A window portion is interposed between the incident light and a heat absorbing portion and at least one of the heat absorbing and window portions has a plurality of spaced apart reflecting surfaces, separate ones of which face each other and transmit the incident light by multiple reflections to the heat absorbing portion. In at least one embodiment, these opposed reflecting surfaces converge in the direction of light travel from the window portion to the heat absorbing portion and infrared reflecting means are interposed between the window portion and the heat absorbing portion to return infrared light emitted by the heat absorbing portion back to the heat absorbing portion.