Patent Publication Number: US-2011075108-A1

Title: Projection display device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a projection display device for displaying images by projecting the images onto a transmissive screen. 
     2. Description of the Related Art 
     Recent trend of a projection display device using a liquid crystal panel or a reflective light modulator (such as a reflective liquid crystal display element) as an image display element is configured to include a high-brightness light source in order to enhance brightness. 
     Extra-high pressure mercury lamps, metal halide lamps and other lamps have been used as light sources of the projection display device. When applied as light sources, these lamps have disadvantageously shortened their lifetimes, and have necessitated frequent maintenance tasks such as changing of lamps. These lamps have also required an optical system for taking red, green and blue colors out of white color of the lamps, resulting in a complicated device structure and degradation of light use efficiency. 
     Laser light sources such as semiconductor lasers have been used in order to solve these problems. Laser light sources have longer lifetimes than those of the conventionally employed lamps as light sources, and do not require any maintenance tasks for a long period of time. Furthermore, laser light sources can directly be modulated according to images to be displayed, thereby simplifying a device structure and increasing light use efficiency. Using laser light sources also advantageously expands a range of color reproduction. 
     In contrast, laser light sources have a high level of coherence. Therefore, using laser light sources as light sources of a projection display device causes interference between a light diffusing material in a transmissive screen and light. This generates glare (speckle noise or scintillation) on images to be displayed, resulting in degradation of image quality. 
     It has been desired that a projection display device has reduced speckle noise or scintillation. The following techniques have been suggested in order to achieve this object. According to one technique, the relationship between the exit pupil diameter and the projection distance of a projection lens, and the number of diffusion layers in a transmissive screen are defined (see Japanese Patent Application Laid-pen No. H8-313865). According to another technique, internal oscillation is caused in at least one of diffusion layers in a transmissive screen (see Japanese Patent Application Laid-open No. 2001-100317). 
     In the conventional technique disclosed in Japanese Patent Application Laid-open No. H8-313865, a ratio between the exit pupil diameter d and the projection distance a of a projection lens (d/a) is set to be no greater than 0.06. However, controlling the ratio d/a at a low level reduces the angle of divergence of light entering a transmissive screen, while worsening speckle noise or scintillation. 
     The conventional technique disclosed in Japanese Patent Application Laid-open No. 2001-100317 requires a mechanism for causing oscillation in a diffusion layer in a transmissive screen. This disadvantageously results in upsizing of a device while entailing high cost. Furthermore, the oscillation in the diffusion layer makes the operation of the device unstable, making it difficult to maintain reliability of image display at a high level. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology, and in order to solve the aforementioned problems, a projection display device for displaying an image by projecting the image onto a transmissive screen from a rear thereof according to one aspect of the present invention is constructed in such a manner as to comprise: a light source for emitting light; an illumination optical system for causing a light beam emitted from the light source to propagate through a predetermined optical path, and guiding the light beam to the transmissive screen; an image display element for forming an image on a region to be illuminated with a light beam guided by the illumination optical system, the image being intended to be displayed on the transmissive screen; a projection optical system for enlarging the image formed on the region of the image display element, and projecting the enlarged image on the transmissive screen; and the transmissive screen including a Fresnel lens for converting incident light to light with substantially parallel rays and causing the converted light to exit the Fresnel lens, the transmissive screen also including a lenticular lens for receiving light exiting the Fresnel lens, and causing the received light to exit the lenticular lens as predetermined diffusion light, wherein the projection optical system is configured such that a product of an F-number and a projection magnification of the projection optical system is less than 400, the Fresnel lens includes a first light diffusion layer for diffusing incident light and causing resultant light to exit the first light diffusion layer as first diffusion light, and the lenticular lens includes a second light diffusion layer for further diffusing the first diffusion light and causing resultant light to exit the second light diffusion layer as second diffusion light, either one of the first and second light diffusion layers having a thickness of less than 300 μm. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the structure of a projection display device according to a first embodiment of the present invention; 
         FIG. 2  explains an idea of light propagation in a projection optical system of the projection display device according to the first embodiment of the present invention; 
         FIG. 3  explains glare observed on a transmissive screen of the projection display device according to the first embodiment of the present invention; 
         FIGS. 4A and 4B  each show the relationship between the magnitude of the angle of divergence of light entering the transmissive screen and the diffusion characteristics of light exiting the transmissive screen in the projection display device according to the first embodiment of the present invention; 
         FIGS. 5 and 6  each show the relationship between the F-number of the projection optical system of the projection display device according to the first embodiment of the present invention and glare (speckle noise or scintillation); 
         FIGS. 7 and 8  each show the relationship between the value of Fp×M of the projection optical system according to the first embodiment of the present invention and glare; 
         FIGS. 9 ,  10 , and  11  each show the structure of the transmissive screen of the projection display system according to the first embodiment of the present invention; 
         FIGS. 12 and 13  each show the relationship between the thickness of a light diffusion layer in the transmissive screen of the projection display device according to the first embodiment of the present invention and glare; 
         FIGS. 14 and 15  each show the relationship among the thickness of the light diffusion layer in the transmissive screen according to the first embodiment of the present invention, glare and a resolution level; 
         FIG. 16  shows the relationship of a distance between diffusion layers of a projection display device according to a second embodiment of the present invention with speckle noise and scintillation; 
         FIG. 17  shows the relationship of a distance between the diffusion layers of the projection display device according to the second embodiment of the present invention with speckle noise and scintillation, and with a resolution level; 
         FIGS. 18 ,  19 , and  20  each show the structure of a transmissive screen of the projection display system according to the second embodiment of the present invention; 
         FIGS. 21 and 22  each illustrate glare observed on the transmissive screen of the projection display device according to the second embodiment of the present invention; 
         FIGS. 23 and 24  each show the relationship between the haze value of a transmissive screen of a projection display device according to a third embodiment of the present invention and glare; 
         FIGS. 25 and 26  each show the structure of an optical system of a projection display device according to a fourth embodiment of the present invention; 
         FIGS. 27 and 28  each show the structure of an optical system of a projection display device according to a fifth embodiment of the present invention; 
         FIGS. 29 ,  30 ,  31 , and  32  each show the structure of an optical system of a projection display device according to a sixth embodiment of the present invention; and 
         FIG. 33  shows the structure of a projection display device according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, embodiments of a projection image display according to the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the embodiments described below are not intended to limit the present invention. 
     First Embodiment 
       FIG. 1  shows the structure of a projection display device according to a first embodiment of the present invention. A projection display device  101  is a rear-projection image display device for projecting images on a screen by using a light bulb. 
     As shown in  FIG. 1 , the projection display device  101  according to the first embodiment includes a light-collecting optical system  1 , an illumination optical system  4 , a reflective light modulator (reflective light bulb)  2  functioning as an image display element, and a projection optical system  3  for enlarging an image on a to-be-illuminated surface (image-forming region)  2   a  of the reflective light modulator  2  illuminated with the illumination optical system  4 , and projecting the enlarged image onto a transmissive screen  5 . 
     The light-collecting optical system  1  includes laser light sources  11  of multiple colors (in  FIG. 1 , three colors), and multiple (in  FIG. 1 , three) light-collecting lenses (light-collecting parts)  12  composed of one or a plurality of lenses or mirrors for collecting light beams emitted from the laser light sources  11 . 
