Screen

Abstract

The invention relates to a screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising an image display device which has a selected brightness and whose brightness is controlled by a control device, an optical element (1) which can be switched into at least two states which correspond respectively to the first and the second operating mode B1, B2, wherein furthermore when switching from the first operating mode B1 to the second operating mode B2 at least a time period t12 with t12 ≥ 50 ms elapses until the second operating mode B2 is reached,wherein during the time period t12 from the start of a switching process from the first operating mode B1 to the second operating mode B2, the brightness of the image display device is dimmed from the selected brightness in such a way that from a selectable viewing direction onto the screen at a selectable angle which deviates by at least 25° from the mid-perpendicular to the optical element (1), a selectable maximum luminance is not exceeded.

Description

Field of the invention

[0001] 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 must 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.

[0002] 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. State of the art

[0003] Additional films based on micro-louvres have already been used for mobile displays to achieve visual privacy. 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.

[0004] 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.

[0005] In WO 2012 / 033583 A1, switching between unobstructed and restricted view is achieved by controlling liquid crystals between so-called "chromonic" layers. This results in a loss of light and is quite complex.

[0006] 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.

[0007] US 2013 / 0308185 A1 describes a special light guide with steps that emits light across a large surface 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 LCD, this makes it possible to create a screen that can be switched between free and restricted view modes. A disadvantage of this is 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.

[0008] The applicant's WO 2015 / 121398 A1 describes a screen with two operating modes, in which scattering particles are essential for switching between the operating modes in the volume of the corresponding light guide. However, the polymer scattering particles selected there generally have the disadvantage that light is coupled out of both large surfaces, resulting in approximately half of the useful light being emitted in the wrong direction, namely toward the backlight, where it cannot be adequately recycled due to the 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.

[0009] Furthermore, WO 2021 / 032735 A1 describes switching between a free and a restricted view mode based on the movement of electrophoretic particles. A disadvantage of this is that switching between the modes can take several hundred milliseconds, depending on the design.

[0010] The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the main screen and / or require a complex and expensive optical element for mode switching and / or reduce the resolution in the freely viewable mode. Another problem is that slow-switching privacy screen variants are currently not considered for safety-critical applications such as in cars, because the privacy screen is insufficient, at least during the initial switching process to the safe mode.

[0011] The applicant's EP 3 570 097 A1 discloses a generic screen with operating modes for unobstructed and restricted viewing using a split illumination device. It is also disclosed that the transition from one operating mode to the other can be made gradually. However, such a gradual transition does not ensure that any privacy requirements can be met until the change in operating mode is complete. Description of the invention

[0012] It is therefore an object of the invention to describe a screen which enables switching between a privacy screen and a free viewing mode, wherein in particular a solution for slowly switching privacy screen variants for safety-critical applications - such as in cars - is to be described, so that the privacy screen is sufficiently performant even during the start of the switching process to the safe mode.

[0013] This object is achieved according to the invention by a screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - an image display device having a selected brightness (e.g. for a white image as a calibration object) and whose brightness is controlled by a control device, e.g. by controlling the brightness of the backlight of an LCD panel, by current regulation in an OLED panel, by suitable image data modification or otherwise, - an optical element which, from the viewer's perspective, is located in front of or behind the image display device and which can be switched into at least two states, each corresponding to the first and second operating modes B1, B2, wherein in the first operating mode B1 the optical element does not restrict the light emanating from or passing through the image display device in its propagation directions up to a selectable tolerance, and wherein in the second operating mode B2 the optical element restricts the light emanating from or passing through the image display device in its propagation directions, - furthermore, when switching from the first operating mode B1 to the second operating mode B2, at least a time period t12 with t12 ≥ 50 ms elapses until the second operating mode B2 is reached (it may even be one to several seconds or, under certain circumstances, many seconds) - wherein during the time period t12 from the start of a switching process from the first operating mode B1 to the second operating mode B2, the brightness of the image display device is dimmed from the selected brightness in such a way that from a selectable viewing direction onto the screen at a selectable angle which deviates by at least 25° (alternatively 30°, 35°, 45° or more than 45°) from the perpendicular bisector to the optical element (e.g. measured from the direction of a viewer in the horizontal relative to the perpendicular bisector), a selectable maximum luminance (i.e. brightness, e.g. 5% of the peak brightness in all directions or max. 35 cd / m 2 or max. 5 cd / m 2 or max. 0.1 cd / m 2 ) is not exceeded.

