Holographic display with suppression of the zero diffraction order

Abstract

In a first aspect, the invention relates to an optical arrangement (1, 12) for a diffractive display. Said display has, in sequence along a direction of extent, a first reflective diffraction grating (15), a polarisation modulator (16), a second reflective diffraction grating (17) and a polarisation filter (18) for the first polarisation, the second polarisation being a transmission polarisation. The polarisation modulator (16) is configured to convert, at least in part, a portion of light having a first linear polarisation and a portion of light having a second polarisation, perpendicular to the first polarisation, into one another, the portions depending on the angle of incidence of the light on the polarisation modulator (16). The second reflective diffraction grating (17) is configured to diffract light with a first angle of incidence on the diffraction grating at least partially in the direction of the first reflective diffraction grating (15) with a first angle of emergence, and the first reflective diffraction grating (15) is configured to diffract diffracted light from the second reflective diffraction grating (17) at least partially with a second angle of emergence in the direction of the polarisation filter (18). The portion of the light which is converted by the polarisation modulator (16) from the first polarisation to the second polarisation or, conversely, from the second polarisation to the first polarisation is not the same for the second angle of emergence as for the first angle of emergence. Further aspects of the invention relate to a display device having the optical arrangement.

Description

[0001] HOLOGRAPHIC DISPLAY WITH NUMBER ZERO DIFFERENCE SUPPRESSION

[0002] DESCRIPTION

[0003] In a first aspect, the invention relates to an optical arrangement for a diffractive display. This arrangement comprises, successively along a direction of extension, a first reflective diffraction grating, a polarization modulator, a second reflective diffraction grating, and a polarization filter for the first polarization with the second polarization as the transmission polarization. The polarization modulator is configured to convert light with a first linear polarization and a second polarization perpendicular to the first into each other, at least to a certain extent, the proportion of which depends on the angle of incidence of the light on the polarization modulator.The second, reflective diffraction grating is configured to diffract light with a first angle of incidence onto the diffraction grating, at least partially, towards the first reflective diffraction grating with a first angle of reflection. The first reflective diffraction grating is configured to diffract the diffracted light from the second reflective diffraction grating, at least partially, towards the polarizing filter with a second angle of reflection. The fraction of light that is converted by the polarizing modulator from the first polarization to the second polarization, or vice versa, is different for the second angle of reflection compared to the fraction for the first angle of reflection.

[0004] In further aspects, the invention relates to a display device with the optical arrangement.

[0005] Background and state of the art:

[0006] Displays, including rear-projection displays, are known from the prior art. In such displays, light from a light source, particularly an image sensor, is projected onto a surface to display information. This surface can then serve as a projection surface. For this purpose, the surface can be equipped with a diffuser. This diffuser can also be diffractive, particularly holographic. In a rear projection, the surface can be at least partially transparent, so that the surface is illuminated from one side, while the display is viewed from the other side.

[0007] Such a transparent display for rear projection is shown in Fig. 1. An image generator 3, also called a PGU (picture generating unit), illuminates an at least partially transparent, planar optical arrangement 1, e.g., a glass disc, with illumination rays 4 from a first side 7, also called the back. This surface forms a projection surface for a (rear projection) display device. This disc 1 includes a diffuser (not explicitly shown) which scatters the rays in the desired manner (scattered light 2) so that an image is visible to a viewer s on the second side 8 of the projection surface 1. The disc 1 can be designed as a so-called "stack" (a stack of layers), in which the diffuser is applied to a substrate (e.g., the glass). The diffuser can also be a diffractive, in particular a holographic, diffuser.This has the advantage that, firstly, the scattering properties are very well adjustable, and secondly, a holographic diffuser only scatters the light for certain wavelengths and / or angles, so that a particularly transparent display device 1 can be realized, which is a transparent disc when the PGU is switched off.

[0008] A disadvantage of the displayed image is that, due to the wavelength-selective behavior of the holographic optical elements and the limited diffraction efficiency, it is not possible to diffract 100% of the light into the desired order. Typical light sources with a spectral width are used, which, for economic reasons, cannot fall below a certain bandwidth. Consequently, a non-negligible portion of the projected light field (partially independent of wavelength) is transmitted without being diffracted as desired. This "undesired" light, in turn, is the source of several phenomena that degrade the quality of the display. The interference order (i.e., undiffracted light in the so-called zeroth order) causes light pollution in the surrounding area. Furthermore, this interference order can be visible to the viewer and, in the worst case, is directly superimposed on the light in the desired order (first order).The problem of zero-order decoupling arises primarily when the illumination of the disk or display device 1 occurs at an angle smaller than the critical angle of total internal reflection within the disk or device. This is often desirable because larger angles of incidence can lead to Fresnel reflections at the outer interface of the projection surface 1. However, even otherwise, broadband scattering of undiffracted light at or within the functionalized disk can contribute to a chromatic aberration and the formation of double images due to the finite substrate thickness (disc thickness).

[0009] Figures 2 and 3 illustrate the aforementioned problems schematically and by means of a simulation, respectively. Figure 2 shows a display device according to Figure 1 (or according to Figure 4, but without a polarization filter), but here, in addition, the stray light 6 from the undiffracted zeroth order as well as from broadband scattering of the light 4 coming from the PGU 3 within or at the disc 1 is shown. This is also visible to the viewer s in addition to the useful light 2.

[0010] Figure 3 illustrates in particular the problem of extracting the undiffracted zeroth order. The spectrum of the light source (intensity shown in relative units [au] versus wavelength) used to illuminate optical arrangement 1 (see top right) with a holographic diffuser is shown in the upper left. It can be seen that this is a broadband light source with a main peak at approximately 420 to 460 nanometers (nm) and a second, broad secondary peak between approximately 520 and 620 nm. Because the holographic diffuser only very selectively diffracts the light into the first order or useful order 9 (see plane of view or "screen" 11 top right) at certain wavelengths, and because the diffraction efficiency is less than 1 in this region, a significant amount of light ends up in the zeroth order.The interference order, which also lands on the screen (reference numeral 10) and might be visible to a viewer, is shown again in the top view. The proportion of light in interference order 10 is even greater than that in useful order 9 (for details on the conditions underlying the simulation, see the description of the invention below, in particular Figure 5). This viewing plane is shown again in top view at the bottom right, as is the spectral composition of the light. The spectrum of the light captured in the viewing plane corresponds approximately to that of the light source in terms of its spectral components. There are so-called edge-lit holograms, where a hologram is illuminated by a side surface of an optical arrangement in such a way that the light undiffracted by the hologram remains within the optical arrangement and does not cause any display interference. However, this type of illumination is not always desirable, especially when a large-area display is required.

[0011] Purpose of the invention:

[0012] It is therefore an object of the invention to provide an optical arrangement for a display device or a display device without the disadvantages of the prior art. In particular, it is an object of the invention to provide an optical arrangement for a display device or a display device for an improved display which is virtually free of interference, color shifts, and double images.

[0013] Summary of the invention:

[0014] The problem is solved by the features of the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0015] In a first aspect, the invention relates to an optical arrangement for a diffractive display. This arrangement comprises, successively along a direction of extension, a first reflective diffraction grating, a polarization modulator, a second reflective diffraction grating, and a polarization filter for the first polarization with the second polarization as the transmission polarization. The polarization modulator is configured to convert light with a first linear polarization and a second polarization perpendicular to the first into each other, at least to a certain extent, the proportion of which depends on the angle of incidence of the light on the polarization modulator.The second, reflective diffraction grating is configured to diffract light with a first angle of incidence onto the diffraction grating (preferably the second one) at least partially towards the first reflective diffraction grating at a first angle of reflection, and the first reflective diffraction grating is configured to diffract the diffracted light from the second reflective diffraction grating at least partially towards the polarizing filter at a second angle of reflection. The fraction of the light that is converted by the polarizing modulator from the first polarization to the second polarization, or vice versa, is different for the second angle of reflection compared to the fraction according to the first angle of reflection.

[0016] It is obvious to those skilled in the art (and this is also clearly described and evident in this document) that the following is meant: the proportion of light that is converted by the polarization modulator from the first polarization to the second polarization, or vice versa, is different for angles of incidence corresponding to the second angle of reflection compared to the proportion corresponding to the angle of incidence corresponding to the first angle of reflection. As described herein, this can primarily mean that the angle of incidence corresponds once to the second angle of reflection and once to the first angle of reflection (or 180° - the first / second angle of reflection, depending on the direction of measurement). Preferably, the proportion is not dependent on the direction of measurement or the direction of the incident light, but only on the angle with the respective surface normal, which can be chosen, for example, according to the direction of incidence of the light.

