Projection type color image display device

Abstract: In a projection type image display device for combining an image by dichroic mirrors in a form of parallel plane plate, a projected image of high picture quality is presented by suppressing the astigmatism. In a projection type image display device comprising a light source for emitting blue light, green light, and red light, liquid crystal panels for modulating individual color lights, dichroic mirrors for combining the modulated lights, and a projection lens for projecting the combined light, the dichroic mirrors are disposed obliquely on a first plane including an optical axis linking the projection lens and liquid crystal panels, and parallel plane plate for correcting astigmatism is disposed obliquely on a second plane orthogonal to the first plane. In particular, preferably, the thickness of the parallel plane plate for correcting the astigmatism should be greater than the thickness of the dichroic mirrors, and the light incident angle into the parallel plane plate for correcting the astigmatism should be smaller than the light incident angle to the dichroic mirrors.

Description: BACKGROUND OF THE INVENTION The present invention relates to a projection type color image display device for combining an image on an image display device by a dichroic mirror, and projecting by a projection lens. Hitherto has been proposed a projection type color image display device for obtaining a picture by projection lenses provided before cathode ray tubes (CRT) for displaying blue, green and red images. On the other hand, a projection type color image display device comprising one projection lens has been also proposed. For example, a conventional projection type color image display device is shown in Fig. 4. In Fig. 4, the conventional projection type color image display device is composed of three liquid crystal panels with transmission type and one projection lens. The light emitted from a light source 101 is reflected by a reflector 102, and is separated into blue light, green light and red light respectively by a first light-separating dichroic mirror 103, a second light-separating dichroic mirror 104, and a first reflecting mirror 105. The separated lights are led into a first liquid crystal panel with transmission type 106a, a second liquid crystal panel with transmission type 106b, and a third liquid crystal panel with transmission type 106c. In each liquid crystal panel, each color light is modulated into light having an image. The color-lights modulated by the liquid crystal panels are combined by a first color-light-combining dichroic mirror 107, a second color-light-combining dichroic mirror 108, and a second reflecting mirror 109. The first color-light combining dichroic mirror 107 and second color-light-combining dichroic mirror 108 are plates having parallel planes. The combined color-light is magnified and projected by a projection lens 110, and a color picture is displayed on a screen 120. In such constitution, however, as the picture elements of the liquid crystal panel are advanced in definition, the picture quality deteriorates significantly. This is caused by the astigmatism occurring in the image of the color-light transmitted through the first color-light-combining dichroic mirror 107 and the image of the color-light passing through the second color-light-combining dichroic mirror 108, thereby extremely impairing the picture quality. Since the first color-light-combining dichroic mirror 107 and second color-light-combining dichroic mirror 108 are disposed obliquely to the optical axis linking the liquid crystal panel and projection lens, such astigmatism occurs. Fig. 5 is a magnified view of essential parts of Fig. 4, and explains the onset mechanism of astigmatism. In Fig. 5, supposing the thickness of the first color-light-combining dichroic mirror 107 to be T, the refractive index of the substrate of the first color-light-combining dichroic mirror 107 to be N, the incident angle of the light transmitted through the first color-light-combining dichroic mirror 107 into the first color-light-combining dichroic mirror 107 to be X 1 , and the angle of approach in the first color-light-combining dichroic mirror 107 to be X 2 , the astigmatism Y is expressed in formula (1). Y = T(1-cos 2 X 1 /cos 2 X 2 ) N · cosX 2 As clear from formula (1), the smaller the thickness of the first color-light-combining dichroic mirror 107, the smaller is also the value of astigmatism, but the thickness of the first color-light-combining dichroic mirror 107 cannot be reduced owing to the following reasons. If the first color-light-combining dichroic mirror 107 is thin, the shape may be deformed due to external force when assembling the first color-light-combining dichroic mirror 107 into a support mechanism component (not shown), and the stress at this time causes to changes the shape gradually. The first color-light-combining dichroic mirror 107 is composed by applying a dichroic coat on the substrate, but if the thickness is small, the substrate may be deformed by the heat in the process of applying the dichroic coat. The first color-light-combining dichroic mirror 107 has the functions of transmitting the image-light from the first liquid crystal panel with transmission type 106a, and also reflecting the image-light from the second liquid crystal panel with transmission type 106b. Therefore, when reflecting the image-light from the second liquid crystal panel with transmission type 106b, in order that the reflected image may not be distorted, the surface of the first color-light-combining-dichroic mirror 107 is demanded to have a flatness of high precision. As for the second color-light-combining dichroic mirror 108, too, the thickness cannot be reduced due to the same reasons as in the first color-light-combining dichroic mirror 107. As explained herein, it is impossible to reduce the thickness of the color-light-combining dichroic mirror, and the thickness of the color-light-combining dichroic mirror is large in constitution, a large astigmatism occurs. By the occurrence of astigmatism, a deviation occurs between the focusing position of