Liquid crystal display apparatus

A liquid crystal display apparatus includes a liquid crystal display panel, a backlight source behind the liquid crystal display panel, and an optical filter interposed between the liquid crystal display panel and the backlight source for controlling a direction of light passing through the liquid crystal display panel. The optical filter has a plurality of horizontal louvers vertically arranged in a column and perpendicularly disposed to a surface of the liquid crystal display panel. The differences in light reflectivity and transmittance between an upper surface and a lower surface of the louver give the optical filter a half mirror function that controls a direction of a passing light therethrough.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2004-91806 filed on Mar. 26, 2004, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display apparatus for a vehicle, more specifically to a liquid crystal display apparatus having an optical filter for controlling a light passing therethrough.

BACKGROUND OF THE INVENTION

A conventional liquid crystal display apparatus used in, for example, a car navigation system for displaying a map or the like typically has a backlight source behind a liquid crystal display panel. It is disposed at an underside of a windshield and at an upper center of an instrument panel. The positioning of the liquid crystal display apparatus makes it easier for a driver to view an image in the apparatus because it minimizes the movement of the driver's sightline. However, an image displayed on the apparatus reflects on an inside surface of the windshield disposed over the instrument panel during night hours and interrupts the driver's view.

Another conventional apparatus disclosed in a Japanese Patent Document JP-A-H7-306411 has an optical waveguide disposed in front of a backlight source to guide a backlight perpendicular to a liquid crystal display panel. However, the optical waveguide does not effectively limit the backlight projected therethrough in an obliquely upward direction. Thus, the apparatus cannot effectively prevent reflection of the displayed image on the inside surface of the windshield.

Still other conventional apparatus disclosed in a Japanese Patent Document JP-A-H5-61034 has a polarizing film disposed between a backlight source and a liquid crystal display panel to direct the backlight perpendicular to the liquid crystal display panel. This type of polarizing film is available as a marketed product of “Light Control Film” released from corporations such as Sumitomo 3M. This apparatus substantially limits a viewing position of the liquid crystal display panel only in a front area of the apparatus for an increased visibility. Thus, it effectively prevents reflection of the displayed image on the inside surface of the windshield during night hours.

While the liquid crystal display apparatus of this kind is primarily used for driving assistance such as a map display apparatus for a car navigation system or the like, the apparatus is more frequently used for displaying different types of image sources provided as a TV program, a video program, a DVD program or the like. On the other hand, the liquid crystal display panel itself is evolving to have a wider viewing angle for viewer's convenience.

Therefore, an image on the display panel of the apparatus may be viewed obliquely upward from a lower position by a viewer (e.g., a driver or a passenger) with the viewer's seat back reclined toward a rear end of a vehicle when, for example, the vehicle is stopping in a rest area. However, the conventional apparatus having the polarizing film described above cannot accommodate this kind of situation, because the viewing position of the apparatus is strictly limited to a small area in front of the apparatus. The visibility steeply decreases if the position of the viewer moves out of the area.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is an object of the present invention to provide a liquid crystal display apparatus for a vehicle that has an improved visibility from an obliquely downward position while effectively preventing a displayed image from reflecting on an inside surface of a windshield of a vehicle.

An automotive liquid crystal display apparatus of the present invention uses an optical filter between a backlight source and a liquid crystal display panel to achieve the above-described object. The optical filter disposed between the backlight source and the liquid crystal display panel limits a projecting direction of a backlight with a plurality of louvers arranged in a column and disposed perpendicular to the surface of the liquid crystal display panel. The louver in the optical filter is characterized by the difference of its light reflectivity and transmittance for each of a downward light and for an upward light passing through the filter. Filtering the passing light in this manner enables the apparatus to limit the upward light and to allow the downward light.

The backlight of the apparatus is filtered to be perpendicular to the surface of the liquid crystal display panel by the louvers of the optical filter. Therefore, the visibility of the apparatus from a front position is improved. Further, the louver of the filter limits a light passing therethrough obliquely upward from behind the filter and allows a light passing therethrough obliquely downward from behind the filter. Therefore, reflection of the displayed image on the windshield is effectively prevented during night hours, and the visibility of the apparatus from the obliquely downward position of the display panel is improved.

The louver of the optical filter may be structured as a half mirror that practically passes a downward light from above the louver and practically reflects a downward light from under the louver. The intensity of the light passing through this louver downward is increased, because the louver passes a downward backlight therethrough and reflects an upward backlight to proceed in a downward direction. As a result, the visibility from the obliquely downward position of the liquid crystal display panel is improved.

More practically, the louver may be made of a transparent base material combined with a film that passes the downward light from above and reflects the upward light from under, or the louver may be made of a transparent base material with an underlying reflection film having many small windows dispersed for transparency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described hereinafter with reference to the drawings. Referring first toFIG. 1, an automotive liquid crystal display apparatus1according to this embodiment includes a liquid crystal display panel2, a backlight source3, and an optical filter4interposed between the display panel2and the backlight source3. These parts are disposed in a case (or in a frame) not shown in the figure. The display panel2has a wide viewing angle. The optical filter4controls a projecting direction of a backlight from the backlight source3.

The case contains, although not shown in the figure, a driver circuit for the liquid crystal display panel2, a control circuit for the driver circuit, and a video conversion circuit or the like for converting a video signal from a TV tuner or a DVD player to a driving signal. The liquid crystal display panel2has a touch panel, not shown in the figure, on its face. A front side of the case has a control switch and the like in an area around the liquid crystal display panel2.

