The angle between the illumination axis and the retroreflector’s normal (perpendicular line). β ranges from in most standards. Example: A road sign mounted high above a roadway has a large entrance angle (light hits at a slant). CIE 54.2 mandates measuring at multiple β values to capture real-world performance.
Outside, the world didn’t change—not yet. But somewhere, a child looked at a stop sign and felt, for the first time, a tiny sliver of doubt. And somewhere else, a fire station began repainting its trucks the color of a winter sky. cie 54.2