[0014] Advantageously, for the screen, for selectable points (e.g., the center of the surface) on the screen, the luminance from the said selectable viewing direction results, up to a selectable tolerance, as the product of the brightness (luminance) of the image display device at a selected point, effective in the said viewing direction, and the transmission of the optical element at the selected point in the said viewing direction at the respective time within the time period t12, wherein the brightness of the image display device is preferably increased with increasing time during a time period t12. This is possible because the transmission of the optical element at such a selected point in the said viewing direction generally decreases continuously during the time period t12.

[0015] Furthermore, it is advantageous if, after the time period t12 has elapsed, the image display device is reset to the (originally) selected brightness. This can also be varied or set by the user.

[0016] For example, the optical element can comprise electrowetting- or microfluidic-based optical means. Other configurations are possible.

[0017] In an advantageous embodiment, the optical element may in turn comprise i. a substantially plate-shaped substrate S with a first large surface designed as a light entry surface and a second large surface designed as a light exit surface, ii. a liquid or a framework matrix F arranged between the first and the second large surface and containing electrophoretically or magnetophoretically movable particles P which interact with light of one or more wavelengths or wavelength ranges, iii. electromagnetic switching means formed in the substrate S in a planar manner on one or both large surfaces and / or between the large surfaces, which generate an electromagnetic field in at least one state, whereby the particles P are moved in the liquid or the framework matrix F, so that an angle-dependent transmission of the optical element for light of the wavelengths or wavelength ranges entering the substrate S via the light entry surface changes due to the interaction with the particles P, wherein the electromagnetic switching means can be switched in at least two states, which respectively correspond to the first and the second operating mode B1, B2, - wherein when switching from the first operating mode B1 to the second operating mode B2, a time period t12 with t12 ≥ 50 ms elapses, after which the positions of more than 80% (preferably 90%, 95%, more than 95% or 100%) of the particles P present no longer change.

[0018] In a first alternative, the particles P can absorb or scatter light of the specified wavelengths or wavelength ranges, the liquid or framework matrix F can contain up to 60 percent by volume of the particles P, and the electromagnetic field can be effective between the large surfaces. Further potential design options can be found in the applicant's aforementioned WO 2021 / 032735 A1, which is cited here as a reference.

[0019] Interaction with light occurs through absorption, reflection, and / or scattering, and possibly also through transmission. Particles that absorb light are also called absorption particles. Particles that reflect, scatter, or—through interaction—transmit light are also called deflection particles.

[0020] In particular, the particles P can either be electrically charged and the electromagnetic switching means are designed as electrodes for generating a static or dynamic electric field, or the particles P can be magnetic and the electromagnetic switching means are designed as electrically conductive layers for generating a static or dynamic magnetic field, so that the electro- or magnetophoretic particles P move in the electric or magnetic field in the liquid or framework matrix F. "Static" can also mean "field-free" here. Furthermore, "dynamic" can mean that the electric field is designed to be variable over time in order to reach a specific operating mode more quickly and / or to maintain it.

[0021] Furthermore, it is possible that by means of the electromagnetic switching means and a control circuit at least the two operating modes B1 and B2 are defined as a function of the position of the particles P, wherein in the first operating mode B1 the angle-dependent transmission is more than 50% and in the second operating mode B2 it is less than 50% in an angular range of more than 30° to 90° relative to a surface normal of the second large surface of the substrate S.

