Simultaneous offset printing, also referred to as see-through register or front-to-back register, is a security printing technique in which offset plate images are applied to both sides of a substrate at the same time. This produces a printed security feature (an “icon” for the purposes of this paper series) in which artwork on the front and back of the sheet are aligned to produce a complete design or visual effect in transmitted light. Replicating this effect is challenging for counterfeiters who only have access to commercial printing equipment not designed to achieve front-to-back register.
Part 1 of this five-part series introduced simultaneous offset technology with a focus on mechanical subjects like registration and plate count. In Part 2 we will instead focus on “soft” design topics like inspection ergonomics, placement near other transmitted light features, integration with offset features and discrete visual effects. Exploration of visual effects sets the stage for introduction of new color and contrast strategies in parts three and four of this series. As this is an academic series, not every strategy will be manufacturable by all issuers or appropriate for all security documents.
Ergonomics and Document Users
Simultaneous offset icons are not always intuitive security features, and it is relevant to ask how document users engage them. For example, how do document users know that a simultaneous offset feature is present? What visual cues can be used to make the icon easier to locate or inspect? Are users really looking for alignment of plate images, or a separate visual effect that is a consequence of accurate registration? What are the training implications? How could issuers describe not just one, but multiple different simultaneous offset features in a brochure? The answers might vary for different document types (banknotes vs. passports) or different user populations (lay cashiers vs. expert border control officials, or even image-based machine inspection systems), but soft design enhancements can be an important vehicle whenever security can be improved without higher costs, increased supply chain complexity or new manufacturing processes.
To begin, one potential problem with simultaneous offset features is that document users may not recognize them. Icon graphics can signal the presence of a simultaneous offset feature. Following the graphics conventions shown in Figure 1, the unprinted areas within the art make the reflected light images in Figures 2 through 7 appear conspicuously incomplete. Consider how document users might perceive these blank spots and how they draw attention to the icon. The missing elements can be non-text artwork, as in Figures 2 through 4. For non-text icons, good examples include reflected light designs that anyone could recognize as incomplete (Figure 2) or which include specific but incomplete national images familiar to a document’s user population, such as a flag (Figure 3) or border (Figure 4). Significantly, Figures 3 and 4 show banknotes intended for domestic user populations that would recognize this imagery. A similar strategy might not work equally in a passport that is inspected primarily by international officials of diverse nationalities.
Figure 1. Model of the figure structure used for all later images in this paper. The left image shows the front of the document in reflected light, the right image shows the back of the document in reflected light, and the center image shows both together in transmitted light (as viewed from the front).
Figure 2. Simultaneous offset icon designed to alert a viewer to its presence. In reflected light, both the front and back images contain artwork that is simultaneously a coherent design but also recognizable as incomplete due to the unprinted spaces within the art. Icon designs that feature conspicuous unprinted space within a reflected light design can help alert or remind document users of the location of a simultaneous offset feature.
Figure 3. In transmitted light, the design of this flag is a familiar symbol to the user population of this banknote. The partial designs on the front and back appear conspicuously incomplete because users of this banknote know the flag should contain two blue stripes, instead of just one shown in reflected light. Selection of artwork recognizable to a document’s user population can help to inform or remind users where to find a simultaneous offset feature and encourage inspection.
Figure 4. The icon outline is the contour of the issuing country’s border and would be a shape familiar to users of this banknote. Like Figures 2 and 3, it contains areas of unprinted paper that make it appear incomplete. However, in transmitted light this icon also forms readable text from front and back reflected light artwork in which no text is legible because the individual character images are partitioned between the front and back.
Figure 5. Like the example in Figure 4, this four-plate simultaneous offset feature produces legible text in transmitted light but in reflected light the images are arbitrary shapes without legible text. In transmitted light the appearance of complete characters helps users determine whether the plate images are all correctly registered, but in reflected light the shapes are not recognizable and users may not understand that readable text can be seen in transmitted light.
