Holographic medium, holographic splitter, optical system, method of reconstructing a holographic image and method of recording a hologram

Abstract

A holographic medium (2) comprises a hologram (14) designed to reconstruct a holographic image by illuminating the hologram (14) with electromagnetic radiation (4) comprising at least two different wavelengths (6, 7). The hologram (14) is formed by an interference pattern recorded by illumination of the holographic medium (2) with electromagnetic radiation comprising an intermediate wavelength. The intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two different wavelengths (6, 7).

Description

[0001] HOLOGRAPHIC MEDIUM, HOLOGRAPHIC SPLITTER, OPTICAL SYSTEM, METHOD OF RECONSTRUCTING A HOLOGRAPHIC IMAGE AND METHOD OF RECORDING A HOLOGRAM

[0002] DESCRIPTION

[0003] The present invention refers to a holographic medium, a holographic splitter comprising a holographic medium, an optical system comprising a holographic splitter, a method of reconstructing a holographic image and a method of recording a hologram .

[0004] This patent application claims priority from German patent application DE 10 2024 120 106 . 1 , the disclosure of which is hereby incorporated by reference .

[0005] Holography is a technique to generate an image which comprises phase and intensity information of light used to generate the image . In contrast , in photography a generated image only comprises intensity information (black and white photography) or intensity and frequency information ( colour photography) . Holography is based on the recording of an interference pattern which is used to reconstruct a three-dimensional light field by di f fraction at the recorded interference pattern, i . e . holography enables to generate three-dimensional images and therefore can be applied in virtual and augmented reality applications .

[0006] In the first step of recording the interference pattern, a coherent light wave is split into an illumination wave and a reference wave . An obj ect is illuminated with the i llumination wave . The illumination wave is reflected and scattered at the obj ect and forms an obj ect wave . The obj ect wave interferes with the reference wave . The generated interference pattern is recorded on a plate or a film comprising a photosensitive layer . Thus , apart from intensity information also phase information is recorded . The recorded interference pattern is called a hologram . In the second step of reconstruc- tion, the hologram is illuminated with a reconstruction wave which is typically identical to the reference wave. By diffraction of the reconstruction wave at the hologram, a reconstructed object wave is generated which is identical to the object wave.

[0007] A hologram can be produced by illumination of a photosensitive film as described. Alternatively, it can be computergenerated, i.e. created by digitally modelling an interference pattern which can be produced by printing, for example. There are different types of holograms such as transmission holograms, reflection holograms, volume holograms, surface holograms, amplitude holograms and phase holograms. A surface hologram only comprises a two-dimensional interference pattern. In a volume hologram, the photosensitive layer comprises a thickness which allows to record information in different planes of the photosensitive film. An amplitude hologram generates diffracted light comprising an amplitude which is proportional to the intensity of the recorded light. Furthermore, a hologram can be generated by using coherent light waves comprising different wavelengths enabling a generation of coloured three-dimensional images.

[0008] In contrast, a phase hologram comprises a phase grating. A phase hologram can be produced by generating a transparent surface relief, as an example, which causes phase shifts due to a varying thickness of the hologram. A phase hologram can be produced by bleaching of an amplitude hologram or by embossing. Also, volume phase holograms (VPH) are known which combine the two described types of holograms. Typically, a VPH can be thicker than a standard phase hologram. In this case, a modulated index of refraction across the VPH is causing diffraction instead of a surface relief.

[0009] Holograms can be used in virtual and augmented reality applications. In this case, an image generator is used to generate image light for the illumination of a hologram which can be part of an eye piece of a wearable heads-up display, for ex- ample. The image generator can be attached to a frame of the wearable device and, e.g., be designed as a scanning laser projector. Thus, a holographic image can be projected on a retina of a user. The user can see the holographic image within the so-called eye box. The eye box defines an area or a space where the user is able to orient his view such that he can see the whole holographic image. A large eye box improves the user experience in contrast to a small eye box limiting the possibility to see the holographic image completely. Apart from wearable devices, holograms can be implemented in automotive applications, e.g., to project a holographic image onto a windshield, or other consumer electronics .

[0010] From the state of the art, a method called pupil replication is known. Here, a plurality of copies of the holographic image, which can be called pupils, are generated and projected to different positions. The user can see the holographic image correctly as long as he is able to see at least one pupil. Consequently, pupil replication is accompanied by an eye box expansion. Pupil replication is achieved by focussing beams of image light emitted by the image projector onto a pupil splitter, i.e. a holographic medium, which spreads the beams of image light by diffraction, wherein each beam defines a plurality of pupils. Pupil replication can be performed for each colour channel, e.g. for a red, a green and a blue channel. As a consequence, the pupil splitter has to comprises at least three matching holograms in order to generate colour images (RGB) .

[0011] A plurality of holograms induces several practical complications. A typical holographic medium can only record a finite number of volume holograms while maintaining a high diffraction efficiency. Furthermore, due to multiple colour channels recorded in the same holographic medium, it is possible to observe an undesired crosstalk, that is e.g. blue light being diffracted by the green hologram or vice-versa. Also, the recording of a hologram can generate parasitic holograms from e . g . interface reflections and stray light . The parasitic holograms can create losses when reconstructing the holographic image close to the recording wavelength .

[0012] From the state of the art , it is also known to design and write a hologram at a first wavelength and to reconstruct the holographic image at a second wavelength, the so-called two- lambda approach . The first and the second wavelengths are very close to each other, typically . A reconstruction based on a volume hologram with a second wavelength which is signi ficantly di f ferent from the first wavelength induces several issues such as aberrations and a drastic decrease of di ffraction ef ficiency . Good ef ficiency can still be achieved at the second wavelength by changing the illumination geometry to match a Bragg condition . These issues are especially problematic when the hologram is used in an imaging configuration . I f the hologram is used in a splitter / di f fuser configuration, aberrations and a decrease in di f fraction e f ficiency don' t impact the final image quality . However, in the split- ter / di f fuser configuration these problems are trans lated into pupil aberrations , instead .

