Abstract:
An apparatus receptive of illumination light from an illumination light source for providing a color image on a projection lens includes three displays responsive to a video signal for modulating incident lights to display respective component images, a first dichroic layer for separating the illumination light into a first component light and a first mixed light of different wavelength regions to give the first component light to one of the displays, a second dichroic layer for separating the mixed light into second and third component lights of different wavelength regions to give the second and third component lights to other two of the displays respectively, a third dichroic layer for synthesizing, out of the first, second and third information lights from the displays, the first and second information light, and a fourth dichroic layer for further synthesizing the light synthesized by the third dichroic layer with the third information light.

Description:
This application is a continuation of application Ser. No. 07/752,202, filed Aug. 21, 1991, now abandoned, which is a continuation of application Ser. No. 07/637,804, filed Jan. 7, 1991, now abandoned, which is a divisional application of Ser. No. 07/147,519, filed Jan. 25, 1988, issued as U.S. Pat. No. 4,989,076, on Jan. 29, 1991. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a liquid crystal display (hereinafter abbreviated to LCD) projection apparatus in which a video signal formed from the entering white light is separated into chrominance components of three primary colors, and then the components after having been modulated by using respective monochrome video image display devices such as LCD ones are synthesized again to permit projection of an image in full color. 
     2. Description of the Related Art 
     Recently, television sets of the projection type have rapidly come into such wide use that they can be seen not only in the public facilities but also in home. This kind of apparatus uses three high-luminance cathode ray tubes in which are made up color component images corresponding to the respective color component lights of red (R), green (G) and blue (B) and these images are projected by projection lenses onto a screen where they are synthesized to display a picture of the original color. FIG. 1 shows the outline of the video projection apparatus. 1, 2 and 3 are cathode ray tubes corresponding to the color components R, G and B respectively and are driven by drive circuits 4, 5 and 6 for R, G and B into which the video signals of the color components R, G, B enter respectively. 7, 8 and 9 are projection lenses, and are each arranged in focus on a screen 10 in front of the cathode ray tubes 7, 8 and 9. Note, in this figure, the projection lens is shown by a single lens, but in actual practice is usually constructed with a plurality of lenses for correction of various aberrations. 
     However, such an apparatus increases in size, and, when the distance to the screen is changed, re-adjustment is required so that the three monochrome projected images overlap one another on the screen. 
     Therefore, as an arrangement for the possibility of projecting by one projection lens P, what is shown in FIG. 2 is considered. In the figure, S is an illumination light source issuing white light, for example, for use in the Koehler illumination system. As means for separating parallel entering light A which is white light into three primary colors, use is made of dichroic mirrors 11 and 12 having two dichroic layers of different wavelength regions crossed to each other. For example, the dichroic mirror 11 reflects the blue color component B, and the dichroic mirror 12 reflects the red color component R. Of the three color components R, G and B separated by this crossed dichroic mirrors 11 and 12, the color component R is reflected by total reflection mirrors 13 and 14, the color component G goes straight as it is, and the color component B is reflected by total reflection mirror 15 and 16, entering LCDs 17, 18 and 19 corresponding to the respective ones. Because in the LCDs 17, 18 and 19, pictures of each color component of red, green and blue are imaged out, when light permeates here, the variation of transmittance due to the pictures of each of the LCDs 17, 18 and 19 is modulated to the variation of intensity of light. 
