Development of a permanent vacuum prism air refractometer for accurate compensation of the refractive index of air as used in dimensional metrology.

Abstract

The refractive index has a wide range of applications, and it is used in many different areas of measurement, such as in thin films, sugar contents in beverages and wine, and the medical industry. Compensation of the refractive index of air has been required since the speed of light was measured accurately enough to require a correction for the medium in which it travels. There have been various research advances to measure the refractive index of numerous mediums more accurately, to keep up with technological advances and the increase in accuracy required. Metrology requires accurate refractive index measurements since it is critical in the definition of the SI units of the metre. Even more so since 2019 with the new definitions of the SI, which is based on physical constants, the kilogram and the Kelvin and the derived unit for pressure, the pascal. This research focussed on length and dimensional metrology where the SI unit, the metre is defined in terms of the speed of light in a vacuum. While the definition of the metre is in terms of the speed of light in a vacuum, most dimensional measurements are performed in the air. The refractive index requires to be measured and correctly applied to the velocity of the light during the measurements compared to the speed of light in a vacuum for more accurate computation of the metre. This research was motivated by technical limitations observed in existing refractometers. Previous studies where the author researched, designed, and built a low-cost tube refractometer with adequate accuracy to perform the required compensation to the velocity of the laser light during dimensional measurements. However, there were drawbacks to this design; mainly the use of a vacuum pump every time a refractive index measurement needs to be made. This led to unwanted heat generation around the measuring area and drawing a vacuum each time a measurement was made increased the likelihood of dust particles entering the vacuum tube, which may cause the refraction of the laser beams through these particles and thereby reducing the accuracy of the measurement. Adding another notable drawback is the deformation of side windows under vacuum conditions, which alters their thickness and, consequently, the optical path length. Such variations can significantly influence the accuracy of refractive index measurements. These limitations highlighted the need for improved refractometer designs, ultimately leading to the development of more stable and accurate measurement techniques researched in this study. This research work focused on the design, building, and testing of a refractometer for the compensation of the velocity of the light compensations that does not require the use of a vacuum pump every time a measurement has to be made. The custom-designed and built refractometer uses a permanent vacuum as the reference etalon. In the first design an additional laser beam was incorporated to improve the zero-measurement stability, while the second design introduces an innovative method for the refractive index measurement based on a Fizeau interferometer configuration. The system is simple, cost-effective and highly accurate for use in everyday dimensional measurements. The results showed that although simple in design, the refractometer is accurate to approximately 2,7 x 10-8, which meets the initial conditions for the design and compares favourably with published values obtained through the use of more expensive and specialised techniques.