Microstructure with optical effects

Abstract

A microstructure with substructures that differ from one another in their shape, each of the substructures having an optically effective surface and being representable by a method comprising the step of rotating a surface element having a straight baseline and a top line non-parallel to the baseline about an axis of rotation perpendicular to the baseline in the plane of the surface element, thereby forming a circular sector from the baseline and the optically effective surface from the top line; a method for producing a microstructure; an optical element supporting the microstructure; the use of the optical element.

Description

Field of invention

[0001] The present invention relates to a microstructure with optical effects, a method for producing a microstructure, an optical element with the microstructure and the use of the optical element. State of the art

[0002] From WO 2009 / 013000 A2, a security element for valuable documents with an optically variable structure is known, comprising a first embossed structure consisting of first embossed elements. The embossed structure is designed such that at least partial areas of the coating are visible when viewed from a first viewing angle and are at least partially obscured when viewed from a second viewing angle. Tilting the security element about a first axis thus creates a first tilting effect. The structure features a second embossed structure, which is arranged within the first embossed structure and generates a second tilting effect in addition to the first.

[0003] WO 2013 / 045056 A1 discloses a safety element with an optically variable structure, which includes a raised embossed element, preferably in the form of a cone or a spherical segment. The embossed element has a partial cutout, preferably in the form of an arc-shaped segment, wherein the remaining cone or spherical segment forms an embossed element that exhibits desired optical effects, such as tilting effects, depending on the viewing angle. Problems to be solved by the invention

[0004] However, the state of the art does not reveal any structures that exhibit high variability in the encoding of optical effects in the surfaces of microstructures and can be produced in a simple manner.

[0005] Therefore, the object of the present invention is to provide optically variable structures with a high number of achievable optical effects when rotating and tilting. Summary of the invention

[0006] The task was solved by providing a microstructure with a circular base, wherein circular sectors of different structure have optically effective surfaces.

