Method for determining the optical properties of a curved reflection hologram

Abstract

The invention relates to a method for determining a reconstruction parameter of a reflection hologram in a curved state, the determination method being based on measurements of the reflection hologram in a planar state. The method comprises the following steps: reconstructing the reflection hologram in the planar state using a light source from an illumination point for projecting the reflection hologram onto a screen; measuring a distribution of a reconstruction parameter of the projected reflection hologram along the screen by means of an eyebox detector; and determining a reconstruction parameter of the reflection hologram in the curved state on the basis of the measured reconstruction parameters of the projected reflection hologram by means of an eyebox overlay function GEB. Alternatively, the reflection hologram is reconstructed in the planar state using a light source from an illumination point. A distribution of a reconstruction parameter of the reflection hologram in the planar state is then measured along an axis of the reflection hologram in the field of view of a field-of-view detector; and a reconstruction parameter of the reflection hologram in the curved state is determined on the basis of the measured reconstruction parameters of the reflection hologram in the planar state by means of a weighting function GL.

Description

[0001] METHOD FOR DETERMINING THE OPTICAL PROPERTIES OF A CURVED REFLECTION HOLOGRAM

[0002] DESCRIPTION

[0003] The invention relates to a method for determining a reconstruction parameter of a reflection hologram in a curved state, wherein the determination method is based on measurements of the reflection hologram in a planar state.

[0004] In one aspect, the procedure comprises the following steps: Reconstruction of the reflection hologram in the planar state with a light source from an illumination point for projection of the reflection hologram onto a screen; measurement of a distribution of a reconstruction parameter of the projected reflection hologram along the screen using an eyebox detector; and determination of a reconstruction parameter of the reflection hologram in the curved state based on the measured reconstruction parameters of the projected reflection hologram using an eyebox superposition function GEB.

[0005] In another aspect, the procedure comprises the following steps: Reconstruction of the reflection hologram in the planar state with a light source from a single illumination point; measurement of a distribution of a reconstruction parameter of the reflection hologram in the planar state along an axis of the reflection hologram in the field of view of a field-of-view detector; and determination of a reconstruction parameter of the reflection hologram in the curved state based on the measured reconstruction parameters of the reflection hologram in the planar state using a weighting function GL.

[0006] Background and state of the art

[0007] The invention relates to the field of hologram replication.

[0008] HOEs (Holographic Optical Elements) are typically optical components that utilize holographic properties to achieve a specific light path, such as transmission, reflection, diffraction, scattering, imaging, deflection, and / or refraction. This allows desired optical functionalities to be implemented compactly in any substrate. The holographic properties preferably exploit the wave nature of light, particularly coherence and interference effects. Both the intensity and phase of the light are taken into account.

[0009] Such holographic elements are used in many areas, such as transparent displays (e.g., in shop windows, refrigerated display cases, vehicle windows), for lighting applications such as indicator or warning signals in glass surfaces, light-sensitive detection systems for example for interior monitoring (eye tracking in vehicles or presence status tracking of people indoors).

[0010] Holograms are created by the interference of a reference beam with light reflected or diffracted from the surface of an object (object rays). Traditionally, three-dimensional objects were used to produce unique, custom-made holograms. Commercially available HOEs, however, are often mass-produced through duplication processes.

[0011] Duplication processes typically use a master hologram containing the image to be copied. The master holograms are often stored on a substrate body that carries the master hologram. This substrate body is preferably transparent and can have various shapes, such as a cuboid, a plate, or a cylinder. The combination of the master hologram and the substrate body forms a master element.

[0012] The master element is illuminated with a coherent light source to replicate the image from the master hologram into a photosensitive composite. For mass production, the photosensitive composite can be provided as a continuous web comprising a photosensitive material and one or more carrier or protective films. The photosensitive web is preferably transported through various workstations to produce the HOEs. For this purpose, several transport rollers are preferably used to facilitate continuous replication of the master hologram into the photosensitive web.

[0013] During exposure, the composite web is placed on or brought into contact with a surface of the master element. The composite web is preferably in a planar state during exposure. This improves handling and contact between the composite web and the master element, which can be configured, for example, as a cuboid or cylinder. To create a reflection hologram, the coherent light can pass through the composite web before reaching the master hologram, from which it is reflected back into the composite web. Alternatively, to create a transmission hologram, the coherent light can first be directed onto the master hologram, through which it is diffracted before reaching the composite web. In both cases, object and reference rays interfere with each other in the photosensitive material and form the replicated hologram.The replication process is sensitive to unwanted stray light, which can also interfere with the object and / or reference beams. For example, if irregularities or gaps occur at the interface between the composite web and the surface of the master element, the reference and / or object beams can be reflected internally. This can lead to either efficiency losses or unwanted interference, which impairs the quality of the replicated hologram. Therefore, ensuring sufficient optical contact between the master element and the composite web is of paramount importance for the quality of the replicated hologram.

[0014] Some end applications require the reproduced hologram to be in a curved state with a high degree of complexity, including one or more axes of curvature. For example, the reproduced hologram can be configured for integration into the three-dimensional shape of a windshield, as described in DE4211728A1. Since the hologram generated in a planar state in the photosensitive composite web is transformed into a three-dimensional shape during integration into the product, e.g., the windshield, the hologram pattern must be exposed during its production at the same angles at which it is to be reconstructed in its three-dimensional form. This may require the use of different exposure angles at different positions of the planar hologram. This is intended to ensure that the replicated hologram exhibits the desired optical function when reconstructed in its curved final shape.

[0015] From WO 2023 148375 A1, for example, it is known to provide a master hologram in a curved target surface shape, wherein, during replication of the master hologram, the substrate material of the master hologram and the substrate material of the replicated hologram are present in a flat exposure surface shape. After the exposure process for the substrate layer of the replicated hologram, it is fixed in the curved target surface shape, which is suitable for a target application, for example, in a motor vehicle. To ensure that the beam path of the light used for exposure, when using the flat exposure surface shape, replicates those rays that maintain the optical functionality of the replicated hologram when a curved target surface shape is present, the provision of a positioning module is proposed.The positioning module advantageously enables the movement of a reference point arranged along a light beam path relative to the master hologram on an (arbitrarily) curved trajectory during the exposure process, while simultaneously setting an exposure angle. In preferred embodiments, the positioning module can, for example, comprise a robotic arm with a two-dimensional galvo scanner at its end.

[0016] However, deviations can occur during the process, so for quality control purposes it is desirable to be able to check during the manufacturing process to what extent the replicated hologram in the curved shape actually has the desired optical function.

[0017] However, simply converting the replicated hologram into its final curved shape for quality control purposes before integration into the product would be complex. In particular, this would interrupt and slow down a continuous roll-to-roll replication process. Such a forming step would also introduce an additional source of error in hologram production if the replicated hologram is returned to its planar state for simple stacking or rolling after quality inspection. This poses a challenge for inline verification of the replicated hologram's quality and suitability for three-dimensional end-use within a continuous duplication process.

[0018] WQ2021094046A1 discloses an alternative method for producing three-dimensional holograms with curved shapes. In this method, a foil-shaped master hologram is provided on one curved mold half. A curved glass pane (in particular, a windshield) is placed on a second mold half, creating a gap between the curved glass pane and the master hologram. A photosensitive material is injected directly onto the curved surface of the glass pane within the gap. The hologram is replicated in place, with both the photosensitive material and the master hologram present in a form corresponding to the final end application. This method can eliminate the need for a photosensitive composite web in a planar state and allows for quality control in the curved final state.However, the process is time- and energy-intensive and therefore only conditionally suitable for the mass production of curved holograms with high production speeds and throughputs.

[0019] In light of the state of the art, there is therefore a need for an efficient quality control technology suitable for use within a continuous roll-to-roll manufacturing process, whose holograms are intended for applications in a curved state.

[0020] Object of the invention

[0021] The object of the invention is to determine a method for testing the quality of a hologram with regard to its application in the curved state, wherein the method can preferably be easily integrated into a roll-to-roll manufacturing process with planar film guidance. It is particularly an object of the invention to provide a method in which a reconstruction parameter of a curved reflection hologram is determined, wherein the determination method is based on measurements that can be easily performed during the production of the reflection hologram in the planar state.

[0022] Summary of the invention

[0023] The problem is solved by the features of the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0024] In a first aspect, the invention relates to a method for determining a reconstruction parameter of a reflection hologram in a curved state, wherein the determination method is based on measurements of the reflection hologram in a planar state. The method comprises the following steps: reconstruction of the reflection hologram in the planar state with a light source from an illumination point for projection of the reflection hologram onto a screen; measurement of a distribution of a reconstruction parameter of the projected reflection hologram along the screen using an eyebox detector; and determination of a reconstruction parameter of the reflection hologram in the curved state based on the measured reconstruction parameters of the projected reflection hologram using an eyebox superposition function GEB.

[0025] Measurements of the reflection hologram, taken while it is in a planar state, can be performed quickly without subjecting the hologram to a precise reshaping process. Reconstructing the planar hologram on a screen offers a rapid and simple way to reliably and repeatably capture the properties of an eyebox generated by the planar hologram. According to the invention, it has been recognized that such measurements can be used to reliably determine the parameter distribution of an eyebox that would be generated by the same hologram if the hologram were transformed into a curved shape.

[0026] Advantageously, the properties of the eyebox of the curved hologram can be determined with surprising precision using the method according to the invention, using a single image captured from the projected eyebox of the planar hologram and a single illumination point, which can be optimized in preferred forms. In particular, it is advantageously possible to take into account, using an eyebox superposition function GEB, as described in more detail elsewhere herein, that the emission characteristics of the hologram from an illumination point in the planar state differ from those of the same hologram in the curved state from an illumination point within the application. Advantageously, the method according to the invention thus provides a sufficiently accurate determination method using a single measurement.

[0027] Alternatively, it would be conceivable to analyze the properties of a planar hologram with regard to its curved state by moving an illumination point along a path to simulate the angles of incidence of the curved hologram while the planar hologram is illuminated. Using such a path for an illumination source, it would, in principle, be possible to measure or evaluate the reconstruction efficiency of the hologram for the curved final state even while the hologram is still in its planar state.

[0028] However, an analysis of the optical parameters of the hologram when it is curved would be further complicated by the fact that the various reconstruction rays falling on the planar hologram from the different illumination points do not create a single, sharply defined eyebox encompassing the diffracted rays from all positions along the hologram. Instead, in such a measurement method, different positions of the hologram in the planar state would generate rays with different diffraction angle ranges. This leads to a fanning out of (partial) eyeboxes of a curved hologram, originating from different positions of the planar hologram, as illustrated by example in Fig. 3.

[0029] To capture the radiation diffracted by the hologram, a projection screen with a varying angle could be moved synchronously with the illumination source to enable imaging of the fanned-out partial eyebox by a detector at a multitude of projection screen positions. However, such a scanning measurement method would require, as illustrated in Fig. 4, traversing a large number of combinations of illumination points and observation positions.

[0030] Such a detection method would therefore be time-consuming and require a high degree of synchronization with tight tolerances. Instead, the detection method according to the invention can be performed with a single image capture on the projection screen, which can be statically positioned. The illumination point can also be fixed, so that the detection method according to the invention can be carried out quickly, with few moving parts, and still delivers precise results.

[0031] Determining or calculating the hologram's properties allows for the assessment of its quality and suitability for the intended end application using a simple measurement method, without requiring the hologram to be laboriously curved for analysis and potentially returned to a planar form for further processing. The solution according to the invention is particularly suitable for measuring holograms produced on flexible materials such as composite webs or flat sheets, as it enables the inventive method to be carried out in-line during the hologram's production process while it is in a planar state.

[0032] Due to its suitability for rapid processing in a continuous process, a photosensitive composite web is frequently used for the production of reflective holograms. A photosensitive composite web can be transported through several processing stations using simple means such as one or more transport rollers. These stations include, for example, a coating or laminating station, an exposure station, a fixing station, a forming station, and / or a cutting station. Simultaneous processing of different sections of the photosensitive composite web at various process stations increases the efficiency and throughput of the hologram manufacturing process.This transport of the photosensitive composite web to and between the various process stations typically occurs with the web in a substantially planar form, for example, while it is drawn over or under one or more transport or deflection rollers. In some replication techniques, such as those using soft or liquid photosensitive materials, it can be important that the photosensitive composite web does not experience any sharp folds or bends before the material has fully solidified, in order to avoid distortion of the diffraction pattern formed within it.

[0033] The invention therefore advantageously enables the analysis of parts of the composite web containing holograms with regard to a curved final shape, without altering the planar shape of the composite web as it is manufactured. This allows the analysis to be performed in-line, without, for example, separating and reshaping the finished holograms from the composite web, which can cause additional mechanical stress. Processed and analyzed sections of the composite web can simply be rolled up for storage. Undesirable distortions of the non-fixed diffraction patterns caused by reshaping are thus avoided. Based on this analysis, holograms can be identified for rejection or directly discarded.Process parameters of a replication process can also be adjusted with regard to detected errors or inefficiencies in order to ensure a high quality of the holograms subsequently produced.

[0034] Investigating the distribution of an optical parameter across or along an eyebox is complicated by the different shape and size of the eyebox produced by a planar hologram compared to a curved one. A further difficulty arises from the fanning out and overlapping of the diffracted rays from different parts of the planar hologram on the screen (described in more detail below with reference to Figures 3-5), which generally prevents the visual recognition of the holographic image's content or optical function.

[0035] The eyebox overlay function GEB advantageously allows for the rectification of the eyebox resulting from the fanned-out and overlapping diffracted rays of the planar hologram, in order to mathematically reconstruct the eyebox that the hologram will exhibit in its curved state. This enables the calculation and evaluation of the distribution of an optical parameter, such as luminance, intensity, efficiency, color, resolution, noise reflection, granularity, homogeneity, and / or image distance, along the eyebox that would be emitted by the curved hologram.

[0036] The determined parameters of the curved hologram allow for an evaluation of its quality. This can lead to the hologram being classified as acceptable or unacceptable, depending on defined quality limits. In the latter case, the hologram can be marked for disposal or disposed of directly. This method ensures a particularly reliable quality standard for the application. The quality assessment can also serve as feedback for adjusting the manufacturing parameters of the reflective hologram, thereby further improving the quality of subsequent holograms.

