Image projection device, image projection method, and image display device

An image projection device includes beam generation means that emits a plurality of beams in one direction such that the plurality of beams are superimposed each other, a convergence angle of each beam being different from each other; and scanning means that scans projection surface with the beams emitted from beam generation means.

TECHNICAL FIELD

The present invention relates to image projection device that scans a projection surface by means of a light beam such as a Gaussian beam (for example, a laser beam) to display an image.

BACKGROUND ART

Patent Document 1 describes an image display device that displays an image by scanning the projection surface by a light beam that is modulated according to a video signal. This image display device includes condensing optics that condense the light beam from a light source and a scanning unit provided with a reflecting mirror that reflects the light beam that has been condensed by these condensing optics toward the projection surface.

The condensing optics form a beam waist at a position farther from the reflecting mirror than the midpoint between the reflecting mirror and the projection surface. In this way, the beam diameter on the reflecting mirror of the scanning unit can be reduced, and moreover, enlargement of the beam diameter on the projection surface can be suppressed, thereby enabling a more compact reflecting mirror and higher-definition image display.

LITERATURE OF THE PRIOR ART

Patent Documents

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-121538

SUMMARY OF THE INVENTION

When the projection surface is scanned by a laser beam to display an image, noise occurs in the form of spots referred to as speckle that originate from the coherence of the laser beam. Speckle is a distraction when viewing a displayed image and detracts from image quality.

In the image display device described in Patent Document 1, a beam waist is formed between the reflecting mirror of the scanning unit and the projection surface at a position that is farther from the reflecting mirror than the midpoint. In this configuration as well, the above-described speckle also occurs, resulting in a decrease of image quality.

When the beam waist is arranged in the neighborhood of the center position, since the beam diameter on the projection surface becomes large, it is difficult to obtain highly precise images.

An object of the present invention is to provide an image projection device, an image projection method, and an image display device that can solve the foregoing problem and reduce speckles without sacrificing resolution of images.

To accomplish the foregoing object, an image projection device according to one aspect of the present invention comprises beam generation means that emits a plurality of beams in one direction such that the plurality of beams are superimposed on each other, a convergence angle of each beam being different from each other; and scanning means that scans a projection surface with the beams emitted from the beam, generation means.

An image projection method according to one aspect of the present invention comprises combining, to form a composite beam, a plurality of beams such that an energy density on a cross plane of the plurality of beams becomes the highest, a convergence angle of each beam being different from each other and the cross plane being perpendicular to an optical axis; and scanning a projection surface with the composite beam.

An image projection method according to another aspect of the present invention, comprises:splitting an incident beam into a plurality of beams;causing a plurality of lenses to condense the plurality of beams, a focal distance of each lens being different from each other;combining individual beams condensed by the plurality of lenses to form a composite beam and scanning a projection surface with the composite beam,wherein a plurality of shutters arranged on individual optical paths of the plurality of beams are successively released for every constant period.

An image projection method according to another aspect of the present invention, comprises:causing a spatial optical modulator that generates image light corresponding to an input drive signal to spatially modulate an incident beam; andscanning a projection surface with the beam obtained by the modulation,wherein a plurality of drive signals that cause image light corresponding to a plurality of Fresnel zone plates s are supplied to the spatial optical modulator for every constant period, a focal distance of each Fresnel zone plate being different from each other.

An image display device according to one aspect of the present invention comprises:a projection surface;light condensing means that condenses an incident beam; andscanning means that scans the projection surface with the beam that passes through the light condensing means,wherein the projection surface is arranged at a position closer to the scanning means than at a position where a beam waist of the beam that passes through the light condensing means is present and in a range defined by a Rayleigh length on a convergence side of the beam that passes through the light condensing means.

DESCRIPTION OF REFERENCE NUMERALS

1Beam generation means

Best Modes That Carry Out The Invention

The inventors of the present application analyzed causes of the occurrence of speckle and gained the new information described hereinbelow.

FIG. 1Ais a schematic view showing the state of a synthesized wavefront that is produced when parallel light is irradiated upon a scattering object. When parallel light100reaches scattering object101, secondary spherical waves are produced at scattering object101. The directions of the wave vectors that are prescribed by the synthesized wavefront of these secondary spherical waves and the wavefront of parallel light100become divergent directions, and the dispersion of wave vectors becomes great. Speckle increases with increase in the dispersion of wave vectors.

FIG. 1Bis a schematic view showing the state of the synthesized wavefront that is produced when divergent light is irradiated upon a scattering object. When divergent light110reaches scattering object111, secondary spherical waves are produced at scattering object111. The directions of wave vectors prescribed by the synthesized wavefront of these secondary spherical waves and the wavefront of divergent light110become directions that diverge (disperse) even more greatly than the case shown inFIG. 1A. The dispersion of wave vectors in this case is even greater than the case shown inFIG. 1A, and speckle is consequently even greater.

