Application of astigmatic method and snell's law on the thickness and refractive index measurement of a transparent plate

In this study, a thickness and refractive index measuring system for the transparent plate has been proposed. The measuring system is composed of an astigmatic measurement module and a light spot measurement module using a DVD optical pickup, a laser diode and a quadrant photodiode detector. Both of the thickness and refractive index of a transparent plate are converted into the focusing error of DVD pickup based on astigmatic method and the light spot displacements based on snell's law in our proposed system. The thickness and the refractive index are simultaneously calculated by means of two nonlinear formulas. The accuracy of the proposed system is 99 % verified by a height Gauge and the refractive index by the TF-166.