Optical element, method for producing same, and illumination device and screen comprising such an optical element

Abstract

The invention relates to an optical element (10) having a light entry side and a light exit side, comprising at least first regions (E1) consisting of a transparent material with a first refractive index (N1) and second regions (E2) consisting of an opaque material and / or transparent and / or scattering or reflective material with a second refractive index (N2), wherein the first regions (E1) are integrated into the second regions (E2) in a one- or two-dimensional pattern, and the first refractive index (N1) is greater than the second refractive index (N2) over the entire wavelength range visible to the human eye, as a result of which the propagation direction of the light exiting the optical element (10) on the light exit side thereof is changed with respect to that of the light incident on the optical element (10) on the light entry side thereof and additionally at least part of the light incident on first regions (E1) of the optical element (10) on the light entry side thereof is bundled after passing through the optical element (10). The invention further relates to a method for producing such an optical element (10) and to an illumination device and screens having such an optical element (10).

Description

title

[0001] Optical element and method for its manufacture as well as lighting device and screen with such an optical element Technical field of the invention

[0002] In recent years, great strides have been made in widening the viewing angle of LCDs. However, there are often situations where this very wide viewing angle of a screen can be a disadvantage. Above all, a wide viewing angle represents a waste of energy and resources when it's not actually needed. State of the art

[0003] US Pat. No. 5,993,940 A describes the use of a film with evenly spaced, small prism strips on its surface to achieve a privacy mode. Development and production are quite complex.

[0004] US 2012 / 0235891 A1 describes a very complex backlight in a display. According to Figs. 1 and 15, not only multiple light guides are used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform the light from the rear illumination on its way to the front illumination. This is expensive and complex to implement and also involves light loss. According to the variant shown in Fig. 17 in US 2012 / 0235891, both light sources 4R and 18 produce light with a narrow illumination angle, with the light from the rear light source 18 first being converted, at great expense, into light with a wide illumination angle. This complex conversion—as already mentioned above—significantly reduces brightness.

[0005] According to JP 2007-155783 A, special optical surfaces 19 are used, which are complex to calculate and manufacture, and which then deflect light into different narrow or wide areas depending on the angle of incidence. These structures resemble Fresnel lenses. Furthermore, there are interference edges that deflect light in undesirable directions. Thus, it remains unclear whether truly meaningful light distributions can be achieved.

[0006] US 2013 / 0308185 A1 describes a special light guide with steps that emits light over a large area in different directions, depending on the direction from which it is illuminated from a narrow side. In conjunction with a transmissive image display device, e.g. an LC display, a screen that can be switched between free and restricted view modes can be created. A disadvantage here is, among other things, that the restricted view effect can only be created for left / right or for top / bottom, but not for left / right / top / bottom simultaneously, as is necessary for certain payment transactions, for example. In addition, even in restricted view mode, residual light is still visible from blocked viewing angles.

[0007] DE 10 2022 134 518 B3, filed by the applicant, describes an optical element, a method for its production, and a lighting device. Differences in refractive indices are used to generate total internal reflection of light at specific angles. This document does not suggest designing the structures used with different contours on the light input and output sides.

[0008] The applicant's WO 2022 / 078942 A1 describes an optical element and a method for its production. Here, differences in refractive indices are used to generate total internal reflection of light at specific angles. This document also does not suggest designing the structures used with different contours on the light input and output sides.

[0009] The applicant's DE 10 2020 008 062 A1 describes an optical element with variable transmission, a corresponding method, and a display with such an optical element. Here, differences in refractive index are again used to generate total internal reflection of light at specific angles. This document also does not suggest designing the structures used with different contours on the light input and output sides.

[0010] Finally, DE 10 2021 110 010 A1 of the applicant discloses an optical element for influencing light directions and arrangements using such elements. However, the use of flexible apertures, which are usually designed as LCD shutters, results in a relatively high light loss.

[0011] The aforementioned methods and arrangements have the disadvantage that they do not allow the selective optimization of the luminance curve of a screen. Description of the invention

[0012] The object of the invention is therefore to develop a planar optical element that can influence incident light in a defined manner along its propagation directions, wherein at least a portion of the light incident on the light entry side of the optical element is focused after passing through the optical element. The optical element should be inexpensive to implement and, in particular, universally usable with various screen types in order to achieve an increased efficiency in luminance distribution.

[0013] Furthermore, the optical element should generally offer the possibility of achieving a top-hat light distribution. This means that the brightness does not decrease by more than 15 percent within an angular range of at least + / -5 degrees around the mean emission angle.

[0014] This object is achieved according to the invention by a planar optical element having a light entry side and a light exit side, comprising at least first regions E1 consisting of a transparent material with a first refractive index N1 and second regions E2 consisting of an opaque material and / or transparent and / or reflective or scattering material with a second refractive index N2, wherein the first regions E1 are embedded in the second regions E2 in a one- or two-dimensional (preferably, but not necessarily periodic) pattern, wherein the first refractive index N1 is greater than the second refractive index N2 over the entire wavelength range visible to a human eye, wherein the first regions E1 each have a larger cross-section on the light exit side than on the light entry side,and further wherein the first regions E1, when viewed in parallel projection perpendicular to the optical element, have different outline shapes on the light incidence side and on the light exit side, which cannot be brought into congruence by scaling, so that on the light entry side of the optical element, light incident thereon, which enters first regions E1 of the optical element, is there, depending on the geometric direction of incidence, polarization and the ratio of the first refractive index N1 to the second refractive index N2, a) propagated unhindered within a first region E1 or totally reflected and is subsequently coupled out again at the light exit side of the corresponding first region E1, or b) penetrates from the first region E1 into an adjacent second region E2 and, if it is not absorbed in the second region E2, propagated in a second region E2 until it is coupled out of a second region E2 on the light exit side or coupled into one or more next first and / or second regions E1, E2 (and so on), and so that in a first alternative, in which the second regions E2 consist of a transparent material, light,which is incident on the light entry side of the optical element into second regions E2 of the optical element, penetrates the latter and is coupled out of the optical element on the light exit side, and so that in a second alternative, in which the second regions E2 at least partially comprise an opaque material, light which is incident on the light entry side of the optical element into second regions E2 of the optical element is at least partially absorbed, and so that in a third alternative, in which the second regions E2 at least partially comprise a reflective and / or scattering material, light which is incident on the light entry side of the optical element into second regions E2 of the optical element is at least partially reflected and / or scattered,whereby the light emerging from the optical element at the light exit side thereof is changed in its propagation directions compared to the light incident on the optical element at the light entry side, and whereby furthermore at least a part of the light incident on the light entry side of the optical element into first regions E1 is bundled after passing through the optical element.