the projected image in the longitudinal direction and the focusing position in the lateral direction. Therefore, the longitudinal line and lateral line cannot be focused at the same time. As a result, clear image cannot be obtained, and only a blurry image is produced. In Fig. 4, the image modulated only by the third liquid crystal panel with transmission type 106c, out of the first liquid crystal panel with transmission type 106a, second liquid crystal panel with transmission type 106b, and third liquid crystal panel with transmission type 106c is led into the screen through reflection by the second reflecting mirror 109 and second color-light-combining dichroic mirror only, so that astigmatism does not occur. However, the image modulated by the second liquid crystal panel with transmission type 106b is transmitted through the second color-light-combining dichroic mirror 108 and hence astigmatism is caused, and the image modulated by the first liquid crystal panel with transmission type 106a is transmitted through the first color-light-combining color-light-combining dichroic mirror 107 and second dichroic mirror 108, and hence astigmatism also occurs. Therefore, when combining the images by projection of these images and adjusting the convergence, the multiplying factor in the longitudinal direction is different from the multiplying factor in the lateral direction owing to the presence of astigmatism. Consequently, the aspect ratio of the projected image varies depending on the value of astigmatism. As a result, it is difficult to match mutually the images projected by three modulating means, that is, the first liquid crystal panel with transmission type 106a, second liquid crystal panel with transmission type 106b, and third liquid crystal panel with transmission type 106c, and a color image having color blurring at end portions is obtained, and a clear image cannot be reproduced. In this way, when the light passes through parallel plane plates disposed obliquely on the optical path, the optical axis on the optical path is included, and the focusing position on the first plane along the incident light and transmitted light includes its optical axis, and hence a position deviation to the focusing position on the second plane orthogonal to the first plane occurs, so that astigmatism occurs. To solve such problems of color blurring and clouding in the projected image caused by occurrence of astigmatism, a projection type image display device using color combining dichroic mirrors of wedge shape is proposed. Generally, however, the dichroic mirrors for color-light-combination are demanded to have a high surface precision and precise spectral characteristic. Therefore, machining the color-light-combining dichroic mirrors into wedge shape is low in the yield of processing and very high in the manufacturing cost because the shape is complicated. Besides, the dichroic mirrors for color-light-combination are composed by applying a dichroic coat on the substrate, and the substrate is exposed to high temperature when applying the dichroic coat on the substrate, which requires to use substrates withstanding high temperature, such as glass and ceramics. Using such glass or ceramics, machining of dichroic mirrors into wedge shape requires a further higher cost. On the other hand, to decrease astigmatism, it is proposed to place a parallel plane plate of glass for correcting astigmatism at a position of distortion to the color-light-combining dichroic mirror, and at a position having a same light incident angle as the color-light-combining dichroic mirror. In such constitution, however, since the astigmatism correcting glass must be placed at a position of distortion to the color-light-combining dichroic mirror, it requires a wide space. Hence, the size of thus assembled projection type image display device is large depending on the angle of distortion. As the material, glass is needed, and the glass processing cost is high. An arrangement of the above type is shown in JP-H5-249408. EP-A-0602732 discloses an LCD image projection system in which light is divided into three colors and recombined in order to form a projected image. In this arrangement an astigmatism correcting means is included in the light path of one of the three colors in the form of a correcting plate inclined at the same angle with respect to the optical axis as a tilted plate which introduces astigmatism but rotated by 90° about the optical axis. The present invention provides a projection type color image display device comprising: a light-source for emitting a light having a first color-light, a second color-light, and a third color-light, a light-separating means for separating the light emitted from the light source into the first color-light, the second color-light, and the third color-light, means for modulating composed of a first modulating means for modulating the first color-light into a first image-light, a second modulating means for modulating the second color-light into a second image-light, and a third modulating means for modulating the third color-light into a third image-light; color-light-combining means having a function of combining the first image-light, the second image-light and the third image light; an astigmatism-correcting means; and a projecting means for projecting the combined first image-light, second image-light, and third image-light on a screen, characterized in that said color-light-combining means comprises a first color-light-combining element arranged to transmit the first image light and to reflect the second image light and a second color-light-combining element arranged to transmit the first image light and the second image light combined by the first color-light-combining element and to reflect the third image light, said first and second color-light-combining elements being disposed on an optical axis linking said first modulation means and the projecting means, each being arranged obliquely with respect to a plane orthogonal to said optical axis and intersecting said plane along respective first lines parallel with each other, said astigmatism correcting means being disposed on said