Lighting device for a free viewing mode and a restricted viewing mode, and screen comprising such a lighting device

Abstract

The invention relates to a lighting device (1a) for a screen (1), said lighting device being capable of being operated in at least two types of operating methods B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted by the lighting device (1a) in an angular range that is restricted in comparison to the free viewing mode, comprising a two-dimensionally extended backlight (2) which emits light in the restricted angular range; a plate-shaped light guide (3) which is located in front of the backlight (2) in the viewing direction and has decoupling elements (6) on at least one of the large surfaces and / or within the volume of the light guide, each decoupling element (6) having, in order to decouple light in a defined manner, at least one functional surface (5) at which light is decoupled from the light guide (3); and lighting means (4) which are provided laterally on at least one narrow face of the light guide (3). The backlight (2) is switched on and the lighting means (4) are switched off in the operating method B2, and at least the lighting means (4) are switched on in the operating method B1. The following applies to at least two percent of the decoupling elements (6): the functional surface (5) has a curvature in at least two directions in order to decouple light in a defined manner, the two shapes of the curvatures of each functional surface (5) differing in the at least two directions, whereby light of different wavelengths, said light originating from the lighting means (4) and being coupled into the light guide (3) at at least one of the narrow faces, is mixed on the respective light path within the light guide (3) up to the respective location where the light is decoupled, whereby visually perceptible color effects are minimized or completely suppressed. The invention further relates to a screen (1b) having a lighting device (1a).

Description

title

[0001] Lighting device for a free and a restricted view mode and screen with such a lighting device 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 the very large viewing area of ​​a screen can be a disadvantage. Information, such as banking details and other personal and sensitive data, is becoming increasingly available on mobile devices such as notebooks and tablet PCs. Accordingly, people need control over who can see this sensitive data; they need to be able to choose between a wide viewing angle in order to share information on their display with others, e.g. when looking at holiday photos or for advertising purposes. On the other hand, they need a narrow viewing angle if they want to keep the image information confidential.

[0003] A similar problem arises in vehicle construction: The driver must not be distracted by image content, such as digital entertainment programs, when the engine is running, while the passenger also wants to consume the same content while driving. Therefore, a screen that can switch between the corresponding display modes is required.

[0004] Additional films based on micro-louvres have already been used for mobile displays to achieve optical data protection. However, these films were not switchable; they always had to be applied and removed manually. They also had to be transported separately from the display when not in use. A major disadvantage of using such louvre films is the associated light loss. State of the art

[0005] US Pat. No. 5,956,107 A discloses a switchable light source that allows a display to operate in multiple modes. The disadvantage here is that all light extraction is based on scattering, resulting in low efficiency and suboptimal light direction effects. In particular, the achievement of a focused light cone is not disclosed in detail.

[0006] CN 1077341 18 A describes a display that uses two backlights to control the viewing angle of a screen. The upper of the two backlights is intended to emit focused light. A grid with opaque and transparent sections is specifically mentioned as a design for this purpose. However, this presumably results in the light from the second backlight, which must penetrate the first toward an LCD panel, also being focused, thus significantly narrowing the viewing angle in the public viewing mode, which is actually intended for a wide viewing angle.

[0007] US 2007 / 030240 A1 describes an optical element for controlling the direction of light propagation from a backlight. This optical element requires liquid crystals in the form of PDLCs, for example, which are both expensive and safety-critical, especially for end-user applications, since PDLC liquid crystals typically require voltages higher than 60V for their circuitry.

[0008] CN 1987606 A, in turn, describes a display that uses two backlights to control the viewing angle of a screen. In particular, a "first light plate" is used, which must be wedge-shaped to enable the intended focused light extraction. Precise details on how to achieve the focused light extraction with the corresponding angle conditions are not disclosed.

[0009] Furthermore, US 2018 / 0267344 A1 describes a setup with two flat lighting modules. The light from the rear lighting module is focused by a separate structure. After focusing, The light still has to pass through the front lighting module, which has diffusing elements. Therefore, a strong light focus for privacy protection is not optimal.

[0010] Finally, US 2007 / 0008456 A1 discloses the division of a light beam angle into at least three areas, with two of these areas typically being illuminated. This means that a privacy screen using such an illuminated display cannot be viewed from only one direction.

[0011] WO 2015 / 121398 A1 by the applicant describes a screen of the type described above. In this case, scattering particles are present in the volume of the corresponding light guide, which are essential for switching between operating modes. However, the polymer scattering particles selected there generally have the disadvantage that light is coupled out of both large surfaces, causing approximately half of the useful light to be emitted in the wrong direction, namely toward the backlight, where it cannot be adequately recycled due to their design. Furthermore, the polymer scattering particles distributed throughout the volume of the light guide can, under certain circumstances, particularly at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

[0012] US2020 / 012129 A1 discloses an illumination device and a display screen, which describe two lights for switching between a narrow and a wide viewing mode. One of the light guides is formed with fibers. The other is that the scattering output structure of a light guide is limited to specific stripes in the projection direction. This is detrimental to homogeneous image illumination and generally also causes unwanted moiré effects in the structure, for example, in interaction with the pixel columns or rows of an LCD panel located above.

