Device for diffracting light

Abstract

The invention relates to a device which has at least one lighting device, at least one color-selective wave plate, and at least two diffracting devices. The lighting device transmits polarized light of different wavelengths. A polarization rotation for only some of the different wavelengths of the light is provided by means of the at least one color-selective wave plate, whereby light with one wavelength can pass through at least one of the diffracting devices with a different polarization state than the polarization state of a different wavelength of the light. Diffracting structures are written into the at least two diffracting devices such that the light can be diffracted at the same angle for the different wavelengths of the light by combining the at least two diffracting devices.

Description

[0001] Device for diffracting light

[0002] The present invention relates to a device for diffracting light, which can preferably be used in a display device for the holographic reconstruction of objects or scenes. In particular, the present invention relates to a device for diffracting light, by means of which light can be deflected or directed to defined locations in space. The use of such devices in display devices could primarily be found in mobile applications such as smartphones, tablet computers, monitors, or televisions, i.e., in the direct-view range, but also in HMD displays or HUD displays, i.e., in projection devices.

[0003] Furthermore, the present invention also relates to a display device, in particular a holographic display device, comprising such a device according to the invention for diffracting light. The display device is intended for generating a two-dimensional and / or three-dimensional representation of a scene, content, or object.

[0004] From the prior art, such as US 2014 / 0055692 A1 , a diffraction device is known which has a controllably variable diffraction structure, as well as a display device with at least one spatial light modulator (SLM) and at least one such diffraction device, in which the light wave field modulated by the spatial light modulator can be diffracted in a predeterminable manner by means of the diffraction device.

[0005] The display device can, for example, be designed as a holographic display device with a virtual viewer window and comprise: at least one light source, at least one spatial light modulator, at least one diffraction device, and at least one position detection system that detects the eye positions of a viewer. A controllably variable diffraction structure is inscribed in the diffraction device such that a light wave field modulated by the spatial light modulator is diffracted toward the detected eye positions by means of the diffraction structure.

[0006] A diffraction device can be embodied as a liquid crystal device and, in this case, is also referred to below as a liquid crystal grating (LCG). By electrically controlling the liquid crystal device or liquid crystal grating, a diffraction structure can be inscribed therein. A liquid crystal grating typically comprises two substrates with electrodes and a liquid crystal layer between them. On at least one of the substrates, the electrodes have a linear structure. By applying electrical signals to the electrodes, a diffraction structure is generated in the liquid crystal layer. The deflection of light then occurs perpendicular to the electrode lines. Various types of diffraction structures are known that can be generated using a liquid crystal grating, depending on the liquid crystal configuration used and the polarization state of the light.Diffractive phase gratings can, for example, be based on sawtooth structures when using linearly polarized light. They can also be polarization gratings, preferably Pancharatnam-Berry phase gratings, which operate under circularly polarized light.

[0007] Typically, a diffraction structure produces a diffraction pattern that depends on the wavelength of the diffracted light. For example, if a diffraction grating is inscribed in a liquid crystal lattice, the diffraction angle of the liquid crystal lattice is approximately proportional to the quotient of wavelength A and the grating period g.

[0008] As is well known, diffraction at a grating occurs according to the equation: sin a_out = A / g + sin a_in, where A is the wavelength of the light, g is the grating constant, and a_in and a_out are the angles at which the light hits and exits the grating. The diffraction angle is then the difference between a_out - a_in.

[0009] The diffraction angle can be changed by controllably varying the grating period g in the diffraction device. However, to set the same diffraction angle for multiple wavelengths of light, different grating periods would have to be written into the diffraction device for these wavelengths, so that the ratio A / g is the same for the different wavelengths.

[0010] In a display device, light of different wavelengths is often mixed spatially or temporally to produce a colorful representation of a scene. For example, the display device may have red, green, and blue light sources in an illumination device.

[0011] In order to diffract the light wave field modulated by a spatial light modulator in a predeterminable manner for different wavelengths of light, it is then necessary to inscribe different diffraction structures one after the other in the diffraction device. This requires the diffraction device to be operated at a high frame rate. A holographic direct-view display device can, for example, have at least one red, green and blue light source, e.g. a laser each, at least one spatial light modulator and at least one liquid crystal grating. If, for example, content is shown in the display device at a frame rate of 60 Hz and a virtual viewer window is generated one after the other for the left and right eye of a viewer, the at least one spatial light modulator can also have color filters in order to display red, green and blue content spatially next to one another. This can then be operated at 120 Hz, for example.i.e. 2 x 60 Hz. However, the at least one liquid crystal grating would have to be operated three times as fast as the at least one spatial light modulator, i.e. at 360 Hz, in order to successively diffract red, green and blue light towards the detected positions of the left and right eyes of the observer.

[0012] A spatial light modulator typically has color filter strips. Only for a liquid crystal grating whose line electrodes run parallel to the color filter strips of the spatial light modulator would a spatial subdivision of colors also be possible with color filter strips, as described in US 2014 / 0055692 A1. However, this would also require precise alignment of the color filter strips on the liquid crystal grating with the color filter strips on the spatial light modulator. For a liquid crystal grating whose line electrodes run at an angle to the color filters of the spatial light modulator, such a spatial subdivision of colors on the liquid crystal grating is not possible without disadvantages.

[0013] However, controlling a liquid crystal grating with a frame rate of 360 Hz is not without its disadvantages. With a liquid crystal grating, the time for a frame is generally divided into a time interval for writing the electrical signals, a time interval for the electrical signals to propagate along the line electrodes, a time interval for the liquid crystals to react to the applied electrical signals, during which the diffraction structure is formed by orienting the liquid crystals, and only when this has happened is there a time interval in which the light source can be switched on and light is actually diffracted by the diffraction structure of the liquid crystal grating. A high frame rate is possible, but it also limits the optical efficiency of the system because the duty cycle, i.e. the relative proportion of the frame in which the light source can be switched on, is reduced.

[0014] A holographic projection device, for example a holographic head-up display, also has at least one light source, at least one spatial light modulator, and at least one liquid crystal grating. Microdisplays, such as LCOS (Liquid Crystal on Silicon), can also be used as the spatial light modulator. For example, a liquid crystal grating can be used in an image plane of the spatial light modulator. An LCOS generally does not have color filters and must therefore also display colored content sequentially, thus requiring the same frame rate as a liquid crystal grating. However, as a reflective display, an LCOS has a thinner liquid crystal layer than a transmissive liquid crystal grating.Due to the known dependence of the liquid crystal switching time on the square of the liquid crystal layer thickness, a high frame rate is easier to achieve in a reflective LCOS than in a transmissive liquid crystal grating. In general, liquid crystal gratings can also be reflective. However, this complicates the combination of multiple liquid crystal gratings, such as those required for a combination of horizontal and vertical diffraction. In a typical arrangement, a reflective spatial light modulator would be combined with a transmissive liquid crystal grating.

[0015] Therefore, it is an object of the present invention to provide a device with which the frame rate of a diffraction device can be reduced when using multiple light wavelengths, so that not all light wavelengths have to be diffracted individually one after the other by means of the diffraction device.

[0016] According to the invention, this object is achieved by a device having the features according to claim 1.

[0017] The device according to the invention comprises an illumination device that emits polarized light of different wavelengths, at least one color-selective waveplate, and at least two diffraction devices. By means of the at least one color-selective waveplate, polarization rotation is provided only for a portion of the different wavelengths of light, whereby at least one of the diffraction devices can be passed through by light of one wavelength with a different polarization state compared to the polarization state of another wavelength of light. Diffraction structures are inscribed in the at least two diffraction devices such that, for the different wavelengths of light, the light can be diffracted at the same angle by combining the at least two diffraction devices.With the diffraction structure, the light wave field modified by a spatial light modulation device or, more generally, the light emitted by the illumination device can be diffracted in a predeterminable manner.

