JP5168386B2 - Image display device and screen used therefor - Google Patents

Description

  The present invention relates to a projection-type image display device using an oblique projection optical system that projects an image generation source image on a rear-projection screen from an oblique direction, and a Fresnel lens sheet used in such an image display device and rear-projection. For mold screens.

  In order to cope with a reduction in the thickness of a projection-type image display device, a configuration in which an enlarged image is projected from an oblique direction on a screen using a reflection mirror, that is, a configuration using a so-called oblique projection optical system is known. At this time, since the enlarged image is projected obliquely with respect to the screen (that is, the perpendicular of the screen main plane and the incident direction of light at the center of the enlarged image form a predetermined angle), the image projected on the screen is Distorted. Therefore, for example, Patent Document 1 below discloses a configuration of an image display apparatus that employs a curved reflecting mirror having a curved surface as a part of the reflecting mirror in order to correct such distortion.

JP 2002-341252 A

  In general, in the above-described image display device, if the screen size is the same, the thinner the depth, the more advantageous in terms of weight, cost, installation space, and the like. That is, in the above-described conventional technology, an enlarged image is projected obliquely with respect to the screen in order to reduce the depth of the image display device. As a result, in order to correct the trapezoidal image distortion caused by the oblique image projection, a configuration using a projection optical system in which the curved reflection mirror described above is provided in the optical path is employed.

  By the way, on the screen, it is desirable to obtain an image with uniform brightness over the entire surface, regardless of the position where this is viewed. For this purpose, the enlarged image light from the projection lens needs to be substantially parallel to the viewing side when it is emitted from the screen. In order to realize this, in general, a Fresnel lens sheet including a concentric Fresnel lens having a prism surface with a spherical original surface is used as one component of the screen. Here, the original surface refers to a virtual surface formed by a curve obtained by connecting the prism surfaces in the entire Fresnel lens sheet, that is, an envelope of a set of all the prism surfaces in the entire Fresnel lens sheet. By making this original surface spherical, the image light projected radially from the exit pupil of the projection lens to each point on the screen is refracted by each prism surface in a direction parallel to the screen normal, over the entire surface of the screen. The emitted light can be made parallel (that is, in a direction parallel to the screen normal).

  However, the following problems may occur in the above-described prior art. That is, when the image light traveling from the exit pupil of the projection lens to the screen is reflected by the curved reflection mirror included in the oblique projection optical system, the angle of the incident light on the screen changes depending on the reflection position of the curved reflection mirror. That is, when the oblique projection optical system is used, the image light traveling from the exit pupil of the projection lens toward the screen is not uniformly emitted. Therefore, it is difficult for the above-described Fresnel lens sheet with the original surface to be a spherical surface to emit the enlarged image light from the projection lens substantially in parallel over the entire surface of the screen. Therefore, the emission angle from the Fresnel lens sheet becomes non-uniform, and when viewed from the viewing side, there is a problem that the brightness of the image projected on the screen is still non-uniform. .

  Therefore, the present invention has been made in view of the above-described problems in the prior art. The object is to provide a technique suitable for making the brightness of the image projected on the screen more uniform while shortening the depth of the apparatus.

  In order to achieve the above object, the present invention is characterized in that the original surface of the Fresnel lens formed on the Fresnel lens sheet has an aspherical shape corresponding to the incident angle of the light beam with respect to the screen. . At this time, the Fresnel angle of each prism surface is larger in the upper part than in the lower part of the Fresnel lens sheet. As a result, the projected image incident on the incident surface of the Fresnel lens sheet can be emitted from the exit surface of the Fresnel lens sheet at an exit angle of approximately 0 degrees over substantially the entire surface of the screen. .

  The Fresnel lens is formed concentrically around the same point, and it is preferable that the center point is located outside the screen (image display area). The projection optical unit for projecting an enlarged image obliquely with respect to the screen has a curved surface for correcting distortion and aberration caused by oblique projection onto the screen, at least one refractive free-form surface and / or reflective free-form surface. It is preferable to include a curved surface.

