Patent Publication Number: US-2012044279-A1

Title: Image Projection Apparatus and Laser Beam Projection Apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an image projection apparatus that projects an image using a laser beam. 
     BACKGROUND ART 
     A display using a laser as a light source can reproduce natural colors with a wider color range, as compared with using a discharge lamp as the light source. Particularly, a laser-beam scan type projector requires no projection optical system, and therefore can be downsized as compared with a method of using a projection optical system to project an illumination light modulated by a two-dimensional spatial modulation device such as a liquid crystal panel or a DMD (digital micromirror device). 
     The laser-beam scan type projector can also reduce power consumption, because it changes the amount of light emission from a light source in accordance with the luminance of a pixel to be displayed, such as stopping the light emission from the light source in a case of displaying a black pixel. In this manner, the laser-beam scan type projector has excellent features as a display device, and is attracting attention as a next-generation high-quality display. 
     On the other hand, a laser display has a problem specific thereto that due to a high coherence of the laser beam, non-uniformity of the luminance on a viewing surface occurs to cause eye flickering called speckle noise (hereinafter also referred to as “speckle”), so that a person viewing the displayed image suffers discomfort and eye strain. Thus, a reduction in the speckle noise is one of technical problems involved in practical use of the laser display. 
     There are generally two methods for reducing speckle noise. In first one of the methods, a generated speckle pattern is changed over time, and a plurality of patterns being overlapped on one another are shown by utilizing an afterimage effect of human eyes, to thereby average the non-uniformity of the luminance and thus reduce the speckle noise. 
     According to Patent Document 3 that adopts the first method, in a rear-projection type television using a laser beam as a light source, a screen is rotated in parallel with a viewing surface, to thereby change a speckle pattern over time and thus reduce speckle noise. 
     In second one of the methods for reducing speckle noise, a plurality of polarization states are overlapped on one another or a plurality of wavelengths are overlapped on one another, to thereby reduce a coherence of a light emitted from a light source and thus reduce speckle noise. 
     According to Patent Document 1 and Patent Document 2 that adopt the second method, a single-mode semiconductor laser (also referred to as “LD”, “laser diode”, or the like) is used as a laser light source, and the LD is used in a multi-mode by using a relaxation oscillation that occurs at a time when a drive current of the LD having a high-frequency signal superimposed thereon exceeds an oscillation threshold current so that the LD starts light emission. In this method, a light-emission wavelength interval is increased by causing the LD to emit a light in the multi-mode, to thereby reduce a coherence of a laser beam emitted from the LD and thus reduce speckle noise. 
       FIG. 1  shows an example of transient response characteristics in a case where the LD is driven by a square-wave current. In  FIG. 1 , the horizontal axis represents a time axis t, and the vertical axis represents the number of photons S. As shown in  FIG. 1 , an ideal response is a square wave Ri in which the number of photons S rises simultaneously with a drive current waveform. Actually, however, the LD starts light emission after elapse of an oscillation delay time period td, and a waveform Rr of the number of photons S repeats a vibration called a relaxation oscillation, and then converges to its steady-state value. Here, a time period tr for which the relaxation oscillation is sustained is referred to as a relaxation oscillation duration. 
     The oscillation delay time period td normally changes within a range of approximately 0 to 5 nsec due to a bias current of the LD prior to the rise of the drive current of the LD, and becomes shorter as the bias current is closer to the oscillation threshold current. Since a shorter oscillation delay time period td is more preferable, the bias current is normally brought closer to the oscillation threshold current and the LD is driven such that the oscillation delay time period td can fall within a range of 0 to 1 nsec. Normally, the relaxation oscillation duration tr is approximately 2 to 3 nsec. While the relaxation oscillation is occurring, the LD emits a light in the multi-mode, and when the relaxation oscillation converges, the LD emits a light in the single-mode. 
     Thus, it is possible that the single-mode LD repeatedly emits a light in the multi-mode, by a method in which the relaxation oscillation exhibited in the waveform Rr is repeatedly generated by repeatedly performing: using a drive current of a continuous square wave to cause the LD being in a light-emission stop state to emit a light; and then setting the drive current lower than the oscillation threshold current of the LD before the relaxation oscillation converges, to thereby stop the light emission from the LD. 
       FIG. 2  shows an example of the transient response characteristics in a case where the LD is driven by a continuous square-wave current. In  FIG. 2 , the vertical axis and the horizontal axis are set in the same manner as in  FIG. 1 . 
     Each of the square waves Ri shown in  FIG. 2  indicates an ideal response of the number of photons S with respect to the drive current of the continuous square wave. The rise and fall of each square wave Ri are coincident with the rise and fall of the continuous square-wave current generated by superimposing a high-frequency signal, respectively. Therefore, in  FIG. 2 , the t 1  represents a time period (also referred to as “semiconductor-laser drive period” or “pulse width”) during one cycle of the drive current of the LD having the high-frequency signal superimposed thereon, in which the current value of the drive current is equal to or more than the current value of the oscillation threshold current of the LD, while the t 0  represents a time period (also referred to as “semiconductor-laser non-drive period”) during one cycle of the drive current of the LD having the high-frequency signal superimposed thereon, in which the current value of the drive current is less than the current value of the oscillation threshold current of the LD. 
     This driving method is called a high-frequency superimposition method, and widely used as a method for suppressing mode hopping noise caused by a return light in pickup of CD/DVD or the like. This is effective as a method for reducing speckle noise in a laser display, too, because a coherence of a projected laser beam is reduced due to the multi-mode. 
     PRIOR-ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. 2001-189520 
         Patent Document 2: Japanese Patent Application Laid-Open No. 2004-70286 
         Patent Document 3: Japanese Patent Application Laid-Open No. 2006-343663 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the method adopted in the Patent Document 3 requires a mechanism for mechanically driving the screen, which causes problems of increased power consumption, occurrence of noise, a lowered reliability, an increased cost due to an increase in the number of component parts, and the like. In an apparatus such as a small-size portable projector (categorized as a so-called pocket projector in recent years) using no special screen, an idea of moving a screen is not possible in the first place. 
     In the high-frequency superimposition method adopted by the Patent Document 1 and the Patent Document 2, it is necessary that the drive current of the LD is lowered to a value equal to or less than the oscillation threshold current in each cycle of the high-frequency signal in order to generate the relaxation oscillation. 
     Although a shorter length of the semiconductor-laser non-drive period t 0  of the drive current of the LD is preferable because it can suppress a reduction in the light intensity, the drive current of the LD in a laser projector is approximately a few hundred mA and therefore it is difficult, in terms of circuit design, to extremely shorten the length of the semiconductor-laser non-drive period t 0 . In actual designing, normally, a time period of about 1 nsec is required. 
     In this manner, performing the high-frequency superimposition creates a time period in which the light emission from the LD is stopped, thus reducing an average amount of light emission from the LD. The reduction in the amount of light emission leads to a reduction in the maximum luminance of an image that is displayed using a single LD, and therefore is a problem in a laser projector requiring a high light intensity. 
     Here, a ratio r of a time period in which the LD emits a light is calculated by the expression (1) using the semiconductor-laser drive period t 1  and the semiconductor-laser non-drive period t 0  of the drive current of the LD, and the oscillation delay time period td of the LD. 
     [Math. 1] 
         r =( t 1 −td )/( t 1 +t 0)  (1)
 
