Counterfeit Currency Detection: Instruments and Field Methods

The 365 nm ultraviolet lamp (long-wave UV) is the primary field instrument for substrate authentication. Genuine banknote paper (cotton-linen substrate) contains no optical brightening agents and appears dark under 365 nm UV. Standard commercial paper (office copier paper, laser printer paper, inkjet paper) contains optical brightening agents added during papermaking and fluoresces bright blue-white. A counterfeit produced on commercial paper is therefore identified in under two seconds under a UV lamp without any further examination needed.

Beyond substrate fluorescence, UV illumination reveals additional authentic features. On MGNS notes, coloured security fibres (randomly distributed red, blue, and green fibres embedded in the substrate during papermaking) fluoresce at their respective colours under UV. On US Federal Reserve Notes from the $5 upward, the embedded security thread is UV-reactive: the $5 thread fluoresces blue, the $10 pink, the $20 green, the $50 yellow, and the $100 pink (the colour is denominated into the thread to prevent an attempt to bleach and reprint a lower-denomination note as a higher one). ECB Europa series notes carry UV-fluorescent constellation patterns visible on both faces. On Bank of England pre-polymer notes, a UV-reactive symbol and serial number element were carried; the polymer series notes carry UV-reactive features within the opacified layers.

UV lamps for field use come in two forms: plug-in ultraviolet inspection lamps (typically 4-watt or 9-watt tubes, used at fixed positions on teller counters) and battery-operated UV pen torches (used by transport police, event security, and mobile traders). The quality of field UV lamps varies: lamps that peak at 365 nm produce cleaner results than broad-spectrum "UV" lamps that emit significant visible light alongside UV, which reduces contrast. Many low-cost counterfeit-detector lamps sold to small businesses emit insufficient UV intensity at 365 nm to reliably differentiate substrate types.

The transmitted-light box (a light box producing even white-light illumination through a diffusion panel) is used to verify watermarks and see-through register. A genuine multi-tone portrait watermark visible in transmitted light against the correct background, with front-to-back register elements aligning correctly, in under five seconds, confirms two of the most difficult-to-replicate genuine features simultaneously. Compact LED light boxes are available for counter use; alternatively, a high-intensity white-light torch held behind the note serves the same function.

The magnification loupe at 10x is the tool for microtext examination. A standard 10x triplet loupe (Carl Zeiss, Peak, Belomo) allows a trained examiner to read the "USA 100" microtext on a genuine $100, the "RBI" microtext on a genuine MGNS thread, and to assess intaglio ridge texture in the portrait area. The loupe is also the instrument for preliminary thread assessment (is it a surface-applied foil strip, or a thread that alternates above and below the surface?).

The counterfeit-detector pen (also called the counterfeit pen, iodine pen, or banknote pen) is a felt-tip applicator containing an iodine solution in an organic carrier. When the pen tip is drawn across paper containing starch (wood-pulp paper, the standard substrate for commercial printers), the iodine reacts with the starch to form the deep blue-black iodine-starch complex. When drawn across cotton-linen banknote substrate (which contains no starch), the pen mark remains amber-yellow or disappears. The result: a mark that stays amber on genuine cotton-linen notes, turns dark on wood-pulp counterfeits.

The chemistry is reliable for the substrate detection it claims to perform. The failure mode is not the chemistry but the assumption it encodes: that a counterfeit will be produced on wood-pulp paper. This assumption was operationally sound when the pens were introduced (most early photocopied counterfeits were on office paper) but is now exploited in three categories of sophisticated counterfeit:

First, counterfeits produced on genuine lower-denomination notes that have been bleached (using bleaching agents that remove the printed design without destroying the cotton-linen substrate) and then reprinted with a higher denomination design. The substrate passes the iodine-starch test because it is genuine banknote paper. This technique, documented in India, the US (the "bleached note" variant), and the UK, defeats both the pen test and the UV substrate check.

Second, counterfeits on cotton-linen paper sourced from non-standard channels (e.g., stationery from a defunct security printer, unissued bank stock, or security paper diverted from the supply chain before printing). These pass the pen test and UV substrate test but fail on other features.

Third, counterfeits produced on polymer substrate simulants: biaxially oriented polypropylene sheets available from industrial suppliers, used to simulate polymer banknotes. The pen mark on polymer is amber (no starch reaction) so the test gives a false "genuine" result.

In India, the Reserve Bank of India and the OCCB (Organised Crime Control Bureau, which handles currency-related offences under state police frameworks) explicitly advise that the pen test is not a standalone authentication method. The specialised imaging tools of forensic physics provide the UV and IR examination capabilities that form the laboratory-tier complement to these field methods. In the US, the United States Secret Service (which has sole federal jurisdiction over counterfeit currency investigations) issues the same advisory. Bank tellers are trained to use the pen test as a supplementary screen alongside UV and transmitted-light inspection, never as the sole criterion.

