Display and display device including upconversion material and photochromic and / or photoelectrochromic material

Abstract

The invention relates to a display device. The display device comprises a display medium and two or more light assemblies. The display medium includes one or more upconversion materials and one or more photochromic materials and / or photoelectrochromic materials. The light assembly is configured to emit photons that excite the upconversion material to emit higher photons suitable for activating the photochromic and / or photoelectrochromic material, thereby enabling the display medium to display an image.

Description

Technical Field

[0001] The present invention relates to displays and display devices. More particularly, the present invention relates to passive displays, light-driven displays, full-color displays, and / or displays based on electrochromic materials and / or photoelectrochromic materials.

[0002] Background Art and Problems to be Solved by the Present Invention

[0003] Currently commercialized displays are mainly light-emitting displays. Today, the market is dominated by LCD (liquid crystal display) and OLED (organic light-emitting diode) displays.

[0004] Currently, in a wide range of different devices including mobile phones, computer screens, TV screens, display panels, and even larger screens, commercialized displays are mainly light-emitting types. Since these displays emit light, they are prone to causing eye fatigue in humans, and have other drawbacks such as high energy consumption, narrow viewing angles, and inability to adapt to changing ambient light (e.g., under outdoor conditions). Another drawback of light-emitting displays is that it is generally difficult to achieve 3D display.

[0005] An object of the present invention is to provide a passive display in which an image is not generated by light emission (such as light-emitting pixels), but by reflection of ambient light on or in the screen. Compared with using a brightness-based screen, viewing a passive display is more similar to viewing a printed medium. The passive screen does not have the above-mentioned drawbacks, and thus can be considered particularly advantageous for outdoor screens (higher contrast under direct sunlight) and screen-based reading (lower eye fatigue). A passive display can also be referred to as a reflective display. Electronic ink displays are known to be one of them.

[0006] An object of the present invention is to provide a display that can be transparent, such that in the case of a 2D display, the displayed content can be seen from two opposite viewing sides of the display, and in the case of a 3D display, the displayed content can be seen from a continuous viewing angle that can span, for example, more than 180° and up to 360° or nearly up to 360°.

[0007] The use of photochromic materials or photoelectrochromic materials (hereinafter referred to as "chromic materials") has also been proposed for displays, including passive displays. Such materials have the property of changing color when exposed to appropriate light, and can thus be used in light-driven displays. A light source is typically provided inside or outside the display, and the light source is used to irradiate the display including the chromic materials in order to generate a desired color at the irradiated positions in the display.

[0008] US 7,410,750 B2 discloses a multicolor screen that includes a substrate having a coating of marker particles, the marker particles including a mixture of two photochromic materials, namely a spiropyran material and a unimolecular dithienylethene. Although the document mentions a screen, it is concerned with images and does not mention the possibility of full color. In addition, the document does not make any statements regarding the driving of the display.

[0009] JP 2004 258474A discloses a photochromic screen that uses light emitted from a light-emitting layer based on electrochemiluminescence, the light-emitting layer being provided between substrates including electrodes, and driving such light by an active matrix provided so as to cause an appropriate color change in the photochromic layer. Due to the complexity of the structure including the active matrix and the electrochemiluminescent layer, the manufacturing process of the device is complex.

[0010] US 6,327,074 discloses a three-dimensional color image display. The display includes a transparent display medium in which particles can emit light of an appropriate color when excited by a laser light source. In this device, the light source is opposite the viewing side of the display, and a filter is used on the display to protect the viewer from the light generated by the laser. Since the particles emit light, this is not a passive display. In addition, it would be advantageous to have more options for providing the driving light source than just opposite the viewing side, because being opposite the viewing side takes up space and is therefore not suitable for most display applications such as, for example, TVs and mobile phones.

[0011] US 5,764,403 discloses the use of two infrared light sources to irradiate a fluorescent visible light-emitting material provided in a transparent host material of a display. The fluorescent material is an active ion or molecule such as erbium ion (Er 3+ ), praseodymium ion (Pr 3+ ), and thulium ion (Tm 3+ ), and the erbium ion (Er 3+ ), praseodymium ion (Pr 3+ ), and thulium ion (Tm 3+ ) are used to obtain green, red, and blue, respectively. However, this display is not a passive / reflective display. In addition, in order to obtain different colors, different layers are usually required, each layer containing different active ions.

[0012] US11335741B2 discloses an active light-emitting display having different display areas, where one display area can be made transparent so as to enable the use of a camera provided below the display area.

[0013] US 5,684,621 and L. Kador, "A Three-Color, Three-Dimensional, Solid-State Display", Adv Mater 1997, 9(1), pp. 83-85, disclose a 3D display in which images appear within the volume of a heavy metal fluoride glass doped with rare earth ions to allow for luminescent display via light-driven two-step upconversion.

[0014] JP2007139928 discloses a similar display in which an organic-inorganic composite material is used as the upconversion luminescent material.

[0015] US2020 / 0251538 A1, US 6,402,037, US2010 / 0085471 A1, WO 2012 / 098511, and US6,402,037 B1 disclose other techniques but do not address the potential objectives of the present invention.

[0016] An object of the present invention is also to provide a new type of display. Summary of the Invention

[0017] In one aspect, the present invention provides a display medium comprising: one or more upconversion materials adapted to convert absorbed light into emitted light of a lower wavelength; and one or more photochromic materials and / or electrochromic materials capable of coloring and / or changing color upon absorption of light.

[0018] In one aspect, the present invention provides a display device comprising two or more optical components and the display medium of the present invention.

[0019] In a preferred embodiment, the display of the present invention is a passive display and / or a reflective display. The display device is preferably a passive display device and / or a display device comprising a passive display.

[0020] In an embodiment, in the display medium, one or more photochromic materials and / or electrochromic materials are coated on the upconversion materials, and / or wherein the upconversion materials are surrounded by one or more photochromic materials and / or electrochromic materials.

[0021] In an embodiment, the display is a photochromic display and / or an electrochromic display.

[0022] In an embodiment, the display is a flat panel display (2D display) or a three-dimensional (3D) spatial display.

[0023] It should be noted that the present invention preferably includes combining the properties of the upconversion material with the characteristics of the color-changing material and utilizing such properties to provide a display that can be driven by light (preferably low-energy light, and in particular infrared (IR) light). Without wishing to be bound by theory, the color of the color-changing material can be generated by the light emitted by the upconversion material. Accordingly, the upconversion material preferably emits light when excited by two or more low-energy light sources (such as NIR light). Then, two photons or light beams from different light sources with relatively low-energy light can be used to sufficiently excite the upconversion material so that it emits photons or light beams with relatively high-energy light, which have sufficient energy to induce a color change in the color-changing material.

