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Author Topic: Luminance meter
Drew King
Film Handler

Posts: 14
From: Valley Centre Drive, San Diego, CA
Registered: Jun 2011


 - posted 07-11-2011 03:36 PM      Profile for Drew King   Email Drew King   Send New Private Message       Edit/Delete Post 
http://www.bhphotovideo.com/c/product/329952-REG/Spectra_Cine_18007SABL_Spot_Meter_System_.html

could this piece of equipment be an acceptable solution for measuring fL in a professional setting?

My instincts tell me no, but curious of your opinions.

What would be the cheapest industry standard piece of equipment for fL measurements?

The minolta LS-100 is out of our price range.

My readings would be of a 182.5 inch silver screen.

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David Zylstra
Master Film Handler

Posts: 432
From: Novi, MI, USA
Registered: Mar 2007


 - posted 07-11-2011 07:39 PM      Profile for David Zylstra   Email David Zylstra   Send New Private Message       Edit/Delete Post 
I use the Sekonic L-758C for most readings, currently available from B&H for about $822 - it works well for everything but 3D through the glasses.

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Marcial Feliciano Ramos
Expert Film Handler

Posts: 103
From: Puerto Rico
Registered: Oct 2003


 - posted 07-11-2011 07:59 PM      Profile for Marcial Feliciano Ramos   Email Marcial Feliciano Ramos   Send New Private Message       Edit/Delete Post 
We have a Minolta CS-200 and for us it works perfectly. But the price is quite high.

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Phil Ranucci
Expert Film Handler

Posts: 227
From: Carpinteria,CA, United States
Registered: May 2006


 - posted 07-11-2011 10:54 PM      Profile for Phil Ranucci   Email Phil Ranucci   Send New Private Message       Edit/Delete Post 
Are you using a film or digital projectors? A few threads on this already, but the shutter in a film projector makes it hard to use a regular photo meter. The Spectra Cine Spot is the film standard, I guess and is about $2500.

I'm curious as to why the Sekonic 758Cine won't work through 3D glasses? I just got one on e-bay and haven't gotten it yet.

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David Zylstra
Master Film Handler

Posts: 432
From: Novi, MI, USA
Registered: Mar 2007


 - posted 07-12-2011 08:51 AM      Profile for David Zylstra   Email David Zylstra   Send New Private Message       Edit/Delete Post 
My thoughts are that reading through the glasses is similar to reading 35mm - there is dark time involved (i.e. at 144hz every other frame is the left eye) - I would welcome any corrections if I am mistaken on this.

A quick test I did a few years ago between Sekonic and a Minolta:
With Z-screen in place, no glasses - Sekonic 14FL, Minolta 14FL
Through the glasses - Sekonic bouncing with a peak of 2.2FL, Minolta 6.5FL

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 11982
From: Annapolis, MD
Registered: Dec 1999


 - posted 07-12-2011 09:59 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Interesting, David.

Out of curiosity...were you measuring using a white test generated from a player or using a projector's built in pattern? You normally can not use the projector's pattern since the 3D device won't be receiving the pulses to sync up. If you use a pattern generated from the player, then the 3D device will sync and the flash rate will be quite high and your meter would have less of an integration issue, in my opinion.

Next time you do your test, wear a set of 3D glasses when looking at the white pattern...if there is a notable flicker to it...then you are not in sync and the reading, regardless of whose meter is invalid.

-Steve

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Scott Norwood
Film God

Posts: 7929
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99


 - posted 07-12-2011 10:09 AM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
Semi-related question: using non-D-cinema video projectors, I have measured "white" using both the internal test pattern generator on the projector and off of a tape (usually the white patch on SMPTE color bars, which should be 100 IRE, but also scenes in which the video fades to white, which I presume is also at 100 IRE). Invariably, the "internal test pattern white" is significantly brighter than "video white." This is with an SD-SDI or HD-SDI link between the deck and the projector, so cable loss (etc.) should not be an issue.

Why is this? It seems to be the case for several different models of video projectors (including the Panasonic 7700 and Christie HD10K-M).

I use a Spectra SC-600 ("Cinespot") that was calibrated about a year ago.

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Robert Minichino
Master Film Handler

Posts: 350
From: Haskell, NJ, USA
Registered: Dec 2005


 - posted 07-12-2011 11:06 AM      Profile for Robert Minichino   Author's Homepage   Email Robert Minichino   Send New Private Message       Edit/Delete Post 
On the SDI signal (assuming 10 bits for luminance), code 64 is black and 940 is white. If the projector is mapping 0 to black and 1023 to white, and using the corresponding values in its internal test patterns, this would explain the discrepancy. I don't know what the DCinema levels are supposed to be; maybe you can find an SDI signal with blacker-than-black and whiter-than-white on it, which should both show up the same as black and white if everything's set properly (64 should look the same as 0, and 940 the same as 1023).

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 11982
From: Annapolis, MD
Registered: Dec 1999


 - posted 07-12-2011 11:14 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
One easy mistake some make is to not ensure that the color correction/color space is correct when making measurements. If you use a projector's uncorrected white, then yes, it is going to be brighter than the white REC709 signal that has to be mapped properly.