Figure 6. Simultaneous offset icon showing complete characters and readable text strings. The full text is partitioned into two complete strings of complete characters, each of which is readable in reflected light (though wrong reading on the back), unlike the examples in Figures 4 and 5 in which individual characters were illegible in reflected light because they were divided between the front and back.
Figure 7. Mockup of a four-plate simultaneous offset text icon. Individual characters are complete and recognizable in reflected light, which signals to document users that legible text should be present and makes them question why it is not. Transmitted light is required to read the text, forcing inspection of registration in the simultaneous offset feature. A design like this, including complete characters but no complete character strings in reflected light, may be a good way to encourage inspection of text icons.
Incomplete icon artwork can also include text, which is different because no familiarity with specific national imagery is needed to interpret text. Text icons can be categorized as to how the text is divided between the front and back. In revisiting Figure 4 and introducing Figure 5, both examples of individual text characters are unrecognizable in reflected light and complete characters only become visible in transmitted light. Prior work advocated for such division of parts of individual characters across multiple plates in microprinting designs, but this may not be the best strategy for simultaneous offset icons if the reflected light art would not necessarily be recognizable as incomplete text. For simultaneous offset, a potentially better approach would visually signal text is present in reflected light (i.e., complete characters) but only allow the complete text string to be read in transmitted light, as shown in Figure 6. The mockup in Figure 7 illustrates a novel approach in which the intact characters ensure the presence of text is conveyed to users while the alternation of characters between front and back prevents legibility in reflected light. This both draws attention to the incompleteness of the icon and forces inspection in transmitted light.
Placement Near Other Transmitted Light Features
Another ergonomic strategy is placing similar security features together so they draw attention to one another and can be inspected at the same time. Simultaneous offset icons can be placed near other transmitted light security features like watermarks, perforations, and security threads. Figures 8 and 9 show some examples of isolated icons placed near a watermark (or over it, which conserves space).
In Figures 8 and 9, the watermark is prominent while the icon is tiny, though this is not a requirement. Future work will explore full-document simultaneous offset designs that challenge this convention.
Figure 8. Single-plate simultaneous offset icon placed in proximity to a watermark to facilitate simultaneous inspection of both features in transmitted light. Like Figures 4, 5, and 7, the text is illegible in reflected light and only becomes readable in transmitted light. The watermark is huge compared to the single tiny simultaneous offset icon. Alternative design strategies could include more icons or larger icons in a pattern covering more of the substrate.
Figure 9. Multiplate simultaneous offset feature placed in proximity to a watermark to facilitate simultaneous inspection of both features in transmitted light. As in Figure 8, the watermark is much larger than the icon, but more or larger icons could be included in a pattern covering more of the substrate.
Similarly, Figure 10 shows a perforation feature in a banknote overlapping a simultaneous offset icon. The icon in Figure 11 is not only intersected with a windowed security thread, but also incorporates a silver metallic ink to deter CMYK-only simulations. Iridescent ink is usually inspected by tilting in reflected light, but the iridescent ink border in Figure 12 also blocks transmitted light, so in transmitted light both features can be checked together. In Figure 13, simultaneous offset is used for the tabbed page numbering in a passport. The page numbers are only visible in transmitted light, which encourages inspection of the simultaneous offset icon and draws attention to the security thread it intersects.
Figure 10. Simultaneous offset feature placed in proximity to a perforation pattern to facilitate simultaneous inspection of both features in transmitted light. This example is a banknote, but simultaneous offset features could also be placed near laser-perforated serial numbers in passports.
Figure 11. Simultaneous offset feature placed in proximity to a security thread to facilitate simultaneous inspection of both features in transmitted light. This example is of a windowed security thread, but simultaneous offset features could also be placed near fully embedded security threads. The back artwork includes silver metallic ink, which contributes high opacity to the icon and the metallic specular reflection defies easy digital counterfeiting by CMYK, so the icon itself is also a hybrid security feature.
