Liquid crystal display device

0001The present invention relates to a liquid crystal display device of a projection type, according to the preamble of claim 1.

0002Such a liquid crystal display device is known from document <patcit id="pcit0001" dnum="EP0509630A"><text>EP 0 509 630 A</text></patcit>.

0003The present invention relates in general to a liquid crystal display device having a liquid crystal panel and, particularly, to a liquid crystal projector which projects an image of the liquid crystal panel. More specifically, the present invention relates to an improvement in a cooling structure for improving a heat resistant reliability.

Description of Related Art

0004A liquid crystal projector has been mainly used as a so-called home theater system for enjoying various movies at a home. In these days, a projection image quality has been improved by a high definition of liquid crystal panel and a high luminance brightness of lamp. Accordingly, the liquid crystal projector has been also used as a presentation system for projecting a display image of a personal computer on a projection screen.

0005A construction of the liquid crystal projector is shown in <figref idref="f0001">FIG. 1</figref>. Light generated from a light source 1 of a high luminance lamp such as a metal-halide lamp, a xenon lamp or a UHP or the like, is reflected by a spherical reflecting mirror 2. Then, light reflected by the spherical reflecting mirror 2 transmits through a filter 3 so that unnecessary infrared ray and ultraviolet ray are removed. Further, light transmitting through the filter 3 transmits through an integrator lens 4 and a condenser lens 5 so as to be condensed, and thereafter, passes through an incidence side polarizing plate 6, and thus, is incident upon a liquid crystal panel 8. Light emitted from the liquid crystal panel 8 transmits through an emission side polarizing plate 7, and thereafter, is enlarged and projected by means of a projection lens 9, and thus, an image is projected onto a screen or the like located ahead of the projector.

0006The aforesaid structure is of a single-panel type using a sheet of liquid crystal panel including color filters. In addition to the above single-panel type structure, a three-panel type structure is also known such that light from a light source is decomposed into decomposition lights of three primary colors of RGB, and these decomposition lights are incident upon three liquid crystal panels.

0007A construction of the liquid crystal projector having the above three-panel type structure is shown in <figref idref="f0001">FIG. 2</figref>. In <figref idref="f0001">FIG. 2</figref>, like reference numerals used in the case of <figref idref="f0001">FIG. 1</figref> are used to designate components having the same function as components shown in <figref idref="f0001">FIG. 1</figref>. In the construction shown in <figref idref="f0001">FIG. 2</figref>, the following components are used; more specifically, dichroic mirrors 10 for transmitting or reflecting an incident light in accordance with a wavelength, a complex prism 11 for synthesizing light, and total reflection mirrors 12. Light, which is generated from the light source 1 and reflected by the spherical reflecting mirror 2, transmits through the filter 3 and the integrator lens 4, and thereafter, is guided to the dichroic mirrors 10 after its optical path is bent by one of the total reflection mirrors 12. Then, light from the light source 1 is decomposed into three primary color lights of red (R), green (G) and blue (B) by means of two dichroic mirrors 10 for reflecting or transmitting light having different wavelength band. Then, these decomposition lights are transmitted through the liquid crystal panel 8 corresponding to each of three primary colors, and thereafter, are synthesized by the complex prism 11, and thus, are projected.

0008In these liquid crystal projectors, there is a tendency for a projected image to be short of a brightness. The first factor is light absorption by polarizers 6 and 7 located front and rear the liquid crystal panel 8. The second factor is that the liquid crystal panel 8 has an area made small up to a size of about 1 inch in order to miniaturize the liquid crystal projector. More specifically, in the case where an image of the liquid crystal panel 8 having a small area is enlarged and projected to several tens of inches to several hundreds of inches, there is the case where the projected image is short of a brightness.

0009In order to solve these problems as described above, high output lamps such as high-luminance metal-halide lamp, Ultra High Pressure Mercury lamp and xenon lamp are used as the light source 1. However, in particular, in the presentation system, demands for further miniaturization, high definition and high luminance are made in the market and a higher output lamp is selected.

0010For this reason, in the liquid crystal projector system, a disadvantage by a heat is a significant problem.

0011For example, in general, an iodine-based polarizer is used as the polarizer constituting a liquid crystal display section. However, the iodine-based polarizer is not sufficient in light resistance, heat resistance and damp and heat resistance. For this reason, in particular, in the liquid crystal projector, a dye-based polarizer is used because of being excellent in light resistance, a heat resistance and a damp and heat resistance (see unexamined Patent Publications (Kokai) No. <patcit id="pcit0002" dnum="JP9022008A"><text>9-22008 </text></patcit>and <patcit id="pcit0003" dnum="JP9022009A"><text>9-22009</text></patcit>)

0012However, in particular, the incidence side polarizer 6 has light transmittance of about 40%, and absorbs most of lights. Further, the polarizer 6 can not maintain its characteristic when becoming a temperature of 70°C or higher.

0013Moreover, the liquid crystal panel 8 itself is weak in a heat, and its characteristic is remarkably deteriorated 5 when becoming a temperature of 60°C or higher.

0014In order to solve the above problem, in the liquid crystal projector, the following various cooling systems have been proposed. <ol id="ol0001" compact="compact" ol-style=""><li>(1) Air cooling system Heat generating sections such as the incidence side polarizer 6, the liquid crystal panel 8 and the emission side polarizer 7, the light source 1 and a power source section, are cooled by means of a cooling fan, and then, an air having a heat is exhausted. However, according to this air cooling system, there are a noise problem and a problem that a dust adheres to the liquid crystal panel 8. More specifically, in the case where an air supply is increased in order to obtain sufficient cooling effect, a fan is rotated at a high speed, and is made into a large size, and thereby, a large noise is generated. For this reason, it is not preferable to apply the above air cooling system to a liquid crystal projector which is used for conducting a presentation in a silent room or for enjoying a movie or the like. </li><li>(2) As shown in <figref idref="f0001">FIG. 1</figref>, the incidence side polarizer 6 is arranged in a state of separating from the liquid crystal panel 8 with a distance of about 1 to 5 mm. By doing so, it is possible to prevent a heat of the polarizer 6 from conducting directly to the liquid crystal panel 8. Further, a cooling air flows between the polarizer 6 and the liquid crystal panel 8, and thereby, a cooling efficiency can be enhanced. In this system, the incidence side polarizer 6, which is a principal heat generating source, is arranged separately and independently, and thereby, an influence to the liquid crystal panel 8 is reduced, and a heat radiating effect can be improved. However, there is a limit in the cooling effect. Moreover, a blue sheet glass orwhite sheet glass is used as a retaining plate of a polarizing substance in the polarizer 6. In the blue or white sheet glass, the heat conductivity is worse and a heat radiating effect is insufficient. Therefore, like the case of the above air cooling system, finally, an output of the cooling fan must increased. As a result, it is impossible to solve the above problems relating a noise and dust adhesion. </li><li>(3) A heat radiating glass plate having a heat conductivity of 1 W/m·K or more is located on an outer surface of the liquid crystal panel 8 with a sealed space interposed therebetween, so as to enhance a heat radiating effect with respect to a heat generation of the liquid crystal panel 8. Further, a cooling air is supplied to the heat radiating glass plate so as to prevent a dust from adhering to the liqu<u>i</u>d crystal panel 8. According to the above system, no dust adheres to the outer surf ace of the liquid crystal panel 8. Since a surface of the heat radiating glass plate diverges from a focal plane, even if a dust adheres to the surface of the heat radiating glass plate, the image of the dust is not imaged on a projection screen, and no influence is given to a projection image. However, a heat conductivity is 2 W/m· K or less at most even though the glass plate has a high heat conductivity and, therefore, sufficient heat radiating effect can not be obtained. </li><li>(4) Liquid cooling system A liquid is encapsulated as a heat exchange medium in a space formed by a transparent panel arranged along the outer surface of the liquid crystal panel, and thus, a cooling effect is enhanced (see Examined Patent Publication (Kokoku) No. <patcit id="pcit0004" dnum="JP6058474A"><text>6-58474</text></patcit>). According to the above liquid cooling system, with a temperature rise, there is the possibility that a pressure reduction is required, or a bubble is generated. Moreover, there is the possibility that a foreign matter gets mixed in the liquid used as a cooling medium, or the cooling medium leaks. For these reasons, in the liquid cooling system, the reliability is worse. In addition, the liquid is used and, therefore, the cooling system is a large scale as a whole. As a result, there is a problem that the whole of liquid crystal projector is made into a large size. </li><li>(5) Electronic cooling system using a Peltier element (solid cooling system) An electronic cooling system with a Peltier element is attached to a heat generating source so as to forcedly cool the heat generating source. According to such electronic cooling system, equipments for the electronic cooling system are required and there is a problem that the cost spent for the whole liquid crystal projector is greatly increased. In addition, a sufficient cooling effect can not be obtained. </li><li>(6) A polarized light converter is arranged just after a light source Before light from the light source 1 is incident upon the polarizer 6, a polarizing direction is aligned with a transmission polarization axis of the polarizer 6 so as to reduce a quantity of light absorbed into the polarizer 6. </li></ol>

