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This topic comprises 2 pages: 1 2
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Topic: Measuring Screen Brightness (FL) without a meter
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-07-2019 07:26 AM
Presuming your Dolby 3D system used a white screen (1.4 - 1.8 gain screen, or less), then the Dolby 3D showing 3.5-4fL in the center will be brighter, overall, than a Real-D showing 5.5fL in the center on a Silver screen (2.2-3.0 gain).
Note, the 3D spec is 4.5fL +/- 1fL in the center and a lot of hand waving for the rest. There is also a "MaxBright" spec that some have offered that targets 6-7fL for center brightness. If your Dolby 3D hits 5fL on a white screen, you can run those without losing the blacks better than Real D running at 6-7fL.
And for the 2D, 14fL in the center with a +/- 3fL is the spec for "Cinemas." Note, the spec doesn't end there, you have a side/corner luminance relative to center of 75%. If your screen gain is higher than 1.3, you'll never hit it. So 3D hurts you even in 2D.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-07-2019 10:25 AM
Dolby 3D running at a lower center fL is brighter over all than Real-D running a brighter center fL...so nothing is ruled out there. In fact, the majority of our 3D cinemas are Dolby 3D.
What rules out Dolby 3D is the fact that it is all but a dead format. Both Dolby and Barco have pulled from production the mechanisms that allowed for it.
Note, when running 2D with Dolby 3D in a mixed program...the 3D mechanism stays in. If one doesn't wear the glasses, the 2D portion isn't horribly dim (typically preshow or trailers). Those houses that have Dolby 3D that we support have 2D and 3D lamps/lamphouses. 3D is such the odd offering now, it isn't a big deal to switch lamps/lamphouses. This way, they don't waste lamp life running the big lamp for 2D shows.
The Real-D XL does allow for 2D and 3D to hit the range of center lumens with a single lamp size...BUT, remember that the 2D center lumens only applies if your sides and corners are at 75% of center...which they won't be on a Real-D system. Furthermore, with the Real-D XL...you lose convergence of the two images (not colors) in the corners...more so on curved screens. So you take a image quality hit to use the Real-D XL too.
Lasers offer some help in the 2D/3D dilemma since they can be run so low for 2D and only need to be cranked for 3D...though the longer you run them hard, the faster they fade.
And then on the top-end, 6P Laser systems and the Dolby-esq 3D is the real way to go because you get bright 3D with whatever bright 2D you want (either 14fL or one of the brighter HDR specs that seem to still be evolving).
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 02-07-2019 05:20 PM
Justin - does Fox tell you any way to confirm screen brightness? Do they expect you to just tell them 'Our screen can do 4.5'? Or do they want some sort of 'certificate' by a tech etc.?
In general, decent screen luminance meters are expensive. Cameras can be used, but there is a bit of calculation involved and no direct readout is possible. I used to measure our screen brightness with my old SLR spot exposure meter. I can't tell you the translation from exposure value to cd/sqm or fl from memory, I used an excel sheet for it.
A common digital photometer can be bought from ebay or amazon from 10-20US$ on. Other than a luminance meter, it measures illuminance, that is, the amount of light shining ONTO the screen, not off the screen. In general, it is easy to calculate the amount of light reflected back from the screen if you know then screen gain.
You can get a decent estimate of screen luminance if you place a photometer probe close to the screen, pointing towards the projector. The readout will be in lux, you need to divide that value by 3.14, and multiply with the screen gain.
If you have a matte white screen (Gain=1), a lux readout of around 150lux will result in a screen luminance of about 48cd/sqm. If you use a gain screen, e.g. a 1.8 silver screen, the peak luminance will be 48cd/sqm*1.8.
Now, for 3D, things may become complicated since you have to measure though the 3D glasses. Some devices may deliver wrong results, depending on your type of 3D system.
The Harkness app has been tested for very good results, but I don't know how well it covers various 3D systems.
Another way to keep an eye on your brightness levels is to establish a reference to a proper brightness measurement by a tech using a properly calibrated instrument.
E.g. you can take a photometer measurement while a tech is measuring the same spot on the screen. From then on, you know the relation between luminance and illuminance for that given screen. While gain suffers a little bit over time, it can still be considered pretty constant. So, bulb aging, 3D loss, etc, can be compared to a proper measurement.
Again, all depends on how Fox wants you to confirm your screen brightness.
- Carsten
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Monte L Fullmer
Film God
Posts: 8367
From: Nampa, Idaho, USA
Registered: Nov 2004
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posted 02-08-2019 01:41 PM
Found out that James Cameron, Exec Producer, is pushing this light requirement to all cinemas doing the 3D playback.