FIG. 1Cis a schematic view showing the state of the synthesized wavefront that is produced when convergent light is irradiated upon a scattering object. When convergent light120reaches scattering object121, secondary spherical waves are produced at scattering object121. The wave vectors that are prescribed by the synthesized wavefront of these secondary spherical waves and the wavefront of convergent light120are substantially parallel. The dispersion of wave vectors in this case is less than the cases shown inFIG. 1AandFIG. 1B, and as a result, there is less speckle.

As can be seen fromFIGS. 1A-1C, the projection surface (scattering object) is preferably scanned by convergent light.

However, in an optical scanning system in which laser light is condensed and the projection surface then scanned by the condensed beam, cases occur in which, depending on the distance from the system to the projection surface, the beam that is projected upon the projection surface does not become convergent light.

FIG. 2Ais a schematic view of the wavefront when the projection surface is scanned by a condensed beam.

In the system shown inFIG. 2A, laser light from laser light source200is condensed by condenser lens201. Scanning means202scans projection surface203by the beam from condenser lens201. The beam emitted from condenser lens201is propagated as convergent spherical waves205and then changes in state from convergent spherical waves205to divergent spherical waves206with the position at which beam waist204forms as the boundary.

When projection surface203is located toward the side of condenser lens201from beam waist204, projection surface203is scanned by a beam of convergent spherical waves205. In this case, projection surface203is scanned by convergent light, whereby speckle can be reduced.

In contrast, when projection surface203is positioned farther from condenser lens201than beam waist204, projection surface203is scanned by a beam of divergent spherical waves206. In this case, projection surface203is scanned by divergent light, and speckle therefore becomes difficult to reduce.

FIG. 2Bshows the change in speckle contrast (%) when a projection surface is moved back and forth with the position of the beam waist as the reference. Speckle contrast indicates the degree of speckle, and more specifically, is a value for a speckle image that is obtained by dividing the standard deviation value of the value of a picture element by the average value of each picture element. InFIG. 2B, the horizontal axis indicates the distance (mm) from the beam waist. The position of the beam waist is assumed to be “0” with the side of condenser lens201being indicated by minus and the opposite side being indicated by plus. In addition, the change of speckle contrast is shown by a broken line (short), a solid line, and a broken line (long) for cases in which the focal length of condenser lens201is 300 mm, 400 mm, and 500 mm, respectively.

As can be seen fromFIG. 2B, when the projection surface is positioned on the side of the condenser lens from the position of the beam waist, the speckle contrast is reduced. In this case, the speckle contrast decreases with increasing proximity of the projection surface to the condenser lens. The speckle contrast decreases as the focal length decreases.

On the other hand, speckle contrast increases when the projection surface is positioned on the side farther from the condenser lens than the position of the beam waist. In this case, speckle contrast increases as the projection surface becomes increasingly remote from the position of the beam waist. Speckle contrast increases with decreasing focal length.

Accordingly, in an optical scanning system in which a projection surface is scanned by laser light (Gaussian beam), the projection surface must be positioned closer to the condenser lens than the position of the beam waist in order to decrease speckle contrast.

In addition, when the beam diameter on the projection surface is large, resolution decreases. Accordingly, a projection surface is preferably positioned in the vicinity of the beam waist in order to limit a decrease of resolution. More specifically, taking the beam waist as a reference, the projection surface is preferably positioned within a range prescribed by the Rayleigh length from the reference (a range of distances in which the beam diameter is a multiple of √2).

From the foregoing, to obtain a speckle reduction effect, a projection surface (screen) needs to be scanned with convergent light, and also to obtain highly precise images, beam diameters need to be suppressed from widening.

Next, as exemplary embodiments of the present invention, image projection devices (of front projection type) and image display devices (of real projection type) that allow a projection surface (screen) to be scanned with convergent light and that suppress beam diameters from widening will be described.

FIG. 3is a schematic diagram showing a structure of principal sections of an image projection device according to a first exemplary embodiment of the present invention.

Referring toFIG. 3, the image projection device has beam generation means1that emits a plurality of beams having different convergence angles in one direction such that the plurality of beams are superimposed each other; and scanning means3that scans projection surface4with the beams emitted from beam generation means1.

Beam generation means1has splitting means11, reflection mirrors12,13, lenses14,15, and compositing means16. Splitting means11splits incident laser light (a beam modulated corresponding to a video signal) into two beams. As splitting means11, a beam splitter or a half mirror can be used.

Reflection mirror12is arranged in the traveling direction of a first beam (transmission light) that is transmitted through splitting means11; and lens14is arranged in the traveling direction of the first beam reflected by reflection mirror12.

Reflection mirror13is arranged in the traveling direction of a second beam (reflection light) reflected by splitting means11; and lens15is arranged in the traveling direction of the second beam reflected by reflection mirror13.