[0015] The “geometric direction of incidence” of a light beam into the first regions refers in particular to its direction vector, which describes the horizontal and vertical angle of incidence onto the light entry surface – also referred to as the “lower surface” – of a first region and, in addition to the polarization state, is essential for the further propagation of the light in the first region E1 or at the interfaces to second regions E2.

[0016] The "periodic sequence" of the first and second areas E1, E2 does not mean that they always have to be the same width and / or height, but rather that the first and second areas simply alternate. Their size can vary, however.

[0017] In general, the smaller the difference in refractive index between the first refractive index N1 and the second refractive index N2, the narrower the light distribution of the light leaving the optical element from the first areas E1. For clarity of physics, it should be noted again here that “refractive index” refers either to the first or second refractive index N1, N2 for a selected wavelength, e.g. 580 nm, or to the respective dispersion curve over the entire wavelength range visible to the human eye. In the case of the dispersion curve, the difference in refractive index means the respective value which corresponds to the difference between the two refractive indices N1, N2 for a selected visible wavelength A.

[0018] The interfaces of the first regions E1 and the second regions E2 of the optical element, when viewed in the section direction perpendicular to the light exit side of the optical element, can be trapezoidal (in particular, but not exclusively, trapezoidal with equally long sides of the trapezoid), funnel-shaped, at least partially or entirely parabolic, parallelogram-shaped, and / or step-shaped. Furthermore, it is possible for said interfaces to generally have the shape of an nth-degree polynomial in the section direction. Furthermore, mixed forms of the aforementioned possibilities are also possible in one and the same interface, for example if, within an interface, a partial step shape is followed by a parabolic portion.

[0019] By means of the above-described forms of formation of the first and second areas E1, E2, a targeted influence on the propagation directions of the optical The light emerging from the element is adjusted: Depending on the design, the light is focused more or less strongly across the surface. Furthermore, it is possible, for example, to use parallelogram-shaped interfaces between the first and second regions E1, E2 to achieve a peak shift through the resulting tilt of the interfaces between the first and second regions E1, E2. The trapezoidal shape, in turn, has the additional advantage of ensuring a good focus of the angular distribution.

[0020] It is advantageous - but not exclusive - that the difference in refractive index between the first refractive index N1 and the second refractive index N2 is less than 0.25.

[0021] If, in the context of the second alternative, opaque material is present in second areas E2, this can optionally consist of an originally transparent material with the second refractive index N2, which, however, is mixed with absorbing particles, resulting in an overall opaque effect.

[0022] The opaque material can, for example, consist of a lacquer or polymer as the transparent component, which is mixed with, for example, graphite particles with a size of less than 500 nm in the direction of their greatest dimension, or with black carbon nanoparticles with a size of less than 200 nm, for example with soot particles, for the opaque component. Alternatively, or in combination, the opaque component of the opaque material can also contain dyes or mixtures of dyes. A suitable dye, for example, is Sudan black, which absorbs all light in the visible range. Typically, the mass fraction of the absorbing particles is at most 50%, preferably less than 20%, particularly preferably less than 10%.

[0023] If, in the context of the third alternative, reflective or scattering material is present in second regions E2, this consists, for example, of a mixture of a lacquer and particles, whereby the particles are now reflective and / or scattering. The particles can be, for example, white-scattering particles such as titanium dioxide (TiO2) or reflective particles such as silver particles or chromium particles with a size of less than 500 nm.

[0024] The third alternative has the following general advantages: Light is not lost in the light balance due to permanent absorption, except for general losses. Compared to the first alternative, in which the second regions E2 are filled with a transparent material, this prevents a type of "crosstalk" of light rays and thus suppresses a type of blending effect, for example when the optical element is used together with an image display unit and its pixels are assigned to first regions E1 of the optical element, because in the third alternative, light from pixels that are not assigned to a specific first region E1 is not simply transmitted through a second region E2 (first alternative), but is reflected or scattered, which also means that it would no longer be available due to absorption, as in the second alternative.

[0025] If such an optical element of the third alternative is used in an arrangement with a backlight, a (large) part of the light incident on the light entry surface of the optical element, which is reflected or scattered at second areas E2, can be recycled, for example, in an underlying light source, i.e. the said backlight.

[0026] Furthermore, the first regions E1 and the second regions E2 can be arranged in strip-like fashion, distributed alternately over the surface of the optical element when viewed in parallel projection perpendicular to the optical element. In contrast, another embodiment provides that the first regions, when viewed in parallel projection perpendicular to the optical element, are arranged in a point-like, circular, oval, rectangular, hexagonal, or other two-dimensional shape, distributed over the surface of the optical element on the light incidence side or the light exit side, and the second regions are each shaped complementarily thereto. Thus, the restriction of the light propagation directions would be effective in at least two planes perpendicular to the surface of the optical element.In practice, the effect of such an optical element is usually such that the light propagation directions for transmitted light are focused at angles close to the perpendicular bisector of the optical element or parallel to it. "Close" in this case means that the deviations from the perpendicular bisector or the parallel to it are less than 20° or 30°, depending on the design.

[0027] Other shapes of the first and second regions E1, E2 are also possible. It is important for maintaining the functionality of the invention that the first and second regions E1, E2 are optically directly adjacent to each other, so that a optical refractive index jump from the first refractive index N1 to the second refractive index N2 (or vice versa) is given as far as possible without an air gap.

[0028] According to the invention, when viewed in parallel projection perpendicular to the optical element, the first regions E1 each have different outline shapes on the light incidence side and on the light exit side of the optical element, which cannot be made congruent by scaling. The same applies to the second regions E2. Due to this configuration, at least two boundary surfaces of the first and second regions each have different angles relative to the light entry or exit side when viewed along the cutting direction - perpendicular to the light exit side of the optical element. This results in the light bundling being of different strengths in at least two directions. This feature allows for a high degree of flexibility in the design, which affects the shaping of the light cone in front of a light source, for example a screen or backlight.In addition, the formation of optical artifacts can be reduced or avoided by influencing the light propagation directions differently in different directions.

