Patent Publication Number: US-6992279-B2

Title: Method for adjusting a multi-beam source unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Application Ser. No. 09/731,793 filed Dec. 8, 2000 now U.S. Pat. No. 6,798,820, and further is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-349145, filed Dec. 8, 1999, and the prior Japanese Patent Application No. 2000-206535, filed Jul. 7, 2000, the entire contents of each of which are hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and device for adjusting a multi-beam source unit used in an image forming apparatus such as a digital copying machine or a laser printer, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit. 
   2. Description of the Prior Art 
   Heretofore, as an image forming apparatus such as a digital copying machine or a laser printer there has been known an image forming apparatus with a laser scanning optical system mounted thereon. With the recent tendency to a higher write accuracy and a higher write speed, a laser scanning optical system using a multi-beam laser diode is becoming most popular. 
     FIG. 1  illustrates such a laser scanning optical system schematically. In the same figure, the reference numeral  1  denotes a multi-beam source unit, numeral  2  denotes a polygon mirror, numeral  3  denotes a fθ lens, and numeral  4  denotes a photosensitive member (also called an image recording medium). The multi-beam source unit  1  substantially comprises a multi-beam laser diode  5  and a collimator lens  6 . The multi-beam laser diode  5  has a plurality of light emitting points to emit multi-laser beams P. The multi-laser beams P are collimated by means of the collimator lens  6 , then are reflected by the polygon mirror  2  and are conducted onto a surface (also called an image recording surface)  4   a  of the photosensitive member  4 . 
   The polygon mirror  2  and the fθ lens  3  constitute a part of a scanning optical system. On the surface  4   a  of the photosensitive member  4 , as shown in  FIG. 2 , the multi-laser beams P are scanned in a horizontal direction Q 1  while leaving predetermined pitches X 1  in a vertical scanning direction Q 2  which is perpendicular to the horizontal scanning direction Q 1 . In this type of a laser scanning optical system, the surface  4   a  of the photosensitive member  4  is scanned over a large number of lines at a time to write data onto the surface  4   a.    
   As the write accuracy and speed have recently become higher, the laser scanning optical system is required to be improved in its accuracy for the diameter of a beam spot  11  of each multi-laser beam P, collimatability of the beams, a pitch X 1  between adjacent beam spots  11  in the vertical scanning direction Q 2 , and a write start position in the horizontal scanning direction Q 1 . The accuracy required is becoming more and more strict as a higher image quality is required. 
   As shown in  FIG. 3 , the multi-beam laser diode  5  has a light emitting section  7  in the interior thereof. In the light emitting section  7  are provided a plurality of light emitting points, which are, for example, four light emitting points  7   a  to  7   d . The light emitting points  7   a  to  7   d  are arranged on a designwise predetermined virtual straight line Q 3  spacedly from one another. The virtual straight line Q 3  is obtained by joining acute points  9   a  and  10   a  of a pair of acute-angled cutout portions  9  and  10  which are formed in a metallic stem  8  of the multi-beam laser diode  5 . 
   In the conventional multi-beam laser diode  5 , the light emitting points  7   a  to  7   d  are spaced widely from one another, so that when multi-laser beams are projected onto the surface  4   a  of the photosensitive member  4 , the pitches X 1  of their beam spots  11  in the vertical scanning direction Q 2  become large and the image quality becomes coarse. For avoiding such an inconvenience, as shown in  FIG. 4 , the multi-beam laser diode  5  is turned around the optical axis of the scanning optical system (not shown) so that an arranged direction (straight line) Q 3 ′ of the beam spots  11  becomes oblique with respect to the horizontal scanning direction Q 1  on the surface  4   a  of the photosensitive member  4 , thereby adjusting the pitches X 1  in the vertical scanning direction Q 2  to enhance the write density (recording density) in the vertical scanning direction Q 2  and improve the image quality. 
   However, if the multi-beam laser diode  5  is turned so that the arranged direction (straight line) Q 3 ′ of the beam spots  11  is deviated obliquely with respect to the vertical scanning direction Q 2 , to improve the write density, then in the case where the light emitting points  7   a – 7   d  are driven simultaneously to effect write, this results in that write start positions in the horizontal scanning direction Q 1  of the beam spots  11  on the surface  4   a  of the photosensitive member  4  become displaced and hence the image quality is deteriorated. 
   In the laser scanning optical system of this type, for making the write start positions of the beam spots on the surface  4   a  of the photosensitive member  4  uniform, for example sensors  12  for detecting scan positions of the laser beams are arranged correspondingly to the laser beams and the emission of beam from each of the light emitting points  7   a  to  7   d  is controlled in accordance with a light receiving timing of each sensor  12 . 
   More particularly, the emission of light from the light emitting points  7   a – 7   d  is controlled upon the lapse of time t 0 ′ after the detection of the head beam spot  11  at time t=t 0 , thereby making write start positions in the horizontal scanning direction Q 1  on the surface  4   a  of the photosensitive member  4  uniform. 
   Moreover, instead of using sensors  12  correspondingly to the light emitting points  7   a – 7   d  a sensor  12  is provided correspondingly to the head beam spot  11  in the horizontal scanning direction Q 1 , as shown in  FIG. 5(   a ), then time lags t 1 , t 2 , and t 3  of beam spots  11  are determined in advance, and as shown in  FIG. 5(   b ), after the head beam spot  11  has been detected by the sensor  12 , the emission of beams from the remaining light emitting points  7   b – 7   d  is delayed correspondingly to the time lags by means of a delay control circuit (not shown), thereby making the beam spots  11  uniform in the vertical scanning direction at the write start positions on the surface  4   a  of the photosensitive member  4 , as shown in  FIG. 5(   a ). 
   In this laser scanning optical system, however, the control circuit used for aligning write start positions is complicated, with consequent increase of cost. 
   At present, a multi-beam laser diode  5  with light emitting points  7   a – 7   d  spaced more narrowly than before is being developed. In a multi-beam source unit having such a multi-beam laser diode  5  it is presumed that positional variations of the light emitting points  7   a – 7   d  will be small. Further, the multi-beam source unit is probably designed so as to be set to a scanning optical system on the assumption that light emitting points  7   a – 7   d  are arranged in the direction of a predetermined standard design line when they are present on the virtual straight line Q 3  defined by a pair of cutout portions  9  and  10 , and is attached as it is to a body portion of an image forming apparatus. 
   Even with such a design, however, due to errors in the manufacturing process of the multi-beam laser diode  5  being considered, it is rarely the case that the light emitting points  7   a – 7   d  are positioned on the virtual straight line Q 3  without error. Even if there exists an arrangement direction (straight line) Q 4  joining light emitting points  7   a – 7   d , as shown in  FIG. 6 , the arrangement direction Q 4  and the virtual straight line Q 3  are slightly inclined with respect to each other and thus aligning the arrangement direction of the light emitting points  7   a – 7   d  with the direction of the standard design line without the need of any adjustment is difficult. The reference mark θ represents the angle of that inclination. 
   When the multi-beam laser diode  5  is to be mounted to a body portion of an image forming apparatus body with a scanning optical system mounted thereon, since there exists a mounting error, it is desirable to turn the multi-beam laser diode  5  around the optical axis so that the arrangement direction Q 4  of the light emitting points is at a predetermined angle relative to the vertical scanning direction Q 2 . 
   SUMMARY OF THE INVENTION 
   The present invention has been accomplished in view of the above circumstances and it is an object of the invention to provide a multi-beam source unit adjusting method and device wherein an arranged direction of light emitting points of a multi-beam laser diode relative to a horizontal scanning direction of a scanning optical system can be aligned with a predetermined standard design line direction, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit, particularly, a multi-beam source unit adjusting method and device wherein an arranged direction of light emitting points of a multi-beam laser diode can be adjusted to a vertical scanning direction of a scanning optical system easily without impairing a required design accuracy, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit. 
   For achieving the above object, according to the present invention, in the first aspect thereof, there is provided a method for adjusting a multi-bean source unit, the multi-beam source unit including a multi-beam laser diode capable of emitting multi-laser beams from a plurality of light emitting points and a collimator lens for collimating the multi-laser beams, the multi-laser diode having a stem formed with a cutout portion, the multi-beam source unit being designed so as to be set to a scanning optical system on the assumption that the plural light emitting points are arranged in the direction of a predetermined standard design line when they are present on a virtual straight line defined by the said cutout portion, the method comprising measuring an arranged state of the light emitting points with respect to the standard design line on the basis of beam spots on an image surface corresponding to an image recording surface and rotating the multi-beam laser diode about an optical axis of the scanning optical system to align the arrangement direction of the light emitting points with the direction of the standard design line. 