The focal distance of lens14is different from the focal distance of lens15. According to this embodiment, the focal distance of lens14is 400 mm, whereas the focal distance of lens15is 300 mm. The focal distances of lenses14and15are not limited to these value, but other values.

Compositing means16is for example a beam splitter or a half mirror. Compositing means16is arranged at the position where the first beam that passes through lens14intersects with the second beam that passes through lens15and compositing means16reflects part of the first beam and transmits part of the second beam. The traveling direction of the first beam that passes through compositing means16coincides with the traveling direction of the second beam that passes through compositing means16and the axis that passes through the center of the cross section of the first beam coincides with the axis that passes through the center of the cross section of the second beam.

Scanning means3is arranged in the traveling directions of the first and second beams that pass through compositing means16. Scanning means3is arranged at a position closer to compositing means16than at positions of their beam waists.

FIG. 4schematically shows states of wavefronts and energy densities of the first and second beams from lenses14,15. For simplicity,FIG. 4shows that the optical axes of lenses14,15are in parallel with each other and lie on one surface and that the states of the first and second beams are viewed from a direction perpendicular to the surface.

InFIG. 4, the scanning means is omitted. The scanning means can be arranged anywhere between the beam generation means and the projection surface. For example, the scanning means can be arranged on the projection surface side at a position 10 mm apart from the beam generation means.

The convergence angle of the first beam is smaller than the convergence angle of the second beam. InFIG. 4, the states of wavefronts and energy densities of the first and second beams are based on position P0represented by a straight line that connects the centers of lenses14,15. In range A from position P0to position P1, which is a focal distance (300 min) of lens15(where a beam waist of the second beam is present), the first and second beams are convergent light (convergent spherical wave beams). In range B from position P1to position P2, which is a focal distance (400 mm) of lens14(where a beam waist of the second beam is present), the second beam is diverged light (diverged spherical wave beam) and the first beam is convergent light (convergent spherical wave beam).

At position P1, the diameter of the first beam is 375 μm, whereas the diameter of the second beam is 200 μm. At position P2, the diameter of the first beam is 300 μm, whereas the diameter of the second beam is 400 μm. At position P2, the energy density of the first beam is around 1.78 times the energy density of the second beam. The beam diameter is defined as the diameter of the beam at a position where the intensity becomes the square of 1/e (where e is the base of the natural logarithms).

The Rayleigh length of the second beam (that is a distance where the diameter of the beam increases √{square root over ( )}2 times) is 59 mm. At position P3that is apart from position P1by the Rayleigh length (59 mm) in the direction of position P2, the diameter of the first beam is 312 μm, whereas the diameter of the second beam is 283 μm.

FIG. 5shows variations of the diameters of the first and second beams as shown inFIG. 4. InFIG. 5, the vertical axis represents the diameters of beams (μm), whereas the horizontal axis represents the distances from position P0shown inFIG. 4. As shown inFIG. 5, in the range from position P0nearly to position P3, the diameter of the first beam is greater than the diameter of the second beam. When the distance nearly exceeds position P3, the diameter of the first beam becomes smaller than the diameter of the second beam. The diameters of beams are defined as their energy densities.

According to this embodiment, at the position defined by the Rayleigh length on the diverged spherical wave side of the second beam having a large convergence angle (position P3shown inFIG. 4), the diameter of the first beam having a small convergence angel coincides with or nearly coincides with the diameter of the second beam. Thus, the range in which speckles can be reduced without sacrificing resolution of images can be widened. Next, this reason will be specifically described.

In the example shown inFIG. 4, the position defined by the Rayleigh length on the convergent spherical wave side of the second beam having a large convergence angel is denoted by P4. In the range from position P0to position P4, since both the first and second beams are convergent light, when the image projection device is arranged such that the projection surface lies in this range, the contrast of speckles can be reduced.

In this case, however, the diameter of the first beam is in excess of the value of √{square root over ( )}2 times the diameter of the beam (300 μm) at position P2where a beam waist is present and the diameter of the second beams is in excess of the value of √{square root over ( )}2 times the diameter of the beam (200 μm) at position P1where a beam waist is present. Thus, the diameter of the beam on the projection surface becomes large and thereby the resolution of an image that appears decreases.

In the range from position P4to position P1, since both the first and second beams are convergent light, when the image projection device is arranged such that the projection surface lies in this range, the contrast of speckles can be decreased.

In the foregoing case, although the diameter of the first beam is in excess of the value of √{square root over ( )}2 times the diameter of the beam (300 μm) at position P2where a beam waist is present in part of the range, the diameter of the second beam is equal to or smaller than the value of √{square root over ( )}2 times the diameter of the beam (200 μm) at position P1where a beam waist is present in all the range. Since the resolution of an image that appears mainly depends on the second beam that has a high energy density, even if the diameter of the first beam is in excess of the value of √{square root over ( )}2 times the diameter of the beam (300 μm) at position P2, high resolutions of images can be maintained.