[0013] An obj ective of the present invention consists in providing an improved holographic medium, a holographic splitter, an optical system and to speci fy a method of reconstructing a holographic image and a method of recording a hologram . This obj ective is solved by a holographic medium, a holographic splitter, an optical system, a method of reconstructing a holographic image and a method of recording a hologram comprising the features of the respective independent claims . Advantageous embodiments are speci fied in the dependent claims .

[0014] A holographic medium comprises a hologram designed to reconstruct a holographic image by illuminating the hologram with electromagnetic radiation comprising at least two di fferent wavelengths . The hologram is formed by an interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising an intermediate wavelength . The intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two di f ferent wavelengths .

[0015] The holographic medium is based on the idea to record a hologram at one wavelength, that is the intermediate wavelength, and to use at least two wavelengths for an ef ficient holographic reconstruction of an image . Since a plurality of wavelengths is used for reconstruction, the hologram can also be called a poly-di f fractive hologram . Note , that in the context of the present description the term electromagnetic radiation denotes coherent electromagnetic radiation . Also , the terms electromagnetic radiation and light both denote radiation comprising any wavelength i f the wavelength is not specified explicitly, i . e . the term light may not only denote visible light , in general .

[0016] The holographic medium can comprise any suitable material which can be illuminated to record an interference pattern, i . e . a photosensitive material . Reconstruction can be performed by using at least two colour channels corresponding to the at least two di f ferent wavelengths . Using a single hologram for two or more colour channels means that the holographic medium accommodates a lower total number of holograms .

[0017] Advantageously, the poly-di f fractive hologram enables a higher achievable di f fraction ef ficiency due to a reduced number of multiplexed holograms of the holographic medium . Also , an inter-channel crosstalk can be signi ficantly reduced or even avoided i f the holographic medium comprises a reduced number or only one hologram . Furthermore , recording a single hologram for multiple colour channels in the holographic medium reduces the risk of exciting parasitic holograms . Thus , a reconstruction of a holographic image at a di f ferent wavelength compared to the wavelength used for the generation of the hologram helps in avoiding losses arising from parasitic holograms , too .

[0018] As a reconstruction with a wavelength which is signi ficantly di f ferent from the wavelength used to generate the hologram induces aberrations and a decrease of di f fraction e f ficiency, the aberrations and the decrease in ef ficiency for the present holographic medium can be minimi zed by choosing the intermediate wavelength appropriately, i . e . to optimi ze di ffraction conditions , for example by choosing the intermediate wavelength such that a di f ference between the intermediate wavelength used to generated the hologram and the at least two wavelengths used for the reconstruction, respectively, is minimi zed .

[0019] In an embodiment the holographic medium comprises a further hologram designed to reconstruct the holographic image by illuminating the further hologram with electromagnetic radiation comprising a further wavelength . The further hologram is formed by a further interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising the further wavelength . In contrast to the hologram, the further hologram is not designed as a poly- di f fractive hologram .

[0020] In an embodiment a first wavelength of the at least two di fferent wavelengths corresponds to light from the blue spectral range . A second wavelength of the at least two di f ferent wavelengths corresponds to light from the green spectral range . The intermediate wavelength corresponds to teal light . In another embodiment the further wavelength corresponds to light from the red spectral range .

[0021] To generate a complete colour gamut three colour channels with the colours red, green and blue (RGB ) can be used for reconstruction . The hologram can be used for reconstruction using the blue channel and the green channel , while the intermediate wavelength corresponds to teal light . The further hologram can be used for reconstruction with red light comprising the further wavelength . This would result in having only two multiplexed holograms for teal and red light instead of three holograms with one hologram per colour channel . Advantageously, parasitic hologram contributions are reduced for the green and blue channels . Furthermore , this method reduces inter-channel crosstalk which typically occurs between channels with neighbouring wavelengths ( i . e . blue / green or red / green) . With this method, the blue / green crosstalk is simply avoided, and the red / teal crosstalk is also lower than the red / green one due to a higher wavelength gap .

[0022] As an example , a first wavelength of 450nm and a second wavelength of 520nm can be chosen which corresponds to the blue channel and the green channel , respectively . In thi s case , the first wavelength and the second wavelength are relatively close to each other . Therefore , it is expedient to split / di f fuse light comprising these wavelengths by the same hologram recorded at the intermediate wavelength .

[0023] In an embodiment the intermediate wavelength is an average of the at least two di f ferent wavelengths . In the example described above , an intermediate wavelength of 485nm can be chosen for recording the hologram which provides a high ef ficiency when the hologram is read out using light comprising wavelengths of 450nm and 520nm . The above embodiment can be modi fied to account for more than two di f ferent wavelengths if more than two light sources are present , e . g . for several sources with respective central wavelengths at 460nm, 480nm, and 500nm and an intermediate wavelength between the minimum and the maximum wavelengths , i . e . between 460nm and 500nm and, e . g . 480nm or close to 480nm .

[0024] In an embodiment the holographic medium comprises an additional hologram designed to reconstruct the holographic image by illuminating the additional hologram with electromagnetic radiation comprising at least two di f ferent additional wavelengths . The additional hologram is formed by an additional interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising an additional intermediate wavelength . The additional intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two di f ferent additional wavelengths .

