In Part 1, we focused on public feature used to protect banknotes from counterfeiting. In this blog, we turn to hidden security features used in banknotes.  These features are often called ‘teller assist’ features or semi-overt. These are the design features or special inks used by cashiers or other people who have a little training or some device to help them see something. I am not talking about the super-secret features that are known only to government agents such as the US Secret Service or Interpol. These hidden features are disclosed by a central bank because they want industry experts to know about them. The hidden features include microprint, watermarks, UV fluorescent inks, and embedded threads.


Banknote printers will use very small type size to write messages that can only be seen with a magnifying glass or under other magnification. This is useful because many printing techniques used by counterfeiters cannot print such small letters. It is also a good feature because sometimes counterfeiters can be lazy and not even notice the small letters and just think it is a line. On the US100, there is written THE UNITED STATES OF AMERICA on Ben Franklin’s Jacket and also in the gold ink of the quill. Sometimes designers will even intentionally spell a work incorrectly in the microprint, qualifying this kind of “error” as a hidden security feature.


Watermark technology is a robust security feature that’s difficult to replicate because it is inside the paper. You can see them when you look at a banknote with light behind the note. This is how a cashier might look at your high denomination note.

Watermarks are created by using screens during the paper marking process to reduce the amount of paper fibers in the watermark image. It is most common to replicate a banknote portrait in the watermark.  On the US$100, the watermark is Ben Franklin’s head.  Watermarks on most US currency are not very detailed or clear. This is because the US uses a longer fiber to give high durability. But the long fiber and manufacturing process does not allow for detailed or multi-tonal watermarks used in most of the world’s banknotes. The US$5 watermark of Abraham Lincoln was so poor and easy to fake with a crude pencil drawing, that the US replaced Ole Abe with a large 5 watermark.  This change significantly reduced some types of counterfeits. Polymer banknotes don’t have watermarks.  Instead, plastic banknotes use windows and images printed (usually in white) in the windows.

United States 5 Dollars | 2013 | P-539
Source: Banknote World

UV Fluorescent inks

Many central banks will use invisible inks in their designs that can only be revealed when the banknote is exposed to ultraviolet light at a very specific wavelength. This hidden security feature is sometimes called a blacklight feature and can be seen when using a Banknote World UV Detector.

UV Venezuela 100,000 Bolivar Fuerte | 2017 | P-New
Source: Banknote World

Many countries will repeat in UV ink the denomination or a large image. The US only uses UV inks in their embedded threads. A ‘thread’ looks more like a metal wire inside the banknote that can only be seen when you have light behind the banknote (like a watermark). Many threads include UV ink and will light up under 365nm ‘black lights’. US banknotes that have threads are all different UV colors. The most common UV color is a greenish-yellow. This is because this type of ink is inexpensive and durable. Some other colors such as red and blue can fade when exposed to the sun or washed in a washing machine. 

Security Threads

Security threads have often been one of the best and most complex security features. The earliest threads were actually pieces of metal embedded in paper. One can see these start to be used in late 1940’s banknotes. Designers learned pretty quickly that the wire could be removed so banknote paper suppliers designed multilayer threads and began adding magnetic inks, holograms, UV inks and even color shifting inks. Threads are often used to authenticate and identify denominations in ATMs and other cash acceptance machines. Take a close look at US currency. You will see that the thread is at a different location on the 20, 50 and 100. Under UV they also light up different colors and show the actual denomination. More recently, security threads have become increasingly complex with the use of micro-optics. The blue strip in the US$100 is a thread that uses micro-optics. These features are extremely hard to mimic or replicate.

Serial Numbers

There are numerous benefits to including serial numbers as a security feature on the American $100 banknotes. Serial numbers ensure that no two individual notes are exactly alike. Each note has its own unique serial number, so a counterfeiter would need to have an extra process to change the number between each note. It would become obvious pretty quickly during banknote sorting and processing if the machines see the same number over and over again. Banks record the serial numbers of the notes that they distribute, which also protects against theft and robbery – notes in a block or sequence of serial numbers can be flagged as lost or stolen.


Protecting the integrity of the money supply is a crucial aspect of maintaining a stable economy and a stable society. The USA has done an excellent job of minimizing counterfeiting and protecting their currency by instituting the latest and best security features available into their bank notes. Look for these features on the American banknotes in your collection, and on other bills from around the world.

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