Illumination optical system and projection display apparatus

An illumination optical system and a projection display apparatus include a beam combining portion and a condensing lens condensing a beam from the beam combining portion. The beam combining portion combines beams from first and second lamp units each including a lamp and a concave mirror, the lamp having a cathode and an anode arranged along an optical axis of the concave mirror. The beam combining portion includes a first reflective section disposed on an optical axis of the first lamp unit and off the optical axis of the second lamp unit and a second reflective section disposed off the optical axis of the first lamp unit. The first and second reflective sections reflect a beam that is emitted from the second lamp unit and that is off an optical axis thereof to combine the beams from the first and second lamp units.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to illumination optical systems and, in particular, to an illumination optical system that illuminates an illumination target surface using a plurality of lamp units.

2. Description of the Related Art

Some illumination optical systems in recent projection display apparatuses use a plurality of lamp units for illuminating an illumination target surface to illuminate it more brightly and obtain irradiance uniformity. The use of a plurality of lamp units in a projection apparatus implies an increase in size of such an apparatus. Meanwhile, miniaturization of a projection display apparatus itself has become an important factor in the current state of the art. Accordingly, there are approaches to narrowing the width of light emitted from a plurality of lamp units.

The invention described in English abstract of Japanese Patent No. 3,992,053 relates to an illumination optical system that combines beams of two lamp units each containing a light-source lamp and a reflector. A large quantity of light in a central portion including the optical axis of the lamp in a light quantity distribution curve of light is emitted from each of the lamp units. Specifically, beams in the central portion containing the optical axes of the lamps emitted from the two lamp units are extracted using a plurality of reflecting mirrors, and they are combined into combined light having substantially the same width of an aperture edge of one lamp unit.

In the configuration disclosed in English abstract of Japanese Patent No. 3,992,053, a beam corresponding to a bright portion is reflected so as to be present in an outer section of combined light, thus resulting in a large quantity of light in the outer section of the combined light. When the combined light is condensed through a fly's eye lens or a condensing lens to illuminate an image modulation element being an illumination target surface, the ratio of incident light with a large angle of incidence to all light incident on an image modulation element is large. As a result, image contrast deteriorates.

SUMMARY OF THE INVENTION

The present invention provides an illumination optical system and a projection display apparatus that are capable of displaying an image brightly with high contrast by reducing the ratio of incident light with a large angle of incidence to all light incident on an image modulation element.

According to an aspect of the present invention, an illumination optical system illuminates an image modulation element. The illumination optical system includes a beam combining portion and a condensing lens. The beam combining portion combines a beam emitted from a first lamp unit and a beam emitted from a second lamp unit, the first and second lamp units each including a lamp and a concave mirror, the lamp having a cathode and an anode arranged along an optical axis of the concave mirror. The condensing lens condenses a beam from the beam combining portion. The beam combining portion includes a first reflective section disposed on an optical axis of the first lamp unit and off an optical axis of the second lamp unit and a second reflective section disposed off the optical axis of the first lamp unit. The first reflective section and the second reflective section reflect the beam emitted from the second lamp unit and being off the optical axis thereof to combine the beams emitted from the first and second lamp units.

Further features of the present invention will become apparent those of ordinary skill in the art from the following description of exemplary embodiments with reference to the attached drawings.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

A configuration according to a first embodiment of the present invention is described with reference toFIG. 1. A light emitting portion1A has a cathode and an anode. The light emitting portion1A is incorporated in a light emitting tube2A. A parabolic mirror (concave mirror)3A reflects a beam emitted from the light emitting portion1A and guides it in a specific direction. The light emitting portion1A emits a beam in wavelength regions of visible radiation. The cathode and the anode inside the light emitting tube2A are arranged along the optical axis of the parabolic mirror3A. The parabolic mirror3A has the focal length f (0<f). The light emitting portion1A is disposed in the vicinity of the focus of the parabolic mirror3A. The light emitting portion1A, the light emitting tube2A, and the parabolic mirror3A are included in a first lamp unit100A. Similarly, a light emitting portion1B having a cathode and an anode, a light emitting tube2B incorporating the light emitting portion1B, and a parabolic mirror (concave mirror)3B are included in a second lamp unit100B.

A combining mirror (beam combining portion or optical combiner)4combines beams emitted from the first lamp unit100A and the second lamp unit100B. The designation of terms “first” and “second” are used for ease of illustration only, and are not intended to indicate a specific order. Accordingly, for the remaining of the description and claims, these terms may be interchanged or not used.