[0022] An advantageous embodiment provides that all electromagnetic switching means are designed as planar electrodes EPN with polarity reversible between positive and negative on the first and second large surfaces, and that in the first operating mode B1 the electrodes EPN on the first large surface have a positive or neutral (for field-free) polarity and the electrodes EPN on the second large surface have a negative or neutral (for field-free) polarity or vice versa, so that more than 70% of the particles P are each located no further than a maximum of one quarter of the thickness of the liquid or the matrix F from the electrodes EPN and / or are diffusely distributed in the liquid or the framework matrix F,and in the second operating mode B2, viewed along a surface normal of the first or second large surface, negatively polarized electrodes EPN on the first large surface are opposite negatively polarized electrodes EPN on the second large surface, and positively polarized electrodes EPN on the first large surface are opposite positively polarized electrodes EPN on the second large surface, wherein, along a preferred direction, a negatively polarized electrode EPN is arranged between two positively polarized electrodes EPN on each of the large surfaces and a positive electrode EPN is arranged between two negatively polarized electrodes EPN, such that more than 70% of the particles P are each located between similarly polarized electrodes EPN, whereby the angle-dependent transmission in the first operating mode B1 is more than 60% and in the second operating mode B2 is less than 5%, in an angular range of more than 30° around a surface normal of the second large surface of the substrate.

[0023] The particles P can be nanoparticles, quantum dots, and / or dyes, which have a spatial dimension of a maximum of 200 nm, preferably a maximum of 100 nm, preferably a maximum of 50 nm, particularly preferably a maximum of 20 nm. However, other configurations are explicitly possible. "Spatial dimension" refers to the maximum dimension in three-dimensional space or the hydrodynamic radius, whichever is larger. For spherical particles, this is the diameter. For chain-like particles, this is the greatest possible distance that two points on the surface of the particle can have from each other.

[0024] The particles P are preferably either electrically charged, and the electromagnetic switching means are designed as electrodes for generating a static or dynamic electric field, or the particles P are magnetic, in particular paramagnetic or superparamagnetic, in which case the electromagnetic switching means are designed as electrically conductive layers for generating a static or dynamic magnetic field. The application of an electric or magnetic field causes the electrophoretic or magnetophoretic particles to move in the electric or magnetic field within the liquid or framework matrix.

[0025] The dominant physical effects for the movement of particles P when an electromagnetic field, especially a static field, is applied are (di)electrophoresis or magnetophoresis. In the absence of an electric or magnetic field, the particles move primarily due to diffusion and thus distribute themselves homogeneously over time. For particles smaller than 50 nm, gravity also plays no role; they do not settle or change their vertical position in the optical element, meaning the particles remain suspended in the liquid or gel matrix.

[0026] A screen according to the invention can further comprise additional (switchable) means for selectively restricting the angular spectrum of the light emitted or transmitted by the image display device in order to enhance the privacy effect in the second operating mode B2. For this purpose, it is possible, for example, for the image display device to comprise an LCD panel and the additional means to comprise a switchable backlight, which emits light in an unrestricted angular spectrum for the B1 operating mode and in a restricted angular spectrum for the B2 operating mode. Alternatively, the additional means can comprise an LC panel for angle-dependently influencing the polarization status of light penetrating the LC panel and a linear polarization filter, which corresponds to a so-called dual-cell structure.Furthermore, it is conceivable that the additional means comprise an LC panel for rotating the polarization of light passing through this LC panel and another optical element with absorbing dipole moments aligned vertically within a certain tolerance. Detailed designs for the latter variant can be found, for example, in US 11,906,828 B2, WO 2024 / 133018 A1, DE 10 2023 110 269 B3, and DE 10 2023 118 933 B3, each of which is owned by the applicant.

[0027] In the event that the optical element allows a partial switching between the operating modes B1 and B2, the said additional means are preferably geometrically matched to the possible ranges of the partial switching of the optical element and can therefore be switched in congruent ranges.