     In order to suppress a reduction in the amount of light emission, the frequency of the high-frequency signal superimposed by the high-frequency superimposition is lowered to thereby increase the length of the semiconductor-laser drive period t 1  of the drive current, thus increasing the ratio r of the time period in which the LD emits a light. However, in a case where the frequency of the high-frequency signal superimposed by the high-frequency superimposition is lowered so that the semiconductor-laser drive period t 1  of the drive current becomes longer than the sum of the oscillation delay time period td and the relaxation oscillation duration tr and thus exceeds the relaxation oscillation duration tr to cause the LD to emit a light, the light emission from the LD is the single-mode and therefore the speckle-noise reduction effect is deteriorated. In this manner, in the high-frequency superimposition method, a trade-off relationship is established between the speckle-noise reduction effect and the amount of light emission from the LD. 
     Moreover, in the laser projector, since a high light intensity is required, the drive current of the LD is large, too. Thus, the high-frequency superimposition method involves a problem that occurrence of EMI (electromagnetic interference) due to an electromagnetic wave caused by use of a high-frequency drive current makes it impossible to satisfy the noise standard when mounting a laser beam projection apparatus on an equipment, and a problem that the size and the cost of the laser beam projection apparatus are increased by a circuit for generating the high-frequency drive current and a shield structure necessary for an anti-EMI product. 
     Particularly, in an apparatus including a plurality of LDs, such as a color projector, if respective LDs of a laser beam projection apparatus including a plurality of LDs are driven by a drive current having the same frequency as in Patent Document 1, the peak of EMI becomes large at a fundamental frequency thereof and each harmonic frequency thereof. This consequently causes a problem that, when the laser beam projection apparatus is mounted on an image projection apparatus or the like, the noise standard cannot be satisfied or alternatively a shield structure or the like serving as an anti-EMI member is required, which increases the cost. 
     Therefore, an object of the present invention is to provide an image projection apparatus and a laser beam projection apparatus that reduce speckle noise by using high-frequency superimposition and that can suppress EMI while reducing a deterioration in the light emission intensity of a semiconductor laser. 
     Means for Solving the Problems 
     To solve the above-described problems, an image projection apparatus according to a first aspect includes: a laser-drive circuit for superimposing each of a plurality of high-frequency signals on corresponding one of a plurality of image signals for different color components to generate drive currents; a plurality of semiconductor lasers for emitting laser beams of different wavelengths in accordance with drive currents respectively; and a deflection section for deflecting each of the laser beams onto the projection surface. In the image projection apparatus, the plurality of high-frequency signals have different fundamental frequencies from one another. 
     An image projection apparatus according to a second aspect is the image projection apparatus according to the first aspect, in which for the plurality of semiconductor lasers, a semiconductor-laser drive period of each of the drive currents is set so as to monotonically increase sequentially from ones of the plurality of semiconductor lasers in which the sum of an oscillation delay time period and a relaxation oscillation duration is shorter. 
     An image projection apparatus according to a third aspect is the image display projection apparatus according to the first aspect, in which for the plurality of semiconductor lasers, a semiconductor-laser drive period of each of the drive currents is set so as to be equal to the sum of an oscillation delay time period and a relaxation oscillation duration. 
     An image projection apparatus according to a fourth aspect is the image projection apparatus according to the first aspect, in which for the plurality of semiconductor lasers, a semiconductor-laser drive period of each of the drive currents is set so as to monotonically increase sequentially from one of the plurality of semiconductor lasers that emits the laser beam having a wavelength to which human&#39;s luminosity factor is higher. 
     An image projection apparatus according to a fifth aspect is the image projection apparatus according to the first aspect, in which the plurality of high-frequency signals have different harmonic frequencies from one another in a range up to a predetermined-order harmonic. 
     A laser beam projection apparatus according to a sixth aspect is a laser beam projection apparatus for projecting a laser beam, including: a laser-drive circuit for superimposing each of a plurality of high-frequency signals on corresponding one of a plurality of image signals for different color components to generate drive currents; and a plurality of semiconductor lasers for emitting laser beams of different wavelengths in accordance with the drive currents, respectively. In the laser beam projection apparatus, the plurality of high-frequency signals have different fundamental frequencies from one another. 
     A laser beam projection apparatus according to a seventh aspect is a laser beam projection apparatus for projecting a laser beam, including: a plurality of semiconductor lasers for emitting laser beams of different wavelengths in accordance with drive currents supplied thereto; and a laser-drive circuit for generating each of the drive currents that serve as cycle signals, such that a semiconductor-laser drive period of each of the drive current can be equal to the sum of an oscillation delay time period and a relaxation oscillation duration corresponding to each of the plurality of semiconductor laser. In the laser beam projection apparatus, each of the drive currents is generated based on a high-frequency signal and an image signal corresponding to each of the plurality of semiconductor lasers, and the drive currents have different cycles from one another. 
     Effects of the Invention 
     In the image projection apparatus according to any of the first to fifth aspects and in the laser beam projection apparatus according to the sixth aspect, the plurality of high-frequency signals to be superimposed on the image signals of the plurality of color components for generating the drive currents of the plurality of semiconductor lasers by the laser-drive circuit have different fundamental frequencies. As a result, spectrums of EMIs caused based on the fundamental frequencies of the high-frequency signals when the plurality of semiconductor lasers are driven can be dispersed to lower a peak value of the EMI, so that the EMI can be efficiently suppressed. 
     In the image projection apparatus according to the second aspect, for the plurality of semiconductor lasers, the semiconductor-laser drive period of each of the drive currents of the plurality of semiconductor lasers is set so as to monotonically increase sequentially from one of the plurality of semiconductor lasers in which the sum of the oscillation delay time period and the relaxation oscillation duration is shorter. This can efficiently suppress speckle noise while reducing a deterioration in the light emission intensity. 
     In the image projection apparatus according to the fourth aspect, for the plurality of semiconductor lasers, the semiconductor-laser drive period of each of the drive currents of the plurality of semiconductor lasers is set so as to monotonically increase sequentially from one of the plurality of semiconductor lasers that emits the laser beam having a wavelength to which human&#39;s luminosity factor is higher. This can efficiently suppress visual speckle noise while reducing a deterioration in the light emission intensity. 
     In the laser beam projection apparatus according to the seventh aspect, the semiconductor-laser drive period of each of the drive currents that serve as cycle signals to be supplied to the plurality of semiconductor lasers is equal to the sum of the oscillation delay time period and the relaxation oscillation duration corresponding to each of the plurality of semiconductor lasers, and the drive currents have different cycles. This can efficiently suppress speckle noise while dispersing spectrums of EMIs caused by the respective drive currents to thereby lower a peak value of the EMI. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A diagram showing an example of response characteristics of an LD responsive to an input of a square wave. 
         FIG. 2  A diagram showing an example of response characteristics of the LD responsive to an input of a continuous square wave. 
         FIG. 3  A block diagram showing an outline of an exemplary functional configuration of an image projection apparatus according to an embodiment. 
         FIG. 4  A block diagram showing an outline of an exemplary functional configuration of a light-source drive section according to the embodiment. 
         FIG. 5  A block diagram showing an outline of a functional configuration of an LD drive circuit according to the embodiment. 
         FIG. 6  A diagram partially showing an example of a signal waveform in a laser-drive circuit according to the embodiment. 
         FIG. 7  A diagram showing an outline of an exemplary functional configuration of an optical mechanism section according to the embodiment. 
         FIG. 8  A diagram showing an example of response characteristics of the LD responsive to an input of a square wave. 
         FIG. 9  A diagram showing an example of response characteristics of the LD responsive to an input of a square wave. 
         FIG. 10  A diagram showing an example of response characteristics of the LD responsive to an input of a square wave. 
         FIG. 11  A diagram for explaining the human&#39;s spectral luminous efficiency with respect to various wavelengths of lights. 
     
