IMAGE CONVERSION METHOD AND DEVICE FOR INTEGRATION OF A SECURITY FEATURE INTO A DIGITAL IMAGE

Abstract

An image conversion method is provided for integrating a security feature in a digital source image to generate a target image secured by the integrated security feature. The image conversion method includes carrying out transverse-wave-shaped distortion of the source image to generate an intermediate image; generating a target image, the intermediate image being scanned by rows of pixels to define, per row of pixels of the intermediate image, a sequence of successive pixels according to the scanning, and each pixel of the sequence being transformed to a corresponding pixel of the target image. The arrangement of the respective pixels of each sequence is a transverse-wave-shaped wave packet in the target image; and the integrated security feature is defined by the wave packets.

Claims

1. An image conversion method (200) for integration of a security feature into a digital source image (305) in order to generate a target image (455) secured by the integrated security feature, wherein the image conversion method (200) comprises: acquiring (205) source image data which represent a digital source image (305) to be protected by means of the security feature, which digital source image comprises pixels (315) arranged in a grid of straight parallel rows of pixels (310), each having at least one pixel value per pixel (315); generating (210) intermediate image data, which represent an intermediate image (400) that results from the source image (305) by applying a distortion rule, according to which, for each row of pixels (310) of the source image (305), the respective pixel values of pixels (315) of the row of pixels (310) are transferred within the grid along a direction which is angled to the row of pixels (310), in particular perpendicular to the row of pixels (310), to a respective other pixel (325; 500. of the grid which, in relation to the respective pixel (315), is determined or determinable by the distortion rule in such a way that the arrangement (405; 410; 415. of these other pixels (325; 500) in the grid has a transverse waveform; generating (220) target image data representing the target image (455), wherein: the intermediate image (400) is scanned in rows of pixels in order to define for each row of pixels of the intermediate image (400) a sequence of pixels (425, 430, 435, 440, 445, 450) that follow each other according to the scanning, the pixel values of which have resulted from the transfer of corresponding pixel values from the source image (305) in accordance with the distortion rule; each pixel (425, 430, 435, 440, 445, 450) of the sequence is transformed into a respective corresponding pixel (425a, 430a, 435a, 440a, 445a, 450a) of the target image (455) by determining its position in the target image (455) based on its position in the intermediate image (400) by compensating for the distortion suffered by applying the distortion rule when generating the intermediate image (400), so that the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) represents a transverse-wave-shaped wave packet; and the integrated security feature is defined by the wave packets.

2. The image conversion method (200) according to claim 1, further including: generating (225) a physical reproduced image (165; 465) of the target image (455) on a surface of a substrate (160), wherein the reproduced image (165; 465) of the target image (455) is generated by serially generating pixels on the substrate (160) by, in order to generate a series of pixels on the substrate (160), which corresponds to a respective sequence of pixels of the target image (455), generating these pixels of the series on the substrate (160) in accordance with the pixel order defined by the sequence of the corresponding pixels of the target image (455).

3. The image conversion method (200) according to claim 2, wherein the physical reproduced image (165; 465) of the target image (455) is generated on the substrate (160) using laser inscription, in which the pixels of the reproduced image (165; 465) are sequentially generated on the substrate (160) using a laser beam (130).

4. The image conversion method (200) according to claim 3, wherein the different positions of the series at which the laser beam (130) strikes the substrate (160) to generate the pixels of the reproduced image (165; 465) are controlled by variable deflection of the laser beam (130) in a mirror-based laser galvanometer (120).

5. The image conversion method (200) according to claim 4, wherein at least one mirror (135; 145) of the laser galvanometer (120) is controlled using a control signal for controlling a respective position of a pixel of the reproduced image (165; 465) to be generated on the substrate (160), which control signal is defined as a function of the compensation that has been determined for that pixel of the intermediate image (400) which corresponds to the pixel of the reproduced image (165; 465) to be generated.

6. The image conversion method (200) according to claim 4 or 5, wherein a laser galvanometer (120) is used for variable deflection of the laser beam (130), in which the inertia of at least one of its mirrors (135; 145) used for deflection is so large that, when imaging the wave packets of the target image (455) by generating the pixels of the reproduced image (165; 465) on the substrate (160), deviations between the reproduced image (165; 465) and the target image (455) arise in the event of abrupt changes in direction along the course of the wave packets.

