Retrochromic articles

#“Retroreflectivity” means the ability, if engaged by a beam of light, to reflect that light substantially back in the direction of the light source. Films, for example, transparent adhesive-backed overlay films, having covert retroreflective patterns have been used to authenticate security articles (e.g., passports, identification badges). Typically, such overlay films have covert retroreflective patterns that are not readily legible under conditions of diffuse illumination, but become readily legible if viewed in retroreflective mode (e.g., with a flashlight or specialized device such as a retroviewer).

#Depending on the application, the addition of covert color to retroreflective articles may provide additional security features (e.g., as in the case of authentication of passports or identification badges), or novel visual effects (e.g., as used in the design and production of graphic articles).

#As used herein, the term “retrochromism” refers to the ability of an object, or region of an object, if viewed in retroreflective mode (i.e., with the illumination source substantially co-linear with the line of sight, typically forming an angle of reflection of about 10 degrees or less), to exhibit a reflected color different from the color exhibited if the object or region is viewed in other than retroreflective mode. Various constructions that exhibit retrochromism are known. For example, in one such construction high refractive index glass beads are partially embedded in a layer of or layers of material(s) (e.g., including a dielectric mirror). A change in color of retroreflected light results from the interaction of light, the bead, and the layer(s) into which it is partially embedded. Such articles may be prone to loss of, or irregularity in, their retrochromism due to coating thickness variations of the layer(s) and differences in the depth of penetration of the beads into the layer(s). Such articles may further be prone to damage caused by flexing the article during usage, or if used as heat-shrink tamper indicating films.

#FIG. 1 is a cross-sectional view of a Type I intrinsically retrochromic bead depicting exemplary paths of retroreflected light.

#FIG. 2 is a flow diagram of an exemplary process, according to the present invention, for making Type I retrochromic beads.

#FIG. 3 is a cross-sectional isometric view of an exemplary embodiment of retrochromic article according to the present invention.

#Articles of the invention are conveniently made by forming a retrochromic pattern utilizing intrinsically retrochromic beads, optionally in combination with non-retrochromic retroflective materials (e.g., glass beads having a high index of refraction). The retrochromic pattern is desirably formed within and/or on the surface of a substrate.

#Retrochromic patterns, useful in practice of the present invention, appear differently if viewed in retroreflective mode than if viewed under diffuse lighting conditions (e.g., fluorescent room lighting). As used herein, a “pattern” is defined by and composed of a plurality of regions.

#In some embodiments, retrochromic patterns comprise retrochromic regions which are each discernible if viewed in both retroreflective and other modes.