DE3310427A1 - Method for optimising the contrast of field-effect liquid crystal cells - Google Patents

Abstract

A method for optimising the contrast of field-effect liquid crystal cells without having to depart from the technically proved, cost-effective technology for the production of large-area liquid crystal display devices having negative contrast representation, such as are applied, in particular, for motor vehicle instrumentation, consists in measuring for a given cell with liquid crystal filling that transmitted light wavelength at which the cell driven electrooptically to block passes through a minimum in its characteristic of the residual transmission over the light wavelength, that is to say experiences maximum absorption. The transmission operation of the liquid crystal cell is then performed by means of light in the spectral region about this wavelength, which light is filtered out from the light of an illuminating arrangement by means of a colour foil. The temperature dependence of the residual transmission is decisively reduced if a dichroitic dye is added at low concentration to the liquid crystal material, being chosen in accordance with the transmitted light colour. Pole filter pairings which optimally block for the light wavelength previously determined can be selected experimentally for the purpose of further contrast optimisation.

Description

Method for optimizing the contrast

of field effect liquid crystal cells The invention relates to a method for optimizing the contrast of field effect liquid crystal cells according to the preamble of claim 1. In particular, it relates to large-area TN liquid crystal cells with negative contrast display in transmissive (i.e. fluoroscopic) operation itself even with great ambient brightness and with information displays on different Surface areas in different colors, such as those in vehicle instrumentation in particular Find use.

Such liquid crystal cells are for example from DE-OS 30 16 396 known. Preferably, however, the present method is intended at Multi-chamber cells according to the parallel registration with the same priority with the designation Multi-color liquid crystal display device to be applied; being on this Parallel registration here in full to avoid repetitions will.

It is known that due to non-ideal properties, technical of available liquid crystal materials, linearly polarized light after passage is elliptically polarized again by the liquid crystal cell; that is, in the cell material again a polarization component arises which is perpendicular to the polarization direction of the light entering the cell. This perpendicular polarization component will thus from the analyzer in front of the cell, its polarization direction is oriented transversely to that of the polarizer behind the cell, is not absorbed. Because of these imperfections in the optical properties of the liquid crystal material the display contrast is worsened because it polarized across the main component Light in the area of not activated symbol segments next to the actually activated (optically transparent) symbol segments shine through, which is particularly annoying low ambient brightness (night operation) of the display device.

To compensate for this system-related loss of contrast, i.e. the To optimize contrast (between controlled and not controlled segments), it is known (DE-OS 30 48 024; DE-OS 31 48 447), two oppositely rotating Liquid crystal cells to be arranged one behind the other to thereby reduce the inside of the cell undesirably occurring transverse polarization due to their opposite orientation overall just to compensate. This optimization measure makes production more expensive of display devices with such liquid crystal cells, however, disproportionately. Because on the one hand, the material coordination for components in the opposite direction is the unwanted cross polarization critical; and most importantly, one is building one out two separate cells existing overall arrangement due to the requirement more exact complex geometrical assignments of the symbol representations. Also enlarged thereby undesirably the need for liquid crystal material and the Installation space required for the combined, i.e. double-layer, cell.

Finally, the display brilliance is due to such a double arrangement deteriorated because the effective liquid crystal layer is thicker and a plurality additional, basically optically lossy components connected in series is.

The invention lies in recognition of these circumstances and deficiencies the object is based on a method for optimizing the contrast of field effect liquid crystal cells of the generic type, in which the advantages of the tried and tested standard technology conventional (single-layer) cell structures are preserved - i.e. no mass production expensive special additional manufacturing processes are required -, nevertheless, compared to the contrast achieved so far with conventional cells, a noticeable increase in contrast and the associated improvement in brilliance is achieved.

According to the invention, this object is essentially achieved by that according to the partial features of the characterizing part of claim 1 proceeded will.

For electro-optical displays under critical conditions (especially in terms of fluctuations in ambient brightness and in terms of concentration diversion of a viewer), as they are given when driving a vehicle, for example for the negative-contrast information display, a contrast between information symbols and the environment (in particular parts of information that are not currently activated) in relation to each other 1: 25 required. Conventional TN liquid crystal cells made for the automotive industry bring it to a contrast ratio of almost 1: 30. According to none of the experts is a further increase only by turning away from conventional manufacturing technologies or cell structures achievable. The mere color matching according to the present, The process according to the invention, on the other hand, yields the same starting materials with unchanged Manufacturing technology has a contrast of at least 1:80 and thus liquid crystal information displays with a brilliance that has been prevalent so far (due to the manufacturing costs liquid crystal cells suitable for mass use could not be expected.

To solve the problem of contrast optimization according to the invention without a transition to more costly manufacturing technologies is thus of the knowledge assumed that the residual transmission of a blocked cell for a certain Wavelength (i.e. color) of the beam that is radiated backwards against the viewing direction Light (i.e. the light for the transmission operation of the cell) over the light wavelength runs through (at least) a minimum. The wavelength at which the absorption curve has a maximum, varies depending on the cell thickness (i.e. on the Thickness of the liquid crystal layer measured in the transmission direction between the to the inside of the cell with symbol electrodes and to the outside of the cell with a polarizer analyzer occupied, cell plates); and this position of the transmission minimum also varies depending on the anisotropy of the optical refractive index of the liquid crystal material (Delta n).