[0029] For special embodiments, it is conceivable that on at least a part of the first regions E1 and / or the second regions E2, advantageously on all first regions E1 and / or all second regions E2, on whose light exit side a lens structure is applied, preferably a convex lens structure.

[0030] The invention also improves the state of the art in that a top-hat distribution can be achieved with such optical elements, since due to the total reflection more useful light is transmitted into the desired angular ranges than - as is usual in the prior art - without the total reflection.

[0031] For special embodiments, it is possible that at least one first region E1 is formed on the optical element, which, when viewed in parallel projection perpendicular to the optical element, is at least twenty times as large in its shortest extent as the shortest extent of all second regions E2 when viewed in parallel projection perpendicular to the optical element, so that within said at least one first region E1, except at its edges and up to There is no restriction on parallel offsets in the propagation directions of the light emerging from the optical element on the light exit side compared to the light incident on the optical element on the light entry side.

[0032] Furthermore, it is possible that, in addition to the first regions E1 and the second regions E2, further third, fourth, etc. regions are formed with different parameters in terms of shape and / or refractive index than those of the first regions E1 and the second regions E2, so that light which penetrates these further third, fourth regions and exits from the optical element experiences different restrictions of the propagation directions than in the first regions E1.

[0033] It should be mentioned that the optical element of the second or third alternative can, in special embodiments, be designed as a very precise, wavelength-selective color filter: If the dispersion curves of both refractive indices N1, N2 intersect as a function of the wavelength, then in the case of opaque second regions E2 according to the second alternative, at those wavelengths where the second refractive index N2 is greater than the first refractive index N1, the corresponding wavelength range would be efficiently extinguished, i.e. not coupled out of the optical element, while the wavelength ranges for which the first refractive index N1 is greater than the second refractive index N2 would be coupled out of the optical element.Depending on the configuration of the dispersion curves, such an optical element with wavelength-selective effect would then have to be operated with obliquely directed light, because in any case, the critical angle of total reflection at the interfaces between the first and second regions E1, E2 must be used to separate the spectra. In an exemplary further development, such a wavelength-selective color filter would then be used to separate two spectra, for example, one in a narrow UV range and one in a white, spectrally broad range.

[0034] Furthermore, it can be helpful to place a polarizer, optionally a reflective polarizer, below and / or above the optical element to optimize the effect. Controlling polarization with a polarizer increases the efficiency of using the refractive index transitions. Furthermore, the p-polarization of the incoming or outgoing light can be used to minimize Fresnel reflections, i.e., to optimize the restriction of the light propagation directions.

[0035] The disclosure furthermore also includes a method for producing an optical element as described above, comprising the following steps: in a first alternative, molding of second regions E2 on a substrate S, wherein the corresponding structures are embossed into an embossable material having the second refractive index N2 - which applies to the cured state - using a tool that has the inverse shape of the structures of the second regions E2 and are subsequently cured, e.g. in a nanoimprint or in a variothermal or isothermal injection molding / injection compression molding process, in a second alternative, molding of first regions E1 on a substrate S, wherein the corresponding structures are embossed into a first embossable material having the first refractive index N1 - which applies to the cured state - using a tool that has the inverse shape of the structures of the first regions E1 and are subsequently cured, e.g.in a nanoimprint or in a variothermal or isothermal injection molding / injection compression molding process, in the first alternative filling the spaces between the second regions E2 with a material having the first refractive index N1 - which applies to the cured state -, whereby the first regions E1 are created, in the second alternative filling the spaces between the first regions E1 with a material having the second refractive index N2 - which applies to the cured state -, whereby the second regions E2 are created, (in both alternatives the filling can take place in one or more steps, whereby one or more intermediate curing steps can also follow between such filling steps). Curing of the second material, e.g. by irradiation with UV light if the second material is UV-curable, or by cooling if it is an injection moulding or embossing process, Optional sealing of the first and second regions E1, E2 on their side not facing the substrate S by applying a lacquer and / or a cover layer.

[0036] It should be noted that for a nanoimprint process, depending on the chosen alternative of the manufacturing process, the embossable material with the second or first refractive index N2 or N1 - which applies to the cured state - is applied to a substrate S (e.g. glass or polymer) as a separate (usually liquid) material, which is then cured, for example, with UV light. In a variothermal or isothermal injection molding / injection-compression molding process, the material of the substrate S simultaneously represents the material into which the structures, i.e., the second or first regions E2, E1, are embossed and then cured by cooling. Other manufacturing processes are possible. In principle, both alternative manufacturing processes can be used, although the first alternative manufacturing process can preferably be used for the first alternative optical element, i.e., when the materials of the first and second regions E1, E2 are transparent. In particular, the second alternative manufacturing process can be used flexibly for all alternative designs of the optical element.

[0037] The aforementioned process for manufacturing an optical element can be implemented as a sheet-to-sheet, roll-to-plate, or roll-to-roll setup, depending on the actual design. This also simplifies filling, for example, with a varnish. In the first alternative, a transparent varnish can be used, making the process more cost-effective and simpler.

[0038] Furthermore, the invention comprises a first screen, comprising an image display unit, an optical element as described above, wherein the optical element is arranged in front of or behind the image display unit in the viewing direction of a viewer, wherein preferably in the case of parallel projection from a perpendicular direction onto the image display unit, the second regions E2 are in front of surface sections of the image display unit which do not (or substantially do not) emit light.

[0039] Non-light-emitting surface sections of the image display unit can, for example, be the black matrix areas of an OLED, microLED, or LCD panel or deactivated pixels (possibly including the respective black matrix areas of an OLED, LCD, or microLED panel). Other configurations are possible.

[0040] The invention can also be advantageously implemented in such a way that the optical element is arranged in front of or behind the image display unit in the viewing direction of a viewer in such a way that in the case of parallel projection from a perpendicular direction onto the image display unit, the first regions E1 of the optical element are in front of Light-emitting surface sections, i.e. in particular pixels (color subpixels, full-color pixels, monochrome or other types of pixels) of the image display unit.

[0041] This is where a particular advantage of the invention becomes apparent: It allows the light emitted by a screen to be focused on a desired viewing or angular range with minimal loss. Thus, the invention can achieve a defined distribution of the luminance curve in one or two dimensions. If the luminance curve is rather broad, the content displayed by the image display unit can be viewed from a wide angle, while still allowing greater efficiency to be achieved compared to the prior art by varying the parameters of the optical element to achieve the desired luminance curve.