7. The image conversion method (200) according to any one of the preceding claims, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is determined so that it represents a transverse-wave-shaped wave packet (460) which is periodic at least in some portions.

8. The image conversion method (200) according to claim 7, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is determined so that it represents a transverse-wave-shaped wave packet (460) which is sinusoidal at least in some portions.

9. The image conversion method (200) according to any one of the preceding claims, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is determined such that each of two adjacent wave packets are separated from each other by a gap.

10. The image conversion method (200) according to claim 9, wherein the substrate (160) is selected or processed in such a way that it has a color that stands out relative to the average of the colors of the pixels that form the wave packets of the target image (455), which are determined according to the pixel values, in such a way that neighboring wave packets can be visually distinguished.

11. The image conversion method (200) according to any one of the preceding claims, wherein each of the wave packets resulting in a respective sequence has at least two inflection points.

12. The image conversion method (200) according to any one of the preceding claims, wherein the grid of pixels of the source image (305) has rows and columns, and the resolution of the rows is different from the resolution of the columns.

13. The image conversion method (200) according to claim 12, wherein the resolution of the source image (305) in the direction orthogonal to the rows of pixels is at most 70% of the resolution in the direction running along the rows of pixels.

14. The image conversion method (200) according to claim 12 or 13, wherein the resolution of the source image (305) in the direction orthogonal to the rows of pixels is at least 200 pixels per inch or per 2.54 cm, PPI.

15. The image conversion method (200) according to any one of the preceding claims, wherein the pixels of the target image (455) are determined in such a way that their respective extents are the same along and orthogonal to the transverse direction of the wave packets.

16. The image conversion method (200) according to any one of the preceding claims, wherein the distortion rule is defined such that when it is applied to rows of pixels of the source image (305), at least for a subset of the rows of pixels, the wave packet respectively resulting from this has a first waveform in one or more portions of its course and a second waveform different from the first waveform in at least another portion of its course.

17. An image conversion device (105) which is configured to carry out the image conversion method (200) according to any one of claims 1 to 16.

18. A computer program or computer program product, comprising instructions which cause the image conversion device (105) according to claim 17 to carry out the image conversion method (200) according to any one of claims 1 to 16.

19. A digital image (700) with an integrated security feature, obtainable by the image conversion method (200) according to any one of claims 1 to 16 as a target image (455) or reproduced image (165; 465) of the same on a substrate.

20. A digital image (700) with an integrated security feature, in particular according to claim 19, having a plurality of mutually parallel rows of pixels, each of which has a transverse-wave-shaped course, wherein adjacent rows of pixels are separated from one another by a gap which, as regards its color, stands out at least in some portions with respect to the rows of pixels separated thereby, wherein the security feature is defined by the wave-shaped course of the rows of pixels and of the gaps between them.

21. The digital image (700) with integrated security feature according to claim 19 or 20, wherein the digital image is formed on a document page serving as a substrate (160) for a value document or a security document.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

[0075] FIG. 1 schematically shows a system for image conversion according to an exemplary embodiment of the security solution with an image conversion device along with a laser galvanometer for laser inscription of substrates.

[0076] FIG. 2 is a flowchart illustrating an exemplary embodiment of the image conversion method in accordance with the security solution.

[0077] FIG. 3 shows a schematic representation to illustrate an exemplary generation of a raster graphic intermediate image from a raster graphic source image as part of the method from FIG. 2.

[0078] FIG. 4 shows a schematic representation to illustrate an exemplary generation of a target image from an intermediate image as part of the method from FIG. 2.

[0079] FIG. 5 shows a schematic representation to illustrate an exemplary generation of a reproduced image of the target image on a substrate by laser inscription as part of the method from FIG. 2.

[0080] FIG. 6 shows a schematic representation to illustrate a wave structure that can be generated in the target image as part of the method from FIG. 2 and, if applicable, the reproduced image of the target image generated therefrom on a substrate.

[0081] FIG. 7 shows a schematic representation to illustrate a resolution adjustment or pixel shape change as part of the method from FIG. 2.

[0082] FIG. 8 is a flowchart illustrating an exemplary embodiment of a verification method for verifying digital images, in particular digital images that can be generated by the image conversion method according to the security solution (such as according to FIG. 2).

