Investigation of error sources in absolute flatness measurement using Fizeau interferometer

Optical flats are essential elements in imaging systems and space applications. Furthermore, high-quality flat surfaces are frequently employed as calibration standards for the devices used in dimensional metrology such as interferometers and autocollimators. Accurate flatness measurement and elimination from errors represent a challenge for modern industries. Flatness can be measured by contact methods like as coordinate-measuring machines or non-contact methods such as laser interferometers. The non-contact methods are divided into 2 types: relative and absolute measurement. The absolute measurement is more accurate because it depends on removing any contribution of form errors from the reference surface. In this study; the absolute flatness for a glass flat of 100 mm in diameter is obtained by a Fizeau laser interferometer (GPI-XP, Zygo) with phase-shifting capability. The measurements are conducted using one of the absolute measurement techniques known as the three-flat test. The average measured flatness is 0.0448 µm, with a reduced measurement uncertainty of ± 0.0071 µm. Also, the long-term deviation for the 2 reference flats is discussed and found to be ~  + 0.4 nm/year. Among eleven error sources that influence the measurement accuracy; air turbulence and alignment represent the main error sources reported by this study. The study proposes effective methods for eliminating the drawbacks of back-surface reflections and mechanical mounting. Uncertainty budget and the optimum conditions for the used system are included in this study.