Solid films with novel optical properties are produced from colloidal suspensions of cellulose crystallites; the colloidal suspensions are prepared by acid hydrolysis of crystalline cellulose under carefully controlled conditions; the solid materials possess a helicoidal arrangement of the constituent crystallites; by appropriate selection of conditions for preparation and treatment of the colloidal suspensions, solid films are produced that reflect circularly polarized visible light; the wavelength of the reflected light can be controlled to give colours across the visible spectrum from red to violet, and if necessary to infrared and ultraviolet wavelengths. The reflected iridescent colours arise from optical interference effects, and change with the viewing angle; this makes the materials ideally suited for optical authenticating devices, since no printing or photocopying technique can reproduce this effect; furthermore, they are easily distinguishable from other optical interference devices since thay have additional optical properties.