Optical: systems and elements – Diffraction – From grating
Patent
1996-02-05
1998-10-20
Henry, Jon W.
Optical: systems and elements
Diffraction
From grating
359569, 359 2, G02B 518, G03H 108, G03H 126
Patent
active
058255478
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a diffractive device. It relates particularly to a diffractive device 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. The device may be used in a number of different applications, and it has particular applicability as an anti-forgery security device on banknotes, credit cards, cheques, share certificates and other similar documents.
Several different types of diffractive devices which, when illuminated, generate diffractive images, are known. In January 1988, an Australian ten dollar banknote was released featuring a diffractive image of Captain Cook. The diffractive grating used in the image was for the most part comprised of substantially continuous lines, and the shapes and configurations of the lines were determined according to optical catastrophe theory in order to generate fine detail in the diffractive image observed.
International patent application PCT/AU90/00395, the contents of which are incorporated herein by reference, discloses an alternative method for generating an optical diffraction image. In this case, the diffractive device is divided into a large number of small diffraction grating structures, each of which diffracts a beam of light which acts as a pixel, with the pixels combining to form an overall image. According to preferred aspects of the arrangement disclosed, the respective diffraction grating of each pixel comprises a plurality of reflective or transmissive grooves or lines which are usually curved across the pixel. Groove or line curvature determines both local image intensity (eg. shading) and local optical structure stability. Groove or line spacing in each pixel grating determines local colour properties, with non-primary colours generated by a pixel mixing. Average groove or line orientation determines movement or colour effects. The overall surface structure of each pixel grating is selected from a palette of different grating types having a limited number of distinct values of average curvature and average spacing.
An advantage of the use of pixel gratings in a diffractive device is that it permits the device to generate more than one diffraction image. Some of the gratings can have diffractive surfaces with particular line spacing curvature and orientation characteristics which contribute to the generation of an image viewable from a particular range of viewing angles, and other gratings have different surface characteristics contributing to the generation of a different image viewable from a different range of viewing angles. This result is much more difficult to achieve in a continuous grating diffractive device.
Another advantage of a pixel grating diffractive device is that it allows storage of picture information in a digital format. However, a predetermined surface area on the diffractive device must be set aside for each pixel, and this is not the most efficient way of storing picture information in a limited space. Accordingly, there is scope for a more efficient manner of storing picture information in a diffraction grating.
Moreover, in a pixel grating diffractive device, there are inevitable discontinuities between adjacent gratings. Diffraction effects occur in these discontinuities. It is normally possible to ensure these extraneous diffraction effects are small relative to the intentional diffraction effects generated by the diffractive device, but the extraneous diffraction effects are still detectable. It is desirable to reduce the extraneous diffraction effects.
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, wherein at least part of the surface relief structure is arranged in a series of tracks, each track having a diffracting surface which generates a component of a diffraction image, su
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Commonwealth Scientific and Industrial Research Organisation
Henry Jon W.
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