Abstract:
In a multicolor electrostatic recording apparatus in which two or more different color toner images are superimposed by respective electrostatic recording units, each of the electrostatic recording units includes an electrostatic latent image carried, a developer for developing an electrostatic latent image formed on the carrier with a color toner, and a detector for detecting a density of the developed image on the basis of a detecting mark which is formed on the carrier as a part of the latent image and developed by the developer. A discriminator which compares the density data with an optional desired density value to discriminate whether the density data falls in an allowable range. A controller for feed-back controlling at least one parameter for determining the density of the developed image so that the density comes to be in the allowable range, when the density falls out of the allowable range. The at least one parameter is memorized as a compensating data for the density of the developed image, when the density falls in the allowable range. Thus, using the compensating data, a process using the parameter for determining the density of the developed image is carried out. The detector for detecting density includes a light emitting section and a light receiving unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multicolor electrostatic recording apparatus in which toner images of at least two colors are superimposed so as to record a multicolor image, and more particularly relates to a multicolor electrostatic recording apparatus in which: an electrostatic latent image of a detection mark is formed on an electrostatic latent image carrier before recording a multicolor image; the detection mark is developed with toner components of a predetermined color; development density data is detected from the developed detection mark; the detected development density data is compared with a predetermined value; and an amount of deposited toner in the development process is subjected to feedback control in accordance with the result of comparison, so that the multicolor recording of a constant hue can be conducted at all times. 
     2. Description of the Related Art 
     In general, the following processes are successively carried out in an electrostatic recording apparatus. In an electrostatic latent image writing process, an electrostatic latent image is written on an electrostatic latent image carrier such as a photoreceptor, a dielectric body or the like. In a development process, the electrostatic latent image is electrostatically developed with electrically charged toner so as to obtain a charged toner image. In a transfer process, the charged toner image is electrostatically transferred onto a recording medium such as a recording sheet. In a fixation process, the transferred toner image is fixed onto the recording sheet. The electrostatic latent image writing process, the development process and the transfer process are repeated at least twice in the case of recording a multicolor image by the above electrostatic recording apparatus. In each development process, an electrically charged toner image of each color is formed using toner of each color, and each toner image is transferred onto the same recording sheet in each transfer process so as to be superimposed. That is, the transferred images of at least two colors are superimposed on the recording sheet. After that, the recording sheet is sent to the fixation process, and the transferred toner images of different colors are simultaneously fixed onto the recording sheet. As is well known, in the case of full color recording, toners of four colors including yellow, cyan, magenta and black are used. In this case, the electrostatic latent image writing process, the development process and the transfer process are repeatedly conducted for each color. 
     Of course, hue is a very important factor when the quality of multicolor recording is evaluated, however, it is difficult to stably maintain such an important factor as hue at a predetermined value at all times. The reason why it is difficult to stably maintain the hue at a predetermined value is described as follows. In order to maintain the hue to be a constant value, it is necessary to regulate an amount of deposited toner (development density) on the electrostatic latent image carrier to be a predetermined value. However, the amount of deposited toner is affected by an amount of electric charge of toner. Further the amount of charge of toner is greatly affected by the environmental temperature and humidity. Furthermore, the hue of an image formed by multicolor recording is greatly affected by the deterioration with time of parts that compose the multicolor electrostatic recording apparatus. For example, when a photoreceptor drum is used as the electrostatic latent image carrier and a semiconductor laser is used as the writing means for writing an electrostatic latent image on the photoreceptor drum, the characteristics of the photoreceptor drum and the semiconductor laser deteriorate with time. Therefore, an amount of deposited toner is changed due to the deterioration with time. 
     In order to solve the above problems, the following feedback control is proposed: 
     Before conducting the actual multicolor recording, an electrostatic latent image of a detection mark is written on the electrostatic latent image carrier, and then the electrostatic latent image is developed with toner of a predetermined color. Development density data is detected using the developed detection mark. The detected development density data is compared with a predetermined value, and it is discriminated whether or not the detected development density data is in an allowable range. When the detected development density data is out of the allowable range, at least one of the parameters to regulate the development density is adjusted, so that an amount of deposited toner in the development process is subjected to feedback control. According to the feedback control described above, an amount of deposited toner of each color can be regulated each time the multicolor recording is conducted. Accordingly, it is possible to maintain the hue constant in the multicolor recording at all times. In this way, it is possible to guarantee the multicolor recording of high quality irrespective of the fluctuation of environmental temperature and humidity. 
     However, in order to conduct the feedback control appropriately, it is necessary that a detector for detecting the development density from the development detection mark, for example, an OD sensor (optical density sensor) is capable of detecting the development density data with high accuracy. In other words, when the detection accuracy of such a detector is low, the feedback control cannot be appropriately conducted, and it is impossible to maintain the hue to be constant in the multicolor recording. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a multicolor electrostatic recording apparatus in which at least two color toner images are superimposed on the recording medium and multicolor printing can always be carried out in a constant hue. 
     Another object of the present invention is to provide a multicolor electrostatic recording apparatus in which the development density of a detection mark is detected by a detector with high accuracy. 
     According to a first aspect of the present invention, there is provided a multicolor electrostatic recording apparatus comprising at least two electrostatic recording units for forming at least two different colors, respectively, and means for superimposing the at least two color toner images obtained by the respective units, each of the electrostatic recording units comprising: an electrostatic latent image carrier; a developing means for developing an electrostatic latent image formed on the carrier with a color toner; a detecting means for detecting a density of the developed image on the basis of a detecting mark which is formed on the carrier as a part of the latent image and developed by the developing means; a discriminating means for comparing the density data detected by the detecting means with an optical desired density value to discriminate whether the density data falls in an allowable range; a control means for feed-back controlling at least one parameter for determining the density of the developed image so that the density becomes to be in the allowable range, when the density falls out of the allowable range; a memory means for memorizing the at least one parameter as a compensating data for the density of the developed image, when the density falls in the allowable range; means for conducting, with the compensating data, a process using the parameter for determining the density of the developed image; and means for conducting, with the compensating data, a process using the parameter for determining the density of the developed image; and the density detecting means comprising a light emitting section from which detecting light is emitted and a light receiving section which receives reflected light of the light emitted from the light emitting section, and the light emitting section comprising a white light source. 
     In this multicolor electrostatic recording apparatus, since a white light source is used in the light emitting section for detecting the developing density, the density detecting means always has an appropriate sensitivity with respect to the detecting marks regardless to the pattern of the detecting mark and its color. 
     According to a second aspect of the present invention, there is provided a multicolor electrostatic recording apparatus in which the density detecting means comprises a light emitting section from which detecting light is emitted and a light receiving section which receives reflected light of the light emitted from the light emitting section, and the light emitting section comprising a predetermined color light source; and at least the detecting mark which is to be developed with a color toner similar to that of the predetermined color light source comprises a solid mark. 
     In this multicolor electrostatic recording apparatus, since the light emitting section comprises a predetermined color light source and at least the detecting mark which is to be developed with a color toner similar to that of the predetermined color light source comprises a dot line pattern, the density detecting means has an appropriate sensitivity with respect to the detecting mark is question. 
     According to a third aspect of the present invention, there is provided a multicolor electrostatic recording apparatus, in which the density detecting means comprises a light emitting section from which detecting light is emitted and a light receiving section which receives a reflection light of the light emitted from the light emitting section, and the light emitting section comprising a light source having a color different from those of toners with which the respective detecting marks in the respective recording units are to be developed. 
     In this multicolor electrostatic recording apparatus, since the light emitting section comprises a light source having a color different from those of toners with which the respective detecting marks in the respective recording units are to be developed, the density detecting means always has an appropriate sensitivity with respect to the detecting marks regardless of the pattern of the detecting mark and its color. 
     According to a fourth aspect of the present invention, there is provided a multicolor electrostatic recording apparatus in which the developing means comprises a spacer engaged with the electrostatic latent image carrier to maintain a predetermined gap between the developing means and the electrostatic latent image carrier so that the density detecting means is mounted on the developing means. 
     According to a fifth aspect of the present invention, there is provided a multicolor electrostatic recording apparatus, in which the developing means comprises a spacer engaged with the electrostatic latent image carrier to maintain a predetermined gap between the density detecting means and the electrostatic latent image carrier. 
     In these multicolor electrostatic recording apparatuses, since a predetermined constant gap is always maintained between the density detecting means and the electrostatic latent image carrier, the density detecting means is always retained in a stable condition. 
     According to a sixth aspect of the present invention, there is provided a multicolor electrostatic recording apparatus comprising: a plurality of electrostatic recording units for forming different colors, each unit comprising: an electrostatic latent image carrier, an optical means for forming an electrostatic latent image on the carrier, a developing means for developing the electrostatic latent image with a color toner, and a position detecting mark being formed on the image carrier as a part of the latent image and developed with the color toner; a belt conveyance means for transporting recording medium along the respective recording units in such a manner that color toner images are transferred to and superimposed on the recording medium, one after another, from the respective electrostatic latent image carriers, so that a plurality of the position detecting marks are transferred, one after another, from the respective carriers to the belt conveyance means by a predetermined time interval; a detecting means for simultaneously detecting the plurality of position detecting marks transferred to the belt conveyance means; and a control means for feed-back controlling relative positions of the electrostatic latent images to be formed on the respective carriers. 
