Abstract:
A transmissive screen is provided that controls moire pattern occurrence to prevent the degradation of image quality. The transmissive screen includes a Fresnel lens sheet having a Fresnel lens unit and a microlens array sheet having a microlens unit that are placed opposite to each other. Projected images enter from one side of the screen and are displayed on the other side of the screen, assuming that the lens pitch on one side of the microlens unit is P, the lens pitch of the Fresnel lens unit is Pf, and the pixel-frame pitch projected on the screen is Pb, the following expressions are satisfied: P≠Pb·Pf/(Pb+Pf); P≠Pb·Pf/2×(Pb+Pf); and P≠Pb·Pf/2×(Pb−Pf).

Description:
RELATED APPLICATION 
   This application claims priority to Japanese Patent Application No. 2003-388172 filed Nov. 18, 2003, which is hereby expressly incorporated by reference herein in its entirety. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a transmissive screen and a rear-projection display device having the transmissive screen. 
   2. Related Art 
   Currently, rear-projection display devices in which projected images enter from the rear of a screen and are displayed on the front of the screen are widely used as rear projectors or the like. Rear-projection display devices have an advantage of obtaining clear images even in a bright ambient environment. Such rear-projection display devices usually have, as a projection device, a built-in liquid crystal projector using a liquid crystal panel, and further, as a display screen, a transmissive screen comprising a plurality of lens sheets combined according to need. Conventionally, this type of transmissive screen is configured by combining a Fresnel lens sheet, a lenticular lens sheet, a prism lens sheet, a diffusion sheet, etc. A combination of a Fresnel lens sheet and a lenticular lens sheet is widely employed in view of advantages such as a bright screen surface and a wide viewing angle. 
   However, since liquid crystal projectors project images by emitting a light source through a liquid crystal panel having segments divided according to pixel frames, there has been a problem of interference occurring between the shades of pixel frames and a striped pattern formed in concentric circles of the Fresnel lens or a vertical striped pattern of the lenticular lens, causing noticeable moire patterns which degrade image quality. To improve such a problematic situation, there is a known solution to adjust the pitch amounts between the pixel-frame pitch of an image projected by the liquid crystal projector and the lens pitch of the Fresnel lens sheet, and also between the projected pixel-frame pitch and the lens pitch of the lenticular lens sheet at a specific range (refer to Japanese Patent Gazette No. 2922233). 
   In the case of applying the above solution to the microlens array sheet, moire patterns occurring between the projected pixel frames and the Fresnel lens sheet and between the projected pixel frames and the microlens array sheet have been improved. However, in the conventional technique, a moire pattern between the Fresnel lens sheet and the microlens array sheet further causes another moire pattern in conjunction with the projected pixel frames, leaving a degraded image quality. 
   The purpose of the present invention is to solve such a problem and provide a transmissive screen which does not degrade image quality and a rear-projection display device with the transmissive screen by controlling moire patterns when images are projected from a projection device such as a liquid crystal projector. 
   SUMMARY 
   The transmissive screen according to the present invention is a screen which comprises a Fresnel lens sheet with a Fresnel lens unit having a plurality of lenses formed at constant pitches and a microlens array sheet with a microlens unit having a plurality of lenses formed at constant pitches, both of which are placed opposite to each other. Further, the transmissive screen in which projected images enter from one side of the screen to be projected on the other side, with an assumption that the lens pitch on one side of the microlens unit is P; the lens pitch of the Fresnel lens unit is Pf; and the projected pixel-frame pitch of an image to be projected on the screen is Pb, satisfies the conditions that: the lens pitch P on one side of the microlens unit is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the lens pitch Pf of the Fresnel lens unit; the lens pitch P on one side of the microlens unit is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the pixel-frame pitch Pb; and the lens pitch Pf of the Fresnel lens unit is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the pixel-frame pitch Pb, and moreover satisfies expressions:
 
 P≠Pb·Pf /( Pb+Pf );
 
 P≠Pb·Pf/ 2×( Pb+Pf ); and
 
 P≠Pb·Pf/ 2×( Pb−Pf ).
 
   With this screen, a moire pattern occurring among the microlens array sheet, the Fresnel lens sheet, and the pixel frames projected on the screen, and another moire pattern caused by a moire pattern between the Fresnel lens sheet and the microlens array sheet in conjunction with the pixel frames can be controlled and quality images with less-noticeable moire patterns can be displayed. 
   Further, in the above screen, all of the following (a), (b) and (c) are satisfied:
         (a) P≦0.9×Pb·Pf/(Pb+Pf) or P≧1.1×Pb·Pf/(Pb+Pf);   (b) P≦0.95×Pb·Pf/2×(Pb+Pf) or P≧1.05×Pb·Pf/2×(Pb+Pf); and   (c) P≦0.95×Pb·Pf/2×(Pb−Pf) or P≧1.05×Pb·Pf/2×(Pb−Pf).       

