Invention:
This technology is a method for measuring heliostats for solar energy applications, as well as any large optical surface composed of second-surface mirrors. A small diffraction grating, which is a 1D, 2D, or 3D periodic phase and/or amplitude modulation, is embedded within the glass substrate of a second-surface mirror. Light from a metrology system propagates through this grating before and after reflection, allowing it to be directed to non-specular directions to be measured by the optical system. Multiple gratings can be written in a single layer or multiple layers in the glass to provide additional diffraction orders or more complex control of the reflected wavefronts. These gratings can enable metrology of large specular surfaces with compact optical setups, such as a point source illumination and reflection into a camera, and can also be used to re-direct a small amount of sunlight to a direction offset from the specular direction to allow measurement of the mirror orientation while it directs most sunlight to a receiver for thermal energy generation. The gratings are written with ultrafast laser pulses, with pulse duration in the femtosecond to picosecond range, to impart index of refraction variation, scattering, or both, providing phase and/or amplitude modulation.
Background:
Large optomechanical structures, such as heliostats used for solar energy concentration, require sub-milliradian precision surface metrology. Due to the large size and long focal lengths of these systems, accurately measuring these surfaces with a compact metrology system is difficult. Several techniques have been developed to overcome these limitations, such as photogrammetry, which is a process of attaching components to diffusely scatter light to the heliostat mirrors; however, attaching components increases measurement uncertainty, degrades robustness, and can be time-consuming and labor-intensive. Phase gratings, which do not scatter or absorb light can be more useful for directing light differently based on wavelength, and gratings can be designed to modify the spectrum of reflected light.
Applications:
- Alignment and measurement of optical systems using second surface mirrors
- Measuring heliostats for solar energy applications
Advantages:
- Reduced cost
- Compact form factor
- Accurate measurements
- Enables operation at higher efficiency
- Does not require additional components
