Reference calibration of white-light interferometers for testing advanced x-ray surfaces with atomic accuracy (Conference Presentation)

White light interferometers (WLIs) have been extensively utilized for testing X-ray surfaces in advanced light sources. The calibration accuracy of the reference error for WLIs must be PV < 0.5 nm, RMS < 0.05 nm. We analyze the impact of test parameters and form accuracy of test flat on the calibration accuracy of the random flats (RF) method and propose a multiple shear-shift (MSS) method that independent of test flat. An absolute test model is established and the reference surface of a 1× objective lens with 8.6 mm aperture was calibrated through the RF and the MSS with test flats of varying accuracy. The results demonstrate the dependency of the RF on the form accuracy of the test flat and confirm that the calibration of the MSS can achieve accuracy with PV 0.232 nm and RMS 0.026 nm. The MSS method, which achieves atomic accuracy calibration independent of the form accuracy of test flat, offers superior universality compared to the RF method and expands the options for calibrating.