[0083] FIG. 9 shows a schematic representation to illustrate a verification of a digital image based on its two-dimensional (2D) Fourier spectrum as part of the method from FIG. 8.

DETAILED DESCRIPTION

[0084] The system 100 for image conversion illustrated in FIG. 1 according to an exemplary embodiment of the security solution has an image conversion device 105, which can in particular have a data processing device with a processor 105a and a memory 105b. The memory 105b can in particular serve as a program memory for a computer program that contains instructions which, when executed on the processor 105a, cause the image conversion device 105 to carry out an image conversion method in accordance with the security solution (for example according to FIG. 2). Accordingly, the image conversion method can in particular be designed as a computer-implemented method.

[0085] The system 100 can further have an image sensor (camera) 110 for recording a digital image, in particular a two-dimensional digital image, of an object, such as a person P. Additionally or alternatively, a data memory 115 may be provided which contains image data that represent an image of such an object. In particular, it is possible for the image data to be generated by the image sensor 110 and temporarily stored in the data memory 115 in order to be able to make it available to the image conversion device 105 from there. The system 100 itself can optionally also be designed in such a way that it already contains the image sensor 110 and/or the data memory 115 as components, in particular in a structural unit. An image provided, in accordance with this, as an input to the image conversion device 105 is referred to below as a source image and the data representing it as source image data.

[0086] If the image conversion device 105 carries out an image conversion method in accordance with the security solution, in particular the image conversion method 200 according to FIG. 2, and, as a result of this, generates a converted image, which in the following is referred to as a target image and the data representing it as target image data, it is also possible to transfer this target image to a substrate 160 as a physical reproduced image 165 of the same. This can be done in particular, as shown in FIG. 1, by laser inscription. The substrate 160 can in particular be a page of a document suitable for inscription by the inscription technology used to generate the reproduced image 165, in the case of laser inscription in particular a page which comprises at least a polymer material which can be modified by laser light in order to achieve a change in properties, in particular to achieve a color change.

[0087] In the system 100, a laser galvanometer 120 is specifically provided as a device for laser inscribing substrates 160. It has a laser 125, which is configured to emit a laser beam 130, which impinges on a first mirror 135 in order there to be deflected onto a second mirror 145 depending on a position of a first mirror 135 that can be adjusted by a first mirror drive 140. The second mirror 145 in turn has a mirror drive 150 in order to be able to variably adjust the position of the second mirror 145. The laser beam 130 deflected by the second mirror 145 then passes through a focusing optics 155, which in particular can be, or comprise, an F-theta lens. From the focusing optics 155, the focused laser beam 130 then hits the substrate 160 to be inscribed in order to generate, at the point where it hits the substrate 160, a pixel 170 of the reproduced image 165 of the target image to be generated. By appropriately controlling the two mirror drives 140 and 150, it can be achieved that the laser beam 130 is variably deflected by the combination of the mirrors 135 and 145, so that its point of impact on the substrate has a linear course over time. Since the target image, as will be explained in detail below, has wave-shaped image components, these wave packets are correspondingly transferred to the substrate 160 during laser inscription in wave-shaped inscription lines in the form of wave packets 175 formed from pixels of the reproduced image 165.

[0088] FIG. 2 illustrates an exemplary embodiment 200 of an image conversion method, which can be carried out in particular by the system 100. In the further FIGS. 3 to 7, individual steps of the image conversion method 200 are illustrated in greater detail.

[0089] As part of the image conversion method 200, a source image 305 is first acquired in a step 205, as already described in more detail with reference to FIG. 1, and, in the form of source image data representing the source image 305, is provided as input for the further steps of the image conversion method 200. In the following, for the purpose of FIGS. 3 to 5, it is assumed, by way of example and in order to simplify the illustrations, that the source image 305 is a digital raster graphic of two-dimensional image points (pixels) arranged in a matrix, which represents a set of parallel straight lines which, in the source image 305, run along the row direction in the source image 305. In FIG. 3, different image values are indicated by different hatching or black or white coloring. By way of example, it is assumed here that the source image 305 and the intermediate image 400 are each a gray value image and each pixel is assigned exactly one gray value as an image value (pixel value). The resolution in the grid can, as assumed here, differ for rows and columns of the grid, so that the pixels have a rectangular, non-square shape, in which, for example, the height of the pixels 315 and 325 is greater than their width. This will be explained in more detail below with reference to FIG. 7.