The liquid crystal display apparatus1is disposed on an upper center face of an instrument panel (not shown in the figure) in front of a driver's seat. The display surface of the liquid crystal display panel2is tilted in order to fully oppose to a face (or an eye) of a driver. A windshield5is disposed above and over the instrument panel. The liquid crystal display apparatus1is connected to a main controller unit of a car navigation system (not shown in the figure), and the liquid crystal display panel2displays an image for navigation such as a map, a current vehicle position or the like. The liquid crystal display apparatus1receives an image signal from the TV tuner or the DVD player as well, and the liquid crystal display panel2displays the image from those sources.

The optical filter4has, as shown inFIG. 2, a substantially thin film shape of the thickness of 0.45 to 0.63 mm, having tiny crosspiece-like louvers6sandwiched by two transparent films7,7made of, for example, PET. The louvers6extend laterally with their faces perpendicular to the surface of the optical filter4. The louvers6are arranged equally spaced in parallel in a vertical column.

The louvers6have a different reflectivity and transmittance for each of a downward light passing therethrough from above and an upward light passing therethrough from under. The downward light from above has a positive entry angle against an upper surface of the louver6, and the upward light from under has a negative entry angle against an upper surface of the louver6. In this embodiment, the louver6having a half mirror structure practically passes a downward light B from above and reflects an upward light C from under, as shown inFIG. 3. That is, the upper surface of the louver6is low in reflectivity (i.e., high in transmittance), and the lower surface of the louver6is high in reflectivity (i.e., low in transmittance). More practically, the louver6is made of a transparent base material (e.g., a transparent plastic film)6awith one side (e.g., lower side) having a selective transparency film6battached thereto. The selective transparency film6bpasses the downward light B therethrough from above but reflects the upward light C therethrough from under.

The liquid crystal display apparatus1is structured to light its display panel2with the backlight source3. The optical filter4(the louver6) in the apparatus makes the backlight directive in a direction perpendicular to the liquid crystal display panel2(as shown by an arrow A inFIG. 1), and viewing position of the display panel2is limited within a front area of the apparatus1with an improved visibility for a viewer in a fully opposing position.

The light C projected obliquely upward from the backlight source3substantially reflects on the lower side of the louver6in the optical filter4because the transmittance of the lower side of the louver6is low. Therefore, the light in a direction obliquely upward (e.g., in a direction of an arrow A′ inFIG. 1) from the liquid crystal display panel2is limited. This structure effectively prevents the image on the liquid crystal display panel2from reflecting on the windshield5when the outside of the vehicle is dark during night hours.

The light B projected obliquely downward from the backlight source3substantially passes the optical filter4to become light B′ as shown inFIG. 3. This makes the liquid crystal display panel2viewable from a downward position, and visibility of the display panel2from this position is secured. Moreover, the intensity of the light projected toward an obliquely downward direction from the display panel2is increased because the reflection of the light C from the lower surface of the louver6becomes light C′ to be added to the light B′. Therefore, the visibility from the downward position of the display panel2is substantially increased.

As a result, a viewer (a driver D) can have an improved visibility of the image displayed on the liquid crystal display panel2in the apparatus1when the viewer watches the image of a TV program, a DVD movie or the like from the downward position of the display panel2, sitting in a position reclined on the seat back of a vehicle while stopping in, for example, a rest area.

The optical filter4may be disposed with its louvers6having an angle other than a right angle to the surface of the display panel2(e.g., parallel to the light B′ and the light C′ inFIG. 3) if the filter4is used only for limiting the light C projected obliquely upward from the backlight source3beside being transparent for the light B. However, this structure limits a viewing angle of the display panel2to an area in a lower position of the display panel2and the visibility from a full frontal position is decreased.

The optical filter4may possibly be disposed in front of the liquid crystal display panel2. That is, the filter4may be disposed between the display panel2and the touch panel. However, this structure results in a low viewing quality such as a decreased contrast of the displayed image, an exposure of the edge of the display panel2and the like. Therefore, the optical filter4disposed between the display panel2and the backlight source3with the louvers6perpendicular to the surface of the display panel2is more suitable.

According to the present embodiment, the optical filter4with louvers6perpendicular to the surface of the display panel2is disposed between the liquid crystal display panel2and the backlight source3. The filter4practically passes the downward light B from above the louver6and practically reflects the upward light C from under the louver6to effectively prevent the displayed image on the display panel2from reflecting onto the windshield5while improving the visibility of the display panel2obliquely from the downward position of the apparatus1.

Another embodiment of the present invention is shown inFIG. 4. The difference of this embodiment from the first one exists in a structure of louvers12in an optical filter11. That is, the optical filter11has many equally spaced small louvers12arranged vertically in a column with their surfaces perpendicular to the surface of the optical filter11. Each louver12is made of a transparent base material12awith an underlying reflection film12battached thereto and having many transparent small holes (i.e., light windows)12cdispersed (evenly spread) therein.

The louvers12have a different reflectivity and transmittance for each of the light passing downward and the light passing upward therethrough. That is, an upper surface of the louver12is low in reflectivity (i.e., high in transmittance), and a lower surface of the louver12is high in reflectivity (i.e., low in transmittance). Therefore, the optical filter11practically passes the downward light B from above and practically reflects the upward light C from under. Thus the reflection of the displayed image in the liquid crystal display panel2onto the windshield5is effectively prevented while the visibility obliquely from the downward position of the display panel2is substantially improved.

In place of adhering a film for a half mirror structure in the above embodiment, a metal foil may be disposed on one side of the base material (e.g., a resin film) by vapor deposition. The base material of the louver may be half-transparent or may be stained to have a color. The louver itself may be made of a material having a different reflectivity and transmittance for each of an obverse surface and a reverse surface.