Method and device for testing a security element for a security document

Abstract

The invention relates to a method and a device for testing a security element for a security document. The method comprises the following: providing a security element with an image comprising a first image element and a second image element, wherein the first image element (8) is visible when illuminated with light from a first wavelength range and when illuminated with light from a second wavelength range different from the first wavelength range, and wherein the second image element is visible when illuminated with light from the first wavelength range but not when illuminated with light from the second wavelength range; capturing an image (20) of the image when the image is illuminated with light from the second wavelength range and determining the second image element by evaluating digital image data displaying the image (20).The following is further provided: dividing the image capture (20) into sub-areas (22, 23), wherein one of the sub-areas (22, 23) encompasses the second image element; determining a respective texture for the sub-areas (22, 23), wherein the texture of the sub-area indicates a distribution of gray-value intensities within the sub-area of ​​the image capture (20); determining a respective texture measure for the sub-areas (22, 23) that indicates the extent to which the sub-area is covered by the texture or is free of such a texture; and determining the presence of the second image element in the image of the security element if, by evaluating the texture measures, it is found that one of the sub-areas (22, 23) which encompasses the second image element is essentially texture-free.

Description

The invention relates to a method and an apparatus for checking a security element for a security document.BackgroundVarious security features for security documents are known which can be optically verified. Optically identifiable security features are considered to be those features which can be identified by a detection of light transmitted through the security feature, reflected at the security feature and / or remitted at the security feature. Light is understood here to mean all electromagnetic radiation which can be in the visible, infrared or ultraviolet wavelength range.In connection with security documents such as documents identifying a person (ID document), provision can be made to apply a printed image on the security document as a security element, which image shows, for example, the face of a person to whom the document is assigned. In this case, provision can be made for producing image elements of the printed image with different colors. For example, four-color printing is known. It can be provided that the color of an image element, for example a fifth color in addition to four-color printing, is visible when illuminated with light in the visible wavelength range, but not when illuminating the same image element with light from another wavelength range, for example with infrared light. In this way, different image recordings can be recorded from the printed image with the aid of a camera device when illuminated with the light of different wavelengths, in which the image element is either visible or not visible, depending on the light with which the printed image is illuminated for the image recording. Such a value or security product is described, for example, in the document DE 10 2016 201 709 A1. The check with regard to the presence of such an image element on the security document can be used for checking the authenticity of the security document.If such a security element with a printed image is provided for a security document, which security element has the picture element visible or invisible depending on the illumination, it is to be checked during the production of the security element for the security document whether the picture element was correctly produced during the production process, in particular during printing. For such a checking process, it has been proposed to check an image recording of the security element with the printed image by means of image analysis when illuminated with light of the wavelength range at which the image element is not visible. In the context of image analysis, respective gray value histograms are determined for the image recording on the one hand and a comparative image recording on the other hand, which shows the printed image when illuminated with the light at which the image element is also visible, in order to compare these for checking the correct production of the image element visible or invisible when illuminated differently. Such a method is known, for example, from the document DE 10 2019 129 491 A1.However, the known test method proves to be not sufficiently reliable in the case of printed images, in particular facial images, if, for example, the image element which is intended to be visible or not visible in the different illumination is incompletely or insufficiently not visible, that is to say still "see through" in the captured image recording used for image analysis.The document DE 10 2006 011 143 A1 relates to a security code for application to packages which are, for example, deposit-containing and are recognized via the so-called barcode and there is the risk that the barcode is easily reproduced, in order thus to manipulate the withdrawal deposit system and in order in this way to purchase deposit money in an unauthorized manner. This security code consists of an IR color or in combination of a color in the visible range, preferably