[0024] Other aspects and preferred embodiments of the present invention are defined below and in the appended claims. From the description of the preferred embodiments given below, additional features and advantages of the present invention will be apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematically shows a display device according to an embodiment of the present invention.

[0026] Figure 2 Schematically shows a display device including a three-dimensional display according to a second embodiment of the present invention.

[0027] Figure 3A and Figure 3B Schematically show embodiments in which the upconversion material and the color-changing material are associated in an island-type configuration and a core-shell type configuration, respectively.

[0028] Figure 4A and Figure 4B Schematically shows the upconversion material and the color-changing material associated in a core-shell type in a full-color display medium according to an embodiment.

[0029] Figure 5A and Figure 5B Schematically show additional examples in which the upconversion material and the color-changing material are associated in an island-type configuration and a core-shell type configuration, respectively, especially for a full-color display according to an embodiment.

[0030] Hereinafter, for the purpose of illustrating the present invention, preferred embodiments of the devices of the present invention are described, but are not intended to limit the scope of the present invention.

[0031] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The present invention relates to a display including one or more upconversion materials and one or more photochromic materials and / or electrochromic materials.

[0033] "Photochromic materials" are materials that can change color upon photon excitation. For the purposes of this specification, "color change" includes changing from a transparent state to an opaque state, and conversely, "color change" includes changing from an opaque state to a transparent state. In addition, black and white are also regarded as colors, such that changing from transparent to black or any other color is also regarded as a "change in color".

[0034] "Photoelectrochromic materials" generally refer to a class of materials with dual functions: they can first absorb light and generate photoelectrons. The generated electrons can then drive the color change of the color-developing part of the photosensitive material. In principle, the electrons inducing the color change can come from an external source. In some cases, two or more compounds with different structures or different materials together constitute a photoelectrochromic material. From different viewpoints, "photoelectrochromic materials" can be regarded as a special case of photochromic materials or as a class of independent materials.

[0035] In this specification, the term "color change" is used for one or both selected from "photochromic" and "photoelectrochromic".

[0036] The color-changing materials as defined herein generally require high-energy photons to change color, particularly photons with a wavelength of 600 nm or lower. Photochromic materials generally have selectivity for the exciting photons. For example, if a specific color-changing material is excited by 500-nm photons, its sensitivity to 400-nm and 600-nm photons is generally lower.

[0037] For the purposes of this specification, when a color-changing material is exposed to radiation that causes a change in the color of the color-changing material compared to its color in the ground state, the color-changing material is "activated", where the color-changing material has a color or is transparent and the color-changing material has not been exposed to the activating radiation. The energy level of the material in the activated state is generally higher than that of the material in the non-activated state.

[0038] "Upconversion materials" are materials that exhibit the phenomenon of emitting high-energy photons when stimulated by relatively low-energy photons. Upconversion materials are generally selected from non-linear optical materials.

[0039] Upconversion materials emit light when exposed to excitation light. For the purposes of this specification, the term "excitation" is used for such a light scenario where such light is suitable for stimulating the upconversion material to emit higher-energy light.

[0040] Using upconversion materials enables the excitation or activation of a sensitizer (such as a color-changing material) that formally requires higher-energy photons by exposing the upconversion material to two or more lower-energy photons.

[0041] "Lower" and "higher", especially when referring to the energy of light or photons and / or the wavelength of light, mainly refer to the relationship between the photons absorbed and emitted by the upconversion material and the photons used to activate the color-changing material. Therefore, it also refers to the relationship between the energy of the drive beams 26, 27 (see below) considered separately and the energy of the light emitted by the upconversion material. In particular, the energy of each of two or more photons (drive beams) suitable for co-exciting the upconversion material is "lower" than the energy of the photons emitted by the excited upconversion material and suitable for activating the color-changing material.

[0042] It should be noted that the term "light of lower wavelength" is equivalent to "light of higher energy", and these expressions can be used interchangeably in this specification.

[0043] Generally, the upconversion material and the color-changing material are selected such that the energy of the light emitted by the upconversion material is sufficient to activate the color-changing material. According to a preferred embodiment, one or more color-changing materials are arranged to change color when absorbing the light emitted by one or more upconversion materials.

[0044] Figure 1 An embodiment of the display device 10 according to the present invention is shown. This embodiment relates to a flat panel display, also known as a two-dimensional (2D) display, which generally means that the displayed content extends on a surface or is displayed on a surface. For the purposes of the present invention, such a display is generally flat, but curved and / or curved screens are also considered 2D displays. Even in the case of a curved display, the viewing surface can be said to extend three-dimensionally, but the displayed content still remains on the surface of the viewing surface, and such a display cannot directly reproduce 3D objects. This does not exclude the possibility of 3D viewing technologies, such as in a stereoscopic display or other displays that use stereoscopic vision to convey depth to the viewer. For the purposes of the present invention, a stereoscopic display is considered a 2D display.

[0045] A 2D display generally includes one (curved or flat) or two opposite (usually flat) viewing surfaces. Further description will be made below with reference to Figure 2 Embodiments of three-dimensional (3D) displays are further described, where the content is displayed in a space that extends in three-dimensional space.

[0046] For the purposes of this specification, "content" in the context of "displayed content" etc. refers to the content on or displayed by the display, and encompasses any displayable content, including written information (such as text, numbers, symbols, information) and graphical representations (such as images, etc.). The content is displayed by reflected visible light.

[0047] The display device 10 includes a display medium 30, which is typically the display itself and in particular includes a part of the viewing area or space (depending on the specific situation). In addition, the display device includes two or more light components 21 and 22. Preferably, there are at least two light components, but there can also be more light components, such as 3 light components, 4 light components, 5 light components or more light components. Each light component can include one or more individual and / or independently addressable light sources. The light emitted by the light source is also referred to as drive light in this text. Each light component includes at least one light source such that two light sources can generate at least two light beams simultaneously.

[0048] In addition to the display medium 30, the display can also include layers and structures, where such layers or structures are not integrated into the display medium. For example, the display can include a reflective layer, such as a background layer that reflects white light and / or any color, and such a reflective layer can be integrated into the display medium or can be provided as a separate layer, close to or attached to one side of the display medium, such as one of the two opposite sides (usually the back side) in a 2D display.

