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screen calc what is gain and how can i adjust it w/ diy screen?

422 Views 1 Reply 2 Participants Last post by  graffixjones
Okay i was going to hang my projector from the ceiling but i realized there is a closet right behind the room where i want to drop it from so i was thinking that i could cut a hole and shoot the image out of that. I would like to do this for the custom install look, like a real movie theater. but i did the pj central calculator with the new throw range and in ordeer for the image to look good i need to adjust the gain of my screen to 2.0. now how can i do that with a diy screen? i was planning on getting blackout cloth and making a screen, and painting it if nessasary. anyone have some help for me.

how can i get this 2.0 gain in my screen? i have a sanyo z3 if anyone needs to know and i am going to throw the image 16' or a little more.
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Essentially, you'll need to add some sort of reflective material to the screen to boost the gain.

Getting a 2.0 boost is going to be tough though, unless you go with one of the aluminum variants, which have a lot of reflectivity, yet have a narrower viewing cone.

Here's the basics when it comes to DIY materials, and how they affect the screen:

1. Flat white - gives you a gain ~1.0. Most choose to use flat white because it's easy to apply and gives good results.

2. Metallics and Pearlescents - gives added gain depending on the amounts present in the mix. Pearlescents give lower gain, but a wider viewing cone, and metallics give higher gain and a narrower viewing cone.

3. Satin and Matte finishes - reduce hot spotting. This is why you'll see these clear components added to many mixes you see here on this forum.

4. Gray screens - sacrifice gain in order to heighten contrast levels... of course some of the aluminum metallics are considered 'high gain' gray screens.

I think your best bet is to probably sacrifice the 'professional' looking install and adjust your projector so that you don't need such a high-gain screen... with DIY materials, you're probably looking at 1.5 gain as the maximum 'easily reachable' gain quotient... after that, the materials become more difficult to apply, and the methods are more complicated.

For example here is what I have done to get a decent >1.0 gain screen:

1. Base coat: Behr UPW Flat latex

2. Top coat: 1:1 mix of Behr *** and CPM

Paint several coats of the base coat onto the material you're using to construct the screen, and sand out any ridges or bumps between coats, so that you have a 'fairly' flat bright white surface. I use a Wagner HVLP sprayer, and I find that it takes about 5 coats before most of the translucency is gone (I apply it to blackout cloth).

Once you have your base finished, mix up your pearl/clear topcoat, and apply between 5-7 coats of that as well (if using a sprayer). It's harder to apply the pearl, because you want it to be evenly spread over the whole surface, otherwise you'll end up with brighter and dimmer spots. When you have even coverage, your screen will be an even off-white cream color... don't let the off-white fool you though... your whites will still be really bright without any cast.

I'd estimate that I get between 1.3 to 1.5 gain with this screen, with a viewing cone that exceeds 90 degrees without drop off (45 degrees to each side from center).

I performed this estimate by using a piece of ultra bright white laser printer paper and taping it to the screen (which has a neutral gain of about 0.9 to 1.0)... the area projected onto the paper was definitely dull compared to the surrounding areas, and I could see the reflectivity in the pearl, so I knew it was doing it's job.

I've tried two screen variants since doing my screen originally with the method above, and I'm currently repainting my screen to go back to it... I should've stopped while I was ahead. :D

You can check out my thread titled "Possible to thin Behr White Opal Pearlescent?" here on the front page of the DIY forum if you want to check out some screenshots.

I'm not saying that this is the best screen for your particular application, only that it gives high(er) gain and is fairly easy to paint, with good picture quality and ambient light performance... it also has a nice viewing cone, which gets tougher the higher the gain.

Also, in my thread I mentioned this screen hot-spotting, when in fact it wasn't the screen, just my projector's contrast adjustment out of whack... once I calibrated it with the THX calibrator things looked really good.

Good luck with your venture, and I hope you had a Merry Christmas.

P.S. This is kind of a late edit, but I forgot to add that the Satin/Matte additions also tend to lower gain, because they diffuse the light, so it's a trade-off between removing the hot spots and keeping a higher gain screen coat.

The final thing to keep in mind, is that gain and viewing cone are mutually exclusive... the higher the gain, the narrower the viewing cone, because light that normally is reflected off-axis giving a wide viewing angle is instead directed directly back at the viewer more toward the center of the screen, at the expense of off-angle viewing. That's mainly why I suggested the readjustment for a lower-gain screen, because I think you'll be much happier with that in the long run... that is unless your screen viewing area is pretty narrow (less than 10 degrees off-axis on either side).
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