[0018] This utilizes the fact that the modulation of light in a diffraction device, for example, in a liquid crystal grating (LCG), is polarization-dependent. Diffraction structures in a diffraction device, preferably a liquid crystal grating, diffract light differently for different polarization states. Liquid crystal gratings based on diffracting sawtooth grating structures diffract light only for a linear polarization state, while light of a perpendicular linear polarization state propagates undiffracted through the liquid crystal grating. Liquid crystal gratings based on Pancharatnam-Berry phase gratings diffract light differently depending on whether it is right- or left-circularly polarized.

[0019] On the other hand, there are color-selective waveplates that can change the polarization state of light for a certain wavelength range of light, while leaving the polarization state unchanged for another wavelength range.

[0020] By using a combination of at least one color-selective waveplate and at least two diffraction devices, preferably in the form of liquid crystal gratings, the polarization for the illumination device, which emits light of different wavelengths, can be rotated and the diffraction structures in the at least two diffraction devices can be adjusted such that the same diffraction angle can be set for different wavelengths of light. The device according to the invention, comprising a combination of at least two diffraction devices and at least one color-selective waveplate, can thus diffract light of multiple wavelengths simultaneously into the same diffraction angle. The at least two diffraction devices can thus then be operated particularly advantageously at a lower frame rate. In this way, it is possible that not all wavelengths of light have to be diffracted individually one after the other.An efficient approach to diffraction of different wavelengths can thus be achieved.

[0021] It is important that the polarization of the light is not rotated in the same way for all wavelengths using at least one color-selective waveplate.

[0022] Further advantageous embodiments and developments of the invention emerge from the subclaims.

[0023] In an advantageous embodiment of the invention, the illumination device can have at least two light sources with at least two different wavelengths or at least one light source with at least two different wavelengths. In a further advantageous embodiment of the invention, the different wavelengths can differ by at least 40 nm.

[0024] According to the invention, an illumination device of the device comprises at least two light sources or at least one light source with at least two different wavelengths that differ by at least 40 nm. Alternatively, the illumination device comprises light sources that emit at least two different wavelengths that differ significantly in color, in a preferred embodiment, differing by at least 40 nm.

[0025] The device according to the invention thus has an illumination device of the type specified above, at least one color-selective wave plate and at least two diffraction devices, wherein by means of the at least one color-selective wave plate a polarization rotation takes place only for some of the light sources, so that at least one of the diffraction devices is passed through by light of one wavelength with a different polarization state compared to the polarization state of another wavelength and diffraction structures are written into the at least two diffraction devices in such a way that for the different wavelengths of the light, light is diffracted at the same angle by the combination of the at least two diffraction devices.

[0026] Furthermore, it can advantageously be provided that the at least two diffraction devices each comprise two substrates between which a liquid crystal layer is provided, wherein a plurality of liquid crystal cells of the liquid crystal layer form a diffractive structure by applying an electric field. The at least two diffraction devices are designed to be controllable.

[0027] In terms of construction, the at least two diffraction devices have essentially linear electrodes that are arranged essentially parallel to one another and are arranged on a first substrate. The electrodes could thus be strip-shaped. The respective diffraction device has a second substrate that is arranged at a distance from the first substrate. The second substrate could have a planar electrode or a plurality of essentially linear electrodes that are arranged essentially parallel to one another. If the second substrate has a plurality of essentially linear electrodes that are arranged essentially parallel to one another, these electrodes could be arranged essentially opposite the linear electrodes of the first substrate or with a predeterminable lateral offset. The alignment of the electrodes of both substrates would be essentially parallel.The electrodes provided on the substrates can be individually electrically controlled in order to create a predetermined electric field between the two substrates. A material is arranged between the first and the second substrate, with which local changes in the refractive index for at least one polarization direction of the light can be achieved by applying voltages to the electrodes and thus by an electric field. The material could therefore comprise liquid crystals, thus representing a liquid crystal layer having a large number of liquid crystal cells. This layer is birefringent, so that the liquid crystal cells can be aligned in their spatial orientation by an electric field. In other words, the alignment of the liquid crystals can be influenced by applying a predetermined electrical voltage to the electrodes.

[0028] It may be advantageous if the diffractive structure of the at least two diffraction devices is designed as a sawtooth-shaped phase profile and for the at least two diffraction devices to be aligned parallel to one another.

[0029] In a further advantageous embodiment of the invention, it can be provided that a first color-selective wave plate is provided between a first diffraction device and a second diffraction device, wherein the color-selective wave plate changes the polarization of the incident light only for a defined wavelength.

[0030] A first color-selective waveplate is provided or arranged in the light direction between the first and second diffraction devices. This color-selective waveplate rotates the polarization of the light by 90 degrees for only one wavelength, e.g., only red light, while it remains unchanged for the other wavelengths of light, e.g., green and blue light. The wavelength of light whose polarization has been changed or rotated then strikes the second diffraction device with a polarization perpendicular to the alignment direction of the liquid crystals and is therefore not diffracted by this diffraction device because this wavelength exhibits the polarization. The second diffraction device thus only diffracts light of the other wavelengths.

[0031] Advantageously, it is thus provided that the first diffraction device is designed to diffract light of all wavelengths, wherein the second diffraction device is designed to diffract light of at least one wavelength only.

[0032] In a particularly advantageous embodiment of the invention, it can be provided that three diffraction devices are provided, wherein a first color-selective wave plate is provided between the first and the second diffraction device and a second color-selective wave plate is provided between the second and third diffraction device.

[0033] It is advantageous if the first color-selective waveplate changes the polarization of the incident light only for a defined wavelength and the second color-selective waveplate changes the polarization of the incident light only for a defined wavelength that differs from the wavelength for which the first color-selective waveplate changes the polarization of the light.

[0034] In this embodiment, the first diffraction device acts as already described above, in that this diffraction device diffracts light of all three wavelengths, so that the first color-selective waveplate changes the polarization of the light by 90 degrees for only one wavelength, e.g. only for red light, while it remains unchanged for the other wavelengths of light, e.g. green and blue light. The wavelength of light whose polarization has been changed or rotated then strikes the second diffraction device with a polarization perpendicular to the alignment direction of the liquid crystals and is therefore not diffracted by this diffraction device because this wavelength has the polarization. The second diffraction device therefore only diffracts light of the other wavelengths. A second color-selective waveplate is now provided or arranged between the second diffraction device and the third diffraction device.which rotates the polarization of only one wavelength, e.g., green light, by 90°. For the third diffraction device, two wavelengths, e.g., green and red light, thus have a polarization perpendicular to the alignment direction of the liquid crystals in the liquid crystal layer of the diffraction device. These two wavelengths, green and red light, are therefore not diffracted by the third diffraction device.

[0035] In one embodiment of the invention, each individual diffraction device of the at least two diffraction devices is designed to diffract only light of one wavelength.

[0036] The advantage here is that only one of the diffraction devices needs to diffract light into large diffraction angles, which requires diffraction structures with fine resolution, i.e., a small grating period. The other two diffraction devices only perform corrections to the diffraction angle for the light sources of the other wavelengths, which requires diffraction structures with coarser resolution, i.e., a longer grating period, which generally results in better diffraction efficiency.

[0037] In a further advantageous embodiment of the invention, three diffraction devices can be provided, wherein a first color-selective waveplate is arranged in front of a first diffraction device in the direction of light, wherein a second color-selective waveplate is arranged between the first and a second diffraction device, and wherein a third waveplate is arranged between the second and a third diffraction device in order to change the direction of polarization of the light, whereby only light of one wavelength can be diffracted by each individual diffraction device. Light with a defined polarization falls from the illumination device onto a first color-selective waveplate. This first color-selective waveplate rotates the polarization only for one wavelength of the light. A first subsequent diffraction device then diffracts only light of this wavelength.A subsequent second color-selective waveplate rotates the polarization for this wavelength, e.g., for red light, and for a further second wavelength, e.g., for green light, but not for a third wavelength, e.g., for blue light. A subsequent second diffraction device then only diffracts light of the further wavelength, i.e., green light. The diffraction structures are adjusted such that the diffraction angle of the second diffraction device for the further wavelength, i.e., green, corresponds to the diffraction angle of the first diffraction device for a first wavelength, i.e., red. A subsequent third color-selective waveplate then rotates the polarization of the light of the second wavelength and the third wavelength, i.e., green and blue light, but leaves the polarization for light of the first wavelength, i.e., red light, unchanged. The third diffraction device then only diffracts light of the third wavelength, i.e., blue light.The diffraction structure in the third diffraction device is therefore chosen such that the diffraction angle for the third wavelength, ie blue light, corresponds to the diffraction angle of the first two diffraction devices for the first and second wavelengths, ie red and green light.

