OVD Foil Microstructures

The first application of OVD technology as an anti-counterfeiting feature was the result of the adaptation of existing holographic technology to high volume manufacture by direct embossing onto foil and the application of that foil onto the security document by a hot stamping process. The security hologram was developed initially by the American Bank Note Company through its subsidiary, American Bank Note Holographics (ABNH) and was first adopted by MASTERCARD in 1982 in the form of a 2D/3D holographic foil image. Two years later VISA used a 3-D variation of the ABNH technology to protect its new credit card series (Fagan 1990). In both cases the use of the hologram resulted in a marked decrease in the rate of credit card counterfeiting and fraud.

7.3.1 The Security Hologram

In the years that followed many other variations of holographic foil images have appeared on various types of financial transaction cards. During the same period attempts were made to apply similar technology to paper substrates, including prototype banknote applications. It then became apparent that the hologram suffered from several serious problems, which reduced its effectiveness as a security feature when applied to these less robust substrates.

Some of these problems already existed in the case of the card applications. The key disadvantages included: (1) image blurring (and therefore reduced recognition) under extended light source illumination, (2) lack of image brightness, (3) very poor brightness and recognition after crumpling of the paper substrate and finally (4) lack of control over the origination technology which sometimes allowed unscrupulous operators to re-originate the security image or simulate the effects from commercially available foils with similar optical effects (Antes 1986).

While some of these problems were eliminated or reduced by the development of new origination processes such as holographic stereography and dot matrix technology (Souparis 1990), the results were usually perceived to be less than satisfactory from a security printer's point of view. This unfavourable perception was mainly due to the lack of a clear optical effects design philosophy as well as an inability to guarantee complete security with respect to the availability of specialised optical effects. In the following sections we shall show how these critical issues were resolved by the development of the first OVD technologies with optical effects exclusively reserved for security printing applications (Lee 1998).

Developed by the Landis & Gyr Corporation in the early 1980's, the Kinegram™ was the first OVD technology to be designed explicitly as a no compromise anti-counterfeiting measure exclusively reserved for banknote and other high security document applications. The Kinegram™ technology (Antes 1982, 1988; Moser 1996; Renesse 1994, 2004) represents probably the most advanced form of OVD microstructure written by laser interference techniques. An example of such a Kinegram™ micro structure is shown in Fig. 7.1. The photomicrograph shows some fundamental microstructure units of a KinegramTM OVD. The small circular 60-micron diameter grating pixels have groove spacings and angles which vary throughout the device, according to the input picture information, thereby providing the means by which the macroscopic artwork of the Kinegram is made optically variable. The photomicrograph is taken from the Kinegram Saudia Arabia passport application of 1987. The initial stimulus for the KinegramTM OVD development came from the Swiss National Bank and followed earlier work by Landis & Gyr on the development of machine-readable optical codes for banknotes. This work in turn drew on Landis & Gyr's still earlier work on optically coded pre-paid telephone cards. The expertise developed on these earlier projects, particularly in relation to the mathematical analysis of inverse scattering problems, provided the critical mathematical tools necessary for calculating the KinegramTM microstructure configuration for particular optical effect designs. In conjunction with Gregor Antes' application of mathematical physics to the design of these unique optical effects, Landis &Gyr also developed a novel system for originating the required Kinegram™ micro structures. In order to satisfy the security requirements of the banknote printing industry, the KinegramTM origination technology was (and still is) maintained as a single site process exclusively reserved for high security anti-counterfeiting applications. Fig. 7.2 shows some applications of the Kinegram OVD technology, including the Swiss 50 Franc and Finnish 500 Mark banknotes.

Carbon Nanotubes Microstructure
Fig. 7.1. Example of a Kinegram OVD microstructure
Fig. 7.2. Kinegram OVD applications, including the OK marketing Kinegram

Underlying this origination development work was a well-defined optical effects design philosophy, which drew heavily on a particular interpretation of the existing knowledge of the psychophysical aspects of visual perception. The two principal psycho-physical Kinegram™ design rules can be stated as follows: (1) use crisp sharp lines in an image to efficiently help memorisation, as shown for example in the 20 mm diameter Kinegram shown in Fig. 7.3 and (2) best memorisation of the variability of an image is achieved when the image change is accompanied by sudden and brisk changes of intensities. As well as these rules of perception, Kinegram™ images are also designed to have high brightness, be easy to observe under a wide range of illumination conditions and to be resistant to image degradation due to crumpling of the foil on the banknote. To complete this unique package of high security design and origination attributes, Landis & Gyr also restricted the availability of the Kinegram™ to banknotes and other official documents. This restriction on availability was intended to ensure that Kinegram optical effects would not be compromised in the way holographic optical effects had been compromised by their appearance on many commercial applications outside of security printing.