[0007] The subject matter of the present invention includes in particular the following aspects: [1] Microstructure with substructures that differ from one another in their shape, wherein each of the substructures has an optically effective surface and can be represented by a method comprising the step of rotating a surface element having a straight baseline and a top line not parallel to the baseline about an axis of rotation perpendicular to the baseline in the plane of the surface element, thereby forming a circular sector from the baseline and the optically effective surface from the top line. [1-1] Microstructure according to Aspect [1], wherein the microstructure has a height of less than 100 µm, preferably less than 20 µm, more preferably less than 10 µm.[1-2] Microstructure according to one of the foregoing aspects, consisting of (i) substructures that can be represented by the said method, or (ii) substructures that can be represented by the said method, as well as structures that cannot be represented by the said method. [1-3] Microstructure according to one of the foregoing aspects, wherein the substructures that can be represented by the said method differ from one another in their form in that they differ from one another in the angle of the top line to the base line, the curvature of the top line, the number of top lines, the distance of the top line from the base line and / or the radius of the circular sector.[1-4] Microstructure according to any of the foregoing aspects, wherein the top line (i) is a single line, which may be straight or curved, or (ii) is a profile line of a profile composed of several independently selected profile lines. [1-5] Microstructure according to any of the foregoing aspects, wherein the optically effective surface is a light-reflecting surface. [1-6] Microstructure according to any of the foregoing aspects, wherein the axis of rotation is located at the midpoint or at an endpoint of the baseline. [1-7] Microstructure according to any of the foregoing aspects, wherein the method comprises a step in which a portion is cut out of the circular sector to form a circular segment.[2] Microstructure according to aspect [1], wherein all substructures of the microstructure can be represented by the said method, wherein rotation is performed about the same axis of rotation, the baselines of the substructures together forming a circular base of the microstructure. [3] Microstructure according to any of the foregoing aspects, in particular according to aspect [2], wherein the method comprises cutting out a part of the microstructure, thereby forming a base other than a circular one from the base of the microstructure. [3-1] Microstructure according to aspect [3], wherein the base formed by cutting out is an angular structure, for example a square or hexagon. [3-2] Microstructure according to aspect [3] or [3-1], wherein the axis of rotation is the center of the structure. [3-3] Microstructure according to any of aspects [3] to [3-2], wherein the cutting out produces cut surfaces perpendicular to the base.[4] Microstructure according to any of the foregoing aspects, wherein the top line of the surface element is a first line and the surface element has a second top line, wherein the first line is inclined upwards towards the axis of rotation and the second line differs from the second line in its inclination towards the axis of rotation. [4-1] Microstructure according to aspect [4], wherein the first line is farther from the axis of rotation than the second line. [4-2] Microstructure according to aspect [4] or [4-1], wherein the endpoint of the first line touches the endpoint of the second line. [5] Microstructure according to aspect [4], wherein the first line and the second line are profile lines of a top profile with multiple peaks and troughs. [5-1] Microstructure according to aspect [5], wherein the height of the profile is 1 to 50 µm. [5-2] Microstructure according to aspect [5] or [5-1], wherein at least one of the troughs lies on the baseline.[6] Microstructure according to aspect [5], wherein the profile has several first profile lines and several second profile lines, wherein the first profile lines have the same or different angles of inclination to the axis of rotation and the second profile lines have the same or different angles of inclination to the axis of rotation, and wherein the first and second profile lines alternate and are preferably arranged immediately one after the other. [6-1] Microstructure according to aspect [6], wherein the angles of inclination of the first profile lines are the same and the angles of inclination of the second profile lines are the same. [6-2] Microstructure according to aspect [6] or [6-1], wherein the first and second profile lines are straight. [6-3] Microstructure according to any one of aspects [6] to [6-2], wherein the upper endpoints of the first and second profile lines are the high points of the profile and the lower endpoints of the first and second profile lines are the low points of the profile.[6-4] Microstructure according to one of aspects [6] to [6-3], wherein the first and second profile lines form a sawtooth profile. [6-5] Microstructure according to one of aspects [6] to [6-4], wherein the axis of rotation is perpendicular to one endpoint of the baseline and the profile extends from the other endpoint of the baseline to a point on the axis of rotation. [7] Microstructure according to aspect [6], wherein the high points of several of the profile lines lie on a straight line perpendicular to the axis of rotation and / or the low points of the profile lines lie on a straight line perpendicular to the axis of rotation. [7-1] Microstructure according to aspect [7], wherein all the high points of the profile lines lie on a straight line perpendicular to the axis of rotation. [7-2] Microstructure according to aspect [7] or [7-1], wherein the straight line on which the low points lie is the baseline of the surface element.[8] Microstructure according to any of the foregoing aspects, which is or includes an embossed and cured polymer structure. [8-1] Microstructure according to aspect [8], wherein the optically effective surface is a surface of the polymer structure. [9] Microstructure according to any of the foregoing aspects, wherein the optically effective surface is the surface of a coating of an embossed and cured polymer structure. [9-1] Microstructure according to aspect [9], wherein the coating comprises an optically effective coating, for example, a colored or reflective metallic coating. [9-2] Microstructure according to aspect [9] or [9-1], wherein the coating comprises a protective layer. [9-3] Microstructure according to any of aspects [9] to [9-2], wherein the coating comprises an optically effective coating and a protective layer applied thereto.

[10] A method for producing a microstructure, comprising the following steps: (i) producing a partial structure representation by a method comprising the step of rotating a surface element having a straight baseline and a top line non-parallel to the baseline about an axis of rotation perpendicular to the baseline in the plane of the surface element, thereby forming a circular sector from the baseline and the optically effective surface from the top line; (ii) producing a microstructure representation comprising the partial structure representation obtained according to step (i) and at least one partial structure representation different in shape therefrom; and (iii) producing the microstructure based on the microstructure representation obtained in step (ii). [10-1] A method according to aspect

[10] , wherein in step (iii) a microstructure according to any one of claims 1 to 9 is produced.

