A transmissive display device

Abstract: A transmissive display device which includes a display panel (4) having multiple picture elements, a first array of microlens (1) disposed on the incident side of the display panel, the microlenses each being disposed in correspondence to respective picture elements (5), a second array of microlenses (2) disposed on the outlet side of the display panel, the microlenses each being disposed in correspondence to respective picture elements (5), wherein the positions of the focal points of the first array of microlenses are identical with those of the focal points of the second array of microlenses, and wherein the focal length of each microlens in the first array is larger than that of each microlens in the second array.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a transmissive-type display device (hereinafter called "transmissive display device"), and more particularly to a dot-matrix type display device having a display panel such as a liquid crystal panel and multiple picture elements arranged in a matrix, wherein the display panel is provided with an array of microlenses. This type of display device is particularly applicable to a large-screen projection TV, information display systems and the like. 2. Description of the Prior Art In the transmissive display panels, non-linear devices such as varistors or MIM (metal insulator metal) are provided in correspondence to the picture elements so as to prevent possible cross talk between the adjacent picture elements. Switching elements such as TFTs (Thin Film Transistor) disposed for the respective picture elements drive picture element by their electrodes. These elements and wiring thereto occupy a space in the display device, thereby reducing the effective area for forming picture elements. This reduces the numerical aperture of the display device. The numerical aperture ratio is expressed by (the effective area of all the picture elements) divided by (total display area). As is evident from the formula, as the areas which do not contribute to display increase, the numerical aperture ratio decreases. The reduced numerical aperture ratio leads to the reproduction of dark picture, and the poor image quality. In order to refine pictures on display, another requirement is to minimize the size of the picture elements on the display panel. If the sizes of the components of all the picture elements are proportionally reduced, the numerical aperture ratio will not change. However, photolithograph and etching has a limit to minimizing the size of the non-linear elements and switching elements, and the wiring thereto cannot be made narrow in its width below an allowable extent. As the spaces are occupied by these elements, the numerical aperture ratio of the display device is reduced. The numerical aperture ratio is the ratio of incident rays to the display panel to a recoverable light. Unrecoverable light is shut out by an untransmissive portion of the display panel, and does not contribute to display. Consequently, the quality of pictures on display depends on the numerical aperture ratio of the display devices when they employ the same source of light; that is, the reduced numerical aperture ratio leads to the reproduction of a dark picture. To solve the problems pointed out above, one proposal is for providing the display panel with an array of microlenses on one side or on both sides thereof, which are disclosed in Japanese Laid-Open Patent Publications No. 60-262131 and No. 61-11788. An advantage of these known display devices is that the incident rays to the portions of display panel that do not contribute to display are focused on the picture elements in the display panel, thereby increasing the numerical aperture substantially. However, these known display devices have the following disadvantages: In a display device whose display panel has an array of microlenses only on the light-incident side, that is, the side where light enters the display panel, after the rays are focused on the picture elements, they are diverged. To use this display panel in a projection type display device, it is required that the numerical aperture (NA) of the projection lens on the light-outlet side is deliberately enlarged. In a display device whose display panel has arrays of microlenses on both sides, the following problems arise: Referring to Figure 3, a typical conventional display panel will be described in detail: The display panel 3 has microlenses 1 and 2 on both sides. The microlenses 1 and 2 are arranged such that their focal lengths are the same. In addition, the focal points of the microlenses 1 on the light-incident side of the display panel 3 and the microlenses 2 on the light-outlet side thereof correspond to each other. As a result, the focal points of these microlenses 1 and 2 are disposed in the cross-sectional center of the display panel 3. The incident parallel rays of the microlens 1 to the light-incident side of the display panel 3 are focused on the focal points, and thereafter tend to be diverged. However, they are changed into parallel rays by passing through the microlenses 2. The state shown in Figure 3 is a hypothetical situation only achievable if idealistic microlenses are produced. However, the reality is that some of the incident rays do not pass through the respective microlenses 2 but pass through other microlenses due to the aberration of them. As a result, the outlet rays are diverged at angles widened by the microlenses through which they pass, as shown by the dotted