   In the second aspect of the present invention there is provided, in combination with the above first aspect, a multi-beam source unit adjusting method wherein the virtual straight line is defined by a concave or convex portion as an engaging portion for positioning formed in the stem. 
   According to the above first and second aspects of the invention, since an arranged state of the light emitting points with respect to the standard design line is measured on the basis of beam spots on an image surface and the multi-beam laser diode is rotated for adjustment about the optical axis of the scanning optical system, the arrangement direction of the light emitting points can be aligned with the direction of the standard design line easily. 
   In the third aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjusting method wherein on the image surface corresponding to the image recording surface an arrangement direction of the light emitting points with respect to the standard design line is determined on the basis of a straight line obtained by joining two beam spots corresponding to two light emitting points located remotest from each other out of the light emitting points. 
   According to the multi-beam source unit adjusting method in the above third aspect of the invention, since an arrangement direction of the light emitting points is determined using only the light emitting points located remotest from each other, it is possible to determine a light emitting points arrangement direction of the multi-beam laser diode easily. 
   In the fourth aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjusting method wherein on the image surface corresponding to the image recording surface there are measured relative positions of beam spots corresponding to the light emitting points to determine an approximate straight line on which the light emitting points can be regarded as being present, and an arrangement direction of the light emitting points is determined by the approximate straight line. 
   In the fifth aspect of the present invention there is provided, in combination with the above fourth aspect, a multi-beam source unit adjusting method wherein the approximate straight line is obtained by a method of least squares. 
   In the sixth aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjusting method wherein, on the image surface corresponding to the image recording surface, relative positions in the horizontal scanning direction of beam spots corresponding to the light emitting points are measured in terms of relative angle positions with respect to the standard design line to determine a maximum deviation in the horizontal scanning direction among the beam spots, then the multi-beam laser diode is rotated to measure relative positions of the beam spots in the horizontal scanning direction at different relative angle positions, thereby determining a maximum deviation in the horizontal scanning direction among the beam spots, these operations are repeated to measure an arranged state of the light emitting points, and an arrangement direction of the light emitting points is determined by a relative angle position corresponding to the smallest maximum deviation. 
   According to the multi-beam source unit adjusting methods in the above fourth to sixth aspects of the invention, the arrangement direction of the multi-beam source unit can be aligned with the direction of the standard design line with a high accuracy. 
   In the seventh aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjusting method wherein the arrangement direction of the light emitting points is substantially parallel to the vertical scanning direction. 
   According to the multi-beam source unit adjusting method in the above seventh aspect of the invention, since the arrangement direction of the light emitting points can be aligned with the vertical scanning direction before mounting the same unit to a body portion of an image forming apparatus, it is possible to dispense with such an operation as adjusting the beam spot pitch in the vertical scanning direction after mounting the multi-beam source unit to the body portion of the image forming apparatus and it is not necessary to adopt a complicated construction such that for correcting write start positions displaced in the horizontal scanning direction due to the beam spot pitch adjustment, sensors for detecting laser beam scan positions are provided for laser beams respectively on the image forming apparatus body side and the write start position is controlled for each laser beam, nor is it necessary to adopt a complicated construction such that a time lag of each laser beam in the horizontal scanning direction is measured and a control is made for driving each laser beam with use of a delay circuit. Thus, it is possible to reduce the number of components used, shorten the assembling time required for mounting to the body portion of the image forming apparatus, and attain the reduction of cost. It is also possible to attain the simplification of control using software. 
   In the eighth aspect of the present invention there is provided, in combination with the first or second aspect, a multi-beam source unit adjusting method wherein the multi-beam source unit is provided with a base member, the base member supporting the multi-beam laser diode rotatably and having a fitting cylinder which defines a rotational center, the multi-beam source unit is also provided with a mounting bracket to be mounted to a body portion of an image forming apparatus, the mounting bracket having a horizontal scanning direction reference plane to be confronted with a horizontal scanning direction reference plane formed in the body portion of the image forming apparatus and also having a fitting hole to be fitted on the fitting cylinder, the multi-beam source unit is further provided with an engaging piece for engagement with an engaging portion for positioning and a pressing spring piece for pressing the stem, the engaging piece being brought into engagement with the engaging portion for positioning, and the base member being rotated while being supported by the mounting bracket to adjust the arrangement direction of the light emitting points substantially in parallel with the vertical scanning direction. 
   According to the multi-beam source unit adjusting method in the eighth aspect of the invention, since the multi-beam source unit can be adjusted alone before being mounted to the body portion of the image forming apparatus, it is possible to simplify the mounting work for mounting the multi-beam source unit to the image forming apparatus body. 
   In the ninth aspect of the present invention there is provided a multi-beam source unit adjusting device comprising a base member adapted to be positioned and fixed, the base member being provided with a multi-beam laser diode having a plurality of light emitting points and capable of emitting multi-laser beams and also provided with a collimator lens for collimating the multi-laser beams; an image pickup device on which the multi-laser beams are projected for measuring an arrangement direction of the light emitting points on the basis of beam spots and for rotating the multi-beam laser diode about an optical axis of an optical system; and a focusing lens disposed between the image pickup device and the collimator lens to condense and focus the multi-laser beams onto an image pickup surface of the image pickup device, wherein a front-side focal position of the focusing lens is substantially coincident with a rear-side focal position of the collimator lens. 
   According to the multi-beam source unit adjusting device in the ninth aspect of the invention, since the multi-laser beams from all the light emitting points can be enlarged, condensed and focused substantially onto the image pickup surface, the position of each beam spot can be detected with a high accuracy. 
   In the tenth aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein the image pickup device is a CCD camera and the focusing lens is disposed so that a focused area of a beam spot of each multi-laser beam on an image pickup surface of the CCD camera is ten times or more as large as a pixel area. 
   According to the multi-beam source unit adjusting device in the tenth aspect of the invention, the resolving performance on the image pickup surface can be improved and a central position of each beam spot can be calculated with a high accuracy. 
   In the eleventh aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein for controlling in such a manner as to give substantially equal respective outputs of the multi-laser beams there is provided a control circuit which controls luminous outputs of N number of light emitting points so that the sum total of outputs of the light emitting points is N times as large as a detected outputs of one of the laser beam from any one of the N number of light emitting points. 
   According to the multi-beam source unit adjusting device in the eleventh aspect, since it is possible to equalize the luminous outputs of the light emitting points, the beam spots on the image pickup surface can be made equal in size and it is possible to calculate a central position of each beam spot with a high accuracy. 
   In the twelfth aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein a central position of each of the beam spots is a centroid position of a CCD pixel output corresponding to the beam spot. 
   According to the multi-beam source unit adjusting device in the twelfth aspect of the invention, the center of each beam spot can be measured with a high accuracy. 
   In the thirteenth aspect of the present invention there is provided, in combination with the above twelfth aspect, a multi-beam source unit adjusting device wherein the centroid position of each of the beam spots is determined by subtracting 1/e of a maximum value of the CCD pixel output and performing calculation using a CCD pixel output larger than zero. 
   According to the multi-beam source unit adjusting device in the thirteenth aspect, the center of each beam spot can be determined with a high accuracy even if the beam spot is out of shape. 