[0025] The additional hologram can also be called a poly-di ffractive hologram as it can be read out by the at least two dif ferent additional wavelengths . In an embodiment a first additional wavelength and a second additional wavelength of the at least two di f ferent additional wavelengths correspond to light from the red spectral range , respectively . Advantageously, i f multiple light sources are used e . g . in the red spectral range , the additional poly-di f fractive hologram can be used in the red spectral range similarly to the hologram in order to reduce a total number of holograms recorded in the holographic medium . Thus , the holographic medium comprises only two multiplexed holograms instead of four holograms for the at least two wavelengths and the at least two additional wavelengths . Similarly, the additional intermediate wavelength can be an average of the at least two di f ferent additional wavelengths , as an example .

[0026] As the hologram is designed for reconstruction with the at least two wavelengths which are shi fted with respect to the intermediate wavelength used to record the hologram, electromagnetic radiation impinging on the hologram during the reconstruction needs to be adj usted to ful fil a Bragg-condition ef ficiently, i . e . the di f fraction ef ficiency can be adj usted by determining an angle of incidence of the electromagnetic radiation impinging on the hologram . For example , this can be achieved at a proj ector level by orienting the light sources of an image proj ector which are designed to emit electromagnetic radiation comprising at least the first and the second wavelengths . Another possibility consists in using intermediate optics to individually redirect electromagnetic radiation generated by the image proj ector . In another embodiment, the holographic medium comprises a deflection hologram designed to deflect electromagnetic radiation such that a Bragg-condition is fulfilled for the deflected electromagnetic radiation impinging on the hologram. This embodiment relies on the use of a preliminary hologram, that is the deflection hologram, positioned before the hologram. The deflection hologram can be designed to receive white light and diffract the blue and green channels to fulfil their respective Bragg conditions for the hologram recorded with teal light, as an example.

[0027] Advantageously, adding the deflection hologram as a preliminary hologram to redirect and Bragg-match the individual wavelength channels on the hologram adds an additional degree of freedom. In this case, the original optical transfer function of the hologram, which is to receive the light from a projector and split it into different pupils, is divided into two parts. The deflection hologram must be Bragg-matched to the electromagnetic radiation of the projector's light, while the hologram needs to efficiently split the electromagnetic radiation into multiple pupils. For this reason, the hologram can also be called a splitting hologram. However, the optical transfer function which can be used to describe the transfer from the deflection to the splitting process can be modelled as desired. For example, the optical transfer function can be kept as a continuity of the projector's optical function, but it can also be changed into a simple plane wave, which can simplify a fabrication process of the holographic medium. Conversely, the optical transfer function can be complexified as to maximize the Bragg-matching .

[0028] In an embodiment the holographic medium comprises a further deflection hologram designed to deflect electromagnetic radiation such that a Bragg-condition is fulfilled for the deflected electromagnetic radiation impinging on the further and / or the additional hologram. In this case, different deflection holograms are used for different (splitting) holo- grams . However, in another embodiment , a single def lection hologram can be used for the hologram and the further hologram or the additional hologram .

[0029] A holographic splitter comprises a holographic medium according to one of the described embodiments and an optical combiner . The holographic medium is designed to split electromagnetic radiation according to a Bragg-condition . The optical combiner is designed to reflect the split electromagnetic radiation to form collimated electromagnetic radiation . The collimated electromagnetic radiation defines an image which can be proj ected towards a user' s eye for example , i f the holographic splitter is part of a wearable VR-device , exemplarily .

[0030] An optical system comprises a holographic splitter and an image proj ector . The image proj ector is designed to emit electromagnetic radiation comprising at least the two di fferent wavelengths or white light and to proj ect electromagnetic radiation on the holographic medium .

[0031] In an embodiment the image proj ector is designed to orient light sources which are designed to generate electromagnetic radiation comprising the at least two di f ferent wavelengths such that a Bragg-condition is ful filled for each wavelength at the hologram . In this case , no deflection hologram is needed . However, in another embodiment the holographic medium comprises the deflection hologram and the proj ector is designed to orient light sources designed to generate electromagnetic radiation comprising at least the two di f ferent wavelengths such that a maximum di f fraction ef ficiency is reached for each wavelength at the hologram .

[0032] In an embodiment the optical system comprises a lightguide arranged between the image proj ector and the holographic splitter and designed to guide electromagnetic radiation emitted by the image proj ector onto the holographic medium . A method of reconstructing a holographic image comprises the following step : A holographic medium comprising a hologram is illuminated with electromagnetic radiation comprising at least two di f ferent wavelengths . The hologram is formed by an interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising an intermediate wavelength . The intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two di f ferent wavelengths .

[0033] In an embodiment the holographic medium comprises a further hologram formed by a further interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising a further wavelength . The method comprises the step of reconstructing the holographic image by illuminating the further hologram with electromagnetic radiation comprising the further wavelength in addition to the illumination of the hologram with electromagnetic radiation comprising at least two di f ferent wavelengths .

[0034] In an embodiment the holographic medium comprises an additional hologram formed by an additional interference pattern recorded by illumination of the holographic medium with electromagnetic radiation comprising an additional intermediate wavelength . The additional intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two di f ferent additional wavelengths . The method comprises the step of reconstructing the holographic image by illuminating the additional hologram with electromagnetic radiation comprising the at least two di f ferent additional wavelengths in addition to the illumination of the hologram with electromagnetic radiation comprising the at least two di f ferent wavelengths . The hologram and the at least one further hologram and / or the at least one additional hologram can be illuminated simultaneously .

