Abstract:
A scanner is provided that has a light-beam emitter for emitting a light beam, a light-beam deflector for deflecting the light beam to scan a scanning surface, a photo-detector provided at a position outside an image-forming scanning range of the scanning surface to detect a scanning light beam before the scanning light beam starts generating a scanning line in the image-forming scanning range, a rotatable member located in front of an incident surface of the photo-detector and positioned in a recess formed on an outer surface of a housing. The rotatable member is rotatable about a rotational axis perpendicular to a plane defined by the scanning light beam by said deflector. The scanner also has an optical member provided on the rotatable member that allows the scanning light beam to pass therethrough to be incident upon the incident surface of the photo-detector, and a device for adjusting rotational position of said rotatable member about said rotational axis. A through hole through which the optical member is inserted in the housing is formed at the bottom of the recess, and the optical member is inserted into the housing through the through hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. patent application Ser. No. 10/098,544, filed Mar. 18, 2002 now U.S. Pat. No. 6,768,568, which is a divisional of U.S. patent application Ser. No. 09/271,455, filed Mar. 18, 1999, now abandoned, the disclosures of which are expressly incorporated herein by reference in their entireties. 

   FIELD OF THE INVENTION  
   The present invention relates to a scanner in which a light beam is deflected to scan a scanning surface, and more specifically to a scanner which is provided with a device for adjusting an incident position of a light beam on a photo-detector used for determining the timing of commencement of writing each scanning line with respect to a scanning surface. 
   DESCRIPTION OF THE RELATED ART  
   A laser-beam printer provided with a laser-beam scanner is well known. In a laser-beam printer, a laser beam which is modulated in accordance with image signals to be output from a laser-beam emitter is deflected by a polygon mirror to scan a photoconductive surface of a photoconductive drum in the main scanning direction to thereby form a main scanning line in the photoconductive surface. The laser emission is turned ON and OFF ion accordance with given image signals to draw a corresponding image (charge-latent image) on the photoconductive surface of the drum and subsequently this image drawn on the photoconductive surface of the drum is transferred to plain paper according to a conventional electrophotographic method. Dry powder (e.g., toner) that adheres only to the charged area is applied to the drum, transferred to the plain paper and fused by heat. Such a laser-beam printer is widely used; e.g., as an output device for a computer. 
   In a laser-beam scanner provided in such a laser-beam printer, a photo-detector (i.e., a laser-beam detector is generally fixed at a position outside the latent-image-forming scanning range to detect the scanning laser beam before it starts generating each scanning line. The photo-detector generates a pulse signal each time the scanning laser beam is incident on the photo-detector. The pulse signals output from the photo-detector are input to a processor, and subsequently the processor generates corresponding horizontal synchronizing pulses (HSYNC) to determine the timing of commencement of writing main scanning data, namely, writing each main scanning line. 
   In such a laser-beam scanner, two types of devices for adjusting the timing of commencement of writing each main scanning line with respect to the photoconductive surface of the drum (i.e., for adjusting the timing of generating horizontal synchronizing pulses) are known. In each type of adjusting device, a reflecting mirror is arranged at a position outside the latent-image-forming-scanning range to detect the scanning laser beam before it starts generating each scanning line, while a photo-detector is arranged at a position on the path of the laser beam reflected by the reflecting mirror. In one type of adjusting device, the reflecting mirror is rotatable so that the incident position of the laser beam on the photo-detector can be adjusted, which makes it possible to adjust the timing of generating horizontal synchronizing pulses. In the other type of adjusting device, the reflecting mirror is fixed while the photo-detector is linearly movable so that the incident position of the laser beam on the photo-detector can be adjusted. 
   In the former type of adjusting device, although the incident position of the laser beam on the photo-detector can be adjusted by rotating the reflecting mirror, it is difficult to finely adjust the incident position of the laser beam on the photo-detector. Furthermore, the reflective mirror needs to be accurately and precisely positioned on a base on which the reflective mirror is to be mounted. In the latter type of adjusting device, the position at which the photo-detector is to be arranged is quite limited. Moreover, in each type of adjusting device, in the case where the base on which the reflective mirror and the photo-detector are mounted is slightly deformed after a long period of use, the respective positions of the reflective mirror and the photo-detector deviate from their original positions. In this case, the respective positions of the reflective mirror and the photo-detector cannot be easily adjusted from outside the laser-beam apparatus. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a scanner provided with a device for adjusting the incident position of a light beam on a photo-detector used for determining the timing of commencement of writing each scanning line with respect to a scanning surface, wherein the adjusting device makes it possible to finely and easily adjust the incident position of the light beam on the photo-detector. 