[0028] Dimming of the selected brightness can also occur only while or when a side viewer, such as the driver in a car, is looking at the screen, which can be verified by suitable eye position tracking. Further details of such a configuration can be found in the applicant's US Pat. No. 11,442,602 B2.

[0029] The display unit can be, for example, an OLED display, an LCD, an SED, an FED, a mini-LED display, a micro-LED display, or a VFD. Since the optical element is effective regardless of the type of display unit, any other screen type is also possible.

[0030] Furthermore, it is possible, for example, to use the optical element according to the invention in an image display unit that has a backlight, such as in an LCD screen. In this case, the optical element would advantageously be arranged between the image display panel (i.e., the LCD panel) and the backlight in order to switch between a first operating state B1 for a free view mode and a second operating state B2 for a restricted view mode, because the light from the backlight is sometimes focused (B2) and sometimes unfocused (B1) due to the optical element. "Focusing" does not mean focusing in the manner of lenses, but rather a narrowing of the emission range according to the respective transmission properties of the optical element according to the invention.

[0031] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0032] 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 on their own, without departing from the scope of the present invention. Short description of the drawings

[0033] 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. They show: Fig. 1 a schematic diagram of an exemplary optical element in operating state B2, Fig. 2 a schematic diagram of an exemplary optical element in operating state B1 in a first variant, Fig. 3 a schematic diagram of an exemplary optical element in operating state B1 in a second variant. Fig. 4a shows an exemplary diagram for a temporal variation of a selected brightness of an image display device in a first embodiment, Fig. 4b shows an exemplary diagram for a temporal variation of a normalized transmission of an optical element for an angle H-45°V0° in a first embodiment, Fig. 4c shows an exemplary diagram for a temporal variation of a resulting brightness (luminance) for an angle H-45°V0° in a first embodiment, Fig. 5a shows an exemplary diagram for a temporal variation of a selected brightness of an image display device in a second embodiment, and Fig. 5b shows an exemplary diagram for a temporal variation of a resulting brightness (luminance) for an angle H-45°V0° in a second embodiment. Detailed description of the drawings

[0034] The drawings are not to scale and merely represent schematic diagrams. Furthermore, for the sake of clarity, only a small selection of electrodes, light beams, particles, or the like is depicted, although in reality, a veritable multitude of these can and does exist. Four different alternatives are described below. These alternatives share commonalities, particularly with regard to the nature of the substrate itself and the nature of the particles, which are not explicitly repeated for each alternative.

[0035] Before the mode of operation according to the invention is explained in more detail, a description of an exemplary optical element follows.

[0036] In Fig. 1 shows a schematic diagram of an exemplary optical element in operating state B2. This includes i. a substantially plate-shaped substrate S with a first large surface designed as a light entry surface (bottom) and a second large surface designed as a light exit surface (top), ii. a liquid or a framework matrix F arranged between the first and the second large surface and containing electrophoretically or magnetophoretically movable particles P which interact with light of one or more wavelengths or wavelength ranges, iii. electromagnetic switching means EPN formed in the form of a surface on one or both large surfaces and / or between the large surfaces in the substrate S, which generate an electromagnetic field in at least one state, whereby the particles P are moved in the liquid or the framework matrix F, so that an angle-dependent transmission of the optical element for light of the wavelengths or wavelength ranges which enters the substrate S via the light entry surface changes due to the interaction with the particles P, wherein the electromagnetic switching means can be switched in at least two states, which respectively correspond to the first and the second operating mode B1, B2.

[0037] Due to the inertia of the particles and the resistance of the liquid or framework matrix F, a time period t12 with t12 ≥ 50 ms elapses when switching from the first operating mode B1 to the second operating mode B2, after which the positions of more than 80% (preferably 90%, 95%, more than 95%, or 100%) of the existing particles P no longer change. The time period t12 can also be a multiple of 50 ms, depending on the design of the optical element.

