Lighting device and projection image display unit

A lighting device, including a first light source emitting light of a first wavelength; a second light source close to the first source, emitting light of a second wavelength in almost a same direction as that of the first source; a third light source located emitting light of a third wavelength in a direction different from that of the first and second sources; a coupling optical system coupling light from the first and second sources; another coupling optical system coupling light from the third source; and a light path synthesizer synthesizing a light path of light from the first, second and third sources, wherein the light path synthesizer includes a first surface reflecting light from the first source and transmitting light from the second and third sources and a second surface unparallel with the first surface, reflecting light from the second source and transmitting light from the third source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light device synthesizing light from plural light sources to form synthesized light and emitting the synthesized light in the same direction, and to a projection image display unit including the light device such as laser scanning projectors.

2. Discussion of the Background

Recently, projection image display units (hereinafter referred to as “projectors”) using solid light sources such as LEDs and lasers are more developed, and they are expected to laptop projectors.

Japanese published unexamined applications Nos. 2006-189573 and 2001-154607 disclose small scanning projectors combined with three primary color lasers and a MEMS (Micro Electric Mechanical System) mirror, which are more developed because of being microminiaturizable.

A conventional scanning projector combined with three primary color lasers and a MEMS mirror is shown inFIG. 26. The projector inFIG. 26includes laser diodes1-R,1-G and1-B emitting laser beams of R (Red), G (Green) and B (Blue), respectively; lenses2-R,2-G and2-B collecting laser beams from the laser diodes1-R,1-G and1-B; dichroic mirrors3-R,3-G and3-B reflecting only red light, green light and blue light, respectively and transmitting other light, respectively; a MEMS (Micro Electric Mechanical System) mirror device501equipped with a mirror having a variable inclined angle; and a control circuit502turning the mirror of the MEMS mirror device501in the horizontal and vertical directions and having the laser diodes1-R,1-G and1-B emit intensity-modulated laser beams.

The control circuit502has a mirror controller and a modulator, and modulates the laser beam intensity to form an image on a screen503.

Such color synthesizing methods need a collection lens for each laser and plural dichroic mirrors for synthesizing a light path for each laser beam, and therefore the number of parts increases, resulting in impairing downsizing and weight saving.

Japanese published unexamined applications Nos. 2007-121899 and 2007-333957 disclose methods of replacing plural dichroic mirrors with a sheet of diffraction element to synthesize a light path from the three light sources.

These methods use a sheet of diffraction element instead of plural dichroic mirrors inFIG. 26, which reduces the number of parts of the devices and downsizes the same.

International publication No. WO2005/073798 A1 discloses a configuration further locating the plural laser light sources on a mount as a package in addition to the above to reduce the number of parts of the devices, downsize the same and save weight thereof.

International publication No. WO2005/073798 A1 discloses a lighting device including three coherent light sources located on a same mount, emitting red, blue and green light; and a diffraction element diffracting the light emitted from the light sources so as to be a coaxial beam to multiplex each of the light.

Japanese published unexamined application No. 2002-207110 discloses a wedge prism synthesizing a light path of light from two light sources located on a mount, and an optical pickup device using the prism.

As mentioned above, the diffraction element can synthesize a light path of light from plural light sources with less number of parts. However, the diffraction element has the following problems.

Since the diffraction efficiency η typically varies according to wavelengths, it is not easy to prepare a diffraction element efficiently affecting plural wavelengths.

For example, a surface relief diffraction optical element is designed with a single wavelength. As shown inFIG. 27, when used for light having a wavelength different from the designed wavelength, the diffraction efficiency η lowers or a flare in imaging optics is caused thereby, resulting in deterioration of image quality.

A lighting device used for projector capable of synthesizing blue, green and red light and emitting the synthesized light is needed. They have a wavelength of from 0.4 to 0.48 μm, 0.5 to 0.55 μm and 0.6 to 0.7 μm, respectively.

Since they have large differences in wavelength, each of the blue, green and red wavelengths are difficult to have a diffraction efficiency η of nearly 100%, and it is difficult to produce only diffracted blue, green and red light having different orders each other.

Therefore, when such a diffraction element is used for a lighting device, the light use efficiency lowers and unnecessary light having no synthesized light path is produced, resulting in flare.