With such a powerful and tightly controlled set of security attributes, it is not surprising that the Kinegram™ quickly became the market leader in OVD protection for high security documents. Appearing first on the Saudia Arabia passport in 1987 and the Austrian 5,000 schilling banknote in 1990, the Kinegram™ later appeared on Finnish banknotes, the new Swiss currency in 1995, German banknotes in 1997 and finally the low denomination euro banknotes of 2002. Fig. 7.4.(a) shows the behaviour of the Kinegram on the Finnish 500 mark banknote. It should be noted however that not all of these projects conformed to the classical Kinegram™ design philosophy of sparkling opto-kinetic effects based on optically variable guilloche line artwork. The "moving 50" Swiss banknote Kinegram as shown in Fig.7.4(b) for example, is composed of arrays of diffractive tracks more reminiscent of an Exelgram™ micro structure than the juxtaposed micrograting elements of the other applications. The acceptance of the

OVD as an essential feature of modern banknote design (Lancaster and Mitchell 2004) has seen the emergence of competitors to the Kinegram™. This is demonstrated by the selection of Hologram Industries' Moviegram™ for the higher denomination banknotes of the new European currency. Some of the competing products also show clear evidence of the adoption of the highly successful Kinegram design and optical effects philosophy. Companies such as ABNH of the US, AOT and De La Rue Holographics of the UK , Hologram Industries of France and the Toppan printing company of Japan are able to offer sometimes similar effects via alternative origination scenarios (Newswager; Aubrecht et al. 2004). While these companies have yet to create opto-kinetic effects of similar brightness to the KinegramTM OVD it is clear that such improvements are only a matter of time.

With the increased availability and reduced control status of these KinegramTM effects, there has developed an increasing interest in OVD technologies which offered additional optical effects unable to be produced by either holographic or opto-kinetic origination systems and which are able to be exclusively restricted to security printing applications. A particularly effective example of an alternative approach is provided by the electron beam lithography origination process as exemplified by the ExelgramTM OVD technology developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia.

Fig. 7.3. Characteristic opto-kinetic Kinegram OVD effects at three angles of view
Fig. 7.4(a). Optical behaviour of the Kinegram on Finnish 500 mark banknote
Fig. 7.4(b). The Swiss 50 Franc banknote Kinegram at three angles of view

7.3.3 The Catpix™ Electron Beam Lithography Microstructure

While the first examples of Exelgram™ technology did not appear until late 1993, the origins of the security philosophy which influenced the development of this OVD technology go back almost twenty years earlier to what was called the CNRD (Currency Note Research and Development) project in the early 1970's. The CNRD project was a joint collaboration between the CSIRO and Reserve Bank of Australia (RBA) and had the aim of developing a new high security banknote containing features that could not be copied by the coming generation of colour photocopiers (Hardwick 1990; Wilson 1998). In contrast to the optically variable guilloche philosophy, which drove the development program of the KinegramTM, the ExelgramTM traces its principal design influence back to the optically variable portraiture requirements of the RBA and similar influences from Bank of England studies during the same early 1970's period. Just as the opto-kinetic line art effects generated by Kinegrams can be regarded as optically variable versions of the complex guilloche effects used in traditional banknote design, so too can the positive to negative image switch of the American Express Centurion ExelgramTM incorporated into the new series of travellers cheques (Wood 1999; McHugh 1999) issued in 1997 be regarded as an optically variable example of the traditional portraits used on the majority of the world's banknotes. In this sense the KinegramTM and the Exelgram can be regarded as alternative and complementary solutions to the same problem of protecting their common banknote heritage from counterfeiting. While portraiture was a critically preferred feature of the CNRD OVD, other design objectives also had to be met. These included the addition of optically variable graphics as well as a requirement that the diffractive properties of the device should be resistant to crumpling of the banknote surface. The final result of this work was the CATPIX™ grating concept developed by CSIRO in 1985 -1987 (RA Lee, EP 0449 893 B1). This Catpix™ groove structure was used on the Captain Cook portrait OVD on the Australian ten-dollar plastic banknote issued in January 1988. This was the first use of an OVD on a banknote anywhere in the world. The OVD on the 1988 polymer banknote took the form of a 20 mm high schematic portrait of Captain Cook, two angles of view of which are shown in Fig. 7.5, and the underlying diffractive microstructure was written by a modified JEOL 5A electron beam lithography system. Interestingly this was the first electron beam lithography system to incorporate an oblique line writing capability as developed by CSIRO and incorporated into the JEOL 5A system installed at the Reserve Bank of Australia note printing department in 1980. The OVD micro structure developed on this machine was designed in the form of a network of continuously connected curvilinear regions (catastrophe pixels) in order that the resultant diffraction pattern from the OVD would be less sensitive to banknote surface crumpling effects. Fig. 7.6 shows a small section of a curvilinear region of this type of OVD.

