Optical inspection of glass substrates is a big challenge. Two very closely spaced, highly parallel surfaces make interferometric metrology difficult. Nevertheless, several companies are offering or developing products to qualify glass disks. Photonics instruments, particularly interferometers, have enjoyed widespread application in the magnetic disc manufacturing process. Recent instruments use glass tooling disks and an interferometer to accurately measure flying height.
Interference microscopes work well here.
The Fizeau interferometer is a workhorse in macroscopic disk flatness measurements. Even the master servo tracks written onto finished disks use distance measuring interferometers. There is a new compact disc format being proposed by an alliance of leading entertainment, electronics, and manufacturing companies called the SD, or Super Density disc. Glass mastering disks are used as in current CDs. To show they are serious, they have already successfully mass produced 650,000 SDs on an actual manufacturing line.