[0017] Advantageously, such an optical arrangement can be illuminated with (preferably linearly) polarized light (e.g., from a PGU) in such a way that this light is at least partially diffracted at the second diffraction grating at the first angle of incidence towards the first diffraction grating and simultaneously exhibits essentially the first polarization there. The polarization transformation of the polarization modulator according to the first angle of incidence can be taken into account, so that the light from the PGU is advantageously polarized such that it exhibits (essentially) the first polarization at the second diffraction grating. The light not diffracted by the diffraction grating is then blocked by the polarization filter, since it exhibits the first polarization, thus preventing the previously described interference orders at this point.The light diffracted towards the first diffraction grating now passes through the polarization modulator, which converts the first polarization into the second polarization proportionally (corresponding to the first angle of incidence). Thus, the light is in a mixed state between the second and first polarizations, with the proportion corresponding to this proportion. This proportion can, in principle, take on a value from zero to one. This light can then be diffracted again at the first diffraction grating towards the polarization filter, this time with the second angle of incidence. It then passes through the polarization modulator once more. Here, the light is again proportionally transformed, so that the portion of the light exhibiting the second polarization is converted into the first polarization according to the second angle of incidence, and likewise, the portion of the light exhibiting the first polarization is converted into the second polarization.Since the proportion now differs from the previous one in any case, it is impossible for the light to again exhibit essentially the first polarization and be blocked essentially completely by the polarizing filter. Diffraction at the second diffraction grating also essentially does not occur, for example, because the second diffraction grating is not designed for this illumination angle and / or at least not for the transmission polarization. This ensures that the light diffracted by the first diffraction grating, which corresponds to the useful order, at least partially passes through the polarizing filter (the part exhibiting the second polarization) and contributes to the display, while the light not diffracted by the second diffraction grating (the perturbation order) is blocked by the polarizing filter as described above.

[0018] In a second aspect, the invention relates to an optical arrangement for a diffractive display. This arrangement comprises, successively along a direction of extension, a first reflective diffraction grating, a polarization modulator, a second reflective diffraction grating, and a polarization filter for the first polarization with the second polarization as the transmission polarization. The polarization modulator is configured to generate a first linear polarization for light with at least one first angle of incidence and a given polarization. Simultaneously, the polarization modulator is configured to convert less light with a first linear polarization to a second, perpendicular polarization for an angle of incidence corresponding to a first angle of reflection than for an angle of incidence corresponding to a second angle of reflection.The second, reflective diffraction grating is configured to diffract light with a first angle of incidence onto the diffraction grating at least partially towards the first reflective diffraction grating with a first angle of refraction, and the first reflective diffraction grating is configured to diffract diffracted light from the second reflective diffraction grating at least partially with a second angle of refraction towards the polarizing filter.

[0019] In a third aspect, the invention relates to an optical arrangement for a diffractive display. This arrangement comprises, sequentially along a direction of extension, a first reflective diffraction grating, a retardation plate, a second reflective diffraction grating, and a polarization filter for the first polarization, with the second polarization serving as the transmission polarization. The second reflective diffraction grating is configured to diffract light with a first angle of incidence onto the grating at least partially towards the first reflective diffraction grating at a first angle of reflection, and the first reflective diffraction grating is configured to diffract the diffracted light from the second reflective diffraction grating at least partially towards the polarization filter at a second angle of reflection. The first angle of reflection is greater than the second angle of reflection. Preferably, the retardation plate can be an A / 2 retardation plate.An A / 2 retardation plate is preferably a retardation plate configured to realize an A / 2 retardation for at least one wavelength of light. Preferably, the A / 2 retardation plate is configured to convert light with a second polarization at the angle of incidence into a first polarization. For example, the A / 2 retardation plate has an optical axis in the plane of incidence as described herein. Preferably, the angle of first reflection is greater than the angle of first incidence.

[0020] It is evident to the person skilled in the art that the advantages, definitions and embodiments of the optical arrangement according to the invention according to the first aspect also apply to the claimed optical arrangement according to the invention according to the second or third aspect.

[0021] An "optical arrangement" preferably describes an arrangement of optical components, e.g., a connected arrangement. An optical arrangement can, in particular, include a stack.

[0022] A "stack" preferably refers to a connected arrangement of optical components or elements. The components or elements are preferably arranged in a compact, integrated configuration. In particular, a stack can comprise a stack of these components, for example, as a layer stack with the individual components as layers. A stack can also be referred to as a "stacked arrangement." The components or elements preferably comprise at least the first diffraction grating, the polarization modulator, the second diffraction grating, and the polarization filter.

[0023] A "display" preferably enables the generation of an image visible to a viewer, e.g., a real and / or virtual image. "Display" preferably describes the function, i.e., the display of (image) information by means of light. The display device preferably describes the device which (among other things) enables such a display.

[0024] A "diffractive display" preferably enables the generation of an image using diffraction. In this case, the first and second diffraction gratings are preferably diffractive and, with suitable illumination, enable the generation of an image.

[0025] "Arranged sequentially along a direction of extension" preferably means that there is at least one direction within the optical arrangement in which the elements are arranged sequentially. Preferably, this means that the elements are arranged sequentially in the aforementioned order, i.e., at least along one direction of extension, first the first diffraction grating, then the polarization modulator, then the second diffraction grating, then the polarization filter. Additional elements may be included in between (e.g., "substrate"), but this does not change the aforementioned order. The elements may also be arranged at least partially directly one after the other, i.e., without intermediate elements. They may also be arranged entirely, i.e., all elements directly one after the other, without intermediate elements.

[0026] A first reflexive "diffraction grating" preferably comprises an element that has a diffractional structure. A diffractional structure can, for example, comprise a diffraction grating or a hologram, or a holographic-optical element (HOE, su).

[0027] "Reflexive" preferably means that the fundamental direction of the diffracted light is opposite to the direction of the light to be diffracted before diffraction. If at least two sides or outer surfaces can be assigned to the diffraction grating, the diffraction takes place in the same space adjacent to one side from which the light to be diffracted originated. If the space in which the diffraction grating is located is divided, for example, into two half-spaces separated by a plane in which the diffraction grating is arranged, then the light originates from the half-space into which it is diffracted before diffraction.

[0028] The nomenclature "first diffraction grating" and "second diffraction grating" preferably serves only to uniquely identify the diffraction gratings. Preferably, this designation clearly assigns the arrangement within the optical system and the respective functionality.

[0029] A "polarization modulator" preferably refers to an element by means of which the polarization state of light can be modulated, i.e., changed, at least if certain conditions are met or within a specifiable range, e.g., the incident polarization state, the spectrum of the light, and / or the angle of incidence. An example of a polarization modulator is a so-called "retardation plate" (see herein).

[0030] “Light” preferably describes electromagnetic radiation, e.g. in the form of electromagnetic waves, in the visible spectrum, especially at wavelengths between 360 nanometers (nm) and 830 nm.

[0031] "Polarization" preferably describes the polarization of an electromagnetic wave, specifically the plane in which the electric field vectors oscillate. A "linear polarization" preferably refers to a polarization that oscillates in an unchanging plane without external influences. An opposite of linear polarization can be, for example, elliptical and especially circular polarization, in which the polarization direction changes continuously.

[0032] A "first polarization" and a "second polarization, perpendicular to the first" define specific preferred polarizations for the device described here. These preferred polarizations are characterized, for example, by their respective transmission behavior in the polarization filter. They are two polarizations perpendicular to each other. The first and second polarizations can also be described as the first and second polarization planes.

[0033] The polarization modulator is "configured to convert light with a first linear polarization and a second polarization perpendicular to the first, at least partially, into one another." This preferably means that the polarization modulator acts on the polarization state of the light during a (single) transmission, such that a portion of the light exhibiting the first polarization state (if present) is partially converted during transmission into the second polarization state, and / or the portion of the light exhibiting the second polarization state (if present) is partially converted into the first polarization state. The polarization of the light itself can advantageously be described in general terms as the superposition of light in the first polarization and light in the second polarization; this is also referred to as the projection of the light into the first and second polarization states, respectively.The second polarization state, so that the transition from the first to the second polarization and vice versa can be described for every (preferably linear) polarization state of light. The projection into or onto the first or second polarization can take values ​​between zero and one, where zero means that no light of the respective polarization is present and one means that all light exhibits the respective polarization. The sum of the two projections can always be one. If one assumes a linear polarization and measures the angle α that the plane of oscillation of the linear polarization forms with the second polarization plane, this projection is given, for example, by the cosine function. 2 a and through sin 2a for the first polarization. The respective projection can, for example, describe the normalized intensity of the light in the respective polarization (the first or the second). If, on the other hand, the projection onto the first and / or second polarization is known in this definition, the angle that the linear polarization has with the second polarization plane can be determined by back-calculation. The proportion of light that is transformed from the first polarization to the second polarization and from the second polarization to the first polarization can be determined, for example, via the respective projections before and after passing through the polarization modulator, in particular by comparing the respective projections before and after passage. An example of such a proportion determination by comparison is given below:

[0034] , where p rher j e p roThe projection onto the first polarization before the passage describes p1, whereas p2 describes the projection onto the second polarization before the passage. achher the projection onto the first polarization after the passage and p™ chher the projection onto the second polarization after the pass.

