Measurement of spherical, aspherical and freeform specular surfaces using experimental raytracing in simulation and measurement

Abstract

With the improved abilities in the manufacturing of optical components, including spherical, aspherical and freeform optics, measurement systems with high potential in accuracy and repeatability are required. In this work, enhanced improvements and abilities of the new measurement technique based on a variation of experimental ray tracing (ERT), recently introduced at the “SPIE – Optifab 2017”, are presented. The idea of ERT is to detect the direction of a ray, deflected by the device under test (DUT), by measuring its location in two known planes. Originally, this has been proposed to measure the function of lenses in transmission. The potential of this original idea is demonstrated in several papers considering the measurement of imaging lenses, objectives and progressive additional lenses. Relying on the experiences from these works, a variation of the original ERT is shown in this work. Therefore, a setup has been created using a ray, represented by a narrow laser beam, introduced into the measurement system under a certain angle. This ray is directed on the DUT. After the ray has been reflected from the specular surface, a camera chip catches this ray and determines its position. By moving the camera chip along a linear axis, the direction of the reflected ray is calculated. Performing this test for a high number of points on the DUT, the surface can measured completely. The advantage of this technique is the simplicity of the measurement principle. In contrast to other non-contact surface measurement techniques, no optical components are needed between the DUT and the measurement sensor. Thus, no errors can be introduced to the measurement results from defective optics in the measurement setup. Furthermore, using only the principle of reflection leads to the advantage, that no reference for the detection of the surface data is needed. Furthermore, the principle of single rays propagated through the measurement system opens up the area of computer vision. High efficient algorithms make the representation of the measurement data in the evaluation process fast and easy. With this measurement setup, the measurement of a huge variety of surfaces is possible. Spheres, aspheres and freeform optics can be measured all in the same way. However, certain limits are given from the measurement setup. The results are compared to the results of commercially available measurement systems.