Backlight device for display apparatus

Abstract: A backlight device suitable for illuminating a transmission-type display device, such as a liquid crystal device, is constituted by a reflection member (10) for reflecting light, a transmission member (11) disposed opposite to the reflection member (10) so as to form a space ( S ) from the reflection member (10), and a light source (1) disposed to emit light into the space ( S ). Light issued from the light source (1) into the space ( S ) is reflected by the reflection member (10) and transmitted through the transmission member (11). The transmission member (11) is preferably provided with a reflection layer (12) having apertures (13) having an aperture ratio which increases with an increase in distance from the light source (1) toward a centre of the space ( S ) as a linear function, a hyperbolic function or a function intermediate therebetween of the distance from the light source (1). Between the transmission member (11) and a display device, a prism sheet (16,17) and/or a diffusion sheet may be disposed so as to improve the frontal illumination performance.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a backlight device for display apparatus, such as those for computers, word processors, television receivers and navigation systems, and view finders for video cameras. Backlight devices or display apparatus may be classified into the direct backing-type and the edge (or H-arrangement)-type. Examples of the former type have been disclosed in Japanese Laid-Open Patent Appln. (JP-A) 2-39118 and JP-A 6-18873, and examples of the latter type have been disclosed in JP-A 63-13202, JP-A 4-71105, JP-A 5-281541 and JP-A 5-323318. A representative example of the H-arrangement-type backlight device for a transmission-type liquid crystal panel has been proposed by JP-A 57-128383 and Figure 10 is a schematic illustration thereof. Referring to Figure 10, a backlight device B 0 includes a fluorescent lamp 1 as a light source and a reflector 2 disposed to surround the fluorescent lamp 1. Extending sidewards from the fluorescent lamp 1 is disposed a light-guide member 3 of a thick transparent plate of, e.g., acrylic resin. Behind the light-guide member 3, light-scattering layers 5 are disposed in a prescribed areal proportion. Light issued from the fluorescent lamp 1 and entering the light-guide member 3 is scattered by the scattering layers 5 and issued from the front face of the light-guide member 3 to illuminate a liquid crystal panel (not shown) at a uniform illuminance. In the above-mentioned backlight device, the light-guide member 3 is composed of, e.g., an acrylic resin, and is therefore heavy, so that the backlight device becomes heavy and suffers from inferior conveyability. Particularly, in recent years, a liquid crystal panel size is enlarged so that the backlight is also enlarged. Accordingly, the light-guide member is thick and enlarged in area, so that the above-mentioned difficulty is promoted. For the above reason, a backlight device using no light-guide member has been proposed as in JP-A 5-323318. The backlight device has resulted in a poor planar illumination uniformity and is unsuitable for a large area display apparatus having a diagonal size of 38 cm (15 inches) or larger. Further, the backlight device is required have a backing reflector shaped in a parabolic curved surface and requires a substantial production cost. JP-A 5-281541 also discloses a backlight device capable of dispensing with a light-guide member, but the device also suffers from a poor planar illumination uniformity, which becomes noticeable in a large-area display apparatus having a diagonal display area size of 38 cm (15 inches) or larger. On the other hand, direct backing-type backlight devices generally require a large thickness behind display apparatus and are unsuitable for flat panel displays. Particularly, uniform illumination light cannot be attained for large area display apparatus unless a large number of light sources (fluorescent lamps) are used. Anyway, direct backing-type backlight devices are liable to be complicated and expensive for a larger size and are believed to be unsuitable for flat panel-type display apparatus of an increasingly large display area. Accordingly, it has been desired to develop an improved edge-type or H-(arrangement-)type device, through elimination of various difficulties of the edge-type backlight device. An edge-type backlight device having an improved illumination efficiency is disclosed in United Kingdom Patent Application GB-A-2259359. The backlight device disclosed therein is provided with spaced parallel panels, namely a transmissive front plate and a reflective backplate. A strip light source is located at each end of the device and is arranged to emit light into the space between these panels. This light is multiply reflected between these panels and emerges from the transmissive front plate. To improve illumination efficiency, the frame housing the strip light sources is provided with inclined inner reflective surfaces which are arranged to redirect light from the light sources to the edges of the panels. Also, to improve illumination uniformity, the transmissive front plate bears a diffused reflection pattern of deflective dots, the density of which is increased in a non-stepwise manner from the strip light sources in the in-plane direction, so that the dot density is maximised at the central region between the light sources. These dots