Despite the introduction of promising new digital security serialization technologies, letterpress numbering machines are still used to serialize most paper documents today.  A letterpress serial number is only a small area of limited artwork complexity, but it plays an important security role distinct from static artwork and security features that vary in selection and form between document types.  A serial number is more than just numerals: it’s also a combination of letterpress print characteristics, one or more security inks, a custom font, and variable artwork applied by an analog printing process.    

This article is the first in a series that explores strategies for optimizing the security value of letterpress serial numbers.  The first three installments of this series will discuss strategies already in common use in security documents, including serial number printing process characteristics in Part 1, color and inks in Part 2 and an introduction to security font design in Part 3. 

All strategies discussed in this series are for informational purposes only.  Issuers must determine what is compatible with their own security priorities, user training goals, design strategy, manufacturing capabilities and/or quality control tolerances.  Although the focus of this series is on letterpress serialization, some of the concepts might be extended to, adapted for, or compared with contemporary digital serialization technologies, which will be addressed in a later part of this series. 

Letterpress print characteristics

Analog and digital printing processes produce distinctive microscopic traits that depend on plate shape, pressure, ink properties, press mechanics, substrate characteristics, and so forth.  Considering a letterpress serial numbering machine like the one in Figure 1, the combination of a relief printing surface, moderate pressure, and viscous ink consistency produces three typical microscopic print characteristics that differentiate letterpress from other printing processes: a halo, voids, and debossing.

Figure 1. A letterpress numbering machine with rotating wheels containing multiple relief printing surfaces.  The raised printing surface and moderate printing pressure produce the distinctive microscopic characteristics of letterpress serial numbers.  Numeral advancement is controlled mechanically in this example, but contemporary numbering machines can be controlled digitally. 

A halo refers to a bead or outline of ink at the edge of a letterpress impression that is produced when ink is squeezed from the edges of the relief plate during printing.  Examples of letterpress serial numbers with prominent haloes are shown in Figure 2 (click to enlarge Figure 2 and all later Figures).

Figure 2. Three letterpress serial numbers featuring prominent halos.  A halo is a bead of ink around the contour of a letterpress impression that is formed when ink squeezes to the edges of the image area as the printing surface is pressed against the substrate.  A halo is typically the most consistent letterpress print characteristic, as voids (see Figure 3) and debossing (see Figure 4) are not always visible.  

Voids are small non-inked regions inside the image area where no ink transfer occurred due to incomplete contact between the relief plate and the substrate surface.  The presence of voids can be highly variable between document types and is affected by the texture of the substrate surface.  Examples of letterpress impressions with exaggerated voids are shown in Figure 3, though letterpress impressions can show few or no voids in some documents. 

Figure 3. Three letterpress serial numbers featuring prominent voids.  Voids are unprinted regions inside the image area resulting from incomplete contact between the relief plate and low areas in the substrate surface.  Voids are common in serial number impressions, but it’s also common for none to be present.  The presence of a halo (see Figure 2) is a more consistent indicator of letterpress.  

Debossing refers to distortion of the substrate from the pressure applied by the relief surface.  Like voids, debossing may be prominent or completely absent in different letterpress serial numbers depending on pressure and other press conditions.  Examples of letterpress serial numbers with a high degree of debossing are shown in Figure 4. 

Figure 4. Three letterpress serial numbers featuring prominent debossing, or indentation resulting from pressure applied to the relief plate, shown in oblique light on the front and back of the substrate.  Serial numbers with prominent debossing are shown for illustration, but many letterpress impressions do not show any debossing.  The presence of a halo (see Figure 2) is a more consistent indicator of letterpress.  

Of these three characteristics, the halo is the most consistent indicator of a letterpress impression. 

Counterfeit detection by printing process

Historically, serial numbers in genuine paper documents have been applied almost exclusively by letterpress numbering machines.  This means bank staff, border control officials, transportation security officers, consular officers and others that work with security documents can expect letterpress print characteristics in serial numbers, even if the document format is unfamiliar, as is often the case in identity and travel contexts where a variety of documents are encountered. 

In contrast, counterfeiters can use printing processes other than letterpress.  Figure 5 shows three counterfeit serial numbers in reflected and oblique light.  Click to enlarge Figure 5 and note the absence of letterpress print characteristics, especially the halo.  The absence of letterpress characteristics in a serial number can be cause for suspicion, even if nothing is known about which other security features should be expected in the document.  This can be an advantage in user training because security features change between document types, but letterpress serialization has historically been constant. 

