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Discussion Starter · #1 ·
Tryg,


A Vutec tech suggested I make an AVS post soliciting your opinion. When the manufacturers are deferring to you--that's impressive!


Here's my situation:


I have Sony G70 CRT projector that I want to set up for rear projection. My home has an open floor plan, and the family room that I'm using for my HT room is open to several other rooms, including a stair case with a roof light. I normally only watch TV or movies at night, so this is usually not a problem. But I considered getting the screen in dark gray so my pj is at least usable during the day. The projector will be in the garage mounted to the ceiling, and to help with the insulation problem a 80" X 45" hole in the wall creates, I had planned on going with a Vutec 1/2" glass screen, which is only available with the Retro-Vu model. I also debated on whether I could get away with using the charcoal gray model, which would make the pj usable during the day, but, may really hurt the overall pq.


I've talked to several people and have received conflicting advice. On my latest call to Vutec, the guy I talked to thinks both dark gray and Retro-Vu are bad ideas, and will not work well in my application. He suggested non-gray PrismaTec, which I understand is a better screen, but it's only available in 1/8" plastic (acrylic?). The PrismaTec is better and cheaper that the Retro-Vu in 1/2" glass, but offers little help with my insulation problem. Not that 1/2" glass is a good insulator, but it's certainly better than a thin sheet of plastic.


Others I have talked to, including Vutec employees, didn't seemed too concerned about using a glass Retro-Vu in my application. One Vutec employee even suggested to one of the venders that called on my behalf that I should go with charcoal gray.


I'm so confused! :)


What do you suggest?
 

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I'm pretty far from being an expert but here's my take on your rear screen. Mine was used and I got it with my projector. I was lucky that it works for my application! Da-Lite has a pretty good description of their front and rear screens here: http://dalite.com/products/selecting.php. Even though its specific to their products, the selection criteria seem useful. Just ignore their product names and look at the gain and type of screen:


1) Fresnel/Lenticular vs. Diffusion

Assuming you don't need to cover a large viewing angle, the fresnel is probably best. It has lots of gain which makes it easy on your CRT and makes it bright enough to use in the daylight, within reason. Also, the finish on the viewing surface is usually ribbed which keeps reflections from being an issue but also means its harder to clean and more easily scratched. I think the fresnels also have a limitation on resolution related to the spacing of the ridges. The manufacturers should know the answer to that one.


2) glass or acrylic

Since you want insulation between the PJ room and the viewing room, glass would be nice! Also, it's harder to scratch so as long as you have lots of friends to help put that heavy thing in, I'd go glass. Beware that most fresnels are only available in acrylic.


3) tint

From what I've read, the tint is usually there to enhance contrast, much like the gray screens used with the lcd / dlp units. Since you're running a CRT ( a nice LC'd one to boot! ), I'd go low tint. Hopefully someone else will chime in here. If it were possible, I'd look at one of the screens with the black stripes applied between the ridges. (I think DNP makes one but Lord knows what that costs.) Supposedly, you get good efficiency AND reduction of ambient light problems.


4) gain

This mainly depends on how large the viewing angle needs to be to cover your audience and how big your screen is. Again, someone with more experience should probably cover this one.


So what do you get? Ideally, probably a DNP fresnel with the black stripes ( I think Vutec calls them DNP Retro-Lens Rear Screens) but your wallet may tell you otherwise. Sorry if that was long winded. Let me re-state my disclaimer about not being an expert! I hope this helps though. There aren't a whole lot of facts and opinions about RP screens like there are with FP screens. I've certainly tried to find them!


-Matt
 

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Discussion Starter · #3 ·
Matt,


Thank you for your response.


1) Fresnel/Lenticular is definitely out of my price range.


2) The last Vutec tech, as well as a couple of other people I talked to, didn't give my insulating problem much importance, and his recommendations never took that into account. Don't get me wrong, he was very helpful and I appreciate it, but insulating was just a non-issue to him. But in reality, it's a HUGE issue, and one that has to be addressed. I don't see how I have any choice but to go with glass. It seems you recognize that too. Thanks for understanding.


