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Author Topic: Black screen on projection  (Read 3593 times)

Brad Weber

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Re: Black screen on projection
« Reply #10 on: March 02, 2009, 09:36:12 am »

Unless you also control any ambient light, you are never going to get a black screen.  In fact, except for some cinema and home theater applications, it is unusual to have an image contrast ratio that is really affected by the projector contrast.  In typical applications a good image contrast ratio is 10:1, a fraction of even a 500:1 projector contrast.

I'm not saying that contrast is not a factor to consider just that between various ways to measure the projector contrast  (such as full on/full off versus ANSI checkerboard) and ambient light on the screen, it is often not a significant factor.
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Brad Weber
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waldo [Casey Williams]

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Re: Black screen on projection
« Reply #11 on: March 03, 2009, 05:43:01 pm »

Brad (or others), maybe you can help educate me...I'm not really a video guy.

I do have total control over all ambient light in my room. (Well, aside from the audience holding up cell phones, or god forbid, lighters.) So my experience has been that when I pull all the lights down for a near-total black, like for a band walk-on, the projector's 'black' is by far the brightest thing in the room.

So here's my question: Looking at two projectors that have the same ratings in terms of lumens -- say, 2500, but different contrast ratios. Projector 1 is 2500 lumens with a contrast ratio of 1000:1. Projector 2 is 2500 lumens with a contrast ratio of 500:1.

Is it not correct to assume the 'black' from projector 2 will be twice as bright as projector 1?

waldo
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Brad Weber

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Re: Black screen on projection
« Reply #12 on: March 04, 2009, 09:09:18 am »

waldo    [Casey Williams

 wrote on Tue, 03 March 2009 17:43]So here's my question: Looking at two projectors that have the same ratings in terms of lumens -- say, 2500, but different contrast ratios. Projector 1 is 2500 lumens with a contrast ratio of 1000:1. Projector 2 is 2500 lumens with a contrast ratio of 500:1.

Is it not correct to assume the 'black' from projector 2 will be twice as bright as projector 1?

Maybe?  If they were both measured the same and set up the same, including black level, then yes.

Contrast in projector terms is an indication of the difference between black and white and not a measurement of the black level.  In theory, if two projectors had the same 'white' level then the higher contrast ratio projector should have a darker black.  But not only are lumens and contrast not actually a measurement of the white and black levels, there are also multiple to ways to measure both.

The ANSI contrast test uses a black and white checkerboard pattern and is measured at multiple points on the screen.  This is a more strict test as it is measuring the actual contrast within an image.  However, some manufacturers rate their projectors at full on/full off, which normally results in larger contrast numbers.  Some manufacturers apparently even 'cheat', such as disabling the clear element of the color wheel on DLP projectors when measuring the 'full off' condition (it is not truly cheating since this can be done, however you can't actually obtain the contrast ratio that results in any real use).  So it can be difficult to directly compare contrast ratings.  Lumens output is usually a bit more standardized, but can also differ.  In addition, how the projector itself is setup and adjusted can have a significant bearing on the resulting black level.

The result is that you could potentially have a single projector with several different lumen and contrast ratings depending on who you adjusted it and how you measured the values.  So without knowing the actual black levels or that everything is equal, it is much more a theoretical than practical comparison.
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Brad Weber
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Ben Lawrence

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Re: Black screen on projection
« Reply #13 on: March 04, 2009, 12:19:44 pm »

For the record not sure if anyone cares but I ended up solving the problem by using a black desktop on my mac and there is a feature that shows your desktop without any folders so I ran that through the a/b switch with the dvd.
The room was light enough so the projected black was not noticeable. Thanks for the suggestions though all.

waldo [Casey Williams]

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Re: Black screen on projection
« Reply #14 on: March 05, 2009, 06:06:15 am »

Thanks for that. Sounds a lot like loudspeaker 'measurements'. I guess I'm not surprised.

best,

waldo
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Paul Lipp

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Re: Black screen on projection
« Reply #15 on: April 21, 2009, 01:59:23 pm »

My personal fix for this is, I use an analog way OctoLogo, for my switching between projector sources, and i have a few still in the frame memory, but you can also use solid black. I know this isn't the cheapest method, but if your looking at adding other features in, it could work for you.
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