[0035] If, for example, the projection onto the second polarization is one before passing through the polarization modulator (and correspondingly the projection onto the first polarization is zero – this would be equivalent to describing the light as being in the second polarization state), and after passing through the polarization modulator the projection onto the first polarization is one ("the light is in the first polarization state"), then the fraction is equal to one (see formula (2)), meaning all the light has been transferred from the second to the first polarization; the fraction is one. Such a "transfer" between the polarization states can also be described by a 90° rotation of the polarization. Conversely, if the projection onto the first polarization is, for example...Before passing through the polarization modulator, the polarization coefficient is one (and correspondingly the projection onto the second polarization is zero), and after passing through the polarization modulator, the projection onto the second polarization is one (see formula (1)). Therefore, the component is also one; all light has now been transferred from the first to the second polarization. This also corresponds to a 90° rotation of the polarization, but in the opposite direction. Such a polarization transformation corresponds to the change in polarization that an ideal A / 2 retarder plate would achieve, whose optical axis lies in the plane of incidence of the light into the polarization modulator, specifically between the first and second polarizations, at an angle of 45° to both polarizations.

[0036] To better understand this, let's describe a case where the fraction is not equal to one. For example, the projection onto the first polarization is one before the passage and 0.75 after the passage. This corresponds to a fraction in the second polarization of zero before the passage and 0.25 after the passage. Then, according to formula (1), the fraction of the light transformed from the first to the second polarization is 0.25, or one-quarter. From this, the angle α after the passage can also be determined; this is 60°. The polarization was therefore only rotated by 30° and not by 90°, as would have been the case with a fraction of one. The fraction can typically take values ​​from zero to one.Such behavior corresponds to the change in polarization that would be achieved by an ideal A / 2 retardation plate whose optical axis lies in the entry plane of the light, but at a different angle, namely at an angle of 75° to the second polarization or 15° to the first polarization.

[0037] The transferred component can be a crucial factor in the filter's operation, as it determines the projections in the first and second polarizations. These projections, in turn, advantageously determine the filter function of the polarizing filter and, if applicable, the diffraction of the diffraction gratings, provided they exhibit a preferred polarization in which they diffract more efficiently.

[0038] Preferably, the described polarizations are linear polarizations. With a polarization modulator, e.g., a retardation plate, varying the angle of incidence can also generate non-linear, e.g., elliptical, polarization states, which, on average, always exhibit a projection in the first and second polarizations. However, this is not problematic for the operation of the optical arrangement described here, as it merely means that a portion of the useful order generated by the first diffraction grating (the projection onto the first polarization) will not be transmitted by the polarization filter and is therefore lost.As long as a sufficiently large proportion of the light diffracted by the first diffraction grating is transmitted, and especially as large as the proportion of the undiffracted light (zero order) from the second diffraction grating, significant advantages of the invention can be realized, even if (slightly) elliptical polarization states occur. Preferably, the incident light, and in particular the first angle of incidence and / or its polarization, can always be adjusted such that after the first passage of the light through the polarization modulator, essentially all the light is in the first polarization (and thus exhibits essentially a linear polarization), and therefore interfering orders are blocked by the polarization filter.

[0039] It has been advantageously found that a retarder plate, in particular an A / 2 retarder plate as described above, inherently possesses the aforementioned properties when the first angle of incidence is greater than the second angle of incidence, and preferably, furthermore, when the first angle of incidence is greater than the first angle of incidence. This applies in particular to the preferred angle ranges described herein.

[0040] “To convert into each other” preferably describes the conversion described above of a first polarization (i.e. preferably a projection into the first polarization) into a second polarization (i.e. preferably a projection into the second polarization) and vice versa.

[0041] The "angle of incidence of the light on the polarization modulator" preferably describes the angle that the light makes with a normal to the entrance plane (preferably measured at the point of entry). Preferably, the first angle of incidence, first angle of reflection, second angle of reflection, and / or second angle of incidence described here are also the angles of incidence of the light on the polarization modulator, because the optical elements are arranged in parallel surfaces. The "angle of incidence of the light on the polarization modulator" can also be an angular range, as described herein for the other angles. In that case, the "angle of incidence of the light on the polarization modulator" can, for example, be defined by the angle of incidence of the principal beam direction on the polarization modulator.

[0042] A "principal beam direction" of light or a beam of light rays is preferably a direction in which the light ray exhibits maximum intensity or an average intensity across all directions. The term principal beam or principal beam direction preferably refers to the central ray of a beam or its direction. The direction of the principal beam, in particular, indicates the direction of the beam. In the case of a collimated beam, the remaining rays of the beam run essentially parallel to the principal beam direction, so that the principal beam direction is preferably representative of the rays of a beam. In the case of a non-collimated beam, the rays of the beam define a solid angle, in the center of which the principal beam direction may lie.

[0043] The functioning of the diffraction gratings and corresponding definitions (e.g., for the first angle of incidence, the first angle of reflection, and the second angle of reflection) are preferably listed below in the description of Figure 4 and are advantageously applicable generally without limitation to the embodiment described therein.

[0044] "First angle of incidence," "first angle of reflection," and / or "second angle of reflection" are preferably quantities that can be determined and therefore considered given for a given optical arrangement, e.g., through manufacturing processes of the diffraction gratings (e.g., by exposure to appropriately directed light). Terms such as "essentially," "approximately," "about," "approximately," etc., preferably describe a tolerance range of less than ± 40%, more preferably less than ± 20%, more preferably less than ± 10%, even more preferably less than ± 5%, and particularly less than ± 1%. Similarly, "approximately" preferably describes quantities that are about equal. "Partially" preferably describes at least 5%, more preferably at least 10%, and particularly at least 20%, and in some cases at least 40%. However, "partially" can also mean 5% or less, 2% or less, or 1% or less.Terms like "essentially" preferably always include the exact value.

[0045] A "first polarization filter" preferably describes a filter that filters out, or in other words, blocks, the first polarization. Blocking or filtering preferably means an attenuation compared to unimpeded transmission by a factor of at least 2, 5, 10, 100, or 1000. The first polarization can preferably also be called "blocking polarization." The second polarization is preferably transmitted and, apart from any losses, can pass through the polarization filter essentially unimpeded. This is therefore also referred to as "passing polarization." Preferably, for the same input intensity, the intensity of the passing polarization at the output is at least a factor of 2, 5, 10, 100, or 1000 compared to the blocking polarization. A polarization filter is, for example, a dichroic polarization filter.

[0046] "Unequal" preferably means a deviation by a factor of at least 1.05, more preferably at least 1.1; even more preferably at least 1.2 and most preferably at least 1.3. This means that the two unequal proportions differ so greatly from each other that they can only be converted into each other by multiplication with the aforementioned or larger factors.

[0047] Preferably, the optical arrangement has the aforementioned properties for at least one wavelength in the visible spectrum, and more preferably for at least one wavelength range, which may be at least 10 nanometers (nm) wide. It is particularly preferred that at least the polarization modulator and / or the polarization filter has the described properties for at least one broad wavelength range, wherein the broad wavelength range is 20 nm or greater, 50 nm or greater, 100 nm or greater, 200 nm or greater, 300 nm or greater, or 400 nm or greater. Preferably, the at least one broad wavelength range covers a wavelength range in the red, green, and / or blue spectral region.

[0048] Advantageously, the inventor recognized that suitable angles and / or input polarizations can always be found for the polarization manipulator, allowing the disturbance order to be greatly reduced while the useful order can at least partially pass through the polarization filter.

[0049] The optical arrangement can be used, for example, for rear-projection applications. When used as a display element, the optical arrangement can also offer the advantages of an edge-lit application (e.g., zero-order suppression) without its disadvantages (illuminating large displays is difficult or only possible with a large arrangement thickness). In a preferred embodiment of the invention, the component corresponding to the second angle of incidence is greater than the component corresponding to the first angle of incidence. If the arrangement is then illuminated such that the light diffracted by the first diffraction grating essentially exhibits a first polarization, this polarization can be at least partially preserved by the component corresponding to the first angle of incidence. This is particularly advantageous if the first diffraction grating is especially efficient at diffracting the first polarization.At the same time, however, a larger portion than before is (back-)transformed into the second polarization according to the second angle of reflection, thereby improving the transmission of the useful order through the polarization filter.

