Frosted perspex for rear projection

[sleepytom]

I'm hoping to build some small perspex screens for rear projection purposes. Does anyone have an experience with different grades / thicknesses of perspex? Is clear frosted perspex OK? I'm hoping for an OK viewing angle with as little hotspotting as possible.

[timsabre]

I've done some stuff with a special projection film (similar to rp screen material) stuck onto clear perspex. See http://www.wedgwood-group.com/projector_screens_holographic.htm

 

Also tried el-cheapo window frosting film from local sign company, this was less successful (didn't really "capture" the projector light) and had bad hotspot problems. I would expect frosted perspex would be the same, you'll need the proper film to make it work right.

[sleepytom]

ouch the film is really expensive.

 

maybe I'll have to use normal soft rear projection screen material.

[jason5d]

Hi tom

I've done some tests with perspex and glass

I found the best was sand blasted glass and

the sand blasted perspex was not quite as

good but still a good image.

 

 

jason

[timsabre]

Why not give the cheap window frosting film a try, it's very similar to the sandblasted effect but with less effort.

A local sign shop should be able to give you an offcut to try out.

 

Basically it works all right until you are in direct line of view of the projector, then you see a hotspot where the projector is.

[sleepytom]

I've never got the sandblasting thing? why not just buy frosted perspex? its no more expensive than clear.

 

I might just order a bit of frosted perspex and see how bad it is. I have located a fastfold rear surface to cut up if the perspex is no good.

[AHYoung]

Ive used polycarb thats been orbital sanded with resonable effect this is probably the same as sandblasted glass, and also used the white film that you use to do large format printing onto which actually works better than youd expect. what size are your Small screens? and are you looking for a floating Holoscreen effect or simply a solid rear screen?

 

The reason you cant use frosted plastic is that you effectively need a thin surface layer on a transparent substrate to project onto rather than a solid translucent material .

 

You could cut up a fastfold, but as you can get the material pretty cheaply off the roll it seems a waste. Its also going to be a challenge to bond the screen material to the substrate not only as its almost impossible to get anything to stick without causing bloom, but the fact that you really need to stretch the screen.

 

If its a budget job, contacting a couple of the manufactures of screens is worth a try as these things get scratched and sold off super cheap.

[rgye]

I really think it's a false economy to try to improvise with frosted perspex as this is a more complex science than it might seem. All the screen manufacturers, whether they sell rigid plastic, stretched flexible or stick-on film, offer different grades of material for different applications - high transmission versions for high ambient light (but poor oblique viewing) through to much denser screens with good oblique viewing or double sided images. The very sophisticated screens have a moulded lens structure in the surface (I don't want to frighten you, but I was quoted £42,000 yesterday for a very large rigid plastic RP screen!). AHY's suggestion of getting an offcut from a damaged screen is a good one, but a small piece of standard grade screen material might anyway be reasonably affordable.

 

I'd google a few screen manufacturers and suppliers if I was you. There's also some useful general info on the subject Gerriets' site.

[sleepytom]

just as a follow up to this thread I thought I'd let you know how it worked out.

 

in the end we didn't use perspex or any of the stuff mentioned in this thread, as the budget was really very small and the installation only ran for 3 days even the cost of frosted perspex would of been too large. We went with a good old fashioned trick and used tracing paper. This is very low cost and provided an acceptable image quality (though getting it off the roll and mounted in the aperture without creases or ripples took a bit of effort.) Its so cheap though than you can mess it up and start again without needing to re-mortgage your house! I was pleasantly surprised by how good the image quality was, we had the projectors set down low with a good deal of keystoning to minimize the hotspotting and the image quality was acceptable, there was a bit of a hotspot sunrise effect but not significantly worse than you get with an RP fastfold.

 

All in all I was very happy with the result. as was the client,