[0037] For the purposes of the invention, "determination" preferably refers to the determination of values ​​that are not directly measured but mathematically calculated. A determined value thus preferably results from a mathematical method or an algorithm. The determined value preferably relates to a curved hologram.

[0038] For the purposes of the invention, a "measurement" preferably refers to the determination of values ​​using a detector, wherein the measurement is preferably based on a change in an electrophysical state within the detector. Values ​​measured according to the invention preferably relate to the planar hologram. While the measured values ​​may have been subjected to mathematical processing or conversion within the detector, this preferably does not go beyond known data processing operations of detectors for optical analysis such as cameras. A measured value has preferably not yet been subjected to mathematical processing using the eyebox superposition function GEB or the weighting function GL of the present invention.

[0039] For the purposes of the invention, a “reconstruction” of the reflection hologram preferably means illumination of the reflection hologram to generate optical content or an optical function by diffraction at an interference pattern which was inscribed in the reflection hologram during its production by replication of a master hologram.

[0040] For the purposes of the invention, a "reconstruction parameter" is preferably a quantifiable optical parameter of a hologram when reconstructed by an illumination source. The optical parameter may have a distribution with respect to positions of the hologram itself or with respect to diffracted radiation from the hologram. Non-limiting examples of the reconstruction parameter may include luminance, luminance distribution, color, color distribution, resolution, noise reflections, granularity, homogeneity, and / or image distance.

[0041] For the purposes of the invention, a "curved state" is preferably a non-planar state, including convex and concave shapes with constant or varying radii, wavy shapes, and angled shapes. Preferably, the "curved state" corresponds to a non-planar state of the hologram in which the hologram will be during its final application. Therefore, for the purposes of the invention, the curved state can also be referred to as the final form and characterizes the relevant state of the hologram for the application, for which a quality assessment is to be carried out.

[0042] For the purposes of the invention, a "planar state" is preferably a state that differs from the non-planar state of the hologram in which it will be in its final application. The hologram in the planar state preferably extends in a single plane and is particularly preferably substantially free of curvatures or folds. In embodiments, a hologram can also be considered planar if a minimum radius of curvature is at least 50 cm, 100 cm, 500 cm, or 1000 cm or more.

[0043] A "radius of curvature" within the meaning of the invention is the radius of a circle that best approximates a curve, in particular the path of a hologram in a plane, at a specific point. The radius of curvature describes how strongly (small radius) or gently (large radius) a curve is curved at this point. In a curved state, the hologram can have different radii of curvature in different directions. It may also be preferred that the hologram, in its curved state, has substantially the same radii of curvature in all directions. In particularly preferred embodiments, a curved state of the hologram refers to a curvature along one (curvature) direction, without any curvature along a direction orthogonal to it.In this case, in the curved state, preferably one radius of curvature in a plane is nearly constant, and a second radius in the plane orthogonal to the previous plane is at least 50 cm, 100 cm, 500 cm, or 1000 cm, where these values ​​represent the radius of curvature of a substantially flat or planar surface. For the purposes of the invention, an "axis of curvature" is preferably the surface normal to a surface in which a hologram has a constant, preferably positive and finite, radius of curvature.

[0044] A "plane of curvature" within the meaning of the invention is preferably a plane that is orthogonal to the axis of curvature. In other words, the plane of curvature is preferably defined by its surface normal, which corresponds to the axis of curvature. A "direction of curvature" lies in the plane of curvature.

[0045] A "light source" within the meaning of the invention is preferably a device configured to emit electromagnetic radiation with a wavelength between 200 nm and 25 pm, preferably 300 nm to 3 pm, and particularly between 400 and 780 nm. The electromagnetic radiation may preferably comprise infrared, visible, and / or ultraviolet radiation, with visible radiation being particularly preferred. In the context of the invention, UV radiation preferably means electromagnetic radiation in the range of 200 pm to 400 pm, particularly preferably 300 pm to 400 pm. Visible radiation particularly means electromagnetic radiation in the range of 400 to 780 nm and infrared radiation from 780 nm to 25 pm, preferably in the near-infrared range, i.e., preferably from 780 nm to 3000 nm, and particularly from 780 nm to 1400 nm.

[0046] The light source may include or be associated with light-deflectoring devices such as lenses. The light source may preferably emit a widened beam and / or a scanning collimated beam and / or multiple collimated beams, particularly with a specific direction. Preferably, the wavelength characteristics of the light source are known so that a wavelength-dependent efficiency of the analyzed hologram can be determined.

[0047] For the purposes of the invention, an "illumination point" is preferably a point from which radiation for the reconstruction of the analyzed hologram strikes a surface of the hologram without further deflection. The illumination point can correspond to the position of the light source itself or be located on a light-reflecting element, such as a mirror.

[0048] In accordance with the invention, a "projection of the reflection hologram onto a screen" preferably involves positioning a screen within a three-dimensional eyebox space of a hologram to reflect or scatter radiation diffracted by the hologram. The screen preferably reflects or scatters the diffracted rays of the hologram in such a way that their distribution and optical properties can be detected equally from multiple directions. Particularly preferably, the screen functions essentially like a Lambertian screen. In accordance with the invention, an "eyebox" is preferably a three-dimensional space defined by the diffracted radiation emitted by a hologram. At least one eyebox region or a cross-section of the entire eyebox is preferably reflected or scattered by the screen. This two-dimensional region is preferably detected by the eyebox detector.

[0049] For the purposes of the invention, an "eyebox detector" is preferably a detector for spatially resolved measurement of optical parameters on a screen. Preferably, the eyebox detector is a camera, in particular an RGB camera. However, alternative types of detectors can also be used, such as photometric sensors based on the spectral sensitivities of the CIE 1931 standard valence system, optical sensor arrays, IR cameras, and spectral cameras.

[0050] For the purposes of the invention, an eyebox superposition function GEB is preferably a mathematical function or algorithm that describes the resulting eyebox projection of a hologram caused by curvature when the hologram is brought into the curved state and would be reconstructed from a target illumination point. Preferably, the eyebox superposition function GEB is a mathematical function that allows the determination of a reconstruction parameter of the reflection hologram in the curved state by unfolding a distribution of the measured reconstruction parameters of the projected reflection hologram in the planar state. An eyebox superposition function can preferably be determined either by simulation or experimentally. An experimental determination of the eyebox superposition function GEB is explained below with reference to Figures 15-22B.

[0051] The determination method according to the invention is preferably carried out in the context of a manufacturing process of a hologram from a planar light-sensitive composite, wherein the hologram is provided for reconstruction in the curved state.

[0052] A "composite" within the meaning of the invention is preferably a multilayered material consisting of two or more different components with different physical properties, which are bonded together at an interface. Preferably, the bond between the individual components is such that it cannot be separated by a slight force and is therefore considered permanent. Preferably, the layers of the composite cannot be separated by a force of less than 1 N / cm, preferably less than 3 N / cm, preferably less than 5 N / cm, preferably less than 10 N / cm, or even more preferably less than 50 N / cm. The composite preferably comprises at least one carrier film and at least one photosensitive layer, which comprises a photosensitive material.A "composite web" within the meaning of the invention is preferably a composite material with a length that is at least twice, preferably at least five times, and more preferably at least twenty times its width. The thickness of the composite web is preferably adjusted to provide a certain degree of flexibility, so that it can, for example, be partially wound around a roller or laminated onto a surface. Preferably, the composite web has a thickness of up to 1000 pm, more preferably up to 500 pm, and particularly preferably up to 150 pm.

[0053] In the context of the invention, "light sensitivity" preferably refers to a material's suitability for holography. A material is preferably considered suitable for holography if, upon exposure to sufficiently coherent electromagnetic radiation (or "light"), the interference fields of the radiation can be stored as microstructures within the material. The suitability for holography is preferably related to the size of the resulting microstructures. Preferably, the resulting microstructures are no larger than the light / dark structures of an interference field.

[0054] The planar hologram analyzed by the determination method according to the invention is preferably present in a photosensitive composite web which has been subjected to exposure using electromagnetic rays and a master hologram.

[0055] For the purposes of the invention, "exposure" preferably means the targeted guidance of electromagnetic rays, preferably in the wavelength range between 200 nm and 3 pm, preferably between 300 and 1600 nm, preferably 400 - 780 nm, onto a photosensitive material, preferably for the formation of a hologram.

[0056] A "master hologram" within the meaning of the invention is preferably a holographic-optical element suitable for serving as the original from which at least one replicated hologram can be produced. The master hologram is designed for an optical function for one or more wavelengths. For this purpose, for example, several holograms, each diffracting light of a single wavelength, and / or multiplex holograms, diffracting light of multiple wavelengths, can be arranged as hologram stacks. The optical function of a master hologram can preferably be an imaging function or a scattering function. The radiation incident on the master hologram is diffracted, particularly at the intended wavelength or angle of incidence. The optical function inscribed in the master hologram (e.g., imaging, scattering, and / or deflection) can be reconstructed by this diffraction.

[0057] The master hologram can, for example, be a diffractive optical element (DOE). Diffractive optical elements (DOEs) utilize a surface relief profile with a microstructure for their optical function. Alternatively, the microstructure can also be present within the element's volume, e.g., in the form of a local difference in the refractive index. The light transmitted by a DOE can be transformed into almost any desired distribution by diffraction and subsequent propagation. This distribution can be an image, a logo, text, a refraction pattern, or similar. Preferably, the master hologram comprises a volume hologram.

[0058] It is particularly preferred that the master hologram is a reflection hologram. Furthermore, it is preferred that the master hologram is a volume hologram, particularly preferably a reflection volume hologram.

[0059] In a further aspect, the invention relates to a method for determining a reconstruction parameter of a reflection hologram in a curved state, wherein the determination method is based on measurements of the reflection hologram in a planar state. The method comprises the following steps: reconstruction of the reflection hologram in the planar state with a light source from a single illumination point; measurement of a distribution of a reconstruction parameter of the reflection hologram in the planar state along an axis of the reflection hologram in the field of view of a field-of-view detector; and determination of a reconstruction parameter of the reflection hologram in the curved state based on the measured reconstruction parameters of the reflection hologram in the planar state using a weighting function GL.

[0060] The invention makes it advantageously possible to draw conclusions about the field-of-view characteristics of the curved hologram by capturing only a single planar image of the hologram. For this purpose, a single illumination point is used to direct the electromagnetic beams onto the entire planar hologram, and a single image of the distribution of the optical parameters is captured using the field-of-view detector. Advantageously, the use of a screen is optional, but not required, so that the investigation method can be carried out in a space-saving and cost-effective manner. This also facilitates the integration of the investigation method into a more comprehensive manufacturing process for the curved hologram.

[0061] The invention advantageously avoids the need to illuminate the planar hologram from different angles by moving the illumination point along a path and capturing a multitude of images of the planar hologram from different eyebox positions to compensate for the deviation of the local angle of incidence of the individual illumination point from the target angle of incidence that would fall locally on the hologram if it were in a curved state. Figure 4 shows a schematic representation of a possible movement of the illumination point for such a measurement method. In this case, the field-of-view detector for capturing the images would also have to be moved along a path to intercept the rays emitted by the hologram (partial eyeboxes). Instead, the invention preferably considers the effect of the locally varying angle of incidence of the radiation from the individual illumination point on the local efficiency of the planar hologram.This relationship is preferably represented by the weighting function GL, which makes it possible to accurately determine the field-of-view properties of the hologram when it is in a curved state.

[0062] When using a single illumination point, the electromagnetic rays arrive at different positions within the field of view (FOV) on the hologram itself at a different angle of incidence than the angle from which the same positions of the hologram would be reconstructed if the hologram were in a curved state. The planar hologram diffracts the incident rays according to a diffraction pattern intended for a curved state. This results in the diffracted radiation being emitted by the hologram in diverging directions due to its planar shape. Consequently, instead of a single, distinct eyebox, several overlapping and fanned-out partial eyeboxes can be created. The eyebox detector according to the invention can thus detect the diffracted radiation from different partial eyeboxes extending from different FOV positions of the planar hologram.The efficiency (or observable intensity, visible color, or other measurable optical parameter) of the hologram when diffracted by electromagnetic rays arriving at a different angle is typically reduced, with the weighting function GL preferably taking this reduction in efficiency into account. Thus, the determination method allows the calculation of the value that an optical parameter such as efficiency would have if the relevant position of the hologram were illuminated from the correct angle.

[0063] Since the field of view (FOV) refers to positions on the hologram itself that can be observed or measured with the field-of-view detector, it is possible to determine what a user or observer might see when viewing the hologram if it were in its curved state and illuminated from a suitable point of light. This detection method can identify not only errors in the hologram but also the positions of those errors. This allows for an assessment of whether such errors are critical for the user.

[0064] As illustrated in Fig. 13, when measuring the hologram in the planar state, the measurement points are twisted or rotated in the feature space of the curved hologram, with the rotation or twisting depending on the hologram curvature.

[0065] As a result, from a single observation position during a measurement in the planar state (e.g., a middle observation position within a screen), a field of view of the hologram can be observed which corresponds to different eyebox positions in the curved state.

[0066] Advantageously, capturing a field of view for a fixed observation point in the planar state allows for statements about the field of view for different observation positions or eyebox positions in the application where the hologram exhibits curvature. This enables a particularly high degree of accuracy in assessing the quality of a hologram in its curved final state with a single measurement. While field of view measurement is possible for different observation positions in preferred forms, even with just one observation position, various eyebox positions for the curved state can be determined as examples.

[0067] The person skilled in the art also recognizes that preferred features or advantages of embodiments of the method in which a field-of-view detector and a weighting function GL are used to determine the hologram reconstruction parameter also apply to embodiments of the method in which an eyebox detector and an eyebox superposition function GEB are used to determine the hologram reconstruction parameter, and vice versa. It is also evident that a combination of the methods may be preferred. That is, a field-of-view detector and an eyebox detector can be used simultaneously to determine the hologram reconstruction parameter. It is particularly preferred that the field-of-view detector is positioned within or behind an opening in the screen, with the planar hologram being projected onto the screen. In this way, both methods can be combined in a compact manner. Furthermore, the screen does not need to be moved or...The eyebox detector must be removed to activate the field-of-view detector for capturing the properties of the planar hologram. Alternatively, both images—from the field-of-view detector and the eyebox detector—can be taken simultaneously.

