James M. Jonza

Multilayer polymeric color-shifting polarizer films

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.

Three-dimensional floating images as overt security features

3M has developed a proprietary laser process for creating three-dimensional images that appear to float above and/or below the plane of a substrate containing an array of microlenses. During the imaging process the laser records a microscopic image of the desired three-dimensional pattern in the material located at the focal point of each microlens in the array. The images exhibit motion parallax comparable to that seen from holograms and are easily visible in a wide range of ambient lighting conditions. The images are therefore similar, but not identical, to integral images, first proposed in 1908 by Lippmann. The fidelity of these floating images requires maintaining exact registration between the microlens array and the corresponding microimage array. In addition, the use of an ablative laser process for the production of the microimages enables the production of microimage features smaller than the diffraction limit (up to approximately 50,000 dpi). The images are therefore very difficult to simulate, counterfeit, or modify and are highly desirable as an overt security feature. 3M has scaled up the floating image process to produce images in ConfirmTM Retroreflective Security Laminate to authenticate passports and drivers licenses and in retroreflective license plate sheeting as the EnsureTM Virtual Security Thread to authenticate vehicle registration. This allows addition of features to a secure document that are easily verifiable, using only the human eye, by a large and widely disperse population to create an identity document that is easily identified as genuine.

Three-dimensional virtual images for security applications

Three-dimensional images may be produced by a number of methods, the earliest being integral photography. The basic concept involves exposing a photographic emulsion to light scattered from an object through a flys eye lens to produce an array of micro-images, one behind each lenslet. An observer viewing the composite image through the lenslet array sees a three-dimensional representation of the object. Over the past 5 years, 3M has applied laser technology to the creation of three-dimensional virtual images using the integral photography approach. The virtual images made by this process can be observed by a viewer with the unaided eye in either reflected or transmitted light. The images display large movement as an observers viewing perspective changes and have a distinct on/off-viewing angle beyond which the image cannot be seen. The fidelity of the virtual images requires maintaining the registration of the substrate lenslets and the micro-images produced by the laser imaging process. This makes the images difficult to copy or modify and an ideal, cost-effective candidate for an overt security feature.

Quarter-wave polymeric interference mirror films

Quarter-wave interference films have seen numerous uses in the field of optical security, due to the strength of reflection, ability to select numerous colors, and especially for their conspicuous color shift. Decorative nanolayer quarter-wave polymeric material of greater than 100 layers have been known for twenty years, but did not have utility in the security market, because of their weak iridescent appearance. Through the use of polymers of higher refractive index difference, 3M has achieved strong reflectors with better band edge control. Extremely efficient broadband mirrors have been made for commercial use. By choosing the reflection to be in the near IR, a unique security laminate film is achieved. It is very transparent at normal incidence, and changes to a brilliant cyan at shallow viewing angles. This material is easily noticed by the typical observer, and is also machine- readable. The polymeric quarter wave mirror films may be fine-line embossed, permanently thinning the layers in that area, and further enhancing their appearance.