Characterization of freeform surfaces by reflective gradient measurement

Abstract

In freeform surfaces, the evolution of optical components has exceeded the next level. After spherical and aspherical surfaces, freeform surfaces do not follow a certain basic shape. Therewith, they offer a much higher degree of freedom for designers and the field of possibilities of optical function is extended. Since verification in the fabrication process is indispensable, adequate measurement techniques for the characterization of freeform surfaces are required. For spherical and aspherical surfaces, highly developed surface measurement techniques area available. However, these techniques exceed their limits, when the basic shape of a freeform is not spherical or aspherical, but flat. To provide manufacturer with a high accurate option to measure freeform surfaces, deviating from a flat basic shape, a gradient-based measurement technique for these kind of freeform surfaces is proposed. This measurement technique is of laser deflectometry type and measures the gradient field of a freeform surface by use of a variation of Experimental Ray Tracing. The proposed measurement technique uses evaluation and calibration techniques, which are implemented using homogeneous coordinates. With a commercially available simulation software, the proposed measurement process and the evaluation and calibration methods are validated. The results for different surfaces models are presented and evaluated. An experimental setup has been built up and described in this work. Using multiple different surface types, the abilities of the experimental measurement setup are shown. Comparing the results from the proposed measurement technique to results from commercially available measurement techniques, good agreement in the sub-micrometer range can be determined. To set the measurement results in context to the expected measurement uncertainty, multiple error sources are evaluated for the experimental measurement setup.