     In this multicolor electrostatic recording apparatus, the respective color marks can be simultaneously read and then the color deviation is determined in accordance with the relative positions of these marks. Therefore, the deviation occurred after the image transferring is not included and thus an accurate color position deviation can thus be attained. Any deviation occurred after the image transferring is represented as the deviation of absolute positions of the resist marks themselves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view showing an outline of the multicolor electrostatic recording apparatus of the present invention; 
     FIG. 2 is a magnified elevation view of one of the electrostatic recording units of the multicolor electrostatic recording apparatus shown in FIG. 1; 
     FIG. 3 is a schematic illustration for explaining the development process of an electrostatic latent image; 
     FIG. 4 is a graph showing the relationship between the development density and the amount of deposited toner of each color; 
     FIG. 5 is a control block diagram of the multicolor electrostatic recording apparatus shown in FIG. 1; 
     FIG. 6 is a block diagram showing the details of a portion of the control block diagram shown in FIG. 4; 
     FIGS. 7 and 8 are a series of flow charts showing the development density correction routine to correct a setting density value in the normal and economical modes, wherein an output level to the laser beam scanner is used as a control parameter; 
     FIG. 9 is a graph showing the relationship between the development density and the output level to the laser beam scanner; 
     FIG. 10((a) and (b)) is a flow chart showing a development density correction routine to correct an arbitrary input setting density value, wherein an output level to the laser beam scanner is used as a control parameter; 
     FIGS. 11 and 12 are a series of flow charts showing a development density correction routine to correct a setting density value in the normal and economical modes, wherein a development bias voltage to the development roller is used as a control parameter; 
     FIG. 13 is a graph showing the relationship between the development density and the development bias voltage impressed upon the development roller; 
     FIG. 14((a) and (b)) is a flow chart showing a development density correction routine to correct an arbitrary input setting density value, wherein a development bias voltage impressed upon the development roller is used as a control parameter; 
     FIGS. 15 and 16 are a series of flow charts showing a development density correction routine to correct a setting density value in the normal and economical modes, wherein a voltage impressed upon the precharger is used as a control parameter; 
     FIG. 17 is a graph showing the relationship between the development density and the electric potential in the charged region on the photoreceptor drum; 
     FIG. 18((a) and (b)) is a flow chart showing a development density correction routine for correcting an arbitrary input setting density value, wherein a voltage impressed upon the precharger is used as a control parameter; 
     FIGS. 19(a) to 19(e) are schematic illustrations showing examples of the pattern of the detection mark; 
     FIG. 20 is a perspective view showing a relation between the photoreceptor drum of the electrostatic recording unit shown in FIG. 2 and the OD sensor; 
     FIG. 21 is a graph showing an example of the relation between the output voltage of the sensor and the OD value in which the detection mark is developed in a solid manner and further the same type color as that of the detection mark is used as the detection light; 
     FIG. 22 is a graph showing an example of the relation between the output voltage of the sensor and the OD value in which the detection mark is developed in one dot line and further the same type color as that of the detection mark is used as the detection light; 
     FIG. 23 is a graph showing an example of the relation between the OD value in which the detection mark is developed in two dot lines and further the same type color as that of the detection mark is used for the detection light, and the output voltage of the sensor; 
     FIGS. 24 and 25 are graphs showing an example of the relation between the output voltage of the sensor and the OD value in which the detection mark is developed in a solid manner and further a different type color from that of the detection mark is used as the detection light; 
     FIGS. 26 and 27 are graphs showing an example of the relation between the output voltage of the sensor and the OD value in which the detection mark is developed in a solid manner and one dot line and further a different type color from that of the detection mark is used as the detection light; 
     FIG. 28 is a graph showing a fluctuation of sensitivity of the OD sensor when a distance from the 0D sensor to the rotational surface of the photoreceptor drum is changed; 
     FIG. 29 is a perspective view showing a development roller removed from the developing unit; 
     FIG. 30 is a plan view showing a state in which the OD sensor is pressed against the photoreceptor drum via a spacer roller; 
     FIG. 31 is schematic illustration of a multicolor electrostatic recording apparatus similar to FIG. 1; 
     FIG. 32 is a plan view illustrating shapes of position detecting marks; 
     FIGS. 33(a) and 33(b) are schematic side and plan views showing an arrangement of the light emitting means and light receiving means constituting position deviation detecting mechanism; 
     FIG. 34 illustrates a controlling method of the charged-coupled device (CCD) sensor array and the laser emission diode (LED) array; 
     FIG. 35 illustrates signals read by the charged-coupled device (CCD) sensor array; 
     FIG. 36 is a schematic plan view showing a second example of an arrangement of LED and CCD; and 
     FIG. 37 is a schematic plan view showing a third example of an arrangement of LED and CCD. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic illustration of a high speed laser printer for full color that is a specific example of the multicolor electrostatic recording apparatus of the present invention. This high-speed laser printer includes an endless belt conveyance means 10 for conveying a recording medium such as a recording sheet. This endless belt conveyance means 10 is composed of an endless belt 10a made of flexible dielectric material, for example, an appropriate synthetic resin. This endless belt 10a is provided around 4 rollers 10b, 10c, 10d, 10e. The roller 10b functions as a drive roller, which drives the endless belt 10a in the arrowed direction by an appropriate drive mechanism not shown in the drawing. The roller 10c functions as an idle roller, which also functions as a charging roller to give an electric charge to the endless belt 10a. Both rollers 10d and 10e function as guide rollers, and the roller 10d is disposed close to the drive roller 10b, and the roller 10e is disposed close to the idle roller 10c. There is provided a tension roller 10f between the idle roller 10c and the guide roller 10e. By the action of the tension roller 10f, the endless belt 10a is given an appropriate amount of tension. The upside running section of the endless belt 10a, that is, the running section defined by the endless belt 10a between the drive roller 10b and the idle roller 10c forms a recording sheet movement path. A recording sheet is introduced into the recording sheet movement path from the idle roller 10c side and discharged outside from the drive roller 10b side. When the recording sheet is introduced into the recording sheet movement path from the idle roller 10c side, the recording sheet is electrostatically attracted onto the endless belt 10a since the endless belt 10a is electrically charged. Therefore, the occurrence of positional deviation of the recording sheet from the endless belt 10a can be prevented. There is provided an AC discharger 10g on the drive roller 10b side. By the action of this AC discharger 10g, the electric charge is removed from the endless belt 10a. Due to the above electrically discharging operation, when the recording sheet is sent outside from the drive roller 10b side, it can be easily separated from the endless belt 10a. 
     The high speed laser printer is provided with four electrostatic recording units Y, C, M and B, which are disposed in series from the upstream to the downstream along the upside running section of the endless belt 10a. In the electrostatic recording units Y, C, M and B, developers having yellow toner components (Y), cyan toner components (C), magenta toner components (M) and black toner components (B) are respectively accommodated. The electrostatic recording units Y, C, M and B have the same structure. The only different point is that the images of yellow, cyan, magenta and black toners are formed on the recording sheet moving along the upside running section on the endless belt 10a. 
     Each of the electrostatic recording units Y, C, M and B includes a photoreceptor drum 12. In the process of recording, the photoreceptor drum 12 is rotated in the arrowed direction shown in the drawing. A precharger 14, which is composed as a corona charger or a scorotron charger, is disposed at an upper position of the photoreceptor drum 12. A rotational surface of the photoreceptor drum 12 is uniformly charged by the precharger 14. An electrostatic latent image is written in the charged region on the photoreceptor drum 12 by an optical writing means, for example, by a laser beam LB emitted from a laser beam scanner 16. That is, the laser beam LB is turned on and off in accordance with the binary image data provided by a computer or a word processor. Due to the foregoing, the electrostatic latent image is written as a dot image. 
     The electrostatic latent image written on the photoreceptor drum 12 is electrostatically developed by a developing unit 18 using a toner of a predetermined color. The developing unit 18 is disposed on the upstream side of the recording sheet moving path with respect to the photoreceptor drum 12. The electrically charged toner image on the photoreceptor drum 12 is electrostatically transferred onto a recording medium such as a recording sheet by a conductive transfer roller 20 disposed below the photoreceptor drum 12. As illustrated in FIG. 1, the conductive transfer roller 20 comes into contact with the photoreceptor drum 12 via the upside running section of the endless belt 10a, so that the recording sheet conveyed by the endless belt 10a is given an electric charge, the polarity of which is reverse to that of the charged toner image. The charged toner image is thus electrostatically transferred from the photoreceptor drum 12 onto the recording sheet. 
     When the recording sheet is introduced from the idle roller 10c side of the conveyance means 10 and successively passes through the electrostatic recording units Y, C, M and B, a toner image of 4 colors is superimposed on the recording sheet so that a full color toner image can be formed. The recording sheet is then conveyed from the drive roller 10b side of the conveyance means 10 to a heat roller type thermal fixation unit 22, in which the full color image is thermally fixed onto the recording sheet. This thermal fixation will be described in detail as follows. The heat roller type thermal fixation unit 22 includes a heat roller 22a, and a backup roller 22b. In the thermal fixing operation, the heat roller 22a and the backup roller 22b are driven in the arrowed direction in FIG. 1, and the recording sheet is sent from the drive roller 10b side of the endless belt conveyance means 10. Then the recording sheet is introduced into a nip portion formed between both rollers 22a, 22b. At this time, the transferred toner image on the recording sheet is pressured and thermally fused. The transferred toner image is thus thermally fixed onto the recording sheet. 
     After the transfer process has been completed, residual toner that has not been transferred onto the recording sheet is deposited on the surface of the photoreceptor drum 12 of each of the electrostatic recording units Y, C, M and B. This residual toner is removed from the surface of the photoreceptor drum 12 by a cleaning unit 24 arranged on the downstream side in the recording sheet moving path with respect to the photoreceptor drum 12. In FIG. 1, reference numeral 26 is a light emitting element for discharging, for example, a light emitting diode array which is used to remove an electric charge from the surface of the photoreceptor drum 12 after the completion of the transfer process. Reference numeral 28 is a developer replenishment container from which an appropriate amount of toner component is replenished to the developing unit 18. Reference numeral 30 is an OD sensor, that is, an optical density sensor. This OD sensor 30 will be explained in detail later. 
     FIG. 2 shows a portion of one of the electrostatic recording units Y, C, M, B arranged on the endless conveyance belt 10. In FIG. 2, the recording sheet moving path formed by the upside running section of the endless conveyance belt 10 is illustrated by a one-dotted chain line. As shown in FIG. 2, the developing unit 18 includes a developer holding container 32. In this developer holding container 32, a two-component developer composed of a toner component (fine particles made of colored resin) and a magnetic component (fine magnetic carrier) is accommodated. The developer holding container 32 includes: a first bottom wall portion 32a; a first rear wall portion 32b extending upward from the rear of this first bottom wall portion 32a; a second bottom wall portion 32c extending horizontally at an upper end of this first rear wall portion 32b; a second rear wall portion 32d extending upward from the rear of this second bottom wall portion 32c; a top wall portion 32e extending horizontally to the front from an upper end of this second rear wall portion 32d; and a front wall portion 32f extending downward from the front end of this top wall portion 32e. Both sides of these wall portions are integrated with the side wall portions (not shown in the drawing). An opening portion is formed between the front end of the first bottom wall portion 32a of the developer holding container 32 and the lower end of the front wall portion 32f. In the opening portion, a magnet roller, that is, a development roller 34 is arranged in such a manner that a portion of the surface of the development roller 34 is exposed. The development roller 34 includes: a shaft 34a fixed and supported by both side walls of the developer holding container 32; a core 34b made of magnetic material fixed onto the shaft 34a; and a sleeve 34c made of nonmagnetic material such as aluminum, rotatably provided around the core 34b. When the developing unit 18 is operated, the sleeve 34c is rotated in the arrowed direction shown in the drawing. When the developing unit 18 shown in the drawing is installed in the electrostatic recording apparatus, an exposed surface of the development roller 34, that is, a surface of the sleeve 34c is opposed to the electrostatic latent image carrier such as a photoreceptor drum. 
     The first bottom wall portion 32a of the developer holding container 32 provides a developer reservoir 36. There is provided a paddle roller 38 in the developer reservoir 36. The paddle roller 38 is rotatably supported by both side walls of the developer holding container 32. When the developing unit 18 is operated, the paddle roller 38 is rotated in the arrowed direction in the drawing. The paddle roller 38 feeds developer stored in the developer reservoir 36 toward the development roller 34. Around the development roller 34, a magnetic brush is formed from the magnetic component of developer, that is, the magnetic carrier. Toner components are electrostatically deposited on the magnetic brush and conveyed by the rotation of the development roller 34 to a region where the development roller 34 is opposed to the photoreceptor drum 12. In order to regulate an amount of developer conveyed to the development region by the development roller 34, a developer regulation blade 40 is attached to the front edge of the first bottom wall portion 32a. 
     A developer stirring section 42 is provided in the second bottom wall portion 32c of the developer holding container 32, wherein the developer stirring section 42 is located above the developer reservoir 36. There is provided a developer stirring unit 44 in this developer stirring section 42. The developer stirring unit 44 is composed of a pair of conveyance screws 44a, 44b provided between both side walls of the developer holding container 32. This pair of conveyance screws 44a, 44b are arranged in parallel with each other. As illustrated in FIG. 2, a pair of curved recess portions to receive the spiral blades of the pair of conveyance screws 44a, 44b are formed on an upper face of the second bottom wall portion 32c. Shafts of the conveyance screws 44a, 44b are rotatably supported by both side walls of the developer holding container 32. When the developing unit 18 is operated, the conveyance screws 44a, 44b are respectively rotated in the arrowed directions illustrated in the drawing, that is, the conveyance screws 44a, 44b are respectively rotated in the opposite directions. In this embodiment, the spiral blades of both conveyance screws 44a, 44b are composed in the manner of a right-handed screw. Therefore, the conveyance screw 44a conveys developer to the rear side of the plane of FIG. 2, and the conveyance screw 44b conveys developer to the front side of the plane of FIG. 2. There are provided a pair of partition boards 46a, 46b between the conveyance screws 44a, 44b, wherein the pair of partition boards 46a, 46b are arranged perpendicular to the second bottom wall portion 32c. The pair of partition walls 46a, 46b are shorter than the conveyance screws 44a, 44b, and predetermined clearances are provided between both ends of the partition walls and both side walls of the developer holding container 32. In this way, a developer circulation passage is formed from the conveyance screws 44a, 44b in the second bottom wall portion 32c of the developer holding container 32. That is, developer is circulated along the pair of conveyance screws 44a, 44b in the following manner. After developer has been conveyed to an end of the conveyance screw 44a, the developer turns around the ends of the pair of partition boards 46a, 46b, so that the developer is moved to the conveyance screw 44b side arranged opposite to the conveyance screw 44a. After the developer has been conveyed to an end of the conveyance screw 44b, it turns around the ends of the pair of partition boards 46a, 46b and is moved to the conveyance screw 44a side. In this way, the developer is circulated along the pair of conveyance rollers 44a, 44b. 
