Transmissive image display device

Abstract: A projection-type image display device includes a transmissive image display panel (1) having a matrix of pixels arranged at different pitches in different directions perpendicular to each other, and a planar microlens array (4) for converging light emitted by a light source (16) onto the transmissive image display panel. The planar microlens array comprises a substrate (3) and a matrix of microlenses (2) disposed on the substrate for converging the light onto the pixels, respectively, each of the microlenses being of an oblong shape. Each of the microlenses may have a longitudinal axis inclined with respect to an axis of the matrix of pixels to cover the pixel aperture of one of the pixels. The oblong shape of each of the microlenses may be composed of a pair of spaced identical semicircles and a rectangle interconnecting the identical semicircles and having sides of the same length as the diameter of the semicircles. Since the oblong microlenses are shaped to fully cover the light source is efficiently converged onto the pixels by the microlenses to produce light display images.

Description: The present invention relates to a projection-type transmissive image display device having a liquid crystal display panel for producing a display pattern based on light applied to and transmitted through the liquid crystal panel, and more particularly to such a transmissive image display device which includes a planar microlens array for converging light onto each pixel of the liquid crystal panel. Some projection-type transmissive image display devices employ a liquid crystal display panel having a matrix of minimum display units known as pixels. The image display devices display desired images and/or characters when independent drive voltages are applied to the pixels to modify optical properties of the liquid crystal of the pixels. Since it is necessary for the pixels to have wires which apply the drive voltages, each of the pixels has a relatively small aperture through which the light passes. If it is assumed that light applied to liquid crystal display panels has the same intensity, then those crystal display panels which have smaller pixel apertures produce darker images on the screen because the light that is applied to the pixel areas other than the apertures does not pass through the apertures. To solve the above problem, there have been proposed planar microlens arrays comprising circular planar microlenses which are rotationally symmetric with respect to their optical axes and disposed in confronting relationship to the respective pixels, for converging the applied light onto the pixels (see, for example, Japanese laid open patent publications Nos. 60-165621, 60-165622, 60-165623, and 60-165624). As shown in FIG. 9 of the accompanying drawings, one conventional planar microlens array is used with a liquid crystal display panel composed of triads of pixels. The planar microlens array comprises circular microlenses 100 confronting the pixels, respectively. The pixels are spaced horizontally with a pitch a and vertically with a pitch b, the pitches a, b being different from each other. If the difference between the horizontal and vertical pitches a, b is larger, then those areas 101 of the pixels which are not covered by the respective microlenses 100 are larger. Therefore, the light applied to the liquid crystal display panel is not effectively utilized for producing a display pattern, and the image projected onto the screen is relatively dark. The same problem arises if the horizontal and vertical pitches of pixels are different from each other in other pixel matrix configurations. As shown in FIG. 5 (a) of the accompanying drawings, a pixel 1b of an active-matrix liquid crystal display panel has an asymmetrically shaped aperture 15 because of an area 16 in which there is located a switching element for applying a voltage to pixel electrodes, such as a TFT (thin-film transistor) or an MIM (metal-insulator-metal) diode. The circular spot of light converged by each circular microlens 100 is positioned on a line M of the pixel aperture 15, as shown in FIG. 5(b) of the accompanying drawings. Therefore, an area S of the aperture 15 is remote from the center of the light spot, and is not as effectively used as the other area of the aperture 15. Consequently, the image projected onto the screen is further made darker. In view of the foregoing drawbacks of the conventional projection-type transmissive image display devices, it is an object of the present invention to provide a projection-type transmissive image display device capable of highly efficiently utilizing light emitted by a light source for producing light display images. EP-A-0425251 discloses an image display