[0013] WO 2012 / 042938 A1 (particularly in Fig. 14 A, B and Fig. 22 A, B) discloses output coupling structures with exactly one direction of curvature within the structures. This can lead to light of different wavelengths being output in different directions from one and the same point on the surface of an output coupling element due to dispersion. White light is thus undesirably split. This, in turn, can often produce disturbing visual "rainbow effects."

[0014] DE 10 2018 1 14 565 A1 discloses a display device for the presentation of monochrome images. The reduction of color artifacts such as rainbow formation is not addressed in DE 10 2018 1 14 565 A1. Rather, DE 10 2018 1 14 565 A1 discloses in paragraph

[0125] This means that two different displays (4B and 5B) simultaneously emit light of different colors in one device. Mixing light of different wavelengths, for example, to reduce disturbing artifacts, is not possible here, because this would negate the desired effect of two different colors.

[0015] The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the basic screen and / or require an active, or at least a special, optical element for mode switching and / or require complex and expensive production and / or reduce the resolution in the freely viewable mode and / or generate optical artifacts. Description of the invention

[0016] It is therefore an object of the invention to describe a lighting device which, in conjunction with a screen, can be used to reliably display information by means of an optionally restricted viewing angle, while in a further operating mode a free view with as little restriction as possible in terms of the viewing angle should be possible. The invention should be implementable using simple means and as inexpensively as possible. In both operating modes, the highest possible resolution, particularly preferably the native resolution of the screen used, should be visible. In particular, the solution should reduce or completely prevent visually perceptible artifacts such as color effects, e.g. rainbow effects. Furthermore, a screen should be specified which uses a corresponding lighting device.

[0017] This object is achieved according to the invention by a lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted by the lighting device in an angular range which is restricted compared to the free view mode, comprising - a surface-like backlight that radiates light into the limited angular range, - a plate-shaped light guide located in front of the background lighting in the viewing direction, with two large surfaces and narrow sides that connect the large surfaces at their edges, whereby - the light guide has decoupling elements on at least one of the large surfaces and / or within its volume, - each coupling-out element has at least one functional surface for the defined coupling-out of light, at which light is coupled out of the light guide, - illuminants arranged laterally on at least one narrow side of the light guide (a coupling side), - where in operating mode B2 the backlight is switched on and the lamps are switched off, and where in operating mode B1 at least the lamps are switched on, - and wherein for at least two percent (alternatively at least four percent, at least ten percent, preferably more than twenty percent or one hundred percent) of the coupling-out elements, the functional surface has a curvature for the defined coupling-out of light in at least two directions, wherein the two forms of the curvature of the respective functional surface (5) differ in the at least two directions, whereby light of different wavelengths originating from the illuminants and coupled into the light guide on at least one of the narrow sides is mixed on the respective light path within the light guide up to the respective location of the coupling-out of the light, whereby visually perceptible color effects, e.g. so-called rainbow effects, are minimized or completely suppressed.

[0018] The inventive design advantageously results in a stronger mixing of the wavelengths contained in white light, compared to the curvature of the functional surfaces of the output elements in only one direction, which is customary in the prior art. In this new way, visually perceptible color effects in the output light are minimized or completely suppressed. In the prior art, it is known to curvature the functional surfaces of the output elements in only one direction, and optionally to vary the angle of incidence if necessary, as described in WO 2023 / 274541 A1. However, this can also be used to compensate for color variations resulting from different output angles on the functional surfaces of the output elements for different wavelengths can only be compensated for within certain limits. In the prior art, three-dimensional, non-scattering output elements with a functional surface that is curved in only one direction often result in perceptible rainbow effects due to locally - unintentionally - concentrated output wavelength ranges. The invention offers a significant improvement here due to the curvature of the functional surfaces of the output elements in at least two directions: The additional curvature ensures greater mixing of the different color components of the output light, which is particularly important for white light in order to avoid producing maxima that are not spatially spectrally resolved and that can be perceived, for example, as undesired color or rainbow effects.

[0019] Advantageously, the illuminants are arranged on at least one of the narrow sides of the light, wherein this coupling side or these coupling sides can be located above, below, left and / or right of the light guide from the perspective of a seated or standing observer.

[0020] For special designs, it may apply that only those output coupling elements have a curvature of their functional surfaces in at least two directions, which are located within the first third of the volume or on the first third of a large surface of the light guide in front of the light coupling side. This third may also assume a different value, such as a quarter or half, or other values. Outside the aforementioned range, the functional surfaces of the output coupling elements may have a curvature in only one direction.