[0068] For the purposes of the invention, a "field of view" is preferably a three-dimensional space within which optical events or changes can be observed or detected. Unless otherwise specified, a user or detector is preferably positioned relative to the hologram such that the hologram is wholly or partially within the user's or detector's field of view. The user or detector can thus observe various positions on the hologram itself, which may extend along the length of the hologram (e.g., from a left edge to a right edge) and along the width of the hologram (e.g., from a top edge to a bottom edge). Field-of-view positions are therefore preferably coordinates of the hologram itself. A two-dimensional area, preferably encompassing wholly or partially the length and width of the hologram, is detected by the field-of-view detector.

[0069] For the purposes of the invention, an “axis of the reflection hologram” is preferably a line or a line array in an arbitrary direction in the two-dimensional space of the hologram coordinates. For example, the axis runs along a length or a width of the reflection hologram. However, it is preferable that the measurement be carried out over a two-dimensional space.

[0070] For the purposes of the invention, a "field-of-view detector" is preferably a detector for the spatially resolved measurement of optical parameters on a hologram. Preferably, the field-of-view detector is a camera, in particular an RGB camera. However, alternative types of detectors can also be used, such as photometric sensors based on the spectral sensitivities of the CIE 1931 standard valence system, optical sensor arrays, IR cameras, and spectral cameras.

[0071] In the context of the invention, a weighting function GL is preferably a mathematical ratio, a conversion curve, a conversion matrix or an algorithm for calculating the distribution of an optical parameter along a field of views of a hologram when the hologram is brought into the curved state and reconstructed from a target illumination point.

[0072] Preferably, the weighting function takes into account a change in the efficiency of the hologram in the case of a reconstruction of a hologram intended for a curved application in the planar state, wherein the change in efficiency is preferably due to the deviation of the angle of incidence of the reconstruction rays from an illumination point onto the planar hologram from a target angle of incidence, as well as a possible edge falloff or limitation in the eyebox distribution due to the assignment shown in Fig. 13. In a preferred embodiment of the invention, the weighting function GL results from a division of the local efficiency of the hologram in the planar state H. p due to the local efficiency of the hologram in the curved state H gThe local efficiencies of the hologram in the planar and curved states can preferably be determined experimentally or by simulation. This is explained further below in light of Figures 8-10.

[0073] In a preferred embodiment of the invention, the determined reconstruction parameter of the reflection hologram in the curved state is compared with a target reconstruction parameter of the reflection hologram in the curved state. This is preferably done to assess the quality of the reflection hologram while it is still in the measured planar state. The comparison of the determined reconstruction parameter with the target reconstruction parameter can be incorporated into the adjustment of one or more manufacturing parameters upstream of a determination process during the production of one or more reflection holograms. For example, the exposure intensity, exposure angle, or exposure rate during the production of a hologram can be adjusted based on a quality assessment of a previous hologram.Thus, during the production of a series of reflection holograms, an immediate (inline) correction of the production parameters can be advantageously carried out if a drop in hologram quality is detected. Alternatively or additionally, the comparison result can be incorporated into a subsequent processing step or a disposal step downstream of the detection process. For example, a hologram can be marked as defective in light of an unacceptable comparison result with the target reconstruction parameter.

[0074] Preferably, the comparison result is compared with a tolerance range. If the comparison result falls outside the tolerance range, a manufacturing parameter is preferably adjusted, or a hologram is classified as defective or of reduced quality. In this way, a quality standard required for the application can be guaranteed with particular reliability. The comparison preferably considers not only the absolute value of a deviation of the determined reconstruction parameter from the target reconstruction parameter, but also the direction of the deviation. This makes it possible, in particular, to adjust the manufacturing parameters appropriately. The quality assessment can thus also be used directly as feedback for adjusting the manufacturing parameters of the reflection hologram.

[0075] In a further preferred embodiment of the invention, the measured distribution of the reconstruction parameter of the projected reflection hologram in the planar state along the screen represents a superposition of displaced eyeboxes (also referred to as "partial eyeboxes") from different emission points of the reflection hologram in the curved state. The eyebox superposition function GEB preferably describes a displacement of eyeboxes from emission points of the reflection hologram in the planar state compared to eyeboxes from corresponding emission points of the reflection hologram in the curved state. This is illustrated schematically in Figures 2 and 3, which are described in more detail below.

[0076] While the planar hologram emits a single general eyebox, this can be divided into various theoretical sub-eyeboxes that function as components of the generated general eyebox. This can be done by dividing the planar hologram, in particular the positions of the field of view on the hologram itself, into individual points or discrete sections.

[0077] Each of the identified points or sections can be considered a theoretical partial eyebox that contributes to the overall eyebox of the planar hologram. When a hologram with a curved shape is illuminated from a target illumination point, the reconstruction rays from the target illumination point strike each of the individual points or discretized sections of the curved hologram at a target angle of incidence.

[0078] Each of the individual points or discretized sections of the curved hologram bends the electromagnetic reconstruction beams to emit a theoretical partial eyebox with a uniform shape and a target beam angle. Ideally, by appropriately designing the curved hologram, all theoretical partial eyeboxes emitted from different points or sections of the curved hologram converge on a well-defined area of ​​the screen (see Fig. 2).

[0079] However, when the curved hologram is converted to a planar state, these theoretical partial eyeboxes no longer converge on the same defined area of ​​the screen. This is primarily due to the difference in the local angles of incidence of the planar hologram compared to the curved hologram, whereby the resulting tilt of the hologram surface shifts the direction of the diffracted electromagnetic rays.

[0080] Furthermore, the intended illumination point no longer converges. This is primarily due to the shifted angle of incidence of the electromagnetic rays arriving at the various positions or sections of the hologram from the illumination point, which in turn depends on the local increases in the curvature of the curved hologram at the different positions.

[0081] To find all positions along the planar hologram H p To directly capture and reconstruct the parameters that the hologram would exhibit in its curved state, a reconstruction beam with a target angle of incidence H should be used at each position of the hologram. p(i.e., every FOV position). This is not possible with a single illumination point. To ensure the desired angles of incidence at all positions of the planar hologram, an illumination source would have to be moved along a path (see Fig. 3). The planar hologram would diffract the incident rays in different directions. Diffracted rays are therefore emitted from different positions of the planar hologram. To detect the rays from the different positions of the planar hologram, the projection screen would have to be moved synchronously with the illumination source along another path, with multiple images of the projection screen being captured by the eyebox detector. This is very complex and would interrupt the production process of a hologram.

[0082] Instead, the planar hologram is preferably reconstructed from a single illumination point and projected onto a single screen. Beyond the existing eyebox shifts when path illumination is maintained (see Fig. 3), the emitted rays (discretized into "partial eyeboxes") are further dispersed from each other due to the deviations in the angles of incidence from the illumination point to the different positions of the hologram (see Fig. 5).

[0083] A central section of the planar hologram can still be illuminated at the target angle of incidence, so that a theoretical partial eyebox emitted from this area will still strike the screen in the same location and with the same shape. However, other positions or sections of the planar hologram will receive reconstruction rays that deviate to varying degrees from the target angle of incidence. This results in the theoretical partial eyeboxes emitted from these positions or sections having different angles and diffraction efficiencies, and striking the screen at different locations, so that the overall eyebox extends over a larger area of ​​the screen. The theoretical partial eyeboxes that make up this overall eyebox can also overlap, so that a central part of the overall eyebox appears brighter, while the edges gradually become darker (see Fig.3 - 4).

[0084] The eyebox superposition function GEB preferably serves to convert an image on the screen, which is generated by the planar hologram and captured by the eyebox detector, into an image that would be generated by reconstructing the curved hologram. Here, an image preferably refers to a two-dimensional intensity distribution, although depending on the type of optical parameter under investigation, other, preferably two-dimensional, distributions of optical parameters may also be meant.

[0085] The eyebox superposition function GEB preferably depends on a local curvature (mpov) of the reflected hologram. In this way, the deviation of the angles of incidence and the shift in the orientation of the FOV coordinates on the hologram, from which the theoretical sub-eyeboxes are emitted, are taken into account when correcting the image projected onto the screen.

[0086] In a further preferred embodiment of the invention, the eyebox superposition function GEB is determined experimentally, wherein the experimental determination of the eyebox superposition function GEB comprises the following steps: Reconstruction of the reflection hologram in the planar state for projection of the reflection hologram onto a screen with a light source from a single illumination point;

[0087] Measurement of the distribution of a reconstruction parameter of the projected reflection hologram in the planar state along the screen using an eyebox detector;

[0088] Reconstruction of the reflection hologram in the curved state for projection of the reflection hologram onto a screen with a light source from a target illumination point, which illuminates all emission points (or FOV positions) of the reflection hologram in the curved state with their target angles of incidence;

[0089] Measurement of the distribution of the reconstruction parameter of the projected reflection hologram in the curved state along the screen; and

[0090] Comparison of measurements of reflection holograms in the planar and curved state, in particular using a fitting algorithm.

[0091] In another preferred embodiment, the fitting algorithm for comparing the measurements of the reflection holograms in the planar and curved states comprises the following steps:

[0092] Optional dewarping of the measurements of the reflection holograms in the planar and curved state, in particular to compensate for an inclination of the eyebox detector to the screen; Fourier transformation of the preferably dewarped measurements of the reflection holograms in the planar and curved state, in particular to transfer the measurements into a frequency domain.

[0093] Division of the Fourier transform of the preferably dewarped measurements of the curved hologram by the Fourier transform of the preferably dewarped measurements of the planar hologram to obtain an eyebox superposition function GEB in the frequency domain.

[0094] To determine the parameter distribution of a curved hologram, measurements taken while the hologram is in the planar state are optionally dewarped and preferably subjected to a Fourier transformation before being multiplied by the eyebox superposition function GEB and the Fourier transformation is then reversed.

[0095] This method enables a simple and reliable determination of the eyebox overlay function GEB without requiring precise mapping of the local rises and / or angles of incidence of the light onto the hologram. The eyebox overlay function GEB can thus be determined quickly and easily without goniometric analysis of holograms. Furthermore, the eyebox overlay function GEB can be easily adjusted if changes are made to the desired curved final state of the hologram.

[0096] Determining the distribution of the reconstruction parameter of the curved hologram using the eyebox superposition function GEB, experimentally derived from the preferred fitting algorithm, is surprisingly precise and reliable. The reliability of such an eyebox superposition function GEB has been investigated with a variety of holograms. It has been found that such an eyebox superposition function can detect and locate deviations and errors that would be present in the eyebox of a hologram if it were reconstructed in a curved state. The same eyebox superposition function can also be applied to slightly different holograms, particularly if the holograms do not differ in the distribution of voids (from which no diffracted light is emitted). Some experimental results are presented in Figures 22A and 22B and discussed below.

[0097] For the purposes of the invention, "dewarping" preferably means the elimination of a distortion present in the spatially resolved data acquired by the eyebox detector due to an oblique orientation between the eyebox detector and the screen. The eyebox detector is preferably considered oblique if it is not centered and orthogonal to the center of the screen. Alternatively, the eyebox detector is preferably considered oblique if a detector plane, in particular a plane comprising a sensor array of the eyebox detector, is not parallel to a screen plane comprising the projection screen. With an obliquely positioned eyebox detector, an image of the screen is preferably acquired from a direction that forms an angle with a normal to the screen.The tilting of the eyebox detector may be necessary for space reasons and can prevent parts of the hologram from being optically blocked by positioning the eyebox detector between the projection screen and the hologram (see Fig. 5).

[0098] Such an oblique angle can, for example, cause a rectangular eyebox in the raw image captured by the eyebox detector to be represented by a trapezoid. By dewarping, the trapezoid, including all intervening matrix coordinates or spatially resolved measurements, is preferably transformed back into a rectangle. The dewarped image is therefore preferably an image of the screen that would have been captured if the eyebox detector had been positioned centrally and aligned orthogonally to the eyebox area on the screen. Image processing techniques are known to those skilled in the art for dewarping the spatially resolved data, in particular the image captured by the eyebox detector.

[0099] In a further preferred embodiment of the invention, the eyebox superposition function GEB is determined by simulation. The simulation-based determination of the eyebox superposition function GEB is preferably carried out by spatially determining a reconstruction parameter of the reflection hologram along the screen when the reflection hologram is projected onto the screen, both in a planar and a curved state. When determining the reconstruction parameter, a local spectral response of the reflection hologram to light from the respective illumination point is preferably taken into account.

[0100] Such a simulation-based determination of the GEB (Geographic Edge Beam) can be used to generate high-resolution images of the eyebox of a curved hologram. This allows for the precise quantification and localization of defects in the hologram while it is still in a planar state, thus ensuring a high quality standard in hologram production.

[0101] Preferably, the simulation-based determination of the eyebox superposition function GEB includes a comparison of the simulated reconstruction parameters from the planar and curved states. This comparison can be performed using an analogous fitting algorithm as in the experimental determination of the eyebox superposition function GEB.

[0102] In a further preferred embodiment of the invention, the determination of a reconstruction parameter of the reflection hologram in the curved state comprises unfolding a distribution of the measured reconstruction parameters of the projected reflection hologram in the planar state using the eyebox superposition function GEB. Such unfolding can take into account the fact that different sections of an eyebox projected onto the screen from a planar hologram originate from different sections of the hologram.

[0103] In particular, a central section of the eyebox preferably encompasses diffracted rays from all FOV positions of the hologram. In other words, all theoretical sub-eyeboxes of the planar hologram can partially overlap in the central section of the entire eyebox. A left and right section of the eyebox preferably encompass the diffracted rays from only a subset of the hologram's FOV positions. From these areas of the eyebox, an observer will thus perceive not the complete hologram, but only a portion of it. Outside these left and right sections, the eyebox ends, so that an optical parameter such as intensity has a value of zero. By dividing the entire eyebox into such sections, each section can be subjected to a different function to reverse-calculate the parameter distribution of the curved state.This is particularly possible when a deployment is used instead of a single function.

[0104] In a preferred embodiment of the invention, the unfolding comprises the following steps:

[0105] Optional unwarping of the captured parameter distribution of the projected hologram in planar state,

[0106] Fourier transform of the optionally dewarped captured parameter distribution of the projected hologram in planar state,

[0107] Multiplication of the Fourier-transformed parameter distribution of the projected hologram in planar state by an eyebox superposition function GEB, inverse of the Fourier transformation to determine the parameter distribution that an eyebox of the hologram would have if it were in a curved state.

