Solar collectors, in particular flat panel solar collectors, are well-known devices which are usually used to absorb and transfer solar energy into a collection fluid. Principally, solar collectors consist of a blackened absorbing cylinder or plate contained in a housing which is frontally closed by a transparent window panel. Due to the diluted nature of solar light, in order to increase the operating temperature by reducing the thermal losses, solar collectors may be evacuated during use to eliminate gas convection and molecular conduction. Very high temperatures could also be achieved by light focusing. However, only direct light is focused, while diffuse light is lost.