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Topic: Measuring Screen Luminance

Manny Knowles
"What are these things and WHY are they BLUE???"

Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002

posted 02-26-2002 04:40 AM

Without spending thousands on the Minolta LS-100 meter or the USL outfit, how can I measure screen luminance?

I remember reading how to do it with a regular spot meter, but I can't remember the conversion techniques.

How are you measuring screen luminance without breaking the bank?

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And, hey! Let's be careful out there.

~Manny.

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Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 02-26-2002 06:10 AM

John P. has posted information on how to do this with only a 35mm still camera and a roll of Ektachrome. A "search" will probably find it.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 02-26-2002 08:31 AM

Here are links to articles I wrote about measuring screen luminance:
http://www.kodak.com/US/en/motion/newsletters/notes/march2000/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/notes/june2000/pytlak.shtml

Yes, it is possible to use an adjustable single lens reflex camera and KODAK ELITE Chrome film to evaluate screen luminance and color temperature. You need to expose a "standard" exposure of an 18% gray card exposed in mid-day daylight (sunlight + skylight). Normal daylight has a color temperature of D5500, and is about 8000 footcandles. So an 18% gray card would have D5500 color temperature and about 1440 footlamberts (8000 x 0.18 = 1440). A theatre screen at D5500 and 16 footlamberts would produce the same color and density on the reversal film with 6.5 stops more exposure in the camera. Using film to measure light levels by comparison to a standard exposure is known as "photographic photometry".
http://www.kodak.com/cgi-bin/webCatalog.pl?product=KODAK+ELITE+Chrome+Slide+Films

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 585-477-5325 Cell: 716-781-4036 Fax: 585-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 02-26-2002 09:00 AM

I think it would be easier (and perhaps more accurate) to simply use the camera's exposure system to read an existing screen of proper brightness and then compare that with the screen in question. Using the same camera would equalize any differences the shutter effect would have although it would be interesting to see if it would be transferable. In other words, at x ISO/ASA setting, y f-stop, z-shutter speed, when the exposure system indicates proper exposure you're at 16 fL. Sufficiently long lens and or close proximity to make it more of a spot meter.

How about this: Long cardboard tube (like from gift wrap), spray paint interior flat black, large solar cell (Radio-Shack) at one end, connected to a cheap meter movement via suitable adjustment potentiometer (perhaps a resistor across the meter as well). Maybe some sort of aiming device. Calibrate to known standard of 16 fL, perhaps some other values higher or lower. The long tube is so as to get a spot reading from a suitably small area of the screen. Actually only the outer end needs the black paint to cut light from parts of the screen not being measured; further down the tube any cross reflection will be light from the area under study and shouldn't matter.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 02-26-2002 09:20 AM

Most built-in camera exposure meters are only accurate to within about 1/2 stop. So they may not be able to distinguish between 12 footlamberts and 20 footlamberts. Also, many camera meters do not have the "Photopic" spectral response specified in SMPTE 196M.

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 04-27-2003 12:54 AM

It's really frustrating that Kodak's webmasters don't seem to have any understanding of the importance of preserving URLs so they always work years later. They don't even seem to work after a few months. I wish you'd could knock some sense into them, John. Is it worth my time to write to the faceless <webmaster@kodak.com> (in the absence of any better-published address). (I know I've complained about this before...sorry to sound like a broken record [Frown]

.)

For the benefit of those trying to follow John's links above,
they are now at:

March 2000: Don't be in the Dark about Measuring Screen Luminance
June 2000: Shedding Light on Screen Luminance Meters

--jhawk

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 04-28-2003 08:15 AM

jhawk: I appreciate your frustration, but Kodak reserves the right to remove outdated material from the website. Kodak's Entertainment Imaging (motion picture) webmaster Tony Amato does a pretty good job of keeping posted material on-line with consistent URLs, and avoiding dead-end URLs. In this case, that set of publications was considered outdated and pulled off the site, except for my articles. He decided to set up a new hierarchy for my articles.

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Duncan Smith
Film Handler

Posts: 50
From: England
Registered: Jan 2003

posted 05-01-2003 02:47 PM

There's a company here in England who are just at the end of trials on a new product called the "screen checker". It will measure the brightness of your white light and display the readings on a led bar so you can set up for 16 ft/l. I haven't used it yet but have spoken to the developers and it sounds very interesting, especially as it'll sell for under £100 ex vat. They are web page . Hope this helps, I know I'll be getting one for my cinema.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 05-01-2003 03:05 PM

The "Screen Checker" looks very interesting. Not sure it is taking a reading of the light REFLECTED by the screen, rather than just incident light, and whether the calibration is traceable to a standard. Standard SMPTE 196M defines the requirements for a screen luminance meter.

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Duncan Smith
Film Handler

Posts: 50
From: England
Registered: Jan 2003

posted 05-01-2003 03:24 PM

Hi John, I guess you'd need to talk to the developers for that. I have been told that it does not use a 1 degree window like a spot meter but in tests against spot meters it performs consistently. Frankly, at a 20th of the price of a spot meter it's the only option I can afford and at least it will give me a guide to what I am only doing by eye at the moment.

Out of interest, what would you set up a video display device (tv or projector?...) to at full white? I seem to remember something about 20 ft/l but don't know where from.

Duncan.

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 05-01-2003 03:37 PM

[bleh, it's hard to link to URLs with spaces. Not sure what's broken...]

Maybe someone should contact Matt Jahans (contact person listed on the page) and invite them to Film-Tech?

--jhawk

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 05-01-2003 03:48 PM

quote:
Out of interest, what would you set up a video display device (tv or projector?...) to at full white? I seem to remember something about 20 ft/l but don't know where from.

Depends upon the type of display and viewing conditions.

Digital Cinema standards are still being developed by the SMPTE DC28 Committee. Right now, 12 footlamberts white level seems to be the consensus for theatre display of feature films using digital projectors.

Here's some links to other types of computer/video displays:

Kodak PCD Monitor Setup

http://www.gossen-photo.de/pdf/Datenblatt_e_Mavomonitor.pdf

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Daryl C. W. O'Shea
Film God

Posts: 3977
From: Midland Ontario Canada (where Panavision & IMAX lenses come from)
Registered: Jun 2002

posted 05-01-2003 05:03 PM

jhawk -- Use %20 instead of spaces.

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 03-31-2004 07:58 AM

I'm bringing this back up because no one ever answered Manny's question about using a standard photographic spot meter to read screen illumination. Since some existing meters are based on spot meters it stands to reason that there ought to be a conversion from an EV reading (exposure value) to fL (foot Lamberts).

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Dick Vaughan
Phenomenal Film Handler

Posts: 1032
From: Bradford, West Yorkshire, UK
Registered: Jul 2000

posted 03-31-2004 08:34 AM

There is a conversion but I can't lay my hands on it at the moment. I seem to recall that the meters the IMAX techs used in the early days measured EV. Perhaps Dave Macaulay might know?

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