Figure 5. Portions of counterfeit serial numbers showing microscopic print characteristics inconsistent with letterpress, particularly the absence of a halo.  Because almost all serial numbers in paper documents are letterpress, the absence of letterpress characteristics in a serial number could be cause for suspicion even if the document is completely unfamiliar. Compare to Figure 6. 

Now consider serial number simulation by letterpress, which is an option for counterfeiters that acquire or manufacture numbering machines.  The counterfeit in Figure 6 shows a halo and some debossing, which are letterpress characteristics, so this counterfeit cannot be detected strictly by serial number printing process identification alone.  Figure 6 illustrates why letterpress print characteristics are just one facet of serial number security that must be paired with a security ink and security font design.  Later parts of this series will explore inks and fonts, but for now the focus will turn to printing process optimizations that make simulations like the one in Figure 6 harder just in terms of print characteristics. 

Figure 6. Portions of a counterfeit serial number showing microscopic print characteristics consistent with letterpress, including a visible halo and some debossing.  Unlike the examples in Figure 5, this serial number is challenging to identify as counterfeit based on its printing process alone.  This example illustrates why ink selection and font design are also important elements of serial number security. 

Optimizing the halo

As is often the case with security document artwork, design optimizations that improve security can be particularly attractive if they are low in cost.  Numbering machine impressions are no exception and are especially significant because a letterpress serial number is one of just a few features usually included even in the lowest-value security documents.  Figures 7 through 9 explore design strategies for optimizing letterpress print characteristics in genuine document serial numbers. 

Figure 7. A serial number in which the left two numerals were applied from textured letterpress surfaces and the right two numerals from flat surfaces.  The left two numerals show what might be described as a halo that appears in the interior of the image.  This distinctive appearance is both a printing process characteristic and design platform distinct from the contour of the font itself. 

Click to enlarge Figure 7, in which the left two numerals were applied by textured relief surfaces and the right two numerals by flat, smooth relief surfaces.  The right numerals show a typical letterpress halo only at the edges, but the left numerals also show what might be termed an “internal” halo produced by ink squeezing out not just around the numeral contour but also inside the image area.  This effect will not distract casual users reading the macro serial number, but when magnified it provides an extra level of distinctiveness and makes simulation more challenging even for counterfeiters that do work with letterpress printing.  Voids may be exaggerated by the presence of such a halo inside the image area. 

Relief plate surface texture art should be seen as a separate canvas that can be designed independently of the serial number font itself.  The font and surface texture designs might complement one another or could be totally uncorrelated, with each approach providing a different security function.  Considerable analysis can be devoted to whether the surface pattern of a given numbering machine character should be the same or different from other characters on the same wheel, or on different wheels of the same numbering machine, or related to the character’s position in the string, or algorithmically determined, and so forth, depending on goals for a particular document.  Surface texture design concepts will be addressed later in this series and are shown here just to illustrate a letterpress optimization.

Optimizing voids

While Figure 7 demonstrated a textured relief plate printing on a flat substrate, Figure 8 illustrates a flat relief plate printing on an embossed substrate.  Void placement is erratic in most letterpress impressions because the surface texture of security paper substrates is naturally variable, with high and low areas appearing pseudo-randomly.  In contrast, in Figure 8 the embossing process overwhelms subtle surface variations and replaces them with bold high and low areas that correspond to the embossed artwork.  Some areas of the embossed surface are less likely to receive ink from the letterpress plate, so these become voids and their placement predictably follows the design of the embossed artwork.  The microscopic pattern in Figure 8 has a different origin from the one in Figure 7, and in some ways the security advantages are similar.  However, no letterpress halo is visible in Figure 8. 

Figure 8. A letterpress serial number applied over embossed intaglio artwork containing a nonrepeating pattern of fine parallel lines, in reflected and oblique lighting.  The left images provide context as to the size and placement of the numerals over the embossing.  The right images are magnified and show how the embossed lines control which parts of the substrate surface are inked by the letterpress plate. 

Figure 8 prompts some further technical considerations.  For example, the fine details in a serial number impression printed over an embossing could persist even if the embossing itself were relaxed post-issuance due to aging, wear, exposure to moisture, etc.  This could make durability of the embossing itself less critical if a partial image of its design were captured in ink during serialization.  Further, if registration between the numbering machine and surface embossing were inexact, each genuine document would have its own distinct microscopic serial number impression, since every numbering machine impression would contact a slightly different region of the complete embossing.  At a macro level the font would appear the same, but under magnification each impression would show image variation, which could be desirable for certain forensic reasons.  Substrate embossing could even be overprinted by a relief plate containing a surface texture like the one in Figure 7.  Optimal design of the underlying embossed artwork is an expansive topic on its own and will be addressed later in this series. 

