Thus, the opacity O is calculated from the reciprocal of the grade of transmission τ. Accordingly, a material that is 100% light permeable (τ=1) has an opacity O=1. ###>L'opacité O se calcule ainsi de l'inverse du degré de transmission τ. Une matiére translucide á 100% (τ=1) posséde une opacité de O=1.