Abstract:
The invention relates to a display, in particular an autostereoscopic or holographic display, for representing preferably three-dimensional information, wherein the stereo views or the reconstructions of the holographically encoded objects can be tracked to the movements of the associated eyes of one or more observers in a finely stepped manner within a plurality of zones of the movement region. In this case, the zones are selected by the activation of switchable polarization gratings.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/373,942, filed Jul. 23, 2014, which claims the priority of PCT/EP2013/051423, filed on Jan. 25, 2013, which claims priority to German Application No. 10 2012 201166.8, filed Jan. 26, 2012, and German Application No. 10 2012 105479.7, filed Jun. 22, 2012, the entire contents of each of which are incorporated fully herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a display, in particular an autostereoscopic or holographic display, for representing preferably three-dimensional information, wherein the stereo views or the reconstructions of the holographically encoded objects can be tracked to the movements of the associated eyes of one or more observers. 
         [0003]    Displays for displaying three-dimensional information are known in a multiplicity of embodiments. Besides embodiments which require additional aids such as shutter or polarization spectacles for viewing the spatial scene, autostereoscopic displays do not require such aids. Without additional measures, however, in the latter displays the 3-D scene can be viewed only in a narrow spatial region, the so-called observer region. In order that the 3-D scene can also be seen conveniently in a large horizontal angular range, devices have been developed which allow said observer region to be tracked to the eyes of the observer. Such a system has been described e.g. by the applicant in the patent specification DE 103 39 076 B4. The applicant has likewise described a series of holographic display devices, e.g. in EP 1 563 346 B1 or EP 1 792 234 B1, wherein a 3-D scene can be perceived in a narrowly delimited observer region as a spatial reconstruction of the intensity distribution by means of holographic diffraction patterns being encoded in a spatial light modulator. Here as well it is expedient for the observer region to be tracked to the eye movements of one or more observers. For this purpose, such devices have a recognition system that determines the positions of the eyes of the observers and forwards the data to a system controller. The recognition systems often operate in a camera-based manner, wherein the eye positions are determined by means of image processing algorithms. In accordance with the respective eye positions, the system controller controls mechanical or electronic light deflection means such that the center of the observer region assigned to the respective eye position as far as possible corresponds to the respective eye position to the greatest possible extent. If desired or necessary, at the same time the image content or the reconstruction of the 3-D scene can be adapted to the new eye position. 
         [0004]    In such a display, an illumination unit generates light that is collimated to the greatest possible extent with the required wavelength spectrum with which the spatial light modulator into which the stereoscopic views are written or the holographic information is encoded is illuminated. In the case of holographic displays, the light must additionally be capable of interference at least in a region required for encoding a pixel of the 3-D scene, i.e. said light must be sufficiently coherent. The light deflection means can be situated upstream and/or downstream of the spatial light modulator. 
         [0005]    The illumination unit can also be configured in a controllable manner such that it can direct light in predefinable spatial directions. For this purpose, for example, displaceable illumination columns can be situated in direct proximity to the focal plane upstream of a cylindrical lens array. The illumination columns can be controllably selected for example from an active light source matrix, which can be embodied e.g. as an OLED matrix. It is also possible to use a planar light source, in front of which is arranged an array of slit diaphragms that are variable in a drivable manner in terms of their horizontal position. Such a diaphragm array can be configured as an LCD matrix, for example. In this case, each column forms a secondary light source which illuminates that cylindrical lens of the cylindrical lens array which is assigned to it. The horizontal position of the illumination columns with respect to the center line of the assigned cylindrical lens determines the horizontal angle—emitted by the respective cylindrical lens—of the partial beam that is collimated to the greatest possible extent. In this case, a plurality of illumination columns can also be activated simultaneously in order to increase the emitted angular range and thus the size of the assigned observer region. Furthermore, the horizontal deviation of the position of the illumination columns with respect to the center line of the assigned cylindrical lens can vary over the area of the cylindrical lens array in order e.g. to realize an additional field lens function and thus to adapt the horizontal diameter of the assigned observer region to the observer distance. The cylindrical lens array can have a diaphragm array that avoids crosstalk between the illumination columns assigned to a cylindrical lens and neighboring cylindrical lenses. Undesired secondary observer regions can thus be avoided. 
