Abstract:
An optical disc recording apparatus can draw an image by radially vibrating a laser beam under stable focus control. A pickup radiates the laser beam onto the optical disc rotated by a spindle motor. A focus servo controller maintains a constant spot diameter of the laser beam on the optical disc by detecting a return light of the laser beam reflected back from the optical disc. An irradiation position controller operates when the pickup opposes a label face of the optical disc for controlling an irradiation trajectory of the laser beam to vibrate in a radial direction of the optical disc while the laser beam runs along circumferential zones defined on a coloring layer of the label face. A modulating section modulates an intensity of the laser beam for forming dots along the circumferential zones so as to draw the image.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates to an optical disc recording apparatus for recording information in a recording layer provided on one surface of the optical disc and forming an image in a coloring layer provided on the other surface off the disc. 
   2. Description of the Related Art 
   Hitherto, recordable optical discs, such as a CD-R (Compact Disc-Recordable) and a CD-RW (Compact Disc-Rewritable) have been extensively used for recording a large amount of information. One surface (recording face) of this type of optical disc is provided with a recording layer, and information is recorded by radiating a laser beam to the recording layer according to the information to be recorded. 
   Meanwhile, in recent years, there has been proposed a technology in which a coloring layer that changes its color in response to heat or light is integrally provided with an optical disc, the coloring layer being provided on a label face opposite from the recording face to draw images in order to indicate the contents recorded on the optical disc. The label face is set to face an optical pickup, and a laser beam is radiated by the optical pickup to cause the coloring layer to change its color so as to form a visible image. 
   Such an optical disc will be explained with reference to the accompanying drawings.  FIG. 4  is a side sectional view showing the construction of the optical disc. As shown in the drawing, an optical disc  200  has a structure in which a protective layer  201 , a recording layer  202 , a reflective layer  203 , a protective layer  204 , a thermo sensitive layer  205  and a protective layer  206  are deposited in this order. Among these layers, the recording layer  202  is formed of a groove (pit)  202   a  and a land  202   b.    
   As shown in  FIG. 6 , the groove  202   a  observed from the recording face is spiraled clockwise from an inner circumference toward an outer circumference. 
   To record information on the optical disc  200 , the recording face is set to oppose an object lens  114  of the optical pickup, as shown in  FIG. 4 , the optical disc  200  is turned counterclockwise as observed from the recording face, as shown in  FIG. 6 , tracking control is carried out to cause a laser beam B to follow along the groove  202   a  from an end point Gs on the inner circumference side, and the laser beam is radiated according to the information to be recorded, thereby recording the objective information. There are various types of tracking control, including one, for example, in which a laser beam is divided into a main beam and an auxiliary beam adjacent before or after the main beam in the radial direction, and the object lens  114  is swung to right or left such that both of return lights of the auxiliary beam coincide when a certain groove  202   a  is aligned with the center of the main beam. These tracking control methods are approximately the same in that the irradiation position of a laser beam is controlled so as to maintain the symmetry of the intensity distribution, including not only the return light in a certain groove  202   a  but also the return lights in the lands  202   b  located on both sides of the groove  202   a.    
   Furthermore when information is recorded, focusing control is also carried out to maintain a constant distance between the object lens  114  and a disc surface even when the optical disc  200  is rotated, the control being accomplished by vertically moving the object lens  114  so as to follow a fluctuated vertical movement taking place as the optical disc  200  is rotated. There are various types of such focusing control, including one, for example, in which an optical system is disposed such that spot image formation of the return light reflected back by the optical disc  200  changes according to the distance with respect to the disc surface, and the object lens  114  is operated so as to maintain a constant condition of the spot image formation. These control methods are approximately the same in that the object lens  114  is operated to maintain the constant condition of the return light of the laser beam. 
   Meanwhile, to form an image on the optical disc  200 , the optical disc  200  is set with its label face opposing the object lens  114  of the optical pickup, the optical disc  200  is rotated, and the laser beam B is applied to the optical disc  200  to perform main scanning by the relative movement as the optical disc  200  is rotated. At the same time, the optical pickup is moved from an inner circumference toward an outer circumference to cause the laser beam B to perform sub scanning. During the scanning, the laser beam B having an intensity that is sufficiently high to change the color of the thermo sensitive layer  205  is applied on the basis of dots (pixel data) so as to form an objective image. 
   When the optical disc  200  is set with its label face opposing the optical pickup, the tracking control becomes difficult for the reason described below. 
   First, when the optical disc  200  is set with its label face opposing the optical pickup, the concavo-convex relationship between the groove  202   a  and the land  202   b  observed from the object lens  114  side is reversed from that  2 in the case where the optical disc  200  is set with its recording face opposing the optical pickup. If, therefore, the tracking control is to be conducted, a laser beam will follow the land  202   b.    
   The material used for all the protective layers  201 ,  204  and  206  is polycarbonate having a refractive index of about 1.5. The protective layer  201  is considerably thicker than the protective layers  204  and  206 . The recording layer  202  is at a point of about 1.2 mm as observed from the recording face, while it is at a point of only about 0.02 mm as observed from the label face. 
   The object lens  114  is designed so that it is focused (or a laser beam forms a spot having a predetermined diameter) on the reflective layer  203  (the recording layer  202 ) when it opposes the recording face to record information thereon. Hence, when the object lens  114  thus designed opposes the label face, the resulting detection range of its intensity distribution makes more extensive than the range applied when the object lens  114  is set to oppose the recording face. This will make it difficult to control the irradiation position of a laser beam to follow the land  202   b.  In addition, a laser beam is absorbed due to the coloration of the thermo sensitive layer  205 , leading to temporarily reduced return light. This is another factor not expected to be encountered when the object lens  114  is set to oppose recording face, and contributes also to the difficulty of tracking control when the optical disc  200  is set with its label face opposing the optical pickup. 
   Thus, if the optical disc  200  is set with its label face opposing the optical pickup in order to form an image, normal tracking control cannot be expected. Rather, therefore, an image must be formed without using the tracking control. 
   However, in a state where the tracking control is disabled, if the optical disc  200  is, for example, eccentrically rotated around a point C 2  slightly away from its central point C 1 , as shown in  FIG. 7 , then an irradiation trajectory Lp of a laser beam will be a circle with its center at the point C 2 . As a result, the circle intersects with the groove  202   a  having its center at the point C 1  a plurality of times (five times in  FIG. 7 ) for each rotation of the optical disc  200 . 
   If a laser beam crosses over the groove  202   a  (or the land  202   b ), then the condition of the return light of the laser beam undesirably varies even when the distance to a disc surface remains constant. More specifically, the condition of the return light varies not only when the distance to the disc surface changes due to the rotation of the optical disc but also when the eccentric rotation causes the laser beam to cross over the groove  202   a  (or the land  202   b ). Furthermore, these two types of variations are both caused by the rotation of the optical disc  200 , so that their frequency components are close to each other and relatively low. 
   Therefore, in the construction for controlling the focus of a laser beam so as to maintain a constant condition of return light, there is no discrimination between the variation attributable to a changed distance to a disc surface caused by the rotation of the optical disc  200  and the variation attributable to the laser beam crossing over the groove  202   a  or the like. This prevents normal focusing control. For instance, when an optical disc  200  that is ideally flat with no undulation is rotated, the distance between the optical disc  200  and the object lens  114  always remains constant; therefore, once a focus is fixed, then there should be no need to adjust the focus thereafter. If, however, a laser beam crosses over the groove  202   a  or the like due to eccentric rotation, then the condition of the return light changes. As a result, the focus is readjusted to cancel such a change, thus preventing the focusing control from being normally carried out. 
   Thus, if the focusing control feature fails to normally function, then the line width of the irradiation of a laser beam varies from one place to another, preventing uniformity from being maintained. This leads to deterioration in the quality of an image to be formed. 
   SUMMARY OF THE INVENTION 
   The present invention has been made with the aforesaid circumstances taken into account, and it is an object of the invention to provide an optical disc recording apparatus and an image forming method that allow focusing control to be normally conducted so as to prevent deterioration in the quality of an image to be formed even when an optical disc is set with its label face opposing an optical pickup to form an image. 
