson (Member # 4308) on 07-25-2017, 07:31 PM:

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.

Posted by Harold Hallikainen (Member # 5405) on 07-25-2017, 10:56 PM:

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

Posted by Steve Guttag (Member # 268) on 07-26-2017, 08:59 AM:

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.

Posted by Harold Hallikainen (Member # 5405) on 07-26-2017, 09:45 AM:

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

Posted by Lucas Iaccarino (Member # 8762) on 10-24-2017, 03:50 PM:

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!

Posted by Marco Giustini (Member # 4544) on 10-24-2017, 04:53 PM:

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.

Posted by Lucas Iaccarino (Member # 8762) on 10-24-2017, 06:05 PM:

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!

Posted by Steve Guttag (Member # 268) on 10-24-2017, 10:25 PM:

Lucas,

the 14fL only has meaning if the rest of the specification is followed (at least 75% in the sides/corners). A flat silver screen will NEVER make that so all bets are off. Anyone that sets 14fL in the middle of a silver screen is going to have a very DARK picture. In fact, anyone that sets 14fL in the center of ANY screen with a gain over 1.3 (and flat) will have a darker picture (overall) than a lower gain screen and will be out of spec.

Through testing, I've found you just can't go above 1.3 gain on a flat screen without excessive hotspotting.

I kindof agree with Marco on this but I took it a step further. I measured a screen (Stewart Ultramatte 130) that measured pretty much right at the limits of SMPTE/DCI spec in terms of uniformity. I measure using a PSA type meter (45 simultaneous readings across the entire surface). I then used that to create what one would have to normalize an out of spec screen (above 1.3 gain) such that it would have an approximately similar overall light level.

If you use a typical Silver screen with about a 2.3 gain, and set the center to 19.6fL you'll get something with a more acceptable overall light level. If you are trying to get the same appearing light level as a matte-white running at 14fL (the goal), then Marco is right, 21.8fL is closest.

Posted by Marco Giustini (Member # 4544) on 10-25-2017, 04:22 AM:

Lucas,
I take measurements on Left, Centre (yes, hotspot), Right, Top and Bottom. Then do an average and adjust the centre to achieve a 14fL.

I appreciate this is not a super-accurate measurement (you will need more readings to achieve a good average) but as mentioned we are just trying to limit the damage here. Your edges will always be completely off and your hotspot too. What you are doing here is to make the picture look pleasant. Setting 14fL on the hotspot/centre is NOT going to achieve that goal!