    
    
     EMBODIMENT FOR CARRYING OUT THE INVENTION 
     Outline Configuration of Image Projection Apparatus 
       FIG. 3  is a block diagram showing an exemplary functional configuration of an image projection apparatus  100  according to an embodiment of the present invention. 
     The image projection apparatus  100  is an apparatus for projecting a moving image to a screen SC serving as a projection surface, and mainly includes an input-image processing section  110 , a drive control section  120 , and an optical mechanism section  130 . The image projection apparatus  100  is equivalent to the “image display projection apparatus” of the invention of the present application. 
     The input-image processing section  110  includes an image input circuit  111 , an image processing circuit  112 , and a frame memory  124 . The image input circuit  111  receives an image signal inputted from an input device IM, and outputs it to the image processing circuit  112 . The image processing circuit  112  appropriately performs image processing such as digitalization on the image signal supplied from the image input circuit  111 , and outputs a resulting signal to an image output circuit  121  of the drive control section  120  (which will be described later). The image signal outputted to the image output circuit  121  is written into the frame memory  124  by the image output circuit  121 . 
     Here, examples of the input device IM include a personal computer (PC), and examples of the image signal include a general NTSC signal. Examples of the image processing performed by the image processing circuit  112  include general γ-correction processing and processing of changing the order of pixel values in a case where a change of the order of scanning pixels is required. 
     The drive control section  120  includes the image output circuit  121 , a deflection control circuit  122 , a light-source drive section  123 , and a sensor-output processing circuit  125 . 
     The image output circuit  121  reads out the image signal from the frame memory  124 , and in response to a horizontal synchronization signal and a vertical synchronization signal of the image signal, outputs to the deflection control circuit  122  a signal (also referred to as “deflection control signal”) for controlling a timing of driving a two-dimensional deflection section  132 , and also outputs to the light-source drive section  123  a signal (also referred to as “pixel data signal”) corresponding each color component (red, green, and blue) of a pixel value of the image signal, a signal (also referred to as “frequency setting signal”) for setting a frequency (fundamental frequency) of a fundamental wave of a high-frequency signal used for high-frequency superimposition to generate a drive current of an LD of each color that emits a light of each color component, and a signal (also referred to as “bias-current setting signal”) for setting a bias current of the LD of each color. The pixel data signal, the frequency setting signal, and the bias-current setting signal will be further described later. 
     The image output circuit  121  receives an output signal (which will be described later) from the sensor-output processing circuit  125 , and in accordance with the strength of the output signal, adjusts the strength of the pixel data signal to be outputted to the light-source drive section  123 . 
     The deflection control circuit  122  supplies, to the two-dimensional deflection section  132 , a drive signal having a potential in accordance with the deflection control signal supplied from the image output circuit  121 . 
     The light-source drive section  123  outputs, to each of a plurality semiconductor lasers (which will be described later) provided in a laser light source section  133 , a drive current in accordance with a pixel data signal supplied from the image output circuit  121 , such that the semiconductor laser can emit a light whose color and luminance are in accordance with a tone corresponding to the pixel data signal supplied from the image output circuit  121 . This output operation is performed in response to the horizontal synchronization signal and the vertical synchronization signal of the image signal. 
     The sensor-output processing circuit  125  performs necessary processes such as an amplification process on the output signal outputted from a light-receiving element  14  ( FIG. 7 ) of the laser light source section  133 , and outputs a resulting signal to the image output circuit  121 . 
     The input-image processing section  110  and the drive control section  120  may be functionally implemented by a CPU reading and executing a predetermined program, or alternatively may be configured as a special electronic circuit. 
     The optical mechanism section  130  includes the two-dimensional deflection section  132  and the laser light source section  133 . 
     The laser light source section  133  has a plurality of semiconductor lasers that emit laser beams of the respective colors of red, green, and blue. The laser light source section  133  outputs to the two-dimensional deflection section  132  a laser beam whose color and luminance are in accordance with a tone corresponding to the pixel data signal supplied from the image output circuit  121 , and also outputs to the sensor-output processing circuit  125  an electrical signal in accordance with the intensity of each laser beam. 
     The two-dimensional deflection section  132  has a part (reflection part) that reflects a luminous flux of a laser beam emitted from the laser light source section  133 . The reflection part rotates around two substantially perpendicular axes, to thereby deflect the luminous flux emitted from the laser light source section  133  in a two-dimensionally reflecting manner, so that the luminous flux is guided onto the screen SC serving as the projection surface, thus projecting a moving image on the screen SC. 
     Here, the light-source drive section  123  and the laser light source section  133  correspond to a “laser beam projection apparatus” of the present invention. In  FIG. 3 , a laser beam traveling from the laser light source section  133  through the two-dimensional deflection section  132  to the screen SC is expressed by the arrow of thick broken line. A specific configuration of the optical mechanism section  130  including the two-dimensional deflection section  132  and the laser light source section  133  will be further described later. 
     In this manner, the image projection apparatus  100 , the input device IM, and the screen SC configure an image projection system that outputs an image data supplied from the input device IM such that the image data can be visible on the screen SC. 
     &lt;Outline Configuration of Light-Source Drive Section&gt; 
       FIG. 4  is a block diagram showing an outline of an exemplary functional configuration of the light-source drive section  123  according to the embodiment of the present invention. 
     The light-source drive section  123  is a circuit for supplying a drive current to each of the semiconductor lasers  1 R,  1 G, and  1 B ( FIG. 7 ) of the laser light source section  133 , and mainly includes a red-LD drive circuit  123 R, a green-LD drive circuit  123 G, and a blue-LD drive circuit  123 B. The light-source drive section  123  corresponds to a “laser-drive circuit” of the present invention. 
     The image output circuit  121  supplies, to the red-LD drive circuit  123 R, a pixel data signal  11 R corresponding to the red color component of the pixel value of the image signal, and a frequency setting signal  12 R and a bias-current setting signal  13 R corresponding to the semiconductor laser  1 R. 
     Likewise, the image output circuit  121  supplies, to the green-LD drive circuit  123 G, a pixel data signal  11 G corresponding to the green color component of the pixel value of the image signal, and a frequency setting signal  12 G and a bias-current setting signal  13 G corresponding to the semiconductor laser  1 G. 
     Thus, the image output circuit  121  supplies, to the blue-LD drive circuit  123 B, a pixel data signal  11 B corresponding to the blue color component of the pixel value of the image signal, and a frequency setting signal  12 B and a bias-current setting signal  13 B corresponding to the semiconductor laser  1 B. 
     Based on the pixel data signal, the frequency setting signal, and the bias-current setting signal thus supplied, the red-LD drive circuit  123 R, the green-LD drive circuit  123 G, and the blue-LD drive circuit  123 B generates drive currents IdR, IdG, and IdB for driving the semiconductor lasers  1 R,  1 G, and  1 B, respectively, and supply them to the semiconductor laser. Each of the drive currents IdR, IdG, and IdB corresponds to “each drive current” of the present invention. 
