Secure hidden NIR dual image technology obtained on stained cotton materials

Rajendrakumar Anayath1, Ana Agić2, Vilko Žiljak2, Darko Agić3

1The Technological Institute of Textil & Science, Birla Colony, Bhiwani, India
2Sveučilište u Zagrebu, Grafički fakultet
3Tehničko Veleuciliste, Zagreb

Summary

For more than ten years ago CMYKIR separation and dual image were performed on standard printing substrate with standard system inks. Principles that were accepted connoted that first image is human visible and second image is hidden, and only instrumentally visualized. In that way perception area is enlarged, while secondary image is hidden and secured. These assumptions opened a wide range of implementation in various reproduction fields, substrates, inks, and gave a wide opportunity o technologists and designers to implement dual image and hidden image technology to unlimited fields of usage. Dual image is performed on cotton fabric, in basic production colored in green, brown and blue color. For colored materials spectrograms and twin pairs are defined. Prints of dual and hidden images performed as infrared technology on cotton are presented.

Keywords: NIR technology, CMYKIR separation, dual image, twin colors, hidden image

Sigurnosna i sakrivena tehnologija NIR dvojne slike na obojenom pamučnom materijalu

Sažetak

Pred vise od deset godina CMYKIR separacija i dvostruka slika su se primijenile na standardni tiskarski substrat sa standardnim sistemskim bojama. Principi koji se se primijenili ukazuju na primarnu sliku koja je vidljiva, te sekundarnu sliku koja je sakrivena, osigurana i može se vizualizirati samo instrumentalno. Te postavke otvaraju niz implementacija u razna područja reprodukcije, razne substrate, bojila, te daje široko područje djelovanja tehnolozima i dizajnerima da primijene dvojne slike i tehnologiju sakrivene slike u neograničena polja primijene. Dualna slika je izvedena na pamuku koji je u bazičnoj proizvodnji obojan u zelenu, smeđu i plavu boju. Za tako obojane materijale su izvedeni spektrogrami i blizanci boja. U radu se pokazuju otisci sa infrared postupkom dvostrukih, sakrivenih slika.

Ključne riječi: NIR tehnologija, CMYKIR separacija, dvojna slika, blizanci boja, sakrivena slika

1. Introduction

During history there were various attempts and methods of hiding variety of information, such as data, image and similar from unwanted observation. Steganographic methods [1] [2], “invisible” inks, painted out pictures, incorporated details and variety of other methods were used [3]. Some of that “hiding systems” were on purpose, some happened casually, but there is a question who and how could or should be aware of them. To day, very often, we have a point of view as from a forensic standpoint. That is because modern technical/instrumental equipment is relatively procurable and exact, and can tell us a lot about some artifact, matter or material. In case of e.g. pictures, often is a question about age, forgery, materials used, reinstate and similar. That technical acceptance allowed secondary, “hidden” image that is incorporated in NIR domain [4] to make it visible rather with simple means, contemporaneously broadening the human experience from visual to near infra red [5] [6]. Broadening the dual image to fabric, stained cotton, gives a new dimension of CMYKIR technology, while for fabric substrate and used inks/dyes very customized and dedicated profiles have to be made. Also, inks/dyes very often are not system based, so arranged mixtures optical and spectral properties for both areas have to be determinated particularly.

2. Reflection and absorption of inks principles

In graphic reproduction as well as in other visual techniques primary interest, except chemistry of colorants, is visual output of practiced colorants. Common methods for that access are optical/spectrophotometric measurements, while various colorimetric models for color description are used [7]. In most graphic reproduction systems, output devices, basic colorants and substrates are described and defined through worldwide accepted specifications, and are supported within color management modules in reproduction applications environment [8]. There is of course a wide range of so called spot colors that can be applied, and also huge number of all other non classified dyes and inks, that to day are not tested and tried out for NIR purposes. Art painters and other artists very often perform their own “palettes” of colors, what might be tested for visual domain, but mainly not for NIR. System dyes widely used are mostly described through their absorption or reflection behavior on selected, also classified, substrate. It can be generally described that cyan absorbs in red visual area, magenta in green area and yellow in blue area. Black ink [9] [10] [11], often applied as “carbon black” usually used in graphic arts reproduction systems has rather high absorption ability, equable through entire visual spectrum. It is considered that black is achromatic from lowest to highest intensity. Achromatic response is of course possible to achieve with three primary colors, that reflection characteristic is not so equalized as pure black. Black is a kind of intensity modulator, similar to complementary color, making possible a hue, or any hue, to perceive in a variety of shades from light to dark experience. Black channel, as fourth color black ink is applied to graphic reproduction, improving color image reproduction. In variety of other situations, such fabrics coloring, other inks/colors with NIR behavior are used.