Figure 12. Simultaneous offset feature surrounded by two iridescent ink borders on the front. The most common use of iridescent ink is to contribute shininess in reflected light, and in this example the iridescent ink is not directly involved in the icon. Despite its low color contrast with the substrate in reflected light, in transmitted light this iridescent ink contributes significant opacity that allows it to be inspected concurrently with the icon it surrounds. The text only becomes legible in transmitted light.
Figure 13. Simultaneous offset feature incorporated into tabbed page numbering in a passport. The page numbers cannot be read whole unless viewed in transmitted light. One side of each page number art must be wrong reading in reflected light to ensure that the complete page number is right reading in transmitted light. This example also shows an embedded security thread intersecting the page number graphics, which is another instance of two transmitted light features being placed in the same location.
Integration With Other Offset Features
Figures 8 through 13 illustrated simultaneous offset icons located near other transmitted light security features. In contrast, Figures 14 through 17 show simultaneous offset integrated with other features applied by offset printing, including some that are not normally inspected in transmitted light. Although some also improve inspection ergonomics, the main relevance of Figures 14 through 17 is about constructing composite offset features that are harder to simulate than their individual components. This can be done by designing artwork that is technically unprintable without access to specific capabilities of security offset presses, including high resolution, split fountains, and compatibility with nonstandard inks. Simulation of such designs cannot be stopped completely, but counterfeiter workflows can be made difficult and counterfeit quality can be suppressed.
Figure 14. Simultaneous offset feature incorporating microprinting on the front and back, which can contribute additional resistance to certain methods of icon simulation. In transmitted light, microprinting on the same side of the sheet as the viewer can be inspected normally, but microprinting on the opposite side of the sheet loses detail and legibility. This icon also includes both complementary and overlapping art, producing two distinct saturations of blue and orange in transmitted light.
Figure 15. Simultaneous offset feature incorporating silver metallic ink on the front and gold metallic ink on the back. In addition to the specular reflectance properties metallics offer in reflected light, metallics with high opacities can make ideal components in high-contrast simultaneous offset features. This example shows front/back registration of metallics and same-side registration of metallics with spot red, but of course incorporating metallics of different colors in same-side register is also an option.
Figure 16. Simultaneous offset feature incorporating split fountains on both the front and back. Attacking this configuration requires not only simulation of the split fountain color transition, but also a printing technology capable of high front/back register.
Figure 17. Simultaneous offset feature incorporating a split fountain between a metallic silver ink and a brown spot color ink in the contour of the bird on the front. The metallic ink contributes specular reflection that cannot be simulated by CMYK alone. The split fountain creates a smooth transition from metallic silver to brown that many counterfeiters would be forced to simulate in two steps. Integration with a simultaneous offset icon requires registration between the front and back images.
To begin, Figure 14 shows a simultaneous offset icon that incorporates microprinting on both sides, in same-side and opposite-side register with other artwork. With magnification and reflected light the microprinting can be inspected in the normal way. When integrated into a simultaneous offset icon, microprinting also complicates methods of simulation that are not capable of both high resolution and accurate registration. As outlined in part one of this series, digital counterfeiters can achieve good same-side spot color registration because CMYK can simulate multiple spot colors in one printing step. However, microprinting can increase the resistance of a simultaneous offset icon to digital simulation because many consumer inkjet devices have difficulty simulating fine microprinting details.
Similarly, the icon in Figure 15 includes a metallic silver ink on the front and a metallic gold ink on the back, both of which are registered to red spot inks. The red spot color could be simulated by CMYK, but the metallics require the counterfeiter to use a separate process to simulate the specular reflection and then register that printing step to the CMYK print. This must be done not only for same-side print, but also front-to-back for both the red and metallics, which increases labor and introduces new registration challenges that are not required for spot color artwork that can be simulated in one step in an all-CMYK digital workflow. An added ergonomic benefit is that metallic inks with high opacity can add contrast, making the icon graphics easy to inspect in transmitted light.