         [0006]    The number of illumination columns assigned to a cylindrical lens determines the number of possible horizontal deflection angles. This number cannot be increased arbitrarily, since the primary and/or secondary light sources have technically dictated minimum dimensions. Moreover, the required luminance increases, the smaller the dimensions become. For a large horizontal movement region in which a plurality of observers can also be situated, however, a large number of finely gradated light deflection angles are required for observer tracking. Therefore, a series of additional measures have already been proposed for increasing said movement region and thus the number of possible deflection angles. By way of example, in the American Patent Specification U.S. Pat. No. 7,791,813 B2, arrays of electrically controllable electrowetting cells were to be used for beam deflection. However, such arrays are complicated to produce and have a restricted aperture on account of the cell height. Mechanical deflection means such as deflection mirrors or rotatable prisms, which were likewise proposed, are sluggish and require a large structural volume. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore, the present invention is based on the object of specifying and developing a display of the type mentioned in the introduction which overcomes the problems mentioned above. In particular, the intention is to enable the observer tracking for one or more observers in a finely stepped manner in a plurality of movement zones, which preferably overlap, without mechanical light deflection means. 
         [0008]    The object is achieved according to the invention by means of the teaching of patent claim  1 . Further advantageous configurations and developments of the invention are evident from the dependent claims. 
         [0009]    According to the invention, a display, in particular an autostereoscopic or holographic display, wherein image views or reconstructions of holographically encoded objects can be tracked to the movements of associated eyes of at least one observer, has an illumination unit comprising light sources for generating light that is collimated to the greatest possible extent with a predefinable emission characteristic, at least one polarization grating stack having a stack of optical components comprising at least two switchable or controllable polarization gratings each comprising a grating structure that can be switched on for light deflection, a spatial light modulator for modulating the light of the illumination unit for displaying image information or for reconstructing encoded hologram information, a recognition system for determining positions of the associated eyes of the at least one observer, and a system controller for driving and synchronizing the switchable and controllable elements, wherein, depending on the current position of the eyes of a observer, the illumination unit and the polarization grating stack are drivable by the system controller in such a way that the light of the illumination unit is deflectable in the direction of the eyes of the observer. 
         [0010]    In this case, the emission characteristic of the illumination unit is determined by the type of display. For holographic displays, use is preferably made of narrowband light sources having good coherence properties, such as are constituted by laser light sources, for example. For autostereoscopic displays, it is possible to use inorganic or organic luminescence diodes, for example, which have a wider wavelength spectrum. 
         [0011]    The polarization gratings of the polarization grating stack are driven by the system controller such that a respective grating is active and directs the light in the predefined direction in order thus to generate a zone of the movement region. The switchable polarization gratings can be configured as switchable liquid crystal polarization gratings, for example. For this purpose, the liquid crystal alignment layer has a periodically recurring structure for locally changing the liquid crystal orientation that determines the polarization of the light passing through. The shorter this period, the greater the diffraction angle and thus the light deflection angle for the predefined wavelength of the light. By applying a voltage to electrodes which are assigned to the liquid crystal layer, it is possible for the alignment of the liquid crystal molecules that is brought about by the liquid crystal alignment layer to be canceled and thus for the polarization grating to be switched off. The light then passes through this grating without being deflected. 
         [0012]    The grating period can be varied locally in order, for example, additionally to enable a field lens function. 
         [0013]    The light sources of the illumination unit can be arranged in an array of primary light sources as a light source matrix, which are switchable or controllable individually or in columns. Said light sources can be LEDs, OLEDs or laser diodes, for example. 
         [0014]    The light sources can be controlled separately in terms of their brightness by the system controller, in order e.g. to compensate for locally different transparencies in the beam path or to improve the contrast when displaying the image information. 
         [0015]    In order to form a color display, different light sources are preferably used, which differ in terms of their emitted wavelength spectrum, in particular the focus wavelength, and which are separately operable. The individual colors can thus be generated in time division multiplex, i.e. in temporal succession. For this purpose, the system controller synchronizes the driving of the respective color with the writing of the associated color information into the spatial light modulator. 
         [0016]    The different image contents or encoding information for the right and left eyes can likewise be written time-sequentially in a synchronized manner by the system controller. In this case, the required deflection directions for generating the associated observer region are respectively set for each eye. It is also possible, in the case of a plurality of observers, to write a dedicated view or encoding information item for each observer eye. Color and view changes can be suitably combined, such that as far as possible no disturbances, in particular disturbing flickering, become visible to the observers. 
         [0017]    A controllable diaphragm array, which can be configured for example as an array of controllable slit diaphragms, can be arranged upstream of the polarization grating stack. This array can be integrated in the illumination unit. 
         [0018]    The diaphragm array is preferably illuminated by a planar light source. 
         [0019]    Such a diaphragm array can consist of a matrix of individually drivable liquid crystal cells, for example. Said cells can be arranged in columns. However, a two-dimensional matrix arrangement of controllable cells can also be used, wherein the correct columns are selected by the driving of the associated cells by the system controller and thus form secondary columnar light sources. 
