Thanks for the challenge, Mike! Here’s what I came up with.
Screen Brightness Defined
Lumens, LUX, and Lamberts
A lumen is standard unit of luminous flux…if you wish, it’s how much light is emitted. When a projector’s light output is measured in lumens, it is an attempt to place a number on the maximum amount of light coming from the projector. It is not now bright your screen will be!
LUX is a measurement of illuminance. It takes into account the area over which the luminous flux (lumens) is spread. For example, 1000 lumens spread over one square meter results in 1000 lux. Back the light off until the same luminous flux now fills 10 square meters, and you get 100 lux over that area.
Screen Brightness, or how bright a screen will look, involves measuring the light reflected from its surface to our eyes. It takes into account the luminous flux (lumens) falling over its entire area (lux) and how reflective the surface of the screen is. It literally is a measure of the light bouncing off the screen. Luminance is measured in foot-lamberts.
How bright a screen is has impact on the image presented, and not in a small way. To help quantify screen brightness, some test method had to be standardized. In film projection, it’s the projector without any film projecting onto the screen. With digital projectors, it’s a 100% white image. It’s interesting to note that the two are not identical. Film base attenuates the light through the projector, so a white film frame would measure lower, but digital projectors use no such film, so 100% is 100%. The target luminance is between 12 and 22 foot-lamberts (fl). The target is 16fl, but a group of surveyed viewers much preferring the 22fl screen brightness. Many movie houses are dimmer, around 7-10fl. Yes, it’s a cost thing. Xenon bulbs are expensive, and last longer if you don’t burn them as bright.
Here's your first answer: The target luminance for a THX Home Theater Screen is 16fl, same as a commercial theater, and brighter would be better...and more expensive!
Note at this point that we are talking Foot-Lamberts, not lumens. They are not the same, and don’t even really relate directly to each other. To reiterate, a lumen is standard unit of luminous flux. A projector that provides 1000 lumens of light will provide that flux regardless of how big the screen is, or how far away it is. To change the luminous flux of a projector you have to do something in the light chain, like boost the lamp current, or get a bigger (faster) lens. Think of it as the total amount of light emitted. The current standard for projectors is know as ANSI Lumens. ANSI, the American National Standards Institue, has standardized the method used to test projectors. The method involves, among other things, testing multiple areas of the light source. Home Theater ANSI Lumens is a measurement standard created by Runco International, and most significantly differs from ANSI lumen measurement in that the projector under test is first calibrated to ISF (Imaging Science Foundation) standards at 6500K, the color temperature required for an accurate video image. The projector is then targeted to a standard screen, and the resulting light falling on the screen is measured at 9 points with a LUX meter, then averaged and multiplied by the surface area of the screen. The resulting measurement is much lower than the standard ANSI lumen equivalent, but is a better indicator of projector performance than the measurement of a projector running wide-open and uncalibrated.
Back to the screen. If everything in your home theater design was correct, you should hit the same 16fl luminance target figure that theaters try for. In fact, the THX Home Theater standard is 16fl, but they talk about trying to duplicate the image seen in mastering houses, which calibrate 100% white to 35fl. That’s quite a range!
Now, how to you know you’ve got it right? The best way is to measure luminance in foot-lamberts directly. If you have the right kind of light meter (Konica-Minolta makes an industrial unit for this purpose), you just aim it at the screen, pull the trigger, and read your meter. Of course, you don’t have that meter. But, like Mike, you may have a DSLR or film SLR camera with built-in light meter. Well, you’re almost there. It takes a bit of math, though.
Second Answer: Measuring Luminance in Foot-Lamberts with a camera
With a camera, it’s best to use a telephoto lens, or get close to the screen. Your object is not to try to measure the entire screen, but try for a small section, ideally, 1/9th. Set your camera for ISO 100, and your shutter speed to 1 second. This places your camera in the range where useful EV figures can be converted to foot-lamberts with our little chart. We picked 1 second because many new zoom lenses only open to f4 or so, and we need the extra sensitivity. With a test DVD (any THX certified DVD has the THX Optimizer on it, which will work fine), put up a 100% white frame, and take a light reading by pointing your camera at the white area and noting the f-stop and shutter speed. Plug them into the formula:
EV = 3.3 Log10 (f²/T)
Where:
f=f stop
T is exposure time
We include a chart for this, if the math is to hard. The chart is limited, but you can get some useful luminance data with it anyway. (For the technologists, we’ve stuck with the Minolta recommended K of 1.3)
Your target is 16fl, which is between EV8 and EV9, or between f16 and f22 (ISO100, 1 sec)
Spot-meter or camera set to ISO100
100% white screen, Table is below...
| EV | 1 sec | .5 sec | ft-L |
| 1 | f-1.4 | f-1.0 | |
| 2 | 2 | 1.4 | |
| 3 | 2.8 | 2 | .33 |
| 4 | 4 | 2.8 | .65 |
| 5 | 5.6 | 4 | 1.3 |
| 6 | 8 | 5.6 | 2.6 |
| 7 | 11 | 8 | 5.2 |
| 8 | 16 | 11 | 10 |
| 9 | 22 | 16 | 21 |
| 10 | 32 | 22 | 42 |
| 11 | 45 | 32 | 84 |
Ok, the challenge is met! The next is finding a projector that will hit 16fl reflected from your screen given its size. Oh, and is 16fl really enough, given a high level of ambient light in the room? At some point, I’ll need to get paid for this stuff….
No comments:
Post a Comment