7.3.4 Structural Stability

As briefly discussed above, the crinkling and flexing of the banknote surface was an issue that arose very early on in the process of developing an effective security microstructure for anti-counterfeiting applications. Curving the rulings of the grating microstructure in an undulatory fashion was found, through earlier work on the mathematical and optical properties of catastrophe gratings (Lee 1983, 1985), to be an effective technique for generating expanded diffraction beams from small surface regions of the banknote OVD. This increased the observational stability of each diffracted beam because each expanded beam could be observed from a broader range of viewing angles than in the case of straight line grating structures, which produced unexpanded diffracted beams. Fig. 7.7 shows the first order Catpix grating Fourier plane diffraction pattern corresponding to the catastrophe grating (Lee 1983) ruling pattern shown in Fig. 7.6. While the Australian Reserve Bank CatpixTM technology shared a similar security philosophy to that of Landis & Gyr's Kinegram technology (unique optical effects, single source origination and controlled availability), there was a fundamental problem with the Catpix™ technology in that its portraiture capabilities were still very limited. Because optically variable greytone line art portraiture had still not been achieved, CSIRO initiated a new research project in late 1988 aimed at securing the optically variable portrait objective by using a radically different underlying grating structure to that used in Catpix™. In the course of this work it was found that the modulated curvature geometry used to improve the observational structural stability of the Catpix OVD could also be adapted to solve the problem of creating a realistic OVD portraiture capability.

1988 Ovd Portrait Captain Cook

Fig. 7.5. Two angles of view of the Captain Cook Catpix OVD; the first banknote OVD

Fig. 7.5. Two angles of view of the Captain Cook Catpix OVD; the first banknote OVD

Fig. 7.6. Photomicrograph of a small section of the Captain Cook Catpix grating OVD
Fig. 7.7. Fourier plane diffraction pattern from a component of a Catpix grating OVD

7.3.5 The Pixelgram™ Palette Concept

An alternative and much more flexible approach to the design of OVD microstructures is based on the use of a palette of independent pixel grating functions of different line curvatures to represent the palette of grayscale or intensity values within a portrait image. The artwork to microstructure mapping process can then be thought of as part of a broader and more flexible mapping concept designed to allow for the conversion of any piece of artwork into optically variable form. The development of this diffractive microstructure palette technology was first implemented in 1989 via a new OVD technology (Lee 1991) developed by CSIRO called Pixelgram™. Each element of a Pixelgram palette is designed to be in one to one correspondence with the RGB palette used in the construction of the input artwork, e.g. as represented on a computer screen. The particular advantage of this microstructure palette concept is that it enables the electron beam lithography data file size to be minimised. In 1989 the data explosion problem was particularly acute for diffractive devices of areas of several square centimetres because in this case the electron beam lithography pattern file consisted of millions of micro trapezoids of different angles, widths and lengths. Converting this pattern file to binary on a Vax computer for the Leica EBMF series of machine took many hours of computer time and resulted in very large data files of several billion e-beam shapes. Disc storage devices at this time also could not very easily store the resultant multi-gegabyte binary pattern file.