3) As you know, we CRT owners normally don't need any help when it comes to contrast or blacks, but the reason I was interested in tint is because it can also lower black levels in ambient light conditions. I imagine the charcoal gray Vutec would really help in that manor, but Tryg has a FP screen demonstration that shows how even light gray and silver noticeably effects color and brightness, so overall tint is bad. But, many RPTVs have dark gray screens (maybe they all do--I haven't looked at any recently) and some of them have a pretty darn good pictures. Because of RPTVs, I wondered if maybe tint doesn't effect RP screens the way it does FP. I'm still confused by this apparent contradiction.


4) My seating angle is pretty narrow. Prismatec (it seems it's probably the best non-fresnel, affordable screen: http://www.vutec.com/vuteccatalog/prismatec.pdf ) has 6.5 gain and would be perfect if it came in glass (it doesn't). Retro-Vu (Vutec's diffusion screen) has gain as high as 2.5 and is quite a step down from Prismatec, but it is available in glass. Other than that, I saw no downside to glass--that is until read your comments about problems with reflections ( http://www.avsforum.com/avs-vb/showt...52#post3287652 ). While no sunlight will shine directly on the screen, there are many windows that could cause reflections.


Yesterday, after a couple of days trying to figure this stuff out, I was ready to order a Retro-Vu with 2.5 gain in 1/2" glass and charcoal gray. My conversation with the Vutec tech caused me to hold off. Your comments today about reflections made me doubt my choice even more.


I know 10 times more about this subject than I did 3 days ago and I'm more confused than ever.:(


PS: Your screen looks pretty dark in your photos. Is it?
 

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I don't have a lot of knowledge of rear projection applications. I've seen a few and I've always wanted to try to build one for fun. My knowledge ranges form standard PVC tensionable material that is lower gain to some of the fresnel options that provide higher gain. Maybe I should try to get some of that Prismatec material from Vutec and build a setup. Of course documenting how cool rear projection can be;)
 

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The reflection thing sounds like another question for Vutec. From their catalog, it says they have an anti-reflective coating available but I couldn't tell you how well it performs. I'd ask if they have a small sample piece or a chunk off of a broken one you could look at. Maybe the Retro-Vu is OK in that regard but you'll never know until you see one.



The tint on mine is actually pretty light. Its the flash from the camera that makes it look dark.


-Matt
 

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Phil,


Not sure why you think glass is better than plastic for insulation. It has a much higher thermal conductivity, but it really doesn't matter much because convective heat transfer dominates the situation.


"so overall tint is bad"


Only if it isn't a neutral gray, and even then it can be calibrated out.


Tint has a singular advantage with RP over FP. If the tint is a layer on the fron surface, ambient light reflecting off the screen goes through the tint twice, vs. once for the image being transmitted from the back. So if you had a 50% gray tint, brightness would be cut in half, but reflected light by 75%, doubling contrast for conditions where the black level is dominated by room light.


I've got a sample of the Prismatec, and while it is amazingly bright, it is also very reflective of ambient light.


Prismatec type technology is popping up from several mfgr's; one in Oregon has three different tint levels, from Prismatec bright to dark. When I get to the office I'll add the link here and you can request a sample pack of all three tints.
 

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Discussion Starter · #7 ·
Tryg,


Hey, you're already THE FP screen expert. Might as well become the RP screen expert too! :) Besides, a RP setup wouldn't be a bad thing to own. At least that's my hope. I've never seen one, but everyone that has seen RP setups rants and raves about them.


Matt,


The anti-reflective coating is a good suggestion. I'll call Vutec today about it. My only worry is that it will be delicate and will require special care, defeating one of the best things about glass--durability and easy to clean.


So your screen is a light tint. Felix, another RP CRT user, says his is a milky white. A light screen still worries me. For my application, I'm afraid it will be completely unusable during the day. Again, because rarely watch TV during the day, that wouldn't be the end of the world. But to have the option would be nice.


I'm going to go to a few electronics stores this evening and check out the screens and pictures on RPTVs. If RPTVs can achieve good pq with a dark screen, it certainly seems to reason that my pj with a dark screen (I'll be using a superior pj and screen) should have an even better picture, most likely a LOT better.
 

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Quote:
Originally posted by Phil Smith
Tryg,


Hey, you're already THE FP screen expert. Might as well become the RP screen expert too! :) Besides, a RP setup wouldn't be a bad thing to own. At least that's my hope. I've never seen one, but everyone that has seen RP setups rants and raves about them.