In the range from position P1to position P3, although the first beam is convergent light, the second beam is diverged light. As shown inFIG. 2B, the intensity of the contrast of speckles linearly varies with the distance from the position where a beam waist is present. Thus, even if a beam is a diverged spherical wave, the contrast of speckles does not largely increase as long as the beam lies in the neighborhood of the position where a beam waist is present (specifically, in the range of the Rayleigh length).

In addition, at position P3, the diameter of the first beam nearly coincides with the diameter of the second beam. When these conditions are satisfied, in the range from position P1to position P3, the energy density of the first beam can be nearly the same as the energy density of the second beam.

Thus, since the increase of the contrast of speckles caused by the second beam (diverged light) is small and since the difference between the energy density of the first beam and the energy density of the second beam is small, the speckle contrast reduction effect of the first beam (convergent light) can be sufficiently obtained in a composite beam of the first and second beams.

In the range from position P3to position P2, although the first beam is convergent light, the second beam is diverged light. In this range, since the energy density of the first beam is equal to or greater than the energy density of the second beam, the speckle contrast reduction effect of the first beam (convergent light) can be sufficiently obtained.

Since the energy density of the first beam (convergent light) is nearly equal to or greater than the energy density of the second beam at position P3, high resolutions of images can be maintained,

As described above, the image projection device according to this embodiment can decrease the contrast of speckles without sacrificing the high resolutions of images when the projection surface lies in the range from position P4to position P2shown inFIG. 4.

In addition, according to this embodiment, the range in which a projection surface can be scanned with convergent light is from position P4to position P2and is sufficiently greater than the range in which projection surface203can be arranged in the system shown inFIG. 2A. Thus, since the range in which the projection surface can be arranged widens, a bothersome installation work for the image projection device can be alleviated.

In the structure shown inFIG. 3, although beam generation means1is structured such that it emits two beams having different convergence angles, beam generation means1may be structured such that it emits three or more beams having different convergence angles. Beams are emitted such that axes that pass through the center of the cross section of each beam coincide with each other.

However, the energy density of each beam needs to satisfy the following conditions. With respect to the first beam and the second beam, the second beam has a next larger convergence angle than that of the first beam in which the energy density of the first beam, at the position where a beam waist of the first beam is present, coincides with the energy density of the second beam at a position in the range from the position where a beam waist of the second beam is present to the position of the Rayleigh length on the divergent side (specifically, in the range from position P1to position P3shown inFIG. 4). In this case, the range in which the projection surface can be arranged can be further widened compared to the structure of two beams. Whatever are the energy densities that coincide with each other, this means that not only the values fully coincide with each other, but also that the values containing deviations caused by production errors or the like nearly coincide with each other.

Next, the overall structure of the image projection device according to this embodiment will be described.

FIG. 6is a block diagram showing the overall structure of the image projection device according to this embodiment. Referring toFIG. 6, the image projection device has beam generation means1, scanning means3, dichroic prisms5a,5b, reflection mirror6, green laser light source7, red laser light source8, blue laser light source9, and light source drive circuit10. Beam generation means1and scanning means3shown inFIG. 6are the same as those shown inFIG. 3.

Light source drive circuit10generates a green laser modulation signal, a red laser modulation signal, and a blue laser modulation signal corresponding to an input video signal. The green laser modulation signal is supplied to green laser light source7. The red laser modulation signal is supplied to red laser light source8. The blue laser modulation signal is supplied to blue laser light source9.

FIG. 7Ashows the structure of a laser light source used as red laser light source8and blue laser light source9.

The laser light source shown inFIG. 7Aincludes current modulation circuit71, semiconductor laser72, and collimate optical system73. Current modulation circuit71controls a current that flows in semiconductor laser72corresponding to the laser modulation signal (for red or blue) supplied from light source drive circuit10. As a result, the intensity of output light of semiconductor laser72is modulated. Laser light emitted from semiconductor laser72is collimated by collimate optical system73.

Red laser light source8uses a semiconductor laser having an oscillation wavelength of 640 nm as semiconductor laser72. Blue laser light source9uses a semiconductor laser having an oscillation wavelength of 440 nm as semiconductor laser72.

FIG. 7Bshows the structure of green laser light source7. The laser light source shown inFIG. 7Bincludes drive circuit74, infrared solid state laser75, second higher harmonic wave device76, acoustic optical device77, collimate optical system78, and convergence optical systems79a,97b.

Second higher harmonic wave device76outputs a second higher harmonic wave (532 nm) of an infrared laser light (1064 nm) that is emitted from infrared solid state laser75through convergence optical system79a. A second higher harmonic wave beam that passes through second higher harmonic wave device76enters acoustic optical device77through convergence optical system79b. Drive circuit74drives acoustic optical device77corresponding to a laser modulation signal (for green) that is supplied from light source drive circuit10. As a result, the intensity of the beam of the second higher harmonic wave that is supplied from second higher harmonic wave device76is modulated. A beam that passes through acoustic optical device77is collimated by collimate optical system78.