   In the fourteenth aspect of the present invention there is provided a method for assembling a multi-beam source unit, the multi-beam source unit including a multi-beam laser diode capable of emitting multi-laser beams from a plurality of light emitting points, the multi-beam laser diode having a stem formed with a cutout portion, the multi-beam source unit also including a collimator lens for collimating the multi-laser beams, the multi-beam source unit being designed so as to be set to a scanning optical system on the assumption that the light emitting points are arranged in the direction of a predetermined standard design line when they are present on a virtual straight line defined by the said cutout portion, wherein an arranged state of the light emitting points with respect to the standard design line is measured on the basis of beam spots formed on an image surface corresponding to an image recording surface and the multi-beam laser diode is rotated about an optical axis of the scanning optical system to align the direction of arrangement of the light emitting points with the direction of the standard design line, the method comprising a fixing step of positioning the multi-beam laser diode to a base member, the base member supporting the multi-beam laser diode rotatably and having a fitting cylinder which defines a rotational center, and fixing the multi-beam laser diode with use of a pressing spring piece, a positioning step of positioning the collimator lens with respect to the multi-beam laser diode, a calculating step of measuring spot positions of the laser beams emitted from the multi-beam laser diode and calculating central positions thereof, and a rotation adjusting step of adjusting the rotation of the base member with the multi-beam laser diode attached thereto in such a manner that the direction of arrangement of the beam spots is aligned with the standard design line direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing the construction of a scanning optical system schematically; 
       FIG. 2  is a schematic diagram showing an arranged state of beam spots on an image recording surface; 
       FIG. 3  is a schematic diagram for explaining an arranged state of light emitting points in a multi-beam laser diode; 
       FIG. 4  is an explanatory diagram for explaining an example of controlling write start positions on the image recording surface; 
       FIG. 5  is an explanatory diagram for explaining another example of controlling write start positions on the image recording surface, in which (a) illustrates a relation between an arranged direction of beam spots and write start positions and (b) is a timing diagram for explaining light emission timings of the light emitting points; 
       FIG. 6  is a diagram for explaining an inclination between an arranged direction of the light emitting points in the multi-beam laser diode and the direction of a standard design line; 
       FIG. 7  is an exploded perspective view of a multi-beam source unit according to the first embodiment of the present invention; 
       FIG. 8  is a front view of a mounting bracket shown in  FIG. 7 ; 
       FIG. 9  is a rear view thereof; 
       FIG. 10  is a plan view thereof; 
       FIG. 11  is an enlarged view of a principal portion of the multi-beam source unit shown in  FIG. 7  as seen from the front side thereof; 
       FIG. 12  is an enlarged view of the principal portion of the multi-beam source unit as seen from the back side thereof; 
       FIG. 13  is an enlarged plan view of a multi-beam laser diode shown in  FIG. 7 ; 
       FIG. 14  is a rear view of the multi-beam source unit shown in  FIG. 7 ; 
       FIG. 15  is a front view thereof; 
       FIG. 16  is a schematic diagram of an adjusting device for the multi-beam source unit shown in  FIG. 7 ; 
       FIG. 17  is an explanatory diagram showing an arranged state of beam spots focused on an image pickup surface shown in  FIG. 7 ; 
       FIG. 18  is an explanatory diagram showing an arranged state of beam spots adjusted by the adjusting device shown in  FIG. 16 ; 
       FIG. 19  is a perspective view showing a schematic construction of a body portion of an image forming apparatus to which the multi-beam source unit shown in  FIG. 7  is to be mounted; 
       FIG. 20  is a perspective view showing a schematic construction of the body portion of the image forming apparatus with the multi-beam source unit shown in  FIG. 7  mounted thereto; 
       FIG. 21  is a schematic diagram of an optical system in the adjusting device shown in  FIG. 16 ; 
       FIG. 22  is a schematic diagram showing an example of a beam spot formed on an image pickup surface shown in  FIG. 16 ; 
       FIG. 23  is an explanatory diagram showing an example of determining a centroid position from a light beam distribution curve shown in  FIG. 22 ; 
       FIG. 24  is a schematic diagram for explaining a relation between a beam spot and the size of the image pickup surface; 
       FIG. 25  is a diagram showing graphically an example of uniforming outputs of light emitting points; 
       FIG. 26  is an explanatory diagram for explaining another example of a method for calculating a centroid position of a beam spot, showing a laser beam distribution curve; 
       FIG. 27  is a diagram showing a principal construction of an automatic adjusting system for the multi-beam source unit according to the present invention; 
       FIG. 28  is a flow chart explaining an adjusting process in the automatic adjusting system shown in  FIG. 27 ; 
       FIG. 29  is a flow chart explaining a detailed procedure of the adjusting process shown in  FIG. 28 ; 
       FIG. 30  is a flow chart explaining another adjusting process in the automatic adjusting system shown in  FIG. 27 ; 
       FIG. 31  is a flow chart explaining a detailed procedure of the adjusting process shown in  FIG. 30 ; 
       FIG. 32  is an explanatory diagram of an edge image obtained by lighting of an illuminating light source, in which (a) shows a case where the edge image is inclined leftwards with respect to a standard design line and (b) shows a case where the edge image is inclined rightwards with respect to the standard design line; 
       FIG. 33  is a plan view showing a schematic construction of a positioning jig, showing a mounted state of a base member before setting of a multi-beam laser diode; and 
       FIG. 34  is a partially enlarged sectional view for explaining positioning of multi-laser beams with use of the positioning jig. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   [Multi-Beam Source Unit and Method for Adjusting the Same According to an Embodiment of the Invention] 
     FIG. 7  is an exploded perspective view of a multi-beam source unit  19  embodying the present invention. In the same figure, the reference numeral  20  denotes a mounting bracket. The mounting bracket  20  has a bottom wall portion  21 , an upright wall portion  22 , and a pair of side wall portions  23 . In the bottom wall portion  21  are formed a pair of positioning holes  21   a  and a pair of screw inserting holes  21   b.    
   On the underside of the bottom wall portion  21  are formed positioning reference portions  24 , as shown in  FIG. 8 . The positioning reference portions  24  are each provided with a horizontal scanning direction reference plane  24   a  for defining a horizontal scanning direction. The positioning reference portions  24  are to be confronted with positioning reference portions of a housing serving as a body portion of an image forming apparatus which will be described later. 
   In the upright wall  22  is formed a circular through hole  25  and on the back side of the upright wall  22  are formed a pair of positioning reference portions  26  on both sides of the through hole  25 , as shown in  FIGS. 9 and 10 . 
   The positioning reference portions  26  are respectively provided with vertical scanning direction reference planes  26   a  substantially perpendicular to the horizontal scanning direction reference planes  24   a  of the positioning reference portions  24 . In the paired positioning reference portions  26  are formed screw inserting holes  27 , respectively. 
   Further, on the back side of the upright wall portion  22  is formed a circular fitting cylinder  28  concentrically with the circular through hole  25  and a rotation restricting pin  29  is projected between the circular fitting cylinder  28  and one positioning reference portion  26 . 
   A base member  30  is attached to the back side of the upright wall portion  22 . The base member  30  holds a multi-beam laser diode  31 . As shown on a larger scale in  FIG. 11 , the base member  30  is provided on its front side with a circular fitting cylinder  32  to be fitted in the fitting cylinder  28 . 
   An inside diameter of the fitting cylinder  28  and the diameter of the circular through hole  25  are equal to each other and an outside diameter of the fitting cylinder  32  is formed a little smaller than the diameter of the circular through hole  25  to the extent that the fitting cylinder  32  can rotate around the circular through hole  25 . 
   The fitting cylinder  32  is formed with an arcuate support portion  34  for supporting a collimator lens  33 . The collimator lens  33  functions to collimate multi-laser beams emitted from the multi-beam laser diode  31 . As to the details of mounting the collimator lens  33  to the circular support portion  34 , it will be described later. 
   The fitting cylinder  32  has a central opening  35 . A pair of cutout portions  32   a  are formed in the fitting cylinder  32  on both sides of the opening  35 . An aperture member  36  for shaping the multi-laser beams is attached to the circular fitting cylinder  32 . The aperture member  36  is formed with a slit  36   a  which extends long horizontally and a pair of engaging pieces  36   b  for engagement with the paired cutout portions  32   a . The multi-laser beams are emitted through the opening  35  toward the collimator lens  33 . 
   A pair of positioning reference portions  37  are formed at both side positions of the base member  30  which positions correspond to the positions of the paired positioning reference portions  26 , with tapped holes  38  being formed in the positioning reference portions  37  respectively. Between one positioning reference portion  37  and the fitting cylinder  32  is formed a loose insertion hole  39  for loose insertion therein of the rotation restricting pin  29 . 
