Abstract:
A projector using coherent light as a light source includes: one or more coherent light sources emitting coherent light beams; a beam scanner which changes a direction of the coherent light beam; and a beam splitter which splits a beam into multiple segments and changes a phase of light of each split segment, the beam splitter being located in an optical path between the beam scanner and the coherent light source. This projector reduces speckle noises. The beam splitter includes a pair of transparent substrates; a plurality of parallel electrodes disposed on each surface of the pair of transparent substrates, the pair of the parallel electrodes being disposed in a perpendicular direction to each other. Voltages are applied to the respective electrodes at predetermined time intervals so as to produce different potentials in a liquid crystal sandwiched by the transparent substrates.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application JP-2010-80103, filed on Mar. 31, 2010, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to display systems which display a video by modulating and scanning a light beam from a coherent light source such as a laser light source in accordance with video signals, and relates to techniques to reduce speckle noises at the time of display. 
     A laser display system is proposed in JP-A-2003-021800, for example, in which a video is displayed on a screen by emitting three color laser beams to an operating mirror and reflecting the three color laser beams. In such a laser display system, the use of the high coherent laser beam causes a speckle phenomenon that a random spotted pattern is generated, and thus the quality as a display becomes an issue. 
     In order to suppress such speckle phenomenon, there is a method of mechanically vibrating an optical path as shown in JP-B-4144713, for example, however, the frequency is insufficient for sufficiently reducing the speckle and thus the speckle reduction effect is small. 
     As the other display systems using the laser beam, a method is also proposed as shown in JP-A-06-208089, in which an image modulated by an optical modulator is enlarged and projected in accordance with image information. This method also has a problem of the speckle. With regard to this problem, JP-A-2007-163702 proposes a speckle canceller comprising a pair of transparent substrates and a liquid crystal sealed therebetween. 
     SUMMARY OF THE INVENTION 
       FIG. 3  illustrates a relationship between a laser beam and a screen when the whole laser beam is fluctuated as in JP-04144713. In the case of a laser scanning display system similar to that of JP-A-2003-021800, speckles are generated reflecting unevenness of a screen  100  in a range where one beam  90  is projected to the screen  100 . If the projection range on the screen  100  shifts from the beam  90  before fluctuation to a beam  91  after fluctuation and overlaps by a length “a”, then the shape of the unevenness does not change in the range of “a” and accordingly the speckle does not change, either. That is, in the method according to JP-A-04144713, the larger the amplitude of the fluctuation and the smaller the overlap of the beam before and after fluctuation, the larger the speckle reduction effect becomes. However, at the same time, there is a problem that the blurring of the image is large as a side effect. 
     It is an object of the present invention to solve the above-described problems and provide a high definition laser display system capable of reducing the speckles of the laser beam. 
     In order to solve the above-described problems, a projector using coherent light as a light source according to the present invention comprises: one or more coherent light sources emitting coherent light; a beam scanner which changes the direction of a beam obtained from the coherent light source; and a beam splitter which splits the beam into a plurality of segments and changes a phase of light of each split segment, the beam splitter being located in an optical path between the beam scanner and the coherent light sources. 
     Here, the beam splitter comprises: a pair of transparent substrates; a plurality of first parallel electrodes disposed on one of the transparent substrates and a plurality of second parallel electrodes disposed on the other one of the transparent substrates, the first parallel electrodes being disposed in a direction perpendicular to the second parallel electrodes; and a liquid crystal enclosed between the transparent substrates, wherein voltages are applied to the respective electrodes at predetermined time intervals so as to produce different potentials in the liquid crystal sandwiched by the transparent substrates. 
     According to the present invention, apparent speckles of a laser beam can be reduced, and therefore a high definition laser projector capable of suppressing degradation in the image quality due to the speckles can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an overall configuration of a display system according to a first embodiment. 
         FIG. 2  is a view illustrating how to split a laser beam. 
         FIG. 3  is an explanatory view when the whole beam is fluctuated. 
         FIG. 4  is an explanatory view when a beam is split and fluctuated. 
         FIG. 5  is a view representing a phase relationship between split beams. 
         FIG. 6  illustrates a configuration diagram of a beam splitter. 
         FIG. 7A  is a diagram representing each electrode potential and a potential difference in each cell in an n-th display frame. 
         FIG. 7B  is a diagram representing each electrode potential and a potential difference in each cell in an (n+1)th display frame. 
         FIG. 8  is a chart of voltage waveforms applied to respective electrodes according to the first embodiment. 
         FIG. 9A  is a diagram representing each electrode potential and a potential difference in each cell in the n-th display frame according to a second embodiment. 
         FIG. 9B  is a diagram representing each electrode potential and a potential difference in each cell in the (n+1)th display frame according to the second embodiment. 