For the conventional liquid crystal cell manufacturing technology is the liquid crystal layer thickness can be technologically optimized; namely in particular in accordance with the required constant cell plate spacing, even with large ones Cell areas by using spacer particles, taking into account certain thicknesses a reaction of the liquid crystal viscosity on the technological possibilities a reproducible filling process and the quadratic dependence of the switching time on the layer thickness. In proven technology with available glass spacer particles and for common liquid crystal materials suitable for automotive dashboards (for example types ZLI 1694 or 1957 from MERCK) the layer thickness is (Cell plate spacing) approx. 8 ym. If only because spacer particles are not freely available it would be inexpedient to vary the layer thickness for contrast optimization want. With the simultaneous display of several adjacent information Symbols with different transmission light colors comes one that is matched to the colors Layer thickness is practically not in any case because of the uniform spacer particles Consider if the multicolor multiple information display for cost reasons to be realized with a single large-area liquid crystal cell, since regional thin glass layer inserts would be impractical.

A practicable way to get a residual transmission minimum for a certain wavelength and one determined by the available spacer particles Achieving layer thickness, however, lies in the adaptation of the refraction anisotropy (An) of the liquid crystal material mixtures used.

According to the present invention, therefore, is used for contrast optimization the path taken that for the desired information color in a production-technical possible layer thickness In each case the liquid crystal material by mixing different Base materials are used with exactly the type with which the minimum of the spectral Residual transmission curve lies in the spectral range of the desired information color.

For a light spectrum from a given lighting device then this is done with the interposition of an appropriately selected color filter Light wavelength mainly within the globally specified display color (e.g. for the requirement of a green display "the corresponding spectral range around 550 nm mean wavelength) and used as backlighting for the transmission operation - preferably via a transflector screen, as in detail explained and illustrated in the parallel application cited at the beginning - used.

For displaying information in different colors The cell is expediently different areas of the liquid crystal cell - as described in detail in the cited parallel application - in several chambers divided with liquid crystal materials which differ from one another according to the color specification (Different tn each time) are filled. The transmission light then becomes for each of these fillings this wavelength range is already assigned to the respective chamber Color filter filtered out of the lighting device.

The exact position of the usable minimum of the transmission curve (i.e. the wavelength of optimal absorption) varies with the ambient temperature, namely with the operating temperature of the liquid crystal material in the cell. To this temperature dependence (the optimized wavelength setting for improved contrast of the negative image) to reduce, it has according to an expedient development of the invention Solution proved to be advantageous, the liquid crystal cell chamber filling in lower Add concentration dichroic dyes, such as those used as guest dyes for GH liquid crystal cells for various basic colors are commercially available. These However, the aggregate concentration here is at most 1% to 2% and is limited on only one dye component, which is its main absorption in the spectral The area of the highest residual transmission of the cell is. So is the concentration much less than in common GH mixtures (6% - 896) which also consist of several Types of color molecules exist; so that the usual, the electrical control options (Switching times) impairing noticeable increase in the viscosity of the liquid crystal material does not enter here.

As a further adjustment measure to optimize the contrast of the TN negative representation it is advisable to use a polar filter pairing that is suitable on average, or even to the Regionally delimited polarizing filter pairings to be selected for the respective chamber areas. Because the absorption of the colored light takes place in the crossed polarization filter arrangement in the color dyes (i.e. in the embedded dichroic dyes) of the polarizing filter structures instead of; which is why an adjustment to the exact set for absorption optimum Light wavelength, as well as that for temperature compensation in the liquid crystal material stored dyes, an even greater increase in contrast that can be easily determined experimentally the presentation of information.

Claims (4)

Claims method for optimizing the contrast of transmissive, especially for negative contrast display, operable field effect liquid crystal cells, in particular of large-area liquid crystal cells for vehicle instrumentation, in which liquid crystal material with anisotropic optical refractive index between transparent plates is bordered, their surfaces facing each other with information symbol electrodes and their surfaces facing each other are covered with polarizing filters (polarizer and analyzer), characterized in that that, for a given color of the transmitted light and thus the display as well for the plate spacing (liquid crystal layer thickness) of a common liquid crystal cell manufacturing technology from commercially available liquid crystal materials of different anisotropy of their optical refractive indices but viscosities that are the same or similar to one another Liquid crystal mixture is obtained, the resulting anisotropy of the refractive indices for the plate spacing at least approximately matched to the desired display color is that then for a cell filled with this mixture the exact wavelength of light is measured in which the course of the transmission light intensity over the Light wavelength passes through a minimum, and that eventually, for operation the liquid crystal cells, by means of a colored film from the non-monochromatic Light e ner lighting arrangement transmission light is filtered out, the spectral mean value at least approximately at precisely that optimal measured value Wavelength lies. 2. The method according to claim 1, characterized in that, in the case of information displays in different basic colors in staggered areas on the Liquid crystal cell, in the cell these areas, locally assigned, chambers are delimited from each other according to the different colors with different liquid crystal material mixtures are filled, their anisotropy of the refractive indices in each case on the mean transmission light color wavelength ge the individual area chambers are matched for minimal residual transmission, and that spatially assigned from a large-area lighting arrangement by means of the chambers Color foils differently colored transmission light bundles with according to specification of the actual chamber fillings optimized spectral mean values of their light wavelengths be filtered out. 3. The method according to claim 1 or 2, characterized in that the Liquid crystal material in accordance with the predetermined basic information display color a dichroic dye whose absorption maximum is in the spectral range of Absorption minimum of the cell, is buried in such a low concentration, that no significant increases in viscosity occur. 4. The method according to any one of the preceding claims, characterized in, that the sub-areas of different display colors on the liquid crystal cell different ones, respectively for the liquid crystal material and the transmission light Experimentally optimized polarizing filter pairings assigned in the relevant area will.