Low-cost reflective Hilger-Chance refractometer used to determine Sellmeier coefficients of bulk polydimethylsiloxane

Refractometry is important for characterizing the optical performance of materials. The refractive index can quickly be assessed using critical angle or thin-film techniques. However, these methods only assess the material surface. Measurement of bulk refractive index is performed by measuring the refracted angle of a transmitted beam but requires precision sample geometry. The method presented here avoids costly sample preparation by measuring the sample geometry and refracted angle simultaneously, using reflections from the front and back surfaces of a wedge of material. The method is demonstrated for polydimethylsiloxane prepared under a range of curing conditions, and no significant dependence was observed. Spectral dependence is characterized, and Sellmeier coefficients are reported.