   To this end, an optical disc recording apparatus according to the present invention is characterized by being equipped with: a rotating section that is provided for rotating an optical disc having a recording layer on one surface of the optical disc and a coloring layer on the other surface of the optical disc, the recording layer being formed with a spiral groove for recording information by radiating a laser beam, the coloring layer having a color changeable in response to heat or light of a laser beam for forming an image in an array of dots arranged along circumferential zones which are defined by concentrically dividing the coloring layer; a light radiating section that is provided for radiating the laser beam onto the optical disc rotated by the rotating section; an irradiation position operating section that is provided for operating an irradiation position of the laser beam radiated onto the optical disc from the light radiating section; a focus operating section that is provided for operating a focus of the laser beam radiated to the optical disc from the light radiating section; a focus controlling section that is provided for controlling the focus operating section so as to maintain a constant spot diameter of the laser beam on the optical disc by detecting a return light of the laser beam reflected back from the optical disc; an irradiation position controlling section that is provided for controlling the irradiation position operating section, the irradiation position controlling section being operative when the light radiating section opposes the one surface of the optical disc for controlling the laser beam radiated by the light radiating section to track the spiral groove in the recording layer on the one surface, and being operative when the light radiating section opposes the other surface of the optical disc for controlling an irradiation trajectory of the laser beam to vibrate in a radial direction of the optical disc while the laser beam runs along the circumferential zones defined on the coloring layer; and a laser beam intensity modulating section being operative when the light radiating section opposes the one surface of the optical disc for modulating an intensity of the laser beam on the basis of the information to be recorded, and being operative when the light radiating section opposes the other surface of the optical disc for modulating the intensity of the laser beam on the basis of the dots along the circumferential zones so as to form the image. 
   With this arrangement, when the optical disc is set with the other surface opposing the light radiating section to form an image, the irradiation position of the laser beam vibrates in the radial direction of the optical disc, so that the laser beam crosses over nearby grooves in the recording layer very frequently while the optical disc is rotating. Hence, the variation component of the return light produced by crossing over the grooves in the recording layer is shifted to a higher frequency that does not interfere with the focusing control so that the variation component is ignored in the focusing control. This makes it possible to realize the focusing control that cancels only the net variation component attributable to a change in the distance to a disc surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a construction of an optical disc recording apparatus according to an embodiment of the present invention. 
       FIG. 2  is a block diagram showing a construction of an optical pickup in the optical disc recording apparatus. 
       FIG. 3  is a top plan view showing a construction of a light receiving element in the optical pickup. 
       FIG. 4  is a sectional view showing a construction of an optical disc on which information is recorded or images are formed by the optical disc recording apparatus. 
       FIG. 5  is a diagram for explaining an dot array of an image to be formed on the optical disc. 
       FIG. 6  is a top plan view showing a groove when the optical disc is observed from a recording face. 
       FIG. 7  is a diagram showing a relationship between the groove and a laser beam irradiation trajectory as observed from the label face of the optical disc. 
       FIG. 8  is a diagram showing a relationship between the groove and a laser beam irradiation trajectory as observed from the label face of the optical disc. 
       FIG. 9  is a diagram showing frequency/gain characteristics of focusing control. 
       FIGS. 10(   a ) and  10 ( b ), respectively, are diagrams for explaining laser beam irradiation trajectories. 
       FIG. 11  is a diagram for explaining contents stored in a frame memory. 
       FIG. 12  is a diagram for explaining a conversion table of a data converter in the recording apparatus. 
       FIG. 13  is a timing chart for explaining the detection of a reference line and the detection of dot arrays of the optical disc. 
       FIG. 14  is a flowchart for explaining an operation for forming an image in the optical disc recording apparatus. 
       FIG. 15  is a flowchart for explaining an operation for forming an image in the optical disc recording apparatus. 
       FIG. 16  is a flowchart for explaining the operation for forming an image in the optical disc recording apparatus. 
       FIG. 17  is a diagram for explaining an example of image contents stored in the frame memory. 
       FIG. 18  is a diagram for explaining an image formed on the basis of the stored contents. 
       FIG. 19  is a diagram for explaining an image formed on the basis of the stored contents. 
       FIG. 20  is a diagram for explaining an example of contents stored in the frame memory. 
       FIG. 21  is a diagram for explaining an image formed on the basis of the stored contents. 
       FIG. 22  is a diagram for explaining a conversion table of a data converter according to an application example of the recording apparatus. 
       FIG. 23  is a diagram for explaining an example of an image in the application example. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following will explain embodiments of the present invention with reference to the accompanying drawings. 
   &lt;Optical Disc Recording Apparatus&gt; 
   An optical disc recording apparatus according to this embodiment (hereinafter referred to simply as “the recording apparatus”) has a newly added image forming feature for forming an image by radiating a laser beam to a coloring layer that is provided on an optical disc and that changes its color in response to heat, in addition to a general information recording feature for recording information by radiating a laser beam to a recording face of the optical disc. The construction of the optical disc itself has already been described; therefore, the descriptions will be given to the construction of the recording apparatus that records information and forms images on the optical disc. The feature for reading out recorded information uses a general technology, so that detailed explanation will be omitted. 
   &lt;Construction of the Optical Disc Recording Apparatus&gt; 
     FIG. 1  is a block diagram showing the construction of the recording apparatus according to the embodiment. As shown in this drawing, a recording apparatus  10  is provided with an optical pickup  100 , a spindle motor  130 , a rotation detector  132 , an RF (Radio Frequency) amplifier  134 , a decoder  136 , a servo circuit  138 , a stepping motor  140 , a motor driver  142 , a PLL (Phase Locked Loop) circuit  144 , a frequency divider circuit  146 , an interface  150 , a buffer memory  152 , an encoder  154 , a strategy circuit  156 , a frame memory  158 , a data converter  160 , a laser power control (LPC) circuit  162 , a laser driver  164  and a main controller  170 . The recording apparatus  10  is connected to a host computer through the interface  150  among the above components. 
   The spindle motor  130  (rotating section) rotates the optical disc  200  on which information is recorded or images are formed. The rotation detector  132  is a type of frequency tacho-generator that utilizes, for example, the back electromotive current of the spindle motor  130  to output a signal FG having a frequency based on the rotational speed of the spindle. 
   The recording apparatus  10  according to this embodiment uses a CAV (Constant Angular Velocity) method to record information while forming an image. Accordingly, feedback control is carried out by the servo circuit  138  also as to set the rotational speed of the spindle motor  130  detected by the signal FG at the angular velocity specified by the major controller  170 . The servo circuit  138  also carries out tracking control and focusing control on the optical pickup  100  in addition to the rotational control on the spindle motor  130 . 
   The optical pickup  100  (the light radiating section) is a block radiating a laser beam to the optical disc  200  that is rotating, the detailed construction thereof being as shown in  FIG. 2 . As shown in the drawing, the optical pickup  100  includes a laser diode  102  that emits laser beams, a diffraction grating  104 , an optical system  110  for condensing a laser beam onto the optical disc  200 , and a light-receiving element  108  for receiving reflected (return) light. 
   The laser diode  102  is driven by a drive signal Liquid crystal from a laser driver  164  (refer to  FIG. 1 ), and emits a laser beam at the intensity based on the current value thereof. The laser beam emitted from the laser diode  102  is separated into a main beam and two sub-beams by the diffraction grating  104 , then the beams pass through a polarizing beam splitter  111 , a collimator lens  112 , a ¼ wavelength plate  113  and an object lens  114 , which constitute an optical system  110 , in order before they are condensed onto the optical disc  200 . 
   Meanwhile, the three laser beams reflected off the optical disc  200  pass through the object lens  114 , the ¼ wavelength plate  113 , and the collimator lens  112  in order again. The laser beams axe reflected at the right angles through the polarizing beam splitter  111 , and pass through a cylindrical lens  115  before entering the light-receiving element  108 . 
   A light-receiving signal Rv by the light-receiving element  108  is amplified by the RF amplifier  134  (refer to  FIG. 1 ), then supplied to the servo circuit  138  or the like. The light-receiving element  108  actually receives the main beam and the two sub-beams, respectively. A detection area for receiving the main beam in the light-receiving element  108  is divided into four sections, as it will be discussed hereinafter, and the light-receiving intensity of an optical image by the main beam is determined for each detection area. For this reason, the light-receiving signal Rv is a generic term of the signals indicating the light-receiving intensities. 
   The object lens  114  is retained by a focus actuator (focus operating section)  121  and a tracking actuator (irradiation position operating section)  122 , and can be moved in the direction of the optical axis of a laser beam (the vertical direction) by the former and in the radial direction of the optical disc  200  (the horizontal direction) by the latter. 
   The details of the constructions of the components will be omitted. The focus actuator  121  vertically moves the object lens  114  in the optical axis direction by a focus coil, while the tracking actuator  122  horizontally moves the object lens  114  in the radial direction of the optical disc  200  by a tracking coll. 