  With the above configuration, even when a projection optical system having a curved surface for correcting trapezoidal image distortion caused by oblique projection is used, the incident angle of the incident light beam on the screen due to reflection from the curved surface It becomes possible to cope with the change of. That is, even if such a change in incident angle occurs, the Fresnel lens having a corresponding Fresnel angle refracts incident light in a direction substantially parallel to the screen normal (perpendicular to the screen main plane). The light can be emitted. Therefore, according to the configuration of the present invention, the emission direction of the emitted light can be aligned substantially parallel to the screen normal over the entire screen, and the depth dimension of the image display device can be reduced and uniform brightness can be achieved. Can be obtained.

  According to the present invention, it is possible to display a high-quality image by reducing the depth of the apparatus and making the brightness of the image projected on the screen more uniform.

1 is a cross-sectional view illustrating an example of a configuration of an image display device according to an embodiment of the present invention. The perspective view of the rear projection type screen which concerns on this embodiment. The figure explaining the method to set the shape of the prism part which comprises a Fresnel lens in the Fresnel lens sheet which concerns on this embodiment. The figure which shows distribution of the incident angle of the incident light ray in the rear projection type screen which concerns on this embodiment. The figure which shows distribution of the outgoing angle of the emitted light in the rear projection type screen which concerns on this embodiment. The figure which showed the relationship between the prism surface of the Fresnel lens sheet which concerns on this embodiment, and an original surface.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  First, FIG. 1 attached herewith is a cross-sectional perspective view of an image display device according to the present invention. In the figure, the image generation source 1 may include, for example, a projection cathode ray tube. Further, it may include an image modulation element such as a reflective / transmissive liquid crystal panel or a display element provided with a plurality of minute mirrors. When such an image modulation element is used, the image generation source further includes a light source such as a mercury lamp for irradiating the image modulation element with light, and the image modulation element receives light from the light source as an input video signal. The image is formed by modulating each pixel according to the above. As a result, a small image is displayed on the display screen of the image generation source 1. The projection optical system for enlarging and projecting this small image onto the rear projection screen 3 is a video enlargement unit for enlarging and projecting the image onto the rear projection screen 3 as is apparent from the figure. A projection lens 2; a first curved reflection mirror 4 that reflects image light emitted from the projection lens; a second curved reflection mirror 5 that reflects image light reflected by the first curved reflection mirror 4; Then, in order to reduce the depth of the image display device, for example, a planar reflecting mirror 6 that reflects the image light reflected by the second curved reflecting mirror 5 is included. The projection lens 2, the first curved reflection mirror 4, the second curved reflection mirror 5, and the planar reflection mirror 6 are provided in the middle of the optical path from the image source 1 to the rear projection screen 3. Yes. These elements are housed inside the housing 7 of the image display device and are fixed at predetermined positions.

  That is, according to the image display apparatus whose configuration has been described above, the image light emitted from the projection lens 2 is projected obliquely onto the rear projection screen 3. That is, a light beam at the center of the image (a light beam emitted from the center position of the image modulation element) is projected from below with a predetermined angle with respect to a normal to the main plane of the projection screen 3. This obliquely projected image light generates aberrations on the rear projection screen 3 together with the trapezoidal distortion described above. Therefore, these are corrected by the first curved mirror 4 and the second curved mirror 5.