   Another object of the present invention is to provide a scanner having such an adjusting device which makes it possible to finely and easily adjust the incident position even from outside the scanner. 
   Other aspects, objects and advantages of the present invention will become apparent to one skilled in the art from the following disclosure and the appended claims. 
   According to an aspect of the present invention, there is provided a scanner including a light-beam emitter for emitting a light beam; a light-beam deflector for deflecting the light beam to scan a scanning surface; a photo-detector provided at a position outside an image-forming scanning range of the scanning surface to detect a scanning light beam before the scanning light beam starts generating a scanning line in the image-forming scanning range; a rotatable member, located in front of an incident surface of the photo-detector, that is rotatable about a rotational axis perpendicular to a plane defined by the scanning light beam by the deflector; an optical member, provided on the rotatable member, that allows the scanning light beam to pass therethrough to be incident upon the incident surface of the photo-detector; and a device for adjusting rotational position of the rotatable member about the rotational axis. 
   Preferably, the light-beam deflector includes a polygon mirror. 
   Preferably, a signal, output from the photo-detector, is used for detecting the timing for commencement of writing the scanning line with respect to the scanning surface. 
   The optical member can include a cylindrical lens or a plane-parallel plate. Preferably, the optical member includes a member having an optical axis which lies in a plane defined by the scanning light beam, and the rotational axis extends perpendicular to the optical axis. 
   The rotatable member can be positioned in a recess formed in a housing to be rotatable about the rotational axis. 
   In an embodiment, the recess is a circular recess, and the rotatable member includes a disc portion which is fitted into the circular recess to be rotatable about the rotational axis. 
   Alternatively, the rotatable member includes a shaft coaxial to the rotational axis, and the rotatable member is positioned in the recess with the shaft being inserted into a hole formed at the bottom of the recess so that the rotatable member is rotatable about the shaft. 
   Further, the recess can be formed on an outer surface of the housing, and a through hole through which the optical member is inserted in the housing is formed at the bottom of said recess, and the rotatable member is positioned in the recess with the optical member being inserted into the housing through the through hole. 
   For holding the rotatable member at an adjusted position, the adjusting device can include at least one set screw which penetrates into the rotatable member through a slot formed thereon to be screwed into the housing. 
   Alternatively, it is possible that the adjusting device includes a member, fixed to the housing, for pressing the rotatable member against the bottom of the recess. Preferably, the pressing member includes a spring. Further, the spring can be a leaf spring fixed to the housing by at least one set screw. 
   Preferably, the scanner further includes a device for rotating the rotatable member about the rotational axis. 
   In an embodiment, the rotating device includes a radial slot formed on the rotatable member to extend in a radial direction thereof; and a rotating tool engageable with the rotatable member to rotate the rotatable member about the rotational axis. Namely, the tool includes an engaging pin engageable with the radial slot, an axis of the engaging pin deviating from a rotational axis of the rotating tool. 
   Alternatively, the rotating device includes a circumferential gear formed on an outer peripheral surface of the rotatable member; and a rotating tool engageable with the rotatable member to rotate the rotatable member about the rotational axis. Namely, the rotating tool includes a pinon gear which is engaged with the circumferential gear. 
   It is preferable that the scanning surface is a photoconductive surface of a photoconductive drum. 
   In an embodiment, the photo-detector and the light-beam emitter are supported on a common circuit substrate and do not relatively move. 
   The scanner can include an fθ reflecting lens that reflects the scanning light beam deflected by the light-beam deflector to the scanning surface. 