optical axis between said first and second color-light-combining elements and arranged obliquely with respect to a plane orthogonal to said optical axis and intersecting said plane along a second line orthogonal to said first lines. Preferably the first and second color-light combining elements are plates having parallel planes with thicknesses t 1 and t 2 respectively, and have light incident to them at angle of incidence `a'. Also, preferably the astigmatism correction means is a plate having parallel planes with a thickness t 3 and having light incident to it at angle of incidence 'b'. The plurality of light-modulating means contain first light-modulating means for modulating the first color-light into a first image-light, second light-modulating means for modulating the second color-light into a second image-light, and third image light-modulating. The first color light passes through the first modulating means, first color-light-combining means, astigmatism-correcting means and second color-light-combining means, and gets into the projecting means to be projected on the screen. The second color-light passes through the second modulating means, reflects the back side of the first color-light-combining means, passes through the astigmatism-correcting means and second color-light-combining means, and gets into the projecting means to be projected on the screen. The third color-light passes through the third modulating means, reflects the back side of the second color-light-combining means, and gets into the projecting means to be projected on the screen. On the screen, a color-image-combining the first image by the first color-light, second image by the second color-light, and third image by the third color-light is displayed. In this embodiment, as the color-light-modulating means, for example, the light valves using liquid crystal panel with transmission type or liquid crystal panel with reflection type are used. As the light-combining means, for example, dichroic mirrors composed of inorganic matter such as glass and ceramics, or plastic materials and dichroic materials are used. As the astigmatism-correcting means, for example, plastics materials such as acrylic resin, or inorganic materials such as glass and ceramics are used. According to this constitution, a first astigmatism occurs in the first image passing through the first color-light-combining means and in the second image passing through the second color-light-combining means. On the other hand, in the first and second images passing through the astigmatism-correcting means, a second astigmatism occurs in a direction orthogonal to the first astigmatism. The first astigmatism and second astigmatism cancel with each other corresponding to the thickness, and the astigmatism is reduced on the image displayed on the screen. In the third image, which only reflects the back side of the second color-light-combining means, astigmatism does not occur. Therefore, the first image and second image small in astigmatism and the third image free from astigmatism are projected on the screen. As a result, a clear color image small in color blurring is displayed on the screen. At the same time, the projected image matched in focus and equal in the multiplying factor in longitudinal and lateral directions is obtained, and a projection type color image display device of further advanced picture quality is obtained. Moreover, since the astigmatism-correcting means is placed on the plane orthogonal to the color-light-combining means, the size of the assembled projection type image display device can be reduced. It is further possible to compose the astigmatism-correcting means and each color-light-combining means having a great thickness, and hence it is possible to compose the astigmatism-correcting means and color-light-combining means having a high surface precision, so that a more excellent clear color image may be obtained. Still more, by injection molding and compression molding using plastics, the astigmatism-correcting means and color-light-combining means having a great thickness can be manufactured, and a more inexpensive image display device is obtained. In the invention, the thickness 't 3 ' of the astigmatism-correcting means is particularly desired to be greater than the thickness 't 1 ' of the first color-light-combining means and thickness 't 2 ' of the second color-light-combining means. In this constitution, the correction amount of astigmatism is larger, and a projection type image display device having further excellent effects may be obtained. It is particularly preferred to constitute so that the thickness 't 3 ' of the astigmatism correcting means may be larger than the thickness 't 1 ' and 't 2 ' of the color-light-combining means, and that the light incident angle 'b' into the astigmatism correcting means may be smaller than the light incident angle 'a' in to each color light combining means . When the light incident angle 'b' into the astigmatism-correcting means is small, the correction a mount by the astigmatism-correcting means is smaller, but by increasing the thickness of the astigmatism-correcting means, the correction amount increases. Therefore, decrease of the correction amount due to light incident angle into the astigmatism-correcting means is canceled by the increase of the correction amount due to the thickness of the astigmatism-correcting means. As a result, the correction effect by the astigmatism-correcting means is maintained. In addition to these effects, since the light incident angle 'b' into the astigmatism-correcting means is small, the space for installing the astigmatism-correcting means is saved, so that the size of the projection type image display device is reduced. In this constitution, the astigmatism-correcting means is disposed between the first color-light-combining means and second color-light-combining means, but instead the astigmatism-correcting means may be disposed between the first color-light-combining means and the first light-modulating means. In such constitution, the first color-light passes through the first modulating means, astigmatism-correcting means, first color-light-combining means and second color-light-combining means, and gets into the projecting means to be projected on the screen. The second color-light passes through the second-modulating means, reflects the back side of the first color-light-combining means, passes through the second color-light-combining means, and gets into the projecting means to be displayed on the screen. The third color-light passes through the third modulating means, reflects the back side of the second color-light-combining means, and gets into the projecting means to be projected on the screen. In this constitution, too, although the effects are slightly inferior to those of the constitution in which the astigmatism-correcting means is disposed between the first color-light-combining means and second color-light-combining means, it has better effects than the prior art. