LCD-cell with color and light filtering layers

Details LCD-cell with color and light filtering layers LCD-cell with color and light filtering layers

2002-05-28

2004-07-13

Chowdhury, Tarifur R. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S074000, C349S110000, C349S113000

Reexamination Certificate

active

06762808

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a cell in a chromatic liquid crystal display (hereinafter an “LCD”).
BACKGROUND OF THE INVENTION
A known cell in a chromatic LCD-display, for instance, from SHARP®, includes two parallel plates, arranged behind one another, between which a liquid crystal is to be found. For each individual image element of the display, one electrode is installed per pair of plates. Upon the application of a potential between the electrodes, the liquid crystal therebetween, first impervious to polarized light, becomes transparent for polarized light, which is known as a “normally-black-mode LCD”, so that the image element appears bright or dark, upon being illuminated from the one surface, for example by a light source or by a layer which reflects incident light.
In the case of color-LCD's, in accord with the state of the technology, there is on one of the two plates (i.e. the first plate) a color filtering layer on the side facing the liquid crystal (inner surface). This surface has the contour of the image element, so that the illuminated image element as seen from outside appears as colored. Advantageously, in this arrangement, every three neighboring image elements are combined with color filters of primary colors into one image point.
Between the color filters of the individual image elements on the inner surface of the first plate an opaque layer (black mask) is placed which completely fills the intervening space between the color filters of the individual image elements. On the one hand, this black mask sharply delimits the individual image elements and so increases the contrast of the entire image. On another hand, the black mask obstructs, upon the control of an image element, the illumination of neighboring elements by random light, which would lead to a falsification of the color.
The color filter and the black mask are normally brought upon the first plate by something like a flex-o-pressure procedure or by photolithography, whereby one after the other, for instance, first all red, then all green, and then all blue color filters and finally, the black mask is laid down. When this is done, the black mask must be placed on the small pixel sizes on the inner surface of the first plate, in order to avoid or limit undesired parallax effects, or to hold as much as possible.
In the case of an LCD-cell of the state of the technology, further disadvantages arise. First, all color filters as well as the black masks must be installed with great precision, have exactly defined contours and be placed in specifically designated areas, since each of them have common delimiting surfaces. This brings about very expensive procedures for manufacturing in clean-room conditions as well as optical quality penalties as a result of manufacturing tolerances. Second, both the color filters as well as the black mask material which are used must be of the highest quality material, since both are separated from the liquid crystal by a thin polyamide layer and otherwise impurities would easily migrate into the liquid crystal by ion diffusion.
BRIEF SUMMARY OF THE INVENTION
The invention provides an LCD-cell for colored liquid crystal displays, which has low sensitivity in regard to the precision of the color filter.
An LCD-cell for a color liquid crystal, in accord with the present invention, comprises a first transparent plate, a transparent second plate arranged parallel to and behind the first plate, a liquid crystal placed between the first and the second plate, at least one color filtering layer of at least a material that transmits specifically light of a definite wave length, and a light impervious layer of light blocking material, which advantageously could also be an opaque material, which possesses openings, whereby the opaque layer and the color filtering layer are placed in planes, essentially parallel to the first and the second plates and outside of the liquid crystal.
By the decoupling of the color filtering and the opaque layer, an LCD-cell in accord with the invention exhibits a number of advantages:
the color filter need not possess the precision of the contour of the current components, that is, be precise as to the placement of the current image element, since this is already sharply deliminated by the black mask. On this account, the color filters can, for example, be installed with a greater manufacturing tolerance and thereby more economical on one or more surfaces,
the color filtering layer does not require any extraordinary purity and/or resistivity to other materials, particularly in regard to liquid crystals, since they no longer come into contact with the liquid crystal,
since the black mask is installed fundamentally alone and upon another surface than the color filter, the black mask can be very precisely installed,
in the case of damage, or for the changing of the colors of the display, principally, the corresponding plane can be exchanged,
due to the defining surface between the color filtering layer and the air, the reflexivity of the color filter increases excellently, and the display appears brighter because of the more strongly changing refractory index.
In a first embodiment of the invention, the LCD-cell includes further a parallel third, transparent plate behind the second plate, whereby the color filtering layer is placed on a surface of this third plate and the opaque layer is placed on another surface lying opposite to this surface.
In a second embodiment of the invention, the LCD-cell possesses likewise a third, transparent plate situated behind the second plate, whereby the color filtering layer is placed on a surface of this third plate and the opaque layer is located on a surface of the second plate which is remote from the liquid crystal.
In an advantageous embodiment of the invention, the openings of the opaque layer are so dimensioned and disposed that they sharply delimit each component to a definite shape, something like a pixel or a segment of an alpha numeric character set, so that the individual components are defined in outline not only by the controlling electrodes, but more exactly by the black mask.
In a further advantageous version of the invention, the color filtering layer possesses various areas of differing materials, which normally allow light of a definite color to pass through, preferably in the colors of red, green and blue. Similarly, in a further advantageous embodiment each color area respectively possesses at least a surface of an opening of the opaque layer and is so arranged, that it covers over this opening in the vision line of the liquid crystal. Thus, varied colored image elements are available, whereby, advantageously, respectively three neighboring image elements exhibit the primary colors.
In a further advantageous embodiment of the invention, spacers are placed between the second plate and the third plate. Behind the second plate for example, in the hollow space so created, is a film for the improvement of the optical characteristics. Such a film is called an “enhancement film”. This film improves, to a certain extent, the luminance of the light, the directional properties thereof, and/or the saturation of the color.
In a further advantageous rendering of the invention, an illumination source is placed behind the second or the third plate and/or an at least partially reflecting layer is placed, which reflects, or allows light to shine in the line of vision of the liquid crystal display.
Finally, a plurality of color filtering layers, advantageously of different materials can be placed in essentially parallel planes, which advantageously only give passage to light of respectively predetermined wave lengths.


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