Abstract: A thermopile type radiation detector for use in radiation pyrometry having small size and high performance in which the thermopiles are formed by evaporating the thermocouple leads onto a thin substrate together with a pattern distribution of thermocouple junctions that produces an output that is more representative of the distributed radiant energy impinging on the hot junctions of the thermopile. The thermopile performance is improved also by the incorporation of relatively large reflecting areas associated with the region of the thermopile where the cold junctions are located.