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a top view showing a constitution of a projection type image display device in an embodiment of the invention; Fig. 2 is a side view of essential composition of the projection type image display device shown in Fig. 1; Fig. 3A is a top view of essential pats for explaining the astigmatism of the projection type image display device shown in Fig, 1; Fig. 3B is a side view of essential parts for explaining the astigmatism of the projection type image display device shown in Fig. 1; Fig. 4 is a top view showing a constitution of a conventional projection type image display device; and Fig. 5 is a magnified view of essential parts in Fig. 4, being a top view for explaining the principle of occurrence of astigmatism due to color-light-combining dichroic mirror having a parallel plane plate shape. Reference Numerals 1 Light source 2 Concave reflector 3 First light-separating dichroic mirror 4 Second light-separating dichroic mirror 5 First reflecting mirror 6a First liquid crystal panel with transmission type 6b Second liquid crystal panel with transmission type 6c Third liquid crystal panel with transmission type 7 First colour-light-combining dichroic mirror 8 Second colour-light-combining dichroic mirror 9 Second reflecting mirror 10 Projection lens 11 Optical axis 12 Astigmatism-correcting means 13a First incident side polarizer 13b Second incident side polarizer 13c Third incident side polarizer 14a First exit side polarizer 14b Second exit side polarizer 14c Third exit side polarizer 20 Screen 101 Light source 102 Reflector 103 First light-separating dichroic mirror 104 Second light-separating dichroic mirror 105 First reflecting mirror 106a First liquid crystal panel 106b Second liquid crystal panel 106c Third liquid crystal panel 107 First color-light-combining dichroic mirror 108 Second color-light-combining dichroic mirror 110 Projection lens 120 Screen DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, preferred embodiments of the projection type image display device of the invention are described in detail below. Fig. 1 is a top view showing a constitution of the projection type image display device of the invention, and Fig. 2 is a side view of essential parts of the projection type image display device shown in Fig. 1. In Fig. 1, a light source 1 has a blue light which is a first color-light, a red light which is a second color-light, and a green light which is a third color-light. Behind the light source 1, a concave light reflector 2 is disposed, and the light emitted from the light source 1 is controlled into a unidirectional light by the light reflector 2. In the direction of this unidirectional light, a first light-separating dichroic mirror 3 is disposed as first light-separating means, on a first plane including the optical axis of the light, obliquely to the first plane. The first light-separating dichroic mirror 3 has a property of reflecting blue light and passing red light and green light. In the direction of red light and green light passing through the first light-separating dichroic mirror 3, there is a second light-separating dichroic mirror 4 which is second light-separating means, disposed on the first plane including the optical axis, obliquely to the first plane. The second light-separating dichroic mirror 4 has a property of reflecting red light and passing green light. Thus, the light emitted from the light source 1 is separated into blue light, red light, and green light bythe first light-separating dichroic mirror 3 and second light-separating dichroic mirror 4 as light-separating means. The blue light reflected by the first light-separating dichroic mirror 3 is reflected by a first reflector 5 disposed in the reflected direction, obliquely to the first plane. The reflected blue light enters first light-modulating means disposed in the reflected direction, orthogonal to the optical axis on the first plane. The first light-modulating means is a light valve composed of a first liquid crystal panel with transmission type 6a, a first incident side polarizer 13a disposed at the light incident side of the liquid crystal panel 6a, and a first exist side polarizer 14a disposed at the light exit side of the liquid crystal panel 6a. First, the blue light reflected by the first reflector 5 is polarized into light having a wavelength limited in the oscillation direction by the first incident side polarizer 13a, and the polarized light enters the first liquid crystal panel 6a. The first liquid crystal panel 6a is composed of a plurality of independent picture elements, and each picture element can be controlled independently by an external signal. The light entering a picture element which is to be displayed in black passes through this picture element, and is absorbed consequently in the first exit side polarizer 14a. On the other hand, the light entering a picture element which is to be displayed in blue passes through this picture element without changing the oscillation direction of its wave, and consequently passes through the first exit side polarizer 14a. In this way, a blue light image is created in the first light-modulating means. The red light reflected by the second light-separating dichroic mirror 4 enters the second light-modulating means disposed in the direction of