     The frequency setting signals  12 R,  12 G, and  12 B, and the bias-current setting signals  13 R,  13 G, and  13 B are adjustable by means of, for example, changing the setting of a DIP switch of predetermined bit provided in the image output circuit  121 . 
     &lt;Outline Configuration of Drive Circuit of Each Light Source&gt; 
     The red-LD drive circuit  123 R, the green-LD drive circuit  123 G, and the blue-LD drive circuit  123 B employ the same circuit configuration. Here, an outline configuration of each LD drive circuit will be described by taking the red-LD drive circuit  123 R as an example. 
       FIG. 5  is a block diagram showing an outline of an exemplary functional configuration of the red-LD drive circuit  123 R, and  FIG. 6  is a diagram partially showing an example of a signal waveform in the red-LD drive circuit  123 R. 
     As shown in  FIG. 5 , the red-LD drive circuit  123 R mainly includes D/A converters (also referred to as “DAC”)  4 ,  5 , and  6 , an analog switch (also referred to as “Analog SW”)  7 , a voltage control oscillator (also referred to as “VCO”)  8 , an LD driver  9 , and a bias current source  10 . 
     The image output circuit  121  supplies the pixel data signal  11 R, the frequency setting signal  12 R, and the bias-current setting signal  13 R to the DAC  4 ,  5 , and  6 , respectively. 
     By the DAC  4 , the pixel data signal  11 R is converted into an analog voltage V 1  ( FIG. 5 ,  FIG. 6 ), and then inputted to the analog switch  7 . 
     Also in the green-LD drive circuit  123 G and the blue-LD drive circuit  123 B, analog voltages V 6  and V 7  (not shown) corresponding to the analog voltage V 1  are generated, respectively. Here, the analog voltages V 1 , V 6 , and V 7  correspond to “a plurality of color components” of the present invention. 
     By the DAC  5 , the frequency setting signal  12 R corresponding to the semiconductor laser  1 R is converted into an analog voltage V 2 , and then supplied to the VCO  8 . In the VCO  8 , a high-frequency signal V 3  ( FIG. 5 ,  FIG. 6 ) having a fundamental frequency in accordance with the analog voltage V 2  is generated and supplied to the analog switch  7 . That is, the high-frequency signal V 3  is a high-frequency signal associated with the semiconductor laser  1 R. 
     Also in the green-LD drive circuit  123 G and the blue-LD drive circuit  123 B, high-frequency signals V 8  and V 9  (not shown) corresponding to the high-frequency signal V 3  that are high-frequency signals associated with the semiconductor lasers  1 G and  1 B, respectively, are generated. Here, each of the high-frequency signals V 3 , V 8 , and V 9  correspond to “a high-frequency signal” of the present invention. 
     The fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  can be adjusted by adjusting the frequency setting signals  12 R,  12 G, and  12 B. 
     The analog switch  7  uses the high-frequency signal V 3  as a control input signal, and if the high-frequency signal V 3  is higher than a predetermined threshold voltage TH 1  ( FIG. 6 ) of a control input, outputs the analog voltage V 1  while if the high-frequency signal V 3  is lower than the threshold voltage TH 1 , outputs a ground (“GND”)—level signal V 0 . 
     In other words, a signal V 4  ( FIG. 5 ,  FIG. 6 ) obtained by chopping the analog voltage V 1  based on the pixel data signal  11 R with the fundamental frequency of the high-frequency signal V 3  supplied from the VCO  8  is outputted from the analog switch  7 , and supplied to the LD driver  9 . Accordingly, as shown in  FIG. 6 , the fundamental frequency of the signal V 4  has the same value as that of the fundamental frequency of the high-frequency signal V 3 , and the signal V 4  is delayed behind the analog voltage signal V 1  and the high-frequency signal V 3  due to a delay in the circuit, for example. 
     Here, the threshold voltage TH 1  can be adjusted by, for example, adjusting a resistance (not shown) that determines the value of the threshold voltage TH 1 , which allows adjustment of a duty cycle between an image portion of the signal V 4 , that is, a signal corresponding to the analog voltage V 1 , and a signal corresponding to the GND-level signal V 0 . 
     The LD driver  9  converts the signal V 4  into a current I 4  in accordance with a signal level thereof. Further, a bias current IbR (which will be described later) is added to the current I 4 , thus generating the drive current IdR for driving the semiconductor laser  1 R. That is, the drive current IdR is a drive current that is generated by superimposing the high-frequency signal V 3  associated with the semiconductor laser  1 R on the analog voltage V 1  and that has the same fundamental frequency as that of the high-frequency signal V 3 . 
     In the same manner, the drive currents IdG and IdB ( FIG. 4 ) are drive currents that are generated by superimposing the high-frequency signals V 8  and V 9  associated with the semiconductor lasers  1 G and  1 B on the analog voltages V 6  and V 7 , respectively, and that have the same fundamental frequencies as those of the high-frequency signals V 8  and V 9 , respectively. 
     By the DAC  6 , the bias-current setting signal  13 R is converted into an analog voltage signal V 5 , and then inputted to the bias current source  10 . The bias current source  10  outputs the bias current IbR in accordance with the inputted analog voltage signal V 5 . As mentioned above, the bias current IbR is added to the current I 4 . 
     An oscillation delay time period td ( FIG. 8 ) of the semiconductor laser  1 R is changed by the bias current IbR. As the bias current IbR becomes lower than the oscillation threshold current of the semiconductor laser  1 R, the oscillation delay time period td is increased to reduce an average amount of light emission from the semiconductor laser  1 R. 
     On the other hand, when the bias current IbR exceeds the oscillation threshold current of the semiconductor laser  1 R, the semiconductor laser  1 R emits a light even at a black level on the image, which deteriorates an image contrast. Therefore, it is desirable to control the bias current IbR to a value as close to the oscillation threshold current as possible but within a range not exceeding the oscillation threshold current. 
     &lt;Outline Configuration of Optical Mechanism Section&gt; 
       FIG. 7  is a diagram showing an example of an outline configuration of the optical mechanism section  130 . As shown in  FIG. 7 , the optical mechanism section  130  includes the two-dimensional deflection section  132  and the laser light source section  133 . In  FIG. 7 , coordinate axes are shown for the indication of directions. 
     The two-dimensional deflection section  132  corresponds to “deflection means” of the present invention, and is formed of, for example, a so-called MEMS (Micro Electro Mechanical Systems) mirror which is obtained by performing microfabrication on a silicon chip. The two-dimensional deflection section  132  includes a movable part that has a plurality of piezoelectric elements, a torsion bar, and the like, and a deflection scanning mirror  132   a  that reflects a luminous flux emitted from the laser light source section  133 . 
     A potential drive signal supplied from the deflection control circuit  122  in accordance with the deflection control signal causes the plurality of piezoelectric elements to expand or contract as appropriate, thus deforming the movable part, so that the deflection scanning mirror  132   a  rotates around the two substantially perpendicular axes (an a-axis substantially parallel to the X axis in  FIG. 7  and a b-axis substantially parallel to the Y-axis in  FIG. 7 ). As a result of this rotation, the luminous flux emitted from the laser light source section  133  is deflected in a two-dimensionally reflecting manner. In this specification, the wording “deflecting in a two-dimensional direction” is used to express a state where the reflection part rotates around the two axes to thereby change a traveling direction of the luminous flux with respect to the vertical direction and the horizontal direction independently of each other, in other words, a state where the luminous flux is deflected in the vertical direction while being also deflected in the horizontal direction. 