[0042] For the first screen, it is advantageous if the optical element is optically bonded or laminated to the image display unit.

[0043] In the event that an optical element according to the invention is arranged in front of an image display unit in the viewing direction in order to limit its light propagation directions, an optical system can optionally be present on the image display unit, which essentially focuses the light emitted by the respective pixels of the image display device onto the surfaces opposite the first regions E1. This is possible, for example, with microlens arrays or lenticular lenses that have approximately the periods of the pixel widths (or, if applicable, pixel heights). The period of the first regions E1 should then ideally correspond to the period of the pixel widths or heights, or to integer multiples thereof.

[0044] Furthermore, the invention also includes an illumination device comprising a backlight, an optical element arranged in front of the backlight in the viewing direction of a viewer, as described above. Here too, the invention allows a defined focusing of the light emitted by the illumination device onto a desired viewing area, ie onto a defined luminance curve, in the same way as described above for the first screen.

[0045] Furthermore, the lighting device can be designed such that at least one partial mirror coating is applied to the first and / or second regions E1, E2 on the light entry side of the optical element. This allows the efficiency of the lighting device to be increased even further.

[0046] Furthermore, the invention comprises a second screen comprising an illumination device as described above, and a transmissive second image display unit, such as an LCD panel, arranged in front of the illumination device in the viewing direction of a viewer.

[0047] In the latter case, the optical element would typically be integrated directly into an illumination device for a transmissive image display unit, such as an LCD panel.

[0048] In a preferred embodiment of this second screen, it comprises a light guide between the lighting device and the transmissive second image display unit. The light guide is generally plate-shaped, with two large surfaces and narrow sides enclosing them. Light from light sources can be coupled into the light guide via the narrow sides, which is then coupled out of the light guide via coupling-out elements or scattering elements, as are known in the art. Depending on whether light is coupled into and out of the light guide, it can be switched between two operating modes: In a first operating mode, no light is coupled into the light guide because, for example, the light sources are switched off; accordingly, no light can be coupled out either. In a second operating mode, however, light is coupled into the light guide and then coupled out again.As a result, the viewing angle in front of the second screen is limited in the first operating mode compared to the second operating mode.

[0049] Finally, the invention also includes a third screen comprising - an image display unit with pixels, each pixel having a pixel outline defined by a luminous or light-transmitting surface, the pixel outlines of all pixels being identical or at least partially different, - a planar optical element with a light entry side and a light exit side, which is located in front of or behind the image display unit from the viewing direction of a viewer, and which comprises: • first regions E1 consisting of at least one transparent material with a first refractive index N1 and second regions E2 consisting of an opaque material and / or transparent and / or reflective or scattering material with a second refractive index N2, wherein the first regions E1 are embedded in the second regions E2 in a one- or two-dimensional pattern, wherein the first refractive index N1 is greater than the second refractive index N2 over the entire wavelength range visible to a human eye, • wherein the first regions E1 each have a larger cross-section on the light exit side than on the light entry side, and wherein at least one location (preferably on at least half of the light entry side and / or the light exit side, particularly preferably on the entire light entry side and / or on the light exit side) on the optical element, the first regions E1, when viewed in parallel projection perpendicular to the image display unit, each have outline shapes on the light entry side and / or on the light exit side which cannot be brought into congruence with a pixel outline present on the image display device by scaling, • so that on the light entry side of the optical element, light incident on the latter, which is incident in first regions E1 of the optical element, there, depending on the geometric direction of incidence, polarization and the ratio of the first refractive index N1 to the second refractive index N2, a) is propagated unhindered or totally reflected within a first region E1 and is then coupled out again at the light exit side of the corresponding first region E1, or b) penetrates from the first region E1 into an adjacent second region E2 and, if it is not absorbed in the second region E2, propagates in a second region E2 until it is coupled out of a second region E2 on the light exit side or coupled into one or more next first and / or second regions E1, E2, • and so that in a first alternative of the optical element, in which the second regions E2 consist of a transparent material, light, which is incident on the light entry side of the optical element into second regions E2 of the optical element, penetrates these regions and is coupled out of the optical element on the light exit side, • and so that in a second alternative of the optical element, in which the second regions E2 at least partially comprise an opaque material, light which is incident on the light entry side of the optical element into second regions E2 of the optical element is at least partially absorbed, • and so that in a third alternative of the optical element, in which the second regions E2 at least partially comprise a reflective and / or scattering material, light which is incident on the light entry side of the optical element into second regions E2 of the optical element is at least partially reflected and / or scattered, • whereby the light emerging from the optical element at the light exit side is changed in its propagation directions compared to the light incident on the optical element at the light entry side, • and whereby at least a part of the light incident on the light entry side of the optical element into first regions E1 is bundled after passing through the optical element, - so that in a first alternative of the third screen, in which the image display device is located behind the optical element in the viewing direction, the luminance curve for the light emitted by the image display device is at least partially changed, and - so that in a second alternative of the third screen, in which the image display device is located in front of the optical element in the viewing direction, the luminance curve for light incident on the rear side of the image display device is at least bundled.

[0050] A key feature of this screen is that the first regions, when viewed in parallel projection perpendicular to the image display unit, have contour shapes on the light incidence side and / or on the light output side that cannot be brought into congruence with a pixel outline present on the image display device by scaling. This leads to the same advantages as those already described for the optical element according to the invention, although the possibility is not excluded that the first regions of the optical element on the light entrance and light exit sides can be brought into congruence when scaling the size.

[0051] In general, for all optical elements, the roughness R a at the interfaces between first and second regions E1, E2 with different refractive indices N1, N2 should preferably be less than or equal to 20 nm. Other configurations, also with higher values ​​of roughness R a , are however possible.

[0052] A screen comprising at least one optical element as described above and an image display unit can be used, for example, in a monitor or a mobile device. Furthermore, it may also be possible to retrofit an image display unit with a first optical element by arranging it in front of the image display unit.

[0053] The invention is particularly advantageous for applications where no air layers or gaps are permitted, i.e., where all layers must be laminated without compromising optical performance. A further advantage, compared to the use of parabolic mirrors for light focusing, is that the structures used in the invention can essentially operate without any reflective coating, since they primarily achieve their advantageous performance through the difference in refractive index.