     In the light-collecting optical system  1 , each of the laser light sources  11  has a one-to-one correspondence with each of the light-collecting lenses  12 . Accordingly, a light beam emitted from one of the laser light sources  11  is transferred through the corresponding light-collecting lens  12  to a wave plate for changing polarization (polarization changing part)  14 . The polarization component of the light beam of each of the laser light sources  11  is subjected to a phase difference introduced by the wave plate  14 , and then the light beam is guided to the illumination optical system  4 . 
     The projection display device  101  changes the polarization directions of the laser light sources  11  by the action of the wave plate  14 , thereby relaxing coherence thereof. While shown to be arranged behind the light-collecting lenses  12  (in a subsequent stage to the light-collecting lenses  12 ) in  FIG. 1 , the wave plate  14  may alternatively be arranged behind the laser light sources  11  (in a subsequent stage to the laser light sources  11 ), or may be provided within the illumination optical system  4 . 
     The illumination optical system  4  includes a light intensity equalizer  41  for providing uniformity in the intensity distribution of light beams emitted from the light-collecting optical system  1 , a relay lens group  42 , a diffusing element  44 , and a mirror group  43  composed of a first mirror  43   a  and a second mirror  43   b . In the illumination optical system  4 , light beams emitted from the light intensity equalizer  41  are guided by the relay lens group  42  and the mirror group  43  to the reflective light modulator  2 . 
     The light intensity equalizer  41  has a function to provide uniformity in the light intensity of light beams emitted from the light-collecting optical system  1  (such as a function to reduce nonuniformity in illumination level). The light intensity equalizer  41  is arranged in the illumination optical system  4  such that its light incident surface (incident end surface) through which light enters faces the light-collecting lenses  12 , and that its light exit surface (exit end surface) through which light exits the light intensity equalizer  41  faces the relay lens group  42 . The light intensity equalizer  41  is made of, for example, a transparent material such as glass or resin. The light intensity equalizer  41  may include a rod of a polygonal columnar shape (columnar member that is polygonal in cross section) configured such that its inner side wall functions as a total reflection surface. Or, the light intensity equalizer  41  may include a pipe that is polygonal in cross section (tubular member) formed into a tubular shape such that its inner surface functions as a light reflecting surface. 
     When the light intensity equalizer  41  is a rod of a polygonal columnar shape, light is caused to reflect several times by using total reflection occurring at the interface between the transparent material and air. Then, the light exits the light intensity equalizer  41  through the exit end (light exit). When the light intensity equalizer  41  is a polygonal pipe, light is caused to reflect several times by using the reflex action of an inward-facing surface mirror. Then, the light exits the light intensity equalizer  41  through the light exit. 
     As long as the light intensity equalizer  41  has an appropriate length in a direction in which light beams travel, light beams having reflected several times inside the light intensity equalizer  41  are superposed and applied to a region near the light exit surface of the light intensity equalizer  41 . This provides substantially uniform intensity distribution in the region near the light exit surface of the light intensity equalizer  41 . The light exiting the light intensity equalizer  41  through the light exit surface and having a substantially uniform intensity distribution is guided by the relay lens group  42  and the mirror group  43  to the reflective light modulator  2 , and are applied to the to-be-illuminated surface  2   a  of the reflective light modulator  2 . 
     The illumination optical system  4  includes the diffusing element (diffusing part)  44  provided in a subsequent stage to the relay lens group  42 . The diffusing element  44  diffuses light propagating through the relay lens group  42  and transfers the diffused light to the mirror group  43 , thereby reducing speckle. The diffusing element  44  may be a holographic diffusing element capable of setting angle of diffusion of light according to a hologram pattern formed on a substrate. The diffusing element  44  relaxes the coherence of the laser light sources  11 . Giving motion of the diffusing element  44  such as rotation or oscillation effectively relaxes coherence of the laser light sources  11 . 
     While shown to be arranged in a subsequent stage to the relay lens group  42  in  FIG. 1 , the diffusing element  44  is not necessarily located in this position. As an example, the diffusing element  44  may be arranged in front of or behind the light intensity equalizer  41 . As another example, a plurality of diffusing elements  44  may be arranged in combination to effectively relax the coherence of the laser light sources  11 . 
     As an example, a DMD (digital micro-mirror device, a registered trademark) is used as the reflective light modulator  2 . The reflective light modulator  2  is composed of a large number of movable micro-mirrors each corresponding to a pixel (hundreds of thousands of micro-mirrors, for example) arranged two-dimensionally, in such a way that the tilt angle of each micro-mirror is changed in response to pixel information. 
     While the illustrated relay lens group  42  is composed of a single lens in  FIG. 1 , the number of lenses constituting the relay lens group  42  is not limited to one, but may be two or more. Likewise, the number of mirrors constituting the mirror group  43  is not limited to two, but may be one, or three or more. 
     Light propagation through an optical path in the projection optical system  3  will be described next.  FIG. 2  illustrates the idea of light propagation in a projection optical system, and conceptually shows the operation of the projection optical system  3  (including the F-number and the projection magnification of the projection optical system). The projection optical system  3  is shown to be a single lens element in  FIG. 2  in order for its schematic illustration. 
     The projection optical system  3  of the present embodiment is configured such that the to-be-illuminated surface  2   a  of the reflective light modulator  2  and the transmissive screen  5  optically conjugate with each other. In  FIG. 2 , the size S 1  and the angle of divergence Ω1 [deg] of the to-be-illuminated surface  2   a  of the reflective light modulator  2 , and the size S 2  and the angle of divergence Ω2 [deg] of the transmissive screen  5  are geometrically and optically related to each other as shown by the following formula (1). The angle of divergence Ω1 is the maximum cone angle of light emitted from the reflective light modulator  2 , and the angle of divergence Ω2 is the maximum cone angle of light received by the transmissive screen  5 . 
         S 1×Ω1 =S 2×Ω2  (1)
 
     The F-number (Fp) of the projection optical system  3  is defined by the following formula (2) by using the angle of divergence Ω1 [deg] of the reflective light modulator  2 : 
         Fp= 1/(2×sin(Ω1/2))  (2)
 
     The projection magnification M of the projection optical system  3  represents a ratio by which the size S 1  of the to-be-illuminated surface  2   a  of the reflective light modulator  2  is magnified to the size S 2  of the transmissive screen  5 , and is determined by using the following formula (3): 
         M=S 2 /S 1  (3).
 
     Next, speckle noise or scintillation observed on the transmissive screen  5  will be described with reference to  FIG. 3 , which illustrates for explaining speckle noise or scintillation observed on the transmissive screen.  FIG. 3  briefly shows how glare (scintillation) is generated on the transmissive screen  5 . 
     Even when entering the transmissive screen  5  through the same position and at different angles, light beams  61  and  62  are caused to exit the transmissive screen  5  in the same direction by a diffusion layer  5   a  (containing diffusing materials  54 ) in the transmissive screen  5 . Some light beams are caused to exit the transmissive screen  5  in different directions by the diffusion layer  5   a  in the transmissive screen  5  even when entering the transmissive screen  5  through different positions and at the same angle. This means that light beams entering the transmissive screen  5  through respective positions and at respective angles are diffused in respective directions by the diffusion layer  5   a  in the transmissive screen  5 , and then exit the transmissive screen  5  through respective resultant positions and at respective resultant angles. Accordingly, the light beams  61  and  62  exiting the diffusion layer  5   a  reinforce each other or cancel each other out, thereby generating brightness difference between their exiting positions. This brightness difference is observed as glare in the form of speckle noise or scintillation. 
