1. Field of the Invention
The present invention relates to a multi-colored liquid crystal display arrangement, especially for motor vehicle instruments, including colored foils with different colorings spatially associated with a plurality of mutually offset and differing information areas of the display, intermediate a liquid crystal cell and a large-surfaced illuminating arrangement.
The invention further relates to a process for the optimization of the contrast of field effect-liquid crystal cells, particularly for optimizing the contrast of transmissively drivable field effect-liquid crystal cells, especially for negative contrast representation, and in particular large-surfaced liquid crystal cells for motor vehicle instrumentation, in which liquid crystal material with an anisotropic optical refractory number is enclosed between transparent plates, the facing surfaces of which are covered with information symbol electrodes, and whose outwardly facing surfaces are covered with polarized filters, such as a polarizer and analyzer. In particular, the invention relates to large-surfaced liquid crystal cells with negative-contrast representation during transmissive (in essence, transilluminating) operation, even at a high environmental brightness, and with information representations on different surfaces areas in different colors, such as is especially utilized for motor vehicle instrumentation.
2. Discussion of the Prior Art
A display arrangement of that type is disclosed in German Laid-open Patent Application No. 30 32 988. The known arrangement incorporates a prismatic light conductor as a large-surfaced illuminating arrangement, which is irradiated through one side surface thereof and in which the opposite light discharging surface is equipped with a reflector. Located between the liquid crystal cell and the light discharging surface of the illuminating prism is a symbol mask including, in the viewing direction of the display arrangement, a color filter arranged therebehind. The color filter incorporates different mutually bounded colored areas which are associated with differently located symbols of the information which is to be represented in toto. The display arrangement thus operates purely transmissively, with fixedly predetermined colors for the individual, mutually displaced and representable symbols. A disadvantage of this known display arrangement resides in the high technological manufacturing requirements for the formation and assembly of the prismatically profiled illuminating arrangement which is critical with respect to its optical coupling. Moreover, it is especially disadvantageous that such a display arrangement provides only an average, and in accordance with the color of the symbol which is to be displayed, even a completely different contrast; so that the contrast, in essence the recognizability, is rendered substantially poorer when, pursuant to the practical conditions of utilization of such a display arrangement, the light of the environmental brightness falls in the viewing direction onto the display arrangement, and the symbol representation of the liquid crystal cell is either obscured or even completely outshined. This can be counteracted through an increase in the transmissive illuminating intensity of the illuminating arrangement on the basis of the losses in the prismatic light conductor, and on the basis of the heat loss radiation of usual light sources, insofar as in a usual manner they are built into already small-sized housing of the display arrangement, and additionally influence in only a limited extent the operating behavior of the liquid crystal cell because of its dependence upon temperature.
A display arrangement which is similar in construction is disclosed in British Published Patent Application No. 20 94 051 for instrumentation as well as for larger-format information panels. For example, provided as a large-surfaced illuminating arrangement is an electroluminescent plate or a fluorescent lamp, which transilluminates a color foil from rearwardly thereof, and which is simultaneously irradiated from the front; in effect in the viewing direction of the display arrangement, by the brightness of the environment. The color foil carries, on its front surface, regional fluorescent color coatings, ahead of which there are located the symbol formations of a liquid crystal cell. This arrangement is subject to the advantage that at an increasing environmental brightness, there must be expected an extremely intense reduction in the color contrast. This, in turn, renders more difficult the easy visual ascertainability of the different displayable symbols; with which there is associated in the mind a predetermined color constellation which, however, becomes always less distinct at an increasing environmental brightness.