[0038] In a first alternative, the particles P can absorb or scatter light of the specified wavelengths or wavelength ranges, the liquid or framework matrix F can contain up to 60 percent by volume of the particles P, and the electromagnetic field can be effective between the large surfaces. Further potential design options can be found in the applicant's aforementioned WO 2021 / 032735 A1, which is cited here as a reference.

[0039] Interaction with light occurs through absorption, reflection, and / or scattering, and possibly also through transmission. Particles that absorb light are also called absorption particles. Particles that reflect, scatter, or—through interaction—transmit light are also called deflection particles.

[0040] In particular, the particles P can either be electrically charged and the electromagnetic switching means are designed as electrodes for generating a static or dynamic electric field, or the particles P can be magnetic and the electromagnetic switching means are designed as electrically conductive layers for generating a static or dynamic magnetic field, so that the electro- or magnetophoretic particles P move in the electric or magnetic field in the liquid or framework matrix F. "Static" can also mean "field-free" here. Furthermore, "dynamic" can mean that the electric field is designed to be variable over time in order to reach a specific operating mode more quickly and / or to maintain it.

[0041] Furthermore, it is possible that by means of the electromagnetic switching means EPN and a control circuit at least the two operating modes B1 and B2 are defined as a function of the position of the particles P, wherein in the first operating mode B1 the angle-dependent transmission is more than 50% and in the second operating mode B2 it is less than 50% in an angular range of more than 30° to 90° relative to a surface normal of the second large surface of the substrate S.

[0042] This shows Fig. 2 a schematic diagram of an exemplary optical element 1 in operating state B1 in a first variant and Fig. 3 a schematic diagram of an exemplary optical element 1 in operating state B1 in a second variant.

[0043] An advantageous embodiment provides that all electromagnetic switching means are designed as planar electrodes EPN with polarity reversible between positive and negative on the first and second large area, and that in the first operating mode B1 the electrodes EPN on the first large area have a positive (see Fig. 3 - second variant) or neutral (for field freedom, see Fig. 2 - first variant) and the electrodes EPN on the second large area have a negative (see Fig. 3 - second variant) or neutral (for field freedom, see Fig. 2 - first variant) polarity or vice versa, so that more than 70% of the particles P are not further than a quarter of the thickness of the liquid or the matrix F from the electrodes EPN (see Fig. 3 - second variant) and / or diffusely distributed (see Fig. 2 - first variant) are located in the liquid or the framework matrix F, and in the second operating mode B2 in the first and second variants (see Fig. 1) viewed along a surface normal of the first or second large surface, negatively poled electrodes EPN on the first large surface are opposite negatively poled electrodes EPN on the second large surface and positively poled electrodes EPN on the first large surface are opposite positively poled electrodes EPN on the second large surface, wherein along a preferred direction a negatively poled electrode EPN is arranged between two positively poled electrodes EPN on each of the large surfaces and a positive electrode EPN is arranged between two negatively poled electrodes EPN, so that more than 70% of the particles P are each localized between similarly poled electrodes EPN, whereby the angle-dependent transmission in the first operating mode B1 is more than 60% and in the second operating mode B2 is less than 5%, in an angular range of more than 30° around a surface normal of the second large surface of the substrate.

[0044] The particles P can be nanoparticles, quantum dots, and / or dyes, which have a spatial dimension of a maximum of 200 nm, preferably a maximum of 100 nm, preferably a maximum of 50 nm, particularly preferably a maximum of 20 nm. However, other configurations are explicitly possible. "Spatial dimension" refers to the maximum dimension in three-dimensional space or the hydrodynamic radius, whichever is larger. For spherical particles, this is the diameter. For chain-like particles, this is the greatest possible distance that two points on the surface of the particle can have from each other.

[0045] The particles P are preferably either electrically charged, and the electromagnetic switching means are designed as electrodes for generating a static or dynamic electric field, or the particles P are magnetic, in particular paramagnetic or superparamagnetic, in which case the electromagnetic switching means are designed as electrically conductive layers for generating a static or dynamic magnetic field. The application of an electric or magnetic field causes the electrophoretic or magnetophoretic particles to move in the electric or magnetic field within the liquid or framework matrix.

