MXPA98003436A - Optically variable device, a paragraph method and a recorder for me - Google Patents

Abstract

In an optically variable device that consists of pixels, there are at least two subsets of pixels (1) that are graphical reproductions of Fourier transform phases of maps (M) of observation angles defined for each of these subsets of pixel pixels ( 1). An optically variable device is recorded by means of illumination, when opening a shutter (2), with an expanded and convergent beam (L) of laser light, an image (6) of the pixel map, obtained as a graphic reproduction of a phase of the Fourier transform of map and observation angles, the map is then reduced to a desired size with a lens (4). Subsequently, the shutter (2) cuts or interrupts the laser beam (L) and photosensitive material where an optically variable device to be engraved moves to the next pixel with the use of a stage moving in XY (5) and the procedure it is repeated until the entire optically variable device is recorded. A recorder for optically variable devices is provided with an image (6) of the pixel map between an expander (3) of the laser beam (L) and the lens (

Description

OPTICAL VARIABLE DEVICE, A METHOD TO RECORD AND A RECORDER FOR THE SAME FIELD OF THE INVENTION The subject of the present invention is an optically variable device (OVD) and a method for recording the OVD and a recorder that performs this method.

BACKGROUND OF THE INVENTION Optically variable devices in the form of diffractive structures and holograms with resolution of about 1000 line pairs per mm, typical of these elements, are frequently used for many purposes, for example, for authentication elements. Diffraction grating mosaics are good examples of these elements. One of the best known types of OVDs is a "Cinegram (Kinegram)" disclosed in Swiss Patent 04576. The images visible in the cinegram change smoothly depending on the angle of observation. This effect is achieved by dividing the flat element into pixels that are further divided into several fragments, imperceptible to the naked eye, the fragments will be assigned to particular images. The fragments of different pixels corresponding to a given image are provided with straight diffraction gratings, in such a way that all the present pixels diffract the light towards one direction, in this way this image is visible at an appropriate observation angle. When the observation angle changes, the pixel fragments that correspond to another image become visible. The main disadvantage of these OVDs is the limitation of the number of images that can be recorded in an OVD up to a certain predetermined number and the limitation of the resolution used in the recording of the OVD. In particular, it is impossible to produce with this mode an OVD containing high resolution images that change smoothly. In Japanese Patent JP 26684/91 and JP 79080, methods for recording OVDs are described in which curvilinear diffraction gratings with precisely defined channels or grooves are used. This allows to obtain the desired effects of optical variability. It is also possible to record an OVD using lithographic techniques. Its main disadvantage is the high cost of the process and the relevant equipment to carry it out. Another method for recording an OVD is presented in U.S. Patent 5262879. In this method a flat image digitized in the computer's memory and which is converted into pixels of the diffraction grating that form an OVD. This grid is recorded pixel by pixel on a photosensitive surface using an optical laser recorder. The disadvantage of this method is its low flexibility, that is, a limited field of potential OVDs possible to manufacture. The method described in the present invention eliminates the aforementioned disadvantages. The OVDs can be recorded by using devices that consist of: a laser, a shutter, an optical beam expander, a rotationally mounted diffraction grating and converging lenses in whose focal point a diaphragm is provided in the form of a point dark that shades undifracted light. The other lens produces an adequately reduced image of the grating pairs of duplicate lines in the photosensitive material located on a stage that moves in computer-operated XY. A disadvantage of this recorder is the limited field of potential OVDs, which can only contain fixed spatial frequency diffraction gratings with variable angular orientations. The recorder described in the present invention eliminates the aforementioned disadvantages.

