Optical screens have a lens system (glass micro beads) that directs off axis (ambient) light into a black vinyl layer, while allowing projected light through to the viewing side. Due to the patented design of 3M's Vikuiti projection screen, light is focused on a single plane, which provides unparalleled off axis viewing. Optical screens also offer higher image brightness, contrast, reduced hot-spotting, and black levels.
Diffusion screens use a combination of particles to "stop" the light while varying tint darkens the image. Diffusion screens have no way of differentiating between sources of light that hit the screen and have contrast issues due to the fact that adding tint darkens both the darks and the lights equally. Diffusion screens typically work well with short throw projectors.