   According to another aspect of the present invention, there is provided a scanner including a light-beam emitter for emitting a light beam; a light-beam deflector for deflecting the light beam to scan a scanning surface; a photo-detector provided at a position outside an image-forming scanning range of the scanning surface to detect a scanning light beam before the scanning light beam starts generating a scanning line, the photo-detector generating an output signal upon detecting the scanning light beam to determine a timing of commencement of writing the scanning line with respect to the scanning surface; and an optical member for deflecting the scanning light beam to be incident on the photo-detector in a direction to vary the timing of the scanning light beam incident upon the photo-detector. 
   The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-92725 (filed on Mar. 19, 1998) which is expressly incorporated herein by reference in its entirety. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described below in detail with reference to the accompanying drawings in which: 
       FIG. 1  is a perspective view of the scanning optical system of a laser-beam scanner to which the present invention is applied; 
       FIG. 2  is a perspective view of an embodiment of a device for adjusting the rotational position of a cylindrical lens with respect to a housing of the laser-beam scanner; 
       FIGS. 3A and 3B  are explanatory views of the cylindrical lens when rotated about a rotational axis; 
       FIG. 4  is a perspective view of the scanning optical system of a laser-beam scanner in which a photo-detector and a light-beam emitter are supported on a common circuit substrate; 
       FIG. 5  is an exploded perspective view of another embodiment of the device for adjusting the rotational position of the cylindrical lens; 
       FIG. 6  is a plan view of still another embodiment of the device for adjusting the rotational position of the cylindrical lens; 
       FIG. 7  is a plan view of yet another embodiment of the device for adjusting the rotational position of the cylindrical lens; 
       FIG. 8  is a plan view of yet another embodiment of the device for adjusting the rotational position of the cylindrical lens; 
       FIG. 9  is a plan view of yet another embodiment of the device for adjusting the rotational position of the cylindrical lens; 
       FIG. 10  is a perspective view of an embodiment of a device for rotating the cylindrical lens; and 
       FIG. 11  is a perspective view of another embodiment of the device for rotating the cylindrical lens. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the scanning optical system of a laser-beam scanner to which the present invention is applied. The laser-beam scanner scans the photoconductive surface of a photoconductive drum  1  (scanning surface). The laser beam scanner and the photoconductive drum  1  are positioned within a laser-beam printer as essential elements. 
   The scanning optical system of the laser-beam scanner is provided with a laser diode (light-beam emitter)  2 , a collimating lens  4   a , a cylindrical lens  4   b , a reflecting mirror  6 , a polygon mirror (light-beam deflector)  8 , an fθ reflecting lens  10 , an fθ lens  12 , a reflecting mirror  14 , a cylindrical lens (optical member)  16 , and a laser-beam detector (photo-detector)  18 . The collimating lens  4   a  and the cylindrical lens  4   b  together constitute an optical system  4  for the laser diode  2 . 
   The laser diode  2  outputs a laser beam L 1  modulated in accordance with image signals. The laser beam emitted from the laser diode  2  is collimated through the collimating lens  4   a . Thereafter, this collimated laser beam is made incident upon the cylindrical lens  4   b  positioned in front of the collimating lens  4   a . The cylindrical lens  4   b  has power in the sub-scanning direction, so that the spot of the laser beam incident thereon is converged therethrough in the sub-scanning direction to be incident upon the reflecting mirror  6 . The laser beam which is incident on the reflecting mirror  6  is reflected thereby to be incident on the polygon mirror  8 . The polygon mirror  8  is driven to rotate at a fast rotational speed by a motor (not shown), so that the laser beam incident on the polygon mirror  8  is deflected in the main scanning direction to be incident on the fθ reflecting lens  10 . 
   The deflected laser beam L 2  which is incident on the fθ reflecting lens  10  to be reflected thereby proceeds to the reflecting mirror  20  through the fθ lens  12 , which is arranged to face the fθ reflecting lens  10 . Subsequently, the laser beam incident upon the reflecting mirror  20  is reflected thereby towards the photoconductive surface of the drum  1 . 