   On the back side of the base member  30 , as shown on a larger scale in  FIG. 12 , there is formed a pressing plate mounting portion  41  for mounting a pressing plate  40 . The pressing plate  40  is centrally provided with four pressing spring pieces  40   a , one engaging piece  40   b  for positioning and a pair of through holes  40   c.    
   In the pressing plate mounting portion  41  is formed a fitting hole  42  which is concentric with the opening  35 . The fitting hole  42  is formed with a mounting reference hole  42   a  for a stem  31 B and an abutting reference plane  42   b  for the stem. In the pressing plate mounting portion  41  is formed a positioning groove  41 A which is in communication with the fitting hole  42 , as shown in  FIG. 12 . The positioning reference hole  41 A is used at the time of positioning and mounting the multi-beam laser diode  31  to the pressing plate mounting portion  42  of the base member  30  by means of a positioning jig which will be described later. If the multi-beam laser diode  31  is mounted to the pressing plate mounting portion  42  with use of a positioning jig, the positioning of the multi-beam laser diode in the rotational direction with respect to the pressing plate mounting portion  42  can be done precisely. But the positioning can be effected using the engaging piece  40   b  for positioning even without using any adjusting device for positioning, provided the use of the positioning jig is essential in the case of using a pressing plate  40  nor formed with the engaging piece  40   b  for positioning. 
   The mounting reference hole  42   a  is formed a little larger in diameter than the stem  31 B and the depth thereof is set to the extent that when the stem  31 B is pushed against the abutting reference plane  42   b , the back of the stem projects from the back of the pressing plate mounting portion  41 , as shown in  FIG. 34 . 
   A cylindrical body portion  31 A of the multi-beam laser diode  31  is fitted in the fitting hole  42 . In the pressing plate mounting portion  41  are formed a pair of tapped holes  41   a  on both sides of the fitting hole  42  correspondingly to the through holes  40   c  formed in the pressing plate  40 . The diameter of each through hole  40   c  is a little larger than the diameter of a shaft portion of a screw  43  with spring washer which will be described later. 
   The pressing plate  40  is pressed and fixed to the pressing plate mounting portion  41  by fitting the cylindrical body portion  31 A of the multi-beam laser diode  31  into the fitting hole  42 , then putting the pressing spring pieces  40   a  on the back of the stem  31 B of the multi-beam laser diode  31 , and further by bringing screws  43  into threaded engagement with the tapped holes  41   a  respectively. 
   [Construction of the Multi-Beam Laser Diode  31 ] 
   In the interior of the cylindrical body portion  31 A of the multi-beam laser diode  31  is formed a pedestal  44 , which is provided with a rectangular light emitting chip portion (light emitting portion)  45 , as shown in  FIG. 13 . In the illustrated construction, four light emitting points  45   a  to  45   d  are provided in the light emitting chip portion  45 . The stem  31 B is formed with a pair of acute-angled cutout portions  46 . The light emitting points  45   a – 45   d  are positioned near a center O 1  of the cylindrical body portion  31 A. 
   The light emitting points  45   a – 45   d  are to be arranged spacedly on a virtual straight line Q 3  joining acute points  46   a  of the paired cutout portions  46 , but due to errors in the manufacture of the multi-beam laser diode  31  an arranged direction Q 4  joining the light emitting points  45   a – 45   d  is inclined with respect to the virtual straight line Q 3 . The stem  31 B is formed with a rectangular cutout portion  47  as an engaging portion for positioning at a position orthogonal to the virtual straight line Q 3 . 
   Although in this embodiment the virtual straight line Q 3  is defined by joining acute points of the paired cutout portions  46 , a pair of convex portions of an acute shape may be formed on the stem  31 B side to define the virtual straight line Q 3 . 
   The cutout portion  47  engages the engaging piece  40   b  of the pressing plate  40  and is used as a positioning reference at the time of fixing the multi-beam laser diode  31  to the base member  30 . As shown in  FIG. 14 , the multi-beam laser diode  31  is positioned relative to the base member  30  by engagement of the engaging piece  40   b  with the cutout portion  47  and is fixed to the base member  30  with screws  43 . Although the rectangular cutout portion  47  is formed as a positioning engaging portion in the stem  31 B, there may be formed a rectangular convex portion for positioning instead. In this case, an engaging recess for engagement with the engaging convex portion for positioning is formed instead of forming the engaging piece  40   b  on the pressing plate  40  side. 
   [Adjusting the Collimator Lens  33  in the Multi-Beam Source Unit  19 ] 
   The position of the collimator lens  33  is adjusted before mounting the base member  30  to the mounting bracket  20 . The collimator lens  33  is arranged by an arranging/feeding device (not shown), then is taken out one at a time from the same device by means of a collimator lens holding arm and is conveyed to the arcuate support portion  34 . The arcuate support portion  34  is coated with a photo-curing type adhesive (an ultraviolet-curing type adhesive) and the collimator lens  33  is disposed at a predetermined initial design position on the thus-coated arcuate support portion  34 . The collimator lens holding arm is provided on a three-axis movable stage capable of being controlled independently in three-axis directions. 
   Next, for aligning the optical axis of the collimator lens  33  with the rotational center, the collimator lens  33  is adjusted its positions in x- and y-axis directions and is also adjusted its position in z-axis direction (position in the optical axis direction). This positional adjustment in the optical axis direction of the collimator lens  33  is conducted for making the light emitting points  45   a – 45   d  coincident with a focal position (focal plane) of the collimator lens  33  to enhance the collimatability and the foregoing three-axis movable stage is used for the adjustment. 
   The positional adjustment of the collimator lens  33  is performed for each of the light emitting points  45   a – 45   d  and a mean value of optimal positions thus obtained is used as a representative characteristic. The collimator lens  33  is positioned at the mean value. 
   After the positional adjustment of the collimator lens  33 , ultraviolet light (UV light) as a parallel beam is radiated from above the collimator lens  33  and the ultraviolet-curing type resin is cooled with air while being cured by the ultraviolet light which has passed through the collimator lens  33 , thereby allowing the collimator lens  33  to be bonded and fixed to the arcuate support portion  34 . 
   As shown in  FIG. 15 , the base member  30  is temporarily fixed to the upright wall portion  22  by means of screws  47 ′ with spring washer, whereby the multi-beam source unit  19  is assembled. 
   [Adjusting the Angle of the Multi-Beam Source Unit] 
   The multi-beam source unit  19  with the collimator lens  33  bonded and fixed thereto is then set to an adjusting device. 
     FIG. 16  is a schematic diagram of an adjusting device for use in the multi-beam laser diode adjusting method. The adjusting device is constituted as an automatic adjusting system, whose detailed construction will be described later. 
   The adjusting device has a condenser lens (focusing lens)  48  and an image pickup device  49 . The adjusting device is further provided with a reference mounting portion  50 . The multi-beam source unit  19  is set to the adjusting device in a state in which horizontal scanning direction reference planes  24   a  thereof are brought face to face with a reference plane of a reference mounting portion  50 . 
   A tip  51  of a micrometer is brought into contact with an upper end face of the base member  30  at a position more outside than the position where the rotation restricting pin  29  is present. In this state, a drive voltage is applied to terminals  31   c  of the multi-beam laser diode  31  simultaneously, whereby the light emitting points  45   a – 45   d  are activated and beam spots  52 – 55  corresponding to the light emitting points  45   a – 45   d  are formed on an image pickup surface  49   a  which corresponds to an image recording surface  4   a , as shown in  FIG. 17 . 
   Assuming that there is no mounting error in design, the virtual straight line Q 3  coincides with the vertical scanning direction Q 2 , namely, the standard design line. The light emitting points  45   a – 45   d  vary due to manufacturing errors and it is presumed that their positions in the horizontal scanning direction Q 1  and intervals in the vertical scanning direction Q 2  on the image pickup surface  49  are not constant. Thus, the arranged direction Q 4  as an arranged state of the beam spots  52 – 55  is not always existent. 
   In this embodiment, therefore, a straight line obtained by joining two light emitting points  45   a  and  45   d , which are remotest from each other out of the four light emitting points  45   a – 45   d , (by joining the beam spots  52  and  55 ), is regarded as being the arranged direction Q 4  and the angle θ of the arranged direction Q 4  relative to the virtual straight line Q 3  is measured. 
   Given that, in the vertical scanning direction, the spacing between the beam spots  52  and  53  is x 2 , the spacing between the beam spots  52  and  54  is x 3 , and the spacing between the beam spots  52  and  55  is x 4 , while, in the horizontal scanning direction, the spacing between the beam spots  52  and  53  is y 2 , the spacing between the beam spots  52  and  54  is y 3 , and the spacing between the beam spots  52  and  55  is y 4 , the angle θ is determined in accordance with the following equation:
 
