Process for realization of an optically variable image

Details Process for realization of an optically variable image Process for realization of an optically variable image

1996-02-15

1998-09-15

Hellner, Mark

Optical: systems and elements

Optical modulator

359 15, 359 22, 283 86, G02B 2600, B42D 1500

Patent

active

058087761

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention concerns a process for realization of an optically variable image serving as the model for reproducing a security element on a document, by insolation of a photosensitive substrate with the aid of two coherent bundles of light.
2. Description of the Related Art
"Optically variable image" is understood to mean an image such that the retinal impressions of an observer vary when, with the observer being immobile and the light source fixed, the document bearing the security element pivots around a vertical axis or a reference axis which is fixed relative to the document.
In the area of security, the most interesting effects are those connected to the variability of grids. Such variability is expressed either by variations in colors or by the appearance and disappearance of juxtaposed elements leading to the effect of animated images obtained by playing upon the directivity of the diffractive light either by pivoting the document around a geometric vertical axis or rocking it along a reference axis of the document.
Among the possible variations of the image as perceived by the observer are the following: the rendering of pure colors and the achromatic; image, allowing, for example, the creation of the effect of animation of one or several parts of the image.
Elements of optical security are used on documents such as bank cards, credit cards, identification papers, or documents having a fiduciary value in order to prevent their falsification.
Historically, the first optical element to be used was the hologram, the product of a technology which allowed the reproduction of images in relief and true colors. However, that technology is very widespread and thus is not well adapted to security documents.
The stereogram is the tridimensional holographic image which is the most complex to elaborate. It is a high level security protection allowing objects, persons, scenes in relief, or three dimensional synthetic images to be recorded. However, under ordinary lighting, the rendering of tridimensional holographic images does not allow perfect verification of the image.
Thus, under certain circumstances, the authentication of high security documents loses its accuracy. EP-A-0 467 601 describes a process for the realization of a holograph by the insolation of a substrate. Insolation of the substrate according to the process described in that document is realized point by point by focusing two bundles of light, and the substrate is moved in increments from the abscissa to the ordinate. The size of the point to be insolated is limited by a mask which has a punctiform opening. The accuracy of the realized image is a function of the size of the pixels and the precision of the incremental movements from the abscissa to the ordinate. The angular arrangement of the bundles of light should be synchronized with the displacement of the substrate. This process does not make it possible to obtain with great accuracy images which consist of fine lines where the desire is for each line to be totally visible or invisible intermittently in the eyes of an observer viewing the image.
EP-A-0 105 099 describes another process for manufacturing an image in which a series of three diaphragms whose relative position can be varied is used, whereby two of the diaphragms make it possible to generate two punctiform sources while masking a larger source. The two punctiform sources are coherent with each other and make it possible to illuminate the entire sensitive surface upon which the optically variable image is to be realized. The third diaphragm makes it possible to isolate a small element of the sensitive surface. Displacement of the first two diaphragms in relation to each other and by rotation around an axis perpendicular to the substrate to be insolated makes it possible to define the interferential structure of the grid, that is, the spatial frequency and angular orientation of the striations on the grid. Displacement of the third diaphragm makes it possible to inscr

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