0015However, in this case, since about 20% of light incident upon the polarizer 6 is absorbed by the polarizer 6, there is the case where a sufficient cooling effect is not always obtained. More specifically, for example, the liquid crystal panel 8 is made into a small size, and the lamp intensity per unit area becomes higher, and thereby, there is the case where a sufficient cooling effect is not obtained.

0016As seen from the above description, even if the aforesaid conventional cooling systems are employed, a sufficient cooling effect can not be obtained with a simple structure.

0017The problem of generating a heat raises in various portions other than the polarizer 6.

0018For example, a pixel electrode and a switching element are formed on an incidence side transparent substrate which is a constituent element of the liquid crystal panel 8. At present, the transparent substrate is constructed with the use of a silica glass substrate having a low heat conductivity of about 1 or 2 W/m·K. For this reason, it is impossible to effectively release a heat accumulated in the liquid crystal panel 8.

0019Recently, a panel size is made small, and thereby, light quantity per unit area increases. In addition, in order to improve an aperture ratio, a micro lens is used so that an incident light is condensed and transmitted for each pixel. Thus, as seen from the above explanation, a thermal load acting on the liquid crystal panel itself becomes greater.

0020In a driving circuit, its operating speed is slow because in a liquid crystal display unit using a polysilicon thin-film transistor as conventionally, a mobility of polysilicon is slow. In addition, since a leakage current is large due to a fault of polysilicon, there is a problem that power consumption is much.

0021On the other hand, in the liquid crystal projector having the construction as shown in <figref idref="f0001">FIG. 2</figref>, the dichroic mirrors 10 are used. The dichroic mirrors 10 are manufactured in a manner of coating a thin film, which selects a wavelength of the light and carries out transmission/reflection, on the surface of a blue sheet glass or white sheet glass. The dichroic mirrors 10 also cause a heat generation by light absorption and, therefore, the temperature of apparatus rises up as a whole.

0022Moreover, there is the case where an infrared ray is previously cut from light incident upon an optical system from the light source 1 so as to restrict a heat generation. The blue sheet glass or white sheet glass is used as the filter 3 (see <figref idref="f0001">FIG. 2</figref>) for cutting an infrared ray. Since these sheet glasses have low heat conductivity, a heat is accumulated. As a result, this is a factor of a temperature rise in the whole of apparatus.

0023Meanwhile, since the single light source 1 is used, the integrator lens 4 is used in order to diffuse a light source intensity and make uniform illuminance of an irradiation surface. In general, the integrator lens 4 has the following structure; more specifically, an optical glass such as Pyrex glass is subjected to mold pressing, and many lenses thus manufactured are made into a single plate. Further, in order to improve a characteristic, there is the case where a material such as silica glass is used, and a total reflection of the side plane of a prism made of silica glass is utilized. In this case, the silica glass has a low refractive index of about 1.46 and, therefore, the total reflection angle becomes large. As a result, a length of the prism rod must be set longer. Moreover, there is a problem that the number of pseudo light sources is small.

SUMMARY OF THE INVENTION

0024It is, therefore, an object of the present invention to provide a liquid crystal display apparatus which includes a simple and effective cooling structure.

0025Another object of the present invention is to provide a projection type liquid crystal display apparatus (liquid crystal projector) which includes a simple and effective cooling structure.

0026Still another object of the present invention is to provide a liquid crystal display apparatus which has a fast operating speed, low power consumption and excellent radiation property, and is adaptable to miniaturization and high definition.

0027The above object is achieved by a liquid crystal display device of projection type according to claim 1.

0028The present invention provides a liquid crystal display device, which includes a liquid crystal panel as one of optical components for transmitting, absorbing or reflecting light, any one of the optical components being comprised of a sapphire substrate.

0029According to the present invention, the liquid crystal display device further includes a light source, and the optical components transmits and projects light from the light source, thereby constructing a projection type display device.

0030More specifically, the optical components include a lens, and a polarizer having a polarizer film and a retaining plate for retaining the polarizer film, and the liquid crystal panel includes a transparent substrate, and further, any one of the lens, the retaining plate and the transparent substrate is comprised of a sapphire substrate, and thus, light from the light source may be transmitted through the lens, the polarizer and the liquid crystal panel so as to be projected.

0031Further, the optical components may further include a wavelength selective optical component for transmitting or reflecting light having a specific wavelength band, a lens , and a polarizer having a polarizer film and a retaining plate for retaining the polarizer film, and the liquid crystal panel includes a transparent substrate. Any one of the wavelength selective optical component, the lens, the retaining plate and the transparent substrate may be comprised of a sapphire substrate, and light from the light source may be transmitted through the lens, the polarizer and the liquid crystal panel so as to be projected.

0032In this case, the wavelength selective optical component may be a filter (for example, an infrared cut filter or an ultraviolet ray cut filter).

0033Moreover, the wavelength selective optical component may be a dichroic mirror.

0034with the above construction, the sapphire substrate having a high heat conductivity is used as a transparent substrate in the liquid crystal projector, and thereby, a heat radiating effect can be enhanced. More specifically, sapphire, which is excellent in heat conductivity, is used as various components of the liquid crystal display device, and thereby, a heat radiating effect can be improved. Therefore, it is possible to realize a high-luminance and small-size liquid crystal projector without causing a problem of characteristic deterioration by a heat generation.

0035Further, a metallic radiation fin may be bonded to the sapphire substrate, and thereby, a heat radiating effect can be improved more.