   A focus signal Fc from the servo circuit  138  (refer to  FIG. 1 ) is applied to both ends of the focus coil. Hence, the position of the object lens  114  with respect to the optical axis direction, that is, the distance between a disc surface and the object lens  114 , is defined by the voltage of the focus signal Fc. In other words, the spot diameter of the laser beam applied to the optical disc  200  is determined by the voltage of the focus signal Fc. 
   Similarly, a tracking signal Tr from the servo circuit  138  is applied to both ends of the tracking coil, so that the irradiation position of the laser beam with respect to the radial direction of the optical disc  200  is defined by the voltage of the tracking signal Tr. 
   The optical pickup  100  has a front monitor diode (not shown), and receives the laser beam emitted by the laser diode  102 , the current based on the light quantity thereof is supplied to a laser power control circuit  162  in  FIG. 1 . 
   The optical pickup is a block that includes these focus actuator  121  and the tracking actuator  122 , and moves in the radial direction with respect to the optical disc  200  as a stepping motor  140  (a feeding section) revolves. The motor driver  142  supplies, to the stepping motor  140 , a drive signal for moving the optical pickup  100  in the direction only for the amount, both being specified by the main controller  170 . 
   The RF amplifier  134  amplifies the light-receiving signal Rv by the optical pickup  100  and supplies the amplified signal to the decoder  136  and the servo circuit  138 . When recorded information is reproduced, the light-receiving signal Rv, which has been subjected to EFM (Eight to Fourteen Modulation), is subjected to EFM demodulation by the decoder  136  and supplied to the main controller  170 . 
   The main beam and the two sub-beams in the optical pickup  100  share a positional relationship in which, when the spot center of the main beam is positioned at the center of the groove  202   a  (refer to  FIG. 4 ), one of the spots of the sub-beams reaches the inner surface of the groove  202   a  (the land  202   b ), while the other spot reaches the outer surface thereof (not shown). Therefore, whether the main beam is shifted to the inner side or the outer side of the objective groove  202   a  and the shifting amount (the tracking error amount) can be known by calculating the value of difference in light-receiving intensity between the sub-beams detected by the light-receiving element  108 . 
   Therefore, when recording information, the servo circuit  138  (the irradiation position controlling section) generates a tracking signal Tr for reducing the shift amount in the shifting direction to zero to operate the tracking actuator  122 . This allows the main beam to be accurately traced along the groove  202   a  even when the optical disc  200  eccentrically rotates (tracking control). 
   To carry out the control for moving the optical pickup  100  in the radial direction by the revolution of the stepping motor  140 , the main controller  170  issues an instruction to move the optical pickup  100  outward by one step each time, for example, the optical disc  200  makes a predetermined number of rotations (thread control). 
   Thus, when recording information, the thread control is carried out to position the optical pickup  100  with respect to the optical disc  200 , while the tracking control is carried out to make the laser beam emitted from the positioned optical pickup  100  trace the groove  202   a.    
   However, when forming an image, the servo circuit  138  only generates the tracking signal Tr according to the instruction of the main controller  170  without conducting such tracking control, as it will be discussed hereinafter. 
   The detection area of the light-receiving element  108  is actually divided into four areas, a, b, c and d, as shown in  FIG. 3 . Meanwhile, the formed image of the main beam in the light-receiving element  108  turns into a vertical ellipse A if the object lens  114  is close to the optical disc  200 , or into a horizontal ellipse B if the object lens  114  is far, or into a circle C in a focused state through a cylindrical lens  115 . 
   Thus, by obtaining the calculation result of (a+c)−(b+d) based on the intensities of the received light in the four areas, it is possible to know whether the object lens  114  is shifted to a closer side or a farther side from the focused point with respect to the optical disc  200 , and also to know the amount of the shift (the focus error amount). 
   When recording information, therefore, even if the optical disc  200  undulates during its rotation, the servo circuit  138  generates a focusing signal Fc that sets the foregoing calculation result to zero so as to allow focusing on the recording layer  202  to be achieved. 
   For the similar reason, when forming an image, it should be possible to maintain a fixed spot diameter of the laser beam applied to the thermo sensitive layer  205  by producing a focusing signal Pc that sets the calculation result to a constant value β (≠0) by the servo circuit  138 . 
   However, as it has been described in the paragraph referring to the related art, when forming an image, it is difficult to implement the tracking control, so that the focusing control cannot be expected to be carried out because it can be implemented on condition that the tracking control is normally carried out. 
   More specifically, when the optical disc  200  is set with its label face opposing the optical pickup to form an image, the laser beam does not accurately trace the land  202   b.  Hence, when the optical disc  200  is eccentrically rotated, the irradiation trajectory of the laser beam crosses the groove  202   a  or the land  202   b.  When this happens, it is impossible to determine whether a change in the image formation of the light-receiving element  108  has been caused by a change in the distance to the disc surface or by intersecting the groove  202   a  or the like. As a result, the focusing control cannot be expected to work for maintaining a constant distance to the disc surface. 
   This aspect will be explained in conjunction with  FIG. 9 .  FIG. 9  is a diagram showing the loop characteristics of a focusing servomechanism required for recording information. The servo circuit  138  is designed to meet the characteristics 
   When the optical disc  200  is set such that its label face faces against the optical pickup to form an image, the variable components of the return light of a laser bean are roughly classified into a variable component Fw attributable to a change in the distance to a disc surface caused by the rotation of the optical disc  200 , and a variable component Fgr attributable to a laser beam striding the groove  202   a  or the like during eccentric rotation. These two types of variations are both due to the rotation of the optical disc  200 , so that their frequency components are close to each other and low. 
   Accordingly, these two components remain in a range Sua covered by the focusing servomechanism, and the focusing control is undesirably engaged merely by the variable component Fgr attributable to the striding of the groove  202   a  or the like. 
   &lt;Irradiation Trajectory of a Laser Beam&gt; 
   This embodiment, therefore, adopts a configuration in which an AC signal, e.g., a triangular wave signal, is produced such that the irradiation position of a laser beam vibrates in the radial direction, as a tracking signal Tr when forming an image. Supplying such a triangular wave signal as the tracking signal Tr causes the laser beam to draw a track Lq- 1 , as shown in  FIG. 8 . More specifically, when the optical disc  200  eccentrically rotates around a point C 2 , the triangular waveform having a trajectory Lp of the central circle as its amplitude reference is produced, causing the laser beam to stride over the groove  202   a  or the like forcibly and frequently. 
   The frequent stride by the laser beam over the groove  202   a  or the like causes the variable component Fgr of the return light attributable to the frequent stride to be shifted to a higher frequency range at once beyond the range Sua covered by the focusing servomechanism, as shown in  FIG. 9 . 
   For instance, when the number of rotations of the optical disc 200 per minute is 600, if no triangular wave signal is supplied as the tracking signal Tr, and if it is assumed that the laser beam strides over the groove  202   a  five times per rotation, as shown in  FIG. 7 , then the frequency of the variable component Fgr will be 50 Hz which is within the range Sua shown in  FIG. 9 . Hence, even if the disc surface is constant, the focusing control undesirably works to cancel the variable component Fgr and fails to normally function. 
   Meanwhile, if a triangular wave signal having a frequency of 40 Hz for causing vibration of a 0.1 mm width in the radial direction is supplied as an example of the tracking signal Tr, then the laser beam strides over the groove  202   a  five thousand times per second (=40×2×0.1/0.0016) if the influences by eccentric rotation are excluded, since the pitch of the groove  202   a  is 0.0016 mm(=1.6 μm). 
   Thus, when the foregoing triangular wave signal is supplied as the tracking signal Tr, the frequency of a variable component Fgr′ obtained as the result of the laser beam striding over the groove  202   a  will be 5050 Hz, which reflects the added influences by the eccentric rotation. The resulting frequency is out of the Sua range in which the focusing servomechanism is valid, as shown in  FIG. 9 , thus being ignored in the focusing control. 
   Accordingly, even if a laser beam strides the groove  202   a  or the like, the focus signal Fc is produced so as to cancel only the variable component Fw attributable to a change in the distance to a disc surface. In this embodiment, therefore, it is possible for the focusing control feature to work so as to maintain a constant spot diameter of a laser beam applied to the thermo sensitive layer  205  by maintaining a constant distance to the disc surface, even if the tracking control feature does not work when forming an image. 
   As it will be explained below, when the irradiation trajectory of a laser beam is to be vibrated with a width of about 0.01 mm, which is substantially equal to the sub scanning pitch of a dot array, to form an image, the frequency of the triangular wave signal may be set to about 400 Hz. 
