a bit technical question regarding retro reflective screens - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 9 Old 06-18-2019, 08:03 AM - Thread Starter
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a bit technical question regarding retro reflective screens

I was wondering how much Gain you would get if the video projection beam was coming out towards a retro reflective screen nearly exactly from your eye position.
In my own home theatre the projector is on a tripod rather than mounted on a wall and the screen is a 120'' retroreflective one. I've always noticed when I crouch to grab something from the floor (the beer), the image gets so bright when my eye is amost in the same angle of incidence as the projection lens that the light dazzles me.


But the question of just how much Gain you get at that angle come to my head when I saw this:
youtube.com/watch?v=XPOgLt_73cY


To summarize the video it's a head-worn, very compact dual projector which directs the projection beam with special optics (a polarizer and QWP) to the screen and back to your eyes. So I'd guess you could expect very bright images even if the source LED wasn't very bright in this. Is 5.0 gain a reasonable estimate?
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post #2 of 9 Old 06-18-2019, 08:38 AM
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a bit technical question regarding retro reflective screens

I have the exact same setup on a tripod. You’re describing this scenario.

I setup a SI Slate 0.8 yesterday and I believe that screen is retro reflective but I haven’t noticed any excessive brightness from it yet. I will certainly be moving around the screen tonight to see if I experience what you’re describing.
Also the YouTube link didn’t work for me so I may have missed the point of your post.

Sent from my iPhone using Tapatalk

Last edited by Gamecock24; 06-18-2019 at 08:42 AM.
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post #3 of 9 Old 06-18-2019, 11:07 AM
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Retroreflective means that incoming light is reflected directly back to the source no matter what angle the light source is from the retroreflective screen. All other screens are angular reflective, meaning that incoming light from any angle is reflected off at an angle and not directly back at the source unless the source is at exactly a 90-degree angle to the screen surface. The only retroreflective screens are those with embedded glass beads or crystals. If your screen doesn't reflect incoming light back at the source regardless of the source's angle from the screen then it's not retroreflective.
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post #4 of 9 Old 06-18-2019, 11:45 AM - Thread Starter
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Originally Posted by Dave in Green View Post
Retroreflective means that incoming light is reflected directly back to the source no matter what angle the light source is from the retroreflective screen. All other screens are angular reflective, meaning that incoming light from any angle is reflected off at an angle and not directly back at the source unless the source is at exactly a 90-degree angle to the screen surface. The only retroreflective screens are those with embedded glass beads or crystals. If your screen doesn't reflect incoming light back at the source regardless of the source's angle from the screen then it's not retroreflective.

Thanks for explaining what a retro reflective screen is.
I wasn't asking for one though.
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post #5 of 9 Old 06-18-2019, 12:28 PM
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When on axis within the advertised viewing cone of a retroflective screen, your gain should be the advertised gain of the screen. So with the old HP 2.8 when set up so the projector beam is level with your head (like shooting between 2 seated viewer just out of the image) it would be very close to 2.8.

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post #6 of 9 Old 06-18-2019, 12:38 PM - Thread Starter
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Originally Posted by airscapes View Post
When on axis within the advertised viewing cone of a retroflective screen, your gain should be the advertised gain of the screen. So with the old HP 2.8 when set up so the projector beam is level with your head (like shooting between 2 seated viewer just out of the image) it would be very close to 2.8.

What I'm trying to say is if you get very close to the exact middle of the cone itself, the difference is huge.
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post #7 of 9 Old 06-18-2019, 12:55 PM
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Yes it is, this is why they were only really good for certain situations like a 2 seater or a very narrow theater with projectors that could be place at center screen. The other issue was the father out of the cone you got the more color shift (HP shifted to red) there was along with the loss of light. Other benefit of the HP was that due to the way the micro beads work, wrinkles would be less obvious than with other screens and the less reflected light hit the side walls and ceiling.

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post #8 of 9 Old 06-18-2019, 01:17 PM
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The highest rated gain retroreflective screen I can recall seeing was Draper's now discontinued Glass Beaded CH3200E at 3.2. Of course the penalty for higher gain was reduced viewing cone, so it was only 3.2 gain at the center of the viewing cone and dropped off quickly for those not seated in the sweet spot. There have been higher gain angular reflective screens using various elements to create that gain but they also suffered from narrow viewing cones plus sparkles, hot-spotting and other assorted artifacts (TANSTAAFL -- there ain't no such thing as a free lunch). You can see a graphic of the CH3200E viewing cone in the document linked to below:

euroscreens.se/backnet/media_archive/original/214cf94331a328f41b25e8c57226f9e5.pdf
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post #9 of 9 Old 06-18-2019, 02:45 PM - Thread Starter
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Quote:
Originally Posted by Dave in Green View Post
The highest rated gain retroreflective screen I can recall seeing was Draper's now discontinued Glass Beaded CH3200E at 3.2. Of course the penalty for higher gain was reduced viewing cone, so it was only 3.2 gain at the center of the viewing cone and dropped off quickly for those not seated in the sweet spot. There have been higher gain angular reflective screens using various elements to create that gain but they also suffered from narrow viewing cones plus sparkles, hot-spotting and other assorted artifacts (TANSTAAFL -- there ain't no such thing as a free lunch). You can see a graphic of the CH3200E viewing cone in the document linked to below:

That's a useful document. The curve shows why I experience so much more brightness from little difference in relative head angle.
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