Method of measuring an inner stress state of disk substrate

A method of measuring an inner stress state in a disk substrate, comprising the steps of:

• projecting a light wave onto a molded disk substrate, the light wave being produced by giving a phase difference to a principal axis of a linearly polarized light from a light source;
• image-processing a birefringent phase difference and a change in a principal axis direction according to an amount of change in polarization produced inside the disk substrate by the birefringence;
• and
• measuring multi-dimensionally an in-plane distribution of birefringence by using a phase shifting technique, said in-plane distribution of birefringence being an inner stress-induced strains in the molded disk substrate and a flow orientation state of a resin being molded;
• providing a retarder made from a liquid crystal element or electro-optic element to give a phase difference to the principal axis of the linearly polarized light emitted from the light source;
• providing a half-wave plate for rotating the principal axis direction of an elliptically polarized light;
• and
• providing a polarizer arranged in a direction perpendicular to the principal axis direction of the light wave rotated by the half-wave plate;
• wherein the elliptically polarized light is expanded into two dimensions by a lens system to produce image information and is projected onto the disk substrate to detect an amount of change in the phase difference of an intensity of the transmitted light and motor-drive the half-wave plate and the polarizer thereby enabling an automatic measurement.