Abstract: Method and apparatus for measuring radiant energy, particularly relatively high energy levels of coherent radiation, in which the energy impinges upon an absorber surface, the energy transferred from the absorber surface to a fluid coolant in thermal association with the absorber surface, and the energy carried away by the coolant measured by means of a temperature difference measuring device, preferably a thermopile, positioned in thermal contact with a portion of the inlet leg of the coolant conduit and the outlet leg thereof, in order that high energy levels may be measured at a cool absorber surface while flowing relatively small amounts of heat energy through the temperature difference measuring means.