JP2006284749A - Scanning type display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized, scanning-type display device in which an optical system is compact, moreover light loss is small, speckle noise can be reduced, while decrease in the resolution is minimized, and the projection distance can be arbitrarily set. <P>SOLUTION: The scanning-type display device displays a two-dimensional image on a surface scanned by a scanning means with the use of a laser beam emitted from a light source means and light-modulated based on image information. The scanning-type display device has one or more birefringence plates in between the scanning means and the screen. The optical axis of each birefringence plate is inclined in the direction of the polarization of the laser beam. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a scanning display apparatus that displays an image on a screen by two-dimensionally scanning a laser beam that is light-modulated based on image information, and is particularly suitable for reducing speckle noise generated on the screen. Is.

  Various scanning display devices that perform two-dimensional scanning on a screen using a laser beam from a laser light source and project and display an image have been proposed (Patent Documents 1 to 3).

Patent Document 1 proposes a scanning display device that displays a color image on a screen by two-dimensionally scanning red, green, and blue laser beams using a deflector.
However, since the laser light has high coherence, so-called speckle noise (granular interference pattern) appears in the display image, causing deterioration of image quality.

  As a method for reducing speckle noise, Patent Document 2 proposes a method using a transparent optical element having a refractive index n composed of N regions having different thicknesses by Δt. Specifically, a divergent light beam of a semiconductor laser is made incident on a transparent optical element as a parallel light beam by a collimator lens, and an optical path difference (n−1) Δt is given to a light beam (divided light beam) that has passed through a different part of the step Δt. , Reduce the coherence of each split beam. Speckle noise is reduced by overlapping a plurality of non-coherent light beams that have passed through the transparent optical element with a lens.

Patent Document 3 proposes a method for reducing speckle noise using a phase hologram. By dividing the laser beam scanned by the diffraction effect of the phase hologram into a plurality of partial laser beams, a plurality of speckle patterns corresponding to the irradiation positions of the partial laser beams on the screen are generated. By superimposing these intensities, speckle noise is reduced.
JP 2003-021800 A JP 2000-206449 A JP-T-2001-509911

  In the speckle noise reduction method proposed in Patent Document 2, it is necessary to increase the step Δt of the transparent optical element in order to give a sufficient optical path difference to the divided light flux. As a result, it becomes difficult to make the optical system compact. In particular, when light having a long coherence length such as a solid-state laser or a single mode semiconductor laser is used, the size of the transparent optical element becomes extremely large.

  Further, in the speckle noise reduction method proposed in Patent Document 3, since the laser beam is transmitted through the phase hologram, a light amount loss due to high-order diffracted light occurs. Furthermore, the scanning beam spreads due to the diffraction effect of the phase hologram, and the resolution of the display image decreases. Even if the scanning beam is condensed using a lens system, only one projection distance can be selected.

  An object of the present invention is to provide a scanning display device capable of reducing speckle noise in a scanning display device that displays an image by two-dimensionally scanning on a screen using a laser beam emitted from a laser light source. To do.

The scanning display device according to the first aspect of the present invention is a scanning type display that displays a two-dimensional image on a surface to be scanned by scanning a laser beam that is optically modulated and emitted from a light source means based on image information. In the device
One or more birefringent plates are provided between the scanning means and the screen;
The optical axis of the birefringent plate is inclined with respect to the polarization direction of the laser beam.

  According to the present invention, a scanning display device capable of reducing speckle noise is obtained.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic diagram of a main part of the configuration of a scanning display device according to a first embodiment of the present invention. In FIG. 1, 101 is a laser light source (light source means), which emits laser light having a polarization component in a predetermined direction.

  The laser light source 101 is electrically connected to the light source driving circuit 120, and the laser output is driven and controlled. Reference numeral 104 denotes a scanning unit, which is electrically connected to the scanning unit control circuit 122, and the scanning is driven and controlled. The light source driving circuit 120 and the scanning means control circuit 122 are connected to the device control circuit 121, and the laser light source 101 and the scanning means 104 are synchronously controlled with respect to a video signal input corresponding to a desired image by a video signal input means (not shown). As a result, a desired image is scanned and displayed on the surface to be scanned (screen) 5.

  The divergent light beam emitted from the laser light source 101 is converted into a parallel light beam or a substantially parallel light beam by the collimator lens 102. A parallel light beam (hereinafter referred to as a laser beam) emitted from the collimator lens 102 enters a condensing optical system (optical system) 103. The laser beam emitted from the condensing optical system 103 enters the scanning unit 104.