black, and the IR security color. One color is visible to the human eye and the other color is invisible as an infrared color, or alternatively, another visible IR color is used. Furthermore, the security mark consists of an individually designed geometry or the EAN code is completely or partially created with the security color.SummaryThe object of the invention is to provide a method and a device for checking a security element for a security document, with which the security element can be determined or checked with improved reliability.The object is achieved by a method and a device for checking a security element for a security document according to the independent claims 1 and 11.According to one aspect there is provided a method of inspecting a security element for a security document, the method comprising: providing a security element having an image comprising a first image element and a second image element, wherein the first image element is visible when illuminated with light from a first wavelength range and when illuminated with light from a second wavelength range which is different from the first wavelength range, and wherein the second image element is visible when illuminated with light from the first wavelength range but not when illuminated with light from the second wavelength range; detecting an image recording of the image with a camera device when illuminating the image with light from the second wavelength range; and determining the second image element by evaluating digital image data which indicate the image recording. In determining the second image element, the following are provided: dividing the image recording into subareas, wherein one of the subareas is formed comprising the second image element; determining a respective texture for the subareas, wherein the texture of the subareas indicates a distribution of gray value intensities within the subarea of the image recording; determining a respective texture measure for the subareas, which indicates to what extent the subarea is captured by the texture or is free of the latter; and determining the presence of the second image element in the image of the security element if it is found by evaluating the texture measures that the one of the subareas comprising the second image element is substantially free of texture.According to a further aspect, there is provided an apparatus for checking a security element for a security document. The apparatus has a camera device, an illumination device and an evaluation device, which is formed, for example, with one or more processors for data processing and associated programming. The apparatus is configured to capture an image recording of an image of a security element using a camera device, wherein the image has a first image element and a second image element, wherein the first image element is visible in each case when illuminated with light from a first wavelength range and when illuminated with light from a second wavelength range which is different from the first wavelength range, and wherein the second image element is visible when illuminated with light from the first wavelength range, but not when illuminated with light from the second wavelength range, and the image recording can be carried out when the image is illuminated by means of the illumination device with light from the second wavelength range. Furthermore, the device is configured to determine the second image element by evaluating digital image data which indicate the image recording. The following are provided here: dividing the image recording into subareas, wherein one of the subareas is formed comprising the second image element; determining a respective texture for the subareas, wherein the texture of the subareas indicates a distribution of gray value intensities within the subareas of the image recording; determining a respective texture measure for the subareas, which indicates to what extent the subareas capture the texture or are free of the latter; and determining the presence of the second image element in the image of the security element if it is found by evaluating the texture measures that the one of the subareas comprising the second image element is substantially free of texture.It is provided to use texture analysis to determine or test the security element with the image. The image comprises picture elements made by different colors, which results in that at least a first picture element is visible when illuminating the image with light of different wavelength ranges, respectively, whereas at least a second picture element, which is different from the first picture element, is either visible or not visible when illuminated with the light of different wavelength ranges. The image can be produced, for example, as a printed image or printed image, wherein different printing inks can be used in particular when producing the first and the second image element. As such, printing inks are known which, however, do not when illuminated with light from a first wavelength range, for example visible light, as well as when illuminated with light from a second wavelength range which is different from the first wavelength range, for example infrared or UV light. Printing inks are also known which are visible in both different wavelength ranges. In the case of a black-and-white image, black color of different composition can be used to implement the corresponding behavior with regard to visibility in the case of light of different wavelength ranges. Such colorants, in particular printing inks, are known per se in