Progress in the design of chromatic null screens to test cylindrical parabolic concentrators

This paper shows the qualitative and quantitative results obtained on the test of cylindrical parabolic concentrators when the screens are designed using dot color changes to guarantee a better dot correspondence assignment and to make the optical test of the concentrator easier; the method allows to measure deformations from tenths of a millimeter to several millimeters, thus demonstrating that null screen method is good enough for testing surfaces with important deformations. On a second approximation, it is designed a null screen corresponding to the surface evaluated with the first one by fitting a fourth grade polynomial with crossed terms; with this, it is shown that the first test gets closer to the shape of the surface validating it.