         FIG. 10  is a chart of voltage waveforms applied to respective electrodes according to a third embodiment. 
         FIG. 11  is a chart of voltage waveforms applied to respective electrodes according to a fourth embodiment. 
         FIG. 12  is a chart of voltage waveforms applied to respective electrodes according to a fifth embodiment. 
         FIG. 13  is a view illustrating the shape of each electrode in a sixth embodiment. 
         FIG. 14  is a view illustrating a glass plate in a seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the present invention, a laser beam  92  to be projected is split into a plurality of beam regions in a beam cross section as illustrated in  FIG. 2 , and the optical phase of each split beam is differentiated from each other, and in addition, their phase states are dynamically varied, thereby scattering speckles generated and also reducing the speckles by averaging the speckles over time. 
     For example, as illustrated in  FIG. 4 , the beam  92  is split into five beam regions b, c, d, e, and f, and a phase relationship among the five regions in an n-th display frame differs from a phase relationship in an (n+1)th display frame as represented in  FIG. 5 . For this reason, a speckle pattern in the n-th display frame differs from a speckle pattern of the (n+1)th display frame, and as a result, a reduced speckle can be seen because these speckle patterns are averaged over time in terms of human&#39;s recognition. 
     In this example, the phase states are varied by a unit of a frame, and as a result, in the case of a video including a moving object, the speckle reduction effect is small because the speckles cannot be averaged over time. Ideally, the speckle reduction effect at the time of a motion picture becomes greater if the phase relationship is varied while a beam is scanning one dot. Although one beam is split into five regions in this example, the more the number of split regions, the greater the effect becomes. 
     For means for splitting the beam and varying the optical phase, a method of using a liquid crystal cell illustrated in  FIG. 6  may be used, or a device to which a MEMS (Mechanical Electro Micro System) is applied or a device using an electrooptic crystal may be used. In addition, a glass plate having uneven cross-section as illustrated in  FIG. 14  may be used. Hereinafter, more specific configurations will be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a view illustrating an overall configuration of a display system according to a first embodiment. The display system of the first embodiment combines laser beams of red and blue and green semiconductor lasers modulated with video signals, and displays a video by scanning the resultant RGB-combined laser beam onto a screen  100  using an MEMS mirror  50 . More specifically, a beam emitted from a green laser  10  is shaped by a collimation lens  20  into a substantially parallel beam, which is then incident upon a beam splitter  40  through a dichroic prism  30  and a dichroic prism  31  and are scanned onto the screen  100  by the MEMS mirror  50 . 
     Similarly, a beam emitted from a red laser  11  is shaped by a collimation lens  21  into a substantially parallel beam, which is then reflected by the dichroic prism  30  and is incident upon the beam splitter  40  through the dichroic prism  31 . A beam emitted from a blue laser  12  is shaped by a collimation lens  22  into a substantially parallel beam, which is then reflected by the dichroic prism  31  and is incident upon the beam splitter  40 . 
     Next, the beam splitter  40  is described. In the beam splitter  40 , as illustrated in  FIG. 2 , the incident RGB mixed laser beam is split into 5×5 segments  401  to  425 . Then, a beam passing through each segment will have an individual phase. For example, the beam size of each color is 1.0 mm in diameter on the beam splitter  40 , and each segment is divided at the pitch of 0.2 mm. 
       FIG. 6  illustrates a configuration diagram of the beam splitter  40 . The beam splitter  40  comprises a pair of substrates  430 ,  431  and a liquid crystal  432  filled therebetween. Electrodes  446 - 450  are formed on a lower surface of the substrate  430 , and electrodes  441 - 445  are formed on a upper surface of the substrate  431  in a direction perpendicular to the electrodes  446 - 450 . The region sandwiched by the upper and lower electrodes corresponds to the 5×5 segments  401  to  425  in  FIG. 2 . 
     The liquid crystal  432  operates as a phase modulation device, in which the voltages applied between the upper and lower electrodes produce a potential difference between the upper and lower electrodes, and this potential difference controls the phase of a laser beam passing through the liquid crystal. 
       FIGS. 7A ,  7 B represent examples of the voltages applied to each electrode of the n-th display frame and the (n+1)th display frame and potential differences between the electrodes of the upper and lower substrates in each segment. Though, for convenience&#39;s sake, an ON state is indicated by 1 and an OFF state is indicated by 0, the actual voltages are in a range of 5-20V. By randomly setting the voltage of each electrode, the potential difference in each segment is also randomly set. Moreover, by randomly setting the voltage of each electrode between the n-th display frame and the (n+1)th display frame, the potential difference in each segment is also randomly set. The direction of the filled liquid crystal varies depending on whether or not there is a potential difference in each segment, and accordingly the refractive index varies, and thus the phase varies when light passes through the filled liquid crystal. 