     In response, the present embodiment defines the angle of divergence Ω2 of light entering the transmissive screen  5  determined by the F-number Fp and the projection magnification M of the projection optical system  3 .  FIGS. 4A and 4B  each show the relationship between the magnitude of the angle of divergence of light entering a transmissive screen and the diffusion characteristics of light exiting the transmissive screen.  FIGS. 4A and 4B  each briefly show the relationship between the F-number of a projection optical system and scintillation.  FIG. 4A  shows the case where the angle of divergence Ω2 [deg] of light entering the transmissive screen  5  is large, whereas  FIG. 4B  shows the case where it is small. 
     As seen from  FIGS. 4A and 4B , light is diffused to a greater degree when exiting the transmissive screen  5  with the greater angle of divergence Ω2 [deg] of the light when entering the transmissive screen  5 . This means the greater angle of divergence Ω2 [deg] of light entering the transmissive screen  5  results in a greater degree of diffusion of the light, thereby relaxing glare. 
     The angle of divergence Ω2 [deg] of light entering the transmissive screen  5  is calculated from the formula (1) using the size S 1  and the angle of divergence Ω1 [deg] of the to-be-illuminated surface  2   a  of the reflective light modulator  2 , and the size S 2  of the transmissive screen  5 . This translates into the fact that the angle of divergence Ω2 [deg] is determined by the F-number Fp and the projection magnification M of the projection optical system  3 . Results of experiment conducted on glare on the transmissive screen  5  are given below that are obtained by changing the F-number Fp and the projection magnification M of the projection optical system  3 . 
       FIGS. 5 and 6  each show the relationship between the F-number of a projection optical system and glare. The results of experiment shown in  FIG. 5  are obtained with the diagonal size S 1  of the to-be-illuminated surface  2   a  of the reflective light modulator  2  being 16.8 mm (0.66 inches), and with the diagonal size S 2  of the transmissive screen  5  being 1651 mm (65 inches). In this case, the projection magnification M is determined as 98.5 from the formula (3).  FIG. 5  shows the levels of speckle noise or scintillation (acceptable levels are indicated by circles and unacceptable levels are indicated by crosses) when the projection magnification M is 98.5 and the F-number Fp of the projection optical system  3  is gradually changed from 2.4 to 6.5. 
     Change in speckle noise and scintillation on the transmissive screen  5  as a result of change in the F-number Fp of the projection optical system  3  was observed. As a result, it has also been found that speckle noise and scintillation are at the allowable levels when the F-number Fp of the projection optical system  3  falls within a range of from 2.4 to 4. 
     In contrast, it has been found that speckle noise and scintillation are worsened when the F-number Fp of the projection optical system  3  is equal to or greater than 4.5. Accordingly, it has been found that speckle noise and scintillation are at the allowable levels when the F-number Fp of the projection optical system  3  falls within a range of from 2.4 to 4. In this case, the product of the F-number Fp and the projection magnification M of the projection optical system  3  is less than 443 and no greater  395 , namely it satisfies a requirement that it should be no greater than 400. 
     The results of experiment shown in  FIG. 6  are obtained with the diagonal size S 1  of the to-be-illuminated surface  2   a  of the reflective light modulator  2  being 17.8 mm (0.7 inches), and with the diagonal size S 2  of the transmissive screen  5  being 1270 mm (50 inches). That is, the results of experiment shown in  FIG. 6  are obtained with the projection display device  101  being configured such that the projection magnification M (71.4) in  FIG. 6  is smaller than that in  FIG. 5 . 
     Like those observed in the projection display device  101  shown in  FIG. 5 , change in speckle noise and scintillation on the transmissive screen  5  as a result of change in the F-number Fp of the projection optical system  3  was observed in the projection display device  101  shown in  FIG. 6 . As a result, it has been found that speckle noise and scintillation are at the allowable levels when the F-number Fp of the projection optical system  3  falls within a range of from 2.4 to 5.5. Accordingly, as in the case of  FIG. 5 , it has been found that speckle noise and scintillation are at the allowable levels when the product of the F-number Fp and the projection magnification M of the projection optical system  3  is less than 429 and not greater than 393, namely when it satisfies a requirement that it should be no greater than 400. 
       FIGS. 7 and 8  show the speckle noise and scintillation shown in  FIGS. 5 and 6  respectively that are represented by numerical values in order to facilitate understanding thereof. Speckle noise and scintillation are evaluated by counting the number of pixels of brightness levels that are the same as or greater than a certain threshold value with respect to an average brightness level on a screen. 
     It is seen from  FIGS. 7 and 8  that change in the evaluated value of speckle noise and scintillation as a result of change in the value of Fp×M clearly differ between the case where the value of Fp×M is greater than 400 and in the case where the value of Fp×M is smaller than 400. More specifically, the evaluated value of speckle noise and scintillation decreases significantly with decrease in the value of Fp×M when the value of Fp×M is greater than 400. In this case, it is seen that decrease in the value of Fp×M is effective in alleviating speckle noise and scintillation. In contrast, the evaluated value of speckle noise and scintillation decreases slightly with a decrease in the value of Fp×M when the value of Fp×M is smaller than 400. In this case, it is seen that a decrease in the value of Fp×M is less effective in alleviating speckle noise and scintillation. 
     As understood from the foregoing, the illumination optical system  4  is allowed to alleviate speckle noise and scintillation effectively when the value of Fp×M satisfies a requirement that it should be no greater than 400, more specifically when it satisfies a requirement that it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. 
     The aforementioned experiments were conducted under conditions that a distance TFLo between the light incident surface of a first diffusion layer and the light exit surface of a second diffusion layer discussed later (distance between the outer surfaces of the diffusion layers, namely maximum distance between the diffusion layers) is 5.1 mm, respective thicknesses DTF and DTL of diffusion layers  51   a  and  52   a  are both 275 μm, and haze values H 1  and H 2  of a Fresnel lens (Fresnel lens sheet)  51  and a lenticular lens (lenticular lens sheet)  52  discussed later are 40% and 90%, respectively. 
     As is described later, speckle noise and scintillation are alleviated to a greater degree with increase in the haze value H 1 . As long as the haze value H 1  is about 40% or higher, speckle noise and scintillation are effectively alleviated when the value of Fp×M satisfies a requirement that it should be no greater than 400, more specifically when it satisfies a requirement that it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. 
     The haze value H 2  set to be greater than the haze value H 1  is effective for avoiding reduction in a resolution level. As long as the haze value H 2  is in a range of from about 80% to 95%, speckle noise and scintillation are effectively alleviated when the value of Fp×M satisfies a requirement that it should be no greater than 400, more specifically when it satisfies a requirement that it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. 
       FIGS. 9 to 11  each show the structures of a transmissive screen. The transmissive screen  5  constituting the optical system of the projection display device  101  includes the Fresnel lens (Fresnel lens sheet)  51  for converting incident light to light with substantially parallel rays, and causing the converted light to exit the Fresnel lens  51 . The transmissive screen  5  also includes the lenticular lens (lenticular lens sheet)  52  for receiving light exiting the Fresnel lens  51 , and causing the received light to exit the lenticular lens  52  as predetermined diffusion light. The transmissive screen  5  includes the diffusion layers  51   a  and  52   a  together with the Fresnel lens  51  and the lenticular lens  52 . 
