Measurement of the crystal birefractive index by conoscopic interference

The conoscopic interference pattern of wafer with optical axis parallel to the surface of a crystal is sensitive to the crystal birefractive index. The symcenter of conoscopic interference pattern is corresponding to vertical incident light and crystal birefractive index is proportional to interference order md. Decimal fraction of interference order md is calculated by the placement of central dark fringe and integer part is ascertained by self-consistent method. The interference fringe is fitted by hyperbola so perfectly that it is more precise to obtain the feature points of interference figure. When different thickness niobate crystal are measured, it is shown that uncertainty of birefractive index is 1×10-4.