Patent Publication Number: US-2011074837-A1

Title: Image display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from Japanese Patent Application No. 2009-229110 filed on Sep. 30, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to an image display device, and more particularly to an image display device provided with a laser beam source which receives inputting of an image signal corresponding to a gradation level of a pixel which constitutes an image and radiates a laser beam having intensity corresponding to the image signal. 
     2. Description of the Related Art 
     Conventionally, there has been known a scanning image display device in which a laser beam whose intensity is modulated in response to an image signal is radiated from a laser beam source, the laser beam is scanned by a scanning part in two-dimensional directions, and the laser beam is projected onto a projecting object thus displaying an image. As this type of image display device, there have been known a retinal scanning display in which a retina of a user is used as the projecting object and a screen scanning image display device in which a screen is used as the projecting object, for example. 
     When an image displayed by the scanning image display device is a color image, it is necessary to provide a plurality of laser beam sources which generate a plurality of laser beams of different wavelengths respectively. In general, the scanning image display device uses a red laser, a green laser and a blue laser. By converging laser beams radiated from these laser beam sources on the same optical path, the laser beams are synthesized thus producing a laser beam of various colors. 
     SUMMARY OF THE INVENTION 
     When a semiconductor laser is used as such a laser beam source, there has been known a semiconductor laser in which an input-output characteristic (current-light output characteristic) is steeply changed so that a bent portion is formed thus giving rise to a region where linearity collapses. It is desirable that the input-output characteristic smoothly continues from a first region where the increase/decrease of output intensity of light with respect to the increase/decrease of an electric current is gentle to a second region where the increase/decrease of the output intensity of the light is steep. However, there has been known a semiconductor laser in which an input-output characteristic has a third region where the increase/decrease of output intensity of light is steeper than the increase/decrease of output intensity of light of the second region between the first region and a second region. Such a third region is called a kink region. This kink region appears conspicuously with respect to a green laser and a blue laser. 
     In a case where the input-output characteristic of the semiconductor laser has such a kink region, when an image signal to which a gradation level is allocated on the presumption that the input-output characteristic has linearity is outputted to the semiconductor laser, a gradation crush occurs at a low gradation level. 
     Accordingly, when the semiconductor laser is used as a laser beam source, it is necessary to allocate an image signal at each gradation level corresponding to the above-mentioned kink region. However, this allocation processing is difficult so that the low gradation level cannot be accurately reproduced. 
     The present invention has been made under such circumstances, and it is an object of the present invention to provide an image display device which can easily and accurately reproduce a low gradation level even when a semiconductor laser having a kink region in an input-output characteristic thereof is used. 
     According to one aspect of the present invention, there is provided an image display device which includes: a signal generation part which generates and outputs an image signal corresponding to a gradation level of a pixel which constitutes an image; a laser beam source which radiates a laser beam having intensity corresponding to the image signal; and a signal adjustment part which superposes a high-frequency signal on the image signal inputted to the laser beam source. The signal adjustment part superposes a high-frequency signal which has a frequency not less than an inverse number of a generation cycle of the image signal and has amplitude not less than a width of a kink region where an input-output characteristic of the laser beam source changes most steeply on the image signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the schematic constitution of an image display device according to one embodiment of the present invention; 
         FIG. 2  is a graph showing an input-output characteristic of a semiconductor laser; 
         FIG. 3  is a graph for explaining the input-output characteristic when a high-frequency signal is superposed on an image signal; 
         FIG. 4  is a graph for explaining the input-output characteristic when a high-frequency signal is superposed on an image signal; 
         FIG. 5  is a graph for explaining the input-output characteristic when a high-frequency signal is superposed on an image signal; 
         FIG. 6  is a graph for explaining the input-output characteristic when a high-frequency signal is superposed on an image signal; 
         FIG. 7  is a schematic view showing the electrical constitution and the optical constitution of a retinal scanning display; 
         FIG. 8  is a view of a scanning range in which a laser beam is scanned by a scanning part shown in  FIG. 7 ; 
         FIG. 9  is a block diagram showing the constitution of a light source part shown in  FIG. 7 ; 
         FIG. 10A and 10B  are flowcharts showing the flow of processing executed by the retinal scanning display shown in  FIG. 7 ; 
         FIG. 11  is a flowchart showing the flow of processing executed by the retinal scanning display shown in  FIG. 7 ; 
         FIG. 12  is a view showing the data constitution of a gradation table; 
         FIG. 13  is a view showing the data constitution of a gradation table; and 
         FIG. 14  is a graph showing an input-output characteristic of a semiconductor laser having hysteresis. 
     
    
    
     DESCRIPTION 
     Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “embodiment”) is explained in conjunction with drawings. 
     The image display device according to this embodiment is an optical scanning type image display device. In the image display device, laser beams whose intensities are modulated in response to an image signal are radiated from laser beam sources, the laser beams are scanned by a scanning part in the two-dimensional directions, and the scanned laser beams are projected on a projecting object thus displaying a color image. 
     [1. Schematic Constitution of Image Display Device  1 ] 
     As shown in  FIG. 1 , the image display device  1  according to this embodiment includes a light source part  2  which sequentially radiates laser beams having intensities corresponding to respective pixels which constitute an image to be displayed, a scanning part  3  which two-dimensionally scans the laser beams radiated from the light source part  2 , and a projection part  4  which projects the laser beams scanned by the scanning part  3  on the projecting object. 
     The light source part  2  includes, for sequentially radiating laser beams having intensities corresponding to respective pixels for three respective primary colors, a plurality of lasers  5   r ,  5   g ,  5   b  which radiate laser beams having different wavelengths corresponding to the respective three primary colors. The laser beams which are radiated from these laser beam sources are converged on the same optical path by a synthesizing part  12  thus synthesizing the laser beams. The laser  5   r  is a red semiconductor laser which radiates a red laser beam, the laser  5   g  is a green semiconductor laser which radiates a green laser beam, and the laser  5   b  is a blue semiconductor laser which radiates a blue laser beam. 
     Further, the light source part  2  includes signal generation part  6   r ,  6   g ,  6   b  which generate image signals of the above-mentioned three respective primary colors. The signal generation part  6   r  generates an image signal  7   r  having a current value (signal value) corresponding to a gradation level of the red component of each pixel, and the image signal  7   r  is inputted to the red laser  5   r . Accordingly, the red laser beam whose intensity is modulated in response to the gradation level of the red component of each pixel is radiated from the red laser  5   r.    
     In the same manner, the signal generation part  6   g  generates an image signal  7   g  having a current value corresponding to a gradation level of the green component of each pixel, and the image signal  7   g  is inputted to the green laser  5   g . Further, the signal generation part  6   b  generates an image signal  7   b  having a current value corresponding to the gradation level of the blue component of each pixel, and the image signal  7   b  is inputted to the blue laser  5   b . Accordingly, the green laser beam whose intensity is modulated in response to the gradation level of the green component of each pixel is radiated from the green laser  5   g , and the blue laser beam whose intensity is modulated in response to the gradation level of the blue component of each pixel is radiated from the blue laser  5   b.    
