An innovative technique was developed for the direct measurement of the absolute radiant flux emitted from transient flames. The design of the experimental device, called FAIRS (Fast Absolute Infra-Red Sensor), is detailed in this work. The main concept of FAIRS is based on the combination of a carbon nano-tube-based black body as a sensitive element, coupled to a fast IR HgCdTe detector via an achromatic optical setup. A specific calibration protocol based on a laboratory blackbody (ambient to 850 °C) allows the qualification of the FAIRS for absolute radiative heat-flux measurements, with a response time less than 1 µs that was checked, thanks to pulsed laser irradiation. It is thus demonstrated that FAIRS is a good candidate for transient measurements, with a simplified calibration procedure. FAIRS was coupled with ultra-fast schlieren imaging on spherical expanding CH4/air and C3H8/air flames. In this condition, it is possible to correlate the real time flame diameter to its absolute radiative heat losses.