Direct measurement of the two principal refractive indexes of a nematic liquid crystal slab

A new technique to measure the principal refractive indexes of a nematic liquid crystal is presented. The method is based on an indirect measurement of the refraction angle by determining the direction of a magnetic field which minimizes the light scattering. With this technique one can perform different light scattering experiments, which require a knowledge of the refractive indexes, on the same crystal slab. The accuracy is better than +/-0.15%. A discussion about the systematic error due to the distortion of the director profile near the holder edges and the relative correction is given.