Abstract:
Image disturbance called speckle noise occurs when an image generator has a small effective screen size and a projection lens has a small effective diameter. The present invention is intended to reduce speckle noise. In an optical projector, a viewing angle enlarging ember ( 60 ) for enlarging viewing angle or a scattering member ( 28 ) for scattering image light is disposed near an image display device ( 18 ), such as a liquid crystal panel. Speckle noise can be reduced, suppressing the deterioration of resolution and contrast.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese applications serial no. P2003-421959, filed on Dec. 19, 2003, and serial no. P2004-132481, filed on Apr. 28, 2004, the contents of which are hereby incorporated by reference into this application.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to an image display apparatus capable of enlarging and displaying an image produced by an image display device as an image generator on a screen by using optical parts including a projection lens, and an optical projector to be used in combination with the image display apparatus.  
         [0003]     Recently, the so-called pixel-selection type image modulators, such as liquid crystal panels, have become prevalently used instead of projection cathode-ray tubes as image generators for projection image display apparatus. The image modulator, for example, forms an image by modulating light emitted by a light source, such as a mercury lamp, for each pixel. The effective screen size of the image modulator is as small as about 1 inch. The image modulator emits light closer to collimated light than that emitted by a projection cathode-ray tube. Therefore, a projection lens to be incorporated into the image modulator is small and has a large f-number, which is the ratio of the lens&#39;s focal length to the lens&#39;s maximum effective diameter. If the incident light on the lens is nearly collimated light, the speed of the lens is high even if the effective diameter D of the lens is small.  
         [0004]     If the effective screen size of the image generator is small and the effective diameter D of the projection lens is small, image disturbance generally called speckle noise (scintillation) occurs; that is, the projection image display apparatus employing the image modulator is liable to generate speckle noise. Techniques intended to reduce speckle noise are disclosed in, for example, Japanese Patent Laid-Open Nos. 2001-228547 (Reference 1), Hei 7-168282 (Reference 2) and Hei 11-38512 (Reference 3).  
       SUMMARY OF THE INVENTION  
       [0005]     Speckle noise occurs when light rays scattered by spatially separate minute diffusing elements interfere with each other at a point. In other words, since light emitted from an optional single point on the image generator passes two points on a screen and interfere with each other, the interference becomes more intense and the contrast of speckle noise becomes higher when the image generator is smaller and the surface density of outgoing light is greater; that is, the smaller the effective diameter D of the lens becomes, the higher the intensity of the interference is.  
         [0006]     Accordingly, speckle noise can be reduced by increasing the effective screen size of the image generator or by using a lens having a large effective diameter D, which, however, runs counter to the trend toward using the image modulator as the image generator. Therefore, measures have been taken to avoid interference between light rays emitted from two points on the screen or to diffuse the interfering light rays into white noise by further diffusion.  
         [0007]     A method of avoiding interference between light rays emitted from two points on the screen mentioned in Reference 2 forms a lenticular lens sheet included in a rear projection screen of a material not containing any diffusing material, and disposes a diffusing layer at distance longer than about three times the focal length of the lenticular lens from the lenticular lens sheet. Thus the lenticular lens increases the directions of light rays falling on the diffusing material and causing speckle noise to avoid interference. Although this method is effective in eliminating causes of speckle noise, the diffusing layer needs to be disposed at a distance longer than about three times the focal length of the lenticular lens from the lenticular lens sheet. Consequently, resolution is deteriorated greatly, which is a new problem.  
         [0008]     A method of converting the interfering light rays into white noise by further diffusion mentioned in Reference 3 uses a lenticular lens sheet of three-layer construction formed by sandwiching a transparent middle layer between first and second diffusing layers. Speckle noise generated by the first diffusing layer is concealed by the third diffusing layer. This method not only deteriorates resolution but also makes the second diffusing layer on the front side look white when external light falls thereon and decreases contrast in a bright place.  
         [0009]     Accordingly it is important to reduce speckle noise that is caused or liable to be caused in an image display apparatus by an image modulator as an image generator, and a projection lens having a large f-number included in the image display apparatus, suppressing the deterioration of resolution and contrast.  
         [0010]     The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide techniques capable of satisfactorily suppressing the deterioration of image quality when an image modulator is used as an image generator.  
