Inks provide low-cost security

April 1, 2000
TOKYO—The Ministry of Finance has developed an inexpensive image-construction technology that can be used for high-security printing. The print shows different views depending on the observation angle relative to a light source.

TOKYO—The Ministry of Finance has developed an inexpensive image-construction technology that can be used for high-security printing. The print shows different views depending on the observation angle relative to a light source. The technology is an inexpensive alternative to high-security printing methods that use special inks or holograms.

The inks used are commercial metal-powder-based metallic inks colored with common dyes. As in conventional color printing, cyan, magenta, yellow, and black inks are used. However, the black ink is a flat nonmetallic ink. Standard offset printing methods are used to produce different colors, where the colors are created by adjusting the relative proportions of the four basic colors on the surface.

The reflections off the printed surface can be separated into two different types. The first component is intralayer reflection, in which light entering the ink layer is scattered. This scattering is isotropic. The other component is mirror reflection, in which the intensity distribution is heavily centered about the angle of reflection. This appears as a white shininess. For conventional inks, mirror reflection is weak at angles other than the angle of reflection. However, metallic inks mirror-reflect in a wider range of angles.

Click here to enlarge image

Characters printed in alternating bands of two sets of inks appear or disappear depending on angle of viewing.

In one case, two samples can look the same when viewed from the angle of reflection, but look different at other angles. The samples have different proportions of cyan, magenta and yellow, so they are of different colors. However, when viewed at the angle of reflection, they both look white because the mirror reflection component dominates. Therefore, if characters and images written using the proportions of one sample are printed against a backdrop of the other sample, the characters and images disappear when viewed at the angle of reflection. They reappear at other angles.

In another case, two samples look different when viewed from the angle of reflection, but look the same otherwise. One sample has a black ink component, while the other sample does not. However, by carefully mixing just cyan, magenta, and yellow inks, it is possible to create colors that are identical to each other at all angles except the angle of reflection. Because the black ink suppresses mirror reflection (shine), the two samples can be differentiated at the angle of reflection. If one sample is used to print characters and images on a backdrop of the other sample, the patterns can be seen only at the angle of reflection. By subdividing a surface into narrow bands and alternating these two schemes, it is possible to print surfaces so that two sets of characters/images can be seen against one backdrop depending on the viewing angle (see figure).

The method has potential for low-cost, high-security printing. Copiers, scanners, and printers are increasing in quality, making it possible for amateurs to copy printed materials. Easily printable, hard-to-copy inks are intended to provide a low-cost solution.

Courtesy O plus E magazine, Tokyo

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