2.1. Achromatic reduction

Originating of colors in graphic arts reproduction is described as autotypic (halvtone) system. It implies mixing or overlapping of primary (subtractive system) dyes by means of screening elements in a defined screening system, obtaining possible colors, or gamut, facilitated by a chosen profile. According to some graphic rules, as mentioned above, also accepted in graphic profiles, not only three subtractive inks/dyes are used, already forth black is used. Most graphic programs perform achromatic reduction principles [12]. This opportunity leads to situation that most colors can be performed in a variety of combinations, but gaining the same visual output, what is one of basic presumptions for double image. Most image manipulation apps do not offer reduction rate yield, but general small, medium or high assets. This is practically not easy to apply, while reduction rate is not under strict control. This reduction system is not particularly convenient, while by multicolor images secondary image differs on its density, and depends on rate of the primary image. By single tone/hue images this problem is minimized. Later investigations brought changes to reduction rate system, and CMYKIR separation module can perform a fixed rate for the whole area [13]. This invention or improvement silenced some visual deviations, and improved possibility of defined differentiation of secondary image, whereby visibility and recognition is improved in contrast achievement described through Z state, and possibility of customizing output printing device parameters. [14].

2.3. Color management, extended and advanced NIR area

Color managed systems are widely used in graphic arts, additive systems and all possible situation where color is reproduced. Color management modules are mostly incorporated in apps dealing with color, providing assistance to optimal reproduction in selected conditions. Widely admitted specifications and regulations are set up defining profiles used in given situation of system media, inks, illuminants and other practiced parameters meaningful for the selected output method. Color management, of course, covers visual area of human vision, but secondary, hidden image is imaged in a human no-visible area, owing to inks specification. Guidance of extended (IR) area specification through visual color management appeared to be non acceptable. In visual corresponding colors/inks have to fit as much as possible, overlapping, and performing as close as possible the same colorimetric and visual output. Domain covering 700 to 1000 nm appears to have two interesting parts: 700-800 nm area, and 900-1000 nm, or advanced area. First area, named distinguishing appears to be crucial for differentiation in NIR domain. When performing and mixing necessarily inks combinations spectrometric curves should not overlap or to be close. This is the assumption for the following domain, called Z domain, what is important for visibility of the secondary hidden image, forming an advanced system. Differentiation impact of inks and visualization Z camera [15] within predicted and/or fulfilled Z parameter will be responsible for secondary image quality [16].

3. Color twins

Color twins or twin colors pairs are crucial asset in CMYKIR separation and achieving secondary hidden image. According to standard graphic separation and achromatic reduction principles numerous colors can be materialized-printed in multiple ways, but preserving the same visual output, that is examined in various ways. Color difference in twin pairs in visual domain has to be as low as possible, optical and instrumental.

slika1
Fig.1: twin pair spectrum in V and Z domain for the specified blue hue

Fig. 1 displays blue twin pair spectrogram used for print purposes on HP plotter 5000 on white cotton.. Twins are defined for visual as CMYK coverages: 98-73-49-0, and for IR (Z) domain as CMYK coverages: 82-48-21-40. K 40 coverage is chosen as optimal value for Z twin as performing unifying absorption in visual and appropriate absorption in Z domain. Twin pair CMY and CMYK spectral curves show very tight folding shapes in visual, distinctly resolving in transient area (700-800 nm), and adequate acceptable differentiation in Z domain. Basic coverages in twins combinations C (98 and 82), M (73 and 48) and Y(49 and 21) inks, spectral curves are also ploted. It is evident that coverage differences in inks combinations do not considerably influent on curves shape, meaning they do not interfear finite combination and common visual experience, but distinguish aborbance in Z domain. Knowledge from spectrophotometric measurements and visual appearance must be proofed, for visual and NIR domain. Spectral differentiation for twin dyes in visual and NIR for pure dyes/inks and possible mixtures for selected output, meaning appropriate profile must be determined. For adjusting twins, profiling, various mathematical procedures are developed [17] [18]. Differentiation must be evident in extended and expanded area above 700 nm. This is important if red part of separation system influences the closest part of NIR domain and reflection/absorption curves are not distinguished enough, Z domain and Z difference of hidden and secured image will not be satisfactory [19].

3.1. Z domain

Z domain covers a part on NIR domain from 900 to 1000 nm. This is the most interesting part for placing secondary hidden image. If we observe spectrophotometric curves of IR inks and other no IR effect specified inks, they must be uncoupled. Difference depends on the amount of IR effect ink/dye. That amount is usually defined through coverage value, what is a compromise between reduction rate, or specified NIR ink in a color mixture combination. As stated before, in visual there has to be no differentiation in twins, and in Z domain the assumed value, that will give adequate Z camera reading. Quantity of NIR effect manifesting ink/dye can be also adjusted through dedicated screening system. ∆Z difference is a numerical value of efficacy and recognizing of the NIR double image separation procedure.

4. Resume

A new technique in infrared art with double, hidden and secured graphic as a protection method is broadened to colored cotton fabrics. One image is human visible at standard light conditions, and second image is hidden, and instrumental visible and separated only by means of Z camera. This hiding and secured image idea allows creation of hidden information, graphic, text or similar performing a Z graphic on cotton fabric using digital or silk screen printing, what is very easy adoptable in various fields, including for military purposes. An example is presented on fig. 1 for a blue twin pair, showing practically the same visual output, and differentiation in Z domain. Two stages are possible, one for visual, and other for infrared domain. These stages are independent, but controlled, and can be performed in variety of hues in visual and monochrome information in infrared domain. This approach is a novelty in info description, and innovation in visual broadening [20]. Of course, broadening information on fabrics is just one of a variety of media, while numerous other substrates will be adopted and practiced, what grants new knowledge of materials and inks, and further technology-design-informatics linking. [21] [22] [23] [24] [25]

5. References

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