         [0020]    With the use of a light source matrix or an array of slit diaphragms, the control of the light sources or the position of the slit diaphragms can be set such that this compensates for the wavelength dependence of the deflection angles of the switchable polarization gratings of the polarization grating stack. In the case of time-sequential illumination with red, green and blue light, for example, the position of the slit diaphragms is set for each color in such a way that for all colors light is directed to the same detected observer position. 
         [0021]    The illumination unit can contain a cylindrical lens array for the purpose of collimation. In this case, the cylindrical lenses are aligned along the illumination columns. Preferably, the illumination columns, which are correspondingly embodied as primary or secondary light sources, are situated to the greatest possible extent in the focal plane of the cylindrical lenses. The cylindrical lenses can also be embodied as gradient-index lenses. A multi-stage construction with a plurality of imaging surfaces arranged one behind another can also be employed, wherein at least one intermediate imaging can also be effected. As a result of the displacement of the illumination columns by the system controller transversely with respect to the cylindrical lenses, finely gradated tracking of the light of the illumination unit in the direction of the eyes of the observer can be effected. 
         [0022]    In this case, the columns can be driven such that the tracking angle varies over the area of the cylindrical lens array, in order to obtain a field lens function and/or to achieve an adaptation of the width of the observer region assigned to the respective eye to the observer distance, in order that from all positions of the movement region a 3-D view can be perceived as far as possible without any disturbances. 
         [0023]    The cylindrical lens array can itself already contain a field lens function, which for example forms a observer region at the preferred observer position. Such a preferred observer position is situated e.g. in the center of the horizontal movement region at the average movement distance. 
         [0024]    The illumination unit can set the circular polarization direction of the light that is required for the downstream polarization grating stack. 
         [0025]    It is also possible to arrange for that purpose a separate means that influences the polarization of the light upstream of the polarization grating stack. Such a means can contain a birefringent retardation layer, for example, which is configured as a quarter-wave plate and converts linear light, emitted purely by way of example by the illumination unit, into circularly polarized light. 
         [0026]    An additionally fixed or variable field lens can be contained in the beam path between the illumination unit and the observer in order to predefine the width of the observer region assigned to the respective eye at the predefined location of the respective observer eye or to set it depending on said location. The field lens can also be part of the illumination unit, situated between the latter and the polarization grating stack, arranged downstream of the polarization grating stack or situated between spatial light modulator and observer. 
         [0027]    The polarization grating stack is preferably arranged in the light path between the illumination unit and the spatial light modulator. However, it is also possible to arrange the polarization grating stack in the light path downstream of the spatial light modulator. 
         [0028]    The polarization grating stack can preferably contain as optical component at least one additional switchable or controllable birefringent retardation layer, preferably configured as a switchable or controllable half-wave plate. In the switched-off state the circularly polarized light that radiates through said layer maintains its direction of rotation. In the switched-on state, the direction of rotation of the circularly polarized light is changed and thus corresponds to the direction of rotation of a switched-on polarization grating of the polarization grating stack for the same direction of rotation of the input polarization. What can be achieved with this birefringent retardation layer is that at the output of the polarization grating stack, for the same input direction of rotation, the same output direction of rotation is always achieved, irrespective of whether or not a grating is switched on, since, with a polarization grating switched off, the light is transmitted without being deflected and without a change in the direction of rotation of the circular polarization. If the birefringent retardation layer is configured in a controllable fashion, for example as a controllable liquid crystal layer, it is possible to compensate for dispersion effects or changes in the effective optical path length in the case of oblique beam passage. For this purpose, the retardation layer is to be synchronized with the other active components by means of the system controller. 
         [0029]    An additional retardation layer can be situated downstream of the polarization grating stack in order to convert circularly polarized light into linearly polarized light. That is advantageous for example if the downstream assembly, for example the spatial light modulator, requires linearly polarized light for its operation. Such a retardation layer can be configured as a quarter-wave plate, for example. For color displays, said retardation layer can advantageously be configured in an achromatic or apochromatic fashion. It is also possible for said birefringent retardation layer to be configured in a controllable fashion, for example as a controllable liquid crystal layer, in order to compensate for dispersion effects or changes in the effective optical path length in the case of oblique beam passage. For this purpose, the retardation layer is to be synchronized with the other active components by means of the system controller. 