     FIG. 7  is intended to merely explain the state in which the irradiation trajectory of a laser beam crosses the groove  202   a  when the triangular wave signal is supplied as the tracking signal Tr, and does not accurately reflect the frequency and amplitude of the triangular wave signal or the pitch of the groove  202   a.    
   If it is assumed that the direction in which the optical disc  200  rotates is defined as the main scanning direction and the radial direction as the sub scanning direction in forming images, then the only section available to accomplish the sub scanning of laser beam irradiation position for a required amount in the radial direction without using the tracking control feature is to move the optical pickup  100  by the revolution of the stepping motor  140 . 
   If the minimum movement resolution of the stepping motor  140  for the optical pickup  100  is about 0.01 mm (=10 μm), then the minimum possible pitch in the sub scanning direction for forming images will be about 0.01 mm, which is the same as the above resolution. 
   Superficially, therefore, the purpose may be considered to be fulfilled by supplying a triangular wave signal as the tracking signal Tr and by carrying out the focusing control to adjust the spot diameter of the laser beam applied to the thermo sensitive layer  205  to about 0.01 mm, which is equal to the resolution, so as to define the intensity of a laser beam according to the dots of the image to be formed. 
   However, if the laser diode  102  designed such that its spot diameter is set to about 0.001 mm (=1 μm) when recording information is used to expand its spot diameter to about 0.01 mm when forming an image, then the intensity of irradiation to the thermo sensitive layer  205  per unit area deteriorates and sufficient coloration cannot be accomplished. 
   On the other hand, however, if a simple construction is used to radiate a laser beam having a spot diameter of about 0.001 mm to the thermo sensitive layer  205  and to carry out sub scanning by shifting the optical pickup  100  in the radial direction by about 0.01 mm, which is the minimum movement resolution, at a time, then the actually colored portion in one dot will be only a linear portion having a width of about 0.001 mm to which the laser beam has been applied, because the laser beam does not applied the remaining 90% of the portion of the dot, leaving it uncolored. Hence, the area of the colored portion in a dot having a lowest density occupies 0%, while the area of the colored portion in a dot having a highest density occupies only about 10%. The difference between these two dots is extremely small, possibly giving rise to a problem in that the contrast ratio in a formed image significantly lowers, resulting in deteriorated visibility. 
   In this embodiment, firstly, in order to form the dots for one line, the optical disc  200  is rotated (circularly moved) a plurality of times with the optical pickup  100  fixed. This, however, may cause irradiation trajectory of a laser beam applied to the optical disc  200  to remain unchanged for the plurality of circular rotations. To avoid this, secondly, the phase of the tracking signal Tr supplied as a triangular wave signal is changed for each round so that the laser beam irradiation trajectory changes for each round. 
   To be more specific, in this embodiment, as it will be discussed hereinafter, if an image is to be formed in eight gradations, then the optical disc  200  is given seven rounds to form the dots for one line. The main controller  170  instructs the servo circuit  138  to generate, as the tracking signal Tr, a triangular wave signal having its phase set to zero for the first round and then delayed by (2π/7) in sequence for the second round and after when the timing for passing a reference line is set to zero of a time axis. 
   When such tracking signal Tr is supplied to the tracking actuator  122 , the irradiation trajectories of the laser beam to the optical disc  200  will be different from each other, track Lq- 1  in the first round to track Lq- 7  in the seventh round, as shown in  FIG. 10(   a ). 
   In  FIG. 10(   a ), a trajectory Lp denotes the laser beam irradiation trajectory obtained when the optical pickup  100  is positioned at a point corresponding to a certain one line among the dot arrays of the image to be formed and the voltage of the tracking signal Tr is presumptively fixed to zero, when the optical disc  200  is eccentrically rotated around a point C 2 . The trajectory Lp is actually an arc, as shown in  FIG. 7  or  FIG. 8 . However,  FIG. 10(   a ) shows a linear development for the convenience of explanation. 
   Referring back to  FIG. 1 , the buffer memory  152  stores the information supplied from a host computer through the intermediary of the interface  150 , that is, the information to be recorded into the optical disc  200  (hereinafter referred to as “data to be recorded”) in an FIFO (first in, first out) form. 
   The encoder  154  carries out the EFM modulation on the record data read from the buffer memory  152  and outputs it to a strategy circuit  156 . The strategy circuit  156  carries out time axis correction processing or the like on the EFM signal supplied from the encoder  154 , and outputs the result to the laser driver  164 . 
   Meanwhile, the frame memory  158  accumulates the information supplied from the host computer through the intermediary of the interface  150 , that is, the information to be formed on the optical disc  200  (hereinafter referred to as “image data”). 
   The image data is a cluster of gradation data that defines the density of dots P to be drawn on the discoid optical disc  200 . The individual dots P are arranged, corresponding to the intersections of the concentric circles of the optical disc  200  and the radial lines extending from the center, as shown in  FIG. 5 . Here, in order to explain the intersection coordinates in the optical disc  200 , the concentric circles are defined as a first line, a second line, a third line, . . . , m-th (last) line in order from the inner circumferential side toward the outer circumferential side, and a certain radial line is defined as a reference line, the remaining radial lines are defined as a first column, a second column, a third column, . . . , n-th (last) column in clockwise order for convenience sake. 
     FIG. 5  is merely a schematic diagram to show the positional relationship among the dots P; actual dots are densely arranged. The same applies to the pitch of the groove  202   a  shown in  FIG. 6  through  FIG. 8 . 
   Here, the arrangement of the dots has been conveniently defined, as described above, for the following reason. 
   In general, the groove  202   a  of the optical disc  200  is spirally formed clockwise from the inner circumferential side when observed from the recording face, as shown in  FIG. 6  described above. When recording information, tracing is required to begin at an end point Gs on the inner circumferential side of the groove  202   a  according to specifications; therefore, the optical disc  200  is rotated counterclockwise, as observed from the recording face, while the optical pickup  100  moves from the inner circumferential side toward the outer side. 
   In this embodiment, based on the construction described above, when the optical disc  200  is rotated with its label face opposing the optical pickup  100 , the main scanning is carried out by the rotation of the optical disc  200 , while the sub scanning is carried out as the optical pickup  100  moves from the inner circumferential side toward the outer circumferential side thereby to form an image. Thus, regarding the relative movement of the optical disc  200  in relation to the optical pickup  100 , the main scanning direction with respect to the optical disc  200  is the clockwise direction, which is opposite from the rational direction, as shown in  FIG. 5 . 
   When defined as described above, the frame memory  158  stores the gradation data on the basis of the arrays of m-th lines, n-th columns, as shown in  FIG. 11 . Here, in this embodiment, it is assumed that an image of 8 (=2 3 ) gradations per dot is formed, the gradation data being 3-bit. To be more specific, among the 3-bit the gradation data, (000) specifies a brightest (low) density, and the density grows darker (higher) in the order of (001), (010), (011), (100), (101), (110) and (111), the densities being thus specified to form dots. 
   The image data accumulated in the frame memory  158  is read as follows. When a particular line is specified by the main controller  170 , the gradation data for the line is read at the same time and used for discrimination in the main controller  170 . If the main controller  170  specifies a line and a column, then the gradation data at the position specified by the line and the column is read for one dot and supplied to the data converter  160 . 
   The image data used in a host computer is usually of a bit map format. For this reason, to form an image in the optical disc  200 , the image data in the bit map format may be converted into the coordinate system as shown in  FIG. 5  by a host computer or the like, and the converted data may be accumulated in the frame memory  158 , as shown in  FIG. 7 . 
   The main controller  170 , a detailed illustration of which will be omitted, is constructed of a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. The component units are appropriately operated according to a program stored in a machine readable medium such as the ROM so as to control the recording of information on the recording face of the optical disc  200  and the formation of an image on the label face of the optical disc  200 . 
   &lt;Gradation Display&gt; 
   As described above, in this embodiment, the irradiation trajectory of a laser beam differs for each round. Hence, the area ratio of a colored portion and an uncolored portion in a dot is changed by conducting control such that the thermo sensitive layer  205  is colored by radiating a laser beam in a certain round, while the layer is not colored in another round, thus making it possible to display a density. 
