The extinction coefficient of a powder mixture of ZnO and beech charcoal is determined at ambient temperature using two experimental set-ups: (a) radiation in the range 500-1000 nm is captured by a 200 mu m-diameter fiber optic and sent to a spectrometer equipped with a Si/PbSe detector; (b) radiation in the range 350-1100 nm is captured by a 5 mm-diameter Si photodiode. Both set-ups measure the attenuation of intensity from a 3273 K blackbody source. The experimental results are implemented in the numerical solution of the equation of radiative transfer, using the Monte-Carlo ray-tracing technique. The extinction coefficient is determined to be 10103 +/- 615 m(-1) at 1000 nm using set up (a), and 7850 +/- 337 m(-1) for the range 350-1100 nn: using the more accurate set-up (b).