  The scanning unit 104 includes a horizontal scanning mirror 104H and a vertical scanning mirror 104V, and scans the incident laser beam in a two-dimensional direction. In this embodiment, the horizontal scanning mirror 104H is a MEMS (Micro Electro Mechanical System) mirror device manufactured using a semiconductor manufacturing technique or the like, and reciprocally swings (vibrates) using an electromagnetic force or the like. Use what you can. The mirror surface of the horizontal scanning mirror 104H is 1.5 mm square. The vertical scanning mirror 104V uses a galvanometer mirror (vibrating mirror).

  Reference numeral 100 denotes a birefringent plate, a surface parallel to both the optical axis and the principal ray of the light beam incident on the center of the field angle of the image displayed on the scanned surface (a surface including the optical axis). Are arranged so that the polarization plane of the laser beam is inclined.

  That is, the birefringent plate 100 is arranged between the scanning means 104 and the screen 105, both the optical axis of the birefringent plate 100 and the principal ray of the light beam incident on the center of the field angle of the image displayed on the scanned surface. The laser beam is incident so as to have both horizontal and vertical polarization components with respect to a plane parallel to.

  The laser beam scanned by the scanning unit 104 scans a screen (projection surface) 105 such as a wall surface via the birefringent plate 100. A laser beam from the laser light source 101 is imaged as a spot on the screen 105 by the collimator lens 102 and the condensing optical system 103. The spot formed on the screen 105 is optically scanned in a two-dimensional direction by the scanning unit 104.

  In FIG. 1, an arrow 110 indicates the swinging direction of the horizontal scanning mirror 104H, and an arrow 111 indicates the rotational direction of the vertical scanning mirror 104V. Horizontal scanning lines 106 and 107 are formed on the screen 105 by the vibration of the horizontal scanning mirror 104H. Further, the vertical scanning mirror 104V vibrates in the direction of arrow 111, so that the laser beam spot formed on the screen 105 is scanned in the direction of arrow 109. Therefore, scanning lines 106 and 107 that reciprocate in the horizontal direction are formed on the screen 105 from the upper end to the lower end of the screen 105. As a result, an image of one frame is displayed on the screen 105 due to the afterimage effect of human eyes.

  When the scanning line 106 is formed at the lower end on the screen 105, the vertical scanning mirror 104V next forms the scanning line 106 at the upper end of the screen 105 to form an image of the next frame. Thereafter, this operation is repeated. For example, when forming an SVGA image with 800 pixels in the horizontal direction and 600 pixels in the vertical direction, if the vertical scanning is repeated at 60 Hz, 300 horizontal scanning lines are required for each of the forward path and the backward path. The mirror 104H requires a resonance frequency of 18 kHz (60 Hz × 300). In FIG. 1, the actual scanning lines are shown as thinned out.

  An area 112 on the screen 105 is an area (effective area) where an image is actually displayed. Since the horizontal scanning mirror 104H performs a resonance operation, the speed decreases as it approaches the maximum amplitude, and is not suitable for displaying an image. Therefore, the horizontal scanning mirror 104H is inward of the region corresponding to the maximum amplitude of the horizontal scanning mirror 104H. The image is displayed only in the area 112.

  In this embodiment, when the laser beam scanned by the scanning unit 104 is optically modulated and emitted based on the image information from the light source unit 101 to display a two-dimensional image on the screen 105, the scanning unit 104 and the screen 105 One or more birefringent plates 100 between the optical axis of the birefringent plate 100 and the center of the angle of view of the image displayed on the scanned surface with respect to the polarization component of the laser beam. A plane parallel to both the principal ray and the principal ray is inclined so that the speckle pattern is reduced.

  Next, the principle of reducing speckle noise generated on the screen 105 in this embodiment will be described. Speckle appears when the scattered light of the laser beam reflected by the screen 105 overlaps with a random phase relationship on the retina.

  A normal screen has a surface roughness sufficiently larger than the wavelength of the laser beam, and a correlation length of the surface roughness (for example, a period of surface irregularities that makes a correlation coefficient described later substantially zero) is sufficiently larger than the irradiation spot diameter of the laser beam. small. For this reason, the phase of the laser light scattered at each point on the screen becomes random corresponding to the microscopic uneven shape of the screen surface, and has a uniform distribution between (−π, π). For this reason, the scattered light of the laser light from the screen overlaps with a random phase relationship on the retina, and an irregular granular interference pattern (speckle pattern), that is, speckle noise appears on the retina.

  One method for reducing speckle noise at this time is a method of superimposing a plurality of speckle patterns with little correlation with each other to smooth the speckle intensity. When N speckle patterns that are not correlated with each other are superimposed, the speckle contrast C is

To reduce.

