Rear projection screen having reduced scintillation

A rear projection screen of the present invention includes lens sheets or optical sheets having an optical function of condensing or diffusing light. The lens sheets or optical sheets have, as a whole, two or more diffusing layers (diffusing parts) separately provided in the light-transmitting direction. It is preferable that one of the two or more diffusing layers be provided on the light-entering-side surface of the outermost lens sheet or optical sheet on the light source side and that another one of the diffusing layers be provided on the light-emerging-side surface of the outermost lens sheet or optical sheet on the observation side. Any two of the two or more diffusing layers are such that the light-source-side diffusing layer has a diffusing power lower than that of the observation-side diffusing layer. Further, it is preferable that the types (refractive indexes or average particle diameters) of diffusers to be respectively incorporated into any two of the two or more diffusing layers be different from each other.

TECHNICAL FIELD
 The present invention relates to rear projection screens chiefly used for
 rear-projection-type projectors such as video projectors and slide
 projectors.
 BACKGROUND ART
 As rear projection screens of this type, there have conventionally been
 known those screens composed of a single lenticular lens sheet comprising
 as a base material a synthetic resin such as polymethyl methacrylate, and
 those screens composed of such a lenticular lens sheet and other lens
 sheets. To form images on these rear projection screens, imaging light is
 projected on the screens by using light sources such as CRTs.
 In recent years, projection tubes having small projection apertures, such
 as liquid crystal projectors and light bulbs came to be used as light
 sources in place of CRTs. However, the conventional rear projection
 screens have such a problem that, when images are formed on these screens
 by the use of projection tubes having small projection apertures,
 scintillation or speckle is caused on the images.
 In order to solve this problem, there have conventionally been proposed a
 method in which the screens are scanned by using laser light sources (see
 Japanese Patent Laid-Open Publication No. 173094/1993); a method in which
 the screens are vibrated (see Reference 1 (J. Opt. Soc. Am., Vol. 66, No.
 11, November 1976, "Speckle-free rear-projection screen using two close
 screens in slow relative motion")); and a method in which large amounts of
 diffusers are incorporated into lens sheets.
 DISCLOSURE OF THE INVENTION
 However, in the aforementioned conventional methods, the modification of
 the projectors themselves is needed, or additional apparatus are required
 in order to prevent images from undergoing scintillation or the like.
 Further, the incorporation of large amounts of diffusers into lens sheets
 causes such troubles that the gain is decreased and that the resolution is
 unfavorably lowered.
 The present invention was accomplished in view of the above-described
 drawbacks. An object of the present invention is therefore to provide a
 rear projection screen capable of forming thereon an image free from
 scintillation or the like with the decrease in gain and resolution
 minimized without using any additional apparatus even when a projection
 tube having a small projection aperture is used
 The first aspect of the present invention is a rear projection screen
 comprising a lens sheet having an optical function of condensing or
 diffusing light, wherein the lens sheet has two or more diffusing parts
 separately provided in a light-transmitting direction.
 In the first aspect of the present invention, it is preferable that one of
 the two or more diffusing parts be provided on a light-entering-side
 surface of the lens sheet and that another one of the diffusing parts be
 provided on a light-emerging-side surface of the lens sheet. Further, it
 is preferable that the two or more diffusing parts be provided on a
 surface of the lens sheet and inside the same.
 The second aspect of the present invention is a rear projection screen
 comprising two or more lens sheets or optical sheets having an optical
 function of condensing or diffusing light, wherein at least one of the two
 or more lens sheets or optical sheets has at least one diffusing part, and
 the two or more lens sheets or optical sheets have, as a whole, two or
 more diffusing parts.
 In the second aspect of the present invention, it is preferable that the
 diffusing parts be provided on surfaces of the two or more lens sheets or
 optical sheets, or inside the same. Further, it is preferable that the
 diffusing part of the outermost lens sheet or optical sheet on a light
 source side be provided on the light-entering-side surface of this lens
 sheet or optical sheet and that the diffusing part of the outermost lens
 sheet or optical sheet on an observation side be provided on a
 light-emerging-side surface of this lens sheet or optical sheet.
 In the above-described first and second aspects of the present invention,
 any two of the two or more diffusing parts are preferably such that a
 light-source-side diffusing part has a diffusing power lower than that of
 an observation-side diffusing part. Further, any two of the two or more
 diffusing parts are preferably such that a light-source-side diffusing
 part is formed by incorporating first diffusive fine particles into a
 first base material, that an observation-side diffusing part is formed by
 incorporating second diffusive fine particles into a second base material
 and that a difference between a refractive index of the first diffusive
 fine particles and that of the first base material is smaller than a
 difference between a refractive index of the second diffusive fine
 particles and that of the second base material.

