Patent Application
20080074575
The invention is explained in more detail by means of the implementation examples shown in the drawings.
A standard film, as has been used until now in liquid crystal displays, is suitable as a front polarization filter 11. Light of one polarization direction is transmitted at a high percentage rate. Light from the perpendicularly aligned polarization direction is blocked or absorbed at a high percentage rate.
The TN type is used, for example, as the liquid crystal layer or liquid crystal cell, respectively 12. It is preferred because it has a construction that yields a voltage-free, bright display. With this mode of operation, incident light is transmitted from all directions from the front onto bright areas (uncontrolled areas) of the liquid crystal cell with the least amount of absorption. A particularly preferred implementation comprises a so-called LTN liquid crystal cell, as defined in WO 02/084 383 A, mentioned in the introduction.
The so-called BEF polarization filter (brightness enhancement filter), with its luminosity-increasing construction, is advantageously used as the rear polarization filter 13. In this type of filter, the light from the blocked polarization direction, which reaches the rear-polarizing filter 13 from the diffusion screen 14, is reflected back, for the most part, in the direction of the diffusion screen 14 or retro-reflection film. Light from one polarization direction is, in turn, transmitted from the light that continuously reaches the BEF filter 13. Radiation polarized in the orthogonal direction is continuously reflected towards the diffusion screen 14. If light incident onto the diffusion screen 14 changes its polarization direction, then a portion of the reflected light can ultimately pass through the liquid crystal cell 12. This increases the luminosity of the liquid crystal cell. This not only applies to light incident as background light HL but also to light incident from the direction of the observer B onto the display unit 1. This light can pass in the luminous areas of the liquid crystal cell 12 and is then thrown back or emitted from the diffusion screen 14 continuously in the direction of the liquid crystal cell 12. A reflective, not diffuse, polarization filter film is used as the luminosity increasing, rear polarization filter 13, especially in conjunction with the retro-reflection film or retro-reflection layer, respectively, because the scattering takes place in the light converter and in the fluorescent dye of the retro-reflection film or retro-reflection layer, respectively. This effect is used largely for the recycling of the reflected, polarized radiation. Prism structures, especially micro-prisms, and/or a diffusion indicatrix (index ellipsoid) that suits specific applications, can also be provided in the area of the diffusion layer 15 or retro-reflection layer. This provision contributes to an increase in luminosity or contrast, respectively, and also produces the desired viewing angle relative to the surface normal of the display (direction and/or f-ratio). Additionally the diffusion layer 15 or retro-reflection layer or film, respectively, can be configured to be wavelength selective, in order to achieve color properties.
The special configuration of the diffusion layer or diffusion screen, respectively 14, or retro-reflection layer or retro-reflection film, respectively, with fluorescent dyes also contributes to the increase in luminosity. As explained above, the use of the BEF polarization filter 13, thus, achieves the increase in luminosity, in that the polarization direction of the light reflected from the BEF filter 13 is changed. The use of fluorescent dyes 14.1 in the diffusion screen 14 or reflective film renders the change in the polarization direction especially efficient. The fluorescent dyes 14.1 cause the shorter-wavelength light to be absorbed and then continuously emitted in the emission wavelength of the fluorescent dyes 14.1, e.g. with consistent disbursement of the polarization direction. For simple application to a rear support layer, the retro-reflection film is coated with a self-adhesive layer on its rear side.
The diffusion layer or diffusion screen, respectively 14, or retro-reflection layer or retro-reflection film, respectively, with highly pure fluorescent dyes, forms an emissive color filter. In one design, it significantly contributes to enhanced readability and contrast under unfavorable outer illumination conditions as a semi-transparent (transflective) optical element (plate, film, sandwich, print, coating): in a monochrome implementation, e.g. as a solid-colored plastic plate, film, sandwich structure, or in polychrome implementation, e.g. as print, coating on a translucent polymer carrier. In so doing it yields an increase in energy efficiency through better utilization of a backlight during transmissive operation. In conjunction with this, a number of principal design variations arise, namely
a transparent or translucent thermoplastic polymer as a matrix, containing one or more photo-stable and thermally stable fluorescent dyes as well as a polymer or inorganic particulate optical diffuser. Alongside these, it can also contain commercial UV-stabilizers, dyes, or process aids. Typical matrix polymer classes that come into consideration here are, among others, polycarbonates, polyesters, poly(methyl) acrylate and its copolymers, polystyrene and polystyrene-copolymers, polyvinylchloride (PVC), polyvinyl fluoride (PVDF), cycloolefin copolymers well as their physical mixtures and blends. Particularly favored among these are polycarbonate (PC), polymethyl acrylate (PMMA), polyethylene terephthalate (PET), methylacrylate-acrylonitrile-butadiene-styrol copolymers (MABS), and polyvinylchloride (PVC).