SUMMARY OF THE INVENTION An OVD comprising a plurality of diffractive pixels, wherein each pixel is a hologram of the pure phase Fourier transform of a map of observation angles associated with this pixel, the plurality of pixels comprising at least two subsets of pixels, each of the pixels of a first subset contained in a first area (OA) is a Fourier hologram of a first observation angle map (PA) and each of the pixels of a second subset contained in a second area (OA) is a Fourier hologram of a second observation angle map (PB). Advantageously, this OVD comprises a third subset of pixels comprised in an area that will be an intersection of the first area (OA) and the second area (OB), and each of the pixels of the third subset is a Fourier hologram of the sum of the first and second observation angles (PA, PB). Advantageously, at least one of these pixels is close to a rainbow hologram. The subject of the method for recording an OVD is that after the opening of the shutter a beam of convergent and expanded laser light illuminates an image of the pixel map obtained as a graphic reproduction of a phase of a Fourier transform of the observation angle map . The map is then photo-reduced to the appropriate size using a lens, the shutter cuts or interrupts the laser light and the photosensitive material where the OVD was recorded is moved or moved to the position of the next pixel using a stage that moves on XY and the procedure is repeated until the pixels included in all the subsets are recorded. Advantageously, the convergent beam of laser light, after passing through the image of the pixel map, modified by a filter, remains in a plane where the Fourier transform of the map is formed by lenses. Advantageously, the Fourier transform of the observation angle map is calculated using the iterative algorithm of the Fourier transform. The matter of the recorder for OVD is that between the expander and the lens an image of the pixel map is provided. Advantageously, in the focal plane of the beam expander there is a filter that modifies the Fourier spectrum of the pixel image.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY The invention is shown in a mode presented schematically in the drawing where, the Figure 1 shows an OVD and a map of the observation angles of a pixel. Figure 2 shows the method of an OVD recording and Figure 3 shows a recorder for OVD that performs the method. An optically variable device (OVD), (Figure 1) contains a diffractive structure consisting of small pixels 1 which are pure phase Fourier holograms of M maps of observation angles of these pixels. Each map M contains a composite image of points of light against a black background. An angle a between the segment of a line from the half 0 of the map of angles to any of the points of light and an axis OX represents an observation direction of the element, the length R of this segment represents the spatial frequency of this pixel and the angle of the inclination of the element necessary to observe this pixel and the value of the brightness of the point represents the relative intensity of the light diffracted in this direction and the spatial frequency. A complete image composed of these points forms a map of observation directions, in which the pixels diffract the light when they are illuminated with monochromatic light. For example, an OVD containing two images that partially overlap OA and OB, which must be visible from different fields of observation angles PA and PB, contains at least 3 types of pixels 1: 1. the pixels contained only in the image area OA, whose map of observation angles contains only the area of light PA, 2. the pixels contained only in the image area OB, whose map of observation angles contains only the area of light PB, 3. the pixels contained in the common part of the image areas OA and OB, whose map of observation angles contains the area of light that will be the sum of the areas PA and PB. The map of observation angles of each pixel is represented in the computer by a matrix of complex numbers whose amplitudes are equal to the brightness of the corresponding points of the map of observation angles and, the phases are assigned in the form of the so-called diffuser, which is a matrix of complex numbers with amplitudes equal to 1, composed in a way that ensures the maximum uniformity of the distribution of the amplitude values in the Fourier transform of this matrix. The simplest diffuser is the random diffuser, which is a matrix of complex numbers with randomly assigned arguments. The matrix composed in this way is then subjected to a Fourier transform and, from the resulting complex matrix only its phase values (arguments) are stored in memory. The matrix of phase values is then represented graphically as a pixel image. Where the subsequent levels in the gray scale represent subsequent phase values. Normally, this pixel image is composed of irregular lines. The distances between these lines are more or less constant, especially in the case of a map that contains only the PA area. The pixel image is recorded, on an appropriate scale (providing resolution lines of around 600-1500 lines / mm), in a photosensitive relief material (photoresist), where the gray levels of the pixel map are reflected in the depth of the relief. An optically invariable, colored and flat image (for example, graphics) or a set of these images is converted into an OVD consisting of pixels described above in a way that ensures the best readability after illumination with white light from appropriate directions . In this example, an OA image is visible at a constant inclination of the vertical of the OVD within a wide field or range of tilt angles of the horizontal and, an OB image is visible when the OVD is properly tilted in relation to the light source and rotates in its plane to any value. further, of the pixels that are Fourier holograms of observation angle maps, the OVD may contain a rainbow hologram, commonly used as an authentication element. The rainbow hologram can be recorded as an analogous hologram in a traditional optical array or as a computer-generated rainbow hologram, using a computer-controlled recorder described later. The inclusion of a rainbow hologram in the OVD described above improves its aesthetic value. The method to record an OVD receives in the following: 1. A Fourier transform of the map of the observation angles is calculated and a map of the gray scale of its phase (argument) that will be of a pixel image 6, is prints on a transparency or is displayed in a spatial light modulator, 2. the pixel image _6 is then illuminated with an expanded and converging laser beam L, 3. when using the lens _4, the pixel map is reduced to the appropriate line density in a photo-sensitive material, 4. the laser light beam L is cut or interrupted using a shutter 2_ and, the photosensitive material is moved to the position of the next pixel using a stage 5_, which is moved in XY, 5. The shutter then opens and the procedure is repeated until all the pixels of the OVD are exposed. It is advantageous if the laser beam L after passing through the image of the pixel map _ß is modified by a filter 1_. The filter located in the focal plane of the laser beam, where the map of the pixel observation angles is reproduced in optical form, stops the light corresponding to angles in which the OVD should not be visible. It is also advantageous, when calculating the Fourier transform (FT) of the observation angle map (MVA), to use an iterative algorithm of the transform of Fourier (IFTA) to obtain a uniform value of the amplitude of the transform, which is impossible with the use of a direct form of the calculation of the transform of Fourier In the IFTA, the complex matrix of the same size as the MVA is filled with complex numbers whose amplitudes correspond to MVA values and the arguments are assigned random numbers (a random diffuser is applied). Then, the direct FT is applied and the amplitudes of the resulting elements of the complex matrix are uniform (that is, they all receive the same value, usually 1) without changing their arguments. Subsequently, the inverse FT is calculated (thus returning to the MVA domain) and the amplitudes of the elements of the matrix are filled with MVA values without changing the phases of the elements. This loop is repeated several times (usually 5-20) and is terminated after the calculation of the direct FT produces a complex matrix with very uniform element amplitudes and a very good quality of MVA restoration. The OVD recorder, Figure 3 consists of a laser that is the source of the L beam of monochromatic light. Behind the (or inside) the laser there is a shutter 2_ and a beam expander 3 ^ that form a convergent and expanded beam of laser light with its focal point located at the front of the lens 4_ which photo-reduces to the image 6 of the map of pixel, which is located between the expander 3 and the slow _4, in the photosensitive material located on the stage that moves in XY. The recorder is controlled by an external computer that opens the shutter 2 and controls the movement of the XY deck. The recorder may also contain the filter 1_ located in front of the lens _4 in the focal plane of the expander _3. This filter modifies the Fourier spectrum of the pixel image, thereby modifying the MVA restoration.

