ALR means selective light management
A normal matte screen scatters light broadly. An ALR surface is engineered to treat light differently depending on its arrival direction. In a UST system, the projector light comes from below at a steep angle, while much of the unwanted room light arrives from above or the sides.
Why orientation matters
Lenticular UST material is directional. Turn it upside down and it can reject the projector light instead of the ceiling light. This is one reason UST-specific screens should be installed according to their intended orientation rather than treated like generic fabric.
ALR percentage and gain answer different questions
ALR percentage describes a claimed ability to suppress unwanted light under a particular test method. Gain compares reflected brightness in a reference direction against a standard diffuse surface. A screen can have strong light rejection and lower gain, or a higher gain with less aggressive rejection.
The useful buying question
Instead of asking “Is ALR good?”, ask “How much uncontrolled light does my room have, from which direction, and how bright is my projector at the image size I want?” That produces a real screen requirement rather than a marketing checklist.