[0038] The concept of diffracting only light of a defined wavelength with each diffraction device makes it possible, in a particularly simple manner, to achieve a time-staggered control of the individual diffraction devices and the illumination device.

[0039] Advantageously, it can be provided that a third or fourth color-selective wave plate is provided, which is arranged after the diffraction device provided last in the light direction.

[0040] Optionally, a third or fourth color-selective waveplate can be provided, which rotates or changes the polarization of the light only for the third wavelength of light, e.g., blue light. Thus, after passing through the device according to the invention, light of three wavelengths is present with the same diffraction angle and the same polarization state. The same polarization state for all wavelengths of light after passing through the device is particularly useful if the described device is to be combined with other elements that are also polarization-selective.

[0041] Advantageously, the diffraction structures are inscribed in the at least two diffraction devices sequentially or simultaneously. In an advantageous embodiment of the invention, it can be provided that a light source of a corresponding wavelength can only be switched on when the liquid crystals of the diffraction device diffracting light of this wavelength are aligned according to the applied field.

[0042] In a further advantageous embodiment of the invention, it can be provided that four diffraction devices are provided, wherein in the light direction an additive diffraction of all wavelengths is provided with the first and second diffraction devices and with the third and fourth diffraction devices the diffraction angle of the first and second diffraction devices can be corrected for defined wavelengths.

[0043] In order to increase the diffraction angle, a device according to the invention with four diffraction devices is also possible, in which, for example, an additive diffraction of all three wavelengths RGB is carried out with the first two diffraction devices, and the diffraction angle of the first two diffraction devices is corrected for, for example, the wavelengths green and blue with the other two diffraction devices.

[0044] The object of the invention is further achieved by a display device having the features of claim 17.

[0045] For this purpose, the display device according to the invention comprises at least one spatial light modulation device, a control device and at least one device according to the invention according to claim 1.

[0046] Advantageously, the control device can be used to switch the lighting device on and off in synchronization with a control of the at least one spatial light modulation device and with a control of the at least two diffraction devices.

[0047] There are now various possibilities for advantageously embodying the teaching of the present invention and / or combining the described embodiments or configurations with one another, where possible. For this purpose, reference is made, on the one hand, to the patent claims subordinate to the independent claims and, on the other hand, to the following explanation of the preferred embodiments of the invention with reference to the drawings, which also generally explain preferred embodiments of the teaching. The invention is explained in principle with reference to the described embodiments, but is not intended to be limited to them.

[0048] The figures show: Fig. 1 : in a principle representation a color-selective wave plate according to the

[0049] State of the art;

[0050] Fig. 2a: in a schematic representation of a diffraction device with a defined

[0051] State-of-the-art orientation of liquid crystals;

[0052] Fig. 2b: a graphic representation of a diffraction structure as a phase profile for two different polarization directions of the light;

[0053] Fig. 3a: a schematic representation of a device according to the invention with three

[0054] Diffraction devices, in plan view;

[0055] Fig. 3b shows a schematic representation of a device according to the invention with two diffraction devices in plan view;

[0056] Fig. 4: a schematic representation of an alternative embodiment of the device according to the invention in plan view;

[0057] Fig. 5: a schematic representation of a spatial light modulation device in a side view;

[0058] Fig. 6: a schematic representation of a diffraction device with vertical

[0059] Line electrodes;

[0060] Fig. 7: a schematic representation of the writing of data into a spatial

[0061] Light modulation device in conjunction with the operation of a diffraction device and an illumination device according to the prior art;

[0062] Fig. 8: a schematic representation of the writing of data into a spatial

[0063] Light modulation device in conjunction with the operation of a diffraction device and an illumination device, wherein each individual diffraction device only diffracts light of one wavelength;

[0064] Fig. 9: a schematic representation of the writing of data into a spatial

[0065] Light modulation device in conjunction with the operation of a diffraction device and an illumination device according to Fig. 8, wherein the spatial light modulation device is controlled at twice the frame rate;

[0066] Fig. 10: a schematic representation of writing data into a spatial

[0067] Light modulation device in connection with the operation of a diffraction device and an illumination device, wherein the spatial light modulation device is designed as an LCoS;

[0068] Fig. 11: a schematic representation of a further alternative embodiment of the device according to the invention in plan view;

[0069] Fig. 12: a schematic representation of the writing of data into a spatial

[0070] Light modulation device in conjunction with the operation of a diffraction device and an illumination device according to Fig. 11;

[0071] Fig. 13: a schematic representation of a further alternative embodiment of the device according to the invention, which has four diffraction devices, in plan view;

[0072] Fig. 14: a schematic representation of a diffraction device with

[0073] Line electrodes in which diffraction structures are inscribed;

[0074] Fig. 15: a schematic representation of more than two diffraction devices for a defined diffraction of light;

[0075] Fig. 16: a schematic representation of a further alternative embodiment of the device according to the invention with four diffraction devices, in plan view;

[0076] Fig. 17: a schematic representation of a further alternative embodiment of the device according to the invention, which has three diffraction devices, for circularly polarized light, in plan view;

[0077] It should be briefly mentioned that identical elements / parts / components may also have the same reference numerals in the figures. Figure 1 schematically shows a color-selective waveplate WP according to the prior art.

[0078] For light with vertical polarization incident on the waveplate WP, the polarization for green light, which is located in the center of the three light beams here, is rotated by 90° from vertical to horizontal, while the polarization of the red and blue light beams, which frame the green light beam, remains unchanged. The right part of Fig. 1 shows the wavelength dependence of the light. There are transition regions from approximately 490 nm to 510 nm and from approximately 580 nm to 610 nm, for which a partial rotation of the polarization would occur. For a holographic display device for displaying two-dimensional and / or three-dimensional scenes that uses narrowband laser light sources, for example with wavelengths of 450 nm, 532 nm, or 640 nm, a good separation of the polarization states of red, green, and blue light is possible with such a color-selective waveplate.

[0079] Fig. 2 shows the result of a liquid crystal simulation for a diffraction device designed as a liquid crystal grating with ECB (electrically controlled birefringence) mode. However, this serves only as an example. The invention is not intended to be limited to the use of this liquid crystal mode.

[0080] Figures 2a and 2b show a diffraction device comprising a lower substrate S1 with line electrodes LE. A section in a plane perpendicular to these line electrodes LE is shown. The diffraction device also comprises an upper substrate S2 provided with a planar electrode FE. A liquid crystal layer LCL comprising a plurality of liquid crystal cells LC is provided between the two substrates S1 and S2. A diffraction structure is created by applying voltages or an electric field to the individual line electrodes LE and the planar electrode FE and aligning the liquid crystals LC in response to the applied voltage.

[0081] Fig. 2a shows the orientation of the liquid crystals LC according to the prior art, whereby Fig. 2b shows the diffraction structure resulting from the orientation of the liquid crystals LC as a phase profile for two different polarization directions of the light. Only for the polarization of the light parallel to the alignment of the liquid crystals LC in the liquid crystal layer LCL without an applied voltage, in this case parallel to the image plane of Fig. 2a, is a sawtooth-shaped phase profile present and light is diffracted. This is shown by the grey curve in Fig. 2b. For the polarization perpendicular to this, i.e. perpendicular to the image plane of Fig. 2a, the phase is constant and independent of the orientation of the liquid crystals LC. This is shown by the black curve in Fig. 2b. Incident light with this polarization would therefore pass through the diffraction device undiffracted. In Fig.Figure 3a shows a top view of a device according to the invention, which has three diffraction devices LCG 1 , LCG 2 and LCG 3 , an illumination device BD for emitting colored light and three color-selective wave plates WP 1 , WP 2 and WP 3 . The diffraction device can preferably be designed as a liquid crystal grating. For this reason, the descriptions of the figures will also assume a liquid crystal grating, although this is of course not limited to such a diffraction device, but other designs are also possible. The illumination device BD can have a light source that emits at least two wavelengths, preferably three wavelengths RGB, or at least two light sources, preferably three light sources, one light source each for red light, green light and blue light. The illumination device BD will be discussed in more detail later.A first diffraction device LCG 1 is followed in the beam direction by the following elements: a first color-selective wave plate WP 1 , a second diffraction device LCG 2 , a second color-selective wave plate WP 2 and a third diffraction device LCG 3 .