[0108] In a further preferred embodiment of the invention, the weighting function GL is determined by comparing a distribution of the reconstruction parameter along at least one axis of the reflection hologram in the planar state with a distribution of the reconstruction parameter along at least the same axis of the reflection hologram in the curved state, preferably for a position of the field-of-view detector. Preferably, the distribution of the reconstruction parameter of the reflection hologram in the planar state is divided by the distribution of the reconstruction parameter of the reflection hologram in the curved state to determine the weighting function GL along the axis of the reflection hologram. The distribution of the reconstruction parameter in the planar and curved states, as well as the corresponding weighting function GL, can be determined by simulation or experiment.

[0109] The weighting function GL can thus be determined by a simple method. After determining the weighting function GL, the planar holograms of a series can be illuminated and detected by a field-of-view detector. The detected reconstruction parameters are then divided by the value of the weighting function GL at the respective FOV position to obtain the determined reconstruction parameters for the curved state. In this way, any deviations or errors in the hologram with regard to its curved state can be reliably detected and located in an end application. For the purposes of the invention, an "axis of the reflection hologram" is preferably any line on the reflection hologram, in particular a length or a width of the hologram.

[0110] In a further preferred embodiment of the invention, the distributions of the reconstruction parameter of the reflection hologram in the planar state and / or in the curved state are determined experimentally using at least one representative sample hologram.

[0111] In a further preferred embodiment of the invention, the distributions of the reconstruction parameter of the reflection hologram in the planar state and / or in the curved state are determined by simulation, preferably taking into account a spectral response of the hologram in the planar state and / or in the curved state to illumination from the respective illumination point during the simulation. The distributions of the reconstruction parameter of the reflection hologram in the respective state preferably include reconstruction parameters on the hologram itself that are distributed in the field of view of the observer or the detector and / or

[0112] Reconstruction parameters of the hologram's eyebox, which are distributed across the screen.

[0113] The representative sample holograms are preferably also referred to as "golden samples." These are preferably holograms of acceptable quality, so that they can be used as a benchmark for the quality of further holograms in the same series. The use of such sample holograms to determine the weighting function GL and / or the eyebox overlay function GEB can be carried out simply without complex calculations or simulation software.

[0114] Preferably, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten or more representative sample holograms are used to determine the weighting function GL and / or the eyebox overlay function GEB. In this way, highly representative and reliable functions can be determined.

[0115] In a further preferred embodiment, different eyebox superposition functions (GEB) are determined for different hologram motifs. This maintains high accuracy when converting the detected parameter distribution of the hologram's eyebox in the planar state to the determined parameter distribution of the hologram's eyebox in the curved state. Although the eyebox superposition function depends primarily on the curvature of the hologram and the relative positions of the eyebox detector, the planar hologram, and the projection screen, it can also vary depending on the content of the hologram pattern in the case of an experimental determination. In particular, the eyebox superposition function can be sensitive to the location of dark areas of the hologram where no radiation is diffracted.It is therefore particularly advantageous to use different eyebox overlay functions for analyzing various hologram patterns. A hologram motif refers specifically to an optical content or function that is to be generated by the reflection hologram.

[0116] Different hologram motifs are therefore preferably characterized by different optical content or functions and exhibit, for example, different color patterns, light or dark zones, which, as explained above, can have an influence on the determination of an eyebox overlay function GEB.

[0117] It is further preferred that a plurality of pattern holograms be used for the experimental determination of each eyebox superposition function. Preferably, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten or more pattern holograms are used to determine each eyebox superposition function.

[0118] In a further preferred embodiment of the invention, when determining the reconstruction parameter of the reflection hologram in the curved state, a plurality of measured reconstruction parameters of the reflection hologram in the planar state are divided by the weighting function GL for the position of the illumination point and preferably also for the position of the field-of-view detector to obtain the determined reconstruction parameters of the reflection hologram in the curved state. The plurality of the measured reconstruction parameters of the reflection hologram can be distributed along a line of the hologram, so that the weighting function GL can be represented as a curve that describes a varying weighting function value along an FOV line.It may also be preferred that the majority of the measured reconstruction parameters of the reflection hologram are distributed in two dimensions, so that the weighting function GL can be represented as a matrix, a multitude of curves, or a three-dimensional curve.

[0119] In a further preferred embodiment of the invention, the distribution of the reconstruction parameter of the reflection hologram in the planar state along one or more axes of the reflection hologram in the field of view of a field-of-view detector is measured from multiple eyebox positions. Measuring the reconstruction parameter of the reflection hologram from multiple eyebox positions preferably means that the field-of-view detector is positioned at different locations within the three-dimensional eyebox space of the hologram and directed at the hologram.

[0120] This can be used to evaluate the quality of the hologram when viewed from different angles. Such an evaluation is particularly important for holograms configured to create multiple separate eyeboxes or eyebox areas when reconstructed in a curved form. For example, some curved holograms are configured to direct diffracted light onto two separate or overlapping eyeboxes corresponding to a user's two eyes. This can be used to provide an immersive visual experience. In this case, it can be especially beneficial to place a field-of-view detector in each of the two separate or overlapping eyeboxes to determine the distribution of optical parameters that would be registered by each eye viewing the hologram in its curved form.Preferably, the eyebox positions in the different areas are chosen with such a distance that, even in a planar state, the respective field-of-view detectors only detect the distribution of optical parameters from eyebox areas which correspond to the separated areas in the curved state.

[0121] In a further preferred embodiment of the invention, the position of the illumination point is optimized to minimize an average local angular deviation of the incident light for the reconstruction of the reflection hologram in the planar state from a local target angle of incidence, wherein the local target angle of incidence preferably corresponds to the angle of incidence with which the reflection hologram is to be reconstructed in the curved state.

[0122] In a wavelength-selective hologram, incident reconstruction rays are only effectively diffracted when they fall within efficiency ranges that lie in one or more reconstruction wavelength ranges. Outside of these efficiency ranges, the hologram preferably does not respond substantially to the incident rays. A deviation of the angle of incidence of the incident light from the target angle can not only lead to a reduction in the maximum efficiency within the relevant reconstruction wavelength range, but also to a shift in the wavelength at which the maximum efficiency occurs within that range. This can result in a change in the color properties of the hologram, which may, for example, arise from a variation in the values ​​and wavelengths of the maximum efficiencies in the different color channels that comprise the holographic image.

[0123] An "efficiency" or "diffraction efficiency" within the meaning of the invention is preferably a ratio of the intensity of diffracted radiation to the intensity of incident radiation during hologram reconstruction. Preferably, the Bragg condition is taken into account for deriving the efficient diffraction case, and the magnitude of the diffraction efficiency is preferably determined using the Kogelnik approximation. The efficiency is preferably measured or determined with respect to a specific wavelength range. This can be achieved, for example, by using one or more monochromatic illumination sources for hologram reconstruction, which preferably emit radiation in discrete reconstruction wavelength ranges. An efficiency range is preferably a wavelength range in which a measurable diffraction efficiency exists at a given hologram position.This can be a wavelength range in which the hologram position has a diffraction efficiency of at least 0.1, preferably at least 0.2 or 0.5, when irradiated with reconstruction beams of the relevant reconstruction wavelength range.

[0124] A local angular deviation of the incident light leads to a reduction in the hologram's efficiency at the affected FOV position. This local angular deviation can result from using a single illumination point to reconstruct the hologram in its planar state (see Fig. 5). Minimizing the average local angular deviation preferably avoids a significant reduction in the efficiency of the reflected hologram during its reconstruction, ensuring that a sufficiently bright image of the reflected hologram is captured by the field-of-view detector or its eyebox by the eyebox detector. Furthermore, the color properties captured by the eyebox or field-of-view detector are closer to the color properties the hologram would exhibit in its curved state if reconstructed from a target illumination point.This improves the precision of the reconstruction parameters determined by the present method.

[0125] In another preferred embodiment of the invention, the optimization of the position of the illumination point comprises the following steps:

[0126] Determination of the local target angles of incidence along at least one axis of the reflection hologram in the planar state in a plane of curvature in which the reflection hologram is curved to transition into the curved state; calculation of the actual angles of incidence along the at least one axis of the reflection hologram in the planar state in the plane of curvature from one or more exposure points, preferably calculating the average deviation of the actual angles of incidence from the target angles of incidence for each exposure point; selection of an optimized illumination point which provides the smallest average deviation from the target angle of incidence along the at least one axis of the reflection hologram in the planar state in the plane of curvature.

[0127] Preferably, when selecting the optimized illumination point, the average deviation from the target angle of incidence in at least two mutually orthogonal planes encompassing the plane of curvature is calculated and minimized. The relative dimensions of the hologram in the two orthogonal planes are preferably taken into account. Using a single illumination point simplifies the determination procedure mathematically and reduces the influence of multiple overlapping angles of incidence on the hologram. This also allows the use of a more compact device for performing the measurements in the determination procedure, saving space and eliminating the need to move or scan the illumination point. As shown in Fig. 2,As explained in section 5 below, for example, reconstructing the planar hologram so that the reconstruction rays arrive at all FOV positions at a target angle of incidence would require the use of a moving illumination point along an illumination path. The field-of-view detector and the screen used to acquire the eyebox data would also have to be moved synchronously with the illumination point within a tight tolerance. This is extremely resource-intensive, both in terms of the equipment used and the time required to acquire the measurements and the necessary mathematical processing.

[0128] Instead, the use of a single optimized illumination point offers a surprisingly good compromise between ease of measurement and sufficient hologram efficiency. The determination procedure can be carried out quickly as part of a continuous process. At the same time, the illumination point ensures that the hologram diffracts light sufficiently to provide measurable values ​​for the reconstruction parameters. The fact that the measurement process can be kept so simple by using a single optimized illumination point, while simultaneously providing usable data for a reliable determination of the optical properties of a curved hologram, was unexpected.

[0129] Optimizing the position of the illumination point with respect to the angle of incidence in the plane of curvature is particularly advantageous because the planar hologram tends to exhibit larger angular deviations along the direction in which it is curved. To illustrate this, consider the case of a target angle of incidence of 90° in the plane of curvature. The angle of incidence preferably refers to the angle in the plane of curvature between a tangent at the point of incidence on the hologram and the incident beam. A concavely curved hologram with a constant radius can, for example, have a target illumination point at the midpoint of the constant radius. The target illumination point is preferably a point from which reconstruction beams emerge at all hologram positions with a target angle of incidence. The target angle of incidence preferably corresponds to the local angle at which a reference beam illuminated the hologram.The hologram is therefore preferably designed for the diffraction of incident radiation with the target angle of incidence (or angle of incidence of the reference beam) and exhibits the highest diffraction efficiency at this angle of incidence.

[0130] In the present example of a concavely curved hologram with a constant radius of curvature and a target illumination point at the center of the radius, this would result in a target angle of incidence of approximately 90° along the axis of the hologram, which runs longitudinally from the right to the left edge of the hologram along the direction of curvature (see Fig. 5 / 6). If the same hologram is rendered planar, light with an angle of incidence of 90° in the plane of curvature can only arrive at the center of the hologram's longitudinal axis, while the left and right edges of the hologram receive light with an acute angle of incidence (< 90°).

[0131] To increase the acute angle of incidence of the light on the left and right edges of the planar hologram, the illumination point can be moved further away from the hologram (see Fig. 6). This brings the angles of incidence along the length of the planar hologram closer to 90°. However, a side effect of increasing the distance between the illumination point and the planar hologram is that the angle of incidence of the light at the top and bottom edges of the planar hologram also increases, thus increasing the deviation of the actual angle of incidence from the desired angle of incidence in these directions. To counteract this, the illumination point can be moved somewhat closer to the planar hologram, proportionally to the relative dimensions of the hologram (see Fig. 7). The optimized illumination point preferably results in the smallest angular deviation in both the length and width of the hologram.

[0132] In a further preferred embodiment of the invention, the weighting function GL is varied along an axis of the reflection hologram and a position of the illumination point is optimized such that the weighting function GL has a minimum size at a start and end point along the axis, wherein preferably during the optimization a central area of ​​the weighting function GL along the axis is assigned a lower weight than outer edge areas to the start and end point of the axis.

[0133] According to the invention, it was recognized that the deviations in the design behavior between a planar and curved state occur most strongly in the edge regions of the hologram.

[0134] By ensuring that the weighting function GL has a minimum value at the edges of the hologram, for example, at least 0.1, 0.2, 0.3, 0.4 or more, a reliable evaluation of the hologram's reconstruction quality can advantageously be guaranteed even at the edges. In central areas of the hologram, deviations in the construction behavior between a planar and curved state are smaller, and, moreover, relatively high values ​​for the weighting function will already be present in this area. Optimizing the illumination point L optTherefore, a central area should be assigned less weight than a peripheral area. By ensuring a minimum value for the weighting function at the edges, these areas can also be analyzed with high precision during production or while the hologram is still in its planar state. Optimizing the weighting function to ensure a minimum size or value in the peripheral area can be performed alternatively or additionally to optimizing for the smallest possible average deviation from the target angle of incidence along at least one axis of the reflection hologram in the planar state within the plane of curvature.For example, it may be preferable to first optimize an illumination point using the methods described above with regard to reducing the average deviation from the target angle of incidence in a plane of curvature and an orthogonal plane to it, and then, starting from the optimized illumination point, to further optimize it with regard to a minimum size of the weighting function in the boundary region.

[0135] Since the efficiency of the hologram often approaches zero at the edges of the hologram, the weighting function GL is preferably adjusted so that the weighting function GL yields a sufficient value at the edges of the hologram to provide an accurate estimate of the efficiency that the hologram edges would have if they were illuminated with the target angle of incidence, i.e. from the target illumination point in the curved state.

[0136] In a further preferred embodiment of the invention, the reconstruction parameter is an intensity or wavelength of the projected reflection hologram along the eyebox area or along the reflection hologram in the planar state within the field of view of the field-of-view detector. Preferably, the reconstruction parameter can also be a parameter derived from one or both of these values. For example, efficiency can be derived from intensity. Such reconstruction parameters are particularly important for determining the visibility and potentially the readability of a hologram configured to diffract visible light and / or transmit information to a user. The preferred reconstruction parameters can also be of great importance for the suitability of functional holograms in the context of their end application.For example, the efficiency of a hologram configured as a compact concave mirror can be crucial for its suitability in a technical application.

