The present invention relates to a direct viewing type liquid crystal display device which is employed in a liquid crystal television set, a liquid crystal display for a computer and the like.
In recent years, the technical progress of a direct viewing type liquid crystal display device is significant especially in a device employing a color display element. There are many displays having a display quality which is comparable to that of a CRT. In the black-white display, until several years before, the main stream of technology had been a reflecting type liquid crystal display element which did not employ a backlight. However, currently, almost all of them are replaced by a transmitting type liquid crystal display element in use of a backlight even in the black-white display. In the color liquid crystal display, the display can not work without a backlight, and the backlight is an indispensable device in the direct viewing type liquid crystal display device.
In a so-called xe2x80x9cnotebook type personal computerxe2x80x9d which has come in use in recent years, the portability is important, and therefore, driving thereof by a battery is indispensable. However, currently, the time capable of driving the device without charging the battery is only several hours, and does not reach a level wherein a day""s operation can continuously be performed. From this viewpoint, the prolongation of the time of continuous use is extremely important. Especially, the illumination device is a device consuming much power in that system, and reducing the power consumption of the illumination device is of great significance.
In the mean time, there is a specific contrast ratio distribution in accordance with the viewing angle in a liquid crystal display element employed in the notebook type personal computer. A representative example is shown in FIG. 5 in case of a super twisted nematic liquid crystal display element. FIG. 5 shows the representative viewing cone of the super twisted nematic liquid crystal display element and a diagram designating a change in the contrast ratio of a section on a horizontal line passing through the center of the contrast ratio of the viewing cone. According to FIG. 5, the viewing angle of the liquid crystal display element is widened from a direction perpendicular to the image plane substantially by 40xc2x0 through 50xc2x0, and it is revealed that there is a region having an especially high contrast ratio in the vicinity of the center.
In an actual use, the device is often set such that the contrast ratio is maximized on the image plane viewed from an operator in case of the notebook type personal computer. Accordingly, the illuminating efficiency can substantially be promoted, when the maximum brightness of illumination is formed in the viewing direction (mostly a direction perpendicular to an image plane or a direction a little deviated therefrom) which causes the maximum contrast ratio.
The viewing angle of the liquid crystal display element employed in the notebook type personal computer is widened substantially to not less than 40xc2x0 through 50xc2x0, and investigations are being performed for widening the viewing angle. Therefore, it is important to adjust the light direction distribution of the illumination device so that the display can be viewed from a more or less oblique direction.
The adapting of the brightness distribution of the illumination device to this contrast ratio distribution in such a manner is significant as a means for promoting substantial brightness.
The color liquid crystal display device is grossly classified into two systems, namely, a twisted nematic (TN) liquid crystal display device by the active matrix driving using TFTs and a super twisted nematic (STN) liquid crystal display device by the multiplex driving. Both are provided with a construction wherein polarizing sheets are disposed on the light-incident side and the light-emitting side of an element wherein the liquid crystal layer is hold by glass substrates, and the liquid crystal display system is operated by modulating the polarization state of an incident linearly polarized light.
However, the direction of polarization of the incident light of the conventional liquid crystal display element, is not uniform due to the randomly polarized light. Therefore, more than half of the incident light is absorbed by the polarizing sheet disposed on the light-incident side of the display element, and does not substantially contribute to the illuminating light. It is proposed as a structure for reusing the light to be absorbed by the polarizing sheet, wherein a polarized light separator is interposed between a light source lamp and a liquid crystal display device in a projecting type liquid crystal display element, for separating a non-polarized light to mutually orthogonal polarized lights, one polarized light is directly emitted from the polarized light separator, and the other polarized light is converged to the light source lamp and is reused as a light source light (Japanese Unexamined Patent Publication No. 184429/1992).
However, this method is performed on the premise of a projector (projecting type), and it is necessary to provide a sufficient distance between the light source and the polarized light separator. Further, the device effectively functions as an illumination for the projecting type liquid crystal display element, only when the light is a considerably collimated parallel ray. Accordingly, the device is not suitable to adopt as the illumination for a direct viewing type display element wherein thinning thereof is an indispensable condition, and the brightness distribution of the illumination device should adapt to the contrast ratio distribution of the liquid crystal display.