FIG. 7Cshows another structure of green laser light source7. The laser light source shown inFIG. 7Cincludes current modulation circuit80, infrared semiconductor laser81, convergence optical system82, second higher harmonic wave device83, and collimate optical system84.

Current modulation circuit80modulates a current supplied to infrared semiconductor laser81corresponding to a laser modulation signal (for green) that is supplied from light source drive circuit10. Infrared laser light that is emitted from infrared semiconductor laser81enters second higher harmonic wave device83through convergence optical system82.

Second higher harmonic wave device83outputs a second higher harmonic wave of the infrared laser light that passes through convergence optical system82. A beam of the second higher harmonic wave that passes through second higher harmonic wave device83is collimated by collimate optical system84.

Referring toFIG. 6again, reflection mirror6is arranged in the traveling direction of laser light (for green) emitted from green laser light source7. Dichroic prisms5a,5b, beam generation means1, and scanning means3are successively arranged in the traveling direction of the laser light (for green) reflected by reflection mirror6.

Dichroic prism5ais arranged at the position where laser light (for green) emitted from green laser light source7intersects with laser light (for red) emitted from red laser light source8. Dichroic prism5atransmits laser light (for green) emitted from green laser light source7and reflects laser light (for red) emitted from red laser light source8. As a result, laser light (for green) and laser light (for red) are combined.

Dichroic prism5bis arranged at the position where the color composite beam (green+red) that passes through dichroic prism5aintersects with laser light (for blue) emitted from blue laser light source9. Dichroic prism5btransmits the combined-color beam (green+red) that passes through dichroic prism5aand reflects laser light (for blue) emitted from blue laser light source9. As a result, the beam (green+red) and the laser light (blue) are combined.

The combined-color beam (green+red+blue) that passes through dichroic prism5bis supplied to beam generation means1. Beam generation means1splits the color composite beam that passes through dichroic prism5binto a plurality of beams having different convergence angles and emits the split beams in one direction such that the split beams are superimposed each other. The beam emitted from beam generation means1enters scanning means3.

Horizontal scanner31is composed of, for example, a resonance micro-mechanical scanning element. The resonance micro-mechanical scanning element is an element capable of reciprocating scanning. The deflection angle is ±20 degrees, and the drive frequency is 15 KHz. In order to enable driving at a drive frequency of 15 KHz in this case, a rectangular mirror having a diameter of 1400 μm is used as the resonant micro-mechanical scanning element.

Vertical scanner32is composed of a galvanometer mirror. A galvanometer mirror has a deflection angle of, for example, ±15 degrees and is driven by a sawtooth wave of 60 Hz.

In the image projection device according to this embodiment shown inFIG. 3toFIG. 6andFIG. 7AtoFIG. 7D, in the condition that projection surface4was arranged at the position of the focal distance of lens14(at the position of 400 mm from P0), when only the second beam was projected to projection surface4, the contrast of speckles was 19.1%; in the same condition for projection surface4, when the first and second beams were projected thereto, the contrast of speckles was 16.4%; and in the same condition for projection surface4, when only the first beam was projected thereto, the contrast of speckles was 16.8%. Thus, these experimental results show that when projection surface4is scanned with the first and second beams having different convergence angles, speckles can be decreased.

When a high frequency current of 300 MHz is superimposed with individual modulation currents of red laser light source8and blue laser light source9, the wavelengths of these light sources can be widened. As a result, the contrast of speckles caused by red laser light and blue laser light can be reduced by 12.0%.

In the foregoing evaluation for the contrast of speckles, a CMOS sensor provided with a lens having a focal distance of 18 mm and a pupil diameter of 2.25 mm was used. The pixel pitch of the CMOS sensor was 2.2 μm. The horizontal and vertical image precisions were 640 pixels and 480 pixels, respectively. The horizontal and vertical screen sizes at a projection distance of 400 mm were 290 cm and 220 cm, respectively.

FIG. 8is a schematic diagram showing the structure of principal sections of an image projection device according to a second exemplary embodiment of the present invention.

Referring toFIG. 8, the image projection device has beam generation means20that emits a plurality of beams having different convergence angles in one direction such that the plurality of beams are superimposed each other; and scanning means3that scans a projection surface with the beams emitted from beam generation means20.

Scanning means3used in this embodiment is the same as that used in the first embodiment. Beam generation means20is a multi-focal lens using Fresnel zone plates (diffraction gratings each having a lens function).FIG. 9schematically shows Fresnel zone plates that compose beam generation means20.