     With respect to the green laser  5   g  and the blue laser  5   b , as shown in  FIG. 2 , an input-output characteristic (current-light output characteristic) has a kink region W where the input-output characteristic is steeply changed so that a bent portion is formed thus collapsing linearity thereof. That is, in addition to a first region where the increase/decrease of output intensity of light with respect to the increase/decrease of an electric current is gentle and a second region where the increase/decrease of output intensity of light with respect to the increase/decrease of an electric current is steep, the input-output characteristic also has the kink region W which is a third region where the increase/decrease of output intensity of light is steeper than the increase/decrease of output intensity of light in the second region between the first region and the second region. Accordingly, with the use of the image signals  7   g ,  7   b  which are outputted from the signal generation parts  6   g ,  6   b  with current values corresponding to gradation levels, the low gradation levels cannot be accurately reproduced. 
     In view of the above, as shown in  FIG. 1 , the light source part  2  includes a signal adjustment part  8   g  which superposes a high-frequency signal  9   g  on the image signal  7   g  inputted to the green laser  5   g , and a signal adjustment part  8   b  which superposes a high-frequency signal  9   b  on the image signal  7   b  inputted to the blue laser  5   b . For the sake of convenience, the explanation is made hereinafter assuming that the green laser  5   g  and the blue laser  5   b  have the same input-output characteristic. However, it is not always necessary that the green laser  5   g  and the blue laser  5   b  have the same input-output characteristic, and these lasers  5   g ,  5   b  usually have different input-output characteristics. Further, either one of the lasers  5   g ,  5   b  may be expressed as “laser  5 ”, and either one of the signal generation parts  6   g ,  6   b  may be expressed as “signal generation part  6 ”. Further, either one of the image signals  7   g ,  7   b  may be expressed as “image signal  7 ”, either one of the signal adjustment parts  8   g ,  8   b  may be expressed as “signal adjustment part  8 ”, and either one of the high-frequency signals  9   g ,  9   b  may be expressed as “high-frequency signal  9 ”. 
     When the high-frequency signal  9  is superposed on the image signal  7  inputted to the laser  5 , the intensity of the laser beam radiated from the laser  5  is changed corresponding to a change of amplitude of the high-frequency signal  9 . For example, assume that the high-frequency signal  9  having current amplitude Ia is inputted to the laser  5  having the input-output characteristic shown in  FIG. 2  such that the high-frequency signal  9  is superposed on the image signal  7  having a current value Ib as shown in  FIG. 3 . Here, a current value of an electric current inputted to the laser  5  is periodically increased or decreased between a current value I 1  (=Ib−Ia/2) and a current value  12  (=Ib+Ia/2). Accordingly, intensity of light radiated from the laser  5  is also changed between an intensity value Y 1  and an intensity value Y 2 . 
     In this manner, although the intensity of the laser beam radiated from the laser  5  is changed corresponding to the change of the amplitude of the high-frequency signal  9  when the high-frequency signal  9  is superposed on the image signal  7 , the brightness of each pixel visually recognized by a user becomes the brightness corresponding to intensity obtained by averaging the changing intensities. 
     Accordingly, when the high-frequency signal  9  is superposed on the image signal  7 , the input-output characteristic of the laser  5  is regarded as a characteristic which changes intensity of light corresponding to a current value of the image signal  7  as indicated by a solid line shown in  FIG. 4 . Hereinafter, such an input-output characteristic is referred to as an apparent input-output characteristic. A broken line shown in  FIG. 4  indicates the input-output characteristic of the laser  5  when the high-frequency signal  9  is not superposed on the image signal  7 . 
     In the image display device  1  according to this embodiment, the influence of the kink region W exerted on the input-output characteristic of the laser  5  is suppressed by superposing the high-frequency signal  9  on the image signal  7  thus approximating the relationship between the current value of the image signal  7  and the intensity of the laser beam to the proportional relationship. Due to such processing, the allocation of the current value of the image signal  7  at the low gradation level can be performed easily. 
     Further, in the image display device  1 , it is necessary to set the current amplitude of the high-frequency signal  9  superposed on the image signal  7  to not less than a width Ic of the kink region W. That is, it is necessary that a current range of the image signal  7  where a current value is changed due to the superposition of the high-frequency signal  9  covers a range of the kink region W. It is because when the width of the high-frequency signal  9  is smaller than the width Ic of the kink region W, as shown in  FIG. 5 , a region W 1  where the influence exerted by the kink region cannot be suppressed is generated. Further, a frequency of the high-frequency signal  9  superposed on the image signal  7  is set to a frequency which is not less than an inverse number 1/T (≧1/T) of a generation cycle T of the image signal  7  generated by the signal adjustment part  8  in accordance with every pixel. The inverse number 1/T of the generation cycle T of the image signal  7  corresponds to a frequency of a reading clock signal for reading image data from a memory not shown in the drawing. 
     In the image display device  1 , the gradation level is allocated such that, in  FIG. 4 , the gradation level assumes a black level when a current value of the image signal  7  is a current value I 1  and the gradation level assumes a white level when the current value of the image signal  7  is a current value I 2 . This is because that, as shown in  FIG. 4 , although the degree of increase of light intensity with respect to the increase of the current value is gently increased from the current value I 1  to the current value I 2 , the degree of increase of the light intensity is suddenly lowered when the current value becomes equal to or more than the current value I 2 , and the degree of increase of the light intensity is suddenly elevated when the current value becomes the current value I 1  from a current value less than the current value I 1 . Due to such processing, it is possible to allocate the gradation level in a region where the current value of the image signal  7  and the intensity of the laser beam exhibit the continuous degree of increase. 
     That is, assuming the intensity of the laser beam outputted from the laser  5  when the high-frequency signal  9  is superposed on the image signal  7  as first intensity and the intensity of the laser beam outputted from the laser  5  when the high-frequency signal  9  is not superposed on the image signal  7  as second intensity, the signal generation part  6  sets the gradation level corresponding to the lower current value I 1  out of the current values I 1 , I 2  of the image signal  7  where the first intensity and the second intensity agree with each other as a black level. On the other hand, the signal generation part  6  sets the gradation level corresponding to the higher current value I 2  out of the current values I 1 , I 2  of the image signal  7  where the first intensity and the second intensity agree with each other as a white level. Here, the black level implies, for example, the gradation level “0” at which the brightness is the lowest when the gradation of the image signal  7  is constituted of 256 gradations (gradation levels: 0 to 255), and the white level implies, for example, the gradation level “255” at which the brightness is the highest when the gradation of the image signal  7  is constituted of 256 gradations. 