     There is provided a communicating passage 48 for communicating the developer reservoir 36 with the developer stirring section 42 between the pair of partition boards 46a, 46b. An upper opening of this communicating passage 48 forms a developer overflow outlet for the developer in the developer stirring section 42. As can be seen from FIG. 2, the partition board 46b is lower than the partition board 46a, so that an upper edge of the partition board 46b forms a developer overflow edge. Specifically, a portion of the developer circulated by the conveyance screws 44a, 44b overflows the upper edge of the partition board 46b, that is, a portion of the developer overflows the developer overflow edge and drops into the communicating passage 48. Due to the foregoing, the developer reservoir 36 is supplied with developer from the developer stirring section 42. 
     As illustrated in FIG. 2, a vertical partition wall portion 32g is integrally formed in the front wall section of the second bottom wall portion 32c of the developer holding container 32. There is provided a developer rising passage 50 between the vertical partition wall portion 32g and the front wall portion 32f. As can be seen from FIG. 2, the developer rising passage 50 is located right above the development roller 34. In the developer rising passage 50, there are provided two magnet rollers 52, 54 being vertically aligned with respect to the development roller 34. That is, the magnet rollers 52, 54 are composed in the same manner as the development roller 34 composed as a magnet roller. The magnet rollers 52, 54 include: shafts 52a, 54a fixed and supported by both side walls of the developer holding container 32; cores 52b, 54b made of magnetic material fixed onto the shaft 34a; and sleeves 52c, 54c made of nonmagnetic material such as aluminum, rotatably provided around the cores. When the developing unit 18 is operated, the sleeves 52c, 54c are rotated in the arrowed directions shown in the drawing. The core 34b of the development roller 34, the core 52b of the magnet roller 52, and the core 54b of the magnet roller 54 are locally magnetized along the respective peripheries as illustrated in FIG. 2. When the cores 34b, 52b, 54b are locally affected by the magnetic field, they can be locally magnetized. The magnetic poles of the core 34b of the development roller 34 are arranged in such a manner that the developer can be conveyed from the developer reservoir 36 to the development region in accordance with the rotation of the sleeve 34c and then the developer can be conveyed to a position on the lower side of the magnet roller 52. The magnetic poles of the core 52b of the magnet roller 52 are arranged in such a manner that the developer can be pulled up from the top of the development roller 34 in accordance with the rotation of the sleeve 52c and conveyed to a position on the lower side of magnet roller 54. The magnetic poles of the core 54b of the magnet roller 54 are arranged in such a manner that the developer can be pulled up from the top of the magnet roller 52 in accordance with the rotation of the sleeve 54c and conveyed to a position on the top of the magnet roller 54. Due to the above structure, after the developer has been conveyed to the development region by the development roller 34, it is raised to the top of the top magnet roller 54 without being directly returned to the developer reservoir 36. 
     A scraper member 56 is provided at the upper end of the vertical partition wall 32g. A fore end of this scraper member 56 comes into contact with the surface of the magnet roller 54 at a position a little behind the top of the magnet roller 54. After the developer has been raised to the top of the magnet roller 54, it is supplied to the conveyance screw 44a side of the developer stirring section 42 by the action of the scraper member 56. 
     In short, the developer is circulated as follows. The developer is supplied from the developer stirring section 42 to the developer reservoir 36 via the communicating passage 48. Then the developer is conveyed from the developer reservoir 36 to the developing region by the development roller 34. After the developer has passed through the developing region, it is successively pulled up by the magnet rollers 52, 54, and returned to the developer stirring section 42 via the scraper member 56. As described above, when the developing unit 18 is operated, the developer is continuously circulated in the developer holding container 32, so that the developer reservoir 36 is supplied with a developer that has been sufficiently stirred. In this connection, when the developer is sufficiently stirred, the toner components and the magnetic components are subjected to a sufficiently high triboelectric charging, and the toner components are uniformly distributed in the magnetic components. 
     As illustrated in FIG. 2, the cleaning unit 24 includes: a toner recovery container 24a having an opening in which a portion of the photoreceptor drum 12 is received; a fur brush 24b arranged at a position close to the opening in the toner recovery container 24a; a toner scraping blade 24c arranged along the upper edge of the opening of the toner recovery container 24a; and a conveyance screw 24d arranged at the bottom of the toner recovery container 24a. Residual toner is removed from the surface of the photoreceptor drum 12 by the fur brush 24b, and toner further remaining on the surface of the photoreceptor drum 12 is scraped off by the scraping blade 24c. After the residual toner has been removed from the surface of the photoreceptor drum 12 by the fur brush 24b and the scraping blade 24c, it is temporarily recovered into the container 24a, however, the thus recovered toner is conveyed from the toner recovery container 24a to a predetermined position by the conveyance screw 24d. 
     The development process carried out in the above developing unit 18 will be described in detail as follows. For example, when the toner components in the developer are negatively charged, a uniformly negatively charged region is formed on the rotational surface of the photoreceptor drum 12 by the precharger 14. When the charged region on the photoreceptor drum 12 is irradiated with a laser beam LB emitted from the laser beam scanner 16, the negative electric charge is discharged from the irradiated portion, so that an electric potential is generated. In other words, an electrostatic latent image is written in the charged region on the photoreceptor drum 12, and the portion in which the electrostatic latent image has been written is generally referred to as &#34;a well of electric charge&#34;. For example, when the electric potential of the charged region on the photoreceptor drum 12 is -600 V as shown in FIG. 3, the electric potential of the electrostatic latent image is lowered to about -15 V as an absolute value. On the other hand, the development roller 34 is impressed with a negative bias voltage, for example, -400 V. In this way, an electric field is formed between the development roller 34 and the photoreceptor drum 12. The negatively charged toner components are drawn toward the photoreceptor drum 12 by the action of the electric field formed between the development roller 34 and the photoreceptor drum 12. At this time, the toner components are deposited at a position of the electrostatic latent image (the well of electric charge), so that the position of the electrostatic latent image is electrically charged. Specifically, the negatively charged toner components are deposited at the position of the electrostatic latent image in such a manner that the electric potential -15 V at the position of the electrostatic latent image is increased to the background electric potential -600 V as an absolute value. Accordingly, the more the amount of electric charge on the toner components is increased, the smaller the amount of toner components deposited at the position of the electrostatic latent image. The less the amount of electric charge of toner components is decreased, the larger the amount of toner components deposited at the position of the electrostatic latent image. That is, in the process of development of the electrostatic latent image, the development density, that is, the amount of deposited toner is affected by the amount of electric charge of toner components. Further, the amount of electric charge of toner components is greatly affected by the environmental temperature and humidity. The amount of deposited toner in the process of development is changed by an intensity of the development bias voltage impressed upon the development roller 34. Also, the amount of deposited toner in the process of development is changed when the characteristic of the photoreceptor drum 12 is deteriorated. 
     In order to maintain a predetermined hue (color balance) when the images of yellow, cyan and magenta toners are superimposed so as to conduct the recording of chromatic colors, it is necessary to regulate an amount of deposited toner (development density) for each dot at a predetermined value when a toner image of each color is developed. Since an amount of deposited toner for each dot is very small, usually, the toner deposition amount is defined as an overall toner deposition amount (weight) in the case where 1 m 2  of recording sheet is subjected to solid recording. For example, in this embodiment, it is possible to provide a predetermined hue when the toner deposition amount is defined as follows. In the case of a yellow toner image, the toner deposition amount is defined to be 4.2±0.4 g/m 2 . In the case of a cyan toner image, the toner deposition amount is defined to be 5.2±0.4 g/m 2 . In the case of a magenta toner image, the toner deposition amount is defined to be 4.7±0.4 g/m 2 . In this connection, for example, when a red toner image is formed from yellow and magenta toners, it is preferable that the weight of deposited yellow toner is the same as the weight of deposited magenta toner. However, when consideration is given to the characteristic of color material of each toner component and the electric charging characteristic, it is actually impossible to make the toner deposition amounts of yellow and magenta toners to be the same for providing a red toner image. Therefore, the predetermined value of deposited toner of each color fluctuates a little as described before. 
     As described above, in this embodiment, when multicolor recording is conducted in the normal mode, the toner deposition amount to obtain a yellow toner image is defined to be 4.2±0.4 g/m 2 , the toner deposition amount to obtain a cyan toner image is defined to be 5.2±0.4 g/m 2 , and the toner deposition amount to obtain a magenta toner image is defined to be 4.7±0.4 g/m 2 . However, when, for example, multicolor recording is conducted at a half density while the predetermined hue is maintained, that is, when multicolor recording is conducted in the economical mode, it is not appropriate that an amount of deposited toner of each color is simply reduced to a half. In this case, the deposition amount of yellow toner is defined to be 3.6±0.4 g/m 2 , the deposition amount of cyan toner is defined to be 4.5±0.4 g/m 2 , and the deposition amount of magenta toner is defined to be 4.0±0.4 g/m 2 . When the deposition amount of toner of each color is defined as described above, it is possible to conduct the multicolor recording at half the density of the normal mode while the predetermined hue is maintained. When the density is changed while the predetermined hue is maintained as described above, it is possible to determine the development density of each color in accordance with the Munsell color system as is well known. Concerning the deposition amount of toner required for obtaining a black toner image, which is not directly related to the hue of a chromatic toner image, for example, the deposition amount of toner is defined to be 5.0±0.4 g/m 2  in the normal mode, and 3.0±0.4 g/m 2  in the economical mode. 
     On the graph shown in FIG. 4, there is shown a relation between the deposition amount of toner of each color and the development density. On the graph, the development density of toner component of each color in the normal mode is defined as 100% which is used as a reference of conversion. As described before, an amount of deposited toner is mainly determined by an amount of electric charge given to the toner itself. Further, the amount of electric charge given to the toner is greatly affected by the environmental temperature and humidity. Consequently, when the environmental temperature and humidity change, the hue of multicolor recording, that is, the color balance of multicolor recording fluctuates. However, according to the present invention, as described below, even when the environmental temperature and humidity change, it is possible to maintain the color balance in the process of multicolor recording. Further, according to the present invention, as described below, it is possible to quickly switch between the normal and economical modes in multicolor recording. Furthermore, it is possible to provide full color images of various development densities while the color balance is maintained. 
     FIG. 5 is a control block diagram of the high speed laser printer shown in FIG. 1. Reference numeral 58 denotes a main control circuit of the high speed laser printer. As can be seen from FIG. 5, the main control circuit 58 is composed of a microcomputer, which includes: a central processing unit (CPU) 58a; a read-only-memory (ROM) 58b in which an operation program and constants to control the entire operation of the multicolor electrostatic recording apparatus are stored; a random-access-memory (RAM) 58c in which data is temporarily stored, wherein the data can be written in and read from the memory; and an input and output (I/O) interface 58d. Reference numeral 60 denotes a main motor of the high speed laser printer shown in FIG. 1. This main motor 60 drives an endless belt conveyance means 10, a photoreceptor drum 12, a developing unit 18 and the like. Reference numeral 62 denotes a power supply circuit of the main motor 60. This power supply circuit 62 is controlled by the main control circuit 58. Reference numerals 64Y, 64C, 64M and 64B respectively denote the control circuits of electrostatic recording units Y, C, M and B. These control circuits 64Y, 64C, 64M and 64B have the same structure, which is illustrated in FIG. 6. As can be seen from FIG. 6, each of the control circuits 64Y, 64C, 64M and 64B includes: a power supply circuit 66 for the precharger 14; an output control circuit 68 for this power supply circuit 66; a laser power supply circuit 70 for the laser beam scanner 16; an output control circuit 72 for the laser power supply circuit 70; a bias power supply circuit 74 for impressing a development bias voltage upon the development roller 34 of the developing unit 18; and an output control circuit 76 for the bias power supply circuit 74, wherein the output control circuits 68, 72 and, 76 are controlled by the main control circuit 58. Each of the control circuits 64Y, 64C, 64M and 64B includes an OD sensor 30. Detection data of this OD sensor 30 is taken into the main control circuit 58 through an A/D converter 78. Each OD sensor 30 detects the optical density (development density) of a detection mark formed on the photoreceptor drum 12. In this connection, the detection mark is obtained when the electrostatic latent image of a predetermined pattern is written on the photoreceptor drum 12 with the laser beam scanner 16 and developed with the toner component in the developing unit 18. When the optical density of the detection mark is detected by the OD sensor 30, the amount of deposited toner can be known. Further, each of the control circuits 64Y, 64C, 64M and 64B includes an electric potential sensor 80. In order to simplify the drawings, this electric potential sensor 80 is omitted in FIGS. 1 and 2, however, this electric potential sensor 80 is arranged between the precharger 14 and the electrostatic latent image writing position (laser beam LB). The electric potential sensor 80 detects an electric potential on the charged region formed on the photoreceptor drum 12 by the precharger 14. Detection data of the electric potential sensor 80 is taken into the main control circuit 58 through the A/D converter 82. In this connection, in FIG. 5, reference numeral 84 denotes a power switch, reference numeral 86 denotes a development density correction switch, reference numeral 88 denotes a mode selection switch, and reference numeral 90 denotes a density setting input key means to which an arbitrary development density correction value is inputted so as to set the density. 