0036The transparent substrate of the liquid crystal panel may perform a function as the retaining plate of the polarizer film. More specifically, the polarizer film may be supported onto the surface of the transparent substrate of the liquid crystal panel.

0037Preferably, the sapphire substrate is constructed in a manner that an angle made by a C-axis direction or C-axis projection line direction and a polarized light transmission axis is set within a range of ±2°, or an angle made by an axis perpendicular to the C-axis and a polarized light transmission axis is set within a range of ±2°, or an angle made by a C-plane and a plane vertical to a transmission direction of a polarized light to be transmitted is set within a range of ±2°.

0038Whereby it is possible to prevent an influence to polarizing characteristic.

0039The lens may be an rod-type integrator lens for diffusing light from the light source, and the integrator lens may be formed of sapphire. More specifically, the sapphire has a high refractive index, so that it is possible to make small a total reflection angle, and to improve a diffusion of light from the light source.

0040A transparent adhesive agent having a Shore hardness of 30 or less may be applied onto the polarizer film or the liquid crystal panel so as to form a thickness of 10 to 70 <i>µ</i>m, and then, the sapphire substrate may be aligned and bonded thereto. Whereby it is possible to prevent a deformation of the liquid crystal panel generated by a difference of thermal expansion, and thereby, to reduce an influence to an image while preventing a failure in a heat conductivity

0041The sapphire substrate may be attached onto an outer surface of the liquid crystal panel so as to be sealed with a space of 0.1 mm or less interposed therebetween. Whereby the sapphire has no influence to the liquid crystal panel, and a generated heat is effectively conducted via a micro space, and thus, a cooling effect can be enhanced.

0042Further, in the case of attaching the sapphire substrate is attached onto an outer surface of the liquid crystal panel, preferably, light shielding layer having a window of a size larger by 0.1 mm or more than an effective pixel area of the liquid crystal panel is applied to the sapphire substrate. Whereby it is possible to prevent a projected image from receiving an influence of scattering light from the surroundings.

0043Further, the liquid crystal panel includes a first sapphire substrate and a second sapphire substrate used as a pair of transparent substrates facing each other with a liquid crystal layer sandwiched therebetween.

0044The first sapphire substrate has a primary plane which is any one of an R-plane, an A-plane, an M-plane and a C-plane. The second sapphire substrate has a primary plane which is any one of an R-plane, an A-plane, an M-plane and a C-plane. The first sapphire substrate and the second sapphire substrate face each other so that each specific crystal axis of them substantially coincide with a polarization transmission axis o polarized light to be transmitted.

0045The liquid crystal panel further includes pixels which are arranged in a matrix on the first sapphire substrate so as to be aligned with a specific crystal axis direction of the first sapphire substrate, a vertical scanning circuit and a horizontal scanning circuit which are formed with the use of a thin film transistor comprised of silicon made by epitaxial growth on the first sapphire substrate, and send a video signal to the pixels, and a transparent electrode formed on the second sapphire substrate.

0046Preferably, the pixels are arranged in a matrix so as to be substantially parallel with or perpendicular to the specific crystal axis direction of the first sapphire substrate.

0047Preferably, the first and second sapphire substrates are constructed in a manner that an angle made by each specific crystal axis of them and a polarization transmission axis of a polarized light to be transmitted is set within a range of ±2°.

0048Each specific crystal axis of the first and second sapphire substrates may be an A-axis or C-axis projection line direction in the case where its primary plane is an R-plane, may be a C-axis or M-axis direction in the case where its primary plane is an A-plane, may be C-axis or A-axis direction in the case where its primary plane is an M-plane, and may be an A-axis or M-axis direction in the case where its primary plane is a C-plane.

0049The liquid crystal panel has a structure in which the polarizers are arranged on both sides of the panel so that their polarizing directions cross at right angle, and a liquid crystal layer is interposed between the polarizers. When an electric field is an on state, a liquid crystal stands up. In the case where the electric field is an off state, on the other hand, the liquid crystal is twisted. By making use of this charcteristic, the liquid crystal performs a function as a switch for shielding or transmitting light. Therefore, an image can be formed by carrying out an on/off control of electric field for each pixel.

0050The specific axis orientation of the sapphire substrate is made coincident with an arrangement direction of the pixels and the polarizing direction of the polarized light to be transmitted, whereby it is possible to prevent the sapphire substrate from giving an influence to a polarizing characteristic. Whereby it is possible to realize a high-luminance and small-size liquid crystal projector which is excellent in a heat radiating effect without causing a problem of characteristic deterioration by a heat generation.

0051Moreover, the specific axis and crystal orientation of the sapphire substrate is controlled as described above, and thereby, it is possible to faithfully maintain a polarizing characteristic, and to realize a liquid crystal projector which can project an image.

0052Moreover, the pixels arranging on the sapphire substrate in a matrix, and the vertical and horizontal scanning circuits are formed of SOS (silicon on sapphire) thin film transistors, and thus, an active matrix type liquid crystal display device is constructed, thereby making it possible to increase an operating speed and to reduce a power consumption. In addition, since an ordinary semiconductor process can be employed, mass production is possible and, therefore, it is possible to realize a liquid crystal display device having various effects.

0053In the above embodiments, it is preferable that antireflection coating is subjected onto a surface of the sapphire substrate. Whereby it is possible to improve the transmittance.

0054The above and further objects, features and effects of the invention will becomes apparent from the following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0055<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a view schematically showing a construction of a transmission liquid crystal single-panel type projector;</li><li><figref idref="f0001">FIG. 2</figref> is a view schematically showing a construction of a transmission liquid crystal three-panel type projector;</li><li><figref idref="f0002">FIG. 3</figref> is a view showing a construction of a polarizer used in a liquid crystal projector not belonging to the present invention;</li><li><figref idref="f0002">FIG. 4</figref> is a view showing a construction of a polarizer used in a liquid crystal projector not belonging to the present invention;</li><li><figref idref="f0003">FIG. 5</figref> is a view showing a liquid crystal panel including the polarizer shown in <figref idref="f0002">FIG. 3 or FIG. 4</figref>;</li><li><figref idref="f0003">FIG. 6</figref> is a view showing a liquid crystal panel including the polarizer shown in <figref idref="f0002">FIG. 3 or FIG. 4</figref>;</li><li><figref idref="f0004">FIG. 7</figref> is a view showing a liquid crystal panel not belonging to the present invention;</li><li><figref idref="f0004">FIG. 8</figref> is a view showing a liquid crystal panel according to an embodiment of the present invention;</li><li><figref idref="f0005">FIG. 9</figref> is an exploded perspective view showing a construction of a liquid crystal display device not belonging to the present invention;</li><li><figref idref="f0006">FIG. 10</figref> is a view showing a circuit configuration of the liquid crystal display device not belonging to the present invention;</li><li><figref idref="f0007">FIG. 11</figref> is a cross sectional view schematically showing a unit pixel and a SOSMOS transistor switching section of the liquid crystal display device shown in <figref idref="f0006">FIG. 10</figref>;</li><li><figref idref="f0008">FIG. 12</figref> is a view showing a liquid crystal panel according to still another embodiment of the present invention;</li><li><figref idref="f0008">FIG. 13</figref> is a view showing a liquid crystal panel according to still another embodiment of the present invention;</li><li><figref idref="f0008">FIG. 14</figref> is a view showing still another embodiment of the present invention; and</li><li><figref idref="f0009">FIG. 15</figref> is a view showing a crystal structure of sapphire.</li></ul>

DESCRIPTION OF THE PREFERRRED EMBODIMENT

0056According to one embodiment in the liquid crystal projector having the construction shown in <figref idref="f0001">FIG. 1 or FIG. 2</figref>, at least one of the optical components is formed of a sapphire substrate. The optical components include: dichroic mirrors 10; a filter 3; a lens 4; a retaining plate of a polarizer film (material) in the polarizer 6, 7; and a transparent substrate constituting the liquid crystal panel 8. A sapphire substrate has high heat conductivity, so that a heat generated from the optical components can be effectively radiated.