Further, it is proposed that a prism array is interposed between a light source for illumination and a display element, as a means for converging light in a direction perpendicular to a display face. However, according to this means, the brightness in the direction perpendicular to the display face is promoted by narrowing the direction of an illuminating light in a specified range, and therefore, the light direction distribution of the illuminating light is narrowed. Further, the brightness in the direction perpendicular to the display face is not sufficient even by this means. Accordingly, an illuminance distribution suitable for the direct viewing type liquid crystal display element can not be provided.
In this invention, attention has been paid to the fact wherein only a polarized light having a specific direction of polarization contributes to the promotion of the illuminance of the directly viewing type liquid crystal display element, and a light in a specific direction is selectively converted its polarization direction, among polarized lights which do not contribute to the promotion of the illuminance of the liquid crystal display element by themselves. In this way, the intensity of light can be enhanced in the specific direction while maintaining a wide light direction distribution, with respect to a polarized light having a direction of polarization which can contribute to the promotion of the illuminance, and the light distribution is suitable for the directly viewing type liquid crystal display.
According to a first aspect of the present invention, there is provided an illumination device for a direct viewing type display element comprising:
a flat light guide;
a light source set such that light is incident on a side portion of said flat light guide;
a polarized light separating sheet set on a first side of a light emitting side of the flat light guide for transmitting a p polarized light component and reflecting at least a portion of an s polarized light component with respect to a light ray substantially having a predetermined direction of incidence; and
a light reflecting sheet disposed on a second side opposite to said light emitting side of the flat light guide in parallel with the light emitting site.
According to a second aspect of the present invention, there is provided the illumination device for a direct viewing type display element according to the first aspect, wherein the polarized light separating sheet is comprising a multi-layered structure wherein light transmitting media having a relatively large refractive index and light transmitting media having a relatively small refractive index are laminated.
According to a third aspect of the present invention, there is provided the illumination device for a direct viewing type display element according to the first aspect, wherein the polarized light separating sheet comprises a transparent supporter and at least one dielectric thin film laminated on said transparent supporter having a thickness which is equal to or smaller than a wavelength of visible light.
According to a fourth aspect of the present invention, there is provided the illumination device for a direct viewing type display element according to the first aspect, wherein the polarized light separating sheet comprises a plurality of laminated transparent polymer layers having different refractive indices.
According to a fifth aspect of the present invention, there is provided a liquid crystal display device, wherein the illumination device according to Claim 1 is disposed on a rear side of a direct viewing type liquid crystal display element such that a principle polarization direction of emitted light from the illumination device substantially agrees with a direction of an optical axis of polarization of a polarizing sheet on a light-incident side of a liquid crystal display element.
According to a sixth aspect of the present invention, there is provided the liquid crystal display device according to the fifth aspect further comprising:
a light reflecting means disposed between the polarized light separating sheet and the liquid crystal display element for deflecting a direction of a light ray maximizing a light intensity among light distributing directions to a direction substantially perpendicular to a display face of the liquid crystal display element.
According to a seventh aspect of the present invention, there is provided the liquid crystal display device according to the fifth aspect, further comprising:
a means for rotating polarization direction disposed between the illumination device and the liquid crystal display element for rotating the principle direction of emitted light.
According to an eighth aspect of the present invention, there is provided an illumination device for a direct viewing type display device comprising:
a flat light emitting means for emitting a diffused light including a first polarized light component having a first direction of polarization and a second polarized light component having a second direction of polarization perpendicular to the first direction of polarization; component and
a polarized light converting means disposed in an optical path of a light emitted from said flat light emitting means for emitting said first polarized light component and for emitting at least a portion of said second polarized light component after converting said portion of the second polarized light component into the first polarized light component selectively with respect to a light ray substantially having a light direction maximizing a brightness thereof.