On some lampshades--most commonly pendant lampshades--you may require a bottom diffuser. The purpose of the bottom diffuser is to "hide" the socket and light bulb of the fixture when viewed from below. A bottom diffuser is often slightly smaller in size than the bottom dimension of your lampshade.
If you order an optional diffuser with your lampshade, the lampshade diffuser will be constructed of an acrylic disc slightly smaller (typically 1/2 inch) in size than the bottom dimension of the lampshade, and the diffuser will rest on the bottom ring of your lampshade.
All of our acrylic lampshade diffusers are constructed from 1/8" thick milky-white acrylic.