Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1997-12-18
2001-02-06
Malinowski, Walter (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S106000, C349S113000
Reexamination Certificate
active
06184949
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a reflective flat-panel color display device having a display panel which comprises a diffusing liquid crystalline material present between a first and a second substrate, the display device including a color filter.
A display panel as used in the color display device of the type described in the opening paragraph is known from, for example the English-language abstract of JP-A 59-10924. This document describes a flat-panel reflective display device in which a dichroic reflector is arranged under the optically active layer. A diffusing and reflecting plate is arranged outside the display panel. The dichroic reflector is transmissive to green light and reflective to red and blue light. When a white beam is incident on the display device and when the liquid crystalline material is in its diffuse state, then red and blue are diffused in the layer and subsequently reflected by the dichroic reflector. They return as diffuse light. Green light is also diffused in the layer and passed by the dichroic reflector to the next reflector. Reflection and diffusion take place on this reflector so that also the green beam returns as a diffuse beam. For the viewer, the display device is in the bright state. When a white beam is incident on the display device and when the layer is in the transparent state, then red and blue light will be reflected on the dichroic mirror without being diffused. The green light is passed and reflected on the diffusing reflector. The green light thus returns in a diffuse state. The viewer now watches a green monochrome display panel.
A drawback of the display device known from the abstract is that an artefact occurs when this configuration is used for a three-color panel. Namely, a transparent pixel will produce a light spot on the diffusing reflector. The edges of this light spot will be visible to the viewer through neighbouring transparent pixels, so that the viewer will observe ghost images. A further drawback is that the diffuser mentioned in this document diffuses through 180°. This light will be enclosed in the substrate until it reaches a pixel with a color filter passing the relevant color to the liquid crystalline layer. However, in this way, a part of this light will not be coupled out when it reaches the sides of the display panel so that this part will be lost. When, for example a pixel is blue in the transparent state, it will be light yellow instead of white in a diffuse state, due to loss at the sides.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a reflective flat-panel display device having a relatively satisfactory white balance and a relatively high brightness, in which the occurrence of disturbing light spots in the viewing angle of the viewer is obviated.
To this end, the reflective flat-panel display has a color filter with a pattern which corresponds to the pixel pattern of the display panel, and a backward-diffusing layer is present between the color filter pattern and the second substrate.
By arranging the diffuser in the display panel, the light will not reach the substrate and ghost images are prevented.
The invention is further based on the recognition that hardly any light is absorbed in the display device but that unsuitable light is redirected and redistributed. Per pixel, at least one-third of the incident light is converted into diffuse light instead of being absorbed.
An alternative embodiment which is based on the same recognition is characterized in that the color filter comprises a pattern which corresponds to the pixel pattern of the display panel, and a forward diffuser is arranged between the reflector and the display panel.
By providing an extra forward diffuser, if the backward diffuser is arranged outside the display panel, the beam passed by a transparent pixel is first forwardly diffused before it reaches the backward diffuser and the light spot is spread so that its edges are no longer visible within the viewer's viewing angle.
A preferred embodiment of the reflective flat-panel display device according to the invention is characterized in that the backward diffuser diffuses within a given angular range &Dgr;&THgr;. This has the advantage that, notably for the embodiment with the external backward diffuser, the light enclosed in the substrate stands a reasonable chance of being coupled out via a pixel before it reaches the side of the display panel and thus cannot contribute to the brightness of the display device.
A further embodiment of the reflective flat-panel display device according to the invention is characterized in that each color filter associated with the color filter pattern is transmissive to at least one primary color and specularly reflective to the other primary colors.
In the transparent state of the liquid crystalline layer, incident light is passed unhindered to the color filter pattern. The color to which the relevant filter element is transmissive will be diffused on the backward diffuser and return as a diffuse beam. The colors to which the filter element is specularly reflective will be reflected away and will not reach the viewer's viewing angle. The viewer will perceive the relevant pixel in a color for which the filter element corresponding to the pixel is transmissive.
In the diffuse state of the liquid crystalline layer, all colors will first be diffused in this layer. One or two colors will be reflected on the filter element, while the other color or colors still pass via the backward diffuser. Since all colors now return as diffuse beams, the viewer will perceive the relevant pixels in a bright state.
A pixel consequently switches between the “white state” and the color to which the relevant filter element is transmissive.
A further embodiment of the reflective flat-panel display device according to the invention is characterized in that the elements of the color filter pattern comprise a dielectric material.
A dielectric stack can easily be provided as a thin layer in the display panel. Moreover, such a material has a relatively high electric resistance and can consequently be combined easily with an active matrix display panel. Dielectric materials further have the advantage that they reflect rotationally symmetrically.
Another embodiment of the reflective flat-panel display device according to the invention is characterized in that the color filters comprise at least two and an even number of layers of a cholesteric liquid crystalline material, one layer of a pair being levorotatory and the other layer of said pair being dextrorotatory, both layers covering the same wavelength range.
A cholesteric layer is a liquid crystalline layer having a cholesteric ordering. This means that the molecules of the material spontaneously order in solution to a helical or helix-like structure with a pitch p. After providing such a solution as a thin, active layer between two parallel substrates, the helix-like structure is directed in such a way that the axis of the helix will be transverse to the layer.
When an unpolarized beam is incident on a cholesteric polarizer, the levorotatory and dextrorotatory circularly polarized beam components will be separated from each other. Namely, a beam component having the direction of rotation corresponding to the direction of the helix will be reflected, whereas the beam component having the other direction of rotation will be passed. However, the fact whether the beam component is passed or not passed is not only determined by the direction of rotation but also by the wavelength of the incident beam. The reflection wavelength &lgr;
0
=½(n
o
+n
e
)p. Of the beam component having the direction of rotation corresponding to the direction of the pitch, only that part will be reflected which is located within the wavelength band &lgr;
o
. Since the light is not to be polarized in the application described here, there should always be two cholesteric layers per wavelength range, namely a levorotatory and a dextrorotatory layer so that both directions of polarizatio
Cornelissen Hugo J.
Neijzen Jacobus H. M.
Paulissen Fransiscus A. M. A.
Malinowski Walter
U.S. Philips Corp.
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