Projector with cross dichroic prism

Abstract: In a cross dichroic prism wherein four square pillar prisms (1,2,3,4) of which the total of the angles including the prism ridge lines is 360° are joined together with the prism ridge lines thereof made coincident with one another and prism sides forming the respective prism ridge lines are joined together so that the joined surface thereof may become two planes intersecting with each other, and dichroic film is provided on the joined surface, when the lengths of perpendiculars from a point on the prism ridge line to the non-joined surfaces of the respective square pillar prisms are defined as L1, L2, L3 and L4, L1 ≠ L2 = L3 = L4 is satisfied.

Description: This invention relates to an image display apparatus including a cross-dichroic prism as a colour combining means for use in a colour liquid crystal projector or the like. A cross-dichroic prism P as shown in Figs. 13 and 14 of the accompanying drawings has heretofore been used as a colour combining means of a colour liquid crystal projector. The cross-dichroic prism P, as shown in Fig. 13, comprises four rectangular prisms 1, 2, 3 and 4 cemented together so that the apex angles of those prisms may intersect with one another. A dichroic surface (film) is provided on the joined surface of each prism. The dichroic surfaces intersect at 90°. The cross-dichroic prism according to the prior art comprises four rectangular prisms cemented together as previously described and therefore suffers from the problem that it is bulky and heavy. Particularly in a liquid crystal projector using a liquid crystal panel provided with a microlens for condensing light for each pixel, a light beam emerging from the liquid crystal panel is great in angle of divergence due to the action of the microlens, and it has been necessary to make the cross-dichroic prism provided between the liquid crystal panel and a projection lens considerably large. The image display apparatus considered hereinbelow is an apparatus of the kind including a cross-dichroic prism comprised of first, second, third and fourth prisms whereby colour image light which is to exit from said first prism is formed by combining colour image light incident via said second, third and fourth prisms, wherein the joined surfaces of the first and second prisms and the joined surfaces of the third and fourth prisms lie in a first plane, the joined surfaces of the second and third prisms and the joined surfaces of the fourth and first prisms lie in a second plane, dichroic films are disposed on the joined surfaces of the first to fourth prisms, and the first plane and the second plane intersect with each other along an intersection line. Examples of image display apparatus of the type just mentioned are to be found described in Japanese Patent Application publication Nos. JP-A-02220040, JP-A-05273404 and in United States Patent US 4,249,205. None of the references above however provide a solution to the aforementioned problem that the cross-dichroic prisms used hitherto have been bulky and heavy. It is thus sought to provide an image display apparatus with a more compact and lightweight cross-dichroic prism. In accordance with the present invention the image display apparatus of the kind just mentioned is characterised in that the lengths of respective lines which extend perpendicularly to the non-joined surfaces of the first, second, third and fourth prisms from the intersection line satisfy the following condition: L1 > L2 = L3 = L4 ; and the area of the non-joined surface of the third prism is smaller than the area of the non-joined surface of the first prism. In a preferred embodiment of the image display apparatus, the lengths of the first, second and fourth prisms, measured in a direction parallel to the intersection line, are longer than the length of the third prism, measured in the same direction. The image display apparatus may include respective liquid crystal display panels arranged on the non-joined surfaces of the second, third and fourth prisms and an illumination optical system for illuminating each of the liquid crystal display panels with light from a light source. This apparatus may be combined with a projection lens to produce a liquid crystal colour projector. In the accompanying drawings: Fig. 1 is a perspective view of a cross-dichroic prism for use as a colour combiner in a first embodiment of the present invention; Fig. 2 is a cross-sectional view of the first embodiment including the cross-dichroic prism of Fig. 1; Fig. 3 is another cross-sectional view of the embodiment taken in the orthogonal first direction; Fig. 4 is the same cross-sectional view as Fig. 2 but includes light ray paths for explanation; Figs. 5 and 6 are orthogonal cross-sectional views of a second embodiment including an alternative cross-dichroic prism; Figs. 7 and 8 are orthogonal cross-sectional views of a third embodiment including another alternative cross-dichroic prism; Fig. 9 is a perspective view of the cross-dichroic prism of