     The transmissive screen  5  will be described next with reference to  FIGS. 9 to 11  to show examples of the diffusion layers  51   a  and  52   a . In the transmissive screen  5  shown in  FIG. 9 , the diffusion layers  51   a  and  52   a  are arranged on the respective light incident sides of the Fresnel lens  51  and the lenticular lens  52 . In the transmissive screen  5  shown in  FIG. 10 , the diffusion layers  51   a  and  52   a  are arranged on the respective light exit sides of the Fresnel lens  51  and the lenticular lens  52 . The transmissive screen  5  shown in  FIG. 11  has an intermediate layer  53  between the Fresnel lens  51  and the lenticular lens  52 . The intermediate layer  53  in  FIG. 11  is formed from a glass or acrylic flat plate, for example, and has a function to enhance rigidity of the transmissive screen  5 . 
     The combination of the Fresnel lens  51  and the lenticular lens  52  is not limited to those shown in  FIGS. 9 to 11 . The Fresnel lens  51  and the lenticular lens  52  may be combined in alternative ways. Further, the transmissive screen  5  may have three or more diffusion layers. 
     In  FIGS. 9 to 11 , the Fresnel lens  51 , the diffusion layer  51   a  of the Fresnel lens  51  as the first diffusion layer, the lenticular lens  52 , and the diffusion layer  52   a  of the lenticular lens  52  as the second diffusion layer have thicknesses called TF, DTF, TL, and DTL, respectively. Change in speckle noise and scintillation on the transmissive screen  5  was observed that was caused as a result of change in the relationship between the thicknesses TF and DTF of the Fresnel lens  51  and the diffusion layer  51   a , or in the relationship between the thicknesses TL and DTL of the lenticular lens  52  and the diffusion layer  52   a.    
       FIGS. 12 and 13  each show the relationship between the thickness of a light diffusion layer of a transmissive screen and glare. The diffusion layers  51   a  and  52   a  each contain a diffusing material, and the amount of the diffusing material contained is expressed by a haze value (in percentage).  FIG. 12  shows the levels of speckle noise or scintillation (acceptable levels are indicated by circles and unacceptable levels are indicated by crosses) when the thickness DTF of the diffusion layer  51   a  is changed from 170 μm to 315 μm while the haze value H 1  of the diffusion layer  51   a  of the Fresnel lens  51  is 40% and the thickness TF of the Fresnel lens  51  is 2000 μm. At this time, the haze value H 2  of the diffusion layer  52   a  of the lenticular lens  52  is 90%, and the thicknesses TL and DTL of the lenticular lens  52  and the diffusion layer  52   a  are 2000 μm and 275 μm, respectively. Furthermore, the aforementioned value of Fp×M is 330. 
     Changes in speckle noise and scintillation on the transmissive screen  5  as a result of change in the thickness DTF of the diffusion layer  51   a  of the Fresnel lens  51  were observed. As a result, it has been found that speckle noise and scintillation are at the allowable levels when the ratio of the thickness (DTF) of the diffusion layer to the thickness (TF) of the Fresnel lens is less than 0.150 and not greater than 0.138, namely when the thickness of the diffusion layer (DTF) is less than 300 μm and not greater than 275 μm. The same results are obtained as long as the haze value H 1  is about 40% or higher, and the haze value H 2  is in a range of about 80 to 95%. 
       FIG. 13  shows the levels of speckle noise or scintillation (acceptable levels are indicated by circles and unacceptable levels are indicated by crosses) when the thickness DTL of the diffusion layer  52   a  is changed from 170 μm to 315 μm while the haze value H 2  of the diffusion layer  52   a  of the lenticular lens  52  is 90% and the thickness TL of the lenticular lens  52  is 2000 μm. At this time, the haze value H 1  of the diffusion layer  51   a  of the Fresnel lens  51  is 40%, and the thicknesses TF and DTF of the Fresnel lens  51  and the diffusion layer  51   a  are 2000 μm and 275 μm, respectively. Furthermore, the aforementioned value of Fp×M is 330. 
     Change in speckle noise and scintillation on the transmissive screen  5  as a result of change in the thickness DTL of the diffusion layer  52   a  of the lenticular lens  52  was observed. As a result, it has been found that speckle noise and scintillation are at the allowable levels when the ratio of the thickness (DTL) of the diffusion layer to the thickness (TL) of the lenticular lens is less than 0.150 and no greater than 0.138, namely when the thickness (DTL) of the diffusion layer is less than 300 μm and no greater than 275 μm. The same results are obtained as long as the haze value H 1  is about 40% or higher, and the haze value H 2  is in a range of from about 80% to 95%. 
       FIGS. 14 and 15  show the speckle noise and scintillation shown in  FIGS. 12 and 13 , respectively, that are represented by numerical values in order to facilitate understanding thereof. Speckle noise and scintillation are evaluated by counting the number of pixels of brightness levels that are the same as or greater than a certain threshold value with respect to an average brightness level on a screen. In the legends of  FIGS. 14 and 15 , speckle noise and scintillation are expressed collectively merely as scintillation due to a lack of space. Furthermore, speckle noise and scintillation are indicated by squares (lower plot), and a resolution level is indicated by rhombuses (upper plot). 
     It is seen from  FIGS. 14 and 15  that changes in the evaluated value of speckle noise and scintillation as a result of change in the thicknesses DTF and DTL of the diffusion layers clearly differs between the case where the thicknesses DTF and DTL are greater than 250 μm and the case where the thicknesses DTF and DTL are smaller than 250 μm. More specifically, the evaluated value of speckle noise and scintillation decreases significantly with a reduction in the thicknesses DTF and DTL of the diffusion layers when the thicknesses DTF and DTL are greater than 250 μm. In this case, it is seen that reduction in the thicknesses DTF and DTL is effective in alleviating speckle noise and scintillation. In contrast, the evaluated value of speckle noise and scintillation decreases slightly with a reduction in the thicknesses DTF and DTL when the thicknesses DTF and DTL are smaller than 250 μm. In this case, it is seen that decrease in the thicknesses DTF and DTL is less effective in alleviating speckle noise and scintillation. 
       FIGS. 14 and 15  each show a resolution level together with the evaluated value of speckle noise and scintillation. Image quality degrades with a reduction in a resolution level, thereby generating severe blur of images. It is seen from  FIGS. 14 and 15  that changes in the thicknesses DTF and DTL of the diffusion layers cause substantially no change in a resolution level. This means that speckle noise and scintillation are alleviated by reducing the thicknesses DTF and DTL without lowering a resolution level. 
     A resolution level is determined by the CTF (contrast transfer function). More specifically, an image with a group of uniformly spaced lines is displayed on a screen. When white and black lines appear alternately on the screen, black levels stand out clearly in a projected image. When the maximum and minimum of the intensity of a projected image are defined as Pmax and Pmin, respectively, CTF indicative of resolution performance is obtained from the following formula (4): 
         CTF= 100×( P max− P min)/( P max+ P min)  (4)
 
     In this case, a CTF value decreases with a reduction in resolution when a difference between the light intensities at Pmax and Pmin becomes small. In contrast, a CTF value increases with an increase in resolution when a difference between the light intensities at Pmax and Pmin becomes large. 
     As understood from the foregoing, speckle noise and scintillation are effectively alleviated by the thicknesses DTF and DTL of the diffusion layers without lowering a resolution level when the thicknesses DTF and DTL both satisfy a requirement that they should be no greater than 250 μm. 
     Requirements for the thicknesses DTF and DTL of the diffusion layers of the Fresnel lens (Fresnel lens sheet)  51  with a haze value of 40% and of the lenticular lens (lenticular lens sheet)  52  with a haze value of 90%, are both such that they should be no greater than 250 μm. This means that a haze value has little influence when the thicknesses DTF and DTL are both no greater than 250 μm. 
