envelope defined by hot plate 50 jacket 54 and window 53 the theoretical basis of the present invention is not understood with certainty however the operation of the radiation cooler of the present invention is believed to depend upon the following theoretical considerations generally heat transfer by infrared radiation occurs between a relatively hot surface and a relatively cold surface in accordance with the following formula f a dimensionless configuration factor that is a direct function of the magnitudes of the areas of both surfaces the degree of parallelism of the surfaces the closeness of the spacing of the surfaces the closeness of the approximation to black body emissivity of the surfaces and ambient conditions u03c3 the stefan boltzman constant 0 171 u00d7 10 8 btu ft 2 h deg r 4 t n the absolute temperature of the hot surface degrees r t c the absolute temperature of the cold surface degrees r the foregoing indicates that cooling by infrared radiation is a direct function of surface area difficulties are encountered in attempting to utilize a large open cooling surface for radiation transfer when its temperature is below freezing because of mechanical problems particularly difficulties associated with frost prevention in accordance with the present invention a geometrically small radiation sink in which frost and other mechanical problems can be easily controlled is converted effectively into a geometrically large radiation sink by disposing it on the axis of an infrared optical condenser of relatively large diameter the configuration of the reflector in various modifications is spherical parabolic elliptical or aspheric in fig5 for example a radiation sink 58 and a subject region 60 of restricted area a 1 to be cooled are positioned at conjugate points along the axis 62 of reflector 64.