   To be more specific, in this embodiment, of the seven rounds required to form the dots for one line, a laser beam is radiated to cause the thermo sensitive layer  205  to color only for the number of rounds equivalent to decimal values of gradation data. For instance, if the gradation data is (101), then a laser beam having an intensity that is sufficiently high to color the thermo sensitive layer  205  is applied for five rounds out of the seven rounds so as to color the track portion. Similarly, if the gradation data is (011), then a laser beam having an appropriate intensity is applied for three rounds out of the seven rounds so as to color the track portion. 
   The data converter  160  (the laser beam intensity modulating section) is a major component unit for defining the intensity of a laser beam for each round until the seven rounds required to form the dots for one line, as described above. More specifically, in a high contrast mode, the data converter  160  converts the gradation data read from the frame memory  158  into ON data (bit) for setting a laser beam intensity to a write level or OFF data for setting it to a servo level on the basis of the number of rounds designated by the main controller  170  according to the table shown in  FIG. 12 . For example, if the gradation data read from the frame memory  158  is (010), then the data converter  160  converts the data into the ON data for a first round and a second round, and into the OFF data for a third round up to a seventh round, respectively, and outputs them. Thus, the target dot receives two shots of the laser beam at the first and second rounds. 
   Here, the write level is a value of irradiation intensity at which the thermo sensitive layer  205  is sufficiently colored, while the servo level is a value of irradiation intensity at which the thermo sensitive layer  205  is hardly colored. The reason for outputting a laser beam at the servo level intensity while the thermo sensitive layer  205  is not colored is to implement the focusing control and light amount control. 
   In a quick mode, which will be explained below, the data converter  160  converts all data into ON data if the gradation data read from the frame memory  158  is other than (000), while it converts into OFF data only If the gradation data is (000). 
   &lt;Image Formation Mode&gt; 
   According to such a method, it is necessary to make seven rounds to form the dots for one line. On the other hand, if the image to be formed is constructed of only characters, such as alphabets, symbols and numerals, it is not always necessary to form an image using multiple gradations or a high contrast ratio. Instead, just two gradations for ON/OFF mode may be adequate for some cases, and shortening the time required for forming an image may be more important than complete gradation display for some users. 
   This embodiment, therefore, has been configured to provide two modes, the high contrast mode for forming an image with a high contrast ratio and the quick mode for giving priority to a shorter time required for forming an image, thus allowing images to be formed in either mode. 
   Mode setting may be accomplished in various methods, including the following: (1) a host computer issues instructions to the main controller  170  through the intermediary of the interface  150 , (2) the main controller  170  analyzes the gradation data accumulated in the frame memory  158  to prepare a histogram to make decisions based on the histogram, and (3) a user makes the setting through the intermediary of a separately provided selecting section. 
   Referring back to  FIG. 1 , the laser power control circuit  162  controls the intensity of the laser beams emitted from the laser diode  102  (refer to  FIG. 2 ). To be more specific, the laser power control circuit  162  controls the current value of drive signal Li such that the value of the emitted light amount of the laser diode  102  detected by a front monitoring diode coincides with an optimum laser power target value supplied by the main controller  170 . 
   Here, the embodiment uses the CAV system in which the angular velocity is constant, as mentioned above, so that the linear velocity increases toward the outer side of the optical disc  200 . For this reason, the main controller  170  sets a higher target value of the write level as the optical pickup  100  is positioned farther outward of the optical disc  200 . 
   The laser driver  164  generates the drive signal Li that reflects the control information supplied by the laser power control circuit  20  on the basis of the modulated data supplied from the strategy circuit  156  when recording information, or on the basis of the converted data supplied from the data converter  160  when forming an image, and the generated drive signal Li is supplied to the laser diode  102  of the optical pickup  100 . 
   Thus, the intensity of the laser beam provided by the laser diode  102  is subjected to feedback control such that it coincides with a target value supplied from the main controller  170 . 
   &lt;Reference Line and Column Detection&gt; 
   As described above, the rotation detector  132  outputs a frequency signal FG based on a spindle rotational speed. The PLL circuit  144  generates a clock signal Dck that synchronizes with the signal FG and has a frequency obtained by multiplying the frequency thereof, then supplies the clock signal Dck to the main controller  170 . Furthermore, the frequency divider circuit  146  generates a reference signal SFG obtained by dividing the signal FG by a predetermined number and supplies the reference signal SFG to the main controller  170 . 
   Here, it it is assumed that, during the period of time in which the spindle motor  130  rotates once, that is, the optical disc  200  rotates once, the rotation detector  132  produces eight pulses as the signal FG, as shown in  FIG. 13 , then the frequency divider circuit  146  divides the frequency of the signal PG into one eighth, and outputs it as the reference signal SFG. This allows the main controller  170  to detect the timing at which the reference signal SFG rises as the timing at which the irradiation position of the laser beam of the optical pickup  100  passes the reference line of the optical disc  200 . 
   In this case, if the multiplying rate of the frequency in the PLL circuit  144  is set to a value of a quotient obtained by dividing a column number n per line by  8 , then one cycle of the clock signal Dck coincides with the period of time during which the optical disc  200  rotates by the angle equivalent to one column of dot arrays. 
   Accordingly, when forming an image, sequentially counting the rise timings of the clock signal Dck from the moment the reference signal SFG rises allows the main controller  170  to detect what number of column the laser beam irradiation of the optical pickup  100  is positioned from the point at which the laser beam irradiation passes the reference line of the optical disc  200 . 
   To be more accurate, the expression “the reference line of the optical disc  200 ” should read “the reference line for the rotating shaft of the spindle motor  130 .” However, when recording information or forming an image, the optical disc  200  rotates while being chucked onto a table directly coupled to the rotating shaft, so that the reference line with respect to the rotating shaft of the spindle motor  130  maintains a constant positional relationship with respect to a certain radial line on the optical disc  200 . Accordingly, as long as this condition is maintained, one radial line on the optical disc  200  may be referred to as the reference line of the optical disc  200 . 
   In this embodiment, the timing at which the reference signal SFG rises is defined as the timing at which the optical disc  200  passes the reference line, and the timing at which the clock signal Dck rises is defined as the timing at which the optical disc  200  is rotated by the angle for one column of the dot arrays. Alternatively, however, fall timings may be used. 
   &lt;Operation&gt; 
   The operation of the recording apparatus  10  in accordance with this embodiment will be explained. The major feature of the recording apparatus  10  is to form images onto the optical disc  200 . Furthermore, the recording apparatus  10  is characteristic in combining the conventional information recording feature and the image forming feature. First, the operation performed for implementing the information recording feature will be briefly explained, then the operation performed for implementing the image formatting feature, which is the major feature of the apparatus, will be explained in detail. 
   &lt;Information Recording Operation&gt; 
   First, to record information, the optical disc  200  is set with its recording face opposing the optical pickup  100 , then the spindle motor  130  is subjected to the feedback control by the servo circuit  138  to obtain the angular velocity instructed by the main controller  170 , as described above. Meanwhile, the optical pickup  100  is moved by the revolution of the stepping motor  140  to the point equivalent to an innermost circumference of the groove  202   a.    
   When the tracing of the groove  202   a  is initiated by the tracking control, the record data stored in the buffer memory  152  is read out in the order in which it was recorded, subjected to the RFM modulation by the encoder  154 , and subjected to the time axis correction processing or the like by the strategy circuit  156 . Then, based on the EFM-modulated data, switching between the write level and the servo level is properly made, and control is carried out so as to cause the intensity to coincide with the target value designated by the main controller  170 . The recording layer  202  irradiated at the write level alters, thereby recording information. 
   When recording information, the aforesaid thread control or focusing control is conducted in addition to the rotation control, the tracking control and the light quantity control. 
   &lt;Image Forming Operation&gt; 
   The descriptions will now be given of the operation performed by the recording apparatus  10  to form an image on the optical disc  200 .  FIG. 14 ,  FIG. 15  and  FIG. 16  are flow charts for explaining the image forming operation. 
   To form an image, the optical disc  200  is set with its label face opposing the optical pickup  100 , as described above. It is assumed that the image data indicating the image to be formed is supplied from the host computer and stored in the frame memory  158 . When forming an image, the optical disc  200  is constantly placed under the focusing control, the light amount control and the rotation control, whereas the tracking control for tracing the land  202   b  is set to be invalid and not carried out, as described above. 
   &lt;Contrast Priority Mode&gt; 
   First, the main controller  170  determines whether the mode has been set to the high contrast mode before actually forming an image (step S 11 ). If the determination result is affirmative, then the main controller  170  outputs an instruction for moving the optical pickup  100  to a point corresponding to the innermost circumference (first line) of the optical disc  200  (step S 12 ). In response to the instruction, the motor driver  142  generates a signal necessary to move the optical pickup  100  to that point. As the stepping motor  140  revolves on the basis of the generated signal, the optical pickup  100  actually moves to that point. 
