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A FRAME FOR A POLARIZATION PRESERVING SCREEN (Originally Posted Sept. 2001) The Stewart Silver400 screen (www.stewartfilm.com) is the best material I have found for polarized stereo projection. It is one stop brighter than super wonderlite (Dalite lenticular) and has much better extinction properties. The material is usually ordered as a sheet with 1" webbing glued to the edges. This webbing has grommet holes nominally on 4" centers (for smaller size screen). Frames from Stewart cost about as much as the material, which must be stretched flat and maintained under weak tension. Here is how to build a frame for about $100. Note on sizes: Slide projectors (e.g. Ektagraphic High Bright) produce about 1800 lumens. Higher intensity (metal halide) projectors can be purchased (2 - 3 X brighter), but you should use "projection dupes" in these because the intense light will ruin originals. Metal halide projectors (Telex Caramate C2000, Simda D5000...) are also 5 to 15 times more expensive than the standard Ektagraphic slide projector. Using the 1800 lumen Ektagraphic with polarizing glasses, the maximum recommended screen size in a dark room is about 10 feet wide (i.e. a 20 foot wide screen will be much too dark). Therefore, if you build a screen for use with standard slide projectors, I recommend the size be under 10 feet wide.
Figure 1. Screen edged with black plastic (top and bottom), and black absorbing felt (sides). Note carrying handles on the sides and screw eyes for hanging on the top. This one is 6'6" wide and sized vertically for full frame 35mm.
Figure 2. The frame is made of angle aluminum. 3/16" bungee cord (20 cents per foot at Lowes Hardware) provides the tension. Small S hooks are crimped onto the cord. The frame is sized 3" larger than the screen size (on each edge, 6" in total). The bungee cord is laced through holes (see figure 3) with about 1 1/2" inverted V's. You don't want to pull the screen too tight. Leave extra cord until you get what you want.
Figure 3. Angle detail. The hole-pairs are close to the back of the angle, to minimize bending of the angle when the screen is tensioned. You have to drill a hundred or more holes for this project. You can do it with an electric hand-drill (after careful layout), but a drill press is better. String the bungee separately in four pieces, one for each side. This gives you more flexibility in adjustment than if you run it all the way around. Leave a couple of feet at each end, so the cord can be loosened if need be.
Figure 4. The hole pattern varies slightly in the corner. Note the corner brace. The Velcro strips to enable attachment of the black border. Frame is assembled using 1/4-20 bolts and lock washers. The frame will twist slightly (i.e. top right corner wants to come out, bottom left corner wants to move back, or similar). So when you hang the frame, a support anchor at the bottom may be necessary. If on a wall, just pull the bottom rail back into the wall. When the frame is stored (with the material on it), I suggest sewing a large cover. The material is expensive, works marvelously, and due care should be taken to keep it clean. The Stewart Silver400 screen material is soft, so try not to "dent it" or "fold it"...... |