lines in Figure 3. This diversion results in the loss of light. In the case of a projection type display device, a relatively large numerical aperture will be required for the projection lens so as to prevent the light from being lost. SUMMARY OF THE INVENTION One transmissive display device of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a display panel having multiple picture elements, a first array of microlenses disposed on the incident side of the display panel, the microlenses each being disposed in correspondence to respective picture elements, a second array of microlenses disposed on the outlet side of the display panel, the microlenses each being disposed in correspondence to respective picture elements, wherein the positions of the focal points of the first array of microlenses are substantially coincident with those of the focal points of the second array of microlenses, and wherein the focal length of each microlens in the first array is larger than that of each microlens in the second array. In a preferred embodiment, the picture elements are provided in the cross-sectional center of the display panel. In a preferred embodiment, the picture elements are disposed at the focal points of the first array of microlenses. In a preferred embodiment, each of the first and second microlenses has a gradient refractive index. In a preferred embodiment, the display panel is a liquid crystal panel. In another aspect, the invention is defined by independent claim 6. Thus, the invention described herein makes possible the objective of providing a transmissive display device capable of reproducing a clear picture irrespective of its relatively small numerical aperture. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a diagrammatic cross-section showing an example of a display device according to the present invention; Figure 2 is a fragmentary cross-section on an enlarged scale showing the relationship between the microlenses and the display panel; Figure 3 is a diagrammatic cross-section showing a conventional display device; Figure 4 is a diagrammatic cross-section showing another example of a display device according to the present invention; Figure 5 is a fragmentary cross-section on an enlarged scale showing the relationship between the microlenses and the display panel in the display device of Figure 4; and Figures 6 and 7 are diagrammatic cross-sections showing further examples of display devices. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figures 1 and 2, the present invention will be described by reference to a projection type display employing a liquid crystal display panel 4 in which picture elements are formed at a ratio of a few picture elements per 10 mm. The numerical aperture ratio of the display panel is 50% to 80%. The incident side of the display panel 4 toward the light source has a first array of microlenses 1, and the outlet side thereof has a second array of microlenses 2. The display panel 4 has multiple picture elements 5 arranged in a matrix. The microlenses 1 and 2 are individually disposed in correspondence to each picture element 5, thereby ensuring that the microlenses 1 and 2 are arranged face to face with each other, with one picture element being interposed therebetween. In the illustrated embodiment, the microlenses 1 and 2 are fabricated with two-dimensionally arranged multiple lenses each having a gradient refractive index. The substrates 4a and 4b constituting the liquid crystal display panel 4 are 1.1 mm thick. The thickness of the picture elements 5 is negligible. The liquid crystal display panel 4 has a thickness (d) of 2.2 mm. The focal length f₁ of each microlens 1 is 1200 µm, and the focal length f₂ of each microlens 2 is 1000 µm. Therefore, the thickness d of the liquid crystal display panel 4 is the sum of the focal lengths f₁ and f₂ becomes 2200 µm. Thus, the focal points of the first array of microlens 1 and those of the second microlens 2 become identical toward the liquid crystal panel. In the illustrated display device, parallel rays passing near the optical axes of the microlenses 1 and 2 are focused at a focus F at a distance of f₁ away from the surface of the liquid crystal display panel 4, as indicated by the full lines in Figure 2. Thereafter, the rays, while diverging, reach the second array of microlenses 2 whereby they are converted into parallel rays. When the parallel rays are to pass through the peripheral portion of each microlens 1, they pass through a point Q toward the first array of microlenses 1 rather than the focal point F thereof owing to the aberration of the microlens 1, as indicated by dotted lines in Figure 2. Since the point Q is disposed toward the light outlet side of the liguid crystal display panel 4 beyond the sectional center of the picture element 5, the rays can enter the second microlens 2. Since the distance between the point Q and the microlens 2 is larger than the focal length f₂ of the microlens 2, the rays passing through the microlens 2 cannot become parallel rays. Advantageously, these rays contribute to display, thereby avoiding the loss of light. Thus, the liquid crystal display device eliminates the necessity of employing a large projection lens, and allows the reproduction of a clean image with the employment of a projection lens having a relatively small diameter. Referring to Figures 4 and 5, wherein like numerals designate similar members and elements to those in Figures 1 and 2, the microlenses 1 and 2, produced by an ion exchange method, have a gradient