Using infrared‐transparent pigments to identify objects

Abstract

AbstractWith the goal of developing an invisible code that can be used to identify objects, we have created several pigments for color printing that share the characteristic of being transparent to near‐infrared light, along with methods to print them. Our recent research has emphasized applications as we continue to pursue our investigations of the characteristics of these infrared‐transparent pigments. In the course of our studies, we have discovered a black pigment with high transparency in the infrared (AM‐BK) (Makino, Yamamiya, and colleagues, 1997). This pigment has visible light characteristics that are the same to the naked eye as ordinary pigments, and like these pigments it is “lightfast,” that is, resistant to bleaching by light. For this reason, we anticipate that there will be applications for AM‐BK in the so‐called “universal design” area, whose goals are to provide information to visually impaired people, to keep track of lost or disoriented elderly people noninvasively, and so on. With this in mind, we have recently attempted to create pigments with colors other than black, and to increase the information density they can encode, so as to extend the range of uses for such pigments. In this article, we describe the optical characteristics of the pigments we have developed, and our efforts to use them in a specific type of invisible code. The inks we used in our code experiments generally were of two types: inks that incorporate carbon black (CB), which absorbs infrared light, and inks that contain AM‐BK pigment, which is transparent to infrared light. By mixing these two varieties of inks with solid‐color pigments, we were able to make pigments in three visible‐light colors—yellow, red, and blue. In an experiment with applications in mind, we then screen‐printed a bright yellow lattice‐like floral pattern on a T‐shirt with a “marker” for identifying an individual—the numbers [911]—hidden in it. Our results confirmed that when these numbers were concealed in the T‐shirt pattern by printing them with the above‐mentioned IR‐transparent pigments AM‐BK and CB, a CCD camera was able to distinguish them with ease although they were invisible to the naked eye. These experiments also revealed that as the amount of CB added to the pigment increases, its transparency to near‐infrared light falls off exponentially. This shows that it is possible to create gray‐scale contrasts, which could be used to encode high‐density two‐dimensional bar codes. © 2002 Wiley Periodicals, Inc. Syst Comp Jpn, 33(10): 74–82, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.1159