     Further, the image display device  1  includes intensity detection parts  11   g ,  11   b  which detect intensities of laser beams radiated from the lasers  5   g ,  5   b . The signal generation parts  6   g ,  6   b  may also perform the allocation of the gradation level corresponding to detection results obtained by the intensity detection parts  11   g ,  11   b  instead of the above-mentioned allocation of preset gradation level. For example, intensities of laser beams at a black level and a white level are preliminarily set, and current values of the image signals  7   g ,  7   b  inputted when intensities of laser beams radiated from the lasers  5   g ,  5   b  assume the preset intensities at the black level are set as current values of the image signals  7   g ,  7   b  at the black level. Further, current values of the image signals  7   g ,  7   b  inputted when intensities of laser beams radiated from the lasers  5   g ,  5   b  assume the preset intensities at the white level are set as current values of the image signals  7   g ,  7   b  at the white level. Further, image signals of the respective gradation levels are allocated corresponding to the apparent input-output characteristic between the image signals  7   g ,  7   b  at the black level and the image signals  7   g ,  7   b  at the white level. 
     Due to such processing, even when the input-output characteristics of the lasers  5   g ,  5   b  are changed in response to a temperature change or the lasers  5   g ,  5   b  have the individual differences, it is possible to properly allocate the gradation levels. Although laser beams whose intensities are changed due to the high-frequency signals  9   g ,  9   b  are incident on the intensity detection parts  11   g ,  11   b , the intensity detection parts  11   g ,  11   b  are configured to output a detection signal corresponding to average intensity of the incident laser beams. 
     Further, the signal adjustment part  8  of the image display device  1  is configured to change amplitude of the high-frequency signal  9  corresponding to a current value of an image signal  7  which the signal generation part  6  outputs. Due to such processing, it is possible to further approximate the relationship between a current value of the image signal  7  and intensity of the laser beam to the proportional relationship. 
     For example, with respect to the laser  5  having the input-output characteristic shown in  FIG. 2 , as shown in  FIG. 6 , the signal adjustment part  8  increases the amplitude of the high-frequency signal  9  which the signal generation part  6  outputs until a current value of an image signal  7  which the signal generation part  6  outputs reaches the center of the kink region W (predetermined value). On the other hand, when the current value of the image signal  7  which the signal generation part  6  outputs passes the center of the kink region W (second predetermined value), the signal adjustment part  8  decreases the amplitude of the high-frequency signal  9 . Depending on the input-output characteristic of the laser, there may be a case where it is preferable to suitably increase or decrease the amplitude of the high-frequency signal  9  rather than increasing the amplitude of the high-frequency signal  9  until the current value of the image signal  7  assumes the predetermined value and decreasing the amplitude of the high-frequency signal  9  from the second predetermined value which is not less than the predetermined value of the image signal  7 . In such a case, the signal adjustment part  8  suitably increases or decreases the amplitude of the high-frequency signal  9  which the signal generation part  6  outputs. 
     In this manner, in the image display device  1  according to this embodiment, the influence of the kink region W exerted on the input-output characteristic of the laser  5  can be suppressed by superposing the high-frequency signal  9  on the image signals  7  respectively thus approximating the relationship between the current value of the image signal  7  and the intensity of the laser beam to the proportional relationship. Accordingly, the allocation of the current value of the image signal  7  at the low gradation level can be performed easily. Accordingly, even when the kink region W is present in the input-output characteristic of the laser  5 , it is possible to reproduce the low gradation level with high accuracy. 
     [2. Application Example of Image Display Device to Retinal Scanning Display] 
     Hereinafter, the specific constitution and the specific manner of operation of the image display device  1  are explained hereinafter in conjunction with the case where the image display device  1  is applied to a retinal scanning display (hereinafter referred to as “RSD”). 
     [2.1. Electrical Constitution and Optical Constitution of RSD  100 ] 
     Firstly, the electrical constitution and the optical constitution of the RSD  100  are explained in conjunction with  FIG. 7 . 
     As shown in  FIG. 7 , the RSD  100  according to this embodiment includes a light source part  110 , an optical fiber  120 , a scanning part  130  and a projection part  140 . 
     The light source part  110  includes a drive signal supply circuit  111 , laser parts  115   r ,  115   g ,  115   b , collimation optical systems  116   r ,  116   g ,  116   b , dichroic mirrors  117   r ,  117   g ,  117   b  and a coupling optical system  118 . 
     Based on the image signal S to be inputted, the drive signal supply circuit  111  generates image signals which respectively constitute elements for forming an image and correspond to respective colors of three primary colors for every pixel. That is, the drive signal supply circuit  111  generates and outputs an R (red) image signal  112   r , a G (green) image signal  112   g  and a B (blue) image signal  112   b  as the image signals for respective colors. Further, the drive signal supply circuit  111  outputs a high-speed drive signal  116  which is used in the high-speed scanning part  132 , and a low-speed drive signal  117  which is used in the low-speed scanning part  134  respectively. Further, the drive signal supply circuit  111  outputs control signals  113   g ,  113   b  for controlling amplitude of high-frequency signals described later which are superposed on the image signals  112   g ,  112   b.    
     The R laser part  115   r , the G laser part  115   g  and the B laser part  115   b  respectively radiate laser beams (also referred to as “optical flux”) whose intensities are modulated in response to an R image signal  112   r , a G image signal  112   g  and a B image signal  112   b  which are respectively outputted from the drive signal supply circuit  111 . 
     An R (red) laser beam Lr, a G (green) laser beam Lg, a B (blue) laser beam Lb radiated from the respective laser parts  115   r ,  115   g ,  115   b  are collimated by the collimation optical systems  116   r ,  116   g ,  116   b  respectively and, thereafter, the collimated laser beams are incident on the dichroic mirrors  117   r ,  117   g ,  117   b  respectively. Thereafter, the respective laser beams of three primary colors are reflected on or are allowed to pass through the dichroic mirrors  117   r ,  117   g ,  117   b  selectively corresponding to wavelengths thereof, arrive at the coupling optical system  118 , and are synthesized by the coupling optical system  118 . Then, the synthesized laser beams are radiated to the optical fiber  120 . In this manner, the laser beams which are radiated to the optical fiber  120  constitute an image light Lc which is obtained by synthesizing the laser beams of respective colors whose intensities are modulated. 
     The scanning part  130  is constituted of a collimation optical system  131 , the high-speed scanning part  132 , a first relay optical system  133 , and the low-speed scanning part  134 . 
     The collimation optical system  131  collimates the laser beams which are generated by the light source part  110  and are radiated through the optical fiber  120 . 
     The high-speed scanning part  132  and the low-speed scanning part  134 , to bring the laser beams incident from the optical fiber  120  into a state where the laser beams can be projected onto the retina  101   b  of the user as an image, scan the laser beams in the main scanning direction as well as in the sub scanning direction. The high-speed scanning part  132  scans the laser beams which are incident on the high-speed scanning part  132  after being collimated by the collimation optical system  131  in the main scanning direction in a reciprocating manner for displaying an image. Further, the low-speed scanning part  134  scans the laser beams which are scanned in the main scanning direction by the high-speed scanning part  132  and are incident on the low-speed scanning part  134  by way of the first relay optical system  133  in the sub scanning direction approximately orthogonal to the main scanning direction. 