θ=tan −1 ( y 4 /x 4)
 
   Instead of regarding the straight line obtained by joining the two remotest light emitting points  45   a  and  45   d  out of the light emitting points  45   a – 45   d  (the straight line obtained by joining the beam spots  52  and  55 ) as the arranged direction Q 4 , there may be adopted a method involving obtaining an approximate straight line by the method of least squares, using the approximate straight line thus obtained as the arranged direction Q 4 , and determining the angle θ of the arranged direction Q 4  relative to the virtual straight line Q 3 .
 
θ=(Σ( x′ ixy′ i )−Σ( x′ ixy′ i )/ N )/(Σ x′ i   2 −(Σ x′ i ) 2 )/ N )
 
where N stands for the number of light emitting points (here, N=4), x′ i and y′ i stand for positions in x and y directions, respectively, of each light emitting point, and i=1−N.
 
   Next, in accordance with the angle θ thus obtained, a pulse motor coupled directly to the micrometer is operated to turn the base member  30  by the angle θ so that the arranged direction Q 4  becomes aligned with the vertical scanning direction Q 2 . 
   Then, the collimator lens  33  is released from the collimator lens holding arm and measurement is made to see whether the arranged state of the beam spots  52 – 55 , namely, spacings y 2 –y 4  in the horizontal scanning direction Q 1  of the beam spots  52 – 55 , are within a specification σ or not. 
   If the spacings y 2 –y 4  in the horizontal scanning direction Q 1  of the beam spots  52 – 55  are output the specification σ, that is, if the arranged state of the beam spots is not proper, the base member  30  is again subjected to the angular adjustment and the collimator lens  33  is adjusted its position. Thereafter, the screws  47  with spring washer are tightened to a further extent to fix the base member  30  to the mounting bracket  20  unrotatably. Then, the laser beams are measured for optical characteristics and arranged state and the multi-beam laser diode  31  is turned OFF to complete the adjusting work. 
   As a result, as shown in  FIG. 18 , the beam spots  52 – 55  are arranged straight in the vertical scanning direction Q 2  and thus the write start positions on the image recording surface  4   a  are aligned. Consequently, it is possible to simplify the configurations of control circuit and drive circuit disposed in the body portion of the image forming apparatus. 
   Thus, in the multi-beam source unit assembling and adjusting process there are performed both measuring an arranged state of the light emitting points  45   a – 45   d  in the multi-beam laser diode  31  and rotating the multi-beam laser diode  31  so as to align the arranged direction of the light emitting points with the direction of the standard design line on the basis of the measurement of the light emitting points  45   a – 45   d  arranged state. The aperture member  36  is mounted after this adjustment. 
   Although the arranged direction Q 4  as the arranged state is here evaluated, there may be adopted a method involving rotating the base member  30  by predetermined angles to determine spacings (deviations) in the horizontal scanning direction Q 1 , regarding the angle θ as the arranged direction Q 4  of the light emitting points  45   a – 45   d  at which angle the maximum deviation out of the deviations y 2 , y 3 , and y 4  becomes minimum, and rotating the multi-beam laser diode  31  in that direction to align the write start positions of the beam spots  52 – 55 . 
   [Schematic Construction of the Body Portion of the Image Forming Apparatus] 
     FIGS. 19 and 20  illustrate a schematic construction of a housing of the image forming apparatus body portion, of which  FIG. 19  illustrates a state prior to mounting of the multi-beam source unit  19  to a housing  56  and  FIG. 20  illustrates a mounted state of the multi-beam source unit  19  to the housing  56 . 
   A scanning optical system  57  is mounted on the housing  56 . The scanning optical system  57  is substantially composed of a polygon mirror  58  and f θ lenses  59 . A positioning reference portion  60  is provided on the bottom wall of the housing  56 . As shown in  FIG. 19 , the positioning reference portion  60  is provided with a pair of positioning pins  61  and a pair of tapped holes  62 . In the multi-beam source unit  19 , as shown in  FIG. 20 , the reference planes of its positioning reference portions  24  are confronted with a reference plane of the positioning reference portion  60 , positioned by the positioning pins  61 , and fixed to the positioning reference portion  60  with screws  63 . 
   In one side wall of the housing  56  is formed an opening  64  which extends in a direction parallel to the horizontal scanning direction Q 1 . When the multi-beam laser diode  31  is driven to activate the light emitting points  45   a – 45   d  simultaneously, multi-laser beams are deflected in the horizontal scanning direction Q 1  by the polygon mirror  58 , then pass through the opening  64  and are radiated to a photosensitive surface  65   a  of a photosensitive drum  65  serving as a recording medium to form four beam spots  11  on the photosensitive surface  65   a . The four beam spots  11  are substantially aligned in the vertical scanning direction Q 2  on the photosensitive surface  65   a  because the light emitting points  45   a – 45   d  of the multi-beam laser diode  31  are adjusted in the vertical scanning direction Q 2 . 
   [Explaining the Details of Adjusting the Multi-Beam Source Unit] 
     FIG. 21  illustrates an optical system of the adjusting device schematically. In the same figure, fco stands for a rear-side focal length of the collimator lens  33 , fco′ stands for a front-side focal length of the collimator lens  33 , f 1  stands for a front-side focal length of the condenser lens  48 , and f 1 ′ stands for a rear-side focal length of the condenser lens  48 . 
   An image pickup surface (an area type image receiving surface)  49   a  of the image pickup device  49  is positioned at the rear-side focal length f 1  of the condenser lens  48 . The front-side focal position of the condenser lens  48  is made substantially coincident with the rear-side focal position of the collimator lens  33 . 
   With the optical system constructed as above, multi-laser beams emitted from the light emitting points  45   a – 45   d  of the multi-beam laser diode  31  are substantially collimated by the collimator lens  33  and are focused onto the image pickup surface  49   a  by the condenser lens  48  substantially on a larger scale. Thus, it becomes possible to measure the position of each beam spot with a high accuracy. 
     FIG. 22  is an enlarged view of the beam spot  52  formed on the image pickup surface  49   a . The position of each beam spot is determined by calculating a centroid position. An example of how to calculate a centroid position of the beam spot  52  will be described below. 
   Each pixel of the image pickup surface  49   a  is defined by the reference mark Zij. The marks Z 1 j, Z 2 j, . . . Zij, . . . , Znj stand for pixels arranged in the horizontal scanning direction Q 1 , while Zi 1 , Zi 2 , . . . , Zij, . . . , Zim stand for pixels arranged in the vertical scanning direction Q 2 , and the mark i (integer from 1 to n) stands for the ith position counted from the left-hand side, while the mark j (integer from 1 to m) stands for the jth position counted from below. 
   If the sum total Wj (Wj=Z 1 j+Z 2 j+ . . . +Zij+ . . . +Znj) of output signals outputted from pixels Z 1 j, Z 2 j, . . . , Zij arranged in the horizontal scanning direction Q 1  is obtained successively from j=1 to j=m in the vertical scanning direction Q 2 , there can be obtained a light beam intensity distribution curve B 1  in the vertical scanning direction Q 2 . Likewise, if the sum total Wi (Wi=Zi 1 +Zi 2 +Zij+ . . . +Zim) of output signals outputted from pixels Zi 1 , Zi 2 , . . . , Zij, . . . , Zim arranged in the vertical scanning direction Q 2  is obtained successively from i=1 to i=n in the horizontal scanning direction Q 1 , there can be obtained a light beam intensity distribution curve B 2  in the horizontal scanning direction Q 1 . 
   In  FIG. 23 , a threshold value P 1   h  is set for the beam intensity distribution curve B 2 , then addresses X 1  and X 2  of pixels in the horizontal scanning direction Q 1  corresponding to the intensities which cross the threshold value P 1   h  are specified, and an address Xim of the pixel corresponding to a mean value of the sum of the addresses X 1  and X 2  is obtained. In this way there is obtained a centroid position (central position) O 1  in the horizontal scanning direction of the beam spot  52 . By performing the same processing for the beam intensity distribution curve B 1  there is obtained a centroid position (central position) in the vertical scanning direction. The threshold value P 1   h  is set at one over the square of e (natural logarithm) from peak Pmax. 
   Thus, since the centroid position of the beam spot  52  is calculated on the basis of the entire shape of the beam spot focused on the image pickup surface  49   a , it is desirable, for enhancing the calculation accuracy, to constitute an optical system so that the focusing area of each beam spot on the image pickup area  49   a  is ten times or more as large as the pixel area. 
   More specifically, as shown in  FIG. 24 , given that the diameter in the horizontal scanning direction of the beam spot  52  on the image pickup surface  49   a  is Wm, the beam spot diameter in the vertical scanning direction on the image pickup surface  49   a  is Ws, the diameter in the horizontal scanning direction of the laser beam after passing the slit  36   a  is Dm′, the beam diameter in the vertical scanning direction after passing the slit is Ds′, and the oscillation wavelength of the multi-beam laser diode  31  is λ, the beam spot diameters Wm and Ws in the horizontal and vertical directions respectively are calculated in accordance with the following equations:
 