[0035] A method of recording a hologram comprises the following step . An interference pattern is recorded by illuminating a holographic medium with electromagnetic radiation comprising an intermediate wavelength . The intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of at least two di f ferent wavelengths . The hologram is designed to reconstruct a holographic image by illuminating the hologram with electromagnetic radiation comprising the at least two di f ferent wavelengths .

[0036] In an embodiment a further interference pattern is recorded by illuminating the holographic medium with electromagnetic radiation comprising a further wavelength . The further hologram is designed to reconstruct the holographic image by illuminating the further hologram with electromagnetic radiation comprising the further wavelength .

[0037] In an embodiment an additional interference pattern is recorded by illumination of the holographic medium with electromagnetic radiation comprising an additional intermediate wavelength . The additional hologram is designed to reconstruct the holographic image by illuminating the additional hologram with electromagnetic radiation comprising at least two di f ferent additional wavelengths .

[0038] Similarly, at least one deflection hologram and / or at least one further deflection hologram can be recorded by illumination of the holographic medium . Here , the recording wavelength can be chosen depending on the recording wavelengths of the hologram and the optional further and / or additional hologram . All speci fied holograms are formed in the holographic medium and stacked along an optical axis .

[0039] The above-described properties , features and advantages of this invention and the way in which they are achieved will become clearer and more clearly understood in association with the following description of the exemplary embodiments which are explained in greater detail in association with the drawings . Here in schematic illustration in each case : Fig. 1: a holographic splitter comprising a holographic medium with a poly-diffractive hologram in a cross-sectional side view;

[0040] Fig. 2: arrangements of holographic pupils generated by the holographic splitter according to Fig. 1 for different wavelengths and angles of incidence;

[0041] Fig. 3: different configurations of holographic splitters comprising deflection holograms in cross-sectional side views ;

[0042] Fig. 4: further configurations of holographic splitters comprising deflection holograms in cross-sectional side views;

[0043] Fig. 5: arrangements of holographic pupils generated by the holographic splitter according to Fig. 4 for different wavelengths and angles of incidence; and

[0044] Fig. 6: an optical system comprising a holographic splitter with a holographic medium having a poly-diffractive hologram.

[0045] Fig. 1 schematically shows a holographic splitter 1 in a cross-sectional side view. The holographic splitter 1 is designed to reconstruct a holographic image. Therefore, the holographic splitter 1 comprises a holographic medium 2 and an optical combiner 3.

[0046] The holographic medium 2 and the optical combiner 3 are arranged next to each other along an optical axis. A distance between the holographic medium 2 and the optical combiner 3 can comprise a few millimetres, e.g. a distance of 5mm to 10mm, as an unlimiting example.

[0047] The holographic medium 2 is designed to split incident electromagnetic radiation 4 according to a Bragg-condit ion or in other words to split or to diffuse incident electromagnetic radiation 4 by diffraction. The holographic splitter 1 can also be called a holographic di f fuser 1 . The optical combiner 3 is designed to reflect electromagnetic radiation 5 split by the holographic medium 2 to form collimated electromagnetic radiation, wherein the collimation is not shown in Fig . 1 for the sake of simplicity .

[0048] The collimated electromagnetic radiation forms the holographic image which can be proj ected into an eye box of a user, for example . In this case , the holographic splitter 1 can be part of an eye piece of a wearable device which can be headmounted in an embodiment . In other embodiments , the holographic splitter 1 is part of an optical system of an automobile and the holographic image can be proj ected onto a windshield of the automobile . The holographic splitter 1 can also be part of other consumer electronics .

[0049] As an optical combiner 3 any combiner can be used which is known from the state of the art . Fig . 1 shows the optical combiner 3 only schematically without showing its elements . An optical combiner 3 typically can comprise a polarizationdependent reflector, a wave plate , e . g . a quarter wave plate , and a reflector comprising a transparent carrier, a dichroic and reflective coating and a circular polari zer . The specified elements can be arranged in the given order, wherein the polari zation-dependent reflector is facing the holographic medium 2 while the circular polari zer is averted from the holographic medium 2 . Typically, an air gap is present between the wave plate and the carrier . The optical combiner 3 can be shaped as a so-called pancake combiner . Some of the speci fied elements of the holographic combiner 3 can be omitted, i f applicable . On the other hand, the optical combiner 3 can also comprise other elements and coatings in addition .

[0050] The holographic medium 2 can comprise any suitable photosensitive material . The holographic medium comprises a hologram formed by an interference pattern designed to reconstruct the holographic image by illuminating the hologram with electromagnetic radiation 4 comprising at least two di f ferent wave- lengths 6, 7. The hologram can be designed as a volume phase hologram (VPH) , as an example.

[0051] Fig. 1 shows that the holographic medium is illuminated with blue light 6 comprising a wavelength of 455nm and green light 7 comprising a wavelength of 515nm, exemplarily. However, the number of different wavelengths 6, 7 used to reconstruct the holographic image is not limited to two wavelengths 6, 7. For example, the holographic medium 2 can be illuminated with electromagnetic radiation 4 comprising three different wavelengths, e.g. 460nm, 480nm, and 500nm to reconstruct the holographic image.

[0052] The holographic medium 2 is based on the idea to record the hologram, i.e. the interference pattern, by using an intermediate wavelength. The intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two different wavelengths 6, 7. The recorded hologram can then be used for reconstruction by illuminating the hologram with electromagnetic radiation comprising the at least two different wavelengths 6, 7. The intermediate wavelength can be an average of the at least two different wavelengths 6, 7, e.g. 485nm if blue light 4 comprising a first wavelength 6 of 455nm and green light 4 comprising a second wavelength 7 of 515nm is used. In this case the intermediate wavelength corresponds to teal light.

