WINDOE™

Type

Diffractive — diffractive optical element (DOE) integrated within a transparent polymer banknote window, transforming the clear window into a recognisable image (typically the denomination numeral) when the note is held in transmitted light against a distant point light source. The trade name is a portmanteau of "Window" and "DOE" (Diffractive Optical Element).

Overview

WINDOE™ is a CCL Secure security feature that places a diffractive optical element directly within the transparent window region of a GUARDIAN™ polymer banknote. In normal reflected light, the window appears as an ordinary clear polymer window with no visible image content. When the banknote is held flat against transmitted light from a distant point source — for example, a ceiling lamp, a torch, or even a night-time street light at distance — the diffractive structure transforms the clear window into a visible image, typically the denomination numeral. The feature is unique among polymer security elements for being best observed under low-light or dim conditions where a single distant point source dominates the lighting environment, making it well suited to verification in restaurants, taxis, bars and other low-illumination retail settings. CCL Secure classifies WINDOE™ as a Level 2 security feature, sitting within the diffractive and optical security family of GUARDIAN™ that exploits the smooth surface and optical clarity of polymer substrate to deliver effects unavailable on paper banknotes. The feature is most prominently deployed on the Papua New Guinea 50 kina banknote.

Technology & Effects

WINDOE™ is built on a diffractive optical element — a precisely engineered microstructure that imposes a calculated phase pattern on light passing through it, causing the transmitted wavefront to reorganise into a defined image at a particular distance from the structure. Unlike conventional holograms (which produce visible imagery via direct reflection or transmission of structured light) and unlike micro-optic lens-array movement features (which require the viewer's eye to integrate the array's output across the structure), a DOE generates its image through diffraction of light from a distant point source — meaning the image only appears clearly when the illumination geometry approximates a point source, and the image effectively "projects" through the DOE onto the viewer's retina. The result is a Level 2 verification cue that is distinctive precisely because of its lighting-dependent behaviour: under bright diffuse light, the window appears clear; under a distant point light source, a denomination numeral or design appears within the window; under intermediate lighting, partial or unclear image. Effect type: Diffractive (primary, by both name and mechanism).

Form Factors & Application

WINDOE™ is integrated into the transparent window of a GUARDIAN™ polymer banknote during the substrate manufacturing stage. The diffractive structure is incorporated into the polymer film itself rather than applied as a foil or applique, taking advantage of CCL Secure's substrate-build production process — in which security features are embedded directly into the GUARDIAN substrate. This approach means WINDOE™ cannot be retrofitted onto polymer banknotes after substrate manufacture, but is permanently integrated into the substrate as supplied to the issuing printer. As a window-integrated feature, WINDOE™ is fully transparent in normal reflected light and does not interfere with other window-based security features (such as CAMEO™ portraits, VIGNETTE™ images, GSWITCH™ inks, or VIVID™ COLOUR window vignettes) that may be deployed within the same window region, provided they are designed to coexist within the available window area.

Security Levels

• Level 2 (cash handler / informed public): Distinctive lighting-dependent image revealed when the banknote is held against a distant point light source. The verification cue is intuitive once the lighting requirement is understood, and the feature is uniquely well suited to verification in low-illumination retail settings where conventional security features may be harder to observe. CCL Secure classifies WINDOE™ explicitly as Level 2 — beyond pure public verification but accessible without specialist equipment.

Notable Deployments

• Papua New Guinea — 50 kina banknote (Bank of Papua New Guinea, 2010 issue, GUARDIAN™ polymer): the most prominently documented WINDOE™ deployment, used as the canonical reference example in Regula Forensics' banknote security feature glossary. The window transforms into a visible denomination numeral image when the note is held against a distant point light source; in reflected light the window appears clear.