[0082] In this example, the diffraction structure in the respective diffraction device LCG 1 , LCG 2 , and LCG 3 is designed as a sawtooth grating with an alignment of the liquid crystal cells LC or liquid crystal molecules horizontally, here in the plane of the figure, and parallel to one another for all three diffraction devices LCG 1 , LCG 2 , and LCG 3 shown. In this embodiment of the invention, the input polarization of the light for all three primary colors RGB or for the three wavelengths of the primary colors RGB is also horizontal, thus parallel to the alignment of the liquid crystal molecules. For better illustration, the line electrodes LE are arranged vertically, i.e., perpendicular to the plane of Fig. 3a.

[0083] The alignment of the liquid crystals LC in the liquid crystal layer LCL can also assume other angles relative to the electrodes LE and FE, for example, 80°. However, what matters is whether the polarization of the light is parallel or perpendicular to the alignment direction of the liquid crystals LC, not the orientation of the polarization relative to the electrodes LE and FE.

[0084] By means of a control device S, switching on and off of the lighting device BD is synchronized with a control of a spatial light modulation device SLM, which is only indicated here, and with a control of the diffraction devices LCG 1 , LCG 2 and LCG 3 .

[0085] Because the polarization of the light for all three wavelengths RGB emitted by the illumination device BD is parallel to the alignment direction of the liquid crystals LC, in this embodiment of the invention according to Fig. 3a, the first diffraction device LCG 1 diffracts light of all three wavelengths RGB. The diffraction angle is proportional to the wavelength, i.e., greatest for light of the red wavelength and smallest for light of the blue wavelength. A first color-selective waveplate WP 1 is arranged between the first diffraction device and the second diffraction device. This first color-selective waveplate WP 1 rotates the polarization of the light by 90 degrees only for light of the red wavelength, while the polarization for light of the green and blue wavelengths remains unchanged.Light of the red wavelength hits a second diffraction device (LCG 2) with a polarization perpendicular to the alignment direction of the liquid crystals (LC) and is therefore not diffracted by it. The second diffraction device (LCG 2) therefore only diffracts light of the green and blue wavelengths.

[0086] A diffraction structure is inscribed in the second diffraction device LCG 2 such that the diffraction angle for light of the green wavelength after passing through the second diffraction device LCG 2 corresponds to the diffraction angle for light of the red wavelength after passing through the first diffraction device LCG 1. The second diffraction device LCG 2 thus corrects the difference angle between light of the green and red wavelengths from the diffraction at the first diffraction device LCG 1.

[0087] A second color-selective waveplate WP 2 is arranged between the second diffraction device LCG 2 and a third diffraction device LCG 3, which rotates only the polarization of light of the green wavelength by 90°. For the third diffraction device LCG 3, light of the green and red wavelengths thus has a polarization that is perpendicular to the alignment direction of the liquid crystals LC. The light of the green and red wavelengths is therefore not diffracted by the third diffraction device LCG 3.

[0088] A diffraction structure is inscribed in the third diffraction device LCG 3 such that the diffraction angle for light of the blue wavelength after passing through the third diffraction device LCG 3 corresponds to the diffraction angle for light of the red and green wavelengths after the first two diffraction devices LCG 1 and LCG 2. With the combination of the three diffraction devices LCG 1, LCG 2 and LCG 3 and the two color-selective wave plates WP 1 and WP 2, light of the red, green and blue wavelengths is diffracted into the same diffraction angle.

[0089] In the light direction downstream of the third diffraction device LCG 3, the light of the blue wavelength exhibits a different polarization than the light of the red and green wavelengths. Therefore, a third color-selective waveplate WP 3 can optionally be used, which rotates the polarization only for the light of the blue wavelength and is therefore arranged downstream of the third diffraction device LCG 3 in the light direction. Thus, after passing through the device, light of three wavelengths RGB with the same diffraction angle and the same polarization state is present. This is particularly useful if the described device is to be combined with other elements that are also polarization-selective.

[0090] As is known, the diffraction of light at a grating occurs according to the equation: sin a_out = A / g + sin a_in, where A is the wavelength of the light, g is the grating constant and a_in and a_out are the angles at which the light hits or leaves the grating.

[0091] For small angles, the sine can also be approximated linearly: a_out = A / g + a_in

[0092] This serves to better illustrate the invention below. The invention is not limited to the linear approximation; the exact equation with sine can also be used.

[0093] For ease of illustration, a perpendicular incidence of light is also used below, ie, a_in = 0, although the invention is not intended to be limited to this case. In the case a_in = 0, a_out is the diffraction angle. In the general case, the difference a_out

[0094] - a_in the diffraction angle.

[0095] For a combination of several gratings or several diffraction devices with grating constants g1 , g2, g3, and if the light falls perpendicularly on the first grating, then: a_out = A ( 1 / g1 + 1 / g2 + 1 / g3).

[0096] The polarization dependence of gratings can be described by additional factors A1 , A2 A3, ie a_out = A (A1 / g1 + A2 / g2 + A3 / g3).

[0097] For a grating with a liquid crystal (LC) mode based on linear polarization, A is either 1 if the grating is active for that polarization or 0 if the grating is inactive. For a grating with an LC mode based on circular polarization, A is either 1 if the grating deflects light into the +1st diffraction order or -1 if the grating deflects light into the -1st diffraction order.

[0098] In this sense, in the embodiment according to Fig. 3, i.e. the first diffraction device LCG 1 is active for light of all three primary colors RGB, the second diffraction device LCG 2 is active for light of the green and blue wavelengths, and the third diffraction device LCG 3 is only active for light of the blue wavelength, the grating periods g1 g2 g3, in order to obtain the same diffraction angle, can be determined such that: a_out = A_red*(1 / g1) = A_green *(1 / g1 + 1 / g2) = A_blue* (1 / g1+1 / g2 + 1 / g3)

[0099] Different diffraction angles a_out for light of the red wavelength can be set by adjusting the grating period g1 in the first diffraction device LCG 1. g1 = A_red / a_out

[0100] Then, according to the above equation, the grating periods g2 and g3 for the second and third diffraction devices LCG 2 and LCG 3 are determined in order to obtain the same diffraction angle a_out for light of the green and blue wavelengths: e.g. g2 = g1 A_green / (A_red - A_green)

[0101] The ratio of the grating periods g2 / g1 and g3 / g1 remains unchanged and is determined by the wavelengths used.

[0102] In a single diffraction device, the diffraction angle can also be varied depending on the location on the diffraction device, for example, to inscribe a lens function instead of a prism. In this case, identical diffraction for different wavelengths of light can also be achieved with multiple diffraction devices by applying the equations described above to the local deflection angle a_out_local and determining the local grating periods g_out_local, g_out_local, and g_out_local from this.

[0103] The advantages of the device according to the invention shown in Fig. 3a are that only one of the diffraction devices needs to diffract light into large diffraction angles, which requires diffraction structures with fine resolution, i.e., a small grating period. The other two diffraction devices only perform corrections to the diffraction angle for the light of the other wavelengths, which requires diffraction structures with coarser resolution, i.e., a longer grating period, which allows for improved diffraction efficiency.

[0104] The embodiment according to Fig. 3a is shown and described here with an illumination device BD having three light sources (red, green, blue) and three diffraction devices, because these wavelengths of the primary colors RGB are frequently used for color representation in display devices.

[0105] In an analogous manner, ie with fewer or more elements or components, the device according to the invention would also function with only two wavelengths or light sources, e.g. only for red and green light, or with more than three wavelengths or light sources, e.g. for red, green, blue, yellow light.