Redefining the OVD image in terms of a palette of 10 or twenty micro grating files solved these problems. It was then only necessary to convert each micro grating file to binary and the resultant binary file for the palette was normally no larger than ten or twenty megabytes. In this new electron beam lithography writing format the image is defined by a special Job control data file consisting of a list of stage positions and exposure conditions for positioning and exposing each micro grating palette element according to the image layout in the original artwork file. A second key advantage of this palette-writing format is the ability to separate the macroscopic properties of the pixellated microstructure array from the microscopic properties of the system. In the case of the diffractive optically variable device this means that many different optically variable images can be constructed from the same microstructure palette. Each image simply corresponds to a different distribution of 30 micron X 30 micron or 60 micron X 60 micron diffractive microstructure palette elements. An example of a four-element diffractive microstructure palette and its corresponding RGB artwork palette is shown in Fig. 7.8 which shows four elements of a 16-element greyscale Pixelgram palette and its corresponding RGB greyscale palette. The diffractive microstructure pixels are 60 microns X 60 microns in size. An example of an optically variable portrait produced from such a palette is also shown at two angles of view. The characteristic OVD effect is a positive to negative tone image switch. Element 16 of the palette also includes a super high-resolution text message as an additional covert security feature. Fig. 7.9 shows a photomicrograph of a small section of an typical Pixelgram OVD.

Fig. 7.9. Small section of a Pixelgram OVD showing pixels of different groove structure

The PixelgramTM technology developed by CSIRO required the use of this new e-beam writing format, which was developed in collaboration with Leica Microsystems of the UK using data file structures developed by CSIRO Australia. The resulting specialised software package, called PATJOB, was implemented on a Leica EBMF 10.5 using a slight modification to some of the operating system routines. The implementation of this new e-beam writing configuration led to the rapid development of the Pixelgram™ OVD technology. All later OVD and security microstructure technologies developed by CSIRO have made use of this proprietary palette based electron beam lithography-writing format. More recently (in 2002) CSIRO and the Central Microstructure Laboratories of the Rutherford Appleton Laboratories in the UK developed an equivalent version of the palette based writing format for the Leica Vectorbeam VB6 series of electron beam lithography machines. This VB6 palette based data format, called PALTEK is a significant improvement over PATJOB as it has three times the resolution and ten times the writing speed of the earlier EBMF 10.5 version.

7.3.6 The Exelgram™ Track based OVD Microstructure

By early 1993 the size of the individual pixel gratings of the Pixelgram™ technology had been reduced to only 30 microns X 30 microns, providing greatly increased image resolution. However this reduction in pixel size also caused some loss in image brightness due to diffuse scattering effects from the edges of the pixel gratings. This diffuse scattering problem was solved by dispensing with the pixel based configuration of the OVD microstructure and replacing it with a surface relief structure based on a multiplicity of very thin diffractive tracks with continuous and smoothly varying groove angles and spacings (Lee, US patent nos. 5825547 and 6088161, 1997). This new alternative technology to Pixelgram™ was named EXELGRAM™; exel being the common term used to describe the fundamental exposure elements of the electron beam writing process. An example of how the diffractive microstructure palette concept can be used to generate application specific specialised optical effects is given for the case of optically variable greyscale portraiture. In this example the 16 greyscales of the input OVD artwork are mapped to 16 different microstructures of different grating groove curvature. Fig.7.10 shows an example of such a microstructure region. American Express US$ and Euro Travellers cheques used Exelgram OVD microstructures of this type. In Fig. 7.10 the portrait dimensions are approximately 8 mm X 10 mm. Horizontal groove regions diffract the negative version of the portrait while sloping groove regions diffract light into angles where the positive version of the portrait can be observed, as shown above.

Fig.7.11 shows two views of the American Express Euro 50 Exelgram OVD at two different angles of observation. The application of the Exelgram to the Euro denomination cheques followed the highly successful application of the technology on the earlier US cheques (McHugh 1999; Wood 1999). These images illustrate the two channel switching capabilities of the technology as well as the highly accurate portraiture representations, which can be achieved. These images are enlarged views. The actual long axis dimension of the foil on the cheque is 19mm. While optically variable portraiture, for example as illustrated in Fig.7.12, was the driving force behind the development of Exelgram, this effect is just one of many unique optical effects available within the ExelgramTM optical effects package. Most of these new effects relate to Exelgram's highly flexible optically variable graphics capability, which is due to the diffractive palette basis of the technology and the electron beam lithography basis of the origination process.