are provided with minuscule through-holes to permit diffuse transmission. The improvement of illumination efficiency is considered herein and accordingly there is now provided a generally rectangular planar backlight device having front and rear principal plane faces, comprising: a light source disposed along a side of the backlight device; a polarization beam splitter disposed along the front principal plane face of the backlight device for selectively transmitting one of a p-polarization component, having an electric field component perpendicular to the polarization beam splitter, and an s-polarization component, having an electric field component parallel to the polarization beam splitter, of the light emitted from the light source, in use, and for reflecting the other of the p-polarization component and the s-polarization component; reflection and polarization conversion means disposed along the rear principal plane face of the backlight device, for reflecting light and effecting polarization conversion between the s-polarization component and the p-polarization component; whereby light emitted from the light source into a region between the polarization beam splitter and the reflection and polarization conversion means is reflected between the polarization beam splitter and the reflection and polarization conversion means so that the p-polarization component or the s-polarization component of the emitted light is selectively leaked outwards through the polarisation beam splitter. The reflection and polarization conversion means comprises a reflection means, for example, may comprise a reflection means disposed along the rear principal plane face of the backlight device arranged to reflect light into the backlight device and a ¼ wavelength plate for effecting polarisation conversion disposed adjacent the reflection means on the side of the reflection means facing the polarization beam splitter. Insofar as the aforesaid backlight device includes a ¼ wavelength plate, it is acknowledged that it has been known to incorporate such plates in direct-backing type devices. An example of such a device is disclosed in European Patent Application EP-A-0573905. That device, which is incorporated as part of an overhead projection (OHP) system, comprises a light source with a curved reflector and a retroreflecting polarizer interposed between a liquid crystal device panel and the light source. The retroreflecting polariser transmits the p-polarisation component of the light, but reflects the s-polarisation component. A ¼ wavelength plate is used to convert the retroreflected s-polarisation component into p-polarised light which is then recycled through the polariser. Additionally, and as means of improving illumination uniformity, a scattering layer may be disposed between the reflection means and the ¼ wavelength plate. In this preferred arrangement, the reflection means is provided with reflectors having a dot pattern or a mesh pattern so as to have an aperture ratio increasing with increase in distance from the light source, and the scattering layer also is formed in a dot or mesh pattern so as to provide a prescribed aperture ratio as a function of distance from the light source. These and other features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, wherein like parts are denoted by like reference numerals. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic exploded illustration of a backlight device according to a comparative example to the present invention. Figures 2 -5 are sectional views for illustrating first and second comparative examples to the invention and first and second embodiments of the invention, respectively, of a backlight device. Figures 6 and 7 are graphs showing an aperture ratio distribution in a reflection layer and a luminance distribution, respectively, of a third comparative example of a backlight device. Figures 8 and 9 are respectively a block diagram of a display apparatus using a backlight device according to the invention. Figure 10 is a schematic sectional view of a conventional backlight device. Figures 11 and 12 are respectively a plan view showing a preferred reflection layer pattern. DESCRIPTION OF THE PREFERRED EMBODIMENTS AND COMPARATIVE EXAMPLES A comparative example to the present invention will be described with reference to Figure 1, an exploded perspective view of a backlight device. Referring to Figure 1, a transmission-type display device P is illuminated by a backlight device according to the invention including light sources 1, reflectors 2, a reflection means 10 and a transmission member 11 having a pattern of reflection layer. The light sources are disposed outside the display device P to form an edge-type or H(-arrangement)-type backlight device. In other words, when the light light sources 1 are constituted by a pair of lamps, the lamps are disposed with a spacing of at least M which is a lateral length of the display device P. The light source 1 may be any light source having a spectral distribution desired by the display device P. Specific examples thereof may include LEDs, halogen lamps, xenon lamps and white fluorescent lamps. Particularly, for a display device having three types of pixels of red (R), green (G) and blue (B), it is preferred to use a three-wavelength type white fluorescent lamp having a spectral characteristic of providing emission peaks in the regions of R, G and B, respectively. The reflectors 2 and reflecting means 10 may for example