         [0011]     The present invention provides, to achieve the object, an image display apparatus including an image display device and characterized by a viewing angle enlarging film disposed on one side of the image display device or by viewing angle enlarging films disposed on the opposite sides of the image display device, respectively, to correct image light such that viewing angle is enlarged. The viewing angle enlarging films may be viewing angle compensating films having different refractive indices with the P-wave and the S-wave, namely, birefringent films.  
         [0012]     According to the present invention, the image display device is a reflection liquid crystal panel or a mirror-reflection optical modulator, and the image display device has a roughened reflecting surface.  
         [0013]     The present invention disposes a scattering member in an optical path between the image display device and the projection lens to scatter light forming an image formed by the image display device. The scattering member may be disposed near the image display device.  
         [0014]     When the image display device is a transmission liquid crystal panel, the scattering member is one of the following members. (1) An exit dustproof glass plate having a haze value greater than that of an entrance dustproof glass plate, (2) a diffusing sheet adhesively attached to an exit dustproof glass plate, (3) a diffusing plate interposed between a transmission liquid crystal panel and an exit polarizing plate, (4) an exit polarizing plate having a haze value greater than that of an entrance polarizing plate and (5) a diffusing sheet adhesively attached to the entrance or the exit surface of a dichroic prism for synthesizing red, blue and green images emitted by a plurality of transmission liquid crystal panels.  
         [0015]     When the image display device is a reflection liquid crystal panel, one of or a combination of some of the following members is used. (1) A dustproof glass plate having a haze value greater than that of a general dustproof glass plate for protecting the reflection liquid crystal panel, (2) a diffusing sheet adhesively attached to the surface of a dustproof glass plate and (3) a diffusing sheet adhesively attached to the exit or the entrance surface of a beam splitter.  
         [0016]     When the image display device is a mirror-reflection optical modulator having a plurality of mirrors, a dust glass plate having a haze value greater than that of a general dustproof glass plate as the diffusing member.  
         [0017]     According to the present invention, the deterioration of image quality due to, for example, speckle noise or the like can be suppressed when an image modulator is used. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a partly cutaway perspective view of an image display apparatus in a first embodiment according to the present invention;  
         [0019]      FIG. 2  is a typical view of an image generator using the transmission liquid crystal panel included in the image display apparatus shown in  FIG. 1 ;  
         [0020]      FIG. 3  is a view of a unit relating to the present invention extracted from  FIG. 2  showing the image generator according to the present invention in the typical view;  
         [0021]      FIG. 4  is a view of a second embodiment according to the present invention;  
         [0022]      FIG. 5  is a view of a third embodiment according to the present invention;  
         [0023]      FIG. 6  is a view of a fourth embodiment according to the present invention;  
         [0024]      FIG. 7  is a view of a fifth embodiment according to the present invention;  
         [0025]      FIG. 8  is a view of a sixth embodiment according to the present invention;  
         [0026]      FIG. 9  is a typical view of an image generator using a reflection liquid crystal panel included in an image display apparatus according to the present invention in a modification;  
         [0027]      FIG. 10  is a typical view of a reflection liquid crystal panel according to the present invention;  
         [0028]      FIG. 11  is a view of a unit relating with the present invention extracted from  FIG. 9  showing the image generator according to the present invention in the typical view;  
         [0029]      FIG. 12  is a view of a seventh embodiment according to the present invention;  
         [0030]      FIG. 13  is a view of an eighth embodiment according to the present invention;  
         [0031]      FIG. 14  is a view of a ninth embodiment according to the present invention; and  
         [0032]      FIG. 15  is a typical view of an image generator using a mirror-reflection optical modulator according to the present invention in a modification. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     Preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0034]     Referring to  FIG. 1  showing an image display apparatus in a first embodiment according to the present invention is a partly cutaway perspective view, an image generator  1  includes a projection cathode-ray tube or a reflection or transmission liquid crystal panel, an image modulator, such as a mirror reflection optical modulator provided with a plurality of small mirrors, and an illuminating system including a lamp. The image generator  1  displays a small image. The small image is projected through a projection lens  2  on a rear projection screen  3 . Generally, projection distance is long and hence a reflecting mirror  4  is disposed in an optical path between the projection lens  2  and the rear projection screen  3  to form the image display apparatus in a small longitudinal dimension. The image generator  1 , the projection lens  2 , the rear projection screen  3  and the reflecting mirror  4  are fixedly held at predetermined positions, respectively, in a cabinet  5 .  