0057First, the following is a description on an embodiment using a sapphire substrate as the retaining plate of the polarizer 6, 7.

0058As shown in <figref idref="f0002">FIG. 3</figref>, the polarizer 6, 7 is manufactured in a manner of bonding a polarizer film 13 to a surface of a retaining plate 15 comprised of a sapphire substrate by means of a sheet-like transparent bonding agent interposed therebetween. In this case, the polarizer film 13 and the retaining plate 15 are bonded together so that an angle made by a transmission polarizing axis 14 of the polarizer film 13 and an axis 16 is set within a range of ±2°, preferably, ±0.5°. The axis 16 is a C-axis of the sapphire substrate constituting the retaining plate 15 or an axis indicating a C-axis projection line direction or an axis (e.g., M-axis) perpendicular to the C-axis. This serves to prevent a polarized light shaped by the polarizer film 13 from generating an optical rotation resulting from a birefringence or double refraction in a sapphire crystal. In the case where the angle made by the two axes is set more than the above angular range, any influence such as disturbance appears in an image projected from the liquid crystal projector.

0059Moreover, as shown in <figref idref="f0002">FIG. 4</figref>, a surface orientation of a primary plane 15a of the retaining plate 15 comprised of the sapphire substrate is set within a range of C-plane ±2°, preferably, ±0.5°. Whereby a stable image can be obtained without receiving an influence by double refraction.

0060<figref idref="f0003">FIG. 5</figref> shows a state that the polarizers 6 and 7 aligned as shown in <figref idref="f0002">FIG. 3</figref> are set front and rear the liquid crystal panel 8, and light is irradiated thereto. In this case, the incidence side polarizer 6 is arranged separating from the liquid crystal panel 8 with a distance of 1 to 5 mm. On the other hand, the emission side polarizer 7 is directly applied to the liquid crystal panel 8 by means of a sheet-like transparent bonding agent in a state that the polarizer film 13 faces the surface of the liquid crystal panel 8.

0061The polarizers 6 and 7 are constructed in a manner of applying the polarizer films 13 to the retaining plates 15 comprised of sapphire substrates having a high heat conductivity of 42 W/m·K. Thus, a heat accumulated by absorption of light incapable of transmitting through the polarizer film 13, is conducted to the sapphire substrate, so that the heat can be effectively radiated. These polarizers are used in combination with a cooling fan, and thereby, a heat of the polarizers 6 and 7 can be effectively radiated.

0062In order to obtain the best display contrast of the liquid crystal panel 8, in the polarizer 6, it is preferable that the polarizer film 13 applied onto the retaining plate 15 is arranged on the liquid crystal panel 8 side so as to face the panel 8. Moreover, in the polarizer 7, it is preferable that the polarizer film 13 applied onto the retaining plate 15 is arranged on the liquid crystal panel 8 side so as to face the panel 8.

0063The reason is because it is significant that change in a polarizing characteristic of light before and after transmitting through the liquid crystal panel 8 is made as little as possible. Even if the retaining plate 15 made of sapphire is accurately set in its crystal axis orientation and crystal orientation, light transmitting through the retaining plate 15 receives some changes in its polarizing characteristic. Therefore, the polarizer film 13 is arranged on the liquid crystal 8 side, and thereby, it is possible to realize a contrast characteristic of the same level as the case of using no sapphire substrate (e.g., the case of using a glass substrate).

0064As shown in <figref idref="f0003">FIG. 6</figref>, the incidence side polarizer 6 may be applied directly onto the surface of the liquid crystal panel 8. rn this case, the sapphire substrate having preferable heat conductivity is us ed as the retaining plate 15, and thereby, a sufficient heat radiating effect can be obtained. In this structure, a cooling air does not reach the surface of the liquid crystal panel 8 and, therefore, no dust adheres to the surface. A dust may adhere to the surface of the retaining plate 15. However, the surface of tne retaining plate 15 diverges from a focal plane of a projection optical system, so that an image of the dust is not vividly imaged on a projection screen. As a result, by the structure as shown in <figref idref="f0003">FIG. 6</figref>, it is possible to prevent a deterioration of quality of the projected image.

0065Preferably, the sapphire substrate used as the retaining plate 15 is subjected to antireflection coat at its one side or both sides so as to improve its transmitivity. In this case, it is preferable that in the antireflection coat, coating characteristic is made different between a bonding surface with the polarizer film and a surface contacting with an air, so as to meet a refractive index of a contacting layer (polarizer film 13 or air). This further improves the transmitivity.

0066For example, a surface of the sapphire substrate (retaining plate 15) contacting with an air may be subjected to antireflection coat with respect to the refractive index of about 1.0. In this case, it is preferable that the antireflection coating has a refractive index within a range of 1.3 ± 0.15. To give an example, MgF₂ having a refractive index of 1.38 is applicable as the antireflection coat. Moreover, preferably, a bonding surf ace with another member (polarizer film 13) is subjected to antireflection coat with respect to a refractive index of 1.38 to 1.55 so as to meet a transparent adhesive agent which will be described later.

0067It is preferable to provide a frame-like light shielding film on a sapphire substrate used as the polarizer 7 arranged on the emission side of the liquid crystal panel 8. The light shielding film has a window. Preferably, the window has a size which is 0.1 mm or more larger than an effective pixel area of the liquid crystal panel 8, and has a transmittance of 1% or less. Whereby it has been confirmed that it is possible to solve the problem that a contrast of a projected image lowers due to an influence of a scattering light from the surrounding. The light shielding film may be formed by silk screen printing, or may be formed of a chrome-based deposition film.

0068In fact, in the case where the polarizers 6 and 7 are constructed with the use of the sapphire substrate as the retaining plate 15, these polarizers 6 and 7 individually have an improvement of 10 to 15°C or more in their cooling effect as compared with the conventional case of using the blue sheet glass or white sheet glass as the retaining plate.

0069Next, the following is a description on an embodiment of using a sapphire substrate as a transparent substrate constituting the liquid crystal panel 8.

0070The liquid crystal panel 8 has a structure in which a liquid crystal is held between an incidence side transparent substrate and an emission side transparent substrate. Pixel electrodes and switching elements are formed on the incidence side transparent substrate. Facing electrodes are formed on the emission side transparent substrate. Thus, the incidence side and/or emission side substrate is composed of a sapphire substrate. Or, apart from the transparent substrate of the liquid crystal panel 8, a transparent substrate comprised of the sapphire substrate may be provided on the incidence side and/or emission side substrate of the liquid crystal panel 8.