Alternatively, as disclosed in international publication No. WO2005/073798 A1, a volume hologram is thought to diffract only a desired wavelength. However, since the volume hologram has a very narrow scope of allowable wavelength having high diffraction efficiency, the diffraction efficiency lowers when the light sources have accidental wavelength errors, resulting in deterioration of light use efficiency and production of unnecessary flare light.

Particularly, a laser diode light source largely varies in wavelength according to the solid and the environmental variation such as temperature variation.

Thus, it is practically difficult to use the diffraction element for synthesizing a light path because of deterioration of light use efficiency and production of flare.

The wedge prism disclosed in Japanese published unexamined application No. 2002-207110 can synthesize a light path of light from two light sources located on a mount. A lighting device usable for laser scanning projector needs efficiently synthesizing light having different wavelengths from three (red, green and blue) light sources to form synthesized light and emitting the synthesized light to a pickup device.

Red and blue laser diodes are available, but a chip-shaped small light source such as a laser diode is not available as a green light source. Therefore, a green light source is difficult to locate close to other light sources and needs another configuration when used for a pickup device as a lighting device.

Because of these reasons, a need exists for a lighting device efficiently synthesizing light from three light sources and emitting the synthesized light, which is downsizable and improving light use efficiency.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a lighting device efficiently synthesizing light from three light sources and emitting the synthesized light, which is downsizable and improving light use efficiency.

Another object of the present invention is to provide a projection image display unit using the lighting device.

These objects and other objects of the present invention, either individually or collectively, have been satisfied by the discovery of a lighting device, comprising:

a first light source configured to emit light having a first wavelength;

a second light source located close to the first light source and configured to emit light having a second wavelength in almost a same direction as that of the first light source;

a third light source configured to emit light having a third wavelength and located so as to emit the light in a direction different from that of the first and second light sources;

a first coupling optical system configured to couple light from the first and second light sources;

a second coupling optical system configured to couple light from the third light source; and

a light path synthesizing element configured to synthesize a light path of light from the first, second and third light sources,

wherein the light path synthesizing element comprises a first surface and a second surface unparallel with the first surface, and wherein the first surface reflects light from the first light source and transmits light from the second and third light source, and the second surface reflects light from the second light source and transmits light from the third light source.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a lighting device efficiently synthesizing light from three light sources and emitting the synthesized light, which is downsizable and improving light use efficiency. Particularly, the present invention relates to a lighting device, comprising:

a first light source configured to emit light having a first wavelength;

a second light source located close to the first light source and configured to emit light having a second wavelength in almost a same direction as that of the first light source;

a third light source configured to emit light having a third wavelength and located so as to emit the light in a direction different from that of the first and second light sources;

a first coupling optical system configured to couple light from the first and second light sources;

a second coupling optical system configured to couple light from the third light source; and

a light path synthesizing element configured to synthesize a light path of light from the first, second and third light sources,

wherein the light path synthesizing element comprises a first surface and a second surface unparallel with the first surface, and wherein the first surface reflects light from the first light source and transmits light from the second and third light source, and the second surface reflects light from the second light source and transmits light from the third light source.

First, a first embodiment of the lighting device of the present invention will be explained, based onFIGS. 1 to 11.

The light device has a first light source1emitting light having a first wavelength λ1and a second light source2emitting light having a first wavelength λ2mounted on a same mount3adjacent to each other in one package11. The first light source1and the second light source2emit light in almost a same direction.

A laser diode emitting light having a blue wavelength range (hereinafter referred to as a “blue LD”) and a laser diode emitting light having a red wavelength range (hereinafter referred to as a “red LD”) can be used as the first and second light sources, respectively.

The blue wavelength range and red wavelength range have a wavelength of from 400 to 480 nm and 600 to 700 nm, respectively. A LD emitting light having a wavelength λ1of 445 nm and a LD emitting light having a wavelength λ2of 630 nm can be used as the blue LD and red LD, respectively. These wavelengths are used in the following explanation. A blue LD1and a red LD2are mounted on the same mount3in separate chips, but may be formed on one chip.

Blue light4from the blue LD1and red light from the red LD2are coupled by a same first coupling optical system6(hereinafter referred to as a “first CL”) and introduced to a light path synthesizing element7.

Light paths of each color light are schematically shown inFIG. 1. In the lighting device of the present invention, a third light source8emitting light having a third wavelength λ3is located at a position from the package11.