[11] A method according to Aspect

[10] , wherein step (iii) comprises: (iii-1) fabricating an embossing tool as a negative of the microstructure representation obtained in step (ii); (iii-2) pressing the embossing tool obtained in step (iii-1) onto an uncured substrate to form an embossed substrate; and (iii-3) curing the embossed substrate obtained in step (iii-2) to form the microstructure. [11-1] A method according to Aspect

[11] , wherein the method is a nanoembossing lithography method. [11-2] A method according to Aspect

[11] or [11-1], wherein the method comprises roll-to-roll embossing of a plurality of the microstructures.

[12] A method according to Aspect

[10] or

[11] , which comprises applying a coating to the optically active surface.

[13] Microstructure obtainable according to a method according to any one of Aspects

[10] to

[12] .

[14] Optical element with a support and a plurality of microstructures arranged on the support according to one of aspects [1] to [9] or

[13] . [14-1] Optical element according to aspect

[14] , wherein the support is a polymer film.

[15] Use of an optical element according to aspect

[14] or [14-1] as a security element on valuable documents or as a decorative element. Advantages of the invention

[0008] The present invention enables the encoding of a large number of different images in the surfaces of microstructures.

[0009] The microstructures can be used to create optical elements that generate motion effects and / or 3D effects of the subject when tilted and rotated.

[0010] The optical elements can be used as security features on valuable documents or as decorative elements. The optical security features on valuable documents can be easily verified without tools and are difficult to copy. Description of the characters

[0011] In the figures, the arrow labeled H_max indicates the height of the microstructure and, in some embodiments, also represents the axis of rotation. Alpha1 and Alpha2, or α1 and α2, indicate the inclination angles of the profile lines of the surface element. H_facet denotes the height of a profile line above the baseline. L_max-Sector denotes the length of the baseline of a surface element or part thereof. When L_max-Sector denotes the total length of the baseline, it indicates the radius of the circular sector of the substructure. Figures 1A and 1B show examples of equidistant ϕ-angle sectorization of square pixels in 4 ( Figure 1A ) and 8 images ( Figure 1B ). Figures 2A to 2C show surface elements for representing microstructures according to the invention as embodiment 1. Figures 3A to 3C show surface elements for representing microstructures according to the invention as described in embodiment 2. Figures 4A to 4C show surface elements for representing microstructures according to the invention as described in embodiment 3. Figures 5A to 5C show surface elements for representing microstructures according to the invention as embodiment 4. Figure 6 shows a microstructure according to the invention in a top view. Figure 7 An example shows an optical element according to the invention with a plurality of microstructures according to the invention. Embodiments of the invention

[0012] The microstructure according to the invention is a three-dimensional structure, i.e., a microbody. Hereinafter, the microstructure according to the invention can simply be referred to as "the microstructure". The straight baseline can be referred to as "the baseline" and the upper line, which is not parallel to the baseline, as "the upper line".

[0013] The microstructure preferably has dimensions below the millimeter range, i.e., in the micrometer range and below. The length and width, or diameter, are 10 µm to 2000 µm, preferably 20 µm to less than 1000 µm. The height can be 1 µm to 2000 µm, preferably less than 1000 µm. Thus, in embodiments, the microstructures can be designed to be tactilely detectable. Preferably, however, the microstructure is very flat with a height of 1 µm to 50 µm (see embodiments 2 to 4). A shape with a length and width, or diameter, of 50 µm to less than 1000 µm and a height of 1 to 20 µm is preferred. The profile height is preferably 1 to 50 µm, more preferably 1 to 20 µm.In this context, the height of the microstructure refers to the distance between the baseline and the highest peak of the profile, viewed in the direction of the rotation axis, while the height of the profile refers to the distance between the lowest point and the highest peak of the profile, viewed in the direction of the rotation axis.