[0049] The display device also includes a driver unit 5 that controls the light components 21 and 22. The driver unit 5 preferably includes a data processing entity, such as a microcontroller or a computer, including a CPU and a memory, and preferably includes software and / or firmware required to drive light sources and / or optical elements of the light components. The driver unit is connected to the light components through communication channels 6 and 7, and the communication channels 6 and 7 can be selected from wired connections or wireless connections.

[0050] The display medium 30 includes one or more upconversion materials and one or more color-changing materials.

[0051] In an embodiment, the display medium 30 includes a host material that is transparent to drive light (preferably NIR light), and preferably the display medium 30 includes a host material that is transparent to visible light, where the upconversion material and the photochromic material and / or the electrochromic material are disposed in the host material. The host material preferably forms a matrix that is suitable for accommodating and preferably fixing the upconversion material and the color-changing material.

[0052] The host material is preferably a transparent matrix material, particularly transparent to the driving light and also preferably transparent to visible light. The host material can be selected from any one or more materials having the required transparency requirements and suitable for accommodating the upconversion material and the color-changing material. In an embodiment, the host material is selected from plastics, resins, and glass. Exemplary host materials can be selected from polymethyl methacrylate (PMMA), epoxy resins, aerogels, and glass. Other exemplary host materials, particularly glass, can be selected from calcium fluoride, fused quartz, halide glasses, sodium chloride, potassium chloride, potassium bromide, and chalcogenide glasses. Other materials include those mentioned in column 4, lines 27 to 44 of US 5,764,403, which are incorporated herein by reference.

[0053] The color-changing material and / or the upconversion material are preferably distributed in the host material, for example, uniformly distributed over the viewing area (or space) of the display medium.

[0054] In a preferred embodiment, one or more upconversion materials are selected from materials including, consisting essentially of, or consisting of one or more selected from the following: Si, SiO2, TiO2, Al2O3, ZrO2, HfO2, SnO2, Fe2O3, ZnO, WO3, Nb2O5, In2O3, Bi2O3, Y2O3, Pr2O3, CeO2, and other rare earth metal oxides, CdS, ZnS, PbS, Bi2S3, CdSe, CdTe, MgTiO3, SrTiO3, BaTiO3, Al2TiO5, Bi4Ti3O 12 and other titanates, CaSnO3, SrSnO3, BaSnO3, Bi2Sn3O9, Zn2SnO4, ZnSnO3, and other stannates, CaZrO3, SrZrO3, BaZrO3, Bi4Zr3O 12 and other zirconates, combinations of two or more of the foregoing, and other multi-element oxides containing at least two of the following: alkali metals, alkaline earth metal elements, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Sc, Y, La, or any other lanthanide element, Ti, Zr, Hf, Nb, Ta, Mo, W, Ni, or Cu.

[0055] In a more preferred embodiment, one or more upconversion materials are selected from transition metal ions, including lanthanide ions. In a preferred embodiment, one or more upconversion materials include materials independently selected from one or more of the following: Pr 3+ (4f2), N d3+ (4f3), Sm 3+ (4f5), Eu 3+ (4f6), Gd 3+(4f7), Tb 3+ (4f8), Dy 3 + (4f9), Ho 3+ (4f10), Er 3+ (4f11), Tm 3+ (4f12), Tm 2+ (4f13), actinide ions (e.g., U 4+ (5f2), U 3+ (5f5)) and transition metal ions (such as Ti 2+ (3d2), Cr 3+ (3d3), Mn 2+ (3d5), Ni 2+ (3d8), Cu 2+ (3d9), Mo 3+ (4d3), Re 4+ (5d3), Os 4+ (5d4)).

[0056] The above ions may exist in the form of salts and / or their corresponding oxides.

[0057] One or more upconversion materials are preferably provided in the form of particles, which are preferably nanoparticles and / or microparticles, most preferably nanoparticles, and / or more generally in the form of a layer with an increased surface area, such as a mesoporous and / or nanoporous 3D scaffold or sintered particles. Nanoparticles are defined herein as particles with an average size less than 1 μm, while microparticles may have an average size of ≥1 μm and up to 1 mm. The particles may have any form and may be selected from particle tubes, particle columns, particle sheets, particle spheres, etc.

[0058] In a preferred embodiment, one or more upconversion materials are selected from materials that can be excited by photons of two or more low-energy lights, such as low-energy visible light, IR light, and preferably NIR light. There is currently no clear wavelength to separate low-energy visible light from NIR light. Depending on the definition of low-energy visible light, the present invention encompasses upconversion materials that can be excited by two photons, where one or both photons may be part of low-energy visible light. Low-energy visible light may be considered to be visible light with a wavelength of 650 nm to 750 nm. According to the definition of NIR light in other parts of this specification, low-energy visible light may include light with a wavelength up to 790 nm or even up to 800 nm.

[0059] The color-changing material may independently be selected from organic, inorganic, and hybrid materials having the desired color-changing properties.

[0060] In an embodiment, one or more color-changing materials are selected from organic materials (such as organic compounds or polymers), metal oxides (especially transition metal oxides), and organometallic compounds. Preferably, the color-changing material is an organic compound. The organic material is preferably selected from small molecules, polymers, and salts of such organic materials. The monomer moiety of the small molecule or polymer may have 5 to 150 carbon atoms and 0 to 60 heteroatoms, where the heteroatoms may be selected from, for example, B, Si, As, O, S, Se, Te, N, P, and halogens. Preferably, the small molecule or monomer moiety has 6 to 100 carbon atoms and 1 to 50 heteroatoms, and most preferably the small molecule or monomer moiety has 7 to 50 carbon atoms and 1 to 30 heteroatoms.

[0061] In an embodiment, the color-changing material includes an anchoring group and is adsorbed on the support material via the anchoring group. Suitable anchoring groups may independently be selected from the following groups (and may also be independently selected from any other anchoring groups): -COOH, -PO3H2, -PO4H2, -P(R 1 )O2H (hypophosphorous acid); -SO3H2, -SO4H2, -CONHOH-, 1,2-hydroxybenzene, 1-hydroxy-2-carboxybenzene, acetylacetonate, the deprotonated forms of the above substances, deprotonated forms of organic salts and / or inorganic salts, and chelating groups having π-conductive properties. R 1 may be an organic substituent including 1 to 50 carbon atoms and 0 to 25 heteroatoms, and the hydrocarbon group is covalently bonded to the P atom of the hypophosphorous acid group through a carbon atom.