     That is, in FIG. 2, color components R, G and B of red, green and blue are modulated by the video signals of the LCDs 17, 18 and 19, respectively, becoming color components (color information light) R&#39;, G&#39; and B&#39;. These lights are synthesized again by a dichroic prism 20 to produce an exiting light A&#39;. Note, the exiting light A&#39; is, as known in the art, projected by the projection lens P onto the screen. On the screen, a picture of full color comes out. In such a manner, in the prior known apparatus, the two dichroic mirrors 11 and 12 are used in the crossed state. Therefore, a portion of the reflection surface of one dichroic mirror 11 is stripped off, giving a drawback that the picture is partly broken down. For example, in FIG. 3, on assumption that the dichroic mirrors 11 and 12 cross each other at right angles, the relationship between the thickness t of the dichroic mirror and the size t&#39; of the broken portion of the reflection surface is expressed by the following equation: 
     
         t&#39;=t/2.sup.0.5 . . .                                       (1) 
    
     The value of t&#39; of this equation (1) cannot be ignored, and a problem arises that by that portion alone, the center of, for example, the blue is broken down to a stripe shape. Also, the dichroic prism 20 used for synthesizing the modulated color components in FIG. 2 is very high in price, and its weight is also large, being unsuited to be used in home. Further, the optical path from the entering light A to the exiting light A&#39; differs from color component to component. Therefore, the problem of aberration etc. also is large. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a projection apparatus in which, by improving such drawbacks of the conventional example, it is made possible that only the dichroic mirrors even separate and synthesize light, and particularly by using such an arrangement that the dichroic mirrors do not cross each other, the problem of the image fracture is eliminated, and, moreover, the optical paths of all the color components can be equalized to each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of arrangement illustrating a conventional example. 
     FIG. 2 is an optical section view of a projection apparatus. 
     FIG. 3 is a view in enlarged scale of a portion of FIG. 2. 
     FIG. 4 is an optical section view illustrating an embodiment of the invention. 
     FIG. 5 is a optical section view illustrating another embodiment. 
     FIG. 6 is an optical section view illustrating an example of variation of the embodiment of FIG. 4. 
     FIG. 7 is a perspective view illustrating one constituent part. 
     FIG. 8 and FIG. 9 are respectively enlarged views of the main parts. 
     FIG. 10 is an exploded perspective view of a liquid crystal display device. 
     FIG. 11 is a diagram illustrating an example of an illumination device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in detail on the basis of an embodiment shown in FIG. 4. In the figure, the illustration of the illumination light source S and the projection lens P is omitted. 
     In FIG. 4, 21 is a first dichroic mirror for spectral light separation reflecting, for example, red and blue color components R and B. A second dichroic mirror 22 for spectral light separation separates such reflected color components R and B. As the second dichroic mirror 22, use is made of the property of reflecting the blue color component B. The first and second dichroic mirrors 21 and 22 for spectral light separation are arranged parallel in a direction of 45° with the incident light A. On both sides of the dichroic mirrors 21 and 22 for spectral light separation are arranged total reflection mirrors 23 and 24 in parallel with the dichroic mirrors 21 and 22. Also, in the directions of prolongation of the first and second dichroic mirrors 21 and 22 for spectral light separation, there are arranged respectively a first dichroic mirror 25 for synthesis to reflect the blue color component B and a second dichroic mirror 26 for synthesis to reflect the red color component R. An LCD 27 is arranged between the total reflection mirror 23 and the first synthesizing dichroic mirror 25 in a direction parallel to the incident light A, and another LCD 28 is arranged between the first and second spectral separation dichroic mirrors 21 and 22 in a direction perpendicular to the incident light A. Yet another LCD 29 is arranged between the second spectral separation dichroic mirror 22 and the total reflection mirror 24 in a direction parallel to the incident light A. 30 is a framework supporting each constituent part. 
     The incident light A impinges on the first spectral separation dichroic mirror 21. The green color component G passed here is reflected by the total reflection mirror 23 and then enters the LCD 27. The blue color component B reflected from the first spectral separation dichroic mirror 21 and then from the second spectral separation dichroic mirror 22 enters the LCD 28, and the red color component R passed through the second spectral separation dichroic mirror 22 enters the LCD 29. The LCDs 27, 28 and 29 have images corresponding to the green, blue and red color components G, B and R, respectively. These images depend on the variations of transmittance of the LCDs 27, 28 and 29, and the respective color components G, B and R are modulated to the variation of intensity of light, becoming color components (color information light) G&#39;, B&#39; and R&#39;. These modulated color components G&#39;, B&#39; and R&#39; are synthesized again and projected as an exiting light A&#39; of full color on the screen. Therefore, all the color components have the same optical length. 