   With this screen, the moire pattern among the microlens array sheet, the Fresnel lens sheet, and the pixel frames projected on the screen, and the moire pattern caused by a moire pattern between the Fresnel lens sheet and the microlens array sheet in conjunction with a moire pattern caused by the pixel frames can be further controlled and quality images with barely-noticeable moire patterns can be displayed. 
   The rear-projection display device according to the present invention, equipped with the transmissive screen and the projection device described in any of the above paragraphs, displays projected images by projecting images from the projection device and letting them enter from the back of the transmissive screen to display them on the front of the screen. Thus, a rear-projection display device displaying quality images can be obtained, making moire patterns virtually non-problematic. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic drawing of a rear-projection display device related to the embodiment of the present invention. 
       FIG. 2  is a schematic drawing describing a transmissive screen related to the embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a configuration drawing of the rear-projection display device related to the embodiment of the present invention.  FIG. 2  is a schematic drawing of the transmissive screen related to the embodiment of the present invention. A rear-projection display device  1  is equipped with a projection optical unit  10 , a mirror  30  for adjusting the path of light projected from the projection optical unit  10 , and a transmissive screen  60  for displaying an image on the front of the screen by allowing the projected light to enter from the back. 
   The projection optical unit  10 , having a built-in liquid crystal panel (not illustrated), projects images formed on the liquid crystal panel to the transmissive screen  60  using an illumination from a light source. 
   The transmissive screen  60  comprises a Fresnel lens sheet  40  with a Fresnel lens unit  41  having a number of jagged lenses formed at constant pitches Pf and a microlens array sheet  50  with a microlens unit  51  having a number of spherical lenses (microlenses) formed at constant pitches P. In this configuration, the lens units  41  and  51  are placed opposite to each other, wherein the Fresnel lens sheet  40  is placed on the side of the projection optical unit  10 , and the microlens array sheet  50  on the side of viewers. 
   In the case of the rear-projection display device with the above configuration, moire patterns may occur due to the relationships between projected pixel frames  70  formed by the projection of pixel frames of the liquid crystal panel and the Fresnel lens unit  41  of the Fresnel lens sheet  40 , between the projected pixel frames  70  and the microlens unit  51  of the microlens array sheet  50 , and also between the Fresnel lens unit  41  of the Fresnel lens sheet  40  and the microlens unit  51  of the microlens array sheet  50 . Further, there may be another moire pattern caused by the moire pattern due to the relationship between the Fresnel lens unit  41  and the microlens unit  51  in conjunction with the projected pixel frames  70 . Here, as shown in  FIG. 2 , the lens pitch of the Fresnel lens unit  41  is assumed to be Pf, the lens pitch of the microlens unit  51  in the direction corresponding to that of the lens pitch Pf of the Fresnel lens unit  41  to be P, and the pitch of the projected pixel frames  70  to be Pb. 
   Now, the following facts are now acknowledged regarding moire patterns. 
   (1) The closer to an integral multiple the pitch ratio of respective related objects becomes, the more noticeable moire patterns become. For example, when the projected pixel-frame pitch and the lens-unit pitch are equal, a noticeable large-cycle moire pattern occurs. Therefore, when the ratio of the two pitches is 2 to 1, 3 to 1, or the like, a large-cycle moire pattern is predicted to occur. Further, the more the relationship between the two pitches deviates from n times or 1/n times (n is an integer), the less noticeable moire patterns become. Also, as the n for the n times or 1/n times becomes larger, the density of moire patterns becomes lower. When the n reaches 4 or more, moire patterns are virtually non-problematic. 
   (2) The moire-pattern pitch is the reciprocal of the remainder obtained by subtracting the respective reciprocals (spatial frequency) of the pitches of related objects. That is, the pitch of the moire pattern between the Fresnel lens unit  41  with the pitch Pf and the microlens unit  51  with the pitch P (hereinafter referred to as the moire pattern between lens units) is given by:
 
1/|1/ m·Pf− 1 /n·P|=m·n·Pf·P/|n·P−m·Pf|.  
 
   Here, m and n are integers. 
   Further, the moire pattern caused by the moire pattern between lens units in conjunction with the projected pixel frames  70  occurs more easily when the pitch of the moire pattern between lens units is equal to the pitch Pb of the projected pixel frames  70 , which is given by:
 
 m·n·Pf·P/|n·P−m·Pf|=Pb.  
 