the reflected light, orthogonal to the optical axis. The second light-modulating means is composed of a second liquid crystal panel with transmission type 6b, a second incident side polarizer 13b disposed at the light incident side of the second liquid crystal panel 6b, and a second exit side polarizer 14b disposed at the light exit side of the second liquid crystal panel 6b. The red light entering the second light-modulating means is modulated by the second light-modulating means, in the same manner as in the case of the blue light, and a red light image is created. The green light passing through the second light-separating dichroic mirror 4 enters the third light-modulating means disposed in the direction of the transmitted light, orthogonal to the optical axis. The third light-modulating means is composed of a third liquid crystal panel with transmission type 6c, a third incident side polarizer 13c disposed at the light incident side of the third liquid crystal panel 6c, and a third exit side polarizer 14c disposed at the light exit side of the third liquid crystal panel 6c. The green light entering the third light-modulating means is modulated by the third light-modulating means, in the same manner as in the case of the blue light, and a green light image is created. The function of the incident side polarizers 13a, 13b, 13c is to transmit only the light having the waves limited in the oscillating direction, among the incident light. The function of the exit side polarizers 14a, 14b, 14c is to limit the direction of polarization by passing only the light having the waves in the oscillating direction, having the axis of transmission distorted by 90 degrees to each incident side polarizer. A first color-light-combining dichroic mirror 7 which is first color-light-combining means is disposed in the direction of the light emitted from the first exit side polarizer 14a, on a first plane including its optical axis 11, obliquely at a light incident angle of 'a' to the first plane, and at the same time the first color-light-combining dichroic mirror 7 is disposed in the direction of the light emitted from the second exit side polarizer 14b, on a first plane including its optical axis 11, obliquely to the first plane. The optical axis 11 is an axis of an optical path linking the liquid crystal panels 6a, 6b, 6c, and a projection lens 10. The first color-light-combining dichroic mirror 7 is a plate with parallel planes having a refractive index 'N 1 ' and a thickness 't 1 '. The image of blue light created by the first light-modulating means passes through the first color-light-combining dichroic mirror 7, and the image of red light created by the second light-modulating means is reflected by the back side of the first color-light-combining dichroic mirror 7. In this way, the first color-light-combining dichroic mirror 7 is disposed so that the transmitted blue light and reflected red light may be same in direction and same in position. Thus, the first color-light-combining dichroic mirror 7 combines the blue light image and red light image. The image of green light created by the third light-modulating means is reflected by a second reflector 9 disposed in the direction of the light and obliquely to its optical axis. The reflected green light is reflected by a second color-light-combining dichroic mirror 8 which is second color-light-combining means. The second color-light-combining dichroic mirror 8 is disposed in the direction of blue light and red light emitted from the first color-light-combining dichroic mirror 7, on a first plane including its optical axis, obliquely at a light incident angle of (a) to the first plane. At the same time, the second color-light-combining dichroic mirror 8 is disposed in the direction of the light reflected by the second reflector 9, obliquely to its first plane. The second color-light-combining dichroic mirror 8 is a plate of parallel planes having a refractive index 'N 2 ' and a thickness 't 2 '. The images of blue light and red light pass through the second color-light-combining dichroic mirror 8, and the image of green light is reflected by the back side of the second color-light-combining dichroic mirror 8. The second color-light-combining dichroic mirror 8 is disposed so that the images of the transmitted blue light and red light and the image of the reflected green light may be same in direction and same in position. The images of the blue light and red light passing through the second color-light-combining dichroic mirror 8 and the image of the green light reflected by the mirror 8 are combined. The combined images get into the projection lens 10. The images of the colors getting-into the projection lens 10 are projected on the screen 20. A color image combined from the blue light, red light and green light is displayed on the screen 20. Astigmatism-correcting means 12 is disposed between the first color-light-combining dichroic mirror 7 and second color-light-combining dichroic mirror 8. The astigmatism- correcting means 12 comprises the optical axis 11 of the light emitted from the first color-light-combining dichroic mirror 7, and is disposed on a second plane orthogonal to the first plane, obliquely at a light incident angle of 'b' to the second plane. The astigmatism-correcting means 12 is a plate of parallel planes having a refractive index 'N 3 ' and thickness 't 3 '. To eliminate distortion in the image displayed on the screen 20, the first color-light-combining dichroic mirror 7, second color-light-combining dichroic mirror 8, second reflecting mirror 9, and astigmatism-correcting means 12 are machined to have a high surface precision. In this constitution, the projection type image display device without astigmatism-correcting means 12 is discussed. In the projection type image display device without astigmatism-correcting means 12, an astigmatism proportional to the plate thickness of the second color-light-combining dichroic mirror 8 occurs in the image on the optical path of red light. Moreover, in the image on the optical path of blue light, an astigmatism proportional to the sum of the plate thickness of the first color-light-combining