     In the illustrated case, the green color component G&#39; modulated by the LCD 27 is produced through the first and second synthesizing dichroic mirrors 25 and 26. Also, the blue color component B&#39; modulated by the LCD 28 is reflected by the first synthesizing dichroic mirror 25 to combine with the green color component G&#39;. Further, the red color component R&#39; modulated by the LCD 29 after having been reflected from the total reflection mirror 24 is reflected by the second synthesizing dichroic mirror 26 where it is combined with the green and blue color components G&#39; and B&#39; to become the exiting light A&#39;. 
     In the second embodiment shown in FIG. 5, the first and second spectral separation dichroic mirrors 31 and 32 are superimposed in a direction of 45 degrees with the incident light A. On the prolongation of these are arranged the synthesizing dichroic mirrors 33 and 34. In the first spectral separation dichroic mirror 31, one reflecting the red color component R is used. Further, as the second spectral separation dichroic mirror 32 for separating the green and blue color components passed through the first spectral separation dichroic mirror 31, use is made of one reflecting the blue color component B. The red color component R enters the LCD 36 passed the total reflection mirror 35, and the green and blue color components G and B separated by the spectral separation dichroic mirror 32 enter the LCDs 37 and 38 respectively. Also, in the first synthesizing dichroic mirror 33 for synthesizing the color components R&#39; and B&#39; modulated by the LCDs 36 and 38, one having the property of reflecting blue is used. Further, in the second synthesizing dichroic mirror 34 for synthesizing the green color component G&#39;, one having the property of reflecting the red and blue color components is used. Note, the green color component G&#39; modulated by the LCD 37 is reflected from the total reflection mirror 39 and then passes through the second synthesizing dichroic mirror 34 and then, as has been described above, is synthesized with the color components R&#39; and B&#39; to become the exiting light A&#39; of full color. 
     In each embodiment described above, for example, the red and blue optical paths may be reversed. In this case, if each dichroic mirror and LCD are so properly chosen as to suit the respective color components, various combinations are possible. 
     FIG. 6 shows the structure of another LCD projection apparatus. Here, a first composite dichroic mirror 41 is obtained by unifying the first spectral separation dichroic mirror 21 and the first synthesizing dichroic mirror 25 which position themselves in the same plane in FIG. 4, and a second composite dichroic mirror 42 is obtained by unifying the second spectral separation dichroic mirror 22 and the second synthesizing dichroic mirror 26 which position themselves in the same plane in FIG. 4 are used. All the other constituent elements are like those of FIG. 4, and are denoted by the same reference characters. 
     The first and second composite dichroic mirrors 41 and 42 are, for example, as shown in FIG. 7, formed on a common glass substrate 43 in separation at the left-hand half S1 and the right-hand half S2 with coatings of different properties from each other. For example, in the case of the second composite dichroic mirror 42, as shown in FIG. 8, a reflection coating 44 for reflecting a blue component is applied on the left half on the common glass substrate 43, and a reflection coating 45 for reflecting a red component on the right half so that the left half constitutes a dichroic mirror which reflects blue, and the right half another dichroic mirror which reflects red. Also, in the case of the first composite dichroic mirror 41, as shown in FIG. 9, after the coating 44 for reflecting blue has been applied over the entire area of the glass substrate 43, then the coating 45 for reflecting red is applied only to the left half in superimposed relation. Thus, a dichroic mirror which reflects blue is formed in the right half, and another dichroic mirror which reflects blue and red is formed in the left half. 
     In other words, the first composite dichroic mirror 41 has both functions of the first spectral separation and the first synthesizing dichroic mirrors 21 and 25 of FIG. 4, and the second composite dichroic mirror 42 has both functions of the second spectral separation and the second synthesizing dichroic mirrors 22 and 26 of FIG. 4. Therefore, the operational principle becomes the same as that of FIG. 4. 
