Do counterfeit detector pens work?
Reviewed on 1 September 2026 · By the Detectalia technical team
The detector pen is the cheapest and most widespread checking method — and in the UK it is close to useless. Understanding how the pen works explains why it tells you nothing at all about polymer banknotes, catches only the worst paper fakes, and lets through precisely the dangerous ones.
What the pen actually tests
The pen's ink contains iodine, which reacts with the starch in ordinary paper and leaves a dark mark. That is the whole test: it does not analyse the note — it analyses the paper. On starch-free material the stroke stays pale or invisible, and the note «passes».
Why it is meaningless on polymer notes
Bank of England notes are polymer — plastic, with no paper and no starch anywhere. The pen will stay pale on a genuine £20, and it will stay just as pale on a counterfeit £20 printed on any plastic film or coated paper. On the notes British tills handle all day, the pen literally cannot distinguish real from fake.
The only scenario where a pen tells you anything is foreign paper currency — and even there, only against the crudest fakes. Euro notes are cotton fibre, starch-free: any counterfeit printed on starch-free stock, including cotton paper, sails through the pen test. Those are the higher-quality forgeries, exactly the ones that do the damage.
And it cries wolf
The pen also fails in the other direction: a genuine paper note that has been in contact with flour, detergent or other starchy products can mark dark and be wrongly rejected. In bakeries and food retail, false positives from the pen are a classic — an awkward scene with an honest customer, over a genuine note.
What to use instead
For a business handling cash daily — in pounds, euros or both — reliable checking is automatic: a tested fake note detector analyses infrared, ultraviolet and magnetic ink, the metallic thread, size and thickness — features no counterfeit can imitate all at once, on polymer or on paper — and decides in half a second, with no interpretation.
The reliability benchmark is the European Central Bank's public list of tested devices: only equipment that passes its tests against real counterfeits withdrawn from circulation appears on it. Detectalia detectors are on that list and verify pounds as well as euros.
Frequently asked questions
Does the pen damage genuine notes?
The pale mark usually fades, but writing on banknotes is not advisable: a heavily marked note may end up refused elsewhere. On polymer notes the mark serves no purpose in the first place.
Is an ultraviolet lamp enough, then?
UV light reveals genuine fluorescent features on both polymer and euro notes, and it is a more reliable step than the pen — but it requires the cashier to know what they are looking at. An automatic detector removes that interpretation.
What check does a bank use?
Certified multi-point authentication machinery — the same philosophy as an automatic counter-top detector: several physical sensors analysing the note at once.