the third embodiment; Fig. 10 is a cross-sectional view of the cross-dichroic prism of a fourth embodiment; Fig. 11 is a cross-sectional view of the cross-dichroic prism of a fifth embodiment; Fig. 12 is a schematic view of a projection apparatus; Fig. 13 is a schematic view of an image display apparatus including a cross-dichroic prism according to the prior art; and Fig. 14 is a typical view of a projection apparatus using the cross-dichroic prism according to the prior art. Preferred embodiments will now be described in detail while reference will be made to the drawings listed above. Embodiment 1 In the figures, reference numeral 1 designates a first prism, reference numeral 2 denotes a second prism, reference numeral 3 designates a third prism, and reference numeral 4 denotes a fourth prism. Reference numeral 5 designates a holding stand for holding the cross-prism CP. LCD denotes each liquid crystal panel of the image display apparatus part of an LCD projector. A first dichroic surface (film) ac reflecting blue light (B light) and transmitting green light (G light) and red light (R light) therethrough is formed on the joined surface "a" of the first prism 1 and the second prism 2 and the joined surface "c" of the third prism 3 and the fourth prism 4, and a second dichroic surface (film) "bd" reflecting red light and transmitting green light and blue light therethrough is formed on the joined surface b of the second prism 2 and the third prism 3 and the joined surface d of the fourth prism 4 and the first prism 1. The joined surface a and the joined surface "c" exist on a plane, and the joined surface b and the joined surface "d" exist on a plane, and these intersect with each other. The red light from LCD, the green light from LCD and the blue light from LCD enter the cross prism CP from the light incidence surface (non-joined surface) S2 of the second prism 2, the light incidence surface (non-joined surface) S3 of the third prism and the light incidence surface (non-joined surface) S4 of the fourth prism, respectively, and acolour light comprising R light, G light and B light combined together emerges from the light emergence surface (non-joined surface) S1 of the first prism 1. Here, when a portion in which the four prisms are proximate to one another is defined as a ridge line portion (prism ridge line) 0, Fig. 2 is a view of a cross-section (xy cross-section) orthogonal to the ridge line portion 0, and Fig. 3 is a view of a cross-section (YZ cross-section) orthogonal to the light emergence surface S1 of the first prism 1. In the present embodiment, each of the first prism 1 and the third prism 3 is an isosceles triangular pillar prism in which the lengths of two sides are equal to each other, and both of them are similar figures differing in size from each other. The second prism 2 and the fourth prism 4 are triangular pillar prisms, and both of them are of the same shape. When the lengths of perpendiculars from a point on the prism ridge line 0 to the non-joined surfaces S1-S4 are defined as: 0, S1a; 0, S2a, 0, S3a and 0, S4a, respectively, 0, S1a > 0, S2a = 0, S3a = 0, S4a. The angles α1, α2, α3 and α4 formed against the prism ridge lines of the four prisms 1, 2, 3 and 4 are as follows: α1 = a3 < 90° α2 = α4 > 90° α1 + α2 = α3 + α4 = 180° (α1 + α2 + α3 + α4 = 360°) The non-joined surface S1 and the non-joined surface S3 are parallel to each other, and the non-joined surfaces S2 and S4 are not parallel to each other. The area S1b of the non-joined surface S1 and the area S3b of the non-joined surface S3 differ from each other. The area S2b of the non-joined surface S2 is equal to the area S4b of the non-joined surface S4. In the present embodiment, S3b < S1b. In the present embodiment, the width k1 of the light emergence surface of the first prism 1 on a cross-section orthogonal to the ridge line portion 0 is made greater than the width k3 of the light incidence surface S3 of the third prism 3 (K1 > k3), whereby the distance (length) RL from the light incidence surface S3 of the third prism 3 to the ridge line portion 0 is RL = k3/(k1 + k3) * PL. PL is the distance (length) from the light incidence surface S3 of the third prism 3 to the light emergence surface S1 of the first prism 1. In the prior-art construction shown in Fig. 13, the distance RLo from the light incidence surface S3 of the third prism 3 to the ridge line portion 0 is RLo = PL/2 and therefore, the difference therebetween is PL - RLo = PL/2 * ((k3 - k1)/(k1 + k3)) k1 > k3, hence RL - RLo < 0 RL < RLo and thus, the ridge line portion 0 becomes proximate to the light incidence surface S3 of the third prism 3. At this time, the apex angle α1 containing the ridge line portion 0 of the first prism 1 and the apex angle α3 of the third prism 3 become acute angles smaller than 90°, and the apex angle α2 containing the ridge line portion 0 of the second prism 2 and the apex angle α4 of the fourth prism 4 become obtuse angles greater than 90°. The light incidence surface S2 of the second prism 2 