 Wm =( f   1 ×λ)/(π× Dm ′)
 
 Ws =( f   1 ×λ)/(π× Ds ′)
 
and the optical system in the adjusting device is designed so as to give:
 
π× Wm×Ws &gt;pixel area×10
 
   Further, if the pitch displacement quantity in the horizontal scanning direction between the remotest light emitting points  45   a  and  45   d  is PLDAm, the pitch displacement quantity in the vertical scanning direction between the light emitting points  45   a  and  45   d  is LDAs, the pitch in the horizontal scanning direction between the beam spot  52  from the light emitting point  45   a  and the beam spot  55  from the light emitting point  45   d  on the image pickup surface  49   a  is Pccdm, and the pitch in the vertical scanning direction between the beam spot  52  from the light emitting point  45   a  and the beam spot  55  from the light emitting point  45   d  on the image pickup surface  49   a  is Pccds, then a pitch displacement is calculated in accordance with the following equations Pccdm=(f 1 /fco)×PLDAm and Pccds=(f 1 /fco)×PLDAs and a magnification of the optical system is set so as to prevent protrusion of both beam spots  52  and  55  of the light emitting points  45   a  and  45   d  from the image pickup surface  49   a  and so as to satisfy the following relationships:
 
 Pccdm ×( N− 1)+ Wm&lt;Lm 
 
 Pccds ×( N− 1)+ Ws&lt;Ls 
 
where Lm stands for an overall length of the image pickup surface  49   a  in the horizontal direction (horizontal scanning direction), Ls stands for an overall length of the image pickup surface  49   a  in the vertical direction (vertical scanning direction), and N stands for the number of light emitting points (N=4).
 
   So setting the magnification of the optical system is efficient because the evaluation of four light emitting points  45   a – 45   d  can be done simultaneously by a single image pickup device  49 . 
   When the light emitting points  45   a – 45   d  are to be turned ON at a time, the following control is made so that the light emitting outputs of the light emitting points  45   a – 45   d  become substantially equal to one another. First, any one of the light emitting points  45   a – 45   d  is turned ON and the output of the image pickup device  49  based on that activated light emitting point is detected and is stored as a reference output P 1 . 
   Next, with the light emitting point thus turned ON left as it is, one of the remaining light emitting points is turned ON and a laser drive control circuit is adjusted so that the output of the image pickup device  49  becomes twice its reference output P 1 . This control adjustment is made successively for the four light emitting points  45   a – 45   d  and the laser drive control circuit is adjusted so as to afford an output four times the reference output P 1 . Generally, in the case where there are N number of light emitting points, the laser drive control circuit is set so as to give an output N times the reference output P 1 , as shown in  FIG. 25 . 
   By so setting the laser drive control circuit it is possible to make constant the intensity of each of the beam spots  52 – 55  on the image pickup sur face  49   a  and hence possible to accurately evaluate the position of each beam spot. 
   [Another Example of Calculating a Centroid Position of Each Beam Spot] 
   In  FIG. 26 , given that a centroid position of a beam spot in the horizontal scanning direction is O 1 , a centroid position thereof in the vertical scanning direction is O 2 , the size of one pixel in the horizontal scanning direction is Gi, the size of one pixel in the vertical scanning direction is Gj, a threshold value obtained from the sum of products in the vertical scanning direction is P 1   h , and a threshold value obtained from the sum of products in the horizontal scanning direction is P 2   h , the centroid positions O 1  and O 2  may be determined in accordance with the following equations, in which the threshold values P 1   h  and P 2   h  are set to 1/e of the maximum value Pmax: 
               O   1     =       ⁢       G   i     ×         ∑     i   =     x   1         x   2       ⁢           ⁢     {       (       W   1     -     P   th       )     ×   i     }           ∑     i   =     x   1         x   2       ⁢           ⁢     (       W   1     -     P     1   ⁢   h         )                         O   2     =       ⁢       G   j     ×         ∑     j   =     y   1         y   2       ⁢           ⁢     {       (       W   j     -     P     2   ⁢   h         )     ×   j     }                 ∑     j   =     y   1         y             2         ⁢     (           ⁢       W   j     -     P     2   ⁢   h         )                                           
 
