The first part of this series discussed microscopic characteristics of serial numbers applied by letterpress numbering machines, detection of counterfeit serial numbers by printing process examination, and strategies for enhancing the value of letterpress print characteristics.  Print characteristics have historically provided an important counterfeit detection function but are only one of the variables that influence serial number security against counterfeiting and alteration.  This second article focuses on serial number ink formulations that facilitate human and/or machine inspection, including visible color, ultraviolet (UV) and infrared (IR) responses, multicolor serialization, and metallic inks.

All strategies discussed in this series are for informational purposes only.  Document manufacturers and issuers must decide which approaches, if any, are appropriate for the needs of a specific document or are compatible with their own manufacturing workflows and quality control practices.  Figure images have been qualitatively optimized to make visual characteristics easier to see and explain.  Accordingly, brightness, contrast and other aspects of the figure images should not be regarded as objective.  Although the focus of this article is on letterpress serialization, some concepts might be adapted for digital serialization technologies and will be addressed in greater detail later in this series. 


A letterpress ink may be described not only by hue but also by opacity, which is a product of characteristics like pigment content and ink layer thickness.  Serial number opacity can be important when serial numbers are applied over background artwork to improve resistance to alteration, and further requires counterfeiters to mimic translucency and not just color. 

Click to enlarge the relatively opaque serial numbers in Figure 1 and the more translucent serial numbers in Figure 2, and their contrast with the background art.  As a group the opaque numbers in Figure 1 are marginally easier to read than the translucent numbers in Figure 2, suggesting higher opacity could present ergonomic and/or accessibility advantages for document users.  However, continuity of the background artwork is easier to see through the serial number impressions in Figure 2, which may help detection of alterations. 

Figure 1. Examples of relatively opaque letterpress serial number inks.  Serial numbers are often applied over offset artwork because serial number alteration may damage the continuity of the background artwork.  Higher-opacity letterpress inks are easier to see but complete obscuration of the underlying art may not be an advantage.  Compare to translucent letterpress examples in Figure 2.

Figure 2. Examples of relatively translucent letterpress serial number inks.  The contrast of numerals against underlying art is lower than the examples in Figure 1, which can make these somewhat harder to read.  However, the background artwork can be inspected through each serial number, which could help reveal alterations more effectively.  Compare to the opaque letterpress examples in Figure 1.

An ideal serial number would be both clearly legible and maximally resistant to alteration, and optimizing for both priorities could be done in various ways.  For example, for banknotes and other documents with two serial numbers, one serial number of high ink opacity could be applied over blank substrate for high contrast and legibility and a second translucent serial number could be applied over high saturation background art for improved alteration resistance.  These serial numbers play different roles but could still be compared with one another to reveal alterations.  For documents with only one translucent serial number, a bold font with a large image area and more substrate coverage per numeral could simultaneously make it easier to see underlying background artwork through the serial number impression while also improving the legibility.  Further, a highly translucent serial number could include UV-reactive components to make it easy to inspect (either by human or machine) in circumstances where UV light is available. 

The background artwork designs in Figures 1 and 2 are also important to serial number security.  Which patterns are continuous lines, and which are disconnected graphics?  Is the serial number of higher saturation than the background artwork, or the reverse, and how does this affect legibility of the macro serial number impression as opposed to continuity of the fine microscopic background lines?  Which contain microprinting, split fountains, or multiple color plate impressions?  Which contain nonrepeating artwork patterns that counterfeiters cannot copy and paste?  Which would be most challenging for a counterfeiter to attempt to restore once damaged in an alteration?  Full discussion of these questions is not undertaken here, but all impact serial number alteration resistance. 


A detailed description of ink formulation and the many possible ink spectral responses is out of scope here, but beyond visible color the most important spectral ranges for security documents include UV and IR.  UV fluorescence occurs when an ink containing a fluorescent component(s) is illuminated by 365 nanometer (nm), 254 nm, or other UV wavelengths, with each discrete wavelength of UV producing a specific visible color response.  Inks may either absorb or transmit various wavelengths of IR illumination to allow for grayscale camera images in which IR-transmitting components disappear and IR-absorbing components darken.  This effect is not fluorescence and does not produce a visible response that can be seen without a camera sensitive to IR.  Accordingly, while UV fluorescence is common in both human visual and machine inspection contexts, IR absorption is more typical in machine inspection. 

To illustrate, in visible light the passport endsheet in Figure 3 shows a typical combination of traditional printing process impressions common to paper security documents, including fine offset background artwork overlaid with high-saturation multicolor intaglio and a letterpress serial number.  The inks in the visible offset art have no UV response, but the lighthouse was printed in two invisible UV-reactive inks: one that fluoresces yellow in 365 nm UV and green in 254 nm UV, and one that fluoresces orange in 365 nm UV with almost no response to 254 nm UV.  Captured through a 980 nanometer longpass IR filter, the brown intaglio transmits IR and disappears, while the blue intaglio absorbs and turns dark.  The serial number includes green fluorescence in both 365 nm and 254 nm UV, and absorbs IR.  In a counterfeit, failure to simulate not just visible color but also the complex UV and IR responses risks detection. 