various embodiments.The different visibility of the colors usable for forming the image makes it possible to form a security element with an image in which different image elements are provided with different visibility depending on the illumination.Provision is made for the image of the security element to be recorded by means of a camera device when the image is illuminated with the light from the second wavelength range, which means that the second image element is then substantially invisible. Such non-visibility of the second image element can be based, for example, on the light of the second wavelength range being substantially completely reflected by the color which was used for producing the second image element. Visibility due to the absorption of light by the color used for the second image element is then omitted, which is why the second image element is not visible on the image recording.The image is then analyzed or determined. In this case, texture analysis is used for subareas of the image recording in order to determine a texture and a texture measure characterizing this texture for the respective subarea. The texture measure can be, for example, a value number, a value range and / or a threshold value. More complex texture masses are also possible, for example a matrix characterizing the texture of the associated sub-surface or a number tuple. Since non-visibility is expected for the second pixel in the captured image capture, it is concluded that the second pixel is present in a partial area comprising the second pixel in the image capture of the security element if a missing texture (freedom from texture) is determined for the partial area with the second pixel. There is thus no texture in this sub-area given by the second picture element.The light used for the illumination in the different wavelength ranges can be narrowband, up to the use of essentially only one wavelength, or broadband, for example white light.The texture of the sub-surface can indicate a planar or a spatial distribution of gray value intensities within the sub-surface. A gray value intensity can be assigned to a single image point or a group of image points.A digital camera can be used to capture the image recording of the security element with the image.The image recording can be divided into subareas in such a way that adjacent subareas abut one another in the region of edges of the subareas. This supports the possibility of determining information relating to the transition of the texture measure between adjacent subareas in the texture analysis and using it for the examination. For example, a minimum difference between the texture measure for adjacent subareas of the image recording may indicate a transition between a pixel visible in the image recording and a substantially invisible (second) pixel.In this or other embodiments, the subareas formed during the subdivision of the image recording can assume different geometric shapes, for example angular or round. Triangles, rectangles or polygons may be provided in forming the sub-surfaces. Edges (edges) of the sub-surfaces may abut each other or be spaced apart from each other for at least some of the sub-surfaces.The image recording can be divided into subareas in such a way that the one of the subareas substantially completely overlaps the second image element. In this embodiment, the partial area formed during the division of the image recording and containing the second picture element substantially completely overlaps with the area that the second picture element assumes in the image of the security element. In an alternative embodiment, the second picture element can be arranged in a larger or smaller partial area, so that there is no complete overlap.The security element may be provided with a printed image comprising the first and second image elements. In forming the printed image, different inks may be used to produce the different behavior of the pixels under different illumination (visibility vs. nonvisibility). Printing inks which are visible in each case when illuminated with light from different wavelength ranges or are not visible when illuminated with light of a specific wavelength, in particular because of their differing reflection and / or absorption behavior for light of different wavelengths, are known as such in various configurations.It may be provided to provide a threshold texture measure and the presence of the second picture element in the image of the security element may be determined if it is found during the evaluation of the texture measures that the texture measure for the one of the subareas comprising the second picture element is less than or equal to the threshold texture measure. This makes it possible, for example, to use the threshold texture measure to define a probability for the essentially given freedom of texture of the subarea with the second image element as a function of the application, for example when the nonvisibility of the second image element in the image recording is only incompletely given, in particular because the second image element still "shines through" in the image recording, which can be the result of incomplete reflection of the radiated light by the color of the second image element, for example. In this way, threshold values or limit values for the freedom from texture to be checked or the probability of freedom from texture can be applied individually in the checking process for the security element.Description data can be provided which indicate properties of the second image element such as edge