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-   -   Guidelines for vertical placement of projector? (http://www.avsforum.com/forum/23-screens/961480-guidelines-vertical-placement-projector.html)

Laserfan 12-26-2007 08:07 PM

I can't seem to find any basic rules for "vertical offset from center" placement of a front projector. I know that if the PJ is too far above-or-below screen center that one encounters keystoning, but I've also seen somewhere that you don't want to place the pj dead-center on the screen either, that there should be SOME offset so light isn't reflected directly back into the projector lens.

Does anyone here know any "rules of thumb" in this regard? Is it a matter of trial & error i.e. do I pick height and look for hotspotting or evenness of light at the seating position? Figure an equal angle between center & pj as the seating position TO center?

I'm new at this but want to try to do it right the first time. TIA for any insights.

Laserfan 12-28-2007 07:23 AM

Hmmm, everyone in here has opinions about everything else, but not about this? I guess the "good news" for me is that my pj is in a rack so I can move it up-and-down and see if there are any differences from doing so. Glad I don't have fixed ceiling mount to guess-at!

budwich 12-28-2007 08:04 AM

not sure that about the question that you ask is.... it is somewhat moot, I think.... because most projectors come with a "built in offset".... so "dead center" is probably never going to happen. As a result, you are "dance with what you brung".... some have more, some have less.

saldog78 12-28-2007 04:00 PM

The answer is, there is no answer. Every projector is different; some have fixed offset (so it needs to be mounted a certain percentage above the centerline), and some have vertical lens shift features, so it's adjustable. You can't really figure out where you'll put the projector until you a pick a projector.

One thing to consider is if you're going with a retro-reflective screen (Dalite High Power, etc), you'll want the projector as close to eye-level as possible to maximize gain.

Laserfan 12-28-2007 08:56 PM

Thanks I was hoping for some science, or at least some rationale, to determine the ideal projector height. At this point I will look for the highest point for the pj where there I still get a perfect rectangle on-screen. The focus of my new HC4900 is so perfect it should be easy to see when I get too far off-axis anyway. Will worry about brightness only if I see any issues there.

McCall 12-31-2007 10:13 AM

in the case of my projector the lens location must be within the screen, not by much but must be within the screen area and dead center horizontally. so mine is about 2" from the top height of the screen. works fine, no keystoning needed. But as stated it is completely projector specific.

FLBoy 01-01-2008 12:26 PM

Quote:
Originally Posted by Laserfan View Post

Thanks I was hoping for some science, or at least some rationale, to determine the ideal projector height. At this point I will look for the highest point for the pj where there I still get a perfect rectangle on-screen. The focus of my new HC4900 is so perfect it should be easy to see when I get too far off-axis anyway. Will worry about brightness only if I see any issues there.

IF your PJ has lens shift, there IS a best height to put it at. That best height depends on your type of screen.

For a retro reflective screen: As saldog768 has said, the best PJ position to maximize the screen gain is just high enough for the light beam to clear your seated viewer's heads. Also, for best brightness uniformity across the screen from side to side for off-center seats, locate the PJ a few inches behind the off-center seats. (If you have two rows, you can only "fix" one of them.) All on-center seats will have near perfect brightness uniformity no matter how high or how far back you place the PJ--as long as the PJ is on-center horizontally.

For an angular reflective screen: To make centerscreen the brightest point (top to bottom), place your PJ so that the angle its beam makes with a line perpendicular to the screen at centerscreen is equal to the angle a line from a centrally seated viewer's eyes to centerscreen makes with the same perpendicular line. When the PJ is too high, the top half of the screen will be brighter than the lower half; and when too low, the lower half will be brighter. Screen brightness uniformity in all directions improves with PJ distance from the screen (but brightness decreases, for optical reasons).

You want science? Try my All Screen Gain Calculator, described here.

Laserfan 01-01-2008 03:11 PM

Many thanks FLboy for this... I will study all of it before I proceed any further!

slicer 10-02-2013 10:20 AM

I would like to know how to calculate the correct vertical offset with Elite Screens 120" diagonal with 24" inch black top? Screen with 96" and height 72" now lets add th 24" black top to that overall height we get 72"+24"=96" total height.

 

Projector is BenQ W1070. On their website specs they say it has:

Projection Offset- Vertical 110%-130% +-5%

 

Here is the artcle which explains how to calculate vertical offset with those previous numbers (total screen height & projector offset).

http://www.digitaltrends.com/home-theater/how-to-install-a-video-projector/

 

The correct height from ceiling is calculated like this, taking into account the projector offset by manufactures specs.

 

Example from the article.

"JVC models offer 80% offset but above and below the screen whereas some models offer different and lesser amounts for up and down.
Here’s how we can make practical use of offset with our JVC projector. Start by multiplying the vertical height of the screen – 56-inches – by 80% which results in 44.8. Then take one-half of the screen height – 56 * .5 – and you get 28. Now subtract 28 from 44.8 and you get 16.8. This is the amount that the center of the lens can be above or below the screen."

 

If I use these calculations with BenQ's 110% offset, I get ridiculous offset! Basically I have to mount projector 57" from the ceiling. I have 118" long walls so the projector will dangle half way down?!

Or am I calculating something wrong?

Help needed, this is my first HT project ever.

 

Cheers,



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