Security Graphics with Multilayered Elements in the Near-Infrared and Visible Spectrum

Abstract

In this paper, the fusion of four graphics into one integrated graphic is selectively observed in the visible and infrared spectrum. Each graphic represents its own information derived from the following sources: vector graphics, drawing, photograph and textual information. One graphic will be visible to the naked eye after the print. The other graphics will be observed with an NIR surveillance camera. These other graphics are nested into the selected visible graphics. All the graphics together make up a security print product with the characteristics of an individual solution with multilayered elements. Reprinting is possible only for the person in possession of the solution created according to the algorithm based on the INFRAREDESIGN^® method. When these graphics are printed on paper, it is impossible to produce an identical graphic prepress (C, M, Y, K) to produce forgery with the same dual properties in the visible and NIR spectrum.

1. Introduction

In this paper, we extend the domain of computer graphics to include two light spectra—the visible and the near-infrared spectrum. The final solution and dyes are visually observed with a double ZRGB camera (Figure 1) in the “Z”-near-infrared and “V”-visible (RGB) spectrum [1]. In this paper, we develop and present the procedure of merging computer graphics with the INFRAREDESIGN^® idea that will manifest itself separately in two light spectra: the visible (V) and near-infrared (NIR-Z).

State-of-the-art INFRAREDESIGN^® is a field of security graphics with the idea of fusing two pictures that will manifest themselves separately in the near-infrared (NIR) and visible spectrum after integration. A one-way hierarchy of pictures was published, where the first picture hides the second picture. Previous solutions have shown the merging of only two graphics for two light spectra [2]: a dress with a hidden portrait or code and clothes with hidden textual information, printed with inkjet inks on a plotter for textiles. The clothing was designed with computer graphics, with the theme “camouflage clothing”. ZRGB cameras are used to observe and photograph double solutions [1]. The Z camera, which photographs the NIR wavelength range, has a filter for 1000 nm. The visible and NIR condition were recorded in daylight without an additional IR source. Spectral analyses of light absorbance for process dyes in two light spectra—the visible (400 to 750 nm) and the beginning of the near-infrared (750 to 1000 nm)—have been performed for several printing techniques [3]. Since our eye does not register the NIR range, it can be said that the graphics in this range are “hidden from the naked eye” [4]. Several models have been published that combine the colors and dyes in the visible spectrum and the coverage of the dyes for those parts of the image that present two spectral ranges. The implementation of the dual image is suitable for the fast creation of highly protected individualized documents [5].

The idea is to introduce INFRAREDESIGN^® in different fields of security graphics. Motivation for this paper is the merging of three and four images whose parts are sorted for detection in the visible and near-infrared spectra. Printing is performed on a toner printer (OKI) for which spectrograms were performed and first published in this paper. These twin dyes are shown in two spectra with an emphasis on equality in the range of 400 to 700 nm. The separation of the dye twins when the measurements are shifted towards the NIR spectrum is also shown. The original digital record is attached as a JPG and PDF four-color graphic preparation for printing on a digital printer. Individual channels for process colors are available with an emphasis on the carbon black channel, which will be viewed separately with an NIR camera. Analysis of the transformation from V to Z spectra is attached as a continuous animated change into MP4 and SWF format.

2. Methodology: Twin Dyes

The merging of different images is based on the idea of twin dyes for dual graphics. Experimental work is performed on an OKI printer with the corresponding set of toners: cyan, magenta, yellow, and black. The first three toners (C, M, Y) absorb light only in the visible spectrum. Toner carbon black (K) absorbs it in two spectra, visible and near-infrared spectrum. By mixing toners, twins of colors and dyes are achieved, with which separate information is created for the visible and NIR spectrum.

A twin dye group consists of several twin dyes (pigments) made up of different components that visually manifest themselves as the same color [6,7]. These twin dyes will absorb light differently only in the NIR wavelength region, which is why they differ from each other.

In the printing practice, prepress for printing is performed in some phases with the use of GCR (Gray Component Replacement), while maintaining the same values of L*a*b*, HSB and RGB [8]. CMYKIR is a special GCR method designed with expansion to NIR spectrum. Near-infrared graphics are designed to be nested in a carbon black channel with the reduction of C, M, Y dyes. Therefore, our CMYKIR (VZ) method is also based on that same GCR theory. From there, routines are generated for our “VZGCR” software written in PostScript code. The focus of the software is data processing, presented in

Table 1. CMYK formulations of twin dyes.

	Measured Data C, M, Y (K0, K20, K40)
	C, M, Y (K = 0%)	C, M, Y (K = 20%)	C, M, Y (K = 40%)	L*a*b*
A	99, 99, 59	88, 85, 41	74, 68, 19	1, 44, −45
B	53, 98, 73	38, 86, 56	20, 70, 36	25, 56, 9
C	70, 46, 99	58, 29, 84	44, 9, 67	45, −25, 43
D	40, 50, 40	28, 40, 24	14, 28, 5	57, 14, 6
	40, 40, 40	28, 30, 25	15, 18, 6	62, 5, 7
	37, 82, 36	26, 71, 23	7, 63, 0	42, 49, −1
	88, 35, 70	70, 24, 65	68, 1, 36	44, −41, 6
	30, 30, 35	19, 21, 20	5, 10, 2	71, 4, 11

.