comprise a metal member having a reflecting inner surface or a light-absorbing or light-transmissive substrate coated with a reflective material forming an inner surface. The transmission member 11 may comprise a light-transmissive substrate surface-coated with pattern of reflection layer. The reflection layer pattern may preferably have apertures (portions free from the reflection layer) occupying an areal ratio (i.e., aperture ratio) which increases with an increase in distance in X-direction from a light source 1. In the embodiment of Figure 1 having two light sources 1 at both ends in the X-direction, the aperture ratio is determined based on distances of 0 to M/2 from the light sources at the left and right ends. The reflection layer may have a pattern of a multitude of reflective portions of various shape, such as circles, ellipses, squares, rectangles, lozenges, parallelograms, trapezoids or stars. Alternatively, as a negative or complementary pattern to the above-mentioned reflective portions, the apertures can assume various discrete shapes as descried above. The display device P may suitably comprise a liquid crystal device, examples of which may include: STN-type and DSTN-type liquid crystal devices using nematic liquid crystals; active matrix-type liquid crystal devices using thin-film transistors or MIM-elements; and ferroelectric liquid crystal devices and anti-ferroelectric liquid crystal devices using chiral smectic liquid crystals. In the case of a liquid crystal device using a chiral smectic liquid crystal, there has been reported a liquid crystal molecular movement resulting in a change in liquid crystal layer thickness and causing a yellow-tinging or a display area (U.S. Patent No. 5,381,256). Accordingly, if the spectral characteristic of illumination light is changed to have a yellowish tint, the yellow tinging is liable to be enhanced. For this reason, it is preferred to use a backlight device of the present invention free from spectral characteristic change in combination with a chiral smectic liquid crystal device for obviating the above difficulty. In addition to the structure shown in Figure 1, the backlight device according to the present comparative example may preferably further comprise a prism sheet so as to have the direction of light issuance therefrom approach a planar normal. In this instance, it is particularly preferred to use a plurality of prism sheets so that their convex surfaces are directed toward a display device side as will be described with reference to Figure 8 or use a prism sheet disposed to have its convex surface directed toward a reflecting means 10 as will be described with reference to Figure 14. Further, it is also possible to provide the reflecting means with a reflecting surface shaped so as to reduce the number of reflections thereat as will be described with reference to Figures 17 and 18, while a shaping step may be additionally required thereby. A backlight device B 1 shown in Figure 2 is provided with a rear reflecting plate (reflection means) 10 composed of an aluminium plate. The rear reflecting plate 10 has a mirror-finished surface so as to reflect light incident thereto. The reflecting plate is integrally formed with reflectors 2 having a concave surface composed also of an aluminum plate. In front of the rear reflecting plate 10, a front transmission plate (transmission member) 11 of a thin transparent acrylic resin plate is disposed with a spacing from and in parallel with the reflecting plate 10, so that a space S is defined by the rear reflecting plate 10 and the front transmission plate 11. At the lateral ends (e.g., the parts surrounded by the reflectors 2), three wavelength-type fluorescent lamps 1 as linear light sources are disposed so that light emitted therefrom enters the space S directly or indirectly after reflection by the reflector 2. On the other hand, on the lower surface (i.e., opposite to the rear reflection plate 10) of the front transmission plate 11, a reflection layer 12 of, e.g., vapour-deposited aluminium is disposed in a pattern of a mesh or various discrete shapes as described above to form a multitude of apertures 13, out of which light is allowed to leak out through the front transmission plate 11. In this example, the apertures 13 are arranged to have an aperture ratio k (x), i.e., a ratio of an area occupied by the apertures 13 per unit region of the front transmission plate 11 at a position disposed at a distance x from the nearest fluorescent lamp 1, which aperture ratio increases with an increase in the distance x , e.g., in a relationship of 1-k(x) = a/x, wherein a is a proportional constant. Above the front transmission plate 11 is disposed a transmission-type liquid crystal panel (not shown) so that the liquid crystal panel is illuminated with the light issued from the backlight device B 1 . When the fluorescent lamp 1 is turned on, light emitted from the fluorescent lamp enters the space S directly or after being reflected by the reflector 2. The light having entered the space is repetitively reflected by the reflecting layer 12 and the rear reflection plate 10 disposed opposite to each other, so that the space S functions as a light-guide space. Then, a portion of the light is leaked out through the apertures 13 of the reflection layer 12 and the transparent plate 11 to illuminate a liquid crystal panel (not shown) disposed above the plate 11. The angle (denoted by " 1 " in Figure 2) of light incident to the