BEST MODE FOR CARRYING OUT THE INVENTION
 By referring now to the accompanying drawings, embodiments of the present
 invention will be described below.
 First Embodiment
 FIG. 1 is an illustration showing a first embodiment of a rear projection
 screen according to the present invention.
 As shown in FIG. 1, a rear projection screen 1 is composed of a single lens
 sheet having on one surface or both surfaces thereof a Fresnel lens or
 lenticular lenses, wherein two or more diffusing parts 1A, 1B are provided
 separately in the light-transmitting direction (in the direction of the
 left and right sides in the figure). In this first embodiment, the
 diffusing parts 1A and 1B are provided on the light-entering-side surface
 (light-entering surface) and light-emerging-side surface (light-emerging
 surface) of the lens sheet, respectively.
 The diffusing parts 1A, 1B are parts for diffusing light, and can readily
 be formed by a conventional method, for example, by using a resin layer
 containing a diffuser (diffusive fine particles) such as microlenses,
 glass beads or organic beads, or by embossing the surfaces of microlenses.
 It is noted that the diffusing parts can be provided not only on the
 surfaces of the lens sheet but also inside the lens sheet like a diffusing
 part 1C.
 The diffusing parts 1A, 1B diffuse light emitted from a light source to
 destroy the coherence of the light, so that they can solve the problem of
 scintillation or speckle. However, when light from a light source is
 diffused, the resolution is lowered. Further, when a large amount of a
 diffuser is incorporated into one diffusing part as in the conventional
 method, the gain is decreased, and the image thus becomes very dark.
 According to the first embodiment of the present invention, since the two
 diffusing parts 1A and 1B are separately provided on the lens sheet, it is
 possible to make the intensity of scintillation or the like low by using a
 diffuser in an amount smaller than that of a diffuser which is required
 for a lens sheet having only one diffusing part to attain the equally low
 intensity of scintillation or the like. Moreover, since the amount of the
 diffuser used is small, the lowering of the gain is prevented, and the
 brightness of the image can thus be prevented from being unfavorably
 decreased.
 Further, since the two diffusing parts 1A and 1B are separately provided on
 the lens sheet, it is enough to incorporate a decreased amount of a
 diffuser into one diffusing part. Therefore, the amount of stray light to
 be produced inside the diffusing parts 1A, 1B can be decreased, and the
 unfavorable lowering of resolution to be caused by flare, ghost or the
 like can thus be prevented.
 Furthermore, by this light-diffusing effect, moires to be formed by the
 interference between Fresnel lenses, lenticular lenses, or pixels of a
 light source can be decreased.
 It is preferable that the diffusing parts 1A and 1B be provided on the
 light-entering-side surface and light-emerging-side surface of the lens
 sheet, respectively. The reason for this is as follows. When the diffusing
 parts 1A and 1B are provided at the above-described positions, the
 distance between the two diffusing parts becomes long, so that light
 emitted from a light source cannot show coherence. Therefore, the
 intensity of scintillation or the like can be decreased, and the lowering
 of the brightness of the image can be minimized while controlling the
 light-diffusing effect at the diffusing parts 1A and 1B to extremely low.
 Further, it is preferable that the diffusing power of the diffusing part 1A
 on the light source side be made lower than that of the diffusing part 1B
 on the observation side. By doing so, the degree of the diffusion of light
 which is caused on the light-entering side by diffusing elements becomes
 low. The intensity of scintillation or the like can thus be decreased
 while preventing the resolution from being unfavorably lowered.
 Scintillation or the like can be evaluated not only by the above-described
 intensity of scintillation or the like, but also by the magnitude
 (roughness) of scintillation or the like which is caused when a dynamic
 picture image is projected. In order to decrease the intensity of
 scintillation or the like, it is preferable to use such a diffuser that
 the difference between the refractive index of the diffuser and that of a
 base material in which the diffuser is dispersed is small. On the other
 hand, in order to decrease the magnitude (roughness) of scintillation or
 the like, it is preferable to use a diffuser whose average particle
 diameter is small.
 For this reason, when only scintillation or the like is taken into
 consideration, it seems to be effective to incorporate, into both the
 diffusing part 1A on the light source side and the diffusing part 1B on
 the observation side, a diffuser whose average particle diameter is small
 and whose refractive index is such that the difference between the
 refractive index of the diffuser and that of a base material in which the
 diffuser is dispersed is small. However, a diffuser whose refractive index
 is such that the difference between the refractive index of the diffuser
 and that of a base material in which the diffuser is dispersed is small,
 or a diffuser whose average particle diameter is small makes the angle of
 visibility narrow. Therefore, in order to solve the problem of
 scintillation or the like and that of the angle of visibility at the same
 time, it is preferable that the type of a diffuser to be incorporated into
 the diffusing part 1A be made different from that of a diffuser to be
 incorporated into the diffusing part 1B.