Fig. 7.10. Exelgram portrait OVD and corresponding diffraction grating microstructure
Fig. 7.11. American Express Euro Exelgram OVD at two angles of view
Fig. 7.12. Exelgram OVD showing two-channel switching and portraiture effects

For example the same curved line and variably spaced groove elements used for the portraiture effects can also used to generate very fast switching two-channel effects. In this case two interlaced micro diffraction grating structures are produced in which one group of grating structures corresponds to the RGB mapping from one input picture while the second interlaced group corresponds to the RGB mapping from a second picture.

By arranging the groove angles and/or spacings of the two sets of microstructures to be quite different, the resultant diffractive images generated by the OVD can be made to appear at quite different angles of view. Fig. 7.13(a) and Fig. 7.13(b) shows examples of the use of multi-channel microstructures (Lee 2000). In Fig. 7.13(a) the separation of the images is due to the different grating groove frequencies. This causes the different images to be diffracted at different angles of view to the OVD. By incorporating a slight degree of curvature into the individual groove elements much faster channel switching speeds can be obtained as well as less cross channel overlap. The Exelgram foils used on the Vietnamese bank cheques issued in 1996 used multi-channel effects of this type.

Examples of other high security Exelgram graphic effects can be seen on the new series of Hungarian banknotes issued in mid 1997. These banknotes contain an ExelgramTM foil stripe, which generates an image switch upon rotation of the banknote by ninety degrees.

Fig. 7.13(a) Fig. 7.13(b)
Microstructure Groove Application

Fig. 7.14. Currency applications; Exelgram microstructures as an anti-counterfeiting feature m-OE-ji OÉii' Vv Mfet- ~ V > I

Fig. 7.14. Currency applications; Exelgram microstructures as an anti-counterfeiting feature

In this case the OVD image switches from the crown of Hungary to the letters MNB when the banknote is rotated by 90 degrees with respect to the normal. A

small part of the OVD microstructure used to achieve this right angle switching effect is shown in Fig. 7. 13(b). Fig. 7.14 shows some sample Exelgram projects, including two Hungarian banknote denominations where the ninety-degree rotation image changes from the crown of Hungary to "MNB" on the foil stripe can be clearly seen. The effectiveness of Exelgram technology as an anti-counterfeiting feature is well documented.

7.3.7 Covert Image Micrographic Security Features

A unique advantage of the electron beam lithography origination process is its ability to fabricate complex non-diffractive anti-copy micrographic features and to integrate these features within the diffractive microstructure of the image. In this case the OVD image is designed to incorporate very small scale graphic elements consisting of combinations of alpha-numeric characters and other graphic elements such as logos and line drawings containing a range of feature sizes from 1 to 30 microns which act to diffusely scatter incoming light so that a palette of such micrographic elements can be used to form optically invariable macroscopic images of the greyscale or line-art form (Lee and Quint, PCT/AU98/00821). The Exelgrams used to protect the new Ukrainian passport/visa and the 2000 special edition New Zealand plastic $10 millenium banknote include micrographic features of this type for enhanced protection against counterfeiting. Fig. 7.15(a) and Fig. 7.15(b) show examples of two types of micrographic elements based on a euro theme test. In Fig. 7.15(b) the micrographic doves forming the background wallpaper pattern are each 20 microns across. The first three letters of the word EURO shown here are comprised of the second type of micrographic element, an enlarged view of which is shown in Fig. 7.15(a). A typical Exelgram OVD of this type may contain up to 100,000 individual micrographic elements embedded within a background diffractive microstructure. Particular advantages of such hybrid diffractive/ micrographic OVD micro structures include; (1) easy microscopic forensic authentication of the smallest piece of embossed substrate, (2) extremely high security against attempted holographic copying and (3) reduced metallic appearance of the OVD foil due to diffuse scattering from the micrographic elements.