[0090] In a further step 210, which is illustrated in more detail in FIG. 3, an intermediate image 400 is generated from the source image 305, which intermediate image is represented by corresponding intermediate image data. The intermediate image 400 results from the source image 305 by applying a distortion rule that defines a transverse-wave-shaped distortion in such a way that the respective image values of pixels 315 of the same row of pixels 310 (in particular row or column of the raster graphic) are shifted within the grid in such a way that the pixels 325 to which the image values of the row of pixels 310 are shifted have an arrangement within the grid that resembles a transverse wave running along a propagation direction 330 parallel to the rows of pixels 310. The shape of the transverse wave can in particular, at least in some portions, be sinusoidal, which can be advantageous in particular with regard to the verification method described below, which is based on a Fourier analysis.

[0091] In the image conversion method 200, a further step 215 follows, which starts from the intermediate image 400 generated in step 210. Depending on the distortion rule used in step 210 to distort the source image 305, correction information is determined which, based on the grid of the intermediate image 400, defines a shift in pixel positions of the grid and which has the effect that the target image 455 appears, or would appear, at least largely undistorted to a viewer of the target image 455. The matrix-shaped grid is thus modified in such a way that rectilinear rows of pixels 310 of the grid are each converted into transverse-wave-shaped wave packets of pixels that correspondingly follow one another in a transverse-wave-shaped course in such a way that, by this, the distortion generated in step 210 is at least largely compensated for.

[0092] Step 215 is illustrated in an exemplary and simplified manner in FIG. 4. For this purpose, the intermediate image 400 is shown again in a simplified representation in the upper region of FIG. 4, with some exemplary transverse-wave-shaped arrangements of pixels each having resulted from the distortion in step 210, in particular the arrangements 405, 410, 415. Each of these transverse-wave-shaped arrangements has emerged in step 210 from a rectilinear row of pixels, in particular row of pixels 310 of the source image 305. Now, in order to get from the intermediate image 400 to the target image 455, the intermediate image 400 is scanned along straight-line scan paths 420, of which only one is illustrated as an example in FIG. 4 to simplify the representation. Along a scanning direction of the scan path 420, the positions of pixels 425 to 450 lying thereon are acquired, each of which belongs to one of the transverse-wave-shaped arrangements of pixels, such as the arrangements 405, 410 and 415 in this example. For pixel 430, a displacement vector v is shown as an example, which corresponds to the shift of the pixel value of a (starting) pixel in the source image 305 to the corresponding other pixel 430 in the intermediate image 400 having taken place as part of the distortion in step 210. This applies accordingly to the other pixels 425 and 435 to 450 individually identified here.

[0093] As part of step 220, this distortion is compensated for, using a displacement vector v that is inverse to v. Unlike distortion, compensation does not involve transferring image values from one pixel to another, but instead shifts the positions of the pixels themselves, which, in the course of this, retain their respective pixel values. Referring to the pixel 430 of the intermediate image 400 as an example, this means that its position is shifted by the displacement vector v, as shown in the lower part of FIG. 4. If this is also done accordingly for the other pixels 425 and 435 to 450 lying on the scan path 420, wherein their individual displacement vector v is respectively determined and used for compensation, then the result is a transverse-wave-shaped wave packet 460 in the target image 455 formed from the correspondingly shifted pixels 425a or 435a to 450a.

[0094] Overall, this displacement process is carried out in step 220 for all defined (parallel) scan paths 420, so that, in the target image 455, a set of transverse-wave-shaped wave packets 460 results.

[0095] In a further step 225, which is illustrated in more detail in FIG. 5, based on the target image 455, a physical reproduced image 465 of the same can be generated on the substrate 160 by laser inscription, for which the incidence point (at pixel 170) of the laser beam is guided by corresponding control of the laser inscription device, in particular of the laser galvanometer 120 of the system 100, along a respective laser path 455a so that the wave packets 460 of the target image 455 are imaged onto the substrate 160 by forming corresponding pixels of the reproduced image along the laser path 455a.