   When this equation is solved for P, the following can be obtained: 
   when n·P−m·Pf is positive, P=Pb·Pf/n·(Pb−Pf); and 
   when n·P−m·Pf is negative, P=Pb·Pf/n·(Pb+Pf)P−b. 
   However, there is no assumption of actually creating a microlens unit having a lens pitch larger than that of the Fresnel lens unit, and therefore m can be fixed to 1. Further, for the moire pattern caused by the moire pattern between lens units in conjunction with the projected pixel frames  70 , only the case where n is 1 or 2 should be considered because moire patterns become less noticeable as n becomes larger. 
   From the viewpoint of (1) above, it is preferable to assume as condition 1 that:
         the lens pitch P on one side of the microlens unit  51  is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the lens pitch Pf of the Fresnel lens unit  41 ;   the lens pitch P on one side of the microlens unit  51  is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the pitch Pb of the projected pixel frames  70 ; and further   the lens pitch Pf of the Fresnel lens unit  41  is 0.8 to 0.55 times, 0.45 to 0.36 times, or 0.3 times or less the pitch Pb of the projected pixel frames  70 .       

   In addition, from the viewpoint of (2) above, by assuming as condition 2 that:
 
 P≠Pb·Pf /( Pb+Pf );
 
 P≠Pb·Pf/ 2×( Pb+Pf ); and
 
 P≠Pb·Pf/ 2×( Pb−Pf ),
         the moire pattern caused by the moire pattern between lens units in conjunction with the projected pixel frames  70  can be made less noticeable.       

   Further by assuming as condition 3 that:
         (a) P≦0.9×Pb·Pf/(Pb+Pf) or P≧1.1×Pb·Pf/(Pb+Pf);   (b) P≦0.95×Pb·Pf/2×(Pb+Pf) or P≧1.05×Pb·Pf/2×(Pb+Pf); and   (c) P≦0.95×Pb·Pf/2×(Pb−Pf) or P≧1.05×Pb·Pf/2×(Pb−Pf),   the moire pattern caused by the moire pattern between lens units in conjunction with the projected pixel frames  70  can be made barely noticeable.       

   Embodiment 
   Assuming the lens pitch Pf of the Fresnel lens unit  41  is 112 μm and the pitch Pb of the projected pixel frames  70 , which attribute to pixel frames of the liquid crystal panel, are 400 μm, 600 μm and 800 μm, respectively, an experiment regarding moire pattern conditions was implemented by projecting a projection light from a liquid crystal projector to the transmissive screen  60  configured by varying the lens pitch P on one side of the microlens unit  51  at 5 μm pitches within a range of 40 μm to 90 μm. The result is summarized in Table 1 below. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Lens pitch of 
                 
                 
                 
             
             
               microlens 
               Pixel-frame 
               Pixel-frame 
               Pixel-frame 
             
             
               array: μm 
               pitch: 400 μm 
               pitch: 600 μm 
               pitch: 800 μm 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               90 
               × 
               × 
               × 
             
             
               85 
               × 
               × 
               Δ 
             
             
               80 
               × 
               ∘ 
               ∘ 
             
             
               75 
               × 
               Δ 
               ⊚ 
             
             
               70 
               Δ 
               × 
               ∘ 
             
             
               65 
               ∘ 
               Δ 
               × 
             
             
               60 
               × 
               × 
               × 
             
             
               55 
               × 
               × 
               × 
             
             
               50 
               Δ 
               Δ 
               × 
             
             
               45 
               × 
               × 
               Δ 
             
             
               40 
               × 
               × 
               × 
             
             
                 
             
             
               Note 1. Lens pitch of Fresnel lens: 112 μm 
             
             
               Note 2. Explanation of symbols 
             
             
               ⊚: Very good; 
             
             
               ∘: Good; 
             
             
               Δ: Practically non-problematic; 
             
             
               ×: Not allowed (Practically problematic) 
             
           
        
       
     
   
   With the combinations indicated by symbols ⊚, ∘ and Δ in Table 1, moire patterns never become noticeable and the quality of displayed images are preferable or practically not problematic. These combinations satisfy condition 1 and condition 2, or condition 1 and condition 3 described above. 
   Whereas, with the combinations indicated by x in Table 1, moire patterns become noticeable and the quality of the displayed images are too degraded to be employed in practical use. These combinations do not satisfy condition 1 or condition 2. 
   FIELD OF INDUSTRIAL APPLICATION 
   The transmissive screen according to the present invention is applicable for various rear-projection display devices such as televisions, recreational machines, etc. that project images from the rear side of the screen and display such images on the front side of the screen. 
   In addition, the projection devices used for such rear-projection display devices are not necessarily limited to liquid crystal projectors, and projection devices using other devices are acceptable.