Biomass particles can experience variable thermal conditions during fast pyrolysis due to differences in their size and morphology, and from local temperature variations within a reactor. These differences lead to increased heterogeneity of the chemical products obtained in the pyrolysis vapors and bio-oil. Here we present a simple, high-throughput method to investigate the thermal history experienced by large ensembles of particles during fast pyrolysis by imaging and quantitative image analysis: We present a correlation between the surface luminance (darkness) of the biochar particle and the-highest temperature that it experienced during pyrolysis. Next, we apply this correlation to large, heterogeneous; ensembles of char particles produced-in a laminar entrained flow reactor (LEFR). The results are used to interpret the actual temperature distributions delivered by the reactor Over a range of operating conditions.