Diffuser plate

The diffuser plate is the stiff, milky-white sheet you'd find if you opened up an LCD TV and lifted out the thin films. It sits a short distance above the LED strips, turns their bright dots into an even glow, and supports the thinner optical films lying on top.

It's made of polystyrene mixed with a small amount of tiny silicone resin beads, a few micrometers across. Polystyrene pellets and beads are melted together and pushed through a wide slot to form a continuous sheet, typically about 1.5 to 2 mm thick, which is cooled between polished rollers and cut to size. Because the beads bend light slightly differently from the polystyrene around them, each one scatters light a little. Millions of them, spread through the plate, scramble the light's direction until the individual LEDs disappear. The surface may be textured for extra spreading.

In the backlight unit, the plate is the main step from points of light to a flat field. Plates have also been made of acrylic or polycarbonate, depending on the manufacturer. Polystyrene is common because it's cheap and absorbs less moisture than acrylic, so it's less likely to warp in humid rooms.