Radiant energy is transmitted from a hot body (surface) to a colder body (surface) through a vacuum or air (not solid or liquid materials) under specific thermodynamic rules and conditions. If the colder body is a reflective surface to that wave length of radiation, a portion (the efficiency of the reflective surface) of the transmitted radiant energy will be reflected back to the hotter emitting surface.
Hence, the reflective surface material must be facing the heat source without intermediate material interference. Thus even dust accumulation on the reflective surface will reduce its reflective efficiency over time.
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