Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Screen other than for cathode-ray tube
Reexamination Certificate
1998-12-10
2001-03-20
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Screen other than for cathode-ray tube
C349S106000
Reexamination Certificate
active
06203951
ABSTRACT:
TECHNICAL FIELD
This invention relates to a color filter for displaying an image in color on a liquid crystal display device, and more particularly to a color filter of the type in which color density is thin and dye is used as a colorant.
BACKGROUND OF INVENTION
As a method for producing a color filter used in a liquid crystal display device, there are known methods for applying a color using pigment, such as a pigment dispersing method or an electrodeposition method, and for applying a color(s) using dye, such as a dyeing method or a dye dissolution method. Usually, a color filter layer consists or three primary colors, red, green and blue. The colored patterns of the color filter layer having a coating thickness of 1 to 2 &mgr;m are formed in a mosaic pattern or in a stripe pattern.
In general, it is said that although dye is superior in transparency and purity of color and more abundant in kind compared with pigment, the former is inferior in light resistance to the latter. Also, it is known that color material such as pigment and dye is usually susceptible to fading on exposure to light due to oxidation and especially due to singlet oxygen produced by light (some colorant acts as a catalyst in this reaction)
However, the abovementioned general knowledge about fading is not applicable to the color filter used in a liquid crystal display device because such a color filter is panelized and then assembled in the display device and therefore, the inside of the panel is completely isolated From air, thus prohibiting progress of fading due to oxidation. This can easily be understood from the following facts. When, for example, a phthalocyanine-based dye, which is widely known as having a good light resistance, is used as a colorant for a color filter used in a liquid crystal display device, the dye is readily changed in color in spite of the fact that it is used in a circumstance completely isolated from air. In contrast, when a xanthene-based dye such as rhodamine, which is generally considered as being inferior in light resistance, is used as a colorant for a color filter, the dye is not readily changed in color and it can exhibit a strong light resistance in spite of the fact that it is used in the same circumstance where the dye is completely isolated from air.
Heretofore, color filters used in liquid crystal display devices have been manufactured by properly selecting dyes having a good light resistance based on experiences, utilizing the aforementioned special circumstance.
Recently, among liquid crystal display devices, the reflection type liquid crystal display devices drew much attention in order to obtain portable terminal devices with a color display capability. Since the reflection type liquid crystal display devices can be advantageously used as portable display devices in view of reduced consumption of power because the reflection type liquid crystal devices utilize natural light surrounding the devices instead of using a back light.
As one way for providing a color display capability to the reflection type liquid crystal display devices, a color filter is used. Such reflection type liquid crystal display devices themselves include, as discussed, for example, in Japanese Patent Unexamined Publication (KOKAI) No. Hei 8-304816 or Hei 9-68608, a color filter disposed on a surface of a liquid crystal layer as an optical shutter and adapted to provide a color display capability, and a reflective layer disposed on the other surface and having a light reflective function. In this method using a color filter, a wider color displaying range can be enjoyed easily by as making the background color paper white. Therefore, this method is advantageous compared with others using no color filter.
Incidentally, with respect to a color filter used in a reflection type liquid crystal display device, usually, a technique of manufacture following the method of manufacturing a transmission type color filter is solely used. However, it is demanded for the reflection type liquid crystal display device that brightness as much bright as possible ((i.e., thin in color) is ensured as a color filter because an incident light coming from the surroundings of the display device passes (penetrates) through the color filter twice and in addition, a color reproduction range is as large as possible. Also, besides the primary colors red (R), green (G) and blue (B), complementary colors yellow (Y), cyan (C) and magenta (M) are used. The color filter itself using dye is known per se. It is also known that the filter using dye is good in light resistance (for example, see U.S. Pat. No. 4,837,098).
DISCLOSURE OF INVENTION
No study has been made on color filters for thin color as 4 to 40% of a minimum value (420 to 610 nm) of a spectral transmission factor for each color, as used in a reflection type liquid crystal display device till today. Also, all findings on light resistance are those obtained by a color filter for thick color.
It was known through experiments that a mere application of dye, which was supposed to have good light resistance, to a colorant of a color filter used in a reflection type liquid crystal display device results in inferior light resistance, thus making such a color filter entirely unusable.
Generally, it is known for colorants such as pigment and dye, those for thin color are decreased in light resistance than those for thick color. However, no comparable findings are so far obtained in such a special circumstance as in a state of no oxygen as in a liquid crystal display device, where a general common knowledge is not applicable.
It was not known that an application of dye, which was supposed to have good light resistance, to a color filter for thin color as used in a reflection type liquid crystal display device, enhances light resistance.
The dye-based color filters have another shortcoming that absorption of light occurs in light transmission regions for respective color patterns under the effect of interference by a coating thickness of the color filter because planarity of the surface of the color filter is good and as a result, the light transmission factor is decreased to darken the color filter in some instances.
Furthermore, in the conventional color filter used in a transmission type liquid crystal display device, this can be neglected without any substantial problem because an interval between maximum values of absorption of light is too small to control (exceeding the control range of the coating thickness) due to the coating thickness so large as 1 to 2 &mgr;m and because chromaticity of the color patterns is, practically, hardly affected.
However, in the color filter used in the reflection type liquid crystal display device contrary to the color filter used in the transmission type liquid crystal display device, absorption of light caused by interference is effective by two powers because light passes through the color filter twice. Moreover, since the color patterns themselves constituting the color filter are thin, color purity is greatly affected.
The present invention aims at providing a color filter capable of exhibiting similar display characteristics to the conventional color filter used in a transmission type liquid crystal display device, by using dye having good transparency and color purity.
It is another object of the present invention to provide a technique capable of enhancing light resistance even in case the colors of the colored patterns constituting the color filter become thinner.
In a color filter for a liquid crystal display device according to the present invention, colored patterns of a color filter layer include resin and dye for dyeing the resin and the thickness of the colored patterns is 0.7 &mgr;m or less. Because the thickness of the colored patterns enhances the coating density of the dye, the thinner the better in view of enhancement of light resistance. However, if the thickness is too small, the colored patterns cannot withstand the manufacturing process. For example, there is a fear that dye is eluded out of the resin. In case of
Furukawa Tadahiro
Murai Tatsuhiko
Takahashi Atsushi
Kyodo Printing Co., Ltd.
McPherson John A.
Pitney Hardin Kipp & Szuch LLP
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