[0046] In the following, with reference to the drawings Fig. 4a to 5b, the effects of the invention are explained in more detail. A screen is used which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, comprising - an image display device having a selected brightness (e.g. for a white image as a calibration object) and whose brightness is controlled by a control device, e.g. by controlling the brightness of the backlight of an LCD panel, by current regulation in an OLED panel, by suitable image data modification or otherwise, - an optical element 1 (e.g. as for the Fig. 1 to Fig. 3 in a first or second variant), which is located in front of or behind the image display device from the viewer's perspective, and which can be switched into at least two states, each corresponding to the first and second operating modes B1, B2, wherein the optical element in the first operating mode B1 does not restrict the light emanating from or passing through the image display device in its propagation directions up to a selectable tolerance, and wherein the optical element in the second operating mode B2 restricts the light emanating from or passing through the image display device in its propagation directions, - furthermore, when switching from the first operating mode B1 to the second operating mode B2, at least a time period t12 with t12 ≥ 50 ms elapses (it may even be one to several seconds or, under certain circumstances, several seconds) - wherein during the time period t12 from the start of a switching process from the first operating mode B1 to the second operating mode B2, the brightness of the image display device is dimmed from the selected brightness in such a way that from a selectable viewing direction onto the screen at a selectable angle which deviates by at least 25° (alternatively 30°, 35°, 45° or more than 45°) from the perpendicular bisector to the optical element (e.g. measured from the direction of a viewer in the horizontal relative to the perpendicular bisector), a selectable maximum luminance (e.g. 5% of the peak brightness in all directions or max. 35 cd / m 2 or max. 5 cd / m 2 or max. 0.1 cd / m 2 ) is not exceeded.

[0047] This shows Fig. 4a shows an exemplary diagram for a temporal variation of a selected brightness of an image display device in a first embodiment. In the observed period from 0 ms to 30 ms, the selected brightness (luminance) at the observed angle H-45°V0° (relative to the perceived horizontal and vertical of a standing or seated viewer in front of the screen) is 1,000 cd / m 2 . When switching from operating mode B1 to operating mode B2, the selected brightness is set to 50 cd / m 2 The corresponding time period t12 from the beginning of the switching process at 30 ms lasts 60 ms and ends in the diagram at 90 ms. Then, in this example, at least 80% of the particles P are in their final position for operating mode B2. In the final period of 90 ms to 100 ms, the selected brightness (luminance) at the considered angle H-45°V0° is again set to 1,000 cd / m 2 set.

[0048] Furthermore, Fig. Figure 4b shows an exemplary diagram for a temporal variation of a normalized transmission of an optical element 1 for an angle H-45°V0° in a first embodiment over the observed period. In operating mode B1 (in this case from 0 ms to 30 ms), the normalized transmission is, for example, 0.7. From the beginning of the switch to operating mode B2 after 30 ms, the normalized transmission drops to approximately 0.01 at 90 ms and remains at approximately this value until the end of the observed time period at 100 ms.

[0049] By multiplying the respective ordinate values ​​from the Fig. 4a and Fig. 4b shows the resulting brightness (luminance). Fig. Figure 4c shows an example diagram for a temporal variation of a resulting brightness (luminance) for an angle H-45°V0° in a first embodiment. It can be seen that the maximum possible brightness is present in the period from 0 ms to 30 ms, even at this angle. This then drops sharply after 30 ms to a maximum of approximately 35 cd / m 2(this value is therefore not exceeded), and therefore less than or equal to 5% of the peak brightness in the specified direction. This means that immediately from the start of the switching process, i.e. at the beginning of the time period t12, a sufficiently reduced luminance at lateral angles can be achieved, even if the actual switching process to operating mode B2 is still in progress and the optimal light angle limitation has not yet been reached. Nevertheless, for a viewer at an angle of, for example, H0°V0°, some residual light still remains, since the standardized transmission is significantly higher there than at lateral angles such as the example angle H-45°V0° considered here. This visible light is significantly increased again from 90 ms after the switch to operating mode B2 is complete, when the selected brightness is reset to its original value.

