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Actually it would be good for the industry if a low cost light meter was available so the theatres would own one.

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Roger that. I've brought up screen brightness several times to managers & tech folks who were honestly interested, and the routine is always:

"How do you check it?"
"With a meter."
"Well, how much does one cost?"
"About two thousand dollars."
"TWO THOUSAND DOLLARS!"

Natural daylight provides a very repeatable source of light. When the sun is at a given height above the horizon (say 45-degrees elevation) on a clear blue-sky day, the intensity and color temperature of daylight is very constant and repeatable, and able to serve as a standard. Say that under those conditions, you would have 5500K (actually D-5500) color temperature and an illuminance of 8000 footcandles. If you took a photograph of a 100% white sheet of paper illuminated by daylight using Kodak ELITE CHROME 100 reversal film, and used a camera exposure that gave a midscale gray on the processed film, you would have a standard representing 8000 footlamberts at D-5500. If you now photographed movie screens with a camera exposure 9 stops greater on the same roll of film, the image of the screens lit to 16 footlamberts and D-5500 should match the "standard" daylight exposure. (i.e., one stop differences in exposure are 8000, 4000, 2000, 1000, 500, 250, 125, 64, 32, 16, giving 9 stops between 8000 and 16).

So, you could expose one "standard" daylight exposure on a roll of color slide film, then photograph each of the screens in a multiplex theatre onto the same roll of film, with 9 stops more exposure than was used for the 8000 footlambert daylight "standard". After processing the reversal film, the daylight exposure would serve as a reference, and the picture of each screen could be compared to it to see if it was darker or lighter, or significantly different in color. You could quantify the luminance, uniformity and color of over 20 screens for the cost of a roll of Kodak ELITE CHROME 100 slide film and processing -- maybe about US $0.50 (50 cents) per screen.

As an example, with Kodak ELITE CHROME 100 film, using an exposure of 1/500 second, f/16 would produce a midscale gray when photographing a 100% white card directly illuminated by natural daylight. Photographing a properly lit screen with an exposure of 1/15 second at f/4 (9 stops more exposure) on the same piece of film should produce an image that matches for density and color.

Is this idea worth developing into a procedure? How many would have the SLR camera and photographic ability to use such a method? Your comments and suggestions are welcome.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Another simple idea would be to develop (sorry for the pun) the design of a lightsource that provided a known 16 fl using an LED or flashlight bulb illuminating a small white square of material. This could be held up to visually compare against the brightness of the screen. The advantage would be portability, inexpensiveness, and instant readout. It wouldn't be useful for color value though.

Speaking of color values, could you speak of the comment I saw elsewhere about film appearing more cold due to poor reflectors? This was the first I'd heard of it, and I assume that dichroic reflectors may be "leaking" reds? Is this a big problem?

Tough to maintain a portable light standard. To do it right would probably cost as much as a good meter.

Dichroic mirrors vary considerably, and will change as the coatings wear.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

I suspect LEDs would give a fairly constant light output.

Take a Velveeta cheese box (or similar-shaped box that is rather long and narrow. Put a flashlight bulb in one end of the box and wire it to the proper voltage battery. Take a paper hole punch and punch a row of holes in the middle of a side, going from one side to the other. Cover the front of the box (and the holes) with a sheet of paper cut to fit. It would look something like this (It's rough, but I drew it very quickly):

Calibrating the device is an issue. The way I did it when I built one of these as part of a science project in spring of 1982 (8th grade) was through the use of a light source of a known intensity and the inverse square law. Get a lamp and take the shade off and put in a 100 watt standard light bulb (the kind that puts out 1750 lumens). Go outside with it and find a place where reflections from the bulb from anything are negligible (at night, of course).

For each hole, the light inside the box will shine through the holes through the paper covering the front. The intensity will be greatest for holes near the bulb and lesser for holes at the far end of the box. Hold the box with its face pointed straight at the 100W light bulb and for each hole, find the distance from the 100W light bulb where the intensity of the hole is the same of the surrouding paper. Based on the distance from the bulb, calculate the foot-candles by dividing 1750 by (4 * pi), then dividing by the distance, in feet, from the bulb, squared). For example, for a particular hole, if the proper distance is 10 feet from the bulb, then the foot-candle value for that hole is about 1.4. 4 feet from the bulb, the value would be about 8.7.

I built one of these long ago and you could measure luminous intensity by placing the meter and checking to see how many holes through which the light source was visible from in the box, and the point at which it was the reading. You write the values underneath the holes. I forget where I saw the description of the construction of this simple meter. It was in an old book that is probably much older than I am, but it was a fun little thing to build. Could something like it be used to measure foot-lamberts?

It's cheesy (literally), but can be built for practically nothing.

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Evans A Criswell
Huntsville-Decatur Movie Theatre Info Site

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When the sun is at a given height above the horizon (say 45-degrees elevation) on a clear blue-sky day, the intensity and color temperature of daylight is very constant and repeatable, and able to serve as a standard.

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"Clear blue sky" is not a regular, reliable occurrence in the spring, summer & fall in southern coastal areas.

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People know better than to take my tools. If they do they are likely to end up with a screwdriver sticking out of their eye socket

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You're supposed to use a socket wrench for that.

As long as toolboxes are being examined, the next question would likely be "What is the most strange & unlikely tool in your tool box?"

I've got a short-handled 5 pound sledge (I forgot what I needed it for). It has a factory label on the side which identifies it as an "Engineering Hammer".

It is useful when problems require extensive engineering.

Likewise, winter in Rochester has few days when you can even see the sun, and there are only a few minutes that it is more than 45 degrees above the horizon. But finding a "standard" illuminant is difficult. For example, the light output of a tungsten lamp varies significantly as the voltage is changed. Another way of using "photographic photometry" would be to photograph a "standard" screen known to have exactly 16 footlamberts, and then compare the photos of other screens taken on the same roll of slide film with exactly the same exposure.

IMHO, the BEST OPTION is to buy that $2000 screen luminance meter, and share it among theatres. If the meter is used to maintain proper screen luminance on 100 screens, the cost is only $20 per screen. And with proper care, the meter should last indefinitely.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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