length, size, shape, area and / or area contour, and the description data can be evaluated in order to form the one of the subareas such that it comprises the second image element when the image recording is divided into the subareas. On the basis of the description data, the subdivision of the image recording into the (image) subareas can be carried out depending on the application, for example in such a way that the subarea comprising the second image element in the image recording overlaps the latter completely or only in specific sections.In one example, the following can be provided: the security element is provided with an image which has a further second image element; the further second image element is visible when illuminated with light from the first wavelength range, but not when illuminated with light from the second wavelength range; the image recording is divided into subareas such that a further of the subareas comprises the further second image element; and the presence of the further second image element in the image of the security element is determined or positively decided if it is found by evaluating the texture measures that the further subarea which comprises the further second image element is substantially texture-free. Testing or analysis of the further second picture element can be carried out in the same way as the handling for the second picture element or in parts different therefrom. A correct production of the image of the security element can be determined in one example only if the substantially given freedom of texture is found for the second and the further second image element. Otherwise, it can be determined that there is no correct production of the image of the security element, i.e. in particular when the freedom from texture or the necessary probability for the freedom from texture is determined only for the second or only the further second image element.Similarly, it can be provided to determine the authenticity of the security element or of a security document with the security element only if both second image elements are categorized or classified as substantially texture-free.For the sub-areas, the texture measure may be determined using a texture filter according to Laws. Within the framework of texture analysis, different methods for determining the texture and the texture measure for the subareas can be used, which are known per se for texture analysis of image data. The methods each provide a characterization of properties or features of the texture of the associated sub-surface, from which a texture measure can then be derived, which indicates the extent or the extent to which the sub-surface has a texture or is substantially free thereof. In one embodiment, Laws texture filters or masks are used for the analysis of the subareas of the image recording (K. I. Laws: Textured Image Segmentation, Department of Electrical Engineering, Image Processing Institute, University of California, January 1980). Alternatively or additionally, texture analysis using co-occurrence matrices can be used (cf., for example, Nanni et al.: Different approaches for Extracting Information from the co-occurrence matrix, PLOS ONE, December 2013, Vol. 8, Issue 12, e83554). In this case, matrices characterizing the texture of the partial surface are determined in particular. The overall matrix can then be used as the texture measure, for example. Alternatively, the trace of the matrix may be determined and used as a texture measure.It may be provided to check correct production of the image of the security element, whereby correct production is determined if at least the presence of the second image element in the image of the security element is determined. The method for determining the correct production of the image of the security element can be used, for example, in the production of ID documents, for example after the image has been applied to the printed image of the security element on a document body, for example a plastic card. If it is found that the image of the security element has not been produced correctly, the produced product is to be assessed from waste and is either corrected or removed from the further production process.It can be provided to use the method for checking a security element for a security document for checking a security document, in particular in order to determine its authenticity, wherein optionally further security features of the document can be evaluated, as is known per se.Artificial intelligence can be used in the evaluation device in texture analysis by means of corresponding software, for example in the form of machine learning. In one example, to determine the freedom of texture in the sub-surface containing the second image element, a neural network may be used such that the elements of a gray scale matrix are given to the input layer of the neural network. Alternatively, a suitable support vector machine can be used for classifying the texture metric; here, for example, a number tuple is used as input, which is formed from the tracks of a plurality of gray value matrices, for example, or is calculated by means of a plurality of Laws filters.Security documents comprise, for example, the ID documents, passports, personal identification cards, driver's licenses, vehicle licenses, vehicle books, access authorisation cards, company identification cards, access cards, bank cards, credit cards, visa, but also labels, entry cards, bank notes, postage stamps, security papers or the like which are secured against forgery. Such security documents