This is another reason to always use the test patterns off of the player when doing 3D calibrations (even non-Dolby). In fact, on some projectors I've found that just asking for "white" will get you the uncorrected white pattern...this will have the resulting picture too dark (or darker than you think it is).

My tests have shown that, on average, color correction alone accounts for about 3% light loss. Screen age, port glass discoloration, notch filter alignment...etc will generally make that number worse.

I have not done any tests to specifically show variances between DCDM XYZ space versus P3 space versus REC709...etc using various patterns versus using internals or brightness variances between each type but it would be interesting (if I could ever find the time).

-Steve

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Bajsic Bojan
Expert Film Handler

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


 - posted 07-12-2011 12:00 PM      Profile for Bajsic Bojan   Email Bajsic Bojan   Send New Private Message       Edit/Delete Post 
but can basic luminance meters measure color reproduction accuracy?

i always thought a spectroradiometer was necessary, like the pr655.

whats the most affordable tool for this?

Just for doing fL measurements i would use much cheaper 'photographer' units, unless you are doing installs on a regular basis, a 800$ unit that does fL readings of luminance on screen is a tad much for something you will probably do once a year or so? And wont you be wanting to get correct color set up at about the same time anyways?

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Drew King
Film Handler

Posts: 14
From: Valley Centre Drive, San Diego, CA
Registered: Jun 2011


 - posted 07-12-2011 12:33 PM      Profile for Drew King   Email Drew King   Send New Private Message       Edit/Delete Post 
NEC1200c throwing to 182.5in silver screen.
Dolby DSS200
realD "Post Production System"

Not doing installs. Work at one location and need to make different macros for different client specs.

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Carsten Kurz
Film God

Posts: 3582
From: Cologne, NRW, Germany
Registered: Aug 2009


 - posted 07-12-2011 06:01 PM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
I think, for simple checks, a digital photometer is sufficient. Of course it does not measure screen brightness, but projector lumens. But most people roughly know their screen gain anyway. A photometer should be okay for comparative measurements, e.g. between bulb changes, adjustments, etc.

Of course, if the money is there for a better instrument...

But I think todays average presentation quality would be better if every projectionist had a 30US$ photometer.

- Carsten

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David Zylstra
Master Film Handler

Posts: 432
From: Novi, MI, USA
Registered: Mar 2007


 - posted 07-13-2011 10:55 AM      Profile for David Zylstra   Email David Zylstra   Send New Private Message       Edit/Delete Post 
Steve - I was hoping you possibly had a better answer than I did on the through the glasses reading. . . . . yes the "bouncing" readings with the Sekonic are evident both on the projector (Christie) test pattern and DCP test content. I just double checked with a series 2 Christie and the built in test pattern gives the RealD XL a sync signal so there is no flicker with glasses on - 20FL from the projector test pattern yeilds a bouncing 1.5-2.2FL through the glasses, then 17FL without glasses from a 3D DCP white pattern also yeilds the same bouncing 1.5-2.2FL reading through the glasses. On one reading I did get the Sekonic to read a semi-solid 6-6.6FL but I could not repeat it.

My initial tests were to prove to our service company that their Sekonic meters did not reliably read 3D through the glasses - we were getting 15+FL without glasses but the tech was reading 1-2FL on his Sekonic with the glasses and telling me I needed a larger lamp, but other tests with better meters proved that 14FL without glasses yeilds about 6FL with.

I recall a comment somewhere about a way to configure one of the older Sekonic models to read 35mm, but since we are all digital I did not pursue doing it (I think it may have been a comment by John Pytlak years ago).

The Sekonic is definitely a good easy tool to use for setting and checking light levels.

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Drew King
Film Handler

Posts: 14
From: Valley Centre Drive, San Diego, CA
Registered: Jun 2011


 - posted 07-14-2011 12:01 PM      Profile for Drew King   Email Drew King   Send New Private Message       Edit/Delete Post 
Seems I should find another solution other than the photo meter since the majority of my calibration is 3D.

If anyone can get consistent 3D luminance readings (through the glasses) with a $1k or under piece of equipment please share with me.

David, I'm curious what yielded your "semi-solid" reading at 6-6.6 and how you CAN repeat it. Let me know if you learn anything new.

Thanks!

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David Zylstra
Master Film Handler

Posts: 432
From: Novi, MI, USA
Registered: Mar 2007


 - posted 07-14-2011 01:52 PM      Profile for David Zylstra   Email David Zylstra   Send New Private Message       Edit/Delete Post 
The Sekonic may still work, it is just a matter of knowing what measurement without glasses equates to your desired target with glasses. It would just take a side by side session with a meter that can read 3D properly with glasses to give yourself a basic chart of what to get without the glasses for various levels with glasses - the tech doing your install might be nice enough to help out while he has his meter out shooting colors.

When I have time I will experiment more with trying to repeat a solid reading on the Sekonic through the glasses.

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