How is measured BRDF data recorded? How is measured BRDF data recorded? Like the MERL BRDF Database.

What kind of equipment is used and what are the key values that need to be recorded?

I know one place this data was used was in the Disney PBR paper.
Section 6 of Microfacet Models for Refraction through Rough Surfaces has a good description of how they did it to validate their own model. That may not be everything needed to build a full database (eg. more automation) but it's an interesting read.