When a counterfeit note passes field examination (either because it is on cotton-linen substrate or because the field examiner lacked adequate tools), laboratory examination becomes necessary. The primary laboratory techniques for ink characterisation are Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy, both of which are non-destructive and require no sample preparation that would destroy the evidence.

Genuine banknote intaglio ink is a specialist product manufactured by licensed suppliers (SICPA SA for many central banks, Sun Chemical for others). Intaglio inks are highly viscous (typically 5,000 to 50,000 cP at working temperature) and contain specific binder resins, pigment complexes, and functional additives that produce a characteristic FTIR absorption pattern. The forensic chemistry analysis of counterfeit currency security features provides the detailed spectroscopic context for these ink-type comparisons. The ink film on a genuine note is also mechanically distinctive: it is a surface-standing raised film rather than a penetrated coating, which means the FTIR spectrum of the ink collected from the surface of a genuine note differs from that of offset-litho or inkjet ink at the substrate interface.

In practice, an FTIR spectrum of the OVI numeral on a genuine US $100 will show absorption bands characteristic of the metallic flake binder chemistry and the thin-film interference coating on the flakes. The same area on an offset-litho counterfeit will show absorption bands from standard offset ink resins (linseed oil alkyd, petroleum resin) and the substrate filler beneath. Comparative spectral databases (including the INTERPOL reference collection and the US Secret Service counterfeit currency reference archive) allow examiners to match unknown spectra against authenticated reference spectra and literature.

Raman spectroscopy complements FTIR by interrogating pigment molecular vibrations rather than functional group absorptions. Many pigments found in counterfeit inks (phthalocyanine blue/green, azo reds, carbon black) produce sharp, reproducible Raman spectra. Genuine banknote pigments from licensed suppliers may carry proprietary modifications (co-crystallisation, surface treatment, encapsulation) that shift Raman band positions or ratios relative to the commercial pigment. Raman mapping across a microprinted area can confirm whether the microtext ink matches genuine intaglio chemistry across its full area or shows lateral variation consistent with inkjet dot printing.

In the UK, the Forensic Science Service (now dissolved, with currency casework transferred to private providers and the National Crime Agency forensic chemistry capability) used FTIR and Raman as standard tools in counterfeit currency casework. In India, the Central Forensic Science Laboratory (CFSL) Delhi and the state FSL Mumbai operate FTIR capabilities; Raman instruments are less uniformly distributed across state FSLs but are present at CFSL Delhi and CFSL Hyderabad.

The digital examination workflow for counterfeit currency begins with acquisition: the suspect note is scanned on a flatbed scanner at a minimum of 1200 dpi under controlled white-light illumination, then re-scanned under UV illumination (365 nm), and (where available) under infrared illumination (850 nm, which reveals differences in ink IR absorption that may be invisible in white light). The three image sets together provide a multi-spectral record of the note.

The reference dataset against which the suspect scan is compared may be sourced from: the central bank's published authentication guides (the RBI publishes denomination-specific security-feature charts; the BEP publishes authenticated reference images for each Federal Reserve Note series; the ECB publishes the Europa series authentication guide); the INTERPOL counterfeit currency reference collection; or the examining laboratory's own authenticated note collection, maintained as physical exemplars.

The comparison is conducted systematically, element by element. Key comparison points include: portrait engraving line spacing and depth (genuine intaglio engraving has a characteristic line separation pattern visible at 300% zoom); microtext character forms (under 500% zoom, genuine microtext shows consistent stroke widths; counterfeit microtext from a scan-and-reprint shows tonal averaging and broken stroke edges); halftone pattern in background printing (genuine security-printed background tints use rosette-pattern or stochastic screening; consumer inkjet and laser printing use regular dot arrays visible at 400% zoom); and colour values (genuine offset-security background printing uses spot colours with defined hue values; counterfeit process-colour printing uses CMYK dot patterns that are apparent at high magnification).

The US Secret Service Counterfeit Division (operating under 18 U.S.C. § 471-476) requires that every submitted counterfeit note be assigned a counterfeit class from the Counterfeit Currency Database (CCD): a classification linking the printing process, the denomination, and the type (offset, inkjet, laser, bleached-genuine, etc.), which is used for criminal intelligence to identify counterfeit "plants" or producers flooding multiple districts.

In India, the OCCB coordinating with the RBI's Department of Currency Management and local state forensic science laboratories uses a similar classification. The RBI issues periodic circulars updating the counterfeit classification data across banking channels; the data is collated at the Reserve Bank's Issue Department.

The term "supernote" (originally applied to extremely high-quality counterfeit US $100 bills attributed to state-sponsored production, first detected in circulation in the late 1980s (1989, at the Central Bank of the Philippines)) has broadened in forensic usage to describe any counterfeit of sufficient quality to pass casual field examination. The original North Korean-attributed supernotes (note class "C-14342" in Secret Service classification) were produced on genuine cotton-linen paper sourced covertly, with intaglio printing on genuine intaglio presses and OVI inks obtained through illicit channels from licensed suppliers, and with security threads inserted during the papermaking stage. These notes passed UV examination, transmitted-light inspection, and pen tests; they were identified only through laboratory spectroscopy of the specific intaglio ink formulation (which differed subtly from the genuine BEP formulation) and through subtle dimensional errors in the Franklin portrait engraving.