[0021] In general, the output coupling elements can have a three-dimensional shape with a maximum extension in their largest dimension that is less than 100 micrometers, preferably less than 50 micrometers. In particular, it is possible for the output coupling elements to have the shape of prismatoids, scutoids, bodies related to these, and / or convex bodies. Other configurations are explicitly possible.

[0022] According to the invention, a lighting device is designed such that the coupling-out elements each have a functional surface for the defined coupling-out of light with a respective curvature in at least two directions, wherein the two forms of curvature of the respective functional surface differ in at least two directions.

[0023] By way of example—and not exclusively—the respective curvature of the functional surfaces of the coupling elements in a first direction and in a second direction perpendicular thereto can be circular, elliptical, exponential, or Bézier curve-shaped, subject to a definable tolerance. Other configurations are explicitly possible.

[0024] Furthermore, it can be helpful for improved producibility, but also for further improving the optical properties of a lighting device, if at least one additional surface of at least part of the output elements, which does not correspond to the respective functional surface, has a curvature in at least one or two directions. This can, for example, be draft angles (e.g., for injection molding or nano-imprinting processes), which are implemented using surfaces of the output elements that do not correspond to the respective functional surface.

[0025] Furthermore, the coupling elements can be formed as depressions or elevations on one (or both) of the large surfaces of the light guide, whereby both depressions and elevations can also be present on one and the same light guide or at least one of its large surfaces. Preferably, however, the coupling elements are formed as depressions on one of the large surfaces of the light guide. It is also possible for coupling elements to be mounted on both large surfaces and / or optionally additionally within the volume.

[0026] Furthermore, it is advantageous for the application if the light guide exhibits a stronger scattering behavior in a selectable direction than in a direction perpendicular thereto. Preferably, this selectable direction corresponds to the vertical direction when a (standing or seated) observer views the illumination device, so that the scattering behavior of the light guide is greater in the vertical direction than in the horizontal direction, with the horizontal direction running parallel to a line between the eyes of said observer.

[0027] Accordingly, the backlight can basically be constructed like an LED backlight, for example as a so-called direct-lit LED backlight, edge LED backlight, OLED or another surface radiator, on which at least one permanent privacy filter (e.g. with micro-louvres or polarization-sensitive) is applied.

[0028] The two operating modes B1 and B2 differ in that in B2 the backlight is switched on and the lamps (on the input side of the light guide) are switched off, whereas in B1 at least the lamps (on the input side of the light guide)—and optionally also the lamps—are switched on. Only light originally emitted by the lamps into the light guide and subsequently re-emitted via the output elements is taken into account, with the emission occurring almost exclusively via the output elements.

[0029] The light guide preferably consists of a transparent, thermoplastic or thermoelastic polymer, e.g., plastic, or glass. For example, the light guide or its substrate can comprise at least 40 percent by weight of polymethyl methacrylate, preferably at least 60 percent by weight of polymethyl methacrylate, based on its weight. Alternatively, it can be polycarbonate (PC), for example.

[0030] Furthermore, it is possible to mitigate any optical artifacts that may arise, for example, from the manufacturing of the light guide or its output structures, using an anisotropic diffuser located in front of the light guide in the viewing direction of an observer. According to the previously described definition of directions, this diffuser should, if possible, scatter significantly less in the horizontal direction than in the vertical direction, in order to scatter light horizontally as little as possible, or ideally not at all, in operating mode B2 for a restricted view.

[0031] In principle, any area smaller than the half-space in front of the background illumination can be considered as a restricted angular range; however, an angular range of + / -20° or + / -30° is preferred. 0horizontal and / or vertical or as a cone around the surface normal or a selectable directional vector on the backlight; small amounts of light of less than 1% to 5% of the maximum brightness can be disregarded when defining the restricted angular range.

[0032] The illumination device may additionally contain a collimation film at a suitable location in the structure, for example a lens or prism grid above or below the plate-shaped light guide.

[0033] It is important for the design of the invention that the targeted coupling of light from the light guide does not occur through scattering, as is usual with most commercially available light guides. For coupling out by means of scattering elements that, so to speak, damage the light guide surface in the prior art, the invention would be ineffective, since the coupling out of the light in this case is not defined or deterministic, but primarily diffuse. Furthermore, coupling out by means of scattering elements would have the negative consequence that the light guide would also have a strongly scattering effect on light penetrating it through the large surfaces, which is an exclusion criterion, particularly for its use in privacy applications (such as for operating mode B2). It can therefore be stated that the invention does not include light guides whose primary approach for coupling out light is the principle of scattering.For this reason, three-dimensional structural elements are particularly preferable because they enable deterministic light extraction—except for avoidable scattering within a specified tolerance. In general, the scattering of the light passing through the light guide should be as minimal as possible, as described in the following two sections.

