Laboratory Demonstration of Grating Embedded Mirrors for Single-Shot Heliostat Optical Metrology

Abstract

Concentrated solar power (CSP) plants need to monitor the surface slope error of thousands of heliostats with sub-milliradian accuracy. Large numbers of heliostats installed in harsh environments mean that measurement speed and durability are key design considerations for metrology systems. We present a compact, accurate, and high-speed heliostat slope error metrology system that is robust to the harsh environmental conditions at CSP plants. The system is composed of (1) grating embedded mirrors (GEMs), which have multiple different phase diffraction gratings written within a glass mirror substrate, and (2) a compact optical system, the diffractive auto-stigmatic Hartmann camera (DASHCam). Using focused ultrafast laser pulses, we write phase gratings within second-surface float glass mirror without damaging the reflective coating on the backside. The gratings direct a small fraction of incident light to non-specular directions, which the DASHCam senses from a virtual center of curvature to measure the mirror slopes at each grating patch. In a CSP plant, each heliostat mirror would be a GEM, and the DASHCam would rapidly measure slope and canting errors during heliostat manufacturing, installation, or operation. We fabricated 0.1 meter-diameter GEMs with 24 grating patches, built a prototype DASHCam system, and tested the system repeatability and accuracy by comparing against a Fizeau interferometer. Our results show, in a laboratory environment, 24μrad root mean square (RMS) measurement repeatability and 47μrad RMS accuracy, with single-shot image capture. The combination of GEMs and DASHCam is a promising metrology approach that could lead to improved optical accuracy of heliostats throughout their life cycle. This work serves as a proof of principle for this system.