  The speckle contrast C is defined as the ratio between the standard deviation σI of the speckle intensity I and the average value <I> (C = σI / <I>). In order to reduce speckle noise using this method, it is necessary to generate a plurality of speckle patterns having little correlation with each other.

  In this embodiment, a method of reducing speckle noise by generating a plurality of speckle patterns with little correlation with each other on the screen 105 and superimposing the speckle intensity is used.

  Here, the relationship between the polarization direction of the laser beam incident on the screen 105 and the speckle pattern of the display image will be described based on experimental results. 2A, 2 </ b> B, 2 </ b> C, and 2 </ b> D, the speckle intensity when the polarization direction of the laser beam is 0 °, 30 °, 45 °, and 90 ° with respect to the horizontal direction of the screen 105. The contour map of is shown.

  The horizontal direction and the vertical direction in FIG. 2 correspond to the horizontal direction and the vertical direction of the screen 105, and the portion of the screen 105 where the speckle strength is high is shown in black.

  As shown in FIG. 2, the speckle pattern of the display image is a different pattern depending on the polarization direction of the laser beam incident on the screen 105. Further, when the correlation between the speckle pattern generated by the linearly polarized light having the vibration plane in the horizontal direction of the screen 105 and the speckle pattern generated by the linearly polarized light having an angle θ with respect to the horizontal direction is observed, the speckle pattern increases as θ increases. The correlation between the patterns was reduced, and when the polarization direction was orthogonal to the horizontal direction (θ = 90 °), the patterns were independent and had no correlation with each other.

  FIG. 3 shows the result of calculating the correlation coefficient of each speckle pattern.

  The correlation coefficient r between the speckle intensities X1 and X2 of each speckle pattern is defined by the following equation using average values <X1> and <X2> of speckle intensity.

  Next, speckle reduction means used in the present embodiment will be described. In this embodiment, a birefringent plate 100 is disposed between the scanning means 104 and the screen 105 as speckle reduction means. The birefringent plate 100 can separate the laser beam into two parallel laser beams whose polarization directions are orthogonal to each other due to the effect of birefringence.

  By scanning two light beams having orthogonal polarization directions and displaying an image on the screen 105, two independent speckle patterns corresponding to the polarization directions of the respective light beams are generated simultaneously. By superimposing these two speckle patterns on the retina, the speckle contrast C is obtained.

Has been reduced.

  The relationship between the direction of the optical axis 100a of the birefringent plate 100 in this embodiment and the polarization direction of the laser beam will be described with reference to FIG.

  FIG. 4 schematically shows a cross section including the optical axis 100 a of the birefringent plate 100. Due to the birefringence effect, the laser beam incident on the birefringent plate 100 is horizontal with a laser beam L1 having a polarization direction in a direction perpendicular to the plane including the optical axis 100a of the birefringent plate 100 (perpendicular to the paper surface). The laser beam L2 having a polarization direction in the direction (in the drawing) is separated. Here, the surface including the optical axis 100a of the birefringent plate 100 and the two separated laser beams is referred to as a beam separation surface of the birefringent plate. Here, the polarization direction of the laser beam incident on the birefringent plate is inclined with respect to the light beam separation surface. In other words, in the scanning display device according to the first embodiment, the polarization direction of the light beam incident on the birefringent plate of the light beam incident on the center of the angle of view of the image displayed on the surface to be scanned has a double direction where the laser beam is incident. The optical axis of the refracting plate (which may be plural as will be described later) and the principal ray of the light beam incident on the center of the angle of view of the image displayed on the scanned surface (or the normal to the incident surface of the birefringent plate) Are inclined with respect to a plane parallel to the two (this is the above-described light beam separation plane). In other words, the plane of polarization parallel to both the polarization direction of the laser beam and the traveling direction of the laser beam is inclined with respect to the optical axis of the birefringent plate.

  A light beam having a polarization direction parallel to the light beam separation surface is called P-polarized light, and a light beam having a perpendicular polarization direction is called S-polarized light.

  In this embodiment, a laser light source 101 that emits linearly polarized light is used. For both the polarization direction of the laser beam incident on the birefringent plate 100, the optical axis of the birefringent plate 100, and the principal ray of the light beam incident on the center of the field angle of the image displayed on the scanned surface. The arrangement direction of the birefringent plate 100 is determined so that the angle formed with the parallel plane is 45 degrees or approximately 45 degrees ± 5 degrees (approximately 45 degrees in the direction perpendicular to the paper surface), and is disposed between the scanning means 104 and the screen 105. To do. As shown in FIG. 4, the laser beam incident on the birefringent plate 100 is separated into a laser beam L1 (S-polarized light) and a laser beam L2 (P-polarized light) by the birefringent effect of the birefringent plate 100. At this time, the intensities of the laser beams L1 and L2 are substantially equal.