[0053] The hologram is designed to diffract electromagnetic radiation comprising the first and the second wavelength 6, 7. Since the hologram is generated by recording an interference pattern using electromagnetic radiation comprising the intermediate wavelength, an angle of incidence of the electromagnetic radiation 4 comprising the at least two wavelengths 6, 7 can be adjusted to fulfil a Bragg-condition efficiently. In the exemplary embodiment of Fig. 1 where the hologram has been generated with light comprising an intermediate wavelength of 485nm, a diffraction efficiency of 44.1% can be achieved for an angle of incidence of 60° measured with re- spect to a plumb line on the holographic medium 2 i f the hologram is read out with electromagnetic radiation 4 comprising the intermediate wavelength .

[0054] For the first and the second wavelengths 6 , 7 a reduced di ffraction ef ficiency can be achieved for the same angle of incidence . However, by adj usting the angle of incidence to 57 . 4 ° for the blue light 6 comprising a wavelength of 455nm, an ef ficiency of 42 . 6% can be achieved . For the green light 7 , by adj usting the angle of incidence to 63 ° , an e f ficiency of 44 % can be achieved, which nearly corresponds to the ideal theoretical case of di f fraction using teal light comprising the intermediate wavelength of 485nm .

[0055] Apart from the hologram, the holographic medium 2 can comprise a further hologram designed to reconstruct the holographic image by illuminating the further hologram with electromagnetic radiation 4 comprising a further wavelength . The further hologram is formed by a further interference pattern recorded by illumination of the holographic medium 2 with electromagnetic radiation comprising the further wavelength . The further wavelength can correspond to light from the red spectral range , as an example .

[0056] In this case , the holographic medium 2 comprises two multiplexed interference patterns , i . e . the hologram and the further hologram, for three colour channels in total which enables the reconstruction of coloured holographic images (RGB ) . However, the further hologram can also be omitted . In other embodiments the holographic medium 2 comprises a plurality of further holograms each recorded by using electromagnetic radiation comprising a further wavelength and designed for reconstruction by using electromagnetic radiation of the same further wavelength, respectively . In any case , the holographic medium 2 comprises a reduced number of multiplexed holograms since the hologram is designed for reconstruction using the at least two di f ferent wavelengths 6 , 7 instead of using di f ferent holograms for the first and the second wavelength 6, 7. In this sense, the hologram can be called a poly- diffractive hologram. In contrast to this, the at least one further hologram is a mono-diffractive hologram as it is recorded and intended for reconstruction using the further wavelength and no other wavelengths.

[0057] The poly-diffractive hologram comprises the advantage that a diffraction efficiency can be maintained in the reconstruction process with the at least two different wavelengths compared to the ideal case of a reconstruction based on the intermediate wavelength. Generally, the efficiency decreases with an increasing number of holograms. This is confirmed by the specified diffraction efficiencies for blue and green light, compared to the ideal case of teal light, which has been used to record the hologram in the exemplary embodiment.

[0058] Also, a reduced number of holograms reduces a cross-talk between different holograms during reconstruction. For the embodiment with three different colour channels (RGB) , the total cross-talk is reduced to cross-talk between the hologram and the at least one further hologram. A cross-talk between the blue and the green channel is omitted. Generally, the cross-talk between the at least two different wavelengths is omitted. In contrast, for a plurality of holograms, crosstalk occurs in pairs. Also, the cross-talk occurs in two directions, i.e. a cross-talk can be observed in a blue channel originating from a green hologram if different holograms are provided for the blue and the green channel. But, the crosstalk can be also observed in the green channel due to the presence of a separate hologram for the blue channel.

[0059] Furthermore, parasitic holograms can be omitted when recording the hologram. Every hologram which is recorded is accompanied by the creation of a parasitic hologram. Parasitic holograms cause diffraction losses during reconstruction. Thus, by providing a single hologram for at least two wavelength channels 6, 7 parasitic diffraction losses can be reduced significantly. The holographic medium 2 can comprise at least one additional hologram which is polydi f tractive which is designed to reconstruct the holographic image by illuminating the additional hologram with electromagnetic radiation comprising at least two di f ferent additional wavelengths . The additional hologram is formed by an additional interference pattern recorded by illumination of the holographic medium 2 with electromagnetic radiation comprising an additional intermediate wavelength . Similarly to the intermediate wavelength, the additional intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two di fferent additional wavelengths , e . g . an average wavelength .

[0060] A first additional wavelength and a second additional wavelength of the at least two di f ferent additional wavelengths can correspond to light from the red spectral range , respectively . The holographic medium 2 can comprise at least one additional hologram instead of the at least one further hologram or in addition to the at least one further hologram . In any case , again, a total number of holograms of the holographic medium 2 is reduced as the additional hologram is used for reconstruction with two additional wavelengths instead of using separate holograms for the additional wavelength channels .

[0061] The term multiplexed holograms means that multiple holograms are recorded or superimposed in the same holographic medium 2 , particularly in the same photosensitive material . Generally, angular multiplexing and wavelength ( colour ) multiplexing are known . For angular multiplexing, the holograms are superimposed using the same wavelength but di f ferent angles of incidence for the obj ect and the reference beams . For wavelength multiplexing, the holograms are superimposed in the same medium but using di f ferent wavelengths .

[0062] Consequently, the holographic medium 2 comprises the poly- di f fractive hologram and optionally at least one further mono-di f fractive hologram and / or at least one additional poly-di f fractive hologram which are superimposed, i . e . multiplexed . Each of the holograms can be a VPH on its own although they are recorded in the same medium .