Novice! :) I would love to make a rear projection set up for fun. But I only have so much space. My HT room is only 1200 square feet
 

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Holodidplay screens:

http://www.holodisplays.com/pages/ho...rey_specs.html


"If RPTVs can achieve good pq with a dark screen, it certainly seems to reason that my pj with a dark screen"


Careful; RPTV's can get away with a lot of tint becuase they use Fresnel/lenticular screens with gains from 3.5 - 5, and are smaller.


I believe the G70 is pretty bright, though; my guess is you'd probably do well with the in-between Hololite tint; not ideal in a bright room, but way better than a white front screen, and likely preserving adequate brightness.
 

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Noah,


In another thread you mentioned that the published gain for these RP screens isn't what we are used to with FP screens in terms of perceived brightness. Can you guestimate what the "apparent gain" in FP terms would be for the HoloLite, Gray and Dark? I'm just trying to get a ballpark handle on how bright these things are.
 

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Discussion Starter · #11 ·
TTryg,


Geez--1200 sf--how many screens does your megaplex have? :)


Noah,


You bring up some interesting points.


I know very little about the subject, but assumed a thick piece of glass would transfer less heat than a thin piece of acrylic, regardless of which was the better conductor. No?


I'm currently projecting on a wall a 45" X 80" picture. I have zero or less gain, I don't push my pj hard and I'm getting adequate brightness. You mention that 50% gray would cut brightness in half, which is what one would assume. Holodisplay claims 4.6 gain for their dark gray screen. That's a lot and really doesn't seem possible. If the dark gray truly has 4.6 gain, I should absolutely no problems with brightness--well maybe too bright!


My G70 is rated at 1,200 lumens. I can't imagine any of the CRT RPTVs having light output anywhere near that. Gain on the Holodisplay dark gray is comparable to RPTV screens, and I would think it would be a better screen. My screen size is larger, but it's still a relatively small screen. I'm not trying to be argumentative (I appreciate the help!), but I'm having a hard time understanding why my setup with a dark screen wouldn't outperform any RPTV in every way.


I called Holodisplay (thanks for the link) and talked to one of the owners (real nice guy). They do mostly commercial stuff, but he said the dark screen really saturated the colors, especially red (a problem color for crt) and I got the impression he liked the dark screen best. He's sending samples that I should get next week. I'll read up on Tryg's work and see if I can figure out how to do a meaningful comparison.
 

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BTW, about the insulation thing, 1/8" acrylic actually insulates slightly better than 1/2" glass.


Thermal Conductivity (watts/meter degC)


Acrylic 0.2

Glass 1.05


Glass has to be 5 times as thick to insulate as well as acrylic. Thick insulating windows work because of the air trapped between the two layers of glass.
 

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Quote:
My G70 is rated at 1,200 lumens
I've read 240 ANSI lumens for the G70 and 350 for the G90. While I haven't measured them, that's about how bright they look to me.
 

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Phil,


catapult's answer is correct.



"...but I'm having a hard time understanding why my setup with a dark screen wouldn't outperform any RPTV in every way."


Maybe it would be; if not it would be because of teh high gain of the optical screen, but maybe the G70's output cancels it and the bigger screen area (your screen is more than 3X the area of a 50" screen).


Looking at the the Holodisplay samples, the Holodark looked slightly less bright than what I assume is a matte white 1.0 pulldown screen in our conference room. Brightness fell off pretty quickly with viewing angle, but it wouldn't be as noticeable w/o a white screen next to it.


The lighter tints are successively several steps brighter, with the Hololite about the same as the Prismatec.


If you'll be happy with the brightness, the Holodark should look great in a lit room; it's even darker than my Toshiba RPTV's screen. It (Holo) has a pretty good antiglare finish, but at just the wrong angle it will reflect a light source pretty badly.


Please let us know your impressions of the samples.


Back to the insulation, it would help a lot if you did something like the Holodark screen with a piece of glass 1/2' in front of it. I don't think AR (anti-reflection) coated piece would be too expensive ($200?), because they're used in commercial applications.


Google on AR glass and you'll get a lot of hits. I'd also be interested to hear what you find out; I've googled but never pursued anything.


It might be as easy as calling up glass suppliers in teh Yellow Pages.


Good luck.


One more thing - the 1200 L rating would be peak (I think it's for 10% of the raster area), 250 ANSI I believe is a checkerboard pattern, which taxes the power supply more by illuminating more phosphor area.
 

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Phil,


I did design & engineering of residential installations before I became a full-time calibrator (man, has it really been 10 years!)