[0014] The microstructure according to the invention comprises substructures that differ from one another in their shape, each of which can be represented by the method defined herein, which can also be referred to as the rotation method. That is to say, the microstructure contains at least two such substructures. Each of these substructures is a partial solid of revolution and, in plan view, a sector of a circle. The other structures of the microstructure may be structures that cannot be represented by the rotation method. If all substructures of the microstructure can be represented by the rotation method, the microstructure as a whole is a solid of revolution.

[0015] According to the invention, the substructure is defined by the expression that it "can be represented by a method comprising the step of rotating a surface element having a straight baseline and a top line not parallel to the baseline about an axis of rotation perpendicular to the baseline in the plane of the surface element, thereby forming a circular sector from the baseline and the optically effective surface from the top line." With this expression, the shape of the substructure is defined intuitively by the representation method.An alternative is a purely structural definition, such that the preceding expression is interchangeable with the definition according to which a substructure "represents a section of a solid body, such that the base of the substructure has the shape of a circular sector consisting of two radii and an arc, wherein in the substructure each section surface is perpendicular to the base, and the line of intersection of each section surface with the base is a baseline which is a radius of the circular sector, and wherein the section surface has a top line that is not parallel to the baseline and which is a profile line of the profile of the optically effective surface." The section surfaces correspond to the cut surfaces when cutting a slice of cake from a cake.

[0016] The microstructure exhibits optically active surfaces. A prerequisite for optical activity is visibility to the observer. An optical surface can be visible from one viewing angle and obscured from another. Therefore, optically active surfaces are those that are visible from at least one viewing angle. The simplest optically active surface has no coating and consists of the microstructure material itself. For example, the microstructure might contain a polymer whose surface reflects light and is thus optically active. However, optical activity can also be imparted or modified by coating the surface. Due to the optically active surfaces on the microstructure, optical effects occur when the microstructure is rotated and tilted, i.e., viewed from different angles (θ, ϕ).Here, ϕ denotes a horizontal angle on the circular surface created by rotating the baseline around the axis of rotation, and θ denotes a vertical angle with respect to this circular surface. Rotating the microstructure changes the viewing angle ϕ, while tilting or skewing it changes the viewing angle θ. Within the surface element, the orientation of the top line with respect to the baseline is also denoted by the angle θ.

[0017] The spatial orientation of the unit consisting of the surface element and the axis of rotation is, of course, not restricted. For the sake of clarity, however, the following spatial orientation is assumed here: The axis of rotation and the surface element are vertical, thus defining "top" and "bottom," with the baseline being the lower side and the upper line part of the upper side of the surface element; and the rotation occurs in a horizontal direction, so that the baseline forms a horizontally oriented sector of a circle.

[0018] The three-dimensional basic shape of the microstructure is not restricted and can be, for example, a tetrahedron, spherical segment, truncated pyramid, cone, truncated cone, cylindrical segment, or pyramid, or it can be composed of parts of these basic shapes. Thus, the base of the microstructure is also not limited to a circular shape. The base of the microstructure can be composed of circular sector-shaped substructures and structures of other geometries. The method for representing the microstructure can also include, for example, cutting out a part of the microstructure, thereby creating a non-circular base from the circular one, e.g., a square. Alternatively, the circular base can be part of the total base of the microstructure, with the other part being arbitrarily shaped, so that the overall result is a non-circular base.Rectangular or square overall base areas allow the microstructures to be arranged next to each other across the entire surface.