         [0030]    A polarization filter can advantageously be arranged in the light path downstream of said retardation layer, said polarization filter suppressing linearly polarized light of the zeroth order of diffraction of a switched-on polarization grating of the polarization grating stack, that is to say light which passes through the polarization grating without diffraction. This light is linearly polarized and is perpendicular to the light which leaves the retardation layer after deflection by a switched-on polarization grating of the polarization grating stack. Said polarization filter can also be part of a downstream spatial light modulator, for example if the latter requires linearly polarized light for its manner of operation. 
         [0031]    Light sources that are controllable in terms of their direction have a high power demand, particularly if they operate with controllable slit diaphragms. It is therefore particularly advantageous, instead of light sources that are controllable in terms of their direction or in order to support said light sources, to arrange a controllable deflection grating having a variable grating period upstream of, downstream of or in the polarization grating stack, with which grating period a finely stepped additional light deflection can be carried out. Such gratings are advantageously embodied as polarization gratings having a variable grating period. Such controllable polarization gratings can be configured for example as electrically controllable liquid crystal cells. In this case, the polarization of the light passing through is influenced locally by a voltage pattern being applied to a finely dimensioned electrode structure, wherein the magnitude of the applied voltage determines the alignment state of the liquid crystal molecules. Like the switchable polarization gratings of the polarization grating stack, these gratings diffract circularly polarized light only in one order of diffraction. The diffraction angle can be set by means of the period of the voltage profile. The diffraction direction is determined by the local voltage profile within a period. Preferably, the voltage profile is saw tooth-shaped, wherein the direction of the pulse ramp determines the deflection direction. Since the variable polarization grating need have only a small deflection angle range, a large angle range is achieved by the zone division with the switchable polarization gratings, and the requirements made of the fineness of the electrode structure can be kept in a realizable range. 
         [0032]    The period of the voltage profile to be applied to the electrode structure can be varied over the area of the grating in order to additionally obtain or support a field lens function. It is particularly advantageous that the focus of said field lens can be changed solely by the electronic driving by means of the system controller. 
         [0033]    Such a controllable variable grating can also advantageously be used to compensate for the wavelength dependence of the deflection angles of the switchable polarization gratings of the polarization grating stack. 
         [0034]    The illumination unit can advantageously also be configured such that it has controllably different light emergence angles. This can be, for example, an array of directional light sources which have different emission directions either in a spatially alternating manner or in a temporally switchable manner. 
         [0035]    At least one optical component in a polarization grating stack used can be embodied as segmented in one or two directions, wherein the individual segments can be separately switched or controlled. Thus, different deflection angles can be realized in a manner dependent on the passage location of the light through the polarization grating stack, in order for example to form or support an additional field lens function in order to better track light to a observer for example in the case of an extensive display. 
         [0036]    The segmentation can for example also be effected in the form of concentric circular or elliptic rings. 
         [0037]    The grating structure of at least one polarization grating in the polarization grating stack can also be arranged in a manner rotated with respect to other polarization gratings in order to enable a two-dimensional deflection. In this case, the individual polarization gratings can differ in terms of their grating constant in order to enable different deflection angles in the directions rotated with respect to one another. It is particularly advantageous in this case if the grating structures of the polarization gratings are aligned perpendicularly to one another. 
         [0038]    Polarization gratings rotated with respect to one other in terms of their grating structure can also be arranged in separate polarization grating stacks in order for example to arrange further components such as controllable deflection gratings having a variable grating period and/or light modulators and/or field lenses between them. In this regard, in each case a separate controllable deflection grating having a variable grating period for a finely stepped deflection of the respective deflection direction can be arranged downstream of the associated polarization grating stack. 
         [0039]    By changing the voltage at a controllable polarization grating, it is possible to influence the deflection effectiveness and thus that proportion of the light which is deflected in the +1st or -1st order of diffraction depending on the circular polarization direction. Besides control of the intensity of the light sources and/or the modulation intensity of the light modulator, it is thus possible to reduce intensity fluctuations, for example, such as can occur for example as a result of different diffraction effectivenesses for individual polarization gratings of the polarization grating stack or for different spectral distributions of the light sources of the illumination device or further angle-dependent optical losses of the optical system. 
         [0040]    Conventional polarization gratings have the property that they alter the polarization direction of the light passing through. By way of example, if left circularly polarized light is incident on such a polarization grating, then the light deflected by said grating emerges again in right circularly polarized fashion. Right circularly polarized light deflected by such a grating correspondingly emerges in left circularly polarized fashion, wherein the signs of the associated first order of diffraction differ. Since the deflection direction, i.e. the sign of the associated first order of diffraction for downstream polarization gratings in a stack of polarization gratings is also influenced by the circular polarization direction incident on said polarization gratings, it is necessary to take account of or set the possible polarization changes in the arrangement of the polarization gratings in the stack, as already described. This is advantageously done by means of switchable or controllable retardation layers between the polarization gratings in order to adapt the direction of rotation of the circularly polarized light to the desired deflection direction, that is to say to the +1st or −1st order of diffraction. 