Claims (8)

1. CLAIMS: 1.
2. An optically variable device comprising: a plurality of diffractive pixels, each pixel is a hologram of the pure phase Fourier transform of a map of observation angles associated with each of the pixels; wherein the plurality of pixels comprises at least two subsets of pixels, each of the pixels of a first subset of the plurality of pixels contained in a first area (OA) is a Fourier hologram of a first map of observation angles (PA); and each of the pixels of a second subset of the plurality of pixels contained in a second area (OA) is a Fourier hologram of a second observation angle map (PB); An optically variable device according to claim 1, wherein the plurality of pixels comprises a third subset of pixels comprised in an area that is an intersection of the first area (OA) and the second area (OB); each of the pixels of the third subset is a Fourier hologram of the sum of the first map and the second map of the observation angle (PA, PB).
3. 3. An optically variable device according to claim 1, characterized in that at least one of these pixels is close to a rainbow hologram.
4. 4. A method for recording an optically variable device, characterized in that after the opening of the shutter. { 2), an expanded and convergent laser beam (L) illuminates an image (o) of the pixel map obtained as a graphic reproduction of a phase of a Fourier transform of the observation angle map associated with this subset of pixels and using a lens (4_), this map is photo-reduced to a desired size and the shutter cuts or interrupts the laser light beam (L); subsequently, the photosensitive material in which an OBD is recorded is moved to the position of the next pixel using a stage that moves in XY (5) and the procedure is repeated until the pixels of all the subsets are exposed.
5. A method according to claim 4, characterized in that the convergent beam (L) of laser light, after passing through the image. { 6) of the pixel map, is modified by a filter. { ! ), which remains in a plane where the image is formed through the lens. { 6) of the Fourier transform.
6. 6. A method according to claim 4, characterized in that the Fourier transforms of the observation angles are calculated using an alterative algorithm of the Fourier transform.
7. 7. A recorder for OVD, which contains a laser, a shutter, the beam expander, the lens and a stage that moves in XY, characterized in that there is an image (_6) of the pixel map obtained as a graphic reproduction of a phase of a Fourier transform of the map of observation angles associated with this subset of pixels between a beam expander (3_) and the lens (_4).
8. 8. A recorder for OVD according to claim 3, characterized in that the filter is in front of the lens (_4). { ! _) that modifies the Fourier spectrum of the pixel image (6).