Characterisation of concentrator surfaces using deflectometry
Coloured pattern deflectometry measurement principle.
A second photo is taken with the pattern rotated by 90°.
Diagram: P. Scott.
Deflectometry systems directly measure surface slopes whilst triangulation techniques such as photogrammetry measure surface positions in 3d space. For concentrating mirror applications, surface slopes are the more important quantity to measure, as a deviation in mirror slope affects the dish optics more than a surface position offset.
A novel method developed at ANU by Paul Scott, known as Coloured Pattern Deflectometry (CPD) was designed to be cheap, fast, easy to operate, compact and to provide not only slope error information, but also the overall shape and principal radii of curvature of a mirror panel. This method differs from other deflectometry systems in that a coloured pattern is used instead of phase shifting techniques. One advantage of the coloured pattern is a reduction in system price and complexity, since it can be a printed sheet of paper instead of the LCD monitor typically used with phase shifting. Also only 2 photographs need to be taken where phase shifting requires upwards of 6. A second key difference of the CPD method is in the way that it calculates mirror surface positions. The surface slopes and positions are recursively calculated by extrapolating the surface out as the calculation proceeds. This has the potential of producing more representative and detailed surface position information.
The software used in this project is also available for download.