Describes an optical element consisting of three specularly reflecting surfaces and an IR reflecting surface facing towards the absorber for use in flat plate solar energy collector. By incorporating a panel of such optical elements in between the absorber and the window of the flat plate solar energy collector, the radiation and convection heat losses are reduced. The design aspects and merits of the optical element are discussed. The concentration factor, radiation trapping capacity and transmission efficiency of the device are computed and presented in the graphical form. The radiation trapping capacity of the device is shown to be better than that of the optical valve with two specularly reflecting surfaces.