     Here, a deflection drive signal for achieving a low-speed rotation of the deflection scanning mirror  132   a  around the a-axis and a deflection drive signal for achieving a high-speed rotation of the deflection scanning mirror  132   a  around the b-axis by using resonance driving thereof are superimposed on each other and then applied to the plurality of piezoelectric elements. This allows the deflection scanning mirror  132   a  to simultaneously perform the low-speed rotation around the a-axis and the high-speed rotation around the b-axis using the resonance driving. Thus, by deflecting the laser beam in two different directions, horizontal scanning and vertical scanning with the laser beam can be simultaneously performed on the screen SC. Two-dimensional scanning in which the horizontal scanning and the vertical scanning are simultaneously performed using a single element is preferable in view of reducing the number of component parts of the two-dimensional deflection section  132 , and also preferable in view of reducing the manufacturing cost and reducing operations required for adjustment of elements. 
     The laser light source section  133  includes the semiconductor lasers  1 R,  1 G, and  1 B that emit laser beams, and a lens (collimator lens) that converts the laser beam emitted from the semiconductor laser into a substantially parallel luminous flux. 
     Here, the laser light source section  133  includes a pair of the semiconductor laser  1 R that produces a red (R) laser beam  15 R and a collimating lens that converts the laser beam  15 R into a substantially parallel luminous flux, a pair of the semiconductor laser  1 G that produces a green (G) laser beam  15 G and a collimator lens  2 G that converts the laser beam  15 G into a substantially parallel luminous flux, and a pair of the semiconductor laser  1 B that produces a blue (B) laser beam  15 B and a collimator lens  2 B that converts the laser beam  15 B into a substantially parallel luminous flux. Here, each of the semiconductor lasers  1 R,  1 G, and  1 B corresponds to a “semiconductor laser” of the present invention. 
     For example, semiconductor lasers that emit laser beams having wavelengths 630 nm, 532 nm, and 445 nm are adopted as the semiconductor lasers  1 R,  1 G, and  1 B, respectively. In accordance with the drive currents IdR, IdG, and IdB supplied from the light-source drive section  123  at a timing responsive to the horizontal synchronization signal and the vertical synchronization signal of the image signal, each of the semiconductor lasers  1 R,  1 G, and  1 B produces and emits a laser beam whose luminance corresponds to each color component (red, green, blue) of the pixel value of the image signal. For example, a semiconductor laser that emits a laser beam in an infrared region may be adopted, and a laser beam emitted from the semiconductor laser may be subjected to wavelength conversion using an SHG (second harmonic generation) element so that a visible light is emitted. 
     The laser beams  15 R,  15 G, and  15 B converted into the substantially parallel luminous fluxes are, as luminous fluxes substantially in parallel with the Z-axis of  FIG. 7 , inputted to a combining section  3 . 
     The combining section  3  includes a dichroic mirror  3 R for reflecting the red laser beam  15 R while passing lights having the other wavelengths, a dichroic mirror  3 G for reflecting the green laser beam  15 G while passing lights having the other wavelengths, and a dichroic mirror  3 B for reflecting the blue laser beam  15 B while passing lights having the other wavelengths. The dichroic mirror  3 B may be an ordinary mirror that reflects any of the laser beams. Instead of each of the dichroic mirrors, a dichroic prism may be adopted, and instead of the ordinary mirror, a ordinary prism that reflects any of the laser beams may be adopted. 
     The position and angle of each semiconductor laser and the position and angle of each dichroic mirror of the combining section  3  are appropriately adjusted. The laser beams  15 R,  15 G, and  15 B inputted substantially in parallel with the Z-axis to the combining section  3  are incident on the dichroic mirrors  3 R,  3 G, and  3 B, respectively, and combined into a single luminous flux  15   a  substantially parallel with the Y-axis due to the function of the dichroic mirrors, and then outputted from the combining section  3 , to be inputted to a branching element  8 . 
     For example, the branching element  8  can be implemented by forming an uncoated portion in a part (for example, a central part) of a reflective mirror coating so that a luminous flux inputted to the uncoated portion can pass therethrough while a luminous flux inputted to the remaining portion can be reflected, or by applying a dielectric multiplayer coating over an entire area of a substrate made of, for example, glass where a luminous flux is inputted to thereby form a semi-transmissive mirror (also referred to as a “leakage mirror”) having both of transmission characteristics and reflection characteristics so that the whole of the inputted luminous flux can pass through with a predetermined transmittance while the whole of the inputted luminous flux can be reflected with a predetermined reflectance. The transmittance value and the reflectance value of the branching element  8  such as a semi-transmissive mirror are appropriately set to be 95% and 5%, respectively, for example. 
     In the former branching element that passes therethrough a luminous flux inputted to the uncoated portion while reflecting a luminous flux inputted to the remaining portion, the intensity distribution of a transmitted light and a reflected light is damaged in a cross-section perpendicular to the luminous flux, and the directivity of the transmitted light and the reflected light is deteriorated due to diffraction. Although this deterioration may not damage the usability of the apparatus, it is rather desirable to adopt the latter leakage mirror as the branching element  8 . 
     The laser beam  15   a  inputted to the branching element  8  is branched into a laser beam  15   b  and a laser beam  15   c . The laser beam  15   b  passes through the branching element  8  toward the +Y direction substantially in parallel with the Y-axis. The laser beam  15   c  is reflected by the branching element  8  toward the −Z direction substantially in parallel with the Z-axis. 
     The laser beam  15   b  is inputted to the two-dimensional deflection section  132 , and deflected in a two-dimensional direction by the deflection scanning mirror  132   a , and then projected onto the screen SC. The laser beam  15   c  is inputted to the light-receiving element  14  that outputs an electrical signal in accordance with the intensity of the laser beam  15   c , then converted into an electrical signal, and outputted to the sensor-output processing circuit  125 . 
     Examples of the light-receiving element  14  include a photodiode of current-output type, and a photodetector sensor of current-output type or voltage-output type. The sensor-output processing circuit  125  of the image projection apparatus  130  is provided with a current-voltage converter, an amplifier, or the like, in accordance with a sensor adopted as the light-receiving element  14 . 
     &lt;Frequency Selection Method&gt; 
     Next, a description will be given of response characteristics of the semiconductor laser at a time of high-frequency superimposition, and a method for selecting a frequency of the high-frequency signal suitable for the response characteristics.  FIGS. 8 ,  9 , and  10  are diagrams showing examples of response characteristics, responsive to an input of a square wave, of the semiconductor lasers  1 R,  1 G, and  1 B ( FIG. 7 ) that emit lights of the wavelengths of red color, green color, and blue color, respectively. 
     Example in which a high-frequency signal having the same fundamental frequency is superimposed: 
     As shown in  FIGS. 8 ,  9 , and  10 , the sum of an oscillation delay time period td and a relaxation oscillation duration tr is smallest in the semiconductor laser  1 G that emits a green light, and is 2.4 nsec. 
     Accordingly, in a case where the high-frequency superimposition is performed on the three semiconductor lasers  1 R,  1 G, and  1 B using the three high-frequency signals V 3 , V 8 , and V 9  having the same fundamental frequency to thereby obtain the drive currents of the respective semiconductor lasers, it is necessary to prevent any of the three semiconductor lasers from oscillating in a single-mode and to set the semiconductor-laser drive period t 1  ( FIG. 2 ) of each of the drive currents IdR, IdG, and IdB of the semiconductor lasers to be 2.4 nsec or less based on the semiconductor laser  1 G, in order to maintain a speckle-noise reduction effect. 
     When a semiconductor-laser non-drive period t 0  ( FIG. 2 ) of each of the drive currents IdR, IdG, and IdB is defined as 1 nsec, a fundamental frequency f of each of the high-frequency signals V 3 , V 8 , and V 9  is calculated based on the expression (2). In this case, therefore, the high-frequency signals V 3 , V 8 , and V 9  having the same fundamental frequency of 294 MHz or more are required. 
     [Math. 2] 
         f =1/( t 1 +t 0)  (2)
 
