A method of making a light converting optical system is described. The method includes providing a layered light trapping structure comprising a first optical layer with a plurality of linear grooves having triangular cross-sections for reflecting and deflecting light through total internal reflection and refraction. A diffuse reflector, made from a thin sheet of diffuse reflective material, is placed parallel to the first optical layer. A light converting film, positioned between the first optical layer and the diffuse reflector, contains an active layer with first and second light converting semiconductor materials of different bandgaps. The thickness of the active layer is below the minimum required to absorb all visible spectrum light in a single passage. The method further involves providing a light source and positioning the layered light trapping structure to receive energy from the light source.