transmission plate 11 becomes smaller as the position leaves away from the fluorescent lamp 1 so that leakage quantity of the light becomes smaller in proportion to almost the angle  1 , if the aperture ratio k is constant. In this example, however, the aperture ratio k(x) of the reflection layer 12 is set to be larger as the distance x increases, so that the leakage light quantities at various positions become substantially equal. As a result, the planar luminance distribution of the leakage light (illumination light) from the backlight device B 1 becomes uniform to provide an improved display quality to the liquid crystal panel. In this example, the light-guide member 3 (Figure 21) required in the conventional device is omitted, so that the backlight device becomes lighter in weight by that much and provides an improved portability to even a large-area display apparatus. A second comparative example to the present invention will now be described with reference to Figure 3, wherein parts identical to those shown in Figures 1 and 2 are denoted by identical reference numerals and descriptions thereof may be omitted. In this example, a front surface (opposite to a transmission plate 11 and a reflection layer 12) of a rear reflection plate 10 is provided with a scattering layer 20. The scattering layer 20 may be composed of a printed layer of a white pigment, such as barium sulphate, calcium carbonate or titanium oxide and, likewise the reflection layer 12, may be formed in a mesh pattern or a discrete dot pattern so as to provide a multitude of apertures 21. The aperture ratio k 1 (x) of the scattering layer 20 is set to be smaller as the distance from the fluorescent lamp increases, in contrast to the case of the reflection layer 12. When the fluorescent lamp 1 is turned on, emitted light is repetitively reflected between the rear reflecting plate 10 and the reflection layer 12, and light incident to the scattering layer 20 is reflected as scattered light. Then, the light having repeated the reflection and scattering is leaked out through the apertures 13 of the reflection layer 12. In this example, similarly as in. the previous example, the aperture ratio k(x) of the reflection layer 12 is set to be larger as the distance x from the fluorescent lamp 1 becomes larger, so that the leakage light quantities at various positions become almost equal. As a result, the leakage light (illumination light) from the backlight device B 2 is caused to have a uniform planar luminance distribution, thereby providing an improved display quality to the liquid crystal panel. Further, in this example, light in the space S is scattered by the scattering layer 20 formed on the surface of the rear reflecting plate 10. As a result, even if the incident angle  2 of light incident to the scattering layer 20 is small, the light scattered from the scattering layer 20 can enter the front transparent plate 11 at a larger incident angle  3 , so that the incident angle  3 can be uniform and less dependent on the distance from the fluorescent lamp 1, and leakage light quantities at various positions become almost uniform because of such uniformized angle characteristic. Accordingly, the planar luminance distribution of the leakage light (illumination light) is uniformized to provide an improved display quality to the liquid crystal panel. In an H-type (or edge-type) backlight device like this example, the incident light angle  2 becomes smaller as the position is left away from the fluorescent lamp 1. However, in this example, the aperture ratio k 1 (x) of the scattering layer is set to be smaller so as to provide a larger occupation areal ratio (1-k 1 (x)) of the scattering layer 20 as the position is left away from the fluorescent lamp 1. As a result, light having a smaller incident angle  2 is more frequently scattered. Then, the scattered light is incident to the transmission plate 11 at a larger incident angle  3 to be more readily leaked out through the apertures 13 of the reflection layer 2. As a result, leakage light quantities at various positions of the backlight device is uniformized, so that the planar luminance distribution of the leakage light (illumination light) becomes uniformized to provide an improved display quality. Thus, the backlight device can exhibit an improved performance for a larger area display apparatus. An embodiment of the backlight device according to the present invention will now be described with reference to Figure 4, wherein parts identical to those shown in Figures 1 to 3 are denoted by identical reference numerals and descriptions thereof may be omitted. In this embodiment, a front surface (upper surface) of a rear reflection plate 10 is provided with a λ/4 plate 30 for effecting plarization conversion between an s-polarization component (light component having an electric field component perpendicular to the drawing sheet of Figure 4) and a p-polarization component (light component having an electric field component parallel to the drawing sheet of Figure 4), and a lower surface of a front transmission plate 11 is provided with a polarization beam splitter layer 31 of an optical multi-layer film, which allows the transmission of only the p-polarization component and reflects the s-polarization component. In this embodiment, a liquid crystal panel (not shown) disposed above the plat 11 is designed to receive the p-polarized light as illumination light. When the fluorescent lamp 1 is turned on, emitted light is