Making a backproj screen for collimated display

[arnolde]

I have 4 collimated display boxes (semitransp mirror and parabolic mirror, old broken monitor removed from top) and i'm trying to think of a way to re-use them. I had the idea of laying a sheet of frosted plexiglass on an old monitor screen and heating it carefully to make it permanently take the curvature of the monitor screen, and then using it with a standard projector as a BP screen instead of the monitor on the display window box.

Has anyone done something like this and could give me a hint what material to use (thickness?) and a method to do the shaping?

[Mike.Powell]

Resolution is an important concern with rear projection screens.The screen must disperse the light from each pixel without making image appear fuzzy. This is particularly the case in a collimated display because the collimating mirror enlarges the image making any fuzziness look worse.

Professional rear projection screens make the light dispersing layer as thin as possible in order to maximize delivered resolution. If you look at the high end rear projection films, you'll see that they disperse light diffractively through a front surface which is composed by random microscopic wrinkles. The dispersion layer is essential reduced to the interface between the film's refractive index and the refractive index of air.

You will likely get a better image by using clear plexiglas and creating some sort for dispersive surface on it. There are rear projection screen paints which might do well. Alternatively, you may get suitable result by wet sanding one surface.

The CRT surface is a spherical section. I seem to remember that a 28 inch radius was common in this sort of application. (Radius is dependant upon the radius of the collimating mirror.) Acrylic can be heated and drap formed, or because it's a spherical section, can be free blown.

At one time I had a plastics forming guide in pdf. If I can find it I'll send you a PM. In the meantime here are a few links to some online info:

http://www.designnews.com/document.asp?doc_id=220362
http://www.plasticsmag.com/thermoforming.asp?fIssue=Jan/Feb-05&aid=4140

[arnolde]

I suppose it would probably be better to frost/sand/coat the outside layer (towards the projector) to reduce the reflectivity of the clear plexiglas?

[Mike.Powell]

I believe so.