A holographic method for matching and measuring the refractive index of a photoelastic material

If a wedge-shaped sample of material is immersed in a tank of fluid and a coherent beam of light passed through it, the ensuing wavefront can be made to interfere with a holographically stored wavefront produced with the wedge removed. The interference is measured by the resulting fringes, which can be reduced by varying the fluid mix until the fluid matches the index of refraction of the material. The number of fringes can also be used to determine the difference between the indices of refraction of the fluid and material, and determine the material index from a known fluid index.