  FIG. 2 attached herewith shows an example of the detailed structure of the rear projection screen 3 in the image display apparatus according to the present invention. That is, FIG. 2 shows a perspective view in which a part of the rear projection screen 3 is cut out and enlarged. As is apparent from the figure, the rear projection screen 3 has a so-called aspherical Fresnel lens sheet having a refractive Fresnel lens in which the original surface for determining the Fresnel angle of the prism surface is aspherical. 8 (hereinafter simply referred to as “Fresnel lens sheet”) and a lenticular lens sheet 9. Here, it is assumed that a plurality of Fresnel lenses are concentrically formed around a certain point on the light emission surface of the Fresnel lens sheet 8 as shown in the figure. The Fresnel angle refers to an angle formed by the prism surface and the principal plane of the Fresnel lens sheet. According to the rear projection screen 3 having such a configuration, the light of an enlarged projection image (not shown) projected from the direction of the arrow a in the figure functions as the Fresnel lens sheet 8, that is, the Fresnel lens constituting the sheet. By the action of the aspherical prism, the light is converted into light substantially parallel to the perpendicular of the main plane of the rear projection screen 3. Alternatively, the light of the enlarged projection image is converted into light that is directed slightly inward, that is, toward the center of the rear projection screen 3. That is, the emission angle of the light emitted from the Fresnel lens sheet 8, that is, the angle with respect to the normal of the rear projection screen 3 is approximately 0 degrees. Thereafter, the light emitted from the Fresnel lens sheet 8 enters the lenticular lens sheet 9. The lenticular lens sheet 9 has a shape in which a plurality of lenticular lenses whose longitudinal direction is the vertical direction of the screen screen are arranged in the horizontal direction of the screen screen, as shown in the drawing. Therefore, it works to diffuse the light emitted from the Fresnel lens sheet 8 in the horizontal direction of the screen screen. Further, a black stripe 10 extending in the vertical direction of the screen is formed on the exit surface of the lenticular lens sheet 9 at a position facing the boundary of the lenticular lens. As a result, external light incident from the screen exit side is absorbed. Further, the lenticular lens sheet 9 is formed of a transparent resin material in which a diffusing material 11 is kneaded. For this reason, the mixed diffusion material 11 functions to diffuse the image light in the horizontal and vertical directions of the screen screen.

  Although not shown, a flat front sheet may be disposed on the light exit side of the lenticular lens sheet 9. Further, the front sheet and the lenticular sheet 9 may be combined and integrated with each other using an adhesive or the like. Further, the diffusion material 11 may not be mixed into the lenticular sheet 9 but may be mixed into the front sheet. Furthermore, an antireflection film or a hard coat may be formed for light emission of the front sheet 9. In FIG. 2, a second lenticular lens is formed at a position facing the lenticular lens on the exit surface of the lenticular lens sheet 9, that is, at the focal point of the lenticular lens. In the case of using an element, the focal portion may be planar. When the lenticular lens sheet 9 and the front sheet are coupled, it is preferable that the coupling surfaces of the lenticular lens sheet 9 and the front sheet are coupled so as not to include air. Next, in the Fresnel lens formed on the Fresnel lens sheet 8 described above, a method for setting the surface shape of a plurality of concentric prism surfaces (so-called Fresnel lens original surfaces) is shown in FIG. This will be described in detail with reference to schematic diagrams. As described above, the prism surface of the Fresnel lens constituting the Fresnel lens sheet 8 is formed concentrically around a certain point (rotation axis). The original surface for determining the Fresnel angle of the prism surface of each Fresnel lens, that is, the angle between the prism surface and the principal plane of the Fresnel lens sheet 8 is an aspherical shape. Here, the original surface is for determining the Fresnel angle of each prism surface as described above, and refers to this lens surface when the entire Fresnel lens sheet 8 is formed as one lens. That is, when setting the Fresnel angle of the prism surface of the Fresnel lens, first, it is assumed that the entire Fresnel lens sheet 8 has one lens characteristic, and the surface shape of the lens is set as the original surface. Then, the shape corresponding to each point of the Fresnel lens sheet 8 on the original surface (for example, the tangent line of the original surface at each point) is developed on the surface of the Fresnel lens sheet 8. Thereby, the Fresnel angle of the prism surface at each point of the Fresnel lens sheet 8 is set. Therefore, a curve obtained by connecting the prism surfaces in the entire Fresnel lens sheet according to the Fresnel angle, that is, an envelope including a set of all the prism surfaces in the entire Fresnel lens sheet represents the original surface. Therefore, the light refraction direction on the prism surface at each point of the angular Fresnel lens sheet is determined by the shape of the original surface corresponding to each prism. In addition, the said rotating shaft shall orthogonally cross the main plane (xy surface of a figure) of the Fresnel lens sheet 8 (surface containing the z axis | shaft of a figure). The rotation axis includes a point 15 where a light beam 13 incident on the Fresnel lens sheet 8 and a surface 14 (a surface parallel to the yz plane) that divides the Fresnel lens sheet 8 equally in the left and right directions intersect. That is, the rotation axis is an axis perpendicular to the main plane of the Fresnel lens sheet 8, that is, the axis 12 in FIG.

