The importance of understanding the design and use of Fizeau transmission spheres, for accurate evaluation of surface irregularities, cannot be understated. There are several characteristics that have a significant impact on the interpretation of the resulting data. Specific tests can be performed on a routine basis to quantify and maintain calibration of the precision masters and as a result, errors attributed to the transmission sphere can be isolated. The overall goal is to obtain accurate evaluations of surface irregularities of the actual surface under test.
"Transmission spheres more than just a reference surface!", Proc. SPIE 10314, Optifab 2003: Technical Digest, 1031415 (19 May 2003); doi: 10.1117/12.2284030; https://doi.org/10.1117/12.2284030