n. The emission and propagation of energy through space or through a material medium in the form of electromagnetic waves (Weast 1973).
The radiation relevant to the wildland fire context is called thermal radiation. Thermal radiation has wavelengths in the range of 0.1μm to 100μm (1μm = 10-6 meters). A black body (ideal radiator) emits radiation over a distribution of wavelengths defined by Plank’s spectral distribution (Incropera and DeWitt 2002). Using Wien’s Displacement Law, a black body (ideal radiator) at a typical flame temperature of 1000C (1273 K) has a peak emissive power at 2.28μm.
Visible light (what humans can detect with their eyes) ranges from approximately 0.4μm to 0.7μm. The peak spectral emissive power of a flame is not visible to human eyes. The percentage of the total radiation emitted by a flame (assumed to be a black body at 1273 K) within the visible spectrum is less than 0.02 percent – more than 99.98 percent of the flame radiation is not visible to humans. Radiation is quantified as the rate of energy transferred per unit area, which is also called radiant heat flux.