Guillaume P. Butel

Aspheric and freeform surfaces metrology with software configurable optical test system: a computerized reverse Hartmann test

Abstract. A software configurable optical test system (SCOTS) based on deflectometry was developed at the University of Arizona for rapidly, robustly, and accurately measuring precision aspheric and freeform surfaces. SCOTS uses a camera with an external stop to realize a Hartmann test in reverse. With the external camera stop as the reference, a coordinate measuring machine can be used to calibrate the SCOTS test geometry to a high accuracy. Systematic errors from the camera are carefully investigated and controlled. Camera pupil imaging aberration is removed with the external aperture stop. Imaging aberration and other inherent errors are suppressed with an N-rotation test. The performance of the SCOTS test is demonstrated with the measurement results from a 5-m-diameter Large Synoptic Survey Telescope tertiary mirror and an 8.4-m diameter Giant Magellan Telescope primary mirror. The results show that SCOTS can be used as a large-dynamic-range, high-precision, and non-null test method for precision aspheric and freeform surfaces. The SCOTS test can achieve measurement accuracy comparable to traditional interferometric tests.

Deflectometry using portable devices

Abstract. Deflectometry is a powerful metrology technique that uses off-the-shelf equipment to achieve nanometer-level accuracy surface measurements. However, there is no portable device to quickly measure eyeglasses, lenses, or mirrors. We present an entirely portable new deflectometry technique that runs on any Android™ smartphone with a front-facing camera. Our technique overcomes some specific issues of portable devices like screen nonlinearity and automatic gain control. We demonstrate our application by measuring an amateur telescope mirror and simulating a measurement of the faulty Hubble Space Telescope primary mirror. Our technique can, in less than 1 min, measure surface errors with accuracy up to 50 nm RMS, simply using a smartphone.

Binary pattern deflectometry

Deflectometry is widely used to accurately calculate the slopes of any specular reflective surface, ranging from car bodies to nanometer-level mirrors. This paper presents a new deflectometry technique using binary patterns of increasing frequency to retrieve the surface slopes. Binary Pattern Deflectometry allows almost instant, simple, and accurate slope retrieval, which is required for applications using mobile devices. The paper details the theory of this deflectometry method and the challenges of its implementation. Furthermore, the binary pattern method can also be combined with a classic phase-shifting method to eliminate the need of a complex unwrapping algorithm and retrieve the absolute phase, especially in cases like segmented optics, where spatial algorithms have difficulties. Finally, whether it is used as a stand-alone or combined with phase-shifting, the binary patterns can, within seconds, calculate the slopes of any specular reflective surface.

Reflectance optimization of second-surface silvered glass mirrors for concentrating solar power and concentrating photovoltaics application

Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrated.

Second-surface silvered glass solar mirrors of very high reflectance

This paper reports methods developed to maximize the overall reflectance second-surface silvered glass. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.4% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 2.7% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric boost, was measured at NREL to have 95.4% solar weighted reflectance. For comparison, non-enhanced, wetsilvered reflectors of the same 4 mm thickness show reflectance ranging from 91.6 - 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending.

Measuring large mirrors using SCOTS: the Software Configurable Optical Test System

Large telescope mirrors are typically measured using interferometry, which can achieve measurement accuracy of a few nanometers. However, applications of interferometry can be limited by small dynamic range, sensitivity to environment, and high cost. We have developed a range of surface measurement solutions using SCOTS, the Software Configurable Optical Test System, which illuminates the surface under test with light modulated from a digital display or moving source. The reflected light is captured and used to determine the surface slope which is integrated to provide the shape. A range of systems is presented that measures nearly all spatial scales and supports all phases of processing for large telescope mirrors.

Optimization of dynamic structured illumination for surface slope measurements

We present a fast and ambiguity-free method for slope measurement of reflective optical elements based on reflectometry. This novel reflectometric method applies triangulation to compute the slope based off projected patterns from an LCD screen, which are recorded by a camera. Accurate, ambiguity-free measurements can be obtained by displaying one pixel at a time on the screen and retrieving its unique image. This process is typically accelerated by scanning lines of pixels or encoding the information with phase using sinusoidal waves. Various classes of solutions exist, centroiding and phase-shifting being the most accepted, but their sensitivities vary with experimental conditions. This paper demonstrates solutions based on various parameters such as accuracy or efficiency. The results are presented in a decision matrix and merit function. Additionally, we propose a new class of solutions – Binary Square screens – in an attempt to address system limitations and compare current systems to our solutions using the decision matrix. Several test conditions are proposed along with the best suited solution.

Design, Optimization and Characterization of Secondary Optics for a Dish-Based 1000x HCPV System

This paper presents a novel design of a solar secondary optics used in a dish-based HCPV system at 1000x. Different optimizations were conducted as well as experiments to determine its optimum configuration.

Development and On‐Sun Performance of Dish‐Based HCPV

The paper describes a new system architecture optimized for utility‐scale generation with concentrating photovoltaic cells (CPV). The system concept is optimized to use predominantly low‐cost materials manufactured by methods proven for high volume production. Triple‐junction cells are used to convert 1000x concentrated sunlight into electricity. Compared to silicon panels, these commercially available cells convert at least twice as much of the incident sunlight energy into electricity, and at 1000x optical concentration, they cost one‐tenth as much per watt of power output. The architecture combines three novel elements: large (3.1 m×3.1 m square) paraboloidal glass dish reflectors to collect and concentrate the sunlight; compact receivers at each dish focus, each one incorporating multiple, actively cooled cells; and a lightweight steel spaceframe structure to hold multiple dish/receiver units in co‐alignment and oriented to the sun. A manufacturing process for replicating the reflector dishes is well ...

Defining System Conversion Efficiency for Dish-Based Solar Concentrator PV

Concentrator system conversion efficiency may be characterized by using measurements of the optical components. Techniques of characterizing radiometric throughput via geometric ray shadowing, optical efficiency, and cell efficiency are presented.