As shown inFIG. 9, beam generation means20is provided with two Fresnel zone plates20a,20bhaving different focal distances, Fresnel zone plates20a,20beach are composed of a plurality of concentric circular rings in which transparent rings and opaque rings are alternately arranged. Fresnel zone plates20a,20bmay be phase-type Fresnel zone plates in which the phase difference of each of the transparent rings and opaque rings is 0 and ρ, respectively. Alternatively, Fresnel zone plates20a,20bmay be transmissive-type Fresnel zone plates in which the transmittance of a transparent ring and the transmittance of an opaque ring are 100% and 0%, respectively, Further alternatively, instead of opaque rings, concentric translucent rings may be used.

By arranging these Fresnel zone plates20a,20bsuch that the center positions of the rings coincide with each other, a multi-focal lens is formed. The multi-focal lens may be composed in such manner that transparent rings and opaque rings of Fresnel zone plates20a,20bare formed together on a single glass plate. Alternatively, the multi-focal lens may be a spatial optical modulator (for example, a liquid crystal panel) that forms an image including transparent rings and opaque rings of Fresnel zone plates20a,20b. Such multi-focal lenses form beam generation means20.

In beam generation means20, Fresnel zone plates20aand20bgenerate first beam21aand second beam21b, respectively. The convergence angle of first beam21ais smaller than the convergence angle of second beam21b.

Scanning means3scans the projection surface with the first and second beams that are emitted from beam generation means20.

As in the image projection device according to the first exemplary embodiment, in the image projection device according to the second exemplary embodiment, the diameter (or energy density) of first beam21a having a smaller convergence angle coincides with or nearly coincides with the diameter of the second beam (or energy density) at the position defined by the Rayleigh length on the diverged spherical wave side of second beam21bhaving a large convergence angle (the position is equivalent to position P3shown inFIG. 4). As a result, the range in which the speckles are reduced without sacrificing resolution of images can be more widened.

In addition, since the range in which the projection surface that is scanned with convergent light can be arranged widens, bothersome installation work for the image projection device can be prevented.

AlthoughFIG. 8shows the structure in which beam generation means20emits two beams having different convergence angles, beam generation means20may emit three or more beams having different convergence angles. In this case, with respect to the first beam and the second beam, the second beam has a next larger convergence angle than that of the first beam in which the energy density of the first beam, at the position where a beam waist of the first beam is present, coincides with the energy density of the second beam at a position in the range from the position where a beam waist of the second beam is present to the position of the Rayleigh length on the divergent side. With three or more beams, the range in which the projection surface can be arranged can be further widened. Whatever are the energy densities that coincide with each other, this means that not only the values fully coincide with each other, but also that the values containing deviations caused by production errors or the like nearly coincide with each other.

The overall structure of the image projection device according to this embodiment is the same as that of the image projection device according to the first exemplary embodiment (refer toFIG. 6andFIG. 7AtoFIG. 7D).

FIG. 10is a schematic diagram showing the structure of principal sections of an image projection device according to a third exemplary embodiment of the present invention.

Referring toFIG. 10, although the image projection device includes beam generation means1and scanning means3shown inFIG. 1, part of beam generation means1is different from that of the first exemplary embodiment. Beam generation means1has shutter switching means40and shutters41,42in addition to the structure shown inFIG. 1.

Shutters41,42each are composed of a liquid crystal shutter, a chopper, or the like. Shutter41is arranged between splitting means11and reflection mirror12. Shutter42is arranged between reflection mirror13and lens15.

Shutter switching means40alternately switches states between a first state in which shutter41is released and shutter42is closed and a second state in which shutter41is closed and shutter42is released corresponding to a vertical synchronization signal. For example, when a vertical synchronization signal for a video signal at a frame frequency of 60 Hz is supplied to shutter switching means40, shutter switching means40alternately switches the states between the first state and the second state every 1/60 seconds.

When shutters41,42lie in the first state, a beam that travels from reflection mirror13to lens15is blocked by shutter42, whereas only the first beam condensed by lens14reaches scanning means3. Scanning means3scans projection surface4with the first beam that passes through lens14.

When shutters41,42lie in the second state, a beam that travels from splitting means11to reflection mirror12is blocked by shutter41, whereas only the second beam condensed by lens15reaches scanning means3. Scanning means3scans projection surface4with the second beam that passes through lens15.

In this embodiment, the first beam that passes through lens14and the second beam that passes through lens15have the relationship as shown inFIG. 4, based on the condition in which projection surface4is arranged in the range from position P1to position P2, when shutters41,42are switched between the first and second states, the first beam that passes through lens14(convergent spherical wave) and the second beam that passes through lens15(diverged spherical wave) are alternately projected to projection surface4on a time division basis. In this case, the intensity of the beam of the convergent spherical wave and the intensity of the beam of the diverged spherical wave are added on projection surface4. This is equivalent to the addition of incoherent beams and thereby a larger speckle reduction effect can be obtained than can be done in the first and second embodiments.