[0096] However, as illustrated in FIG. 5, in practice the transfer of the target image 455 into the reproduced image 465 is usually not perfect. This may be due in particular to the fact that the mirrors 135 and 145 have a non-zero inertia and, at high laser inscription speeds, are no longer able to accurately image abrupt changes in direction along the courses of the wave packets 460 onto the substrate 160. Therefore, at points of strong changes in direction, deviations can regularly arise between the exact shape of the respective wave packet 460 and the associated laser path 455a derived therefrom. In particular, angular portions of the course of a wave packet 460 will generally have a rounded shape in the reproduced image 465, particularly in the sense of an overshoot. This is shown as an example in FIG. 5, wherein the pixels 425a to 450a of the target image 455 are displayed as a reference in order to make the deviations clearly visible. However, this imperfect mapping from the target image 455 into the reproduced image 465 is actually advantageous because it represents a further aspect of the security feature defined by the wave packets of the reproduced image 465, which makes forgery even more difficult.

[0097] FIG. 6 illustrates, using an enlarged representation 470a of an exemplary image portion 470, a wave structure that can be generated within the framework of the method 200 in the target image 455 or the reproduced image 465 generated therefrom on a substrate 160 (without showing the deviations mentioned between target image 455 and reproduced image 465). The notable mouth portion contained here in the image portion 470 is represented in the target image 455 or equally in the reproduced image 465 by corresponding gray values of the pixels (here forming the thick line regions of the wave packet portions) on the wave packets, which contrast with neighboring image regions. I adjacent wave packets are separated frI another by a space of a different color (such as white or in the background color of the substrate 160), so that they can be individually optically recognized and detected, at least when using appropriate magnification.

[0098] FIG. 7 illustrates a resolution adjustment or pixel shape modification as part of the method 200. As previously described in detail with reference to FIG. 3, an image grid of the source image 305 and of the intermediate image 400 was used here as an example, in which the rows and columns have a different resolution and therefore different edge lengths of the pixels in the two orthogonal directions (x and y, respectively). In order to be able to put into effect the aforementioned gaps between adjacent wave packets in the target image 455 and, if applicable, the subsequently generated reproduced image 465, without thereby having to accept undesirable distortion (not to be confused with the desired temporary distortion in step 210 according to the distortion rule) of the respective image, the pixels, when transitioning to the target image 455, are trimmed so that they have a symmetrical, in particular square, shape with respect to width and height, so that the portion or portions that have been cut away are available for the gaps to be formed.

[0099] This is shown in FIG. 7, where a pixel 500 of the intermediate image 400 is broken down into a square portion 505 that is to be transferred into the target image, and two remaining portions 510 that are not to be transferred into the target image. Instead of the two remaining portions 510 that are not to be transferred into the target image, a (half) gap to the respective neighboring pixel is formed in the target image 455.

[0100] In addition, exemplary dimensions and resolutions (in dpi) are given in FIG. 7, whereby these exemplary dimensions and resolutions refer specifically to the source image 305 or equally to the intermediate image 400.

[0101] FIG. 8 illustrates, with the aid of a flowchart, an exemplary embodiment 600 of a verification method, in particular a computer-implemented verification method, for verifying digital images, in particular digital images that can be generated by the image conversion method according to the security solution (such as according to FIG. 2). FIG. 9 shows a schematic representation to illustrate a verification of a digital image based on its two-dimensional (2D) Fourier spectrum as part of the method 600 from FIG. 8.

[0102] In the verification method 600, in a step 605, image data are acquired that represents a digital image 700 to be verified. In the following, it is assumed by way of example that this digital image 700 represents a person P (see FIG. 9). Similar to the case of the system 100 from FIG. 1, the acquiring of the image 700 can take place in particular using a camera, by receiving image data via a communication connection or by reading out from a memory. The image 700 can in particular be present as a physical image on an object surface, in particular on a sheet-shaped substrate 160, in particular on a document page of a document (such as a passport document).

[0103] In a further (optional) step 610 of the verification method 600, an image analysis takes place, in which the digital image 700 is analyzed, in particular for particularly relevant image components. This can in particular include image segmentation. In the case of a person P represented at least in portions by the image 700, the image analysis can in particular include a facial analysis in which biometric features of the face, such as the position of the eyes, in particular the pupils, the nose, ears, corners of the mouth, etc. are localized. This can in particular have the purpose, in a further step 615 of the method 600, in which the digital image is subdivided into different image sectors 705, to define the image sectors 705 depending on the localized biometric features, for example in such a way that at least one image sector 705 is defined for each biometric feature. In FIG. 9, for example, the image sector 710 represents such a selected image sector from the set of image sectors 705.

