MIKES’ modernized phase stepping interferometer for gauge block calibration (PSIGB) will be described. The instrument is based on the well-regarded NPL-TESA gauge block interferometer from 1994. The decision to upgrade the instrument resulted from several components, such as the PC and charge-coupled device (CCD) camera, having reached the end of their lifetime. In this paper modernized components, measurement method and analysis will be explained. The lasers are coupled to the instrument using single mode fiber. The instrument uses phase stepping generated by an added optical window on a controllable rotatory table in the reference arm with a recently developed nine-position phase stepping algorithm. Unwrapping is done with a robust path following algorithm. Procedures for adjusting the interferometer are explained. Determination and elimination of wavefront error, coherent noise and analysis of their influence on the results is described. Flatness and variation in length are also important parameters of gauge blocks to be characterized, and the corresponding analysis method is clarified. Uncertainty analysis for the central length, flatness and variation in length is also described. The results are compared against those of the old hardware and software. The standard uncertainty for central length measurement is u = [(9.5 nm)2 + (121 × 10−9 L)2]½, where L is measured length.

Small angle generators are simple devices for providing small angles traceable to the definition of the SI-unit radian. The most accurate ones use a laser interferometer for measurement of angular displacement. Small angle generators used for autocollimator calibration usually create angles around a single axis. A two-directional angle generator would be preferable, as it could more efficiently reveal artefacts related to angular displacement across both axes, such as orthogonality of the autocollimator's measuring axes. Characterizing errors depending on one axis only would also be easier, as the setup needs to be aligned only once for studying both axes of the autocollimator. We describe a novel interferometric two-directional small angle generator which we have built and tested for autocollimator calibration. The range is ±1000″ for both axes. The estimated standard uncertainty for full range is 0.0036″ for the horizontal and 0.0053″ for the vertical direction.

Double-ended interferometry has several benefits over single-ended gauge block interferometry: there is no need for wringing, which wears surfaces and requires expertise, and there is improved repeatability, since there is no variation due to inconsistent wringing conditions or form errors of the gauge block surfaces. Some disadvantages of double-ended interferometry are that absolute phase change correction is needed for the gauge block and its uncertainty has a double effect on total uncertainty. In addition, elimination of the wavefront error is more complicated than with single-ended interferometry. A simple optical modification that enables double-ended interferometer (DEI) measurements with the MIKES interferometer for long gauge blocks is presented. This modification is applicable to almost any single-ended interferometer (SEI). A procedure for evaluating the wave front correction for different parts of the interferogram of DEI is explained, and a modification and software with capability for nine-point phase stepping is presented. Three independent methods for evaluation of the phase correction were studied. One of them uses integrating sphere for the surface roughness correction and literature values for the phase change due to complex refractive index of material correction. The second evaluates the phase correction from the difference between DEI and SEI results obtained with a quartz platen. The third uses differences-from separate measurements-between the results obtained with quartz or steel auxiliary platens. Only a few gauge blocks per set need testing to obtain phase correction. SEI and DEI results with different phase correction determination methods are presented and evaluated. The uncertainty 2 9 2 .

Step gauges are convenient transfer standards for the calibration of coordinate measuring machines. A novel interferometer for step gauge calibrations implemented at VTT MIKES is described. The four-pass interferometer follows Abbe’s principle and measures the position of the inductive probe attached to a measuring head. The measuring head of the instrument is connected to a balanced boom above the carriage by a piezo translation stage. A key part of the measuring head is an invar structure on which the inductive probe and the corner cubes of the measuring arm of the interferometer are attached. The invar structure can be elevated so that the probe is raised without breaking the laser beam. During probing, the bending of the probe and the interferometer readings are recorded and the measurement face position is extrapolated to zero force. The measurement process is fully automated and the face positions of the steps can be measured up to a length of 2 m. Ambient conditions are measured continuously and the refractive index of air is compensated for. Before measurements the step gauge is aligned with an integrated 2D coordinate measuring system. The expanded uncertainty of step gauge calibration is .