1. Field of the Invention
The present invention relates to reflectors and reflective liquid crystal displays, and more specifically relates to a reflector having reflection characteristics such that it appears brighter when reflection light is observed at a specific viewing angle than when it is observed at other viewing angles, and to a reflective liquid crystal display using the reflector.
2. Description of the Related Art
Liquid crystal displays can be generally classified into two types: transmissive liquid crystal displays and reflective liquid crystal displays. In reflective liquid crystal displays, external light is used for illumination and a front light is used for ensuring visibility. Reflective liquid crystal displays are commonly used as display units for electronic devices such as mobile computers, calculators, digital watches, communication equipment, game machines, measuring devices, electronic display boards, etc.
An example of a reflective liquid crystal display is shown in FIG. 8. With reference to FIG. 8, a display-side substrate 20 and a reflector-side substrate 10 oppose each other with a liquid crystal layer 30 therebetween. The display-side substrate 20 is transmissive and the reflector-side substrate 10 is reflective. The external surface of the display-side substrate 20 serves as a display surface, and the reflector-side substrate 10 is provided with a reflective layer 12. In this reflective liquid crystal display, external light incident on the display surface passes through the display-side substrate 20 and the liquid crystal layer 30, is reflected by the reflective layer 12 in the reflector-side substrate 10, passes through the liquid crystal layer 30 again, and is emitted from the display surface, thereby making an image visible.
In FIG. 8, the reflector-side substrate 10 is formed by laminating a glass substrate 11, the reflective layer 12, an intervening layer 13, a color-filter layer 14, a planarizing layer 15, a transparent electrode layer 16 formed of an Indium Tin Oxide (ITO) film, a Nesa film, etc., and an alignment layer 17, in that order from the bottom. In addition, the display-side substrate 20, which opposes the reflector-side substrate 10 across the liquid crystal layer 30, is formed by laminating an alignment layer 21, an insulating layer 22, a transparent electrode layer 23 formed of an ITO film, a Nesa film, etc., a glass substrate 24, and a light-modulating layer 25 (a polarizing plate, a retardation plate, etc.) in that order from the liquid crystal layer 30.
In the above-described liquid crystal display, the color-filter layer 14 in the reflector-side substrate 10 includes red (R), green (G), and blue (B) color films which are sequentially arranged in parallel to each other in a striped pattern, and the transparent electrode layer 16 includes transparent electrodes disposed in parallel to each other in a striped pattern at positions corresponding to the color films. In addition, the transparent electrode layer 23 in the display-side substrate 20 includes transparent electrodes which are arranged in parallel to each other and perpendicularly to the transparent electrodes of the transparent electrode layer 16. Parts of the liquid crystal layer 30 at intersections of the transparent electrodes of the transparent electrode layer 23, which is disposed at the display-side, and the transparent electrodes of the transparent electrode layer 16, which is disposed at the reflector-side, are formed as pixels, each pixel corresponding to one of the colors.
In addition, in the above-described liquid crystal display, a front light (not shown) is disposed outside the display-side substrate 20 as required. In such a case, similarly to external light, light emitted from the front light passes through the display-side substrate 20 and the liquid crystal layer 30, is reflected by the reflective layer 12 in the reflector-side substrate 10, passes through the liquid crystal layer 30 again, and is emitted from the display surface.
The reflective layer 12 in the reflector-side substrate 10 can be generally classified into the specular-reflection type and the diffuse-reflection type. FIG. 9A shows a specular-reflection type reflective layer 12′ and the reflective surface of this reflective layer 12′ is made flat so that the absolute value of the incidence angle and the absolute value of the emission angle with respect to the normal of the display surface are the same. Accordingly, when the display surface is observed, there are problems in that the brightness of the display surface varies depending on the positional relationship between the light source and the viewpoint and visibility is degraded due to back reflection, that is, reflection of the light source, the observer's face, etc., in the display surface. In order to solve such problems, in a diffuse-reflection type reflective layer 12″ shown in FIGS. 9B and 10, a plurality of small concavities and convexities (concave portions 31 in FIG. 10) are irregularly formed next to each other on the reflective surface of the reflective layer 12″. Thus, in the diffuse-reflection type reflective layer 12″, external light incident at a certain angle is diffusely reflected by the surface of the reflective layer 12″. Accordingly, it is possible to obtain a reflective liquid crystal display having a wide viewing angle in which brightness does not vary even when the viewpoint is moved and back reflection is reduced.
With regard to the material of the diffuse-reflection type reflective layer 12″, the shape and distribution of the concavities and convexities, and the method for forming the concavities and convexities, various suggestions have been made from the viewpoints of reflection characteristics and productivity.
Regarding the method for forming the concavities and convexities, a method is known in which light is radiated on the surface of a plate-shaped resin substrate formed of a photosensitive resin layer, etc. through a pattern mask, and a plurality of small, spherical concave portions 31 are formed next to each other by a development process. In order to obtain a mirror-finished surface, a layer of aluminum, silver, etc. is formed on the surface on which the concave portions are formed by vapor deposition or plating. In addition, another method is also known in which a plurality of small, spherical concave portions 31 are formed next to each other by pressing a punch (stamping tool) having a hemispherical end portion against the surface of a flat substrate such as an aluminum plate, a silver plate, etc.
The concave portions 31 are generally formed in a spherical shape whose depth varies in the range of 0.1 μm to 3 μm, and distances between the concave portions 31 are set such that the pitch between the concave portions 31 (distance between the central points of the concave portions 31) varies in the range of 5 to 50 μm.
An example of a desk calculator is shown in FIG. 11A, and an example of a mobile computer is shown in FIG. 11B. As shown in FIGS. 11A and 11B, when an observer actually views the display surface of a liquid crystal display, he or she often looks up at the display surface from the lower side thereof. More specifically, the viewpoint Ob of the observer is inclined toward the lower side of the display surface by an angle θ relative to the normal X perpendicular to the display surface.
On the other hand, in reflective liquid crystal displays, external light is used for illumination; however, the intensity of the external light is greatly reduced as it passes through the light-modulating layer 25 formed of a polarizing plate, etc., the two transparent electrode layers 16 and 23, the liquid crystal layer 30, the color-filter layer 14, etc., and returns. In addition, when the diffuse-reflection type reflective layer 12″ is used, incident light is widely diffused, so that the display surface appears substantially dark when viewed from the viewpoint Ob. Accordingly, when the intensity of external light is small, the visibility is substantially reduced. In the reflective liquid crystal display of the known art, the shape and arrangement of the concave portions are determined such that variations in brightness caused by the difference in viewing angle are made as small as possible. Thus, there is a problem in that sufficient brightness cannot be obtained when the display surface is observed in a specific viewing-angle range, for example, from the lower side relative to the normal of the display surface. In addition, also in the case in which a front light is used, there are problems in that the intensity of the light is reduced and incident light is diffused as in the case of external light. Accordingly, it has been difficult to ensure sufficient brightness in a specific viewing-angle range without increasing the consumption of electrical power for illumination more than necessary.
Accordingly, reflective liquid crystal displays in which the display surface appears especially bright when viewed in a specific viewing-angle range and back reflection is suppressed over a wide viewing-angle range, have been required.