Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Screen other than for cathode-ray tube
Reexamination Certificate
1999-11-29
2001-01-23
McPherson, John A. (Department: 2871)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Screen other than for cathode-ray tube
C349S106000, C349S115000
Reexamination Certificate
active
06177216
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a color filter layer of a cholesterically ordered material, in which the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer. The invention also relates to a liquid crystal color display device of the reflective type, provided with such a color filter layer.
Color filter layers, also referred to as cholesteric mirrors, are known per se. They comprise a relatively thin layer of a liquid crystalline material having a cholesteric (or chirally nematic) order. The liquid crystalline molecules of this material have such a structure that they order to a spiral or helix-like structure in a solution—spontaneously or under the influence of given doping agents. After such a solution has been provided between two parallel substrates, this helix-like structure is aligned in such a way that the axis of the molecular helix extends transversely to the layer thus obtained. A better alignment of the helix is obtained if an orientation layer is provided on the facing surfaces of the substrates.
When unpolarized light is incident on such a color filter layer, that part of the light which “fits” the (dextrorotatory or levorotatory) direction and pitch of the helix is reflected, whereas the rest of the incident light is transmitted. If desired, this transmitted light is absorbed on an absorption layer which is present behind the color filter layer. The (reflected) color of the layer is mainly determined by the pitch of the cholesterically ordered material. Such filter layers may be used as coating layers or as mirror layers, for example for decorative purposes.
The known color filter layers have a major drawback. It appears that the color (=reflection band) of the color filter is strongly dependent on the angle at which the filter is viewed. This phenomenon is known as color shift. This color shift is caused by the strong angle-dependent anisotropic properties of the cholesterically ordered material. Such a color shift is unacceptable in a large number of applications.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to obviate the above-mentioned problem. More particularly, it is an object of the invention to provide a color filter layer in which there is hardly any or no color shift.
These and other objects of the invention are achieved by means of a color filter layer of the type described in the opening paragraph which, according to the invention, is characterized in that the color filter layer comprises at least one or more dyes which absorb unwanted colors caused by color shifts.
Due to the presence of one or more dyes in the color filter layer, unwanted colors can be absorbed. This inhibits color shift of the filter. The dye(s) should therefore preferably be chosen to be such that its (their) absorption wavelength(s) correspond(s) to the wavelength(s) of the unwanted color(s). Per desired color (=reflection band) two dyes are preferably present in the filter. The absorption band of both dyes should have at least some overlap with both edges of the reflection band of the cholesterically ordered material. A filter of this type has the important advantage that also the intensity of the color does not have any angle dependence. It will be evident that in this case a single dye having two or more absorption bands may also be used.
A favorable embodiment of the color filter layer according to the invention is characterized in that the cholesterically ordered material comprises a polymer network. Such a network consists of a polymer material having a three-dimensional structure. At least a part of the liquid crystalline molecules present in the layer is preferably incorporated in the network. The presence of such a network causes the reflection characteristic of the color filter to be relatively insensitive to mechanical tensions and temperature changes.
A further preferred embodiment of the color filter layer according to the invention is characterized in that the pitch of the molecular helix is varied in such a way that the difference between the maximum pitch and the minimum pitch is at least 20 nm. Cholesteric color filter layers ordered in such a way have a relatively broad reflection band which, dependent on the refractive indices of the layer, may have a width of 80 nm or more. The possibility of adjusting the width of the reflection band provides great advantages in the eventual color setting of the color filter. The pitch difference is preferably 50 nm or more. U.S. Pat. Nos. 5,506,704 and 5,793,456 in the name of the applicant describe a method of manufacturing such a broadband color filter.
A further preferred embodiment of the color filter layer according to the invention is characterized in that the dye(s) is (are) incorporated in the network via chemical bonds. This measure promotes the stability and durability of the color filter layer according to the invention. Separating one or more of the dyes from the cholesterically ordered layer (for example, by phase separation) is thereby inhibited.
The invention also relates to a liquid crystal color display device of the reflective type, provided with such a color filter layer. More particularly, the invention relates to a liquid crystal color display device of the reflective type, comprising two substantially parallel substrates provided with an electrode layer and an orientation layer between which a layer of liquid crystalline material is present, one substrate being provided with a broadband absorption layer, a patterned color filter layer of cholesterically ordered material, and a ¼&lgr; layer, and the other substrate being provided with a polarizer.
Display devices of the type described above are known. For example,
FIGS. 3 and 4
of European patent application EP-A 600 349 show two such display devices. In principle, display devices of the reflective type do not require any backlight. Consequently, the reflective display devices have a relatively low energy consumption.
The operation of a liquid crystal color display device of the reflective type with a cholesteric color filter is based on the following principle. An incident ray of light of unpolarized white light is polarized by the polarizer to linearly polarized light and is subsequently incident on a pixel of the LC material. If this pixel is not driven by means of an electric voltage, the light ray passes through the LC material and its direction of polarization is rotated, for example 90°. Subsequently, upon passing through the ¼&lgr; layer behind this material, it is converted into levo-circularly polarized light.
The part of this polarized light having a wavelength outside the reflection band of the color filter layer of the ordered cholesteric material completely passes through the color filter layer and is absorbed in the broadband absorption layer. The part of the light having a wavelength within the reflection band of the color filter layer is, however, completely reflected on the color filter layer. Upon passage through the ¼&lgr; layer, the direction of polarization is converted again into levo linearly polarized light. The direction of polarization of the light ray is again rotated in the layer of LC material, for example 90° again, but now in the opposite direction so that the total rotation is 0°. As a result, this light ray has the correct direction of polarization for passing through the polarizer. A viewer now sees the reflection color of the color filter layer of the relevant pixel.
The color filter layer consists of a pattern of three types of pixels associated with cholesteric materials having different reflection bands. It is possible to manufacture liquid crystal color display devices of the reflective type with such filters. When driving the pixel, the direction of polarization of the linearly polarized light is not rotated in the LC material. Upon passage through the ¼&lgr; layer, the direction of polarization is now converted into dextro-circularly polarized light. The color filter reflect
Broer Dirk J.
Leenhouts Frans
Mol Grietje N.
Fox John C.
McPherson John A.
U.S. Philips Corporation
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