Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-12-14
2004-12-21
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S002000
Reexamination Certificate
active
06833956
ABSTRACT:
This invention relates to diffractive devices which generate diffraction images when illuminated by a light source. It relates particularly to diffractive devices or “optically variable devices” (that is, devices which appear different when observed from different angles or under different illumination conditions) which incorporate in their surface relief structures a new type of diffractive element, hereinafter referred to as “interstitial elements”.
Throughout this specification including the claims, the terms “diffraction”, diffractive” and “diffracted” have the meaning that includes the process whereby electromagnetic radiation in the form of light waves is scattered either coherently, incoherently, diffusely, reflectively and/or specularly from surface relief elements of the device in the form of ridges, grooves, troughs, indentations and/or protrusions of spatial and depth and/or height dimensions of extent greater than 0.05 microns, and the wavelength range of the incident and diffracted light is any single or multiple combination of wavelengths in the range of from 0.1 microns to 15 microns.
There are numerous different uses for diffractive devices of the type to which this invention relates. One common use is as a security element on banknotes, cheques, credit cards, share certificates, computer software and other valuable documents and objects. Diffractive devices are typically created by embossing a diffractive surface relief structure into metallised foil which is then adhered to the document or object, in such a way that the device cannot be removed without destroying it. In some cases, the diffractive structure can be embossed directly into the document or object. Optically variable structures cannot be photocopied, because a photocopy does not possess the optically variable characteristics of the original.
One common type of optically variable device is a hologram. Coherent laser radiation is split into two beams, one of which is directed at an object. Light reflected from the object is combined with the other beam to create an interference pattern, which is then embossed into a surface. When the surface is subsequently illuminated, an image of the object is observed. Holograms have been used on Visa™ and MasterCard™ credit cards since 1984.
Holograms are best observed under a single point source of light. However, credit cards are normally used in diffuse lighting conditions, such as in general daylight or under multiple fluorescent lights. As a consequence, the image observed is typically blurred and indistinct. A poor quality counterfeit hologram could therefore conceivably be passed off as an original.
Another type of optically variable device is a diffractive device composed of numerous straight-line grating elements. Straight-line gratings (and concentric circular gratings) can be produced by interference between two coherent laser beams. Parallel lines are written concurrently by the interference pattern. Kinegram™ devices provide examples of optically variable devices using such technology. Relevant patent publications include European Patents 105099, 330738 and 375833. Kinegram™ devices were used on the Saudi Arabian passport in 1987 and on the Austrian 5000 Schilling banknote in 1990.
An advantage of straight-line grating devices is that they provide much brighter colours than holograms, and they have considerably clearer visual effects. However, because of the diffractive properties of straight line gratings, each effect is observable only from a very narrow range of viewing angles. Moreover, standard straight line gratings are useful only for producing line-art graphics and not for portraiture which requires more flexibility in terms of grey-scale and colour effect selection.
Another type of diffractive device is the Catpix™ device, used on the Australian ten dollar plastic banknote issued in 1988. The Catpix™ diffractive structure consists essentially of continuous undulating lines which extend throughout the device or a substantial portion of it. The surface area is divided into notional pixels, and at each pixel the undulating lines come together to form an optical catastrophe region, which is observed by the viewer as a point of light with a fixed brightness value which depends on the spatial frequency of the lines at the catastrophe region. The points of light combine to form a diffraction image which is seen by the observer. In the case of the 1988 ten dollar note, the image was a portrait of Captain Cook.
Catpix™ diffractive devices, details of which were published in European Patent Publication EP 044 9893 B1 (the contents of which are hereby incorporated herein by reference) are made using computer-controlled electron beam lithography. Each line or groove in the diffraction pattern is written individually by an electron beam. This enables precise control over the positioning and shape of each diffractive element.
Pixelgram™ diffractive devices are based on an actual division of a surface relief structure into separate squares or pixels. Each pixel forms a separate diffraction grating, which may consist of straight lines or curved lines, and is responsible for generating a single point of light with a particular grey-scale and/or colour value in the diffraction image. Details of Pixelgram™ technology have been published in European Patent Publication 049 0923 B1 (the contents of which are hereby incorporated herein by reference). As is the case for Catpix™ devices, Pixelgram™ devices are constructed using computer-controlled electron beam lithography.
An advantage of Pixelgram™ devices over Catpix™ devices is that the whole of each pixel area on the Pixelgram™ device contributes to the light point generated by that pixel, whereas only the catastrophe region of a Catpix™ notional pixel contributes to the corresponding light point. Another advantage is that pixels may have radically different orientations from those of surrounding pixels, so that more than one diffraction image can be generated simultaneously. However, there are inevitable discontinuities between adjacent pixel gratings, and these result in extraneous diffuse scattering effects. Diffuse scattering from the edges of pixels has the effect of reducing the brightness of the observed optical effects, particularly for very small pixel sizes. Moreover, because of the clear separation between pixels, it is potentially easier for a person to decode the optical effects mechanisms of Pixelgram™ structures than Catpix™ structures, using microscopic examination of the structures.
Exelgram™ diffractive devices consist of diffractive elements arranged in tracks. Diffractive elements, which may be grooves, ridges, or polygonally shaped indentations or protrusions, are arranged in substantially continuous configuration along the tracks. Details of Exelgram™ technology have been published in U.S. Pat. No. 5,825,547 (the contents of which are hereby incorporated herein by reference). As is the case for Catpix™ and Pixelgram™ devices, Exelgram™ devices are typically created using electron beam lithography.
Exelgram™ devices, which were first used on Australian opal stamps in 1995 and American Express Travellers' cheques in 1997, overcome part of the extraneous diffuse scattering effect problem apparent in Pixelgram™ devices; however, there are still extraneous diffuse scattering effects caused at the interface between tracks.
According to the present invention, there is provided a diffractive device having a surface relief structure which, when illuminated by a light source, generates one or more diffraction images which are observable from particular ranges of viewing angles around the device, including:
background diffractive structural elements; and
interstitial diffractive structural elements;
wherein the interstitial elements are interspersed between the background elements such that the diffractive action of the background elements is modulated by the interstitial elements, with differing interstitial element configuration in differing parts of the surface relief structure producing di
Amari Alessandro
Commonwealth Scientific and Industrial Research Organisation of
Ladas & Parry LLP
Robinson Mark A.
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