[0106] Fig. 3b shows a device in which two diffraction devices LCG 1 and LCG 2 are provided, wherein a color-selective waveplate WP 1 is arranged between these two diffraction devices LCG 1 and LCG 2 in order to rotate or change the polarization of the light of one wavelength. The mode of operation in Fig. 3b corresponds to the mode of operation of the device according to Fig. 3a, but only with two diffraction devices and one color-selective waveplate, i.e. corresponds to the mode of operation of the first part of the device according to Fig. 3a, shown in the direction of light. In this case, only light of the red and green wavelengths is used and accordingly an illumination device BD only emits light of these wavelengths.

[0107] Figure 4 shows an alternative embodiment of the device according to the invention, in which each individual diffraction device diffracts light of only one wavelength. This embodiment has the advantage that the diffraction structures can be generated separately for each individual wavelength used.

[0108] From an illumination device BD, which here again has three light sources, light with a polarization that is perpendicular to the alignment direction of the liquid crystals of the liquid crystal layer of a first diffraction device LCG 1 falls onto a first color-selective waveplate WP 1. This color-selective waveplate WP 1 rotates the polarization only for light of the red wavelength. The first subsequent diffraction device LCG 1 then diffracts only light of this red wavelength. A subsequent second color-selective waveplate WP 2 now rotates the polarization for light of the red and green wavelengths, but not light of the blue wavelength. Since the polarization for light of the red wavelength is rotated by 90° for the second time, it is then again perpendicular to the alignment direction of the liquid crystals of the diffraction devices. The polarization for light of the blue wavelength has not yet been rotated and is still perpendicular to the alignment direction.The second diffraction device LCG 2, which now follows the second color-selective waveplate WP 2 in the light direction, then only diffracts light of the green wavelength. The diffraction structures of the second diffraction device LCG 2 are adjusted so that the diffraction angle of the second diffraction device LCG 2 for light of the green wavelength corresponds to the diffraction angle of the first diffraction device LCG 1 for light of the red wavelength. A third color-selective waveplate WP 3 following the second diffraction device LCG 2 now rotates the polarization of light of the green and blue wavelengths, but leaves the polarization for light of the red wavelength unchanged. The third diffraction device LCG 3 then only diffracts light of the blue wavelength.The diffraction structure in the third diffraction device LCG 3 is therefore selected so that the diffraction angle for light of the blue wavelength corresponds to the diffraction angle of the first two diffraction devices LCG 1 and LCG 2 for light of the red and green wavelengths. Optionally, a fourth color-selective waveplate WP 4 can subsequently rotate the polarization for light of the blue wavelength so that all colors of light have the same polarization at the device's output.

[0109] In this case, the three grating periods of the three diffraction devices LCG 1 , LCG 2 and LCG 3 are determined with the following equation: a_out = A_red*(1 / g1) = A_green *(1 / g2) = A_blue* (1 / g3)

[0110] By means of the device according to the invention, in which only light of one wavelength of light is diffracted with each diffraction device, it is also particularly easy to control the individual diffraction devices LCG 1, LCG 2 and LCG 3 and the illumination device BD at different times by means of a control device S.

[0111] In the following, a display device, for example a holographic display device for displaying two-dimensional and / or three-dimensional objects or scenes, with such a device according to the invention is described.

[0112] In one embodiment of a display device according to the invention, the device thus has at least one spatial light modulation device (hereinafter referred to as SLM), which is provided with color filters, and at least one device shown in Fig. 4, which has an illumination device, diffraction devices and color-selective wave plates, such that the modulation values ​​for the pixels of the SLM that are intended to emit light of the red, green and blue wavelengths are written into the at least one SLM with a time offset. This means that first the modulation values ​​for all pixels of one wavelength or color, for example red, are written into the SLM one after the other, followed by the modulation values ​​of the next color, for example green, and then the modulation values ​​of the third color, for example blue, are written into the SLM, wherein a light source of the illumination device is assigned to the respective previous wavelength or color.color is switched on to display a hologram of that color, e.g. red, while the pixels are written with modulation values ​​to emit the blue wavelength or color.

[0113] The diffraction structures for the three diffraction devices LCG 1, LCG 2, and LCG 3 are also written at different times, each for the same wavelength or color as for the SLM. For example, the diffraction structure for the LCG diffraction device, which is active for light of the red wavelength, is written simultaneously with the SLM pixels that are intended to emit the red wavelength, and the diffraction structure for the LCG diffraction device, which is active for light of the green wavelength, is written simultaneously with the SLM pixels that are intended to emit the green wavelength.

[0114] In a further embodiment of the invention, the display device has at least one SLM, for example an LCOS, for the color-sequential display of holograms and at least one device of diffraction devices and color-selective waveplates shown in Fig. 4. An illumination device of the device according to Fig. 4 has at least one red, green, and blue light source. Diffraction structures are or will be written into the diffraction devices, offset in time from the display on the SLM. A hologram of the red, green, and blue wavelength is thus written one after the other into the SLM and then displayed by switching on the associated light source of the illumination device. For example, while the hologram with the red wavelength is being written and displayed in the SLM, a diffraction structure is simultaneously written into the diffraction device that is active for light of the blue wavelength.In this way, for example, diffraction devices can be operated at 120 Hz to combine them with a color-sequential SLM operating at 360 Hz. This embodiment can also be easily adapted to a different number of light sources of different wavelengths, as few as two or even more than three wavelengths. This embodiment is not limited to the use of an LCOS as an SLM. Other types of SLM, such as phase-modulated MEMS, can also be used.

[0115] However, the invention is not limited to the embodiments described in detail above. Other combinations are also possible.

[0116] Below, the first three examples describe further possibilities for the detailed design of the first embodiment of a display device mentioned above, namely the combination of an SLM with color filters and diffraction devices. A fourth example describes a further possibility for the detailed design of the above-mentioned further embodiment of a display device, namely the combination of an SLM designed as an LCOS with diffraction devices.

[0117] An SLM can, for example, have horizontally arranged color filter strips. Fig. 5 shows an SLM from the front, i.e., from the viewpoint of a viewer of the display device. The SLM can be controlled by a control device S such that new content is written into the SLM line by line in a scan, and such that all lines of one color or wavelength are scanned sequentially, followed by all lines of a second color or wavelength and then a third color or wavelength.

[0118] One or more diffraction devices may, for example, comprise vertical line electrodes as shown in Fig. 6.

[0119] In a display device with such an SLM according to Fig. 5 and / or Fig. 6 and with diffraction devices according to the state of the art, the diffraction devices for the individual colors or wavelengths would have to be operated sequentially.

[0120] This is shown schematically in Fig. 7, with the time axis running horizontally. The individual bars shown describe how long the individual processes last for the different colors or wavelengths, i.e. how much time is required for the processes. In this case, in the SLM, all green pixels would be scanned first (left bar in medium gray), then all red pixels (following bar in dark gray), and then all blue pixels (following bar in light gray). A specific time interval is then required in which the liquid crystals in the SLM react to the written signal via the applied voltage by means of a control device S. This time interval only begins for the last line after the scan has ended. For the lines scanned first, the time interval begins earlier; however, for the sake of clarity, this is not shown separately in Fig. 7.

[0121] Since the SLM has color filters, for example, a green hologram can be displayed by switching the light source to emit the green wavelength, while at the same time new signals are written into the blue pixels of the SLM and the red pixels still respond to the previously written signals.

[0122] However, the diffraction structures for the individual colors or wavelengths would have to be written into the LCG diffraction device one after the other. Furthermore, the liquid crystals in the liquid crystal layer in the LCG diffraction order have less time to react to these signals than those in the SLM. Only when the liquid crystals for both the LCG diffraction device and the pixels of the respective color of the SLM have aligned according to the applied voltages via the control device S can the light source of that color or wavelength be switched on. The light sources of the individual colors or wavelengths are switched on only one after the other and only for a portion of the time.

[0123] Preferably, the signals for the LCG diffraction device for a specific color or wavelength are written at a time offset from the SLM. In the example shown in Fig. 7, the diffraction structures for the red wavelength are written into the LCG diffraction device simultaneously with the scanning of green pixels in the SLM. This is evident from the different gray levels displayed in the individual bars.

