MY GOD, IT’S FULL OF SMALL GLASS BEADS
For 2001: A Space Odyssey (1968), the director Stanley Kubrick established a rarely used technique in filmmaking—front-screen projection. While other films projected the background of the scene onto the back of a translucent screen, Kubrick projected it onto the front of an opaque screen. In order to eliminate the shadows that the actors would inevitably cast on the screen in this lighting and to keep the shot from looking washed out, he used a screen made of Scotchlite. This material was made of millions of tiny glass beads which reflected the light back to its source, thus resolving both issues. Later, Star Wars would use Scotchlite for its light sabers, which were just sticks wrapped in this material.
(Miller, Ron. Special Effects: An Introduction to Movie Magic. Page 89.)