The experimental investigation of radiation in large combustion chambers is challenging due to the harsh environment and various radiation effects. This chapter presents experimental findings from several laboratory-scale studies that examined these effects separately. One goal was to determine the emissivity of the solid fuels and of ash utilising two setups: (1) observing a particle streak in a laminar flow reactor (LFR) and (2) studying laser-ignited fuel particles. The emissivity of both investigated solid fuels (Colombian bituminous coal and walnut shells) decreased with increasing temperature and progressing conversion, where mineral content has also an influence. For particles at higher levels of conversion, a final increase in emissivity was noticed. A spectroscopic experiment examined the emissivity of typical ash components and their mixtures. Carbonates and sulphates exhibited characteristic temperature- and wavelength-dependent emission trends, with specific emission bands attributed to CO \(_3\) and S–O functional groups. Another goal was to determine the index of refraction of the solid fuels experimentally. It was measured at room temperature and for burning particles applying inverse Mie-theory.