Figure 16 shows a simultaneous offset icon incorporating split fountains on both the front and back. Prior work explored new methods of integrating split fountain printing in novel monochromatic design strategies 1,2,3,4,5, some of which could be adapted to include simultaneous offset art. However, split fountain spot-to-spot color transitions like those in Figure 16 are not very resistant to digital counterfeiting because CMYK can simulate them in one printing step. Just as in Figure 15, this can be addressed by integrating one or more inks that are hard to simulate by CMYK. The example in Figure 17 combines metallic ink within both a split fountain and a simultaneous offset icon. In addition to the extra printing step and registration challenge that the metallic ink forces on digital counterfeiters even without the split fountain (revisit Figure 15), the gradual split fountain transition from metallic to spot brown must also be simulated and registered to other art, further increasing difficulty. Again, the complexity of simulating these features in combination is greater than any one in isolation.
The examples in Figures 15 and 17 included metallic ink, but also consider the potential of split fountain simultaneous offset icons with iridescent, color shifting, matte spot, glossy spot, white or clear inks, to the extent that these ink types are compatible with offset application. A more complete discussion of this topic will be explored later in this series.
Perceptual and Visual Effects
Another aspect of ergonomics is the design of simultaneous offset to produce specific visual transitions when viewing conditions are changed from reflected light to transmitted light. Figures 18 through 25 illustrate some visual effects, which for the purposes of this paper are grouped into four categories: image completion, image disappearance (or contrast reduction), contrast increase, and color change.
Figure 18. Simultaneous offset icon containing only complementary artwork, where a complete design is partitioned into geometric art between the front and back images, without overlap. The numbers are illegible in reflected light on either side and the complete design can only be seen in transmitted light. Poor registration would result in fracturing, rather than doubling, of the text in transmitted light.
Figure 19. Simultaneous offset icon containing only complementary artwork. The text is legible on both the front and back in reflected light, but when viewed in transmitted light the text disappears. This visual effect is the opposite of the more common icon design approaches illustrated in most other examples used in this paper, which cause the complete artwork to appear instead of disappearing in transmitted light.
Figure 20. Simultaneous offset icon incorporating a split fountain in both the front and back images. Because the front and back feature complementary artwork and because the split fountain transitions incorporate similar colors but in opposite directions, this is another example of a design that is visible in reflected light but disappears in transmitted light. Compare to the disappearing image with no split fountain shown in Figure 18.
Figure 21. Simultaneous offset icon featuring identical front and back artwork inside and including the boundary line around the icon. Outside the boundary line the front and back artwork are different. In transmitted light, the interior images and boundary line reinforce to produce a darker image of higher saturation. The different front and back images outside the boundary compete in transmitted light for a blurry, indistinct blend. With poor registration the icon would appear to double in transmitted light.
Figure 22. Simultaneous offset icon featuring (mostly) identical front/back split fountain artwork within the interior of the dolphin, but different front/back artwork outside the dolphin. In transmitted light, the dolphin images reinforce to produce a darker split fountain image of higher saturation. Outside the dolphin the front and back images compete with one another in transmitted light to produce a blurry, indistinct blend of reduced saturation.
Figure 23. Simultaneous offset pattern incorporating a contrast change. In reflected light the darkest parts of the front image are the fine diagonal line patterns, and the thick border lines are the lightest. Viewed in transmitted light from the front, the diagonal line patterns lighten and the borderlines darken. This contrast change is enhanced by the unprinted white space adjacent to the artwork. Consider what the transition from reflected to transmitted would look like if viewed from the back.
Figure 24. Simultaneous offset icon incorporating both complementary artwork and overlapping artwork. Viewing the complete image of the elephant requires complementary artwork from both the reflected light front and back images, which are fully differentiated and do not overlap. In contrast, the circle surrounding the elephant is a part of both the front and back designs, so the circle darkens in transmitted light when the two images overlap. The gray on the front is a silver metallic.
