Abstract
Demands are increasing for highly-precise shape measurement of large curved surfaces such as lens and reflecting mirrors for telescopes. In this paper, a design concept of a new surface shape measuring method is proposed for the purposes using an improved sequential three-point method. The method has distinctive features described as follows. In three-point method, if a zero position error between sensors exists, a large quadratic error component is superimposed on measured shape profile. In the method proposed, the quadratic component is compensated using the difference between the inclination angles of sensor head at the starting and finishing points of shape measurement. When the specimen surface has large unevenness, the distance between surface and displacement sensors exceeds the working range of the sensors during measuring operation. Therefore, the distance from the specimen surface and the inclination angle of sensor head are adjusted so that the specimen surface always comes within the working range of all sensors. The surface shape profile of a curved surface the length of which is 700mm is evaluated by the experimental measuring equipment assembled based on the concept. The experimental result shows that the measuring error of the equipment is estimated to be about 0.40μm if the inclination angle of sensor head is measured properly.
