Optical: systems and elements – Holographic system or element – Authentication
Patent
1993-07-15
1996-01-09
Dzierzynski, Paul M.
Optical: systems and elements
Holographic system or element
Authentication
359 22, 359 24, 359 25, 283 86, 283 93, G03H 100
Patent
active
054833639
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to security devices, methods for constructing such devices, and methods and apparatus for authenticating the devices.
Security devices such as holograms and diffraction gratings have become well known for protecting identification articles such as credit cards and the like.
A typical example is described in U.S. Pat. No. 4,269,473 in which a hologram is incorporated into a layer of an identification card. In this case the hologram includes machine readable characters as well as a visual representation of the other features of the card.
U.S. Pat. No. 4,184,700 describes the provision of a relief structure on a thermoplastic coating of an identification card or document which responds to incident, visible light to generate an interference pattern.
U.S. Pat. No. 4,544,266 describes a special diffraction pattern which is provided on an identity card or the like, the pattern comprising for example a hologram or diffraction grating. The diffraction pattern will diffract light at different wavelengths in different directions and this is used to provide an indication of whether a security device under test is authentic.
U.S. Pat. No. 3,542,448 discloses the recording of a number of holograms in different sub-areas of a storage medium so that upon exposure to light, coded information within the holograms can be determined.
GB-A-2016775 describes the provision of two optical markings on a substrate which cause a reading light be to deflected in different directions.
U.S. Pat. No. 4,140,373 describes a composite hologram which generates two machine readable codes which can be read at respective, different wavelengths. This suffers from the disadvantage that the existence of machine readable information is readily apparent and thus is likely to be fraudulently copied.
SUMMARY OF THE PRESENT INVENTION
There is a continuing need to increase the security of devices of this type and in accordance with a first aspect of the present invention, we provide a security device comprising first and second diffractive structures contained within a surface relief structure, the structures being such that the device responds to illumination at a first, visible wavelength, by a white light source to generate a first, visible pattern while any pattern generated by the second structure is not substantially visible at that wavelength, and that the device responds to illumination at a second wavelength substantially different from the first wavelength to generate a second pattern suitable for machine reading while any pattern generated by the first structure is substantially suppressed.
This new security device involves the provision of first and second diffractive patterns to form a composite structure, which patterns are individually generated (eg. reconstructed) upon illumination at two different wavelengths and alternately predominate depending upon the wavelength of the incident radiation used for reconstruction of the images.
This device appears to the normal observer to be conventional in that the first pattern is visible (ie. human readable) upon normal illumination. It has enhanced security not only because of the presence of the second structure but also because the machine readable pattern is not apparent upon illumination with the first wavelength.
Typically, at the second wavelength the first structure pattern is angularly or spatially separated from the first pattern.
The patterns may be fully superposed (ie. added to form a single, combined relief structure), partially overlap, or be positioned side by side.
In one example, the second wavelength may comprise infra-red radiation and the first wavelength is a band of white light. The references to illumination at the second wavelength includes illumination at a wavelength band. In this context, visible wavelengths are regarded as lying in the range 400 nm to 700 nm. Usually, the second wavelength will be longer than the first and preferably will lie in the infrared range, particularly the near infrared. Typical wavelength
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Andreassen Jon
Drinkwater Kenneth J.
Ezra David
Holmes Brian W.
De La Rue Holographics Limited
Dzierzynski Paul M.
Jr. John Juba
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