  However, in the above, since the incident light ray 13 has its incident angle (angle with respect to the normal of the incident surface) changed depending on the position on the screen, there are a plurality of the axes 12 obtained as described above. . Of the plurality of axes, the axis at the center is the rotation axis of the Fresnel lens (that is, the center position of the concentric prisms constituting the Fresnel lens).

  Subsequently, the Fresnel angle shape (angle) of each prism surface is obtained as follows. First, for each point of the Fresnel lens sheet 8, the angle of the prism to be emitted in the perpendicular direction (the exit angle is 0 degree) while refracting the incident light to the screen by the prism surface of the Fresnel lens sheet 8 is set. Obtained by Sumer's law. Next, the obtained prism surface is made continuous to form the original surface (aspheric surface) of the Fresnel lens. The obtained original surface is approximated by an aspherical expression. At this time, the approximate aspheric coefficient is compared with the actual light beam exit angle, and the position of the rotation axis and the aspheric coefficient are appropriately modified and / or appropriately adjusted so that the exit angle is approximately 0 degrees. Or make a change.

  Thus, depending on the required element, that is, the position of the rotation axis serving as the rotation center of the concentric prism portion constituting the Fresnel lens, and the aspherical coefficient of the original surface formed by the set of the prism surfaces, A Fresnel lens sheet 8 is configured. If the Fresnel lens sheet 8 is configured in this way, a curved reflection mirror (the first curved reflection mirror 4 or the second curved reflection mirror 5 provided in the middle of the optical path of the image light in the projection optical system of the image display device). ) Through the screen can be emitted with an exit angle of approximately 0 degrees from the screen. Therefore, according to the Fresnel lens sheet according to the present embodiment, the brightness of the image can be made substantially uniform over the entire surface of the screen.

  Next, an example of numerical values of each optical component in the case where the Fresnel lens sheet 8 described above is applied to an image display device having a screen diagonal of 50 inches (length: aspect ratio = 9: 16) is shown. Table 1 below shows the arrangement of optical components after the projection lens 2 in terms of (x, y, z, angle, distance). Here, the angle of the projection lens represents the lens exit angle, while the angles of the curved reflecting mirror, the reflecting mirror, and the screen represent the incident angle.

  Here, the center position of the screen is the origin of coordinates ((x, y, z) = (0, 0, 0)). In these coordinates, the left-right direction (horizontal direction) of the screen is the x-axis, and the right direction is + (positive). Further, the vertical direction of the screen is the y axis, and the upper direction is + (positive). Further, the depth direction is taken as the z-axis, and the direction toward the rear of the image display device is taken as-(negative). The angle is an angle formed with the x axis of the xz cross section. The distance is the optical path length between the optical components of the light beam from the center point of the image modulation element included in the image generation source 1 to the center of the screen. The unit of x, y, z and distance is [mm].

  The following equation 1 is a polynomial representing a free-form surface, and the reflection surface shapes of the first curved reflection mirror 3 and the second curved reflection mirror 4 are obtained according to this equation 1. An example of each coefficient of a polynomial representing the free curved surface shape of the first curved reflecting mirror 3 according to the present embodiment is shown in Table 2 below, and an example of each coefficient of the polynomial representing the free curved surface shape of the second curved reflecting mirror 4 is shown in Table 2 below. It is shown in Table 4 below.

Here, the coordinate system in the above equation 1 is such that the horizontal direction is the u coordinate, the vertical direction is the v coordinate, and the direction perpendicular to the surface and the same direction as the z axis is the w coordinate. In the above equation 1, c _j is a coefficient of u ^m · v ⁿ obtained by using the equation shown in the following equation 2, and j is an integer of 2 or more.