         [0035]     Referring to  FIG. 2  showing the image generator  1  including a transmission liquid crystal panel according to the present invention in a typical view, light emitted by a lamp  6 , namely, a light source, falls on a multilens unit consisting of an entrance multilens array  7   a  and an exit multilens array  7   b . The multilens unit makes the distribution of the incident light uniform. A beam splitter  8  transmits light beams of the same polarization direction. A condenser lens  9  gathers the light beams transmitted by the beam splitter  8 . A first mirror  10  is used to form the image generator  1  in a small overall size. The first mirror  10  reflects the light beams gathered by the condenser lens  9  to change the traveling direction of the light beams. The light beams reflected by the first mirror  10  fall on a first collimator lens  11  for focusing the light emitted by the lamp  6  on panels, which will be described later. A first dichroic mirror  12  transmits red light beams and reflects blue and green light beams of the light beams traveled through the first collimator lens  11 . A second dichroic mirror  13  transmits the blue light beam reflected by the first dichroic mirror  12  and reflects the green light beam reflected by the first dichroic mirror  12 . A relay lens unit including a first relay lens  14   a  and a second relay lens  14   b  is disposed on a blue optical path for the blue light beam longer than red and green optical paths for the red and the green light beam. A second mirror  15  disposed between the relay lenses  14   a  and  14   b  reflects the blue light beam traveled through the second dichroic mirror  13  to change the direction of the blue light beam. A third mirror  16   a  and a fourth mirror  16   b  are placed on the red and the blue optical path for the red and the blue light beam, respectively, to change the respective traveling directions of the red and the blue light beam such that the red and the blue light beam fall on a red liquid crystal panel  18 R and a blue liquid crystal panel  18 B, respectively. A second collimator lens  17   a , a third collimator lens  17   b  and a fourth collimator lens  17   c  are placed in the red, the green and the blue optical path, respectively. The second collimator lens  17   a , the third collimator lens  17   b  and the fourth collimator lens  17   c  work in cooperation with the first collimator lens  11  to focus the light emitted by the lamp  6  on panels, which will be described later. The red, the green and the blue light beam reflected by the third mirror  16   a , the second dichroic mirror  13  and the fourth mirror  16   b  fall on the red liquid crystal panel  18 R, the green liquid crystal panel  18 G and the blue liquid crystal panel  18 B, respectively. The red liquid crystal panel  18 R for displaying red images is combined with a first entrance polarizing plate  19   a  and a first exit polarizing plate  20   a . Similarly, the green liquid crystal panel  18 G for displaying green images is combined with a second entrance polarizing plate  19   b  and a second exit polarizing plate  20   b . The blue liquid crystal panel  18 B for displaying blue images is combined with a third entrance polarizing plate  19   c  and a third exit polarizing plate  20   c . The light beams traveled through the exit polarizing plates  20   a ,  20   b  and  20   c  fall on a dichroic prism  21 , namely, optical modulator. The dichroic prism  21  includes a red light reflecting film  22  and a blue light reflecting film  23 . The red light reflecting film  22  reflects the red light beam passed through the red liquid crystal panel  18 R in the direction of the arrow A toward a projection lens, not shown. The blue light reflecting film  23  reflects the blue light beam passed through the blue liquid crystal panel  18 B in the direction of the arrow A toward the projection lens. The green light beam passed through the green liquid crystal panel  18 G passes through the red light reflecting film  22  and the blue light reflecting film  23  and travels in the direction of the arrow A toward the projection lens. Those optical components are housed in the cabinet  24 .  
         [0036]      FIG. 3  shows a unit included in the image generator  1  in the first embodiment according to the present invention in a typical view, in which parts corresponding to those shown in  FIG. 2  are denoted by the same reference characters. The red, the green and the blue optical path shown in  FIG. 2  are basically the same in construction and function, and hence only one of those optical paths is shown in  FIG. 3 . The liquid crystal panel  18  consists of a TFT substrate  25  and an opposite substrate  26 , and is protected by an entrance dustproof glass plate  27  and an exit dustproof glass plate  28 . A first viewing angle enlarging film  60  is interposed between the liquid crystal panel  18  and the entrance polarizing plate  19 . A second viewing angle enlarging film  61  is interposed between the liquid crystal panel  18  and the exit polarizing plate  20 . Each of the first viewing angle enlarging film  60  and the second viewing angle enlarging film  61  is a viewing angle enlarging film formed, for example, by coating a transparent carrier with a discotic liquid crystal compound and crosslinking the discotic liquid crystal compound and generally used for enlarging the viewing angle of electron-flood-beam liquid crystal panel or a viewing angle enlarging film equivalent to the same.  