[0050] In a further preferred embodiment of the invention, the first and / or the second diffraction grating is configured to diffract light with the first polarization. This preferably means that the diffraction efficiency for the first polarization is high or highest. It may be preferred that essentially only the first polarization is diffracted. However, it is also possible that the second polarization is diffracted, but with a lower efficiency, preferably by a factor of at least 1.2, 1.3, 1.4, 1.5, or 2.

[0051] In a further preferred embodiment of the invention, the proportion of light at the first angle of incidence is greater than the proportion at the first angle of reflection. This allows the illumination to exhibit an advantageous proportion of light with the second polarization before entering the optical arrangement, which is then converted to a greater extent into the first polarization. This prevents Fresnel reflections when the light enters the optical arrangement.

[0052] In a further preferred embodiment of the invention, the portion of the beam with a larger angle of incidence onto the polarization modulator is reduced. This advantageously allows the second angle of reflection to be chosen to be sufficiently large. It has been found that the portion with a larger angle of reflection is reduced. While the path through the modulator may be longer, if the polarization modulator and the incident beam are tilted relative to each other, the refractive index difference (from the beam's perspective) can be reduced, resulting in less delay.

[0053] The angle or proportion can also be determined for delay plates or phase modulators in general as follows: The path difference increases when tilted about the slow axis, while the change is negative when tilted about the fast axis, whereby the change can be approximately determined by:

[0054] A = ß (cp e 2- cp0 2 )

[0055] Here, β is preferably a constant that depends on the respective retarder (β ~ I nm / degree). 2 ), see also https: / / www.b-halle.de / produkte / verzoegerer.html, accessed on 12.07.2024 at 15:00.

[0056] Generally, the optical axis can coincide with the slow axis. However, in some cases, the optical axis can also coincide with the fast axis.

[0057] Similarly, the angle-of-incidence-dependent delay in the case of a delay plate can be described as a polarization modulator, as described in the publication by Honggang Gu et al. 2018 J. Opt. 20 015401, which is hereby incorporated into the disclosure. In a further preferred embodiment of the invention, the component corresponding to the second angle of incidence is at least 1.5 times, more preferably at least 2 times, even more preferably at least 3 times, and most preferably at least 4 times greater than the component corresponding to the first angle of incidence. It has been found that this allows for a particularly good signal-to-noise ratio (SNR, see below) to be achieved for an optical display. Preferably, a plurality of corresponding angles of incidence and reflection are suitable for this purpose. For example, the first angle of incidence and / or the second angle of reflection can be kept correspondingly small (e.g.,The first angle of incidence is 10° or less, and the first angle of reflection is correspondingly large, e.g., 60° or greater or 75° or greater. It has also been shown that a corresponding ratio can be achieved even with a larger first angle of incidence (e.g., 35° or greater, but preferably smaller than the first angle of reflection), for example, by appropriately orienting the optical axis and / or by appropriately polarizing the incident light. Advantageously, suitable angles of incidence and reflection can also be found through appropriate simulations.

[0058] In a further preferred embodiment of the invention, the proportion corresponding to the first angle of incidence and / or the second angle of reflection is essentially 1. This can also be advantageously achieved by the measures described above. This allows, for example, illumination with a second polarization at the first angle of incidence. If the second polarization corresponds to a p-polarization, this can be particularly effective in preventing Fresnel reflections at the optical arrangement, depending on the first angle of incidence.

[0059] In a further preferred embodiment of the invention, the proportion corresponding to the first angle of incidence is 0.5 or less, more preferably 0.25 or less. Surprisingly, it has been found that corresponding proportions can be achieved, particularly for larger first angles of incidence (preferably 60° or greater, more preferably 70° or greater, and especially 75° or greater), and are also stably attainable over larger angular ranges.

[0060] In a further preferred embodiment of the invention, the first polarization is an s-polarization. "s-polarization" preferably denotes the polarization direction of an electric field that oscillates perpendicular to the plane of incidence into the optical arrangement. The plane of incidence is defined by the incident vector of the light into the arrangement and the surface normal to the arrangement at the point of incidence. "p-polarization" denotes an electric field oscillating parallel to the plane of incidence and perpendicular to the s-polarization. If the first polarization is an s-polarization, the second polarization is preferably a p-polarization.

[0061] In a further preferred embodiment of the invention, the first and / or the second diffraction grating comprises a holographic optical element (HOE), in particular a volume HOE.

[0062] In holography, unlike conventional imaging techniques such as photography, not only the intensity of an object being imaged is recorded, but also the phase relationships of the light emanating from the object. These phase relationships contain additional spatial information, allowing for the creation of a three-dimensional impression of the image. This is achieved through the interference of light rays during the recording or exposure. The object being exposed is illuminated with coherent light, which is reflected and scattered by the object. The resulting wave field, the so-called object wave, is superimposed with light coherent to the object wave (the so-called reference wave – typically from the same light source, e.g., a laser), and the wave fields interfere with each other as a function of their phase relationship. The resulting interference pattern is then used, for example, to create a three-dimensional image.The image is captured by a light-sensitive layer (also called exposure), thus storing the information contained in the phase. To reconstruct the image, the resulting hologram is illuminated with a light wave identical or similar to the reference wave, which is then diffracted by the recorded interference patterns. In this way, the original wavefront of the object wave can be reconstructed.

[0063] There are various types of holograms, such as volume holograms. Volume holograms preferably have a thickness sufficient to realize multiple Bragg planes. This allows volume holograms to be particularly wavelength- or angle-selective. Holograms can be, for example, transmission or reflection holograms, each generating this reconstruction either through transmission or reflection. For instance, if one is on the opposite side of a transmission hologram from the light source and views it, the depicted object appears three-dimensional. With a reflection hologram, one preferably needs to be on the same side as the light source. Reflection holograms preferably exhibit a particularly wavelength-selective efficiency in diffracting light in a specific direction (along a specific angle).The word hologram is used here primarily as a synonym for the holographic structure that generates light diffraction. Colloquially, "hologram" is sometimes used to refer to the generated, especially three-dimensional, image. However, an expert understands from the context what is meant by the term "hologram" in each case.

[0064] Besides three-dimensional representation, holograms can be used in the form of so-called holographic-optical (construction) elements (HOEs), whose holographic properties can be used for the optics of devices. For example, HOEs can replace conventional lenses, mirrors, and prisms. In other cases, HOEs are used as special diffraction gratings. HOEs exhibit, for example, spectral selectivity and / or angle-of-incidence selectivity. Simultaneously, they can be completely or partially transparent for other spectral ranges and / or angles of incidence. Holograms also enable a combination of representation and light shaping. Furthermore, HOEs can be used as diffusers with tailored scattering properties. Holograms, especially technical holograms, can be recorded directly using various holographic methods or with the aid of wavefront printers or similar technologies.Stereoholographic printers can print from computer-generated data. While these manufacturing methods are suitable for the mass production of optical functions in the form of holograms, they are not practical due to the high time required. For this purpose, suitable methods, particularly optical replication, are available, in which a holographic "master" can be replicated many times using a suitable copying process. A holographic optical element, especially a volume hologram, enables a substantially or partially transparent first and / or second diffraction grating in a particularly simple manner. Holographic optical elements, and especially volume holograms, allow for wavelength- and / or angle-selective diffraction, so that light scattering occurs only for suitable wavelengths and / or angles, while other wavelengths and / or angles are advantageously transmitted undiffracted.By appropriately matching the wavelength(s) and / or illumination angle of the PGU to the optical arrangement, a (partially) transparent display with the desired optical properties can be implemented particularly easily. A holographic diffuser can be designed in such a way that the desired (preferably) directional scattering properties are achieved.

[0065] In a further preferred embodiment of the invention, the first diffraction grating comprises a diffuser, in particular a holographic diffuser, wherein the diffuser is configured for diffraction in the direction of the polarization filter as a directed scattering.

[0066] The fact that the dispersion is "directional" can mean that the dispersion has a preferred direction, i.e., that the intensity distribution deviates from Lambert's law. Furthermore, it can mean that, in addition to the dispersion, a deflection occurs.

[0067] In particular, the scattering can be directed in such a way that a large proportion of the scattered light intensity falls within a designated eyebox, e.g., 80% or more, 90% or more, or 95% or more. Likewise, the intensity can drop sharply at an outer boundary of the eyebox due to the directed radiation, e.g., by 60% or more, 70% or more, or 80% or more.

[0068] In a further preferred embodiment of the invention, the polarization modulator comprises a delay plate, in particular an A / 2 plate.

[0069] A "retarding plate" is an optical element that can modulate the polarization and phase of transmitted light. This often incorporates a birefringent material, causing the light to propagate at different speeds depending on the position of the polarization plane.