 Specifically, as will be described later in Example 7, it is preferable
 that the difference between the refractive index of a diffuser to be
 incorporated into the diffusing part 1A on the light source side and that
 of a base material in which the diffuser is dispersed be made smaller than
 the difference between the refractive index of a diffuser to be
 incorporated into the diffusing part 1B on the observation side and that
 of a base material in which the diffuser is dispersed, and that a diffuser
 having an average particle diameter not greater than a predetermined size
 (e.g., 15 micrometers) be incorporated into the diffusing part 1B on the
 observation side.
 A variety of methods have been known as the method for controlling the
 degree of the diffusion of light. Specifically, when a diffusing part is
 formed by means of embossing, irregularities to be produced by embossing
 are changed; and when a diffuser is employed, the particle diameter,
 refractive index, or amount of the diffuser to be used is changed. The
 relationship between the particle diameter, refractive index or the like
 of a diffuser and the light-diffusing effect is described in, for example,
 Reference 2 (J. Opt. Soc. Am. A, Vol. 2, No. 12, December 1985,
 "Diffraction analysis of bulk diffusers for projection-screen
 applications").
 Second Embodiment
 FIG. 2 is an illustration showing a second embodiment of a rear projection
 screen according to the present invention.
 As shown in FIG. 2, a rear projection screen 2 is composed of two or more
 lens sheets or optical sheets 2-1, 2-2, 2-3, . . . , and diffusing parts
 2A, 2B, 2C, . . . are provided on the lens sheets or optical sheets 2-1,
 2-2, 2-3, . . . , respectively.
 In this figure, the diffusing part 2A is provided on the
 light-emitting-side surface (light-entering surface) of the outermost lens
 sheet or optical sheet 2-1 on the light source side; the diffusing part 2B
 is provided on the light-emerging-side surface (light-emerging surface) of
 the outermost lens sheet or optical sheet 2-2 on the observation side; and
 the diffusing part 2C is provided on the light-entering-side surface
 (light-entering surface) of the lens sheet or optical sheet 2-3.
 As the lens sheet for use in this embodiment, it is possible to employ a
 linear or circular Fresnel lens sheet, a lenticular lens sheet having on
 one surface or both surfaces thereof lenticular lenses, a lens sheet
 having on each surface thereof a combination of Fresnel lenses or
 lenticular lenses, or the like.
 As the optical sheet, a panel made from polymethyl methacrylate or the
 like, whose both surfaces are flat, or the like can be used.
 The properties of the diffusing parts 2A, 2B, 2C, . . . , and the
 properties related to the configuration of the diffusing parts 2A, 2B, 2C,
 . . . , are the same as those of the diffusing parts 1A, 1B, 1C in the
 above-described first embodiment. Therefore, detailed explanations for
 these properties are herein omitted.
 EXAMPLES
 Specific examples of the rear projection screens as shown in FIGS. 1 and 2
 will be given below.
 Example 1
 FIG. 3 is an illustration showing a rear projection screen of Example 1
 according to the present invention. Example 1 corresponds to the first
 embodiment shown in FIG. 1, and, in this rear projection screen of Example
 1, two diffusing layers (diffusing parts) are separately provided on both
 surfaces of a single lens sheet.
 Namely, in this Example, a rear projection screen 10 was produced, as shown
 in FIG. 3, by respectively providing diffusing layers 10A and 10B on the
 light-entering surface 11a and light-emerging surface 11b of a lens sheet
 11 made from polymethyl methacrylate, having a thickness of 5 mm. The
 diffusing layer 10A was formed on the light-entering surface 11a by
 embossing a Fresnel lens part on the light-entering surface 11a. On the
 light-emerging surface 11b of the lens sheet 11 was formed the diffusing
 layer 10B having a thickness of 500 micrometers, in which 15 parts by
 weight of glass beads having an average particle diameter of 11
 micrometers and a refractive index of 1.535 were dispersed. It is noted
 that, in this Example and also in the following Examples 2 to 7 and
 Comparative Example, the amount (parts by weight) of the diffuser such as
 glass beads is a value based on 100 parts by weight of the base material
 into which the diffuser is incorporated.
 As the base material of the lens sheet 11, an impact-resistant methacrylic
 resin (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd.,
 Japan was used. As the glass beads having an average particle diameter of
 11 micrometers and a refractive index of 1.535, "EMB20" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 10 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 2
 FIG. 4 is an illustration showing a rear projection screen of Example 2
 according to the present invention. Example 2 corresponds to the second
 embodiment shown in FIG. 2, and, in this rear projection screen of Example
 2, two diffusing layers (diffusing parts) are separately provided on two
 lens sheets, one diffusing part on one lens sheet. One of the two
 diffusing layers is provided on the surface of the lens sheet (the
 light-entering-surface of a Fresnel lens sheet).
