The secret symbols are embedded in the reflective sheeting that covers the plate. The sheeting has a layer of tiny glass beads embedded in clear plastic. Behind the beads is a mirror-like coating. When viewed from most angles, the beads reflect back just about all the light that strikes them.
However, the beads in the pattern of the secret image are treated so that they don't reflect any light when viewed from an angle of 30 degrees. The non-reflecting beads make up the dark areas of the secret pattern.
The reflective sheeting is provided to the states by the 3M Corporation, the only company that makes it. The manufacturing process is so sophisticated that it is unlikely that crooks will be able to duplicate it. Holograms on credit cards serve the same purpose as the secret images on license plates.
From DON'T TRY THIS AT HOME: Science Fun for Kids on the Go, co-authored with Kathy Darling, illustrated by True Kelley. Morrow Junior Books, 1998