[0070] Preferably, the relationship between the angle of incidence and the delay between the planes or axes of the delay plate is nonlinear. This has been found to be the case for a large number of standard delay plates (e.g., polymer delay plates). Thus, surprisingly large angular ranges can be advantageously found for commercially available delay plates in which the behavior of the delay plate, in conjunction with the polarization filter, enables good suppression of the disturbance order and at least acceptable transmission of the useful order through the filter.

[0071] An "A / 2 plate" retards light polarized parallel to an axis (preferably referred to here as the "optical axis," which can also be called the "crystal-optical axis" and specifically describes the slow axis) or plane by half a wavelength relative to light polarized perpendicular to it. It can thus rotate the polarization of linearly polarized light by a selectable angle. The term "A / 2 plate" is preferably used synonymously with the term "A / 2 retardation plate." In particular, the retardation plate is a retardation plate comprising a polymer. Polymer retardation plates exhibit, in particular, desired angle-of-incidence-dependent properties and are advantageously at least partially achromatic over a broad wavelength range.

[0072] In a further preferred embodiment of the invention, the A / 2 plate has an optical axis whose orientation is configured to maximize the proportion corresponding to the second angle of reflection and / or the first angle of incidence. Thus, for example, it is possible to illuminate the arrangement with p-polarized light, thereby reducing Fresnel reflections. Furthermore, it is advantageously possible to convert s-polarized light, which is diffracted particularly efficiently by the first diffraction grating, as far as possible into p-polarized light, which can pass through the polarizing filter particularly efficiently.

[0073] Preferably, the optical axis lies substantially between the first and second polarizations, preferably at an angle between 40° and 50° between the first and second polarizations, and in particular at 45° ± 3° between the first and second polarizations. For example, the optical axis (the slow axis) can be rotated 47° relative to the second polarization.

[0074] In a further preferred embodiment of the invention, the first and / or second diffraction grating is configured to diffract light with more than one wavelength, wherein the first and / or second diffraction grating in particular comprises at least one RGB-HOE.

[0075] This embodiment is explained in detail below in connection with Figure 4.

[0076] In another preferred embodiment of the invention, the A / 2 plate is at least partially achromatic. Surprisingly, it has been found that a standard A / 2 plate can adequately realize the advantages of the invention. Nevertheless, it can be particularly advantageous, especially for broadband light sources and / or RGB applications, to use an achromatic A / 2 plate.

[0077] Achromatic A / 2 plates can, for example, comprise so-called zero-order double plates, in which two zero-order delay plates made of different materials are combined, e.g., quartz and MgF2. Due to their different material dispersions, a nearly constant phase delay can be achieved over a broad spectral range.

[0078] In a further preferred embodiment of the invention, the first angle of incidence is larger than a critical angle of total internal reflection within the optical arrangement. This effectively prevents the light diffracted by the second diffraction grating from escaping the optical arrangement, even if it should not be diffracted by the first diffraction grating. This prevents the emission of interference patterns on both sides of the optical arrangement.

[0079] In a further preferred embodiment of the invention, the arrangement is at least partially transparent for at least one wavelength range of light.

[0080] In a further preferred embodiment of the invention, the arrangement further comprises a substrate. The substrate can preferably comprise glass, e.g., BK7 or PMMA. A substrate can make the arrangement particularly mechanically stable. The substrate is preferably at least one substrate; it can also comprise more than one substrate.

[0081] In a further preferred embodiment of the invention, the arrangement has a planar extent perpendicular to the direction of extent, wherein the first diffraction grating, the polarization modulator, the second diffraction grating and the polarization filter are each extended along the planar arrangement, wherein the arrangement preferably has a constant (layer) thickness in the direction of extent.

[0082] The arrangement can be flat. However, it can also be curved.

[0083] The layer thickness can also vary in other embodiments.

[0084] In a further preferred embodiment of the invention, the first angle of incidence is preferably between 20° and 65°, more preferably between 30° and 60°, and even more preferably between 40° and 50°, and in some embodiments can essentially correspond to a Brewster angle. It is also advantageous to select a first angle of incidence which represents a Brewster angle for the incidence surface (the illuminated outer surface of the optical arrangement) (i.e., forms a Brewster angle with the surface normal to this surface). Particularly in conjunction with illumination of the optical arrangement with p-polarized light, Fresnel reflections upon entry into the arrangement can be prevented.

[0085] In another preferred variant, the initial angle of incidence is between 20° and 30°. This allows, for example, incident p-polarized light to be converted particularly well into s-polarized light by the polarization modulator.

[0086] In a further preferred embodiment of the invention, the first angle of incidence is at least 50°, more preferably at least 60°, even more preferably at least 70°, and particularly at least 75°. In particular, the first angle of incidence is between 55° and 77°. It has been shown that under such angles of incidence, and also over larger angular ranges, the proportion of the first polarization that is converted into the second polarization can advantageously be kept relatively low, preferably while still maintaining a linear polarization.

[0087] In a further preferred embodiment of the invention, the second angle of incidence is 30° or less, more preferably 20° or less, even more preferably 10° or less, and yet more preferably 5° or less. In particular, the second angle of incidence can be essentially 0°. In this way, the proportion transformed from the first to the second polarization can advantageously be kept large.

[0088] In a further aspect, the invention relates to an arrangement as described herein for an edge-lit display device. In particular, the first angle of incidence is configured for illumination by a light source through a side surface of the arrangement. The side surface can preferably be arranged transversely to the planar extent of the optical axis.

[0089] In other cases, illumination can occur through an outer surface along the planar extent and / or the first angle of incidence can be set for illumination along such a surface. Thus, the optical arrangement can be used like an edge-lit display device, but without the need for illumination through the side surface.

[0090] In a further aspect, the invention relates to an edge-lit display device comprising an arrangement as described herein and a light source configured to couple light into a side surface of the arrangement at an angle corresponding to the first angle of incidence on the diffraction grating. This angle is greater than the angle of total internal reflection in the arrangement.

[0091] It is evident to those skilled in the art that the advantages, definitions and embodiments of the arrangement according to the invention also apply to the claimed edge-lit display device according to the invention.

[0092] In a preferred embodiment of the invention, the proportion according to the second angle of reflection is greater than the proportion according to the first angle of incidence.

[0093] In another aspect, the invention relates to an arrangement as described herein for a rear projection display device.

[0094] In another aspect, the invention relates to a rear-projection display device comprising an arrangement as described herein and an image generator (PGU) which is configured to illuminate the arrangement with light at an angle such that the light strikes the second diffraction grating at a first angle of incidence.

[0095] It is evident to those skilled in the art that the advantages, definitions and embodiments of the arrangement and the edge-lit display device according to the invention also apply to the claimed rear-projection display device according to the invention.

[0096] Preferably, the PGU sends the image to be displayed in reverse.

[0097] The PGU can be "monochrome". Preferably, however, it is a spectrally broadband PGU and / or a PGU that emits light in the red, green and blue spectral range ("RGB").

[0098] In a preferred embodiment of the invention, the image sensor emits light with a second polarization, wherein the component corresponding to the first angle of incidence is preferably substantially 1. In particular, the second polarization is a p-polarization.

[0099] In a further preferred embodiment, the polarization of the image sensor, the first angle of incidence, and / or the arrangement of the optical axis of the retardation plate are optimized to achieve a maximum signal-to-noise ratio (SNR). The polarization of the image sensor, e.g., a linear polarization, can be rotated using simple means, and an optimal polarization can be found based on the SNR. Those skilled in the art know how to measure the SNR, e.g., with a light detector or a luminous flux meter.

[0100] For given angles of incidence and reflection, and a given polarization, an optimal position of the optical axis can be determined to maximize the signal-to-noise ratio (SNR). Each of the aforementioned parameters can be optimized while the other parameters are held constant to find an optimum for that parameter. For example, the arrangement of the optical axis can be optimized for different angles of incidence, thus finding an optimum for both parameters. Alternatively, the aforementioned parameters can be varied and optimized within predefined ranges, where these ranges are determined by external constraints. For example, polarizations and / or angles can be varied within a range of ±15°, preferably ±10°. This can be achieved, for example, through a computer-implemented method, such as a simulation.

[0101] Both parameters can also be varied simultaneously with the aim of optimizing them.

[0102] In a further aspect, the invention relates to a computer-implemented method for determining the polarization of an image sensor and / or for determining the arrangement of an optical axis for an optical arrangement and / or a display device described herein, wherein the signal-to-noise ratio (SNR) is optimized for this determination. This computer-implemented method is particularly advantageous for the fabrication of an optical arrangement described herein.