         [0037]     A prior art contrast improving film for improving the contrast of a liquid crystal panel is formed by coating a transparent carrier with a discotic liquid crystal compound and crosslinking the discotic liquid crystal compound. A technique using the contrast improving film improves the contract through the improvement of the orthogonality of the liquid crystal panel with the S-wave and the P-wave. This technique is effective in improving contrast, but not effective at all in reducing speckle noise. Although enlargement of viewing angle is not necessary for a liquid crystal panel included in a projection optical system, improvement of contrast is important for such a liquid crystal panel. Therefore, a viewing angle enlarging film developed for the enlargement of viewing angle is used for improving contrast. The viewing angle enlarging film and the contrast improving film are formed from the same materials. The viewing angle enlarging film and the contrast improving film are different from each other. Whereas the viewing angle enlarging film is designed so that contrast inversion may not occur in a wide viewing angle range, the contrast improving film is designed so as to improve the orthogonality of the liquid crystal panel with the S-wave and the P-wave in a narrow viewing angle range. The viewing angle enlarging film corrects light not to improve contrast, but to enlarge viewing angle. The contrast improving film according to the present invention is black and cuts oblique light by a correction plates to prevent light leakage. The viewing angle enlarging film according to the present invention is white and corrects image light to make oblique light pass the correction plates.  
         [0038]     When the viewing angle of the liquid crystal panel  18  is enlarged by placing the viewing angle enlarging films  60  and  61  on the opposite sides of the liquid crystal panel  18  as shown in  FIG. 3 , the collimation of the light beam transmitted by the liquid crystal panel  18  can be eased. Consequently, the present invention is able to reduce speckle noise that appears on the rear projection screen  3 .  
         [0039]     A unit included in an image generator in a second embodiment according to the present invention will be described with reference to  FIG. 4 , in which parts like or corresponding to those shown in  FIG. 3  are denoted by the same reference characters. A liquid crystal panel  18  consists of a TFT substrate  25  and an opposite substrate  26 , and is protected by an entrance dustproof glass plate  27  and an exit dustproof glass plate  28 . The exit dustproof glass plate  28  has a haze value greater than that of the entrance dustproof glass plate  27 , and serves as a scattering member for scattering image light. Generally, it is desirable that both the entrance and the exit dustproof glass plate have a high transmittance. However, if it is possible that speckle noise is generated, the exit dustproof glass plate  28  having a high haze value is used. Thus, speckle noise that appears on the rear projection screen  3  can be reduced by greatly scattering the light transmitted by the liquid crystal panel  18 . Speckle noise can be thus reduced by making light beams that cause speckle noise fall on the diffusing member of the rear projection screen  3  from irregular directions to make interference between the light beams difficult. Although it is possible that speckle noise is generated on the exit dustproof glass plate  28 , this speckle noise scarcely causes a trouble because the exit dustproof glass plate  28  is very close to the image display part of the liquid crystal panel  18 . However, polarization is disturbed by scattering and, consequently, the contrast of the image is deteriorated. Therefore, the haze value of the exit dustproof glass plate  28  must be determined such that the reduction of speckle noise and the deterioration of the contrast are properly balanced. The scattering of the image light that generates speckle noise to prevent speckle noise like that done by the present invention is more effective than the improvement of only the screen on which speckle noise appears directly like that done by prior art techniques mentioned in References 2 and 3. The exit dustproof glass plate  28  having a high haze value may be formed from glass containing a dispersed diffusing material or may be formed by coating a glass plate with a film of a binder containing a material having a low refractivity by spray coating.  
         [0040]     A unit included in an image generator in a third embodiment according to the present invention will be described with reference to  FIG. 5 , in which parts like or corresponding to those shown in  FIG. 4  are denoted by the same reference characters. Generally, a liquid crystal panel  18  is a common part and hence, in some cases, it is difficult to form a special unit as shown in  FIG. 4 . In such a case, an exit dustproof glass plate  28  provided with a diffusive sheet  29  adhesively attached to the surface of the exit dustproof glass plate  28  may be employed. If the liquid crystal panel  18  needs cooling, the diffusive sheet  29  may be formed so as to have a necessary strength, and the diffusive sheet  29  may be interposed between the liquid crystal panel  18  and an exit polarizing plate  20 .  