     The high-speed scanning part  132  includes a resonance-type deflecting element  132   a  having a reflection mirror  132   b  which scans the laser beams in the main scanning direction by swinging, and a high-speed scanning drive circuit  132   c  which, based on a high-speed drive signal  116 , generates a drive signal for resonating the deflecting element  132   a  so as to swing the reflection mirror  132   b  of the deflecting element  132   a . On the other hand, the low-speed scanning part  134  includes a non-resonance-type deflecting element  134   a  having a reflection mirror  134   b  which scans the laser beams in the sub scanning direction by swinging, and a low-speed scanning drive circuit  134   c  which, based on a low-speed drive signal  117 , generates a drive signal for forcibly swinging the reflection mirror  134   b  of the deflecting element  134   a  in a non-resonant state. The low-speed scanning part  134  scans the laser beams for forming the image in the sub scanning direction toward a final scanning line from a first scanning line for every 1 frame of an image to be displayed. Here, “scanning line” implies one scanning in the main scanning direction performed by the high-speed scanning part  132 . In this embodiment, a galvanometer mirror is used as the deflecting elements  132   a ,  134   a . However, any one of a piezoelectric drive method, an electromagnetic drive method, an electrostatic drive method and the like may be used as a drive method of the deflecting elements  132   a ,  134   a  provided that the drive method can swing or rotate the reflection mirrors  132   b ,  134   b  for scanning the laser beams. 
     The first relay optical system  133  is arranged between the high-speed scanning part  132  and the low-speed scanning part  134 , and relays the laser beams. The first relay optical system  133  converges the laser beams which are scanned in the main scanning direction by the reflection mirror  132   b  of the deflecting element  132   a  on the reflection mirror  134   b  of the deflecting element  134   a . Further, the converged laser beams are scanned in the sub scanning direction by the reflection mirror  134   b  of the deflecting element  134   a . Here, the horizontal direction of the image to be displayed is assumed as the main scanning direction and the vertical direction of the image to be displayed is assumed as the sub scanning direction. However, the vertical direction of the image to be displayed may be assumed as the main scanning direction and the horizontal direction of the image to be displayed may be assumed as the sub scanning direction. 
     The projection part  140  includes a second relay optical system  135  and a half mirror  136 . The laser beams which are scanned by the deflecting element  134   a  passes through the second relay optical system  135  in which two lenses  135   a ,  135   b  having a positive refractive power are arranged in series, are reflected on the half mirror  136  positioned in front of the eye  101 , and are incident on a pupil  101   a  of the user. Due to such an operation, the image corresponding to the image signal S is projected on the retina  101   b  and hence, the user recognizes the laser beams (image light Lc) which is incident on the pupil  101   a  as an image. The half mirror  136  also allows the external light La to pass therethrough and to be incident on the pupil  101   a  of the user. Accordingly, the user can visually recognize an image which is obtained by superposing the image based on the image light Lc on the scenery based on the external light La. 
     In the second relay optical system  135 , using the lens  135   a , the respective laser beams have center lines thereof arranged approximately parallel to each other, and are respectively converted into converged laser beams. Then, using the lens  135   b , the converged laser beams are arranged approximately parallel to each other and, at the same time, are converted such that the center lines of these laser beams are converged on the pupil  101   a  of the user. 
     Further, the RSD  100  according to this embodiment includes a light detection part  141  for adjusting radiation timing of a laser beam radiated from the light source part  110 . The light detection part  141  is constituted of a photodiode, an I-V converter and the like, and outputs a detection signal  143  when a laser beam is incident on the light detection part  141  in  FIG. 8 . 
       FIG. 8  shows the relationship between a maximum scanning range G and an effective scanning range Z obtained by the deflecting elements  132   a ,  134   a  of the high-speed scanning part  132  and the low-speed scanning part  134 . Here, the “maximum scanning range G” implies a maximum range where laser beam can be scanned by the deflecting elements  132   a ,  134   a . The image light Lc which is the laser beam whose intensity is modulated in response to an image signal S is radiated from the light source part  110  at timing where the scanning positions of the deflecting element  132   a ,  134   a  fall in the effective scanning range Z within the maximum scanning range G Due to such processing, the image light Lc is scanned within the effective scanning range Z by the deflecting element  132   a ,  134   a , and the image light Lc for 1 frame is scanned. This scanning is repeated for every image of 1 frame. In  FIG. 8 , a trajectory γ of the laser beam scanned by the deflecting element  132   a ,  134   a  assuming that the laser beam is constantly radiated from the light source part  110  is virtually shown. However, the number of scanning lines in the main scanning direction in the scanning performed by the deflecting element  132   a  is several hundreds to several thousands for every 1 frame so that the trajectory γ of the laser beam is described in a simplified manner. 
     The high-speed scanning drive circuit  132   c  generates a drive signal by amplifying a high-speed drive signal  116 , and a reflection mirror  132   b  of the deflecting element  132   a  is swung in response to the drive signal. Further, the low-speed scanning drive circuit  134   c  generates a drive signal by amplifying a low-speed drive signal  117 , and a reflection mirror  134   b  of the deflecting element  134   a  is swung in response to the drive signal. However, swing trajectories γ 1 , γ 2  (see  FIG. 8 ) of the reflection mirrors  132   b ,  134   b  do not completely agree with signal waveforms of the drive signals  116 ,  117  thus giving rise to the generation of the phase difference or the like. Particularly, it is necessary for the deflecting element  132   a  to swing the reflection mirror  132   b  at a high speed and hence, the deflecting element  132   a  is a resonance-type deflecting element whereby the phase difference between the swing trajectory γ 1  of the reflection mirror  132   b ,  134   b  and the signal waveform of the high-speed drive signal  116  is increased. 
     Accordingly, in the RSD  100  according to this embodiment, as shown in  FIG. 7  and  FIG. 8 , a light detection part  141  for detecting scanning timing of laser beams reflected on the reflection mirrors  132   b ,  134   b  is arranged. The drive signal supply circuit  111  detects laser beams radiated from the light source part  110  by the light detection part  141 , and adjusts the radiation timing of laser beams radiated from the light source part  110 . 