[0103] In some embodiments, the means for performing the steps of the method described herein may include a data processor and / or a storage device. In some implementations, the execution of the method may include one or more computers containing one or more data processors configured to execute one or more programs containing a variety of instructions for performing the method. Each data processor may contain one or more processor cores, and each processor core may contain logic circuits for processing data. For example, a data processor may contain an arithmetic and logical unit (ALU), a control unit, and various registers. Each data processor may contain a cache memory.Each data processor can contain a system-on-a-chip (SoC), which includes multiple processor cores, random-access memory, graphics processing units, one or more controllers, and one or more communication modules. Each data processor can contain millions or billions of transistors.

[0104] The computer-implemented method preferably serves to realize a physically real optical arrangement or display device based on the determination of the optical axis and / or the polarization of the image sensor.

[0105] Another aspect concerns the use of an optical arrangement as described herein for a window pane of a motor vehicle.

[0106] Another aspect concerns a window pane encompassing an optical arrangement as described herein.

[0107] It is clear to the expert how an optical arrangement as described herein can be integrated into a window pane of a motor vehicle.

[0108] Description of the invention:

[0109] The invention will be explained below with reference to further figures and examples. The examples and figures serve to illustrate preferred embodiments of the invention without limiting them.

[0110] Figure 1 describes a transparent display for rear projection.

[0111] Figure 2 describes the problems of the disturbance order using a simulation. Figure 3 describes the problem of coupling out the undiffracted zeroth order in a setup largely according to the invention, but lacking the polarization filter.

[0112] Figure 4 schematically describes a possible embodiment of the optical arrangement.

[0113] Figure 5 describes a simulation with a basic setup corresponding to that shown in Figure 3, but with a polarization filter.

[0114] Figure 6 describes an optical arrangement in a rear projection display device with PGU.

[0115] Figure 7 describes in detail the beam path described in Figure 6 within the optical arrangement.

[0116] Figure 4 schematically shows a possible embodiment of the optical arrangement 12 and a rear-projection display device 13 comprising the optical arrangement 12. The optical arrangement 12 comprises, in the following order along a direction of extension, a first diffraction grating 15, a polarization modulator 16, a second diffraction grating 17, and a polarization filter 18. The exemplary arrangement 12 shown has a planar extension perpendicular to the direction of extension along which the aforementioned components of the optical arrangement 12 are arranged. These components are also planar in their form perpendicular to the direction of extension. Along the direction of extension, the optical arrangement 12 has a constant (layer) thickness. However, there may also be embodiments in which the arrangement 12 has a varying (layer) thickness.Furthermore, the arrangement 12 does not have to be flat as shown here; curved embodiments of the arrangement 12 are also possible.

[0117] The arrangement 12 is illuminated by a PGU (not shown) with a beam of light, of which beam 14 is shown. This beam 14 can, for example, correspond to a main direction of the beam. The beam can essentially consist of parallel beams. However, it is typically the case that the PGU is compact and the beam therefore consists of diverging beams that cover a certain angular range in order to illuminate the arrangement 12 over as large an area as possible.

[0118] In the example shown, the first polarization corresponds to s-polarization. The beam of light is initially polarized in the second polarization, i.e., p-polarized. This is represented in the figure by the dashed line of the light beam 14. The beam of light, or light beam 14, is directed towards the optical arrangement 12 such that, after passing through the first diffraction grating 15 and the polarization modulator 16 (see below for details), it strikes the second diffraction grating 17 at an angle of incidence 19 (measured at the surface normal 20), which corresponds to the angle of incidence of the first. The second diffraction grating 17, in turn, diffractes this light at least partially at an angle of reflection 21, which corresponds to the angle of reflection of the first, in the direction of the first diffraction grating 15. The diffracted light beam shown as an example has the reference symbol 22.The second diffraction grating 17 is also called a reflective diffraction grating, since it reflects the light beam back into the same half-space (to the left of the second diffraction grating 17) from which it originated. However, unlike with a mirror, the angle of incidence 19 does not necessarily have to correspond to the angle of reflection 21.

[0119] Before the beam path through the arrangement 12 is explained in detail, the passage of the still undiffracted light beam 14 through the first diffraction grating 15 and the polarization modulator 16, i.e., before diffraction at the second diffraction grating 17, will first be described in more detail. The light beam 14 initially passes through the first diffraction grating 15 unaffected, since the first diffraction grating 15 is also reflective and is designed to diffract light diffracted by the second diffraction grating 17, thus exhibiting a high diffraction efficiency for the corresponding angle(s). When the light beam 14 passes through the polarization modulator 16, the initial angle of incidence is such that the portion transformed from the second polarization (p-polarization) to the first polarization (s-polarization) is essentially one.The light from beam 14 is therefore in the s-polarization state after passing through the polarization modulator 16, indicated by the now dotted line of the light beam 14. This prevents light that is not diffracted by the second diffraction grating 17 (zero order, reference numeral 27) from reaching an observer (not shown) who would be located to the right of the optical arrangement 12 and would view it from there. This is achieved by the polarization filter 18, which is a filter for the first polarization (in this case, s-polarization) and thus blocks it.

[0120] The second diffraction grating 17 can also exhibit a particularly high diffraction efficiency for s-polarization. This can be the case, especially for holographic diffraction gratings, due to manufacturing processes. At the same time, the diffraction efficiency for p-polarized light can be low or even essentially zero. In any case, as shown, the second diffraction grating 17 diffracts the light arriving at the first angle of incidence, at least partially, towards the first diffraction grating 15. Before it reaches the first grating, the light passes through the polarization modulator 16 again (see light beam 22). The polarization modulator 16 converts only a portion of the light with the first polarization (here: s-polarization) into the light with the second polarization (here: p-polarization). This portion is determined by the first angle of incidence 21 at which the light passes through the polarization modulator 16. This angle of incidence 21 determines the angle of incidence on the polarization modulator 16.In this case, the angle of incidence 21 is equal to the angle of incidence on the polarization modulator 16. This angle is very steep, or large. As a result, the path of the light beam 22 through the polarization modulator 16 is longer, but at such steep angles, it is advantageously less effective. This can result from the operating principle of the polarization modulator 16. For example, it is a retardation plate (e.g., an A / 2 plate) comprising at least one birefringent material with two mutually perpendicular distinguished (refractive index) directions (ordinary and extraordinary, or fast and slow). These directions typically lie in the plane along which the polarization modulator 16 is arranged.If the incident beam onto the modulator, as in the example shown (light beam 22), is strongly tilted, i.e., if the angle is very steep, the refractive index difference of the retardation plate can become smaller from the beam's perspective, resulting in less overall retardation. Thus, after passing through the polarization modulator, beam 22 exhibits both s- and p-polarizations, corresponding to the proportion of polarizations transferred by the polarization modulator 16 (according to the first angle of incidence). This is represented by the dashed line of beam 22. The first diffraction grating 15 now diffractes this light, at least partially, towards the polarization filter 18 and thus ultimately also towards the viewer to the right of the optical arrangement 12. The first diffraction grating 15 is also reflective, as it diffractes the light into the same hemisphere (to the right of the first diffraction grating 15) from which it originated before diffraction.The light is diffracted with a second angle of incidence 23 in the direction of the polarizing filter 18. The image shows several diffracted light rays 24, 25, and 26. Such diffraction in multiple directions occurs particularly when the first diffraction grating 15 is a (preferably holographic) diffuser that scatters the light in various directions. This allows for image generation (a display) in the plane of the first diffraction grating 15, which is visible from the viewer position described above (to the right of the arrangement 12). The second angle of incidence 23 is preferably defined as the angle between a surface normal to the first diffraction grating 15 and a principal ray direction of the diffracted light, represented in this case by light ray 25. Thus, the second angle of incidence 23 is approximately 0°.According to the invention, the proportion of light converted from s-polarization to p-polarization and from p-polarization to s-polarization is now different from the proportion converted according to the first angle of incidence 21 before diffraction at the first diffraction grating 15, due to the second angle of incidence 23 at which the light now transmits to the polarization modulator 16 (where the second angle of incidence 23 determines the angle of incidence on the polarization modulator 16 and, in particular, corresponds to the angle of incidence on the polarization modulator 16 in this case). This ensures that the light is not converted back to the polarization state it had previously at the second diffraction grating 17 after passing through the polarization modulator again. Thus, at least not all of the light diffracted by the first diffraction grating 15 is in the first or s-polarization state before passing through the polarization filter 18 and is blocked by it, which would prevent a display.In this way, a display can be realized which suppresses a possible interference order caused by light undiffracted by the second diffraction grating 17 (so), but allows light diffracted by the first diffraction grating 24, 25, 26 (useful order for the display) to pass at least partially (the part which is in the second or p-polarization) through the polarization filter 18, which transmits the second polarization (transmission polarization).