  The laser beams L1 and L2 emitted from the birefringent plate 100 are imaged as S-polarized and P-polarized spots on the screen 105 and simultaneously scanned two-dimensionally by the scanning unit 104.

  In the present embodiment, a green image having a wavelength of 532 nm is used as the laser light source 101 and calcite is used as the birefringent plate 100 to display a still image on the screen 105.

  In this embodiment, the birefringent plate 100 is disposed between the scanning means 104 and the screen 105, and the laser beam is separated into two parallel laser beams whose polarization directions are orthogonal to each other, so that the speckle contrast C is evaluated. As a result, C = 0.22.

  On the other hand, when the birefringent plate 100 is not inserted, the speckle contrast C = 0.30. Therefore, by arranging the birefringent plate 100, the speckle contrast C can be reduced by 0.73 times.

  Next, the spot separation direction on the screen 105 will be described. The spot scanned on the screen 105 is separated in a direction determined by the polarization direction of the laser beam from the incident laser light source 101 and the optical axis 100 a of the birefringent plate 100. Therefore, in order not to reduce the resolution of the scanned image, it is better to separate the spots in a direction perpendicular to the horizontal scanning lines 106 and 107 shown in FIG.

  That is, the birefringent plate is arranged so that the direction of separation of the laser beam emitted from the birefringent plate is perpendicular or substantially perpendicular to the scanning line formed on the screen surface.

  Since the effect of reducing the speckle pattern in the present embodiment is an effect of superposition of the speckle pattern with respect to incidence of two different polarized lights, the separation distance of the spots is not related to the effect. Considering a decrease in resolution of the formed scanned image, it is better to set the separation distance small.

  In this embodiment, an example in which a green laser having a wavelength of 532 nm is used as a light source has been described. However, even when a monochromatic image is displayed as a light source that emits a blue laser or a red laser, or when a color image is formed using these. However, the same speckle noise reduction effect as when using a green laser can be obtained.

  FIG. 5 is a schematic diagram of a main part of the configuration of the scanning display device according to the second embodiment of the present invention. The difference between the present embodiment and the first embodiment is that a plurality of birefringent plates are arranged between the scanning unit 104 and the screen 105.

  In this embodiment, a plurality of birefringent plates are provided, and adjacent birefringent plates are arranged so that their optical axes are different from each other.

  FIG. 5 shows a case where two birefringent plates A1 and A2 are arranged. Hereinafter, the unit of the two birefringent plates is referred to as speckle reduction means 200.

  Similar to the first embodiment, the divergent light beam emitted from the laser light source 101 is converted into a parallel light beam by the collimator lens 102 and is incident on the scanning unit 104 via the condensing optical system 103. The laser beam scanned by the scanning unit 104 passes through the speckle reduction unit 200 and is two-dimensionally scanned on the screen 105 to form an image on the screen 105.

  FIG. 6 is a schematic diagram of the configuration of the speckle reduction means 200 in the present embodiment. The speckle reduction means 200 is composed of birefringent plates A1 and A2. When the laser beam is incident on the birefringent plate A1, it is separated into two beams L1 (S-polarized light) and L2 (P-polarized light) within the light beam separation surface of the birefringent plate A1.

  The birefringent plate A2 is arranged so that the light beam separation surface of the birefringent plate A2 forms an angle of 45 degrees with respect to the light beam separation surface of the birefringent plate A1. As a result, the polarization directions of the laser beams L1 and L2 emitted from the birefringent plate A1 can have an inclination of 45 degrees or 45 degrees ± 5 ° with respect to the light beam separation surface of the birefringent plate A2.

  As a result, the laser beams L1 and L2 incident on the birefringent plate A2 are separated into two beams having polarized light in directions perpendicular to and parallel to the light beam separation surface of the birefringent plate A2, and a total of four beams are separated. The birefringent plate A2 is emitted as laser beams L11 (S-polarized light), L12 (P-polarized light), L21 (S-polarized light), and L22 (P-polarized light).

  Thus, the speckle reduction means 200 has a function of separating the scanned laser beam into four parallel laser beams. The four separated laser beams are imaged as spots on the screen 105. FIG. 7 shows the arrangement and polarization direction of the four separated laser beams L11, L12, L21, and L22 on the screen 105. In FIG. 7, the P-polarized spot is indicated by a broken line, and the S-polarized spot is indicated by a solid line.

  In the present embodiment, a green image having a wavelength of 532 nm is used as the laser light source 101 and calcite is used as the birefringent plates A1 and A2 constituting the speckle reduction means 200 to display a still image on the screen 105.

  Since the laser beam to be scanned is separated into four parallel laser beams by the speckle reduction means 200, the speckle contrast C is evaluated to be speckle contrast C = 0.19.

