Amongst various types of display device, a practical application of a liquid crystal display device (Hereinafter, LCD) has been developed in recent years. This is because the LCD which uses liquid crystal is capable of performing a screen displaying with a low power consumption. As a display mode of the LCD and as a driving method, two methods are suggested. These methods are a simple matrix method and an active matrix method. Meanwhile, along with a progress in the development of multimedia technology for information communications, there is an increasing needs for a higher resolution, a higher contrast, and a wider view-angle of a display, and an ability to display multiple grayscale levels (i.e., an ability to perform multiple color displaying or full color displaying). Under such circumstances, it appears that such needs are difficult to be satisfied by using the simple matrix method. In view of that, there is suggested the active matrix method in which a switching element (active element) is provided to each pixel, for increasing the number of drivable scanning line electrodes.
With this technology of active matrix method, the resolution, contrast, and view-angle of the display have been improved, and the number of grayscale levels to be produced has been increased. A liquid crystal display device of the active matrix method includes: pixel electrodes arranged in a matrix manner; scanning lines extended nearby the pixel electrodes, the scanning lines electrically connected with the pixel electrodes via switching elements. The switching element can be a nonlinear element having two terminals, or a nonlinear element having three terminals. A representative switching element currently adopted is a thin film transistor (Hereinafter, TFT) which is a three terminal element.
Further, in recent years, the active matrix LCD is rapidly growing its popularity amongst mobile devices such as a mobile phone.
Incidentally, a conventional transmissive LCD performs a transmissive display by using a backlight unit arranged on a back surface of a liquid crystal display panel. This method results in a good screen displaying, when used under an indoor environment or the like, where surrounding light is relatively weak. However, when using such a transmissive LCD under an outdoor environment or nearby a window, where the transmissive LCD is directly exposed to strong surrounding light such as the sunlight, the surrounding light is reflected from a surface of the liquid crystal display panel and/or internal wiring. Since an amount of the reflected light surpasses the light emitted from the backlight unit and transmitted through the liquid crystal display panel, it has been virtually impossible to view a displayed item (i.e. to obtain a good visibility) under an environment where the surrounding light is strong.
In view of that problem, a reflective LCD device and semi transmissive LCD device have been developed, so as to obtain a good visibility even under the strong surrounding light environment. The reflective LCD device and the semi transmissive LCD device have a reflecting section (reflecting plate) which performs the screen displaying by reflecting the surrounding light within the liquid crystal panel. In such a configuration, the brightness is acquired, but the color purity is deteriorated. Further, the brightness will become inadequate, if a density of a color filter corresponding to the reflecting section is raised. Accordingly, it has been virtually impossible to realize a beautiful displaying of an image, by using the reflecting section.
For example, Patent document 1 (hereinafter, referred to as conventional example 1) or Patent document 2 (Hereinafter, referred to as conventional example 2) discloses an LCD device with a light converging mechanism. This LCD device includes means for converging external light (surrounding light) on to a light-guiding plate which is provided on a back surface of the transmissive LCD device. Each of the LCD devices disclosed in these patent documents has a light converging section (light inlet section) which is formed, in a lens-like shape, on an end portion of the light-guiding plate.
In the LCD device of conventional example 1, a first light irradiating section serving as a wedge-shaped converging section and a second light irradiating section serving as a wedge-shaped light source section are laminated on an LCD panel, so that the respective thicknesses are complemented. On the other hand, the LCD device of the conventional example 2 is provided with a light-guiding section behind an LCD, and the light converging section for connecting the light path with this light-guiding section. The light-guiding section has on its periphery a mirror finished surface, except for (a) a surface from which light is emitted toward the LCD and (b) an aperture to which light from the light converging section enters. In both of the LCD devices of the conventional examples 1 and 2, the external light converged by the light converging section is guided and diffusely reflected by the light-guiding section (light-irradiating section). Thus the external light is irradiated as a plane light source to the LCD.