     The light detection part  141  is arranged at a position where a scanned timing detection laser beam is incident in an ineffective scanning range N outside the effective scanning range Z within the maximum scanning range G. Behind the light detection part  141  in the ineffective scanning range N, a light blocking part  142  is provided for preventing a laser beam scanned outside the effective scanning range Z from being incident on an eye  101  of a user. The light blocking part  142  is formed of a non-transmissive plate-shaped member having an opening which allows a laser beam scanned in the effective scanning range Z within the maximum scanning range G to pass therethrough as shown in  FIG. 7 , for example. Further, the light blocking part  142  is used as a fixing member for fixing the light detection part  141 . In this embodiment, an intermediate image surface is formed by a lens  135   a , and the light blocking part  142  is arranged at a position of the intermediate image surface. Due to such a constitution, the light blocking part  142  can be miniaturized and, at the same time, the intermediate image surface position is a position where a beam diameter of the laser beam scanned by the scanning part  130  becomes minimum and hence, it is possible to enhance the detection accuracy of the laser beam at the light detection part  141 . 
     [2.1. Specific Constitution of Light Source Part  110 ] 
     Next, the specific constitution of the light source part  110  which is a characterizing part of the RSD  100  according to this embodiment is further explained in conjunction with  FIG. 9 . The light source part  110  is, as described previously, constituted of the drive signal supply circuit  111 , the laser parts  115   r ,  115   g ,  115   b , 
     The drive signal supply circuit  111  includes a CPU  201 , a ROM  202 , a RAM  203 , a VRAM  204 , D/A converters  205   r ,  205   g ,  205   b ,  206   g ,  206   b ,  208 ,  209 , A/D converters  207   g ,  207   b ,  210 ,  211  and the like. These parts are respectively connected with a data communication bus  212 , and various information is transmitted and received via the bus  212 . In the drive signal supply circuit  111 , the CPU  201  is operated in response to execution of a control program stored in the ROM  202 , and operates respective parts which constitute the RSD  100  thus executing various functions which the RSD  100  possesses. The ROM  202  stores, for example, a gradation table described later besides the above-mentioned control program. When the CPU  201  executes the control program stored in the ROM  202 , the drive signal supply circuit  111  functions as a signal generation part which outputs an image signal corresponding to a gradation level of a pixel which constitutes an image and as a signal adjustment part which superposes a high-frequency signal on the image signal inputted to the laser. 
     The CPU  201  converts the image signal S inputted from the A/D converter  211  into image data which is constituted of a plurality of pixel data, writes the image data in the VRAM  204  and develops the image data in the VRAM  204 . The VRAM  204  is used as a frame buffer. The respective pixel data stored in the VRAM  204  are constituted of R pixel data for a red component, G pixel data for a green component and B pixel data for a blue component. The CPU  201  sequentially reads the R pixel data, the G pixel data and the B pixel data developed in the VRAM  204  in accordance with every pixel, converts these pixel data into analogue data by the D/A converts  205   r ,  205   g ,  205   b , and sequentially outputs the analogue data as image signals  112   r ,  112   g ,  112   b.    
     Further, the CPU  201  convents control signals  113   g ,  113   b  for controlling amplitude of a high-frequency signal described later to be superposed on the image signals  112   g ,  112   b  into analogue signals by the D/A converters  206   g ,  206   b  and outputs the control signals  113   g ,  113   b  as the analogue signals. The drive signal supply circuit  111  includes a first mode to a third mode as operation modes thereof. Upon receiving inputting of a mode setting signal from an input I/F not shown in the drawing, based on the mode setting signal, in the drive signal supply circuit  111 , a mode in which the drive signal supply circuit  111  is operated is set out of the first mode to the third mode. When the second mode is set, the CPU  201  looks up the gradation table stored in the ROM  202 , and changes the amplitudes of the control signals  113   g ,  113   b  corresponding to gradation levels of the image signals  112   g ,  112   b.    
     Further, the CPU  201  converts a high-speed drive signal  116  used in a high-speed scanning part  132  and a low-speed drive signal  117  used in the low-speed scanning part  134  into analog signals by the D/A converters  208 ,  209 , and outputs these drive signals  116 ,  117 . Further, the CPU  201  receives inputting of voltage signals  119   g ,  119   b  corresponding to electric currents which flow in photodiodes (PD)  308   g ,  308   b  described later via the A/D converters  207   g ,  207   b . Further, the CPU  201  receives inputting of a detection signal  143  outputted from the light detection part  141  via the A/D converter  210 , and detects a scanning position of the scanning part  130  based on timing at which the detection signal  143  is inputted. The CPU  201 , based on the detected scanning position of the scanning part  130 , determines radiation timing of a laser beam whose intensity is modulated in response to an image signal S from the light source part  110 , and allows the light source part  110  to radiate the laser beam. 
     (R laser part  115   r ) 
     The R laser part  115   r  is constituted of an R laser driver  301   r  and an R laser diode  303   r.    
     The R laser driver  301   r  generates an image signal  302   r  having a current value corresponding to a voltage value of the R image signal  112   r  outputted from the drive signal supply circuit  111 , and supplies the image signal  302   r  to the R laser diode  303   r.    
     A red laser beam having intensity corresponding to the image signal  302   r  is radiated from the R laser diode  303   r . That is, the R laser diode  303   r  radiates the red laser beam having intensity corresponding to the R image signal  112   r  outputted from the drive signal supply circuit  111 . 
     (G laser part  115   g ) 
     The G laser part  115   g  is constituted of a G laser driver  301   g , a G laser diode  303   g , a high-frequency oscillator  305   g , a variable capacitor  307   g , a photodiode  308   g , and an I-V converter  309   g.    
     The G laser driver  301   g  generates an image signal  302   g  having a current value corresponding to a voltage value of the G image signal  112   g  outputted from the drive signal supply circuit  111 , and supplies the image signal  302   g  to the G laser diode  303   g.    
     The high-frequency oscillator  305   g  generates and outputs a high-frequency signal  306   g  having a current value of predetermined amplitude. The frequency of the high-frequency signal  306   g  is a frequency of not less than an inverse number 1/Ts of a generation cycle Ts of a G image signal  112   g  generated by the drive signal supply circuit  111  (a cycle during which the G image signal  112   g  of one pixel is outputted). That is, the frequency of the high-frequency signal  306   g  is set to a frequency not less than a frequency of a reading clock signal for reading image data from the VRAM  204 . By setting the frequency fa of the high-frequency signal  306   g  to the frequency of the reading clock signal or more, it is possible to reproduce the input-output characteristic of the laser whose intensity is changed as indicated by the solid line in  FIG. 4  with high accuracy. However, since the reflection mirror  132   b  is swung due to resonance oscillation, a scanning speed is not fixed. In this case, it is also necessary to change the reading clock signal for reading the image data corresponding to the scanning speed. To reduce the influence exerted by the change of the reading clock signal, it is desirable to set the frequency of the high-frequency signal  306   g  higher than the frequency of the reading clock signal as much as possible. The frequency of the high-frequency signal  306   g  may also be referred to as a frequency which is more than two times as high as the maximum frequency within a frequency band of image signals continuously outputted from the drive signal supply circuit  111 . Usually, the reading clock signal is set to the frequency which is more than two times as high as the maximum frequency. 
