Evaluation of parallelism by ultrasonic echo method

The determination of parallelism in a transparent plate is usually done either by the movement of a stylus or by an optical method, such as interference fringes. For opaque objects, the only method appears to be the use of stylus. The authors have developed an acoustic method to evaluate the parallelism between faces of any block even if it is opaque to light. It is based on the echoes received from the two surfaces when incident upon by a short duration high frequency ultrasonic pulse. The paper describes the basic principle and also the methodologies for evaluation in laboratory and in industry. The method requires essentially only one side to be accessible. Theoretical and experimental results show excellent agreement up to 1° of arc. The uncertainty of measurements of results is ±0.1°. It is expected that the new method would find applications in several industries.