Counterfeit Currency: Security Features and Chemical Analysis

Optically variable inks (OVIs) produce a visible colour shift when the viewing angle is changed, without any electronic components. The technology, commercialised primarily by SICPA Security Inks and Coatings SA (Prilly, Switzerland) under the trade name SICPA OVI, depends on thin-film optical interference: a stack of thin dielectric and metal films produces constructive interference at a specific wavelength (colour) that shifts as the angle of incident light and observation changes.

The ink particle structure is the key. Each OVI flake consists of a metallic layer (typically aluminium or chromium, tens of nanometres thick), sandwiched between two layers of a dielectric material (magnesium fluoride or silicon dioxide) of precisely controlled thickness. The dielectric layer thickness determines the base colour of the interference; by varying this thickness, SICPA and competing manufacturers (Sun Chemical, Viavi Solutions) produce colour-shifting effects across the visible spectrum. The flakes are suspended in a carrier ink and applied by intaglio printing, which aligns the flat flakes parallel to the paper surface, maximising the angular shift effect.

On genuine notes, the observed shifts are: Indian ₹500 (2016 Mahatma Gandhi New Series) Devanagari numeral "500" at bottom left, green to blue shift at 45-degree angle change; Indian ₹2000 (now being withdrawn from circulation), Devanagari "2000" numeral, magenta to green; US $100 (Federal Reserve Note 2013 series), copper to green on the "100" numeral and Liberty Bell in the inkwell; EUR €50 (Europa series, second issue 2017), emerald green to deep blue; GBP £20 (Bank of England polymer note, Queen Elizabeth II and later King Charles III series), shifting patch on the front; CHF 100 (Swiss National Bank), combined intaglio and OVI elements with gold-to-green shift. The broader VSC and IR spectral workflow for examining all ink types on documents is described in the ink chemistry: TLC, HPLC, Raman and the VSC workflow topic.

In the laboratory, OVI examination is performed with the VSC at multiple angles and illumination conditions, combined with Raman microspectroscopy to identify the metallic and dielectric components of the flakes. The angular colour shift can be quantified using a goniospectrophotometer (an instrument that measures reflectance spectra at multiple viewing and illumination angles simultaneously), though this instrument is specialised and not present in all forensic labs. Simpler angle-dependent reflectance measurements at 0 and 45 degrees, logged on the VSC, are sufficient for a qualitative comparison against authenticated references.

Counterfeit OVI elements almost always fail the Raman test: instead of the metallic flake stack, counterfeiters typically apply iridescent thermochromic ink, holographic foil, or colour-shifting paint (commercially available for automotive custom painting, not engineered for the specific interference spectrum of genuine OVI). These substitutes may produce a superficially similar colour shift to the naked eye but produce immediately distinguishable Raman and FTIR-ATR spectra.

Fluorescent security inks are invisible or subdued under ordinary white light and fluoresce at a specific emission wavelength when excited by UV illumination. On genuine notes, the fluorescent elements are precisely defined in colour, position, size, and intensity by the issuing central bank's specification, and they are printed with UV-fluorescent inks that are not commercially available to the general public.

On the Indian ₹2000 note (Mahatma Gandhi New Series, 2016), the numeral "2000" in the right-hand area fluoresces bright green under 365 nm UV, while the bank logo, serial numbers, and certain background motifs show orange-red and yellow-green fluorescence. The paper substrate itself is treated to show no UV fluorescence (optically brightener-free paper), so any fluorescent element is by deliberate design. On the US $20 Federal Reserve Note, a vertical security strip reading "USA TWENTY" is embedded in the paper and fluoresces green under 365 nm UV. On EUR €50 (Europa series), the EU flag fluoresces green, the map of Europe fluoresces orange-red, and the denomination numeral and ECB initials fluoresce in specific colours depending on the language group.

In the forensic laboratory, fluorescent element examination is performed on the VSC at 254 nm (short-wave UV, examining quenching and weak luminescence), 365 nm (long-wave UV, examining primary fluorescence), and at specific visible excitation wavelengths if the VSC is equipped with bandpass filters. Emission spectra can be measured with a fibre-optic spectrophotometer coupled to the VSC illumination system to capture the full emission profile, enabling comparison with authenticated reference notes.

Infrared-absorbing inks are a third category: security elements that are invisible in visible light and also non-fluorescent, but absorb strongly in the near-infrared (800-1000 nm range) and therefore appear dark when viewed through an IR-sensitive camera or the VSC in IR mode. On some banknotes, specific patches or patterns of security ink are IR-opaque, while the surrounding printed design is IR-transparent. An ATM machine's IR sensor detects these patterns as part of its authentication protocol; a counterfeit note lacking the correct IR-absorbing elements fails the sensor check.

Intaglio printing is the process used for the major visual elements (portrait, architectural motifs, denominational numerals) on most high-denomination banknotes worldwide. In intaglio, the design is engraved into a steel plate, the plate is inked with a high-viscosity ink, the excess ink is wiped from the surface leaving ink only in the engraved recesses, and the paper is then pressed against the plate under very high pressure (up to 600 MPa for modern sheet-fed intaglio presses). The paper fibres are forced into the recesses, picking up the ink, and the dried ink line forms a slightly raised ridge standing above the paper surface.