[0034] During the manufacture of the light guide, the output coupling elements can be distributed in a variety of ways within or on the light guide, depending on the adaptable and predeterminable conditions for the light output. Output coupling elements are locally limited structural changes in the volume and / or on the surfaces of the light guide. Therefore, the term output coupling element expressly excludes additional optical layers applied to the surfaces of the light guide, e.g., diffusion layers, reflection layers, (dual) brightness-enhancing, collimating brightness enhancement films (BEFs), or polarization-recycling layers, such as polarization-selective Bragg mirrors (dual) brightness enhancement films (D)BEFs) or wire-grid polarizers.These additional layers, which do not fall under the term "outcoupling element," are only connected to the light guide at the edges—if at all. However, they are usually only loosely bonded to the large surfaces and do not form a physical unit with the light guide. In contrast, lacquers applied to the large surfaces form a bond with the light guide through chemical reactions or other processes. Forces (e.g. van der Waals forces) bind together, forming a physical unit, and can no longer be separated; such coatings therefore do not count as an additional layer in the above-mentioned sense.

[0035] The number of output elements per surface and their extent are selected such that the light guide has an average haze value of less than 20%, preferably less than 15%, particularly preferably less than 10%, measured according to ASTM D1003 over at least 50%, preferably 80% of its surface, particularly preferably over its entire surface - whereby the measurement according to the more common procedure A with a hazemeter is used as a reference. As a result, light penetrating the light guide through its large surfaces is only slightly scattered. By "slightly" is meant, for example, that (due to the low haze value) in an angular range of, for example, horizontally + / -40 0A maximum of 1% to 5% of the luminance is added to the surface normal due to scattering of light from the light guide, which is radiated into the light guide at a perpendicular angle across a large area. Commercially available light guides that use scattering as their primary mechanism for coupling out the light typically do not achieve the aforementioned haze values.

[0036] Alternatively, the number of output elements per surface and their dimensions can be selected such that the light guide scatters a maximum of 25 percent, but preferably a maximum of 10 percent, of the light penetrating its large surfaces by more than 10 degrees (preferably only 7°, particularly preferably only 5°) over at least 80% of its surface. Commercially available light guides that use scattering as their primary output mechanism typically do not achieve the aforementioned maximum scattering values.

[0037] In a further embodiment, the large surface of the light guide from which the light emerges can be divided into sub-areas of a predetermined size, and the ratio of the (summed) surface areas of the functional surfaces in a sub-area to the surface area of ​​the respective sub-area can be different for different sub-areas, so that the scattering behavior of the light guide varies across the large surface from which the light emerges. It should be noted that the large surface from which the light emerges does not necessarily have to correspond to the large surface on which the output coupling elements are located. Rather, for example, a large surface can contain the output coupling elements. elements as output coupling elements directed toward the volume of the light guide, which then redirect the coupled light and thus output it. However, light rays output in this way first traverse the volume of the light guide or parts of it before exiting the light guide at the other large surface. In general, the scattering behavior of the output coupling elements is defined in particular by their shape and size, but especially by their functional surface, which is taken into account in the optical design.

[0038] The backlight consists, for example, of a planar radiator, preferably a further light guide with additional light sources arranged on the side or on the rear, as well as at least one light collimator integrated into the planar radiator and / or arranged in front of it, such as at least one prism film and / or at least one privacy filter (leaf filter). Alternatively, instead of a leaf filter, an optical element with absorption dipole moments, the majority of which are aligned with a maximum tolerance of 20° to the vertical on the surface of the optical element, can be used to limit the light direction in conjunction with a linear polarization filter, for example, a rear polarization filter of an LCD panel used with the lighting device.In addition, a so-called focused backlight unit can be used as background lighting, in which light from a (different) light guide is already coupled out into a limited angular range and, if necessary, directed, redirected or reshaped.

[0039] The coupling-out elements themselves can also be designed, for example, as cavities formed within the volume of the light guide. The cavities can be airless, but are preferably filled with a gaseous, liquid, or solid material. The material has a refractive index that differs from that of the material used for the light guide; preferably, it is lower. The filling with material and the choice of material can influence the light transmission or coupling-out. Alternatively or additionally, the haze value of the material preferably differs from that of the material used for the light guide and is preferably higher. The advantages of these designs are greater efficiency in light extraction.

[0040] Alternatively and technically more simply, the cavities can also be formed if the light guide is made of two interconnected substrate layers, The substrate layers are preferably of the same type. The bonding can be chemical, physical, or adhesive. The cavities are then formed as material recesses at at least one of the interfaces between the substrate layers.