An optical axis12A of the first lamp unit100A reflected by the combining mirror4and an optical axis12B of the second lamp unit100B passing through the combining mirror4are spaced apart by a distance D. In other words, as illustrated inFIG. 1, the distance D indicates the distance between optical axes of the lamp units seen from a reflected-light side of light from one of the lamp units. The reflected light is reflected by the combining mirror4. Alternatively, the distance D represents the distance between the optical axes of the lamp units seen from a transmission side of light emitted from the other lamp unit. Accordingly, it should be note that for the remainder of the following description one of the optical axes of the lamp units is considered to be reflected by the combining mirror and the other of the optical axis of the lamp units is considered to pass through the combining mirror.

The first lamp unit100A is arranged such that its optical axis12A is at an approximately 45-degree angle with respect to the combining mirror4. The second lamp unit100B is arranged substantially orthogonal to the first lamp unit100A, and is also arranged such that its optical axis12B is at an approximately 45-degree angle with respect to the combining mirror4. The combining mirror4is arranged such that the distance D between the optical axis12A of the first lamp unit100A (reflected by the combining mirror4) and the optical axis12B of the second lamp unit100B (passing through the combining mirror4) satisfies 0<D<4f, where f is the focal length of either one of the parabolic mirrors3A and3B. The reason this condition is described below in greater detail.

Each of a first fly's eye lens5and a second fly's eye lens6are a lens array in which minute spherical lenses are two-dimensionally arranged. A polarization conversion element7converts unpolarized light into substantially linearly polarized light.

The combining mirror4, the first fly's eye lens5, the second fly's eye lens6, the polarization conversion element7, and a condensing lens8are included in an illumination optical system200along an optical axis13.

Combined light output from the combining mirror4is divided into beams and the beams are condensed by the first fly's eye lens5. The beams are condensed in the vicinity of the second fly's eye lens6. The beams condensed in the vicinity of the second fly's eye lens6enter the polarization conversion element7and are converted into respective substantially linearly polarized beams. The beams converted by the polarization conversion element7are condensed by the condensing lens8, pass through a polarizing beam splitter9, and are superimposed by a liquid-crystal display element (image modulation element)10. In the case of full-white displaying, p-polarized light entering the liquid-crystal display element10is modulated into s-polarized light. The s-polarized light is reflected at a polarization separating surface of the polarizing beam splitter9. The reflected light reaches a projection lens (projection optical system)11, and an enlarged image is projected on a projected plane of, for example, a screen.

The above-described illumination optical system performs köhler illumination. The light emitting portions1A and1B and the second fly's eye lens6are disposed at conjugate positions. The first fly's eye lens5and the liquid-crystal display element10are also disposed at conjugate positions.

Next, an illuminance distribution of beams emitted from each lamp unit is described.

An illuminance distribution at an aperture plane of each of the first lamp unit100A and the second lamp unit100B is described with reference toFIGS. 2A,2B, and2C.

FIG. 2Aillustrates beams emitted from the first lamp unit100A or the second lamp unit100B. The description of components having the same reference numerals as in the lamp units illustrated inFIG. 1is omitted. Beams emitted from the light emitting portions1A and1B disposed in the vicinity of the parabolic mirrors3A and3B, respectively, are reflected at the parabolic mirror and are output to the aperture plane of the parabolic mirror as substantially parallel beams. Of light emitted from the light emitting portion, a beam reflected by the parabolic mirror at a position at which the distance to the optical axis of the lamp unit is2fis indicated by the line with the arrow. In other words, it is a beam emitted from the light emitting portion1A or1B in the direction of approximately 90 degrees with respect (orthogonal) to the optical axis of the lamp unit.

FIG. 2Billustrates a gray-scale image of an illuminance distribution at the aperture plane of the lamp unit.FIG. 2Breveals that the illuminance distribution of beams emitted from the lamp unit is annular.

FIG. 2Cillustrates a light quantity distribution (intensity distribution) at a cross section IIC ofFIG. 2B. The cross-section IIC is the same as a plane that passes through the optical axis12A of the first lamp unit100A or the optical axis12B of the second lamp unit100B. The horizontal axis of the graph ofFIG. 2Cindicates the position along the intensity distribution of light emitted from a lamp unit, and the vertical axis indicates brightness in arbitrary units. The brightness of the vertical axis represents the ratio of 1 (highest intensity), at the position of the intensity distribution where the brightest portion is located. This light quantity distribution graph reveals that, in the cross section IIC, the light quantity is significantly low in the vicinity of the optical axis of the lamp unit and is highest in the vicinity of the position spaced away from the optical axis of the lamp unit by the radius2f. Hereinafter, the position where the highest light quantity is concentrated in the light quantity distribution is referred to as a peak. The beam corresponding to that peak is indicated by the line with the arrow illustrated inFIG. 2A. Accordingly, fromFIG. 2C, it can be seen that a beam emitted from a given lamp unit is off the optical axis thereof.

A factor of a small light quantity (low intensity) in the vicinity of the optical axis of the lamp unit is that a hole of the parabolic mirror for use in arranging the light emitting tube2A or2B or the light emitting tube2A or2B itself serves as a light-shielding portion for emitted light.

