The Index Interferometer is a novel instrument being developed by Northeast Photosciences. The instrument is a breakthrough in the high-accuracy measurement of the index of refraction, the dispersion, and the index profile of materials. The instrument accurately measures the index of refraction of materials to one or two more significant figures than previous instruments. Material slices polished moderately flat are sufficient, without any requirement for special or complicated material shapes, such as prisms. The index profile at any chosen wavelength can be measured using a simple color filter. No special laser sources or carefully collimated parallel beams are required. The index profile over an entire sample can be directly obtained at any desired wavelength. This instrument is remarkable in that it greatly increases the accuracy of measurement, eliminates the need for high-quality, extremely narrow sources and for fabrication of special-geometry samples, and adds additional features, such as index profile measurements. The technique compares the fringe pattern from the top surface with that from a reference mirror to determine the thickness. Then, with the aid of a filtered white light source, the interference pattern from the back surface is compared with that from the front to yield the optical thickness of the sample. The combination of the two measurements gives the index. The back surface fringe pattern itself gives the index profile.