A standard Ambient Light Rejecting (ALR) screen is not a UST screen. A standard ALR screen rejects light from the sides and/or above and/or below, depending on it’s intended purpose. The one thing it always does is reflect the majority of light from a medium or long throw projector back into the room and hence in the direction of the viewer where it is required for the best image quality. It will mess up the image from a UST projector, not enhance it.
A UST screen is a special TYPE of ALR screen (called a Ceiling Light Rejecting (CLR) screen) because it reflects the majority of the light from the projector located below it into the middle of the room and rejects light from above it (ceiling reflections and/or overhead lighting). This yields the maximum image quality from your UST projector.
See ‘Video: How A UST Screen Works’ below the FAQ.