[0124] Fig. 8 now relates to a display device according to the invention in which an SLM, as shown schematically in Fig. 5, is combined with an arrangement of diffraction devices and color-selective waveplates according to Fig. 4, in which each individual diffraction device only diffracts light of one wavelength. In this case, the writing into the diffraction device LCG for the individual colors or wavelengths takes place synchronized with the scanning of these colors in the SLM. Since the diffraction devices LCG are only active for one wavelength, for example, the light source emitting the blue wavelength can also be switched on while new signals are being written into the diffraction device LCG for the deflection of light of the red wavelength and while the liquid crystals in the diffraction device LCG for light of the green wavelength are still aligning.

[0125] Overall, this provides more time for the alignment of the liquid crystals in the diffraction devices (LCG). Additionally, in this embodiment, as shown in Fig. 8, the light sources of the illumination device can be switched on for a longer time.

[0126] A further embodiment of the invention is shown in Fig. 9 and again relates to an SLM with an arrangement of color filters according to Fig. 5, but which is now electrically controlled by a control device S at twice the frame rate compared to Fig. 8, for example, 240 Hz, but in which the same content is written twice in succession, so that content is updated, for example, at 120 Hz. After the first scan for each color or wavelength, the time interval for the last line begins, in which the liquid crystals in the SLM can react to the written signal.

[0127] In one embodiment of the display device, this allows, for example, the combination of an SLM, as described in the previous section, with an arrangement of LCG diffraction devices as described in Fig. 3, in which the first diffraction device LCG 1 deflects all three colors RGB, the second diffraction device LCG 2 also deflects green and blue light, and the third diffraction device LCG 3 deflects only blue light. The diffraction devices LCG 1, LCG 2, and LCG 3 can be controlled at half the frame rate of the SLM, for example, at 120 Hz, if the SLM is operated at 240 Hz.

[0128] In this case, the diffraction devices LCG 1, LCG 2, and LCG 3 are all written simultaneously, and the liquid crystals within them align simultaneously. In this example, as shown in Fig. 9, light sources of multiple colors or wavelengths can be switched on simultaneously. The times for which the individual light sources are switched on can also differ for the colors or wavelengths. In the example, the switch-on time for the blue wavelength is shorter than for the red and green wavelengths. This arrangement also allows for relatively long response times of the liquid crystals, both in the SLM and in the diffraction device.

[0129] The embodiment according to Fig. 10 now shows a combination of an SLM designed as an LCOS with diffraction devices LCG 1 , LCG 2 and LCG 3 as in Fig. 4, in which each diffraction device LCG 1 , LCG 2 and LCG 3 deflects only light of one color or wavelength.

[0130] The SLM or LCOS operates at a high frame rate. Here, too, a line-by-line scan with the same content is performed multiple times, for example, three times for the same color or wavelength, but for the individual colors or wavelengths one after the other. The signals are written to the LCG diffraction device offset from the SLM. For example, signals for the diffraction of light at the green wavelength are written to the LCG diffraction device simultaneously with the red scan of the SLM, or signals for the diffraction of light at the blue wavelength are written simultaneously with the green scan of the SLM.

[0131] In an SLM, there is little time for the liquid crystals to react to the inscribed signal, whereas in the LCG diffraction devices, there is a lot of time for the liquid crystals to react to the inscribed diffraction structure. However, an SLM or LCoS as a reflective display typically has shorter liquid crystal response times than a transmissive display or SLM. Instead of an LCoS, other types of SLMs could also be used, such as MEMS, which can respond very quickly.

[0132] The invention is not intended to be limited to the aforementioned configurations of diffraction devices. For example, in a device comprising three diffraction devices and color-selective waveplates, each diffraction device can diffract light of two wavelengths, for example, the first diffraction device can diffract light of the red and blue wavelengths, the second diffraction device can diffract light of the green and blue wavelengths, and the third diffraction device can diffract light of the red and green wavelengths. Such a device is advantageous in that it allows the largest possible diffraction angles to be set with the smallest possible number of diffraction devices. Typically, the adjustable grating period for a single diffraction device has a lower limit. The maximum diffraction angle of a single diffraction device then corresponds to a_out = A / g_min, where g_min is the minimum grating period.

[0133] Advantageously, compared to other embodiments described above with the same number of diffraction devices, i.e., with three diffraction devices, the set diffraction angle can be increased if each diffraction device diffracts light of two wavelengths. Furthermore, the inscription of diffraction profiles or diffraction structures into the diffraction devices can be advantageously combined with an SLM with color filters, in which the individual colors are scanned sequentially.

[0134] It is then: a_out = A_red*(1 / g1+1 / g2) = A_green *(1 / g1 + 1 / g3) = A_blue* (1 / g2+1 / g3)

[0135] From this equation the grating periods g1 , g2 and g3 can be calculated for an angle a_out.

[0136] Fig. 11 schematically shows a device with three diffraction devices LCG 1 , LCG 2 and LCG 3 and four color-selective waveplates WP 1 , WP 2 , WP 3 and WP 4 . In this case, a first diffraction device LCG 1 diffracts light of the red and blue wavelengths. A second diffraction device LCG 2 diffracts light of the green and blue wavelengths. And a third diffraction device LCG 3 diffracts light of the green and red wavelengths. As shown schematically in Fig. 11, light of the blue wavelength can be diffracted twice at a large angle. In the other embodiments of the invention with three diffraction devices, there is either only a single diffraction device that diffracts light of the blue wavelength, as in Fig. 4, or the second and third diffraction devices only compensate for the difference in angles between the light of the blue and red wavelengths from the first diffraction device, as shown in Fig. 3.

[0137] In the device shown and described here in Fig. 11, the diffraction angle for light of the blue wavelength can be increased compared to a single diffraction device and also increased compared to the other described embodiments with three diffraction devices. In the example already mentioned above with the wavelengths of 450 nm, 532 nm and 640 nm used, 1.75 times the maximum diffraction angle of a single diffraction device can be achieved. This value 1.75 is obtained from the above equation by increasing the angle a_out and calculating the grating periods g1, g2 and g3 until one of the three grating periods assumes the minimum permissible value g_min.

[0138] For other wavelengths of red, green and blue light, the factor would be calculated as:

[0139] 2 * [ 1 / ( 1 / A_blue + 1 / A_green - 1 / A_red) ] / A_blue

[0140] Fig. 12 schematically shows a time sequence for a display device with an SLM with color filters and a device with diffraction devices according to Fig. 11.

[0141] As in Figures 7 to 10, the SLM first scans pixels that are to emit green light (light gray bar), then all pixels that are to emit red light (dark gray bar), and finally all pixels that are to emit blue light (medium gray bar). A specific time interval is then required for the liquid crystals in the SLM to react to the input signal, i.e., the applied voltage via a control device S. The light source of the respective color or wavelength of the illumination device is then switched on.

[0142] Since an LCG diffraction device diffracts light of two of the three colors or wavelengths from the primary colors RGB, but does not diffract the third color or wavelength, new diffraction structures can be inscribed into this LCG diffraction device. The liquid crystals in this LCG diffraction device react to the inscribed signal while either all light sources of the illumination device are turned off or the light source of the other, third color or wavelength is turned on.

[0143] For example, the diffraction structures are written into the LCG diffraction device, which diffracts light of red and green wavelengths. The liquid crystals in this LCG diffraction device react to the written signal while the light source of the blue wavelength is switched on.

[0144] The diffraction structures are then written into the LCG diffraction device, which diffracts light of the green and blue wavelengths after the blue wavelength light source has been turned off and the red wavelength light source has been turned on instead. The diffraction structures are then written into the LCG diffraction device, which diffracts light of the red and blue wavelengths after the red wavelength light source has been turned off and the green wavelength light source has been turned on instead.

[0145] In this exemplary embodiment, large diffraction angles are advantageously combined with relatively long times for orienting the liquid crystals in the diffraction device and a relatively good duty cycle. To increase the diffraction angle of the light, a device with four diffraction devices is also possible, in which, for example, additive diffraction of all three wavelengths of light occurs with the first two diffraction devices, and the diffraction angle of the first two diffraction devices is corrected for light of the green and blue wavelengths with the other two diffraction devices, as illustrated in Fig. 13.