[0054] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0055] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations indicated, but also in other combinations or in isolation, without departing from the scope of the present invention. Short description of the drawings

[0056] The invention will be explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as limiting. For example, a description of an exemplary embodiment with a plurality of elements or Components should not be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other embodiments may also include alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the embodiments may also be applicable to other embodiments. To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. They show: Fig.1 shows the schematic diagram (sectional view) of an optical element in an exemplary first embodiment, Fig.2 shows the schematic diagram (sectional view) of an optical element in an exemplary second embodiment, Fig.3 shows the schematic diagram (sectional view) of an optical element in the exemplary first embodiment, combined with pixels and a black matrix of an image display unit, Fig.4 shows the schematic diagram (sectional view) of an optical element in the exemplary second embodiment, combined with pixels and a black matrix of an image display unit, Fig.5 shows the schematic diagram (sectional view) of an optical element in an exemplary third embodiment, Fig.6 shows the schematic diagram (sectional view) of an optical element in an exemplary fourth embodiment, combined with pixels and a black matrix of an image display unit, Fig.7a shows the schematic diagram (perspective view) of an optical element in the exemplary fifth embodiment, combined with pixels and a black matrix of an image display unit, Fig.7b the schematic diagram (perspective view) of an optical element in the exemplary sixth embodiment, combined with pixels and a black matrix of an image display unit, as well as Fig.8 exemplary simulation results for various parameters of exemplary optical elements in the first embodiment in comparison with a reference value. Detailed description of the drawings

[0057] The drawings are not to scale and merely represent schematic diagrams. Furthermore, for clarity, only a few light rays are shown, although in reality there are many.

[0058] Fig. 1 shows a schematic diagram of an optical element 10 in an exemplary first embodiment, in a sectional view and as an enlarged detail. The optical element 10 shown therein, which is flat (the surface is conceptually perpendicular to the plane of the paper) and has a light entry side (here: lower surface) and a light exit side (here: upper surface), comprises at least first regions E1 consisting of a transparent material with a first refractive index N1 and second regions E2 consisting of an opaque material and / or transparent and / or reflective or scattering material with a second refractive index N2. The first regions E1 are embedded in the second regions E2 in a one- or two-dimensional (preferably, but not necessarily periodic) pattern, the first refractive index N1 being greater than the second refractive index N2 across the entire wavelength range visible to the human eye.wherein the first regions E1 each have a larger cross-section on the light exit side than on the light entry side, and wherein furthermore (not shown in the drawing) the first regions E1, when viewed in parallel projection perpendicular to the optical element 10, each have different outline shapes on the light incidence side and on the light exit side, which cannot be brought into congruence by scaling, so that light incident on the light entry side of the optical element 10, which light enters first regions E1 of the optical element 10, is there, depending on the geometric direction of incidence, polarization and the ratio of the first refractive index N1 to the second refractive index N2 a) propagated unhindered within a first region E1 or totally reflected and is subsequently coupled out again at the light exit side of the corresponding first region E1,or b) penetrates from the first region E1 into an adjacent second region E2 and, if it is not absorbed in the second region E2, in a, second region E2 until it is coupled out of a second region E2 on the light exit side or coupled into one or more next first and / or second regions E1, E2 (and so on), and so that in a first alternative, in which the second regions E2 consist of a transparent material, light that is incident on the light entry side of the optical element 10 into second regions E2 of the optical element 10, penetrates this and is coupled out of the optical element 10 on the light exit side, and so that in a second alternative, in which the second regions E2 at least partially comprise an opaque material, light that is incident on the light entry side of the optical element 10 into second regions E2 of the optical element 10 is at least partially absorbed, and so that in a third alternative, in which the second regions E2 at least partially comprise a reflective and / or scattering material,Light incident on the light entry side of the optical element 10 into second regions E2 of the optical element 10 is at least partially reflected and / or scattered, whereby the light exiting the optical element 10 at the light exit side is changed in its propagation directions compared to the light incident on the optical element 10 at the light entry side, and wherein furthermore at least a portion of the light incident on the light entry side of the optical element 10 into first regions E1 is bundled after passing through the optical element 10.

[0059] The “geometric direction of incidence” of a light beam into the first regions E1 refers in particular to its direction vector, which describes the horizontal and vertical angle of incidence onto the light entry surface – also referred to as the “lower surface” – of a first region E2 and, in addition to the polarization state, is essential for the further propagation of the light in the first region E1 or at the interfaces to second regions E2.

[0060] The "periodic sequence" of the first and second areas E1, E2 does not mean that they always have to be the same width and / or height, but that the first and second areas simply alternate. Their size can, however, vary. In Fig. 1 (and also Fig. 2 and other drawings), only two are shown. First regions E1 and three second regions E2 (or possibly fewer) are shown. However, in the physical design of an optical element 10, significantly more such first and second regions E1, E2 are present.

[0061] In general, the smaller the difference in refractive index between the first refractive index N1 and the second refractive index N2, the narrower the light distribution of the light leaving the optical element 10 from the first regions E1. For clarity of physics, it should be noted again here that “refractive index” refers to either the first or second refractive index N1, N2 for a selected wavelength, e.g. 580 nm, or the respective dispersion curve over the entire wavelength range visible to the human eye. In the case of the dispersion curve, the difference in refractive index means the respective value which corresponds to the difference between the two refractive indices N1, N2 for a selected visible wavelength A.

[0062] The interfaces of the first regions E1 and the second regions E2 of the optical element 10 can assume various configurations when viewed in the sectional direction perpendicular to the light exit side of the optical element 10. In the exemplary first configuration according to Fig. 1, these interfaces are trapezoidal, e.g., funnel-shaped. In contrast, in Fig. 2, which shows an exemplary second configuration of the optical element 10, the interfaces are approximately parabolic (more precisely, as sections of parabolas).

[0063] The previously described configurations of the first and second areas E1, E2 allow for a targeted influence on the propagation directions of the light emerging from the optical element 10: Depending on the configuration, the light is focused more or less strongly across the surface. The trapezoidal and parabolic shapes have the advantage of providing good focus for the angular distribution.

[0064] Advantageously, but not exclusively, the difference in refractive index between the first refractive index N1 and the second refractive index N2 is less than 0.25.

[0065] If, in the context of the second alternative, opaque material is present in second areas E2, this can optionally consist of an originally transparent material with the second refractive index N2, which, however, is mixed with absorbing particles, resulting in an overall opaque effect.