  On the other hand, when the speckle reduction means 200 is not inserted, the speckle contrast C = 0.30.

  Therefore, the speckle contrast C can be reduced by 0.61 times by arranging the speckle reduction means 200.

  As a result, a speckle reduction effect greater than the speckle contrast C reduction rate (0.73 times) shown in the first embodiment can be obtained.

  Next, the principle of speckle reduction in the speckle reduction means 200 of this embodiment will be described. By separating the laser beam into four parallel laser beams, an image is displayed by optical scanning at the same time with four spots on the screen 105. Each spot is irradiated at a different position on the screen 105, and the surface roughness of the screen 105 varies depending on the position of the spot, so that a plurality of different speckle patterns corresponding to each separated beam are generated. Speckle noise is reduced by superimposing the intensity of these speckle patterns.

  In order to generate an extremely large speckle reduction effect due to the separation of the laser beam shown in this embodiment, the separation distance d of the laser beam by one birefringent plate and the spot diameter w of the laser beam on the screen 105 are: The following relational expression should be satisfied.

d> w
In this example, the experiment was performed under the conditions (d> w) where the spot diameter w was 350 μm and the separation distance d was 500 μm, and the speckle contrast C was 0.61 times (C = 0.30 to C = 0.19). It was confirmed that it can be reduced.

  In this embodiment, the condition is d> w. However, even when d <w, the effect of reducing speckles can be obtained.

Next, in this embodiment, the condition of the thickness (optical path length) t of the birefringent plate necessary for satisfying the condition of d> w will be described. Separation distance d of the laser beam by one birefringent plate, the refractive index n ₀ of ordinary ray of the birefringence plate, the refractive index of extraordinary ray n _e, the angle between the optical axis and the laser beam of the birefringent plate theta, 1 Using the thickness (optical path length) t of the two birefringent plates, it can be calculated as follows:

  The spot diameter w of the laser beam is expressed by the following equation using the truncation constant k (k = 1.64 in the case of a circular aperture), the F number Fno on the image side of the optical system (condensing optical system 103), and the wavelength λ of the laser beam. It can be calculated as follows.

w = k × Fno × λ
Therefore, the condition of the thickness (optical path length) t of the birefringent plate for satisfying the condition of d> w is expressed by the following equation.

In this embodiment, the spot diameter w (= k × Fno × λ) is 350 μm, the angle θ between the optical axis of the birefringent plate and the laser beam is 45 degrees, and the refractive index n ₀ of calcite ordinary light at a wavelength of 532 nm is 1.6629, the refractive index n _e of the extraordinary ray experiments were conducted under the conditions of 1.4885.

  In the case of this condition, if the thickness (optical path length) t of the birefringent plate is set to 3.17 mm or more from the above equation 5, the effect of speckle reduction by beam separation can be extremely increased. it can.

  It has been confirmed that the effect of speckle reduction by laser beam separation shown in this embodiment appears to be extremely large if the laser beam separation distance d is set longer than the spot diameter w. If the pixel size is set to be larger than this, the problem that the resolution is lowered occurs. In order to prevent the resolution from decreasing, the F number on the image side of the optical system (condensing optical system) 3 may be set bright so that the spot diameter w of the separated laser beam becomes small.

  By making the spot diameter w sufficiently small with respect to the pixel size, the four spots of the laser beam separated within the range of one pixel can be arranged without overlapping. As a result, speckle noise can be reduced without reducing the resolution of the scanned image.

  In this embodiment, an experimental example using a green laser having a wavelength of 532 nm as a light source has been described. However, even when a monochromatic image is displayed as a light source that emits a blue laser or a red laser, a color image is formed using these images. Even in this case, the same speckle noise reduction effect as in the case of using the green laser can be obtained.

  FIG. 8 is a schematic diagram of a main part of the configuration of the scanning display device according to the third embodiment of the present invention. The difference between the present embodiment and the first embodiment is that three birefringent plates A 1, A 2, A 3 are arranged between the scanning means 104 and the screen 105. Hereinafter, a total of three birefringent plate units will be referred to as speckle reduction means 300.

  Similar to the first embodiment, the divergent light beam emitted from the laser light source 101 is converted into a parallel light beam by the collimator lens 102 and is incident on the scanning unit 104 via the condensing optical system 103. The laser beam scanned by the scanning unit 104 passes through the speckle reduction unit 300 and is scanned two-dimensionally on the screen 105 to form an image on the screen 105.

