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 they have additional optical properties .