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EP-4642652-B1 - OPTICAL VARIABLE DEVICE

EP4642652B1EP 4642652 B1EP4642652 B1EP 4642652B1EP-4642652-B1

Inventors

  • MONOVSKI, Valentin Kostantinov
  • IVANOVSKYI, Andrii Albertovich
  • BALDZHIEV, Angel Georgiev

Dates

Publication Date
20260513
Application Date
20240726

Claims (8)

  1. An optical variable device comprising a planar base (1), on one side of which is formed a micro-lens diffraction matrix (2) with a period P 1 consisting of at least two cells (5) with a center O in each of which there is a diffraction micro-lens L with a center C consisting of alternating concentric protrusions (6) with an asymmetric triangular profile and distance d between them decreasing radially from the center C to the periphery, where said optical variable device also includes a micro-object matrix with a period P 2 , said micro-object matrix containing at least two micro-objects comprising regions T 1 and T 2 where upon observation at a certain angle the optical variable device forms at least one magnified image of the micro object comprising light and dark areas R 1 and R 2 ,, and the micro-lens diffraction matrix (3) is modulated with the micro-object matrix (3) so that the regions T 1 and T 2 of the micro-object matrix form corresponding regions M 1 and M 2 in the micro-lens matrix containing respective diffractive micro-lenses L and L', and said diffractive micro-lens L' has a center C' and consists of alternating concentric protrusions (6') with an asymmetrical triangular profile and distance d' between them decreasing radially from the center C' to the periphery, characterised in that the period P 2 is different from the period P 1 , and d and d' are in the range of 0.4-100µm, the center C of the diffraction micro-lens L is offset relative to the center O of the corresponding cell (5) and the length of the vector OC is greater than or equal to P 1 /4 and the length of the vector OC ' is greater than or equal to P 1 /4, the at least one magnified image is a three-dimensional black-and-white magnified image, and wherein the diffractive micro-lenses L and L' have opposite signs of the optical power and/or the values of d and d' at the boundary between two adjacent regions M 1 and M 2 differ by more than twofold or the angle φ is between 45° and 180°.
  2. The optical variable device according to claim 1, wherein the diffractive micro-lenses L and L' have the same sign of the optical power, the angle φ < 45° and the values of d and d' at the boundary between two adjacent regions M 1 and M 2 differ more than twofold.
  3. The optical variable device according to claim 1, wherein the diffractive micro-lenses L and L' have opposite signs of the optical power, the angle φ < 45° and the values of d and d' at the boundary between two adjacent regions M 1 and M 2 differ more than twofold.
  4. The optical variable device according to claim 1, wherein the diffractive micro-lenses L and L' have the same sign of the optical power, the angle φ is between 45° and 180° and the values of d and d' at the boundary between two adjacent regions M 1 and M 2 differ less than twofold.
  5. The optical variable device according to claim 1, wherein the diffractive micro-lenses L and L' have the same sign of the optical power, the angle φ is between 45° and 180° and the values of d and d' at the boundary between two adjacent regions M 1 and M 2 differs more than twofold.
  6. The optical variable device according to claims 4-5, wherein the angle φ is close to 90°.
  7. The optical variable device according to claims 4-5, wherein the angle φ is close to 180°.
  8. The optical variable device according to claims 1-7, wherein the length of the vector OC is greater than P 1 and the length of the vector OC ' is greater than P 1 .

Description

FIELD OF INVENTION The invention relates to an optical variable device forming a magnified three-dimensional black-and-white image of a micro-object with different optical effects such as three-dimensional images with full parallax and contrast change observed at different viewing angles, and finds application in the protection of documents and securities, authentication of goods as well as for decorative purposes. PRIOR ART Security devices that reproduce an enlarged image and/or an image with optical effects are well suited for authentication and protection against unauthorized reproduction of documents and goods, as optical effects make imitation difficult. It is known that when a matrix of micro-lenses and a matrix of micro-figures having different periods are superimposed, a magnified image of the micro-figures is observed. An optical variable device for protection against counterfeiting using this principle is disclosed in US 2010/0208036. The optical variable device includes a first layer on which a matrix of refractive lenses is formed, and a second layer spaced from the refractive lenses on which a matrix of micro-figures is formed. The two matrices are of different periods. The optical variable device reproduces a magnified three-dimensional image of the micro-figures including at least two planes. When tilting the security device a magnified three-dimensional black-and-white image with a full parallax motion effect is observed above or below the plane of the security device. The production of such a multilayer device to reproduce a clear and contrast-enhanced three-dimensional black-and-white image is complex and expensive, and increases the cost of the security device. In addition, the presence of refractive lenses results in the relatively large thickness of this optical variable device, and complicates and limits its application. The use of diffractive lenses instead of refractive ones in optical variable devices is known. It is preferable the diffractive lenses to have a triangular asymmetric relief since in this way the brightest and most contrasting image is obtained. US 8,488,242 discloses an optical variable device comprising a planar base of a suitable material such as polyester, polystyrene, polyamide or cellulose acetate. On the one side of the base a diffraction structure in the form of a micro-lens matrix is formed, including a multitude of cells with micro-relief diffraction micro-lenses which can have an arbitrary profile, incl. an asymmetrical triangular profile. On the same side of the base is formed a matrix of micro-objects (micro-figures) having micro-relief, said matrix being integrated into the micro-lens matrix. The micro-lens matrix and the micro-object matrix are of different periods. When illuminated with directional white light at a certain viewing angle this optical variable device reproduces an enlarged black-and-white three-dimensional image of at least one micro-figure consisting of light and dark areas. When changing the viewing angle an inversion of image contrast is observed - dark areas become light and vice versa. The optical variable device is small in thickness and low in cost, but the formed magnified image is low in brightness because the micro-lenses distribute the light in all directions along the azimuth angles, and the micro-figures integrated into the diffraction structure block part of the incident light. This disrupts the structure of the diffractive micro-lenses and deteriorates their optical characteristics. When viewed in scattered light the micro-relief regions of the micro-figures form a blurring cloud visible in the zeroth order of diffraction which reduces the brightness of the reproduced magnified image and degrades its visual characteristics. KR 2009 0066826 A discloses a security film with a two-dimensional array of diffraction lenses for reducing the film's thickness. The structure includes a lens layer which is a transparent substrate having a diffractive lens array portion in which diffractive lenses are arranged; an image layer positioned below the lens layer and having patterns or images formed thereon; and transparent functional layers located below the image layer and providing reflection and transmission of light, adhesion of the film to the document, and additional properties such as fluorescent, infrared, or magnetic features. The diffractive lenses have a circular shape with a diameter of 2-30 µm formed by concentric protrusions with a standard distance d between them and a triangular relief profile. The depth of the micro-relief of the diffractive lens is about 1 µm. The focal length of the lenses is 20-30 µm which means that the total thickness of thesecurity film is about 30 µm.The thickness of the image layer is selected so that the images are in the focus of the diffraction lenses. When illuminated with directional white light the security film reproduces the images formed on the image layer. When changing the viewing angle