  FIG. 9 is a schematic diagram of the configuration of the speckle reduction means 300 in the present embodiment. The speckle reduction means 300 is constituted by birefringent plates A1, A2, A3. When the laser beam is incident on the birefringent plate A1, it is separated into two beams L1 and L2 within the light beam separation surface of the birefringent plate A1. When the birefringent plate A2 is arranged so that the light beam separation surface of the birefringent plate A2 forms an angle of 45 degrees with respect to the light beam separation surface of the birefringent plate A1, the laser beams L1 and L2 emitted from the birefringent plate A1. The polarization direction can be inclined at an angle of 45 ° or 45 ° ± 5 ° with respect to the light beam separation surface of the birefringent plate A2.

  As a result, the laser beams L1 and L2 incident on the birefringent plate A2 are both separated into two beams, and the birefringent plate A2 is emitted as a total of four separated laser beams L11, L12, L21, and L22. The birefringent plate A3 is arranged in a direction that forms an angle of −45 degrees with respect to the light beam separation surface of the birefringent plate A2 (a direction parallel to the light beam separation surface of the birefringent plate A1). The four separated laser beams L11, L12, L21, and L22 emitted from the birefringent plate A2 are incident on the birefringent plate A3, and then separated into two beams, which are combined into a total of eight separated laser beams. The light exits from the refractive plate A3.

  The eight separated laser beams L111, L112, L121, L122, L211, L212, L221, and L222 are imaged as spots on the screen 105.

  FIG. 10 shows the arrangement and polarization direction of eight separated laser beam spots on the screen 105. In FIG. 10, the P-polarized spot is indicated by a broken line, and the S-polarized spot is indicated by a solid line.

  In this embodiment, a green image having a wavelength of 532 nm is used as the laser light source 101, and calcite is used as the birefringent plates A1, A2, and A3 constituting the speckle reduction means 300, and a still image is displayed on the screen 105.

  Since the scanning beam was separated into eight parallel laser beams by the speckle reduction means 300, the speckle contrast C was evaluated, resulting in C = 0.15. On the other hand, when the speckle reduction means 800 is not inserted, the speckle contrast C = 0.30. Therefore, the speckle contrast C can be reduced by 0.5 times by arranging the speckle reduction means 200.

  This is an effect of speckle reduction larger than the speckle contrast C reduction rate (0.61 times) by the beam separation (separation number: 4) shown in the second embodiment. Thus, the effect of reducing speckles can be increased by increasing the number of beam separations.

  Also in the present embodiment, as in the case of the second embodiment, when the laser beam separation distance d and the laser beam spot diameter w satisfy the relation of d> w, the effect of reducing speckles can be extremely increased. Can do.

  In the examination of this example, the spot diameter w was 350 μm and the separation distance d was 500 μm (d> w). In this embodiment, the condition is d> w. However, even when d <w, the effect of reducing speckles can be obtained.

In this embodiment, the speckle reduction means 300 is configured by using three birefringent plates A1, A2 and A3. However, if one additional birefringent plate A4 is added, the number of beam separations is doubled, resulting in a total of 16 Since a parallel beam of books can be obtained, it is possible to obtain a speckle reduction effect that is greater than the aforementioned effect (separation number: 8). Thus, when the number of birefringent plates constituting the speckle reduction means 300 is N, a total of 2 ^N separated laser beams can be generated. As the number N increases, the speckle decreases. The effect of reduction can be further increased.

  An example in which the spot separation direction is characterized will be described with reference to FIGS. 11 and 12. FIG. 11 shows a configuration of speckle reduction means using four birefringent plates A1 to A4. FIG. 12 shows the arrangement and polarization direction of 16 separated laser beam spots on the screen 105.

  The birefringent plate A4 is arranged so as to be in the direction of −45 degrees (the direction orthogonal to the light beam separating surface of the birefringent plate A2) with respect to the light beam separating surface of the birefringent plate A3. When arranged in such a direction, the arrangement of the separated laser beam spots on the screen 105 is as shown in FIG. In FIG. 12, the P-polarized spot is indicated by a broken line, and the S-polarized spot is indicated by a solid line. By arranging the laser beam spots separated in such a symmetrical arrangement close to a circle, by reducing the spot diameter w, it is possible to easily create a state where the spots do not overlap within the range of one pixel. Accordingly, speckle noise can be effectively reduced without reducing the resolution of the scanned image.

  Further, in this embodiment, an experimental example using a green laser with a wavelength of 532 nm as a light source has been described, but even when a monochromatic image is displayed as a light source that emits a blue laser or a red laser as in Embodiments 1 and 2, Alternatively, even when a color image is formed using these, the same effect of reducing speckle noise as when using a green laser can be obtained.