[0062] In a preferred embodiment, the color-changing material is selected from: (i) 1-ethyl-1'-(2-phosphonoethyl)-[4,4'-bipyridine]-1,1'-diium, (ii) 1-phenyl-1'-(4-(2-phosphonoethyl)phenyl)-[4,4'-bipyridine]-1,1'-diium, and (iii) 1-ethyl-4-(4-(1-(2-phosphonoethyl)pyridin-1-ium-4-yl)phenyl)pyridin-1-ium. The cationic compound may exist in the form of a salt, such as a halide salt, especially a bromide salt.

[0063] Irrespective of its chemical nature and composition, one or more color-changing materials may also be provided in the form of particles, for example as a form of distributed molecules and / or polymers.

[0064] In a preferred embodiment, the color-changing material has at least a first state (transparent or white) and a second state (colored or dark). Preferably, the second state is the activated state.

[0065] The upconversion material and the color-changing material are preferably associated in such a way that the color-changing material can be effectively activated by the radiation emitted by the upconversion material. Suitable examples of associations, such as the sea-island configuration and the core-shell configuration, will be discussed below with respect to Figures 3A to 5B be discussed further.

[0066] According to the present invention, one or more color-changing materials determine and / or correspond to the coloring of pixels, and / or the generation of the content on the viewing surface or in the space of the display, and are preferably generated on substantially all parts (e.g., at least 80%) of the viewing surface or in the space of the display. Preferably, the content to be displayed is directly generated by the color-changing material, such that what the viewer sees is the color-changing material in the medium forming the content to be displayed. The color-changing material is not used to provide background reflection or background transparency.

[0067] Preferably, the color-changing material is substantially contained everywhere (e.g., ≥80%) in the display medium 30 and / or is substantially distributed throughout the display medium. Preferably, the color-changing material is necessary for the display content of the display according to the present invention.

[0068] In a preferred embodiment, the light reflected by one or more of (1) a photochromic material and / or an electrochromic material, (2) a display, and (3) a display medium satisfies one condition, multiple conditions, or all of the following conditions:

[0069] - The reflected light provides the content displayed by the display medium, such as an image, text, symbol, and / or number,

[0070] - The reflected light is visible to the viewer viewing the display medium, and / or

[0071] - The reflected light does not provide a reflective background for the display of the content displayed by the display medium.

[0072] The light source will be described in more detail below.

[0073] According to an embodiment, the display device includes two or more light components 21, 22. The light components preferably include light sources, preferably low-energy visible light and / or invisible light, and most preferably near-infrared light (NIR). For the purposes of this specification, NIR light is defined as light having a wavelength in the range of 750 nm to 2500 nm, preferably NIR light is defined as light having a wavelength in the range of 790 nm to 2500 nm, and most preferably NIR light is defined as light having a wavelength in the range of 800 nm to 2000 nm. As described above, depending on the viewing angle, some of these light components may be considered to overlap with low-energy visible light. Note that two or more light components may be configured to emit light having the same or different energies, for example, a light beam including two NIR light beams having different energies, two low-energy visible light beams, or one NIR light beam and one low-energy visible light beam.

[0074] The light components are preferably disposed outside the display medium 30, for example, disposed adjacent to the display medium laterally.

[0075] Each light component includes one or more light sources, and optionally includes optical elements such as lenses, mirrors, filters, etc., in order to generate a light beam having desired parameters. If the light component includes multiple light sources, the light component is preferably configured to address each light source independently.

[0076] In Figure 1 In the illustrated embodiment, the two light components 21 and 22 are preferably adapted to generate a light beam and direct the light beam into the display medium 30.

[0077] In a preferred embodiment, two or more light components 21, 22 are configured to emit light beams into the display medium independently of each other, such as NIR light beams.

[0078] In a preferred embodiment, the display device includes a first light component 21 and a second light component 22, wherein the first light component and the second light component are disposed at a first position and a second position in the device, and the first light component and the second light component are configured to emit a first light beam 26 and a second light beam 27 into the display medium 30 from the first position and the second position, such that the first light beam and the second light beam intersect at an intersection point 40 in the display medium.

[0079] Preferably, the optical components can be adjusted with respect to one or more of the following parameters: (1) the direction of the light beam, (2) the position of the light beam, (3) the intensity of the light, (4) the wavelength, (5) the exposure time or the duration of irradiation. With respect to the wavelength of the light, the light is preferably in the NIR range. Preferably, each of the optical components 21, 22 is adapted to generate NIR light of two or more different and / or separate wavelengths, and more preferably is adapted to generate NIR light of three or more different and / or separate wavelengths.

[0080] For the purposes of this specification, the expression "position of the light beam" is intended to mean the position where the light beam enters the display medium.

[0081] Preferably, two or more optical components can independently adjust the above parameters of the emitted light. Preferably, the driver unit 5 controls and / or adjusts the above parameters according to the information to be displayed by the display medium 30.

[0082] As Figure 1 shown, the driver unit causes the optical components 21, 22 to emit drive beams 26, 27 respectively, which intersect at the intersection point 40.

[0083] In a preferred embodiment, at the intersection point 40 of the first light beam 26 and the second light beam 27, one or more upconversion materials are adapted to convert the light into light of a lower wavelength and / or higher energy.

[0084] The two drive beams from the two optical components together provide sufficient energy to excite the upconversion material located at the intersection point. The reference numeral 45 indicates that the upconversion material located at the intersection point 40 has been excited to emit higher energy photons, which are suitable for activating the color-changing material in the immediate vicinity of the upconversion material.

[0085] The intersection point 40 covers a region or space of a defined size and / or a finite size, in which there are upconversion materials and associated color-changing materials, and these materials are activated by the light beam. For the purposes of this specification, such a defined region or space is herein referred to as a pixel (or voxel) of the display medium. In some embodiments, the pixel is defined by the size of the light beam at the intersection point and the materials provided at the intersection point. If the color-changing material and the upconversion material are uniformly distributed in the medium, the position of the pixel in the display medium is not necessarily predefined, but corresponds to the position of the intersection point 40 of the drive beams.