[0041] If the output coupling elements are attached to at least one of the large surfaces of the light guide, they are advantageously formed from a plastic or glass whose structure has been embossed using a tool. This is possible, for example, in mass production by applying a UV-curing material - e.g. a lacquer, a monomer, etc. - to a light guide substrate, which is then structured using a tool and cured, e.g. polymerized, by UV radiation. Other radiation-curing materials can also be used. The formation of the recesses for the output coupling elements can be achieved, for example, mechanically, lithographically or printing-technically, or by material application, conversion, removal or dissolution.In particular, variants of injection molding (variothermal / isothermal, injection compression molding / injection molding) can be used with the aid of appropriate structural inserts (see also DE102020134055 B4 of the applicant).

[0042] This allows for the cost-effective implementation of three-dimensional structural elements—either convex with the plastic portion facing outward on the surface, and / or concave as an embossed or recessed surface layer of the structured plastic—with mass production capability. Concave and convex structures can be used equally well.

[0043] The structure of the decoupling elements is specified as described above according to the criteria mentioned, whereby the effect of each decoupling element is at least approximately known and properties of the light guide or of the light emerging from the light guide can be specifically determined by a predeterminable structure and distribution of the decoupling elements, whereby the ratio of the sum of the surface areas of the functional surfaces to the surface area of ​​the total surface of the large surface from which light is coupled out as well as the shapes of the functional surfaces are particularly important.

[0044] The required properties for the decoupling elements that are essential for the invention with regard to their number per unit area, their shape including the functional area, their orientation and extension in three dimensions as well as their Distribution on at least one of the large surfaces and / or within the volume of the light guide can be determined, for example, using optical simulation software such as “LightTools” from Synopsis or other providers and then physically implemented accordingly.

[0045] Advantageously, the distribution of the coupling-out elements on at least one of the large surfaces and / or within the volume of the light guide is specified such that the coupled-out light achieves a luminance homogeneity (particularly with respect to white light) of at least 50%, preferably at least 60%, over at least 50% (preferably 95% or 100%) of the entire (light-coupling area of) the light guide. The luminance homogeneity can be defined as Lv min / Lv maxbe defined as the ratio of the smallest value of the luminance to the largest value of an area under consideration (so-called “area scan” approach, in which each determined value on the area under consideration is included in the evaluation).

[0046] Furthermore, the invention comprises a screen according to the invention which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted by the lighting device in an angular range which is restricted compared to the free view mode, comprising - a lighting device as described above, - a transmissive imager, preferably an LCD panel, in front of the illumination device in the viewing direction.

[0047] Furthermore, it is advantageous for some applications if the said restricted angular range is designed asymmetrically around the surface normal of the background lighting. The asymmetric design preferably occurs in one of the preferred directions. This is particularly helpful for applications in vehicles, for example when a screen to be combined with the lighting device according to the invention is arranged as a so-called center information display in the dashboard approximately midway between the driver and front passenger. In this case, the restricted angular range of vision, which is released exclusively for the front passenger in operating mode B2, must be designed asymmetrically, i.e. directed towards the front passenger. The preferred direction in which the asymmetry is designed corresponds to the horizontal.

[0048] The lighting device according to the invention with a screen is particularly advantageous for use in a vehicle for selectively displaying image content only for the front passenger in operating mode B2 or simultaneously for the driver and front passenger in operating mode B1. The former is helpful, for example, when the front passenger is watching entertainment content that could distract the driver.

[0049] Other variants are conceivable and within the scope of the invention. Advantageously, the light guide is at least 50% transparent to the light penetrating it due to its large surface area. The light sources can be, for example, LEDs (preferred) or LED arrays or laser diodes, whose emission surface is particularly preferably at least approximately rectangular. Other variants are conceivable and within the scope of the invention.

[0050] Furthermore, the desired restricted angle ranges for mode B2 for a restricted view can be defined and implemented independently for the horizontal and vertical directions. For example, a larger angle (or possibly no restriction at all) might be appropriate in the vertical direction than in the horizontal direction, such as when people of different heights need to see an image at ATMs, while the side view should remain severely or completely restricted. For POS payment terminals, however, security regulations often require restricted views in mode B2 in both the horizontal and vertical directions.

[0051] A lighting device according to the invention with a screen can also be used to enter or display confidential data, for example PIN numbers, e-mails, SMS or passwords, at ATMs, payment terminals or mobile devices.

