Slope Measurements of Parabolic Dish Concentrators Using Color Codified Targets

Abstract

A new short, yet highly accurate method for measuring the slope errors of parabolic dish concentrators has been developed. This method uses a flat target with colored stripes that is placed close to the focal plane and a digital camera located at an observation point on the optical axis at some distance from it. A specially developed image analysis algorithm detects the different colors in the images of the reflection of the target in the concentrator and assigns them their known position on the color target. This information, along with the geometric relationship between the components of the measurement setup and the theoretical parabolic shape of the concentrator, is used to calculate the normal vectors of the concentrator surface. From these normal vectors the radial and tangential slopes can be calculated and compared to the design values of the concentrator. The resulting slope errors not only give the total concentrator error for general characterization of the dish, but also indicate systematic errors in fabrication and mounting with high spatial resolution. In order to verify the quality of the results obtained, a ray-tracing code was developed that calculates the flux distribution on planes perpendicular to the optical axis. Measured slope errors of a DISTAL-2 dish concentrator are presented and the calculated flux distributions are compared to measured flux distributions. The comparison shows excellent agreement in the flux distribution on the absorber plane. This verifies the promising potential of this method for fast, highly precise measurement of imperfections in dish concentrator shape.