[0146] For this purpose, no waveplate is required between a first diffraction device LCG 1 and a second diffraction device LCG 2, since both diffraction devices LCG 1 and LCG 2 deflect light of all three wavelengths. Between the second diffraction device LCG 2 and a third diffraction device LCG 3, a first color-selective waveplate WP 1 rotates the polarization for light of the blue and red wavelengths, but leaves the polarization unchanged for light of the green wavelength. The third diffraction device LCG 3 then only diffracts light of the green wavelength. Between the third diffraction device LCG 3 and a fourth diffraction device LCG 4, a second color-selective waveplate WP 2 rotates the polarization for light of the green and blue wavelengths, so that the fourth diffraction device LCG 4 then only diffracts light of the blue wavelength.

[0147] The grating periods of the diffraction devices LCG 1 to LCG 4 can be determined in this embodiment as: a_out = A_red*(1 / g1+1 / g2) = A_green *(1 / g1 + 1 / g2 + 1 / g3) = A_blue* (1 / g1+1 / g2+1 / g4)

[0148] With the same minimum grating period g_min, larger angles a_out can be realized by increasing the number of diffraction devices.

[0149] In a display device, in particular a holographic display device, several diffraction devices for horizontal and vertical diffraction of light are often used for viewer tracking in order to track a virtual viewer window or viewer area both horizontally and vertically to a viewer when the viewer moves.

[0150] Fig. 14 shows a schematic view from the front of Fig. 14 at the top, a diffraction device LCG-V with vertical line electrodes LE, into which diffraction structures for diffracting the light in the horizontal direction are inscribed. Below this diffraction device LCG-V, a diffraction device LCG-H is shown, which has horizontal line electrodes LE and into which diffraction structures for deflecting the light in the vertical direction are inscribed. For illustration purposes, these two diffraction devices LCG-V and LCG-H are shown one below the other. For a combination of horizontal and vertical diffraction, the diffraction devices LCG-V and LCG-H would be arranged in the light path so that light rays pass through them one after the other. Here, too, wave plates are required that rotate the polarization of the light between the first diffraction device and the second diffraction device.Typically, however, this would be done in the same way for all wavelengths.

[0151] Fig. 15 shows a further embodiment that combinations of more than two diffraction devices can also be used for horizontal and vertical diffraction.

[0152] In this case, two diffraction devices, LCG-S1 and LCG-S2, are provided, which diffract the light by +30° and -30° from the horizontal, respectively, and one diffraction device, LCG-H, which diffracts the light vertically. Of course, other combinations of angles are also possible, such as 90°, +45°, and -45°.

[0153] In further embodiments of the invention, each of these diffraction devices can be replaced by a combination of multiple diffraction devices and color-selective waveplates according to the examples described above in Figures 3 to 5, in order to achieve identical diffraction both horizontally and vertically for light sources of multiple wavelengths. However, this has the disadvantage that the total number of diffraction devices and waveplates used then becomes very large. In the example shown in Figure 15, a total of nine diffraction devices would then be required for the colors or wavelengths red, green, and blue.

[0154] Holographic displays with a virtual viewing window can optionally feature full-parallax holographic coding or single-parallax holographic coding. With full-parallax holographic coding, a small virtual viewing window is created in the horizontal and vertical directions, typically approximately 10 mm in size.

[0155] With single-parallax encoding, a virtual viewer window is generated in only one direction, e.g., horizontally. A sweet spot can also be created perpendicular to this, e.g., vertically, which can be significantly larger than the virtual viewer window.

[0156] When a diffraction device with a diffraction structure is used for observer tracking in a holographic display device, it is necessary that there is a good overlap of the tracked red, green and blue parts of the virtual observer window at the eye position of an observer so that the observer sees a colored hologram reconstruction as desired.

[0157] Often, the vertical range of motion of a viewer with respect to a display is smaller than the horizontal range of motion. For example, a seated viewer in front of a monitor may move sideways or turn their head, but their seat height typically does not change significantly. Similarly, with a head-up display (HUD) in a vehicle, a driver's seat height typically does not change significantly while driving. The vertical range over which a HUD display is visible to the driver usually only needs to be adjusted for drivers of different heights.

[0158] An embodiment of the invention therefore also relates to a holographic display device with a single-parallax coding of a hologram in the SLM, preferably a single-parallax coding of a hologram in the horizontal direction, in which a diffraction for all wavelengths of light is set at the same diffraction angle in the coding direction by means of the device according to the invention, which has diffraction devices and color-selective wave plates.

[0159] In the direction perpendicular to this, preferably the vertical direction, a sweet spot is created over the full height of the tracking area or the viewer area, so that no active tracking of the virtual viewer window or visibility area in this direction is necessary for the viewer of a scene.

[0160] In another embodiment of a holographic display device, a device is used in which a single diffraction device for vertical diffraction is used, which diffracts light of different wavelengths into different vertical angles and in which the difference in the tracking position for the individual colors or wavelengths is compensated by a sufficiently large sweet spot.

[0161] Both of the above cases can optionally be combined with a mechanical adjustment of the sweet spot, preferably vertically, to accommodate viewers of different heights. For example, in a HUD (Head-Up Display) with a mirrored optic, a 50 mm high or large sweet spot can be created for a viewer located at a fixed position. To accommodate viewers of different heights in a scene, this sweet spot can be adjusted vertically by mechanically tilting a mirror.

[0162] The above-mentioned device with the single diffraction device for vertical diffraction can, for example, be designed similarly to the embodiment shown in Fig. 4, wherein the third color-selective waveplate is then no longer optional but necessary, and wherein an additional diffraction device for vertical diffraction is provided, as shown in Fig. 16.

[0163] The device according to Fig. 16 again has an illumination device BD, here four diffraction devices LCG 1 , LCG 2, LCG 3 and LCG 4, and three color-selective wave plates WP1 , WP 2 and WP 3. Since the diffraction of the light by means of a fourth diffraction device LCG 4 is proportional to the wavelength, it is advantageous, for example with a given target of 50 mm vertical observer tracking in an observer plane and the use of light sources of wavelengths 450 nm, 532 nm and 640 nm in the illumination device BD, if the diffraction structures are inscribed in the diffraction devices LCG 1 , LCG 2, LCG 3 and LCG 4 in such a way that the diffraction angle is suitable for light of the green wavelength, the green part of the sweet spot is shifted up or down by 50 mm, the red part of the sweet spot is shifted by 60 mm and the blue part of the sweet spot is shifted by 42 mm.If a 28 mm sweet spot is generated in the display device with a one-dimensional (1D) diffuser or a lenticular element by adjusting the scattering properties or by adjusting the pitch, radius, and refractive index of the lenticular element, the overlap for the three RGB colors with a 50 mm vertical shift of the sweet spot would still be 10 mm (28 mm minus the difference between 60 and 42), which is significantly larger than the pupil of an eye. Thus, a total tracking range of 100 mm with a 28 mm sweet spot could be created.

[0164] Since less light power is required to create the smaller sweet spot compared to a 100 mm high sweet spot, this device has significant advantages in terms of power consumption and the brightness of the displayed scene.

[0165] Although the device of the diffraction devices for horizontal single parallax coding and vertical sweet spot is described here as a preferred embodiment, the invention is of course not intended to be limited to this case.

[0166] The entire device with the four diffraction devices LCG 1 , LCG 2 , LCG 3 and LCG 4 according to Fig. 16, as described above, can also be rotated by 90°, for example, for vertical single-parallax coding and a horizontal sweet spot, so that the first three diffraction devices LCG 1 , LCG 2 and LCG 3 achieve the same diffraction angle vertically for all colors or wavelengths and the fourth diffraction device LCG 4 achieves different diffraction angles horizontally, so that there is still an overlap of all colors or wavelengths within the sweet spot. With a horizontal sweet spot, the sweet spot can be prevented from hitting the neighboring eye of a viewer of a scene by suitable selection of the sweet spot size, for example by selecting a scatterer with a suitable scattering angle, and limiting the deflection range.In the above numerical example with a 28 mm sweet spot, for example, even light of the green wavelength could diffract 50 mm horizontally. The blue portion of the sweet spot would then be shifted horizontally by only 42 mm, while the red portion would be shifted by 60 mm. If a viewer of a scene moves 50 mm to the left, for example, the blue portion of the sweet spot would move 8 mm closer to the viewer's right eye, but would not reach it at an interpupillary distance typically greater than 55 mm, or on average, 65 mm.