[0052] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0053] 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

[0054] The invention is 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 restrictive. For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary 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. Furthermore, none of the drawings are to scale. They show: Fig. 1 is a schematic diagram of an illumination device with a transmissive imager, Fig.2 a two-dimensional schematic diagram as a top view of exemplary coupling elements, Fig.3 shows a cross-sectional view and a schematic diagram of a light guide with decoupling elements in the state of the art, as well as Fig.4 shows a light guide shown in cross section and as a schematic diagram with coupling-out elements, the functional surfaces of which have a curvature in at least two directions for the defined coupling-out of light. Detailed description of the drawings

[0055] Fig.1 shows a schematic diagram of a lighting device 1 a for a screen 1 b, which can be used in at least two operating modes B1 for a free view mode and B2 for a restricted view mode in which light is emitted by the lighting device in an angular range restricted compared to the free view mode, comprising - a surface-like backlight 2, which radiates light into the restricted angular range, - a plate-shaped light guide 3 located in front of the background lighting 2 in the viewing direction, with two large surfaces and narrow sides that connect the large surfaces at their edges, wherein - the light guide 3 has decoupling elements 6 on at least one of the large surfaces and / or within its volume (the decoupling elements 6 are not shown in Fig.1), - each coupling-out element 6 has at least one functional surface 5 (the functional surfaces are also not shown in Fig.1) for the defined coupling-out of light, at which light is coupled out of the light guide 3, - illuminants 4 arranged laterally on at least one narrow side of the light guide 3 (a coupling side), - wherein in operating mode B2 the backlight 2 is switched on and the lamps 4 are switched off, and wherein in operating mode B1 at least the lamps 4 are switched on, - and wherein for at least two percent (alternatively at least four percent, at least ten percent, more than twenty percent or one hundred percent) of the coupling-out elements 6, the functional surface 5 has a curvature for the defined coupling-out of light in at least two directions, wherein the two forms of the curvature of the respective functional surface (5) differ in the at least two directions, whereby light of different wavelengths originating from the lighting means 4 and coupled into the light guide 3 on at least one of the narrow sides is mixed on the respective light path within the light guide 3 up to the respective location of the coupling-out of the light, whereby visually perceptible color effects, e.g. so-called rainbow effects, are minimized or completely suppressed.

[0056] Furthermore, Fig. 2 shows a two-dimensional, stylized schematic diagram as a top view of exemplary coupling elements. Their functional surfaces 5 (not shown in Fig. 2) The coupling-out elements 6 (shown in the drawing) can each be located on the convex or concave side, wherein it can be seen that these functional surfaces have a first curvature in plan view from this viewing or projection angle. In general, the coupling-out elements 6 can have a three-dimensional shape with a maximum extension in their largest dimension that is less than 100 micrometers, preferably less than 50 micrometers. In particular, it is possible for the coupling-out elements 6, as shown here, to have the shape of (curved) prismatoids, (curved) scutoids, bodies related to these, and / or other convex bodies.

[0057] Furthermore, Fig. 3 shows a prior art light guide with output coupling elements. The output coupling elements, which are shown here as being significantly oversized in relation to the thickness of the light guide (which can be, for example, a few tens of μm to a few millimeters), have functional surfaces that, by design, have no curvature in this sectional view, thus exhibiting a total curvature in only one direction (at most). The two left-hand output coupling elements schematically show what happens to two light beams guided approximately parallel to the two output coupling elements by total internal reflection due to the output coupling by means of the functional surfaces: Both are output coupled approximately parallel.This, in turn, means that with such functional surfaces in the state of the art, three-dimensional, non-scattering output elements with a functional surface that exhibits curvature in only one direction, perceptible rainbow effects can occur due to locally—unintentionally—clustered wavelength ranges being extracted. This is because, due to the wavelength dependence of the refractive index, parallel incident rays become colored, fanned-out rays—as in a prism. These, in turn, lead to the aforementioned locally clustered wavelength ranges, which can be visible, for example, as rainbow effects.

[0058] In contrast, Fig. 4 shows a light guide 3 with output elements 6, whose functional surfaces 5 for the defined output of light have a curvature in at least two directions. Similar to Fig. 3, the output elements 6 are shown here as being significantly exaggerated in relation to the thickness of the light guide 3 (which can be, for example, a few 100 pm to a few millimeters). For better clarity, only two of the four functional surfaces 5 of the four output elements 6 shown are labeled "5" in Fig. 4. The two left-hand output elements 6 schematically show what happens to two light beams directed approximately parallel to the two output elements 6 due to the output by means of the functional surfaces 5: Both Due to the curvature present in this top view (corresponding to a second direction), the light is no longer coupled out approximately parallel. Rather, the presence of curvatures of the functional surfaces 5 in two directions results in a greater light mixing than in the prior art, especially with regard to different wavelengths.

[0059] It should be noted that despite the few light rays shown in Fig.3 and Fig.4, in reality there is a very large number of different light rays.