         [0041]    A publication “Twisted nematic liquid crystal polarization grating with the handedness conservation of a circularly polarized state” by Honma and Nose, Optics Express Vol. 20, pages 18449-18458, 2012, describes a specific type of polarization grating in which the deflected light emerging from the polarization grating has the same circular polarization state as the incident light. This type of polarization grating is based on a liquid crystal structure having a periodic twist. The setting of such a twist is achieved by means of a periodic surface orientation of the liquid crystal molecules in the liquid crystal layer which has an opposite direction of rotation on both substrates. What is disadvantageous about this type of polarization grating is that relatively thick liquid crystal layers are required, such that small grating periods and short switching times can be realized only with difficulty. Therefore, the majority of the exemplary embodiments mentioned below relate to conventional polarization gratings. Generally, the invention is also applicable to the type of grating described in the publication by Honma and Nose. A polarization grating stack can also contain gratings of both types in mixed form. In these gratings, too, the desired deflection direction can be selected by means of the direction of rotation of the circularly polarized light entering the respective polarization grating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    There are, then, various possibilities of advantageously configuring and developing the teaching of the present invention and/or of combining the above-described embodiments—insofar as possible—with one another. In this respect, reference should be made firstly to the patent claims subordinate to patent claim  1  and secondly to the following explanation of the preferred exemplary embodiments of the invention with reference to the drawings. Generally preferred configurations and developments of the teaching are also explained in conjunction with the explanation of the preferred exemplary embodiments of the invention with reference to the drawing. In the drawing, in each case in a schematic illustration: 
           [0043]      FIG. 1 a    shows an explanation of the symbols used in the drawings for the polarization directions, 
           [0044]      FIG. 1 b    shows a first configuration variant with displaceable light source columns in a controllable slit diaphragm array  120  in which a second polarization grating  340  of a polarization grating stack  300  is switched on, 
           [0045]      FIG. 1 c    shows a first configuration variant with displaceable light source columns in a controllable slit diaphragm array  120  in which a first polarization grating  310  of a polarization grating stack  300  is switched on, 
           [0046]      FIG. 1 d    shows a first configuration variant with displaceable light source columns in a controllable slit diaphragm array  120  in which a switchable half-wave plate  320  of a polarization grating stack  300  is switched on, 
           [0047]      FIG. 1 e    shows a first configuration variant with displaceable light source columns in a controllable slit diaphragm array  120  in which two polarization gratings  310 ,  340  and a switchable half-wave plate  320  of a polarization grating stack  300  are switched on, 
           [0048]      FIG. 2  shows a second configuration variant with a controllable variable polarization grating  200  and a field lens  600  in which a second polarization grating  340  of a polarization grating stack  300  is witched on, 
           [0049]      FIGS. 3 a  and 3 b    show a third configuration variant, wherein the illumination unit  100  additionally has controllably different light emergence angles, 
           [0050]      FIG. 4  shows a fourth configuration variant with an additional field lens  600 , 
           [0051]      FIG. 5  shows a fifth configuration variant, in which the diaphragms in the slit diaphragm array  120  are set such that a field lens function is realized with the cylindrical lens array  150 , 
           [0052]      FIG. 6  shows a sixth configuration variant, which additionally contains a controllable deflection grating having a variable grating period  700 , 
           [0053]      FIG. 7  shows a seventh configuration variant similar to  FIG. 5 , but with a spatial subdivision of the polarization gratings  310 ,  340  and the half-wave plate  320  in the polarization grating stack  300 , 
           [0054]      FIG. 8  shows an eighth configuration variant similar to  FIG. 5 , in which the polarization grating stack  300  contains an additional switchable half-wave plate  350  and an additional polarization grating  360  for a two-dimensional light deflection, 
           [0055]      FIG. 9  shows a ninth configuration variant similar to  FIG. 6 , which contains an additional switchable or controllable polarization grating stack  305  and an additional controllable deflection grating having a variable grating period  705  for a two-dimensional light deflection, and 
           [0056]      FIG. 10  shows a tenth configuration variant similar to claim  6 , in which the polarization grating stack  300  has polarization gratings  380 ,  390  which are based on a twisted structure and which do not change the polarization direction in the case of a light deflection. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]      FIG. 1 a    shows the symbols—used in  FIGS. 1 to 6 —of the polarization direction leaving the respective illustrated optical element for the light passing through. The symbols are situated above the relevant optical elements in the drawings. Right and left circular polarization and vertical and horizontal linear polarization are illustrated. 