   According to this calculation method a central position of each beam spot can be determined with a high accuracy, particularly even when the beam spot is out of shape. 
   [Principal Construction of the Automatic Adjusting System] 
     FIG. 27  is a perspective view showing a principal construction of the automatic adjusting system. In the same figure, the numeral  70  denotes a control circuit (a personal computer for control) and numeral  71  denotes a CCD camera. The CCD camera  71  has the focusing lens  48  and a CCD as the image pickup device  49 . The control circuit  70  has an image processing control section  70   a , a mechanical drive section  70   b , and a pulse motor control section  70   c . An output signal from the CCD camera  71  is applied to a CCD camera drive unit  72  and is then inputted as an image signal to the image processing section via the CCD camera drive unit  72 . 
   Mechanically, the automatic adjusting system is substantially composed of solenoid valves  73 ,  74  and  75 , air cylinders  76  and  77 , pressing arms  79  and  80 , and an air chuck unit  81 . The pressing arms  79  and  80  are respectively provided with pressing pieces  79   a  and  80   a  for pressing the base member  30  against the upright wall portion  22  of the mounting bracket  20 . The air chuck unit  81  has gripping arms  82  and  83  to grip a side portion: of the base member  30  vertically. The air chuck unit  81  is fixed onto a support stand  84 , which stand is actuated vertically by means of a pulse motor  85 . A micrometer  78  is provided on the pulse motor  85  and the support stand  84  is mounted to an upper end of the micrometer  78 . 
   In the construction of the micrometer shown in  FIG. 16 , the upper end thereof is in direct abutment against a side portion of the base member  30  from above, but it is to be understood that this is because the illustration of the micrometer in  FIG. 16  is schematic. 
   Likewise, although a spatial layout relation between the base member  30  and the mounting bracket  20  is different between  FIGS. 27 and 7 , this is for the convenience of easily drawing an explanatory diagram of holding the base member  30  on the mounting bracket  20  through the pressing arms  79  and  80 . 
   The mechanical drive section  70   b  outputs a valve ON-OFF signal to each of the solenoid valves  73 ,  74  and  75  and the air cylinders  76  and  77  are switched over from one to the other air feed direction, whereby the pressing arms  79  and  80  are switched over between a direction of pressing the base member  30  against the mounting bracket  20  and the opposite direction. Likewise, the supply of air for the air chuck unit  81  is switched over by the solenoid valve  75 , whereby the gripping arms  82  and  83  are actuated between a direction in which the gripping arms grip side portions of the base member  30  vertically and a direction in which the side portions of the base member  30  are released from the gripped state. 
   The pulse motor control section  70   c  controls a pulse motor drive unit  86  and the pulse motor  85  is controlled by the pulse motor drive unit  86 . 
   In the illustrated example an illuminating light source  87  and an illuminating lens barrel  88  are provided in the automatic adjusting system. The illuminating lens barrel  88  is provided with an illuminating lens  89 . Illuminating light from the illuminating light source  87  is conducted to the illuminating lens  89  through an optical fiber  90 . The illuminating lens  89  converges the illuminating light to illuminate the light emitting chip portion  45 . 
   There may be adopted an adjusting method wherein a reflected image of the light emitting chip portion  45  resulting from reflection by the illuminating light of the light emitting chip portion  45  is received by the CCD camera  71  and the adjustment of rotation for the mounting bracket  20  of the base member  30  is performed by calculating an inclination of an edge image (to be described later) of the light emitting chip portion  45 . A description will be given later about the procedure of this adjustment. Here, reference will be made first to the procedure for making adjustment of rotation by making the light emitting points  45   a – 45   d  emit light. 
     FIGS. 28 and 29  are explanatory views of the said rotation adjusting procedure. 
   As shown in  FIG. 28 , the base member  30  is fixed temporarily to the mounting brackets  20  with screws  47 ′ and the mounting bracket  20  is set to the adjusting system (S.  1 ). Next, a start switch (not shown) is turned ON (S.  2 ), whereby the air cylinders  76  and  77  are actuated and the base member  30  is pressed to the mounting bracket  20  and is held thereby (S.  3 ). Then, the base member  30  is gripped by the gripping arms  82  and  83  (S.  4 ). Next, a laser drive connector (not shown) is connected to each terminal  31 C of the laser diode  31  (S.  5 ). The laser drive connector may be connected to each terminal  31 C automatically with use of an air cylinder (not shown). As a result, the light emitting points  45   a – 45   d  are turned ON (S.  6 ). 
   Upon turning ON of the light emitting points  45   a – 45   d , a shift is made to the procedure for adjustment processing shown in  FIG. 29  (S.  7 ). 
   As shown in  FIG. 29 , a CCD image is fetched upon turning ON of the light emitting points  45   a – 45   d  (S.  71 ). More specifically, a CCD image is introduced into the image processing control section  70   a  and central positions O 1  and O 2  of each beam spot are calculated (S.  72 ). It is optional whether the central positions O 1  and O 2  are to be calculated in accordance with the centroid position calculating method described above with reference to  FIG. 22  or by the centroid position calculating method described above with reference to  FIG. 26 . Next, an inclination angle θ relative to the virtual straight line Q 3  in the arrangement direction Q 4  is calculated (S.  73 ). For this calculation of the inclination angle there may be adopted any of the methods described previously. 
   Then, it is judged whether the inclination angle θ is within the specification or not (S.  74 ). If the answer is affirmative, the adjustment is ended and the processing flow shifts to step S.  8 , in which the screws  47 ′ are re-tightened and the base member  30  is fixed to the mounting bracket  20 . It is optional whether this operation is to be done automatically or manually. 
   Unless the inclination angle θ falls under the specification, the image processing control section  70   a  calculates a rotation drive quantity of the pulse motor on the basis of for example, the distance LO from a rotational center O 1 ′ to a central axis of the micrometer  78  and the inclination angle θ. The data on the rotation drive quantity of the pulse motor is sent to the pulse motor control section  70   c , whereby the pulse motor drive unit  86  is controlled and the pulse motor  85  is rotated by the rotation drive quantity to move the micrometer  78  up or down (S.  76 ). 
   With the vertical movement of the micrometer  78 , the gripping arms  82  and  83  also move vertically, whereby the base member  30  is rotated about the rotational center O 1 ′. 
   Then, the processing flow shifts to S.  71  and the processings of S.  71  to S.  74  are repeated. If the inclination angle θ falls under the specification, the flow shifts to S.  8 , while if the inclination angle θ is still outside the specification, the processings after S.  75  are repeated. 
   When the fixing of the base member  30  to the mounting bracket  20  is completed by re-tightening the screws  47 ′ in S.  8 , the base member  30  is released from the gripping arms  82  and  83  (S.  9 ). 
   Thereafter, a check is made to see if the inclination angle θ is within the specification or not (S.  10 ) and if the answer is negative, the screws  47 ′ are loosened, the flow again shifts to S.  7  and the processings of S.  7  to S.  10  are repeated. 
   If the inclination angle θ is found to be within the specification in S.  10 , the flow shifts to S.  11  to turn OFF the light emitting points  45   a – 45   d  and thereafter the laser drive connector is removed from each terminal  31   c  (S.  12 ). Subsequently, the base member  30  is released from the mounting bracket  20  (S.  13 ), followed by removal of the mounting bracket  20  from the adjusting system (S.  14 ). 
     FIGS. 30 and 31  are explanatory diagrams explaining the procedure of rotation adjustment for the base member  30  relative to the mounting bracket  20  which adjustment is based on an inclination measurement of an edge image of the light emitting chip portion  45 . 
   As shown in  FIG. 30 , the base member  30  is fixed temporarily to the mounting bracket  20  with screws  47 ′ and the mounting bracket  20  with base member  30  is set to the adjusting system (S.  1 ′). Next, a start switch (not shown) is turned ON (S.  2 ′). Then, the base member  30  is pressed against the mounting bracket  20  (S.  3 ′) and is gripped by the gripping arms  82  and  83  (S.  4 ′). Next, the illuminating light source  87  is turned ON (S.  5 ′). Images of the light emitting chip portion  45  shown in  FIGS. 32(   a ) and ( b ) are received by the illuminating light source  87  (S.  6 ′). 
   The image processing control section  70   a  fetches the image of the light emitting chip portion (S.  61 ′), then performs a binarization processing (S.  62 ′) and subsequent edge detection (S.  63 ′). As a result, edge images  45   p ,  45   q ,  45   r , and  45   s  of the light emitting chip portion  45  are highlighted. The inclination angle θ of the straight line Q 4  is calculated on the basis of coordinate values (x 1 ′, y 1 ′), (x 2 ′, y 2 ′), (x 3 ′, y 3 ′), and (x 4 ′, y 4 ′) of corner points  45   p ′,  45   q ′,  45   r ′, and  45   s ′ of the edge images  45   p ,  45   q ,  45   r , and  45   s , respectively, and in accordance with the following equations: 
         x3   &gt;     x4   ⁢           ⁢   θ       =       tan     -   1       ⁡     (         y   3     -     y   2           x   3     -     x   2         )           
         x3   &gt;     x4   ⁢           ⁢   θ       =       (     -   1     )     ⁢   x   ⁢           ⁢       tan     -   1       ⁡     (         y   1     -     y   3           x   1     -     x   3         )             
 