     Next, alleviation of speckle noise and scintillation by the thicknesses DTF and DTL of the diffusion layers, and alleviation of speckle noise and scintillation by the value of Fp×M are compared. The value of Fp×M, and the thicknesses DTF and DTL have their respective effects in alleviating speckle noise and scintillation. However, in the first embodiment, speckle noise and scintillation are alleviated first by decreasing the value of Fp×M, and then by reducing the thicknesses DTF and DTL. 
     The reason for doing so is as follows. Regarding alleviation by the value of Fp×M, decreasing the value of Fp×M from its normally applied value that is around 600 to the aforementioned value that is around 400 results in a difference of about 9700 in speckle noise and scintillation. In terms of ratio, alleviated speckle noise and scintillation are about 0.024 times those before alleviation. Regarding alleviation by reducing the thicknesses DTF and DTL of the diffusion layers, reducing the thicknesses DTF and DTL from their normally applied values that are around 300 μm to 250 μm results in a difference of about 44 to 61 in speckle noise and scintillation. In terms of ratio, alleviated speckle noise and scintillation are about 0.3 to 0.4 times those before alleviation. 
     When the alleviation is considered in terms of ratio, speckle noise and scintillation alleviated by the value of Fp×M is 0.024 times those before alleviation. Speckle noise and scintillation alleviated by the thicknesses DTF and DTL of the diffusion layers are about 0.3 to 0.4 times those before alleviation. If the thicknesses DTF and DTL results in the alleviation of about 0.35 times, the relation of 0.35/0.024=14.6 is established. That is, it is seen that the value Fp×M alleviates speckle noise and scintillation is about 15 times more effectively than the thinned thicknesses DTF and DTL. It is understood accordingly that, when speckle noise and scintillation are alleviated first by the value of Fp×M and then by reducing the thicknesses DTF and DTL of the diffusion layers, adverse effect to be caused by decreasing the value of Fp×M and reducing the thicknesses DTF and DTL in designing the structure of the projection display device  101  is minimized. It is also understood that speckle noise and scintillation are alleviated without requiring excessive cost. 
     By the way, decreasing the value of Fp×M increases the size of the projection optical system  3  and makes formation of a lens difficult, thereby making downsizing of the projection optical system  3  difficult. In contrast, reducing the thicknesses DTF and DTL of the diffusion layers makes formation of the Fresnel lens  51  and the lenticular lens  52  difficult. Thus, in considering the overall structure of the projection display device  101 , it is important to first take an action that may be more effective in alleviating speckle noise and scintillation within a range that allows formation of the lenses, and then to take less effective action in order to make up for a deficit to achieve a target. 
     Accordingly, in the present embodiment, the projection display device  101  is configured such that the value of Fp×M satisfies a requirement that it should be no greater than 400, more specifically it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. As a result, the angle of divergence of light entering the transmissive screen  5  falls within a predetermined range. 
     In the present embodiment, the ratio of the thickness (DTF) of the diffusion layer to the thickness (TF) of the Fresnel lens, that is the relationship between the thickness TF of the Fresnel lens  51  and the thickness DTF of the first diffusion layer  51   a , is set to be less than 0.150 and no greater than 0.138. Namely, the thickness (DTF) of the diffusion layer is set to be less than 300 μm and no greater than 275 μm. Further, in the present embodiment, the ratio of the thickness (DTL) of the diffusion layer to the thickness (TL) of the lenticular lens, that is the relationship between the thickness TL of the lenticular lens  52  and the thickness DTL of the second diffusion layer  52   a , is also set to be less than 0.150 and no greater than 0.138. Namely, the thickness (DTL) of the diffusion layer is set to be less than 300 μm and no greater than 275 μm. In the present embodiment, the reflective light modulator  2  is used as a light bulb of the projection display device  101 . A light bulb of different types such as a transmissive or reflective liquid crystal display element may alternatively be used in the projection display device  101 . Furthermore, while the laser light sources  11  are used in the first embodiment, light sources of different types such as extra high pressure mercury lamps and metal halide lamps may be used as well. 
     As described above, in the first embodiment, the value of Fp×M, that is the product of the F-number Fp and the projection magnification M of the projection optical system  3 , is set to satisfy a requirement that it should be no greater than 400, more specifically it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. In addition to this, the ratio of the thickness (DTF, DTL) of the diffusion layer  51   a  or  52   a  to the thickness (TF, TL) of the Fresnel lens or the lenticular lens of the transmissive screen  5 , is set to be less than 0.150 and no greater than 0.138. Namely, the thickness (DTF, DTL) of the diffusion layer is set to be less than 300 μm and no greater than 275 μm. Thus, speckle noise and scintillation on the transmissive screen  5  are effectively reduced even when a high-brightness light source is used. 
     Furthermore, by the use of the laser light sources  11 , the first embodiment realizes a long-lasting optical system that achieves good color reproducibility. Still further, by the provision of the light diffusing element  44  in the illumination optical system  4 , the first embodiment efficiently reduces speckle noise and scintillation. Also, by the use of the light intensity equalizer  41  for providing uniformity in light beams emitted from the light-collecting optical system  1 , the first embodiment provides favorable images with no nonuniformity in illumination level while reducing speckle noise and scintillation. 
     As described above, the first embodiment makes the following setting in order for the angle of divergence of light entering the transmissive screen  5  to fall within a predetermined range. That is, the product of the F-number Fp and the projection magnification of the projection optical system  3  (value of Fp×M) is set to satisfy a requirement that it should be no greater than 400, more specifically it should be no greater than 395 or 393 in light of the values obtained by the results of experiment. 
     In addition to this, the ratio of the thickness (DTF, DTL) of the diffusion layer, that is the thickness of the diffusion layer  51   a  or  52   a  of the transmissive screen  5 , to the thickness (TF, TL) of the Fresnel lens or the lenticular lens, is set to be less than 0.150 and no greater than 0.138. Namely, the thickness (DTF, DTL) of the diffusion layer is set to be less than 300 μm and not greater than 275 μm. Thus, even when high-brightness light is projected onto the transmissive screen  5 , speckle noise or scintillation is effectively reduced to thereby achieve high quality of images to be displayed. 
     Second Embodiment 
     A second embodiment of the present invention will be described by referring to  FIGS. 9 to 11  and  16 . In the second embodiment, at least two diffusion layers are arranged in the transmissive screen  5 , and a distance TFLo between the light incident surface of a first diffusion layer and the light exit surface of a second diffusion layer (distance between the outer surfaces of the diffusion layers, namely maximum distance between the diffusion layers) is set to fall within a predetermined range. 
       FIG. 16  shows the relationship of the distance TFLo between the light incident surface of the first diffusion layer  51   a  and the light exit surface of the second diffusion layer  52   a  shown in  FIGS. 9 to 11  with speckle noise or scintillation observed on the transmissive screen  5 . As seen from  FIG. 16 , scintillation is reduced effectively when the distance TFLo is set to be no less than 5 mm. In the present embodiment, the distance TFLo is defined such that scintillation is lower than a predetermined value (scintillation allowable range), namely, such that scintillation falls within its allowable range. 
     The values shown in  FIG. 16  are obtained from experiments conducted under conditions that the product of the F-number Fp and the projection magnification M of a projection optical system is 420, the thickness DTF and the haze value of the diffusion layer  51   a  of the Fresnel lens  51  are 275 μm and 43%, respectively, and the thickness DTL and the haze value of the diffusion layer  52   a  of the lenticular lens  52  are 275 μm and 90% respectively. 
