Absorptivity and its dependence on heat source temperature and degree of thermal breakdown

The spectral absorptivity of 62 products has been measured in the wavelength region of 0.3-20 mu m. Effective absorptivity for fire-induced heat radiation typically lies in the range of 0.75-0.95. It was found that the effective absorptivity varies significantly with the temperature of the heat source. This has implications on the heating of a surface. The effect is more important when the absorptivity is used as input for calculations of ignition temperature and thermal inertia. It was also found that the absorptivity of radiation from fires for products exposed to irradiation in many cases decreased with increased exposure time. This is surprising since, for example, wood that is darkened when exposed to heat obviously has a higher absorptivity in the visual part of the spectrum than fresh non-darkened wood. The reason that was identified for this is because the absorptivity in the IR drops, and measurement results are given which clearly illustrate this.