The light quantity distribution illustrated inFIG. 2Ccan be obtained at positions other than the cross section IIC. For example, a similar light quantity distribution may be obtained at a cross-section on a line that intersects the optical axis of the lamp unit on the aperture plane of the lamp unit.

Next, a shape of the combining mirror4for combining beams emitted from the lamp units is described with reference toFIG. 3.

FIG. 3illustrates the combining mirror4as seen from the III direction indicated by the arrow illustrated inFIG. 1.

The combining mirror4according to the present embodiment is made of a single glass plate and includes a reflective section and a transmissive section. The reflective section is indicated by the dark-gray section, and the transmissive section is indicated by the clear section.

InFIG. 3, line B indicates a line along which a plane that passes through both of the optical axis12A of the first lamp unit100A (FIG. 1) and the optical axis12B of the second lamp unit100B (FIG. 1) intersects the combining mirror4.

The combining mirror4includes a first transmissive section31A, a first reflective section31B, a second transmissive section32A, and a second reflective section32B arranged in this order from a first end to a second end, for this drawing, from the right to left, along the line B. The first reflective section31B is present on the optical axis12A of the first lamp unit100A (FIG. 1), and the second transmissive section32A is present on the optical axis12B of the second lamp unit100B (FIG. 1).

The shape of each of the first reflective section31B, the first transmissive section31A, the second reflective section32B, and the second transmissive section32A of the combining mirror4is not limited to the one illustrated inFIG. 3. The first reflective section31B or the second transmissive section32A may have a substantially semicircle shape corresponding to the annular shape of the illuminance distribution of emitted beams. Alternatively, it is not necessary for the first reflective section31B and the second transmissive section32A to share one side thereof; they may be separated from each other.

Next, operation occurring when the above-described combining mirror4is used in an illumination optical system200to combine light beams emitted from lamps units100A and100B is described with reference toFIG. 4.FIG. 4illustrates the two lamp units100A and100B arranged substantially orthogonal to each other and the combining mirror4arranged along optical axis13of the illumination optical system200. The same reference numerals are used in components previously described, and the description thereof is omitted. For the sake of simplifying the description, the combining mirror4is illustrated inFIG. 4such that its cross-section taken along the line B inFIG. 3is shown. The dark sections of the combining mirror4are the reflective sections, whereas the sections surrounded by the dotted lines are the transmissive sections. As previously described, the combining mirror4(beam combining portion) includes the first transmissive section31A, the first reflective section31B, the second transmissive section32A, and the second reflective section32B arranged in this order from a first end to a second end. Beams41and42indicated by the solid-line arrows correspond to two peaks of the first lamp unit100A, whereas beams43and44indicated by the dotted-line arrows correspond to two peaks of the second lamp unit100B.

Referring the reflective sections of the combining mirror4, it is noted that the first reflective section31B is disposed on the optical axis of the first lamp unit100A and off the optical axis of the second lamp unit100B. The second reflective section32B is off the optical axis of the first lamp unit100A. The first reflective section31B and the second reflective section32B reflect light that is emitted from the second lamp unit100B and that is off the optical axis thereof, and the reflected light is combined with light emitted from the first lamp unit100A. On the other hand, when referring to the transmissive sections of the combining mirror4, the second transmissive section32A is disposed on the optical axis of the second lamp unit100B and off the optical axis of the first lamp unit100A. The first transmissive section31A is off the optical axis of the second lamp unit100B. In other words, the second transmissive section32A and the first transmissive section31A transmit light that is emitted from the first lamp unit100A and that is off the optical axis thereof, and the transmitted light is combined with light emitted from the second lamp unit100B.

The above description is described on the basis of the light quantity distribution of beams emitted from the lamp units illustrated inFIGS. 2A,2B, and2C. Of the peaks of the light quantity distribution of beams emitted from the first lamp unit100A, the first peak beam41and the second peak beam42are arranged in this order from top to bottom in the drawing. Of the peaks of the illuminance distribution of emitted light from the second lamp unit100B, the third peak beam43and the fourth peak beam44are arranged in this order from left to right in the drawing. First, the first transmissive section31A of the combining mirror4allows the first peak beam41of the first lamp unit100A to be transmitted therethrough. The second transmissive section32A on the optical axis of the second lamp unit100B allows the second peak beam42of the first lamp unit100A to be transmitted therethrough. The third peak beam43of the second lamp unit100B is reflected by the second reflective section32B. The fourth peak beam44of the second lamp unit100B is reflected by the first reflective section31B on the optical axis of the first lamp unit100A. That is, as seen from an observation plane A illustrated inFIG. 4, beams from the lamp units are combined such that the first peak beam41, the fourth peak beam44, the second peak beam42, and the third peak beam43are arranged in this order from top to bottom in the drawing. The observation plane A used here indicates a plane that is positioned immediately after an exit surface of the combining mirror4. The observation plane A is substantially perpendicular to a plane containing the optical axis of the first lamp unit100A and the optical axis of the second lamp unit100B (where the optical axis of the second lamp unit100B has been reflected by combining mirror4). Stated in another manner, the observation plane A is substantially parallel to the liquid-crystal display element10.