[0167] Diffraction devices with liquid crystal modes based on phase modulation with circular light are also feasible, as described, for example, in US 2016 / 0004126 A1. In such diffraction devices, right-hand circularly polarized light and left-hand circularly polarized light are diffracted into opposite diffraction orders, +1st diffraction order or -1st diffraction order. Using color-selective waveplates and multiple diffraction devices, diffraction into the same diffraction angle for multiple wavelengths can also be achieved.

[0168] As shown in Fig. 17, a device with three diffraction devices LCG 1 , LCG 2 , and LCG 3 and three color-selective waveplates WP 1 , WP 2 , and WP 3 is used. A first diffraction device LCG 1 diffracts light of the red wavelength at a larger angle than the target angle for the three wavelengths. Light of the blue wavelength, on the other hand, is diffracted at a smaller angle.

[0169] Using a first color-selective waveplate WP 1, the polarization state for light of the red wavelength is then changed, e.g., from right-hand to left-hand circular light, but the polarization state is retained for light of the green and blue wavelengths. Using a second diffraction device LCG 2, the diffraction for light of the blue and green wavelengths then occurs in the same direction as before, but for light of the red wavelength in the opposite direction, as can be seen in Fig. 17. Thus, the diffraction angle for the blue and green wavelengths is increased by the second diffraction device LCG 2, but the diffraction angle for the red wavelength is reduced. The polarization rotation for light of the green wavelength then occurs using a second color-selective waveplate WP 2.By means of a third diffraction device LCG 3, the diffraction angle for light of the blue wavelength is further increased, while the diffraction angle for light of the red and green wavelengths is reduced. With a suitable selection of the diffraction structures inscribed in the diffraction devices LCG 1, LCG 2, and LCG 3, the same diffraction angle can be set for all three wavelengths RGB for the three diffraction devices LCG 1, LCG 2, and LCG 3.

[0170] In contrast to the previously described designs of devices with diffraction devices based on linearly polarized light, it is therefore not possible here to select the polarization of the light such that individual diffraction devices become inactive for specific wavelengths. Instead, each diffraction device diffracts all three colors (RGB) or wavelengths. By adjusting the polarization of the light, however, it is possible to direct light of specific wavelengths into the +1st or -1st diffraction order, so that with multiple diffraction devices and color-selective waveplates, identical diffraction angles can also be set for different wavelengths.

[0171] Since half-wave plates have the property that they either rotate linearly polarized light or, alternatively, convert left-circularly polarized light into right-circularly polarized light, the color-selective wave plates in the illustrated embodiments with linear and circular polarization of the light would in principle not differ.

[0172] Previously described embodiments of the device with regard to diffraction of light in the horizontal direction and vertical direction are also transferable to these types of diffraction devices.

[0173] For a grating with a liquid crystal mode as a diffraction device based on circular polarization, as shown in Fig. 17 by the half-turned arrows, the same diffraction angle of the light would be obtained using the equations: a_out = A_red*(1 / g1 - 1 / g2 -1 / g3) = A_green *(1 / g1 + 1 / g2 -1 / g3) = A_blue* (1 / g1+1 / g2 + 1 / g3)

[0174] Again, the angle a_out of the light emerging from a diffraction device can be changed by adjusting the grating period g1 of the diffraction device, while the ratio of the grating periods g2 / g1 and g3 / g1 remains unchanged and is determined by the wavelengths RGB.

[0175] By means of a control device S, switching on and off of the illumination device is provided in synchronization with a control of the spatial light modulation device or the SLM and with a control of the at least two diffraction devices.

[0176] The invention is not intended to be limited to the exemplary embodiments presented here. Further embodiments or exemplary embodiments and combinations of the described exemplary embodiments are also possible, where possible. Finally, it should be particularly noted that the exemplary embodiments described above serve merely to describe the claimed teaching, but this is not intended to be limited to the exemplary embodiments.

Claims

Patent claims 1. Device comprising - a lighting device that emits polarized light of different wavelengths, - at least one color-selective wave plate, - at least two diffraction devices, wherein by means of the at least one color-selective wave plate a polarization rotation is provided only for a part of the different wavelengths of the light, whereby at least one of the diffraction devices can be passed through by light of one wavelength with a different polarization state compared to the polarization state of another wavelength of the light, wherein diffraction structures are inscribed in the at least two diffraction devices in such a way that for the different wavelengths of the light the light can be diffracted at the same angle by the combination of the at least two diffraction devices.

2. Device according to claim 1, characterized in that the illumination device has at least two light sources with at least two different wavelengths or at least one light source with at least two different wavelengths.

3. Device according to claim 2, characterized in that the different wavelengths differ by at least 40 nm.

4. Device according to claim 1, characterized in that the at least two diffraction devices each have two substrates between which a liquid crystal layer is provided, wherein a plurality of liquid crystal cells of the liquid crystal layer form a diffractive structure by applying an electric field.

5. Device according to one of the preceding claims, characterized in that the at least two diffraction devices are designed to be controllable.

6. Device according to claim 4 or 5, characterized in that the diffractive structure is designed as a sawtooth-shaped phase profile and for which at least two diffraction devices are aligned parallel to each other.

7. Device according to one of the preceding claims, characterized in that a first color-selective wave plate is provided between a first diffraction device and a second diffraction device, wherein the color-selective wave plate changes the polarization of the incident light only for a defined wavelength.

8. Device according to one of the preceding claims, characterized in that the first diffraction device is designed to diffract light of all wavelengths, wherein the second diffraction device is designed to diffract light of at least one wavelength only.

9. Device according to one of claims 1 to 6, characterized in that three diffraction devices are provided, wherein a first color-selective wave plate is provided between the first and the second diffraction device and a second color-selective wave plate is provided between the second and third diffraction device.

10. Device according to claim 9, characterized in that the first color-selective waveplate changes the polarization of the incident light only for a defined wavelength and the second color-selective waveplate changes the polarization of the incident light only for a defined wavelength which differs from the wavelength for which the first color-selective waveplate changes the polarization of the light.

11. Device according to one of the preceding claims 1 to 6, characterized in that each individual diffraction device of the at least two diffraction devices is designed to diffract only light of one wavelength.

12. Device according to claim 11, characterized in that three diffraction devices are provided, wherein a first color-selective wave plate is arranged in front of a first diffraction device in the direction of light, wherein a second color-selective wave plate is arranged between the first and a second diffraction device, and wherein a third wave plate is arranged between the second and a third diffraction device in order to change the direction of polarization of the light, whereby only light of one wavelength can be diffracted by each individual diffraction device.

13. Device according to one of the preceding claims, characterized in that a third or fourth color-selective wave plate is provided, which is arranged after the last diffraction device provided in the light direction.

14. Device according to one of the preceding claims, characterized in that the diffraction structures are written into the at least two diffraction devices sequentially or simultaneously.

15. Device according to one of the preceding claims, characterized in that a light source of a corresponding wavelength can only be switched on when the liquid crystals of the diffraction device diffracting light of this wavelength are aligned according to the applied field.

16. Device according to one of the preceding claims, characterized in that four diffraction devices are provided, wherein in the light direction an additive diffraction of all wavelengths is provided with the first and second diffraction devices and with the third and fourth diffraction devices the diffraction angle of the first and second diffraction devices can be corrected for defined wavelengths.

17. A display device comprising at least one spatial light modulation device, a control device and at least one device according to one of claims 1 to 16.

18. Display device according to claim 17, characterized in that by means of the control device, switching on and off of the lighting device is provided synchronized with a control of the at least one spatial light modulation device and with a control of the at least two diffraction devices.

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