 Namely, in this Example, a rear projection screen 20 was produced, as shown
 in FIG. 4, by the combination use of a Fresnel lens sheet 21 having a
 thickness of 2 mm, made from polymethyl methacrylate, and a lenticular
 lens sheet 22 having a thickness of 1 mm, made from polymethyl
 methacrylate. On the light-entering surface 21a of the Fresnel lens sheet
 21 was formed a diffusing layer 20A having a thickness of 150 micrometers,
 in which 7.0 parts by weight of organic beads (cross-linked polymer beads)
 having an average particle diameter of 12 micrometers and a refractive
 index of 1.59 were dispersed. Further, into the lenticular lens sheet 22
 (diffusing layer 20B) were homogeneously incorporated 0.75 parts by weight
 of organic beads having an average particle diameter of 12 micrometers and
 a refractive index of 1.59.
 As the base material of the Fresnel lens sheet 21 and that of the
 lenticular lens sheet 22, an impact-resistant methacrylic resin
 (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd., Japan
 was used. As the organic beads having an average particle diameter of 12
 micrometers and a refractive index of 1.59, "PB3011" (styrene beads)
 manufactured by Sumitomo Chemical Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 20 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 3
 FIG. 5 is an illustration showing a rear projection screen of Example 3
 according to the present invention. Example 3 corresponds to the second
 embodiment shown in FIG. 2, and, in this rear projection screen of Example
 3, two diffusing layers (diffusing parts) are separately provided on three
 lens/optical sheets. One of the two diffusing layers is provided on the
 surface of the outermost lens sheet on the light source side (the
 light-emitting-surface of a Fresnel lens sheet), and the other diffusing
 layer is provided on the surface of the outermost optical sheet on the
 observation side (the light-entering-surface of a flat face panel).
 Namely, in this Example, a rear projection screen 30 was produced, as shown
 in FIG. 5, by the combination use of a Fresnel lens sheet 31 having a
 thickness of 2 mm, made from polymethyl methacrylate, a flat face panel 32
 having a thickness of 2 mm, made from polymethyl methacrylate, and a
 lenticular lens sheet 33 having a thickness of 1 mm, made from polymethyl
 methacrylate, containing no diffuser, provided between the Fresnel lens
 sheet 31 and the flat face panel 32. On the light-entering surface 31a of
 the Fresnel lens sheet 31 was formed a diffusing layer 30A having a
 thickness of 150 micrometers, in which 45 parts by weight of glass beads
 having an average particle diameter of 11 micrometers and a refractive
 index of 1.535 were dispersed. On the light-entering surface 32a of the
 flat face panel 32 was formed a diffusing layer 30B having a thickness of
 150 micrometers, in which 45 parts by weight of glass beads having an
 average p article diameter of 11 micrometers and a refractive index of
 1.535 were dispersed.
 As the base materials of the Fresnel lens sheet 31, of the flat face panel
 32 and of the lenticular lens sheet 33, an impact-resistant methacrylic
 resin (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd.,
 Japan was used. As the glass beads having an average particle diameter of
 11 micrometers and a refractive index of 1.535, "EMB20" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 30 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 4
 FIG. 6 is an illustration showing a rear projection screen of Example 4
 according to the present invention. Example 4 corresponds to the second
 embodiment shown in FIG. 2, and, in this rear projection screen of Example
 4, two diffusing layers (diffusing parts) are separately provided on two
 lens sheets. One of the two diffusing layers is provided on the surface of
 the outermost lens sheet on the light source side (the
 light-entering-surface of a Fresnel lens sheet), and the other diffusing
 layer is provided inside the outermost lens sheet on the observation side
 (inside a lenticular lens sheet). Further, the diffusing layer provided on
 the light source side has a diffusing power lower than that of the
 diffusing layer provided on the observation side.
 Namely, in this Example, a rear projection screen 40 was produced, as shown
 in FIG. 6, by the combination use of a Fresnel lens sheet 41 having a
 thickness of 2 mm, made from polymethyl methacrylate, and a lenticular
 lens sheet 42 obtained by forming transparent lenses 42b on both surfaces
 of a film 42a having a thickness of 200 micrometers, made from polymethyl
 methacrylate. On the light-entering surface 41a of the Fresnel lens sheet
 41 was formed a diffusing layer 40A having a thickness of 100 micrometers,
 in which 35 parts by weight of glass beads having an average particle
 diameter of 11 micrometers and a refractive index of 1.535 were dispersed.
 Further, into the film 42a (diffusing layer 40B) of the lenticular lens
 sheet 42 were homogeneously incorporated 10.0 parts by weight of organic
 beads having an average particle diameter of 12 micrometers and a
 refractive index of 1.59.