  FIG. 13 is a schematic diagram of a main part of the configuration of the scanning display device according to the fourth embodiment of the present invention. The difference between the present embodiment and the third embodiment is that three laser light sources 401r, 401g, and 401b that emit three colors of red, green, and blue are used as light source means, and a color image is formed on the screen 105. It is. A laser light source that emits red and blue light uses a semiconductor laser. The laser light source that emits green light uses a laser light source obtained by wavelength-converting an infrared semiconductor laser.

  In FIG. 13, 401r is a red semiconductor laser, 401b is a blue semiconductor laser, and 401g is an infrared semiconductor laser. The laser light sources 401r, 401b, and 401g are electrically connected to the light source driving circuit 120 as in the first embodiment.

  The scanning unit 104 is electrically connected to the scanning unit control circuit 122. The light source driving circuit 120 and the scanning means control circuit 122 are connected to the device control circuit 121, and the laser light sources 401r, 401b, 401g and the scanning means are applied to a video signal input corresponding to a desired image by a video signal input means (not shown). A desired image is displayed on the screen 105 by synchronously controlling 104.

  The divergent light beams emitted from the red semiconductor laser 401r and the blue semiconductor laser 401b are converted into parallel light beams by the collimator lenses 402r and 402b.

  The divergent light beam emitted from the infrared semiconductor laser 401b is incident on the LN crystal 403g having a polarization inversion distribution through the condenser lens 402g, and is wavelength-converted into green laser light that is the second harmonic. The green laser light emitted from the LN crystal 403g with the polarization inversion distribution is converted into a parallel light beam by the collimator lens 404g.

  The parallel light beams of red, blue, and green emitted from the collimator lenses 402r, 402b, and 404g reach the cross dichroic prism 405, and are combined into one composite light beam by the cross dichroic prism 405.

  The light beam emitted from the cross dichroic prism 405 enters the scanning unit 104 via the condensing optical system 103. The laser beam scanned by the scanning unit 104 passes through the speckle reduction unit 400 formed of one or more birefringent plates, and optically scans the screen 105.

  Even in the case of a scanning display device that displays a color image on the screen 105 as in this embodiment, the speckle reduction means 400 can provide the same speckle reduction effect as in the first to third embodiments.

  In this embodiment, laser light (SHG light) obtained by wavelength conversion of an infrared semiconductor laser is used as the green light source. Since the green laser (SHG light) oscillates in a single mode and has a very narrow wavelength width, the coherence is higher than that of a semiconductor laser and a speckle pattern is likely to be generated.

  In order to greatly reduce the speckle pattern of the green laser, the separation distance dg of the laser beam of the green laser and the spot diameter wg on the screen 105 satisfy the following relational expression as described in the second embodiment. good.

dg> wg
Further, as shown in the third embodiment, the effect of reducing the speckle pattern can be increased by increasing the number of laser beam separations.

  In the first to fourth embodiments described above, the laser beam to be scanned is separated into a plurality of laser beams due to the effect of birefringence by transmitting the laser beam through speckle reduction means (birefringent plate).

  In the case of using a conventional phase hologram, high-order diffracted light is generated and the light quantity loss is a problem.

  On the other hand, in the configuration of each embodiment of the present invention, no diffracted light is generated, so no light loss occurs. Further, since the plurality of laser beams emitted from the speckle reduction means are parallel, there is no beam spread as in the case of using a conventional phase hologram, and the projection distance can be arbitrarily set.

  In addition, an exit window is provided at the exit (laser exit portion) of the laser beam in the scanning display device of each embodiment. The exit window is made of a transparent member typified by glass and prevents dust from entering the apparatus.

  Since the speckle reduction means in each embodiment has a size that covers the entire area for two-dimensional scanning with a laser beam, it can also be used as an exit window of a scanning display device. Therefore, the number of parts can be reduced, and speckle noise in the projected image can be reduced with a compact and low-cost optical system configuration.

  In each embodiment, examples of the one-dimensional scanning type MEMS mirror device and the galvanometer mirror are shown as the scanning unit 104. However, other examples such as a MEMS mirror device that can vibrate one mirror in a two-dimensional direction. The scanning means may be used.

  In each embodiment, an example using calcite was described as the birefringent plate constituting the speckle reduction means, but even when using a birefringent plate such as quartz or rutile, and using calcite A similar speckle noise reduction effect can be obtained.

  As described above, according to each embodiment, in a scanning display apparatus that displays an image on a screen by two-dimensionally scanning a laser beam, the speckle noise of a projected image is achieved even though the optical system is compact and has no light loss. Can be effectively reduced, and a high-quality image can be displayed. Further, the laser beam to be scanned has little spread, and the projection distance can be set arbitrarily.

