A light converting optical system employing a planar light trapping optical structure illuminated by a source of monochromatic light. The light trapping optical structure includes a photoabsorptive layer including quantum dots. The photoabsorptive layer is configured at a relatively low thickness and located between a broad-area optically transmissive surface configured to reflect light using a total internal reflection and an opposing broad-area reflective surface formed by a thin sheet of material configured to diffusely reflect light. The opposing surfaces confine and redistribute light within the light trapping structure, causing multiple transverse propagation of light through the photoabsorptive layer and enhanced absorption and light conversion. The light trapping optical structure may further incorporate an array of lenses or other optical elements located on a light path between the light source and the photoabsorptive layer.