device comprising: a light source for emitting light; a transmissive image display panel having a matrix of pixels arranged at different pitches in different directions perpendicular to each other; and a planar microlens array arranged to converge the light emitted by said light source onto said transmissive image display panel, said planar microlens array comprising a substrate and a matrix of oblong microlenses disposed on said substrate and arranged to converge the light onto the pixels. A further example of an image display device can be found in EP-A-0 366 462. The present invention is characterised over the disclosure of EP-A-0425251 in that each of said oblong microlenses comprises two end portions and a central portion, said end portions being at either end of said central portion in a longitudinal direction of said microlens, said end portions being constructed to each converge light in both the transverse and longitudinal directions of the microlens and said central portion being constructed to converge light only in the transverse direction of the microlens. The oblong microlenses are shaped to fully cover the pixel apertures of the corresponding pixels, so that the light from the light source is efficiently converged onto the pixels by the microlenses to produce light display images. Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:- FIG. 1 is a schematic side elevational view of an image display device according to an embodiment of the present invention; FIG. 2 is a fragmentary perspective view of a planar microlens array used in the image display device shown in FIG. 1; FIG. 3 is an enlarged fragmentary plan view of the planar microlens array shown in FIG. 2; FIG. 4(a) is a plan view of an oblong microlens of the planar microlens array; FIG. 4(b) is a transverse cross-sectional view of the microlens, showing the manner in which incident light is converged by the microlens; FIG. 4(c) is a longitudinal cross-sectional view of the microlens, showing the manner in which incident light is converged by the microlens; FIG. 4(d) is a view showing a converged image on a focal plane of the microlens; FIG. 5(a) is a plan view of a pixel and a pixel aperture thereof; FIG. 5(b) is a plan view of the pixel aperture; FIG. 6 is a plan view of the pixel aperture; FIG. 7(a) is a fragmentary plan view of a planar microlens array in an image display device according to a second embodiment of the present invention; FIG. 7(b) is a fragmentary plan view of the planar microlens array shown in FIG. 7(a) in combination with respective pixel apertures; FIG. 8(a) is a fragmentary plan view of a planar microlens array in an image display device according to a third embodiment of the present invention; FIG. 8(b) is a fragmentary plan view of the planar microlens array shown in FIG. 8(a) in combination with respective pixel apertures; FIG. 9 is a plan view of a conventional planar microlens array; and FIG. 10 is a graph showing the relationship between the refractive index of an adhesive and the transmittance of an assembly of a microlens and a transparent substrate that are bonded by the adhesive. As shown in FIG. 1, a projection-type transmissive image display device according to one embodiment of the present invention comprises a transmissive liquid crystal display panel 1, a planar microlens array 4 composed of a two-dimensional matrix of microlenses 2 disposed on a glass substrate 3, a condensing lens 5, a source 6 of white light, a reflecting mirror 7 for reflecting white light emitted by the source 6 toward the liquid crystal display panel 1, a projection screen 8, and a projection lens 9 for projecting the light that has passed through the liquid crystal display panel 1 onto the projection screen 8. The liquid crystal display panel 1 comprises a pair of transparent substrates 1a with a two-dimensional matrix of pixels 1b disposed therebetween and supported thereon. The liquid crystal display panel 1 has an effective display area size of about 45.6 mm x 62.0 mm, a vertical pixel pitch of 190 µm, a horizontal pixel pitch of 129 µm, and an aperture ratio (the proportion of the area of the apertures to the entire area of the pixels) of 41%. The transparent substrates 1a have a refractive index of 1.53, and each have a thickness of 0.7 mm. The pixels 1b of the liquid crystal display panel 1 are arranged in triads each in the shape of an elongate triangle, but not a regular triangle. In view of such a pixel triad configuration, each of the microlenses 2 is of an oblong shape, as shown in FIGS. 2 and 3, to fully cover an area that includes the pixel aperture of the corresponding pixel 1b. The microlenses 2 of the planar microlens array 4 are spaced at a vertical pixel pitch of 190 µm, which are the same as the