     Here, if it is assumed that the semiconductor-laser non-drive period t 0  of the drive current of each semiconductor laser is 1 nsec and any of the fundamental frequencies f of the high-frequency signals V 3 , V 8 , and V 9  corresponding to the drive currents of the respective semiconductor lasers is 294 MHz, a ratio r of a time period in which each of the semiconductor lasers  1 R,  1 G, and  1 B emits a light is obtained as the value indicated in Table 1, based on the expression (1). In Table 1, the upper row of “Red LD”, “Green LD”, “Blue LD”, and “Average” indicate the semiconductor lasers  1 R,  1 G,  1 B, and the average of the semiconductor lasers, respectively, and the lower row of numerical values indicate the ratios of the time periods of the respective semiconductor lasers emit lights, and the average of the ratios of the time periods in which the respective semiconductor lasers emit lights. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Red LD 
                 Green LD 
                 Blue LD 
                 Average 
               
               
                   
                   
               
             
            
               
                   
                 53% 
                 59% 
                 59% 
                 57% 
               
               
                   
                   
               
            
           
         
       
     
     In a case where the semiconductor lasers are driven by using the drive currents IdR, IdG, and IdB of the semiconductor laser that are generated by superimposing the high-frequency signals V 3 , V 8 , and V 9  having the same fundamental frequency on the analog voltages V 1 , V 6 , and V 7 , respectively, the peak of EMI becomes large at frequencies of the fundamental wave each harmonic wave of the drive current, as mentioned above. 
     Therefore, in an example described below, the frequency of the high-frequency signal to be used for the high-frequency superimposition for generating the drive current of each LD is selected in accordance with the response characteristics of the LD such that suppression of the EMI and suppression of a deterioration in the light emission intensity can be obtained while maintaining the speckle reduction effect in the semiconductor lasers  1 R,  1 G, and  1 B. 
     Example 1 of superimposing high-frequency signals having different fundamental frequencies: 
     Firstly, an example of setting will be described in which a condition (also referred to as “condition  1 ”) is satisfied that the drive currents IdR, IdG, and IdB have different fundamental frequencies, that is, the high-frequency signals V 3 , V 8 , and V 9  used for high-frequency superimposition have different fundamental frequencies, and in addition a condition (also referred to as “condition  2 ”) is satisfied that the semiconductor-laser drive period of the drive current monotonically increases sequentially from one of the semiconductor lasers  1 R,  1 G, and  1 B in which the sum of the oscillation delay time period and the relaxation oscillation duration is shorter. 
     In an exemplary case where the drive currents IdR, IdG, and IdB satisfy the conditions  1  and  2 , as indicated by the response characteristics of the respective LDs shown in  FIGS. 8 to 10 , while the sum of the oscillation delay time period td and the relaxation oscillation duration tr differs among the semiconductor lasers  1 R,  1 G, and  1 B, the frequency setting signals  12 R,  12 G, and  12 B supplied from the image output circuit  121  are adjusted to thereby adjust the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9 , so that the semiconductor-laser drive periods t 1  of the drive currents IdR, IdG, and IdB can be set equal to the sum of the oscillation delay time period td and the relaxation oscillation duration tr of the corresponding semiconductor lasers, respectively. Thereby, in the response characteristics of the respective LDs shown in  FIGS. 8 to 10 , the semiconductor-laser drive period&#39;s t 1  of the drive currents IdR, IdG, and IdB are 3.6 nsec, 2.4 nsec, and 2.8 nsec, respectively. 
     Accordingly, if the semiconductor-laser non-drive period t 0  of each of the drive currents IdR, IdG, and IdB is set to 1 nsec similarly to the above-described example, the fundamental frequencies of the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B, that is, the fundamental frequencies f of the high-frequency signals V 3 , V 8 , and V 9 , are 217 MHz, 294 MHz, and 263 MHz, respectively, based on the expression (2). 
     In this case, the ratio r of a time period in which each of the semiconductor lasers  1 R,  1 G, and  1 B emits a light is obtained as the value indicated in Table 2, based on the expression (1). The denotations of Table 2 are the same as those of Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Red LD 
                 Green LD 
                 Blue LD 
                 Average 
               
               
                   
                   
               
             
            
               
                   
                 65% 
                 59% 
                 63% 
                 62% 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 shows the fundamental frequency and each harmonic frequency (unit: MHz) of each of the high-frequency signals V 3 , V 8 , and V 9  used for the high-frequency superimposition on the analog voltages V 1 , V 6 , and V 7  for generating the drive currents IdR, IdG, and IdB in this case. In Table 3, the items “Fundamental Wave” to “Seventh Harmonic Wave” represent a fundamental wave through a seventh harmonic of a high-frequency signal corresponding to each semiconductor laser, and the items “Red LD”, “Green LD”, and “Blue LD” represent the semiconductor lasers  1 R,  1 G, and  1 B, respectively. In the table, the frequencies appearing in italic type represent frequencies being higher than the highest frequency among the fifth harmonic frequencies of the high-frequency signal used for high-frequency superimposition with respect to each of the semiconductor lasers  1 R,  1 G, and  1 B. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Red LD 
                 Red LD 
                 Green LD 
                 Blue LD 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Fundamental Wave 
                 217 
                 294 
                 263 
               
               
                   
                 Second Harmonic Wave 
                 435 
                 588 
                 526 
               
               
                   
                 Third Harmonic Wave 
                 652 
                 882 
                 789 
               
               
                   
                 Fourth Harmonic Wave 
                 870 
                 1176 
                 1053 
               
               
                   
                 Fifth Harmonic Wave 
                 1087 
                 1471 
                 1316 
               
               
                   
                 Sixth Harmonic Wave 
                 1304 
                 1765 
                 1579 
               
               
                   
                 Seventh Harmonic Wave 
                 1522 
                 2059 
                 1842 
               
               
                   
                   
               
            
           
         
       