[0066] Furthermore, Fig. 3, corresponding to Fig. 1 , shows the schematic diagram (in a sectional view and as a detail) of an optical element 10 in the exemplary first embodiment, combined with pixels 1 and a so-called “black matrix” 2 of an image display unit. In the actual embodiment, there are considerably more pixels 1. The black matrix 2 is shown exaggeratedly large here in order to underline its effect, namely that no light is actively emitted from there. In the left-hand first region E1 it can be seen that, due to the functioning of the optical element 10, the exemplary beams S1 and S2 emitted by a left-hand pixel 1 directly below are directed closer to the normal. In the right-hand pixel 1, above which a first region E1 is located, an exemplary light beam S3 incident obliquely into the first region E1 is refracted further away from the normal and penetrates into an adjacent second region E2.Depending on the alternative, it can propagate there (e.g. in the first alternative with transparent material for the second areas E2) and would then be coupled out at the light exit side or alternatively passed on via total reflection, is absorbed (e.g. in the second alternative with opaque material for the second areas E2), or it is scattered or reflected in the (e.g. in the third alternative with reflective and / or scattering material for the second areas E2).

[0067] Furthermore, Fig. 4, corresponding to Fig. 2, shows the schematic diagram (in a sectional view and as a detail) of an optical element 10 in the exemplary second embodiment, combined with pixels 1 and a so-called “black matrix” 2 of an image display unit. In the right-hand pixel 1, above which a first region E1 is located, an exemplary light beam S3 incident obliquely into the first region E1 is refracted further away from the normal and penetrates into an adjacent second region E2. Depending on the alternative, it can propagate there (e.g. in the first alternative with transparent material for the second regions E2), is absorbed (e.g. in the second alternative with opaque material for the second regions E2), or it is scattered or reflected (e.g. in the third alternative with reflective and / or scattering material for the second regions E2).The other explanations given in section 3 for the exemplary beams S1 and S2 are applicable here mutatis mutandis and are therefore not repeated here.

[0068] Insofar as the design is the second alternative with opaque material, this can, for example, consist of a lacquer or polymer as a transparent part, which for the opaque part can be made up of, for example, graphite particles, which have a size of less than 500 nm in the direction of their greatest dimension, or nanoparticles of black carbon which have a size of less than 200 nm, for example with soot particles.

[0069] If, in the context of the third alternative, reflective or scattering material is present in second regions E2, this consists, for example, of a mixture of a lacquer and particles, whereby the particles are now reflective and / or scattering. The particles can be, for example, white-scattering particles such as titanium dioxide (TiO2) or reflective particles such as silver particles or chromium particles with a size of less than 500 nm.

[0070] If such an optical element of the third alternative is used in an arrangement with a backlight, a (large) part of the light incident on the light entry surface of the optical element 10, which is reflected or scattered at second areas E2, can be recycled, for example, in an underlying light source, as the said backlight.

[0071] Furthermore, the first regions E1 and the second regions E2 can be arranged in strip-like fashion, distributed alternately over the surface of the optical element 10 when viewed in parallel projection perpendicular to the optical element 10, such as for the exemplary first and second embodiments according to Fig.1 to Fig.4.

[0072] In addition, Fig. 5 shows the basic sketch (sectional view and as a detail) of an optical element 10 in an exemplary third embodiment. The explanations for the drawing Fig. 1 apply here to the arrangement and mode of operation of the first and second regions E1, E2. However, here there are transparent and opaque sections 4, 5 between a transparent substrate S (e.g. made of glass or polymer) and the first optical element 10, with transparent sections 4 being arranged parallel to the surface center perpendicular between the substrate S and the first regions E1 and opaque sections 5 being arranged between the substrate S and the second regions E2 in the projection direction. This third embodiment ensures that on the light entry side of the optical element 10 only light enters through the first regions E1.

[0073] Furthermore, Fig.6 shows the schematic diagram (sectional view and as a detail) of an optical element 10 in an exemplary fourth embodiment, combined with pixels 1 and a black matrix 2 of an image display unit. On the light exit side of the optical element 10, opaque sections 3 are present exclusively in the second regions E2 in the projection direction parallel to the surface center perpendicular. This configuration serves to optimize the light direction and avoids crosstalk, for example, from exemplary rays S3, as shown in Fig. 3.

[0074] A particularly advantageous, exemplary fifth embodiment of an optical element 10, again combined with pixels 1 (which are shown here in a simplified circular shape and can correspond to color subpixels, full-color pixels, monochrome or other types of pixels) and a black matrix 2 of an image display unit, is shown as a schematic diagram (here as a perspective view and in detail) in Fig. 7a. It is provided that the first regions E1 (which have a funnel shape in section) are arranged in a circular shape distributed over the surface of the optical element 10 on the light incidence side or the light exit side when viewed in parallel projection perpendicular to the optical element 10, and the second regions E2 (not shown in the drawing) are each shaped complementarily thereto.This restricts the light propagation directions in at least two planes perpendicular to the surface of the optical element 10. In practice, the effect of such an optical element 10 is generally such that the light propagation directions for light emitted (or transmitted) by the pixels 1 are focused at any angle close to the perpendicular bisector of the optical element 10 or parallel to it. "Close" in this case means that the deviations from the perpendicular bisector or the parallel to it are less than 20° or 30°, depending on the design.

[0075] An advantageous, exemplary sixth embodiment is shown in Fig. 7b, a modification of Fig. 7a. The change from Fig. 7a is that the pixels 1 according to Fig. 7b are now shown in rectangular form. In practical implementation, completely different shapes or outlines of the pixels 1 are also possible, such as chevron structures made of IPS / FFS cells or various sizes and shapes for R, G, B color subpixels, for example in the case of OLED panels as image display devices.

[0076] The invention also improves the state of the art in that with such optical elements 10, a top-hat distribution can be achieved, since due to the total reflection more useful light is transmitted into the desired angular ranges than - as is usual in the prior art - without the total reflection.

[0077] For this purpose, Fig.8 gives exemplary simulation results for various parameter configurations of exemplary optical elements 10 in the first embodiment in comparison with a reference value.

[0078] By varying the input parameters, various defined distributions of the transmission of the optical element 10 can be achieved, such as a so-called top-hat distribution (see Example 1). The graphs are normalized to the reference viewing angle of 0° and show the relative luminance at the various horizontal viewing angles. Basically, the relationships shown in Fig. 1 were used here (i.e., trapezoidal or funnel shape of the interfaces of the first and second regions E1, E2 in section). The simulated parameters are as follows:

[0079] Reference: Lambertian light source (e.g. OLED), each with one optical element, but where N1 =N2.