     As represented in  FIG. 8 , the electrode potential and the potential difference of a segment continue to randomly vary in the (n+2)th frame and thereafter.  FIG. 8  represents a timing of a voltage waveforms applied to respective electrodes, wherein the voltage waveforms of all the electrodes are switched at each frame. Moreover, the voltage of one electrode is not varied within the display time of one frame. 
     In this way, the phase of the laser beam passing through the adjacent segments varies spatially or temporally at random, whereby a different speckle pattern appears in the unit of display frame and therefore the speckle is diffused and reduced in terms of the human visual sense. 
     Embodiment 2 
     Next, a second embodiment is described with reference to  FIGS. 9A ,  9 B. The second embodiment is an example, in which the laser beam phase control of the beam splitter  40  is different from the first embodiment. The configuration of a display system applied is the same as that of the first embodiment, and the description thereof is omitted. 
       FIGS. 9A ,  9 B represent an example of voltages applied to each electrode of the n-th display frame and the (n+1)th display frame and a potential difference between the electrodes of the upper and lower substrates in each segment. The second embodiment differs from the first embodiment in that a multivalued voltage is applied to each electrode. In  FIGS. 9A ,  9 B, if four values 0 to 3 are used for each electrode voltage, the potential difference of each segment can be controlled in multiple values and the phase of a laser beam can be controlled more finely than the first embodiment. Moreover, in the second embodiment, the above-described beam splitting and phase control of the laser beam are performed in the unit of a frame as in the first embodiment. 
     Embodiment 3 
     In the first embodiment and second embodiment, the examples have been described in which the beam splitting and phase control of a laser beam is performed in the unit of a frame. However, if a ferroelectric liquid crystal is used for the liquid crystal  432  of the beam splitter  40 , the unit of phase control can be more finely divided. Thus, the speckle noise can be further diffused in time scale, and as a result, the speckle noise is effectively reduced. 
       FIG. 10  represents voltage waveforms applied to respective electrodes when a ferroelectric liquid crystal is used for the beam splitter to increase the modulation frequency and the waveform is changed at each 10 μS. When the waveform is changed at each frame, the speckle is changed by the time integration of a plurality of frames and therefore the speckle reduction effect is small in the case of a video including motions. However, by changing the phase of a laser beam a plurality of times within one frame, the speckle reduction effect in the case of the video including motions can be increased. 
     Embodiment 4 
     In Embodiment 3, a voltage is applied to each electrode at the same timing, however, as represented in  FIG. 11 , the voltage application timing may be shifted for each electrode. Since the phase of the voltage waveform applied to the electrode is shifted, the variation of the speckle generated in one frame further increases, so that even in the case of the video including motions, the speckle reduction effect can be increased more than the third embodiment. 
     Embodiment 5 
     Although Embodiments 1 to 4 represented the examples in which the voltage waveform applied to each electrode utilizes square waves as represented in  FIG. 12 , sine waves whose phases are shifted for each electrode may be used. In this case, the direction of the liquid crystal  432  of the beam splitter  40  smoothly varies, and therefore the same effect as the case where the phase of a laser beam is shifted by a number of times equal to or greater than the number of splits, can be obtained. Note that, in  FIG. 12 , although the phase is gradually shifted in the order of the rows or columns, the amount of phase shift may not be fixed. 
     Embodiment 6 
     Although Embodiments 1 to 5 showed the examples of the beam splitter  40  illustrated in  FIG. 6 , the electrodes may be formed only on one side as illustrated in  FIG. 13 . In this case, preferably, the direction of the electrodes is substantially perpendicular to the direction in which the laser beam is scanned. This is because the split beams whose phases vary according to a voltage waveform are sequentially emitted when observed from one point on the screen  100 . 
     Embodiment 7 
     Although Embodiments 1 to 6 represented the example of the beam splitter  40  having electrodes, a glass plate having uneven cross-section without electrodes as illustrated in  FIG. 14  may be used. This embodiment uses an effect that the phase varies if there is a difference in the thickness of glass when a beam passes through the glass. Differences between the unevenness are preferably equal to or greater than a half of the maximum wavelength of the beam passing through the glass. In this view, the glass plate has a striped unevenness in the vertical direction, however, the shape of the glass plate is not particularly limited, and the glass plate may have unevenness in a matrix. When the glass plate has the striped unevenness, preferably the direction of the unevenness is substantially perpendicular to the direction in which the laser beam is scanned. This is because the split beams whose phases vary according to a voltage waveform are sequentially emitted when observed from one point on the screen  100 . In addition, a method may be used, wherein instead of using glass, a mirror having the same shape as that of the glass is used and a laser beam is reflected by the mirror and the resultant reflected laser beam is applied. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.