A bilateral comparison of optical glass refractive index between NMIJ and INRiM for the validation of the measuring systems

A bilateral comparison of refractive index measurements between the National Metrology Institute of Japan (NMIJ) and the Istituto Nazionale di Ricerca Metrologica (INRiM) was carried out in 2008. Each laboratory developed a measurement system based on different principles. NMIJ’s system was based on the interferometric technique and INRiM’s system was based on a modified method of the traditional refractive technique. A comparison using a prism-shaped glass made of OHARA BSL7Y was carried out in order to evaluate the newly developed methods and confirm the consistency of the results. The details of the comparison artefacts, measurement conditions and uncertainty factors and their degrees of freedom of each laboratory are described here. The measurement results for the glass at 633 nm at 20 ◦C and 23 ◦C are within each laboratory’s claimed expanded uncertainty (coverage factor k = 2) which was of the order of 10−6. The agreement between the two different measurement approaches shows that both systems can be used to obtain accurate measurements.