Abstract
We propose a new method for measuring c-axis orientations of optically uniaxial crystals with conoscopic figures. The fundamental operation in this method is that a crystal is rotated on the microscope stage under conoscopic illumination until extinction occurs at a test point specified in a conoscopic field of view and that the rotation position is measured on the microscope stage scale. This operation is done for two test points in order to obtain a c-axis orientation of the crystal. We set the two test points at positions equidistant from the conoscopic field center in both directions along the E-W hair. The distance from the field center to these test points can be connected with the wave normal direction within the crystal for the light passing each test point. When the rotation position satisfies the condition that extraordinary light along the wave normal vibrates at right angles to the E-W hair, the test point will be in extinction state. The extinction condition was formulated as equations in terms of c-axis orientation, distance from the field center to the test points, principal refractive indices of the crystal, and stage readings for the two test points in extinction state ; c-axis orientations can be calculated using the equations. The conoscopic method is superior to the conventional U-stage method in its accuracy (i.e. the measurement error in c-axis orientation could be less than 0.2° for quartz).