is provided in symmetrical relationship with the light incidence surface S3 of the third prism 3 with respect to a second dichroic surface b, d and the light incidence surface S4 of the fourth prism 4 is provided in symmetrical relationship with the light incidence surface S3 of the third prism 3 with respect to a first dichroic surface a, c, whereby the optical path lengths of the optical paths of the respective colour lights on the optical axis from the light incidence surface to the ridge line portion 0 can be made equal to one another. At this time, the angles formed by the light incidence surfaces S2 and S4 of the second prism 2 and the fourth prism 4 and the light emergence surface S1 of the first prism 1 can be set to angles smaller than 90° and therefore, the shape of the cross-section orthogonal to the ridge line portion 0 becomes a trapezoidal shape, and by a shape in which for the divergent light beam travelling from the light incidence surface S3 of the third prism 3 to the light emergence surface S1 of the first prism 1, an area D not used is made small as shown in Fig. 4, three light beams can be combined together and the volume of the cross prism CP can be curtailed. When the angle of expanse of the outermost ray of the light transmitted through the cross prism CP is defined as 2β (> 0) and the angle formed by the first and second dichroic surfaces and the light incidence surface S3 of the third prism 3 is defined as ,  = 45 + β/2, whereby the light incidence surfaces S2 and S4 of the second prism 2 and the fourth prism 4 and the outermost ray of the light transmitted through the prism can be made parallel to one another, and a more compact shape is provided. Further, when the length of the prism from the light incidence surface S3 of the third prism 3 to the light emergence surface S1 of the first prism 1 is defined as RL and the width of the light incidence surface S3 of the third prism 3 is defined as k3, if RL is determined so as to satisfy RL = k3/(tan (45-β/2)-tan (β)), the cross prism can be constructed so that the cross-sectional shapes on the cross-sections orthogonal to the ridge lines 0 of the first to fourth prisms 1-4 may be triangular and the vertexes of the respective prisms may coincide with one another, and a more compact shape can be provided. The cross dichroic prism CP used in the present embodiment comprises four prisms 1, 2, 3 and 4 joined together, and dichroic surfaces are provided on the joined surfaces of the four prisms. When the area of the light incidence surface S3 of the third prism 3 on the optical path of light transmitted through the first dichroic surface a,c and the second dichroic surface b,d is defined as S3b and the area of the light emergence surface S1 of the first prism 1 is defined as S1b, S3b < S1b is satisfied, and when the distance from the light incidence surface S3 to the position 0 at which the dichroic surfaces intersect with each other is defined as L3 and the distance from the light emergence surface S1 to the position 0 at which the first and second dichroic surfaces (films) intersect with each other is defined as L1, L3 < L1 is satisfied, whereby the position 0 at which the first and second dichroic surfaces intersect with each other can be made proximate to the position of each liquid crystal panel LCD. The two prism surfaces constituting a dichroic surface must be flat (the angle formed thereby must be 0). When these are inclined, there will be provided the left and right positional deviation of the image on a screen, as shown in Fig. 14. This amount of deviation Δ, when the distance converted into air from the position at which the dichroic surfaces intersect with each other to each liquid crystal panel LCD is defined as m and the amount of angular deviation is defined as , becomes Δ = B• m•tan. Here, B is the projection magnification of the projection lens. To make the amount of deviation small at this time, the distance m can be made small, and the position 0 at which the first and second dichroic surfaces intersect with each other is made proximate to the position of the liquid crystal panel LCD, whereby m can be made small. Further, the light transmitted through the liquid crystal panel LCD emerges as a divergent light beam from the liquid crystal panel to the projection lens and therefore, the effective range S1a of the light emergence surface S1 is large relative to the effective range S3a of the light incidence surface S3 and heretofore, there has been a useless space (a portion D in Fig. 13) on the light incidence surface S3 side, but here the size of the light incidence surface S3 is made smaller than the size of the light emergence surface S1 and is made approximate to the effective range to thereby decrease this useless space. When such a cross dichroic prism is applied to a colour liquid crystal projector using three liquid crystal panels, if it is used in combination with a so-called liquid crystal panel with a microlens array provided with a minute lens (microlens) corresponding