embodying value, such as postage stamps, security papers or banknotes, are also referred to as value documents. A distinction between value documents and security documents is not in any case unambiguously possible and is also not essential with regard to the present disclosure. In the sense of the articles described here, value documents are also always understood as security documents.DESCRIPTION OF EMBODIMENTSFurther exemplary embodiments are explained below with reference to figures of a drawing. The following are shown here: FIG. 1 shows a schematic representation of a device for checking a security element of a security document; FIG. 2 shows a schematic representation of an image recording for a facial image, wherein a second image element is not visible; FIG. 3 shows a schematic illustration of an image recording for the facial image from FIG. 2, wherein the second image element is also visible on account of other illumination; FIGS. 4a to 4d are schematic diagrams in connection with texture analysis for image recording with facial image using texture filters according to Laws; FIGS. 5 aand 5 b show schematic representations in connection with a further texture analysis for different image recording with facial image using texture filters according to Laws; FIG. 6 shows a schematic illustration in connection with texture analysis of an image recording with facial image using texture filters according to Laws, and FIG. 7 is a schematic illustration in connection with texture analysis of another image recording with facial image using texture filters according to Laws.FIG. 1 shows a schematic representation of an arrangement with a device 1 for checking or determining an image 2 of a security element 3 on a security document 4, which is for example a document for identifying a person (ID document). The security document 4 is formed with a document body 5, which is for example a plastic card. In other embodiments, the security document 4 is designed, for example, as an identity card or passport.The apparatus 1 comprises a camera device 6, for example a digital camera, which is configured to capture an image record of the image 2. For this purpose, the security element 3 is illuminated with the image 2 with the aid of an illumination device 7.In the exemplary embodiment shown, the image 2 has a first picture element 8 with an upper half face and a second picture element 9 with a lower half face.In the example shown, the first and the second picture elements 8, 9 abut each other. In other embodiments, a distance may be formed between the first and second picture elements 8, 9.In the exemplary embodiment, the image 2 is embodied as a printed image. Here, the first picture element 8 and the second picture element 9 were formed with different printing inks. The image 2 may be a color image or a black-and-white image. In the case of the black-and-white image, different black colors are used in forming the first and second picture elements 8, 9. The color of the first image element 8 can be selected such that the first image element 8 is visible in each case when illuminated with light from a first wavelength range and also when illuminated with light from a second wavelength range, which is different from the first wavelength range, in particular for the camera device 6.In contrast, the second image element 9 is visible only when illuminated with the light from the first wavelength range, but not when illuminated with the light from the second wavelength range (nonvisibility). The light from the respective wavelength range can be narrowband or broadband. In the case of narrowband light, the illumination comprises only light of fewer wavelengths or essentially only one wavelength, whereas in the case of broadband light a plurality of wavelengths contributes to the illumination, as is the case, for example, with white light. In one embodiment, the light from the first wavelength range is white light, whereas the light from the second wavelength range is infrared or UV light. By way of example, the second image element 9 is not visible when illuminated with the infrared or UV light. Non-visibility in the sense of the present disclosure means here that the second image element 9 is visible not at all or only completely weakened ("translucent") when recorded with the camera device 6. In contrast to this, the first image element 8 is visible in an image recording of the camera device 6 not only when illuminated with the light from the first wavelength range, but also when illuminated with the light from the second wavelength range.With the aid of the apparatus 1 shown in FIG. 1, it is possible to check for different purposes whether the image 2 has the second image element 9 in a correct manner. For this purpose, digital image data representing the image recorded by the camera device 9 is evaluated with the aid of an evaluation device 10. For this purpose, the evaluation device 10 is equipped with hardware and corresponding software, in particular programs for digital image analysis. The programs can be configured to apply artificial intelligence in the image analysis.The check by means of the image analysis can be used to check correct production of the image 2 in the production process, that is to say for example after the application of the image 2 as a printed image on the document body 5.When checking the image 2 of the security element 3, an image recording 20 of the image 2 according to FIG. 2 is captured by means of the camera device 6. In this case, the security element 3 is illuminated with light from the second