repetitively reflected between the rear reflecting plate 10 and the polarization beam splitter layer 31, and a p-polarization component of the light incident to the polarization beam splitter layer 31 is transmitted through the layer 31 and the front transmission plate 11 to be leaked outwards. On the other hand, the remaining s-polarization component is reflected by the polarization beam splitter layer 31 and then incident to and reflected by the rear reflection plate 10 where the s-polarization is converted to p-polarization because it passes twice the λ/4 plate 30. The resultant p-polarization component is transmitted through the polarization beam splitter layer 31 and the front transmission plat 11 to be leaked outwards. As the liquid crystal panel is designed to receive a p-polarized light in this embodiment, the illumination efficiency of the backlight device can be remarkably improved. Next, a second embodiment of the backlight device according to the present invention will be described with reference to Figure 5 wherein parts identical to those shown in Figure 4 are denoted by identical reference numerals. In this embodiment, a front transmission plate 11 is composed of a transparent glass plate and the front surface of the front transmission plate 11 is provided with a reflection layer 12. Further, between a rear reflection plate 10 and a λ/4 plate 30, a scattering layer 20 is disposed. The reflection layer 12 is formed in a dot pattern or a mesh pattern so as to have an aperture ratio k(x) increasing with an increase in distance from the fluorescent lamp 1 similarly as in the first embodiment, and the scattering layer 20 is formed in a dot or mesh pattern so as to provide a prescribed aperture ratio k 1 (x) similarly as in the second embodiment. When the fluorescent lamp 1 is turned on, an s-polarization component is reflected by a polarization beam splitter layer 31 and is then reflected by a rear reflection plate 10 where it is converted into a p-polarization component. On the other hand, a p-polarization component is transmitted through the polarization beam splitter 31, a portion thereof is leaked out of the apertures 13 to illuminate a liquid crystal panel (not shown) disposed above, and the remaining portion thereof is reflected by the reflection layer 12 to enter the space S again. The portion of the p-polalization component reffected by the reflection layer 12 is converted into an s-polarization component when it is reflected by the rear reflection plate 10. The s-polarization component is again reflected by the polarization beam splitter layer 31 and the rear reflection plate 10, where it is again converted into a p-polarization component allowed to be leaked outwards similarly as the above for illuminating the liquid crystal panel. As the aperture ratio k(x) of the reflection layer 12 is set to be larger as thesdistance x from the fluorescent lamp 1 increases, the light leakage quantities at various positions become almost equal, so that the leakage light (illumination light) from the backlight device is caused to have a uniform planar luminance distribution, thereby providing an improved display quality to the liquid crystal panel. Further, as light from the fluorescent lamp 1 is scattered by the scattering layer 20, the illumination light angle characteristic is uniformized regardless of the distance from the fluorescent lamp 1 and the leakage light quantities at various positions are made almost equal, so that the planar luminance distribution is uniformized also in this respect to provide an improved display quality of the liquid crystal panel. Further, as the aperture ratio k 1 (x) of the scattering layer 20 is set to be smaller as the distance x from the fluorescent lamp 1 increases, a fraction of light incident at a smaller incident angle  2 is more easily scattered, so that the leakage light quantities from various positions of the backlight device are equalized also in this respect, leading to a uniform planar luminance distribution of the backlight device B 4 and an improved display quality of the liquid crystal panel. Further, by disposing the liquid crystal panel so as to receive p-polarized light, the utilization efficiency of the backlight illumination light can be remarkably improved. While no particular explanation has been made regarding the above-mentioned embodiments, it is possible to provide the upper surface of the front transmission plate 11 with a diffusion treatment or dispose a separate diffusion plate on the upper surface. Particularly, in the second embodiment (Figure 5), it is possible to dispose a diffusion plate above the reflection layer 12, whereby light issued from the front transmission plate 11 is diffused to alleviate an light intensity distribution due to a dot-pattern or mesh-pattern arrangement of the reflection layer 12, so as to moderate the pattern of the reflection layer 12. In the embodiments of Figures 4 and 5, only the fluorescent lamp 1 is indicated but it is possible to dispose an additional fluorescent lamp at a position opposite to the fluorescent lamp 1 so as to dispose the reflection plate 10 and the transmission plate 11 between the resultant pair of fluorescent lamps similarly as shown in Figure 1. Further, the number of fluorescent lamps is not limited to 2 but may be 4 so as to surround the space S. By this arrangement, it is possible to provide a backlight device with an increased luminous intensity. Further, in the second embodiment (Figure 