[0019] The microstructure according to the invention enables the encoding of a number of different images directly into the optically effective surfaces. This results in optical features that become visible under specific viewing angles relative to the surface and relative to the light source. By selectively choosing the images, a sequence of images can be generated in this way, which, when the surface is tilted and / or rotated, produces motion effects of the subject, similar to a film. The surface element

[0020] A substructure can be represented by rotating a surface element, which has a baseline and a top line that is non-parallel to the baseline, around an axis of rotation. Non-parallel means that a line is curved, or that a straight line is perpendicular or inclined upwards or downwards towards the axis of rotation. In the simplest case, the top line is a line connecting the baseline to the axis of rotation. This line is either straight, so that the surface element is a right-angled triangle, or curved. When rotated, the non-parallel top line creates an optically active surface. However, it is not impossible that an optically active surface could also be created from a parallel profile line.

[0021] The surface element used in the rotation process corresponds to the cutout surface described above.

[0022] The top surface can have any number of lines that are independent of each other and non-parallel to the baseline. It can also have any number of parallel lines. A top surface with several different lines is referred to here as a profile. A profile can consist of several profile lines that are straight or curved and / or differ from each other in their length and / or their angle or orientation to the baseline and thus also to the axis of rotation. The profile can also be a combination of straight and curved profile lines, such that a profile has high points and low points connected by straight and / or curved profile lines. The high points can lie on one or more straight lines perpendicular to the axis of rotation. The same applies to the low points. It is preferred that all high points lie on a first straight line and all low points lie on a second straight line parallel to the first.The second straight line can lie on the baseline.

[0023] The height of the surface element, i.e., the distance between the baseline and the profile, can be arbitrary. It may be preferred that the distance between the baseline and the lowest point of the profile is small, e.g., 50 nm to 1 µm, or even zero. An example of a top surface is sawtooth-shaped, where in embodiments the individual sawtooths lie on the baseline and do not touch each other, or only touch at one point. By rotating about the axis of rotation, the profile forms a solid body whose surface has elevations with climaxes and indentations with troughs, as well as flanks between the climaxes and troughs. The flanks are created by rotating the profile lines about the axis of rotation and form the optically effective surfaces of the image element.

[0024] The structure of the top surface of the surface element can be selected depending on the desired intensity values ​​of the optically effective surfaces. The relative intensity values ​​of the individual substructures and microstructures from the respective θ,ϕ viewing angles can be calculated. The base of a circular microstructure can be subdivided into ϕ sectors, i.e., circular sector-shaped bases of the substructures. Each sector of each microstructure is assigned an intensity value between 0 and maximum intensity in the image to be encoded, depending on the previously calculated intensity values ​​of the microstructure. Based on these sector intensity values, suitable surface elements are calculated, and the substructures of the microstructure are then represented from them. These substructures are subsequently assembled to form the shape of the microstructure.

[0025] A substructure produced by the rotational process forms a circular sector in plan view. The profile of a surface element representing a substructure or sector is calculated such that certain surface areas [A_ref] of the resulting total surface area [A_ges] in the microstructure reflect incident light in at least one desired θ viewing direction, while other surface areas [A_aus] block incident light from the desired viewing direction (e.g., by reflection in a different viewing direction or in a direction that cannot be viewed). The proportion of these surface areas depends on the intensity value of the sector of the image to be encoded [I_sector] and the maximum achievable intensity value [I_max] of the sector and is determined by the following formulas: A _ ref / A _ ges = I _ sector / I _ max A _ aus = A _ ges − A _ ref

[0026] This creates a brightness impression of the microstructure for the viewer in the desired θ viewing direction, corresponding to the encoded images. According to the encoded brightness impression of the neighboring images, the brightness impression changes due to the different shapes of the substructures, while maintaining the θ viewing direction and simultaneously changing the ϕ viewing direction, i.e., when rotating the device.

[0027] The high number of optical effects of the microstructure results from the combination of sectors and the profiles of the sectors. The optical element

[0028] The optical element ( Figure 7The optical element comprises a plurality of microstructures according to the invention, arranged on a substrate and representing the pixels of the motif of the optical element. The optical element can, for example, be a security feature on a valuable document. Any substrate can be used, such as paper or a film. Materials for the film can be plastics, in particular PC (polycarbonate), PMMA (polymethyl methacrylate), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PEN (polyethylene naphthalate), PP (polypropylene), PA (polyamide), PE (polyethylene), or biaxially oriented polypropylene (BOPP). The film can be translucent.