The chemistry of genuine intaglio ink is a closely guarded central-bank specification. However, FTIR-ATR analysis of authentic notes consistently identifies a high-resin-loading alkyd or modified polyamide binder with a low-solvent-content formulation (the ink must be stiff enough to remain in the recesses without bleeding). The tactile relief of authentic intaglio is measurable by profilometry (a contact surface-roughness instrument) or by atomic force microscopy: genuine intaglio lines have a profile height of typically 5 to 20 µm above the paper surface, depending on the element.

Counterfeit notes attempting to simulate intaglio almost universally use offset lithography or high-resolution inkjet printing, which deposits ink on the paper surface with no measurable relief. Examined notes from USSS and Europol PEX investigations show a flat FTIR-ATR profile where the genuine ink binder resin peaks are replaced by a thinner lithographic or inkjet ink signature. FTIR-ATR makes this distinction chemically definitive and court-defensible.

In the FTIR-ATR workflow for currency examination, the diamond ATR crystal is pressed directly against the denomination numeral or portrait area of the note (the highest-relief area of genuine intaglio) with a controlled contact pressure. A background spectrum is collected from an unprinted area of the note, and the resulting absorbance spectrum is compared against a reference library of authenticated-note spectra. Key discriminating peaks include the carbonyl stretch of the alkyd binder resin at approximately 1720-1740 cm-1 and the characteristic pattern of aliphatic C-H stretches at 2850-3000 cm-1.

The security thread in modern banknotes is a strip of biaxially oriented polyethylene terephthalate (PET) film, typically 1 to 4 mm wide, embedded in the paper during the papermaking process so that it alternately appears and disappears (windowed thread) or remains fully embedded. The PET film is metallised on one face by vacuum evaporation of aluminium in a pattern that creates alternating reflective and transparent segments. Colour-shifting magnetic pigments may be incorporated into the thread coating (as in some EUR and CHF denominations), and microprinted text (readable only under magnification) runs along the thread surface.

The magnetic properties of the thread, created by magnetite (Fe3O4) or barium ferrite particles in the coating, are detected by the magnetic sensors in ATM machines and currency-processing machines. FTIR-ATR of genuine threads identifies the PET polymer (sharp ester carbonyl peak at 1720 cm-1, characteristic fingerprint in the 1000-1100 cm-1 region) alongside the metallic coating. Raman microspectroscopy at 785 nm on the metallised segments identifies the aluminium and, in magnetic threads, the Fe3O4 Raman signature (peaks at 670 and 550 cm-1).

The paper substrate is itself a primary security feature. Genuine notes are printed on paper manufactured to central-bank specification, not commercially available to the public. The composition is typically 75% cotton and 25% linen by fibre weight (US Federal Reserve Notes, Indian Reserve Bank notes, most EU member-state note papers), though the exact ratio varies by denomination and issuer. Cotton-linen blend paper has a distinctive FTIR-ATR spectrum dominated by cellulose I crystalline structure (C-O-C stretching at 1160 cm-1, O-H stretching at 3200-3500 cm-1) and lacks the filler peaks (calcium carbonate at 1430 cm-1, titanium dioxide at 910 cm-1) present in most commercial printing paper. Photocopy paper and laser printer paper, the substrate for most low-grade counterfeits, can be distinguished from genuine currency paper by FTIR-ATR within seconds.

Polymer-substrate notes (Australian, New Zealand, Canadian, and UK Bank of England polymer notes, Indian ₹10 polymer pilot series) are printed on biaxially oriented polypropylene (BOPP) film. The FTIR-ATR spectrum of BOPP is entirely unlike cellulose: the dominant peaks are the methyl C-H symmetric stretch at 2872 cm-1, the asymmetric stretch at 2960 cm-1, and the C-C backbone stretch at 997 cm-1. Any attempt to counterfeit a polymer note on conventional paper is immediately identified by FTIR-ATR.

The forensic examination of a suspected counterfeit note follows a tiered, non-destructive-first protocol. No feature that can be characterised non-destructively should require destructive sampling: the note is evidence, may need to be exhibited in court, and must be preserved for the investigative agency's intelligence use.

The examination sequence begins with macroscopic examination under white light in transmitted and reflected modes on the VSC, documenting the note's overall condition, any printing defects, substrate quality, and the presence and position of security elements. The VSC examination then proceeds through UV (254 nm short-wave, 365 nm long-wave) and IR (850, 950, 1000 nm) imaging, capturing the fluorescence pattern, UV quenching, and IR absorptance pattern of each security element.

Raman microspectroscopy is then applied to the OVI element, the security thread, and selected ink areas. At 785 nm excitation, the paper background contributes minimal fluorescence, and the OVI flakes yield their metal and dielectric Raman signatures within seconds of data acquisition. The thread metallisation is examined by placing the Raman objective over the thread window areas and collecting spectra from both the metallised (aluminium D and G bands absent; aluminium has no Raman active modes, but the dielectric coating MgF2 and the PET substrate contribute peaks) and the clear film regions. The magnetic coating, if present, shows the Fe3O4 A1g mode at 670 cm-1.