   Next, a check is made to see if the inclination angle θ is within the specification or not (S.  65 ′) and if the answer is affirmative, the adjustment is ended and the screws  47 ′ are re-tightened (S.  7 ′) to fix the base member  30  to the mounting bracket  20 . 
   On the other hand, if the inclination angle θ does not fall under the specification, a rotation drive quantity of the pulse motor is calculated (S.  66 ′) and the micromotor  78  is moved up or down by the rotation drive quantity of the pulse motor (S.  67 ′). 
   As a result, the gripping arms  82  and  83  move vertically and the base member  30  is turned about the rotation center O 1 ′. Next, the processing flow shifts to S.  61 ′ and the processings of S.  61 ′ to S.  65 ′ are repeated. If the inclination angle θ falls under the specification in S.  65 ′, the flow shifts to S.  7 ′, while if the inclination angle θ is still outside the specification, the processing of S.  66 ′ and subsequent processings are repeated. 
   After the fixing of the base member  30  to the mounting bracket  20  by re-tightening of the screws  47 ′ in S.  7 ′ is completed, the base member  30  is released from the gripping arms  82  and  83  (S.  8 ′) and then a check is made to see if the inclination angle θ is within the specification or not (S.  9 ′). If the answer is negative in S.  9 ′, the screws  47 ′ are loosened and the processing flow again shifts to S.  6 ′, followed by repetition of the processings S.  6 ′ to S.  9 ′. 
   If the inclination angle θ is within the specification in S.  9 ′, the flow shifts to S.  10 ′ to turn OFF the illuminating light source  87  and thereafter the base member  30  is released from the mounting bracket  20  (S.  11 ′). Subsequently, the mounting bracket  20  is removed from the adjusting system (S.  12 ′). 
   Although in the above rotation adjustment based on edge images the images are obtained using a CCD camera and the rotation adjustment is made automatically, there may be adopted a construction wherein the same adjustment is made visually. 
   [Adjusting the Mounting Position of the Multi-Beam Laser Diode with a Positioning Jig] 
     FIG. 33  is a plan view of the positioning jig and  FIG. 34  is an explanatory diagram explaining in what state positioning is effected using the positioning jig. 
   In  FIG. 33 , the numeral  100  denotes the positioning jig. The positioning jig  100  has a positioning jig body  101  and a base member rest portion  102 . The base member rest portion  102  has a base member abutting portion  103 , a base member rest surface  104 , and a base member guide portion  105 . The reference planes of the positioning reference portions  37  of the base member  30  are brought into abutment against the base member rest surface  104 , one longitudinal end portion of the base member  30  is abutted against the base member abutting portion  103 , and both side portions thereof perpendicular to the longitudinal direction are guided by base member guide portions  105  until the base member is placed on the base member rest portion  102 . 
   A pair of guide rails  106  are disposed on the positioning jig body  101  and a slide member  107  is mounted sidably on the guide rails  106 . A positioning member  108  is secured to the slide member  107  removably with screws  109 . The positioning member  108  comprises a guide portion  108 A which is guided by a positioning groove  41 A and an engaging lug  108 B for positioning which is formed at a free end of the guide portion  108 A and which comes into close engagement with the cutout portion  47 . 
   As shown in  FIG. 34 , the base member  30  is positioned and placed on the base member rest portion  102 , the multi-beam laser diode  31  is inserted into the fitting cylinder  32 , then the pressing plate  40  is put on the stem  31 B of the multi-beam laser diode  31  and is fixed temporarily with screws  43 . 
   Subsequently, the slide member  107  is allowed to slide toward the base member  30 , with the result that the positioning member  108  is guided along the positioning groove  41 A and the engaging lug  108 B for positioning comes into engagement with the cutout portion  47 . If the base member  30  and the multi-beam laser diode  31  are displaced from each other in the rotational direction, the engaging lug  108 B for positioning abuts the peripheral wall of the cutout portion  47  as it advanced into the cutout portion, whereby the multi-beam laser diode  31  is rotated with respect to the base member  30  and is adjusted to a predetermined positional relation in the rotational direction with respect to the base member  30 . 
   By re-tightening the screws  43  in this state the multi-beam laser diode  31  is pressed against the pressing plate mounting portion  41  by means of the pressing plate  40  and is fixed to the base member  30  unrotatably. If the slide member  107  is allowed to slide in the opposite direction in this fixed state of the diode, the cutout portion  47  and the engaging lug  108 B for positioning are disengaged from each other. 
   In this way the multi-beam laser diode  31  and the base member  30  are rendered integral with each other and the base member  30  with the multi-beam laser diode is mounted to the mounting bracket  20  and is adjusted for rotation, as described earlier. 
   As set forth above, in the adjusting method for the multi-beam source unit according to the present invention, an arranged state of the light emitting points relative to the standard design line is measured on the basis of beam spots on the image surface corresponding to the image recording surface and the multi-beam laser diode is rotated for adjustment around the optical axis of the optical system. Consequently, the arranged direction of the light emitting points can be aligned with the standard design line direction easily. 
   Particularly, if the arranged direction of the light emitting points is aligned with the vertical scanning direction in advance of the mounting of the multi-beam source unit to the body portion of the image forming apparatus, then the adjusting operation of adjusting the beam spot pitch in the vertical scanning direction after the mounting of the multi-beam source unit to the image forming apparatus body, a complicated construction such that, for correcting write start positions displaced in the horizontal scanning direction by the beam spot pitch adjustment, a sensor for detecting the scanning position of each laser beam is provided for each laser beam in the image forming apparatus body and a write start position is controlled for each laser beam, and a complicated construction such that a time delay in the horizontal scanning direction of each laser beam is measured and a laser beam drive control is performed using a delay circuit, can all be avoided and hence it is possible to reduce the number of components used and also possible to shorten the assembling time required for the mounting to the image forming apparatus body, with consequent attainment of the reduction of cost. The simplification of control using software can also be attained. 
   In the multi-beam source unit adjusting device according to the present invention, since multi-laser beams from all the light emitting points can be enlarged and substantially condensed and focused onto an image pickup surface, it is possible to detect beam spot positions with a high accuracy. 
   In the multi-beam source unit assembling method according to the present invention, both adjustment in the arranged direction of beam spots and positional adjustment of the collimator lens can be done simultaneously in the multi-beam source unit assembling process and thus the multi-beam source unit assembling work can be done efficiently. 
   In the image forming apparatus according to the present invention, at the time of mounting the adjusted multi-beam source unit to the body portion of the image forming apparatus, it is possible to simplify the position adjusting work for the scanning optical system.