Fig. 4

Bézier curve as description of a concentrator surface in the LightTools program (https://www.synopsys.com/optical-solutions/lighttools.html). The curve has three control points of which only two parameters (‘Weight’ and ‘Position’) of the center point are changed, whereas ‘Front Size’, ‘Rear size’ and length are kept constant. The ‘Size’ of the center point is kept equal to the ‘Rear size’ to ensure a zero end slope, like in a CPC