to each pixel of the liquid crystal panels on the light incidence side, it will be more effective, but even when use is made of liquid crystal panels having no microlens array, it is still effective. Embodiment 2 A second embodiment will now be described with reference to Figs. 5 and 6. In this embodiment, the setting of the dichroic surface and the shape of the cross-section orthogonal to the ridge line portion are the same as those in Embodiment 1. The present embodiment achieves a lighter weight of the cross-prism as compared with Embodiment 1. In a cross-section containing a ridge line portion 0 shown in Fig. 6 and orthogonal to the light emergence surface S1 of a first prism 1, the end surfaces U and T of the prism are obliquely cut in conformity with the range of light diverging from a liquid crystal panel LCD to thereby make the shape more compact. In this case, the holding portion of a holding stand 5 holding the cross-prism CP is provided inclinedly with respect to the emergence surface S1 of the first prism 1 so that the cross-prism CP may be slid and fixed relative to the inclination. Embodiment 3 A third embodiment will now be described with reference to Figs. 7 to 9. Again this embodiment, like Embodiment 2, achieves a lighter weight of the cross-prism CP. The present embodiment is easy to work. As shown in Fig. 9, the height of the third prism 3 in the direction of a ridge line portion o is made lower than the heights of the other three prisms 1, 2 and 4. Embodiment 4 In the cross-dichroic prism of this embodiment, as can be seen in Fig. 10, a first prism 1 and a third prism 3 are isosceles triangle pole prisms of which the lengths of the two sides are equal to each other, and both of them are similar figures. A second prism 2 and a fourth prism 4 are triangle pole prisms of which the lengths of the sides differ from one another, and both of them are of the same shape. When the lengths of perpendiculars from a point on the prism ridge line 0 to the non-joined surfaces S1-S4 of the respective prisms are defined as 0, S1a, 0, S2a, 0, S3a and 0, S4a, respectively, 0, S1a > 0, S2a = 0, S3a = 0, S4a. The angles α1, α2, α3 and α4 formed against the prism ridge lines 0 of the four prisms 1, 2, 3 and 4 are as follows: α1 = α3 < 90° α2 = α4 > 90° α1 + α2 = α3 + α4 = 180° (α1 + α2 + α3 + α4 = 360°) The non-joined surface S1 and the opposite non-joined surface S3 are parallel to each other. The area S1b of the non-joined surface S1 and the area S3b of the non-joined surface S3 differ from each other. The area S2b of the non-joined surface S2 is equal to the area S4b of the non-joined surface S4. In the present embodiment, S3b < S1b. Embodiment 5 In the cross-dichroic prism of this embodiment, as seen in Fig. 11, a first prism 1 and a third prism 3 are isosceles triangle pole prisms of which the lengths of the two sides are equal to each other, and both of them differ in size from each other, but are similar figures. A second prism 2 and a fourth prism 4 are both of the same shape. When the lengths of perpendiculars from a point on the prism ridge line 0 to the non-joined surfaces S1-S4 of the respective prisms are defined as 0, S1a, 0, S2a, 0, S3a and 0, S4a, respectively, 0, S1a > 0, S2a = 0, S3a = 0, S4a. The angles α1, α2, α3 and α4 formed against the prism ridge lines o of the four prisms 1, 2, 3 and 4 are as follows: α1 = α3 < 90° α2 = α4 > 90° α1 + α2 = α3 + α4 = 180° (α1 + α2 + α3 + α4 = 360°) The non-joined surface S1 and the opposite non-joined surface S3 are parallel to each other. The area S1b of the non-joined surface S1 and the area S3b of the non-joined surface S3 differ from each other. The area S2b of the non-joined surface S2 is equal to the area S4b of the non-joined surface S4. In the present embodiment, S3b < S1b. Fig. 12 is a schematic view of a liquid crystal projector (projection apparatus) using a cross-dichroic prism. In Fig. 12, reference numeral 11 designates a white light source, reference numeral 12 denotes a reflector, reference numeral 13 designates an integrator portion comprising two fly-eye lenses 131 and 132 in which a plurality of lenses are constructed on an array, reference numeral 14 denotes a polarization converting portion comprising a combination of a minute polarizing beam splitter 141 and a phase plate 142 and uniformizing non-polarized light in a predetermined direction of polarization, reference numeral 15 designates a condensing lens, reference numeral 16 denotes a reflecting mirror, reference numeral 17 designates a dichroic mirror for separating, for example, white light into blue (B) light, red (R) light and green (G) light, the reference numeral 18 denotes a dichroic mirror for separating, for example, red light and the green light from each other, the reference numeral 19 designates a mirror for reflecting the blue light, and reference numeral 20 denotes a relay illuminating portion for relaying the red light separated by the dichroic mirrors 17 and 18 by lenses 201, 202, 203 and mirrors 204, 205, and forming