     
     Firstly, from Table 3, it can be found that spectrums of EMIs caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed without overlapping each other at the fundamental frequencies of the respective drive currents. 
     That is, as shown in Table 3, the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  paired with the semiconductor lasers  1 R,  1 G, and  1 B, respectively, satisfy the condition  1 . In other words, the fundamental frequencies of the respective high-frequency signals are set to be different from one another. Thereby, in an EMI caused by the fundamental wave and each harmonic wave of each of the drive currents IdR, IdG, and IdB that are generated by superimposition of the high-frequency signals V 3 , V 8 , and V 9  on the analog voltages V 1 , V 6 , and V 7 , respectively; a spectrum of the EMI caused by the fundamental wave, which may produce the maximum EMI peak, is dispersed without overlap. Therefore, a peak value of the EMI caused by the fundamental waves that occur when the plurality of semiconductor lasers  1 R,  1 G, and  1 B are driven is lowered as compared with, for example, a case where all of the high-frequency signals used for high-frequency superimposition have the same fundamental frequency. Thus, the EMI can be suppressed efficiently. 
     Thus, from the viewpoint of suppression of EMI, it is more desirable that high-frequency signals used for high-frequency superimposition for generating the drive currents of the plurality of LDs have different fundamental frequencies, than that all of the high-frequency signals have the same fundamental frequency. 
     Table 1 and Table 2 reveal the fact that, in a case of Table 2, the ratios of the time periods in which the semiconductor lasers  1 R and  1 B emit lights increases so that a deterioration in the average amount of total light emission from each semiconductor laser is reduced as compared with the case of Table 1 where all of the high-frequency signals V 3 , V 8 , and V 9  to be superimposed on the analog voltages V 1 , V 6 , and V 7  for generating the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B have the same fundamental frequency. 
     That is, the semiconductor lasers  1 R,  1 G, and  1 B are set such that the semiconductor-laser drive period t 1  of the drive current can monotonically increase sequentially from one of the semiconductor lasers  1 R,  1 G, and  1 B in which the sum of the oscillation delay time period td and the relaxation oscillation duration tr is shorter. Thereby, while efficiently suppressing speckle noise in each semiconductor laser in accordance with differences in the response characteristics among the respective semiconductor lasers responsive to the drive currents, a deterioration in the light emission intensity of each LD can be reduced as shown in Table 2. In addition, a deterioration in the average amount of total light emission of each semiconductor laser can also be reduced. 
     Moreover, Table 3 reveals that the spectrums of the EMIs caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed without overlapping each other at least up to the fifth harmonic wave. 
     Here, in the EMI standard, a measurement frequency range is defined as 1 GHz or lower according the VCCI standard, and as a fifth harmonic wave of the maximum frequency or lower according to the FCC standard. Therefore, in the example shown in Table 3, no frequency overlap occurs within the range defined by the VCCI standard and the FCC standard. 
     Merely when the high-frequency signals V 3 , V 8 , and V 9  have different fundamental frequency values, an advantageous effect can be obtained. However, when the frequencies of the high-frequency signals V 3 , V 8 , and V 9  are set so as not to overlap each other in a range from the fundamental wave to a predetermined-order harmonic wave, for example, up to the fifth harmonic wave, not only the spectrums of EMIs caused by the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  but also the spectrums of EMIs caused by the harmonic frequencies up to a predetermined-order harmonic frequency are dispersed. This can further lower the peak value of the EMI caused when the plurality of semiconductor lasers  1 R,  1 G, and  1 B are driven, to allow efficient suppression of the EMI. 
     Example 2 of superimposing high-frequency signals having different fundamental frequencies: 
     Next, an example will be shown in which the frequency of the high-frequency signal used for high-frequency superimposition is selected in accordance with the response characteristics of each LD such that a greater speckle reduction effect can be exhibited in an LD in which speckle noise tends to be prominent. 
       FIG. 11  is a diagram for explaining the human&#39;s spectral luminous efficiency (also referred to as “luminosity function”) with respect to lights of various wavelengths.  FIG. 11  reveals that, in a case of adopting LDs that emit laser beams having wavelengths of 645 nm, 532 nm, and 445 nm as a red light, a green light, and a blue light, respectively, the luminosity factors for the red light and the blue light are lower than the luminosity factor for the green light. The green light corresponding to a high luminosity factor causes the most prominent speckle noise to the human eye, and therefore it is important to reduce the speckle noise caused by the LD that emits a green laser beam. On the other hand, in the blue laser beam having the lowest luminosity, less speckle noise is caused. Therefore, putting priority on the suppression of the deterioration in the light intensity, the semiconductor-laser drive period t 1  ( FIG. 2 ) of the drive current of the semiconductor laser that emits the blue light can be set to be longer than the sum of the oscillation delay time period td and the relaxation oscillation duration tr. 
     Next, an example will be shown in which the frequencies of the high-frequency signals V 3 , V 8 , and V 9  to be superimposed on the analog voltages V 1 , V 6 , and V 7  for generating the drive currents IdR, IdG, and IdB of the respective semiconductor lasers are selected in consideration of the human&#39;s luminosity factor with respect to the semiconductor lasers  1 R,  1 G, and  1 B that have the transient response characteristics shown in  FIGS. 8 to 10  and that emit laser beams of wavelength of red-color, green-color, and the blue-color. 
     Here, an example of setting will be described in which the condition  1  mentioned in Example 1 is satisfied, in other words, the high-frequency signals V 3 , V 8 , and V 9  used for high-frequency superimposition have different fundamental frequencies while a condition (also referred to as “condition  3 ”) is satisfied that the semiconductor-laser drive period t 1  of the drive current monotonically increases sequentially from one of the semiconductor lasers  1 R,  1 G, and  1 B that emits a laser beam having a wavelength to which the human&#39;s luminosity factor is higher. 
     The frequency setting signals  12 R,  12 G, and  12 B supplied from the image output circuit  121  are adjusted to thereby adjust the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9 , so that the semiconductor-laser drive period t 1  of each drive currents can be set equal to the sum of the oscillation delay time period td and the relaxation oscillation duration tr of each of the semiconductor lasers  1 R,  1 G, and  1 B shown in the response characteristics of  FIGS. 8 to 10 . Thereby, the semiconductor-laser drive periods t 1  of the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B are 3.6 nsec, 2.4 nsec, and 2.8 nsec, respectively. Here, in order that the drive currents IdR, IdG, and IdB can satisfy the conditions  1  and  3 , the semiconductor-laser drive periods t 1  of the drive currents of the semiconductor lasers  1 R and  1 G causing more prominent speckle noise are set to be slightly shorter for priority on the speckle-noise reduction effect, while the semiconductor-laser drive period t 1  of the semiconductor laser  1 B causing less prominent speckle is set to be longer for priority on ensuring of the light intensity. In this case, for example, if time periods of 3.3 nsec, 2.2 nsec, and 4.0 nsec are adopted as the semiconductor-laser drive periods t 1  of the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B, respectively, while a time period of 1 nsec is adopted as the semiconductor-laser non-drive period t 0  of each drive current; the fundamental frequencies of the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B, that is, the fundamental frequencies f of the high-frequency signals V 3 , V 8 , and V 9 , are 233 MHz, 313 MHz, and 200 MHz, respectively, based on the expression (2). 
     In this case, the ratio r of a time period in which each of the semiconductor lasers  1 R,  1 G, and  1 B emits a light is obtained as the value indicated in Table 4, based on the expression (1). The denotations of Table 4 are the same as those of Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Red LD 
                 Green LD 
                 Blue LD 
                 Average 
               
               
                   
                   
               
             
            