     The frequency fa of the high-frequency signal  306   g  may preferably be set to a frequency higher than an inverse number of a cycle for fetching a detection signal  143  by the drive signal supply circuit  111 . That is, the drive signal supply circuit  111  fetches the detection signal  143  by digital conversion by operating the A/D converter  210  in response to a predetermined clock signal, and the frequency fa of the high-frequency signal  306   g  is set to a frequency higher than a frequency of the clock signal. Due to such frequency setting, the drive signal supply circuit  111  can reduce the erroneous detection of the detection signal  143 . 
     The high-frequency oscillator  305   g  has an output end thereof connected between the G laser driver  301   g  and the G laser diode  303   g  via a variable capacitor  307   g . The variable capacitor  307   g  is a variable capacitor which has a capacitive value corresponding to a voltage value of a G control signal  113   g  outputted from the drive signal supply circuit  111 . When the capacitive value of the variable capacitor  307   g  becomes small, amplitude of the high-frequency signal  306   g  which is superposed on the image signal  302   g  becomes small, while when the capacitive value of the variable capacitor  307   g  becomes large, the amplitude of the high-frequency signal  306   g  which is superposed on the image signal  302   g  becomes large. 
     The G laser diode  303   g  receives inputting of an electric current which is formed by superposing the high-frequency signal  306   g  on the image signal  302   g  outputted from the G laser driver  301   g , and radiates a green laser beam having intensity corresponding to the electric current. When the G laser diode  303   g  exhibits the input-output characteristic shown in  FIG. 2 , for example, by superposing the high-frequency signal  306   g  on the image signal  302   g , the input-output characteristic of the G laser diode  303   g  is regarded as the characteristic in which intensity of light is changed as indicated by the solid line in  FIG. 4 . 
     The photodiode  308   g  is arranged at a position where the photodiode  308   g  receives a portion of a laser beam radiated from the G laser diode  303   g , and an electric current  310   g  having a current value corresponding to intensity of the laser beam radiated from the G laser diode  303   g  flows in the photodiode  308   g . The I-V converter  309   g  is connected to the photodiode  308   g  so that the voltage corresponding to the electric current  310   g  is outputted to the drive signal supply circuit  111  from the I-V converter  309   g.    
     (B laser part  115   g ) 
     The B laser part  115   b  is constituted of a B laser driver  301   b , a B laser diode  303   b , a high-frequency oscillator  305   b , a variable capacitor  307   b , a photodiode  308   b , and an I-V converter  309   b.    
     The B laser part  115   b  has the substantially equal constitution as the G laser part  115   g . That is, the B laser driver  301   b  generates an image signal  302   b  having a current value corresponding to a voltage value of a B image signal  112   b  outputted from the drive signal supply circuit  111 . On the other hand, the high-frequency oscillator  305   b  generates and outputs a high-frequency signal  306   b  having a current value of predetermined amplitude. The high-frequency oscillator  305   b  is connected to the B laser driver  301   b  via the variable capacitor  307   b  so that the image signal  302   b  on which the high-frequency signal  306   b  is superposed is supplied to the B laser diode  303   b . When the B laser diode  303   b  exhibits the input-output characteristic shown in  FIG. 2 , for example, by superposing the high-frequency signal  306   b  on the image signal  302   b , the input-output characteristic of the B laser diode  303   b  is regarded as the characteristic in which the intensity of light is changed as indicated by the solid line in  FIG. 4 . The variable capacitor  307   b  is a variable capacitor having a capacitive value corresponding to a voltage value of the B control signal  113   b  outputted from the drive signal supply circuit  111  and, in the same manner as the G laser part  115   g , amplitude of the high-frequency signal  306   b  which is superposed on the image signal  302   b  is changed corresponding to magnitude of the capacitive value of the variable capacitor  307   b.    
     It is necessary that the frequency fb of the high-frequency signal  306   b  is a frequency sufficiently higher than the maximum frequency of the B image signal  112   b  and hence, the frequency fb of the high-frequency signal  306   b  is set to a frequency not less than a frequency of a reading clock signal for reading image data from the VRAM  204 , for example. However, the reflection mirror  132   b  is swung due to resonance oscillation and hence, a scanning speed is not fixed. In this case, it is also necessary to change the reading clock signal for reading the image data corresponding to the scanning speed. To reduce the influence exerted by the change of the reading clock signal, it is desirable to set the frequency of the high-frequency signal  306   b  higher than the frequency of the reading clock signal as much as possible. 
     The photodiode  308   b  is arranged at a position where the photodiode  308   b  receives a portion of a laser beam radiated from the B laser diode  303   b , and an electric current  310   b  having a current value corresponding to intensity of the laser beam radiated from the B laser diode  303   b  flows in the photodiode  308   b . The I-V converter  309   b  is connected to the photodiode  308   b  so that the voltage corresponding to the electric current  310   b  is outputted to the drive signal supply circuit  111  from the I-V converter  309   b.    
     [2.2 Control Processing in RSD  100 ] 
     Control processing executed by the CPU  201  of the light source part  110  which constitutes the RSD  100  having the above-mentioned constitution is explained in conjunction with  FIG. 10A ,  10 B and  FIG. 11 . With respect to the following processing executed by the CPU  201 , the processing is started due to a power source supply operation in which a user pushes a power source button of the RSD  100  not shown in the drawing, for example. 
     Firstly, when the user pushes the power source button, the CPU  201  executes initial setting processing such as the initialization of a working area of the RAM  203  (step S 10 ), and advances the processing to step S 11 . In the initial setting processing, the CPU  201  outputs a low-speed drive signal  117  so as to change an angle of the reflection mirror  134   b  (angle position Y indicated in  FIG. 8 ) of the low-speed scanning part  134  thus allowing a laser beam reflected on the reflection mirror  134   b  of the low-speed scanning part  134   b  to be incident on the light detection part  141 . 
     In step S 11 , the CPU  201  determines whether or not the RSD  100  is set in the first mode by reference to the ROM  202 . Here, by inputting a mode setting signal from an input I/F not shown in the drawing thus setting a mode flag corresponding to the mode setting signal in the ROM  202 , the user can set the RSD  100  in any one of the first mode to the third mode. 
     In this processing, when the CPU  201  determines that the RSD  100  is set in the first mode (step S 11 : YES), the CPU  201  reads a gradation table and set amplitude values of the high-frequency signal in the first mode from the ROM  202 , and stores the gradation table and the set amplitude values of the high-frequency signal in the RAM  203  (step S 12 ). The gradation table in the first mode is, as shown in  FIG. 12 , a table in which respective gradation levels and voltage levels of image signals  112   r ,  112   g ,  112   b  are associated with each other and is constituted of gradation tables for three respective primary colors. 