Specifically, the third light source8is located so as to emit light in a direction different from that of the first and second light sources (almost perpendicular thereto).

A green light source emitting light having a wavelength of from 500 to 550 nm can be used as the third light source8. A laser diode is most preferably used as the green light source, but a stably-usable green laser diode is not available at present. Therefore, a solid laser and a double wavelength of an infrared laser diode are used in many cases. A specific configuration of the green light source will be mentioned later, and a light source emitting light having a wavelength of 532 nm is used in this embodiment.

In the optical pickup device disclosed in Japanese published unexamined application No. 2002-207110, the light sources are typically located in the wavelength order (blue<green<red). In this embodiment, the third light source8emitting light having a green wavelength range is not entered into the same surface of the light path synthesizing element7as a same package with the first and second light sources, and entered in a surface different therefrom (a second surface mentioned later). Namely, when the green light source not having the shape of a chip such as a laser diode is combined in the same package of the first and second light sources, the compactness of the package of the first and second light sources is impaired.

In this embodiment, the lighting device which needs efficiently synthesizing light from the three light sources compacts the conventional method inFIG. 26while maintaining the compactness of the package of the first and second light sources. The compactness (of capacity) of the lighting device largely influences the compactness of a laser scanning projector. A practical priority of the compactness of the conventional method is mentioned later.

Green light9from a green light source8is coupled by a second coupling optical system10(hereinafter referred to as a “second CL”) and introduced to the light path synthesizing element7. Red light4, blue light5and the green light9entered into the light path synthesizing element7are synthesized in a same light path and emitted.

Thus, a lighting device emitting light having three red, blue and green wavelength ranges in a same light path is formed.

In the above explanation, the first light source having a blue wavelength range, the second light source having a red wavelength range and the third light source having a green wavelength range are used, but may have different wavelength ranges or different combinations. The light source synthesizing three red, blue and green light and emitting the synthesized light is most useful because it can be used as a light source for a scanning projector.

In addition, as mentioned above, a laser diode emitting green light is not available and the green light source is difficult to locate in a same mount of the other light sources. Therefore, a configuration where the first light source is red or blue, the second light source is blue or red and the third light source is green is easy to prepare.

The first and second coupling optical systems are plane-convex lenses inFIG. 1, and may have other shapes or plural optical elements. Particularly, the first coupling optical system is preferably formed of plural lenses to reduce chroma aberration to the red and blur light. Alternatively, it may have a diffraction structure in addition to conventional refractive lenses.

The function of a light path synthesizing element is explained in detail, referring toFIG. 2. The light path synthesizing element7is a wedge-shaped flat plate formed of a first surface21reflecting light having a blue wavelength range and transmitting light having a red wavelength range and a green wavelength range, and a second surface reflecting light having a red wavelength range and transmitting light having a green wavelength range. The first and second surfaces are relatively inclined at an angle of α.

The light path synthesizing element7is located such that the peak of the wedge points to the light source. “Points to the light source” does not mean “surfaces the light source” but means “approaching the light source”.

The blue light4and red light5enter the first surface, and reflected by the first and second surfaces, respectively, and emitted. The green light9enters the second surface, transmits the second and first surfaces, and emitted. The first and second surfaces are selectively reflecting or transmitting light according to the wavelength. Such a surface is formed of an optical multiple layer and called a dichroic mirror.

FIG. 3is a spectral refractive index of a polarization component s entered into the first and second surfaces at an incident angle of 45°. The first surface is drawn with a full line and the second surface with a dotted line therein. The first surface has a high refractive index to the blue light having a wavelength λ1(445 nm), and a dichroic mirror having a low refractive index to the red light having a wavelength λ2(630 nm) and the green light having a wavelength λ3(532 nm). The second surface has a high refractive index to the red light having a wavelength λ2(630 nm) and a dichroic mirror having a low refractive index to the green light having a wavelength λ3(532 nm).

Specific layer structures of the first surface are shown inFIG. 4Aand those of the second surface are inFIG. 4B. A low-refractive index material silicon dioxide (SiO2) and a high-refractive index material tantalum pentoxide (Ta2O5) are used, besides, materials such as magnesium fluoride titanium oxide which are typically used in an optical multiple layer can be used. The substrate there in means a wedge-shaped flat plate.