dichroic mirror 7 and plate thickness of the second color-light-combining dichroic mirror 8 occurs. However, no astigmatism occurs in the image on the optical path of green light. Therefore, the projected image of red light and blue light differs in the focal position in the longitudinal direction and lateral direction, and an off-focus blurry image is obtained on the screen 20, and clear image is not obtained. Moreover, in the projected images of red light, blue light and green light on the screen 20, the longitudinal multiplying factor and lateral multiplying factor differ. Therefore, it is hard to combine the three images of blue light, red light and green light by convergence adjustment, and the color image displayed on the screen 20 is blurry and cloudy in color, and clear image is not obtained. By contrast, by installing the astigmatism-correcting means 12 as in this embodiment, a clear image is obtained. The action and effect of the astigmatism-correcting means 12 are explained below by referring to Fig. 3A and Fig. 3B. Fig. 3A is a top view of essential parts of the projection type image display device shown in Fig. 1, and Fig. 3B is its side view. In Fig. 3A and Fig. 3B, on the first plane including the optical axis 11, the first color-light-combining dichroic mirror 7 with thickness 't 1 ', and second color-light-combining dichroic mirror 8 with thickness 't 2 ' are disposed obliquely at light incident angle of 'a' to the first plane. Between the first color-light-combining dichroic mirror 7 and second color-light-combining dichroic mirror 8, the astigmatism-correcting means 12 with thickness 't 3 ' is disposed, orthogonal to the first plane, on a second plane including the optical axis, obliquely at light incident angle of 'b' to the first plane. The thickness 't 3 ' of the astigmatism-correcting means 12 is greater than the thickness 't 1 ' of the first color-light-combining dichroic mirror 7 and thickness 't 2 ' of the second color-light-combining dichroic mirror 8. The image of blue light emitted from the first light-modulating means enters the first color-light-combining dichroic mirror 7 at angle 'a 1 '. The image of the entering blue light passes through the first color-light-combining dichroic mirror 7 at angle of approach 'a 2 ' corresponding to the refractive index 'N 1 ', and leaves at angle 'a 1 ' to the first plane. At this time, in the image of the emitted blue light, astigmatism 'Y 1 ' t 1 (1-cos 2 (a 1 )/cos 2 (a 2 )) [N 1 · cos(a 2 ) = Y 1 occurs. The image of the blue light leaving the first color-light-combining dichroic mirror 7 enters the astigmatism-correcting means 12 at angle 'b 1 '. The image of the entering blue light passes through the astigmatism-correcting means 12 at angle of approach 'b 2 ' corresponding to the refractive index 'N 3 ', and is emitted to the second plane at angle 'b 1 '. At this time, in the image of the emittedblue light, astigmatism 'Y 3 ' t 3 (1-cos 2 (a 1 )/cos 2 (a 2 )) [N 3 · cos(a 2 ) = Y 3 occurs. The image of the blue light leaving the astigmatism-correct-ing means 12 enters the second color-light-combining dichroic mirror8 at angle 'a 1 '. The image of the entering blue light passes through the second color-light-combining dichroic mirror 8 at angle of approach 'b 2 ' corresponding to the refractive index 'N 2 ', and is emittedat angle 'a 1 ' to the first plane. At this time, in the image of the emitted blue light, astigmatism 'Y 2 ' t 3 (1-cos 2 (a 1 )/cos 2 (a 2 )) [N 2 · cos(a 2 ) = Y 2 occurs. The image of the blue light leaving the second color-light-combining dichroic mirror 8 enters the projection lens 10, and is displayed on the screen 20. That is, the astigmatism of the image of the blue light displayed on the screen 20 is as follows. Y 1 + Y 2 - Y 3 The image of the red light reflected by the first color-light-combining dichroic mirror 7 enters the astigmatism-correcting means 12 at angle b 1 . The image of the incident red light passes through the astigmatism-correcting means 12 at angle of approach 'b 2 ' corresponding to the refractive index 'N 3 ', and leaves at angle 'b 1 ' to the second plane. At this time, astigmatism 'Y 3 ' occurs in the image of the emitted red light. The image of the red light leaving the astigmatism-correcting means 12 enters the second color-light-combining dichroic mirror8 at angle 'a 1 '. The image of the entering red light passes through the second color-light-combining dichroic mirror 8 at angle of approach 'a 2 ' corresponding to the refractive index 'N 2 ', and leaves at angle 'a 1 ' to the first plane. At this time, astigmatism 'Y 2 ' occurs inthe image of the emitted red light. The image of the red light leaving the second color-light-combining dichroic mirror 8 enters the projection lens 10 and is displayed on the screen 20. That is, the astigmatism of the image of red light displayed on the screen 20 is as follows. Y 2 - Y 3 The image of the green light created by the third light-modulating means is reflected by the second reflecting mirror 9, and is further reflected by the back side of the second color-light-combining dichroic mirror 8, and enters the projection lens 10. That is, the image of the green light created by the third light-modulating means does not pass through any plate with parallel planes. Therefore, the image of the green light displayed on the screen 20 is free from astigmatism. In the projection type color image display device for displaying a color image by combining images of colors, by disposing plates of parallel planes of a greater plate thickness than the plate thickness of the color-light-combining dichroic mirror in the optical paths of the color-lights generating astigmatism, the difference of astigmatism extents of the blue image, red image and green image can be decreased. As a result, cloudiness due to astigmatism can be decreased, so that a clearer image may be obtained. (Example 1 and Comparison 1) In the constitution of the projection type image display device without astigmatism-correcting means, supposing: thickness 't 1 ' of first color light-combining dichroic mirror 7 to be 2 mm, refractive index 'N 1 ' of first color-light-combining