         [0058]    A first configuration variant of the invention is illustrated purely schematically in  FIGS. 1 b  to 1 e   . An illumination unit  100  that is collimated to the greatest possible extent and is controllable in the emission direction contains a light source matrix  110 , a controllable slit diaphragm array  120 , which can be embodied as a controllable liquid crystal matrix, a linear polarization filter  130 , which can advantageously be configured as an output polarizer of the liquid crystal matrix, a birefringent retardation layer  140 , for generating the required circular polarization from the linear polarization, which can advantageously be configured as a quarter-wave plate, and a cylindrical lens array  150  for collimating the light stripes emerging from the slit diaphragms. The birefringent retardation layer  140  or the birefringent retardation layer  140  and the linear polarization filter  130  can also be situated in the light path downstream of the cylindrical lens array  150 . Purely by way of example, left circularly polarized collimated light leaves the cylindrical lens array  150  and impinges on the polarization grating stack  300 , which contains here purely by way of example a first switchable polarization grating  310 , a switchable birefringent retardation layer  320 , which can advantageously be configured as a switchable half-wave plate, and a second switchable polarization grating  340 . The polarization grating stack  300  is followed here by a birefringent retardation layer  410 , which can advantageously be configured as a quarter-wave plate, for generating linearly polarized light from the right circularly polarized light leaving the polarization grating stack. In a downstream linear polarization filter  420 , this light is converted into horizontally polarized light. At the same time, vertically polarized light that arises in the zeroth order of diffraction of a switched-on polarization grating is blocked. The polarizer  420  is followed by a spatial light modulator  510 , which is followed here purely by way of example by a polarization filter  530  serving as an analyzer. The polarizer  420 , as illustrated in  FIG. 2 , can also serve as an analyzer for the spatial light modulator. 
         [0059]    In  FIG. 1 b   , the second polarization grating  340  is switched on, i.e. this diffraction grating is active and directs the light into its zone (not illustrated here) of the movement region of the observers. At this grating, no voltage that destroys the grating-shaped alignment of the liquid crystal molecules is present at the electrodes. Accordingly, right circularly polarized light leaves the second switchable polarization grating  340 . The first switchable polarization grating  310  and the switchable half-wave plate  320  do not influence the polarization state. 
         [0060]    In  FIG. 1 c   , the first switchable polarization grating  310  is activated and generates that zone of the movement region of the observers which is assigned to it. Here right circular light leaves this polarization grating  310  and its polarization direction is not altered in the downstream non-activated switchable birefringent retardation layer  320  and the second non-activated switchable polarization grating  340 . 
         [0061]    In  FIG. 1 d   , neither of the two switchable polarization gratings  310  and  340  is activated. The light passes through the polarization grating stack  300  without being deflected and forms a third zone of the movement region for the observers. The activated birefringent retardation layer  320  here provides for the necessary rotation of the direction of rotation of the polarization from left circular to right circular, in order that the correct linear polarization direction arrives at the polarization filter  420 . 
         [0062]    In  FIG. 1 e   , both polarization gratings  310  and  340  are activated. A combination of the light deflection of both polarization gratings  310  and  340  can thus be used for observer tracking. In this case, too, the activated birefringent retardation layer  320  provides for the rotation of the direction of rotation of the polarization, in order that the correct linear polarization direction arrives at the polarization filter  420 . 
         [0063]    The controllable birefringent retardation layer  320  is activated whenever an even number of polarization gratings is activated. The controllable birefringent retardation layer  320  is not activated when an odd number of polarization gratings is activated. As a result, an identical direction of rotation of the circular polarization downstream of the polarization grating stack  300  is always obtained. 
         [0064]      FIG. 2  shows a second exemplary embodiment. Here, an illumination unit  100  generates purely by way of example collimated left circularly polarized light. The latter illuminates a downstream controllable polarization grating  200 , the grating period of which is used to set the beam direction in the activated zone of the downstream polarization grating stack  300 , which here likewise purely by way of example contains a first switchable polarization grating  310 , a switchable birefringent retardation layer  320  and a second switchable polarization grating  340 . The polarization filter  520  downstream of the birefringent retardation layer  410  is assigned here to the spatial light modulator  510 , which likewise has a polarization filter  530  as an analyzer. Downstream in the beam path, a field lens  600  is provided here purely by way of example, said field lens advantageously being configured as a flat Fresnel lens. 