The dichroic mirror can make the refractive index close to 100% in a desired wavelength range and a transmission close to 100% in a different wavelength range, and has better light use efficiency than a method of using a diffraction element as a light path synthesizing element.

A difference between luminous points of the blue LD1and red LD2does not conform a light path of the blue light4and red light5coupled by the first CL6. For example, the luminous points of the blue LD1and red LD2have a gap of 110 μm, and when the first CL6has a focal distance f of 2.1 mm, the blue light and red light emitted from the first CL6have aberrational light axes at an angle about 3°.

The light path synthesizing element7can correct the angle disagreement because the second surface is inclined with respect to the first surface at an angle of α. This will be explained with formulae.

When the blue light enters the light path synthesizing element7at an incident angle of θB1, the blue light has an output angle of θB1to a normal line of a first surface21of the light path synthesizing element7.

Since the red light has an output angle of θR5to a normal line of the first surface21of the light path synthesizing element, the light axes of the red and blue light are preferably emitted in parallel when θR5and θB1are the same. Therefore, the best embodiment of the present invention is that a light path synthesizing element has the first and second surfaces at angle of α such that θR5is equal to θB1.

For example, when the light path synthesizing element has a refractive index of 1.5 and θB1is 45°, α is about 0.82°.

Since the green light has an output angle of θG4to a normal line of the first surface21of the light path synthesizing element7, the light axes of the green, red and blue light are preferably emitted in parallel when θg4, θB1and θR5are the same. Therefore, in this embodiment of the present invention, a green light source is located such that green light enters the second surface of the light path synthesizing element at an incident angle of θG1satisfying θG4=θB1=θR5.

For example, when the light path synthesizing element has a refractive index of 1.5, θB1is 45° and a is about 0.82°, the green light source is located such that the incident angle θG1is 43.5°.

Further, in a lighting device emitting red, blue and green light in the same light path, a position between a first CL and a light path synthesizing element and a position between a green laser and the light path synthesizing element are preferably located such that central light axes of each colored light emitted from a first surface of the light path synthesizing element are conformed.

Hereinafter, an embodiment in which the central light axes of the emitted blue light4and red light5are almost conformed is explained.

InFIG. 5, an incident light axis has a position displacement x to conform the central light axes when emitted and the light path synthesizing element7has a thickness d at a position where the red light enters. When an angle α between the first and second surfaces of the light path synthesizing element is small, and the red light5and blue light4enter the light path synthesizing element7at almost a same angle, the red light5has a position displacement x having the following formula at a position where the red light5enters the light path synthesizing element7.
x=2dtan θR3·cos θB(8)

For example, when the light path synthesizing element7has a refractive index of 1.5, α is 0.82°, d is 0.5 mm and θB1is 45°, the position displacement x is 0.365 mm.

The locations of the blue LD1, red LD2, first CL6and the light path synthesizing element7will be explained, usingFIG. 6. When the blue LD1and red LD2have a gap a therebetween and the first CL6has a focal distance f, the first CL6and the light path synthesizing element7have a gap L therebetween having the following formula and satisfying the incident light position displacement x when entering the light path synthesizing element7.
L=f+f·x/a(9)

When the gap a between the blue LD1and red LD2is too small, the heat release efficiency deteriorates. When too large, the first CL deteriorates in coupling efficiency or produces an aberration in coupling. Therefore, they need a suitable gap therebetween.

For example, when the gap a is 0.1 mm and the focal distance is 2 mm, the gap L between the first CL6and the light path synthesizing element7is 9.3 mm.

Thus, the central light axes of the emitted blue light4and red light5are almost conformed.

Thus, the light path synthesizing element synthesizes the light paths of the blue, red and green light.

The above-mentioned light path synthesizing element is a dichroic mirror formed of an optical multiple layer including a first surface and a second surface. Either of them may be a polarization separation surface. The polarization separation surface divides a polarization component s and a polarization component p into reflection and transmission to the entrance surface, respectively. The polarization separation surface can be formed with an optical multiple layer or a wire grid element having a fine lattice structure. For example, inFIG. 2, the red and green light having different polarized light take the light path inFIG. 2even when the second surface is a polarization separation surface.

As shown inFIG. 7, a wedge-shaped flat plate having a inclination reverse to that inFIG. 1can be used as the light path synthesizing element7. Namely, in the formulae (1) to (7), a is a negative value. Then, the light path can be synthesized asFIG. 2is explained.