     According to the high speed laser printer of the present invention, even when the constitutive parts such as a photoreceptor drum and a semiconductor laser are subjected to the deterioration with time so that the characteristics of the parts are changed, and even when the environmental temperature and humidity fluctuate, it is possible to guarantee the multicolor recording in which the hue is kept constant at all times. It can be accomplished when the development density of toner component of each color, that is, the amount of deposited toner is corrected in the multicolor recording in accordance with the change in the characteristics of parts or the change in the environmental temperature and humidity. 
     There is shown a development density correction routine in FIGS. 7 and 8. With reference to the development density correction routine, the development density correction of the present invention will be explained below. In this connection, the development density correction routine shown in FIGS. 7 and 8 is carried out when the power switch 84 is turned on. 
     In step 701, in order to drive the main motor 60, the main control circuit 58 outputs an ON-signal to the power circuit 62 through the I/O 58d. Due to the foregoing operation, the photoreceptor drum 12 is rotated and the developing unit 18 is operated. Next, in step 702, in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharger 14 by the power circuit 66 is adjusted when an output control value of the output control circuit 68 sent from the main control circuit 58 is controlled. In this way, an electric potential of the charged region on the photoreceptor drum 12 can be maintained at a predetermined value. That is, the electric potential of the charged region on the photoreceptor drum 12 is detected by the potential sensor 80, the detection data is taken in by the main control circuit 58 through the A/D converter 82, this potential data is compared with a predetermined setting value, and an output control value sent to the output control circuit 68 is subjected to feedback control. Due to the foregoing operation, the electric potential of the charged region on the photoreceptor drum 12 can be maintained at a predetermined value, for example, -600 V. As a result, even when the characteristic of the photoreceptor drum 12 is deteriorated with time, a predetermined electric potential level can be guaranteed in the charged region on the photoreceptor drum 12. 
     In step 703, in each of the electrostatic recording units Y, C, M, B, an electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 with the laser beam LB sent from the laser beam scanner 16. In this case, an output control value outputted from the main control circuit 58 to the output control circuit 72 is set in such a manner that an output level of each laser power supply circuit 70 to the laser beam scanner 16 corresponds to a predetermined density value in the normal mode. That is, when the main control circuit 58 outputs a control signal to the output control circuit 72 in accordance with the output control value that has been set in the above manner, the output level of the laser power supply circuit 70 to the laser beam scanner 16 corresponds to a predetermined density value in the normal mode. For example, under the condition that an electric potential of the charged region on the photoreceptor drum 12 is -600 V and further the development bias voltage impressed upon the development roller 34 is -400 V, the output control value to the output control circuit 72 is set so that the laser beam scanner 16 can be operated by the laser power supply circuit 70 with an electric power of 1.5 mW. At this time, the laser beam scanner 16 generates a laser beam LB, the intensity of which corresponds to the control value. As shown in FIG. 9, there is a relation between the output level of the laser beam scanner 16 and the development density. In this embodiment, it is defined that the development density obtained when the laser beam scanner 16 is operated with an electric power of 1.5 mW is the density value 100% in the normal mode. However, even if the laser beam scanner 16 is operated with an electric power of 1.5 mW, the development density value 100% is not necessarily obtained in the normal mode. The reason is that an amount of deposited toner corresponding to the development density value 100% can not be necessarily obtained due to the fluctuation of electric charge of toner of each color as described before. In any case, in step 703, when the laser beam scanner 16 is operated by the electric power of 1.5 mW, an electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12. 
     In step 704, an output control value to the output control circuit 72 in the case of writing the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 705, the electrostatic latent image of the detection mark is developed in each developing unit 18 with the toner component of each color. At this time, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is controlled to be -400 V by the output control circuit 76 as described before. 
     In step 706, the optical density value of the development detection mark is detected by the OD sensor 30, and the detection optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data of the detection mark which represents an amount of deposited toner. Next, in step 707, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the normal mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 4.2±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 5.2±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.7±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 5.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range (±0.4 g/m 2 ). When the detection development density data does not coincide with the predetermined density value in the allowable range in the normal mode, the program advances to step 708. In step 708, the correction of an output level from the laser power supply circuit 70 to the laser beam scanner 16 is conducted when the output control value to the output control circuit 72 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the normal mode, an output control value to the output control circuit 72 is raised by a predetermined value so that the laser beam scanner 16 can be operated with an electric power, the intensity of which is higher than 1.5 mW. When the detection development density data is higher than the predetermined density value in the normal mode, an output control value to the output control circuit 72 is lowered by a predetermined value so that the laser beam scanner 16 can be operated by electric power, the intensity of which is lower than 1.5 mW. In this connection, the predetermined density value in the normal mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 703, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the normal mode in step 707. At this time, the output control value to the output control circuit 72, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as a normal mode correction value for the output control value to the output control circuit 72. 
     In step 707, when the detection development density data coincides with the predetermined density value in the allowable range in the normal mode, the program advances to step 709. In step 709, in each of the electrostatic recording units Y, C, M and B, an electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 with the laser beam LB sent from the laser beam scanner 16. In this case, the output control value to the output control circuit 72 is set in such a manner that the output level of the laser power supply circuit 70 to the laser beam scanner 16 corresponds to the predetermined density value in the economical mode. In this embodiment, the predetermined density value in the economical mode is a half of the density value in normal mode. That is, under the above condition, the output control value to the output control circuit 72 is set so that the laser beam scanner 16 can be operated with an electric power of 0.5 mW. As illustrated in FIG. 9, when the laser beam scanner 16 is operated with an electric power of 0.5 mW, an amount of deposited toner corresponding to the density value 50%, which is a half of the development density value 100% in the normal mode, can be provided. However, as described before, due to the fluctuation of electric charge of toner component of each color, an amount of deposited toner corresponding to the development density value in the economical mode is not necessarily provided. In any case, when the laser beam scanner 16 is operated with an electric power of 0.5 mW in step 709, an electrostatic latent image is written in the charged region on the photoreceptor drum 12. 
     In step 710, an output control value to the output control circuit 72 in the case of writing the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 711, the electrostatic latent image of the detection mark is developed by each developing unit 18 with a toner of each color. 
     In step 712, an optical density value of the development detection sensor is detected by the OD sensor 30. The detected optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data of the detection mark, wherein the development density data represents an amount of deposited toner. Next, in step 713, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the economical mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 3.6±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 4.5±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.0±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 3.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range. When the detection development density data does not coincide with the predetermined density value in the allowable range in the economical mode, the program advances to step 714. In step 714, the correction of an output level from the laser power supply circuit 70 to the laser beam scanner 16 is conducted when the output control value to the output control circuit 72 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the normal mode, an output control value to the output control circuit 72 is raised by a predetermined value so that the laser beam scanner 16 can be operated by electric power, the intensity of which is higher than 1.5 mW. When the detection development density data is higher than the predetermined density value in the normal mode, an output control value to the output control circuit 72 is lowered by a predetermined value so that the laser beam scanner 16 can be operated by electric power, the intensity of which is lower than 1.5 mW. In this connection, the predetermined density value in the economical mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 709, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the normal mode in step 713. At this time, the output control value to the output control circuit 72, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as an economical mode correction value for the output control value to the output control circuit 72. Next, the program advances to step 715. In step 715, the main motor 60 is temporarily stopped, and the high speed laser printer is ready for an actual recording operation. 
     When the mode selection switch 88 is in a condition of OFF, multicolor recording is conducted in the normal mode. In this case, when an electrostatic latent image is written by each of the electrostatic recording units Y, C, M and B, an output level of the laser power supply circuit 70 to the laser beam scanner 16 is determined in accordance with the correction value in the normal mode stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the normal mode of multicolor recording so that the hue can be appropriately maintained. On the other hand, when the mode selection switch 88 is in a condition of ON, multicolor recording is conducted in the economical mode. In this case, when an electrostatic latent image is written by each of the electrostatic recording units Y, C, M and B, an output level of the laser power supply circuit 70 to the laser beam scanner 16 is determined in accordance with the correction value in the economical mode stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the economical mode of multicolor recording so that the hue can be appropriately maintained. 
     In the development density correction routine shown in FIGS. 7 and 8, the predetermined density values in the normal and economical modes may be set as the predetermined amounts of deposited toner. In this case, the graph shown in FIG. 4 is held in the main control circuit 58 as a ROM table. An output value of the 0D sensor 30 is inputted onto the ROM table and converted into a toner deposition amount. The thus converted toner deposition amount is compared with the predetermined toner deposition amount. 
     The development density correction routine shown in FIGS. 7 and 8 is carried out when the operation of the high speed printer starts, that is, when the power supply switch 84 is turned on. However, when an operator turns on the development density correction switch 86, the routine may be appropriately carried out. When the recording operation is not carried out by the high speed laser printer over a predetermined period of time under the condition that the power supply switch 84 of the high speed laser printer is turned on, for example, when the recording operation is not carried out over one hour, the development density routine shown in FIGS. 7 and 8 may be automatically carried out. 
     In the development density correction routine shown in FIGS. 7 and 8, the development density correction of multicolor recording is conducted only in the two cases of the normal and economical modes. However, according to the multicolor recording apparatus of the present invention, even when arbitrary density data is inputted by the density setting input key means 90, it is possible to correct the development density with respect to the arbitrary input density data. With reference to the development density correction routine shown in FIG. 10, the development density correction to correct the arbitrary input density data will be explained as follows. In this connection, the development density correction routine shown in FIGS. 10(a) and 10(b) is carried out when the development density correction switch 86 is turned on after the arbitrary density data has been inputted using the density setting input key means 90. 
     In step 1001, it is judged whether or not the density data has been inputted by the density setting input key means 90. After the density data has been inputted, the program advances to step 1002. In step 1002, in order to drive the main motor 60, the main control circuit 58 outputs an ON signal to the power supply circuit 62 through the I/O 58d. Due to the foregoing, the photoreceptor drum 12 is rotated and the developing unit 18 is operated at the same time. In this connection, when the image density data has not been inputted by the density setting input key means 90, the development density correction routine shown in FIGS. 7 and 8 is carried out by switching ON of the development density correction switch 86. Next, in step 1003, in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharge 14 by the power supply circuit 66 is adjusted by controlling the output control value of the main control circuit 58 to the output control circuit 68. In this way, an electric potential in the charged region on the photoreceptor drum 12 can be maintained at -600 V. 
     In step 1004, in each of the electrostatic recording units Y, C, M and B, the electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 by the laser beam LB sent from the laser beam scanner 16. In this case, the output control value inputted into the output control circuit 72 is set in such a manner that the output level of the laser power supply circuit 70 sent to the laser beam scanner 16 corresponds to the input density data. That is, when a control signal is outputted to the output control circuit 72 by the main control circuit 58 in accordance with the output control value that has been set as described above, the output level sent to the laser beam scanner 16 from the laser power supply circuit 70 corresponds to the input density data. For example, when the input density data inputted by the density setting input key means 90 is a density value 75% with respect to the development density value 100% in the normal mode, the output control value given to the output control circuit 72 is set, as can be seen from the graph in FIG. 9, so that the laser beam scanner 16 can be operated by the electric power of 1.0 mW in the laser power supply circuit 70. At this time, it is possible to provide an amount of deposited toner corresponding to the development density value 75% with respect to the development density value in the normal mode. However, for the reasons described above, an amount of deposited toner corresponding to the development density value 75% can not be necessarily provided. In any case, in step 1004, when the laser beam scanner 16 is operated by the electric power corresponding to the input density data, the electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12. 