I would like to interject something that I feel is extremely important to the discussion of rear-projection installation. IMO, the idea of putting quality electronics and optics (ie: your G70) in an environment that is subject to dust, dirt, humidity, etc. is asking for trouble. You should treat your projector as if it was a special-needs child. In this case the special needs are:


Temperature & humidity control

Completely blacked-out surroundings

Floor treatment (dust & dirt management)

Access to all sides (for setup)


Good luck with whatever you decide to do.


Ken Whitcomb
 

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Discussion Starter · #16 ·
I know how insulated windows work. That was not my inspiration for thick glass. I can't double pain for optic reasons, so I assumed a thick piece of whatever would be better than a thin piece of whatever, especially if it's 4 times as thick. I understand now that's not necessarily the case.


I knew that about the lumens. That was a brain fade moment. While writing that post I couldn't remember what the light output rating was, so I quickly looked it up at PSI's website, which list peak lumens. I'm not sure, but I think the numbers are actually even lower, maybe 1000 peak and 220 ansi. As low as it sounds, my G70 is brighter than the 2000 ansi data grade digital pj it replaced.


Back to screens!


Noah,


catapult mentioned you had previously commented on the published gain, the apparent gain and perceived brightness of RP screens. You mention in this thread that the Holodark, rated at 4.6 gain, appeared to be slightly less than 1.0 gain. Why such a huge discrepancy?


Ken,


I'm very familiar with your well respected work.


I run an ebay based mail order business out of my home and the garage is my "shipping dept." I keep it clean, in fact it's often cleaner than inside my house. Dirt and dust will not be an issue. The pj will be mounted on the ceiling with the same level of access as any ceiling mounted pj. I'm in Dallas, so the winters are not too bad. I can use a space heater to keep the temp from dropping below 45 or 50. Humidity may be a problem, but don't think any of the stuff that's been in my garage for years has any signs of moisture related damage, so I think I'll be all right. Summer's are HOT, and this may be problem.


Not only do I want a RP setup, I really don't have a choice. My family room has an 18' ceiling. Plus, RP should be better suited for my ambient light problem. I'll just have to figure out how to deal with whatever problems arise.


PS: Ken, you have an opinion on the white, gray, dark gray issue?


PSS: Noah, I Googled AR glass and didn't find much. I'll make some local phone calls next week.
 

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Phil,


"I can't double pain for optic reasons, "


If you use AR coated glass with, IIRC, a fraction of a % reflectivity, it won't be an issue.


"You mention in this thread that the Holodark, rated at 4.6 gain, appeared to be slightly less than 1.0 gain. Why such a huge discrepancy?"


Beats me. It's not just the Holo material; I have a dozen different kinds of rear screen samples and apparent gain seems to bear no relation to claims.
 

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Discussion Starter · #18 ·
Noah,


"If you use AR coated glass with, IIRC, a fraction of a % reflectivity, it won't be an issue."


I previously ask one of the Vutec techs about it and was advised not to do it, which was the answer I expected. It would be the best way to go if feasible, but I'd hate to go to the trouble and expense only to find out when I'm done it's a problem. I'll check into it further.


You would think that the specs for such expensive, niche market products would be free of false, marketing-driven claims. I guess not.


I looked at RPTVs at Ultimate Electronics last night. I turned off a few of the top of the line Pioneers and Mitsubishis, and the screens were extremely dark, even darker than I expected. The pictures were only so-so, but none of them were dialed in very good, so they were not a good indicator of what a dark RP screen is capable of. They had a pretty narrow field of view, so I guess the screen gain was pretty high.


I feel confident from the RPTV trip that I'll be ok with a gray screen. Still not sure about light gray or dark gray. As you pointed out, I may not have enough light output for dark gray. Hopefully I can determine that with the samples.


PS: I appreciate everyone's help! Thanks to your advice, I've gone from completely lost to at least stumbling in the right direction.
 

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Phil,


"I previously ask one of the Vutec techs about it and was advised not to do it,"


He may or may not be knowledgable about the subject; in any case he has nothing to gain by reccommending it and he covers his ass by not doing so.


AR glass will be essentially invisible. I was even considering getting the Holo screen and bonding AR a piece to the fron and/or rear surfaces of the screen to cut reflections and increase transmission (the former would be a lot more noticeable than the latter because the % change is much greater).
 
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