Passive stereo in rear-projected systems has been difficult in the past because standard rear-projection screens do not preserve polarization; after evaluating numerous screen materials, we have identified one that produces very little stereo crosstalk (a.k.a. ghosting).
We obtained a number of different screen material samples from two manufacturers (Stewart Filmscreen and Da-Lite), and tested them with polarizing filters to determine how bright images would appear and how visible the stereo crosstalk would be. Table 1 summarizes the results of these tests. The tests were performed using a 35mm slide projector as directional light source 1 meter from the screen sample; a cropping slide was used to limit the illumination to a 2" x 3" area of each sample. A Gossen LunaPro light meter set to ASA 50 was used to take readings, with the diffuser in place, less than 1" from the screen's surface. The ambient light level reading was EV = -5 (less than 0.010 footcandles).
For the first column of readings, a left hand circular filter was placed at the projector lens. This is a general indicator of the relative brightness of each screen material. For the second reading, a second left hand circular filter was placed at the viewer's side of the screen sample. This is a relative measure of the amount of light reaching the eye through the passive glasses. The third reading is taken with a right hand circular filter replacing the left hand filter at the viewer's side of the screen sample. This is a relative measure of the amount of extinguished light reaching the eye through the passive glasses. The delta number is obtained by subtract reading 3 from reading 2. It is presented in f-stops. The larger the f-stop difference, the greater the extinguishing effect (less crosstalk).
The material we chose for our screen is referred to as ``Disney black diffusion film'' (Dbd). The Dbd material's performance is superior to all the other tested offerings, both because of its minimal stereo crosstalk and because of its contrast qualities. Specifically, the matte black surface of Dbd affords excellent ambient light absorption. This, coupled with brighter LCD projectors, allows acceptable image contrast with only subdued room illumination. For publicly attended showings, increasing the general room lighting has long been considered a desired safety improvement. Further, the material's neutral density effect on the transmitted light masks the ``light bleed of black'' typical of LCD based projectors. Image contrast is thereby further enhanced.
| Material | Screen | View | Ghost | Delta |
| Stewart ``Disney'' black | 5.0 [16fc] | 4.5 [12fc] | -1.0 [0.26fc] | 5.5 f/stops |
| [0.020" membrane] | ||||
| Stewart '96 sample, (black) | 4.0 [8fc] | 3.5 [6fc] | -1.5 [0.2fc] | 5.0 f/stops |
| [0.020" membrane] | ||||
| Stewart FilmScreen 200 regular (gray) | 6.5 [48fc] | 6.0 [32fc] | 1.3 [1.3fc] | 4.7 f/stops |
| [membrane on 3/8" plexiglass] | ||||
| Stewart FilmScreen 200 HiGain (gray) | 6.5 [48fc] | 6.0 [32fc] | 1.0 [1fc] | 5.0 f/stops |
| [membrane on 3/8" plexiglass] | ||||
| Stewart FilmScreen 150 (gray) | 6.5 [48fc] | 5.7 [28fc] | 2.0 [2fc] | 3.5 f/stops |
| [membrane on 3/8" plexiglass] | ||||
| DA-WAN-HC | 4.5 [12fc] | 4.0 [8fc] | 0.5 [0.35fc] | 3.5 f/stops |
| [gray coating on 3/8" black glass] | ||||
| DA-TEX | 6.5 [48fc] | 6.0 [32fc] | 4.0 [8fc] | 2.0 f/stops |
| [gray 0.011" membrane] | ||||
| DA-KS150 | 5.3 [20fc] | 4.3 [10fc] | 3.0 [4fc] | 1.3 f/stops |
| [gray coating on 3/8" black glass] | ||||
| DA-KS150 | 5.5 [24fc] | 4.5 [12fc] | 3.5 [6fc] | 1.0 f/stops |
| [gray coating on 3/8" plexiglass] |