A method for calibration of radiant heat flux meters (thermal radiometers, heat flux gages) of the Gardon and Schmidt-Boelter types has been devised. It is based on a method previously developed by the author, which has been further improved and slightly modified. The method now enables calibration from 2 to 100 kW/m2, for gages with a maximum housing diameter of 50 mm and with a sensitive element diameter of up to 10 mm. The method virtually avoids influence of convective heat transfer to the gage and has a relative inaccuracy of less than +-2.8% over the whole range. The method uses a spherical furnace, electrically heated, with a water cooled aperture device inserted in the bottom. The gage to be calibrated is mounted inside the water cooled device and is thus protected from direct contact with the hot furnace gas. The equipment is easily calibrated to traceable standards. The water cooled aperture could also be used with other types of black body cavities. This report describes the equipment and the theoretical basis, and gives a detailed error analysis.