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 means 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 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 float glass mirror facets without damaging the reflective coating. 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 facet slopes at each grating patch. In a CSP plant, each heliostat facet 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, built a prototype DASHCam system. Our results show, in a laboratory environment, 24 μrad RMS measurement repeatability and 47 μrad 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.