[0063] The hologram and the at least one further hologram and / or the at least one additional hologram can be recorded in di f ferent planes / layers of the holographic medium 2 such that the hologram and the at least one further hologram and / or the at least one additional hologram are arranged in paral lel planes . The hologram and the at least one further hologram and / or the at least one additional hologram can be right in contact with each other or separated . For example , the hologram and the at least one further hologram and / or the at least one additional hologram can be separated from each other by tens to a few hundreds of micrometres . Thus , the hologram and the at least one further hologram and / or the at least one additional hologram form a stack of paral lelly arranged holograms , e . g . VPHs .

[0064] As already explained, a reconstruction of a holographic image using electromagnetic radiation comprising a wavelength which deviates from a wavelength that was used to record the hologram can cause pupil aberrations in the splitter / di f fuser configuration . Fig . 2 schematically shows arrangements 9 , 10 , 11 of holographic pupils 8 generated by the holographic splitter 1 according to Fig . 1 for di f ferent wavelengths 6 , 7 and angles of incidence .

[0065] The arrangements 9 , 10 , 11 of holographic pupils 8 of Fig . 2 were reconstructed by using the exemplary hologram of Fig . 1 which has been recorded with light comprising an intermediate wavelength of 485nm . A first arrangement 9 of pupil s 8 is shown in the middle of Fig . 2 which depicts the ideal case of a reconstruction with teal light 4 comprising an angle of incidence of 60 ° and a wavelength of 485nm which corresponds to the intermediate wavelength . However, the hologram is intended for a reconstruction with electromagnetic radiation com- prising at least two different wavelengths which are different from the intermediate wavelength.

[0066] A second arrangement 10 of pupils 8 is shown on the left side of Fig. 2. In this case, the pupils 8 were reconstructed with blue light comprising a wavelength of 455nm and an angle of incidence of 57,4°. A third arrangement 10 of pupils 8 is shown on the right side of Fig. 2, wherein the pupils 8 were reconstructed with green light comprising a wavelength of 515 nm and an angle of incidence of 63°.

[0067] A slight pupil shift can be observed with respect to the first arrangement 9 of pupils 8 for the second and the third arrangement 10, 11. The pupils 8 of the second arrangement 10 comprise bigger distances to each other than the pupils 8 of the first arrangement 9. On the other hand, the pupils 8 of the third arrangement 11 comprise smaller distance to each other than the pupils 8 of the first arrangement 9. However, the pupils 8 of the second and the third arrangement 10, 11 don't comprise significant aberrations. Also, the pupils 8 of the second and the third arrangement 10, 11 are not shifted outside an aperture 12 arranged within the eye box. The aperture 12 can comprise a diameter of 10mm, for example. Thus, the holographic medium 2 enables a reliable reconstruction of a holographic image within the eye box with electromagnetic radiation comprising the at least two wavelengths 6, 7 which are different from the intermediate wavelength used to record the hologram.

[0068] Fig. 3 schematically shows two different exemplary configurations of holographic splitters 1 according to further embodiments comprising deflection holograms 13 in cross-sectional side views. The side views of Fig. 3 correspond to the side view of Fig. 1, wherein the optical combiner 3 is not shown in Fig. 3 as a detailed view on the holographic medium 2 is shown. The holographic splitters 1 shown in Fig. 3 comprise similarities to the holographic splitter 1 according to Fig. 1. In the following description, only differences of the hoi- ographic splitter 1 according to Fig. 3 with respect to the holographic splitter 1 of Fig. 1 are described. The reference numerals are maintained for similar and identical elements. Also, reference numerals which are introduced in Fig. 3 for elements which are described in connection with Fig. 1, can also refer to similar and identical elements of Fig. 1.

[0069] The holographic media 2 of Fig. 3 comprise a poly-diffractive hologram 14 which is designed according to one of the described embodiments, respectively. In addition, the holographic media 2 of Fig. 2 comprise a deflection hologram 13, respectively. The deflection holograms 13 are designed to deflect electromagnetic radiation such that a Bragg-condition is fulfilled for the deflected electromagnetic radiation impinging on the poly-diffractive holograms 14 which can be called splitting holograms 14.

[0070] The deflection holograms 13 and the splitting, poly- diffractive holograms 14 are formed in the same holographic medium 2 which means they are multiplexed. The deflection holograms 13 and the poly-diffractive holograms 14 are arranged in parallel planes perpendicular to an optical axis.

[0071] Electromagnetic radiation impinging on the holographic media 2 is refracted at surfaces 15 of the holographic media 2, diffracted at the deflection holograms 13 such that a deflection occurs to fulfil a Bragg-condition for all wavelength channels used for reconstruction and diffracted at the poly- diffractive hologram 14, i.e. split, diffused, or fanned out to be projected onto the optical combiner 3. The holographic splitter 1 of Fig. 3 comprise the advantage that an adjustment of electromagnetic radiation such that a Bragg-condition is fulfilled for all wavelength channels used for reconstruction with the poly-diffractive hologram 14 can be omitted on a projector level. The alignment of electromagnetic radiation to read out the splitting hologram 14 is implemented in the holographic medium 2 or the holographic splitter 1 instead. The holographic media 2 can comprise a further deflection hologram 13 for every poly-diffractive hologram 14 and / or mono-diffractive hologram, wherein the further deflection hologram 13 is designed to deflect electromagnetic radiation such that a Bragg-condition is fulfilled for the deflected electromagnetic radiation impinging on the further mono- diffractive hologram or the additional poly-diffractive hologram. Since the hologram 14 and the at least one further hologram and / or the at least one additional hologram are multiplexed they are arranged also in parallel planes stacked along the optical axis.

