Small-angle measurement by use of a single prism

A new method of angle measurement based on the internal reflection effect is proposed that uses a single right-angle prism. We measure the angular displacement between a laser beam and the prism by detecting the changes in reflectance as a function of the angle of incidence. We achieve high linearity of measurement by taking the inverse of reflectance as the output. The inverse of reflectance is obtained from the intensities of the reflected and the transmitted beams measured by two photodiodes. Experiments with a prototype device have demonstrated that angle measurement with a range of ?500 arc sec, a nonlinearity error of ?0.1%, and a resolution of 0.1 arc sec can be readily achieved. The measurement range can be further increased with some sacrifice of linearity.