refractive index. The focal length f₁ of the first microlens 1 is 1100 µm, and the focal length f₂ is 700 µm. A pair of substrates 4a and 4b constituting the liguid crystal panel 4 are respectively 1.1 mm and 0.7 mm thick. The thickness of the substrates 4a and 4b are equal to the focal lengths f₁ and f₂ of the microlenses 1 and 2. The focal points of the first microlens 1 and those of the second microlens 2 become identical with each other toward the liquid crystal panel. In this way, each picture element 5 formed between the substrates 4a and 4b is disposed at the focal point of the microlens 1 toward the liguid crystal display panel 4, which means that it is disposed at the focal point of the microlens 2 toward the liquid crystal display panel 4. Parallel rays passing near the optical axes of the microlenses 1 and 2 are focused at a focal point F on the picture element 5 of the liquid crystal display panel 4, as indicated by the full lines in Figure 5. Thereafter, the rays, while diverging, reach the second microlens 2 whereby they are converted into parallel rays. When the parallel rays are to pass through the peripheral portion of the microlens 1, they pass through a point Q nearer the microlens 1 than the focal point F thereof owing to the aberration of the microlens 1, as indicated by dotted lines in Figure 5. This point Q is disposed nearer the light outlet side of the display panel than in the conventional display device shown in Figure 3, thereby allowing the rays to enter the microlens 2. Since the distance between the point Q and the microlens 2 is larger than the focal length f₂, the rays passing through the microlens 2 do not become parallel rays. Advantageously, these rays converge and contribute to display, thereby reducing the loss of light rays. In this example, the focal points of the microlenses 1 and 2 are situated on the picture element 5, thereby reducing the amount of rays which are shut out by other portions than the picture element 5 as compared with the conventional device shown in Figure 3 . Thus, a clear image is displayed. As referred to above, the microlenses 1 and 2 are fabricated by an ion exchange method, but they can be made by a swelling method, a heating method or a mechanical method. In the swelling method, photosensitive monomer is polymerized by ultraviolet ray, and then an exposed portion in swollen into lenses by the osmotic pressure between the exposed portion and the non-exposed portion. In the heating method, after a photosensitive resin is patterned in circles, it is heated at a higher temperature than its melting point, and the resulting molten resin is formed into lenses under surface tension. In the mechanical method, a raw material is mechanically cut into lenses. The materials usable for fabricating the microlenses can be clear resins such as acrylic resin, polycarbonate resin, polystyrene resin, glass or the like. The microlenses can be plano-convex or double convex. In the illustrated embodiments, the flat surfaces of the microlenses are faced to the liquid crystal display panel 4, but the convex surfaces thereof can be faced thereto. As an alternative method, it is possible to fabricate the substrates 4a and 4b, and the microlenses 1 and 2 in one piece whether they may be within the panel as shown in Figure 6 or on the panel as shown in Figure 7. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention as defined by the appended claims.

Claims: A transmissive display device comprising a display panel (4) having multiple picture elements (5), a first array of microlenses (1) disposed on the incident side of the display panel, the microlenses each being disposed in correspondence to respective picture elements, a second array of microlenses (2) disposed on the outlet side of the display panel, the microlenses each being disposed in correspondence to respective picture elements, wherein the positions of the focal points of the first array of microlenses are substantially coincident with those of the focal points of the second array of microlenses, and wherein the focal length of each microlens in the first array is larger than that of each microlens in the second array. A transmissive display device according to claim 1, wherein the picture elements are provided in the cross-sectional center of the display panel. A transmissive display device according to claim 1, wherein the picture elements are disposed at the focal points of the first array of microlenses. A transmissive display device according to claims 1, 2 or 3, wherein each of the first and second microlenses has a gradient refractive index. A transmissive display device according to claim 1, wherein the display panel is a liquid crystal panel. A transmissive display device comprising an array of picture elements (5), an array of first microlenses (1) disposed on one side of said picture element array, each first microlens being disposed to transmit incident light to a corresponding picture element, and an array of second microlenses (2) disposed on the other side of said picture element array, each second microlens being disposed to receive light transmitted through a corresponding picture element, wherein the focal points of each first and corresponding second microlens are substantially coincident, and wherein the focal length of each first microlens is greater than that of the corresponding second microlens.

Inventors: HAMADA HIROSHI, NAKANISHI HIROSHI, HAMADA, HIROSHI, NAKANISHI, HIROSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G02F1
IPC codes: G02F1