  Thus, each reflective surface of the 1st curved reflective mirror 3 and the 2nd curved reflective mirror 4 concerning this embodiment makes free curved surface shape. In this embodiment, the free-form surface shape is symmetric with respect to the y axis, but is asymmetric with respect to the x axis. That is, each reflecting surface of the first curved reflecting mirror 3 and the second curved reflecting mirror 4 according to the present embodiment has a rotationally asymmetric free curved surface shape. At least one of them is curved so as to be convex with respect to the reflection direction, and the curvature of the portion that reflects the light incident on the lower end of the transmissive screen 3 is changed to the light incident on the upper end of the screen. Is larger than the curvature of the reflecting part. In addition, any one of the above portions that reflects the light incident on the lower end of the screen has a convex shape with respect to the reflection direction, and the portion that reflects the light incident on the upper end of the screen has a concave shape in the reflection direction. It may be a thing to do.

  The following formula 3 is a polynomial representing an aspheric surface, and the shape of the original surface of the Fresnel lens sheet 8 is obtained according to this formula 3. An example of each coefficient of the polynomial representing the shape of the original surface of the Fresnel lens sheet 8 according to the present embodiment is shown in Table 4 below.

Here, in the equation 3 above, z is the sag of the surface parallel to the z-axis, r is the distance from the axis of rotation, c is the vertex curvature, k is a conic constant, d _{n (n} = 2,4, 6, 8, 10, 12...: Multiple of 2) indicates an n-order aspheric coefficient.

  In the Fresnel lens sheet 8 according to the present embodiment, the center axis of the Fresnel lens (see reference numeral 12 in FIG. 2) is separated downward from the screen center by 545 mm from the distribution of the incident angles of the image light on the screen. Set to position. In other words, according to the present embodiment, the Fresnel lens sheet 8 does not have the central axis on the surface of the sheet, but rather the central axis of the Fresnel lens deviates from the surface of the Fresnel lens sheet 8. Located in. In other words, in the present embodiment, the center point of the concentric circles forming the prism surface is outside the region for displaying the image of the image display device. And each prism which comprises a Fresnel lens will be formed along the concentric circular arc centering on the central axis located in this screen outer side.

Here, FIG. 6 shows the relationship between the original surface 21 of the Fresnel lens sheet 8 obtained by the aspheric coefficient in Table 4 and each prism surface 22 of the Fresnel lens. FIG. 6 shows a cross section of the Fresnel lens sheet 8 that is parallel to the perpendicular of the Fresnel lens sheet 8 and includes the rotation axis. In FIG. 6, r corresponds to r in Equation 3 above, and represents the distance from the rotation axis. Fresnel angle theta ₁ of the prism surface 22 of the Fresnel lens at a distance r _{1 (angle} formed with the main plane and the prism surface of the Fresnel lens sheet 8) is approximately equal to the inclination of the original surface 21 of the distance r ₁ (tangent). That is, when the aspherical expression of the original surface represented by Equation 3 is Z (= f (r _n )) and n is an integer of 1 or more, the Fresnel angle θ _n at each position of the Fresnel lens sheet 8 is It is expressed by Equation 4.
(Equation 4) θ _n = f (r _n ) ′
Therefore, θ ₁ = f (r ₁ ) ′, θ ₂ = f (r ₂ ) ′ θ ₃ = f (r ₃ ) ′. As described above, the Fresnel angle θ _n at each position of the Fresnel lens sheet 8 substantially corresponds to the differential value at each position (each distance r _n ) of the aspherical expression. In this way, the Fresnel angle θ _n at each position of the Fresnel lens sheet 8 is set. As described above, the light beam incident on the Fresnel lens sheet 8 is refracted by each prism surface 22 of the Fresnel lens. As described above, if the original surface 21 of the Fresnel lens sheet 8 is formed into an aspherical shape corresponding to the incident angle of the incident light beam on each position of the Fresnel lens sheet 8, each light beam refracted by each prism surface 22 is reflected. The line is substantially parallel to the normal of the Fresnel lens sheet 8. In this embodiment, the Fresnel angle θ of the prism surface 22 located at the upper part of the Fresnel lens sheet (that is, the position farthest from the rotation axis) is the prism surface located at the lower part of the Fresnel lens sheet (that is, the position closest to the rotation axis). 22 is larger than the Fresnel angle θ. This is because in the oblique projection optical system according to the present embodiment, the incident angle of light rays is larger at the upper part of the screen than at the lower part of the screen. Moreover, in this embodiment, since the rotating shaft is located outside the Fresnel lens sheet 8 as described above, the prism surfaces 22 are inclined in the same direction.