[0121] The first diffraction grating 15 can also exhibit a particularly high diffraction efficiency for s-polarized light. It is possible, in particular, that essentially only s-polarized light is diffracted, while p-polarized light is transmitted undiffracted. In this case, it is especially advantageous if the polarization modulator 16 is configured to convert the first and second polarizations into each other in proportions, depending on the angle of incidence, with the proportion corresponding to the second angle of incidence being greater than the proportion corresponding to the first angle of incidence. This ensures that, firstly, a smaller proportion of the first polarization is converted into the second polarization after diffraction at the second diffraction grating 17. As a result, a significant proportion of the light 22 remains in the first polarization after passing through the polarization modulator 16 and can also be effectively diffracted by the first diffraction grating 15.Therefore, it may be desirable for the proportion corresponding to the first angle of incidence to be as small as possible. After diffraction at the first diffraction grating 15, essentially only s-polarized light is present, see the dotted light rays 24, 25, and 26 before passing through the polarization modulator 16. At the same time, it may be desirable for the proportion corresponding to the second angle of incidence to be as large as possible (where 1 represents the maximum). This is because a particularly large proportion of the initially s-polarized light of the light rays 24, 25, and 26 is then converted into p-polarized light (dashed light rays 24, 25, and 26 after passing through the polarization modulator 16). This light can now pass through the polarization filter 18 unimpeded and thus contribute to a display visible to an observer.

[0122] The light 24, 25, 26 diffracted by the first diffraction grating 15 is diffracted at the second angle of incidence 23 and now strikes the second diffraction grating 17 at this angle. Since the second angle of incidence 23 preferably does not correspond to the first angle of incidence, the second diffraction grating 17 preferably does not diffract at this angle. Furthermore, the light 24, 25, 26 exhibits a second polarization, particularly at the second diffraction grating 17, which is preferably also not diffracted or only minimally diffracted by it.

[0123] The undiffracted light (zeroth diffraction order) of the first diffraction grating 15 and its interference potential have not yet been discussed. In principle, undiffracted light from the first diffraction grating 15, coming from the direction of the second diffraction grating 17, would either exit the optical arrangement 12 on the side facing away from the observer (here: the left side of the optical arrangement 12) and / or be reflected at the outer surface of the optical arrangement 12 on that side. In many applications, exiting light has little or no interference potential, since it exits on the side facing away from the user and is therefore not visible to them. However, in some applications, for example in a (side) window of a motor vehicle, interference potential could also exist on this side.Reflected light could at least partially return to the polarizing filter 18 and be present there, at least partially, in the second polarization, thus being transmitted and causing interference. Therefore, it can be advantageous for the first angle of reflection to be large enough to exceed a critical angle for total internal reflection within the optical arrangement 12 (e.g., at the left outer surface). Thus, this undiffracted light (if it is not diffracted by the first diffraction grating 15) is "trapped" within the arrangement 12 and can cause little or no interference to the outside.

[0124] The angle of incidence 19 on the second diffraction grating 17 ideally corresponds (and in the example shown) essentially to the first angle of incidence. The angle of incidence 19 on the second diffraction grating is measured, for example, starting from the surface normal 20, with the first angle of incidence also being defined with reference to this surface normal. If the second diffraction grating 17 has a curvature, this surface normal 20 can, for example, lie at the geometric centroid of the second diffraction grating 17 or at the respective point of incidence of the light beam. The angles defined for the optical arrangement 12 (first angle of incidence, first angle of reflection, second angle of reflection 23) can be defined identically for all elements or result in the same angles of incidence and reflection, because these elements can be arranged parallel to each other and the surface normals for the respective elements are essentially the same or parallel.The first angle of incidence can essentially be a single angle, particularly if the light rays of the beam are essentially parallel, but also if the light rays occupy an angular range as also described herein (in which case the second diffraction grating 17 can preferably be described as a plan-to-plan hologram). However, the diffraction efficiency is typically reduced for angles deviating from the first angle of incidence, so that the second diffraction grating 17 diffracts these light rays with lower efficiency. This diffraction efficiency is typically greatest at the first angle of incidence.

[0125] However, if the light rays of the beam, as described, have an angular range, the first angle of incidence can also encompass an angular range, which is specifically adapted to the angular range of the beam. In this case, the second diffraction grating 17 can preferably be described as a point-to-plan hologram. This can mean that the second diffraction grating 17 exhibits a sufficiently high diffraction efficiency everywhere over the angular range (e.g., everywhere more than 0.4 or 0.6, where the possible minimum diffraction efficiency is 0 and the possible maximum is 1). However, it is also possible that different locations on the second diffraction grating 17 exhibit a maximum diffraction efficiency for different angles of incidence 19, so that the first angle of incidence is defined as location-dependent. For example,For the various light rays which emanate from the PGU in different directions, the first angle of incidence must be adjusted at their designated point of impact on the second diffraction grating 17 so that it essentially corresponds to the actual angle of incidence 19.

[0126] Similar considerations can apply, mutatis mutandis, to the first diffraction grating 15. The second diffraction grating 17 can, on the one hand, be configured to diffract the diffracted light essentially into a single, constant first angle of incidence. Then the first diffraction grating 15 must essentially be configured to diffract light incident at the first angle of incidence at a second angle of incidence. However, the first angle of incidence can, of course, also be an angular range, so that the first diffraction grating 15 can be configured to diffract light for this angular range at a second angle of incidence. The second angle of incidence can also be an angular range, especially if the first diffraction grating 15 is a diffuser (so).

[0127] Diffraction gratings, especially (volume) holographic diffraction gratings, are not only optimized for specific angles but also wavelength-selective. Diffraction gratings 15 and 17 can therefore be essentially monochromatic. However, they can also exhibit the aforementioned functionalities for a wavelength range. This range is often relatively narrow (e.g., 20 nanometers or less, or 10 nanometers or less). To enable, for example, multicolored and white displays, the first and second diffraction gratings 15 and 17 can each be configured as so-called RGB holograms (for "red," "green," and "blue"). RGB holograms exhibit the desired diffraction properties for a specific wavelength or wavelength range within the red, green, and blue spectral regions, respectively, allowing for the creation of colorful displays by mixing appropriate color components.The RGB properties can be realized by individual holograms in a so-called hologram stack, or by inscribing the respective spectral diffraction properties into a single, so-called multiplex hologram. Figure 5 shows a simulation with a basic setup corresponding to that of Figure 3. This largely corresponds to the optical arrangement 12 according to the invention, e.g., according to Figure 4, but the polarization filter 18 is missing here (in the setup according to Figure 3). (More on the setup underlying the simulation is also below.) In contrast to the intensities that were visible on the screen in Figure 3, essentially the useful order is visible here, which uses only a relatively narrow part of the spectrum of the light source, in this case approximately from 635 to 655 nm. The interference order visible in Figure 3 is almost completely suppressed.A numerical comparison of the simulations from Figure 3 and Figure 5 showed a suppression of the interference order of 57 dBW (decibel watts) for a first angle of incidence of 60°. Simultaneously, there was also a certain attenuation of the useful order (6 dBW). This is because, although the proportion of the light converted from the first to the second polarization is reduced by the large angle of incidence, a significant portion of the light exhibits p-polarization. Consequently, it is not diffracted as efficiently by the first diffraction grating 15. Furthermore, after diffraction and passage through the polarization modulator 16, it is converted back into the first polarization, which is subsequently not transmitted through the polarization filter 18. The signal-to-noise ratio (SNR), i.e., the comparison of the light power between the useful and interference orders, is 1.3 x 10⁻⁶ in the example shown. 3 .

[0128] Increasing the first diffraction angle to 75° compared to Figures 3 and 5 significantly reduces the amount of light converted from s-polarization to p-polarization by the polarization modulator. This allows for a greater increase in the useful order (attenuation reduced to only 1.37 dBW), as more light is diffracted by the first diffraction grating 15 and more light is transmitted through the polarization filter 18. The signal-to-noise ratio (SNR) is 5.63 x 10⁻⁶. 7 .