     In step 1005, an output Control value given to the output control circuit 72 in the case of writing the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 1006, the electrostatic latent image of the detection mark is developed by the toner component of each color in each developing unit 18. 
     In step 1007, an optical density value of the development detection mark is detected by the OD sensor 30. The detected optical density value is taken into the main control circuit 58 through the converter 78 as the development density data of the detection mark, wherein the development density data represents an amount of deposited toner. Next, in step 1008, the thus detected development density data is compared with the input density data, and it is judged whether or not the detected development density coincides with the input density data in the allowable, range. When the detected development density data does not coincide with the input density data in the allowable range, the program advances to step 1009. In step 1009, the output level of the laser power supply circuit 70 to the laser beam scanner 16 is corrected when the output control value given to the output control circuit 72 is changed by a predetermined value. For example, when the input density data inputted by the density setting input key means 90 is a density value 75%, and when the detected development density data is lower than the input density data, the output control value given to the output control circuit 72 is increased by a predetermined value so that the laser beam scanner 16 can be operated by the electric power higher than 1.0 mW. On the other hand, when the detected development density data is higher than the input density data, the output control value given to the output control circuit 72 is lowered by a predetermined value so that the laser beam scanner 16 can be operated by the electric power lower than 1.0 mW. 
     After that, the program is returned to step 1004, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the input density data in the allowable range in step 1008. At this time, the output control value to the output control circuit 72, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as an input density data correction value for the output control value to the output control circuit 72. 
     In step 1008, when the detected development density data coincides with the input density data in the allowable range, the program advances to step 1010. In step 1010, it is judged whether or not a recording command has been given in a predetermined period of time. When a recording command has been given in a predetermined period of time, a recording operation routine (not shown) is carried out, and the actual multicolor recording is started. In this case, when the electrostatic latent image is written in each of the electrostatic recording units Y, C, M and B, the output level given to the laser beam scanner 16 by the laser power circuit 70 is determined in accordance with the input density data correction value stored in RAM 58c. Due to the foregoing, it can be guaranteed that the amount of deposited toner of each color in the multicolor recording, that is, the development density is provided with an appropriate hue. On the other hand, when a recording command is not given in a predetermined period of time, the main motor 60 is temporarily stopped in step 1011, and the high speed laser printer is put in a waiting condition with respect to the multicolor recording operation. 
     In the development density correction routine shown in FIGS. 10((a) and (b)), the density data inputted by the density setting input key means 90 may be replaced with the amount of deposited toner. In this case, the graph shown in FIG. 4 is held in the main control circuit 58 as a ROM table, and an output value of the OD sensor 30 is inputted into the ROM table so that it is converted into an amount of deposited toner, and the converted toner deposition amount is compared with the input toner deposition amount inputted by the density setting input key means 90. 
     As can be seen from the above descriptions, in the development density correction routine shown in FIGS. 7 and 8, and also in the development density correction routine shown in FIGS. 10(a) and 10 (b), output level given to the laser beam scanner 16 by the laser power supply circuit 70 is used as a control parameter for correcting the development density. However, it is possible to use other control parameters for correcting the development density. In FIGS. 11 and 12, there is shown a development density correction routine in which a development bias voltage impressed upon the development roller 34 by the bias power supply 76 is used as a control parameter. Multicolor recording in which the hue is maintained constant at all times can be also accomplished by this development density correction routine. In this connection, the development density correction routine shown in FIGS. 11 and 12 is also carried out when the power switch 84 is turned on. 
     In step 1101, in order to drive the main motor 60, the main control circuit 58 outputs an ON signal into the power supply circuit 62 through the I/O 58d. Due to the foregoing, the photoreceptor drum 12 is rotationally driven, and at the same time the developing unit 18 is operated. Next, in step 1102, in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharger 14 by the power supply circuit 66 is adjusted when the output control value given to the output control circuit 68 by the main control circuit 58 is controlled. In this way, an electric potential in the charged region on the photoreceptor drum 12 is maintained at a predetermined value. That is, the electric potential in the charged region on the photoreceptor drum 12 is detected by the electric potential sensor 80. The detected data is taken into the main control circuit 58 through the A/D converter 82. When this electric potential data is compared with a predetermined setting value and the output control value to the output control circuit 68 is subjected to feedback control, the electric potential in the charged region on the photoreceptor drum 12 can be maintained, for example, at -600 V. In this way, even if the characteristic of the photoreceptor drum 12 is deteriorated with time, the predetermined electric potential can be guaranteed in the charged region on the photoreceptor drum 12. 
     In step 1103, in each of the electrostatic recording units Y, C, M and B, the electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 by the laser beam LB sent from the laser beam scanner 16. At this time, an output control value given to the output control circuit 72 by the main control circuit 58 is determined so that the laser beam scanner 16 can be operated by the laser power supply circuit 70 at the electric power of 1.5 mW. 
     In step 1104, the electrostatic latent image of the detection mark is developed by the toner component of each color in each developing unit 18. At this time, an output control value given to the output control circuit 76 by the main control circuit 58 is determined so that the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 corresponds to a predetermined density value in the normal mode. That is, when the main control circuit 58 outputs a control signal to the output control circuit 76 in accordance with the output control value that has been set in the above manner, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 becomes a value corresponding to the predetermined density value. For example, under the condition that the electric potential in the charged region on the photoreceptor drum 12 is -600 V and the operational electric power given to the laser beam scanner 16 is 1.5 mW, the output control value sent to the output control circuit 76 is determined so that the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 can be -400 V. Between the development bias voltage and the development density, there is a relationship shown in FIG. 13. In this embodiment, the development density provided when the development bias voltage of -400 V is impressed upon the development roller 34 is defined to be a density value of 100% in the normal mode. However, even when the development bias voltage of -400 V is impressed upon the development roller 34, the development density of 100% is not necessarily provided. The reason is that an amount of deposited toner corresponding to the development density value 100% can not be necessarily obtained due to the fluctuation of electric charge of toner of each color as described before. In any case, in step 1104, when the development bias voltage -400 V is impressed upon the development roller 34, the electrostatic latent image of the detection mark is developed. 
     In step 1105, the output control value given to the output control circuit 76 in the development process of the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 1106, an optical density value of the development detection mark is detected by the OD sensor 30. The optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data which represents an amount of deposited toner. Next, in step 1107, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the normal mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 4.2±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 5.2±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.7±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 5.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range. When the detection development density data does not coincide with the predetermined density value in the allowable range in the normal mode, the program advances to step 1108. In step 1108, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is corrected when the output control value given to the output control circuit 76 by the main control circuit 58 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is increased by a predetermined value so that an absolute value of the development bias voltage -400 V impressed upon the development roller 34 can be increased. When the detection development density data is higher than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is decreased by a predetermined value so that an absolute value of the development bias voltage -400 V impressed upon the development roller 34 can be decreased. In this connection, the predetermined density value in the normal mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 1103, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the normal mode in step 1107. At this time, the output control value to the output control circuit 76, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as a normal mode correction value for the output control value to the Output control circuit 76. 
     When the detected development density data coincides with the predetermined density value of the normal mode in the allowable range in step 1107, the program advances to step 1109. In step 1109, the electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 by the laser beam LB. At this time, the output control value given to the output control circuit 72 is determined so that the laser beam scanner 16 can be operated by the laser power supply circuit 70 by the electric power of 1.5 mW. 
     In step 1110, the electrostatic latent image of the detection mark is developed by each developing unit 18 with toner of each color. In this case, the output control value given to the output control circuit 76 by the main control circuit 58 is determined in such a manner that the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 corresponds to the predetermined density value in the economical mode. In this embodiment, the predetermined density value in the economical mode is a half of the density value in the normal mode. In other words, under the above condition, the output control value given to the output control circuit 76 by the main control circuit 58 is set so that the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 can be -350 V. As shown in the graph of FIG. 13, when the development bias voltage of -350 V is impressed upon the development roller 34, it is possible to provide an amount of deposited toner, the value of which corresponds to the development density value 50% that is a half of the development density value 100% in the normal mode. However, as described before, due to the fluctuation of the electric charge of toner of each color, it is not always possible to provide an amount of deposited toner that corresponds to the development density value in the economical mode. In any case, when the development bias voltage of -350 V is impressed upon the development roller 34 in step 1110, the electrostatic latent image of the detection mark is developed. 
     In step 1111, the output control value given to the output control circuit 76 in the development process of the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 1112, an optical density value of the development detection mark is detected by the OD sensor 30. The optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data which represents an amount of deposited toner. Next, in step 1113, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the economical mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 3.6±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 4.5±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.0±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 3.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range. When the detection development density data does not coincide with the predetermined density value in the allowable range in the economical mode, the program advances to step 1114. In step 1114, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is corrected when the output control value given to the output control circuit 76 by the main control circuit 58 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is increased by a predetermined value so that an absolute value of the development bias voltage -350 V impressed upon the development roller 34 can be increased. When the detection development density data is higher than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is decreased by a predetermined value so that an absolute value of the development bias voltage -350 V impressed upon the development roller 34 can be decreased. In this connection, the predetermined density value in the economical mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 1109, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the economical mode in step 1113. At this time, the output control value to the output control circuit 76, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as an economical mode correction value for the output control value to the output control circuit 76. Next, the program advances to step 1115, and the main motor is temporarily stopped here. At this time, the high speed laser printer is ready for the actual multicolor recording operation. 
     When the mode selection switch 88 is turned off, the multicolor recording is carried out in the normal mode. In this case, when the electrostatic latent image is developed by each of the electrostatic recording units Y, C, M and B, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is determined in accordance with the normal mode correction value stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the normal mode of multicolor recording so that the hue can be appropriately maintained. On the other hand, when the mode selection switch 88 is turned on, the multicolor recording is carried out in the economical mode. In this case, when the electrostatic latent image is developed by each of the electrostatic recording units Y, C, M and B, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is determined in accordance with the economical mode correction value stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the normal mode of multicolor recording so that the hue can be appropriately maintained. 
     In the development density correction routine shown in FIGS. 11 and 12, the predetermined density values in the normal and economical modes may be set as the predetermined amounts of deposited toner. In this case, the graph shown in FIG. 4 is held in the main control circuit 58 as a ROM table. An output value of the 0D sensor 30 is inputted onto the ROM table and converted into a toner deposition amount. The thus converted toner deposition amount is compared with the predetermined toner deposition amount. 
     The development density correction routine shown in FIGS. 11 and 12 is carried out when the operation of the high speed printer starts, that is, when the power supply switch 84 is turned on. However, when an operator turns on the development density correction switch 86, the routine may be appropriately carried out. When the recording operation is not carried out by the high speed laser printer over a predetermined period of time under the condition that the power supply switch 84 of the high speed laser printer is turned on, for example, when the recording operation is not carried out over one hour, the development density routine shown in FIGS. 11 and 12 may be automatically carried out. 
     In the development density correction routine shown in FIGS. 11 and 12, the development density correction of multicolor recording is conducted only in the two cases of the normal and economical modes. However, in the same manner as the development density correction routine shown in FIGS. 10(a) and 10(b) even when arbitrary density data is inputted by the density setting input key means 90, it is possible to correct the development density with respect to the arbitrary input density data. With reference to the development density correction routine shown in FIGS. 14((a) and (b)), the development density correction to correct the arbitrary input density data will be explained as follows. In this connection, the development density correction routine shown in FIGS. 14(a) and 14(b) is carried out when the development density correction switch 86 is turned on after the arbitrary density data has been inputted using the density setting input key means 90. 
     In step 1401, it is judged whether or not the density data has been inputted by the density setting input key means 90. After the density data has been inputted, the program advances to step 1402. In step 1402, in order to drive the main motor 60, the main control circuit 58 outputs an ON signal to the power supply circuit 62 through the I/O 58d. Due to the foregoing, the photoreceptor drum 12 is rotated and the developing unit 18 is operated at the same time. In this connection, when the image density data has not been inputted by the density setting input key means 90, the development density correction routine shown in FIGS. 11 and 12 is carried out by switching ON of the development density correction switch 86. Next, in step 1403, in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharger 14 by the power supply circuit 66 is adjusted by the output control circuit 68. In this way, an electric potential in the charged region on the photoreceptor drum 12 can be maintained at -600 V. 