Each of the first transmissive section31A and the second transmissive section32A of the combining mirror4may be a transparent glass plate or a free-space aperture as long as it has the function of allowing light to be transmitted therethrough.

The above-described operation of the combining mirror4is described in further detail with reference toFIGS. 5A to 5F.

FIG. 5Ashows an image of an illuminance distribution of combined light in the observation plane A (FIG. 4), andFIG. 5Billustrates a light quantity distribution at a cross section taken along the line VB-VB ofFIG. 5A. These drawings reveal that the combining mirror4combines beams such that a bright portion is adjacent to the optical axis13of the illumination optical system200by having the reflective section provided in consideration of the light quantity distribution of beams emitted from the lamp units. The cross section used inFIG. 5Bis obtained by cutting the illuminance distribution along a plane containing the optical axes of the first and second lamp units100A and100B, and the same applies toFIGS. 5D and 5F.

For reference purposes,FIG. 5Cshows an image of an illuminance distribution in the observation plane A after beams pass through the combining mirror4when only the first lamp unit100A (FIG. 4) is used in illumination, andFIG. 5Dillustrates a light quantity distribution at a cross section taken along the line VD-VD ofFIG. 5C. Similarly,FIG. 5Eshows an image of an illuminance distribution in the observation plane A after beams pass through the combining mirror4when only the second lamp unit100B is used in illumination, andFIG. 5Fillustrates a light quantity distribution at a cross section taken along the line VF-VF ofFIG. 5E.

Appropriately arranging the combining mirror4having the shape illustrated inFIG. 3and the lamp units with reference to the combining mirror4enables beams to be combined such that a bright portion of beams that are emitted from the lamp units and that are off the optical axis thereof is adjacent to the optical axis of the illumination optical system. When the bright portion is adjacent to the optical axis of the illumination optical system, in condensing combined light using the condensing lens8(FIG. 1) and illuminating the liquid-crystal display element10(FIG. 1), the proportion of light obliquely incident on the liquid-crystal display element10can be reduced. A reduced proportion of obliquely incident light enables the liquid-crystal display element as the image modulation element to modulate a large quantity of light into an appropriate polarization state, so high image contrast can be achieved.

As described in the beginning, when the concave mirror is a parabolic mirror having the focal length f, it is useful that the distance D between the optical axes of the two lamp units satisfies 0<D<4f. This is because, when 0<D<4f, the beams corresponding to the peaks of the lamp units can be superimposed.

Preferably, the distance D may be set so as to satisfy f≦D≦3f. This enables beams to be combined such that a portion of a large quantity of light from a second lamp unit lies in a portion of a small quantity of light in the vicinity of the optical axis of a first lamp unit, so the liquid-crystal display element or other illumination target surfaces can be uniformly illuminated.

With a system that includes, in addition to the liquid-crystal display element10, the polarizing beam splitter9disposed between the condensing lens8and the liquid-crystal display element10, like the present embodiment, there are more advantageous effects in terms of bright illumination. This is because, if light with a large angle of incidence enters the polarization separating film of the polarizing beam splitter9, which also has incident angle characteristics, light that should be transmitted would be reflected and the quantity of light entering the liquid-crystal display element10would be reduced.

For the present embodiment, a plane containing the optical axes12A and12B of the lamp units and a normal to the polarization separating surface of the polarizing beam splitter9are substantially parallel. However, they may not be in parallel.

For the first embodiment, a fly's eye lens is used as an integrator. However, the present invention is also applicable to an illumination optical system using a rod integrator. In this case, light combined by a combining mirror can be condensed so as to enter the rod integrator. Also in this case, high contrast can be achieved.

Here, quantitative effects are described below.

FIG. 6is a graph that illustrates a relationship between the diagonal length of an image display region of the liquid-crystal display element10and contrast. The image display region is a region for use in actually displaying images in the liquid-crystal display element10. The solid line indicates results for the configuration according to the first embodiment of the present invention; the dotted line indicates results for the configuration according to Japanese Patent No. 3,992,053, which is mentioned above; and the alternate long and short dashed line indicates results for a configuration using one lamp unit. The horizontal axis denotes the diagonal length of the image display region of the liquid-crystal display element10in millimeters, and the vertical axis denotes the contrast. The contrast of the vertical axis is represented as the ratio to 1, where the brightness for a configuration in which the diagonal length of the liquid-crystal display element is approximately 17.8 mm and one lamp unit is used is 1. The brightness is constant for any of the systems, and the results were obtained with an arc length of the lamp of approximately 1.1 mm. The arc length of the lamp indicates the distance between the electrode of the anode and that of the cathode.