 As the base material of the Fresnel lens sheet 41, an impact-resistant
 methaciylic resin (refractive index 1.51) manufactured by Sumitomo
 Chemical Co., Ltd., Japan was used. As the glass beads having an average
 particle diameter of 11 micrometers and a refractive index of 1.535,
 "EMB20" manufactured by Toshiba-Ballotini Co., Ltd., Japan was used.
 Further, as the organic beads having an average particle diameter of 12
 micrometers and a refractive index of 1.59, "PB3011" (styrene beads)
 manufactured by Sumitomo Chemical Co., Ltd., Japan was used. The
 lenticular lens sheet 42 was obtained by covering, with the film 42a, a
 UV- (ultraviolet-) or EB- (electron beam-) curable resin poured into a
 mold in a shape reverse to the shape of the transparent lenses 42b, and
 applying ultraviolet rays or electron beams to the UV- or EB-curable
 resin.
 An image was projected on the thus-produced rear projection screen 40 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 5
 FIG. 7 is an illustration showing a rear projection screen of Example 5
 according to the present invention. Example 5 corresponds to the second
 embodiment shown in FIG. 2, and, in this rear projection screen of Example
 5, two diffusing layers (diffusing parts) are separately provided on two
 lens sheets. One of the two diffusing layers is provided on the surface of
 the outermost lens sheet on the light source side (the 10
 light-emitting-surface of a Fresnel lens sheet), and the other diffusing
 layer is provided on the surface of the outermost lens sheet on the
 observation side (the light-emerging surface of a lenticular lens sheet).
 Further, the diffusing layer provided on the light source side has a
 diffusing power lower than that of the diffusing layer provided on the
 observation side.
 Namely, in this Example, a rear projection screen 50 was produced, as shown
 in FIG. 7, by the combination use of a Fresnel lens sheet 51 having a
 thickness of 2 mm, made from polymethyl methacrylate, and a lenticular
 lens sheet 52 having a thickness of 1 mm, made from polymethyl
 methacrylate. On the light-entering surface 51a of the Fresnel lens sheet
 51 was formed a diffusing layer 50A having a thickness of 100 micrometers,
 in which 35 parts by weight of glass beads having an average particle
 diameter of 11 micrometers and a refractive index of 1.535 were dispersed.
 On the light-emerging surface 52b of the lenticular lens sheet 52 was
 formed a diffusing layer 50B having a thickness of 100 micrometers, in
 which 12.0 parts by weight of organic beads having an average particle
 diameter of 12 micrometers and a refractive index of 1.59 were dispersed.
 As the base material of the Fresnel lens sheet 51, and that of the
 lenticular lens sheet 52, an impact-resistant methacrylic resin
 (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd., Japan
 was used. As the glass beads having an average particle diameter of 11
 micrometers and a refractive index of 1.535, "EMB20" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan was used. Further, as the organic beads
 having an average particle diameter of 12 micrometers and a refractive
 index of 1.59, "PB3011" (styrene beads) manufactured by Sumitomo Chemical
 Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 50 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 6
 FIG. 8. is an illustration showing a rear projection screen of Example 6
 according to the present invention Example 6 corresponds to the second
 embodiment shown in FIG. 2, and, in this rear projection screen of Example
 6, three diffusing layers (diffusing parts) are separately provided on
 three lens/optical sheets, one diffusing layer on one lens or optical
 sheet. The three diffusing layers are respectively provided on the
 surfaces (the light-entering surfaces) of the three lens/optical sheets.
 Namely, in this Example, a rear projection screen 60 was produced, as shown
 in FIG. 8, by the combination use of a Fresnel lens sheet 61 having a
 thickness of 2 mm, made from polymethyl methacrylate, a flat face panel
 having a thickness of 2 mm, made from polymethyl methacrylate, and a
 lenticular lens sheet 63 having a thickness of 1 mm, made from polymethyl
 methacrylate, provided between the Fresnel lens sheet 61 and the flat face
 panel 62. On the light-entering surface 61a of the Fresnel lens sheet 61
 was formed a diffusing layer 60A having a thickness of 100 micrometers, in
 which 3.5 parts by weight of glass beads having an average particle
 diameter of 11 micrometers and a refractive index of 1.535 were dispersed.
 On the light-entering surface 62a of the flat face panel 62 was formed a
 diffusing layer 60B having a thickness of 100 micrometers, in which 3.5
 parts by weight of glass beads having an average particle diameter of 11
 micrometers and a refractive index of 1.535 were dispersed. On the
 light-entering surface 63a of the lenticular lens sheet 63 was formed a
 diffusing layer 60C having a thickness of 300 micrometers, in which 5.0
 parts by weight of organic beads having an average particle diameter of 30
 micrometers and a refractive index of 1.49 were dispersed.
