Phase change correction (PCC) is an important correction value of the end effect in an optical interferometry system. Normally, this value is used to compensate for gauge block measurement by an optical interferometry system based on ISO 3650:1998. Two different interferometric measurement systems in terms of fringe fraction measurement were performed to determine the phase change correction by a five-stacking method. These results are used to determine the length measurement of gauge blocks in an optical interferometer technique and consequently, to evaluate the uncertainty of gauge blocks measurement. The preliminary results for steel gauge block are shown that the value of phase change correction in a phase shift gauge block interferometer (PSGBI) system and a standard uncertainty are 35.2 nm and 5.8 nm, respectively. In contrast, the values from an average slits gauge block interferometer (ASGBI) system and a standard uncertainty are 66.0 nm and 6.0 nm, respectively. We found that the phase correction from the PSGBI system is lower than ASGBI about 0.53 - 0.56 times because the different of wave front correction in two interferometric systems. However, the lengths of gauge blocks of all materials measured by the two systems were consistent as assessed by En number. According to the study, we can conclude that phase change correction is based on the characteristics of each GBI system, surface texture characteristic in term of wringing condition and material types of gauge block and optical plates such as the fringe fraction measurement technique, and wave front error compensation. Consequently, measurements that require a high accuracy should determine the phase change correction before each measurement due to this value is not interchangeable.
The non-linearity of probes is one of the important components in gauge block calibration by the mechanical comparative method of two gauges blocks at the same nominal length. However, an advanced method for gauge block calibration is a mechanical direct measurement method of two gauge blocks showing the greatest difference in nominal length of 25 mm. This method uses a special probe based on the interferential scanning principle to produce the signals to measure the displacement. In this paper, non-linearity and error due to measurement position were investigated as they related to the accuracy of measurement results. The differences in central length of pairs of standard gauge blocks made of steel were measured by optical interferometry with the measurement uncertainty (k=2) 23 nm. Length in the range of 5 μm to 25 mm was used in the experiment. Non-linearity of the probe was evaluated by the simple linear regression model. Various factors such as origin setting point, temperature, and vibration have been analysed. In the preliminary experiment, the non-linearity, position error, repeatability and retrace error over the measuring range 25 mm are 13 nm, -18 nm, 15 nm, and 10 nm respectively. The standard uncertainty of direct measurement type caused by non-linearity is 4 nm.
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