Cube beam splitters are made from two triangular glass prisms glued together, as seen in Fig.2A-C. The thickness of the glue is carefully adjusted so that, for certain wavelengths of light, half the light that enters the cube is reflected 90° and the other half is transmitted through the cube in the same direction as it entered, as seen in Fig.2A. http://www.jxzb-mechanical.com/color-sorter/