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A method to amplify the rotation angle of a mirror, based on multiple reflections between two quasi-parallel mirrors, is presented. The method allows rotations of fractions of nanoradians to be measured with a simple setup. The working principle, the experimental setup, and the results are presented.

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At the angle metrology laboratory of the Istituto Nazionale di Ricerca Metrologica (INRiM, formerly IMGC), an instrument to generate very small angles has been recently developed. It is a sine-bar angle generator based on an elastic hinge and a piezocapacitive device. The facility can be used to calibrate precise angle measuring instruments (levels, autocollimators, etc) in a range of 120 µrad with an uncertainty of about 20 nrad and a sensitivity of fractions of a nanoradian. The working principle and metrological characterization are presented.

The comparison of two methods of measurement or instruments is addressed from a statistical point of view. Several techniques for assessing the degree of conformity between two methods/instruments measuring the same quantity are discussed and compared. Their application in the framework of angle measurements is presented.

A novel angle comparator has been built and tested at INRIM. The device is based on a double air bearing structure embedding a continuously rotating encoder, which is read by two heads: one fixed to the base of the comparator and a second fixed to the upper moving part of the comparator. The phase measurement between the two heads' signals is proportional to the rotation of the moving table. The advantage of this solution is to reduce the encoder graduation errors and to cancel the cyclic errors due to the interpolation of the encoder lines. By using only two pairs of reading heads, we have achieved an intrinsic accuracy of ± 0.04'' (rectangular distribution) that can be reduced through self-calibration. The residual cyclic errors have shown to be less than 0.01'' peak-to-peak. The random fluctuations are less than 0.01'' rms on a 100 s time interval. A further advantage of the rotating encoder is the intrinsic knowledge of the absolute position without the need of a zeroing procedure. Construction details of the rotating encoder angle comparator (REAC), characterization tests, and examples of practical use are given.

Five Pt/Pd thermocouples, constructed and calibrated at IMGC at fixed points in the temperature range from 0 °C to the Ag point, were calibrated by comparison with the local primary standard radiation thermometer with the aim of replacing the presently used Pt/Pt–Rh alloy thermocouples as secondary reference standards up to 1500 °C. To fully exploit accuracy of Pt/Pd thermocouples, high-level calibration techniques need to be adopted. For this purpose, a new high-temperature three-zone furnace was arranged and characterized in order to obtain the best axial uniformity and a specially designed blackbody cavity was used as a transfer source for calibrating the thermocouples in the temperature range from 962 °C up to 1500 °C. At the end of the comparison measurements, additional calibrations at the Ag fixed point were made, in order to check the stability of the thermocouples' signals. A comparison between experimental results and the reference function is presented and an extrapolation of the fixed-point calibration data is analysed.

A simple method to measure the refractive index of a glass prism with very low uncertainty was developed at INRiM. The method is a modification of the classical minimum deviation method. A brief description of the methods used to measure the vertex angles of the prism and the angle of minimum deviation is reported together with the uncertainty evaluation. The technique is going to be validated by a comparison between INRiM and two other laboratories. A relative standard uncertainty better than 1 ppm has been obtained.