wavelength range with the aid of the illumination device 7, with the aim of capturing an image record in which the first image element 8 is visible by means of the camera device 6, but not the second image element 9 with the lower half of the face and neck (cf. FIG. 2 ). FIG. 3 shows, for comparison, a further image recording 21 of the image 2 when illuminated with the light from the first wavelength range. Here, first and second picture elements 8, 9 can be seen.The image recording 20 is then analyzed with the aid of the evaluation device 9. In this case, description data can be provided in the evaluation device 10 which indicate properties of the first and / or of the second image element 8, 9 such as edge length, size, shape, area and / or area contour. The description data can be used in the checking process to subdivide the captured image recording 20 into subareas. Alternatively, the division of the image capture 20 into the subareas can be carried out without reference or use of the description data. In one embodiment, it is provided, for example, according to FIG. 3 to divide the image recording 20 into a first partial area 22 with the first image element 8 and a second partial area 23 with the second image element 9 (not visible here as expected), wherein the associated partial area completely captures the respective image element.For the digital image data representing the image recording 20, a texture analysis is carried out by means of the evaluation device 10, in which a respective texture is determined for the first and second subareas 22, 23 formed for this purpose, wherein the texture of the subareas displays a distribution of gray value intensities within the subareas, for example with a gray value intensity for each pixel or each image pixel in the subareas. For the first and the second sub-surface 22, 23, a respective texture measure is furthermore determined, which indicates to what extent the sub-surface is captured by the texture or is free of such. The texture measure can be, for example, a value number, a value range and / or a threshold value. More complex texture masses are also possible, for example a matrix characterizing the texture of the associated sub-surface or a number tuple.The texture analysis thus determines on the one hand the texture that may be present in the associated sub-surface and provides a texture measure which at least makes it possible to determine whether the associated sub-surface is substantially free of a texture or has such a texture. If it is found for the second partial surface 23 of the image recording 20 with the (expected) missing, i.e. invisible, second image element 9-as shown in FIG. 3-that the associated texture measure indicates a substantially present freedom from texture, the correct presence of the second image element 9 in the image 2, i.e. the correct color production or formation, is deduced.Alternatively, further second picture elements can be provided in the image 2 which are configured comparable to the second picture element 9, at least with regard to the use of the same colour, and can thus be analyzed in a similar manner in order to check the correct formation of the second picture elements.Of course, for checking the authenticity of the security document 4, further security elements (not shown) on the security document 4 can be checked, as is known per se in various embodiments.With reference to FIGS. 4 to 6, further aspects of the texture analysis in checking the image 2 of the security element 3 are explained below. In this case, the texture analysis is carried out by way of example using texture filters according to Laws (K. I. Laws: Textured Image Segmentation, Department of Electrical Engineering, Image Processing Institute, University of Southern California, January 1980).After Laws, a texture transformation of the image recording is carried out by means of convolution with filter cores in order to determine the texture. The filter cores are, for example, 5x5 matrices formed from vectors l, e, s, w and r for emphasizing specific texture properties ("level", "edge", "spot", "wave", "ripple"):The texture filter mask "sl" then has the following appearance, for example:FIG. 4 ashows an image record 40 of a face image of a security element in which-unlike in FIG. 3 for example-the first image element 8 is missing, which mostly but not completely captures the upper face half. The first picture element 8 is not visible in the illumination for the image recording 40, in contrast to the second picture element 9 with the lower half of the face. Edges of the first and second picture elements 8, 9 abut each other in a region 41.Within the framework of the texture analysis, different filter masks according to Laws can now be used to determine the texture. In FIGS. 4 band 4 c, this is shown for the filter masks "Is" and "sl". It follows that for FIGS. 42, 43 filtered within the framework of the texture analysis, the region with the first picture element 8, which covers the upper half of the face for the most part, but not completely, is essentially texture-free, whereas the second picture element 9 with the lower half of the face and the neck projection clearly has a texture. In the example shown, the first image element 8 is formed with a tile shape (rectangle) when dividing the image recording 40 for the texture analysis, such that the tile largely comprises the upper face half, but not completely. A partial region 45 of the upper half of the face is visible outside and in the image recording 40, i.e. it was not produced with the color which