     That is even in FIG. 6, the incident light A on the left half of the first composite dichroic mirror 41 reflects the red color component R and the blue color component B, while the green color component G goes straight as it is and enters the LCD 27 through the total reflection mirror 23. Also, of the red and blue color components reflected by the left half of the first composite dichroic mirror 41, the blue color component B is reflected by the left half of the second composite dichroic mirror 42 and enters the LCD 28. The red color component R goes straight as it is and enters the LCD 29. Of the entering and modulated color components G&#39;, B&#39; and R&#39; which enter into and by the LCDs 27, 28 and are modulated 29, the color component G&#39; passes through the right half of the first composite dichroic mirror 41, and the right half of the second composite dichroic mirror 42. When passing through the first composite dichroic mirror 41, it is synthesized with the blue color component B&#39;, and further when passing through the second composite dichroic mirror 42, it is synthesized with the red color component R&#39; coming from the total reflection mirror 24. The three color components G&#39;, B&#39; and R&#39; are synthesized to become an exiting light A&#39; of full color which is then projected by the projection lens onto the screen. 
     In the above-described embodiment, the characteristics concerning red and blue of the dichroic mirror may be exchanged by each other, of course. 
     As has been described above, in the LCD projection apparatus according to this embodiment, whilst in the embodiment of FIG. 4, the necessary number of dichroic mirrors of high price is 4, only two can suffice. Therefore, the cost can largely be cut down. Also, when assembling, because the only necessary operation is to adjust the position of the two dichroic mirrors, the adjustment is easy and the number of steps in the assembling process can be reduced. Further, because of the decrease of the positioning factor, there is another advantage that the quality is stable and the difference of the items can be lessened. 
     By the way, the liquid crystal display 50 is constructed, as shown in FIG. 10, from a polarizer 50a, a liquid crystal layer 50b and an analyzer 50c. At the polarizer 50a, only the vertical component (linearly polarized light) of the incident white light is permitted to pass through, while the parallel component is absorbed or reflected. This vertical component is rotated 90° in passing through the liquid crystal layer 50b so that it can pass through the analyzer 50c orthogonal to the polarizer 50a. About a half of the white light after having been modulated by the liquid crystal display 50 is projected onto a screen (not shown), but the remaining half is shut out by the polarizer 50a. Hence it is impossible to set the efficiency of light utilization at higher than 50%. 
     FIG. 11 shows an example of an illumination system free from the above-described loss of light amount. 60 is assumed to be the assembly shown in FIG. 4 or FIG. 5, except that the liquid crystal display of this case does not have the polarizer in front of the liquid crystal layer. 
     51 is a light polarizing assembly which converts white light A from a light source to a linearly polarized light to be used in the color separation-synthesizing assembly 60. This light polarizing assembly 51 is formed from polarizing beam splitters 51a and 51b and a TN (twist nematic) liquid crystal 51c of 90° rotation directly installed in the prism boundary face of the polarizing beam splitter 51a. 
     In the arrangement described above, the white light A is entered through a condenser lens (not shown) to the polarizing beam splitter 51a, the vertical component S of the white light is reflected by its prism boundary face to enter the color separation-synthesizing assembly 60. Meanwhile, the parallel component P of the white light passes through the prism boundary face, but becomes vertically polarized light by the TN liquid crystal 51c of 90° rotation which constitutes part of the multi-layer coating on the prism boundary face of the polarizing beam splitter 51a. The thus-converted vertical component S is reflected by the next polarizing beam splitter 51b, while a component α generated due to the defect etc. of the TN liquid crystal 51c passes therethrough as it is. This reflected vertical component S is entered into the color separation-synthesizing assembly 60 like the vertical component S reflected by the polarizing beam splitter 51a. Hence, nearly 100% of the incident light is transmitted out of the device. Thus, the efficiency of use of the light rises.