5), the reflection layer 12 is disposed on the upper surface of the front transmission plate but this is not limitative. For example, a reflection layer may also be disposed on a lower surface (confronting with the λ/4 plate 30) of the front transmission plate 11. Next, a third comparative example will now be described, which is a modification of the first comparative example wherein the aperture ratio of the reflection layer 12 is changed in a manner described hereinafter. Regarding the aperture ratio distribution of a reflection layer 12, we have made an examination of three types as shown in Figure 6 wherein a curve α representing a linear relationship, a curve γ representing a hyperbolic relationship and a curve β representing a relationship intermediate therebetween (with respect to a left half of a laterally symmetrical structure), wherein the abscissa represents a distance x from a lamp 1 and the ordinate represents an aperture ratio Ap of the reflection layer 12. More specifically, in this instance, a structure as shown in Figure 2 was supplemented with a symmetrical right half so that two fluorescent lamps were disposed at both lateral sides between which a light-guide space S was disposed. As a result, luminance distributions as shown in Figure 7 were obtained for the respective cases (only left halves being indicated). The results show that the relationship β representing an intermediate function between a linear one and a hyperbolic one regarding a distance x and an aperture ratio thereat, resulted in a best uniformity of luminance distribution. However, the linear relationship a and the hyperbolic relationship γ could also be regarded as practically acceptable without providing a detrimental difference. As described above, according to the first and second embodiments of the present invention, it has been possible to provide a backlight device which is light and excellent in portability. Further, by adopting an arrangement allowing repetitive reflection of light between a reflection means and a reflection layer, so as to have the light leak out of apertures formed in the reflection layer, it has become possible to provide a backlight device which provides a uniform planar luminance distribution even without a light-guide member, is light and is excellent in portability. Further, in case where the apertures in the reflection layer are arranged to have an aperture ratio which increases with an increase in distance from a light source, the light leakage quantities at respective positions are substantially equalized to provide a uniform planar luminance distribution of leakage light (illumination light). Further, in case where the reflection means is provided with a scattering layer for scattering and reflecting light incident thereto, the angle characteristic of issued light can be uniformized to equalize the leakage light quantities, thereby providing a uniform planar luminance distribution of illumination light. Further, in case where the scattering layer is provided with a multitude of apertures providing an aperture ratio which decreases with an increase in distance from the light source, a light fraction having a smaller incident angle is subjected to a promoted scattering to increase the light leakage through the apertures of the reflection layer thereat. This also promotes the equalization of leakage lights from the backlight device leading to a uniform planar luminance distribution of illumination light. Further, in case where emitted light is reflected between a polarization beam splitter layer and a reflection means and only one type of polarization component (e.g., p-polarization component) is allowed to issue as illumination light, the utilization efficiency of illumination light can be remarkably increased. Further, by using a backlight device described above for illuminating a liquid crystal display device, it is possible to provide an information transmission apparatus or display apparatus with excellent display quality. Figures 11 and 12 are respectively a plan view showing a preferred reflection layer pattern (aperture pattern) used in the present invention. The X-Y coordinates in these figures correspond to that shown in Figure 1, and a larger x represents a larger distance from a light source 1. In these figures, the reflection layer pattern (aperture pattern) is depicted only one-dimensionally in the X-direction but actually the pattern may be repeated in the Y direction. The reflective layer patterns shown in Figures 11 and 12 respectively include a sub-pattern 12a and a sub-pattern 12b. The sub-pattern 12a comprises a continuous masking layer (shown with a hatching, illustrated as discrete square patterns for convenience of illustration but may actually constitute a continuous layer) and discrete apertures having an aperture ratio increasing with an increase in X-coordinate. The sub-pattern 12a is suitable for providing aperture ratios in a range of, e.g., 0 - 50 %. The sub-pattern 12b is a negative (or complementary) pattern with respect to the sub-pattern 12a, i.e., comprises discrete masking layers and a continuous aperture (or opening) area surrounding the masking layers. The sub-pattern 12b is suitable for providing aperture ratios in the range of, e.g., 50 - 100 %. It is clear that the boundary aperture ratio for selection of the sub-patterns 12a and 12b is not necessarily 50 %. In case where a pair of light sources 1 are used as shown in Figure 1, the patterns of Figures 11 and 12 