         [0065]      FIGS. 3 a  and 3 b    show a further exemplary embodiment similar to that in  FIG. 2 . In contrast to  FIG. 2 , it contains an illumination unit  100  having controllably different light emergence angles. In this case,  FIG. 3 a    shows schematically, in a manner indicated by arrows, one controllably set light emergence angle and  FIG. 3 b    shows another controllably set light emergence angle of the illumination unit  100 . 
         [0066]    In this example, the set light emergence angles both lie in the plane of the drawing. In general, an illumination unit  100  can also have additional settable light emergence angles for example perpendicular to the plane of the drawing. 
         [0067]      FIG. 4  shows a further exemplary embodiment. In contrast to the first exemplary embodiment according to  FIGS. 1 b  to 1 e   , an additional field lens  600  is provided here as in the second exemplary embodiment according to  FIG. 2 . In this case,  FIG. 4  schematically depicts light beams which emerge from the slit diaphragm array  120  and are focused by the field lens  600 . Both polarization gratings  310  and  340  are illustrated as not activated, as in  FIG. 1 d   , with the result that the light passes through the polarization grating stack  300  without being deflected. 
         [0068]      FIG. 5  shows an exemplary embodiment corresponding to the exemplary embodiment shown in  FIGS. 1 b  to 1 e   . Here, however, the positions of the diaphragms in the slit diaphragm array  120  are set such that, in combination with the cylindrical lens array  150 , the light emerging from different slit diaphragms is focused onto a common position. In this case, therefore, the cylindrical lens array also contains the field lens function, such that, in contrast to the exemplary embodiments according to  FIGS. 2 to 4 , an additional field lens can be dispensed with. Here as well, both polarization gratings  310  and  340  are illustrated as not activated, as in  FIG. 1 d   , such that here as well the light passes through the polarization grating stack  300  without being deflected. 
         [0069]      FIG. 6  shows a configuration variant similar to  FIG. 2 . In addition thereto, it contains a controllable deflection grating having a variable grating period  700 . In this example, the grating is arranged downstream of the polarization grating stack  300 , the light modulator  510  and the field lens  600 . The illustration shows that the second polarization grating  340  of the polarization grating stack  300  is activated analogously to  FIG. 1 b   . Such a variable deflection grating  700  enables an additional finely stepped light deflection and/or compensation of the wavelength dependence of the light deflection in the polarization grating stack  300 . 
         [0070]    It is also possible to arrange the variable deflection grating upstream of the polarization grating stack  300  or between two individual components  310 ,  320 ,  340  of the polarization grating stack  300 . The function of the deflection grating can also be divided among a plurality of components which can be situated at different locations in the beam path. 
         [0071]      FIG. 7  shows a further configuration variant similar to  FIG. 5 . In this case, the polarization gratings  310  and  340  and the switchable half-wave plate  320  are subdivided into a plurality of separately switchable segments. In the example shown, purely by way of example, each optical component ( 310 ,  320 ,  340 ) was subdivided into two segments ( 311 ,  312 ,  321 ,  322 ,  341 ,  342 ). The polarization grating  310  is illustrated as activated in the upper segment ( 312 ); the switchable half-wave plate  320  is illustrated as activated in the lower segment. Downstream of the polarization grating stack  300 , the emerging polarization state is the same in both segments, but the deflection angle differs in the upper and lower segments. This subdivision can be used for tracking a observer in the case of an extensive display. The segmentation can also be effected two-dimensionally or, for example, concentrically. 
         [0072]      FIG. 8  shows a configuration variant similar to  FIG. 5 . In contrast to  FIG. 5 , the polarization grating stack contains an additional switchable half-wave plate  350  and an additional polarization grating  360 . These elements are illustrated as activated in the figure. In the polarization grating  350 , the grating structure is arranged in a manner rotated by 90 degrees in comparison with the polarization gratings  310  and  340 . As a result, the deflection direction is also rotated by 90 degrees relative to the deflection direction of the gratings  310  and  340 . Such a polarization grating stack can be used for example in combination with a two-dimensional arrangement of spherical lenses  155  and an array  125  of square or round diaphragms which are controllable in two directions. Observer tracking both in the horizontal direction and in the vertical direction thus becomes possible. 
         [0073]    The grating structures of the two polarization gratings  310 ,  340  need not be arranged orthogonally with respect to the grating structure of the polarization grating  350 , rather they can be arranged such that the deflection can be effected in two arbitrarily chosen directions. 
         [0074]    Other illumination devices  100  that are controllable in two directions can also be employed. 