[0086] As can be seen from the above, in the context of the present invention, the term "pixel" does not necessarily have the same definition as in a light-emitting display in all cases. In a light-emitting display, a pixel is typically defined by the presence of a light-emitting element or a group of light-emitting elements, where each light-emitting element in the group is considered a sub-pixel. In particular, the position of a pixel according to the present invention is not necessarily defined by the presence of one or more electronic components. However, it should be noted that in some embodiments, as discussed below with respect to Figure 4B locally predefined pixels and / or sub-pixels are also covered by the present invention.

[0087] Therefore, Figure 1 it is shown how a light assembly can be used to determine the coloring at a pixel, i.e., by guiding the beams of two independent drive lights to the pixel at the intersection point 40 and thereby causing a change in the color of the color-changing material at that location.

[0088] Preferably, the light assembly is jointly adapted to address any pixel of the display medium 30. Preferably, an information display is generated over the entire display viewing area by addressing all pixels with write light as required for generating an image. The pixels can be addressed sequentially and / or simultaneously by the intersecting light beams generated by the light assembly. It should be noted that in addition to the position of the intersecting light beams, the intensity, duration, and / or wavelength of the intersecting light beams can also be adjusted in order to generate a desired result at a given pixel. These parameters can be used to determine the pixel characteristics of a given pixel, such as color, intensity, contrast, sharpness, etc.

[0089] There are different possibilities for doing this, and all of these possibilities are covered by the present invention. In an embodiment, each light assembly includes a plurality of light sources, such as an array of individually and independently addressable unit light sources, such that the position and orientation of the light beams will be simply determined by turning on and / or off specific unit light sources in the array.

[0090] In another embodiment, one or more light sources, or optical elements working in cooperation with the light sources, can be movably arranged within the light assembly, for example, moving along a guide rail. In this case, a drive unit moves the light source and / or its optical element in order to control the position and / or direction of the light beam. In this case, each light assembly can include only a single light-emitting unit, but due to the movable arrangement of at least part of the light source itself or the optical element for guiding the light beam generated by the light source, the light beam can be guided to any pixel of the display medium.

[0091] In yet another embodiment, the optical component includes a light source and / or an optical element, and the light source and / or the optical element are arranged to be adjustable to change the orientation and / or position of the light beam so as to direct the light beam to any specific part in the display medium. For example, the light source and / or the optical element can be arranged to be rotatable to enable control of the direction of the light beam. In other embodiments, different forms of motion can be combined to achieve the desired aiming of light at a specific part in the display medium.

[0092] In some embodiments, the optical component includes a motor as needed to control the direction and / or position of the light beam generated by one or more light sources of the optical component.

[0093] In an embodiment, each optical component is configured to address more than one pixel.

[0094] In an embodiment, the light source of the first optical component is suitable for addressing more than one pixel of the display medium. "Addressing" herein means irradiating with appropriate light such that the upconversion material at that position can be excited together with the light emitted by another light source, where the other light source is preferably arranged in the second optical component. Similarly, the light source of the second optical component is suitable for addressing more than one pixel of the display medium to achieve excitation together with the light emitted by another light source. In other words, the light source is preferably suitable for addressing multiple pixels.

[0095] In an embodiment, each of the two optical components 21, 22 is arranged outside the display medium 30. Preferably, the optical component and / or the light source are not integrated in the part including the viewing area or space. On the other hand, considering the display 10 as a whole (including the optical component), the optical component can be integrated in the display device as a whole, while preferably still being located outside the display medium containing the pixels, for example, fixed at the boundary of the display medium. The present invention also encompasses that one or more optical components can be separated from the display medium such that the excitation at the pixels can occur at least partially through the blank space and / or the light traveling a certain distance that separates the corresponding optical component from the display medium.

[0096] In an embodiment, each of the two optical components 21, 22 is arranged laterally with respect to the display medium 30, preferably near and / or in contact with the side or top / bottom boundary of the display medium.

[0097] In Figure 1 In the embodiment shown, the optical components 21, 22 extend horizontally and vertically respectively, substantially parallel to the corresponding boundaries of the display medium. Each of the two or more optical components can be arranged on one of the boundaries, preferably on adjacent boundaries. In an embodiment, the two optical components are preferably arranged perpendicular to each other.

[0098] In the illustrated embodiment, the first optical component 21 emits light 26 extending in the vertical direction, while the second optical component 22 emits light 27 extending in the horizontal direction within the display medium.

[0099] In this and other embodiments, the direction of the light beam generated by the optical component remains unchanged, such as vertical or horizontal, but the horizontal and vertical positions of the light emission are adjustable. By determining the vertical position of the horizontal beam 27 and the horizontal position of the vertical beam 26 relative to the display medium, any given pixel can be addressed at any position within the display medium.

[0100] Preferably, the display medium 30 has a rectangular and / or square profile common to currently available displays. The light sources 21, 22 can be arranged to extend along two of the four boundaries of the rectangular display medium, for example, as shown, the right boundary and the top boundary.

[0101] In another embodiment (not specifically shown), the optical components can be arranged adjacent to one of the boundaries of the display medium. For example, the two optical components can be arranged at the top or bottom, or on the left or right side.

[0102] In the illustrated embodiment, the optical component is in contact with the display medium. In other embodiments, the light source is not connected to the display medium but is spaced a certain distance from the display medium.

[0103] In a preferred embodiment, the display device includes a three-dimensional (3D) display medium, such as a cubic display medium.

[0104] Figure 2 An embodiment of a display device 100 including a 3D or spatial display is shown. The device includes a display medium 90, which in this case defines a 3D display space rather than just a display surface. One characteristic and / or advantage of a 3D display is that objects can be displayed in a 3D manner naturally (or at least partially), depending on the extent of the three-dimensional space. In a 3D display, the view changes according to the position of the viewer relative to the display. For example, in a 3D display showing different objects, a smaller object may be occluded by a larger object shown in the display, but if the viewer moves around the display, thus causing a relative displacement relative to the 3D display, the smaller object can become visible.

[0105] The above indications regarding the components of the display medium 30 (such as the host material, upconversion material, and color-changing material) are also applicable to the display medium 90 of the 3D display.

[0106] In the illustrated embodiment, the display medium 90 has the form of a cube, but the display medium of the 3D display of the present invention can have any other 3D shape, such as a sphere, a cylinder, a cuboid, or an irregular 3D shape, to name just a few exemplary forms.