     On the other hand, when the CPU  201  determines that the RSD  100  is not set in the first mode (step S 11 : NO), the CPU  201  determines whether or not the RSD  100  is set in the second mode (step S 13 ). In this processing, when the CPU  201  determines that the RSD  100  is set in the second mode (step S 13 : YES), the CPU  201  reads a gradation table in the second mode from the ROM  202 , and stores the gradation table in the RAM  203  (step S 14 ). The gradation table in the second mode is, as shown in  FIG. 13 , a table in which voltage levels of the image signals  112   g ,  112   b  and voltage levels of control signals  113   g ,  113   b  for controlling amplitudes of the high-frequency signals  306   g ,  306   b  are associated with each other with respect to each gradation level, and is constituted of a gradation table for green and a gradation table for blue. On the other hand, the red gradation table is, in the same manner as the gradation table shown in  FIG. 12 , a table in which respective gradation levels and a voltage level of the image signal  112   r  are associated with each other. 
     On the other hand, when the CPU  201  determines that the RSD  100  is not set in the second mode (step S 13 : NO), the CPU  201  executes third mode processing (step S 15 ). The processing in step S 15  is constituted of processing in steps S 30  to S 33  shown in  FIG. 11  and is described later in detail. 
     When the processing in steps S 12 , S 14 , S 15  is finished, the CPU  201  starts driving of the scanning part  130  (step S 16 ), In this processing, the CPU  201  outputs a sinusoidal high-speed drive signal  116  while maintaining the angle of the reflection mirror  134   b  set in step S 10  thus starting swinging of the reflection mirror  132   b . Next, the CPU  201  outputs the G image signal  112   g  having a predetermined voltage value thus allowing the G laser diode  303   g  to radiate a laser beam having predetermined intensity as a timing detection laser beam. The reflection mirror  132   b  scans the timing detection laser beam. Here, the timing detection laser beam scanned by the reflection mirror  132   b  passes through the light detection part  141  and is incident on the light detection part  141 . The light detection part  141  outputs a detection signal  143  at timing that the timing detection laser beam is incident on the light detection part  141 . The CPU  201  detects a scanning position in the main scanning direction determined by the deflecting element  132   a  based on the detection signal  143 . Next, the CPU  201  stops outputting of the G image signal  112   g , stops the radiation of the timing detection laser beam, and outputs a saw-tooth low-speed drive signal  117  thus starting swinging of the reflection mirror  134   b . When the angle of the reflection mirror  134   b  of the low-speed scanning part  134  falls within the ineffective scanning range N, even when the laser beam is radiated from the light source part  110 , the laser beam outputted from the scanning part  130  is blocked by the light blocking part  142  so that the laser beam is not incident on an eye  101  of the user. 
     Next, the CPU  201  determines whether or not the image signal S is inputted to the CPU  201  from the outside (step S 17 ). In this processing, the CPU  201  determines that the image signal S is inputted to the CPU  201  from the outside when inputting of the image signal S from the outside is started or when inputting of the image signal S from the outside is being continued. When the CPU  201  determines that the image signal S is inputted to the CPU  201  from the outside (step S 17 : YES), the CPU  201  generates and outputs the image signal and the high-frequency signal (step S 18 ). 
     In such processing in step S 18 , the CPU  201  stores the inputted image signal S in the VRAM  204  in a developed form in accordance with every pixel, and outputs the R image signal  112   r , the G image signal  112   g ,  13  image signal  112   b  in accordance with every pixel. Here, the CPU  201  outputs the image signals  112   r ,  112   g ,  112   b  based on gradation tables for respective colors stored in the RAM  203 , Further, the CPU  201  outputs control signals  113   g ,  113   b  based on set values of the high-frequency signals or the gradation tables stored in the RAM  203 . Due to such processing, high-frequency currents having amplitudes corresponding to amplitude setting are superposed on the image signal  302   g  and the image signal  302   b.    
     In the first mode, the fixed high-frequency signals  306   g ,  306   b  are superposed on the image signals  302   g ,  302   b . Accordingly, there is no possibility that the high-frequency signals  306   g ,  306   b  are changed in accordance with every pixel and hence, it is possible to prevent the processing from being complicated thus reducing a load imposed on the drive signal supply circuit  111 . Here, with respect to the current values of the image signals  302   g ,  302   b  where intensities of the laser beams outputted from the laser diodes  303   g ,  303   b  when the high-frequency signals  306   g ,  306   b  are superposed on the image signals  302   g ,  302   b  and intensities of the laser beams outputted from the laser diodes  303   g ,  303   b  when the high-frequency signals  306   g ,  306   b  are not superposed on the image signals  302   g ,  302   b  agree with each other, the gradation level corresponding to a higher current value out of the current values of the image signals  302   g ,  302   b  is allocated to a white level, and the gradation level corresponding to a lower current value out of the current values of the image signals  302   g ,  302   b  is allocated to the black level. Further, the gradation levels between the white level and the black level are allocated corresponding to the current values of the image signals  302   g ,  302   b . Due to such processing, it is possible to allocate the gradation levels to a region where the current value of the image signal and the intensity of the laser beam exhibit the continuous degree of increase. 
     Further, in the second mode, the voltage values of the control signals  113   g ,  113   b  are set in accordance with every gradation level based on the gradation table. In this manner, by changing the amplitudes of the high-frequency signals corresponding to the current values of the image signals  302   g ,  302   b , it is possible to further approximate the relationship between the current values of the image signals  302   g ,  302   b  and the intensity of laser beams to the proportional relationship. Accordingly, even the laser which has the kink region in the input-output characteristic thereof can easily reproduce the low gradation level with high accuracy. 
     Further, in the third mode, as described later, based on the detected input-output characteristics of the respective laser diodes  303   g ,  303   b , within the preset intensity ranges, voltage values of the respective image signals  112   g ,  112   b  are set for respective gradation levels and, at the same time, voltage values of the control signals  113   g ,  113   b  are set for every gradation levels. By setting the voltage values in this manner, the amplitudes of the high-frequency signals are changed corresponding to the current values of the image signals  302   g ,  302   b  thus further approximating the relationship between the current values of the image signals  302   g ,  302   b  and intensities of the laser beams to the proportional relationship. Accordingly, even the laser having the kink region in the input-output characteristic can easily reproduce the low gradation level with high accuracy. Further, even when the individual difference or a temperature change is recognized between the respective laser diodes  303   g ,  303   b , it is possible to reproduce the low gradation level with high accuracy. 
     In the processing in step S 17 , when the CPU  201  determines that the image signal S is not inputted from the outside (step S 17 : NO) and the processing in step S 18  finishes, the CPU  201  determines whether or not the scanning position detection timing arrives (step S 19 ). In this processing, the CPU  201  determines that the scanning position detection timing arrives when the angle of the reflection mirror  134   b  assumes an angle corresponding to a position of the light detection part  141  or another position immediately before such a position in the sub scanning direction. That is, the CPU  201  determines that the scanning position detection timing arrives when the timing detection laser beam radiated from the light source part  110  reaches a position in the sub scanning direction at which the timing detection laser beam passes through the light detection part  141  (an angular position Y shown in  FIG. 8 ) or another position immediately before such a position. 