For example, when the light path synthesizing element7has a refractive index of 1.5, θB1is 45° and α is 0.79°, the green light source is located so as to have an incident angle θG1of 46.5°.

InFIG. 7, the central axes of the emitted blue light4and red light5can almost be conformed as they are inFIGS. 5 and 6. This will be explained, usingFIG. 8. Similarly toFIG. 6, the blue LD1and red LD2have a gap a therebetween, the first CL6has a focal distance f, the first CL6and the light path synthesizing element7have a gap L therebetween.

Similarly toFIG. 5, when an angle α between the first and second surfaces of the light path synthesizing element is small, and the red light5and blue light4enter the light path synthesizing element7at almost a same angle, the red light5has a position displacement x having the formula (8) at a position where the red light5enters the light path synthesizing element7.

For example, when the light path synthesizing element7has a refractive index of 1.5, α is 0.79°, d is 0.5 mm and θB1is 45°, the position displacement x is 0.39 mm.

The first CL6and the light path synthesizing element7have a gap L therebetween having the following formula (10) and satisfying the incident light position displacement x determined by the formula (8).
L=f−f·x/a(10)

For example, when the gap a is 0.5 mm and the focal distance is 10 mm, the gap L between the first CL6and the light path synthesizing element7is 2.2 mm.

Thus, the central light axes of the emitted blue light4and red light5are almost conformed.

InFIG. 8, the blue LD1and red LD2cannot have a small gap a as the formula (10) shows. When the focal distance f is small, L is very small, resulting in difficulty of locating the light path synthesizing element7and the first CL6.

A first CL having a short focal distance f is preferably used to downsize the light source and heighten the light use efficiency. In this respect, the configuration inFIG. 6is more preferably used than that inFIG. 8because the location is easy even with a first CL having a short focal distance f.

InFIG. 1, when the red light5entering the light path synthesizing element7is convergent light or diverging light, an astigmatism is produced when the red light5transmits the wedge-shaped flat plate. In addition, an astigmatism is produced when the red light is located out of the light axis of the first CL.

The lighting device of the present invention preferably has a luminous point at a position with a distance d1from the central light axis of the first collimated optical system (CL) in a direction to reduce the astigmatism caused by the second light source transmitting the wedge-shaped flat plate. An embodiment thereof is shown inFIG. 9. When the first CL6is a flat convex lens, the light path synthesizing element7is a wedge-shaped flat plate inFIG. 2and the red light5entering the light path synthesizing element7is diverging light, the red LD is located so as to have a luminous point above in the drawing relative to a light axis23of the first CL6.

Further, inFIG. 1, when the green light9entering the light path synthesizing element7is convergent light or diverging light, an astigmatism is produced when the green light9transmits the wedge-shaped flat plate as when the red light does. Therefore, as the above-mentioned red light, the luminous point is preferably located at a position with a distance d2from the central light axis of the second collimated optical system (CL) in a direction to reduce the astigmatism caused by the green light transmitting the wedge-shaped flat plate.

An embodiment thereof is shown inFIG. 10. When a second CL10is a flat convex lens, a light path synthesizing element7is a wedge-shaped flat plate inFIG. 2and the green light9entering the light path synthesizing element7is diverging light, the red LD is located so as to have a luminous point left in the drawing relative to a light axis24of the second CL10.

The laser diode typically produces an oval beam, and the shape of an oval possibly deteriorates the performance of a lighting device, depending on the constitution and the application. Therefore, a beam shaping optical system is typically needed to encircle the oval beam.

The lighting device of the present invention, using a laser diode emitting an oval beam as a second light source can improve the shape of a beam without a shaping optical system. This will be explained usingFIG. 11.

InFIG. 11, the red light5having a beam diameter W1enters the light path synthesizing element7in a direction parallel to a paper surface. When emitted from the light path synthesizing element7, the beam diameter W1parallel to the paper surface changes to W2. The beam diameter ratio W2/W1is determined by the following formula (11).
W2/W1=cos(θR2)/cos(θR1)×cos(θR5)/cos(θR4)  (11)
wherein θR2, θR1, θR5and θR4are the same inFIG. 2, and satisfy the formulae (1), (2), (3) and (4). On the other hand, a beam diameter vertical to the paper surface does not change when passing the light path synthesizing element7. Therefore, the light path synthesizing element7formed of a wedge-shaped flat plate can shape a beam. In the light path synthesizing element in which W2/W1>1, the red light source is located such that the oval red light4has a short axis parallel to the paper surface. In the light path synthesizing element in which W2/W1<1, the red light source is located such that the oval red light4has a long axis parallel to the paper surface. For example, W2/W1is 0.97 when θR5is 45°, α is 0.82° and n is 1.5, and therefore the red light source is located such that the oval red light has a long axis parallel to the paper surface.

