Improvements in the evaluation of parabolic trough solar collector using a dynamic flat null-screen

We use the null-screen method to evaluate in a qualitative and quantitative way the shape of a parabolic trough solar collector. When a parabolic trough solar collector is fabricated can occur that the manufacturing errors are large. In some cases, when using the null screen method, the objects in the image plane are overlapped. This problem is solved adequately by placing the CMOS sensor and LCD null-screen in off-axis positions. Also, we propose to display a suitable null-screen on the LCD screen. In the first evaluation the null screen is composed of few object points, in the next evaluation, we change the distribution of objects that compose the null screen with the aim of increasing the measurement points. Finally, the data corresponding to each evaluation is merged in only one set of evaluation data to calculate the surface sagitta from slope measurements of the test surface in the x and y directions.