It is clear from the graph ofFIG. 6that the contrast according to the present embodiment is higher than the contrasts of the other systems. In particular, when the diagonal length of the liquid-crystal display element10is short, more advantageous effects are obtainable. This is because illuminating the liquid-crystal display element10having a short diagonal length involves an increased proportion of light obliquely incident on the liquid-crystal display element10.

As described above, with the present embodiment, beams can be combined such that a bright portion is adjacent to the optical axis of the illumination optical system, so images can be displayed with high contrast.

For the present embodiment, the first and second lamp units100A and100B are arranged such that a plane containing the optical axes12A and12B thereof and a normal to the polarization separating surface of the polarizing beam splitter9are substantially parallel. However, the present invention is not limited to this configuration. Specifically, it is useful that the polarizing beam splitter9be rotated approximately 90 degrees about the optical axis13of the illumination optical system200. In other words, it is useful that a plane containing a normal to the polarization separating surface of the polarizing beam splitter9and the optical axis13of the illumination optical system200and a plane containing the optical axes12A and12B of the first and second lamp units100A and100B be substantially perpendicular to each other. The same applies to variations described below.

As one example with the parabolic mirror in the first embodiment being modified, a variation in which an elliptic mirror is used as the concave mirror is described with reference toFIG. 7A. The configuration other than a lamp unit using an elliptic mirror is substantially the same as the configuration according to the first embodiment (FIG. 1), so only the lamp unit is illustrated inFIG. 7A. A lamp unit100C includes a light emitting portion1C having a cathode and an anode, a light emitting tube2C incorporating the light emitting portion1C, an elliptic mirror3C, and a concave lens4C. The lamp unit100C has an optical axis12C. Here, the focal lengths of the elliptic mirror3C are f1and f2(0<f1<f2). The light emitting portion1C is disposed in the vicinity of the focuses of the elliptic mirror3C.

With the above-described configuration, beams emitted from the light emitting portion1C in vicinity of the focuses are reflected by the elliptic mirror3C and condensed at the position corresponding to the focal length f2. The condensed beams are output by the concave lens4C as substantially parallel beams. The output beams are combined by the combining mirror4illustrated in the first embodiment such that the peak of the light quantity distribution of each lamp unit is adjacent to the optical axis of the illumination optical system, so high-contrast illumination can be achieved.

As one example using an elliptic mirror as the concave mirror, a variation that uses an elliptic mirror having a long focal length f2without using the concave lens4C can be made.

When the elliptic mirror (concave mirror) has the focal lengths f1and f2(0<f1<f2), where the distance between the optical axes of the two lamp units is D, the lamp units100C and the combining mirror4(FIG. 3) can be arranged such that 0<D<4×f1×f2/(f1+f2) is satisfied. This is because when 0<D<4×f1×f2/(f1+f2) is satisfied the beams corresponding to the peaks of light emitted from the lamp units can be superimposed.

Preferably, the distance D between the optical axes may be set such that f1×f2/(f1+f2)≦D≦3×f1×f2/(f1+f2) is satisfied. This enables a bright portion of light emitted from a second lamp unit to be appropriately combined with a portion of a small light quantity in the vicinity of a first lamp unit, so an illumination target surface can be illuminated more uniformly.

The concave mirror is not limited to a parabolic mirror or an elliptic mirror. Any concave mirror can be used as long as it can guide light emitted from a light emitting tube to a specific direction.

As an example with the combining mirror4in the first embodiment being modified, a variation in which a combining prism includes a reflective section at its joint surface is described with reference toFIG. 7B.

For the reference numerals illustrated inFIG. 7B, only71and72, which are not illustrated inFIG. 1, are described.

A combining prism71is an example of the beam combining portion and includes a joint surface72. The combining mirror4is used for combining beams emitted from the two lamp units100A and100B in the first embodiment. Alternatively, the combining prism71including the joint surface72provided with a reflective section having the same function as in the combining mirror4may also be used, as illustrated inFIG. 7B. With this, images can be displayed with high contrast, as in the case of the first embodiment.

The combining mirror4and the combining prism71for obtaining the advantageous effects of the present embodiment are not limited to the shapes described above. For example, the reflective section can have a substantially semicircular shape corresponding to a light quantity distribution of beams emitted from the lamp units. In this case, the shape is fitted for the illuminance distribution, so brighter illumination can be achieved.

