OPTICAL SECURITY: Printing system creates fluorescent security without fluorescent inks

The idea of embedding an authenticating fluorescent pattern—invisible under normal light but visible under UV illumination—for document security is not new, according to Raja Bala, a principal scientist at the Xerox Research Center (Webster, NY). But doing so without the use of fluorescent inks is new. And this is the essence of a technique developed by Bala and Reiner Eschbach, a research fellow in the Xerox Innovation Group. The company expects this method to prove particularly suited for providing security in relatively low-cost and short-run applications such as the printing of checks, coupons, contract, or concert tickets.

Rather than add fluorescing inks to a document, the technique is based on the use of standard inks to suppress fluorescing optical brighteners that are placed as a standard practice in all digital printing papers to make and keep them white. The Xerox researchers have developed a way to choose combinations of the cyan, magenta, yellow, and black (CMYK) inks—used to create color in printed documents—that selectively suppress the fluorescence that is manufactured into standard digital paper. “Imagine two CMYK colors that look the same under normal illumination, but that show a marked difference under pure UV illumination,” Bala said.

Color reproduction is based on the function of the human eye, which has three types of bright-light color sensors (cones) sensitive to long wavelengths (red), medium wavelengths (green), and short wavelengths (blue). It is thus possible to reproduce colors found in the natural world by mixing just three primary colors. In the case of an emissive desktop monitor, the three primaries are red, green, and blue lights, while on a reflective sheet of paper, we have subtractive mixing of the cyan, magenta, and yellow pigments that absorb the red, green and blue photons, respectively, from incident illumination.

The color that enables the Xerox innovation, however, is the black in the CMYK. “It adds an additional degree of freedom that allows you to find two different CMYK mixtures with the same color under normal lighting conditions, but different fluorescence properties under pure UV light,” Bala said. Creating a working system posed some difficult engineering challenges, however.

For instance, if printing-parameter settings vary slightly during printing, the printer may give a slightly different color match. Also, while incandescent illumination is relatively free of UV, sunlight and various forms of artificial illumination do have a UV component that could compromise the effectiveness of the method. Bala and Eschbach deal with potential situations such as these by adding “noise” in the form of additional colors or textures to a document that render minor discrepancies invisible. “For instance, a document might contain four colors: A, B, C, and D,” Bala said. “A and B might convey information, while C and D create distracting patterns and retain security under illumination with a small UV component.”

The business implication of this approach is the potential to increase document security with no additional cost from the standpoint of requiring expensive specialty materials such as fluorescent inks, Bala said. The implementation cost would consist primarily of expenses associated with development of the enabling software for high-end commercial printing systems that would carry this capability.

In addition, because it is becoming common for printing systems to broaden the available color gamut by including more inks than just the primary colors, the newly developed Xerox method would likely acquire more degrees of freedom in which several, not just a pair of, color mixtures could provide the desired characteristics. “What we are essentially doing is creating an inexpensive security feature system based on standard colorants and paper,” Bala said.