I suppose you would want a partially reflective surface so that most of the light bounces back toward the audience rather than completely scattering. That scatt

I suppose you would want a partially reflective surface so that most of the light bounces back toward the audience rather than completely scattering. That scatter would otherwise raise the ambient light of the screen, and therefore make blacks worse. You wouldn't want a mirror like finish either, though, because then any given viewer would only see a narrow beam of the overall image. My guess is that a reflective cone of about 60 degrees would be optimal? That's probably what the glass beads are doing, reflecting the light and defocusing it in a consistent way. I vaguely recall that the optimal wall paint recommended by the internet was not only gray, but also satin finish.