Optically diffractive structure

Details Optically diffractive structure Optically diffractive structure

1997-12-03

2001-08-07

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Diffraction

From grating

C359S569000, C359S575000, C359S002000, C283S085000, C283S086000

Reexamination Certificate

active

06271967

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a structure arrangement comprising surface regions having one or more structures with an optical-diffraction effect, in particular for visually identifiable, optical security elements for value-bearing documents, for example banknotes, credit cards, passes or check documents, or other items to be safeguarded.
2. Description of the Prior Art
When using a structure arrangement of that kind visually perceptible items of information can be communicated to a viewer by diffraction and/or refraction of incident ambient light. It is however also possible to envisage detection of optical items of information of that kind by machine, using suitable items of equipment. In the simplest case a structure arrangement of that kind is afforded by an in particular rectilinear wave-shaped relief structure which is provided on the surface of a surface region of a carrier element and at which incident ambient light is reflected with diffraction and/or refraction. In this respect the term wave or relief structure does not necessarily denote a structure with a surface line which is steady in terms of the cross-section of the surface region and which in particular is sinusoidal, but this may also involve rectangular, step-shaped or wedge-shaped surface structures. Those surface structures can be of a periodic or aperiodic configuration. It is also conceivable that the structures having an optical-diffraction effect are not formed exclusively by relief structures, but that there are provided variations in the refractive index in structured form.
Diffraction of incident light or light which passes through the structure arrangement, at the structures of the surface regions, and therewith the information which is emitted therefrom in the form of an optical diffraction image are determined by the grating or structure parameters. In the case of relief structures reference is to be made here to the number of wave or grating lines per unit of length of a surface region, the so-called spatial frequency, as well as the orientation and the cross-sectional shape of the relief structure. The cross-sectional shape is determined inter alia by the differences in respect of height in the relief structure, more specifically both by the differences in respect of height between the individual raised portions relative to each other, and also between raised portions and troughs or depressions of the relief structure. In the case of structures which are not formed by relief structures but by variations in refractive index which are arranged in a structured manner, the structure parameters are defined in accordance with the foregoing, while in addition the refractive indices of the optically effective layer or layers are to be taken into account. By virtue of a suitable configuration and arrangement of the structures, it is possible to obtain a structure arrangement which influences the phase relationships of the incident light in such a way that a given item of optical information can be emitted in a given viewing angle range and thus perceived by a viewer while in another viewing angle range another item of information can be emitted. A variation in the phase relationship by virtue of the structure arrangement results from the product of the refractive index and the geometrical wavelength within or at the structure arrangement. The optical phase difference (OPD) in the case of a wave which is diffracted or reflected at a location x
1
(for example a raised portion of the structure) and at a location x
2
(for example a depression of the structure) would be:
OPD
(
x
1
, x
2
)=∫
n
(
x
1
, z
)
dz−∫n
(
x
2
, z
)
dz.
It also follows from that relationship that it makes a difference whether a reflection relief grating is covered with a lacquer, as is current practice, or not, as it is not just the difference in respect of height alone but also the refractive index of the cover layer, that is relevant. An item of visually perceptible information which corresponds to the structures of the surface regions and which is dependent inter alia on the lighting or viewing angle, in particular information in regard to authenticity of the safeguarded item, can therefore be communicated to a viewer in the form of the reflected light or the light which passes through the structure.
By virtue of the use of per se known security elements with a structure arrangement having an optical-diffraction effect, in regard to the articles to be safeguarded as were referred to in the opening part of this specification, it is possible for items of authenticity information in respect of the safeguarded article to be rendered visible even to the unpracticed lay person. At the same time it is possible for forgery, for example in the form of duplication, having regard to known forgery procedures, in particular optical duplication procedures, to be rendered impossible or made sufficiently difficult.
Structure arrangements for example are known in which, by virtue of a specific variation in the above-mentioned structure parameters—spatial frequency, orientation and cross-sectional shape of a relief structure, differences in respect of height or phase in the relief structure—a given item of visually perceptible optical information which originates from one surface region can be communicated to a viewer in dependence on the lighting direction, in a given viewing angle range, while no or another item of optical information can be perceived in the same viewing angle range, originating from another surface region of the structure arrangement. Pivotal movement of the carrier element which carries the structure arrangement, about an axis which is in the plane of the carrier element or about an axis which extends perpendicularly to the plane of the carrier element, causes a change in the information which originates from the surface region which is first viewed—in particular that surface region can appear dark—while another surface region which initially appeared dark imparts optical information, for example in the form of a color impression.
SUMMARY OF THE INVENTION
The object of the present invention is to make it more difficult to forge and in particular copy a structure arrangement of the above-described kind, and in particular to increase the multiplicity of encoding options for the optical information which can be perceived within a viewing angle range.
In accordance with the invention that object is attained in that for the production of given items of optical information in given viewing directions in one or more of the surface regions of the structure arrangement there are provided sub-regions with a structure that is identical except for the structural parameter of optical depth and that the optical depth of the structure is constant over the extent of a sub-region but is different from the optical depth of the structure of another sub-region.
In the case of pure relief structures the optical depth is determined by the geometrical depth thereof; it corresponds to the optical wavelength difference between two beams which are reflected at raised portions or depressions respectively of the relief structures. In the case of structures with local variations in refractive index, the optical depth which is responsible for the phase relationships of the light which is diffracted at the structure is given by the different refractive indices and additionally by different thicknesses of the coating. The optical depth of a structure determines inter alia the amount of light which is diffracted away from the geometrical reflection direction, that is to say the diffraction efficiency of that structure. Two structures which are identical except for the parameter of optical depth thus communicate items of optical information which differ from each other, in a given viewing direction. The association according to the invention of a structure of a given first optical depth with an item of wave-optical information which is wanted within a given viewing angle range, in conjunction

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