Figure 3. The front endsheet of a passport, including static multicolor offset and intaglio designs and a variable letterpress serial number.  The reflected light, 365 nm UV, 254 nm UV, and IR filter images illustrate how inks applied by these processes produce specific color and/or contrast effects depending on viewing conditions.  These can be intended for human inspection, machine inspection or both. 

Serial number inks can encompass similar diversity to the static artwork inks shown in Figure 3.  While Figure 3 shows a larger security document for context, Figure 4 shows only letterpress serial numbers.  In Figure 4 consider which serial numbers do not fluoresce in UV, which fluoresce more in 365 nm UV than in 254 or the reverse, which absorb IR and which do not, and so on. 

Figure 4. These serial numbers from various security documents show some ways inks feature a) UV fluorescence of the same hue or a different hue than the visible ink color (or no UV response at all), b) a brighter UV response in 365 nm UV than in 254 nm (or the opposite), and c) IR opacity vs. IR transparency.  Many counterfeiters will have difficulty mimicking such ink properties.

Some security documents are only serialized once, but most banknotes have two serial numbers and different inks can be used for each.  Adjacent rows in Figure 5 show serial number pairs in banknotes.  For each pair the font is the same, but the ink properties are different.  Part 3 of this series, which covers font design, will include comparison of serial number pairs with the same ink but different font artwork. 

Figure 5. Letterpress serial number pairs in four banknotes.  In these examples the same font is used for both serial numbers, but the inks feature different visible color(s) and different UV and IR responses.  The additional serial number becomes a carrier for a second ink formulation, increasing the resistance of the banknote to both counterfeiting and alteration attacks.  Compare to Figure 4. 

Finally, the serial number at the bottom of the passport endsheet in Figure 6 was applied in visible black ink exhibiting only a faint UV fluorescence, but the second serial number at the top of the page was applied in invisible UV-reactive ink that may be overlooked by counterfeiters.  If only the visible number is altered, the two numbers can be compared to one another to reveal a mismatch.  If a counterfeiter does attempt to alter both numbers, the extra work provides more opportunity for errors or damage. 

Figure 6. The visible serial number located at the bottom of this passport endsheet responds more brightly in 254 nm UV than it does in 365 nm UV.  A second serial number located at the top of the endsheet is invisible in reflected light but glows brightly in both 365 nm UV and 254 nm UV.  Besides the anti-counterfeiting functions, the two numbers can be compared to detect alterations. 


The serial number inks illustrated in Figures 1-6 show spectral responses spanning visible color, UV, and IR.  These improve security since simulating only the macro visible hue of a serial number ink, but not its microscopic color composition or its UV/IR spectral responses, makes detection more likely. 

For example, Figure 7 shows a genuine serial number and its simulation by cyan, magenta, yellow and black (CMYK) toner and Figure 8 shows a genuine serial number and its simulation by CMYK inkjet.  In both cases the counterfeiter closely mimicked the font, but both counterfeits are CMYK process color simulations of the true spot color, and in UV neither counterfeit responds with the expected fluorescence.  For these examples IR is less useful than UV, but this is not always the case.

Figure 7. A true letterpress serial number on a genuine document (top row) and the CMYK toner serial number on a counterfeit of the same type (bottom row).  The genuine ink is a solid spot color in reflected light and responds to UV.  Under magnification the process color counterfeit shows misregistration between yellow and magenta, and no fluorescence in either 365 nm or 254 nm UV. 

Figures 7 and 8 also illustrate why a translucent additive primary (RGB) or other non-CMYK spot color might be an ideal choice for genuine serial numbers.  Process color simulations of spot colors are easier to see when extensive color mixing is required (e.g. simulations of RGB) and CMYK dots can be seen more easily in areas of low saturation (e.g. higher translucency), suggesting that visible serial number color and translucency choices do matter and can be optimized in serial number color selection.  Conversely, black is a native process color in inkjet and toner devices, so counterfeiting black serial numbers using black inkjet or toner does not force the printer to combine multiple colors. 

Figure 8. A true letterpress serial number on a genuine document (top row) and the CMYK inkjet serial number on a counterfeit of the same type (bottom row).  The genuine ink is a solid spot color in reflected light and responds to UV.  Under magnification the process color counterfeit shows colored dots, and no fluorescence in either 365 nm or 254 nm UV.   

The counterfeits in Figures 7 and 8 could also be detected by printing process characteristics as described in Part 1 of this series.  Neither method is better in all cases, but printing process and ink optimizations (plus font design) are each important and complementary parts of a secure serial number. 

Figure 9. A serial number containing two altered numerals, which the counterfeiter applied by letterpress with an ink that fluoresces in UV, similar to the genuine serial number ink.  Although the alteration can be detected by microscopic examination, the altered numerals also stand out in UV because they do not fluoresce with the same intensity as the genuine numerals. 

