Evaluation of Measurement Uncertainty for Absolute Flatness Measurement by Using Fizeau Interferometer with Phase-Shifting Capability

Abstract

It is important to precisely measure flatness of the optical flats, as many industries use these as reference standards to ensure the quality of precision measurements and fabricated components. This paper describes identification of sources of error and measurement uncertainty evaluation for three flat test. Three flat test is used for absolute flatness measurement of optical flats, with the help of Fizeau interferometer (VerifireXP/D, with phase shift interferometry) established recently at National Physical Laboratory, India (NPL-I). The absolute profile of reference flat with higher accuracy can be determined using liquid level reference but liquid flat reference is more difficult to realize practically. Therefore three flat test is frequently adopted in standard interferometric measurements and traceability of this test can also be established by using a traceable laser head. This paper describes three flat method in detail along with observations and evaluation of measurement uncertainty as per ISO GUM is also done. Factors contributing to uncertainty of measurement of surface flatness have been indentified and detailed evaluation of uncertainty in measurements has been reported here.