For example, based on the condition in which projection surface4lies at position P2shown inFIG. 4, when projection surface4was scanned with the second beam that passed through lens15, the contrast of speckles was 19.1%. In contrast, based on the same condition for projection surface4, when the first beam that passed through lens14and the second beam that passed through lens15were alternately switched on a time division basis and projection surface4was scanned with these beams, the contrast of speckles was 14.8%.

As in the image projection device according to the first exemplary embodiment, in the image projection device according to the third exemplary embodiment, since the range in which the projection surface can be arranged can be widened in the range from position P4to position P2shown inFIG. 4, installation work for the image projection device can be easily performed.

With respect to the range from position P1to position P2, when the first beam that passes through lens14(convergent spherical wave) and the second beam that passes through lens15(diverged spherical wave) are alternately switched on a time division basis, more speckles can be reduced than can be done in the first exemplary embodiment.

As choppers for use in shutters41,42, revolving choppers may be used.FIG. 11shows an example of a revolving chopper.

Revolving chopper43shown inFIG. 11is formed in a disc shape and has first to fourth regions circumferentially split for every 90 degrees. The first and third regions are non-transparent regions. In contrast, the second and fourth regions are transparent regions. The transparent regions and the non-transparent regions are alternately arranged.

A center portion of revolving chopper43is supported by a rotating axis of a motor (not shown). The number of revolutions of the motor is synchronized with a video signal by a PLL (Phase-Locked Loop) circuit or the like. While a beam that passes through splitting means11is blocked by a non-transparent region of the revolving chopper that comprises shutter41, a beam that is reflected by reflection mirror13passes through a transparent region of the revolving chopper that comprises shutter42. By contrast, while a beam that is reflected by reflection mirror13is blocked by a non-transparent region of the revolving chopper that comprises shutter42, a beam that passes through splitting means11passes through a transparent region of the revolving chopper that comprises shutter41. As a result, the first beam that passes through lens14and the second beam that passes through lens15can be alternately switched on a time division basis.

Next, the overall structure of the image projection device according to this embodiment will be described.

FIG. 12is a block diagram showing the overall structure of the image projection device according to this embodiment. Referring toFIG. 12, the image projection device includes beam generation means1, scanning means3, dichroic prisms5a,5b, reflection mirror6, green laser light source7, red laser light source8, blue laser light source9, and light source drive circuit10. The structure except for beam generation means1and light source drive circuit10is the same as that shown inFIG. 6.

Light source drive circuit10supplies a laser modulation signal to each of green laser light source7, red laser light source8, and blue laser light source9corresponding to a video signal and also supplies a synchronization signal that synchronizes with the video signal to beam generation means1.

Beam generation means1is the same as that shown inFIG. 10. In beam generation means1, shutter switching means40controls shutters41,42corresponding to the synchronization signal supplied from light source drive circuit10.

FIG. 13is a schematic diagram showing the structure of principal sections of an image projection device according to a fourth exemplary embodiment of the present invention.

Referring toFIG. 13, the image projection device has beam generation means22that emits a plurality of beams having different convergence angles in one direction such that the plurality of beams are superimposed each other; and scanning means3that scans projection surface4with the beams emitted from beam generation means22.

Scanning means3is the same as that of the first exemplary embodiment. Beam generation means22has spatial optical modulator22asuch as a liquid crystal panel; and drive circuit22bthat drives spatial optical modulator22a.

Drive circuit22balternately outputs a first drive signal that causes a first image corresponding to Fresnel zone plate20ashown inFIG. 9to be formed and a second drive signal that causes a second image corresponding to Fresnel zone plate20bshown inFIG. 9to be formed. The first and second drive signals are supplied to spatial optical modulator22a.

Spatial optical modulator22ais a multi-focal lens and forms the first image and second image corresponding to Fresnel zone plates20aand20bbased on the first and second drive signals supplied from drive circuit22b, respectively.

According to this embodiment, the first and second drive signals are alternately supplied from drive circuit22bto spatial optical modulator22afor every frame identified by the vertical synchronization signal.

When the first drive signal is supplied to spatial optical modulator22a, spatial optical modulator22aforms the first image. As a result, first beam21acondensed by Fresnel zone plate20ais supplied to scanning means3. Scanning means3scans the projection surface with first beam21a.

When the second drive signal is supplied to spatial optical modulator22a, spatial optical modulator22aforms the second image. As a result, second beam21bcondensed by Fresnel zone plate20bis supplied to scanning means3. Scanning means3scans the projection surface with second beam21b.

According to this embodiment, the first and second beams have the relationship as shown inFIG. 4. When projection surface4is arranged in the range from position P1to position P2, the first beam (convergent spherical wave) and the second beam (diverged spherical wave) are alternately projected to projection surface4on time division basis. Thus, as in the image projection device according to the third exemplary embodiment, in the image projection device according to the fourth exemplary embodiment, the intensity of the convergent spherical wave and the intensity of the diverged spherical wave are added on projection surface4and thereby a larger speckle reduction effect can be obtained than can be realized in the first exemplary embodiment and the second exemplary embodiment.

