This method is intended for cellulose acetate having an acetic acid yield of 54 % ± 2.5 %. It may also be applicable to other transparent plastics which are not strongly coloured and which can be moulded under the specified conditions. Two measurements of absorption, one at the red end and one at the blue end of the visible spectrum, are sufficient to characterize the absorption of light by the material. Describes the preparation of the material, two methods for the incorporation of plasticizers and die moulding of the test specimen. For the determination of the initial optical density, specimens given the smallest practicable amount of heating are used. The optical density at 25 mm thickness is calculated as the initial light absorption. The light absorption after further heating is obtained by similar measurements on specimens prepared using a longer period of heating during moulding.