International Comparison of Refractive Index Measurements

The refractive index of glasses is an important measurement quantity. Uncertainties in the visible range of 1E-5 to 1E-6 are reported. To guarantee traceability, it is important to compare the different measuring systems. A new international comparison with PTB as pilot laboratory has been initialized. Participants are national metrology institutes and optics companies and new participants are welcome. A former high accuracy comparison with 13 participants ten years ago was based on prismatic samples, since the “minimum deviation method,” which uses prisms in the orientation of minimum beam deviation, reached the lowest uncertainty. For this comparison prisms were circulated among the participants. For the new comparison, also glass blanks can be specimen, which the participants have to process. Thus the manufacturing of the prisms (e.g. surface quality), which can influence the uncertainty, is included. The concept of the international comparison and the highly accurate refractive index measuring setup of PTB will be presented and the different influence quantities will be discussed with a special view to the demands of this new ‘ab initio’ refractive index comparison.