[0137] In a further preferred embodiment of the invention, the determined reconstruction parameter of the reflection hologram in the curved state is compared with a target reconstruction parameter of the reflection hologram in the curved state to evaluate the following properties: luminance, luminance distribution, color, color distribution, resolution, holographic interference reflections, granularity, homogeneity, and / or image distance. In a further aspect, the invention relates to a manufacturing process for a reflection hologram intended for use in a curved state, wherein the reflection hologram is exposed onto a photosensitive composite film and is in a planar state after exposure.The manufacturing process includes an evaluation of the quality that the reflection hologram would exhibit in the curved state, wherein the evaluation of the quality is based on a measurement of the provision of a reconstruction parameter of the reflection hologram in the planar state.

[0138] Preferably, a quality assessment method such as the one described here is used. This advantageously allows for the rapid verification of the acceptable quality of the planar hologram before complex shaping steps are carried out to convert the planar hologram into its curved state. Furthermore, the quality of the hologram can be determined, preferably with regard to predefined tolerance thresholds, before the hologram is incorporated into a larger device, such as a camera or a windshield. In this way, it can be avoided that the curved hologram is defective due to insufficient quality and therefore has to be rejected.

[0139] Since quality assessment can be performed during the manufacturing process, an immediate (inline) correction of the manufacturing parameters can be made if a drop in hologram quality is detected. For example, the exposure intensity or the exposure angle can be adjusted immediately. The master hologram used can also be examined and identified as the cause of the problem, allowing it to be replaced as quickly as possible. In this way, high-quality replicated holograms for the intended application can be ensured with simple means and high throughput.

[0140] Preferably, the manufacturing process is continuous. This preferably means that different process steps can be carried out in parallel at different process stations, particularly on a light-sensitive composite web, which can be provided, for example, in roll form. In this way, a high throughput or high process speed can be achieved.

[0141] In a preferred embodiment of the manufacturing process, a composite web is guided and processed through various stations of a replication device. These preferably include an exposure station and a fixing station.

[0142] In a preferred embodiment of the manufacturing process, a photosensitive composite layer is brought into contact with a master hologram in an exposure station. Preferably, electromagnetic beams are directed onto the master hologram to expose the photosensitive composite layer. A reference beam is preferably directed at the master hologram at a predetermined angle of incidence such that it passes through the photosensitive composite layer before reaching the master hologram. The master hologram preferably diffractes the reference beam to form an object beam that passes through the photosensitive composite layer and interferes with the reference beam in the photosensitive material. This process preferably forms a diffraction pattern within the photosensitive material.

[0143] In a preferred embodiment of the manufacturing process, the master hologram is mounted in or on a master element.

[0144] The master element is preferably planar, particularly in the form of a plate or cuboid. Exposure would then preferably be carried out with varying local angles of incidence that match the desired angles of incidence of the curved hologram. This is particularly suitable for an intermittent replication process in which successive sections of the photosensitive composite are applied to a planar surface of the master element and removed after exposure.

[0145] Alternatively, the master element can be prismatic or cylindrical and mounted for axial rotation. The photosensitive composite web is preferably guided along the lateral surface of the axially rotatable master element, so that a section of the photosensitive composite web temporarily assumes the shape of a region of the master element's lateral surface. Exposure beams are preferably directed onto the section of the photosensitive composite web that is in contact with the master element in order to replicate the master hologram into the photosensitive composite web. Advantageously, such a manufacturing process can be carried out continuously at high throughput rates.

[0146] A "master element" is preferably a three-dimensional unit comprising a master hologram in a form that ensures that a movement of the master element directly results in a corresponding movement of the master hologram. When the "master element" is described as "axially rotatable," this preferably means that the master element is mounted rotatably along an axis within the exposure station. An axially rotatable mounting thus characterizes a mounting that allows the master element to rotate about its axis. Preferably, the axis is located in the center of a cross-section of the master element, allowing for space-saving rotation of the master element. The rotatable master element is preferably prismatic, i.e., it has a constant cross-section of any shape, e.g., square, polygonal, elliptical, or circular.The ends of the master element, which have the shape of the cross-section, can be referred to as the "base surface". The elongated surface of the master element, which lies between the two ends, can be referred to as the "shell surface".

[0147] In another preferred embodiment of the manufacturing process, the photosensitive composite web is fixed after exposure. This fixing improves the repeatability of the replication process. Since an exposed photosensitive material could still be susceptible to mechanical or optical disturbances, fixing prevents potential mechanical or electromagnetic distortions.

[0148] The term "fixation" preferably refers to a process step for curing a liquid material, in particular a liquid photopolymer, wherein electromagnetic and / or thermal energy is preferably applied to the material. Preferably, the energy can be applied uniformly to a surface of the sensitive material to ensure simultaneous curing. Preferably, all layers of the photopolymer composite, in particular the photopolymer layer, are solidified in this stage.

[0149] In a further preferred embodiment of the invention, the quality assessment takes place after the hologram has been fixed but before it has been transformed into its curved shape. This has the advantage that disturbances to the diffraction pattern in the photosensitive material caused by the illumination of the hologram for its reconstruction are avoided. Furthermore, the exposure station and / or fixing station can remain compact and lightproof. Instead, the devices for performing the quality assessment can be housed in a separate quality assessment station. In particular, the photosensitive composite web can run through the quality assessment station in a planar form, for example, between two transport rollers or on a conveyor belt. While the photosensitive composite web moves through the quality assessment station, the data can be acquired by the eyebox detector or the field-of-view detector.Alternatively, the light-sensitive composite track can be briefly stopped while the data is being captured. This improves the quality of the captured data and allows the hologram to be immediately flagged as faulty if necessary.

[0150] A person skilled in the art will recognize that preferred features and advantages of the determination method according to the invention also apply to the manufacturing process, and vice versa. Different embodiments of the determination method and the manufacturing process can, of course, be combined. Terms such as "essentially," "approximately," "about," "about," "nearly," etc., preferably describe a tolerance range of less than ± 20%, preferably less than ± 10%, particularly preferably less than ± 5%, and especially less than ± 1%, and include the exact value.

[0151] Detailed description

[0152] The invention will be explained in more detail below using examples and illustrations, without being limited to these.

[0153] Brief description of the illustrations

[0154] Fig. 1 is a schematic representation to illustrate a field-of-view and an eyebox based on any hologram.

[0155] Fig. 2 is a schematic representation of the reconstruction of a curved hologram from a target illumination point.

[0156] Fig. 3 is a schematic representation of the reconstruction of the hologram of Fig. 2 while it is (still) in a planar state, maintaining the angles of incidence for the curved reconstruction to each position on the planar hologram, requiring a corresponding number of different illumination points.

[0157] Fig. 4 illustrates a possible measurement method for a hologram while it is in a planar state with regard to a curved state in the application case, whereby a multitude of light direction directions or illumination points as well as observation positions or eyebox positions are traversed.

[0158] Fig. 5 is a schematic representation of the different emission behavior of the hologram in the curved and planar states when the hologram is projected onto a screen from a single illumination point.

[0159] Fig. 6 is a schematic representation of the optimization of the illumination point in a plane of curvature of the curved hologram.

[0160] Fig. 7 is a schematic representation of the optimization of the illumination point for the curved hologram from Fig. 6 in another plane, which is oriented orthogonally to the plane of curvature of the curved hologram.

[0161] Fig. 8 is a representation of the diffraction efficiency of a hologram at different wavelengths during its reconstruction in the curved state.

[0162] Fig. 9 shows the diffraction efficiency of a hologram at different wavelengths during its planar reconstruction. Fig. 10 is a graphical representation of a weighting function GL.

[0163] Fig. 11 is a schematic representation of measurement points of a hologram in the curved state in the feature space (eyebox vs. field of view) for the hologram in the curved state.

[0164] Fig. 12 is a schematic representation of the reconstruction of the hologram in the planar state from an optimized illumination point and the projection of the reconstructed holographic emission onto a screen.

[0165] Fig. 13 is a schematic representation of measurement points of a hologram in the planar state in the feature space (eyebox vs. field of view) of the hologram for the curved state.

[0166] Fig. 14 is a graphical representation of the eyebox overlay function GEB.

[0167] Fig. 15 is a schematic illustration of the calibration of a de-warping of an eyebox image from an obliquely oriented eyebox detector DEB.

[0168] Figures 16A and 16B show eyebox images of a hologram in the curved state H. g and in the planar state H p .

[0169] Figs. 17A and 17B show one dewarp step of the eyebox images from Figs. 16A and 16B.

[0170] Figs. 18A and 18B show the dewarped eyebox images from Figs. 16A and 16B.

[0171] Figs. 19A and 19B show Fourier transformations of the dewarped eyebox images from Figs. 18A and 18B.

[0172] Fig. 20 shows an experimentally determined eyebox superposition function GEB, which results from dividing the function of Fig. 19A by the function of Fig. 19B.

[0173] Fig. 21A shows a back-calculation of the distribution of optical parameters of a curved hologram based on an eyebox image of the hologram in the planar state and the eyebox superposition function GEB.

[0174] Fig. 21 B shows an approximation deviation between the determined distribution of the optical parameters of the curved hologram according to Fig. 21A and the recorded dewarped distribution of the optical parameters of the curved hologram according to Fig. 18A.

[0175] Figures 22A and 22B show eyebox images and back-calculations of the distribution of optical parameters of a plurality of different holograms using an experimentally determined eyebox superposition function GEB. Detailed description of the figures

[0176] Figures 1-4 illustrate background concepts that contribute to understanding the invention. Figure 5 illustrates an exemplary setup for measuring the optical parameters of a planar hydrogram. p and compares this with the case in which the Hclcgram is curved H g Figures 6-10 refer to the optimization of the lighting point L. O pt and the determination of vcn optical parameters along the Field View (FOV) of a hologram H. Figures 11-22B refer to the determination of optical parameters along an eyebox of the hologram H.

[0177] Figure 1 schematically shows the reconstruction of an arbitrary reflection hologram H. The hologram H diffractes incident light rays to perform an optical function. The diffracted light rays are emitted by the hologram H. These diffracted light rays define a three-dimensional area called the eyebox EB. The eyebox EB thus preferably denotes optical features that appear on the side of the hologram H in the direction of an observer.

[0178] On the other hand, the field of view (FOV) encompasses the optical features that can be perceived by an observer within a holographic image. The figure shows a field-of-view detector (DFOV) in the form of a camera pointed at the hologram H. The field-of-view detector (DFOV) acts as the observer. Specifically, the field of view (FOV) includes the portion of the hologram H visible to the field-of-view detector (DFOV). The field-of-view detector (DFOV) can observe various positions on the hologram, so field-of-view positions preferably correspond to coordinates of the hologram itself.

[0179] The parameters of the field-of-view (FOV) and the eyebox (EB) are interconnected and form a feature space. The invention preferably utilizes the fact that inferences about the entire feature space for a specific hologram H can be drawn from the data of a limited part of this feature space.

[0180] Figure 2 schematically shows the reconstruction of a curved hologram H g from a target illumination point L. The curved hologram H g It is a reflection hologram. Light rays from the illumination point L strike all points of the curved hologram H. g at a target angle of incidence, for which the curved hologram H g The incoming light rays are diffracted at the various positions of the curved hologram in such a way that they form a holographic image. The diffracted rays from all positions of the curved hologram Hg form a single eyebox EB g The holographic image can be viewed by placing an observer (or a detector) in this eyebox EB. g in the direction of the hologram H g The same is observed. For purely illustrative purposes, the diffracted light rays are observed from three positions along a longitudinal axis of the curved hologram H. g These correspond to a left edge, a center, and a right edge of the hologram H. g From each of these three positions, the diffracted light rays (or partial eyeboxes) extend over the entire length of the eyebox EB. g Ideally, an eyebox EB is created. g with a clearly defined shape, which can be rectangular, for example.

[0181] Since the eyebox EB is in every position g diffracted rays from all positions of the curved hologram H g upon arrival, the entire curved hologram H gfrom all positions along the eyebox EB g Observable. In other words, the field of view (FOV) from any position within the eyebox (EB). g includes the complete hologram H g If the observer or detector is positioned outside the right or left edge of the eyebox EB g , so the curved hologram H g no longer visible. The illumination of a curved hologram H g Ideally, a single, optimally positioned lighting point L creates an eyebox EB. g , the bent rays from the entire curved hologram H g includes the Eyebox EB. g is preferably uniformly bright.

[0182] Figure 3 shows the case where the curved hologram H gfrom Figure 2 in the planar state. This can preferably be the state of the hologram during its production, before the planar hologram transitions into the curved final shape. The planar hologram is represented by H p designated. A simple reconstruction of the planar hologram H p From a single illumination point L, the holographic image of the curved hologram H can be generated. g They cannot be reproduced. This is partly because the light rays from the single illumination point L are directed to different positions on the planar hologram H. p The images would be incident at different angles, most of which deviate significantly from the desired angle of incidence. Reconstruction from a significantly deviating angle of incidence is only possible with limited diffraction efficiency of the hologram H. p take place.

[0183] To find all positions along the planar hologram H pTo reconstruct with high efficiency, a reconstruction beam with a target angle of incidence at each position of the hologram H should be used. p (i.e., each FOV position). This can theoretically be achieved by guiding a light source along a path that includes the target illumination points Li - LM and from the target illumination points Li - LM for each position along the planar hologram H p This ensures a target angle of incidence. Using such a path for the light source, it would therefore be fundamentally possible to measure or evaluate the reconstruction efficiency of the hologram for the curved final state even in the planar state of the hologram.

[0184] An analysis of the optical parameters that the hologram would exhibit if it were in the curved state H gHowever, this is further complicated by the fact that the various reconstruction beams, which are directed from the different illumination points Li - LM onto the plane hologram H p fall, not creating a single, sharply defined eyebox that encompasses the diffracted rays from all positions along the hologram.

