Studies on Measurement of Angle by Diffraction Pattern (2nd Report)

A Method of Measurement Using Lightfan

Abstract

When an object has generally a straight line-shaped aperture, a similar straight-shaped pattern appears in Fraunhofer diffraction pattern which is obtained by it. The pattern and the aperture are perpendicular to each other. The pattern thus produced is termed a lightfan. So Fraunhofer diffraction pattern produced by wedge-shaped angle has two lightfans. The angle where they intersect is equal to the wedge-shaped angle. Using this phenomenon, this paper describes the method to measure the angle optically. This method is based on the fact that the photoelectric element detects the positions of lightfans, rotating on lightfans. And the basic problems and the main cause of error of measurement by this method are also considered. The examples of measurement are shown, using an experimental apparatus and the specimens as a model. The main results are as follows :
(1) The method of measurement using lightfan can measure the angle with considerable accuracy, except for the angle where two lightfans overlap each other. In comparison between the results of examples of measurement and those of a profile projector, these results agree within 0.5% in the region where wedge-shaped angles are above 20°.
(2) The effect of errors caused by two directional inclinations of specimen is small because the errors work so as to compensate each other.