Further, as a liquid crystal display device which performs a displaying process by using the surrounding light, there is an LCD device (Hereinafter, conventional example 3) whose back surface side is made transparent by removing a reflection sheet arranged on an opposite side (back surface side to) to the side of a liquid crystal panel in the light-guiding plate, used in the transmissive LCD device. In this configuration, sufficient aperture for taking in the light is acquired, because the surrounding light enters from the back surface of the liquid crystal display panel. As a result, a good display can be performed under a strong surrounding light environment.
Further, for example, Patent document 3 discloses another example of the LCD device which performs the displaying operation by using the surrounding light, the LCD device including a semi-transmissive plate on a back surface of a light-guiding member. The LCD device disclosed in the Patent document 3 (Hereinafter, conventional example 4) is a transmissive LCD device which performs the displaying operation by using a backlight. In this transmissive LCD device, a semi-transmissive plate (semi-transmissive member) and a light-shielding-use liquid crystal element (TN type liquid crystal element) are arranged at the back of the light-guiding member. That is, in the LCD device, the light-shielding-use liquid crystal element is provided on an opposite side to the side of the light-guiding member in a semi-transmissive plate, so as to allow switching over operation between (i) a light-transmitting mode which transmits the external light, and (ii) a light-shielding mode which blocks the external light. By switching over the light-shielding-use liquid crystal element to the light-transmitting mode, the external light from the back surface can be used. Further, by switching the light-shielding-use liquid crystal element to the light-shielding mode, viewing of a displayed item from the back surface side is prevented, thus protecting privacy of a user. Further, with the use of the semi-transmissive plate, it is possible to use the light emitted from the back surface side of the light-guiding member.
(Patent document 1)
Japanese Unexamined Patent Publication No. 11-52374/1999 (Tokukaihei 11-52374; published on Feb. 26, 1999)
(Patent document 2)
Japanese Unexamined Patent Publication No. 11-95199/1999 (Tokukaihei 11-95199; published on Apr. 9, 1999)
(Patent document 3)
Japanese Unexamined Patent Publication No. 9-265069/1997 (Tokukaihei 9-265069; published on Oct. 7, 1997)
However, in the above described LCD devices, there are following problems.
First, in the LCD device of the conventional examples 1 and 2, a lens-like shape is formed on the end portion of the respective light-guiding section (light irradiating section). As such, an amount of the surrounding light taken in by using the lens is proportional to an area of the lens section. Accordingly, the area of the lens section must be increased, in order to take in a larger amount of the surrounding light. This necessitates an increased thickness of the light-guiding section having the lens section. However, since an actual thickness of the light-guiding section can be increased up to several millimeters, it is impossible to use sufficient amount of surrounding light. Accordingly, the LCD devices are not capable of effectively using the surrounding light for performing a good screen displaying.
Further, in the LCD device of the conventional example 3, the back surface side of the liquid crystal display panel is transparent. Thus, it is possible to view, from the back surface side, the displayed screen on the liquid crystal display panel. This is disadvantageous in terms of privacy protection.
On the other hand, the LCD device of the conventional example 4 includes the light-shielding-use liquid crystal element, so as to use the surrounding light while protecting the privacy. However, under the environment where the surrounding light is strong, the problem regarding the privacy is not solved when the light-shielding-use liquid crystal element is switched to the light-transmitting mode. That is, the LCD device of the conventional example 4 is not able to protect the privacy, while improving visibility under the environment of the strong surrounding light at the same time. Further, the LCD device of the conventional example 4 includes the semi-transmissive plate arranged between the light-guiding member and the light-shielding-use liquid crystal element, so as to more efficiently use, while the light-shielding-use liquid crystal element is in the light-shielding mode, the light emitted from the light-guiding member in a direction toward the back surface. However, the transmissivity of the semi-transmissive plate is low. Accordingly, the efficiency of using the surrounding light is deteriorated, while the light-shielding-use liquid crystal element is in the light-transmitting mode.
In view of the above problems, the present invention is made, and it is an object of the present invention to provide a liquid crystal display device which allows a good screen displaying even under a strong surrounding light environment, while protecting privacy of a user.