         [0041]     A unit included in an image generator in a fourth embodiment according to the present invention will be described with reference to  FIG. 6 , in which parts like or corresponding to those shown in  FIG. 4  are denoted by the same reference characters. An exit polarizing plate  20  has a haze value greater than that of an entrance polarizing plate  19  to use the exit polarizing plate  20  as a scattering member for scattering image light. Although a diffusing material contained in a polarizing plate decreases the degree of polarization, the use of the diffusive exit polarizing plate  20  is effective in reducing the number of component parts. The exit polarizing plate  20  may be formed by bonding a diffusive sheet to a polarizing plate as mentioned in connection with  FIG. 5 . Polarized light is not disturbed when such a polarizing plate is disposed with the polarizing plate facing the entrance side and the diffusive sheet facing the exit side.  
         [0042]     A unit included in an image generator in a fifth embodiment according to the present invention will be described with reference to  FIG. 7 , in which parts like or corresponding to those shown in  FIG. 4  are denoted by the same reference characters. A diffusive sheet  30 , namely, a scattering member, is attached adhesively to an entrance surface of a dichroic prism  21 . Separation of a diffusive layer from a liquid crystal panel  18  deteriorates resolution. Since a diffusive layer is outside two polarizing plates, the deterioration of contrast due to the disturbance of polarized light by the scattering effect of a diffusing material does not occur. The diffusive sheet  30  may be formed so as to have a necessary strength, and the diffusive sheet  30  may be interposed between the liquid crystal panel  18  and the dichroic prism  21 .  
         [0043]     A unit included in an image generator in a sixth embodiment according to the present invention will be described with reference to  FIG. 8 , in which parts like or corresponding to those shown in  FIG. 4  are denoted by the same reference characters. A diffusive sheet  31 , namely, a scattering member, is adhesively attached to the exit surface of a dichroic prism  21 . Although the separation of a diffusive layer from a liquid crystal panel deteriorates resolution, the red, the green and the blue light beam can be diffused by the single diffusive sheet  31  because the red, the green and the blue light beam pass the exit surface. The diffusive sheet  31  may be formed so as to have a necessary strength and disposed apart from the dichroic prism  21 .  
         [0044]      FIG. 9  is a typical view of an image generator using a reflection liquid crystal panel included in an image display apparatus according to the present invention in a modification of the foregoing image generator  1 . Light emitted by a lamp  6 , namely, a light source, falls on a multilens unit consisting of an entrance multilens array  7   a  and an exit multilens array  7   b . The multilens unit makes the distribution of the incident light uniform. A beam splitter  8  polarizes the light passed the multilens unit in S-polarized light beams. A condenser lens  9  gathers the S-polarized light beams transmitted by the beam splitter  8 . A first mirror  10  and a second mirror  32  are used to form the image generator  1  in a small overall size. The first mirror  10  and the second mirror  32  reflect the light beams gathered by the condenser lens  9  to change the traveling direction of the light beams. The light beams reflected by the first mirror  10  and the second mirror  32  fall on a first collimator lens  33  for focusing the light emitted by the lamp  6  on panels, which will be described later. A dichroic mirror  34  transmits blue light beams and green light beams and reflects red light beams of the light beams traveled through the first collimator lens  33 . A first wavelength-selective wave plate  35  changes only the blue light beam, namely, S-polarized light beam, transmitted by the dichroic mirror  34  into a P-polarized light beam. The green light beam, namely, the S-polarized light beam, and the blue light beam, namely, the P-polarized light beam, fall on a first beam splitter prism  36 . The first beam splitter prism  36  reflects the green light beam (S-polarized light beam) toward a green reflection liquid crystal panel  38 G for the green light beam, and transmits the blue light beam (P-polarized light beam) toward a blue reflection liquid crystal panel  38 B for the blue light beam. The red light beam (S-polarized light beam) reflected by the dichroic mirror  34  is reflected again by a second beam splitter prism  37  toward a red reflection liquid crystal panel  38 R for the red light beam.  