     In step 1404, in each of the electrostatic recording units Y, C, M and B, the electrostatic latent image of the detection mark is written in the charged region on the photoreceptor drum 12 by the laser beam LB sent from the laser beam scanner 16. At this time, the output control value given to the output control circuit by the main control circuit 58 is determined so that the laser beam scanner 16 can be operated by the laser power supply circuit 70 with the electric power of 1.5 mW. 
     In step 1405, the electrostatic latent image of the detection mark is developed by the toner component of each color in each developing unit 18. At this time, an output control value given to the output control circuit 76 by the main control circuit 58 is determined so that the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 corresponds to the input density data. That is, when the main control circuit 58 outputs a control signal to the output control circuit 76 in accordance with the output control value that has been set in the above manner, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 becomes a value corresponding to the input density data. For example, when the input density data inputted by the density setting input key means 90 is determined to be a density value of 75% with respect to the development density 100% in the normal mode, as can be seen from FIG. 13, the output control value given to the output control circuit 76 is determined so that a development bias voltage of -375 V can be impressed upon the development roller 34. At this time, it is possible to provide an amount of deposited toner corresponding to the development density 75% with respect to the development density in the normal mode. However, from the reasons described before, it is not always possible to provide an amount of deposited toner corresponding to the development density 75%. In any case, when the development bias voltage corresponding to the input density data is impressed upon the development roller 34 in step 1405, the electrostatic latent image of the detection mark is developed. 
     In step 1406, the output control value given to the output control circuit 76 at the time of developing the electrostatic latent image of the detection mark is stored in RAM 58c. Next, in step 1407, the optical density value of the development detection mark is detected by the OD sensor 30, and the detected optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data of the detection mark which represents an amount of deposited toner. Next, in step 1408, the detected development density data is compared with the input density data, and it is judged whether or not the detected development density data coincides with the input density data in the allowable range. When the detected development density data does not coincide with the input density data in the allowable range, the program advances to step 1409. In step 1409, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is corrected when the output control value given to the output control circuit 76 is changed by a predetermined value. For example, in the case where the input density data is a density value of 75%, when the detected development density data is lower than the input density data, the output control value given to the output control circuit 76 by the main control circuit 58 is increased by a predetermined value so that the development bias voltage -350 V impressed upon the development roller 34 can be increased. When the detected development density data is higher than the input density data, the output control value given to the output control circuit 76 by the main control circuit 58 is decreased by a predetermined value so that the development bias voltage -350 V impressed upon the development roller 34 can be decreased. 
     After that, the program is returned to step 1404, and the same operation is repeated. The repetition is continued until the detected development density data coincides with the input density data in the allowable range in step 1408. At this time, the output control value to the output control circuit 76, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as a correction value of the input density data for the output control value given to the output control circuit 76. 
     When the detected development density data coincides with the input density data in step 1408, the program advances to step 1410. In step 1410, it is judged whether or not a recording command has been given in a predetermined period of time. When a recording command has been given in a predetermined period of time, a recording operation routine (not shown) is carried out, and the actual multicolor recording is started. In this case, when the electrostatic latent image is written in each of the electrostatic recording units Y, C, M and B, the development bias voltage impressed upon the development roller 34 by the bias power supply circuit 74 is determined in accordance with the input density data correction value stored in RAM 58c. Due to the foregoing, it can be guaranteed that the amount of deposited toner of each color in the multicolor recording, that is, the development density is provided with an appropriate hue. On the other hand, when a recording command is not given in a predetermined period of time, the main motor is temporarily stopped, and the high speed laser printer is put in a waiting condition with respect to the multicolor recording operation. 
     In the development density correction routine shown in FIGS. 14(a) and 14 (b) the density data inputted by the density setting input key means 90 may be set as a predetermined amount of deposited toner. In this case, the graph shown in FIG. 4 is kept in the main control circuit 58 as a ROM table. An output value of the OD sensor 30 is inputted onto the ROM table and converted into a toner deposition amount. The thus converted toner deposition amount is compared with the input toner deposition amount inputted by the density setting input key means 90. 
     FIGS. 15 and 16 show another development density correction routine in which a voltage impressed upon the precharger 14 by the power supply circuit 66 is used as a parameter. By this development density correction routine, it is possible to guarantee the multicolor recording provided with a constant hue. In this connection, the development density correction routine shown in FIGS. 15 and 16 is also carried out when the power supply switch 84 is turned on. 
     In step 1501, an ON signal to drive the main motor 60 is outputted from the main control circuit 58 to the power supply circuit 62 through I/O 58d. Due to the 0N signal, the photoreceptor drum 12 is rotated and the developing unit is operated. Next, in step 1502, in each of the electrostatic recording units Y, C, M and B, a voltage is impressed upon the precharger 14 by the power circuit 66. Due to the foregoing, a charged region is formed on the photoreceptor drum 12. An output control value given to the output control circuit 68 by the main control circuit 58 is determined in such a manner that a voltage impressed upon the precharger 14 by the power supply circuit 66 corresponds to the predetermined density value in the normal mode. In other words, in accordance with the output control value determined in the above manner, the main control circuit 58 outputs a control signal to the output control circuit 68, and then the voltage impressed upon the precharger 14 by the power supply circuit 66 becomes the predetermined density value in the normal mode. For example, under the condition that the operational electric power of the laser beam scanner 16 is 1.5 mW and the development bias voltage impressed upon the development roller 34 is -400 V, the output control value given to the output control circuit 68 by the main control circuit 58 is determined so that the electric potential of the charged region on the photoreceptor drum 12 can be -600 V in accordance with the voltage impressed by the power supply circuit 66. There is a relation shown in FIG. 17 between the electric potential of the charged region on the photoreceptor drum 12 and the development density. In this embodiment, the development density obtained when the electric potential of the charged region on the photoreceptor drum 12 is -600 V is defined to be the density value 100% in the normal mode. However, even if the electric potential of the charged region on the photoreceptor drum 12 is kept to be -600 V, the development density value in the normal mode is not necessarily 100%. The reason is that an amount of deposited toner corresponding to the development density value 100% can not be necessarily obtained due to the fluctuation of electric charge of toner of each color. In any case, the charged region, the electric potential of which is -600 V, is formed on the photoreceptor drum 12 in step 1502. 
     In step 1503, the output control value given to the output control circuit 68 in the case of forming the charged region on each photoreceptor drum 12 is stored in RAM 58c. Next, in step 1504, the electrostatic latent image of the detection mark is written in the charged region on each photoreceptor drum 12 by the laser beam scanner 16. At this time, the operational electric power of the laser beam scanner 16 is 1.5 mW. Next, in step 1505, the electrostatic latent image of the detection mark is developed with toner components of each color. At this time, the development bias voltage impressed upon the development roller 34 is -400 V. 
     In step 1506, an optical density value of the development detection mark is detected by the OD sensor 30. The optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data which represents an amount of deposited toner. Next, in step 1507, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the normal mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 4.2±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 5.2±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.7±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 5.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range. When the detection development density data does not coincide with the predetermined density value in the allowable range in the normal mode, the program advances to step 1508. In step 1508, the development bias voltage impressed upon the precharger 14 by the power supply circuit 66 is corrected when the output control value given to the output control circuit 68 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is decreased by a predetermined value so that an absolute value -600 V of the electric potential of the charged region on the photoreceptor drum 12 can be decreased. When the detection development density data is higher than the predetermined density value in the normal mode, the output control value given to the output control circuit 76 is increased by a predetermined value so that the absolute value of -600 V of the electric potential of the charged region on the photoreceptor drum 12 can be increased. In this connection, the predetermined density value in the normal mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 1502, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the normal mode in step 1507. At this time, the output control value to the output control circuit 68, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as a normal mode correction value for the output control value to the output control circuit 68. 
     When the detected development density data coincides with the predetermined density value of the normal mode in the allowable range in step 1507, the program advances to step 1509. In step 1509, a voltage is impressed upon the precharger 14 by the power supply circuit 66 in each of the electrostatic recording units Y, C, M and B. Due to the foregoing, a charged region is formed on the photoreceptor drum 12. At this time, the output control value given to the output control circuit 68 is determined in such a manner that a voltage impressed upon the precharger 14 by the power supply circuit 66 corresponds to the predetermined density value in the economical mode. That is, under the above conditions, the output control value given to the output control circuit 68 is determined so that the electric potential of the charged region on the photoreceptor drum 12 can be -700 V in accordance with the voltage impressed by the power supply circuit 66. As shown in FIG. 17, when the electric potential of the charged region on the photoreceptor drum 12 is -700 V, it is possible to provide an amount of deposited toner, the value of which corresponds to the development density value 50% that is a half of the development density value 100% in the normal mode. However, as described before, due to the fluctuation of the electric charge of toner of each color, it is not always possible to provide an amount of deposited toner that corresponds to the development density value in the economical mode. In any case, in step 1509, an electrically charged region, the electric potential of which is -700 V, is formed on the photoreceptor drum 12. 
     In step 1510, the output control value given to the output control circuit 68 in the case of forming the charged region on each photoreceptor drum 12 is stored in RAM 58c. Next, in step 1511, the electrostatic latent image of the detection mark is written in the charged region on each photoreceptor drum 12 by the laser beam scanner 16. At this time, the operational electric power of the laser beam scanner 16 is 1.5 mW. Next, in step 1512, the electrostatic latent image of the detection mark is developed with toner components of each color. At this time, the development bias voltage impressed upon the development roller 34 is -400 V. 
     In step 1513, an optical density value of the development detection mark is detected by the OD sensor 30. The optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data which represents an amount of deposited toner. Next, in step 1514, the detection development density data is compared with a predetermined density value corresponding to the amount of deposited toner of each of the electrostatic recording units Y, C, M, B in the economical mode. In this case, the amount of deposited toner of each electrostatic recording unit is described as follows. In the electrostatic recording unit Y, the predetermined amount of deposited toner is 3.6±0.4 g/m 2 . In the electrostatic recording unit C, the predetermined amount of deposited toner is 4.5±0.4 g/m 2 . In the electrostatic recording unit M, the predetermined amount of deposited toner is 4.0±0.4 g/m 2 . In the electrostatic recording unit B, the predetermined amount of deposited toner is 3.0±0.4 g/m 2 . Then it is judged whether or not the detection development density data coincides with the predetermined density value in the allowable range. When the detection development density data does not coincide with the predetermined density value in the allowable range in the economical mode, the program advances to step 1515. In step 1515, the development bias voltage impressed upon the precharger 14 by the power supply circuit 66 is corrected when the output control value given to the output control circuit 68 is changed by a predetermined value. For example, when the detection development density data is lower than the predetermined density value in the economical mode, the output control value given to the output control circuit 76 is decreased by a predetermined value so that an absolute value -700 V of the electric potential of the charged region on the photoreceptor drum 12 can be decreased. When the detection development density data is higher than the predetermined density value in the economical mode, the output control value given to the output control circuit 76 is increased by a predetermined value so that an absolute value -700 V of the electric potential of the charged region on the photoreceptor drum 12 can be increased. In this connection, the predetermined density value in the economical mode is previously stored in ROM 58b as a constant. 
     After that, the program is returned to step 1509, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the predetermined density value in the allowable range in the economical mode in step 1514. At this time, the output control value to the output control circuit 68, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as an economical mode correction value for the output control value to the output control circuit 68. Next, the program advances to step 1516. In step 1516, the main motor 60 is temporarily stopped. At this time, the high speed laser printer is prepared for the actual multicolor recording. 
     When the mode selection switch 88 is in a condition of OFF, multicolor recording is conducted in the normal mode. In this case, when a charged region is formed on the photoreceptor drum 12 in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharger 14 by the power supply circuit 66 is determined in accordance with the correction value in the normal mode stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the normal mode of multicolor recording so that the hue can be appropriately maintained. On the other hand, when the mode selection switch 88 is in a condition of ON, multicolor recording is conducted in the economical mode. In this case, when a charged region is formed on the photoreceptor drum 12 in each of the electrostatic recording units Y, C, M and B, a voltage impressed upon the precharger 14 by the power supply circuit 66 is determined in accordance with the correction value in the economical mode stored in RAM 58c. Due to the foregoing, the amount of deposited toner of each color, that is, the development density is guaranteed in the economical mode of multicolor recording so that the hue can be appropriately maintained. 