[0107] The display device 100 includes at least two optical components 51, 52, which are arranged laterally, and preferably the at least two optical components 51, 52 are arranged outside the display medium. Each optical device includes one or more light sources suitable for directing a light beam into the display medium. Preferably, two or more optical components 51, 52 are arranged such that each optical component can address any voxel within the medium.

[0108] As described above with respect to Figure 2 the 2D display shown, the optical components can be arranged in any suitable manner, for example in the form of an array of light sources, such that the position of the light beam can be controlled by turning on specific light sources within the optical components. Other possibilities include shiftable light sources, shiftable optical elements, and / or changes in the orientation of the light sources, their optical elements, and / or parts of the optical elements.

[0109] In particular, the present invention places no restrictions on the manner in which the direction and / or position of the light beam generated by the optical components is adjusted.

[0110] For any given content to be displayed, some voxels may not need to be addressed at all, while other voxels need to be addressed by generating intersecting light beams. The overall 3D information display is generated by addressing the voxels sequentially and / or simultaneously as needed. Note that in addition to the position and / or orientation of the intersecting light beams, the intensity, duration, and / or wavelength of the light beam can also be adjusted in order to generate the desired result at a given voxel. These parameters can be used to determine the characteristics of a given voxel (or pixel), such as color, intensity, contrast, and sharpness.

[0111] In addition, although two optical components are shown, there can be only one optical component including a number of light sources, or more than two optical components. The number of optical components is irrelevant as long as two light sources can be controlled to address the pixels in the medium.

[0112] Figure 3AShows upconversion material 36 and color-changing material 31 associated in a sea-island configuration in display media 30, 90. According to the present embodiment, these materials are uniformly and / or randomly distributed in the host material, thereby defining the shape of the display medium. Such a configuration can be obtained by dispersing the upconversion material and the color-changing material in the host material and then hardening the host material by, for example, polymerization and / or solvent evaporation. In this embodiment, the upconversion material is surrounded by the color-changing material. Under these conditions, the color-changing material may or may not be in direct contact with the upconversion material and is generally in a sufficiently close proximity to ensure that the light emitted by the upconversion material reaches the color-changing material and activates the color-changing material.

[0113] Figure 3B Shows upconversion material 36 and color-changing material 31 associated in a core-shell configuration. In this embodiment, the color-changing material is coated, deposited, and / or adsorbed on the outer surface of the upconversion material. It should be noted that the color-changing material may be provided in the form of a compound including an anchoring group, which enables the compound to be adsorbed on the surface of the upconversion particles.

[0114] In Figure 3A and Figure 3B In the two embodiments shown, the upconversion material 31 may be provided in the form of particles, particularly the upconversion material 31 may be provided in the form of nanoparticles, while the color-changing material 36 may be an organic compound or polymer deposited and / or adsorbed on the particles ( Figure 3B ) or near the upconversion material ( Figure 3A ).

[0115] In an embodiment, the color-changing material and the upconversion material may be included in one particle, such as a coated particle for example.

[0116] Figure 3B and Figure 4A The embodiments shown in and show that a single upconversion component (such as a particle) can be larger than a single color-changing material component (which can be an organic material). One or more color-changing materials may also be provided in the form of particles coated or adsorbed on the upconversion material. Figure 3B and Figure 4A Are schematic diagrams, and the single color-changing material component covered by the present invention is larger than the single upconversion component associated therewith. Still in another embodiment, the single color-changing material and upconversion component may have similar or the same average size.

[0117] In an embodiment, the concentration of the color-changing material 36 in the display media 30, 90 is higher than the concentration of the upconversion material in the display medium.

[0118] Preferably, any entity of upconversion material, such as nanoparticles, is surrounded by multiple entities of the color-changing material. By selecting an appropriate concentration, the probability that photons emitted by the upconversion material are captured by the color-changing material can be adjusted as desired.

[0119] In an embodiment, the display of the present invention is a multi-color display, preferably the display of the present invention is a full-color display.

[0120] The full-color function can be implemented in any suitable manner. In an embodiment, there are two, three, four or more different color-changing materials, thereby providing the corresponding number of different colors. For example, different color-changing materials can be selected to achieve additive or subtractive color mixing.

[0121] In an embodiment, the color-changing material can be upconverted to present a color that will directly be the color seen by the viewer, or combined with other colors in an additive or subtractive mixing manner. In other embodiments that can be used in combination with the embodiments just mentioned, some colors are generated by the reflection of light not absorbed by the color-changing material. For example, a white background reflective layer can be provided inside the display medium or in a separate layer, and the color-changing material can be provided to absorb some white light so as to present a color produced by subtracting one or more absorbed wavelengths from the white light. In this embodiment, the color-changing material is typically provided between the reflective layer and the viewing surface of the display. In this embodiment, the color-changing material can act as a filter. The color of the light not absorbed by one or more color-changing materials is obtained by subtractive removal of the absorbed light, and the resulting (remaining) light can, but does not need to, be independently combined with other colors in an additive and / or subtractive mixing manner. For example, other colors can also be generated by removing the absorbed light or by direct reflection of light having a specific color.

[0122] In one embodiment, different color-changing materials are distributed at specific positions in the display medium, thereby forming limited regions that can be regarded as sub-pixels. To generate colors, each sub-pixel can be individually addressed by intersecting beams, thereby generating the colors required to obtain the overall color of the pixel at each sub-pixel. In this embodiment, a single upconversion material can be used to activate any one of the different color-changing materials.

[0123] Figure 4A and Figure 4B Taking the core-shell type association between the upconversion material 31 and different color-changing materials 36 to 38 as an example, the above embodiments are illustrated. As Figure 4A shown, the particles 31 of a suitable upconversion material are coated with any one of the three different color-changing materials 36, 37, 38. In Figure 4BAmong them, particles including the same color-changing materials 36, 37, or 38 are respectively aggregated in different regions 41, 42, and 43, and these regions form sub-pixels 41, 42, 43 in the display medium. The three sub-pixels 41, 42, 43 together form a pixel 45, and this pixel can present colors through additive color mixing (such as RGB color mixing). It should be noted that the light beams 26, 27 are preferably emitted by the optical components 21, 22 with sufficient precision to individually address only one sub-pixel by precisely intersecting any one of the three regions 41, 42, 43, so as to generate local colors at each sub-pixel. Different sub-pixels can be addressed sequentially and / or simultaneously.