   The main controller  170  reads ahead the gradation data of the line at which the optical pickup  100  is positioned among the image data stored in the frame memory  158  (step S 13 ). When step S 13  is carried out for the first time, all the gradation data of the first line, which is the innermost circumference, of the optical disc  200  is advance-read. 
   Then, the main controller (the first determining section)  170  determines whether all the gradation data of the line that has been advance-read is (000)(step S 14 ). If all the gradation data of the line is (000), it section that it is not required to color the thermo sensitive layer  205  for any one round out of the seven rounds required for forming the dots of the line. 
   Hence, if the determination result is affirmative, then the main controller  170  skips all the processing steps to step S 28 , which will be discussed hereinafter, thereby to omit the processing required for forming an n number of dots making up the line. 
   Meanwhile, if the determination result is negative, the main controller  170  sets a variable p to “1” (step S 15 ). Here, the variable p is used to indicate at what number of rounds the optical pickup  100  is positioned out of the seven rounds necessary for forming the dots of the line. Hence, setting “1” at the variable p indicates the first round. 
   Subsequently, the main controller  170  scrutinizes the first column to process the first to the last n-th column in order on the line where the optical pickup  100  is positioned (step S 16 ). Then, the main controller  170  stands by until the reference line of the rotating optical disc  200  passes a particular position, i.e., until the rise timing of the reference signal SFG is reached (step S 17 ). 
   Here, when the reference signal SFG rises, the main controller  170  instructs the servo circuit  138  to output the tracking signal Tr of the phase equivalent to the round number indicated by the variable p (step S 18 ). This causes the servo circuit  138  to start outputting the tracking signal Tr if the phase corresponding to the round number indicated by the variable p. Actually, therefore, the light beam of the optical pickup  100  starts librating in the radial direction of the optical disc  200  while tracing the track corresponding to the variable p among tracks Lq- 1  to Lq- 7  shown in  FIG. 10(   a ). For example, if the variable p is “1”, then the light beam traces the track Lq- 1  of the optical disc  200 . 
   The following series of processing from step S 19  through step S 24  is carried out in synchronization with one cycle of the clock signal Dck while the foregoing tracking signal Tr is being generated. 
   More specifically, the main controller  170  reads from the frame memory  158  the gradation data of the dots corresponding to the target column of the line where the optical pickup  100  is currently positioned. Alternatively, of the gradation data for one line that has been advance-read, the data corresponding to the dots in the line and column may be output. Thus, the gradation data is converted by the data converter  160  into the ON data for setting the intensity of a laser beam to the write level or the OFF data for setting it to the servo level according to the round number indicated by the variable p (step S 19 ). 
   The laser driver  164  discriminates the converted data (step S 20 ) and outputs the drive signal Li corresponding to the write level only if the data is the ON data (step S 21 ). This causes the laser diode  102  in the optical pickup  100  to emit light at the write level, thus coloring only the track portion corresponding to the round number indicated by the variable p among the dots in the line opposing the optical pickup  100  and corresponding to the column currently in interest in the thermo sensitive layer  205  of the optical disc  200 . 
   Meanwhile, the laser driver  164  outputs the drive signal Li corresponding to the servo level if the converted data is the OFF data or in a non-ON data case, such as when no converted data is supplied (step S 22 ). Thus, the laser diode  102  in the optical pickup  100  emits light at the servo level, so that the thermo sensitive layer  205  is not colored. 
   Thereafter, the main controller  170  determines whether the target columned is the last n-th column (step S 23 ), and if the determination result is negative, then moves to the next column (step S 24 ). Then, the similar processing is repeated on the new column. Thus, the processing is repeatedly carried out up to the last n-th column so that the laser beam is radiated along the track of the round number corresponding to the variable p on the line where the optical pickup  100 , is positioned. 
   As described above, one cycle of the repetitive processing is synchronized with one cycle of the clock signal Dck, as discussed above. Hence, the laser beam is radiated according to the ON data or OFF data converted on the basis of the line and round number each time the optical disc  200  rotates for the angle corresponding to one dot from the reference line. 
   Meanwhile, If the main controller  170  determines that the target column is the last n-th column, then it further determines whether the current variable p is “7” (step S 25 ), and if the determination result is negative, then it increments the variable p by “1” (step S 26 ) to prepare for the next round. 
   Furthermore, the main controller (a second determining section)  170  scrutinizes the gradation data for one line that has been advance-read to determine whether the laser beam should be radiated at the write level for the round indicated by the variable p after the increment (step S 27 ). 
   When, for example, the variable p following the increment is “4”, if, for instance, the gradation data for one line is all (011) or less, then it can be determined that there is no case where the laser beam should be radiated at the write level for the fourth round, referring to  FIG. 12 . It can be also determined that there is a case where the laser beam should be radiated at the write level for that particular round it there is gradation data of (100) or more for even a single dot. 
   If the determination result in step S 27  is negative, then the processing procedure returns to step S 25  again to determine whether the variable p after increment is “7.” As in the case of this embodiment, when the converted data in the data converter  160  is as shown in  FIG. 12 , if the determination result in step S 27  is switched to negative when the variable p in a certain line is a value α (α being an integer satisfying 2≦α&lt;7), then the determination result will continue to be negative until the variable p becomes “7.” 
   On the other hand, if the determination result in step S 27  is affirmative, then the processing procedure returns to step S 16  again. Thus, the processing from step S 16  to step S 25  will be implemented based on the round indicated by the variable p after increment. 
   Furthermore, if the main controller  170  determines in step S 25  that the variable p is “7” or if the determination result in step S 14  is affirmative, then it further determines whether the line on which the optical pickup  100  is positioned is the last m-th line (step S 28 ). If the determination result is negative, then the main controller  170  issues an instruction for moving the optical pickup  100  for the distance corresponding to one line on the optical disc  200 , i.e., the minimum movement resolution of the optical pickup  100  by the stepping motor  140 , to a point on the outer circumference side (step S 29 ). This instruction causes the motor driver  142  to generate a signal necessary for moving the optical pickup  100  to that point. The stepping motor  140  rotates according to the signal, thus actually moving the optical pickup  100  to the point. Thereafter, the processing procedure returns to step S 13  again. In this way, the processing from step S 13  to step S 28  is carried out on the line following the movement of the optical pickup  100 . 
   Meanwhile, if it is determined that the line where the optical pickup  100  is positioned is the last m-th line, then it section that the formation of the image of the first line to the last m-th line on the set optical disc  200  has been completed. The main controller  170 , therefore, terminates the formation of the image, and carries out, for example, ejection processing (not shown) for ejecting the optical disc  200 , as necessary. 
   Thus, according to this embodiment, in the high contrast mode, the image for one line (one round) is formed on the optical disc  200  by overwriting seven rounds, each round tracing a different laser beam irradiation trajectory. For the seven rounds, the number of overwriting times is increased as the density level indicated by the gradation data increases. 
   In this embodiment, prior to one-line overwriting, the gradation data for the one line is scrutinized. If all the gradation data for the one line is (000), i.e., if there is no need to radiate a laser beam at the write level for any one of the seven rounds required for forming the image of the one line, then the optical pickup  100  is immediately moved outward for one line without actually rotating the optical disc  200  for seven rounds. More specifically, if the determination result in step S 14  is affirmative, then the processing procedure skips over to step S 28 , and if the determination result in step S 28  is negative, then the processing in step S 29  is carried out. Hence, the processing is omitted for the line requiring no image formation (no coloration on the thermo sensitive layer  205 ), so that the time required for forming an image can be reduced. 
   In the high contrast mode, it is determined beforehand whether there is a case requiring the irradiation of a laser beam at the write level in the second round and after, excluding the first round, out of the seven rounds necessary for forming the one-line image. If the determination result is negative, then the rounds thereafter are skipped. More specifically, if the determination result in step S 27  is negative, then the processing procedure returns to step S 25  rather than step S 16 . Furthermore, as in this embodiment, if the converted data in the data converter  160  is as shown in  FIG. 12 , once the determination result in step S 27  is switched to negative, the determination result continues to be negative thereafter until the variable p reaches “7.” 
   For instance, when the variable p in a certain line is, for example, “4” and the gradation data for this line is all (011) or less, if the determination result in step S 27  changes to negative, then the determination result in step S 27  continues to be negative thereafter until the variable p is incremented to “7.” Therefore, the optical pickup  100  moves outwards by one line from the fourth round to the seventh round without carrying out the processing from step S 16  to step S 24 . 