A second embodiment will be explained, based onFIG. 12or14. The same parts have the same numbers, and only main parts will be explained, omitting explanations already made on the configurations and functions unless particularly necessary (this is same for following other embodiments).

The light path synthesizing element may be formed with two or more optical elements. An embodiment formed of two prisms is shown inFIG. 12as a light path synthesizing element40. InFIG. 12, a first prism41and a second prism42are bonded to each other through an adhesion layer43. An adherend of the first prism is a first surface44and an adherend of the second prism is a second surface45, and the first surface44and the second surface45are bonded to each other at a predetermined angle α on a decline.

In the lighting device inFIG. 1, it is difficult to precisely adjust a gap between the blue LD1and the red LD2, and a positional adjustment error is possibly made. When a wedge-shaped flat plate is used as the light path synthesizing element7as shown inFIG. 1, since the first and second surfaces are fixed, the positional adjustment error becomes a light path error after synthesized.

Meanwhile, when the two prisms shown inFIG. 12are used, the first and second surfaces have separate optical elements and can be adjusted when assembled. Therefore, even when there is a positional error on the locations of the red LD and blue LD, the light path error can be reduced when the light path synthesizing element is assembled.

As shown inFIG. 13, an angle adjuster for adjusting a relative angle of the first surface and second surfaces is preferably present between the two prisms. InFIG. 13, a spacer48is located between the first prism41and the second prism42, and which is moved in a direction indicated by an arrow to adjust the relative angle of the first surface44and second surface45.

In addition, according to a relative positional relation among the first light source, the second light source and the first CL, the location of a spacer50is adjusted and fixed with an adhesive to fix the relative angle of the first and second surfaces.

In the light path synthesizing element inFIG. 12, a first entrance surface46light from the third light source enters and an output surface47the green light9from the third light source emits from are preferably located in parallel.

The first entrance surface46and the output surface47located in parallel can minimize astigmatism produced when the green light9transmits the light path synthesizing element40.

InFIG. 12, a second entrance surface (the second surface45) is more preferably located such that either of the red or the blue light has a minimum astigmatism produced when passing the light path synthesizing element40.

Further, another embodiment of the light path synthesizing element formed of two or more optical elements is shown inFIG. 14. A light path synthesizing element50is formed of laminated first and second wedge-shaped flat plates51and52.

The first and second wedge-shaped flat plates51and52are formed of materials having different refractive index and refractive index wavelength dispersion from each other.

The Laser diode typically varies its wavelength due to an environment such as a temperature, and the lighting device inFIG. 1possibly shifts the light path due to the refractive index wavelength dispersion of a material forming the light path synthesizing element when the red LD varies its oscillation wavelength.

The light path synthesizing element formed of plural materials having different refractive index wavelength dispersion from each other as shown inFIG. 14can reduce the shift of the light path when the red LD varies its oscillation wavelength.

A third embodiment will be explained, based onFIG. 15or17.

A first light source1emitting light having a first wavelength λ1, a second light source2emitting light having a first wavelength λ2and a third light source60emitting light having a first wavelength λ3are mounted on a same mount61as one package62.

As mentioned in the explanation ofFIG. 1, a blue LD having a wavelength of 445 nm, a red LD having a wavelength of 630 nm and a green light source having a wavelength of 532 nm can be used as the first, second and third light sources, respectively. A green LD is most preferably used as a green light source (the third light source60). However, since a green LD is not available at present, as shown inFIG. 16, a green light source8formed with a start-up mirror63and on a same mount64where the blue LD1and red LD2are formed on may be used.

The blue light4from the blue LD1, the red light5from the red LD2and the green light65from the green light source60(equivalent to the light green light9inFIG. 1) are coupled by a same coupling lens (CL)66and introduced to a light path synthesizing element67.