[0080] Example 1: a. Thickness of the optical element 10: 70 pm b. Width of the first regions E1 on the light entrance side: 10 pm c. Amount of the critical angle between first and second regions E1, E2: 8° d. Offset between the end of a first region E1 and an edge of an assumed pixel 1 of an image display unit: 0.0 pm e. Difference N2 to N1: 0.140

[0081] Example 2: a. Thickness of the optical element 10: 60 pm b. Width of the first regions E1 on the light entrance side: 14 pm c. Amount of the critical angle between the first and second regions E1, E2: 8° d. Offset between the end of a first region E1 and an edge of an assumed pixel 1 of an image display unit: 5.0 pm e. Difference N2 to N1: 0.120

[0082] Example 3: a. Thickness of the optical element 10: 60 pm b. Width of the first regions E1 on the light entrance side: 15 pm c. Amount of the critical angle between the first and second regions E1, E2: 7° d. Offset between the end of a first region E1 and an edge of an assumed pixel 1 of an image display unit: 5.0 pm e. Difference N2 to N1: 0.140

[0083] These parameters are selected as examples and may need to be adjusted to the size of the pixels or subpixels when using a specific image display unit.

[0084] Fig. 8 shows that different optical elements 10 focus the light to different degrees. This is a great advantage if you want to increase the light output of screens, for example, because in certain applications no viewing angle greater than + / -40 0is sensible or necessary. Examples 1 to 3 according to Fig. 8 increase the central luminance by a factor of 4 to 7. The power consumption of a screen consisting of an image display device with an optical element 10 can be correspondingly lower, which also has a positive effect on the service life of the screen.

[0085] At this point, a particular advantage of the invention becomes apparent again: It allows the light emitted by a screen to be focused on a desired viewing area with minimal loss. Thus, the invention allows a defined distribution of the luminance curve in one or two dimensions to be achieved. If the luminance curve is rather broad, the content displayed by the image display unit can be viewed from a wide angle, while still allowing greater efficiency to be achieved compared to the prior art by varying the parameters of the optical element 10 to achieve the desired luminance curve.

[0086] Furthermore, the invention comprises a first screen, comprising an image display unit, an optical element 10 as described above, wherein the optical element 10 is arranged in front of or behind the image display unit in the viewing direction of a viewer, wherein in the case of parallel projection from a perpendicular direction onto the image display unit, the second regions E2 are in front of surface sections of the image display unit which do not (or substantially do not) emit light, for example a black matrix 2. Such a configuration has been described earlier in Fig.3 and Fig.4. It is also advantageous that the optical element 10 is arranged in front of or behind the image display unit in the viewing direction of a viewer in such a way that in the case of parallel projection from a vertical direction onto the image display unit, the first regions E1 of the optical element 10 lie in front of light-emitting surface sections 1, i.e. in particular pixels (color subpixels, full-color pixels, monochrome or other types of pixels) of the image display unit.

[0087] Non-light-emitting surface sections 2 of the image display unit can, for example, be the black matrix areas 2 of an OLED, microLED, or LCD panel or deactivated pixels (possibly including the respective black matrix areas of an OLED, LCD, or microLED panel). Other configurations are possible.

[0088] For the first screen, it may be advantageous if the optical element 10 is optically bonded or laminated to the image display unit.

[0089] Furthermore, the invention also includes a lighting device comprising a backlight, an optical element 10 arranged in front of the backlight in the viewing direction of a viewer, as described above, and preferably in the first, particularly preferably in the third alternative. Here too, the invention allows a defined focusing of the light emitted by the illumination device onto a desired viewing area, ie onto a defined luminance curve, in the same way as described above for the first screen.

[0090] Furthermore, the invention comprises a second screen comprising an illumination device as described above, and a transmissive second image display unit, such as an LCD panel, arranged in front of the illumination device in the viewing direction of a viewer.

[0091] In the latter case, an optical element 10 would typically be integrated directly into an illumination device for a transmissive image display unit, such as an LCD panel.

[0092] Finally, the invention also comprises a third screen having an image display unit with pixels, each pixel having a pixel outline defined by a luminous or light-transmitting surface, the pixel outlines all pixels can be identical or at least partially different. The third screen comprises an optical element as described above, with the only difference being that here it is not the first regions E1 on the light entry side and the light exit side that have different outline shapes that cannot be made congruent by scaling, but rather the first regions on the light entry and / or light exit side, on the one hand, and the pixel outlines of the pixels of the image display device, on the other.

[0093] A first or second screen comprising at least one optical element as described above and an image display unit can be used, for example, in a monitor or a mobile device. Furthermore, it is also possible to retrofit an image display unit with an optical element by placing it in front of the image display unit to increase its efficiency.

[0094] The invention described above solves the stated problem: Optical elements have been described that can influence incident light in a defined manner in its propagation directions, wherein at least a portion of the light incident on the light entry side of such an optical element is focused after passing through the optical element. Such optical elements can be implemented inexpensively and, in particular, can be used universally with various screen types to achieve increased efficiency in luminance distribution. Furthermore, they fundamentally offer the possibility of achieving a top-hat light distribution.

Claims

Patent claims 1. A planar optical element (10) having a light entry side and a light exit side, comprising first regions (E1) consisting of at least one transparent material having a first refractive index (N1) and second regions (E2) consisting of an opaque material and / or transparent and / or reflective or scattering material having a second refractive index (N2), wherein the first regions (E1) are embedded in the second regions (E2) in a one- or two-dimensional pattern, wherein the first refractive index (N1) is greater than the second refractive index (N2) over the entire wavelength range visible to the human eye, wherein the first regions (E1) each have a larger cross-section on the light exit side than on the light entry side,and further wherein the first regions (E1), when viewed in parallel projection perpendicular to the optical element (10), each have different outline shapes on the light incidence side and on the light exit side, which cannot be brought into congruence by scaling, so that light incident on the light entry side of the optical element (10) and incident on the first regions (E1) of the optical element (10) is, depending on the geometric direction of incidence, polarization, and the ratio of the first refractive index (N1) to the second refractive index (N2), a) propagated unhindered within a first region (E1) or is totally reflected and is subsequently coupled out again at the light exit side of the corresponding first region (E1), or b) penetrates from the first region (E1) into an adjacent second region (E2) and, if it is not absorbed in the second region E2, propagated in a second region (E2),until it is coupled out of a second region (E2) on the light exit side or coupled into one or more next first and / or second regions (E1, E2), and so that in a first alternative, in which the second regions (E2) consist of a transparent material, light which is emitted at the light entry side of the, optical element (10) is incident on second regions (E2) of the optical element (10), penetrates these regions and is coupled out of the optical element (10) on the light exit side, and so that in a second alternative, in which the second regions (E2) at least partially comprise an opaque material, light which is incident on the light entry side of the optical element (10) on second regions (E2) of the optical element (10) is at least partially absorbed, and so that in a third alternative, in which the second regions (E2) at least partially comprise a reflective and / or scattering material, light which is incident on the light entry side of the optical element (10) on second regions (E2) of the optical element (10) is at least partially reflected and / or scattered,whereby the light emerging from the optical element (10) at the light exit side thereof is changed in its propagation directions compared to the light incident on the optical element (10) at the light entry side, and whereby furthermore at least a part of the light incident on the light entry side of the optical element (10) into first regions (E1) is bundled after passing through the optical element (10).