Fig. 7.15. Micrographic doves forming the background
Fig. 7.16. CSIRO high security electron beam lithography laboratory established in 1993

7.3.8 Kinegram™ and Exelgram™: Comparison

The foregoing comparison of the attributes of the ExelgramTM and KinegramTM technologies shows a very high degree of agreement with respect to satisfying security printing industry requirements. Both technologies are based on high cost single source origination systems and each technology generates its own unique set of tightly controlled optical effects. A summary of Kinegram and Exelgram optical effects and security attributes is shown in Table 7.1. The above discussion relating to the particular advantages of using electron beam lithography in the fabrication of specialised diffractive microstructures for anti-counterfeiting applications shows that this technique has much greater flexibility, resolution and geometric precision than previous techniques based on laser interference or holographic imaging. This is particularly true in the case of fast switching graphic effects, optically variable greyscale and line art portraiture and complex anti-copy micrographic effects. The origination of these effects is not possible using the earlier optical techniques. The mathematically defined input structure functions combined with the polygon based sequential writing mode of the EBL origination technique offers an almost unlimited range of possibilities for the generation of security microstructures that can be applied to security documents as specialised "inks", by direct embossing or as embossed hot stamping foils.

7.3.9 VectorgramTM Image Multiplexing

As a further example of the versatility inherent in the electron beam lithography method for the micromanufacturing of optical security devices we will now discuss the technique of image multiplexing. The VectorgramTM multiplexing concept (Lee, US patent 6,342,969) is best understood by considering a PixelgramTM or ExelgramTM microstructure palette as a set of optical field generators. For example a typical palette of 26 elements might consist of two main groups. By element here we mean a 30 micron X 30 micron pixel containing a miniature diffraction grating of curved or straight grooves. Exelgram pixels are constrained by the requirement to smoothly connect to other pixels vertically above or below in the image. Elements 1 to 10 in a typical palette might consist of horizontal grooves of different spatial frequency; element 1 being of the lowest frequency and element 10 of the highest frequency. These elements are used for the generation of kinematic effects within the image. Elements 11 to 26 on the other hand might have a fixed spatial frequency, but varying degrees of groove curvature between the different elements. This group of 16 elements can therefore be used for positive to negative switch portraiture effects. Two channel effects can be produced by putting pixels of different spatial frequency next to each other in the image or by splitting the pixels in half and using different spatial frequencies in each half pixel. Micrographic effects can be produced with other palettes by putting little pictures into each 30-micron pixel. Different degrees of diffuse scatter between the different micro pictures can be used to generate optically invariable image effects. A two channel Pixelgram or Exelgram palette using split pixels can be regarded as a multi-component pixel palette because each pixel is a generator of two different optical fields.

Table 7.1. A summary of Kinegram and Exelgram security elements and optical effects

Security elements and optical effects

Kinegram™

Exelgram™

Single source origination

Yes

Yes

Origination technology

Analogue by proprietary laser exposure system

Digital by vector scan electron beam lithography

Technology availability

Tightly controlled

Tightly controlled

IP protection

Yes, various patents

Yes , various patents

Origination costs

High

High

Microstructure geometry

Arrays of 60 micron diameter pixels or 60 wide lines

Based on arrays of 15 or 30 micron wide diffractive tracks with internal groove angle modulations

Image resolution

60 micron

30 micron

Image brightness

Very high

High

Kinematic effects

Yes, observed by rotating

Yes, observed by rotating the

OVD about axis

OVD about an axis within

perpendicular to the plane of the OVD.

the plane of the OVD.

+/- greyscale portraiture

No

Yes

Greyscale effects

No

Yes

Sharp colour effects

Limited

Yes, sharp and highly visible

Multichannel effects

Yes

Yes

Channel switching speed

Slow

Very fast

Diffuse scattering efects

No

Yes

Microtext

Yes

Yes

Micrographic effects

No

Yes

Covert image effects

Yes. Fourier plane covert image observed under laser illumination

Yes. OVD plane image observed through coded transparent screen.

While the total image field generated by the two channels Exelgram may be regarded as a vector object, the local image field (i.e. the image field observed at a particular angle of view) is not a vector object because the two channels are not overlapping at the observer's eye. This is a preferred requirement as it is usually desirable to avoid cross talk between channels so as to generate a fast and clean switch from one image in channel 1 to another image in channel 2. However sometimes it is desirable to deliberately maximise cross talk between channels in order to create new types of optical effects. This multiplexing of optical fields to generate new types of optical effects is the key idea behind the Vectorgram™ process.

