Infrared (IR) imaging is a standard technique for the detection of forest fires but its use to measure the physical parameters of fires faces difficulties due to their spatial and spectral complexity. Multi-spectral images in the Medium IR make possible to classify the scene in different fire regions, thus enabling a region-specific processing and providing values for the instantaneous radiated power and the total radiated energy. Comparison with energy release determined from fuel consumption results in an estimation of a radiative energy fraction of 14 ± 3 %. It is the first time that this parameter is measured by IR imaging in the field.