[0185] Instead, such a measurement method would determine different positions of the hologram in the planar state H p Generate rays with different diffraction angle ranges. This leads to a fanning out of the individual theoretical partial eyeboxes EB of the curved hologram H. g into various overlapping partial eyeboxes, which are positioned at different points on the planar hologram H poriginate. In other words, the series of illumination points Li - LM would lead to the formation of a corresponding series of partial eyeboxes EBi - EBM. This is shown in discretized form in Fig. 3. A complete analysis of the optical (reconstruction) properties that define the planar hologram H p would have been if it were in a curved state H g If this were to be done, the analysis of all or most of these partial eyeboxes would require EBi - EBM as well as the separation of the overlapping partial eyeboxes from each other.

[0186] For example, a projection screen with a varying angle could be moved synchronously with the light source to reflect the partial eyeboxes EBi-EBM. A detector could capture the projected eyeboxes EBi-EBM on the screen at a variety of positions. However, the overlap between the partial eyeboxes complicates the analysis of the captured images. Furthermore, such a capture method is time-consuming and requires a high degree of synchronization with tight tolerances.

[0187] Figure 4 shows in further detail a possible, hypothetical investigation method according to Figure 3. Three illumination points are identified and described in more detail as examples. One illumination point Li is located on a left edge of the planar hologram H. p directed, an illumination point LM is aimed at a right edge of the planar hologram H pdirected and an illumination point LM / 2 is on a center of the planar hologram H p Directed, each with a target angle of incidence and optical path length. The corresponding reconstruction rays are designated kj,i, kj,M / 2 and kj,M. For example, the reconstruction ray k,,i is directed from the left edge of the planar hologram H p Diffraction occurs within an angular range that begins at a first edge EBi,i and ends at a second edge EBI,N of the first partial eyebox EBi. However, the first partial eyebox comprises several diffracted rays kd,i,i to kd,i,N distributed between EBi,i and EBI,N. A complete analysis of the optical parameters, and thus the quality, of the left edge of the planar hologram H pIdeally, this would require an analysis of all eyebox positions between and including EBi,i and EBI,N. This would preferably be repeated for all fanned-out and superimposed sub-eyeboxes from EBi to EBN of the planar hologram. Thus, a multitude of combinations of a matrix of dimension MxN of illumination points (Li to L) would have to be analyzed. m ) and observation positions in the corresponding sub-eyeboxes (EBi to EBN). This results in a long measurement time during the scanning process and a high level of effort required to provide the measurement components, including the corresponding actuators, evaluation, and data volume.

[0188] Integrating such a measurement method into a replication process, especially a continuous roll-to-roll replication process, is difficult to implement in practice.

[0189] Against this background, it can be seen that the possibility of using only one measurement for the eyebox and / or the field of view in the inventive method offers significant advantages in terms of the efficiency of the method and its integration into a manufacturing process.

[0190] Fig. 5 schematically shows a setup for carrying out the investigation method according to a preferred embodiment of the invention. A planar hologram H p It is produced using a replication process, e.g. between transport rollers or on a conveyor belt, which carries the planar hologram H p transported from an exposure station. Although the hologram was in a planar state during and after exposure, the exposure process and the master hologram used were set to achieve a desired optical function for the hologram H. gto ensure the desired optical function is only directly observable when the hologram H p into the curved state H g is brought.

[0191] The dashed lines show the hologram in a curved state H g and the way the hologram H g The hologram would react to light from a target illumination point Li in its curved state to form a single eyebox. However, such an observation would only be possible in the specific application. Alternatively, the hologram could be temporarily transformed from the planar state into the curved state. This process, however, is both complex and can lead to additional distortions.

[0192] The determination method according to the invention can advantageously perform measurements in the planar state of the hologram H instead. p perform. A screen 12 is placed in front of the hologram H poriented so that a reconstruction of the hologram H p The projected hologram is shown on screen 12. An eyebox detector DEB in the form of a camera is directed at screen 12 to capture at least one section of the projected hologram. The captured section preferably comprises an eyebox area 10, which is used for the detection method. A field-of-view detector DFOV is placed on the screen, preferably in or behind an opening in screen 12, within the eyebox area 10 and directed towards the planar hologram H. p oriented. In this embodiment, the field-of-view detector DFOV is a camera which is used to detect a multitude of positions on the hologram H. p is designed. Furthermore, a light source is provided at the illumination point Li to illuminate the planar hologram H. p to reconstruct.

[0193] As can be seen from Fig. 5, the image projected onto the screen 12 differs when the hologram is in the planar state H p is located, clearly from the image that is observed when the hologram is in the curved state H g The determination method according to the invention preferably comprises a step to reduce the effect of the position of the individual illumination point L on this difference, as explained in more detail below.

[0194] Fig. 6 schematically shows an optimization of the position of the illumination point L in a plane of curvature xy of the hologram H, corresponding to a top view along the axis of curvature, while Fig. 7 shows an optimization of the illumination point L in a plane yz orthogonal to it of the hologram H, corresponding to a side view along the axis of curvature z. Preferably, in the application, the hologram will exhibit curvature in the plane of curvature xy, but not in the orthogonal plane yz. In other words, the hologram can preferably have a curvature analogous to a (partial) lateral surface of a cylinder, where the z-direction corresponds to the cylinder axis.

[0195] The influence of the angle of incidence of the radiation is taken into account from both planes in order to determine an optimal illumination point L. opto determine t in three dimensions. When optimizing the illumination point L, the deviations of the incidence angles of the reconstruction rays at different positions of the planar hologram H are taken into account. p from the angles of incidence of the reconstruction rays at the corresponding positions of the curved hologram H g minimized. The effect of the angle of incidence deviation due to the use of a single illumination point is thus minimized. When optimizing in the curvature plane xy (along, for example, the length of the hologram), the illumination point is separated from the planar hologram H. p preferably further away in order to optimize a target angle of incidence for the left and right edges of the hologram.

[0196] This distance of the illumination point from the planar hologram H p However, it has an effect on the angles of incidence at other positions of the planar hologram H pin the orthogonal plane zy (along e.g. a width of the hologram) in which the hologram will not be curved even in the application case.

[0197] In this process, by optimizing the illumination angle in the curvature plane xy by moving the illumination point, a deviation of the angles of incidence from the target angles of incidence is created, especially at the upper and lower edges of the hologram.

[0198] To minimize this deviation, the optimal lighting point L op t somewhat closer to the planar hologram H p shifted, taking into account the proportions between the dimensions of the hologram and its degree of curvature. The selected optimal illumination point L op t is therefore further from the planar hologram H premoved, but not as far as would be the case if the optimization in the orthogonal plane zy were disregarded. The determination method is preferably carried out from an optimized illumination point L. op t carried out, as will be explained in more detail with reference to Fig. 12.

[0199] Figures 8 and 9 show the influence of the illumination point on the diffraction efficiency of the hologram in the curved state H g and in the planar state H pat different wavelengths. Figure 8 shows the spectral response of the curved hologram to reconstruction beams at three wavelengths and at one position on the hologram. The dashed line shows the intensity distribution of the illumination source in the visible electromagnetic spectrum (“emission projector”). The three peaks correspond to the RGB wavelengths, for example, of an RGB laser or an RGB LED. The solid line shows the intensity distribution of the light emitted by the curved hologram H. g diffracted radiation in the visible electromagnetic spectrum ("diffracted emission"). A dotted line shows a distribution of the local efficiency of the curved hologram H. gin the visible spectrum ("curved efficiency"). This is the ratio of diffracted radiation (solid line) to incident radiation (dashed line). This distribution exhibits three narrower peaks, indicating wavelength selectivity of the hologram in narrow wavelength ranges.

[0200] Figure 9 shows the spectral response of the planar hologram to reconstruction beams at three wavelengths and at one position on the hologram. The dashed line shows the intensity distribution of the illumination source in the visible electromagnetic spectrum (“emission projector”). This is identical to the dashed line in Figure 8. The solid line shows the intensity distribution of the radiation emitted by the planar hologram H. pDiffracted radiation in the visible electromagnetic spectrum ("diffracted emission"). The three peaks are clearly lower than those for the curved hologram, indicating reduced diffraction efficiency. A dotted line shows the distribution of the local efficiency of the planar hologram H. p in the visible spectrum ("curved efficiency"). This is the ratio of diffracted radiation (solid line) to incident radiation (dashed line). The local efficiency results in a more pronounced shift in the efficiency peaks of the planar hologram H. pThis is noticeable in comparison to the efficiency peaks of the curved hologram. This shift is influenced by the actual angle of incidence of the reconstruction rays in the planar state, which deviates from the desired angle of incidence. Therefore, the planar hologram diffracts different wavelengths of light most effectively compared to the hologram in the curved state.

[0201] The determination method according to the invention can preferably take this shift in the efficiency peaks into account by providing a weighting function GL for the illumination point.

[0202] In a preferred embodiment of the invention, the weighting function GL results from the division of the efficiency of the hologram in the planar state H. p due to the efficiency of the hologram in the curved state H gSince this ratio varies across the FOV positions of the hologram with the deviation of the angle of incidence, it can be calculated experimentally or by simulation for different coordinate points of the hologram. For example, the weighting function GL is preferably calculated as a curve along an axis of the hologram. Such an axis can run from the left edge to the right edge of the hologram, preferably along the direction of a curvature (in relation to the examples above, for instance, approximately in the direction of the x-axis). Such a weighting function GL is shown in Fig. 10.

[0203] As can be seen in Fig. 10, the analyzed hologram extends over a shorter length compared to the positions for which a local hologram efficiency was plotted (-2.0 to 2.0). This profile can theoretically be calculated from the continuation of the hologram surface, but in practical measurements, it is limited to the actually structured area of ​​the hologram. Instead, the usable efficient sub-region appears in the interval [-1, 1]. These coordinates are preferably maintained for both the planar and curved cases, e.g., by considering a warping or de-warping function when assigning coordinates to the hologram (field of view, FOV). The dashed line preferably represents a local efficiency of the hologram in the curved state H. g dar (“curved”), while the solid line represents the local efficiency of the hologram in the planar state H p represents (“plan”).

[0204] As can be seen in the figure, both holograms in the curved and planar states exhibit similar efficiency at the center of the hologram (at a FOV position of approximately 0.0). The efficiency of the planar hologram H p The value decreases more sharply with increasing distance from the center of the hologram. This is due to the fact that the edges of the planar hologram H p a larger deviation of the actual angle of incidence of the light from the individual illumination point L op are exposed to the target angle of incidence.

[0205] The weighting function GL will therefore typically have a smaller (preferably non-zero) value at the edges of the hologram to account for the greater deviation in the angle of incidence and the hologram's efficiency in these areas, assuming the hologram is planar. Since the hologram's efficiency H pSince the efficiency approaches zero at the edges of the hologram, it can be advantageous to extrapolate the curve of the weighting function GL slightly beyond the hologram's edges. Additionally or alternatively, it can be advantageous to ensure that low efficiency values ​​at the hologram's edges are not reduced to zero, so that dividing the efficiency of the planar hologram by the efficiency of the curved hologram at the edge always yields a sufficient GL value. Optimization of the weighting function GL can preferably also be performed with respect to the outer edges of the field of view (FOV), since deviations in the design behavior between a planar and curved state are most pronounced there. Mathematical techniques are known to those skilled in the art for appropriately adjusting the best-fitting curve.

[0206] Preferably, the weighting function GL varies along the FOV coordinates of the hologram. Preferably, the position of the illumination point is also optimized such that the weighting function GL has a minimum value at a start and end point along the axis (e.g., from the left to the right edge of the hologram), wherein, preferably, during optimization, a central region of the weighting function GL along the axis is assigned a lower weight than outer edge regions at the start and end points of the axis.

[0207] This preferably ensures that the weighting function GL has a minimum value at the edges of the hologram, thus guaranteeing an evaluation of the hologram's reconstruction quality, particularly at the edges. Firstly, the weighting function will usually already have high values ​​in the center of the hologram, making weighting less significant when optimizing the illumination point with regard to a central area. Secondly, the reconstruction properties at the edges of the hologram are typically most affected by changes in shape, making it particularly desirable for quality assurance to obtain meaningful data for these areas as well, using measurements taken in the planar state.

[0208] By ensuring a minimum value for the weighting function at the edges, for example, at least 0.1, 0.2, 0.3, 0.4 or more, these areas can be analyzed with high precision even during production or when the hologram is still in its planar state. In this way, the values ​​H at the edges of a hologram in the planar state can be determined. p The measured efficiency values ​​are divided by a weighting function GL that has a sufficient value at the relevant coordinate to determine the efficiency value at the same edge of the hologram in the curved state H. g to determine precisely.

[0209] To illustrate this using the example of Fig. 10, the FOV coordinate of the planar hologram H can be p The value is taken at -1.0. Based on the curve's shape, it can be determined that the planar hologram H pAt this point, it exhibits an efficiency of approximately 0.3. This value can be determined by measurement during quality control. At -1.0, the weighting function GL has a value of approximately 0.45. This GL value is preferably predetermined by sample analysis or calibration using a golden sample hologram. Dividing the measured efficiency value of the planar hologram H p (0,3) divided by the predetermined value of the weighting function GL (0,45), one obtains an estimated efficiency of the curved hologram H. g at the same FOV coordinate of 0.67. The dashed line in Fig. 10 does indeed show that the curved hologram H g at the same FOV coordinate (-1 ,0) it would have an efficiency of approximately 0.67.

[0210] Based on the efficiency values ​​determined by such a method and with knowledge of the intensity and / or optical path length with which the hologram is projected in the curved state H g It could also be reconstructed from different positions on the curved hologram H g The emitted intensity can be determined. In other variations of this method, GL can be specified for different wavelength ranges. This preferably allows a wavelength characteristic or color mixture to be determined that is visible from different positions on the curved hologram H. g is emitted. Thus, the method can be adapted to determine values ​​and / or a distribution of various optical parameters that define the hologram in the curved state H. g would exhibit, based on measurements taken in the planar state H pThese optical parameters relate to positions on the hologram itself (field of view positions). Further embodiments of the invention relate to the values ​​and distribution of optical parameters in or along an eyebox.

