Surface pattern including light-diffracting relief structures

Details Surface pattern including light-diffracting relief structures Surface pattern including light-diffracting relief structures

1998-09-03

1999-10-19

Henry, Jon

Optical: systems and elements

Diffraction

From grating

359572, 359576, 359 2, 283902, G02B 518, G02B 2744, B42D 1510

Patent

active

059698632

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a surface pattern of the kind set forth in the generic part of claim 1 and 12.
Such surface patterns with a microscopically fine relief structure are suitable for example for increasing the level of safeguard against forgery and/or the conspicuous identification of articles of all kinds and can be used in particular in relation to value-bearing papers or bonds, passes, payment means and similar articles to be safeguarded, as optical information carriers. Such surface patterns are also suitable for packaging foils.
The EP-A 0 467 601 discloses a light diffracting surface pattern divided into regions with different gratings. The regions may comprise overlays of two different gratings, e.g. as shown in the EP-A 0 357 837 the gratings may be parallel and differ in their spatial frequency. On the other hand, the WO 95/02200 teaches how to superimpose several diffractive structures so that each of the diffractive structure gives rise to a distinct diffraction image or component thereof.
The object of the present invention is to provide a surface pattern having conspicuous patterns of optical grating structures, which is difficult to forge.
In accordance with the invention the specified object is attained by the characterising features of claim 1 and claim 12. Particular embodiments of the invention are characterised in the dependent claims.
A complete understanding of the present invention will be accomplished by reading the detailed description of a preferred embodiment thereof in conjunction with the drawings, wherein is shown in
FIG. 1: a grating,
FIG. 2: a hemisphere,
FIG. 3: a scaled grating vector circle,
FIG. 4: squared Bessel functions,
FIG. 5a: first surface pattern
FIGS. 5b+c: the vector circles of the first surface pattern,
FIG. 6: a second surface pattern,
FIGS. 6b+c: the vector circles of the second surface pattern,
FIG. 7: graphic elements,
FIG. 8a: the vector circle of the grating composed of two parallel gratings,
FIG. 8b: the vector circle of a reference grating,
FIG. 9: light diffracted by the superimposed grating GS,
FIG. 10a: the vector circle of the grating composed of two perpendicular gratings,
FIG. 10b: the vector circle of the reference grating,
FIG. 11a: the vector circle of the grating G5,
FIG. 11b: the vector circle of the grating G6,
FIG. 11c: the vector circle of the grating composed of the perpendicular grating G5 and G6,
FIG. 12a: a pattern made of discrete closed lines,
FIG. 12b the vector circle of the gratings composed of a first set of pairs and
FIG. 13: the vector circle of the gratings composed of a second set of pairs.
For understanding of the invention, some fundamental facts of the light-diffracting properties of gratings reflecting the incident light in the context of the Fraunhofer diffraction theory are briefly described with reference to FIGS. 1 to 4. A grating G1 which is arranged in a plane and which is in the form of a relief and which is formed with rectilinear, regularly arranged furrows 3 can be characterised by the parameters line spacing d, profile shape, profile height h and orientation j of the grating furrows 3 in the plane. The angle j is referred to as the azimuth angle. A monochromatic light beam 1 of the wavelength I which impinges onto such a grating G1 with microscopically fine dimensions is diffracted into a finite number of discrete diffraction orders in accordance with the equation: diffraction angle q.sub.j denote the intermediate angles between the line 4 which is normal to the plane of the grating G1 and the incident beam 1 or the reflectedly-diffracted beam 2 respectively, and the integral index j denotes the diffraction order. Equation (1) applies for the situation where the light beam 1 is in a plane which is perpendicular to the furrows 3 of the grating G1. Only a single diffracted beam 2 is shown in FIG. 1. Varying the line spacing d makes it possible to determine the maximum number p of the possible diffraction orders which occur for example in the event of perpendicular incidence of the light, that is to say q.sub

REFERENCES:
patent: 734134 (1903-07-01), Porter
patent: 4568141 (1986-02-01), Antes
patent: 5058992 (1991-10-01), Takahashi
patent: 5483363 (1996-01-01), Holmes et al.
patent: 5627663 (1997-05-01), Horan et al.
patent: 5886798 (1999-03-01), Staub et al.

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