The lightboard is a 4x8 foot sheet of heat-tempered PPG Starphire architectural glass, 10.0mm thick (also called 3/8 inch). The glass is lit internally from the upper and lower edges by bright white LEDs. Light travels to all parts of the glass by total internal reflection (TIR).
Getting unmarred glass can be a problem. See Build or Buy. And no, it can't be plastic. See FAQ.
Ordinary plate glass looks dark green from the edge. Because of the dark tint, not much light from the edge can reach the center of a panel of ordinary glass. Starphire glass has very low iron content so it is very clear. It looks pale blue from the edge. As a demonstration of how clear Starphire glass is, I shined a laser pointer in from the edge. You can see the beam bouncing up and back through the glass, reflecting from the inside surfaces by TIR, all the way across the panel: