Liquid crystal display device in which generation of moire fringes is prevented

Abstract: Disclosed is a liquid crystal display device in which it is possible to prevent the generation of moire fringes, and to realize an improvement in color clarity, a reduction in color inconsistency, clearness in color separation, etc., with the result that it is possible to improve the color image quality of the display screen. A liquid crystal display device includes a liquid crystal display module in which a liquid crystal layer 15 and a color filter 10 are formed between a pair of opposing transparent substrates 2 and 3, and a front light 24 mounted to the front side of the liquid crystal display module 16, wherein the stripe pattern of the color filter 10 is substantially perpendicular to the stripe pattern of the front light 24.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device suitable for use in a reflection type liquid crystal display device such as an STN type color liquid crystal display device utilizing the STN (super-twisted nematic) effect. 2. Description of the Related Art Conventionally, there exists, as a light reception type (non-light-emission type) flat (flat panel) display device utilizing changes in the optical natures of a medium, a liquid crystal display device having a liquid crystal display module in which a liquid crystal layer is formed between transparent substrates such as two glass substrates. This liquid crystal display device is of a transmission type in which a backlight is provided on the back side, a reflection type in which there is provided a reflection plate reflecting light impinging upon the front side, and a semi-transmission type which functions as both transmission type and reflection type. In particular, in business machines and apparatuses for home use, a simple matrix structure reflection type liquid crystal display device, in which the element construction is simple, the driving mechanism is simple, and the cost is low, is widely used. Fig. 4 is a partially cutaway perspective view showing an example of the conventional simple matrix structure STN reflection type liquid crystal display device. In this reflection type liquid crystal display device 1, a pair of glass substrates (transparent substrates) 2 and 3 are opposed to each other, and, on the upper surface of the glass substrate 2 which serves as the upper substrate, a phase difference plate 4 and a polarizing plate 5 are sequentially formed, and, on the lower side thereof, an insulating layer 6 consisting of SiO 2 or the like, a transparent electrode 7, and an orientation layer 8 are sequentially formed, and, on the glass substrate 3, which serves as the lower substrate, a reflection layer 9 consisting of a metal layer, a stripe-shaped color filter 10, a flattening layer 11 making the upper surface of the color filter 10 a flat surface, an insulating layer 12 consisting of SiO 2 or the like, a transparent electrode 13 consisting of an ITO layer, a NESA layer or the like, and an orientation layer 14 are sequentially formed, a liquid crystal layer 15 being formed between the orientation layers 8 and 14 to provide a liquid crystal display module 16. Above this liquid crystal display module 16, front lights 19, in which a large number of grooves 18 having a substantially V-shaped sectional configuration that is of a stripe pattern formed on the upper surface of the glass substrate (transparent substrate) 17, at fixed intervals. In this reflection type liquid crystal display device 1, as shown in Fig. 5, the angle  made by the stripe-shaped color filter 10 and the grooves 18 of the front lights 19 is set to be in the range of 0 to 50 degrees. Further, in an active matrix type liquid crystal display device using a TFT (thin film transistor) which is another example of a reflection type liquid crystal display device, the angle made by the stripe patter of the color filter and the strip pattern of the front lights is set to be 22.5 degrees. Incidentally, in the above-described reflection type liquid crystal display device, the stripe-shaped color filter 10 is inclined by the angle  with respect to the grooves 18 of the front lights 19, so that when light L is projected onto the liquid crystal layer 15 from the front light 19, a fringe-like diffraction image, that is, a moire fringe M, is generated, resulting in a deterioration in the color image quality. In these moire fringes M, the inter-fringe distance is larger than the width of the groove 18, so that, in particular, when generated in a color image, the color image is greatly deteriorated due to these moire fringes M, and in some cases, it is completely impossible to obtain a clear color display. In this way, the moire fringes M greatly influence the hue, brightness, saturation, etc. of the liquid crystal display module, so that they lead to generation of problems such as a deterioration in clarity in color display, generation of color inconsistency, and unclearness in color separation, resulting in a deterioration in the color image quality. Further details in relation to a conventional reflective display device can be found in EP-A-0 942 228. The present invention has been made in view of the above problem. Accordingly, it is an object of the present invention to prevent the generation of moire fringes to thereby realize an achievement in color clarity, a reduction in color inconsistency, clearness in color separation, etc., thereby making it possible to provide a liquid crystal display device capable of improving the color image quality of the display screen. According to an aspect of the present invention there is provided a liquid crystal display device comprising a liquid crystal display module in which a liquid crystal layer and a color filter are formed between a pair of opposing transparent substrates and a front light guide plate mounted to the front side of the liquid crystal display module, characterised in that the color filter has a stripe-like structure and the front light guide plate includes a plurality of grooves forming a stripe pattern, and in that the stripe pattern resulting from the stripe-like structure of the color filter is substantially perpendicular to the stripe pattern formed by the front light guide plate grooves. Preferably, the angle made by the stripe pattern of the color filter and the stripe pattern of the front light is in the range of 90±10 degrees. Optionally, the stripe pattern of the color filter consists of a plurality of stripe-shaped color filters arranged in the short side direction. In this liquid crystal display device, the stripe pattern of the color filter is substantially perpendicular to the stripe pattern of the front light, whereby even when light is projected from the front light toward the liquid crystal layer, no stripe-like diffraction image, that is, no moire fringes will be generated. Thus, there is no influence on the color hue, brightness, saturation, etc. of the liquid crystal display module due to moire fringes, and there is no fear of problems being generated such as a deterioration in clarity in color display, generation of color inconsistency, unclearness in color separation, etc., making it possible to improve the color image quality of the display screen. When the angle made by the stripe pattern of the color filter and the stripe pattern of the front light is in the range of 90±10 degrees, there is no fear of moire fringes being generated, making it possible to further improve the color image quality of the display screen. Here, the angle made by the stripe pattern of the color filter and the stripe pattern of the front light is in the range of 90±10 degrees because no moire fringes are to be visually recognized in the range of 90±10 degrees. Out of this range, moire fringes are to be visually recognized, resulting in a substantial deterioration in the color image quality of the display screen. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partially cutaway perspective view of an STN type reflection type liquid crystal display device according to an embodiment of the present invention; Fig. 2 is a schematic diagram showing the angle made by the stripe-shaped color filter and the front light groove of the reflection type liquid crystal display device according to an embodiment of the present invention; Fig. 3 is a sectional view of a liquid crystal display device provided with a front light; Fig. 4 is a partially cutaway perspective view showing an example of a conventional simple matrix structure STN type reflection type liquid crystal display device; and Fig. 5 is a schematic diagram showing the angle made by the stripe-shaped color filter and the front light groove of the conventional reflection type liquid crystal display device. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the liquid crystal display device of the present invention will now be described with reference to the drawings. Fig. 1 is a partially cutaway perspective view of an STN type reflection type liquid crystal display device 21 according to an embodiment of the present invention. In Fig. 1, the components which are the same as those of the liquid crystal display device shown in Fig. 4 are indicated by the same reference numerals. In this reflection type liquid crystal display device 21, there is formed between a pair of opposing glass substrates 2 and 3 a liquid crystal layer 15, and there is mounted a front light 24 in which a large number of grooves 23 having a substantially V-shaped sectional configuration in stripe pattern is mounted to the upper surface of the glass substrate 22, opposing to the upper portion of the liquid crystal display module 16 in which a plurality of stripe-like color filters 10 are arranged in the short side direction. The angle  made by the stripe-like color filters 10 and the grooves 23 of the front light 23 is, as shown in Fig. 2, substantially a right angle or, preferably, in the range of 90±10 degrees (80 to 100 degrees). Fig. 3 is a sectional view of a liquid crystal display device provided with a front light (surface light emitting device). In this diagram, the liquid crystal display device 25 substantially comprises a liquid crystal display unit 26, and a front light 27 arranged on the front side of the liquid crystal display unit 26 and adapted to illuminate the liquid crystal display unit 26. The front light 27 comprises a transparent light guide plate 28 and a light source 29, the light source 29 being arranged on the side end surface 28a for guiding light to the light guide plate 28. The light guide plate 28 is arranged on the front side of the display region of the liquid crystal display unit 26 (the upper surface side in Fig. 3) and adapted to apply the light from the light source 29 to the liquid crystal display unit 26. It comprises a base member 30 consisting of a transparent acrylic resin or the like and transparent reflection preventing layers 31 and 32 provided on the lower surface and the upper surface of the base member 30. As shown in Fig. 3, the lower surface of the light guide plate 28 (the surface opposed to the liquid crystal display unit 26) is a light output surface 28b outputting light for illuminating the liquid crystal display unit 26. The upper surface (outer surface of the light guide plate 28) on the opposite side of the output surface 28b is a reflection surface 28c for changing the direction of the light propagated through the interior of the light guide plate 28. As the material of the base member 30 of the light guide plate 28, it is possible to use, apart from acrylic resin, transparent resin material such as polycarbonate type resin or epoxy type resin, or glass. The output surface 28b of the light guide plate 28 is a surface opposed to the liquid crystal display unit 26 and adapted to output light for illuminating the liquid crystal display unit 26. It is a smooth surface whose surface roughness (Ra) is not more than 10 nm. On this output surface 28b, there is provided a reflection preventing layer 31 for preventing the reflection of light. Formed on the reflection surface 28c are a plurality of stripe-like wedge-shaped grooves 35 consisting of a first inclined surface portion 33 inclined with respect to the output surface 28b in order to change the light propagating direction by reflecting the light propagated through the interior of the light guide plate 28, and a second inclined surface portion 334 connected to the first inclined surface portion 33. Of the first inclined surface portion 33 and the second inclined surface portion 34 constituting the grooves 35, the first inclined surface portion 33 has a steeper inclination angle. Further, there is provided on the reflection surface 28c a reflection preventing layer 32 for preventing reflection of light. While in the light guide plate 28 shown in Fig. 3 the configuration of the reflection surface 28 is a triangular wave configuration in which wedge-shaped grooves 35 are successively formed, it is also possible to adopt a stripe-like configuration in which there is provided between adjacent grooves 35 a flat portion substantially parallel to the output surface 28b, with successive trapezoidal forms, a configuration in which a plurality of strip-like recesses constituting a part of a spherical surface are formed on the reflection surface 28c, or a stripe configuration in which there are formed a plurality of strip-like protrusions constituting a part of a spherical body. The light source 27 may be a cold cathode tube, organic EL element, LED or a combination of LED and a bar-like light guide member. Any type of light source will do as long as it allows light to be uniformly applied to the side end surface 28a of the light guide plate 28. The liquid crystal display unit 26 comprises a first substrate 37 holding the liquid crystal layer 36 and consisting of glass or the like, and a second substrate 38 joined by a seal material 39. Sequentially stacked on the surface on the liquid crystal layer 36 side of the first substrate 37 are a reflection layer 38 including a metal reflection film, a color filter layer 39, an orientation layer, etc. Formed on the surface on the liquid crystal layer 23 side of the second substrate 22 are an orientation layer 40, etc. Thus, the liquid crystal display unit 26 is a reflection type liquid crystal display unit provided with a reflection layer 38 for reflecting light coming from outside. The color filter layer 39 is formed as a plurality of stripes such that it is substantially perpendicular to the stripe configuration of the grooves 34 in the longitudinal direction of Fig. 3. The reflection layer 38 is formed, for example, by forming by sputtering or the like a metal reflection layer consisting of aluminum or silver or the like on an organic layer consisting of acrylic resin or the like on whose surface protrusions and recesses are formed, and is flattened by the color filter layer 39 so as to cover the reflection layer and the organic layer. In this liquid crystal display device, the stripe-like color filter 10 is substantially perpendicular to the grooves 23 of the front light 24, preferably, the angle range is 90±10 degrees, whereby, even when light L is projected from the front light 24 toward the liquid crystal layer 15, there is no fear of moire fringes being generated on the color image, and there is no fear of the color image quality being deteriorated. Thus, there is no fear of problems being involved such as a deterioration in the color clarity in the color display, generation of color inconsistency, and unclearness of color separation. As described above, in the reflection type liquid crystal display device of this embodiment, the angle  made by the stripe-shaped color filter 10 and the grooves 23 of the front light 24 is substantially a right angle or, preferably, in the range of 90±10 degrees, so that there is no fear of moire fringes being generated on the color image, and there is no fear of the color image quality being deteriorated. Thus, there is no fear of problems being involved such as a deterioration in the color clarity in the color display, generation of color inconsistency, and unclearness of color separation, and it is possible to obtain a color image which is clearer, free of color, with the hues being clearly separated. While an embodiment of the reflection type liquid crystal display device of this invention has been described with reference to the drawings, the specific construction is not restricted to that of this embodiment. Modifications in design, etc. are possible without departing from the scope of the present invention. For example, it is sufficient that the angle  made by the stripe-shaped color filter 10 and the grooves 23 of the front light 24 is set to be in the range of 90±10 degrees. Specifically, an optimum angle  is appropriately set taking into consideration the width of the stripe-like color filter 10, the distance between it and the adjacent color filter 10, the width of the grooves 23 of the front light 24 and the distance between it and the adjacent groove 23 so that no moire fringes may not be generated. Further, while in the above embodiment the stripe pattern of the color filter is the stripe-like color filter 10 a plurality of which are arranged in the short side direction, and the stripe pattern of the front light is the substantially V-shaped grooves 23 formed on the upper surface of the glass substrate 22, these are only given as and example of the stripe pattern of the color filter and the stripe pattern of the front light. They can be realized in various configurations according to the kind of the liquid crystal display device. Further, while in the above-described embodiment an STN type reflection type liquid crystal display device is given, it is only necessary for the construction of this reflection type liquid crystal display device to be such that the stripe pattern of the color filter is substantially perpendicular to the stripe pattern of the front light. Apart from the STN type reflection type liquid crystal display device, the present invention is also applicable, for example, to an active matrix type liquid crystal display device using TFT elements. As described above, in the liquid crystal display device of the present invention, the stripe pattern of the color filter is substantially perpendicular to the stripe pattern of the front light, so that even when light is projected toward the liquid crystal layer from the front light, there is no fear of moire fringes being generated, making it possible to prevent the light interference from influencing the hue, clarity, saturation, etc. of the color of the liquid crystal module. Thus, it is possible to prevent problems from being involved such as a deterioration in the color clarity in the color display, generation of color inconsistency, and unclearness of color separation, making it possible to improve the color image quality of the display screen. Further, the angle made by the stripe pattern of the color filter and the stripe pattern of the front light is in the range of 90±10 degrees, whereby it is possible to reliably prevent the generation of moire fringes, making it possible to further improve the color image quality of the display screen. Due to the above arrangement, it is possible to prevent the generation of moire fringes, and it is possible to realize an improvement in color clarity, a reduction in color inconsistency, clearness in color separation, etc., with the result that it is possible to provide a liquid crystal display device capable of improving the color image quality of the display screen.

Claims: A liquid crystal display device (25) comprising a liquid crystal display module (26) in which a liquid crystal layer (36) and a color filter (39) are formed between a pair of opposing transparent substrates (37, 38), and a front light guide plate (28) mounted to the front side of the liquid crystal display module, characterised in that the color filter has a stripe-like structure and the front light guide plate includes a plurality of grooves (23) forming a stripe pattern, and in that the stripe pattern resulting from the stripe-like structure of the color filter is substantially perpendicular to the stripe pattern formed by the front light guide plate grooves. A liquid crystal display device according to Claim 1, wherein the angle made by the stripe pattern of the color filter and the stripe pattern of the front light guide plate is in the range of 90±10 degrees. A liquid crystal display device according to Claim 1, wherein the stripe pattern of the color filter consists of a plurality of stripe-shaped color filters (10) arranged in the short side direction.

Inventors: KOBAYASHI MASAKAZU, KOBAYASHI, MASAKAZU, Kobayashi, Masakazu, c/o Alps Elect. Co., Ltd.
Applicants: ALPS ELECTRIC CO LTD, ALPS ELECTRIC CO., LTD.
Application year: 2001
Main classification: G02F1
IPC codes: G02F1, G09F9