The blue light4, red light5and green light65coupled by the CL66shift their light axes due to displacements of their light sources. For example, the luminous points of the blue LD1and the red LD2have a shift length of 110 μm, and the luminous points of the red LD2and the green light source60further have a shift length of 110 μm. When the CL has a focal distance f of 2.1 mm, the blue light4and red light5shift their light axes at an angle of 3°, and the red light5and green light65further shift their light axes at an angle of 3°.

Light paths of color light having shifted light axes are synthesized by the light path synthesizing element67. This is explained, usingFIG. 17.

The light path synthesizing element67is formed of a first wedge-shaped flat plate68and a second wedge-shaped flat plate69. The light path synthesizing element has a first surface71reflecting light having a blue region, and transmitting light having a red region and light having a green region; a second surface72reflecting light having a red region and transmitting light having a green region; and a third surface73reflecting light having a green region. The first surface71and the second surface72form the first wedge-shaped flat plate68relatively inclined at an angle of α. The second surface72and the third surface73form the second wedge-shaped flat plate69inclined at an angle of β.

Dichroic mirrors can be used for the first and second surfaces as mentioned in the explanation ofFIG. 2. A reflection surface simply formed of a metal, etc. can be used as the third surface.

The light path of each color light is explained.

When the blue light enters the light path synthesizing element67at an incident angle of θB1, the blue light has an output angle of θB1to a normal line of the first surface71of the light path synthesizing element.

When the first wedge-shaped prism68has a refractive index n, the light path of the red light can be shown by the formulae (1), (2), (3) and (4). The red light has an output angle of θR4relative to a normal line of the first surface71of the light path synthesizing element.

The green light has an output angle of θG9relative to a normal line of the first surface71of the light path synthesizing element.

When the first and second wedge-shaped prisms68and69have angles of α and β respectively such that θB1, θG4and θG9are almost same, the light emitted from the light path synthesizing element have an equivalent angle to form the best configuration.

For example, when the first and second wedge-shaped prisms68and69have a refractive index of 1.5 and θB1is 45°, α and β are 0.82° and 0.84°, respectively.

InFIGS. 15 to 17, light from the first, second and third light sources have light axes in a same plane, but which the lighting device of the present invention not limited to. The light axes may not be in a same plane, and normal lines of the first, second and third surfaces may not be in a same plane.

FIG. 18is a fourth embodiment. This is almost equal to that ofFIG. 15, and a birefringent light path synthesizing element80is located instead of the light path synthesizing element67. The birefringent light path synthesizing element80is formed of a birefringent element having different refractive indices according to polarization directions of incident light. In addition, the red LD2and the green light source60are formed such that the output light have mutually-perpendicular polarization directions. Having different refractive indices according to polarization directions of incident light, the birefringent element can reduce a mutual light axes angle error of the red light5and the green light9when the polarization directions thereof are different from each other.

A scanning projector can be configured with the light devices shown inFIGS. 1,7,15and18. As an example, an embodiment of a scanning projector using the lighting device shown inFIG. 1will be explained. A scanning projector inFIG. 19includes a scanner85two-dimensionally scanning the light emitted from the lighting device and a controller86controlling the scanner85and the light source of the lighting device in addition to the light device inFIG. 1.

The controller86adds a modulation to the light source so as to add a desired image in synchronization with the movement of the scanner85. The scanner85can shake a reflection surface in a direction indicated by an arrow, and further in a direction of the paper depth.

This forms a two-dimensional projection image on a screen87. Galvanometer mirrors, polygon mirrors and MEMS mirrors prepared by semiconductor process technologies, etc. can be used as the scanner. Particularly, the MEMS mirrors being very compact and consuming low power is most suitably used for a small projector.

One biaxially-movable mirror is shown inFIG. 19, but two monoaxially-movable mirrors may be used.

The configuration of a MEMS mirror usable as a scanner is shown inFIG. 20. A MEMS mirror90has a structure where a microscopic mirror91having a reflection surface is supported by a torsion bars92and93. The microscopic mirror91is resonantly reciprocated almost on an axis94by the torsion of the torsion bar92. In addition, it is resonantly reciprocated almost on an axis95by the torsion of the torsion bar93. The reciprocations almost on the axes94and95two-dimensionally vary a normal line direction of the deflected surface of the microscopic mirror91.