As another variation of the first embodiment, a configuration in which a beam that was not used in combined light is returned to a parabolic mirror using a reflecting member is described with reference toFIG. 7C. For the reference numerals illustrated inFIG. 7C, only73, which is not illustrated inFIG. 1, is described. A reflecting member73is disposed opposite to the second lamp unit100B, and the combining mirror4is arranged between the reflecting member73and the second lamp unit100B. The thick dotted line arrow indicates a partial beam that was emitted from the second lamp unit100B, was not incident on the first or second reflective section of the combining mirror4, and was not used in combined light. The beam that was not incident on the first or second reflective section of the combining mirror4is then reflected by the reflecting member73in substantially the same direction as the direction of the incident beam and is returned to the parabolic mirror3B again, as indicated by a thin dotted line arrow. The beam returned to the parabolic mirror3B is directed in the direction of the focus of the parabolic mirror3B (the direction to the light emitting portion1B), is reflected at a location different from the location at which the beam has been previously reflected, and is directed to the combining mirror4again. The beam directed to the combining mirror4is reflected by the reflective section of the combining mirror4and is used in combined light.

The provision of the above-described reflecting member73enables beams emitted from the lamp units to be effectively used, so the brightness of illumination can further increase.

The position of the reflecting member73is not limited to that illustrated inFIG. 7C. For example, the reflecting member73may have a short length in the vertical direction in the drawing and may be arranged between the second lamp unit100B and the combining mirror4or arranged in contact with the concave mirror. With such configurations, similar advantageous effects are obtainable.

Second Embodiment

FIG. 8illustrates a projection display apparatus using three reflective liquid-crystal display elements for red (R), green (G), and blue (B) as an image modulation element.

InFIG. 8, the lamp units100A and100B and the illumination optical system200are the same as in the first embodiment, so the description thereof is omitted. Only different portions are described here.

Beams emitted from the light emitting portions1A and1B enter the illumination optical system200, and their p-polarized light enters a dichroic mirror801. The dichroic mirror801reflects light corresponding to blue (B) and red (R) and allows light corresponding to green (G) to be transmitted therethrough. A first polarizing beam splitter802allows p-polarized light to be transmitted therethrough and reflects s-polarized light and has a polarization separating surface.

Reflective liquid-crystal display elements803R,803G, and803B reflect incident beams, perform image modulation, and correspond to red, green, and blue, respectively. Quarter-wave plates804R,804G, and804B correspond to red, green, and blue, respectively. An exit side polarizer805allows s-polarized light of green light to be transmitted therethrough. An incident side polarizer806allows p-polarized light to be transmitted therethrough. A color selective retardation plate807rotates the direction of polarization of red light by approximately 90 degrees and does not rotate the direction of polarization of blue light. A second polarizing beam splitter808allows p-polarized light to be transmitted therethrough and reflects s-polarized light and has a polarization separating surface.

An exit side polarizer809deals with blue light and allows only s-polarized light of blue light to be transmitted therethrough; it allows red light to be transmitted therethrough irrespective of its polarization direction. A combining prism810has characteristics of the function of a dichroic mirror for blue and green light and the function of a polarizing beam splitter that allows p-polarized light of red light to be transmitted therethrough and reflects s-polarized light thereof. The components from the dichroic mirror801to the combining prism810described above are included in a color separation and combination optical system300. A projection lens811serves as a projection optical system.

The projection display apparatus according to the second embodiment includes the lamp units100A and100B, the above-described illumination optical system200, the color separation and combination optical system300, and a projection optical system400.

Combining beams from a plurality of lamp units using the combining mirror4in this projection display apparatus enables peaks of the lamp units of light quantity distributions to be concentrated in the vicinity of the optical axis of the illumination optical system. Therefore, image can be displayed with high contrast.

The second embodiment describes an example that uses three reflective liquid-crystal display elements. However, the number of reflective liquid-crystal display elements is not limited to three; it may be two or four, for example.

For the present embodiment, a plane that contains the optical axes12A and12B of the first and second lamp units and a color separation and combination surface (surface that contains light before being separated and light after being separated) of the color separation and combination optical system300are the same (or parallel). However, other configurations may be used. Specifically, it is useful that the plane that contains the optical axes12A and12B of the first and second lamp units and the color separation and combination surface of the color separation and combination optical system300may be substantially perpendicular to each other (the color separation and combination surface is substantially perpendicular to the sheet surface ofFIG. 8). In other words, a plane that contains the color separation surface of the dichroic mirror801, the polarization separating surface of each of the first and second polarizing beam splitters802and808, a normal to the color combining surface of the combining prism810, and the optical axis13of the illumination optical system200may be substantially perpendicular to a plane that contains the optical axes12A and12B of the first and second lamp units. It is useful that a plane containing a normal to the polarization separating surface of at least one of the first and second polarizing beam splitters802and808and the optical axis13of the illumination optical system200be substantially perpendicular to a plane that contains the optical axes12A and12B of the first and second lamp units. In further other words, a plane that contains the optical axis of each of the lamp units may be substantially perpendicular to a normal to at least one liquid-crystal display element.