[0029] The entire surface of the optical element is divided into individual microstructures with any base area (e.g. square, rectangular, hexagonal or generally a combination thereof), with geometries that are arranged side by side to cover the entire surface are preferred.

[0030] Depending on the number of images to be encoded, the base area of ​​the microstructures is divided into sectors ( Figures 1A and 1B The area of ​​the images to be encoded is also divided into the same microstructures, whereby the relative intensity values ​​of the individual microstructures of the images are calculated.

[0031] The inventive approach enables the encoding of any number of different images directly into the structures of the surface of the optical security features, which become visible under specific viewing angles relative to the surface and relative to the light source. By selecting appropriate images, a sequence of images can be generated that, similar to a film, creates motion effects of the motif when the surface is tilted and / or rotated. The number of images is essentially arbitrary; however, the maximum intensity and distinguishability of the images decrease with increasing number, depending on the light source under which the images are viewed. Highly parallelized light, such as sunlight, allows for greater distinguishability, while more diffuse light, such as ambient light, allows for less distinguishability.Therefore, depending on the task and the subject matter, there is a practical maximum number of images. Production

[0032] The substructures can be fabricated based on the structures obtained through the fabrication process. Known microstructuring methods can be used to generate the microstructures, e.g., embossing processes such as nanoembossing lithography (NIL). Known methods can be employed in which thermoplastic films or films coated with radiation-curing varnishes are embossed. Microstructures containing or consisting of polymers are preferred, thus enabling fabrication via embossing processes, e.g., roll-to-roll processes.

[0033] Alternatively, known forming processes such as injection molding or extrusion can be used to create the microstructures. These processes involve injecting molten material into a negative mold to create the desired shape. Microstructures can be obtained by using suitable inserts that contain the negative of the microstructure's surface relief.

[0034] The embossing tool has the negative of the surface relief of the microstructure and can be produced by known methods such as photolithography, electron beam lithography or laser beam lithography. coating

[0035] The optically active surfaces may have at least a partial reflective or reflection-enhancing coating, in particular a metallic or high-refractive-index coating. This can be a metallic coating, for example, vapor-deposited to a thickness of 50 nm. Suitable coating materials include, in particular, aluminum, gold, silver, copper, palladium, chromium, nickel, and / or tungsten, as well as their alloys. Alternatively, the reflective or reflection-enhancing coating can be formed by coating it with a material with a high refractive index.

[0036] The optically active surfaces of the microstructures can be metallized after embossing and curing with an embossing lacquer applied to a carrier film. To prevent copying of the security feature by molding the structures, the coated structures can be sealed with another polymer. An additional lacquer layer can also be applied as a protective or adhesive lacquer.

[0037] Four exemplary embodiments for the construction of surface elements with profiles are described below. Embodiments 3 and 4 may be preferred because they allow more degrees of freedom. embodiment 1 (Figures 2A to 2C)

[0038] Depending on the desired viewing direction (θ), a structural angle α 1 is calculated so that incident rays from a light source are reflected by the optically effective surfaces (2) in the direction of the viewing angle (θ). For surface areas that are intended to block the light, a line (3) parallel to the baseline is used.

[0039] Depending on the required intensity value (0 to I_max) of the microstructure sector, the following profile lines result: (i) In the case intensity = 0 ( Fig. 2A ) no structure results; the solid of the sector consists only of the base area (or the surface of the support). (ii) In the case intensity = I_max ( Fig. 2B ) results in a straight line with slope = α 1 ; the volume of the sector corresponds to a conic sector with H = H_max. (iii) For the case intensity = l_sector ( Fig. 2CIn the range between 0 and I_max, a combination of the two functions results, where the proportion of the straight line is calculated according to the required area ratio from formula 1. The solid of the sector corresponds to a truncated cone sector with H = H_facet.