may be formed laterally symmetrically with a line (extending longitudinally) of aperture ratio of 100 % at the center. Finally, a data transmission apparatus 400 including a backlight device B as described above will now be described with reference to Figure 8. Referring to Figure 8, the data transmission apparatus 400 includes a liquid crystal panel (display device) P so as to display various data or information by the panel P. As is known, such a liquid crystal panel includes a pair of oppositely disposed substrates, and a liquid crystal disposed between the substrates. The pair of substrates are provided with scanning electrodes and data electrodes so as to form an electrode matrix. The liquid crystal panel P is connected to a scanning signal application circuit 402 and a data signal transmission circuit 403, which are in turn further connected to a scanning signal control circuit 404 and a data signal control circuit 406, a drive control circuit 405 and a graphic controller 407, sequentially. In operation, data and scanning scheme signals are supplied from the graphic controller 407 via the drive control circuit 405 to the scanning signal control circuit 404 and the data signal control circuit 406, where the data are converted into address data and display data and the other scanning scheme signals are sent as they are to the scanning signal application circuit 402 and the data signal application circuit 403. The scanning signal application circuit 402 applies a scanning signal having a waveform determined by the scanning scheme signals to a scanning electrode determined by the address data, and the data signal application circuit 403 applies data signals having waveforms determined by both display contents of white or black based on the display data and the scanning scheme signals. Next, a liquid crystal display apparatus 50 provided with a backlight device 10 as described above will now be descried with reference to Figure 9. A liquid crystal display apparatus 50 is provided with a backlight device according to any one of the above-descried embodiments. The backlight device 10 is connected to a backlight lighting circuit (backlight drive means) 51 so as to drive the backlight device 10. In front of and opposite to the backlight device 10 is disposed a liquid crystal panel P, which comprises a pair of oppositely disposed glass substrates provided with scanning electrodes and data electrodes, and a ferroelectric liquid crystal disposed between the glass substrates. The scanning electrodes and data electrodes are connected to X-drivers 52 and a Y-driver 53, respectively, which in turn are corrected to a panel drive controller (liquid crystal device drive means) 55. The panel drive controller 55 and the backlight lighting circuit 51 are connected to a power supply unit 56 so as to supply powers. Further, panel drive controller is connected to a host computer (not shown) to be supplied with display signals. On the other hand, based on the display signals, the panel drive controller 55 supplies signals to the liquid crystal panel P via the drivers 52 and 53. Further, the panel drive controller 55 supplies ON/OFF signals and lighting control signals to the backlight lighting circuit 51, which drives the backlight device 10 based on such signals.

Claims: A generally rectangular planar backlight device having front and rear principal plane faces, comprising: a light source (1) disposed along a side of the backlight device; a polarization beam splitter (31) disposed along the front principal plane face of the backlight device for selectively transmitting one of a p-polarization component, having an electric field component perpendicular to the polarization beam splitter (31), and an s-polarization component, having an electric field component parallel to the polarization beam splitter (31), of the light emitted from the light source (1), in use, and for reflecting the other of the p-polarization component and the s-polarization component; reflection and polarization conversion means (10,30) disposed along the rear principal plane face of the backlight device, for reflecting light and effecting polarization conversion between the s-polarization component and the p-polarization component; whereby light emitted from the light source (1) into a region (S) between the polarization beam splitter (31) and the reflection and polarization conversion means (10,30) is reflected between the polarization beam splitter (31) and the reflection and polarization conversion means (10,30) so that the p-polarization component or the s-polarization component of the emitted light is selectively leaked outwards through the polarisation beam splitter (31). A backlight device according to claim 1, wherein the reflection and polarization conversion means comprises a reflection means (10) disposed along the rear principal plane face of the backlight device arranged to reflect light into the backlight device and a ¼ wavelength plate (30) for effecting polarisation conversion disposed adjacent the reflection means (10) on the side of the reflection means (10) facing the polarization beam splitter (31).

Inventors: KANDA TOSHIYUKI, KUREMATSU KATSUMI, ONITSUKA YOSHIHIRO, KANDA, TOSHIYUKI, KUREMATSU, KATSUMI, ONITSUKA, YOSHIHIRO, KANDA, TOSHIYUKI, C/O CANON K.K., KUREMATSU, KATSUMI, C/O CANON K.K., ONITSUKA, YOSHIHIRO, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: F21V7
IPC codes: F21V11, G02B27, F21V5, G02B6, G02F1, F21V13, F21V7, G09F9, F21V8