Spectral characteristics also help reveal serial number alterations.  Figure 9 shows an altered serial number containing two counterfeit numerals.  Although the counterfeiter closely matched the visible spot color and even applied the substituted numerals by letterpress, the UV fluorescence of the counterfeit serial numbers is different than the adjacent genuine numbers.  Figure 9 shows completely substituted numerals, but alterations within a single numeral could be detected similarly. 


The typical security document serial numbers in Figures 1, 2, 4 and 5 each contain only one spot color ink with uniform visible, UV and IR characteristics throughout all numerals.  However, innovative serial numbers with more complex ink characteristics can improve resistance to counterfeiting and alteration. 

For example, Figure 10 shows serial numbers containing two ink formulations that differ not only in visible color but also in UV and IR characteristics.  These depend on the mechanical capabilities of the numbering machine and whether different wheels or parts of numerals can be selectively inked. 

Figure 10. Letterpress serial numbers containing two ink colors: segregated between the left and right numerals (top row), alternating numerals (middle row) or across each numeral (bottom row).  Here all the black inks are IR opaque and only the red inks glow in UV in the bottom two examples; these are typical formats but not absolute rules.  Compare to Figures 4 and 5. 

In security printing split fountains are usually associated with offset printing, but split fountains can also be used with letterpress.  The serial numbers shown in Figure 11 include both visible color transitions and UV-reactive split fountain color transitions.  Like selective placement of two inks in Figure 10, applying split fountain serial numbers requires specific hardware capabilities in a numbering machine.

Figure 11. Split fountain color transitions in security printing are usually applied by offset, but some letterpress serial numbers also feature a rainbow effect.  These include a visible split without UV (top row), both visible and UV splits (middle row) and a UV-only split (bottom row). 

Figures 12 and 13 show what is often termed bleeding ink or penetrating ink.  Bleeding inks contain chemical components that migrate into the substrate from the letterpress impression to impede removal or alteration.  Bleeding ink components may be visible as in Figure 12 or UV-reactive as in Figure 13, and the UV response of the serial number impression may be different than that of the bleeding components.  Unlike the multicolor effects in Figures 10 and 11 that require special press hardware to control the placement or blending of two inks, Figures 12 and 13 each include only one ink. 

Figure 12. Letterpress serial numbers applied in visible bleeding ink.  After printing, some components of the ink migrate into the substrate and can be seen around the serial number impression, or on the back of the substrate.  Bleeding inks are also an anti-counterfeiting technology, but their main purpose is to improve alteration resistance by increasing serial number permanence.  Compare to Figure 13. 

Figure 13. Letterpress serial numbers applied with UV-reactive bleeding ink.  The UV-reactive components of bleeding ink may include the numeral impression or the bleeding components, or both, as shown in the bottom row.  The middle and bottom rows show examples in which the UV response was customized to be different in 365 nm UV and 254 nm UV.  Compare to Figure 12. 

Finally, Figure 14 shows a metallic serial number.  Serial numbers are variable data, so counterfeiters want a serial numbering workflow that is both 1) capable of emulating metallic specular reflection and 2) digital or otherwise changeable so numerals can be easily varied between counterfeits.  Options exist, but none are as simple as simulating spot color serial numbers by CMYK, which is why metallics can contribute security value even though metallic consumables are nonproprietary materials. 

Figure 14. A letterpress serial number applied with a metallic effect: flat (left image) and tilted to show specular reflection (right image).  Although digital counterfeiters can capture and manipulate letterpress font art, process color printers cannot simulate metallics.  Digital counterfeiters must adopt more complex workflows to simulate changeable serial numbers and metallic effects in the same art. 

Although Figures 10 through 14 show various serial number innovations independently, these strategies might be joined with one another in new ways to produce combination features.  For example, a split fountain might blend two inks that differ not only in visible and/or UV color, but also in the bleeding component colors to produce parallel but different color transitions in the serial number impression and surrounding bleed.  Other permutations are conceivable. 


This article explored some strategies for formulation of serial number inks to achieve better resistance to digital CMYK counterfeiting, traditional spot color counterfeiting, and alteration.  Just as a serial number is more than the letterpress print optimizations explored in Part 1 of this series, a serial number is also more than the hue and spectral responses of its ink(s).  A third critical variable for serial number security, font design, complements printing process and ink characteristics.  Part 3 of this series will explore contemporary serial number font design strategies, including how novel numbering can produce an expanded numeral character set and help serial number graphics resist transposition between string positions.  After Part 3, later work will explore strategies for producing an expanded numeral set, ensuring counterfeiter transposition of numeral graphics leaves visual indicators, and creating algorithmic serial number graphics that facilitate both machine readability and human readability.

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.


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Joel Zlotnick is employed by the US Department of State, Bureau of Consular Affairs, Counterfeit Deterrence Laboratory as a physical scientist. His current work involves research in security artwork and design techniques in security printing. He is an instructor on counterfeit detection at the US Department of State Foreign Service Institute.

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Tyra McConnell is a Forensic Document Examiner at the US Department of State, Bureau of Consular Affairs, Counterfeit Deterrence Laboratory. She provides training on security documents and develops presentations and e-learning courses regarding counterfeit detection.

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