dichroic mirror 7 to be 1.52, installation angle (a) of first color-light-combining dichroic mirror 7 to be 45 degrees, thickness 't 2 ' of second color-light-combining dichroic mirror 8 to be 2 mm, refractive index 'N 2 ' of second color-light-combining dichroic mirror 8 to be 1.52, and installation angle 'a' of second color-light-combining dichroic mirror 8 to be 45 degrees, then: the astigmatism of blue light image is 1.076 mm, the astigmatism of red light image is 0.538 mm, and the astigmatism of green light image is 0 mm. That is, the maximum difference among images is 1.076 mm. By contrast, in the constitution with astigmatism-correcting means 12 of parallel plane plate, supposing: the thickness of astigmatism-correcting means 12 is greater than the thickness of color-light-combining dichroic mirror, and the installation angle of astigmatism-correcting means 12 is smaller than the installation angle of color-light-combining dichroic mirror, that is, thickness 't 3 ' of astigmatism-correcting means 12 is 7.3 mm, refractive index 'N 3 ' of astigmatism-correcting means 12 is 1.49, and installation angle 'b' of astigmatism-correcting means 12 is 30 degrees, then the correction value of astigmatism by astigmatism-correcting means 12 is 0.805 mm. Therefore, the astigmatism of blue light image is 0.271 mm, the astigmatism of red light image is -0.267 mm, and the astigmatism of green light image is 0 mm. That is, the maximum difference among images is 0.538 mm. Thus, by installing the astigmatism-correcting means 12 of parallel plane plate in which the thickness of astigmatism-correcting means is greater than the thickness of the color-light-combining dichroic mirror, and the installation angle of astigmatism-correcting means is smaller than the installation angle of the color-light-combining dichroic mirror, the astigmatism of the images of blue light and red light can be notably decreased. Therefore, the difference in astigmatism of the images of blue light, red light and green light projected on the screen is decreased. As a result, in the color image displayed on the screen, occurrence of blurry color and cloudiness is decreased, and a clear color image is obtained. (Example 2) When the installation angle of the astigmatism-correcting means 12 is smaller than the installation angle of the color-light-combining dichroic mirror, and the thickness of the astigmatism-correcting means 12 is nearly same as the thickness of the color-light-combining dichroic mirror (in the astigmatism-correcting means of which thickness is smaller than the thickness of the color-light-combining dichroic mirror, it is hard to obtain high precision on the parallel planes owing its manufacturing process, and hence the thickness of the astigmatism-correcting means is set nearly same as the thickness of the color-light-combining dichroic mirror), for example, instead of the example illustrated above, if the thickness of the astigmatism-correcting means is 2.0 mm, the correction value of astigmatism by the astigmatism-correcting means 12 is 0.221. Therefore, the astigmatism of blue light image is 0.855 mm, the astigmatism of red light image is 0.317 mm, and the astigmatism of green light image is 0 mm. That is, the maximum difference among images is 1.172 mm, and hence the maximum difference among images is decreased by installation of the astigmatism-correcting means. As described herein, when the light passes through the color-ight-combining dichroic mirror of parallel plane shape disposed obliquely on the optical path, the imaging position on the first plane parallel to the incident light and transmitted light including the optical axis is deviated from the imaging position on the second plane orthogonal to the first plane and including the optical axis, so that astigmatism occurs. As in the invention, when the astigmatism-correcting means of parallel plane shape is installed on the second plane, astigmatism 'Y 1 , Y 2 ' due to the color-light-combining dichroic mirror of parallel plane shape and astigmatism 'Y 3 ' due to the astigmatism-correcting means of parallel plane shape occur, but since the astigmatism 'Y 3 ' occurs in a direction orthogonal to the astigmatism 'Y 1 , Y 2 ', the astigmatism 'Y 3 ' and astigmatism 'Y 1 , Y 2 ' cancel each other. Therefore, vagueness of the image due to astigmatism 'Y 1 , Y 2 ' caused by the color-light-combining dichroic mirrors can be reduced. In particular, in the constitution of installing the astigmatism-correcting means having parallel planes between the first color-light-modulating means and projecting means, on the second plane including the optical axis, with the thickness of the astigmatism-correcting means greater than the thickness of the light-color-combining dichroic mirrors, the correction amount of astigmatism is larger. Therefore, the difference of astigmatism among images of color-lights is much smaller, so that a superior clear color image may be obtained. What is particularly preferred is a constitution in which the thickness of the astigmatism-correcting means is greater than the thickness of the color-light-combining dichroic mirrors and the light incident angle into the astigmatism-correcting means is smaller than the light incident angle into the color-light-combining dichroic mirrors. That is, when the light incident angle into the astigmatism-correcting means is small, the correction amount by the astigmatism-correcting means is small, but by increasing the thickness of the astigmatism-correcting means, the correction amount increases. Therefore, the decrease of correction amount due to the light incident angle into the astigmatism-correcting means is canceled by the correction amount due to the thickness of the astigmatism-correcting means. As a result, the space for installing the astigmatism-correcting means is saved, and the size of the projection type image display device is reduced. (By contrast, when the light incident angle into the astigmatism-correcting means is large, although the correction amount by the astigmatism-correcting means increases, it requires a wider space for installing the astigmatism-correcting means and the size