vertical and horizontal pixel pitches of the liquid crystal display panel 1. Each of the microlenses 2 has a focal length of 0.7 mm which is equal to the thickness of each of the transparent substrates 1a. (In air, 0.7/1.53 = 0.46 mm). FIG. 4(a) shows in plan each of the microlenses 2 as viewed in a direction normal to the glass substrate 3. The microlens 2 is shown in a transverse cross section in FIG. 4(b) along an axis X - X' in FIG. 4(a), and in a longitudinal cross section in FIG. 4(c) along an axis Y - Y' in FIG. 4(a). The microlenses 2 are fabricated according to the ion exchange process, as follows: First, the glass substrate 3 is coated with a metal diffusion mask of Al, Ti, Ni, or Cr which has minute holes or windows corresponding to the pixels of the liquid crystal panel 1. Then, the glass panel 3 is dipped, for a predetermined period of time, in a melted salt containing ions that contribute more to refractive indices than one-valence ions contained in the glass substrate 3. As a result, graded-index microlenses 2 whose refractive index is gradually lower radially outwardly from the center thereof are formed in the glass substrate 3, as shown in FIG. 2. In FIGS. 4(b) and 4(c), points of equal refractive index in the microlens 2 are indicated by the reference numeral 11. FIG. 4(a) also illustrates each of the minute holes or windows, represented by the reference numeral 12, of the diffusion mask. The minute hole 12 is also of an oblong shape corresponding to the microlens 2. More specifically, the oblong shape of the minute hole 12 is composed of a pair of spaced identical semicircles inter-connected by a rectangle whose sides are of the same length as the diameter of the semicircles. When the glass substrate 3 is dipped in the melted salt, its ions are isotropically diffused through the minute holes 12 of the diffusion mask so that a diffusion front (a foremost edge of the diffusing ions) 13 is represented by a curve that is equidistant from the oblong shape of the hole 12, i.e., spaced a diffusion length L from the oblong shape of the hole 12. Therefore, the diffusion front 13 is also of an oblong shape. The diffusion front 13 represents the outer boundary of the oblong microlens 2. The oblong microlens 2 has a longer axis extending along the longer pixel pitch of the liquid crystal display panel 1. As shown in FIG. 4(a), each microlens 2 comprises a pair of spaced hemispherical lenses A, A' sandwiching a lenticular lens B which is capable of converging light only in the transverse direction of the microlens 2. When light rays R (see FIG. 1) are incident on the microlens 2, the lens A converges the light rays R to the center 0 thereof, and the lens A' converges the light rays R to the center 0' thereof, as viewed in a direction normal to the microlens 2 in FIG. 4(a). Actually, the light rays R are converged onto a focal plane behind the microlens 2 as shown in FIGS. 4(b) and 4(c). The lenticular lens B does not converge the light rays in the longitudinal direction (Y - Y') of the microlens 2, but converges the light rays in the transverse direction (X-X'). The light rays R converged by the microlens 2 form an oblong image 10 on the focal plane, as shown in FIG. 4(d), the centers 0, 0' being at the respective centers of opposite semicircles of the oblong image 10 as viewed in a direction normal to the image 10 in FIG. 4(d). The image 10 which is converged onto the image plane by the microlens 2 is composed of a pair of spaced identical semicircles interconnected by a rectangle whose sides are of the same length as the diameter of the semicircles. The image 10 has a semicircle radius rs and a width which are determined by the angle in which the light emitted from the light source 6 (FIG. 1) spreads and the focal length of the microlens 2 was 0.46 mm, the radius rs of the semicircles of the image was about 35 µm and the width of the image 10 was about 70 µm. The distance W (see FIG. 4(a)) between the centers 0, 0' of the semicircles of the hole 12 is determined such that the oblong image 10 is positioned in the aperture of each pixel 1b of the liquid crystal display panel 1. If the radius rm of each semicircle of the hole 12 and the radius r of each semicircle of the diffusion front 13 are selected such that the ratio r/rm is in the range of from 1.8 to 3.0, then the microlens 2 has relatively small aberrations and can converge the light efficiently. As described above, the pixel 1b which has a width a and a length b that are equal to the pixel pitches a, b, respectively, has an asymmetrical aperture 15, as shown in FIG. 5(a), because of an area 16 in which there is located a switching element for applying a voltage to pixel electrodes, such as a TFT (thin-film transistor) or an MIM (metal-insulator