  The incident angle (vertical axis) of the light beam (transmission light) with respect to the distance (horizontal axis) from the central axis of the Fresnel lens actually observed in the magnifying projection optical system configured as described above. The distribution state is shown in the attached graph of FIG.

  In addition, FIG. 5 attached is a graph showing the distribution state of the emission angle (angle between the ray and the screen perpendicular) of the light ray (transmission light) emitted from the screen with respect to the distance (horizontal axis) from the central axis of the Fresnel lens. Is indicated by In this figure, for comparison, the outgoing angle of a light beam when a Fresnel lens sheet 8 having a Fresnel lens whose original surface is spherical is used is indicated by a broken line.

  That is, as is apparent from the graph shown in the above figure, in the case of a Fresnel lens having a spherical original surface, the difference in incident angle is about 5 degrees at maximum depending on the distance (horizontal axis) from the central axis of the Fresnel lens. Has occurred. For this reason, in the Fresnel lens sheet whose original surface is spherical, the luminance on the screen is non-uniform. On the other hand, according to the Fresnel lens sheet 8 according to the present embodiment, since the original surface is aspherical, the light beam is emitted regardless of the distance (horizontal axis) from the central axis of the Fresnel lens. The angle can be suppressed within about 0.8 degrees. In other words, since the exit angle of the light beam emitted from the screen can be made substantially parallel to the normal of the screen over the entire screen, the brightness of the display image can be made uniform over the entire screen.

  In addition, in an image display apparatus with a screen diagonal of 50 inches (vertical: aspect ratio = 9: 16) employing the enlarged projection optical system configured as described above, the thickness of the apparatus is about 300 mm. That is, according to this embodiment, it can be seen that the apparatus can be further thinned. Note that the projection lens in the above-described embodiment has a projection distance of 1300 mm and a function capable of enlarging and projecting an image on a diagonal screen of 50 inches (length: aspect ratio is 9:16). However, the present invention is not limited to those shown in the above-described embodiments, particularly with respect to the numerical values indicating the dimensions and characteristics of the components described above. For example, those skilled in the art will naturally understand that the above dimensions and characteristics can be appropriately obtained by optical design together with the curved reflecting mirror in accordance with specifications such as an oblique projection angle and a projection distance.

  In the above-described embodiment, the optical axis of the projection lens is in the yz plane, and the optical axis of the image light is reflected by the two curved reflecting mirrors and the one planar reflecting mirror in the plane. Arranged. However, these quantities and positions are not limited to the above-described embodiments.

  For example, assuming that the optical axis direction of the projection lens is the x-axis direction, the optical axis is bent in the z-axis direction by a reflecting mirror or a curved reflecting mirror, and the light is reflected by the curved reflecting mirror and the planar reflecting mirror in the yz plane. Also good. If arranged in this manner, the projection lens and the image source can be stored compactly below the screen in the housing. Therefore, there is an effect that the height of the image display device can be suppressed.

  In addition, the projection optical system used in the present embodiment only needs to have at least one curved surface for correcting aberrations together with trapezoidal distortion, and in this embodiment, a projection optical system using two curved reflecting mirrors is used. Not limited. For example, the curved surface can be one or more reflective free-form surfaces. Further, instead of the reflective type, a rotationally asymmetric lens having at least one refracting type free-form surface, that is, a free-form surface may be used. Furthermore, it goes without saying that both a reflective free-form surface (free-form surface reflection mirror) and a refraction-type free-form surface (free-form surface lens) may be combined. Here, the free-form surface lens is curved so as to make a concave in the light emission direction, and the curvature of the portion through which the light beam incident on the lower end of the screen passes is the same as the light beam incident on the upper end of the screen. It is good also as what makes a shape larger than the curvature of the part to do.

