An ambient light rejection (ALR) screen selectively reflects light back to the audience. This effect is achieved by positioning the projector and screen in such a way that the projector's light is bounced towards the audience while the other ambient light in the room is reflected back in some other direction out of the view of the audience. ALR screens only work if the ambient light is not hitting the screen from the same direction the projector.
ALR fabrics are engineered to furnish directional acceptance and re-reflectance of light within defined limits. The goal of an ALR surface is to respond well to light from orthogonal or perpendicular sources — like the light from a projector — and to limit the re-reflection of light coming from off-axis sources such as lighting and windows.