metal) diode, in an active-matrix liquid crystal display panel. In the conventional image display devices, the circular spot of light converged by a circular microlens is positioned on a substantially central vertical line M of the pixel aperture 15, as shown in FIG. 5(b). According to another embodiment of the present invention, in order to effectively utilize the entire area of the pixel aperture 15, the pixel aperture 15 is divided into two rectangles having respective centers d, d', as shown in FIG. 6, and a planar microlens array 20 as shown in FIG. 7(a) is arranged such that each oblong microlens 2 thereof has a major or longitudinal axis P inclined to a vertical axis T of the liquid crystal display panel 1 for aligning the vertical central axis V of the oblong image 10 (see Fig. 4(d)) with a straight line N passing through the centers d, d'. In the embodiment shown in FIGS. 7(a) and 7(b), the longitudinal axis P is inclined to the vertical axis T by an angle of 12°. The microlenses 2 thus inclined and the corresponding pixels 1b of the liquid crystal display panel 1 are shown in FIG. 7(b). The pixel aperture 15 may be of another asymmetrical configuration, and each of the microlenses 2 may be inclined a suitable angle to cover the asymmetrical pixel aperture 15 as fully as possible. As shown in FIG. 1, the microlenses 2 are aligned with the respective pixels 1b in a direction along the optical axis of the image display device. The planar microlens array 4 is bonded to the liquid crystal display panel 1 by an optical adhesive 25 therebetween which has a refractive index that is substantially equal to the refractive index of the transparent substrates 1a of the liquid crystal display panel 1. The transparent substrates 1a have a refractive index of 1.53 at their surfaces, and the microlenses 2 have a refractive index of about 1.6 at their surfaces. As shown in FIG. 10, if the refractive index of the optical adhesive 25 is in the range of from 1.4 to 1.7, then a reflection loss caused due to the difference between the refractive indices in the optical path from the microlens 2 to the transparent substrates 1a can be held in a range of 1%. The reflection loss is further minimized if the refractive index of the optical adhesive 25 is of an intermediate value between the refractive index (= 1.53) of the surface of the transparent substrates 1a and the refractive index (= about 1.6) of the surface of the microlens 2. The microlenses 2 are not of an elliptical shape. If each of the microlenses 2 were elliptical in shape, then it would have different focal lengths in its longitudinal and transverse directions due to astigmatism. When the corresponding pixel 1b would be positioned at the focal point associated with one of the focal lengths, then the pixel 1b would not be positioned at the focal point associated with the other focal length, with the result that the microlenses 2 would not effectively converge the applied light. In the above embodiments, the microlenses 2 are fabricated such that adjacent microlenses 2 are held in contact with each other partially along their boundaries. However, insofar as aberrations of the microlenses are kept within an allowable range, the ions may be excessive diffused into the glass substrate 3 so that adjacent microlenses 2 are held in contact with each other fully along their boundaries, i.e., the glass substrate 3 is fully covered with the microlenses 2. According to still another embodiment shown in FIGS. 8(a) and 8(b), each microlens 2 is of a substantially rectangular oblong shape, and is inclined to cover the corresponding pixel aperture 15. Adjacent microlenses 2 have boundaries held in full contact with each other for converging almost 100% of the applied light. The planar microlens array 4 or 20 may be fabricated of plastics according to the diffusion polymerization process. The image display device has been shown as being incorporated in a Köhler illumination system, but may be combined with a critical illumination system or a telecentric illumination system. The image display system may employ three liquid crystal panels for forming images of three primary colors which may be combined into a fully colored image. Any other transmissive display panels than the liquid crystal display panel 1 may be used in the image display device according to the present invention. The principles of the present invention may be applied to a direct-viewing-type transmissive image display device. Although there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that the invention may be embodied in other specific forms without departing from the scope of the appended claims. The present embodiments are therefore to be considered in all respects as illustrative, and not restrictive.