[0124] In an embodiment, the display medium includes a variety of different upconversion materials, and these upconversion materials can convert a variety of low-energy lights with different wavelengths and / or emit lights with different wavelengths of lower wavelengths and / or higher energies. In some embodiments, different upconversion materials and different color-changing materials are used, and generally a specific upconversion material is associated with one of the different color-changing materials. Different upconversion materials are selected so that they can be excited by light beams with different wavelengths, and preferably, lights with different energies are required for excitation.

[0125] In an embodiment, the upconversion material 31 is the first upconversion material 31, and one or more color-changing materials 36 are the first color-changing materials, where when the first color-changing material absorbs the radiation emitted by the first upconversion material, it changes from transparent to the first color or changes its color to the first color.

[0126] In an embodiment, the display medium includes a second upconversion material 32 and a second color-changing material 37, where when the second color-changing material absorbs the radiation emitted by the second upconversion material, it changes from transparent to the second color or changes its color to the second color.

[0127] In an embodiment, the display medium includes a third upconversion material 33 and a third color-changing material 38, where when the third color-changing material absorbs the radiation emitted by the second upconversion material, it changes from transparent to the third color or changes its color to the third color.

[0128] Preferably, the first color, the second color, and possibly more colors are different colors. Preferably, different colors can achieve multi-color display or full-color display, preferably through additive and / or subtractive color mixing.

[0129] Figure 5A and Figure 5B respectively show an exemplary island-type configuration and a core-shell-type configuration of an embodiment based on three or possibly more upconversion materials and associated color-changing materials. In Figure 5AIn [the display medium], different particles of different upconversion materials 31, 32, 33 are randomly and / or evenly distributed in the host material of the display medium 30.1. Different color-changing materials 36, 37, 38 are also distributed in the host material. To locally generate a desired color, only the upconversion materials suitable for activating the color-changing materials suitable for presenting the desired color are excited. This is preferably achieved by adjusting the wavelength and / or other parameters of the addressing beams 26, 27 as needed to selectively excite the corresponding upconversion materials. For example, the overall color of a region can be achieved by additive color mixing.

[0130] In Figure 5B [the display medium], different coated particles 11, 12, 13 suitable for generating three different colors are shown. Particle 11 includes an upconversion material 31 and a color-changing material 36, particle 12 includes an upconversion material 32 and a color-changing material 37, and particle 13 includes an upconversion material 33 and a color-changing material 38. The upconversion materials 31, 32, 33 are different upconversion materials, and the color-changing materials 36, 37, 38 are also different color-changing materials, which will display different colors after activation. In addition, each upconversion material is suitable for activating the corresponding color-changing material associated therewith, for example, the color-changing material coated on the particle including the appropriate upconversion material.

[0131] In an embodiment, the display medium includes a fourth upconversion material and a fourth color-changing material, wherein the fourth color-changing material changes from transparent to a fourth color or changes its color to the fourth color when absorbing the radiation emitted by the second upconversion material. The embodiment using four different color-changing materials and upconversion materials is a reasonable extension of the foregoing embodiments and is not specifically shown in the figure.

[0132] It should be noted that the color change of the color-changing material is preferably reversible. The reversal of the color change (also referred to as "decolorization", i.e., the color-changing material returns to its original basic state and is preferably a colorless state through "decolorization") can be caused independently by one or several processes. Such processes include, for example, applying a voltage between electrodes, by irradiation, or via an electrolyte. For example, an electrode layer can be provided, wherein the display medium is sandwiched between the electrode layers. At least one of the opposite electrodes located on the viewing side should be transparent so that the image can be viewed through the electrode. In other embodiments, the decolorization of the color-changing material is achieved by irradiating with appropriate light suitable for reversing the light-induced color change. For example, the light devices 21, 22 can be equipped with light sources that allow the decolorization of the color-changing material.

[0133] The display may include additional layers, such as additional layers adjacent to and / or laminated over the display medium. The additional layer may be, for example, selected from among a light filtering layer, a reflective layer, a light scattering layer, and a background color layer. For example, in the case of a 2D display, the display device may include a back substrate or a cover layer and / or a white layer such that there is only one viewing surface. In embodiments where the display can be viewed from two opposite main sides (two viewing surfaces) of the display, it is preferred that such a white layer and / or back cover layer is absent, and preferably the entire display medium is at least transparent to visible light when the color-changing material is not activated.

[0134] It is also noted that different colors may be provided in different layers of the stacked display medium, thereby also enabling the possibility of subtractive color mixing. In this case, the combination of the color-changing material and the upconversion material provided to generate a specific color is located in one of several layers. This can help to address these specific combinations with drive light and reduce the risk of accidentally generating colors at locations that have not been specifically addressed.

[0135] The display of the present invention is preferably a passive reflective display. In particular, the color-changing material presents a color and thus reflects external visible light of the corresponding color. Although the upconversion material emits light, as long as the light is visible light, preferably such light is absorbed by the surrounding color-changing material. The sea-island configuration and / or the core-shell configuration are examples of how to ensure that there is sufficient color-changing material near any upconversion entity to ensure absorption of the light emitted by the upconversion material. The present invention does not exclude, and thus also covers, the fact that some of the light emitted by the upconversion material includes visible light that may leave the display medium. In such an embodiment, the display may not be considered completely passive, but rather, for example, partially passive or substantially passive. In this regard, the display of the present invention can be regarded as a new type of display that can emit light partially, but preferably this new type of display is mainly passive.

[0136] In an embodiment, the display and / or the display medium of the present invention preferably do not have light-emitting elements (such as LEDs, OLEDs) and / or do not have a backlight (such as the backlight present in an LCD). On the other hand, any one, several, or all of the optical components in the optical assembly may include one or more selected from among LEDs, OLEDs, and LCDs. Therefore, such a light source may be present in the display device of the present invention.

[0137] The display of the present invention is preferably a light-driven display, and the information displayed on the display is generated by drive light, such as the intersecting light beams 26, 27 generated by the light devices 21, 22.

[0138] In the present invention, the combination of the upconversion material and the color-changing material offers several advantages: 1) Color-changing materials typically require high-energy photons (such as, for example, blue light or ultraviolet (UV) light) to be activated, and these high-energy photons are harmful to the human body. By using the upconversion material, the mentioned color-changing materials can be activated with harmless low-energy photons; 2) The low-energy photons for driving the display are in the lower visible to IR region. The excitation light in these regions is mostly or completely invisible to the human body, thus reducing light pollution during display use; 3) Low-energy photons have better penetration; therefore, a spatial display can also be used to locate three-dimensional space coordinates; 4) Since upconversion requires multiple photon interactions, this enables the use of two interacting drive beams to address specific coordinates.