0071<figref idref="f0004">FIG. 7</figref> shows an embodiment of providing transparent substrates 18 comprised of sapphire substrates on the incidence side and emission side of the liquid crystal panel 8 apart from the liquid crystal panel 8. The transparent substrate 18 is bonded to the liquid crystal panel 8 in a state of forming a space of 0.1 mm or less between the outer surface of the transparent substrate of the liquid crystal panel 8 and the transparent substrate 18, and closing the formed space.

0072In the liquid crystal panel 8 used in the liquid crystal projector, no beads is encapsulated in a liquid crystal layer in order to keep the space constant. Because, if the beads were encapsulated in the space, a quality of an enlarged and projected image would be lost. Therefore, in a pair of transparent substrates constituting the liquid crystal panel 8, a mutual interval is kept by only seal layer of an outer peripheral portion surrounding the liquid crystal layer. In the case where the sapphire transparent substrate is mounted to the outer surface of the liquid crystal panel 8 in a state of contacting therewith, the surface of the liquid crystal panel 8 is deformed by a thermal expansion due to a temperature change. For this reason, ununiformity is generated in the liquid crystal layer. As a result, a quality of a projected image is worsened.

0073So, in this embodiment, the transparent substrate 18 are arranged with spaces from the surfaces of the liquid crystal panel 8. On the other hand, in order to effectively conduct an accumulated heat of the liquid crystal panel 8 to the transparent substrates 18, it is preferable to make narrow spaces between the liquid crystal panel 8 and the transparent substrates 18 as much as possible. The space may be set to 0.1 mm or less, and thereby, it is possible to prevent a surface deformation of the liquid crystal panel 8, while securing a heat radiating effect.

0074Moreover, in this embodiment, the transparent substrates 18 have been bonded to the liquid crystal panel 8 with the closed spaces sandwiched therebetween. In place of the spaces, the transparent substrates 18 may be stuck onto the surfaces of the liquid crystal panel 8 with a sheet-like transparent adhesive agent or bonding agent interposed therebetween that has a flexibility. In this case, as shown in the Table 1, the transparent adhesive agent having a Shore hardness of 30 or less may be used, and thereby, it is possible to preferably secure an image quality. Moreover, in the case of taking an influence to the image quality into consideration, if a thickness of the transparent adhesive agent is set to 10 µm or more, there is no problem. An experiment was conducted in order to confirm a cooling performance by heat conduction. As a result, as shown in the Table 2, it can be seen that a preferable cooling effect is obtained if the thickness of the transparent adhesive agent is set to 70 µm or less. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="41mm" /><colspec colnum="2" colname="col2" colwidth="38mm" /><thead><row><entry align="center" valign="top">Adhesive agent hardness</entry><entry align="center" valign="top">Image quality</entry></row><row><entry align="center" valign="top">(Shore hardness)</entry><entry align="center" valign="top">(including bond quality)</entry></row></thead><tbody><row><entry align="center">10</entry><entry align="center">○</entry></row><row><entry align="center">20</entry><entry align="center">○</entry></row><row><entry align="center">30</entry><entry align="center">Δ</entry></row><row><entry align="center">40</entry><entry align="center">×</entry></row><row><entry align="center">50</entry><entry align="center">×</entry></row><row><entry align="center">60</entry><entry align="center">×</entry></row></tbody></tgroup></table></tables><tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="53mm" /><colspec colnum="2" colname="col2" colwidth="55mm" /><colspec colnum="3" colname="col3" colwidth="26mm" /><thead><row><entry align="center" valign="middle">Thickness of adhesive layer (mm)</entry><entry align="center" valign="middle">Image quality (including bond quality)</entry><entry align="center" valign="middle">Cooling effect</entry></row></thead><tbody><row><entry align="center" valign="middle">5</entry><entry align="center" valign="middle">×</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">10</entry><entry align="center" valign="middle">Δ</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">15</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">20</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">30</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">40</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">50</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">60</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">○</entry></row><row><entry align="center" valign="middle">70</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">Δ</entry></row><row><entry align="center" valign="middle">80</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">Δ</entry></row><row><entry align="center" valign="middle">90</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">Δ</entry></row><row><entry align="center" valign="middle">100</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">Δ</entry></row><row><entry align="center" valign="middle">110</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">×</entry></row><row><entry align="center" valign="middle">120</entry><entry align="center" valign="middle">○</entry><entry align="center" valign="middle">×</entry></row></tbody></tgroup></table></tables>

0075In Table 1 and Table 2, a symbolic mark "○" is given in the case where a preferable result is obtained, a symbolic mark "Δ" is given in the case where the result within a allowable limit range is obtained, and a symbolic mark "×" is given in the case where the result not being allowable is obtained.

0076The transparent substrates 18 are arranged on both incidence side and emission side of the liquid crystal panel 8, and thereby, a higher heat radiating effect can be realized. It is possible to obtain a high heat radiating effect even if the transparent substrate is arranged on only one side. Moreover, an antireflection coat may be subjected to one surface or both surfaces of the sapphire substrates constituting the transparent substrates 18, and thereby, it is possible to improve a transmission characteristic.

0077In the sapphire substrates constituting the transparent substrates 18, an angle made by a C-axis of the sapphire substrate or a C-axis projection line or an axis perpendicular to the C-axis is set within a range of ±2°, preferably ±0.5°, with respect to a polarization axis of a polarized light to be transmitted. Whereby it is possible to prevent a deterioration of a projected image quality due to light rotation. In addition, a surface orientation of the sapphire substrate may be set within a range of ±2°, preferably ±0.5°, with respect to the C-surface, and thereby, it is possible to prevent from giving an influence to the projected image quality.

0078Further, in this embodiment, the surface of the liquid crystal panel 8 is covered with heat radiating plates comprised of the sapphire transparent substrates 18. Even if a dust adheres to the transparent substrates 18, no hindrance is generated in an image quality. Because a focal plane of a projection optical system is aligned with the liquid crystal panel 8, and the surface of the transparent substrate 18 diverges from the focal plane. According to the experiment, a distance of 1 mm or more was required between the liquid crystal panel 8 and the surface of the transparent substrate 18 (the surface opposite to the liquid crystal panel 8).

0079In addition, with the use of the retaining plates 15 constituting the polarizers 6 and 7 composed of the sapphire substrate, the following effect was obtained; more specifically, a liquid crystal panel surface temperature lowered about 5°c or more.

0080<figref idref="f0004">FIG. 8</figref> shows an embodiment in which an incidence side transparent substrate 19 and an emission side transparent substrate 20 constituting a transmission type liquid crystal panel 8 are composed of sapphire substrates.

0081In first and second sapphire substrates 19 and 20, the following construction is required; more specifically, an angle made by a polarization axis of a polarized light to be transmitted and an axis is set within a range of ±2°, preferably ±0.5°. The axis is a C-axis of a crystal axis of sapphire substrate or a C-axis projection line or an axis perpendicular to the C-axis. Alternatively, a surface orientation of the sapphire substrate may be set within a range of ±2°, preferably ±0.5°.