[0060] The inventive design of a light guide 3, as shown in Fig. 4 as a schematic diagram, advantageously results in a stronger mixing of the wavelengths contained in white light, compared to the curvature of the functional surfaces of the output elements in a maximum of only one direction, as shown in Fig. 3, which is common in the prior art. In this new way, visually perceptible color effects in the output light are minimized or completely suppressed. The invention therefore offers a significant improvement here due to the curvature of the functional surfaces 5 of the output elements 6 in at least two directions: The additional curvature ensures a stronger mixing of the different color components of the output light, which is particularly important for white light in order to avoid producing maxima that are not spatially spectrally resolved and which can be perceived, for example, as undesirable color or rainbow effects.

[0061] Advantageously, the illuminants 4 are arranged on at least one of the narrow sides of the light guide 3, wherein this coupling side or these coupling sides can be located above, below, left and / or right of the light guide 3 from the perspective of a seated or standing observer.

[0062] Advantageously, a lighting device 1 a is designed such that the coupling-out elements 6 each have a functional surface 5 for the defined coupling-out of light with a respective curvature in at least two directions, wherein the two forms of the curvatures of the respective functional surface 5 differ in the at least two directions.

[0063] Preferably, as shown in Fig.4 as a schematic diagram, the coupling-out elements 6 are formed as depressions on one of the large surfaces of the light guide 3.

[0064] The background lighting 2 consists, for example, of a planar radiator, preferably a further light guide with further light sources arranged laterally or on the rear, as well as at least one light collimator integrated into the planar radiator and / or arranged in front of it, such as at least one prism film and / or at least one privacy filter (louvre filter).

[0065] Accordingly, the backlight 2 can basically be constructed like an LED backlight, for example as a so-called direct-lit LED backlight, edge LED backlight, an OLED or as another surface radiator, to which, for example, at least one permanent privacy filter (e.g. with micro-louvres or polarization-sensitive) is applied.

[0066] The two operating modes B1 and B2 differ in that in operating mode B2, the background illumination 2 is switched on and the lamps 4 (on the input side of the light guide 3) are switched off, while in operating mode B1, at least the lamps 4 (on the input side of the light guide 3)—and optionally also the lamps 4—are switched on. Only light originally emitted by the lamps 4 into the light guide 3 and subsequently re-emitted from the light guide via the output elements 6 is taken into account, with the emission occurring almost exclusively via the output elements 6.

[0067] The light guide is preferably made of a transparent, thermoplastic or thermoelastic polymer, e.g. polycarbonate (PC).

[0068] In principle, any area smaller than the half-space in front of the background illumination can be considered as a restricted angular range; however, an angular range of + / -20° or + / -30° is preferred. 0 horizontal and / or vertical or as a cone around the surface normal or a selectable directional vector on the backlight; small amounts of light of less than 1% to 5% of the maximum brightness can be disregarded when defining the restricted angular range.

[0069] It is important for the design of the invention that the targeted coupling of light from the light guide 3 is not carried out by scattering, as is usual with most commercially available light guides. For coupling by means of scattering elements that damage the light guide surface, as is the case with the prior art, the invention would be ineffective, since in this case the coupling out of the light is not defined or deterministic, but primarily diffuse. Furthermore, coupling out by means of scattering elements would have the negative consequence that a light guide would also have a strongly scattering effect on light penetrating through the large surfaces, which is an exclusion criterion in particular for its use in privacy applications (i.e. for operating mode B2). It can therefore be stated that the invention does not include light guides whose primary approach for coupling out light is the principle of scattering. For this reason, three-dimensional structural elements are to be preferred in particular, because they enable a deterministic coupling out of light - except for avoidable scattering within a predeterminable tolerance. In general, the scattering of the light penetrating the light guide 3 should be as low as possible.

[0070] The output coupling elements 6 can, in principle, be distributed in different ways in or on the light guide 3 during the design or manufacture of the light guide 3, depending on adaptable and predeterminable conditions for the output of the light. The output coupling elements 6 with functional surfaces 5 are locally limited structural changes in the volume and / or on the surfaces of the light guide 3. Therefore, the term output coupling element 6 expressly does not include additional optical layers that are applied to the surfaces of the light guide 3, e.g. diffusion layers, reflection layers, (dual) brightness-enhancing, collimating brightness enhancement film (BEF) or polarization-recycling layers, such as polarization-selective Bragg mirrors (dual) brightness enhancement film (D)BEF) or wire grid polarizers.These additional layers, which do not fall under the term "coupling element," are connected to the light guide 3 only at the edges, if at all. However, they are usually only loosely bonded to the large surfaces and do not form a physical unit with the light guide 3. In contrast, lacquers applied to the large surfaces, which bond with the light guide through chemical reactions or other forces (e.g., van der Waals forces), form a physical unit and can no longer be separated from each other; such lacquers therefore do not count as an additional layer in the above-mentioned sense.