The process of arranging different types of optical microstructure elements within a Vectorgram™ pixel in order to cause a combined optical effect at the observer's eye is analogous to the process of combining the elementary directional aspects (Vector components) of a physical field to produce a resultant directional effect (Vector); hence the name Vectorgram™. Two examples of optical effects, which can be generated by this process, are described.

By splitting each pixel of a pixellated diffractive device into three areas corresponding to the red, green and blue components of a full colour image of a face or scene it is possible to create a device, which under point source illumination, generates a reproduction of that face or scene at one particular angle of view. The input artwork here involves three pictures corresponding to the red, green and blue components of the image. Fig. 7.17 shows a small section of a Vectorgram OVD with this type of structure. The area inside the black square represents a pixel of dimensions 30 microns X 30 microns. Each triplet of tracks contains diffraction grating structures of three slightly different spatial frequencies so that a red, green and blue triplet of spectra is allowed to overlap at one angle of view of the observer. The intensity of the red, green and blue components is controlled by varying the width of the pixel tracks according to the intensity required by the input artwork pixel.

In the optically variable device known under the trademark Pixelgram portraiture effects are achieved by mapping the greyscale palette of an image into a palette of miniature diffraction gratings (e.g. each of size 30 microns X 30 microns) where the grating rulings are curved in such a way as to modulate the intensity of the diffracted light in accordance with the level of greyness in the original portrait artwork. Lighter shades of grey correspond to greater degrees of groove curvature and more expansion of the diffracted beam. In the case of the Exelgram technology the modulation of the diffracted light intensity is achieved by varying the angle of the grooves within each grating track.

By combining both groove modulation techniques it is possible to generate optically variable portrait effects with increased brightness and a higher degree of variability in the transition from positive tone image to negative tone as the angle of view is changed. Fig. 7.18 shows a small section of the microstructure of such a Vectorgram portrait structure. A photograph of a corresponding example Vectorgram OVD at one angle of view is also shown. As can be seen from the micrograph, the combination process is a result of interlacing columns of Pixelgram type grating structures with Exelgram type microstructures. Fig. 7.19

shows a comparison of Exelgram positive to negative switching portraiture effects with the corresponding Vectorgram equivalent. Note the increased brightness of the Vectorgram positive tone image when compared to the Exelgram version. In each case the OVD size is 22mm X 22mm.

Fig. 7.17. Micrograph of a small section of a Vectorgram OVD
Fig. 7.18. Vectorgram microstructure for producing high brightness dynamic portraiture
Fig. 7.19. Comparison of Exelgram images (A , B) and Vectorgram images (C , D)

7.3.10 Interstitial Groove Element Modulation

In the Pixelgram, Exelgram and Vectorgram scenarios, the techniques for image modulation of the input artwork include groove frequency modulation, groove angle modulation, curvature modulation and combinations thereof. In all of these cases the fundamental modulation unit is the pixel palette element consisting of a fixed number of diffraction grating grooves of particular angles, spacings and curvatures. However it is also possible to extend the resolution of the modulation unit by considering modifications to the diffracted wavefront due to the "doping" of individual pixel palette elements with particular groove elements. The principle idea (Lee PCT/AU99/00520) here is the concept of using a continuous background groove pattern to generate a diffracted carrier wave and then to modulate this carrier wave with optically variable information by "doping" the background groove pattern with a multiplicity of interstitial groove elements to modulate both the spatial frequency and amplitude from each interstitial groove region according to the requirements defined by the optically invariable input picture information. In particular, the angles of diffraction from any particular interstitial groove are given by the local spatial frequency in that region and this is related to the number of parallel interstitial groove elements in that region. The intensity of diffracted light from the particular interstitial groove region under consideration is proportional to the length, angle, shape and degree of curvature of the interstitial groove elements. Hence the interstitial groove element concept provides a possible mechanism for modulating both diffracted light intensity and diffraction angle at a much higher degree of resolution. Fig. 7.20 shows a small section of an OVD microstructure of this type.

Fig. 7.20. Small section of an interstitial groove element optical security microstructure

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