[0040] A disadvantage of this embodiment compared to the particularly preferred embodiments is that the blocked light is not controlled, which can lead to interference images appearing near the desired θ viewing direction. These interference images can also disrupt the image sequence of the subject when rotating or tilting. Furthermore, the desired θ viewing direction, the maximum height of the structures, and the base area of ​​the pixels cannot be selected independently. Only two of these parameters can be freely chosen, while the third parameter is determined by the other two. embodiment 2 (Figures 3A to 3C)

[0041] Depending on the desired viewing direction (θ), an angle α1 is calculated so that incoming rays from a light source are reflected in the direction of the viewing angle. Additionally, a second viewing direction (θ) with a corresponding angle α2 is chosen, under which a negative image of the image to be encoded is to appear. For areas that are to block the light, a straight line with a slope of α2 is used to direct the blocked light into the viewing direction (θ) of the negative image.

[0042] Depending on the required intensity value of the pixel sector, the following profile lines result: (i) In the case intensity = 0 ( Fig. 3A) results in a straight line with slope = α 2 and a straight line with slope = 0 to fill the sector. The solid corresponds to a conical sector with H = H_Max that does not completely cover the base. (ii) For the case intensity = I_max ( Fig. 3B ) results in a straight line with slope = α 1 . The solid of the sector corresponds to a conic sector with H = H_max. (iii) For the case intensity = l_sector ( Fig. 3C In the range between 0 and I_max, a combination of the two lines results, with their proportion of the profile calculated according to the required area ratio from Formula 1. Additionally, a straight line with a slope of 0 is used to connect the two straight lines. The solid body of the sector corresponds to a truncated cone sector with a conical sector attached to it.

[0043] One advantage of this embodiment is that the blocked light is used to generate another image (a negative image of the encoded image) under a different θ viewing direction. However, flat sections remain, which can lead to interference images. A disadvantage is that the desired θ viewing direction, the maximum height of the structures, and the base area of ​​the microstructure cannot be selected independently. Only two of these parameters can be freely chosen, while the third parameter is determined by the other two. Fabricated microstructures can be measured using a Keyence laser scanning microscope. embodiment 3 (Figures 4A to 4C)

[0044] Depending on the desired θ viewing direction, an angle α1 is calculated so that incoming rays from a light source are reflected in the direction of the 8° viewing angle. Additionally, a second θ viewing direction with a corresponding angle α2 is chosen, under which a negative image of the image to be encoded is to appear. For areas that are to block the light, a curve with a slope of α2 is used to direct the blocked light into the θ viewing direction of the negative image. Furthermore, in this approach, straight lines are subdivided if they exceed H_max. This allows for the decoupling of the three parameters—namely, θ viewing direction, maximum height of the structures, and base area of ​​the microstructure—as well as the avoidance of straight lines with a slope of 0.

[0045] Depending on the required intensity value (0 - I_max) of the microstructure sector, the following profiles result: (i) In the case intensity = 0 ( Fig. 4A (ii) Several straight lines with slope = α 2 result, which are subdivided by other straight lines, creating a sawtooth profile. (ii) In the case intensity = I_max ( Fig. 4B ) several straight lines with slope = α 1 result, which are subdivided by other straight lines, also creating a sawtooth profile. (iii) For the case intensity = l_sector ( Fig. 4C ) in the range between 0 and I_max a combination of the two areas results, whereby their share of the profile is calculated according to the required area ratio from formula 1. embodiment 4 (Figures 5A to 5C)

[0046] Starting with embodiment 3, a structural angle α 2 = 45° is chosen for the blocking surfaces and their subdivisions. The combination of two surfaces that are perpendicular to each other enables an effect similar to that of a retroreflective structure, which reflects incident light back towards the source. This ensures that the portion of the light to be blocked is reflected in a direction different from the viewing direction, and no unwanted negative images are generated. Examples