     In the development density correction routine shown in FIGS. 15 and 16, the predetermined density value in the normal or economical mode may be set as a predetermined amount of deposited toner. In this case, the graph shown in FIG. 4 is maintained in the main control circuit 58 as a ROM table. An output value of the OD sensor 30 is inputted onto the ROM table and converted into a toner deposition amount. The thus converted toner deposition amount is compared with the predetermined toner deposition amount. 
     The development density correction routine shown in FIGS. 15 and 16 is carried out when the operation of the high speed printer starts, that is, when the power supply switch 84 is turned on. However, when an operator turns on the development density correction switch 86, the routine may be appropriately carried out in the same manner described above. When the recording operation is not carried out by the high speed laser printer over a predetermined period of time under the condition that the power supply switch 84 of the high speed laser printer is turned on, for example, when the recording operation is not carried out over one hour, the development density correction routine shown in FIGS. 15 and 16 may be automatically carried out. 
     In the development density correction routine shown in FIGS. 15 and 16, the development density correction of multicolor recording is conducted only in the two cases of the normal and economical modes. However, in the same manner as that of the development density routine shown in FIGS. 10(a), 10(b), 14(a) and 14(b), even when arbitrary density data is inputted by the density setting input key means 90, it is possible to correct the development density with respect to the arbitrary input density data. With reference to the development density correction routine shown in FIGS. 18(a) and 18(b), the development density correction to correct the arbitrary input density data will be explained as follows. In this connection, the development density correction routine shown in FIGS. 18(a) and 18(b) is carried out when the development density correction switch 86 is turned on after the arbitrary density data has been inputted using the density setting input key means 90. 
     In step 1801, it is judged whether or not the density data has been inputted by the density setting input key means 90. After the density data has been inputted, the program advances to step 1802. In step 1802, in order to drive the main motor 60, the main control circuit 58 outputs an ON signal to the power supply circuit 62 through the I/O 58d. Due to the foregoing, the photoreceptor drum 12 is rotated and the developing unit 18 is operated at the same time. In this connection, when the image density data has not been inputted by the density setting input key means 90, the development density correction routine shown in FIGS. 15 and 16 is carried out by switching ON of the development density correction switch 86. In step 1803, in each of the electrostatic recording units Y, C, M and B, a voltage is impressed upon the precharger 14 by the power supply circuit 66. Due to the foregoing, a charged region is formed on the photoreceptor. At this time, an output control value given to the output control circuit 68 is determined so that the voltage impressed upon the precharger 14 by the power supply circuit 66 corresponds to the input density data. That is, when a control signal is outputted to the output control circuit 68 by the main control circuit 58 in accordance with the output control value determined in the above manner, the voltage impressed upon the precharger 14 by the power circuit 66 becomes a value corresponding to the input density data. For example, when the input density data inputted by the density setting input key means 90 is a density value 75% with respect to the development density value 100% in the normal mode, the output control value given to the output control circuit 68 is set, as can be seen from the graph in FIG. 17, so that the electric potential of the charge region on the photoreceptor drum 12 can be -650 V in accordance with the voltage impressed by the power supply circuit 66. However, for the reasons described before, an amount of deposited toner corresponding to the development density value 75% can not be necessarily provided. In any case, in step 1803, when a voltage corresponding to the input density data is impressed upon the precharger 14 by the power supply circuit 66, a charged region is formed on the photoreceptor drum 12. 
     In step 1804, the output control value to the output control circuit 68 in the case of forming the charged region on the photoreceptor drum 12 is stored in RAM 58c. Next, in step 1805, the electrostatic latent image of the detection mark is written in the charged region on each photoreceptor drum 12 by the laser beam scanner 16. At this time, the operational electric power of the laser beam scanner 16 is 1.5 mW. Next, in step 1806, the electrostatic latent image of the detection mark is developed by the toner component of each color in each developing unit. At this time, the development bias voltage impressed upon the development roller 34 is -400 V. 
     In step 1807, an optical density value of the development detection mark is detected by the OD sensor 30. The detected optical density value is taken into the main control circuit 58 through the A/D converter 78 as the development density data of the detection mark, wherein the development density data represents an amount of deposited toner. Next, in step 1808, the thus detected development density data is compared with the input density data, and it is judged whether or not the detected development density coincides with the input density data in the allowable range. When the detected development density data does not coincide with the input density data in the allowable range, the program advances to step 1809. In step 1809, the output level of the laser power supply circuit 66 to the precharger 14 is corrected when the output control value given to the output control circuit 68 is changed by a predetermined value. For example, when the input density data is a density value 75%, and when the detected development density data is lower than the input density data, the output control value given to the output control circuit 76 is decreased by a predetermined value so that an absolute value of the electric potential -650 V of the charged region on the photoreceptor drum 12 can be lowered. When the detected development density data is higher than the input density data, the output control value given to the output control circuit 76 is increased by a predetermined value so that an absolute value of the electric potential -650 V of the charged region on the photoreceptor drum 12 can be raised. 
     After that, the program is returned to step 1803, and the same operation is repeated. The repetition is continued until the detection development density data coincides with the input density data in the allowable range in step 1808. At this time, the output control value to the output control circuit 68, which is stored in RAM 58c, is rewritten and renewed at any time, and the last renewed value is employed as an input density data correction value for the output control value to the output control circuit 68. 
     In step 1808, when the detected development density data coincides with the input density data in the allowable range, the program advances to step 1810. In step 1810, it is judged whether or not a recording command has been given in a predetermined period of time. When a recording command has been given in a predetermined period of time, a recording operation routine (not shown) is carried out, and the actual multicolor recording is started. In this case, when the charged region is formed on the photoreceptor drum 12 in each of the electrostatic recording units Y, C, M and B, the voltage impressed upon the precharger 14 by the power supply circuit 66 is determined in accordance with the input density data correction value held in RAM 58c. Due to the foregoing, it can be guaranteed that the amount of deposited toner of each color in the multicolor recording, that is, the development density is provided with an appropriate hue. On the other hand, when a recording command is not given in a predetermined period of time, the main motor 60 is temporarily stopped, and the high speed laser printer is put in a waiting condition. 
     In the development density correction routine shown in FIGS. 18(a) and 18 (b) the density data inputted by the density setting input key means 90 may be replaced with the amount of deposited toner. In this case, the graph shown in FIG. 4 is held in the main control circuit 58 as a ROM table, and an output value of the OD sensor 30 is inputted into the ROM table so that it is converted into an amount of deposited toner, and the converted toner deposition amount is compared with the input toner deposition amount inputted by the density setting input key means 90. 
     In the above embodiments, the output level to the laser beam scanner 16, the development bias voltage impressed upon the development roller 34, and the voltage impressed upon the precharger 14 are respectively individually adjusted so as to correct the development density. However, it is possible to correct the development density when at least two of theses parameters are combined. For example, when a predetermined development density correction can not be accomplished in a range of output level adjustment of the laser beam scanner 16, further the development bias voltage impressed upon the development roller 34 or the voltage impressed upon the precharger 14 may be combined so as to accomplish the predetermined development density correction. Also, in the above embodiments, a plurality of detection marks may be continuously formed, and an average of the plurality of pieces of detected data is used as the detection data to be compared with the predetermined density value. Due to the foregoing, it is possible to enhance the detection accuracy. Further, when a plurality of pieces of data are obtained, the maximum and minimum may be omitted, and an average of the detection data except for the maximum and minimum may used as the detection data. In this connection, in the above development density correction routine, when the detected development density data does not coincide with the predetermined density value in the allowable range even if the parameters are adjusted by a plurality of times, it is preferable that the occurrence of an error is displayed. 
     In FIGS. 19(a) to 19(e), there are shown patterns of the detection mark. The detection mark shown in FIG. 19(a) is formed to be a pattern in which lateral lines of one dot are arranged at regular intervals. The detection mark shown in FIG. 19(b) is formed to be a pattern in which longitudinal lines of one dot are arranged at regular intervals. The detection mark shown in FIG. 19(c) is formed to be a pattern in which lateral lines of two dots are arranged at regular intervals. The detection mark shown in FIG. 19(d) is formed to be a pattern in which longitudinal lines of two dots are arranged at regular intervals. The detection mark shown in FIG. 19(e) is formed to be a solid mark. Of course, the detection mark is not limited to the patterns illustrated in FIGS. 19(a) to 19(e), but other patterns may be adopted. 
     As described above, the development density is detected before the multicolor recording. In accordance with the detected value, the development density in the development process of the multicolor recording is subjected to feedback control. Due to the above operation, it is possible to keep the hue constant in the multicolor recording at all times. Therefore, it is necessary to highly accurately detect the development density of the detection mark using the OD sensor 30. 
     As illustrated in FIG. 20, a reflection type sensor is employed as the OD sensor 30 in this embodiment. Specifically, the OD sensor 30 includes: a light emitting section from which detection light DL is emitted to a surface of the photoreceptor drum; and a light receiving section which receives reflection light RL on the surface of the photoreceptor drum 12. For example, a light emitting diode (LED) may be used for the light emitting section. Alternatively, a white light source containing all visible spectra may be used for the light emitting section. The light receiving section is composed of, for example, a photo-diode or CCD element. Of course, as illustrated in FIG. 20, the light emitting and receiving sections of the OD sensor 30 are arranged at the position through which the detection mark passes. 
     In order to detect the detection mark by the 0D sensor 30 with high accuracy, it is necessary to enhance the sensitivity of the OD sensor 30. For example, when the detection mark is developed with magenta toner in a solid manner (shown in FIG. 19(e)), and also when a red light emitting diode, which emits light of the same color as that of red, is used, there is a relation between the OD value of the detection mark and the output voltage of the OD sensor 30 as shown in FIG. 21. As can be seen from FIG. 21, when the OD value of the detection mark is approximately 0.9, the output voltage of the OD sensor 30 is approximately 4.5 V. When the OD value of the detection mark is approximately 1.55, the output voltage of the OD sensor 30 is approximately 4.2 V. As can be seen from the graph in FIG. 21, when the detection mark is developed in a solid manner, and also when detection light, the color of which is the same as that of the detection mark, is used, the sensitivity of the OD sensor is very low. 
     On the other hand, when the detection mark is developed by magenta toner with one-dot lines (shown in FIGS. 19(a) and 19(b)), and also when a red light emitting diode, which emits light of the same color as that of magenta, is used for the light emitting section of the OD sensor 30, there is a relation between the 0D value of the detection mark and the output voltage of the OD sensor as shown in FIG. 22. Also, when the detection mark is developed by magenta toner with two-dot lines (shown in FIGS. 19(c) and 19(d)), and also when a red light emitting diode, which emits light of the same color as that of magenta, is used for the light emitting section of the OD sensor 30, there is a relation between the OD value of the detection mark and the output voltage (V) of the OD sensor as shown in FIG. 23. As can be seen from FIGS. 22 and 23, the sensitivity of the 0D sensor is remarkably improved compared with the case shown in FIG. 21. As a result, it could be concluded that the detection mark pattern must be composed of one-dot lines or two-dot lines when the optical density of the detection pattern is detected with detection light of the same color. 
     In FIG. 24, there is shown a relationship between the OD value of the detection mark and the output voltage (V) of the OD sensor 30 when the detection mark is developed with yellow toner in a solid manner and further a green light emitting diode is used for the light emitting section of the OD sensor 30. In FIG. 25, there is shown a relation between the OD value of the detection mark and the output voltage (V) of the 0D sensor 30 when the detection mark is developed with magenta toner in a solid manner and further a green light emitting diode is used for the light emitting section of the OD sensor 30. As can be seen from both graphs, even if the detection mark is developed in a solid manner, when detection light of a different color from that of the detection mark is used, the sensitivity of the 0D sensor is high. 