[0211] In preferred embodiments of the invention, the weighting function GL is determined by simulation and / or experiment. In an experimental determination of the weighting function GL, various sample holograms are preferably used. These are reconstructed and recorded in a planar state using a light source and field-of-view detector positioned analogously or identically to the determination method. The recording includes capturing the spatial distribution of a reconstruction parameter, e.g., the intensity. Each sample hologram is then brought into a curved state. The light source is positioned so that the sample hologram is reconstructed with a target angle of incidence. The field-of-view detector records the spatial distribution of the reconstruction parameter while the sample hologram is in the curved state.Preferably, the distribution of the reconstruction parameter of the pattern hologram in the planar state is divided by the distribution of the reconstruction parameter of the pattern hologram in the curved state to determine the weighting function GL along the axis of the reflection hologram. If there are multiple weighting functions GL, the reconstruction parameters are preferably summed or averaged.

[0212] A simulation-based determination of the weighting function GL is preferably carried out analogously, whereby the optical behavior of a diffraction pattern with a known geometric function in the planar and curved states is analyzed from a known illumination point and a known observer position. This can be performed with suitable software.

[0213] Referring to Fig. 11, the optical properties, for example the efficiency q, of a curved hologram H are given. gaccording to Fig. 2 in feature space EB g -FOV g (Eyebox - Field of View for the curved state) shown schematically. The one with FOV g The x-axis represents positions along an axis, in particular a length, of the hologram in its curved state, wherein the positions have been normalized such that they range from -1 to 1. The length of the hologram preferably runs along a dimension of the hologram in which the curvature exists, preferably approximately in the direction of the x-axis shown in Fig. 6. Thus, FOV represents g ,.i at the beginning of the x-axis a left edge of the curved hologram H g This represents a central position on the x-axis, FOV. g ,o represents a central position along the length of the curved hologram H g The position FOV g ,i at the end of the x-axis represents a right edge of the curved hologram H gThe dotted shading in the graphic is intended to represent an efficiency value or the presence of radiation from the positions on the curved hologram H. g represent those that differ from FOV g ,.i to FOV g ,i extend.

[0214] Those with EB g The y-axis represents positions along an axis, in particular a length, of an eyebox that is projected onto a screen 12 when the curved hologram H g is reconstructed from its target illumination point L. The eyebox can occupy part of the screen 12, the screen 12 preferably being larger than the eyebox. The positions on the eyebox are normalized such that they are independent of EB. g ,i at a first end of the eyebox to EB g,.i at a second end of the eyebox. The dotted shading in the graphic represents the efficiency or presence of projected light on screen 12 at the different positions of the eyebox. The graphic can be used to illustrate the relationship between the light projected from different positions of the curved hologram H g self-emitted light (FOV) g ) and the light projected onto different positions of the screen (EB g ) to represent visually.

[0215] Ideally, the reconstruction light falls from the single illumination point L shown in Fig. 2 onto all positions FOV. g along the length of the curved hologram H g with a target angle of incidence. Ideally, the hologram is uniformly efficient, with a constant efficiency of 1 along its entire FOV. g Thus, all points along the length of the curved hologram H bend. gthe incident light with the same efficiency, represented by the same density of the dot fill pattern between FOV g ,-i and FOV g ,i.

[0216] Assuming the regular curvature shown in Fig. 2, the diffracted light is reflected from all positions along the FOV. g The light is emitted into an infinite number of theoretical sub-eyeboxes, all of which overlap precisely on screen 12 to form a single eyebox area 10 with uniform brightness. All areas of screen 12 outside the individual eyebox remain dark. This is achieved by the equal density of the dot fill pattern between the EB. g ,i and EB g ,.i in the case of the reconstruction of the curved hologram H g illustrated. At one end EB g ,i Eyebox 10 therefore receives light coming from all positions along the hologram (from FOV). g ,.i to FOV g,i) is emitted, on screen 12. At the second end EB g .i hits light coming from all positions along the hologram (from FOV) g ,.i to FOV g ,i) is also projected onto screen 12.

[0217] To calculate the efficiency with which the curved hologram H g The efficiency values ​​between FOV can be reconstructed at any position or any part of the projected eyebox 10. g ,.i and FOV g ,i along a corresponding part of the y-axis EB g to be integrated: Ji

[0218] A preferred embodiment of the invention aims to determine the local efficiency values ​​of an eyebox of the hologram in the curved state H g theoretically, from a planar hologram H pto determine. The local efficiencies at different FOV positions could be determined using the weighting function GL. However, if the hologram is in its planar state H p Once reconstructed, the simple integration described above can no longer be used to accurately determine the local efficiency distribution, which is an Eyebox EB. g would have been the case if the hologram were in curved form H g would be. This will be illustrated with the help of figures 12 and 13.

[0219] Fig. 12 shows a setup for analyzing the distribution of an optical parameter along an eyebox EB. P , which are represented by a hologram in the planar state H p is formed. The planar hologram H p is intended for use in a curved state as shown in Fig. 2. For example, the planar hologram H is located pin a workstation of a continuous replication process. The hologram H p is illuminated by an optimized lighting point L op t illuminated. The hologram H p The diffracted rays are projected onto a screen 12. Due to the fanning out of the theoretical partial eyeboxes, which are created from different FOV positions along the planar hologram H p When emitted (see Fig. 3 - Fig. 5), these no longer produce a single uniformly bright area on the screen 12. Instead, the light striking the screen 12 appears more diffuse or fanned out.

[0220] That from the center of the planar hologram H p Diffracted light is preferably least affected by the variation of the illumination point to L. op t is influenced. A theoretical partial eyebox, which is determined by the center of the planar hologram H pThe emitted image has approximately the same properties as the theoretical partial eyebox that is projected from the center of the curved hologram H. g would be broadcast (center, green). This theoretical partial eyebox therefore appears in approximately the same position on screen 12 as it would if the hologram were in the curved state H. g would be (see Fig. 2 and Fig. 5).

[0221] In contrast, one from the left edge of the planar hologram H p emitted partial eyebox (blue) opposite the center of the planar hologram H p The emitted partial eyebox has shifted. The partial eyebox that extends from the left edge of the planar hologram H p The broadcast area overlaps with the partial eyebox that extends from the center of the planar hologram H. pThe light is emitted in such a way that a brighter section appears on screen 12 in the area of ​​overlap. Outside the overlap area, screen 12 is less brightly illuminated until it becomes dark. This causes the light projected onto screen 12 to spread and appear more diffuse. In a narrow central area of ​​screen 12, the partial eyeboxes from all FOV positions of the planar hologram overlap. Adjacent to this area are regions to the right and left where light from only some, but not all, FOV positions of the planar hologram reaches screen 12. Outside these regions, screen 12 remains dark.

[0222] To analyze the properties that define the Eyebox EB gIn the determination method according to the invention, an eyebox detector DEB, in particular a camera, is preferably provided if the hologram were in its curved state. The eyebox detector DEB captures an image of at least one area of ​​the screen 12, which is here referred to as the eyebox area 10, on which the reconstruction of the planar hologram H is projected. p is projected. The optical properties of the eyebox EB are determined based on the data acquired by the eyebox detector DEB. g The function is determined in a curved state. This is done using a conversion algorithm based on an eyebox superposition function GEB. The eyebox superposition function GEB can be determined experimentally or through simulation beforehand.

[0223] The eyebox overlay function GEB preferably takes into account the changing ratio between the light coming from different positions (FOV). P ) of the hologram H pis emitted itself, and the light that is projected onto different positions of the eyebox area 10 (EB P ). Fig. 13 illustrates this relationship.

[0224] Similar to Fig. 11, Fig. 13 illustrates the distribution of the efficiency of an ideal hologram in the feature space FOV. g -EB g Shown are the dot-shaded rectangle based on ideal efficiency and the sharp eyebox of the curved hologram H. g . Superimposed on this, as a shaded parallelogram, is shown the distribution of the hologram efficiency of the same hologram when it is in the planar state H. p is reconstructed. This is represented by a denser dot shading.

[0225] One with FOV g The labeled y-axis shows the normalized positions along the length of the hologram when it is in the curved state H gis located. However, since the hologram is actually in the planar state H p When the eyebox detector DEB measures the distribution of optical parameters along the eyebox area 10, the FOV positions along the hologram are also fanned out. Comparing Fig. 12 with Fig. 5, it can be seen that a central position of the hologram, in which the FOV g The field of view (FOV) remains constant if the hologram is rendered planar. p ,o is therefore the same point in the feature space of Fig. 13 as FOV g ,o. This is due to the fact that the curved hologram H g at this point, it exhibits a local increase from 0. With increasing distance from the center and towards the edges of the curved hologram H g The absolute value of the local increase increases. Simultaneously, the corresponding positions of the planar hologram (FOV) also change. Pshifted further and further within the feature space. The positions of FOV P In the feature space of Fig. 13, preferably for large radii of curvature of the curved hologram H, the following correspond g in relation to the hologram extent along the FOV g approximately a product of FOV g -mFov, where rriFov is determined by the curvature of the hologram.

[0226] This is represented by a curve defined by FOV g ,o,EB g ,o in Fig. 13. This can be approximated by a dotted-dashed diagonal line passing through the same point FOVg,o,EB. g The local slope of the hologram can be calculated by differentiating the function of the hologram's curvature when it is brought into its curved state. The slope function can be determined by further differentiation.

[0227] A simple example is the case of a rectangular hologram curved in a single plane with a constant radius R, such that it assumes the shape of part of the lateral surface of a cylinder. The constant-radius curvature can be mapped in an xy-plane. In this case, the curvature has the following function:

[0228] (y + R) 2 + x 2 = R 2

[0229] By differentiating the curve function, a slope can be calculated:

[0230] The slope can be approximated, for example, by a Taylor polynomial, whereby an approximate change in slope can be obtained as follows:

[0231] Amf v

[0232] This allows the eyebox shift (AEB) in the x-axis during the reconstruction of the hologram in a planar state to be estimated as follows: EB ~Lm ■ d E ~ x ■ dE

[0233] 4 / t = m F0V ■ x

[0234] , where dE denotes a distance between the hologram and the screen. In this way, the fanning out of the eyeboxes, in particular for a simulation-based calculation of the eyebox superposition function GEB, can be determined for the case where the direction of incidence of the light still corresponds to the target specification from the curved state. Figures 12 and 13 show that FOV p ,o with FOV g ,o always coincides. FOV p ,i and FOV p ,-i , which correspond to the edges of the planar hologram H p They correspond, but are shifted upwards or downwards along the EB axis. This shift results from the fanning out of the partial eyeboxes that originate from these FOV positions. P of the planar hologram H p are emitted. Three theoretical partial eyeboxes of the planar hologram H are given as examples. pShown with bold vertical lines: an eyebox (blue, shifted downwards, from approximately 0.5 to -1.5) from the left edge of the hologram, an eyebox (green, not shifted, from approximately 1 to -1) from the center and an eyebox (orange, shifted upwards, from approximately 0.5 to 1.5) from the right edge of the hologram Hp.

[0235] In other words, as shown in Fig. 13, when measuring the hologram in the planar state, the measurement points are twisted or rotated in the feature space of the curved hologram, with the rotation or twisting depending on the hologram curvature rriFov.

[0236] As a consequence, with a single observation position during a measurement in the planar state (e.g., a mean observation position EB), p ,o ) a field of view of the hologram (along the dashed diagonal) can be observed, which originates from different eyebox positions in the curved state.

[0237] Referring to Fig. 13, a measuring point FOV can be defined for a single measuring position in the planar state. p ,.i can be observed, which is a measurement FOV g ,.i at an eyebox position of the curved hologram EB g of approximately -0.5 would correspond to observing the left edge of the curved hologram from a position shifted upwards and to the left of the center within eyebox EB. This would be analogous to observing the left edge of the curved hologram from a position shifted upwards and to the left of the center. g A measurement point FOV p ,o allows an estimation of the reconstruction quality of the FOV g ,o from a central eyebox position (EB g = 0). A measurement point FOV p ,+i reflects a measurement point FOV g ,+i at an eyebox position of the curved hologram EB gof approximately +0.5. With regard to Fig. 2, this would correspond to an observation of the right edge of the curved hologram from a position shifted downwards and to the right from the center within the eyebox EB. g .

[0238] Advantageously, the acquisition of a field of view for a fixed observation point in the planar state (e.g., in EB) allows p ,o) thus statements about a field of view for different observation positions or eyebox positions for the application case in which the hologram has a curvature.

[0239] Along the EB axis, there are several regions with denser point shading (corresponding to the reconstructed and projected planar hologram H). p ) can be recognized. In a central area of ​​the EB axis, extending approximately from EB position 0.5 to -0.5, the FOV axis is located between the FOV gPositions 1 and 1 are completely shaded with a dense dot pattern. This area corresponds to the area of ​​screen 12 onto which the light from all FOVp positions of the planar hologram H is directed. p is projected, that is, the brightest area or the highest integral of its efficiency values ​​or intensity. Above this region of the EB axis, which extends approximately from EB position 0.5 to 1.5, the more densely shaded area roughly corresponds to a triangle. This would result in a lower value when integrating the efficiency in this region, indicating lower brightness. This area no longer fully encompasses the light diffracted from the left edge of the planar hologram. Thus, the light intensity on screen 12 decreases.

[0240] If, for example, the brightness is measured as an expression of the diffraction efficiency from a position (FOVi), a dependence on an EB coordinate is eliminated, since a detector (DFOV) is located at a fixed position and detects the brightness along the FOV via this detector.

[0241] Based on this theoretical consideration, it can be seen that a local efficiency of the planar hologram at any FOV position, here FOVi, corresponds to the following:

[0242] This can be written in a shortened form as: rKFOVO = 7] p (FOV p )

[0243] In the transition from planar efficiency to a desired curved-state efficiency, a linear approximation of the eyebox shift EB with increasing slope IYIFOV occurs. Additionally, an approximation of FOV takes place. p ~FOV g .

[0244] The efficiency in the curved state can therefore be approximated as follows:

[0245] VlFOVj ~ G L i] g (FOV g ,EB g = FOV g m F0V )

[0246] The overall efficiency at a point along the eyebox area is 10 when the hologram is in the planar state H p The location can be approximated as follows, where GEB is the eyebox overlay function: z / CEBi) = G EB * rj(EB g )

[0247] Fig. 14 shows an example of such an eyebox superposition function GEB for a sample hologram. The eyebox superposition function GEB is represented as a dotted line. It has a high value in a range between the eyebox positions around -0.5 and 0.5 and then drops off more sharply outside this eyebox range. This may be because the eyebox superposition function GEB takes the form of a deconvolution algorithm (in the case of a curved hologram with constant radius R):

[0248] In a preferred embodiment of the invention, the eyebox overlay function GEB is experimentally tested using one or more sample holograms ("golden samples") of a curved hologram H. g determined that the planar form H p The procedure for the experimental determination of the eyebox superposition function GEB is described with reference to Figures 15-22B.