   Thus, the processing for the rounds involving no image formation on the optical disc  200  is skipped (the rounds being skipped), resulting in a further reduced time required for forming an image due to the combination with the line skipping described above. 
   In the high contrast mode, the first round is excluded from the rounds to be skipped out of the seven rounds necessary for forming the one-line image. This is because skipping the first round causes the determination result in step S 14  to be affirmative, so that the line is skipped. 
   &lt;Quick Mode&gt; 
   The descriptions will now be given of the operation for a case where the determination result in step S 11  is negative, that is, the image formation mode has been set to the quick mode. In the quick mode, the image formation of one line (one round) on the optical disc  200  is implemented only by one round on the optical disc  200 . Hence, in the quick mode, the processing related to the variable p does not exist, as it will be explained hereinafter, and image formation by overwriting cannot be implemented. Accordingly, in the quick mode explained here, only binary display such as ON/OFF display is possible. However, since the gradation data itself is 3-bit in this embodiment, a laser beam of the write level will be applied to color the thermo sensitive layer  205  if gradation data is other than (000), while the laser beam of the servo level will be applied so that the thermo sensitive layer  205  remains uncolored if gradation data is (000). 
   When the mode has been set to the quick mode, the main controller  170  outputs an instruction for moving the optical pickup  100  to a point corresponding to the innermost circumference (first line) of the optical disc  200  (step S 30 ). This instruction causes the optical pickup  100  to move to the point, as in the case of the high contrast mode, as described, above. 
   Next, as in the case of the high contrast mode, the main controller  170  reads ahead the gradation data of the line at which the optical pickup  100  is positioned among the image data stored in the frame memory  158  (step S 31 ). Then, the main controller  170  determines whether all the gradation data of the line that has been advance-read is (000)(step S 32 ). If all the gradation data of the line is (000), it section that it is not required to color the thermo sensitive layer  205  for at all during one round required for forming the dots of the line. Accordingly, if the determination result is affirmative, the main controller  170  skips all the steps of processing procedure to step S 42 , which will be discussed later, omitting the processing necessary to form the n number of dots making up the line. 
   If, on the other hand, the determination result is negative, the main controller  170  focuses its attention on the first one column to process from the first column to the last n-th column in sequence in the line where the optical pickup  100  is positioned (step S 33 ). The main controller  170  then stands by until the reference line of the rotating optical disc  200  passes a particular position, that is, until the rise timing of the reference signal SFG is reached (step S 34 ). 
   Here, when the reference signal SFG rises, the main controller  170  instructs the servo circuit  138  to output the tracking signal Tr of the phase for the first round (step S 35 ). This causes the servo circuit  138  to start outputting the tracking signal Tr of the phase for the first round. Actually, therefore, the light beam of the optical pickup  100  starts librating in the radial direction of the optical disc  200  while tracing the track Lq- 1  shown in  FIG. 10(   b ). 
   The following series of processing from step S 36  through step S 41  is carried out in synchronization with one cycle of the clock signal Dck. More specifically, the main controller  170  reads from the frame memory  158  the gradation data of the dots corresponding to the target column of the line where the optical pickup  100  is currently positioned. The data converter  160  converts the gradation data into the OFF data for setting the intensity of a laser beam to the servo level if the gradation data is (000) or into the ON data for setting it to the write level if the gradation data is other than (000)(step S 36 ). 
   The laser driver  164  discriminates the converted data (step S 37 ) and outputs the drive signal Li corresponding to the write level only if the data is the ON data (step S 38 ). This causes the laser diode  102  in the optical pickup  100  to emit light at the write level, thus coloring only the track portion corresponding to the dots in the line opposing the optical pickup  100  and corresponding to the column currently in interest in the thermo sensitive layer  205  of the optical disc  200 . 
   Meanwhile, the laser driver  164  outputs the drive signal Li corresponding to the servo level if the converted data is the OFF data or in a non-ON data case, such as when no converted data is supplied (step S 39 ). Thus, the laser diode  102  in the optical pickup  100  emits light at the servo level, so that the thermo sensitive layer  205  is not colored. 
   Thereafter, the main controller  170  determines whether the target columned is the last n-th column (step S 40 ), and if the determination result is negative, then moves to the next column (step S 41 ). Then, the similar processing is repeated on the new column. Thus, the processing is repeatedly carried out up to the last n-th column so that the laser beam is radiated on the line where the optical pickup  100  is positioned according to the converted ON data or OFF data. 
   As described above, one cycle of the repetitive processing is synchronized with one cycle of the clock signal Dck, as discussed above. Hence, the laser beam is radiated according to the converted ON data or OFF data each time the optical disc  200  rotates for the angle corresponding to one dot from the reference line. 
   Meanwhile, if the main controller  170  determines that the target column is the last n-th column or the determination result in step S 32  is affirmative, then it further determines whether the line where the optical pickup  100  is positioned is the last m-th line (step S 42 ). If the determination result is negative, then the main controller  170  issues an instruction for moving the optical pickup  100  to a point on the outer circumference side by the distance corresponding to one line of the optical disc  200  (step S 43 ). This instruction causes the optical pickup  100  to actually move to the point. After that, the processing procedure returns to step S 31  so as to carry out the processing from step S 31  to step S 42  on the new line. 
   Meanwhile, if it is determined that the line where the optical pickup  100  is positioned is the last m-th line, it section that the formation of the images from the first line up to the last m-th line on the set optical disc  200  has been completed. Hence, the main controller  170  terminates the formation of the image. 
   Thus, in the quick mode, the image formation for one line (one round) on the optical disc  200  is accomplished by one writing cycle along the track Lq- 1 . Therefore, the time required for forming an image can be dramatically reduced although the contrast of the formed image is inferior to that formed in the high contrast mode. 
   Prior to one-line single writing, the gradation data for the one line is scrutinized. If all the gradation data for the one line is (000), then the optical pickup  100  is immediately moved outward for one line. More specifically, if the determination result in step S 32  is affirmative, then the processing procedure skips over to step S 42 , and if the determination result in step S 42  is negative, then the processing in step S 43  is carried out. Hence, as in the case of the high contrast mode, the processing is omitted for the line requiring no image formation (no coloration on the thermo sensitive layer  205 ) on the optical disc  200 , so that the time required for forming an image can be reduced. 
   &lt;Specific Example of a Formed Image&gt; 
   The following is a specific example used to explain an image formed by the recording apparatus  10 . 
   When the mode has been set to the high contrast mode, the dots in each line are represented by repeating the overwriting for the number of times indicated by a decimal value of the gradation data. More specifically, the area corresponding to the dots in the thermo sensitive layer  205  of the optical disc  200  is subjected to a laser beam of the write level for the number of times indicated by the decimal value of the dot gradation data, the laser beam being radiated along a different track for each round. Hence, the ratio of the colored area to the dot area substantially increases as the number of irradiations at the write level increases. 
   When the gradation data on which the image is formed is stored in the frame memory  158 , as shown in  FIG. 17 , the image formed in the high contrast mode will be as shown in  FIG. 18 . More specifically, in the high contrast mode, for a dot whose gradation data is (111), a laser beam of the write level is radiated along a different track for each round from the first round to the seventh round. Therefore, the ratio of the area colored by the irradiation to the area of the dot will be maximum. 
   When the contents stored in the frame memory  158  is as shown in  FIG. 20 , the image formed in the high contrast mode will be as shown in  FIG. 21 . More specifically, in the high contrast mode, for a dot whose gradation data is (000), the number of times of the irradiation of a laser beam of the write level is zero, while the number of times of the irradiation of the laser beam of the write level increases 1, 2, 3, . . . , 7 as the value of the gradation data increases (001), (010), (011), . . . , (111). Hence, the ratio of the area colored due to the irradiation of the laser beam to the area of the dot gradually increases with the gradation data, eventually making it possible to form an image of eight gradations respectively corresponding to the individual pieces of 3-bit gradation data. 
   Meanwhile, when the quick mode has been set, the dots in each line are represented by one irradiation of a laser beam of the write level if the gradation data is other than (000) in this embodiment. Here, when the gradation data as shown in  FIG. 17  is stored in the frame memory  158 , the image formed in the quick mode will be as shown in  FIG. 19 . More specifically, in the quick mode, the dots whose gradation data is other than (000), will be merely expressed by the coloration caused by only one irradiation of the laser beam of the write level. Hence, the contrast ratio of a formed image degrades, as compared with the high contrast mode. 