 As the base materials of the Fresnel lens sheet 61, of the flat face panel
 62, and of the lenticular lens sheet 63, an impact-resistant methacrylic
 resin (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd.,
 Japan was used. As the glass beads having an average particle diameter of
 11 micrometers and a refractive index of 1.535, "EMB20" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan was used. Further, as the organic beads
 having an average particle diameter of 30 micrometers and a refractive
 index of 1.49, "XC01" (acrylic beads) manufactured by Sumitomo Chemical
 Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 60 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was low
 and that the resolution of the image was excellent.
 Example 7
 FIGS. 9A and 9B are illustrations showing rear projection screens of
 Example 7 according to the present invention. Example 7 corresponds to the
 second embodiment shown in FIG. 2, and, in these rear projection screens
 of Example 7, two diffusing layers (diffusing parts) are separately
 provided on two lens sheets. Further, the type (refractive index and
 average particle diameter) of the diuser incorporated into the diffusing
 layer provided on the light source side is different from that of the
 eiffuser incorporated into the diffusing layer provided on the observation
 side.
 Namely, in this Example, rear projection screens 70, 80 were produced, as
 shown in FIGS. 9A and 9B, by the combination use of Fresnel to lens sheets
 71, 81 made from polymethyl methacrylate, and lenticular lens sheets 72,
 82 having a thickness of 1 mm, made from polymethyl methacrylate. As shown
 in FIGS. 9A and 9B, the shape and structure of the rear projection screen
 70 are identical with those of the rear projection screen 80 except that
 the structure of the Fresnel lens sheet 71 is different from that of the
 Fresnel lens sheet 81.
 As the base material of the Fresnel lens sheets 71, 81, an impact-resistant
 methacrylic resin (refractive index 1.51) manufactured by Sumitomo
 Chemical Co., Ltd., Japan was used. This resin was subjected into
 extrusion molding, and a Fresnel lens part was formed on one surface of
 the molded product by coating thereto a UV-curable resin containing no
 diffuser, followed by curinhg the UV-curable resin by the application of
 ultraviolet rays, thereby obtaining the Fresnel lens sheets 71, 81. It is
 noted that the boundary between the substrate and the Fresnel lens part
 formed thereon by the use of the UV-curable resin is not shown in FIGS. 9A
 and 9B.
 The Fresnel lens sheet 71 shown in FIG. 9A is a lens sheet obtained by
 forming a Fresnel lens part on one surface of a single layer (diffusing
 layer 70A), serving as a substrate, into which a diffuser having a
 predetermined average particle diameter and refractive index is
 homogeneously incorporated. On the other hand, the Fresnel lens sheet 81
 shown in FIG. 9B is a lens sheet obtained by forming a Fresnel lens part
 on one surface of a co-extruded two-layered substrate having, on the
 light-emitting surface 81Ia thereof, a diffusing layer 80A in which a
 diffuser having a predetermined average particle diameter and refractive
 index is dispersed.
 Into the diffusing layers 70A, 80A was incorporated, as the diffuser, one
 of (1) acrylic beads having an average particle diameter of 30 micrometers
 and a refractive index of 1.49 ("XC01" manufactured by Sumitomo Chemical
 Co., Ltd., Japan), (2) acrylic beads having an average particle diameter
 of 11 micrometers and a refractive index of 1.49 ("MBX" manufactured by
 Sekisui Chemical Co., Ltd., Japan), (3) glass beads having an average
 particle diameter of 17 micrometers and a refractive index of 1.535
 ("EGB210" manufactured by Toshiba-Ballotini Co., Ltd., Japan), and (4)
 styrene beads having an average particle diameter of 12 micrometers and a
 refractive index of 1.59 ("PB3011" manufactured by Sumitomo Chemical Co.,
 Ltd., Japan).
 These diffusers (1) to (4) were incorporated into the diffusing layers 70A,
 80A of the Fresnel lens sheets 71, 81 in a manner as shown in the
 following Table 1.
 TABLE 1
 Concentration of Diffuser Thickness of
 on Light- on Observation Diffusing Layer
 Entering Side Side (mm)
 XC01 2.5t 2.0P --
 XC01 1.8t 3.0P --
 XC01 1.8t (2) 17P Clear 0.2
 MBX 1.8t 1.15P --
 MBX 1.8t (2) 9.0P Clear 0.2
 EGB 1.8t (2) 6.8P Clear 0.2
 EGB 2.5t (2) 3.4P Clear 0.4
 PB3011 1.8t 0.17P --
 In the above Table 1, the numeral (e.g., "2.5t") shown next to the type of
 the diffuser (e.g., "XC01") indicates the thickness (mm) of the substrate
 of the Fresnel lens sheet 71, 81. The thickness of the Fresnel lens sheet
 71, 81 is the sum total of the thickness of the substrate and 0.2 mm, the
 thickness of the Fresnel lens part formed by using a UV-curable resin.