There are many different cameras around us: security cameras in the streets and in protected areas, IR money detectors in banks, IR reflectography in galleries. A, B, C and D spectra of dyes (Table 1) are derived with the forensic camera Projectina, model PAG [9] and with “XRITE–iONE”. The light absorbance value at 1000 nm is called “Z”. Our experimental work is based on process colorants C, M, Y and K is thus marked as “VZ”. For example, the IR technology has been applied in the process of designing banknotes, but only with spot dyes whose material formulation is not published. On today’s banknotes, the visible and NIR graphics are printed separately, one next to the other. There is no “hiding” of one image within the other, which is the innovation and purpose of this paper.

2.1. Twin Dyes Components

The recipes (%) for eight groups of twin dyes have been given in Table 1. The A dyes are the same color but with three completely different formulations. The same happens with all other groups of dyes.

2.2. Spectroscopy of Twin Dyes

Figure 2 and Figure 3 show spectrograms of A, B, C, and D dyes (Table 1) in the area from 400 to 900 nm. The measured reflected light is presented inversely as the absorbance of light in the process printing dyes: cyan, magenta, yellow and black [9]. The graphs of light absorbance show the pigment in its corresponding nanometer range. The maximum, “saddle” and “back” are displayed, which, in addition to the spectrograms of the components, shows the direction of the color twin repair [10].

Conventionally, the visible range is defined as the range from 260 to 760 nm and it follows the near-infrared spectrum. In this paper, the range “V” is marked as a part of the visible spectrum from 400 to 700 nm. The dyes made up from twin dyes have the same values of light absorbance in the V area. The range between 700 and 800 nm is called Z1, and the values of light absorbance in it differ exceedingly. Z2 refers to the range from 800 to 900 nm. This range of the near-infrared spectrum is crucial for the clarity of the appearance of the hidden image detected by the NIR camera.

In this paper, the discussion of twin dyes is presented with the spectroscopy of the first four dyes from Table 1. The correct definition of the range between 800 and 1000 nm is the main subject of dual design and dual recognition of different graphics in the same place.

3. Results

This chapter demonstrates the fusion of different images as a “picture within the picture” technology. The result of printing is interpreted as a unique preparation for four-color printing. Some images were created by photographing, and some of them are computer graphics from the field of security graphics.

3.1. Experiment Plan for Creating Security Graphics

We present four graphics in color in order to demonstrate new ideas in the field of security graphics. The first graphic is a portrait en face (Figure 4). The second graphic is a vignette (Figure 5). These elements are usually found in securities. The lines are generated as vector Bézier lines. In our examples, the vignette is prepared in high resolution, but it is equalized with pixel graphics, such as images in Figure 4, Figure 6 and Figure 7. However, the completely different images are mutually connected through content: en face, profile and the text in the vignette.

Figure 6 is composed of alphanumeric characters. Each letter and number is a separate font that has 30 different values of thickness. An alphanumeric character is a simulation of a raster (screen) form, and it simulates the portrait en face from Figure 4. The text creates a surface with micro-text that can often be found in the design of securities. Here the text is connected to the information about the person: name and surname, date, place and country of birth (Figure 6). The fourth picture (Figure 7) is a profile of that same person. The pictures are diverse; they derive from vector graphics, pixel graphics or a combination. All the colors are set in the RGB system of colors (only the region visible to our eyes).

Two solutions were conducted for the same group of images. It is shown that the solution is independent of which images are assigned to the visible spectrum, and which to the NIR spectrum.

Design plan no. 1: Figure 5 and Figure 7 will be joined in the visible spectrum. Figure 4 (en face) will be joined in the near-infrared spectrum. After merging and printing, the security graphic becomes visible to the naked eye, and the hidden graphic will be recognized with an NIR (near- infrared) security camera.

Design plan no. 2: the portrait en face (Figure 4) will be joined in the visible spectrum. Three images will be joined to the near-infrared spectrum: the profile (Figure 7), vignette (Figure 5) and micro-text (Figure 6). A separate region is planned for each of these three images. Since there are four different images that will manifest themselves in two ways and in two schedules, the graphic size in pixels of each image was additionally adjusted.

For each image a (geometry) range is specified to be included in the common, nested Z picture. The plan is to print the merged picture with process CMYK dyes.

3.2. A Security Graphic with Three Nested Images

In the first example for the visible spectrum, two graphics were highlighted: the profile of the person and protective vignette. Those two graphics hide the portrait, which is captured by the NIR (Z) camera. The VZ merging and separation for the duality of the visible and near-infrared spectra have been limited according to the conventional printing separation practice–GCR. In Figure 8, gray scale is added. This image will be the final solution for the visible spectrum. Individuality of the graphic solution is introduced through different algorithms of gray tone replacement.

Figure 8 was prepared and printed. After the printing of Figure 8, the visible graphic is checked with an NIR–Z camera and the portrait en face in gray tone appears on the screen of the Z camera (Figure 9).