Figure 25. Simultaneous offset icon featuring both complementary and overlapping artwork and incorporating a color change. The center design includes red and green on the front and red and blue on the back, resulting in a brown/purple effect from overlapped art in transmitted light. In user training, this could be described as a transmitted light color-changing feature. The outline of the little circle is overlapping art, but the interior is complementary art, so the circle interior does not change color.
First, consider how the artwork is partitioned in the simple icon in Figure 18. This artwork might be referred to as “complementary” since the front and back reflected light images show no overlap and were created by dividing a single complete image into front and back elements. The corresponding visual effect might be described as “image completion” because neither reflected light image is meaningful and the text is only legible in transmitted light. This icon is still a registration feature and counterfeits could be detected if the plates were misaligned, but it could also be described in terms of readable text appearing from unintelligible reflected light artwork, with consequences for user training.
Complementary artwork can also make images disappear. In Figure 19, the front and back components of the text are each readable in reflected light (unlike Figure 18) but are essentially negatives of one another. In transmitted light, the density of the text interior is approximately the same as the density of the patterns of lines surrounding the icon, causing image disappearance (or, alternately, a reduction in contrast). Consider how this icon would appear if out of register in a counterfeit and how this visual effect could be described in user training. Figure 20 illustrates a similar example of image disappearance, but with the integration of a split fountain.
While Figures 18 through 21 feature a single complete image partitioned between the front and back, the icon in Figure 21 has the same complete artwork on both sides. This produces two consequences. First, in reflected light neither side seems incomplete and the icon may not attract attention in the same way as other designs that appear unfinished. Second, instead of image completion or disappearance, in transmitted light the effect is of increasing saturation as the front and back layers overlap and opacity increases. Outside the icon the front and back art are different, so in transmitted light the images compete, reducing clarity. Consider how misregistration of the front and back of Figure 21 could result in a doubling of the icon image in transmitted light, a different effect than would be expected for the complementary design in Figure 18, in which poor registration would fracture the image. Figure 22 shows another example of a saturation-increasing icon with complete art on both sides, but with a split fountain. Figure 23 illustrates another contrast example; consider how regions of high and low contrast change between reflected and transmitted light and how the art and ink opacities facilitate the effect.
Icons can be also designed with both complementary and identical art, such as the icon in Figure 24. The elephant design inside the circle is complementary art and the complete image is only visible in transmitted light. The circular border is part of the art on both the front and back, so contrast in the border increases as the images overlap in transmitted light. Front/back misregistration of this icon might result in a breakup of the elephant image, or a doubling of the circular border, or both, depending on exactly which plate images were out of register. Like Figures 15 and 17, metallic ink is included.
Finally, simultaneous offset can even be used to create color changing features like the one shown in Figure 25. The front of Figure 25 is red and green and the back red and blue in reflected light, but in transmitted light red, green and blue become brown and purple. The small circle in the corner is different, with complementary red and blue art in the interior of the circle but the border art around the circle is identical between the front and back designs. Figure 25 is another example of an icon that incorporates several strategies and effects in a single design.
This paper has illustrated various strategies for optimizing simultaneous offset icons relevant to ergonomics, placement, security feature integration and perceptual effects. These strategies were discussed largely in isolation because simultaneous offset features are typically included as a single tiny, localized icon. However, because simultaneous offset is primarily a design feature, multiple icons with different visual effects, sizes, plate counts, feature integrations and so forth might be added without incurring large increases in cost. This suggests that simultaneous offset may be an underutilized security feature which, if prioritized differently, could instead be a highly flexible foundational technology capable of integrating multiple visual effects throughout security offset artwork. Accordingly, parts three and four of this series will explore novel color and contrast applications and the potential for full-document simultaneous offset artwork will be considered in Part 5 of this series.
Disclaimer: This document represents the opinions of its authors and not necessarily the opinions of the U.S. government. The technologies and strategies described may not be available, appropriate or manufacturable for all document issuers. The examples shown do not imply anything about the quality of a document, its designer, its manufacturer, or the issuing authority. For informational purposes only.