What is the difference between ground glass and opal glass diffusers?

Ground glass is produced by taking clear float glass and sandblasting it with a grit. Grit refers to the size of the sand particle impacting on the glass. The process of sandblasting creates divots in the glass, which then diffuse any light that passes through it. Ground glass has the advantage of being able to control the relative amount of diffusion that occurs depending upon the grit of the sandblast. For example, a 220-grit ground glass will diffuse more than a 120-grit glass. On the other hand, opal glass is float glass where one side is flashed with a milky white coating called "opal". It is this opal coating that acts as a diffuser for light passing through the glass. While both will diffuse transmitted light, opal glass diffuses more efficiently but causes a significant amount of scattering loss. Also, due to the thickness specification of the coating, opal glass should not be used in highly-toleranced opto-mechanical systems. Ground glass is best used in projection systems as a screen.