               
                   
                 63% 
                 56% 
                 72% 
                 64% 
               
               
                   
                   
               
            
           
         
       
     
     Table 5 shows the fundamental frequency and each harmonic frequency (unit: MHz) of each of the high-frequency signals V 3 , V 8 , and V 9  used for the high-frequency superimposition on the analog voltages V 1 , V 6 , and V 7  for generating the drive currents IdR, IdG, and IdB in this case. The denotations of Table 5 are the same as those of Table 3. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Red LD 
                 Red LD 
                 Green LD 
                 Blue LD 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Fundamental Wave 
                 233 
                 313 
                 200 
               
               
                   
                 Second Harmonic Wave 
                 465 
                 625 
                 400 
               
               
                   
                 Third Harmonic Wave 
                 698 
                 928 
                 600 
               
               
                   
                 Fourth Harmonic Wave 
                 930 
                 1250 
                 800 
               
               
                   
                 Fifth Harmonic Wave 
                 1163 
                 1563 
                 1000 
               
               
                   
                 Sixth Harmonic Wave 
                 1395 
                 1875 
                 1200 
               
               
                   
                 Seventh Harmonic Wave 
                 1628 
                 2188 
                 1400 
               
               
                   
                 Eighth Harmonic Wave 
                 1860 
                 2500 
                 1600 
               
               
                   
                   
               
            
           
         
       
     
     Firstly, from Table 5, it can be found that spectrums of EMIs caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed without overlapping each other at the fundamental frequencies of the respective drive currents. 
     This is, similarly to the result shown in Table 3 in Example 1, because of the following. That is, the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  paired with the semiconductor lasers  1 R,  1 G, and  1 B, respectively, satisfy the condition  1 , and in other words, they are set to be different from one another. Thereby, in an EMI caused by the fundamental wave and each harmonic wave of each of the drive currents IdR, IdG, and IdB that are generated by superimposition of the high-frequency signals V 3 , V 8 , and V 9 , respectively; a spectrum of the EMI caused by the fundamental wave, which may produce the maximum EMI peak, is dispersed without overlap. Therefore, a peak value of the EMI caused by the fundamental waves that occur when the plurality of semiconductor lasers  1 R,  1 G, and  1 B are driven is lowered as compared with a case where all of the high-frequency signals used for high-frequency superimposition have the same fundamental frequency. Thus, the EMI can be suppressed efficiently. 
     Thus, from the viewpoint of suppression of EMI, it is more desirable that high-frequency signals used for high-frequency superimposition have different fundamental frequencies, than that all of the high-frequency signals have the same fundamental frequency. 
     As shown in Table 2 and Table 4, regarding the semiconductor lasers  1 R and  1 G, the ratio of the light-emission time period is shorter in the result shown in Table 4 than in the result shown in Table 2 because of priority on the speckle-noise reduction effect, while regarding the semiconductor laser  1 B, the ratio of the light-emission time period is performed increases in the result shown in Table 4 as compared with the result shown in Table 2 because of priority on increase in the ratio of the light-emission time period. 
     From Table 1 and Table 4, it can be found that an average deterioration in the light emission intensity of the semiconductor lasers can be reduced in Table 4 showing the result of setting the fundamental frequencies of the high-frequency signals used for high-frequency superimposition for the respective semiconductor lasers such that visual speckle noise can be reduced in consideration of the human&#39;s luminosity factor to the wavelength of the laser beam emitted from each semiconductor laser, as compared with Table 1 showing the result of setting the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  to be superimposed on the analog voltages V 1 , V 6 , and V 7  for generating the drive currents of the semiconductor lasers  1 R,  1 G, and  1 B such that all of them can have the same fundamental frequency. 
     That is, the setting is made such that the semiconductor-laser drive period t 1  of the drive current can monotonically increase sequentially from one of the semiconductor lasers  1 R,  1 G, and  1 B that emits a laser beam having a wavelength to which the human&#39;s luminosity factor is higher. Thereby, in accordance with the human&#39;s luminosity factor to the wavelength of the laser beam emitted from each semiconductor laser, the visual speckle noise of each semiconductor laser can be suppressed in an efficient manner while a deterioration in an average amount of light emission of each semiconductor laser can also be reduced as shown in Table 4. 
     Moreover, Table 5 reveals that the spectrums of the EMIs caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed without overlapping each other at least up to the fifth harmonic wave. 
     Therefore, in the example shown in Table 5, similarly to the result shown in Table 3 of Example 1, no frequency overlap occurs within the range defined by the VCCI standard and the FCC standard. 
     Merely when the high-frequency signals V 3 , V 8 , and V 9  have different fundamental frequency values, an advantageous effect can be obtained. However, when the frequencies of the high-frequency signals V 3 , V 8 , and V 9  are set so as not to overlap each other in a range from the fundamental wave to a predetermined-order harmonic wave, for example, up to the fifth harmonic wave, not only the spectrums of EMIs caused by the fundamental frequencies of the high-frequency signals V 3 , V 8 , and V 9  but also the spectrums of EMIs caused by the harmonic frequencies up to a predetermined-order harmonic frequency are dispersed. This can further lower the peak value of the EMI caused when the plurality of semiconductor lasers  1 R,  1 G, and  1 B are driven, to allow efficient suppression of the EMI. 
     Although there is no overlap at the harmonic wave of the drive signal of each LD in Example 1 and Example 2, overlap may occur in the fundamental wave or the harmonic wave depending on the transient response characteristics of an LD used. In such a case, if the EMI arises a problem at the frequency where the overlap occurs, frequency overlap can be avoided by shifting a drive frequency for any of LDs causing the frequency overlap. Here, which of the LDs should be the target of changing the frequency of the high-frequency signal corresponding to the drive current may be appropriately set in consideration of the speckle-noise reduction effect and a deterioration in the light emission intensity. 
     Although in Example 1, the frequency of the high-frequency signal to be superimposed in each LD is selected such that the semiconductor-laser drive period t 1  can be equal to the sum of the oscillation delay time period td and the relaxation oscillation duration tr, the frequency of the high-frequency signal for each LD may be adjusted in accordance with a demand concerning the speckle-noise reduction effect and the light emission intensity. 
     In the same manner, in Example, the frequency of the high-frequency signal for each LD may be adjusted in accordance with a demand concerning the speckle-noise reduction effect and the light emission intensity. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
         
           
               1 R,  1 G,  1 B semiconductor laser 
               2 R,  2 G,  2 B collimator lens 
               3  combining section 
               3 R,  3 G,  3 B dichroic mirror 
               4 ,  5 ,  6  D/A converter 
               7  analog switch 
               8  voltage control oscillator 
               9  LD driver 
               10  bias current source 
               11 R,  11 G,  11 B pixel data signal 
               12 R,  12 G,  12 B frequency setting signal 
               13 R,  13 G,  13 B bias-current setting signal 
               14  light-receiving element 
               100  image projection apparatus 
               110  input-image processing section 
               111  image input circuit 
               112  image processing circuit 
               120  drive control section 
               121  image output circuit 
               122  deflection control circuit 
               123  light-source drive section 
               124  frame memory 
               125  sensor-output processing circuit 
               130  optical mechanism section 
               132  two-dimensional deflection section 
               132   a  deflection scanning mirror 
               133  laser light source section 
             IM input device 
             SC screen