an illuminating area equivalent to the blue light and the green light at a predetermined position. Reference characters 21b and 21g designate condenser lenses for blue and green illuminating lights to a projection lens 24, and reference characters 22r, 22g and 22b denote liquid crystal panels with microlenses for forming red, green and blue image lights, and a polarizing plate, a phase plate, etc., not shown, are provided forwardly and rearwardly of the main body of each liquid crystal panel. Reference numeral 23 designates the cross-dichroic prism of the present invention used for colour combination. The reference numeral 24 denotes a projection lens. As can be seen from this figure, the cross-dichroic prism 23 is provided as a prism for colour combination, whereby the liquid crystal panels 22r and 22b can be constructed in proximity to the liquid crystal panel 22g, and this also contributes to the downsizing of the entire apparatus. In Fig. 12, unpolarized white light emitted from the light source 11 is forwardly reflected by the reflector 12, and becomes linearly polarized light through the fly-eye lens 131, the fly-eye lens 132 and the polarization converting element 14, and enters the first dichroic mirror 17 through the intermediary of the condensing lens 15 and the total reflection mirror 16. The white linearly polarized light is resolved, for example, into two colours (here, blue and (green + red) lights), and the blue light is transmitted through the mirror 17 and is reflected by a total reflection mirror 19 and enters the condenser lens 21b, and illuminates the liquid crystal panel 22b for a blue image. The other (green + red) color light (G, R) is reflected by the first dichroic mirror 17 and enters the second dichroic mirror 18, and is further resolved into two colours i.e., green and red, by this second dichroic mirror 18, and the green light (G) is reflected by the mirror 18 and enters the condenser lens 21g, and illuminates the liquid crystal panel 22g for a green image. The red light (R) is transmitted through the mirror 18 and illuminates the liquid crystal panel 22r for a red image through the relay illuminating system 20. The lights having entered the liquid crystal panels 22b, 22g and 22r are light-modulated by the liquid crystal panels corresponding to the respective colours and become lights having the image information of the respective colours and emerge from the liquid crystal panels 22b, 22g and 22r. Those image lights of the respective colours having emerged from the liquid crystal panels 22b, 22g and 22r are combined into a light by the cross-prism 23 of the present invention for colour combination, and the light is projected onto a screen (a predetermined surface) by the projection lens 24.

Claims: An image display apparatus (CP, LCD; 23,22b,22g,22r) including a cross-dichroic prism (CP;23) comprised of first, second, third and fourth prisms (1-4) whereby colour image light which is to exit from said first prism (1) is formed by combining colour image light incident via said second, third and fourth prisms (2-4), wherein the joined surfaces (a) of the first and second prisms (1,2) and the joined surfaces (c) of the third and fourth prisms (3,4) lie in a first plane, the joined surfaces (b) of the second and third prisms (2,3) and the joined surfaces (d) of the fourth and first prisms (4,1) lie in a second plane, dichroic films are disposed on the joined surfaces (a-d) of the first to fourth prisms (1-4), and the first plane and the second plane intersect with each other along an intersection line (o); which image display apparatus is characterised in that the lengths (L1 to L4) of respective lines which extend perpendicularly to the non-joined surfaces (S1-S4) of the first, second, third and fourth prisms (1-4) from the intersection line satisfy the following condition: L1 > L2 = L3 = L4 ; and the area (S3b) of the non-joined surface (S3) of the third prism (3) is smaller than the area (S1b) of the non-joined surface (S1) of the first prism (1). An apparatus according to claim 1 wherein the lengths of the first, second and fourth prisms (1,2,4), measured in a direction parallel to the intersection line, are longer than the length of the third prism (3), measured in the same direction. An apparatus according to either preceding claim, including: respective liquid crystal display panels (LCD; 22b,22g,22r) arranged on the non-joined surfaces (S2,S3,S4) of the second, third and fourth prisms (2-4) of the cross-dichroic prism (CP;23); and an illumination optical system (13-20) for illuminating each of said liquid crystal display panels with light from a light source (11,12). An apparatus according to claim 3 including said light source (11,12). A liquid crystal colour projector comprising an image display apparatus according to either of claims 3 and 4 and a projection lens (24).

Inventors: OKUYAMA ATSUSHI, OKUYAMA, ATSUSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1999
Main classification: G02B27
IPC codes: H04N9, G02B27, G02B5