A reflective liquid-crystal display element is used as an image modulation element. However, the present invention is not limited to a reflective liquid-crystal display element. In the drawings, for the sake of simplifying the description, the fundamental configuration of the projection display apparatus is illustrated; it may include other components, such as an infrared-ray cut filter and a polarizer.

Third Embodiment

The third embodiment is an example of a projection display apparatus that uses three micro-mirror devices (image modulation element) and is described with reference toFIG. 9.

The description of substantially the same configuration as in the first embodiment is omitted, and only different portions are described.

The lamp units100A and100B and the illumination optical system200are substantially the same as in the first embodiment.

Light combined by the combining mirror4from beams emitted from the lamp units100A and100B passes through the first fly's eye lens5, the second fly's eye lens6, and the condensing lens8. Light output from the condensing lens8is fully reflected by a prism91and is guided to a color separation and combination prism93. The color separation and combination prism93separates color light and guides separated light components to micro-mirror devices94R,94G, and94B.

In white displaying, color light components reflected by the micro-mirror devices94R,94G, and94B pass through the color separation and combination prism93for combining beams again. The combined light passes through a prism92and is directed to a projection lens (projection optical system)95. White displaying used here indicates a state in which a projection display apparatus displays a white image on a projection target surface.

In black displaying, minute mirrors in each of the micro-mirror devices94R,94G, and94B are tilted, so light having entering the micro-mirror devices94R,94G, and94B through the illumination optical system200and the prism91is returned to the prism91and the illumination optical system200without entering the projection lens95. Here, the direction in which beams entering the micro-mirror devices94R,94G, and94B in black displaying is guided is not limited to the direction toward the illumination optical system200(toward the light source); it may be any direction that differs from the direction toward the projection lens95.

Each of the micro-mirror devices changes the direction of reflection of incident light by changing the inclination of its minute mirrors to adjust brightness of pixels. Accordingly, if incident light is not incident at a proper angle, the light would not be reflected in a desired direction and contrast would degrade. The use of the beam combining portion (combining mirror4) according to the present embodiment can reduce the proportion of obliquely incident light, so images can be displayed brightly with high contrast.

The present embodiment describes an example that uses three micro-mirror devices as an image modulation element. However, the number of micro-mirror devices may be one or two, for example.

In the foregoing, the embodiments of the present invention have been described. In addition to the above-described advantageous effects, with the present invention, an advantageous effect of illuminating an illumination target surface more brightly is obtainable. The technical details are described with reference toFIGS. 10,11A, and11B.

FIG. 10illustrates a basic configuration from a lamp unit to an image modulation element of a projection display apparatus. For the sake of simplifying the description, only the basic configuration is illustrated.

The projection display apparatus includes a lamp unit102including a lamp101and a parabolic mirror, a first fly's eye lens103, a second fly's eye lens104, a polarization conversion element105, a condensing lens106having the focal length F, and a liquid-crystal display element107as an image modulation element.

FIG. 11Ais an enlarged schematic view of the second fly's eye lens104and the polarization conversion element105illustrated inFIG. 10.FIG. 11Bis an enlarged schematic view of a part that contains a minute lens of the second fly's eye lens104illustrated inFIG. 11A.

Substantially parallel beams emitted from the lamp unit102illustrated inFIG. 10are divided and collimated by the first fly's eye lens (in which minute spherical lenses are two-dimensionally arranged)103. The divided beams are condensed in the vicinity of the second fly's eye lens104and form an image of the light source (secondary light source image). Each of the minute lenses included in the fly's eye lenses103and104has a substantially rectangular shape that is similar to the liquid-crystal display element being an illumination target surface (FIG. 11B). The divided beams output from the second fly's eye lens104are converted by the polarization conversion element105into substantially linearly polarized light. As illustrated inFIG. 11A, the polarization conversion element105includes polarization separating films arranged in rows. The divided beams converted into substantially linearly polarized light by the polarization conversion element105are condensed by the condensing lens106and superimposed on the liquid-crystal display element107. The liquid-crystal display element107is typically substantially rectangular and is typically used such that it is horizontally oriented on a screen. In the specification, hereinafter, the direction along a long side of the image display region of the liquid-crystal display element (horizontal on the screen) is referred to as the long direction, and the direction along a short side thereof (vertical on the screen) is referred to as the short direction.

Here, the following relationship is approximately satisfied:
B=a×(F/d)  (1)
where a is the width of a minute lens included in the second fly's eye lens104, d is the distance between the first fly's eye lens103and the second fly's eye lens104, F is the focal length of the condensing lens106, and B is the width of the liquid-crystal display element107in the short direction, as illustrated inFIGS. 10,11A, and11B.

An incident angle θ of light on the liquid-crystal display element107is represented by the following expression:
θ=tan−1(A/2F)  (2)
where A is the width of the aperture of the lamp unit102.