Therefore, a beam entering the microscopic mirror91changes in its reflection direction to two-dimensionally scan with the beam.

Such a projector modulates the light source intensity in synchronization with movement of the scanner, and a light source endurable to high-speed modulation is needed to display an image natural to the human eye.

An embodiment of a green light source modulatable at high speed is shown inFIG. 21. A LD-excited solid laser is used as a green light source. Light having a wavelength of 808 nm from a LD for exciting is collected by a collecting lens102to a Nd:YA crystal103to be excited. The Nd:YA crystal103emits light having a wavelength of 1064 nm, and which is converted to have a double wavelength of 532 nm in a green wavelength region by a nonlinear optical crystal104.

A filter105blocking light which is not converted to have double wavelength and light of the LD for exciting is located at the light emitting part. These optical elements are contained in one package106to increase stability thereof and form a LD-excited solid laser107.

Further, an output from the solid laser is introduced by a lens108to an acoustooptic element109. The acoustooptic element109is capable of modulating an incident laser beam to have a high speed according to an input signal. The LD-excited solid laser is not ordinarily capable of modulating light to have a high speed. However, with another modulation element such as the acoustooptic element, the D-excited solid laser can be used as a light source for the projector of the present invention.

An electrical optical element can be used as the modulation element as well.

Another embodiment of a green light source is shown inFIG. 22. The green light source inFIG. 22includes a laser diode201emitting light having a wavelength of 1060 nm, a wavelength stabilizing element202stabilizing the output wavelength of the laser diode, a collecting lens203, and a nonlinear optical crystal204converting the light having a wavelength of 1060 nm to light having a double wavelength.

A volume hologram can be used as the wavelength stabilizing element202. Cyclic-polarization-reversed lithium niobate is preferably used as the nonlinear optical crystal204to increase the conversion efficiency to the light having a double wavelength. A filter205blocking the light having a wavelength of 1060 nm and all the elements are contained in one package206.

The configuration having less parts is preferably used because of being capable of directly modulating the laser diode101to have a high speed to modulate the output light having a green wavelength. This laser configuration is disclosed on pages 85 to 89 in Nikkei Micro Device of November, 2007 in detail. (High Efficiency Green Laser Realizes One Hour Operation of Ultracompact Projector: A green laser which can be expected to have increased electrical efficiency by not less than 10% has been developed by Corning Inc. from US. The green laser has a volume less than 1 cm3aiming at a light source of “an ultracompact color projector” for mobile phones. At present, a laser diode directly oscillating green is difficult to realize, and devices converting the wavelength from laser diodes emitting near infrared light to obtain green light are being developed. Corning Inc. expects the efficiency of 15% equivalent to existing many blue laser diodes by this wavelength conversion method. The ultracompact projector can be operated for 1 hr or more with a battery included in a mobile phone.)

The size comparisons between the projector of the present invention and the conventional laser scanning projector inFIG. 26are shown inFIGS. 23 and 24.FIG. 23shows a layout of the projector inFIG. 19, andFIG. 24shows a layout of the projector inFIG. 26.

The projector of the present invention includes a blue light source and a red light source on a same mount, where a green light source emits light almost perpendicular to the blue and red light emitted from the blue and red light sources, and synthesizes a light path using two surfaces of one light path synthesizing element7. The projector is more apparently compact (has smaller capacity) than a conventional method of locating one dichroic mirror for each light source. The optical system inFIG. 23has a capacity about 5 cc, and that inFIG. 24about 7 cc. Obviously, the projector of the present invention is more compact and has less parts than conventional ones, and is smaller by about 30%.

InFIG. 19, an optical system is not present between the scanner and a screen, and may be a projector having a projection optical system (sixth embodiment). The projection optical system can reduce image distortion on the screen.

An embodiment is shown inFIG. 25. In addition to the projector shown inFIG. 19, a projection optical system300reducing image distortion is installed.

The image distortion can be improved by adjusting time of emitting light, but image area becomes small or images become dark, and which can be prevented by an optical control.

A scanner such as MEMS mirrors has a limited scanning angle because of being difficult to have a large deflection angle, but the scanning angle can be enlarged with a projection optical system as shown inFIG. 25.

This application claims priority and contains subject matter related to Japanese Patent Application No. 2008-204661, filed on Aug. 7, 2008, the entire contents of which are hereby incorporated by reference.