[0050] In Fig. 5a is an exemplary diagram for a temporal variation of a selected brightness of an image display device in a second embodiment and in Fig. 5b shows an exemplary diagram for a temporal variation of a resulting brightness (luminance) - in each case - for an angle H-45°V0° in a second embodiment.

[0051] The to Fig. The explanations given in section 4a apply mutatis mutandis to Fig. 5a, which Fig. 4c given explanations for Fig. 5b. The Fig. Figure 4b shows the exemplary normalized transmission for the second embodiment, which is also applicable here, as described above.

[0052] The difference between the first ( Fig. 4a-4c) and the second ( Fig. 5a, Fig. 4b, Fig. 5b) The embodiment consists in that with increasing time during the time period t12 the brightness of the image display device ( Fig. 5a). This is possible because the normalized transmission ( Fig. 4b) of the optical element 1 in the said direction below H-45°V0° during the time period t12 is generally continuously reduced.

[0053] Furthermore, it is advantageous if, after the time period t12 has elapsed, the image display device is reset to the (originally) selected brightness. This can also be varied or set by the user.

[0054] The display unit can be, for example, an OLED display, an LCD, an SED, an FED, a mini-LED display, a micro-LED display, or a VFD. Since optical element 1 is effective regardless of the type of display unit, any other screen types are also possible.

[0055] The invention solves the stated problem: A screen has been described which enables switching between a privacy screen and a free viewing mode, wherein in particular a solution for slowly switching privacy screen variants for safety-critical applications - such as in cars - has been described, so that the privacy screen is sufficiently performant even during the start of the switching process to the safe mode.

[0056] The invention can be advantageously used in conjunction with other systems wherever confidential data is displayed and / or entered, such as when entering a PIN or displaying data at ATMs or payment terminals. As described above, it can also be used in cars.

