On some lamp shades--most commonly pendant lamp shades--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 lamp shade.
If you order an optional diffuser with your shade, the diffuser will be constructed of an acrylic disc slightly smaller (typically 1/2 inch) in size than the bottom dimension of the shade, and the diffuser will rest on the bottom ring of your shade.
All of our acrylic diffusers are constructed from 1/8" thick milky-white acrylic.