 Further, the numeral in parentheses, "(2)", shown next to the numeral
 (e.g., "1.8t") indicating the thickness (mm) of the substrate represents
 that the substrate of the Fresnel lens sheet into which the diffuser is
 incorporated has a two-layered structure (the structure as shown in FIG.
 9B). The unit (P) of the concentration of the diffuser is parts by weight
 (number of grams) of the diffuser incorporated into 100 parts by weight
 (100 g) of polymethyl methacrylate, the base material.
 On the other hand, as the base material of the lenticular lens sheets 72,
 82, an impact-resistant methacrylic resin (refractive index 1.51)
 manufactured by Sumitomo Chemical Co., Ltd., Japan was used as in the case
 of the above-described Fresnel lens sheets 71, 81. The entire lenticular
 lens sheets 72, 82 were obtained by subjecting the resin to extrusion
 molding. As shown in FIGS. 9A and 9B, diffusing layers 70B, 80B having a
 thickness of 600 micrometers, in which a diffuser having a predetermined
 particle diameter and refractive index was dispersed were respectively
 formed on the light-emerging surfaces of the lenticular lens sheets 72,
 82.
 Into the diffusing layers 70B, 80B was incorporated, as the diffuser, one
 of (1) glass beads having an average particle diameter of 11 micrometers
 and a refractive index of 1.535 ("EMB20" manufactured by Toshiba-Ballotini
 Co., Ltd., Japan), (2) glass beads having an average particle diameter of
 17 micrometers and a refractive index of 1.535 ("EGB210" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan), (3) a 6:1 mixture of acrylic beads
 having an average particle diameter of 30 micrometers and a refractive
 index of 1.49 ("XC01" manufactured by Sumitomo Chemical Co., Ltd., Japan)
 and the above "EGB210" manufactured by Toshiba-Ballotini Co., Ltd., Japan
 (XC01+EGB-1), and (4) a 2:3 mixture of the above "XC01" manufactured by
 Sumitomo Chemical Co., Ltd., Japan and the above "EGB210" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan (XC01+EGB-2).
 These diffusers (1) and (4) were incorporated into the diffusing layers
 70B, 80B of the lenticular lens sheets 72, 82 in the following manner.
 Namely, among the above-described diffusers (1) and (4), the diffuser (3),
 which is a 6:1 mixture of "XC01" manufactured by Sumitomo Chemical Co.,
 Ltd., Japan and "EGB210" manufactured by Toshiba-Ballotini Co., Ltd.,
 Japan, "XC01+EGB-1", was taken as a standard, and the concentration of the
 "EG210" contained in the "XC01+EGB-1" was made 2.0 P. The concentrations
 of the other diffusers (1), (2) and (4) were so adjusted that the gain of
 a rear projection screen obtainable by the combination use of a lenticular
 lens sheet containing the diffuser (1), (2) or (4), and the Fresnel lens
 sheet having the diffusing layer indicated by "XC01 2.5t" in the above
 Table 1 would be almost equal (within .+-.0.2) to the gain of a rear
 projection screen obtainable by the combination use of the lenticular lens
 sheet having the diffusing layer containing the above-described diffuser
 "XC01+EGB-1", and the above-described Fresnel lens sheet having the
 diffusing layer indicated by "XC01 2.5t".
 Rear projection screens 70, 80 were respectively produced by assembling, in
 a frame (not shown in the figure), the above-described various Fresnel
 lens sheets 71, 81 and lenticular lens sheets 72, 82. White images were
 projected on the thus-produced various rear projection screens 70, 80 by
 using an LCD projector, and evaluated in terms of the intensity and
 magnitude (roughness) of scintillation or the like caused on the images.
 The results obtained are as shown in the following Table 2. The magnitude
 (roughness) of scintillation or the like was evaluated by the roughness of
 speckle perceived by an observer when he/she moved his/her eyes. The
 intensity and magnitude (roughness) of scintillation or the like were
 evaluated according to 6 ranks of "5" (most excellent) to "0" (poorest).
