Radiant heat flux
n. The heat transfer rate per unit area as thermal radiation.
Radiant heat flux is typically calculated or measured for a specific surface to determine the heat transfer at that surface. Because all objects above a temperature of absolute zero radiate thermal energy, the net radiant heat transfer between two objects of the same radiation characteristics depends on their temperature difference, as shown in the diagram below (see "more").
Related to wildland fire, the assumption is commonly made that flames (the hotter “surface”) and fuel have similar radiation characteristics. The following equation describes the factors that determine the radiant heat flux from surface 1 incident at surface 2.