Interferometry still only acts as a comparison (to a reference mirror). So there's still the 3 plate method at the heart of it, although I expect most commercia

Interferometry still only acts as a comparison (to a reference mirror). So there's still the 3 plate method at the heart of it, although I expect most commercially produced surface plates are just machine-ground and then measured against a master, rather than hand-made three at a time. Edit to elaborate -- to make those ultraflat mirrors, you can also use an interferometric variant of the 3-plate method when, instead of granite or cast iron surface plates, you're making optical flats. Instead of looking at contact patterns with dye, you can observe the interference bands that show up between two optically flat, smooth surfaces to compare them, which is generally more sensitive. But it's still a comparison process. You still proceed by lapping the high spots and checking again...