     As already mentioned, the distance TFLo defines a distance between the outer surfaces of the first and second diffusion layers  51   a  and  52   a . Scintillation is investigated next by defining a distance TFLi between the inner surfaces of the first and second diffusion layers  51   a  and  52   a.    
     As shown in  FIGS. 9 to 11 , at least two diffusion layers are arranged in the transmissive screen  5 . 
     Furthermore, the diffusion layer  51   a  of the Fresnel lens  51  nearest the light incident surface of the transmissive screen  5  is defined as a first diffusion layer, and the diffusion layer  52   a  of the lenticular lens  52  nearest the light exit surface of the transmissive screen  5  is defined as a second diffusion layer. Then, the relationship among the distance TFLi between the light exit surface of the first diffusion layer and the light incident surface of the second diffusion layer (distance between the inner surfaces of the diffusion layers, namely minimum distance between the diffusion layers), speckle noise or scintillation, and a resolution level is considered. The internal distance TFLi mentioned in the present embodiment is optically expressed in the form of air thickness. 
       FIG. 17  shows the relationship among the distance TFLi between the light exit surface of the first diffusion layer  51   a  and the light incident surface of the second diffusion layer  52   a  shown in  FIGS. 9 to 11 , speckle noise or scintillation observed on the transmissive screen  5 , and the resolution level of the transmissive screen  5 . The values shown in  FIG. 17  are obtained from experiments conducted under conditions that the product of the F-number Fp and the projection magnification M of a projection optical system is 420, the thickness DTF and the haze value of the diffusion layer  51   a  of the Fresnel lens  51  are 275 μm and 43%, respectively, and the thickness DTL and the haze value of the diffusion layer  52   a  of the lenticular lens  52  are 275 μm and 90%, respectively. In the legends of  FIG. 17 , speckle noise and scintillation are expressed collectively merely as scintillation due to a lack of space. Furthermore, speckle noise and scintillation are indicated by squares (lower plot), and a resolution level is indicated by rhombuses (upper plot). 
     It has been found from  FIG. 17  that speckle noise or scintillation is reduced (favorable images are obtained) with an increase in the internal distance TFLi between the diffusion layers. However, it has also found that increase in the internal distance TFLi in turn causes reduction in a resolution level. Reduction in resolution levels increases the degree of blurring. Accordingly, a resolution level is lowered as shown in  FIG. 17 . That is, image quality degrades with reduction in a resolution level. 
     Accordingly, increasing the distance TFLi between the diffusion layers is effective for reducing speckle noise or scintillation. However, a projection display device should be configured such that the distance TFLi is optimized in order to suppress reduction in a resolution level.  FIG. 17  shows that the internal distance TFLi is preferably from 4.5 mm to 7.5 mm. 
     As described above, in the second embodiment, the transmissive screen  5  is configured such that the distance TFLo between the light incident surface of the first diffusion layer  51   a  and the light exit surface of the second diffusion layer  52   a  is no less than 5 mm, or such that the distance TFLi between the light exit surface of the first diffusion layer  51   a  and the light incident surface of the second diffusion layer  52   a  is from 4.5 mm to 7.5 mm. Accordingly, speckle noise or scintillation is reduced to thereby achieve high quality of images to be displayed. 
     Next, the relationship between the internal distance TFLi between the diffusion layers with the thicknesses of the diffusion layers  51   a  and  52   a  is considered.  FIGS. 18 to 20  each show how the internal distance TFLi is defined with the increased thicknesses DTF and DTL of the diffusion layers  51   a  and  52   a  with respect to the diffusion layers  51   a  and  52   a  shown in  FIG. 9  that are arranged on the respective incident sides of the Fresnel lens  51  and the lenticular lens  52 . In  FIG. 18 , the diffusion layers  51   a  and  52   a  are increased in thickness but the thicknesses TF and TL of the Fresnel lens screen  51  and the lenticular lens  52  do not change. In this case, the internal distance TFLi is shortened. In  FIG. 19 , the diffusion layers  51   a  and  52   a  are increased in thickness and the thicknesses TF and TL are also increased. In this case, the Fresnel lens screen  51  and the lenticular lens  52  come closer to each other and the internal distance TFLi is shortened accordingly. In  FIG. 20 , the diffusion layers  51   a  and  52   a  are increased in thickness as in the structure shown in  FIG. 18 , and the thicknesses TF and TL are also increased. The internal distance TFLi is, however, maintained at the same level as that in  FIG. 9 . 
     As shown in  FIGS. 18 to 20 , the internal distance TFLi is shortened with increase in the thicknesses DTF and DTL of the diffusion layers  51   a  and  52   a . Or alternatively, with increase in the thicknesses DTF and DTL of the diffusion layers  51   a  and  52   a , the total thickness of the transmissive screen  5  is increased when the internal distance TFLi is maintained at the same level. In the structures shown in  FIGS. 18 to 20 , the cases where the diffusion layers  51   a  and  52   a  are arranged on the respective incident sides of the Fresnel lens  51  and the lenticular lens  52  are considered. The same results are obtained when the diffusion layers  51   a  and  52   a  are arranged on the respective light exit sides thereof as shown in  FIG. 10 , or when they are arranged on a different combination of the incident side and the light exit side. 
     The internal distance TFLi between the diffusion layers, speckle noise and scintillation, and a resolution level are closely related to one another as shown in  FIG. 17 . The internal distance TFLi should be from 4.5 mm to 7.5 mm in order to achieve favorable performance. In this case, the total thickness of the transmissive screen  5  is increased when the diffusion layers  51   a  and  52   a  are increased in thickness as shown in  FIG. 20 . 
       FIGS. 21 and 22  conceptually illustrate how speckle noise and scintillation are reduced in accordance with a change in the thickness of the diffusion layer  51   a  or  52   a . The diffusion layer  51   a  or  52   a  contains a diffusing material, and the amount of the diffusion material contained is expressed by a haze value (in percentage). The diffusion layer  51   a  or  52   a  has a thickness that is different between  FIGS. 21 and 22 , and has a haze value that is the same between  FIGS. 21 and 22 . Again, the diffusion layer  51   a  or  52   a  has a thickness (that is, an optical path length) that is different between  FIGS. 21  and  22 . This means that the smaller thickness of the diffusion layer  51   a  or  52   a  shown in  FIG. 22  results in a shorter optical length path. In order to compensate for reduction in an optical path length, a rate of filling with a diffusing material should be increased to maintain a haze value at the same level. 
     When the diffusion layer  51   a  or  52   a  is reduced in thickness so as to increase a rate of filling with a diffusing material as shown in  FIG. 22 , light beams  61  and  62  entering the diffusion layer  51   a  or  52   a  are diffused by diffusing elements  54  with higher probability. Accordingly, the light beams  61  and  62  emitted to an observer have more irregularities than those in the case of  FIG. 21  where the diffusion layer  51   a  or  52   a  has a greater thickness, thereby more effectively reducing speckle noise and scintillation. 
     Increase in the haze value H 1  or H 2  achieves increase in a rate of filling with a diffusing material despite the greater thickness of the diffusion layer  51   a  or  52   a . However, increase in the haze value H 1  or H 2  is undesirable as it in turn reduces brightness as well as a resolution level. 
       FIG. 17  shows the results obtained with change in the internal distance TFLi between the diffusion layers as a result only of the changes in the thicknesses of the diffusion layers  51   a  and  52   a  as shown in  FIG. 18 . However, the same results are obtained from the structures shown in  FIGS. 19 and 20  about the relationship among a distance TFL between diffusion layers, speckle noise or scintillation, and a resolution level. 