  According to the present embodiment, the optical system is compact, the light amount loss is small, speckle noise can be reduced while suppressing a decrease in resolution, and a small scan capable of arbitrarily setting the projection distance. A mold display device is obtained.

1 is a schematic configuration diagram of a scanning display device according to a first embodiment of the present invention. Explanatory drawing which shows the mode (experimental example) of the change of the speckle pattern in Example 1. FIG. 3 is a table showing a correlation coefficient (experimental example) of a speckle pattern in Example 1. FIG. FIG. 3 is an explanatory diagram showing an optical path of a cross section including the optical axis of the birefringent plate in Example 1. FIG. 5 is a schematic configuration diagram of a scanning display device according to a second embodiment of the present invention. Explanatory drawing which shows the structure of the speckle reduction means in Example 2. FIG. FIG. 6 is an explanatory diagram showing the arrangement and polarization direction of four separated laser beam spots on a screen in Example 2. FIG. 6 is a schematic configuration diagram of a scanning display device according to a third embodiment of the present invention. Explanatory drawing which shows the structure of the speckle reduction means in Example 3. FIG. FIG. 10 is an explanatory diagram showing the arrangement and polarization direction of eight separated laser beam spots on a screen in Example 3. FIG. 6 is an explanatory diagram showing the configuration of speckle reduction means using four birefringent plates in Example 3. FIG. 11 is an explanatory diagram showing the arrangement and polarization direction of 16 separated laser beam spots on a screen in Example 3. FIG. 6 is a schematic configuration diagram of a scanning display device according to a fourth embodiment of the present invention.

Explanation of symbols

100 birefringent plate 101 laser light source 102 collimator lens 103 condensing lens 104 scanning means 104H horizontal scanning means 104V vertical scanning means 105 screen 106 forward scanning line 107 backward scanning line 108 scanning return line 109 vertical scanning direction 110 fluctuation of horizontal scanning means Movement direction 111 Swing direction of vertical scanning means 112 Scanned surface effective portion 120 Light source modulation circuit 121 Image signal supply device 200, 300, 400 Speckle reduction means 401-r Red semiconductor laser 401-g Infrared semiconductor laser 401 -B Blue semiconductor lasers 402-r, 402-b, 404-g Collimator lens 402-g Condensing lens 403-g LN crystal 405 Cross dichroic prism

Claims (10)

In a scanning display device that displays a two-dimensional image on a surface to be scanned by scanning a laser beam that is light-modulated and emitted from a light source unit based on image information.
One or more birefringent plates are provided between the scanning means and the screen;
An optical axis of the birefringent plate is inclined with respect to a polarization direction of the laser beam.   2. The scanning display according to claim 1, wherein a polarization plane parallel to both the polarization direction of the laser beam and the traveling direction of the laser beam is inclined with respect to the optical axis of the birefringent plate. apparatus.   2. The one birefringent plate is disposed so as to be separated into two or more laser beams whose polarization directions are orthogonal to each other after the laser beam passes through the birefringent plate. Or the scanning display device according to 2 above.   The one birefringent plate has a plane parallel to both the optical axis and the principal ray of the light beam incident on the center of the field angle of the image displayed on the scanned surface. 4. The scanning display device according to claim 1, wherein the scanning display device is disposed at an angle of 45 degrees or substantially 45 degrees with respect to the direction.   The one birefringent plate is arranged such that the direction of separation of the laser beam emitted from the birefringent plate is perpendicular or substantially perpendicular to the scanning line formed on the scanned surface. The scanning display device according to claim 1, wherein:   6. The scanning display device according to claim 1, wherein a plurality of the birefringent plates are provided, and the adjacent birefringent plates are arranged so that their optical axes are different from each other. . When the separation width of the laser beam emitted from the one birefringent plate is d and the spot diameter of the laser beam on the surface to be scanned is w, d> w
The scanning display device according to claim 1, wherein: The laser beam from the light source means is guided to the scanning means through an optical system, and the thickness t of the one birefringent plate is k as a truncation constant (k = 1.64 for a circular aperture), Fno the image-side F-number of the optical system, the wavelength of the laser beam lambda, ordinary refractive index of the n ₀ birefringent plate, the refractive index of the extraordinary ray of the n _e birefringent plate, a θ of the birefringent plate When the angle between the optical axis and the polarization direction of the laser beam
The scanning display device according to claim 1, wherein: The light source means has three light source portions that emit laser beams of three color lights, and the separation width of the green laser beam emitted from the one birefringent plate is set to dg on the screen surface of the green laser beam. Dg> wg where the spot diameter in w is wg
The scanning display device according to claim 1, wherein:   10. The scanning display device according to claim 1, wherein the one or more birefringent plates also serve as an emission window of a laser beam emitted from the scanning display device. 11.