  In the embodiment described in detail above, the prism surface is described in order to calculate the aspherical surface of the lens constituting the Fresnel lens sheet 8. However, at that time, optimization may be difficult depending on, for example, an oblique projection distance or a projection distance. Hereinafter, a method for obtaining a suitable prism surface even in such a case will be described.

  Generally, an image display device has a wider viewing angle in the horizontal direction than vertical. Therefore, the diffusivity in the horizontal direction is improved by the action of the lenticular lens in the horizontal direction. On the other hand, in the vertical direction, the viewing angle is broadened mainly by the diffusion material of the lenticular lens (see reference numerals 9 and 11 in FIG. 2). Therefore, when obtaining the shape of the prism surface of the Fresnel lens, the following is performed. That is, instead of optimizing the light emitted from the screen in parallel with the normal of the screen as described above, instead, the light emitted from the screen is divided into horizontal and vertical components, The angle for making the emission angle of the vertical component about 0 degree is calculated according to Sumer's law. Thus, the prism surface at each screen position is obtained.

  Also with the original surface obtained by continuing the prism surfaces obtained in this way, a vertical component with low diffusion performance can be emitted at an emission angle of approximately 0 degrees as described above. For this reason, it is possible to sufficiently diffuse even with a conventional diffusion material. On the other hand, since the horizontal component has good diffusion performance, it can be sufficiently diffused by the lenticular lens even when the emission angle is large. As a result, it is possible to obtain uniformity in the brightness of the projected image obtained on the screen. In this case, the prism surface of the Fresnel lens constituting the Fresnel lens sheet 8 may be a curved surface, and is not necessarily limited to an aspherical shape. However, as described above, according to the Fresnel lens sheet 8 having a prism surface whose original surface is aspherical, it is possible to obtain a translucent image with a more uniform brightness on the entire screen. Each of the prism surfaces may be a flat surface or a curved surface in the cross section of the Fresnel lens sheet 8 passing through the rotation axis. This curved surface may be an aspherical surface.

  In the above embodiment, the prism surface of the Fresnel lens whose original surface is aspherical is formed concentrically around the rotation axis, and the rotation axis is outside the image display area of the image display device. It is in. However, the rotation axis may be inside the image display area of the image display device, and is not limited to the above-described embodiment. However, the center point of the concentric circle forming the prism surface is adjusted to the degree of refraction required for emitting the exit angle of the screen exit surface at approximately 0 degrees when the image display area of the image display device is used. It becomes easy. The reason will be described below. Hereinafter, the adjustment of the degree of refraction will be referred to as “refraction adjustment”.

  First, consider the case where the rotation axis is located inside the Fresnel lens sheet. The prism surface in the horizontal line including the rotation axis refracts light only in the horizontal direction, that is, the left-right direction, and cannot refract light in the vertical direction, that is, the vertical direction. Therefore, when a light beam including a vertical component is incident on a horizontal line including the rotation axis, the incident light including the vertical component is favorably converted into parallel light (a screen main component) on the prism surface on the horizontal line. Light that is parallel to the normal of the plane).

  In the present embodiment, as described above, a projection optical system that projects an image obliquely from below the screen is used, and the oblique projection optical system has a curved surface for correcting trapezoidal distortion and aberration on the screen. I have. When such an optical system is used, in many cases, all rays incident on the screen have a vertical component. Therefore, it is preferable that the Fresnel lens sheet 8 has no prism surface that does not refract light in the vertical direction so that refraction adjustment in both the horizontal direction and the vertical direction can be performed at each position of the Fresnel lens sheet 8.