In the system illustrated inFIG. 10, brightness of illumination largely depends on the size of an image of a light source formed in the vicinity of the second fly's eye lens104(secondary light source image), or alternatively, on the width a of the minute lens of the second fly's eye lens104and the distance between an effective section and an ineffective section arranged in rows (FIG. 11A) of the polarization conversion element105.

A secondary light source image in the vicinity of the second fly's eye lens104is formed by the first fly's eye lens103condensing substantially parallel beams emitted from the lamp unit102. Because of this, the size of the secondary light source image is substantially proportional to the focal length of the first fly's eye lens103or the distance d between the first fly's eye lens103and the second fly's eye lens104.

Because a secondary light source image is formed by condensed beams emitted from the lamp in the lamp unit, when the arc length of the lamp in the lamp unit102is L, the size of the secondary light source image is also substantially proportional to the arc length L of the lamp in the lamp unit102.

Accordingly, the size of the secondary light source image can be represented by the following expression using the arc length L of the lamp and the distance d between the first fly's eye lens and the second fly's eye lens:
<Size of Secondary Light Source Image>=∝L×d(3)

The liquid-crystal display element107is illuminated with only light passing through a corresponding minute lens of the second fly's eye lens104and effective section of the polarization conversion element105out of light forming the secondary light source image. Therefore, as illustrated inFIG. 11B, brightness increases with a reduction in the size of a secondary light source image with respect to the width of a minute lens of the second fly's eye lens104and the width of one row of the polarization separating films arranged in rows of the polarization conversion element105. In contrast, if the size of the secondary light source image is large, it extends off the region of the minute lens of the second fly's eye lens104, or alternatively, it is subjected to vignetting by the ineffective section of the polarization conversion element105, so brightness decreases.

Accordingly, the value obtained by dividing the width of a minute lens of the second fly's eye lens by the size of a secondary light source image can be a variable that represents brightness of illumination. The following relationship is satisfied:
γ∝a/(L×d)  (4)
where γ is the value obtained by dividing the width of a minute lens of the second fly's eye lens by the size of a secondary light source image. That is, when γ is large, brightness of illumination increases, whereas, in contrast, when γ is small, it decreases.

From the expressions (1) and (2), the following relationship is satisfied:
γ∝(2B×tan θ)/A×L(5)

Because the light quantity distribution of beams emitted from the lamp unit102is not uniform, the expression (5) and brightness of illumination are not proportional. However, the expression (5) reveals that, when the width A of the aperture of the lamp unit and the arc length L of the lamp are large or the width B of the liquid-crystal display element is small, brightness decreases.

In other words, in order to maintain the contrast above a certain degree without changing the width B of the liquid-crystal display element, the width A of the aperture of the lamp unit, and the arc length L of the lamp, it is necessary to reduce the incident angle θ on the liquid-crystal display element107. If the value of θ is reduced, the value of γ is inevitably reduced because of the expression (5), so brightness decreases.

Next, quantitative effects relating to brightness according to the first embodiment are described below.

FIG. 12is a graph that illustrates a relationship between the diagonal length of an image display region of the liquid-crystal display element10and brightness. The solid line indicates results for the configuration according to the first embodiment of the present invention; the dotted line indicates results for the configuration according to Japanese Patent No. 3,992,053, which is mentioned above; and the alternate long and short dashed line indicates results for a configuration using one lamp unit. The horizontal axis denotes the diagonal length of the image display region of the liquid-crystal display element10in millimeters, and the vertical axis denotes the brightness. The brightness of the vertical axis is represented as the ratio to 1, where the brightness for a configuration in which one lamp unit is used and the diagonal length of the liquid-crystal display element is approximately 17.8 mm is 1. The contrast is constant for any of the systems, and the results were obtained with an arc length of the lamp of approximately 1.1 mm.

The results illustrated inFIG. 12reveal that the brightness of the present embodiment on a screen is higher than other systems. Preferably, 0<P/L<23 may be satisfied, where L is the arc length of the lamp of the lamp unit along the optical axis and P is the diagonal length of the image display region of the liquid-crystal display element. This is because, with other systems, if the length of the light source is long or the diagonal length P of the image display region of the liquid-crystal display element is short, the quantity of light that does not enter the lens array or light that is subjected to vignetting by the ineffective section of the polarization conversion element would be increased.

With the present embodiment, a beam combining portion that combines beams emitted from a plurality of lamp units such that their bright portions are adjacent to the optical axis of an illumination optical system can be used. Accordingly, an illumination optical system and a projection display apparatus capable of brightly displaying a high-contrast image can be provided.

This application claims the benefit of Japanese Patent Application No. 2009-191191 filed Aug. 20, 2009, which is hereby incorporated by reference herein in its entirety.