     In the processing in step S 19 , when the CPU  201  detatuines that the scanning position detection timing arrives (step S 19 : YES), the CPU  201  executes timing adjustment processing (step S 20 ). In this timing adjustment processing, firstly, the CPU  201  outputs the G image signal  112   g  having a predetermined voltage value thus allowing the G laser diode  303   g  to radiate a laser beam having a predetermined intensity for a predetermined period as a timing detection laser beam. Here, the timing detection laser beam scanned by the deflecting element  132   a  passes through the light detection part  141  and is incident on the light detection part  141 . The light detection part  141  outputs a detection signal  143  at timing that the timing detection laser beam is incident on the light detection part  141 . The CPU  201 , based on the detection signal  143 , detects a scanning position in the main scanning direction brought about by the deflecting element  132   a  and a scanning position in the sub scanning direction brought about by the deflecting element  134   a , and adjusts timing at which the laser beam whose intensity is modulated in response to the image signal S is radiated. That is, the laser beam whose intensity is modulated in response to the image signal S within the effective scanning range Z is radiated from the light source part  110 . 
     Next, the CPU  201  determines whether or not an image display is stopped due to pushing of the power source button or an image stop button not shown in the drawing (step S 21 ), When the CPU  201  determines that the image display is not stopped (step S 21 : NO), the CPU  201  returns the processing to step S 17 . On the other hand, when the CPU  201  determines that the image display is stopped (step S 21 : YES), the CPU  201  finishes the processing. 
     Next, the third mode processing executed in step S 15  is explained in conjunction with  FIG. 11 . 
     When the third mode processing is started, the CPU  201  executes processing for detecting the input-output characteristic of the G laser diode  303   g  (step S 30 ). In this processing, the CPU  201  outputs the G image signal  112   g  while sequentially increasing a voltage level of the G image signal  112   g , while sequentially acquiring a voltage signal  119   g  outputted from the I/V converter  309   g  via the A/D converter  207   g . Due to such processing, the CPU  201  acquires the input-output characteristic of the G laser diode  303   g.    
     Next, the CPU  201  executes processing for detecting the input-output characteristic of the B laser diode  303   b  (step S 31 ). In this processing, the CPU  201  outputs the B image signal  112   b  while sequentially increasing a voltage level of the B image signal  112   b , while sequentially acquires a voltage signal  119   b  outputted from the I/V converter  309   b  via the A/D converter  207   b . Due to such processing, the CPU  201  acquires the input-output characteristic of the B laser diode  303   b.    
     Next, the CPU  201  executes the allocation of the gradation level (step S 32 ). In this processing, the CPU  201  sets the apparent input-output characteristic based on the input-output characteristic of the G laser diode  303   g  detected in step S 30 , and decides voltage values of the G image signal  112   g  for respective gradation levels ranging from the black level to the white level of the gradation level based on the apparent input-output characteristic. Here, the CPU  201  determines a current value to be supplied to the G laser diode  303   g  when the intensity of the laser beam radiated from the G laser diode  303   g  assumes the lowest intensity preset in the ROM  202 . The CPU  201  allocates the image signal  302   g  having the current value determined in this manner as the image signal of black level. Further, the CPU  201  determines a current value to be supplied to the G laser diode  303   g  when the intensity of the laser beam radiated from the G laser diode  303   g  assumes the highest intensity preset in the ROM  202 , The CPU  201  allocates the image signal  302   g  having the current value determined in this manner as the image signal of white level. Thereafter, the CPU  201  allocates the respective gradation levels between the black level and the white level to the voltage levels of the G image signal  112   g . For example, assume a case where the apparent input-output characteristic is a linear characteristic, the voltage level of the image signal corresponding to the black level is V 1 , the voltage level of the image signal corresponding to the white level is V 2 , and the number of gradations is 256. In this case, the CPU  201  allocates the voltage levels of each pixel signal to the respective gradation levels such that the voltage level of the image signal corresponding to the gradation level n(1≦n≦254) becomes n×(V 2 −V 1 )/255. 
     Further, in step S 32 , the CPU  201 , in the same manner as the allocation of the gradation level with respect to the G image signal  112   g , executes the allocation of the gradation level with respect to the B image signal  112   b . That is, the CPU  201  sets the apparent input-output characteristic based on the input-output characteristic of the B laser diode  303   b  detected in step S 31 , and decides voltage values of the B image signal  112   b  for respective gradation levels ranging from the black level to the white level of the gradation level based on the apparent input-output characteristic. The specific allocation is executed in the same manner as the allocation of the gradation levels with respect to the G image signal  112   g.    
     Next, the CPU  201  executes setting of amplitude of the high-frequency signal (step S 33 ), and finishes the third mode processing. In the processing in step S 33 , the CPU  201  sets the apparent input-output characteristic based on the input-output characteristic in the G laser diode  303   g  detected in step S 30 , and sets the amplitude of the high-frequency signal such that the apparent input-output characteristic is acquired. For example, when the input-output characteristic of the G laser diode  303   g  assumes the characteristic indicated by a broken line in  FIG. 6 , the amplitude of the high-frequency signal is allocated to respective gradation levels such that the apparent input-output characteristic assumes the characteristic indicated by a solid line in  FIG. 6 . In the same manner, the CPU  201 , based on the input-output characteristic of the B laser diode  303   b  detected in step S 31 , sets the apparent input-output characteristic, and sets the amplitude of the high-frequency signal such that the apparent input-output characteristic is acquired. 
     As has been explained heretofore, according to the RSD  100  of this embodiment, by superposing the high-frequency signals to the respective image signals, the influence of the kink region W exerted on the input-output characteristic of the laser can be suppressed thus approximating the relationship between the current value of the image signal and the intensity of the laser beam to the proportional relationship. Due to such processing, the allocation of the current value of the image signal at the low gradation level can be performed easily. Accordingly, it is possible to reproduce the low gradation level with high accuracy even when the kink region W is present in the input-output characteristic of the laser. 
     In the above-mentioned embodiment, the explanation has been made with respect to the laser which has no hysteresis in the input-output characteristic thereof. However, in a laser shown in  FIG. 14  which has hysteresis in the input-output characteristic thereof, a high-frequency oscillator sets amplitude of a high-frequency signal to a value not less than a width of hysteresis. By adopting such a high-frequency oscillator, even when a laser which has hysteresis in the input-output characteristic thereof is used, the influence of the kink region W exerted on the input-output characteristic of the laser can be suppressed thus approximating the relationship between the current value of the image signal and the intensity of the laser beam to the proportional relationship. Here, the hysteresis implies a characteristic in which the kink region W changes between an input-output characteristic of a laser when an electric current supplied to the laser is decreased and the input-output characteristic of the laser when the electric current supplied to the laser is increased. The hysteresis width implies a shift of the kink region W when an electric current supplied to a laser is decreased and when an electric current supplied to a laser is increased.