         [0075]      FIG. 9  shows a configuration variant similar to  FIG. 6 . However, it additionally has a second switchable or controllable polarization grating stack  305 , whose grating structure of the switchable or controllable polarization gratings  315 ,  345  illustrated is rotated relative to the grating structure of the two polarization gratings  310 ,  340  in the polarization grating stack  300 . This configuration contains two controllable deflection gratings having a variable grating period  700  and  705 . In this case, purely by way of example, the additional controllable deflection grating  705  having a variable grating period is assigned to the polarization grating stack  305 , the diffraction direction of said deflection grating being coordinated with the diffraction direction of the polarization grating stack  305 . The controllable deflection grating having a variable grating period  700  is assigned to the polarization grating  300  and is coordinated with the deflection direction thereof. Purely by way of example, it is arranged between the two polarization grating stacks  300 ,  305 . In this case, the deflection grating  700  serves for example for horizontal light deflection, and the grating  705  for vertical light deflection. The variable deflection gratings  700  and  705  enable an additional finely stepped light deflection and/or compensation of the wavelength dependence of the light deflection in the polarization grating stacks  300  and  305 . 
         [0076]    The configuration variants in  FIGS. 1 to 9  relate to the use of conventional polarization gratings. These polarization gratings have the property that they alter the polarization of the incident light, for example from left circular to right circular. 
         [0077]      FIG. 10  shows a configuration variant of the invention in which the polarization grating stack  300  contains switchable or controllable polarization gratings  370 ,  380  which are based on a periodically twisted structure and in which the light passing through maintains the direction of rotation of its polarization. The configuration variant is constructed in a manner similar to  FIG. 6 . However, the first polarization grating  370  of the polarization grating stack  300  is illustrated as activated, that is to say that the light passes through this polarization grating in a diffracted and thus deflected manner. When the light passes through this polarization grating  370 , the light maintains its direction of rotation of the polarization in the same way as when it passes through the second polarization grating  380 , which is illustrated here as not activated, such that no further light deflection takes place therein. 
         [0078]    If the intention is to use only one deflection direction, that is to say that only one fixed input polarization is selected for each grating  370 ,  380 , it is possible to dispense with a switchable or controllable retardation layer between two successive switchable or controllable polarization gratings  370 ,  380 , as is illustrated purely by way of example here in  FIG. 10 . Since the polarization gratings  370 ,  380  used here do not change the direction of rotation of the polarization as light passes through, advantageously in this configuration variant no polarization rotating layers are required between the polarization gratings  370 ,  380  if polarization gratings having an identical twist are used in the layer stack. In this example, the illumination unit  100  generates right circularly polarized light. This right circular polarization state is maintained during passage through both polarization gratings  370 ,  380 . For a spatial light modulator  510  that requires linearly polarized light, as in  FIG. 6  a birefringent retardation layer  410  can be included and undesired stray light of the zeroth order of diffraction can be suppressed by means of a linear polarizer  520 . 
         [0079]    Finally, it should be pointed out very particularly that the exemplary embodiments discussed above serve merely to describe the claimed teaching, but do not restrict the latter to the exemplary embodiments. In particular, the exemplary embodiments described above could—insofar as is possible—be combined with one another. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
         
           
               100  Illumination unit 
               110  Light source matrix 
               120  Controllable slit diaphragm array 
               125  Controllable diaphragm array 
               130  Polarization filter 
               140  Birefringent retardation layer 
               150  Cylindrical lens array 
               155  Lens array 
               200  Variable controllable polarization grating 
               300  Polarization grating stack 
               305  Rotated polarization grating stack 
               310  1 st  switchable polarization grating 
               311  1 st  segment of the 1 st  switchable polarization grating 
               312  2 nd  segment of the 1 st  switchable polarization grating 
               315  1 st  switchable polarization grating in the rotated polarization grating stack 
               320  switchable birefringent retardation layer 
               321  1 st  segment of the switchable birefringent retardation layer 
               322  2 nd  segment of the switchable birefringent retardation layer 
               325  Switchable birefringent retardation layer in the rotated polarization grating stack 
               335  Switchable birefringent retardation layer in the rotated polarization grating stack 
               340  2 nd  switchable polarization grating 
               341  1 st  segment of the 2 nd  switchable polarization grating 
               342  2 nd  segment of the 2 nd  switchable polarization grating 
               345  2 nd  switchable polarization grating in the rotated polarization grating stack 
               350  Switchable birefringent retardation layer 
               360  Rotated switchable polarization grating 
               370  1 st  switchable polarization grating maintaining the direction of polarization 
               380  2 nd  switchable polarization grating maintaining the direction of polarization 
               410  Birefringent retardation layer 
               420  Linear polarization filter 
               510  Spatial light modulator 
               520  Linear polarization filter 
               530  Linear polarization filter 
               600  Field lens 
               700  Controllable deflection grating 
               705  Rotated controllable deflection grating