is not visible in the illumination for the image recording 40. A region of the filtered FIGS. 42, 43 assigned to the subregion 45 then also has a recognizable texture.FIG. 4 d shows a FIG. 44 of maximum gray value intensities from FIGS. 42, 43 in FIGS. 4 band 4 c. For each image pixel, the maximum of the gray value intensities from the images is shown in FIGS. 4 band 4 c. Here too, the freedom from texture is shown in the region with the first picture element 8.A bar 46 in FIG. 4a serves only for formal anonymisation of the represented face image.FIGS. 5 aand 5 b show representations for a captured image capture 50, 51 (left representation) and FIGS. 52, 53, respectively, which show the result of the determination of the texture (right representation). In the construction of FIGS. 52, 53, filter masks according to Laws were again applied for the image recordings 50, 51. It follows that the filtered FIGS. 52, 53 on the right side in FIGS. 5 aand 5 b respectively clearly emphasize and make them determinable the texture-free regions, even if, as in FIG. 5 a, the facial image is not arranged centrally in the image recording 50 in the image recording 50. For the image recording 51 in FIG. 5 b, in which the facial image "looks through", the texture-comprising and the texture-free regions likewise result.Bars 54, 55 in FIGS. 5 aand 5 beach serve only for formal anonymization of the represented face image.For checking for essentially given freedom of texture of a partial surface of the image recording, an application-related defined texture measure is used. For this purpose, a histogram can be used, for example, for the images filtered according to the texture analysis. FIGS. 6 and 7 show respective FIGS. 60, 70-based on previously acquired image recordings-with four subareas 60,..., 63; 70,..., 73 formed for the texture analysis.For each of the subareas 60,..., 63; 70,..., 73, a texture measure P is determined as part of a texture dimension determination as a function of the number N of image pixels having a gray value intensity above a specific threshold value (here, for example: 127): σ is an arbitrary scaling parameter here, as a result of which it can be made possible in embodiments that the result P lies better in the interval [0;1]. Since N is the number of pixels in the corresponding sub-area with a gray value intensity (after filtering) above a certain threshold value, σ is on the order of N. In the exemplary embodiment, σ is σ=0.04·A, wherein A denotes the size of the sub-area in pixels.The following results for the subareas 60,..., 63: P 60= 0.989, P 61= 0.248, P 62= 0.000, P 63= 0.987.The subareas 60, 63 have a high number of pixels with gray value intensity. The picture elements located therein are therefore not visible during the illumination for the underlying image recording (freedom from texture). The image elements contained in the subareas 60, 63 are each not visible, that is to say are printed, for example, with a color which is not visible in the illumination which was used for the image recording.For FIG. 7, the following results: P 70= 0.989, P 71= 0.984, P 72= 0.000, P 73= 0.987.The high values for the subareas 70, 71, 73 each indicate freedom from texture. If, for the facial image which is recorded here and checked by means of texture analysis, a visible image element in the partial surface 71 would also be expected in the image recording (for example comparable to FIG. 6 ), then it is determined on the basis of the texture analysis that the image (in the example here: facial image) of the security element was not correctly produced. The reason for this may be, for example, that the image element whose visibility is expected in the partial surface 71 has not been printed with the correct color. Also, the pixel could simply not have been printed.The features disclosed in the above description, the claims and the drawing can be important for the realization of the various embodiments both individually and in any combination.List of reference characters1 Device 2 Image 3 Security element 4 Security document 5 Document body 6 Camera device 7 Lighting device 8 First image element 9 Second image element 10 Evaluation device 20, 21 Image recording 22, 23 First and second subareas in image recording 40 Image recording 41 Edge of image elements 42, 43, 44 for texture analysis filtered image 50, 51 Image recordings 52, 53 for texture analysis filtered image 60,..., 63 Subareas for texture analysis 70,..., 73 Subareas for texture analysis

Claims

A method for checking a security element for a security document, the method comprising: - providing a security element (3) having an image (2) comprising a first image element (8) and a second image element (9), wherein the first image element (8) is visible in each case when illuminated with light from a first wavelength range and when illuminated with light from a second wavelength range which is different from the first wavelength range, and wherein the second image element (9) is visible when illuminated with light from the first wavelength range, but not when illuminated with light from the second wavelength range; acquiring an image recording (20) of the image (2) with a camera device (6) when the image (2) is illuminated with light from the second wavelength range, and determining the second image element (9) by evaluating digital image data which indicate the image recording (20), wherein the following is provided: dividing the image recording (20) into subareas (22, 23), wherein one of the subareas (22, 23) is formed comprising the second image element (9); determining a respective texture for the subareas (22, 23), wherein the texture of the subarea indicates a distribution of gray value intensities within the subarea of the image recording (20); determining a respective texture measure for the subareas (22, 23) which indicates to what extent the subarea acquires from the texture or is free of such a texture; and - determining the presence of the second picture element (9) in the image (2) of the security element (3) if it is found by evaluating the texture measures that the one of the subareas (22, 23) which comprises the second picture element (9) is substantially texture-free.Method according to Claim 1, characterized in that the image recording (20) is divided into subareas (22, 23) in such a way that adjacent subareas abut one another in the region of edges of the subareas (22, 23).Method according to Claims 1 and 2, characterized in that the image recording (20) is divided into subareas (22, 23) in such a way that the one of the subareas substantially completely overlaps the second image element (9).Method according to at least one of the preceding claims, characterized in that the security element (3) is provided with a printed image which comprises the first and the second image element (8, 9).Method according to at least one of the preceding claims, characterized in that a limit value texture measure is provided and the presence of the second picture element (9) in the image (2) of the security element (3) is determined if it is found during the evaluation of the texture measures that the texture measure for the one of the subareas (22, 23) which comprises the second picture element (9) is smaller than or equal to the limit value texture measure.Method according to at least one of the preceding claims, characterized in that description data are provided which indicate properties of the second image element (9) such as edge length, size, shape, area and / or area contour, and the description data are evaluated in order, when dividing the image recording (20) into the subareas (22, 23), to form the one of the subareas (22, 23) such that it comprises the second image element (9).Method according to at least one of the preceding claims, characterized in that - the security element (3) is provided with an image which has a further second image element; - the further second image element is visible when illuminated with light from the first wavelength range, but not when illuminated with light from the second wavelength range; - the image recording is divided into subareas such that a further of the subareas comprises the further second image element; and - the presence of the further second image element in the image of the security element is determined if it is found by evaluating the texture measures that the further subarea which comprises the further second image element is substantially texture-free.Method according to at least one of the preceding claims, characterized in that the texture measure is determined for the subareas (22, 23) using a texture filter according to Laws.Method according to at least one of the preceding claims, characterized in that a correct production of the image (2) of the security element (3) is checked, wherein a correct production is determined if at least the presence of the second image element (9) in the image (2) of the security element (3) is determined.Method for checking a security document, in which a security document (4) having a security element (3) is provided and the security element (3) is checked according to a method according to at least one of Claims 1 to 8.Apparatus for checking a security element for a security document, comprising: - a camera device (6); - an illumination device (7) and - an evaluation device (10); wherein the apparatus is configured to - capture an image recording (20) of an image (2) of a security element (3) with a camera device (6), wherein - the image (2) comprises a first image element (8) and a second image element (9), wherein the first image element (8) is visible in each case when illuminated with light from a first wavelength range and when illuminated with light from a second wavelength range which is different from the first wavelength range, and wherein the second image element (9) is visible when illuminated with light from the first wavelength range, but not when illuminated with light from the second wavelength range; and - the image recording (20) can be carried out with light from the second wavelength range when the image (2) is illuminated by the illumination device (7); and - the second image element (9) can be determined by evaluating digital image data which indicate the image recording (20), wherein the following is provided here: - dividing the image recording (20) into subareas (22, 23), wherein one of the subareas (22, 23) is formed to comprise the second image element (9); - determining a respective texture for the subareas (22, 23), wherein the texture of the subareas indicates a distribution of gray value intensities within the subarea of the image recording (20); - determining a respective texture measure for the subareas (22, 23) which indicates to what extent the subarea acquires from the texture or is free of such a texture; and - determining the presence of the second picture element (9) in the image (2) of the security element (3) if it is found by evaluating the texture measures that the one of the subareas (22, 23) which comprises the second picture element (9) is substantially texture-free.

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