Heard also that FOX will be sending out test clips of both 4.5 and 6fL to ensure the brightness requirements. Linky
quote:
February 7, 2019 Martin Samoylov
“Alita” Filmmakers Send Theaters Details For How Not To Mess Up 3D Projection
As we previously reported, Alita: Battle Angel is being heavily marketed as a “cinematic 3D event”. Shot in native 3D and produced by 3D mastermind James Cameron, it’s an important part of the film’s theatrical run.
To make sure theaters don’t screw up the actual 3D projection, director Robert Rodriguez andProducers James Cameron and John Landau have taken a rare step by leaving a message to projectionists about which 3D file to play.
To add some context, movies get distributed in hard drives. Those hard drives contain various digital files which can then be projected onto the big screen. Since the film won’t be projected the same way in all auditoriums, there are separate files for 5.1 and 7.1 audio, for 2D and 3D projections and for open and closed subtitles.
However, not all 3D projectors are the same. Some can project 6 and some only 4.5 units of luminance, depending on the technology. So the studio sends out separate files for each. If the file intended for weaker lighting is projected on the stronger one, the image will look dim. Vice-verse, the one intended for the stronger projector played on a weaker one will cause an overly bright image.
That’s the scenario the filmmakers want to avoid, which is why they attached the following message on the projectionist letter.
IMPORTANT 3D NOTE: The key to the 3D experience is the light. You have been provided with both a general release 4.5 FL 3D DCP and a premium 6 FL-XBrite 3D DCP. Projecting an XBrite 3D DCP at the standard 4.5 Foot Lamberts of light will result in a dark, hard-to-see presentation. It was especially color graded at 6 foot lamberts and must be run at 6 FL. Projected in this way, it will look amazing!
Conversely, if a general release 3D DCP is shown at 6 FL it will look overly bright. It must be run at 4.5 Foot Lamberts, and it will look terrific at that light level. Since you may have some auditoriums capable of running at 6 FL 3D with others that can only hit 4.5 FL 3D, please confirm that the correct DCP file is loaded in the appropriate theater and that each is run at its proper light level.
This will make a huge difference in the presentation and to audience enjoyment. Thanks so much from Robert Rodriguez, Jon Landau, and Jim Cameron.
Although it would be easy to dismiss the filmmakers as paranoid and overly attentive to levels of brightness, their concern isn’t unfounded. In the age of digital projection, many theaters don’t monitor for proper projection as most of it is automized. One of the few steps that must still be done by a human is the step of inserting files into a theater’s server. Attaching the message in the paper intended for that human is a logical step to avoid theaters accidentally projecting the wrong file and causing a dim or bright 3D experience.
It’s not the first time filmmakers have left notes on digital projectionist sheets. Oliver Stone indicated his preferred audio level for Snowden. Warner Bros. indicated how theaters can zoom in properly to avoid black bars for Dunkirk, which had an unusual aspect ratio. And famously, Disney let theaters know that those few seconds of silence during hyperspeed in Star Wars: The Last Jedi were intentional and not sound failure.
Overly dim projections are an issue, especially with movies such as Alita which have setpieces that take place at night. Last year, multiple theaters complained about dimness in screening of Solo: A Star Wars Story, which has similar scenes set in dark environments.
AMC told IndieWire last year that the chain has “Digital Cinema Managers” in some of their locations. However, at those that don’t as well as other theater chains there will often only be a corporate phone number available to help in case projection is faulty.
Although the studio really wants you to see Alita: Battle Angel in 3D and even took out a Super Bowl spot to remind you that the film was one of those that “must seen on the big screen” and anew calling it the “3D event of the year”, you might not find too many 3D showtimes in U.S. theaters. A certain other 3D movie called The Lego Movie 2 is coming out the same weekend and will be taking some 3D screenings.
As a silver lining for Fox, 3D is more popular overseas anyways where it has already opened in a few markets.
If you’re in the U.S. and want to see the film in 3D before it opens, there will be “3D fan events” on February 11 and 12. On Tuesday, T-Mobile will also offer Atom Ticket codes for the film which reduce a ticket cost to $4 as part of their T-Mobile Tuesdays series on the February 12.
Alita: Battle Angel officially opens in the U.S. on February 14. The film is produced by James Cameron’s production company Lightstone Entertainment.
Photo Credit: Courtesy Twentieth Century Fox TM
Read more about Alita: “Alita: Battle Angel” Being Promoted as “3D Event”, “Alita: Battle Angel” Early 3D Fan Screenings Set For 02/11 and 02/12
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