         [0045]     An optical modulator  39  is a third beam splitter prism capable of synthesizing optical images provided by the red reflection liquid crystal panel  38 R, the green reflection liquid crystal panel  38 G and the blue reflection liquid crystal panel  38 B. A half-wave plate  40  is attached to the exit surface of the second beam splitter prism  37 . The S-polarized red light beam is converted into an P-polarized red light beam by the second beam splitter prism  37 , and the P-polarized red light beam is converted into an S-polarized red light beam by the half-wave plate  40 . The S-polarized red light beam falls on the third beam splitter prism  39 . The third beam splitter prism  39  reflects the incident red light beam in the direction of the arrow B. The S-polarized green light beam fallen on the green reflection liquid crystal panel  38 G is converted into a P-polarized green light beam by the first beam splitter prism  36 , and the P-polarized green light beam travels through the third beam splitter prism  39  in the direction of the arrow B. The P-polarized blue light beam fallen on the blue reflection liquid crystal panel  38 B is converted into an S-polarized blue light beam, the S-polarized blue light beam is reflected by the first beam splitter prism  36  and is converted into a P-polarized blue light beam by a second wavelength-selective wave plate  41  that converts only the blue light beam from an S-polarized light beam into a P-polarized light beam. The P-polarized blue light beam travels through the third beam splitter prism  39  in the direction of the arrow B. Those optical components are housed in the cabinet  24 .  
         [0046]      FIG. 10  is a typical view of the reflection liquid crystal panel  38  employed in the image generator included in the image display apparatus according to the present invention. Since the red liquid crystal panel  39 R, the green liquid crystal panel  38 G and the blue reflection liquid crystal panel  38 B are basically the same in construction and function, the reflection liquid crystal panel  38  shown in  FIG. 10  is any one of the reflection liquid crystal panels  38 R,  38 G and  38 B. Referring to  FIG. 10 , the reflection liquid crystal panel  38  consists of a display unit  43 , a transparent electrode layer  44 , and a dustproof glass plate  45  covering the transparent electrode layer  44 . The display unit  43  consists of a silicon wafer  64 , a reflecting mirror  63  having a rough surface, and a liquid crystal layer  62 . Generally, the surface of the reflecting mirror  63  is finished in the smoothest possible surface because the surface having a higher reflectance has a higher reflecting ability. However, it is expected that speckle noise is generated, the surface of the reflecting mirror  63  is roughened. Consequently, speckle noise that appears on the rear projection screen  3  can be reduced by greatly scattering light emerging from the reflection liquid crystal panel  38 .  
         [0047]      FIG. 11  is a view of a unit relating to the present invention extracted from  FIG. 9  showing the image generator. In  FIG. 11 , parts like or corresponding to those shown in  FIG. 9  are denoted by the same reference characters. The red reflection liquid crystal panel  38 R, the green reflection liquid crystal panel  38 G and the blue reflection liquid crystal panel  38 B have display units  43 R,  43 G and  43 B and transparent electrode layers  44 R,  44 G and  44 B, and dustproof glass plates  45 R,  45 G and  45 B covering the transparent electrode layers  44 S,  44 G and  44 B, respectively. The dustproof glass plates  45 R,  45 G and  45 B has a haze value greater than those of general dustproof glass plates to use the dustproof glass plates  45 R,  45 G and  45 B also as scattering members. The dustproof glass plate  45 R,  45 G and  45 B having a high haze value may be formed from glass containing a dispersed diffusing material or may be formed by coating a glass plate with a film of a binder containing a material having a low refractivity by spray coating.  
         [0048]     A seventh embodiment according to the present invention will be described with reference to  FIG. 12 , in which parts like or corresponding to those shown in  FIG. 11  are denoted by the same reference characters. Generally, a reflection liquid crystal panel  38  is a common part and hence, in some cases, it is difficult to form special units as shown in  FIGS. 10 and 11 . In such a case, diffusive sheets  46 R,  46 G and  46 B may be attached adhesively to the surfaces of exit dustproof glass plates  45 R,  45 G and  45 B covering the reflection liquid crystal panels  38 R,  38 G and  38 B, respectively, as shown in  FIG. 12 . The diffusive sheets  46 R,  46 G and  46 B serve as scattering members, respectively. If the reflection liquid crystal panels  38 R,  38 G and  38 B need cooling, the diffusive sheets  46 R,  46 G and  46 B may be formed so as to have a necessary strength, and may be formed so as to have a necessary strength, and the diffusive sheet  46 R may be interposed between the reflection liquid crystal panel  38 R and a beam splitter prism  37 , and the diffusive sheets  46 G and  46 B may be interposed between the reflection liquid crystal panel  38 G and a beam splitter prism  36  and between the reflection liquid crystal panel  38 B and the beam splitter prism  36 , respectively.  