of the projection type image display device increases.) Simultaneously with these effects, the difference in astigmatism of the images of blue light, red light and green light projected on the screen is decreased. As a result, in the color image displayed on the screen, occurrence of blurry color and cloudiness is decreased, and a clear color image is obtained. Moreover, as the astigmatism-correcting means and color-light-combining means, plates of parallel planes of relatively large thickness are composed, so that plates of parallel planes having a high precision may be easily obtained. Besides, the astigmatism-correcting means of parallel plane shape of high precision having a relatively large thickness can be easily obtained by molding, such as injection molding and compression molding, by using plastic materials. Therefore, the cost can be reduced. In addition, since the astigmatism-correcting means is installed on the plane orthogonal to the color-light-combining means, the size of the assembled projection type image display mans is small and compact. In the invention, other various modified examples are possible. For example, is the light-modulating means, instead of the liquid crystal panel, components capable of controlling the optical path by electric signal may be used. Depending on the optical paths of images of color lights, a plurality of astigmatism correction means may be placed at optimum positions. Moreover, as the color light-combining means, instead of the color-light-combining dichroic mirrors, filters for transmitting, cutting off or reflecting the light selectively depending on the wavelength may be used. As the light-separating means, instead of the two light-separating dichroic mirrors, a combination of an arbitrary number may be possible, or filters for controlling selectively the wavelength of light may be used. Further depending on the optical paths of images of color-lights, an arbitrary number of color light-combining means may be placed at optimum positions.

Claims: A projection type color image display device comprising: a light-source (1) for emitting a light having a first color-light, a second color-light, and a third color-light, a light-separating means (3) for separating the light emitted from the light source into the first color-light, the second color-light, and the third color-light, means for modulating (6a, 13a, 14a; 6b, 13b, 14b; 6c, 13c, 14c) composed of a first modulating means (6a, 13a, 14a) for modulating the first color-light into a first image-light, a second modulating means (6b, 13b, 14b) for modulating the second color-light into a second image-light, and a third modulating means (6c, 13c, 14c) for modulating the third color-light into a third image-light; color-light-combining means (7, 8) having a function of combining the first image-light, the second image-light and the third image light; an astigmatism-correcting means (12); and a projecting means (10) for projecting the combined first image-light, second image-light, and third image-light on a screen (20), characterized in that said color-light-combining means comprises a first color-light-combining element (7) arranged to transmit the first image light and to reflect the second image light and a second color-light-combining element (8) arranged to transmit the first image light and the second image light combined by the first color-light-combining element and to reflect the third image light, said first and second color-light-combining elements (7, 8) being disposed on an optical axis linking said first modulation means (6a, 13a, 14a) and the projecting means, each being arranged obliquely with respect to a plane orthogonal to said optical axis and intersecting said plane along respective first lines parallel with each other, said astigmatism correcting means (12) being disposed on said optical axis between said first and second color-light-combining elements (7, 8) and arranged obliquely with respect to a plane orthogonal to said optical axis and intersecting said plane along a second line orthogonal to said first lines. A projection type color image display device of claim 1, wherein the shape of each of the astigmatism-correcting means (12), the first color-light-combining element (7) and the second color-light-combining element (8) is a parallel plane shape, and a thickness of the astigmatism-correcting means is greater than each thickness of the first color-light-combining element and the second color-light-combining element. A projection type color image display device of claim 1, wherein the shape of each of the astigmatism-correcting means (12), the first color-light-combining element (7), and the second color-light-combining element (8) is a parallel plane shape, and a light incident angle into the astigmatism-correcting means (12) is smaller than each light incident angle into the first color-light-combining element (7) and the second color-light-combining element (8). A projection type color image display device of claim 1: wherein the astigmatism-correcting means (12) has a parallel plane shape, a thickness of the astigmatism-correcting means (12) is greater than each thickness of the first color-light-combining element (7) and the second color-light-combining element (8), and a light incident angle into the astigmatism-correcting means (12) is smaller than each light incident angle into the first color-light-combining element (7) and the second color-light-combining element (8). A projection type color image display device of any of claims 1-4, wherein the first color-light-combining element (7) is a first dichroic mirror containing a dichroic coat, and the second color light combining element (8) is a second dichroic mirror containing a dichroic coat. A projection type color image display device of any of claims 1-5, wherein the means for modulating is composed of a liquid crystal panel. A projection type color image display device of any of claims 1-6, wherein the astigmatism-correcting means includes moulded plastic.

Inventors: YAMAGISHI SHIGEKAZU, YAMAGISHI, SHIGEKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H04N9
IPC codes: G03B33, G02F1, G03B21, H04N9