Simple gravity-directed laser methods for CSP mirror metrology

Abstract

Concentrating solar mirrors require high optical precision to concentrate sunlight onto a target receiver. This motivates the need for metrology systems to measure mirror quality. Surface normal slope is the key functional characteristic of a concentrating solar mirror, because the surface normal defines the reflection angle. This observation led to the development of several types of reflection-based instruments capable of measuring mirror slope maps. These include computer-controlled laser systems, deflectometry systems, and scanning target reflection systems. These sophisticated systems rapidly produce maps of slope observed for a large number of sample points distributed across the mirror. However, their complexity raises the question: How do we know these systems are accurate? This raises the need for ground truth to cross-check these systems against a trusted reference. Here we present a simple, inexpensive mirror measurement method utilizing a low-cost self-leveling laser. This gravity-directed device will point consistently regardless of its location, and when combined with a suitable optical target and measurement method, provides a rapid, easy-to-use, and low-cost method for measuring slope at a selection of points across the mirror, without the need for complex calculations and calibrations. This can then be used for very rapid basic characterization of mirror overall curvature, or through more careful manual scanning, to obtain a primary measurement of mirror shape and slope error. We present the technique, including both data collection and analysis, and show example results. Data analysis is performed using a single interactive spreadsheet, which is available with documentation on OpenCSP.