How do you use a Light meter to get On/off contrast & Ansi - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 4 Old 10-27-2009, 11:32 AM - Thread Starter
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post #2 of 4 Old 10-27-2009, 12:51 PM
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Originally Posted by SOWK View Post

Looking to buy the AEMC CA813

When taking measurements for on/off contrast and Ansi contrast

Do you take it off the screen?
How far from the screen should the sensor be to take the readings?

Do you take it off the Front Projector itself?
How far from the lens should the sensor be placed?

What range and mode (lux vs fc) is the most accurate for Front Projection?

Do you change it when you read 109% window vs 0% window?

109% = klux / kfc
0% = lux / fc

or just keep it the same?

I assume read from the projector itself about 12 inches and keep it set to Klux mode for both and take the value from 109% and divide the value from 0% for on/off contrast number.

I found this guide:
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post #3 of 4 Old 10-28-2009, 07:08 AM
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Knowing how to make a contrast reading and actually being able to do it with any vague accuracy at all means having a meter that is excellent at measuring the black produced by the projector (either directly or off the screen).

Knowing what the projector is capable of is one thing, but measuring what you are viewing off the screen is a completely different thing. In fact, the projector (no screen) contrast ratio could very possibly be massively different than the contrast ratio measured off the screen since the room-and-screen and projector-and-screen are going to interact in some way and the amount of interaction has a lot to do with the type of screen you have.

We use "light meters" to measure grayscale and color. They are good at measuring light, but pretty poor when it comes to measuring the lack of light (black). And the less you spend on a meter, the worse it is going to be at measuring black. I've seen $10,000 meters that can't be trusted for measurements below 1 foot-lambert and that's not very dark. That means if you were getting 12 foot-lamberts for the peak white level from the screen, the highest possible contrast ratio you could measure would be about 12:1.

Remember, just because your software is telling you the meter is measuring .08 foot-lamberts... that doesn't mean the measurement is accurate - it may not even be remotely accurate. And taking 10 black measurements and averaging them isn't goingto make the measurement accurate either - you will just be making 10 inaccurate measurements and averaging them to get 1 inaccurate average.

Many projector-screen-room combinations produce black levels so dark that few, perhaps no, meters in general use by even pro calibrators are going to be able to get a black measurement off the screen. So you are left with having to measure black directly from the projector and using that to calculate what you are seeing from the screen (and that's going to be an estimate since the screen gain as specified by the manufacturer may or may not be accurate and each room produces different interactions from reflections.

Bottom line -- there's a pretty low liklihood of capturing a reasonably accurate contrast ratio (that represents what you are actually seeing) without lab-grade instrumentation that is very expensive, and not very portable.

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post #4 of 4 Old 10-28-2009, 11:03 AM
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I always felt calculating the ansi was a bit redundant. I guess if you felt you were a little low and wanted to buy a higher gain screen it might be useful. Its kind of rare that people complain they have too much.

digital film janitor
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