A null instrument for the measurement of intense beams of radiant energy (from about 5 to 300 cal cm−2sec−1) is described. The instrument utilizes two identical hollow blackbody receivers whose walls are provided with coils carrying a stream of cooling water. The inflowing water divides equally between each chamber. The temperature difference between the two outflows is determined by a differential thermopile. One of the chambers is exposed to the radiant energy beam, while the other chamber is heated electrically until a null is obtained on a galvanometer connected to the differential thermopile. It is shown that measurements of the electrical power dissipated in the chambers under null conditions and a measurement of the aperture areas of the chambers is sufficient to give the radiant flux density.