3M™ Diamond Grade™ DG³ Reflective Sheeting

Entrance angle is the angle formed between a light beam striking a surface at some point and a line perpendicular to the surface at the same point.

This is the angle referred to when discussing the "angularity" of reflective sheeting. And it is especially important when you consider that signs are often positioned far off the shoulder, on the left hand side of the road, on curves, and can become misaligned due to accident or snowplow abuse.