[0104] The subdivision of the image into several image sectors 705 illustrated in FIG. 9 serves in particular to carry out a transformation from the spatial domain into the frequency domain by a two-dimensional transformation, here, by way of example, specifically a two-dimensional Fourier transformation, in a further step 620 based on individual image sectors (such as image sector 710). In this context, the image sectors taken into account can in particular be all of the defined image sectors 705 or only a specific selection thereof. This selection can be determined in particular by the fact that only image sectors 710 defined for the biometric features, in particular all of them, belong to this selection.

[0105] As an example, a matrix-shaped (discrete) 2D Fourier spectrum 715 of the selected image sector 710 resulting from step 620 is shown in FIG. 9, in which each point of the matrix represents, via its point value (which is characterized in FIG. 9 by different colors or hatchings), the value of a specific Fourier coefficient or spectral value of the spectrum 715. The center of the spectrum corresponds to the frequency zero and the position of the respective point in the matrix identifies the respective wave vector or Fourier coefficient associated therewith, so that the distance of a respective point from the center corresponds to the frequency (or wavelength) represented by the point. The direction vector represented by the center and the point characterizes the direction of propagation in the two-dimensional spatial domain represented by the point.

[0106] In the present example, it is assumed that the image sector 710-similar to the image portion 470a from FIG. 6-comprises a plurality of wave packets. The bright line in the spectrum, which indicates that the Fourier coefficients corresponding to the points of the line in the spectrum 715 are heavily occupied, is essentially due to the very frequent, approximately horizontal regions of the minima and maxima of the wave packets (in the spatial domain) that are due to the wave packets that are present in the image sector 710 several times.

[0107] As part of a further step 625 of the method 600, several comparisons are now made, in which a spectrum 715 obtained from a selected image sector in step 620 is compared with a respective reference spectrum 720, which results from the authentic original image for the relevant image sector when the same transformation is applied. The reference spectra 720 can in particular be stored in advance in a memory in a manner secured against unauthorized access in order to be read from them and made available for the purpose of comparison. It is possible, for example, to design this so that the reference spectra 720 can be retrieved from a remote server via a secure communication connection.

[0108] In the example of FIG. 9, a comparison of the spectrum 715 derived from the image 700 to be verified with the corresponding reference spectrum 720 derived from the authentic original image shows a significant deviation. A corresponding comparison is now carried out individually for all selected image sectors. In order to finally determine whether the image 700 is the original image or a fake, in particular a test criterion can be used that is based on a number N of the selected image sectors of the image 700, in which, as part of their respective assigned spectrum comparison, a deviation beyond a defined deviation threshold has been detected. This number N can now be compared with a predetermined threshold M, with which the sensitivity of the test method can be adjusted. If the result of this is that N is smallerthan M (630-yes), then the digital image 700 is classified as authentic and this result is output in a step 635. Otherwise, (630-no) the image 700 is classified as fake and this result is output in a step 640. In particular, M=1 can be chosen, so that a deviation in the spectrum 715 of a single image sector is sufficient to classify the image 700 as inauthentic or fake.

[0109] The comparison of the two spectra 715 and 720 for the respective image sector can be carried out in particular on a Fourier coefficient basis, i.e. in such a way that the Fourier coefficients of the two spectra 715 and 720, which Fourier coefficients correspond to one another in a one-to-one relationship, are respectively compared with one another, whereby a check is carried out as to whether their values differ from one another by more than a permitted threshold. The total number of values lying beyond the threshold can then be compared with an acceptance threshold in order to determine whether there is a (significant) deviation between the two spectra in the respective image sector.

[0110] While at least one exemplary embodiment has been described above, it is to be noted that a large number of variations thereto exist. It is also to be noted that the exemplary embodiments described only represent non-limiting examples, and that it is not intended to thereby restrict the scope, the applicability, or the configuration of the devices and methods described herein. Rather, the preceding description will provide the person skilled in the art with guidance for implementing at least one exemplary embodiment, wherein it is to be understood that various modifications in the operation and arrangement of the elements described in an exemplary embodiment may be made without thereby departing from the scope of the subject matter respectively defined in the appended claims, as well as its legal equivalents.