FTIR-ATR is applied last in the non-destructive phase, because the diamond crystal pressure may slightly flatten the intaglio relief (a reversible effect but worth documenting before and after with profilometry). The diamond ATR crystal is pressed against the portrait area, the denomination numeral, and an unprinted substrate area. The three spectra, along with their reference spectra from authenticated notes, constitute the chemical identity of the note's intaglio ink and paper substrate.

If these non-destructive methods are insufficient for a definitive conclusion (unusual circumstance, novel counterfeiting technique), a micro-sample may be taken from the margin of the note for TLC or HPLC-DAD analysis of the ink dyes. The ISO 17025 and NABL accreditation framework governs chain-of-custody documentation and method validation requirements that apply to both destructive sampling and the non-destructive spectroscopic methods used throughout this workflow. This is carried out only under chain-of-custody documentation approved by the submitting agency and the case officer.

Counterfeit currency operations are organised across national boundaries, and the forensic chemistry report feeds directly into currency intelligence networks. Understanding these networks contextualises the significance of the analytical findings.

In India, the primary counterfeiting threat involves Fake Indian Currency Notes (FICN), predominantly high-denomination notes (₹500, ₹1000 before demonetisation, ₹2000 from 2016). The physical security features that make these notes difficult to counterfeit are catalogued in the currency security features: substrate, threads, watermarks, intaglio, OVI and microprinting topic. The Government of India's intelligence agencies and the NIA have publicly attributed a significant proportion of high-quality FICN, particularly "super" FICN (quality approaching genuine notes), to printing operations in Pakistan with alleged Pakistan intelligence agency (ISI) logistical support. The print quality of such notes, including partially replicated OVI and authentic-looking intaglio simulation, makes forensic chemistry analysis essential rather than optional for definitive identification. The FICN Coordination Group (FCORD), a multi-agency body chaired by the Intelligence Bureau and including the NIA, CBIC, and banking sector representatives, coordinates seizure intelligence and analysis.

In the United States, the USSS Counterfeit Division has investigated "supernotes" (extremely high quality counterfeit $100 notes) since the 1990s. The USSS and US intelligence community have publicly attributed the origin of the most sophisticated supernotes to state-sponsored operations in North Korea (the DPRK). Iran was an early suspect theory in the 1990s but the settled official U.S. government position identifies the DPRK as the primary source. The supernotes replicated genuine intaglio printing and cotton-linen paper more closely than any commercially available counterfeiting operation, yet Raman and FTIR-ATR analysis of seized supernotes still distinguished them from authentic Federal Reserve Notes by subtle differences in the intaglio ink binder chemistry and OVI flake dimensions. The shift of the US $100 to the 2013 redesign with 3D Security Ribbon and colour-shifting Liberty Bell, partly motivated by DPRK supernote intelligence, illustrates how forensic chemistry findings directly drive currency redesign policy.

In the European Union, Europol's Project Ester (PEX) coordinates counterfeiting intelligence across member states. The European Central Bank's Counterfeit Monitoring System (CMS) receives data from national central banks on detected counterfeits, categorised by the "CUIS" identification system (which assigns a number to each detected counterfeiting source, analogous to a criminal dossier). ECB forensic chemists and the Bundesbank's currency laboratory (Mainz) lead technical analysis of high-quality EUR counterfeits. The Interpol Counterfeit Currency Group (CCG) extends this coordination globally, enabling forensic chemistry findings from a note seized in Singapore to be cross-referenced against a printing plate seized in Albania or a seizure in Karachi.

1. Note intake and macroscopic documentation
   The suspect note is photographed under white, transmitted, oblique, and raking light before handling. Serial number, denomination, series year, and physical condition are recorded. The note is handled with nitrile gloves.
2. VSC white-light and UV examination
   VSC examination at white light (both sides), 254 nm UV (quenching), and 365 nm UV (fluorescence). All OVI, fluorescent, and IR-absorbing elements are imaged and compared against authenticated reference notes of the same series.
3. VSC near-infrared examination
   IR examination at 850, 950, and 1000 nm. The IR transmittance pattern of the security thread and IR-absorbing patches is imaged and compared against references.
4. Raman microspectroscopy
   785 nm Raman spectra collected from OVI element, security thread window, selected ink areas. Spectra matched against reference library (NFI / ECB / USSS Raman database where accessible).
5. FTIR-ATR examination
   Diamond ATR crystal pressed against intaglio portrait area, denomination numeral, and unprinted substrate margin. Spectra compared against authenticated reference notes. Substrate fibre composition determined.
6. Intelligence report submission
   Full examination report (VSC images, Raman and FTIR spectra, comparison table, conclusion) submitted to the submitting agency. If FICN (India), simultaneous notification to NIA liaison. If USD or EUR, USSS or Europol PEX intelligence report initiated.