2. Optical element (10) according to claim 1, characterized in that the interfaces of the first regions (E1) and the second regions (E2) are trapezoidal, funnel-shaped, at least partially or completely parabolic, parallelogram-shaped, and / or step-shaped when viewed in the sectional direction perpendicular to the light exit side of the optical element (10).

3. Optical element (10) according to one of claims 1 or 2, characterized in that the first regions (E1) and the second regions (E2) are arranged in strip-like fashion distributed alternately over the surface of the optical element (10) when viewed in parallel projection perpendicular to the optical element (10).

4. Optical element (10) according to one of claims 1 or 2, characterized in that the first regions (E1) are each arranged on the light incidence side or the light exit side when viewed in parallel projection perpendicular to the optical Element (10) has a point-shaped, circular, oval, rectangular or hexagonal outline and is arranged distributed over the surface of the optical element (10) and the second regions (E2) are each shaped complementarily thereto.

5. First screen, comprising an image display unit, an optical element (10) according to one of claims 1 to 4, wherein the optical element (10) is arranged in front of the image display unit in the viewing direction of a viewer such that in the case of parallel projection from a perpendicular direction onto the image display unit, the second regions (E2) are in front of non-light-emitting surface sections (2) of the image display unit.

6. First screen, comprising an image display unit, an optical element (10) according to one of claims 1 to 4, wherein the optical element is arranged in front of or behind the image display unit in the viewing direction of a viewer such that in the case of parallel projection from a perpendicular direction onto the image display unit, the first regions (E1) of the optical element (10) are in front of light-emitting surface sections (1) of the image display unit.

7. Lighting device comprising a backlight, an optical element (10) according to one of claims 1 to 4 arranged in front of the backlight in the viewing direction of a viewer.

8. Second screen comprising an illumination device according to claim 7, a transmissive second image display unit arranged in front of the illumination device in the viewing direction of a viewer.

9. Second screen according to claim 8, further comprising a plate-shaped light guide with two large surfaces, which are enclosed and connected by narrow sides, between the lighting device and the transmissive second image display unit, wherein light from light sources can be coupled in at the narrow sides and is then coupled out again at the large surfaces, so that depending on whether light is coupled into the light guide and then coupled out of the light guide or not, it is possible to switch between two operating modes, wherein in a first operating mode in which no light is coupled out of the light guide, a viewing angle in front of the second screen is restricted compared to a second operating mode in which at least light is coupled out of the light guide.

10. Third screen comprehensive - an image display unit with pixels, each pixel having a pixel outline defined by a luminous or light-transmitting surface, the pixel outlines of all pixels being identical or at least partially different, - a planar optical element (10) having a light entry side and a light exit side, which is located in front of or behind the image display unit from the viewing direction of a viewer, and which comprises: • first regions (E1) consisting of at least one transparent material with a first refractive index (N1) and second regions (E2) consisting of an opaque material and / or transparent and / or reflective or scattering material with a second refractive index (N2), wherein the first regions (E1) are embedded in the second regions (E2) in a one- or two-dimensional pattern, wherein the first refractive index (N1) is greater than the second refractive index (N2) over the entire wavelength range visible to a human eye, • wherein the first regions (E1) each have a larger cross-section on the light exit side than on the light entry side, and wherein at least one location on the optical element (10) the first regions (E1) when viewed in parallel projection perpendicular to the image display unit each have outline shapes on the light entry side and / or on the light exit side which cannot be brought into congruence with a pixel outline present on the image display device by scaling, • so that on the light entry side of the optical element (10) incident light which enters first regions (E1) of the optical element (10) is incident, there, depending on the geometric direction of incidence, polarization and the ratio of the first refractive index (N1) to the second refractive index (N2), a) is propagated unhindered or totally reflected within a first region (E1) and is then coupled out again at the light exit side of the corresponding first region (E1), or b) penetrates from the first region (E1) into an adjacent second region (E2) and, if it is not absorbed in the second region E2, propagates in a second region (E2) until it is coupled out of a second region (E2) on the light exit side or coupled into one or more next first and / or second regions (E1, E2), • and so that in a first alternative of the optical element, in which the second regions (E2) consist of a transparent material, light which is incident on the light entry side of the optical element (10) into second regions (E2) of the optical element (10), penetrates these and is coupled out of the optical element (10) on the light exit side, • and so that in a second alternative of the optical element, in which the second regions (E2) at least partially comprise an opaque material, light which is incident on the light entry side of the optical element (10) into second regions (E2) of the optical element (10) is at least partially absorbed, • and so that in a third alternative of the optical element, in which the second regions (E2) at least partially comprise a reflective and / or scattering material, light which is incident on the light entry side of the optical element (10) into second regions (E2) of the optical element (10) is at least partially reflected and / or scattered, • whereby the light emerging from the optical element (10) at the light exit side is changed in its propagation directions compared to the light incident on the optical element (10) at the light entry side, • and whereby at least a part of the light incident on the light entry side of the optical element (10) into first regions (E1) is bundled after passing through the optical element (10), - so that in a first alternative of the third screen, in which the image display device is located behind the optical element (10) in the viewing direction, the luminance curve for the light emitted by the image display device is at least partially changed, and - so that in a second alternative of the third screen, in which the image display device is located in front of the optical element (10) in the viewing direction, the luminance curve for light incident on the back of the image display device is at least bundled.

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