[0072] In other embodiments, deflection holograms 13 can be omitted for all wavelength channels. Instead, an orientation of electromagnetic radiation comprising the at least two wavelengths 6, 7 can be realized by an image projector comprising adjustable light sources. In yet another embodiment deflection holograms 13 can be combined with an adjustment on the projector level to achieve maximum diffraction efficiencies for all mono- and poly-diffractive splitting holograms 14. Generally, an alignment with at least one deflection hologram 13 and an alignment on the projector level can be omitted.

[0073] Fig. 3 shows two exemplary configurations of holographic splitters 1 with different deflection holograms 13 for the poly-diffractive holograms 14, respectively. For both configurations, a cross-sectional side view onto an yz-plane is shown corresponding to the side view of Fig. 1, wherein the incidence angle is in the yz-plane. The angle of incidence is 60° as an example in both cases measured with respect to a plumb line on the deflection holograms 13.

[0074] On the left side, the holographic splitter 1 comprises a first deflection hologram 16 designed for an on-axis wavelength channel reorientation. In this case, electromagnetic radiation impinging on the first deflection hologram 16 is separated slightly before the diffraction at the poly- diffractive splitting hologram 14 without changing the direc- tion of the electromagnetic radiation at the deflection hologram 13 signi ficantly in the yz-plane .

[0075] On the right side of Fig . 3 , the holographic splitter 1 comprises a second deflection hologram 17 designed for an on- axis opposite redirection . In this case , electromagnetic radiation impinging on the second deflection hologram 17 is redirected signi ficantly within the yz-plane .

[0076] Fig . 4 schematically shows two further di f ferent exemplary configurations of holographic splitters 1 according to further embodiments comprising deflection holograms 13 in cross- sectional side views . The side views of Fig . 4 correspond to cross-sections onto an xz-plane perpendicular to the yz- plane . The incidence angle is in the yz-plane and the angle of incidence is 60 ° , again . The optical combiner 3 is not shown in Fig . 4 . The holographic splitters 1 of Fig . 4 comprise similarities to the holographic splitters 1 according to Fig . 3 . In the following description, only di f ferences of the holographic splitters 1 according to Fig . 4 with respect to the holographic splitters 1 of Fig . 3 are described . The reference numerals are maintained for similar and identical elements .

[0077] On the left side of Fig . 4 , the holographic splitter 1 comprises a third deflection hologram 18 designed for an upwards redirection in the xz-plane . On the right side of Fig . 4 , the holographic splitter 1 comprises a fourth deflection hologram 19 designed for a downwards redirection in the xz-plane . In other words , the holographic splitters 1 of Fig . 4 are designed to redirect electromagnetic radiation in opposite directions within the xz-plane . The redirection of electromagnetic radiation is not limited to the examples of Fig . 3 and Fig . 4 . Rather than that , a person skilled in the art may derive other configurations comprising deflection holograms 13 designed for a desired redirection in the yz- and / or in the xz-plane . Fig. 5 schematically shows arrangements 20, 21, 22 of holographic pupils 8 generated by the holographic splitter 1 according to one of the embodiments described in connection with Fig. 3 and Fig. 4 for different wavelengths 6, 7 and angles of incidence. The arrangements 20, 21, 22 of holographic pupils 8 of Fig. 5 were reconstructed by using a holographic splitter 1 comprising a deflection hologram 13 apart from at least one poly-diffractive hologram 14. As an example, the deflection hologram 13 is designed for a downwards redirection .

[0078] A fourth arrangement 20 of pupils 8 is shown in the middle of Fig. 5 which depicts the ideal case of a reconstruction with teal light 4 comprising an angle of incidence of 60° and a wavelength of 488nm which corresponds to the intermediate wavelength, as an example. Here, a diffraction efficiency of 38.1% can be achieved. A fifth arrangement 21 of pupils 8 is shown on the left side of Fig. 5. In this case, the pupils 8 were reconstructed with blue light comprising a wavelength of 455nm and an angle of incidence of 57°. A diffraction efficiency of 34.3% can be achieved. A sixth arrangement 22 of pupils 8 is shown on the right side of Fig. 5, wherein the pupils 8 were reconstructed with green light comprising a wavelength of 515 nm and an angle of incidence of 62,7°, wherein the diffraction efficiency can be 36.8%.

[0079] Again, a slight pupil shift can be observed with respect to the fourth arrangement 20 of pupils 8 for the fifth and sixths arrangement 21, 22 of pupils 8. Also, the pupils 8 of the fifth arrangement 21 comprise bigger distances to each other than the pupils 8 of fourth arrangement 20, while the pupils 8 of the sixth arrangement 22 comprise smaller distance to each other than the pupils 8 of the fourth arrangement 20. Again, no significant aberrations can be observed when using a deflection hologram 13 in addition to the poly- diffractive hologram 14. Also, the pupils 8 are not shifted outside the aperture 12 arranged within the eye box. Fig. 6 schematically shows an optical system 23 comprising a holographic splitter 1 according to Fig. 1, Fig. 3 or Fig. 4. Apart from the holographic splitter 1, the optical system 23 comprises an image projector 24. The image projector 24 is designed to emit electromagnetic radiation comprising at least the two different wavelengths 6, 7 or white light and to project electromagnetic radiation on the holographic medium 2. The image projector 24 can be designed to emit white light if the holographic medium 2 is provided with at least one deflection hologram 13, for example.

[0080] The image projector 24 can be designed to orient light sources which are designed to generate electromagnetic radiation comprising at least the two different wavelengths 6, 7 such that a maximum diffraction efficiency is reached for each wavelength at the hologram 14. This feature of the image projector 24 can be omitted, e.g., if the holographic medium 2 is provided with at least one deflection hologram 13 but may also be combined with at least one deflection hologram 13.