[0129] For the simulation, it was assumed that the PGU emits p-polarized light, which strikes the optical arrangement 12 at an angle of incidence close to the Brewster angle, specifically 35° in this case. With an assumed refractive index of 1.52 (BK7 was assumed), this results in a corresponding first angle of incidence of approximately 22°. The polarization modulator 16 is an A / 2 retarder plate (monochromatic) whose fast axis is oriented at an angle of 43° to a vertical, i.e., the plane of incidence of the light. The polarization of the light is rotated by the polarization modulator 16 so that, after passing through the modulator 16, it exhibits almost complete s-polarization (see also Fig. 7). The light is diffracted at the second diffraction grating 17 at a first angle of incidence of either 60° or 75° in the direction of the first diffraction grating 15.The polarization is converted from the first to the second polarization by the polarization modulator 16 to a proportion of approximately 0.25 (for a first angle of incidence of 60°), see also Fig. 7. The light (especially that which is in the s-polarization) is then diffracted by the first diffraction grating 15 towards the polarization filter 18 at the second angle of incidence 23. Upon passing through the polarization modulator 16, the proportion of light converted from s- to p-polarization is essentially one, since the second angle of incidence is 0°. This light is transmitted through the polarization filter 18 essentially unimpeded. This does not apply to all rays to the same extent, of course, since the first diffraction grating 15 has a scattering effect and only the main beam direction has a second angle of incidence of 0°.Other rays passing through the polarization modulator 16 at an angle other than 0° may be transformed differently after passing through and therefore may not be completely transmitted through the polarization filter 18.

[0130] Figure 6 shows the optical arrangement 12 in a rear-projection display device with an additional PGU 3 for illuminating the optical arrangement 12. In the example shown, the illumination beams 4 of the PGU 3 are initially unpolarized before passing through a polarization filter 28. A suitable linear polarization can be selected for the specific device. Advantageously, in this example, p-polarization is chosen with respect to the optical arrangement 12, i.e., a polarization that oscillates parallel to the plane of incidence of the illumination 4 on the optical arrangement 12. The parallel polarization is also shown by the diagram above the illumination beams 4, whose y- and x-axes represent the p- and s-polarizations in their orientation. The polarization vector 29, representing the polarization, is drawn along the y-axis in the diagram (thick line) and is thus fully p-polarized.Also shown (and in all subsequent diagrams) is the Stokes vector (SO, S1, S2, S3, reference numeral 38, always to the left of the diagram), which can completely describe the polarization. The beam path 2 to the right of the optical arrangement 12 shows the useful order 2 diffracted from the optical arrangement 12. This is also completely p-polarized (see polarization vector 30, also a thick line in the diagram above the useful order 2).

[0131] Figure 7 shows in detail the beam path described in Figure 6 within the optical arrangement 12, focusing on the different polarization states as the beam passes through it (the various "layers" of the optical arrangement 12 shown can also represent additional substrate layers besides the optical components mentioned, although these need not be included in every case). The illumination beam 4 initially exhibits p-polarization, see polarization vector 31 in the diagram at the top left, where polarization vector 31 is represented by the thick line in the diagram, as in the following diagrams. Upon entering the optical arrangement 12, the illumination beam is refracted depending on the angle of incidence (beam section 4'). As it passes through the polarization modulator 16 with a first angle of incidence, the polarization is rotated by 90° at that first angle of incidence, see Figure 7.Polarization vector 32 (diagram top right), which now lies along the x-axis. This corresponds to a fraction of 1; all light is converted from p- to s-polarization. After diffraction at the second diffraction grating 17, the polarization state of the diffracted light beam 22 does not change, see polarization vector 33 in the diagram center left. Non-diffracted light is blocked by the polarization filter 18 and cannot contribute to the perturbation order. Upon passing through the polarization modulator 16, the polarization state changes again, see polarization vector 34. In contrast to the first pass, however, the polarization is not completely reversed by 90°, but only rotated by approximately 30° towards p-polarization. This is due to the first angle of reflection, which is, for example, larger than the first angle of incidence (and also larger than the second angle of reflection, su). The rotation by approximately30° corresponds to a fraction of 0.25 that has been transferred from s-polarization to p-polarization. Thus, a large portion of the light remains in s-polarization. The light is now diffracted by the first diffraction grating 15 towards the polarization filter 18, whereby the polarization does not change in this example (see polarization vector 35) and passes through the polarization modulator 16 again (see light beam 25), this time at the second angle of incidence, which is approximately zero. The polarization is then rotated further towards p-polarization by approximately another 30° (see also polarization vector 36). This corresponds to a fraction of almost 1 that is converted from s- to p-polarization and vice versa. The light now has a high proportion of p-polarization. This portion can transmit essentially unhindered through the polarization filter 18 and thus contributes to the usable order 25'.This utility order 25' is completely p-polarized due to the polarization filter 18, see polarization vector 37.

[0132] REFERENCE MARK LIST

[0133] 1, 12 optical arrangement

[0134] 2, 9, 25' scattered light / usage regulations

[0135] 3 Imagers / PGU

[0136] 4.4' lighting beams

[0137] 5 viewers

[0138] 6, 10, 27 Stray light / striking order / zeroth order

[0139] 7 first page

[0140] 8 second page

[0141] 11 umbrella

[0142] 13 Rear projection display device

[0143] 14 Light beam of the illumination beams

[0144] 15 first diffraction grating

[0145] 16 Polarization modulator

[0146] 17 second diffraction grating

[0147] 18 polarization filters

[0148] 19 Angle of incidence at the second diffraction grating

[0149] 20 Surface normals

[0150] 21 Angle of incidence

[0151] 22 Light beam diffracted at the second diffraction grating

[0152] 23 second angle of reflection

[0153] 24, 25, 26 Light rays diffracted by the first diffraction grating

[0154] 28 polarization filters for PGU

[0155] 29 - 37 Polarization vector

[0156] 38 Stokes vector

Claims

PATENT CLAIMS 1. Optical arrangement (1 , 12) for a diffractive display, comprising successively along a direction of expansion: A first reflexive diffraction grating (15) A polarization modulator (16) is configured to convert light with a first linear polarization and a second polarization perpendicular to the first, at least to a certain extent, into each other, wherein the proportion depends on the angle of incidence of the light on the polarization modulator (16). A second reflective diffraction grating (17) configured to diffract light with a first angle of incidence onto the second diffraction grating (17) at least partially towards the first reflective diffraction grating (15) with a first angle of refraction. A polarization filter (18) for the first polarization with the second polarization as the transmission polarization, wherein the first reflective diffraction grating (15) is configured to diffract diffracted light from the second reflective diffraction grating (17) at least partially with a second angle of refraction (23) towards the polarization filter (18), wherein the component according to the second angle of refraction (23) is unequal to the component according to the first angle of refraction.

2. Optical arrangement (1 , 12) according to the previous claim, wherein the proportion according to the second angle of incidence (23) is greater than the proportion according to the first angle of incidence.

3. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the first and / or the second diffraction grating (15, 17) are configured to diffract light with the first polarization, wherein the first polarization is preferably an s-polarization.

4. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the proportion according to the first angle of incidence is greater than the proportion according to the first angle of reflection.

5. Optical arrangement (1, 12) according to one or more of the preceding claims, wherein the proportion according to the second angle of incidence (23) is greater by at least a factor of 2, preferably by at least a factor of 4, than the proportion according to the first angle of incidence, wherein preferably the proportion according to the first angle of incidence and / or second angle of incidence (23) is substantially 1 and / or wherein the proportion according to the first angle of incidence is 0.25 or less.

6. Optical arrangement (1, 12) according to one or more of the preceding claims, wherein the first and / or the second diffraction grating (15, 17) comprise a holographic optical element (HOE), in particular a volume HOE, wherein the first diffraction grating (15) in particular comprises a holographic diffuser, wherein the diffuser is configured for diffraction in the direction of the polarization filter (18) as a directed scattering.

7. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the polarization modulator (16) comprises an A / 2 plate, wherein the A / 2 plate preferably has an optical axis whose orientation is configured to maximize the proportion according to the second angle of reflection (23) and / or the first angle of incidence.

8. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the first and / or second diffraction grating (15, 17) is configured to diffract light with more than one wavelength, wherein the first and / or second diffraction grating (15, 17) in particular comprises at least one RGB-HOE.

9. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the A / 2 plate is an at least partially achromatic A / 2 plate.

10. Optical arrangement (1 , 12) according to one or more of the preceding claims, wherein the first angle of reflection is greater than a critical angle of total internal reflection within the optical arrangement (1 , 12).

11. Optical arrangement (1, 12) according to one or more of the preceding claims, which has a planar extent perpendicular to the direction of extent, wherein the first diffraction grating (15), the polarization modulator (16), the second diffraction grating (17) and the polarization filter (18) are each extended along the planar arrangement, wherein the optical arrangement (1, 12) preferably has a constant thickness in the direction of extent.

12. Rear projection display device comprising an optical arrangement (1 , 12) according to one or more of the preceding claims 1-11 and an image generator (PGU) (3) which is configured to illuminate the optical arrangement (1 , 12) with light at an angle such that the light strikes the second diffraction grating (17) at a first angle of incidence.

13. Rear projection display device according to one or more of the preceding claims, wherein the image transmitter (3) emits light with a second polarization, wherein the proportion according to the first angle of incidence is preferably substantially 1.

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