Optimizing debossing

Unlike the halo and voids that characterize Figures 2, 3, 7 and 8, debossing is a pressure distortion of the substrate that is not related to ink density or placement.  From this point of view, one unlikely possibility for optimizing debossing could be to apply the serial number as a dry process, in which the numbering machine distorts the substrate without ink.  Without ink-dependent features like a halo or voids, debossing would be the only print characteristic.  Such a serial number might look like the examples shown in Figure 9, which are actually inked serial numbers on issued documents and captured in oblique light using an infrared filter to show texture without color.  The strategy could be expanded upon with higher pressures or matching male/female front and back dies to improve embossing legibility. 

Figure 9. Serial number impressions with heavy debossing, captured in oblique light with an infrared filter that removes ink color but shows texture.  This simulates how an inkless serial number might look, with exaggerated debossing but no halo or voids.  Colorless debossing cannot be simulated by CMYK, but without a visible hue it would be challenging to see or inspect, limiting its utility. 

In concept, a dry serial number impression could be attractive because it cannot be counterfeited by conventional color printing methods, but there are some clear drawbacks.  Without ink color blind debossing could be hard to read or even locate, so it might only be appropriate as a second serial number intended to supplement a conventional inked serial number.  Additionally, wear, aging, moisture, or other post-issuance exposures might cause the debossing to relax, obscuring serial number legibility.  Possibly, blind letterpress serial number debossing applied over an embossed fine line pattern like the one shown in Figure 8 could flatten the embossing in the serial number image areas, providing improved legibility.  Similarly, the serial number could be applied in invisible ultraviolet (UV) reactive ink, such that in visible light the primary characteristic of the serial number impression would be debossing but in UV a halo or voids might be visible. 

Front to back registration

Simultaneous offset, alternately called see-through registration or front/back registration, is a printing technology that applies images on opposite sides of a substrate such that alignment between the front and back artwork can be examined in transmitted light.  As the name suggests, in contemporary security printing simultaneous offset is associated with static artwork applied by offset printing and not with variable serial numbers applied by letterpress numbering machines.  However, an analogous technology for letterpress serial numbers exists.  Printing static images in front/back register without a specialty press designed for this purpose is already challenging for counterfeiters, but printing letterpress serial numbers in front/back register presents different challenges because serial numbers are dynamic. 

Figure 10. Front and back images of a document containing serial number impressions registered front-to-back, where the back font is a wrong-reading version of the front font.  Today, printing in front/back register is usually associated with static offset art and not variable letterpress serial numbers.  Nonetheless, this document was issued in 1982, so front/back registered serial numbering is not new. 

Figure 10 shows a document containing multiple letterpress serial number impressions on both the front and back, including wrong reading numerals on the back, and with the front and back images in register when viewed in transmitted light.  In Figure 10, all 36 red numerals show the expected letterpress characteristics including a halo and voids.  Design possibilities within the scope of front/back serial number design are not presented in detail here but could be extrapolated from strategies for simultaneous offset[1],[2],[3],[4],[5] to the extent that such strategies could be modified for numbering machine capabilities and applications.


This article discussed printing process characteristics associated with letterpress numbering machines, detection of counterfeits serialized by non-letterpress printing processes, and options for enhancing the security value of letterpress print characteristics.  Beyond letterpress printing technology, ink selection and font design are also cornerstones of serial number security.  Accordingly, Part 2 of this series will explore letterpress serial number inks, including ink properties like translucency, spot color, process color simulations, ultraviolet/infrared responses, multicolor and specialty inks.  Part 3 and later installments will address font design, including novel numbering and other strategies that expand the total character set available across a full serial number string, tie unique character designs to a specific string position to prevent counterfeiters from transposing serial numerals, and facilitate serial number machine readability without compromising visual readability (and vice versa). 

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.


1 Simultaneous Offset: Strategies for Optimization (Part 1)

2 Simultaneous Offset: Strategies for Optimization (Part 2)

3 Simultaneous Offset: Strategies for Optimization (Part 3)

4 Simultaneous Offset: Strategies for Optimization (Part 4)

5 Simultaneous Offset: Strategies for Optimization (Part 5)

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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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