Since 2000, the class of high-quality counterfeits has expanded. Commercially available intaglio printing equipment (second-hand from closed security printers in Eastern Europe and South Asia) has enabled a tier of counterfeit production that replicates the tactile intaglio profile and defeats the fingertip test. High-quality watermark simulation using genuine watermarked paper from non-bank sources (security paper diverted from stationery manufacturers) replicates the UV and transmitted-light signature.

The current forensic response in high-stakes cases (large-quantity seizures, suspected organised production) involves: FTIR and Raman spectroscopy of ink at multiple loci; scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) to compare the elemental composition of intaglio ink against reference standards (the metallic flake pigment in genuine OVI contains specific elemental ratios from the manufacturing process); and 3-D surface profilometry to measure the actual topographic height of ink ridges (genuine intaglio on MGNS notes has a measurable Z-height of 15 to 40 micrometres depending on the element; a very good inkjet-raised simulation may reach 5 to 8 micrometres but cannot reach the genuine intaglio height without access to intaglio equipment).

INTERPOL's Counterfeit Currency group (Operation Jupiter and its successors, which coordinate across member states on counterfeit currency and goods intelligence) coordinates across member states for intelligence on production infrastructure. In India, the Economic Offences Wing (EOW) and OCCB of major city police forces coordinate with the NIA (National Investigation Agency) for cases involving organised transnational counterfeit distribution.

The lifecycle of a detected counterfeit note has a specific enforcement pathway in each jurisdiction, and the forensic examiner's report is a node in that pathway. In India, any bank or financial institution detecting a suspected counterfeit is required under RBI Directive (Master Circular on Counterfeit Notes, updated periodically) to: impound the note (not return it to the depositor); record the depositor's details; submit the note to the nearest Currency Verification Processing Centre (CVPC) or police station; and report the detection on Form-I to the local police and to the RBI. The RBI collates this data and publishes annual counterfeit currency statistics by denomination and detecting institution.

The Reserve Bank's Department of Currency Management receives the impounded notes and, for notes referred for forensic analysis, transmits them to the CFSL or state FSL. The forensic laboratory's examination report (production method, ink classification, printing process) feeds into the RBI's counterfeit intelligence database, which is shared with INTERPOL's Counterfeit Currency Working Group.

In the US, the US Secret Service has exclusive federal investigative jurisdiction over counterfeit currency under 18 U.S.C. § 470-476. Any financial institution receiving a suspected counterfeit note is required by law to report it to the local USSS field office. The Secret Service Counterfeit Division in Washington DC maintains the CCD (Counterfeit Currency Database), assigns class codes to each distinct printing-process signature detected, and tracks production origins of counterfeits through the classification system. High-volume seizures of the same class code in multiple districts indicate an active counterfeit plant.

The Bank of England coordinates with the National Crime Agency (NCA) in the UK for currency counterfeiting investigations. Post-Brexit, intelligence exchange with Europol (coordinating counterfeit euro cases across EU member states) continues under the UK-EU Trade and Cooperation Agreement law-enforcement provisions.

INTERPOL's database of authentic banknote specifications, maintained at its Lyon headquarters and accessible to member-state forensic laboratories through the I-24/7 secure network, provides the reference standard for genuine-note comparison. A forensic examiner in Mumbai or Hyderabad can access the ECB's authenticated reference data for a suspect euro note through this channel.

1. Initial field screening
   UV lamp at 365 nm (substrate and thread fluorescence); transmitted-light box (watermark and register); counterfeit-detector pen (substrate starch test, if appropriate); 10x loupe (microtext, intaglio, OVI). Record pass/fail per feature.
2. Document impoundment and chain of custody
   Record denomination, serial number, issuing institution, detecting party, date/time. Photograph note in reflected white light, UV, and transmitted light before any further handling. Seal in evidence bag.
3. Forensic laboratory referral
   Transmit note to CFSL/state FSL (India), USSS laboratory (US), NCA forensic capability (UK), or designated national laboratory. Request: substrate analysis, ink FTIR/Raman, production-method classification.
4. FTIR and Raman ink characterisation
   Acquire FTIR spectrum of intaglio and background inks. Acquire Raman spectra of pigment-bearing areas. Compare against reference database spectra for genuine ink and known counterfeit ink profiles.
5. Digital high-resolution comparison
   Scan at 1200 dpi (white, UV, IR). Compare portrait engraving, microtext, OVI, halftone pattern, and serial-number font metrics against authenticated reference images from central bank or INTERPOL database.
6. Production-method classification and reporting
   Classify production method (offset-litho, inkjet, laser, intaglio simulant, bleached-genuine, etc.). Issue forensic examination report with class code. Transmit to enforcement agency and to the central bank counterfeit intelligence database.