3D-Shape Measurement of Parabolic Trough Mirror Panels:

First Results of the SFERA-III Round Robin

Abstract

The 3D-shape round-robin initiative aims to compare the main geometric parameters of 3D shape measurements for parabolic-trough mirror panels, assessed using equipment developed and employed by each participating institution: ENEA, F-ISE, DLR, NREL, and SNL. Except ENEA equipment, all the other are based on deflectometry, also call fringe-reflection method. The round-robin is based on circulating 6 trough mirror panels (3 inner and 3 outer) of RP3 dimensions, with a focal length of 1710 mm, between participating laboratories; a simple and rugged supporting fixture together with precise instruction on how to use it have been provided to make the comparison more reliable. ENEA wrote a custom evaluation software for comparing the results. We observe a reasonable agreement among the mean values of the deviations of height and slopes from the ideal parabola: standard deviation better than 0.1 mm, 0.5 mrad and 0.3 mrad for z, slopeX and slopeY, respectively. The agreement is improved when a software realignment procedure for setting the height values on the support points to the expected ideal values is applied. The absolute difference between pairs of evaluators is sometimes greater than the declared experimental uncertainty; investigation into these deviations is still ongoing.