[0139] Although certain preferred embodiments of the present invention have been described and specifically illustrated above, this does not mean that the present invention is limited to these embodiments. Various modifications can be made to these embodiments without departing from the scope and spirit of the present invention as set forth in the appended claims.

Claims

1. A display device (10, 100), the display device (10, 100) comprising: (a) Two or more light components (21, 22) capable of generating near-infrared (NIR) light, and (b) A display medium (30, 90), the display medium comprising: - One or more upconversion materials (31) adapted to convert the absorbed NIR radiation (26, 27) into emitted lower wavelength light, and - One or more photochromic materials and / or electrochromic materials (36) capable of coloring and / or changing color upon absorption of light.

2. The display device according to claim 1, wherein, One or more of the photochromic materials and / or electrochromic materials (36) are arranged to change color upon absorption of light emitted by one or more of the upconversion materials (31).

3. The display device according to any one of claims 1 or 2, the display device comprising a plurality of different upconversion materials capable of converting a plurality of NIR radiations having different wavelengths and / or emitting a plurality of lower wavelength and / or higher energy lights having different wavelengths.

4. The display device according to any one of claims 1 to 3, wherein, The upconversion material (31) is a first upconversion material (31), and one or more of the photochromic materials and / or electrochromic materials are first photochromic materials and / or electrochromic materials (36), wherein the first photochromic materials and / or electrochromic materials change from transparent to a first color or change color to the first color upon absorption of the radiation emitted by the first upconversion material.

5. The display device according to claim 4, wherein, The display medium comprises a second upconversion material (32) and a second photochromic material and / or electrochromic material, wherein the second photochromic material and / or electrochromic material changes from transparent to a second color or changes color to the second color upon absorption of the radiation emitted by the second upconversion material.

6. The display device according to any one of the foregoing claims, the display device comprising a first light assembly (21) and a second light assembly (22), wherein, The first light component and the second light component are disposed at a first position and a second position in the device, and the first light component and the second light component are configured to emit a first NIR beam (26) and a second NIR beam (27) from the first position and the second position to the display medium (30) such that the first NIR beam and the second NIR beam intersect at an intersection point (40) in the display medium.

7. The display device according to claim 6, wherein, At the intersection point (40) of the first NIR beam (26) and the second NIR beam (27), one or more of the upconversion materials are adapted to convert the NIR light into the lower wavelength and / or higher energy light.

8. The display device according to any one of the preceding claims, wherein, The two or more light components (21, 22) are configured to emit NIR beams into the display medium in an independent manner from each other.

9. The display device according to any one of the preceding claims, wherein, One or more of the upconversion materials are provided in the form of particles, preferably one or more of the upconversion materials are provided in the form of nanoparticles.

10. The display device according to any one of the preceding claims, wherein, One or more of said upconversion materials include one or more selected from the following: Pr 3+ (4f2), N d3+ (4f3), Sm 3+ (4f5), Eu 3+ (4f6), Gd 3+ (4f7), Tb 3+ (4f8), Dy 3+ (4f9), Ho 3+ (4f10), Er 3+ (4f11), Tm 3+ (4f12), Tm 2+ (4f13), actinide ions, and transition metal ions; preferably, said actinide ions are selected from U 4+ (5f2) and U 3+ (5f5); preferably, said transition metal ions are selected from Ti 2+ (3d2), Cr 3+ (3d3), Mn 2+ (3d5), Ni 2+ (3d8), Cu 2+ (3d9), Mo 3+ (4d3), Re 4+ (5d3) and Os 4+ (5d4).

11. The display device according to any one of the preceding claims, wherein, One or more of said photochromic materials and / or electrochromic materials (36) are selected from organic materials and inorganic materials. Preferably, one or more of said photochromic materials and / or electrochromic materials (36) are selected from organic compounds.

12. The display device according to any one of the preceding claims, wherein, The display medium (30) includes a host material that is transparent to NIR light and preferably also to visible light, wherein the upconversion material and the photochromic material and / or electrochromic material are disposed within the host material.

13. The display device (100) according to any one of the preceding claims, the display device (100) including a three-dimensional (3D) display medium (90), the three-dimensional (3D) display medium (90) being, for example, a cubic display medium.

14. The display device according to any one of the preceding claims, wherein, In the display medium, one or more of said photochromic materials and / or electrochromic materials (36) are coated on the upconversion material, and / or wherein, The upconversion material is surrounded by one or more of said photochromic materials and / or electrochromic materials.

15. The display device (100) according to any one of the preceding claims, wherein, Light reflected by one or more of (1) said photochromic material and / or electrochromic material, (2) said display, and (3) said display medium: - Provides content displayed by the display medium, the content being, for example, an image, text, symbol, and / or number, - Is visible to a viewer viewing the display medium, and / or, - Does not provide a reflective background for the display of the content displayed by the display medium.

16. The display device according to any one of the preceding claims, the display device being a passive display device and / or a display device for passive display.

17. A display medium (30), the display medium (30) including: - One or more upconversion materials (31), the upconversion materials (31) being adapted to convert absorbed near-infrared (NIR) radiation (26, 27) into emitted radiation of a lower wavelength, and, - One or more photochromic materials and / or electrochromic materials (36), the photochromic materials and / or electrochromic materials (36) being capable of coloring and / or changing color upon absorption of light.

18. The display medium according to claim 17, wherein: - The upconversion material (31) is a first upconversion material, and one or more of said photochromic materials and / or electrochromic materials are a first photochromic material or electrochromic material (36), wherein the first photochromic material and / or electrochromic material changes from transparent to a first color or changes color to the first color upon absorption of photons emitted by the first upconversion material, Wherein: - The display medium includes a second upconversion material (32) and a second photochromic material or electrochromic material, wherein the second photochromic material and / or electrochromic material changes from transparent to a second color or changes color to the second color upon absorption of photons emitted by the second upconversion material, and wherein: the first color and the second color are different colors.

19. The display medium according to claim 17 or 18, wherein, One or more of said upconversion materials (31) are provided in particulate form, preferably, one or more of said upconversion materials (31) are provided in the form of nanoparticles.

20. The display medium according to any one of claims 17 to 19, wherein the display medium is a passive display medium and / or the display medium is a medium for passive display.

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