Abstract: Apparatus for measuring laser power includes a target disc of a predetermined thickness located within, and in thermal contact with, a heat-sink. The heat-sink has an opening therein for directing laser radiation to be measured onto the target disc. A thermal gradient is established in the target disc when laser radiation to be measured is directed onto one surface of the disk. The opposite surface of the disc includes means for detecting the thermal gradient. The heat-sink includes a cylindrical core having an opening in one end thereof for admitting laser radiation to be measured. Cooling-vanes are disposed around the cylindrical core extending radially outward therefrom and extending therealong. A housing surrounding the heat sink includes a fan which causes air to be drawn into the housing, flow between the cooling-vanes, and the be expelled from the housing.