Claims: An image display device comprising: a light source (6) for emitting light; a transmissive image display panel (1) having a matrix of pixels (1b) arranged at different pitches (a,b) in different directions perpendicular to each other; and a planar microlens array (4) arranged to converge the light emitted by said light source (6) onto said transmissive image display panel (1), said planar microlens array (4) comprising a substrate (3) and a matrix of oblong microlenses (2) disposed on said substrate (3) and arranged to converge the light onto the pixels (1b), characterised in that each of said oblong microlenses (2) comprises two end portions and a central portion, said end portions being at either end of said central portion in a longitudinal direction of said microlens, said end portions being constructed to each converge light in both the transverse and longitudinal directions of the microlens and said central portion being constructed to converge light only in the transverse direction of the microlens. An image display device according to claim 1, wherein each of said pixels (1b) has an asymmetrical pixel aperture (15), each of said microlenses (2) having a longitudinal axis (Y-Y') inclined with respect to an axis of said matrix of pixels to effectively utilise the entire area of the pixel aperture (15). An image display device according to claim 1 or 2 wherein said oblong shape of each of said microlenses (2) is composed of a pair of spaced identical semicircles and a rectangle interconnecting said identical semicircles and having sides of the same length as the diameter of the semicircles. An image display device according to claim 3, wherein each of said microlenses (2) comprises a pair of spaced hemispherical lenses (A, A') and a lenticular lens (B) sandwiched between said hemispherical lenses (A, A'). An image display device according to any preceding claim, wherein said image display panel (1) includes a transparent substrate (1a) on which said pixels (1b) are supported, said microlenses (2) being arranged in pitches (a,b) corresponding to said different pitches (a,b), respectively, in different directions perpendicular to each other, each of said microlenses (2) having a focal length equal to the thickness of said substrate (1a) of the image display panel. An image display device according to claim 5, wherein said microlenses (2) are aligned with said pixels (1b), respectively, said microlenses (2) being bonded to said transparent substrate (1a) of the image display panel (1) by an optical adhesive (25) having a refractive index which is substantially the same as the refractive index of said transparent substrate (1a). An image display device according to claim 6, wherein said refractive index of said optical adhesive (25) is of an intermediate value between the refractive index of a surface of said microlenses (2) and the refractive index of a surface of said transparent substrate (1a). An image display device according to any preceding claim, wherein adjacent ones of said microlenses (2) contact each other partially along their boundaries only at the surface of said substrate. An image display device according to any of claims 2 to 7, wherein adjacent ones of said microlenses (2) contact each other fully along their boundaries. A method for manufacturing an image display device according to any of claims 3 to 9, wherein each of said microlenses (2) is fabricated as a diffusion front (13) of ions diffused into said substrate (3) of the planar microlens array (4) through a hole (12) defined in a diffusion mask, said hole (12) in the diffusion mask being of an oblong shape including a pair of spaced identical semicircles, each of the semicircles of said diffusion front (13) having a radius r and each of said semicircles of said hole (12) in the diffusion mask having a radius rm, the ratio r/rm being in the range 1.8 to 3.0.

Inventors: FUNADA FUMIAKI, HAMADA HIROSHI, HAMANAKA KENJIRO, KISHIMOTO TAKASHI, NEMOTO HIROYUKI, OIKAWA MASAHIRO, SHIBATANI TAKASHI, FUNADA, FUMIAKI, HAMADA, HIROSHI, HAMANAKA, KENJIRO, KISHIMOTO, TAKASHI, NEMOTO, HIROYUKI, OIKAWA, MASAHIRO, SHIBATANI, TAKASHI, Funada, Fumiaki, c/o Sharp K.K., Hamada, Hiroshi, c/o Sharp K.K., HAMANAKA, KENJIRO, C/O NIPPON SHEET GLASS CO., LTD, Kishimoto, Takashi, c/o Nippon Sheet Glass Co.,Ltd, Nemoto, Hiroyuki, c/o Nippon Sheet Glass Co., Ltd., Oikawa, Masahiro, c/o Nippon Sheet Glass Co., Ltd., Shibatani, Takashi, c/o Sharp K.K.
Applicants: NIPPON SHEET GLASS CO LTD, SHARP KK, NIPPON SHEET GLASS CO., LTD., SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02F1
IPC codes: G02B3, G02F1