High Power and Ceiling mounted PJs ? - AVS Forum
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post #1 of 6 Old 06-11-2001, 06:50 AM - Thread Starter
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My Davis 450 clone is ceiling mounted and there is no getting around it. My question is whether this setup would benefit from the Da-lite high power. Would the High Power
be brighter than a matte screen even at this non ideal configuration?

Gerald
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post #2 of 6 Old 06-13-2001, 06:06 AM
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Gerald,

The image of any high gain screen will be brighter but the brightness will not be constant across the screen surface. As an average you will probably get around twice the brightness of a matte screen. The problem is that the brightest sections of the screen will be some 30-60 % (depending on the reflective properties of the screen and the geometry of the setup) brighter than the darkest sections. This is what is commonly known as hotspotting. Some viewers do not find this objectionable, others do.

Another positive aspect of a high gain screen is the ability of suppressing the screen reflected background light if the sources are located to the sides of the screen. The effective contrast may this way increase even more than the brightness.

The drawbacks of a high gain screen can be minimized by using a curved screen, preferably a torus screen with a two-dimensional curvature. Unfortunately, this usually requires a CRT projector for getting the geometry right.
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post #3 of 6 Old 06-13-2001, 10:47 AM
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What Iceman says applies to the majority of high gain screens. However, in the case of the Da-lite High Power, you have an additional problem, in that the screen is retro-reflective and really only acts as a high gain screen within a narrow viewing cone near the vertical height of the projector. Once you move outside of this cone, the brightness falls off rapidly to a level at or below a matte white screen. I have tested samples of the High Power and some of Da-Lites other screens (Da-Mat; Cinevision, etc.) with gains from 1.0 to 2.0 with my floor mounted CRT. The High Power definately outshined the others in terms of brightness, but I found the viewing angle and color shift to be too extreme for me (color shift will not be a problem with a digital projector) to deal with. Unless the Davis is mounted on a very low ceiling, you'll lose all your brigtness at the seating position. You might want to check out some other high gain screens like the Draper M2500 that reflect back towards the floor.

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[This message has been edited by Sean Max (edited 06-13-2001).]

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post #4 of 6 Old 06-13-2001, 08:15 PM - Thread Starter
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Thanks for the replies guys! I've finally got a sample of the High Power and tried it out in my setup. I must say... WOW! It really works, even in my config, but I didn't know about the extreme drop off in gain from side to side! In my config, sitting outside the width of my screen yielded less gain than my bedsheet! This to me is just unacceptable! So the search for a screen is still on. Sean, do you know if Draper provides samples on their screens? Thanks!

Gerald
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post #5 of 6 Old 06-14-2001, 01:35 AM
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Sean Max,

Thanks for pointing out that the High Power is retro reflective. I suspected as much, but I was not completely up to date on the Da-Lite products.

A retro reflective screen reflects light back to the source. This means that your viewing position has to be close to the projector. Unless you like hanging from the ceiling, this is not a good choice for your setup http://www.avsforum.com/ubb/smile.gif.

When the angle from the viewing position to the screen normal is larger than some 20-30 degrees, the gain will drop below unity for a high gain screen with the projector placed directly in front of the screen center, just as Gerald noted. Placing the projector below or above this position makes things slightly more complicated in the vertical plane.

I would much prefer if the term gain could be substituted for directivity, which is a much better technical description of what is really going on. The term gain really seems to make many people assume that they get more light output from certain screens than from others. This is, of course, impossible for a passive device such as a screen.

What is actually happening is that a wide reflective cone is concentrated to a very narrow one with higher 'brightness'. Remember that total light output has to be the same (neglecting reflective losses) regardless of screen material. In other words, the area under the gain vs angle curve has to remain constant. If it does not (as I have seen in some cases) - well, either someone has formed the basis for a perpetuum mobile or the same someone is full of B.S.
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post #6 of 6 Old 06-14-2001, 11:30 AM
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Gerald,

Draper will provide samples of screen material for you. Visit their site www.draperinc.com and drop them an email or call them. They will send out samples of screen material.

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