   However, in the quick mode, a one-line image can be formed by just one rotation of the optical disc  200 , making it possible to reduce the time required for forming an image to about one seventh, as compared with the high contrast mode, in a case where the gradation data of (111) exists for at least one dot or more in each line. 
   Thus, the embodiment enables a user to set the mode to the high contrast mode when he or she wishes to form an image with a high contrast ratio, or to the quick mode when he or she wishes to quickly form an image. This feature makes it possible to properly use the modes according to user&#39;s needs or various conditions, such as image quality, in forming images. 
   In  FIG. 18  through  FIG. 21 , i is a symbol used for general explanation of each line from 1 to m, and j is a symbol used for general explanation of each line from 1 to n (the same being applied to  FIG. 23 , which will be discussed hereinafter). 
   &lt;Applications and Modifications&gt; 
   The present invention is not limited to the embodiment described above, and can be embodied by the following application and modification. 
   &lt;Prevention of Unevenly Colored Portion&gt; 
   The embodiment described above has been configured such that, when the high contrast mode has been set, gradation data is converted into the ON data or the OFF data according to the number of rounds by using the conversion table shown in  FIG. 12 , the converted data being continuous among adjoining rounds. Therefore, if gradation data of a certain value or more does not exist throughout one line in a certain round, then the irradiation of a laser beam from that round and after is skipped, thus making it possible to shorten the time required for forming an image accordingly 
   However, in the above construction, the tracks of the irradiation of a write level laser beam are adjacent to each other. For instance, if the gradation data is (100), the write level laser beam traces the track Lq- 1  for the first round, the track Lq- 2  for the second round, the track Lq- 3  for the third round and the track Lq- 4  for the fourth round, respectively, as shown in  FIG. 21 . These tracks will be adjacent to each other both in the direction of lines and the direction of columns. Therefore, even for the same gradation data, the portion colored by the irradiation of the laser beam may have the dots concentrated on an upper side or a lower side of the colored portion, depending on the column. This may be visually recognized as a difference in display. 
   For example, the dots in the (i+4)th line and the (j+2)th column and the dots of the (i+4)th line and the (j+5)th column share the same gradation data (100)(refer to  FIG. 20 ); however, the colored portions are concentrated on the upper side of a dot in the former, while they are concentrated on the lower side of a dot in the latter (refer to  FIG. 21 ). 
   A conceivable application example that corrects the uneven coloration discussed above is constructed to define the conversion by the data converter  160  such that the irradiation trajectories of a write level laser beam are disposed at equal intervals as much as possible from the first round to the seventh round. 
   More specifically, as shown in  FIG. 22 , the conversion by the data converter  160  for one piece of gradation data may be carried out such that the ON data or the OFF data is disposed at equal intervals as much as possible for individual rounds. For such conversion, if the gradation data has been stored in the frame memory  158 , as shown in  FIG. 20 , the image formed in the high contrast mode will be as shown in  FIG. 23 , which indicates that the uneven coloration can be restrained to a certain extent. 
   In addition to the above approach in which the conversion by the data converter  160  is changed, there is another approach for improving such an unevenly colored portion. The shifting amount or order of the phase of the tracking signal Tr may be changed for each round. 
   &lt;Forcible Insertion of the Servo Level&gt; 
   In the embodiment described above, if thick dots continue in a certain line, the write level laser beam is continuously radiated. 
   Meanwhile, when the write level laser beam is applied, the thermo sensitive layer  205  is colored by the energy of the laser beam. The energy used for the coloration transiently and constantly changes from the moment the irradiation is started, and also varies according to diverse conditions, including individual differences or the like of the optical disc  200 . For this reason, it is considered that the return light when the write level laser beam is applied is not stabilized, easily leading to unstable focusing control. 
   Therefore, when the write level laser beam is continuously radiated, the focusing control may fail to normally function. 
   As a conceivable application example for preventing such failure, even when the write level laser beam should be continuously radiated, the servo-level laser beam may be periodically radiated for a short time (of course to an extent that does not affect the coloration) and the focusing control may be carried out by using the light receiving signal Rv in the irradiation period as a return value. 
   &lt;Another Example of the Tracking Signal&gt; 
   In this embodiment, the triangular wave signals have been supplied as the tracking signal Tr; however, any other type of signal can be adequately used as long as the irradiation trajectory of a laser beam crosses the groove  202   a  or the like of the rotating optical disc  200 . Therefore, in addition to the triangular wave signals, various ac signals, including sine wave signals, may be supplied as the tracking signal Tr. 
   &lt;Number of Irradiations of Laser Beam and Number of Gradations&gt; 
   In the aforesaid embodiment, the number of irradiations of the laser beam for coloring the thermo sensitive layer  205  has been set to 0 to 7 to form an image with eight gradations in the high contrast mode. The number of irradiations of the laser beam may be increased as the density is increased. For example, if the gradation data is (000), (001), (010), (011), . . . , (111), the number of irradiations of the write level laser beam per line may be set to 0, 2, 4, 6, . . . , 14. Increasing the number of irradiations of the laser beam makes it possible to form images with a further higher contrast ratio. The increments of the number of irradiations need not be fixedly set. 
   Furthermore, the descriptions have been given by taking, as an example, the case where the image of eight gradations per dot, the gradation data being 3-bit; however, the present invention is not limited thereto. For instance, an image may be formed by 8-bit gradation data and 256 gradations. 
   Also in the embodiment, one line of the image has been formed by one travel (feed) of the optical pickup  100 . Alternatively, however, one line of the image may be formed by repeating the feed a plurality of times. Thus, to form one line of an image by feeding the optical pickup  100  a plurality of times, e.g., 64 times, the image can be formed in 256 gradations (=4×64) by representing a density of 4 gradations per feed and changing the density for each of the 64 times of feed. 
   &lt;Forming an Image With a Reduced Number of Colors in the Quick Mode&gt; 
   Meanwhile, in the aforesaid embodiment, the image has been formed in the quick mode by the binary method wherein it is simply controlled to irradiate or not irradiate the write level laser beam. Alternatively, however, the number of the basic gradations indicated by gradation data may be reduced to form an image. For instance, the number of irradiations of the write level laser beam per line may be set to 0 if the gradation data is (000), (001), or one if the gradation data is (010), (011), or two if the gradation data is (100), (101), or three if the gradation data is (110), (111), carrying out three rounds per line and reducing the number of gradations to four to form the image. The irradiation trajectory of the laser beam is of course set so that it strides over the groove  202   a  in all three rounds and differs in each round. Thus, by reducing the original number of gradations indicated by the gradation data in forming an image, the time required for forming the image itself can be also shortened although the effect for shortening the time may be smaller than that in the high contrast mode. 
   When forming an image in the quick mode, the same gradation data as that in the high contrast mode has been stored in the frame memory  158 . Alternatively, however, the gradation data may be processed by a host computer to store, in the frame memory  158 , binary gradation data or the gradation data of a reduced number of gradations to decrease the number of colors, and an image may be formed on the basis of the gradation data in the same manner as that in the high contrast mode. This modification achieves the same advantage in that the time required for forming one line of image is shortened because the number of colors of gradation data is binary or reduced from the original number. 
   &lt;CLV Method&gt; 
   The aforesaid embodiment has adopted the CAV method wherein a laser beam is radiated while rotating the optical disc  200  at a predetermined angular velocity to form an image. Alternatively, a CLV method using a constant linear velocity may be adopted. Unlike the CAV method, the CLV method does not require the control for increasing the write level of a laser beam as the irradiation position of the laser beam shifts toward an outer circumference. This section that the quality of an image to be formed is not deteriorated due to the changes of the target value of laser power. 
   &lt;Arrangement of Dots&gt; 
   In the aforesaid embodiment, the number of columns has been set to the same m from the first line to the last m-th line. Alternatively, however, the number of columns may be increased toward an outer circumference. In other words, the number of the columns may be different in each line. 
   In the aforesaid embodiment, if the multiplying rate of the frequency in the PLL circuit  144  is set to a value of a quotient obtained by dividing a column number n per line by 8, then one cycle of the clock signal Dck coincides with the period of time during which the optical disc  200  rotates by the angle equivalent to one column of dot arrays. Hence, the multiplying rate of the PLL circuit  144  may be set on the basis of the number of columns for each line so as to permit an arrangement wherein the number of columns is different in each line. 
   As explained above, according to the present invention, even when an optical disc is set with its label face opposing an optical pickup when forming an image, focusing control can be properly carried out, making it possible to prevent deterioration in the quality of an image to be formed.