  For this reason, in this embodiment, the rotation axis of the Fresnel lens is configured to be located outside the Fresnel lens sheet 8. According to this, all the prism surfaces of the Fresnel lens sheet 8 can have a function of refracting light in the vertical direction. Therefore, refraction adjustment in both the horizontal direction and the vertical direction at each position of the Fresnel lens sheet 8 is facilitated. In other words, if the rotation axis is positioned outside the Fresnel lens sheet 8, the incident light on the screen can be made into parallel light satisfactorily.

  As described above, the present embodiment uses an oblique projection optical system to shorten the depth, and an image display device having a curved reflection mirror for correcting trapezoidal image distortion caused by oblique projection. Applies to In such an image display device, in order to emit incident light having various incident angles from the screen substantially vertically, in this embodiment, the original surface of the Fresnel lens formed on the Fresnel lens sheet is aspherical. It is what. As a result, the emission angle of the light emitted from the screen can be made substantially parallel to the screen normal over almost the entire surface of the screen, so that an image with uniform brightness can be obtained, and Thinning of the image display device can be achieved at the same time.

  DESCRIPTION OF SYMBOLS 1 ... Image generation source, 2 ... Projection lens, 3 ... Rear projection type screen, 4 ... 1st curved reflection mirror, 5 ... 2nd curved reflection mirror, 6 ... Reflection mirror, 7 ... Housing, 8 ... Fresnel lens Sheet, 9 ... lenticular lens sheet, 10 ... black stripe, 11 ... diffusing material, 12 ... rotating shaft, 13 ... light incident on the screen, 14 ... surface that divides the screen equally in the left and right directions, 15 ... light incident on the screen A point that intersects a plane that divides the screen equally vertically.

Claims (8)

In an image display device,
An image source, and
A video enlargement unit for enlarging the video from the image source;
A screen on which a magnified image is projected from the back;
A projection optical unit having a rotationally asymmetric free-form surface, and projecting an enlarged image of the image generation source from the image enlarging unit obliquely with respect to the screen;
The screen includes at least a Fresnel lens sheet on which a Fresnel lens formed concentrically with the prism surface as the center is the same point,
The center point of the Fresnel lens is outside the image display area of the screen,
An image display device, wherein an original surface of a Fresnel lens formed by a set of prism surfaces of the Fresnel lens has an aspherical shape. The image display device according to claim 1,
The projected image incident on the incident surface of the Fresnel lens sheet by the Fresnel lens including the prism surface exits from the exit surface of the Fresnel lens sheet over the substantially entire surface of the screen with an exit angle of approximately 0 degrees. An image display device. The image display device according to claim 1,
The image display apparatus, wherein the projection optical unit includes a curved surface for correcting distortion and aberration caused by oblique projection onto the screen. The image display device according to claim 1,
The image display device, wherein the projection optical unit includes at least one refractive free-form surface and / or reflective free-form surface. In a screen used in a projection image display apparatus including a projection optical unit that performs oblique projection by correcting trapezoidal distortion by a reflection mirror having a rotationally asymmetric free-form surface or a rotationally asymmetric refractive lens having a free-form surface,
The prism surface includes at least a Fresnel lens sheet formed with a Fresnel lens formed concentrically around the same point,
The center point of the Fresnel lens is located outside the image display area of the screen,
A projected image is incident on the incident surface of the Fresnel lens from an oblique direction with respect to the perpendicular of the Fresnel lens,
A screen characterized in that an original surface of a Fresnel lens formed by a set of prism surfaces of the Fresnel lens has an aspherical shape. The screen according to claim 5.
The projected image incident on the incident surface of the Fresnel lens sheet by the Fresnel lens including the prism surface exits from the exit surface of the Fresnel lens sheet over the substantially entire surface of the screen with an exit angle of approximately 0 degrees. Screen characterized by being made. The screen according to claim 5.
A lenticular lens sheet in which a lenticular lens is formed is disposed on a light emitting side of the Fresnel lens sheet. The screen according to claim 7 ,
The lenticular lens sheet is characterized in that a diffusing material is mixed therein, and a light absorber is provided on its exit surface.

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