[0047] Various microstructures according to the invention were produced using the inventive method. 1. Circle_V2: Profile according to embodiment 3; h_max = 2.5 µm; base area 80 x 80 µm; α 1 = 14°; α 1 = 28° 2. Qube_V1: Profile according to embodiment 3; h_max = 2.5 µm; base area 100 x 100 µm; α 1 = 14° 3. Qube_V1: Profile according to embodiment 4; h_max = 5 µm; base area 100 x 100 µm; α 1 = 14°

[0048] In all three experiments, microstructures with the desired motifs and effects could be produced by changing the viewing angle. Reference symbol list

[0049] (1) Baseline (2) Profile line (non-parallel top line) (3) Profile line (4) Axis of rotation

Claims

1. Microstructure with substructures which differ from one another in their shape, wherein each of the substructures has an optically effective surface and can be represented by a method which comprises the step in which a surface element which has a straight base line and an upper side line which is not parallel to the base line is rotated about an axis of rotation which is perpendicular to the base line in the plane of the surface element, whereby a circular sector is formed from the base line and the optically effective surface is formed from the upper side line.

2. Microstructure according to claim 1, wherein all partial structures of the microstructure can be represented by the said method, wherein the rotation takes place around the same axis of rotation, whereby the partial structures obtained together form a circular base area of ​​the microstructure.

3. A microstructure according to claim 1 or 2, wherein the method comprises cutting out a portion of the microstructure, thereby forming a base surface of the microstructure other than a circular base surface.

4. Microstructure according to one of the preceding claims, wherein the top line of the surface element is a first line and the surface element has a second top line, wherein the first line is inclined upwards towards the axis of rotation and the second line differs from the second line in the inclination towards the axis of rotation.

5. The microstructure of claim 4, wherein the first line and the second line are profile lines of a top-side profile having multiple peaks and troughs.

6. Microstructure according to claim 5, wherein the profile has a plurality of first profile lines and a plurality of second profile lines, wherein the first profile lines have the same or different angle of inclination to the axis of rotation and the second profile lines have the same or different angle of inclination to the axis of rotation and wherein first and second profile lines alternate with one another.

7. Microstructure according to claim 6, wherein the high points of several of the profile lines lie on a straight line perpendicular to the rotation axis and / or the low points of the profile lines lie on a straight line perpendicular to the rotation axis.

8. A microstructure according to any one of the preceding claims, which is or contains an embossed and cured polymer structure.

9. Microstructure according to one of the preceding claims, wherein the optically active surface is the surface of a coating of an embossed and cured polymer structure.

10. A method for producing a microstructure, comprising the following steps: (i) producing a partial structure representation by a method comprising the step of rotating a surface element having a straight base line and an upper side line non-parallel to the base line about an axis of rotation which is perpendicular to the base line in the plane of the surface element, whereby a circular sector is formed from the base line and the optically effective surface is formed from the upper side line; (ii) producing a microstructure representation which comprises the partial structure representation obtained according to step (i) and at least one partial structure representation which differs in shape therefrom; and (iii) producing the microstructure on the basis of the microstructure representation obtained in step (ii).

11. The method according to claim 10, wherein step (iii) comprises the following steps: (iii-1) producing an embossing tool as a negative of the microstructure representation obtained in step (ii); (iii-2) pressing the embossing tool obtained in step (iii-1) onto an uncured substrate to form an embossed substrate; and (iii-3) curing the embossed substrate obtained in step (iii-2) to form the microstructure.

12. A method according to claim 10 or 11, which comprises applying a coating to the optically active surface.

13. Microstructure obtainable according to a process according to any one of claims 10 to 12.

14. An optical element comprising a carrier and a plurality of microstructures arranged on the carrier according to one of claims 1 to 9 or 13.

15. Use of an optical element according to claim 14 as a security element on valuable documents or as a decorative element.

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