I was trying to figure out how to quantify glare the other day and found this:
Unified Glare Rating UGR basic explanation.
Pretty clear, right? If not, just buy the poster.
A more useful page for the laity, perhaps, is this explanation of light diffusers:
Light Diffusion and Light Diffuser Types
There are more easy-to-follow explanations on the Fusion Optix blog, but sadly it appears they’ve stopped updating it.