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Author Topic: Photographic photometry for screen luminance? John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 04-27-2003 02:12 AM                     This has come up before in at least 3 threads, but neither seemed terribly well-focussed on this issue, and so I figured starting a new topic seemed worth it. In posts over the past few years, John Pylak has suggested using photographic photometry to measure screen luminance, by exposing some film to a known source, and then exposing film to the screen, and evaluating the closeness of the match. Specific threads are: Feb 2002: Measuring Screen Luminance Feb 2002: Changing the lamp's color temperature Feb 2003: Kollmoorgen [sic] light meter... Specifically, I'd like to look at doing this with a nice digital camera and a laptop, rather than having to expose film and develop it. I assume that there's nothing in John's calculations that is unique to Kodak ELITE Chrome 100. I am a bit puzzled at the disparity in recommended reference sources. Originally John recommends an 18% gray card in "mid-day daylight" (8000fc), for 1440 footlamberts, and then 6.5 stops down from that is 16 foot-lamberts (actually, 1400/(2^6.5) => 15.47). But then this year he suggests using a 90% white card in "sunlit daylight" (9000fc) for 8100 fl, and then 9 stops down to 16fl (8100/(2^9) => 15.82). Which is better and why? I'm a bit confused as to the measurement of daylight being 9000fc versus 8000fc, and find myself wondering if there might not be a better way to find a reference than the sun, which seems awfully variable to me, but I'm not really a photographer and don't think in those terms. Maybe something with lasers? Anyhow, the goal of doing this with a digital camera is the ability to get rapid turnaround and quantitative results. I'm under the impression I can simply sum the values of all pixels in the reference image to get a brightness value; does that then scale linearly with foot-lamberts? (e.g., if I get a brightness of X at 8100fl, and then measure a brightness of 2/3X at 9 stops down, is that 2/3*15.82 => 10.55fl?) Can I then chunk up the image into sections in order to evaluate the luminance in sections of the screen, like the USL computerized photometer does? What about exposure times, and how do they interact with the 48fps shutter? John suggests a 1/8sec (125ms) exposure, but that's going to have 6 shutter interruption cycles -- wouldn't something longer be better. Elsewhere [some other thread] it's suggested the Minolta LS-100 can take 400ms exposures. Lastly, how does the issue of spot angle apply? Should I make an effort to choose a lens that results in a 2-degree angle on the screen? [doing so would appear to toss out the potentially useful sectionalization methods] If not, are the numbers comparable? I guess since a footlambert is candela/ft^2, measurements should be comparable regardless of the area measured? Thanks for any and all assistance. --jhawk  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 04-27-2003 10:26 PM                        A digital "camera" and laptop are already available as a very useful light measuring system from UltraStereo Labs: http://www.uslinc.com/products/psa.htm The sun is a very reproducable light source, but you must specify the elevation angle and other variables like atmospheric haze. The 8000 to 9000 footcandles I suggested was just a starting point, and would have to be researched and specified further.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 04-27-2003 11:47 PM                     Yes, I'm aware of the USL meter...I even mentioned it in my 8th paragraph. But the goal of this was to make the measurement with available tools, not to spend $4000. Do you have pointers to information on footcandles as a function of elevation angle and atmospheric haze? Thanks! --jhawk  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 04-28-2003 05:47 AM                        quote: Do you have pointers to information on footcandles as a function of elevation angle and atmospheric haze? I recall there was quite a bit of work done by either the National Bureau of Standards or National Institute of Science and Technology several decades ago. Quick search found these: http://www.soluxtli.com/edu13.htm http://www.energydesignresources.com/publications/skylighting/pdfs/2-SkyDesign.pdf  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-28-2003 07:56 PM                        If it is this complicated to figure out if your screen image is as bright as it should be, then no wonder screen brightness is the one parameter that varies so widely. All the lamphouse and xenon manufactures have charts that will give you a rough idea of how large a lamp you should be burning for the size screen you are trying to fill. Assuming modern lenses and a screen with a given gain, just the choice of the lamp wattage should get you a brightness level that is in the ballpark of your 16ftl. After that, the more important question would be, what's the uniformity of light across the entire screen. I would think for that, any good spot meter would do the job as it need not take into consideration all those other variables like shutter rate, color temp or conversion from lumens for that matter -- all it need to is show you the percentage of variation in light intensity from point to point. This is the really important factor, no? Frank  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 04-28-2003 10:35 PM                        quote: If it is this complicated to figure out if your screen image is as bright as it should be, then no wonder screen brightness is the one parameter that varies so widely. It isn't complicated if you have a proper screen luminance meter! If cost is an issue, share its use with other theatres in your city.  |  IP: Logged

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