Both these new stamps were produced by Anglo-American company OpSec Security (OpSec), which supplied them as part of a comprehensive issuance, tracking and monitoring system. The stamps themselves carry an optical security feature that includes the Equinox technology, intended for easy verification by the public, which was also introduced on the Maltese tax stamp in 2019.
Equinox was adopted by OpSec as the name of its reflective security feature in 2014. It had previously been called the Ink Security Imaging System, ISIS for short, but this began to have negative connotations as events unfolded in Iraq and Syria. In the optical security field, ISIS is also the acronym for Interference Security Image Structure (ref: Optical Document Security, van Renesse), so the change of name was anyway a useful differentiation.
Equinox – the two days in the year when day and night are equal at 12 hours – refers to one of the optical device’s key visual features: the ability to make a distinct switch between black and white as it is rotated through 90°, as illustrated in the high-contrast image of Mona Lisa (Figure 1). This provides an easy verification feature for non-specialists.
Figure 1 – Reversing black and white effect as Equinox is rotated 90°.
Equinox varies from many familiar optical security devices in that it works through reflection (where light bounces off the surface of an object) rather than diffraction (where light bends around an object or spreads out as a result of passing through a narrow opening).
Figure 2 – difference between refracted, reflected and diffracted light (courtesy of www.sciencestruck.com).
Equinox comprises tiny shaped mirrors, where the size and shape of the mirror determines the image that is seen by the viewer. These mirrors range in size from a few to tens of microns, while their shape varies between flat, parabolic, symmetrical and asymmetrical. It is this variation in shape and the ability to control it that separates Equinox from other micromirror techniques, and which is patented under WO 2014152119, titled variable device exhibiting non-diffractive three-dimensional optical effect (see inset).
Figure 3 – the Equinox concept of reflecting light to show a different image to each eye.
Dr Paul Dunn, Director of Technology Innovation at OpSec, told Tax Stamp & Traceability News™ that the mirrors work to deliver what are in effect stereo pairs of images to each eye of a viewer (Figure 3), but where typical stereo pairs give a left-eye and right-eye view of the image to create a 3D effect, Equinox offers much more variability. 3D images are one of the choices available, but so are variations of an image, such as the reversing black/white effect and a full range of grey-scale intensities.
This grey-scale capability allows the use of photographic-quality images, such as the image of the snow leopard that OpSec uses to demonstrate the capability – note the blue eyes in an otherwise grey image in Figure 4.SnowLeopard GENEQX
Figure 4 – example of Equinox grey-scale image.
Equinox’s versatility also allows the use of image switches (two distinct images on rotation) and kinetic designs, with dynamic movement linking two images as the feature is rotated. Some of these elements can be machine read, a useful feature for tax stamps. Dr Dunn explained that these multiple effects are achieved by overlaying mirrors in what OpSec terms multilayer complex reflectors.
The original concept for this feature, which dates back to the late 1990s, was that the parabolic micromirrors could be produced as printed structures, in the way that intaglio prints ink structures with depth – hence the original name of Ink Security Imaging System.
Now, however, the structures are originated using e-beam technology – which gives the greatest control of the structure at the micro- and nano-level – and then mass-produced using embossing and UV casting techniques. As this is the method for production of holograms, Equinox can be combined with holograms and similar optical features. They are mainly metallised to ensure a high reflectivity, but they can also be specified to work in transmission using a high refractive index (HRI) coating.
Michigan, Mozambique and Malta tax stamps all feature holographic stripes or patches which in turn contain Equinox elements (Figure 5). It is only fair to point out that the improvements in collection and enforcement are not solely the result of using these sophisticated optical features, but flow from the complete system solution of which the stamp is a part.