Transferring freeform lens design into phase retrieval through intermediate irradiance transport

Freeform lens design for the prescribed irradiance without thin element and paraxial approximations is very complicated. We propose to transport a point source irradiance before the required freeform lens into an intermediate irradiance estimate next to the lens. In this way, the complicated freeform lens design problem could be transferred into a simpler problem of retrieving the phase from the intermediate and target irradiances. The phase retrieval solution can help construct an approximate lens, which is subsequently used to update the intermediate irradiance. Several iterations could bring a good performance, as demonstrated by two challenging designs.