Optical: systems and elements – Diffraction – From grating
Patent
1991-08-19
1994-08-02
Ben, Loha
Optical: systems and elements
Diffraction
From grating
359572, 283 72, 283 91, G02B 518, G09F 300, B42D 1500
Patent
active
053351134
DESCRIPTION:
BRIEF SUMMARY
This invention relates to diffraction gratings and is especially but not exclusively concerned with diffraction gratings which may be employed as security devices, for example, in currency notes and credit cards.
The traditional security device employed to assist in distinguishing genuine bank notes from counterfeits is a metal foil insert. This device is not a great challenge to professional counterfeiters and it has been considered desirable for some time to develop satisfactory security inserts which are more sophisticated and less easily reproduced than metal foil strips. Past proposals, some of which have been made with particular regard to the recent development of longer life currency notes comprising of plastic laminates, have included diffraction structures such as multiple film devices, straight line gratings, image holograms, and line gratings with precisely determined variable depth grooves. None of these proposals have reached fruition however, because the optically variable patterns produced could not sustain an acceptable level of structural stability as the notes became heavily crinkled in day-to-day use. It is one objective of this invention to overcome this problem in a device of adequate complexity for use as a currency note security insert.
In one aspect, the invention essentially comprises a significant practical use of the application of the present inventor's theory of generalized curvilinear diffraction gratings to optical diffraction catastrophes. The theory is outlined in Optica Acta 1983, Vol. 30 Nos. 3 and 4, and the application to optical diffraction catastrophes is disclosed in Vol. 30, No. 4, 449-464 and in Vol. 32, No. 5, 573-593. In particular, it has been realized that the aforementioned objective can be achieved by imposing a matrix of diffraction catastrophe pixels on a regular diffraction grating.
The invention accordingly provides a generalized diffraction grating of reflective or transmissive lines formed by a regular matrix of pixels each containing a respective curvilinear portion of one or more of said lines, which pixels when illuminated each generate a two-dimensional optical catastrophe image diffraction pattern whereby the total image diffraction pattern of the grating is optically variable but structurally stable.
By "image diffraction pattern" in the context of this specification is meant the optical image observed by the naked eye focused on the grating when it is illuminated by an arbitrarily extended diffuse source of finite width such as a fluorescent tube. A pattern is described herein as "optically variable" where it varies, according to the position of observation and is "structurally stable" if its broad form at any given position of observation is not materially altered by distortion of the grating surface.
Expressed in mathematical terms, the reflective/transmissive lines of the grating are advantageously such that they are defined, in terms of coordinates x,y in the plane of the grating, by the equation S(x,y)=kN where k is a scaling factor, N is an integer and the function S(x,y) is given by: illuminated normally by a collimated monochromatic light wave, of the image diffraction pattern, and is piecewise relatively slowly varying with respect to x and y, and rapidly with respect to x and y and whose Hessian is not identically zero except along certain characteristic lines corresponding to the caustics in the Fresnel or Fraunhofer diffraction pattern of the grating.
The Hessian of C(x,y) is a standard complex derivative expressed as follows: C(x,y)/.differential.y.sup.2 ]-[.differential..sup.2 C(x,y,)/.differential.x.differential.y].sup.2 ( 2)
The condition that the Hessian be not identically zero except along certain characteristic lines is a condition for an optical catastrophe image diffraction pattern in accordance with the above theory of generalized curvilinear diffraction patterns. The function C(x,y) may thus be described as the catastrophe function for the grating.
The right side of equation (1) may include a further summed term F(x,y), be
REFERENCES:
patent: 4130348 (1978-12-01), Minami
patent: 4520387 (1985-05-01), Cortellini
patent: 4984824 (1991-01-01), Antes et al.
patent: 5032003 (1991-07-01), Antes
patent: 5145212 (1992-09-01), Mallik
R. A. Lee "Generalized Curvilinear Diffraction Gratings I-V" Optica Acta vol. 30, #s 3 and 4, 1983.
R. A. Lee et al, "Diffraction Patterns of Generalized Curvilinear Diffraction Gratings, etc." Optica Acta, vol. 32, #5, 1985.
Lee, R. A., 1983, vol. 30, No. 4, 449-464 Generalized Curvilinear Diffraction Gratings.
Goodman Richard A.
Jackson Wayne K.
Ben Loha
Collins Darryl
Reserve Bank of Australia
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