Diffusion bodies and fluorescent dyes can be present together in one layer as well as in separate layers (in multi-layer designs). In the first case the diffusion body must be photo-inactive, i.e. it may not exhibit any photo-catalytic or photochemical activity. Barium sulfate, zinc sulfide, or zirconium oxide as well as all polymer diffusion materials, such as the Paraloid™ product line by Arkema (preferably barium sulfate and zinc sulfide or mixtures of them) are named as examples of photo-inactive diffusion bodies. Titanium dioxide as well as doped and undoped stannous oxide (preferably titanium dioxide) are named as examples of preferred photoactive diffusion bodies. Examples of preferred photo-stable and thermally stable fluorescent dyes are the Lumogen® F product line by BASF as well as the Hostasol ® product line by Clariant.
The fluorescent dyes are generally dosed so that the optical density has a maximum absorption range of 1.5 to 2. This produces an effective dosing level of 0.001 to 10% by weight, depending on the density of the colored layer and the absorption efficiency of the dye. Typically for thick through-colored systems (e.g. plates, sandwiches) >2 mm thick it is 0.001 to 0.1% by weight, while for thin, through-colored systems (films, sandwiches) <0.5 mm thick, it is 0.05 to 2% by weight, and for print or coating applications in the range of 0.5 to 10% by weight.
The particulate diffusion materials generally are dosed so that the average transmission values of the overall system lie outside of the absorption bands of the dye by approximately 50%. This produces an effective dosing level of 0.1 to 40% by weight, depending on the thickness of the additive-containing layer and the scattering efficiency of the diffusion body. For thick, whole-body-additive containing systems (e.g., plates, sandwiches) >2 mm thick this typically lies in the range of 0.1 to 5% by weight and for thin, whole-body-additive containing systems (films, sandwiches) <0.5 mm thick between 2 and 40% by weight. For print or coating applications, the attainable layer thickness (of a few micrometers), is generally not sufficient to produce the desired diffusion effect. In this case, the diffusion materials should be integrated completely, or at least predominantly, in the substrate that is to be imprinted or coated.
In the non-transparent, reflecting, optical implementation, which is advantageously used in the construction of highly legible displays without backlighting (reflective operation), different implementation variations likewise come into consideration. These variations are in the form of multilayer film, sandwich structure, print, or coating. In a monochrome implementation, e.g. a film that is dyed in the surface layer, a sandwich structure made of at least a through-colored, transparent layer, and an optional reflecting layer positioned behind the sandwich or a print come into consideration. In a polychromatic implementation, a print, a coating on a reflective polymer or metallic support come into consideration. A preferred implementation comprises a micro-prismatic sandwich film, as described in EP 0 853 646 or EP 0 862 599.
In an especially preferred design, the emissive color filter serves simultaneously as an assembly/LC glass support.
For example, in a first implementation, a green-yellow emissive color filter in plate format is used. In this implementation, the structure comprises a PMMA panel, which is single-layered (e.g. approximately 2-6 mm, e.g. 3-5 mm thick). It is implemented using 0.01 Lumogen F yellow 083 (BASF) and 2% Barium sulfate (Blanc Fixe; Sachtleben). Further implementation examples involve a construction analogous to example 1, however, using Lumogen F yellow 170 (BASF: yellow), Lumogen F orange 240 (BASF; orange), Lumogen F pink 285 (BASF; orange-red), Hostasol red GG (Clariant; s.o. 63; orange-red), Lumogen F red 305 (BASF; red) and/or Lumogen F green 850 (BASF; green).
In an additional implementation the use of an optical brightener instead of a fluorescent dye produces an emissive performance.
The combination of the three components, liquid crystal cell 12 operating in voltage-free light mode, BEF polarization filter as rear polarization filter 13, and a diffusion layer or diffusion screen, respectively 14, or retro-reflection layer or retro-reflection film, respectively, with fluorescent dyes, contributes advantageously to an especially effective increase in luminosity and also contrast. In this manner the luminosity of liquid crystal displays is considerably improved. Even with relatively little ambient light, clearly legible displays are attained even without additional backlighting.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20 2004 017 411.1 | Nov 2004 | DE | national |
| 20 2005 016 373.2 | Oct 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/11939 | 11/8/2005 | WO | 00 | 11/27/2006 |