         [0049]     An eighth embodiment according to the present invention will be described with reference to  FIG. 13 , in which parts like or corresponding to those shown in  FIG. 11  are denoted by the same reference characters. Diffusive sheets  47 , namely, scattering members, are disposed on the exit side of beam splitter prisms  36  and  37 . The diffusive sheets  47  disposed behind the beam splitter prisms  36  and  37 , which separates image light from unnecessary light, are more effective then those shown in  FIG. 12  in improving contrast. Since the diffusive sheets  47  are spaced from the reflection liquid crystal panels  38 , resolution is deteriorated. Although the diffusive sheets  47  are adhesively attached to the respective exit surfaces of a half-wave plate  40  and a second wavelength-selective wave plate  41  in the eighth embodiment, the diffusive sheets  47  may be adhesively attached to the respective entrance surfaces of a half-wave plate  40  and a second wavelength-selective wave plate  41  for the same effect. The diffusive sheets  47  may be formed so as to have a necessary strength and disposed apart from the half-wave plate  40  and the second wavelength-selective wave plate  41 .  
         [0050]     A ninth embodiment according to the present invention will be described with reference to  FIG. 14 , in which parts like or corresponding to those shown in  FIG. 11  are denoted by the same reference characters. The eighth embodiment is characterized by a diffusive sheet  48 , namely, scattering member, adhesively attached to the entrance surface of a third beam splitter prism  39 . Since the diffusive sheet  48  for separating image light from unnecessary light is disposed below beam splitter prisms  36  and  37  and the third beam splitter prism  39 , the contrast ability of the ninth embodiment is higher than that of the eighth embodiment shown in  FIG. 13 . However, resolution is further deteriorated because the diffusive sheet  48  is spaced a long distance apart from the reflection liquid crystal panels  38 .  
         [0051]     The image display apparatuses in the preferred embodiments shown in FIGS.  10  to  14  have both advantages and disadvantages in contrast ability, resolution and the number of parts. Therefore, those image display apparatuses may be selectively used taking into consideration the total ability of the image display apparatuses.  
         [0052]      FIG. 15  is a typical view of an image generator using a mirror-reflection optical modulator according to the present invention in a modification. Light emitted by a lamp  6  is uniformed by a light pipe  49 . Light emerging from the light pipe  49  is periodically separated into red, green and blue light beams by a color wheel  50 . Then, the right, the green and the blue light beam travel through a condenser lens  51 , and are reflected toward a mirror-reflection optical modulator  53  by a mirror  52  used for forming an image generator  1  in a small size. The mirror-reflection optical modulator  53  is protected by a dustproof glass plate  54 . A diffusive sheet  55  is adhesively attached to the exit surface of the dustproof glass plate  54 . Those optical components are housed in a cabinet  56 . This image generator  1  is characterized in that the dustproof glass plate  54 , the diffusive sheet  55  or the combination of the dustproof glass plate  54  and the diffusive sheet  55  is used as a scattering member. Since the mirror-reflection optical modulator  53  does not deal with polarized light, the diffusive sheet  55  may be disposed at any position on the exit side of the mirror-reflection optical modulator  53 . It is the simplest method of using the diffusive sheet  55  to adhesively attach the diffusive sheet  55  to the exit surface of the dustproof glass plate  54 . Since all the pixels of the mirror-reflection optical modulator  53  are minute mirrors, not shown, the surfaces of all the minute mirrors may be roughened. The mirror-reflection optical modulator  53  having the minute mirrors having roughened surfaces is a special type and there are many difficulties in manufacturing such a special mirror-reflection optical modulator. However, since such a mirror-reflection optical modulator causes secondary scattering scarcely, the deterioration of contrast is limited to the least extent and resolution is not deteriorated.  
         [0053]     Although the image display devices provided respectively with the transmission liquid crystal panels, the reflection liquid crystal panels and the mirror-reflection optical modulator have been described, the same principle of speckle noise reduction applies to other image display devices. Therefore, it goes without saying that the present invention is applicable to image display devices other than those specifically described herein.