[0249] Figure 15 shows a calibration procedure for dewarping an image of the eyebox area 10 captured by the eyebox detector DEB. The upper image shows a mapping of various eyebox coordinates of the eyebox area 10, captured by a camera positioned obliquely to the screen 12 onto which the hologram is projected. The lower image shows the same eyebox coordinates after dewarping. This calibration procedure can be set for a specific relative position and orientation of the eyebox detector DEB to the screen 12, regardless of the type of projected and captured hologram. Dewarping the captured image can be performed using known mathematical methods from the field of image processing.

[0250] Figures 16A-20 show an exemplary procedure for the experimental determination of an eyebox superposition function GEB using a sample hologram (golden sample) of a curved hologram H. g Fig. 16A shows the eyebox emission of the curved hologram H g , which is projected onto a screen 12. An image of the projected hologram H g The image is captured by an eyebox detector DEB, which is aligned at an angle to the screen 12. The image includes data on an optical parameter distribution, in particular the intensity, efficiency, and color of the eyebox. The curved hologram H g is then converted into a planar state H p brought, with the same relative distances and orientations between the hologram H pThe screen 12 and the eyebox detector DEB are maintained. The eyebox detector DEB then captures an image of the screen 12, which includes at least the eyebox area 10. The defined coordinate system for each of the curved and planar projected holograms is subsequently mathematically superimposed on the captured images of the projected holograms. This is shown in Fig. 17A for the curved hologram H. g and in Fig. 17B for the planar hologram H p schematically illustrated. Using a calibration procedure based on the relative positions and orientations of the respective holograms H g and H p Once the image from screen 12 and eyebox detector DEB has been created, a dewarping process is performed. The dewarped images are shown in Fig. 18A for the curved hologram H. g and in Fig. 18B for the planar hologram H p depicted.

[0251] Each of the dewarped images is subsequently subjected to a Fourier transform. The Fourier transform of the projected and dewarped curved hologram is shown in Fig. 19A, while the Fourier transform of the projected and dewarped planar hologram is shown in Fig. 19B. The eyebox superposition function GEB is calculated by dividing the Fourier-transformed parameter distribution of the curved hologram (i.e., Fig. 19A) by the Fourier-transformed parameter distribution of the planar hologram (i.e., Fig. 19B).

[0252] The resulting eyebox overlay function GEB is shown in Fig. 20.

[0253] Fig. 21A shows a back-calculation of the distribution of the optical parameters (intensity and color) of the same curved hologram based on the image of the planar hologram (Fig. 16B) taken using this method, as well as the eyebox superposition function GEB from Fig. 20.

[0254] The reverse calculation is performed by dewarping the image from Fig. 16B and Fourier-transforming the dewarped image. This Fourier-transform is then multiplied by the eyebox superposition function GEB to obtain the Fourier-transform that would produce the same hologram if it were in the curved state H. g A reverse transformation of the Fourier transform of the image, which would generate the curved hologram, yields the image shown in Fig. 21A. This process can be considered an unfolding algorithm. As can be seen, the image determined on the basis of the eyebox superposition function GEB in Fig. 21A essentially reflects the actual, measured image of the curved hologram H. gfrom Fig. 18A. A comparison of Fig. 18A with Fig. 21A allows the calculation of an approximation deviation, which can be used to optimize the eyebox superposition function GEB.

[0255] The approximation deviation can preferably be calculated by subtracting the determined reconstruction parameter values ​​of the hologram's eyebox in the curved state from the recorded reconstruction parameter values ​​of the hologram's eyebox in the curved state (or vice versa). This is illustrated, for example, in Fig. 21B. The scale for the differences from -10 to 10, which, compared to the eyebox intensity values ​​of approximately 1000 (see Fig. 21A), is therefore less than 1%, indicates the clear agreement between the determined and recorded values.

[0256] The quality of the Eyebox Overlay function (GEB) can be improved by increasing the number of sample holograms (golden samples) used to create the function. The Eyebox Overlay function (GEB) was tested on a variety of holograms with different intensity and color distributions.

[0257] Fig. 22A shows a series of dewarped images captured by projecting various holograms onto a screen 12. The same holograms were then converted into a planar form HP. From images of the eyebox areas 10 of the planar holograms H p The optical properties of the eyeboxes of the holograms in curved state H were investigated. gThe image is recalculated using the GEB superposition function, as explained above with reference to Fig. 21A. The recalculation is preferably performed using an adaptation algorithm that includes dewarping the captured images, Fourier transforming the dewarped images, multiplying them by the eyebox superposition function GEB, and inverting the Fourier transform.

[0258] Fig. 22B shows the determined images and confirms a high degree of agreement between the determined images and the recorded images in the curved state H. gIn other words, a small approximation deviation was determined. The method according to the invention thus enables inline monitoring of the quality of the eyeboxes EB of planar holograms intended for use in curved form. Holograms whose reconstructions exhibit an excessive deviation from the target parameter distribution in curved form can be rejected before they reach an end user. Errors in the manufacturing process can also be quickly corrected to ensure the reliable production of high-quality holograms.

[0259] REFERENCE MARK LIST

[0260] H Hologram

[0261] H p Hologram in planar state

[0262] H g Hologram in a curved state

[0263] EB Eye box

[0264] EB P Eyebox of the planar hologram

[0265] EB gEyebox of the curved hologram

[0266] FOV Field of View

[0267] DEB Eyebox Detector

[0268] DFOV Field-of-View Detector

[0269] L Lighting point

[0270] L O pt Optimized illumination point kj Incident vector kd Emission vector

[0271] 10 Eyebox area

[0272] 12 umbrella

Claims

PATENT CLAIMS 1. Method for determining a reconstruction parameter of a reflection hologram in a curved state (H g ), wherein the determination method is based on measurements of the reflection hologram in a planar state (H p ) is based, characterized in that the procedure comprises the following steps: Reconstruction of the reflection hologram in the planar state (H p ) with a light source from an illumination point (L) for the projection of the reflection hologram onto a screen (12); Measurement of the distribution of a reconstruction parameter of the projected reflection hologram (H) p ) along the screen (12) using an eyebox detector (DEB); Determination of a reconstruction parameter of the reflection hologram in the curved state (H g ) based on the measured reconstruction parameters of the projected reflection hologram (H p) using an eyebox overlay function GEB.

2. Method for determining a reconstruction parameter of a reflection hologram in a curved state (H g ), wherein the determination method is based on measurements of the reflection hologram in a planar state (H p ) is based, characterized in that the procedure comprises the following steps: Reconstruction of the reflection hologram in the planar state (H p ) with a light source from a single illumination point (L); Measurement of a distribution of a reconstruction parameter of the reflection hologram in the planar state (H p ) along an axis of the reflection hologram in the field of view (FOV) of a field-of-view detector (DFOV); determination of a reconstruction parameter of the reflection hologram in the curved state (H g ) based on the measured reconstruction parameters of the reflection hologram in the planar state (H p) using a weighting function GL.

3. Method according to claim 1 or 2 characterized in that the determined reconstruction parameter of the reflection hologram in the curved state (H g ) with a target reconstruction parameter of the reflection hologram in the curved state (H g ) is compared to determine the quality of the reflection hologram in the measured planar state (H p ) to record.

4. Method according to claim 1 characterized in that the measured distribution of the reconstruction parameter of the projected reflection hologram in the planar state (H p ) along the screen (12) a superposition of displaced eyeboxes from different emission points of the reflection hologram in the curved state (H g ) represents, wherein the eyebox superposition function GEB preferably represents a displacement of eyeboxes from emission points of the reflection hologram in the planar state (Hp ) compared to eyeboxes of corresponding emission points of the reflection hologram in the curved state (H g ) describes and depends on a local curvature (I IFOV) of the reflection hologram.

5. A method according to one of claims 1, 3 or 4 characterized in that the eyebox superposition function GEB is determined experimentally, wherein the experimental determination of the eyebox superposition function GEB comprises the following steps: Reconstruction of the reflection hologram in the planar state (H p ) for projecting the reflection hologram onto a screen (12) with a light source from a single illumination point (L); Measurement of a distribution of a reconstruction parameter of the projected reflection hologram in the planar state (H p ) along the screen (12) using an eyebox detector (DEB); Reconstruction of the reflection hologram in the curved state (H g) for projecting the reflection hologram onto a screen (12) with a light source from a target illumination point, which illuminates all emission points of the reflection hologram in the curved state (H g ) illuminated with their target angles of incidence; Measurement of a distribution of the reconstruction parameter of the projected reflection hologram in the curved state (H g ) along the screen (12); comparison of the measurements of the reflection holograms in the planar (H p ) and curved state (H g ), in particular by means of an adjustment algorithm.

6. Method according to one of claims 1 or 3-4 characterized in that the eyebox superposition function GEB is determined by simulation, wherein the simulation-based determination of the eyebox superposition function GEB is carried out in particular by simulation-based spatial determination of a reconstruction parameter of the reflection hologram along the screen (12) when this is projected onto the screen (12), both in a planar (H p ) as well as in a curved state (H g ), wherein the determination of the reconstruction parameter takes into account a local spectral response of the reflection hologram to light from the respective illumination point (L), and the simulation-based determination of the eyebox superposition function GEB involves a comparison of the simulated reconstruction parameters from the planar (H p ) and the curved state (H g ) includes.

7. Method according to one of claims 1 or 3-6, characterized in that the determination of a reconstruction parameter of the reflection hologram in the curved state (H g ) by unfolding a distribution of the measured reconstruction parameters of the projected reflection hologram (H p ) in the planar state using the eyebox overlay function GEB.

8. Method according to claim 2 characterized in that the weighting function GL is determined by comparing a distribution of the reconstruction parameter along at least one axis of the reflection hologram in the planar state (H p ) with a distribution of the reconstruction parameter along at least the same axis of the reflection hologram in the curved state (H g) for a position of the field-of-view detector (DFOV), preferably the distribution of the reconstruction parameter of the reflection hologram in the planar state (H p ) by the distribution of the reconstruction parameter of the reflection hologram in the curved state (H g ) is divided to determine the weighting function GL along the axis of the reflection hologram.

9. Method according to the previous claim, characterized in that the distributions of the reconstruction parameter of the reflection hologram in the planar state (H p ) and / or in the curved state (H g ) determined experimentally using at least one representative sample hologram.

10. Method according to claim 8 characterized in that the distributions of the reconstruction parameter of the reflection hologram in the planar state (H p ) and / or in the curved state (H g) can be determined by simulation, preferably a spectral response of the hologram in the planar state (H p ) and / or in the curved state (H g ) the illumination from the respective lighting point (L) is taken into account in the simulation.

11. Method according to one of the preceding claims 2 or 8-10, characterized in that when determining the reconstruction parameter of the reflection hologram in the curved state (H g ), a plurality of measured reconstruction parameters of the reflection hologram in the planar state (H p ) are divided by the weighting function GL for the position of the illumination point (L) and for the position of the field-of-view detector (DFOV) to obtain the determined reconstruction parameters of the reflection hologram in the curved state (H g ) to obtain.

12. Method according to one of the preceding claims 2 or 8-11 characterized in that the distribution of the reconstruction parameter of the reflection hologram in the planar state (H p ) is measured along one or more axes of the reflection hologram in the field of view (FOV) of a field-of-view detector (DFOV) from multiple eyebox positions.

13. Method according to one of the preceding claims characterized in that the position of the illumination point (L) is optimized to minimize the average local angular deviation of the incident light for reconstructing the reflection hologram in the planar state (H). p ) to minimize from a local target angle of incidence, wherein the local target angle of incidence preferably corresponds to the angle of incidence with which the reflection hologram is reflected in the curved state (H g ) is to be reconstructed.

14. Method according to the preceding claim characterized in that the optimization of the position of the illumination point (L) comprises the following steps: Determination of the local target angles of incidence along at least one axis of the reflection hologram in the planar state (H p ) in a plane of curvature in which the reflection hologram (H p ) to transition into the curved state (H g ) is curved, Calculation of the actual angles of incidence along at least one axis of the reflection hologram in the planar state (H p ) in the plane of curvature one or more exposure points, preferably calculating the average deviation of the actual angles of incidence from the target angles of incidence for each exposure point, Selection of an optimized lighting point (L opt) , which has the smallest average deviation from the target angle of incidence along at least one axis of the reflection hologram in the planar state (H p ) in the plane of curvature.

15. Method according to the preceding claim, characterized in that when selecting the optimized illumination point (L) op t) the average deviation from the target angle of incidence in at least two orthogonal planes including the plane of curvature is calculated and minimized.

16. Method according to one of the preceding claims characterized in that the weighting function GL is varied along an axis of the reflection hologram and a position of the illumination point (L) is optimized such that the weighting function GL has a minimum size at a start and end point along the axis, wherein preferably during the optimization a central area of ​​the weighting function GL along the axis is assigned a lower weight than outer edge areas to the start and end point of the axis.

17. Method according to one of the preceding claims characterized in that the reconstruction parameter is an intensity or wavelength of the projected reflection hologram along the eyebox area (10) or along the reflection hologram in the planar state (H p ) within the field of view (FOV) of the field-of-view detector (DFOV).

18. Method according to one of the preceding claims characterized in that the determined reconstruction parameter of the reflection hologram in the curved state (H g ) with a target reconstruction parameter of the reflection hologram in the curved state (H g ) is compared to evaluate the following properties: Luminance, luminance distribution, color, color distribution, resolution, interference reflections, granularity, homogeneity and / or image distance.

9. Manufacturing process of a reflection hologram suitable for use in a curved state (H g ) is determined, wherein the reflection hologram is exposed onto a light-sensitive composite track and, after exposure, in the planar state (H p ) is present, the manufacturing process includes a quality assessment that evaluates the reflection hologram in the curved state (H g) would exhibit, with the evaluation of the quality based on a measurement of the distribution of a reconstruction parameter of the reflection hologram in the planar state (H p ) according to one of the preceding claims.

Images