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Topic: Measuring Hot Spots Screens
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-19-2018 02:32 PM
There is indeed an SMPTE (DCI) standard and if your gain is above 1.3, you'll never hit it. The 14fL number is only valid if the uniformity is valid. One cannot pick and choose which part of the standard to adhere to and expect proper results.
Class, if you open your DCI handbook to section 8.3.4...
quote:
8.3.4.3.
Peak White Luminance
Using the white field test pattern (X’=3794, Y’=3960, Z’=3890), adjust the peak white luminance, as measured at screen center, to 48 cd/m2 (14 ft-L), with the measurement made at the reference viewing position.
8.3.4.4.
Luminance Uniformity
Using the white field test pattern (X’=3794, Y’=3960, Z’=3890), align the lamp house to minimize luminance fall-off from center to corners. The measured luminance of the corners and sides in a 3 x 3 grid shall be at least 75% of the center, as measured from the reference viewing position.
Follow manufacturer’s recommendations for digital uniformity correction (if applicable).
Measure center to corner uniformity as described in [SMPTE 196E].
DCI being DCI has Table 11 that contradicts the above paragraphs and goes for a more stringent requirement of 85% of center as the goal with review rooms allowed to drop to 80% and normal theatres to be within 70% of center.
So, you getting ~19fL in the center (which is out of spec since normal theatres are allowed to be between 11 and 17fL) and ~10fL in the corners would be...bzzzzt out of spec.
Thank you for playing "Is your screen too gainy?"
Note, I'm not saying that what you are doing is necessarily wrong for the situation you are in (too high a screen gain). I have done a bit of a study on trying to make the total quantity of light coming back to the audience to be the same regardless of screen gain and yes, you have to go out of spec in the middle to do it but the results are always more pleasing than too dark. Those that set for 14fL in the center on a silver screen are putting out a VERY dark picture. Much darker than a matte white screen out putting 11fL. It is why Dolby 3D often looks brighter/better than Real-D despite normally being darker in the center...it is brighter most everywhere else.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-19-2018 05:50 PM
quote: Lucas Iaccarino
So DCI-SMPTE has still not updated - or make a different rule - for 1.8 or Silver Screens?
No! It is not a matter of "updating" or even accommodating. If you have a screen above 1.3, you are out of spec. That is it. That is the bottom line. There is no negotiating it away. As Carsten has mentioned, you can curve a higher gain screen to get the side-to-side evenness to be within spec, even on a silver screen (I've done it). But on a cylindrical curve (one dimension), you still will have dark corners and darker top/bottom. The net result will be a "band" of light in the middle.
That is a curved silver screen and the light from edge to edge is within 1fL and it is evident in the picture. Clearly, as you move away from the center to the corners, things get worse. Note too, once you curve the screen, you introduce geometric distortions and will likely start cropping a bit form the top/bottom due to the barrel distortion.
There is no chart or guideline, at this time and SMPTE will never provide such at thing as it would be codifying an inferior presentation.
If you want to put out a proper presentation, light wise, you need to have a 1.0 (preferred) to 1.3 gain screen and that is it.
If your 3D system requires a silver or even a high gain white screen, well, you are going to have an inferior 2D show in order to accommodate the 3D show (which also will suffer from light issues too.
The closest to a 3D spec I've seen is that the "standard" light spec is 4.5fL +/- 1fL for 3D and use the same percentages on the sides/corners. Which isn't realistic on a silver screen, if you are 4.5fL in the center, you are down in the 1 or so fL in the corners. There are some brighter 3Ds out there with the 6-7 fL target (some called "MAX BRIGHT"). Again, that is just the center. Don't use those unless you can hit those light levels because the blacks will be crushed even more than they already are. That said, Dolby 3D can get away with the 6-7 fL files on a 4-5 fL center image if they are on a 1.5-1.8 gain screen because they have more light in the sides/corners. The higher the gain, regardless of format, the worse you are.
Screen gain is such a misrepresentation. Is is a light concentrator, not a light gain. The screen doesn't gain you light on anything, just distributes what you have differently.
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