     The values shown in  FIG. 17  are obtained from experiments conducted under conditions that the product of the F-number Fp and the projection magnification M of a projection optical system is 420, the haze value of the diffusion layer  51   a  of the Fresnel lens  51  is 43%, and the haze value of the diffusion layer  52   a  of the lenticular lens  52  is 90%. Even when the product of the F-number Fp and the projection magnification M of the projection optical system is smaller, speckle noise and scintillation are reduced more effectively as long as the distance TFL between the diffusion layers is from 4.5 mm to 7.5 mm. 
     Third Embodiment 
     A third embodiment of the present invention will be described by referring to  FIGS. 23 and 24 . In the third embodiment, at least two diffusion layers are arranged in the transmissive screen  5 , and the diffusion rate (haze value) of each of the diffusion layers is set to fall within a predetermined range. 
     The diffusion layers  51   a  and  52   a  of the Fresnel lens  51  and the lenticular lens  52  each contain a diffusing material, and the amount of the diffusing material contained may be expressed by a haze value (in percentage). Here, the haze value of the diffusion layer  51   a  is defined as a haze value H 1 , and that of the diffusion layer  52   a  is defined as a haze value H 2 . 
     Increase in the haze value H 1  or H 2  of the diffusion layer  51   a  or  52   a  reduces speckle noise or scintillation observed on the transmissive screen  5 . However, increase in the haze value H 1  disadvantageously reduces forward brightness and lowers a resolution level. Increase in the haze value H 2  also reduces forward brightness. Furthermore, the haze value H 2  reaching or exceeding a predetermined value is less effective in reducing speckle noise or scintillation. 
     Described next are a peak gain (PG) indicative of the brightness of the transmissive screen  5 , and to which degree glare (speckle noise and scintillation) is reduced when the haze values H 1  and H 2  of the diffusion layers  51   a  and  52   a  are varied.  FIG. 23  shows results of examination of PG and a degree of reduction in glare obtained by varying a haze value.  FIG. 23  represents the relationship among the haze value H 1  of the diffusion layer  51   a , PG, and a degree of reduction in glare (speckle noise and scintillation) with the corresponding haze value H 1 . The haze value H 1  of the diffusion layer  51   a  used in the examination was specifically 40% as a smaller haze value, and was 72% as a larger haze value. At this time, the lenticular lens  52  combined with the Fresnel lens  51  had five variations defined by the haze value H 2  of the diffusion layer  52   a  that ranges from 80% to 95%. 
     The two variations of the Fresnel lens  51  including the one with the smaller haze value H 1  and the other with the higher haze value H 1  of the diffusion layer  51   a  were combined in various ways with the five variations of the lenticular lens  52  with different haze values H 2  of the diffusion layer  52   a . Then, PG, and a degree of reduction in speckle noise and scintillation in these combinations were observed. 
     As a result, when the haze value H 1  of the diffusion layer  51   a  was smaller, PG was reduced in some cases according to the variation of the lenticular lens  52 . However, when the haze value H 1  of the diffusion layer  51   a  was smaller, speckle noise and scintillation were never reduced to an allowable level (allowable limit L 1  of speckle noise and scintillation) even in the combinations of the Fresnel lens  51  with any variations of the lenticular lens  52 . 
     In contrast, when the haze value H 1  of the diffusion layer  51   a  was larger, speckle noise and scintillation were reduced significantly to reach the allowable limit L 1  in many cases as long as the haze value H 2  of the diffusion layer  52   a  was greater than a predetermined value (as long as PG was small). Accordingly, it has been found that increase in the haze value H 1  of the diffusion layer  51   a  more effectively reduces speckle noise and scintillation. 
     Next, the lenticular lens  52  with the haze value H 2  of the diffusion layer  52   a  being 80% is combined in various ways with five variations of the Fresnel lens  51 . Then, PG, a degree of reduction in speckle noise and scintillation, and a resolution level in these combinations were observed.  FIG. 24  shows results of examination of PG, a degree of reduction in glare, and a resolution level obtained by varying a haze value. Specifically,  FIG. 24  shows the relationship among the haze value H 1  of the diffusion layer  51   a , PG, a degree of reduction in speckle noise or scintillation, and a resolution level with the corresponding haze value H 1 . 
     The Fresnel lens  51  has five variations defined by the haze value H 1  of the diffusion layer  51   a  that ranges from 40% to 82%. Speckle noise and scintillation are reduced effectively with the haze value H 1  of 82% (higher than the haze value H 2  of the diffusion layer  52   a  that is 80%). In this case, however, a resolution level is severely lowered. Accordingly, it has been found that, while increase in the haze value H 1  of the diffusion layer  51   a  effectively reduces speckle noise and scintillation, increase in the haze value H 1  to a level higher than the haze value H 2  of the diffusion layer  52   a  lowers a resolution level. Accordingly, setting the haze value H 1  of the diffusion layer  51   a  to be smaller than the haze value H 2  of the diffusion layer  52   a  effectively reduces speckle noise and scintillation while providing a favorable resolution level. Thus, in the present embodiment, the haze value H 1  is increased to a level that does not exceed the level of the haze value H 2 . 
     As described above, in the third embodiment, the transmissive screen  5  is configured by using the diffusion layer  51   a  the haze value H 1  of which is set to a high level but that does not exceed the level of the haze value H 2  of the diffusion layer  52   a . As a result, speckle noise or scintillation is reduced to thereby achieve high quality of images to be displayed. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will be described by referring to  FIGS. 25 and 26 . In the fourth embodiment, the diffusion layer  52   a  of the lenticular lens  52  is configured to contain at least two types of diffusing materials (diffusing elements). 
       FIGS. 25 and 26  each show the structure of an optical system of a projection display device according to the fourth embodiment of the present invention.  FIGS. 25  and  26  each conceptually show the structure of the diffusion layer  52   a  of the lenticular lens  52 . The transmissive screen  5  functioning as an optical system of the projection display device  101  includes the lenticular lens  52 , and the diffusion layer  52   a  of the lenticular lens  52  contains at least two types of diffusing materials. The diffusion layer  52   a  is configured by combining diffusing materials of different substances (different refractive indexes), different sizes, different shapes and the like. 
       FIG. 25  shows an example of a diffusion layer containing diffusing materials of different sizes.  FIG. 26  shows an example of a diffusion layer containing diffusing materials of different sizes and shapes. The diffusion layer  52   a  of the lenticular lens  52  shown in  FIG. 25  contains at least two types of diffusing materials  55 A and  55 B of different sizes. The diffusion layer  52   a  of the lenticular lens  52  shown in  FIG. 26  contains at least two types of diffusing materials  56 A and  56 B of different sizes and different shapes. According to this configuration, light beams  61  and  62  entering the lenticular lens  52  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  52   a  contains a single type of diffusing material. 
     As described above, in the projection display device  101  of the fourth embodiment, the diffusion layer  52   a  of the lenticular lens  52  is configured to contain at least two types of diffusing materials. Accordingly, the light beams  61  and  62  entering the lenticular lens  52  and emitted as diffusion light to an observer have irregularities. As a result, speckle noise and scintillation are more effectively reduced to thereby achieve high quality of images to be provided. 
     Fifth Embodiment 
     A fifth embodiment of the present invention will be described by referring to  FIGS. 27 and 28 . In the fifth embodiment, the diffusion layer  51   a  of the Fresnel lens  51  is configured to contain at least two types of diffusing materials (diffusing elements). 