In addition, as in the image projection device according to the first exemplary embodiment, in the image projection device according to the fourth exemplary embodiment, the range in which the projection surface can be arranged can be widened in the range from position P4and position P2(or from position P1to position P2) shown inFIG. 4, therefore, installation work for the image projection device can be easily performed.

The overall structure of the image projection device according to this embodiment is the same as the overall structure shown inFIG. 12except that beam generation means1is substituted with beam generation means22.

Although beam generation means22is structured to emit two beams having different convergence angles, beam generation means22may be structured to emit three or more beams having different convergence angles. In this case, with respect to the first beam and the second beam, the second beam has a next larger convergence angle than that of the first beam in which the energy density of the first beam, at the position where a beam waist of the first beam is present, coincides with the energy density of the second beam at a position in the range from the position where a beam waist of the second beam is present to the position of the Rayleigh length on the divergent side. With three or more beams, the range in which the projection surface can be arranged can be further widened. Whatever are energy densities that coincide with each other, this means that not only the values fully coincide with each other, but also that the values containing deviations caused by production errors or the like nearly coincide with each other.

Although the image projection devices according to the first to fourth exemplary embodiments are of front projection type, they can be applied to rear projection type.

For example, when the image projection devices according to the first and second embodiments are applied to the rear projection type, the overall structure shown inFIG. 6including the screen used as the projection surface is accommodated in a housing. When the image projection device according to the third exemplary embodiment is applied to the rear projection type, the overall structure shown inFIG. 12(including the screen) is accommodated in a housing. When the image projection device according to the fourth exemplary embodiment is applied to the rear projection type, the structure shown inFIG. 12(in which beam generation means1is substituted with beam generation means22and that includes the screen) is accommodated in a housing. In any case, the screen has a diffusing characteristic. The inner surface of the screen (inner plane of the housing) is scanned with a light beam that passes through scanning means3. When the light is transmitted through the screen, the light diffuses and thereby the observers observe the diffused light.

In the foregoing rear projection type image display device, when the beam generation means generates n (where n is an integer equal to or greater than 2) beams having different convergence angles, the screen is arranged between a position where a beam waist of the first beam having the lowest convergence angle is present and the position of the Rayleigh length on the convergence side of an n-th beam having the largest convergence angle.

The image production devices or image display devices according to the first to fourth embodiments each superimpose a plurality of beams having different convergence angles and thereby widen the range in which the projection surface on their optical path can be scanned with the convergent light and also suppress the beams from widening in the range. Thus, the speckle reduction effect and high precision images can be obtained by scanning the projection surface with convergent light.

FIG. 14is a block diagram showing the structure of an image display device according to a fifth exemplary embodiment of the present invention.

Image display device50shown inFIG. 14is different from the image display device shown inFIG. 6in that beam generation means1is substituted with light condensing means51and in that screen52having a diffusion characteristic is provided as projection surface4. Scanning means3, dichroic prisms5a,5b, reflection mirror6, green laser light source7, red laser light source8, blue laser light source9, and light source drive circuit10for use in image display device50are basically the same as those shown inFIG. 6.

Light condensing means51condenses a light beam containing individual colors (red, green, and blue) composited by dichroic prisms5a,5b. Scanning means3scans screen52with the light beam that passes through light condensing means51. When light transmits through screen52, the light diverges and observers can observe the diverged light.

Screen52is arranged at a position closer to scanning means3than the beam waist of the light beam that passes through light condensing means51. More preferably, screen52is arranged at a position closer to scanning means3than at a position where a beam waist of the light beam that passes through light condensing means51is present and in the range of the Rayleigh length on the convergence side of the light beam. As a result, screen52is always irradiated with a convergent beam and the diameter of the beam on screen52is √{square root over ( )}2 times the diameter of the beam where the beam waist is present

Thus, since image display device50according to this embodiment always scans screen52with convergent light, image display device50can reduce speckles compared to the device in which the screen is arranged at a position where a beam waist is present or on the divergent side of the beam.

Since the diameter of the beam on screen52is at most √{square root over ( )}2 times the diameter of the beam where the beam waist takes place, highly precise images can be provided.

The foregoing individual embodiments are just examples of the present invention and thereby their structures may be changed without departing from the scope of the present invention.

In the individual embodiments, images projected or displayed on the projection surface include those that can be projected or displayed on the projection surface based on electronic data such as characters, figures, tables, and so forth as well as image data such as images and pictures.

The present invention can be applied to not only luster scanning, but also vector scanning.

With reference to the embodiments, the present invention has been described. However, it should be understood by those skilled in the art that the structure and details of the present invention may be changed in various manners without departing from the spirit of the present invention.

This application claims the benefits of priority based on Japanese Patent Application No. 2009-238088 for which application was submitted on Oct. 15, 2009 and incorporates by citation all of the disclosures of that application.