0082In this construction, the transparent substrates 19 and 20 constituting a transmission type liquid crystal panel 8 are composed of sapphire substrates having a sufficient heat radiating effect. Thus, there is no need of specifically providing the polarizing plate, because the polarizer film 13 may be bonded directly to each outer surface side of the transparent substrates 19 and 20. By doing so, the transparent substrates 19 and 20 also functions as a retaining plate of the polarizer film 13. Therefore, it is possible to provide a compact structure at a low cost.

0083In this case, an antireflection coat may be subjected to one surface or both surfaces of the sapphire substrate, and thereby, it is possible to further improve a transmission characteristic. However, in general, the transparent adhesive agent for bonding the polarizer film 13 has a refractive index of 1.4 to 1.5, and also, performs a function of partially antireflection. Thus, even if no antireflection coating is subjected in particular, it is possible to realize a preferable transmission characteristic. Of course, antireflection coating may be subjected to the sapphire substrates according to a refractive index of the transparent adhesive agent, and thereby, the transmission characteristic can be further improved.

0084In the structure shown in <figref idref="f0004">FIG. 8</figref>, in the case where a cooling structure combining a cooling fan is employed, it is possible to realize a temperature reduction effect of about 15 to 20°C and a temperature distribution uniformity as compared with the case of forming the same structure as above with the use of quartz substrates. Further, an optical path of an optical system can be shortened about 5%.

0085<figref idref="f0005">FIG. 9</figref> shows a structure of a liquid crystal panel 8 according to another embodiment. In <figref idref="f0005">FIG. 9</figref>, like reference numerals are used to designate portions corresponding to the components shown in <figref idref="f0004">FIG. 8</figref>.

0086The first sapphire substrate 19 is formed with pixels 36, in a matrix pattern on its inside surface, each including a pixel electrode and switching element, and the polarizer film 13 is stuck onto the outer side of the sapphire substrate 19 so as to be aligned with the matrix pattern. Moreover, the second sapphire substrate 20 facing the first sapphire substrate 19 is formed with a transparent facing electrode 37, and another polarizer film 13 is stuck onto the outer side of the sapphire substrate 20. The polarization axis directions of the two polarizer films 13 are parpendicular to each other. A liquid crystal is encapsulated between the first and second sapphire substrates 19 and 20 faced in the aforesaid manner, and thus, the liquid crystal panel is formed.

0087It is assumed that a polarized light 40 shaped into a polarization direction 38 is incident from the outside of first sapphire substrate 19. A liquid crystal vertically stands in a pixel whose electric field is in an on state and, therefore, the polarized light 40 straight advances through the pixel with the polarization direction remain unchanged. The polarized light 40 is therefore absorbed by the emission side polarizer film 13, and then, the pixel becomes a black indication. In a pixel whose electric field is in an off state, since the liquid crystal becomes in a twisted state, the polarized light 40 is rotated by an angle of 90° in its polarization direction, and then, has a polarization direction 39. Thus, the polarized light 40 passes through the emission side polarizer film 13, so that the pixel becomes an operating or lighting state.

0088In the first and second sapphire substrates 19 and 20, one of R-plane, A-plane, M-plane and C-plane is used as a primary plane. A specific crystal axis direction of each of sapphire substrates 19 and 20, is substantially coincident with an arrangement direction of the pixels 36 and a polarization axis direction of polarized light to be transmitted. In this case, the above specific crystal axis direction is an A axis or C axis projection line direction in the case where the primary plane is the R-plane, is a C-axis or M-axis direction in the case where the primary plane is the A-plane, is a C-axis or A-axis direction in the case where the primary plane is the M-plane, and is an A-axis or M-axis direction in the case where the primary plane is an M-plane.

0089More specifically, the specific crystal axis direction of the first sapphire substrate 19 and the arrangement direction of the pixels 36 are set substantially parallel or perpendicular, and an angle made by the specific crystal axis direction of the first sapphire substrates 19 and 20 and the polarization axis direction of polarized light to be transmitted is set within a range of ±2°, preferably ±0.5°. In the first and second sapphire substrates 19 and 20, since a direction of the polarized light to be transmitted is different, these sapphire substrates 19 and 20 are arranged, accordingly.

0090This arrangement ensures that a polarized light shaped by the polarizer film 13 does not generate light rotation resulting from a birefringence in sapphire crystal. For example, in the case where an angle exceeds the aforesaid angular range, an influence such as disturbance is caused in an image projected from the projector.

0091Moreover, in the case where the primary plane of the sapphire substrates 19 and 20 is the R-plane, a silicon film can be formed on the surface by epitaxial growth, and thereby, the silicon film becomes a single crystal (SOS = Si ON SAPPHIRE). Therefore, it is possible to obtain an effect such that a thin-film transistor characteristic using the silicon film is greatly improved. Whereby a display speed can be greatly improved, and also, a power consumption can be reduced.

0092The first and second sapphire substrates 19 and 20 constituting the liquid crystal panel 8 has a sufficient heat radiating property or effect. Thus, there is no need of specially providing a polarizing plate, and the polarizer films 13 may be bonded directly to the outer surfaces of the first and second sapphire substrates 19 and 20. By doing so, these first and second sapphire substrates 19 and 20 can be also used as the retaining plates of the polarizer films 13, thereby making it possible to make compact an optical path, and to provide a low cost structure.

0093In addition, by employing a structure in which the polarizer film 13 is separated from the liquid crystal panel, a more sufficiently cooling effect can be obtained. The first and second sapphire substrates 19 and 20 may be set to a thickness of 2 mm or more, and thereby, it is possible to improve a deterioration of image quality due to a dust adhered to the panel surface by means of a defocus effect.

0094Next, <figref idref="f0006">FIG. 10</figref> shows a circuit configuration of a liquid crystal panel in the case of using the first sapphire substrate 19 whose primary plane is an R-plane. As shown in <figref idref="f0006">FIG. 10</figref>, pixels 41 are arranged like a matrix on the first sapphire substrate 19. Each pixel 41 is provided with a SOSMOS transistor switch 42 which is formed of a single crystal silicon by epitaxial growth on the sapphire substrate 19. Further, the first sapphire substrate 19 is formed with a vertical scanning circuit 43 for driving a gate of the SOSMOS transistor switches 42 , and a horizontal scanning circuit 44 for transferring a video signal to each signal line 45 in order. Also, MOS transistors required for these vertical scanning circuit 43 and horizontal scanning circuit 44 are formed of SOSMOS transistors.

0095<figref idref="f0007">FIG. 11</figref> shows a schematically cross section of the unit pixel 41 and the SOSMOS transistor switch 42 in the liquid crystal display device.

0096A single crystal silicon layer 46 is formed on the primary plane of the first sapphire substrate 1 by epitaxial growth so that a crystal orientation of the primary plane becomes an R-plane, and then, impurities are doped in the formed single crystal silicon layer so as to form a thin film transistor. Further, a gate 48 insulated by a SiO₂ layer 47 is formed on the thin film transistor, and thus, a SOSMOS transistor 42 is formed as a whole.

0097One end of the SOSMOS transistor switch 42 is connected to the signal line 45 to which the video signal from the horizontal scanning circuit 44 is transferred. Another end of the SOSMOS transistor switch 42 is connected to a transparent electrode 33 (pixel 41) formed of an ITO and the like, and thus, a pixel 41 is constituted. Further, a transparent electrode 37 is formed on the sapphire substrate 20 facing the sapphire substrate 19 on which the pixels 41 and SOSMOS transistor switches 42 are formed, and liquid crystal alignment layers 49 are interposed so that a liquid crystal 50 is held between these alignment layers.

