Abstract: A hollow spherical casing is provided with an inlet opening for admitting e full intensity of the laser beam. A primary substrate formed of a bank of small tubes is mounted in the sphere to intercept the beam. The tubes partially reflect the beam and also conduct a predetermined fraction of the beam intensity into a water coolant. Beam reflections are evenly distributed over the sphere's interior surface by the tube geometry and the sphere applies the reflected beam energy to a surrounding coolant. Temperature measurements are made for both coolants. The predetermined fraction of the energy applied to the primary substrate coolant is a function of the amount of heat needed to produce a substantially instantaneous heat flux time constant that permits a substantially instantaneous measurement of beam power. Integration of the measurements derived from both the tube and the sphere coolants provides a self-calibration capability.