Claims

[1] Screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - an image display device having a selected brightness and whose brightness is controlled by a control device, - an optical element (1) which, from the viewer's perspective, is located in front of or behind the image display device and which can be switched into at least two states, each corresponding to the first and second operating modes B1, B2, wherein the optical element (1) in the first operating mode B1 does not restrict the light emanating from or passing through the image display device in its propagation directions up to a selectable tolerance, and wherein the optical element (1) in the second operating mode B2 restricts the light emanating from or passing through the image display device in its propagation directions, - wherein, when switching from the first operating mode B1 to the second operating mode B2, at least a time period t12 with t12 ≥ 50 ms elapses until the second operating mode B2 is reached, - characterized bythat during the time period t12 from the start of a switching process from the first operating mode B1 to the second operating mode B2, the brightness of the image display device is dimmed from the selected brightness in such a way that a selectable maximum luminance is not exceeded from a selectable viewing direction onto the screen at a selectable angle which deviates by at least 25° from the mid-perpendicular to the optical element (1). [2] Screen according to claim 1, characterized by that the optical element (1) in turn comprises i. a substantially plate-shaped substrate (S) with a first large surface designed as a light entry surface and a second large surface designed as a light exit surface, ii. a liquid or a framework matrix (F) arranged between the first and the second large surface and containing electrophoretically or magnetophoretically movable particles (P) which interact with light of one or more wavelengths or wavelength ranges, iii. electromagnetic switching means formed in the substrate (S) in a planar manner on one or both large surfaces and / or between the large surfaces, which generate an electromagnetic field in at least one state, whereby the particles (P) are moved in the liquid or the framework matrix (F) so that an angle-dependent transmission of the optical element for light of the wavelengths or wavelength ranges entering the substrate (S) via the light entry surface changes due to the interaction with the particles (P), wherein the electromagnetic switching means can be switched in at least two states, which respectively correspond to the first and the second operating mode B1, B2, - whereby when switching from the first operating mode B1 to the second operating mode B2, a time period t12 with t12 ≥ 50 ms elapses, after which the positions of more than 80% of the particles present no longer change. [3] Screen according to claim 1, characterized by that the optical element (1) comprises optical means based on electrowetting or microfluidics. [4] Screen according to one of the preceding claims, characterized by that for selectable points on the screen, the luminance from said selectable viewing direction results, up to a selectable tolerance, as the product of the brightness of the image display device at a selected point, effective in said viewing direction, and the transmission of the optical element (1) at a selected point in said viewing direction at the respective time within the time period t12, wherein the brightness of the image display device is preferably increased with increasing time during a time period t12. [5] Screen according to one of the preceding claims, characterized bythat after the time period t12 has elapsed, the image display device is reset to the selected brightness. [6] Screen according to claim 2, characterized by that in a first alternative the particles (P) absorb or scatter light of the said wavelengths or wavelength ranges, the liquid or framework matrix (F) contains up to 60 volume percent of the particles (P) and the electromagnetic field is effective between the large surfaces. [7] Screen according to claim 2, characterized bythat the particles (P) are either electrically charged and the electromagnetic switching means are designed as electrodes for generating a static or dynamic electric field or the particles (P) are magnetic and the electromagnetic switching means are designed as electrically conductive layers for generating a static or dynamic magnetic field, so that the electro- or magnetophoretic particles (P) move in the electric or magnetic field in the liquid or framework matrix (F). [8] Screen according to claim 2, characterized bythat by means of the electromagnetic switching means and a control circuit at least the two operating modes B1 and B2 are defined as a function of the position of the particles (P), wherein in the first operating mode B1 the angle-dependent transmission is more than 50% and in the second operating mode B2 it is less than 50% in an angular range of more than 30° to 90° relative to a surface normal of the second large surface of the substrate. [9] Screen according to claim 2, characterized bythat all electromagnetic switching means are designed as planar electrodes EPN with polarity that can be changed between positive and negative on the first and second large surfaces, and that in the first operating mode B1 the electrodes EPN on the first large surface have a positive or neutral polarity and the electrodes EPN on the second large surface have a negative or neutral polarity or vice versa, so that more than 70% of the particles (P) are each located no further than a maximum of one quarter of the thickness of the liquid or the matrix (F) from the electrodes EPN and / or are diffusely distributed in the liquid or the framework matrix (F),and in the second operating mode B2, viewed along a surface normal of the first or second large surface, negatively polarized electrodes EPN on the first large surface are opposite negatively polarized electrodes EPN on the second large surface, and positively polarized electrodes EPN on the first large surface are opposite positively polarized electrodes EPN on the second large surface, wherein, along a preferred direction, a negatively polarized electrode EPN is arranged between two positively polarized electrodes EPN on each of the large surfaces and a positive electrode EPN is arranged between two negatively polarized electrodes EPN, such that more than 70% of the particles (P) are each located between similarly polarized electrodes EPN, whereby the angle-dependent transmission in the first operating mode B1 is more than 60% and in the second operating mode B2 is less than 5%, in an angular range of more than 30° around a surface normal of the second large surface of the substrate. [10] A screen according to any one of the preceding claims, further comprising additional means for selectively restricting the angular spectrum of the light emitted or transmitted by the image display device. [11] Screen according to claim 10, characterized by that the image display device comprises an LCD panel and the additional means comprise a backlight which emits light in an unrestricted angular spectrum for the operating mode B1 and in a restricted angular spectrum for the operating mode B2. [12] Screen according to claim 10, characterized by that the additional means comprise an LC panel for angle-dependent influencing of the polarization status of light penetrating the LC panel and a linear polarization filter. [13] Screen according to claim 10, characterized bythat the additional means comprise an LC panel for rotating the polarization of light passing through this LC panel and a further optical element with absorbing dipole moments aligned vertically within a tolerance.

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