0098Then, when the gate of each SOSMOS transistor switch 42 is driven by means of the vertical scanning circuit 43, a video signal flows through the transparent electrode 33 (pixel 41) from the signal line 45, and thus, the pixel 41 is operated.

0099Although not illustrated, MOS transistors used in the horizontal scanning circuit 44 and the vertical scanning circuit 43 are also comprised of SOSMOS transistor switches. More specifically, in the same manner as described above, the MOS transistors are comprised of thin film transistors which are formed of a single crystal silicon formed by epitaxial growth on the sapphire substrate 19.

0100In the liquid crystal display device of this embodiment, the single crystal silicon layer 46 is used for forming thin film transistors. The single crystal silicon layer 46 has almost no defect and, therefore, a leakage current is small, and a power consumption can be reduced. Further, since the single crystal silicon layer 46 has a high current response, it is possible to make high an operating speed, and to prevent a pixel shift in a displayed image. For example, in comparison with a mobility of electron, as shown in Table 3, a single crystal silicon shows a mobility of about three times as much as polysilicon. <tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="37mm" /><colspec colnum="2" colname="col2" colwidth="45mm" /><colspec colnum="3" colname="col3" colwidth="46mm" /><thead><row><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle">Electron mobility (cm²/V.sec)</entry></row></thead><tbody><row><entry align="center" valign="middle">Present Invention</entry><entry align="center" valign="middle">SOS single crystal silicon film</entry><entry align="center" valign="middle">300∼400</entry></row><row><entry align="center" valign="middle">Comparative Example</entry><entry align="center" valign="middle">Polysilicon</entry><entry align="center" valign="middle">40∼150</entry></row></tbody></tgroup></table></tables>

0101In addition, the liquid crystal display device of this embodiment can be readily manufactured by a conventional C-MOS process technology of single crystal silicon because the sapphire substrate 19 is used. Further, because the sapphire substrate is usable in a high temperature process of 1000°C or more, production line of a semiconductor may be employed.

0102The above embodiment has described the process and structure in which a single crystal silicon film is formed, and thus, a liquid crystal panel is manufactured. The sapphire substrate is applicable to a high temperature process of a polycrystal silicon (p-Si) TFT manufacturing process without problems, and therefore, a liquid crystal panel can be manufactured without problems.

0103<figref idref="f0008">FIG. 12</figref> shows an embodiment in the case where the present invention is applied to a reflection type liquid crystal panel 8. In this case, the liquid crystal panel 8 is constructed in a manner that a liquid crystal is encapsulated between a transparent substrate 21 on which reflective electrodes and switching elements are formed, and an incidence/emission side transparent substrate 22 on which a facing electrode is formed. Sapphire substrates are used as these transparent substrates 21 and 22. This construction can also improve a heat radiating effect.

0104Since the sapphire substrate used as the transparent substrate 21 has no influence to a polarized light characteristic, its axis and crystal orientation may be arbitrary. Only reflective surface side of the transparent substrate may be subjected to mirror polishing. A radiation heat sink 23 may be attached directly to the back side of the transparent substrate 21. On the other hand, in the sapphire substrate used as the incidence/emission side transparent substrate 22, since an incident light and a reflected emissive light are different in a polarizing direction, the primary plane is set within C-plane ±2°, preferably, ±0.5° so that no light rotation is generated with respect to a polarized light.

0105Another embodiment of the present invention will be described below.

0106In the case where a high definition image display is required, the projector for enlarging and projecting an image employs the following system regardless of being any of a transmission liquid crystal type, a reflective liquid crystal type and a DLP type. More specifically, as shown in <figref idref="f0001">FIG. 2</figref>, the light source color is separated into three color lights of RGB, and an image corresponding to each color of RGB is displayed, and further, is synthesized by using a prism or the like.

0107In this case, the dichroic mirrors 10, which perform a function separating a color of the light source 1, are formed of sapphire substrates, and then, the surface of each of the sapphire substrate is formed with a film which selects a wavelength of the light source 1, and carries out transmission/reflection.

0108Whereby it is possible to effectively radiate a heat accumulated in dichroic mirrors 10, and thus, to reduce a temperature of the whole system. In the sapphire substrates used as described above, a C axis or the C-axis projection line direction or an axis perpendicular to the C axis is set within ±2°, preferably, ±0.5° with respect to a polarizing transmission axis adjusted after or before transmission. Or the primary plane of the sapphire substrate is set within a C-plane ±2°, preferably, ±0.5°.

0109In still another embodiment, a sapphire substrate is used in an infrared ray cut filter 3 for preventing an infrared ray from the light source 1 of unnecessary heating factor from being incident upon an optical system, and an infrared cut film may be formed on the surface of the sapphire substrate. In this case, since the filter 3 is arranged just after the light source 1 which is cooled by using a cooling fan, it is possible to effectively improve a heat radiating effect by cooling the filter 3 by means of the cooling fan. As a result, a temperature reduction effect of the whole system can be greatly improved.

0110In the sapphire substrate used as described above, a C axis or the C axis projection line direction or a direction perpendicular to the C axis is set within ±2°, preferably, ±0.5° with respect to a polarizing transmission axis adjusted after or before transmission. Or the primary plane of the sapphire substrate is set within a C-plane ±2°, preferably, ±0.5°.

0111A metallic radiation fin may be bonded to the above sapphire substrate. For example, as shown in <figref idref="f0008">FIG. 13</figref>, metallized layers 24 such as of Mo-Mn is formed on outer peripheries of the sapphire substrates used as the retaining plates of the polarizers 6 and 7, and then, metallic radiation fins 25 are brazed thereto.

0112In this case, a heat accumulated in the polarizer film 13 is conducted to the retaining plate 15 made of sapphire, and further, is effectively conducted to the metallic radiation fin 25 having a high heat conductivity. Then, a surface area of the metallic radiation fin 25 is made large so as to improve a heat radiating effect, and the radiation fin 25 may be cooled by a cooling fan, and thereby, a cooling effect can be further improved.

0113According to an experiment, a temperature reduction effect of 5 to 10 °C was improved as compared with the case that only sapphire substrates were used. Whereby it is possible to satisfy a stable operating condition further lower than 70°C which is a characteristic guarantee temperature of the polarizer film 13.

0114In this embodiment, Mo-Mn metallization method has been employed. In the case of manufacturing the metallic radiation fin by employing an active metalizing method, the same effect can be also obtained.

0115A general resin frame (mold) used for positioning an optical system may be replaced by one formed of a material such as aluminum having high heat conductivity, and thereby, a heat is effectively conducted and, as a result, the whole system can have a heat radiating structure.

0116Still another embodiment of the present invention will be described bellow.

0117In the projector for enlarging and projecting an image, as shown in <figref idref="f0008">FIG. 14</figref>, a rod type integrator lens 26 for diffusing light from the light source 1 of high luminance lamp is formed of sapphire.