[0071] The number of coupling elements 6 per surface, their shape (including the respective functional surface 5) and their extent are selected such that the light guide 3 has an average haze value of less than 20% over at least 50%, preferably over 80% of its surface, particularly preferably over its entire surface. preferably less than 15%, particularly preferably less than 10%, measured according to ASTM D1003 - whereby the measurement according to the more common Procedure A with a hazemeter is used as a reference. As a result, light penetrating the light guide through its large surfaces is scattered only slightly.

[0072] The structure and distribution of the decoupling elements 6 is specified as described above according to the criteria mentioned, wherein the effect of each decoupling element 6 and its functional surface 5 is at least approximately known and properties of the light guide 3 or of the light emerging from the light guide 3 can be specifically determined by a predeterminable structure and distribution of the decoupling elements 6, wherein the ratio of the sum of the surface areas of the functional surfaces 5 to the surface area of ​​the total area of ​​the large area from which light is coupled out, as well as the shapes of the functional surfaces 5 of the decoupling element 6 are particularly important.

[0073] The required properties for the decoupling elements 6, which are essential for the invention, with regard to their number per unit area, their shape including the functional area 5, their orientation and extension in three dimensions as well as their distribution on at least one of the large areas and / or within the volume of the light guide 3 can be determined, for example, using optical simulation software such as “LightTools” from Synopsis or other providers and then physically implemented accordingly.

[0074] With reference to Fig.1, a screen 1b which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted by the illumination device 1a in an angular range restricted compared to the free view mode, - a lighting device 1 a as described above, - in the viewing direction in front of the illumination device 1 a, a transmissive image generator 1 , preferably an LCD panel.

[0075] Of course, there is a corresponding control system for all required components (e.g. image sensor 1, light source 4, background lighting 2, etc.).

[0076] In all of the aforementioned embodiments, the said illuminants can be 4 LEDs or LED arrays or laser diodes. Other variants are conceivable and within the scope of the invention.

[0077] The above-described inventive lighting device and the screen implementable therewith solve the stated problem: They allow for practical, easily implementable solutions for reliably displaying information through an optionally restricted viewing angle, while allowing a free view with unrestricted viewing angle in another operating mode. The invention can be implemented inexpensively using simple means. The native resolution of the screen used can be used in both operating modes. Furthermore, the solution reduces or completely eliminates visually perceptible artifacts such as color effects, e.g., rainbow effects.

[0078] The invention described above can be advantageously applied wherever confidential data is displayed and / or entered, such as when entering a PIN or displaying data at ATMs or payment terminals, or when entering a password or reading emails on mobile devices. As described above, the invention can also be applied in cars. List of reference symbols 1 transmissive imager 1a Lighting device 1b Screen 2 backlight 3 light guides 4 bulbs 5 Functional area 6 Decoupling element

Claims

Patent claims 1 . Lighting device (1 a) for a screen (1 b), which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted by the lighting device in an angular range that is restricted compared to the free viewing mode, comprising a planar extended backlight (2) that emits light in the restricted angular range, a plate-shaped light guide (3) located in front of the backlight (2) in the viewing direction, with two large surfaces and narrow sides that connect the large surfaces at their edges, wherein - the light guide (3) has decoupling elements (6) on at least one of the large surfaces and / or within its volume, - each decoupling element (6) has at least one functional surface (5) for the defined decoupling of light, at which light is coupled out of the light guide (3), illuminating means (4) arranged laterally on at least one narrow side of the light guide (3), wherein in operating mode B2 the background lighting (2) is switched on and the illuminating means (4) is switched off, and wherein in operating mode B1 at least the illuminating means (4) are switched on, characterized in that for at least two percent of the decoupling elements (6) the functional surface (5) for the defined decoupling of light has a curvature in at least two directions, wherein the two forms of the curvature of the respective functional surface (5) differ in the at least two directions,whereby light of different wavelengths originating from the illuminants (4) and coupled into the light guide (3) at least on one of the narrow sides is mixed on the respective light path within the light guide (3) up to the respective location of the light coupling, which minimizes or completely suppresses visually perceptible color effects.

2. Lighting device (1 a) according to claim 1, characterized in that the coupling-out elements (6) have the shape of prismatoids, scutoids and / or convex bodies.

3. Lighting device (1 a) according to claim 1 or 2, characterized in that the respective curvature of the functional surfaces (5) of the decoupling elements (6) in a first direction and in a second direction perpendicular thereto is circular, elliptical, exponential or Bezier curve-shaped.

4. Lighting device (1 a) according to one of claims 1 to 3, characterized in that the coupling-out elements (6) are designed as depressions or elevations on one of the large surfaces.

5. Screen (1 b) which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, comprising an illumination device (1 a) according to one of claims 1 to 4, a transmissive image generator (1 ), preferably an LCD panel, in front of the illumination device (1 a) in the viewing direction.

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