A power measuring sensor for an optical beam which utilizes the temperature difference across a thin layer of heat insulating material, generated by the axial flow of the absorbed beam, from an absorber layer on which the beam impinges, to a cooled heat sink which dissipates the heat after passage through the sensor. The axial heat flow is measured by means of a continuous matrix of adjacent thermocouple junctions over the heat flow region of the sensor disc, with the thermal insulating layer, which generates the temperature drop, having thicker and thinner regions at alternate junctions. The junctions on the thicker regions of the insulator thus become the hot junctions, and those on the thinner regions of the insulating layer become the cold junctions, and the sum of the voltages generated by the thermocouples is proportional to the flow of heat, and thus to the incident optical power.