0118The sapphire is a transparent material having a high refractive index of No = 1.768. Thus, in the case where a quadrangular prism integrator lens 26 is manufactured with the use of sapphire, it is possible to set a total reflection angle smaller. Therefore, it is possible to greatly improve diffusion as compared with the case where the integrator lens is manufactured with the use of an optical glass such as quartz or the like. Further, it is possible to make a design such that a dimension of the integrator lens is made compact.

0119Whereby it is possible to improve a uniformity of image luminance by the liquid crystal projector, and also, to improve a heat radiating effect. An antireflection coat may be applied to end faces of the quadrangular prism, which are an incident face and an emissive face of the integrator lens 26, and thereby, it is possible to improve a transmission efficiency.

0120Next, the following is a description on a manufacturing process of sapphire used in the present invention.

0121A single crystal sapphire is a single crystal of alumina (Al₂O₃), and an aluminum atom and an oxygen atom are arranged so as to form a crystal. As shown in <figref idref="f0009">FIG. 15</figref>, sapphire has a hexagonal system, and its center axis is a C-axis, and a plane vertical to the C-axis is a C-plane (0001). A plane vertical to an A-axis (a1, a2, a3) radially extending from the C-axis is an A-plane (11<o ostyle="single">2</o>0). An R-plane exists in a state of having a predetermined angle (about 32.382°) with respect to the C-axis. These axes and planes can be analyzed by an X-ray diffraction.

0122The single crystal sapphire used in the present invention is manufactured by an EFG (Edge-defined Film-fed Growth) method, for example. More specifically, high purity alumina is melt in an inert atmosphere, and then, a molybdenum die for a single crystal growth of ribbon-like sapphire including a slit in its interior is positioned so as to contact with the alumina molten solution. Then, the alumina molten solution is guided to an upper end portion of the molybdenum die by a capillary action so as to contact with a seed crystal. Subsequently, the seed crystal is upwardly pulled up, and then, sapphire which is a single crystal alumina is grown. When pulling up and growing a substrate raw material, a primary plane of the seed crystal is set as a growth crystal orientation, and its growth axis is set as a pull-up axis, and thus, the seed crystal is pulled up. By pulling up the seed crystal in this manner, it is possible to readily obtain a desired plane and crystal axis orientation of the primary plane, and thus, to accurately grow a single crystal sapphire.

0123The single crystal sapphire growth method is not limited to the EFG method, and other methods such as a Czochralski method may be employed. In the case of obtain an angular-shaped crystal, the EFG method of accurately obtaining a plate-shaped crystal is adaptable because a cylindrical-shaped crystal has a worse efficiency.

0124The single crystal sapphire thus obtained is subjected to grinding by a diamond wheel or the like so as to obtain a desired shape, and thereafter, is subjected lapping with the use of diamond abrasives.

0125Next, the single crystal sapphire and an abrasive cloth are relatively slid while supplying a liquid in which SiO₂ colloidal ball particles having a grain size of 50 nm or less are scattered, as an abrasive slurry, and then, CMP (Chemical Mechanical Polishing) is carried out. This polishing process advantageously removes a fracture layer generated by the aforesaid grinding and lapping without newly causing a strain, so as to obtain a preferable smooth surface which is excellent in a transmission property even in the case of using the single crystal sapphire as a substrate.

0126Next, the following Table 4 shows characteristic values of the result of comparing the sapphire with a silica glass, BK-7 (trade name: made of Hoya glass Co. , Ltd.) and a blue sheet glass (float glass), which are a currently used transparent material. <tables id="tabl0004" num="0004"><table frame="all"><title>Table 4</title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="35mm" /><colspec colnum="2" colname="col2" colwidth="52mm" /><colspec colnum="3" colname="col3" colwidth="23mm" /><colspec colnum="4" colname="col4" colwidth="29mm" /><colspec colnum="5" colname="col5" colwidth="29mm" /><thead><row><entry namest="col1" nameend="col5" align="center" valign="top">Characteristic data</entry></row><row><entry align="center" valign="middle" /><entry align="center" valign="middle">Sapphire</entry><entry align="center" valign="middle">Silica glass (synthesis)</entry><entry align="center" valign="middle">BK7</entry><entry align="center" valign="middle">Blue sheet glass</entry></row></thead><tbody><row><entry valign="middle">Melting point (°C)</entry><entry align="center" valign="middle">2053</entry><entry align="center" valign="middle">1600</entry><entry align="center" valign="middle">870</entry><entry align="center" valign="middle">720</entry></row><row><entry valign="middle">Bending strength (MPa)</entry><entry align="center" valign="middle">690</entry><entry align="center" valign="middle">670</entry><entry align="center" valign="middle">590</entry><entry align="center" valign="middle">643</entry></row><row><entry valign="middle">Young's modulus (MPa)</entry><entry align="center" valign="middle">4.7 × 10⁵</entry><entry align="center" valign="middle">7.3 × 10⁴</entry><entry align="center" valign="middle">7.2 × 10⁴</entry><entry align="center" valign="middle">7.2 × 10⁴</entry></row><row><entry valign="middle">Thermal conductivity (W/m.K)</entry><entry align="center" valign="middle">42.0</entry><entry align="center" valign="middle">1.2</entry><entry align="center" valign="middle">1.0</entry><entry align="center" valign="middle">0.8</entry></row><row><entry valign="middle">Vickers hardness</entry><entry align="center" valign="middle">2300</entry><entry align="center" valign="middle">900</entry><entry align="center" valign="middle">570</entry><entry align="center" valign="middle">548</entry></row><row><entry valign="middle">Refractive index at 589 nm</entry><entry align="center" valign="middle">No = 1.768 Ne = 1.760</entry><entry align="center" valign="middle">Nd = 1.459</entry><entry align="center" valign="middle">Nd = 1.517</entry><entry align="center" valign="middle">Nd = 1.52</entry></row><row><entry valign="middle">Thermal expansion coefficient</entry><entry align="center" valign="middle">C-axis parallel: 5.3 × 10^-6/K C-axis vertical : 4.5 × 10^-6/K (25°C)</entry><entry align="center" valign="middle">0.51 × 10^-6/K (0 - 100°C)</entry><entry align="center" valign="middle">9.8 × 10^-6/K (25 - 350°C)</entry><entry align="center" valign="middle">8.5 × 10^-6/K (25 - 350°C)</entry></row></tbody></tgroup></table></tables>

0127As is apparent from the above Table 4, the sapphire is superior in heat conductivity and is also superior in heat radiating effect. It is also apparent that the sapphire has a high refractive index and, therefore, a high characteristic value can be obtained in the case where the sapphire is used as an integrator lens.

0128Moreover, the sapphire has a high strength and, therefore, it is possible to make a design of making the sapphire thin as compared with other materials, and to make a design of a compact optical system as a whole, In addition, the sapphire is excellent in a heat resistance and, therefore, the sapphire is applicable to a high temperature process of a manufacturing process of a polycrystal silicon (p-Si) TFT constituting a liquid crystal panel without problem.

0129Since the scope of the invention is defined by the appended claims, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.

0130This application claims the convention priority on the basis of Patent Application No. <patcit id="pcit0005" dnum="JP11052061A"><text>11-52061 filed on March 1, 1999 with the Japanese</text></patcit> Patent Office, and Patent Application No. <patcit id="pcit0006" dnum="JP10371720A"><text>10-371720 filed on December 28, 1998 with the Japanese </text></patcit>Patent Office.