[0081] In addition, the optical system 23 can comprise a lightguide 25 arranged between the image projector 24 and the holographic splitter 1 which is designed to guide electromagnetic radiation emitted by the image projector 24 onto the holographic medium 2. The optical system 23 can also comprise further elements such as a frame designed to support the image projector 24 and the holographic splitter 1, depending on the application of the optical system 23.

[0082] The invention has been illustrated and described in detail with the aid of the preferred exemplary embodiments. Nevertheless, the invention is not restricted to the examples disclosed. Rather, other variants may be derived therefrom by a person skilled in the art without departing from the protective scope of the invention. REFERENCE SYMBOLS holographic splitter / di f fuser holographic medium optical combiner incident electromagnetic radiation split electromagnetic radiation first wavelength second wavelength holographic pupil first arrangement of pupils second arrangement of pupils third arrangement of pupils aperture deflection hologram polydi f f ractive hologram surface of the holographic medium first deflection hologram second deflection hologram third deflection hologram fourth deflection hologram fourth arrangement of pupils fifth arrangement of pupils sixth arrangement of pupils optical system image proj ector lightguide

Claims

CLAIMS1. Holographic medium (2) comprising a hologram (14) designed to reconstruct a holographic image by illuminating the hologram (14) with electromagnetic radiation (4) comprising at least two different wavelengths (6, 7) , wherein the hologram (14) is formed by an interference pattern recorded by illumination of the holographic medium (2) with electromagnetic radiation comprising an intermediate wavelength, wherein the intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two different wavelengths (6, 7) .

2. Holographic (2) medium according to claim 1, comprising a further hologram designed to reconstruct the holographic image by illuminating the further hologram with electromagnetic radiation (4) comprising a further wavelength, wherein the further hologram is formed by a further interference pattern recorded by illumination of the holographic medium (2) with electromagnetic radiation comprising the further wavelength.

3. Holographic medium (2) according to claim 1 or 2, comprising an additional hologram designed to reconstruct the holographic image by illuminating the additional hologram with electromagnetic radiation (4) comprising at least two different additional wavelengths, wherein the additional hologram is formed by an additional interference pattern recorded by illumination of the holographic medium (2) with electromagnetic radiation comprising an additional intermediate wavelength, wherein the additional intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two different additional wavelengths.

4. Holographic medium (2) according to one of the preceding claims , wherein the intermediate wavelength and / or the additional intermediate wavelength is an average of the at least two different wavelengths (6, 7) and / or the at least two different additional wavelengths.

5. Holographic medium (2) according to one of the preceding claims , wherein a first wavelength (6) of the at least two different wavelengths (6, 7) corresponds to light from the blue spectral range, wherein a second wavelength (7) of the at least two different wavelengths (6, 7) corresponds to light from the green spectral range, wherein the intermediate wavelength corresponds to teal light .

6. Holographic medium (2) according to one of the claims 2 to5, wherein the further wavelength corresponds to light from the red spectral range.

7. Holographic medium (2) according to one of the claims 2 to6, wherein a first additional wavelength and a second additional wavelength of the at least two different additional wavelengths correspond to light from the red spectral range, respectively.

8. Holographic medium (2) according to one of the preceding claims , comprising a deflection hologram (13) designed to deflect electromagnetic radiation (4) such that a Bragg-condition is fulfilled for the deflected electromagnetic radiation impinging on the hologram (14) .

9. Holographic medium (2) according to one of the preceding claims 2 to 8, comprising a further deflection hologram (13) designed to deflect electromagnetic radiation (4) such that a Bragg- condition is fulfilled for the deflected electromagnetic radiation impinging on the further or the additional hologram.

10. Holographic splitter (1) comprising a holographic medium (2) according to one of the preceding claims and an optical combiner (3) , wherein the holographic medium (2) is designed to split electromagnetic radiation according to a Bragg-condition, wherein the optical combiner (3) is designed to reflect the split electromagnetic radiation to form collimated electromagnetic radiation.

11. Optical System (23) comprising a holographic splitter (1) according to claim 10 and an image projector (24) , wherein the image projector (24) is designed to emit electromagnetic radiation (4) comprising at least the two different wavelengths (6, 7) or white light and to project electromagnetic radiation (4) on the holographic medium (2) .

12. Optical system (23) according to claim 11, wherein the image projector (24) is designed to orient light sources which are designed to generate electromagnetic radiation (4) comprising the at least the two different wavelengths (6, 7) such that a Bragg-condition is fulfilled for each wavelength (6, 7) at the hologram (14) .

13. Optical system (23) according to claim 11 or 12, comprising a lightguide (25) arranged between the image projector (24) and the holographic splitter (1) and designed to guide electromagnetic radiation (4) emitted by the image projector (24) onto the holographic medium (2) .

14. Method of reconstructing a holographic image comprising the following step:- illuminating a holographic medium (2) comprising a hologram (14) with electromagnetic radiation (4) comprising at least two different wavelengths (6, 7) , wherein the hologram (14) is formed by an interference pattern recorded by illumination of the holographic medium (2) with electromagnetic radiation comprising an intermediate wavelength, wherein the intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of the at least two different wavelengths (6, 7) .

15. Method of recording a hologram (14) comprising the following step:- recording an interference pattern by illuminating a holographic medium (2) with electromagnetic radiation comprising an intermediate wavelength, wherein the intermediate wavelength comprises a value between a minimum wavelength and a maximum wavelength of at least two different wavelengths (6, 7) , wherein the hologram (14) is designed to reconstruct a holographic image by illuminating the hologram (14) with electromagnetic radiation (4) comprising the at least two different wavelengths (6, 7) .

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