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The transmission properties of aluminium oxynitride (AlON) ceramics were studied. AlON samples with different transmittances were prepared. SEM detected two types of pores with a different range of diameters in the samples; the porosity was obtained by counting the number of pores in a certain area. Mie theory was applied to illuminate the effects of pores on the transmittances. The calculated transmittances were compared with the experiments. The existence of pores with sizes in the 1.1-1.6 mu m range and porosities above 10(-4) accounted for the degradation of the transmittance spectra at 2.5 mu m. Discrepancies of the spectra between the samples were caused mainly by different densities of the A-type pores with sizes above 1 mu m. The concepts of critical size and critical concentration were proposed and were quantitatively characterized by critical curves, which divided the regions of translucency and transparency into three parts: R(1), R(2) and R(3) depending on porosity. New approaches on how to control the pore sizes and porosities quantitatively for transparent ceramics design were presented.