Multilayer Polymeric Color-shifting Polarizer Films

摘要

Color-shifts have been used in the field of optical security for many years. Through the use of birefringent polymers, 3M has pioneered highly reflective, multilayer, all-polymeric interference optical films for use as mirrors and polarizers. Polarizer and mirror multilayer films with reflectance bands covering all of the visible wavelengths have found uses in LCD displays and solar light pipes. Color-shifting polarizer (CSP) films may be made by uniaxially orienting a multilayer stack that has sharp band edges and does not cover all of the visible wavelengths. By judicious choice of polymers, the refractive indices of the two polymers have a large difference in refractive index in the stretch direction and match in the transverse direction. The resulting film has a noticeable color shift to the unaided eye, and a readily verifiable feature when viewed with both polarization states. In the pass condition, the film becomes colorless; in the block direction, the color is very saturated and noticeably shifts in hue when the viewing angle is changed. The films may have reverse printing under the CSP films, which hides during verification. The indelible marking of the film for the intended end use and the tamper resistance of labels made from these films will also be discussed.