P3D Drafting Mylar for Rear-Projection Screen?

[Bill Davis <bd3d@xxxxxxxxxxxxx>]

In Digest 2407, John Fairstein wrote this about replacing the
rear-projection screen in a TDC Project-or-View:

>
>I replaced the missing screen with a sheet of thin window glass backed
>with drafting mylar. This material is frosted on one side. It shows good
>extinction so I don't believe it depolarizes the image. It would
>probably be better to sandwich the mylar between two sheets of glass.
>
>

This was very interesting information to me, as I have been looking for
suitable material for a large rear-projection screen for some time and have
had minimal success.

I picked up a sheet of drafting mylar at the local art supply house.  As
described, it has one frosted side and one shiny.  The material I bought
was not individually packaged, just one sheet from a bundle in a cabinet
drawer, so there was no product or grade information other than "5 mil one
side frosted".  They also carry 3 mil as well as a two-side-frosted
material.  Cost was $3.00 for a 24 by 18 inch sheet.

I taped the mylar to a large window pane then used it as a screen for my
TDC 116. Very sharp images were possible, with nearly invisible grain.  

While enough polarization was maintained to present a decent stereo effect,
there was also enough depolarization to cause significant ghosting in most
images. This was especially evident in my main test slide, which includes a
dark red barn with white-trimmed windows.  Testing with various other
slides showed the same effect in various degrees. Careful adjustment of
left and right to overlap the brightest spots in each image helped minimize
the ghosting in some slides, but this isn't the way we like to project.

Would this ghosting be minimized with the 300 watt lamps of the
Project-or-View?  Black would still be black, but bright spots would be
less so? After polarization, is it relative or absolute brightness most
responsible for ghosting?

There was no difference in image quality whichever side of the mylar was
facing the projector.

Light fall-off to the sides was rapid, not good for a roomful but probably
not a problem for Project-or-View with its somewhat fixed viewing position.

Hot spotting was evident (500 watt lamps) but not too bad.  One could look
straight back through the screen into the lenses without being blinded.
This worked out well with the adapter for View-Master reels installed on
the TDC, where the hot spot is centered on the images. 

Pros: Sharp image; bright image; inexpensive; easy to cut

Cons: Depolarization

So, is there a particular grade of drafting mylar I should be looking for?
Has anyone else done some testing with this material with better success?  

Sam, have you exhumed your P-o-V for experimentation?

So far, the bast material I've found for rear-projection stereo is
Da-Lite's PolyLens material.  For technical info see:
http://www.da-lite.com/Tech/Specs/specification16.html

Don't have the price info, but I think it's in the $30-$40 per sq. ft. range.
Best regards,
Bill Davis
mailto:bd3d@xxxxxxxxxxxxx



------------------------------