     In FIG. 26, mark &#34;&#34; shows a relation between the 0D value of the detection mark and the output voltage (V) of the OD sensor 30 when the detection mark is developed by black toner with one-dot lines and further a red light emitting diode is used for the light emitting section of the OD sensor 30. Also, in FIG. 26, mark &#34;▪&#34; shows a relationship between the OD value of the detection mark and the output voltage (V) of the OD sensor 30 when the detection mark is developed by black toner in a solid manner and further a red light emitting diode is used for the light emitting section of the OD sensor 30. In FIG. 27, mark &#34;&#34; shows a relation between the OD value of the detection mark and the output voltage (V) of the OD sensor 30 when the detection mark is developed by cyan toner with one-dot lines and further a red light emitting diode is used for the light emitting section of the 0D sensor 30. Also, in FIG. 27, mark &#34;▪&#34; shows a relation between the OD value of the detection mark and the output voltage (V) of the OD sensor 30 when the detection mark is developed by cyan toner in a solid manner and further a red light emitting diode is used for the light emitting section of the OD sensor 30. As can be seen from both graphs, when detection light of a different color from that of the detection mark is used, even when the detection mark is developed as a dot-line pattern or in a solid manner, the sensitivity of the OD sensor is high. 
     In this connection, when a white light source is used for the light emitting section of the OD sensor 30, the detection sensitivity is high irrespective of the type of the detection mark and the color. 
     In the graph of FIG. 28, there is shown a relationship between the distance from the detection mark to the OD sensor and the output voltage (V) of the 0D sensor 30. At this time, the detection mark is developed by black toner so that the OD value of the detection mark can be 1.2, and a red light emitting diode is used for the light emitting section of the OD sensor 30. As can be seen from FIG. 28, the output voltage of the 0D sensor 30 is stabilized when a distance from the detection mark to the OD sensor 30 is approximately in a range from 4.7 to 5.3 mm. On the other hand, when the distance from the detection mark to the OD sensor 30 is different from the optimum value 5.0 mm by not less than 0.5 mm, the output voltage of the OD sensor 30 fluctuates greatly. Accordingly, the distance from the OD sensor 30 to the surface of the photoreceptor drum 12 must be maintained at the optimum value at all times. However, as a matter of fact, it is impossible to provide a truly circular photoreceptor drum 12. Accordingly, even if the OD sensor 30 is arranged at a predetermined position with respect to the surface of the photoreceptor drum 12, when the photoreceptor drum 12 is rotated, there is a possibility that a distance from the rotational surface to the OD sensor 30 fluctuates by not less than 0.5 mm. 
     Therefore, in this embodiment, the distance from the OD sensor 30 to the surface of the photoreceptor drum 12 is maintained constant in such a manner that the 0D sensor 30 is attached to the developer holding container 32 via an appropriate mount as illustrated in FIG. 2. 
     The detail will be described as follows. In order to accomplish uniform development, it is necessary to maintain the distance from the development roller 34 to the photoreceptor drum 12 at a constant value. Therefore, the developing unit 18 is supported in such a manner that it can be moved to the forward and rearward with respect to the photoreceptor drum 12, and annular spacers 34d are provided on both sides of the development roller 34 as illustrated in FIG. 29. These annular spacers 34d are integrally attached onto the sleeve 34c of the development roller 34. On the other hand, the developing unit 18 is elastically pushed toward the photoreceptor drum 12 in such a manner that the annular spacers 34d come into contact with the photoreceptor drum 12. Thus, the distance from the development roller 34 to the rotational surface of the photoreceptor drum 12 is maintained constant as described above. Therefore, when the OD sensor 30 is supported by the developer holding container 32 of the developing unit 18, the distance from the OD sensor 30 to the rotational surface of the photoreceptor drum 12 can be also maintained constant. 
     In the case where the OD sensor 30 is not supported by the developer holding container 32, it is possible to provide spacer rollers 30a on both end sides of the 0D sensor 30 as illustrated in FIG. 30. The OD sensor 30 is supported so that it can be moved to the forward and rearward with respect to the photoreceptor drum 12, and at the same time, the OD sensor 30 is elastically pushed toward the photoreceptor drum 12. Due to the foregoing, the distance from the OD sensor 30 to the rotational surface of the photoreceptor drum 12 can be maintained constant. In this connection, reference numeral 30b denotes a bearing element for rotatably supporting the spacer roller 30a. 
     Referring now to FIGS. 31 to 37, a mechanism for detecting the deviation of the printing position will now be described. FIG. 31 is a schematic illustration of an embodiment of a multicolor electrostatic recording apparatus, which is similar to that shown in FIG. 1. The recording apparatus comprises, in the same manner as shown in FIG. 1, an endless belt conveyance means, i.e., a light permeable transparent, electrostatic attraction belt 10, a photoreceptor drum 12, a precharger 14, an optical writing means, i.e., a laser beam scanner 16, a developing unit 18, a transferring unit 20 and a thermal fixation unit 22. FIG. 31 also illustrates a recording sheet supply cassette 21 and a recorded sheet stacker 23. According to this embodiment, there is provided a detector 25 for detecting the deviation of the printing position. The detector 25 is located at a predetermined position along the recording sheet movement path constituted by the endless belt conveyance means 10, particularly at a downside running section of the endless belt 10. 
     FIG. 32 is a plan view illustrating the shapes of marks for detecting the deviation of the printing position. Marks indicated by (a), i.e., A1, A2, A3 and A4, and (c), i.e., C1, C2, C3 and C4, are used for detecting the deviation in the sub-scanning direction, i.e., the sheet conveyance direction shown by an arrow in FIG. 32 and the marks (b), i.e., B1, B2, B3 and B4, are used for detecting the deviation in the scanning direction, i.e., the direction perpendicular to the sheet conveyance direction. As shown in FIG. 32, since there are two series of marks (a) and (c) for detecting the deviation in the sub-scanning direction which are arranged adjacent to the respective side edges and exactly aligned with respect to each other in the sub-scanning direction, the amount of skew can also be detected. The marks A1, B1 and C1 indicate the first color, for example, yellow (Y), the marks A2, B2 and C2 indicate the second color, for example, cyan (C), the marks A3, B3 and C3 indicate the third color, for example, magenta (M), and the marks A4, B4 and C4 indicate the fourth color, for example, black (B). 
     Such marks for detecting the deviation of the printing position are preferably written on the electrostatic attraction belt 10 when the printer is in the initial position, such as when an elective power is supplied to the printer, so that any position deviation can be detected, before a printing operation is effected on the recording sheet on the light permeable transparent belt 10. Otherwise, if such marks are written when the printer is in the printing operation, these marks should be recorded on the electrostatic attraction belt 10, out of the region of the printing area, i.e., the region other than the recording sheet. Thus, these deviation detecting marks are formed as follows. 
     First, in the respective electrostatic units Y, M, C and B, electrostatic latent images of these marks are formed on the respective photoreceptor drum 12 by the respective optical unit 16, in quite the same manner as the print images. Then, these electrostatic latent images on the respective photoreceptor drum 12 are developed by the respective developing unit 18 with the respective color toners. Then, the developed color images formed on the respective photoreceptor drums 12 are transferred, one after another, on the electrostatic attraction belt 10 by the respective transfer unit 20. 
     As mentioned above, the electrostatic latent marks for detecting the deviation of the printing position are written on the photoreceptor 12 in both directions, i.e., in the main-scanning and sub-scanning directions, by the signals triggered in quite the same manner as the electrostatic latent images for actual printing. Therefore, such marks are transferred to the electrostatic attraction belt 10 so that these marks are deviated if any, from the regular positions in quite the same manner as the deviation of the transferred printed images. Therefore, the amount of deviation of the actual printed images can be determined by detecting and measuring the deviation of these marks transferred onto the electrostatic attraction belt 10. 
     Since the respective color marks A1 to A4, B1 to B4, and C1 to C4 are arranged, one after another, at a predetermined time interval in a state where there is no substantial deviations between the respective color marks, it becomes possible to specify which color is deviated. Therefore, according to this embodiment, by applying such marks, the position deviation can be detected by the position deviation detector 25 which only detects the positions of the image of these marks. Thus, the position deviation detector 25 needs no complicated means, such as a color determination mechanism or the like. 
     In FIGS. 33(a) and 33(b), there is shown a first example of the position deviation detecting mechanism 25, which comprises three pairs of light emitting diode (LED) arrays 27 and charge-coupled device (CCD) linear image sensor arrays 29. The diode arrays 27 and sensor arrays 29 are located opposite to each other and, at the lower and upper sides, respectively, of the electrostatic attraction belt 10 and their sensing elements are arranged in conformity with the arrangements of the respective color marks A1 to A4, B1 to B4, and C1 to C4. Since these marks on the endless belt 10 which should be detected are moving in the sub-scanning direction indicated by an arrow, the laser emission diode arrays 27 emit a pulse beam toward the CCD sensor arrays 29 through the electrostatic attraction belt 10 to catch an instant pulse beam. The beam indicating the positions of the marks are projected onto the light receiving surface of the CCD sensor arrays 29 and thus the positions of the marks are detected. 
     Any other optical means, such as a rod lens array, may be used for projecting the light image onto the light receiving surfaces of the CCD sensor arrays 29. Otherwise, the light receiving surfaces of the CCD sensor arrays 29 may be arranged in the close vicinity of the electrostatic attraction belt 10 so that the diffusion of light can be minimized and the light image is directly received on the light receiving surfaces of the CCD sensor array 29 without using any rod lens or the like. 
     FIG. 34 illustrate a controlling method of the charge-coupled device (CCD) sensor array 29 and the light emitting diode (LED) arrays 27. In this example, the trigger is a VS signal of the first color photoreceptor drum (VS 1). The transfer pulse signals, i.e, the clock signals, are always given to the CCD sensor array 29. After a predetermined time has passed since the VS signal as the trigger, the mark comes to a position which corresponds to the position of the CCD sensor array 29 and, at this time, a pulse optical signal is emitted from the LED arrays 27. On the other hand, the shift pulse signal is given twice, i.e., before and after the light emission from the LED arrays 27. The first one serves to eliminate the remaining images in the light receiving elements of the CCD sensor array 29 and the second one serves to input the light image of the mark which is generated by the light emission from the LED arrays 27 into the shift resistor of the CCD sensor arrays 29. 
     The images input into the CCD sensor arrays 29 are put onto the transfer clock signal and read, one after another, as the height of the voltage potential. The 
     signal which has been read by the CCD sensor array 29 is shown in FIG. 35. This information is calculated and therefore the amount of color deviation can be determined. 
     FIG. 36 shows a second example of the position deviation detecting mechanism 25 comprising light emitting diode (LED) arrays 27 and charge-coupled device (CCD) image sensor arrays 29, which is constituted as an area image sensor. In the same manner as the previous example, the LED arrays 27 and the CCD sensor arrays 29 are located at the lower and upper sides, respectively, of the electrostatic attraction belt 10 and arranged to cover the respective color marks A1 to A4, B1 to B4, and C1 to C4. Therefore, position deviation in the main-scanning direction, in the sub-scanning direction, and skew can be detected in the same manner as the above example. 
     FIG. 37 shows a third example of the position deviation detecting mechanism comprising light emitting diode (LED) arrays 27 and charge-coupled device (CCD) linear image sensor arrays 29, in the same manner as the first example. The LED arrays 27 and the CCD sensor arrays 29 are located at the lower and upper sides, respectively, of the electrostatic attraction belt 10 and, however, their respective sensor elements are arranged so as to be inclined about 45° with respect to the main-scanning direction. In such an arrangement, the marks in both of the main-scanning direction and sub-scanning direction can be detected by the same sensor array. 
     Thus, according to the above-mentioned embodiments, the relative position deviations of the respective color images on the electrostatic attraction belt 10 can be read and, therefore, the amount of color deviations can be determined in accordance with the detected relative positions. Thus, the color deviations can be feedback controlled on the basis of the amount of detected color deviations and thus a printing image with a high print quality can be obtained. 
     Particularly, according to the above-mentioned embodiments, the respective color marks can be simultaneously read and then the color deviation is determined in accordance with the relative positions. Therefore, the deviation occurred after the image transferring is not included and thus an accurate color position deviation can thus be attained. According to the above-mentioned embodiments, any deviation occurred after the image transferring is represented as the deviation of absolute positions of the resist marks themselves. Such deviations will give arise no problems since the CCD sensor has sufficient margins in the detection areas. Thus, the position accuracy of the CCD sensors is not so strictly required. In addition, in these embodiments, monochromatic-CCD sensors can be used and therefore the manufacturing cost can be reduced.