If, for some reason, Figure 8 does not get published in the article as a CMYK model, we are providing the link of the original (CMYK) Figure 8 on the author’s website: http://www.jana.ziljak.hr/portret82.jpg (accessed on 10 January 2022).

The digital record was created as a four-color (CMYK) print. All images for INFRAREDESIGN^® have a special (their own) procedure, from preparing the images, connecting the computer with the printer, to printing an image with NIR content. If such a record is sent to a digital printer, it is necessary to set the printing parameters. Printing (on the printer from, for example, Photoshop) is performed with the following options:

• P1: No color management (to disable the internal process of image separation);
• P2: In Page setup/Color, select: CMYK link profile—True Black.

The discernment of pictures in four channels, prepared in this way, with CMYKIR–VZ separation, is impossible even with a 24-filter forensic system [9]. This means that only the one who has the original image (C, M, Y, K) can reproduce this CMYKIR image.

3.3. A Security Graphic with Four Nested Images

A security graphic is composed with four merged color graphics. The portrait en face (Figure 4) will be visible to the naked eye, while the remaining three graphics are hidden and can be recognized with an NIR camera. The vignette (Figure 5) is intended for the horizontally lower part of the graphic.

The visible portrait en face (Figure 4—RGB) remains in color, with the goal that the initial portrait is visually the same as the portrait that has three nested images after VZ separation (CMYK). The photograph shows the eye color, hair color and skin tone. The advantage of the picture is the recognizability of the person. Eye color and skin tone are important elements of the portrait en face. Image Z is shown as a black-and-white micro-text, protective vignette and a part of the portrait in profile. After printing, our eyes see only Figure 4 although the print also contains the remaining three graphics.

In this example, it was decided that, after merging, the color of the portrait en face would be visible to the naked eye (in the visible spectrum, 400 nm blockage). After the merging of all four gray channels (Figure 10, Figure 11, Figure 12 and Figure 13) in, for example, Photoshop, a pure color picture equal to the picture in Figure 4 can be seen. The check is performed by inserting the corresponding gray graphic into each of the C, M, Y, K channels (Figure 10, Figure 11, Figure 12 and Figure 13).

In Photoshop (for example), each CMYK channel is written as a shade of gray. Together, they give a color solution for the visible graphic (identical to the picture in Figure 4) that hides the planned Z picture.

The final, hierarchical graphic and print is a simulation of the color portrait (Figure 4), while the other images (Figure 5, Figure 6 and Figure 7) are hidden within that portrait. The portrait, after the four images have been merged, is given in a C, M, Y, K, record.

The IRD security graphic represents the merging of four gray images to be printed with C, M, Y, K (1) colors, Figure 10, Figure 11, Figure 12 and Figure 13. With this kind of graphic prepress, color printing is performed in an unlimited number of prints. The print on the paper is a color graphic that contains two different images: one for observation with the naked eye and one for observation with an NIR camera. It is not possible to use the color imprint on the paper and go back to creating four C, M, Y, K (2) channels that are equal to the channels (1) at the beginning of the paragraph using any existing scanning techniques. The intention of such a process in the opposite direction would be to make counterfeits. New colors (2) will not give the same visual and NIR image. The portrait in Figure 14 and the original merged image are recorded at the following address: http://www.jana.ziljak.hr/portret81.jpg (accessed on 10 January 2022).

3.4. Blockage in the Visible and Near-Infrared Spectrum

The discernment of the merged picture is performed with light blockages (filters in cameras). Blockages at 400, 600, 700 and 850 nanometers are shown (Figure 15, Figure 16, Figure 17 and Figure 18).

Animations of portrait changes were taken with a PAG camera that shoots in 24 light blocks. The display is located at the addresses: http://www.jana.ziljak.hr/portret81.mp4 (accessed on 10 January 2022). http://www.jana.ziljak.hr/portret82.mp4 (accessed on 10 January 2022).

4. Conclusions

Control systems, such as NIR cameras, are all around us: cameras in road traffic, security night cameras, IR money detectors in banks, etc.

Outside the visible color range (400 to 750 nm), the graphic is shown instrumentally. Already after the first light blockage (shown in multilayer light presentation video at 600 nm) in the visible spectrum, images appear partially as “hidden”.

Multiple pictures that are shown in the end make one picture and significantly raise the security of personal portraits and biometric portraits, as well as other pictures that require a high level of protection against forgery. Portrait images prepared in this manner cannot be subject to photo manipulation that is nowadays omnipresent. The image prepared in this way is designed to be implemented in extremely protected documents and identity cards.

The massiveness of NIR cameras in our environment initiated the idea for the launch of IRD design in new areas with materials: textiles, polypropylene, cardboard, postage stamps, new paper money design. A new information area of duality is opening up, such as the expansion of data on clothing, packaging, scenography, and costume design in the film and theater industry. Each color for artistic painting has its Z value. A new look at the information provided to us by fine artists is opening up. InfraRedArt is becoming a new “multilayeredness” in the creative field of multi-media art.