       FIGS. 27 and 28  each show the structure of an optical system of a projection display device according to the fifth embodiment of the present invention.  FIGS. 27 and 28  each conceptually show the structure of the diffusion layer  51   a  of the Fresnel lens  51 . The transmissive screen  5  functioning as an optical system of the projection display device  101  is configured to include the Fresnel lens  51 , and the diffusion layer  51   a  of the Fresnel lens  51  contains at least two types of diffusing materials. The diffusion layer  51   a  is configured by combining diffusing materials of different substances (different refractive indexes), different sizes, different shapes and the like. 
       FIG. 27  shows an example of a diffusion layer containing diffusing materials of different sizes.  FIG. 28  shows an example of a diffusion layer containing diffusing materials of different sizes and shapes. The diffusion layer  51   a  of the Fresnel lens  51  shown in  FIG. 27  contains at least two types of diffusing materials  57 A and  57 B of different sizes. The diffusion layer  51   a  of the Fresnel lens  51  shown in  FIG. 28  contains at least two types of diffusing materials  58 A and  58 B of different sizes and different shapes. According to this configuration, light beams  61  and  62  entering the Fresnel lens  51  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  51   a  contains a single type of diffusing material. 
     As described above, in the projection display device  101  of the fifth embodiment, the diffusion layer  51   a  of the Fresnel lens  51  contains at least two types of diffusing materials. Accordingly, the light beams  61  and  62  entering the Fresnel lens  51  and emitted as diffusion light to an observer have irregularities. As a result, speckle noise and scintillation are more effectively reduced to thereby achieve high quality of images to be provided. 
     Sixth Embodiment 
     A sixth embodiment of the present invention will be described by referring to  FIGS. 29 to 32 . In the embodiments described so far, the diffusion layers  51   a  and  52   a  of the Fresnel lens  51  and the lenticular lens  52  each contain diffusing materials of different substances, shapes and sizes. In the present embodiment, a diffusing element is added to the light exit surface of the Fresnel lens  51  or the lenticular lens  52 . 
       FIGS. 29 to 32  each show the structure of an optical system of a projection display device according to the sixth embodiment of the present invention.  FIGS. 29 to 32  each conceptually show the structure of the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52 . The transmissive screen  5  functioning as an optical system of the projection display device  101  is configured to include the Fresnel lens  51  or the lenticular lens  52 , and the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  contains at least two types of diffusing materials (in  FIGS. 29 to 32 , three types of diffusing materials are shown). The diffusion layer  51   a  or  52   a  is configured by combining diffusing materials of different substances (different refractive indexes), different sizes, different shapes and the like with a diffusion layer having a function of a lens. 
     The diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  shown in  FIG. 29  contains two types of diffusing materials  155 A and  155 B of different sizes, and a diffusing element  155 C having a lens structure. According to this configuration, light beams  61  and  62  entering the Fresnel lens  51  or the lenticular lens  52  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  51   a  or  52   a  contains a single type of diffusing element. 
     Likewise, the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  shown in  FIG. 30  contains two types of diffusing materials  156 A and  156 B of different sizes and shapes, and a diffusing element  156 C having a lens structure. According to this configuration, light beams  61  and  62  entering the Fresnel lens  51  or the lenticular lens  52  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  51   a  or  52   a  contains a single type of diffusing element. 
     Likewise, the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  shown in  FIG. 31  contains two types of diffusing materials  157 A and  157 B of different sizes, and a diffusing element  157 C having a prism lens structure. According to this configuration, light beams  61  and  62  entering the Fresnel lens  51  or the lenticular lens  52  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  51   a  or  52   a  contains a single type of diffusing element. 
     Likewise, the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  shown in  FIG. 32  contains two types of diffusing materials  158 A and  158 B of different sizes and shapes, and a diffusing element  158 C having a prism lens structure. According to this configuration, light beams  61  and  62  entering the Fresnel lens  51  or the lenticular lens  52  and emitted as diffusion light to an observer have more irregularities than those in the case where the diffusion layer  51   a  or  52   a  contains a single type of diffusing element. 
     In each of  FIGS. 29 to 32 , the diffusion layer  51   a  or  52   a  is shown to contain three different types of diffusing elements. However, the number of types of diffusing elements is not limited to three. Furthermore, while shown to be arranged on the light exist side of the Fresnel lens  51  or the lenticular lens  52  in each of  FIGS. 29 to 32 , a diffusing element is not necessarily arranged on the light exit side. Also, the diffusing elements  155 C and  156 C shown in  FIGS. 29 and 30  each have a lens structure, and the diffusing elements  157 C and  158 C shown in  FIGS. 30 and 31  each have a prism lens structure. However, the lens structures of the diffusing elements are not limited to those shown in  FIGS. 29 to 32 . 
     As described above, in the projection display device  101  of the sixth embodiment, the diffusion layer  51   a  or  52   a  of the Fresnel lens  51  or the lenticular lens  52  is configured to contain at least two types of diffusing elements. Accordingly, the light beams  61  and  62  entering the Fresnel lens  51  or the lenticular lens  52  and emitted as diffusion light to an observer have irregularities. As a result, speckle noise and scintillation are more effectively reduced to thereby achieve high quality of images to be provided. 
     Seventh Embodiment 
     A seventh embodiment of the present invention will be described by referring to  FIG. 33 . In the seventh embodiment, optical fibers  13  are added to the light-collecting optical system  1 .  FIG. 33  shows the structure of a projection display device according to the seventh embodiment. Constituent elements shown in  FIG. 33  having the same functions as those of the corresponding elements of the projection display device  101  of the first embodiment shown in  FIG. 1  are designated by the same reference numerals, and the same description thereof is not given repeatedly. 
     A light-collecting optical system  1 X of a projection display device  102  is configured to include laser light sources  11  of multiple colors (in  FIG. 33 , three colors), a plurality of (in  FIG. 33 , three) light-collecting lenses (light-collecting parts)  12  composed of one or a plurality of lenses or mirrors for collecting light beams emitted from the laser light sources  11 , and a plurality of (in  FIG. 33 , three) optical fibers  13  for guiding light beams emitted from the light-collecting lenses  12  to an illumination optical system  4 . 
     In the light-collecting optical system  1 X, each of the laser light sources  11  has a one-to-one correspondence with each of the light-collecting lenses  12  and each of the optical fibers  13 . Accordingly, a light beam emitted from one of the laser light sources  11  is transferred through the corresponding light-collecting lens  12  and the corresponding optical fiber  13  to the illumination optical system  4 . 
     The illumination optical system  4  includes a light intensity equalizer  41  for providing uniformity in the intensity distribution of light beams emitted from the light-collecting optical system  1 X (optical fibers  13 ), a relay lens group  42 , a diffusing element  44 , and a mirror group  43  composed of a first mirror  43   a  and a second mirror  43   b . In the illumination optical system  4 , light beams emitted from the light intensity equalizer  41  are guided by the relay lens group  42  and the mirror group  43  to the reflective light modulator  2 . 
     As described above, in the seventh embodiment, light beams emitted from the laser light sources  11  are guided through the optical fibers  13  to the illumination optical system  4 . This provides flexibility in the arrangement of an optical system, and the resultant structure of the optical system allows admission of light beams at a high rate. Also, as a result of multiple reflection of light beams inside the optical fibers  13 , speckle noise and scintillation are reduced to thereby provide uniformity in images. 
     A projection display device may be constructed by combining the structures of the first to seventh embodiments. In this case, the projection display device is formed of a simple structure capable of reducing speckle noise and scintillation to achieve high quality of images to be displayed. 
     Although the invention has been described with respect to specific preferred embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.