A two-reflection divergent differentiating critical angle refractometer

A new critical angle refractometer (CAR) for high accuracy refractive index measurement of liquid has been developed. The instrument improves the accuracy by two reflections in an elongated parallelogram prism, and acquires the angular reflectivity without any angle scanning parts through introduction of a point source with a divergent beam and a charge coupled device. In addition, it employs a simple and robust measurement method that gets the critical angle by differentiating the angular reflectivity. Through investigating absorbing media with absorption index κ (the imaginary part of refractive index) from 0 to 10(-2.1), the theoretical calculation shows that the proposed two-reflection CAR would outperform the traditional one-reflection CAR on lowering the principal error from the differentiation method and improving the ability of getting the critical angle. By testing two typical liquids-salt-water solution and milk, the preliminary experiment indicates that this two-reflection divergent differentiating critical angle refractometer is feasible and of high accuracy.