Additionally, in DEIs it is possible to measure the length or thickness of bodies which are not suitable for wringing, e.g. due to long term drifts of the materials or the coefficient of thermal expansion. Especially for measuring relative changes in length, e.g.
Not having to take the wringing effects into account is particularly useful for the comparison of lengths.
Although the included wringing in the length definition in ISO 3650 is of significance on the shop floor, the limited repeatability of the wringing process limits the accuracy of high-accuracy length measurements. Several double-ended interferometers (DEIs) have been developed to avoid the need for wringing and therefore be able to neglect these influences. Furthermore, wringing the gauge blocks to reference plates can lead to wear or damage the measuring faces. The measured length can change after repeating the wringing process and, due to form deviations, often depends on which end face was chosen.
The central length is defined as 'the length (.) taken at the center point of the free measuring face'. In accordance with ISO 3650, a length of a gauge block is defined as a 'perpendicular distance between any point of the measuring face and the planar surface of an auxiliary plate of the same material and surface texture upon which the other measuring face has been wrung'. Twyman–Green interferometers, for which one side of the gauge block must be wrung onto a reference plate, are commonly used to perform such measurements. Interferometric length measurement is a technique necessary for primary calibration of gauge blocks. The results suggest that the developed alignment method is highly accurate and is expected to yield an uncertainty contribution to the final length measurement in the sub-nanometer range. Finally, Monte Carlo runs of the virtual experiment are performed to quantitatively explore the size of different sources of uncertainty on the developed alignment method. In addition, theoretical relations are confirmed. The virtual experiment is validated by a comparison to experimental data. In order to explore the accuracy of the developed procedure and to estimate the size of the uncertainty caused by deviations from perfect gauge block shapes, virtual experiments are carried out using the PTB library SimOptDevice.
In this work, we develop a precise alignment method for the double-ended interferometer and systematically study the contributions of misalignments to the uncertainty of the measured length.
Compliance with the defined gauge-block length in ISO 3650 is also challenging, especially for non-perfect shaped gauge blocks. However, because the setup of this interferometer is complex and additional optical components are required the alignment process is challenging. PTB's double-ended interferometer allows high-accuracy length measurements that are traceable to the International System of Units to be performed without a reference plate. Furthermore, it limits the use of such interferometers to bodies that are suitable for wringing. The quality of contact affects the measured length and also the wringing process wears down or damages the measuring faces. To perform these measurements, the gauge block must be wrung to a reference plate. gauge blocks) are usually performed by means of single-ended interferometers. High-accuracy length measurements of prismatic bodies (e.g.