Calibration of the z-axis for large-scale scanning white-light interferometers

The calibration of white-light interferometers is an important requirement to make the results of these fast and accurate measurement tools comparable to the well-established tactile techniques. In this paper we present an approach for the calibration of the z-axis of a large-scale scanning white-light interferometer (SWLI) covering the full scanning range by utilizing a newly developed step-height calibration standard. The standard was specially adapted to the requirements of the large-scale WLI with an x-y-field of view of 38 x 28 mm2 and a z-scan range of 70 mm. We have developed a new procedure to calibrate the z-scale by using the measured WLI data of the step heights of the standard to obtain an inverse characteristic to compensate non-linearities of the length reference, which is in our case simply the lead screw of the step-motor-driven linear stage. We describe the development of our standard, present measurement results before and after the calibration and discuss the uncertainties of the procedure.