 TABLE 2
 XC01 + EGB-1 XC01 + EGB-2 EGB
 EMB
 magnitude magnitude
 magnitude magnitude
 intensity (roughness) intensity (roughness) intensity
 (roughness) intensity (roughness)
 XC01 2.5t 5 4 5 4 5 4
 5 5
 XC01 1.8t 3 2 3 2 2 2
 2 3
 XC01 1.8(2) 4 4 4 4 3 4
 5 5
 MBX 1.8t 3 2 3 2 2 2
 2 3
 MBX 1.8t(2) 4 4 4 4 3 4
 5 5
 EGB 1.8t(2) 3 2 2 2 2 2
 2 4
 EGB 2.5t(2) 4 4 3 4 3 4
 3 4
 PB3011 1.8t 1 1 1 0 0 0
 0 1
 XC01: acrylic beads (30 .mu.m, 1.49)
 MBX: acrylic beads (11 .mu.m, 1.49)
 EGB: glass beads: (17 .mu.m, 1.535)
 PB3011: styrene beads (12 .mu.m, 1.59)
 EMB: glass beads: (11 .mu.m, 1.535)
 XC01 + EGB-1: a 6:1 mixture of "XC01" and "EGB"
 XC01 + EGB-2: a 2:3 mixture of "XC01"and "EGB"
 The results shown in the above Table 2 demonstrate that those screens
 produced by using acrylic beads having an average particle diameter of 30
 micrometers and a refractive index of 1.49 ("XC01" manufactured by
 Sumitomo Chemical Co., Ltd., Japan) as the diffuser to be incorporated
 into the Fresnel lens sheet (FL) on the light source side, and glass beads
 having an average particle diameter of 11 micrometers and a refractive
 index of 1.535 ("EMB20" manufactured by Toshiba-Ballotini Co., Ltd.,
 Japan) as the diffuser to be incorporated into the lenticular lens sheet
 (LL) on the observation side show good results in terms of both the
 intensity and magnitude (roughness) of scintillation or the like.
 Further, from the results shown in the above Table 2, the following
 tendency is confirmed: the intensity of the scintillation or the like
 becomes lower and the magnitude (roughness) of the same becomes smaller as
 the difference between the refractive index of the diffuser to be
 incorporated into the Fresnel lens sheet (FL) on the light source side and
 that (1.51) of the base material in which the diffuser is dispersed
 becomes smaller (for instance, compare the "MBX 1.8t" series results with
 the "PB3011 1.8t" series results, where the average particle diameter of
 "MBX" is almost equal to that of "PB3011"). Furthermore, it is also
 confirmed that the magnitude (roughness) of scintillation or the like
 becomes smaller as the average particle diameter of the diffuser
 incorporated into the lenticular lens sheet (LL) on the observation side
 becomes smaller (for instance, compare the "EGB" series results with the
 "EMB" series results, where the refractive index of "EGB" is equal to that
 of "EMB"). With respect to the average particle diameter of the diffuser
 to be incorporated into the lenticular lens sheet (LL), a great
 improvement is found in the magnitude (roughness) of scintillation between
 the average particle diameter of 17 micrometers ("EGB210") and that of 11
 micrometers ("EMB20"), especially in the vicinity of the average particle
 diameter of 15 micrometers.
 The following is also confirmed from the results shown in the above Table
 2: the intensity of scintillation becomes lower and the magnitude
 (roughness) of the same becomes smaller as the thickness of the Fresnel
 lens sheet (FL) becomes greater, and when the Fresnel lens sheet is not
 composed of a single layer but composed of two layers.
 COMATIVE EXAMPLE
 FIG. 10 is an illustration showing a comparative rear projection screen.
 In this Comparative Example, a rear projection screen 70 was produced, as
 shown in FIG. 10, by the combination use of a Fresnel lens sheet 91 having
 a thickness of 2 mm, made from polymethyl methacrylate, containing no
 diffuser, and a lenticular lens sheet 92 having a thickness of 1 mm, made
 from polymethyl methacrylate. Into the lenticular lens sheet 72 (diffusing
 layer 90B), 5 parts by weight of glass beads having an average particle
 diameter of 11 micrometers and a refractive index of 1.535 were
 homogeneously incorporated.
 As the base material of the Fresnel lens sheet 91, and that of the
 lenticular lens sheet 92, an impact-resistant methacrylic resin
 (refractive index 1.51) manufactured by Sumitomo Chemical Co., Ltd., Japan
 was used. As the glass beads having an average particle diameter of 11
 micrometers and a refractive index of 1.535, "EMB20" manufactured by
 Toshiba-Ballotini Co., Ltd., Japan was used.
 An image was projected on the thus-produced rear projection screen 90 by
 using an LCD projector, and observed for evaluation. As a result, it was
 confirmed that the intensity of scintillation caused on the image was high
 and that the quality of the image was poor.
 According to the present invention, since at least two diffusing parts are
 separately provided on one lens sheet or optical sheet, or on a plurality
 of lens sheets or optical sheets, it is possible to make the intensity of
 scintillation low by using a diffuser in an amount smaller than that of a
 diffuser which is required for a lens sheet having only one diffusing part
 to attain the equally low intensity of scintillation. Further, by
 respectively incorporating diffusers of different types into two diffusing
 parts, not only the intensity of scintillation or the like can be made
 low, but also the magnitude (roughness) of the same can be made small.
 Scintillation or the like to be caused on an image can thus be effectively
 decreased without lowering the resolution and brightness of the image.