   The polygon mirror  8  rotates in a counterclockwise direction (shown by an arrow “A”), as viewed in  FIG. 1 . The reflecting mirror  14  is fixed at a position to receive the scanning laser beam emitted from the polygon mirror  8  before the scanning laser beam is incident on the fθ reflecting lens  10  at each scanning sweep while the polygon mirror  8  rotates. The laser beam L 3  reflected by the reflecting mirror  14  is incident on the laser-beam detector  18  through the cylindrical lens  16 . The laser-beam detector  18  is fixed at a position facing to the reflecting mirror  14  with the cylindrical lens  16  being positioned between the reflecting mirror  14  and the laser-beam detector  18 . Namely, the cylindrical lens  16  is located in front of an incident surface of the laser-beam detector  18 . 
   The laser-beam detector  18  outputs a pulse signal for detecting the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1  each time the laser beam L 3  is incident on the laser-beam detector  18 . 
   As shown in  FIG. 2  the cylindrical lens  16  is fixed onto a rotatable base (rotatable member)  22  which is mounted on the housing  26  of the laser-beam scanner to be rotatable about a rotational axis  16   a  relative to the housing  26 . The scanning optical system shown in  FIG. 1  is enclosed in the housing  26 . The rotational axis  16   a  extends perpendicular to the optical axis of the cylindrical lens  16  and the direction (path) of the laser beam L 3 . Note that, in this embodiment, the optical axis of the cylindrical lens  16  lies in a plane that is defined by the scanning light beam emitted from the polygon mirror  8 . 
   The cylindrical lens  16  can be rotated about the rotational axis  16   a  to deflect the laser beam L 3  which passes therethrough so as to shift the same substantially in parallel on a plane which is perpendicular to the rotational axis  16   a  to thereby either delay or advance the timing of the incident laser beam L 3  on the laser-beam detector  18 . Accordingly, the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1  can be adjusted by rotating the cylindrical lens  16 . 
   The rotatable base  22 , onto which the cylindrical lens  16  is mounted, is provided with a disc portion  221  and a shaft  224  which is formed integral with the disc portion  221 . The rotatable base  22  is connected to the housing  26  so that the disc portion  221  is rotatably fitted in a circular recess  222  with the shaft  224  being rotatably fitted into a hole  223  formed at the center of the bottom of the circular recess  222 . With this structure, the rotatable base  22  is rotatable about the shaft  224  with respect to the housing  26  so that the cylindrical lens  16  can rotate about the rotational axis  16   a.    
   The rotatable base  22  is provided with a circumferential slot  241  which extends circumferentially about the rotational axis  16   a . A set screw  242  is inserted into the circumferential slot  241  so that the set screw  242  is screw-engaged with a female screw hole  243  formed at the bottom of the circular recess  222 . The rotatable base  22  can be rotated about the rotational axis  16   a  on the housing  26  when the set screw  242  is loosened while the rotatable base  22  cannot be rotated about the rotational axis  16   a  on the housing  26  when the set screw  242  is tightly fastened. Accordingly, the circumferential slot  241 , the set screw  242  and the female screw hole  243  together constitute an adjusting device  24  for adjusting the rotational position of the cylindrical lens  16  about the rotational axis  16   a  and for fixing the same with respect to the housing  26 . 
   In the laser-beam scanner having such a structure, the laser beam L 1  emitted from the laser diode  2  is incident upon the reflected mirror  6  via the collimating lens  4   a  and the cylindrical lens  4   b . Subsequently, the laser beam L 1  is reflected by the reflected mirror  6  to be incident upon the polygon mirror  8 . The polygon mirror  8  has a regular hexagonal cross section and is provided along a circumference thereof with six reflecting surfaces (scanning laser beam deflecting surfaces). The laser beam reflected by the reflecting mirror  6  to be incident on the polygon mirror  8  is reflected by each of the six reflecting surfaces while the polygon mirror  8  rotates. The laser beam reflected by the polygon mirror  8  is incident on the fθ reflecting lens  10 . The laser beam L 2  reflected by the fθ reflecting lens  10  to proceed towards the fθ lens  12  passes therethrough to be reflected by the reflecting mirror  20  to thereby proceed towards the photoconductive surface of the drum  1 . The laser diode  2  is controlled to turn its laser emission ON and OFF in accordance with given image data to draw a corresponding image (charge-latent image) on the photoconductive surface of the drum  1 ; and subsequently, the image drawn on the photoconductive surface of the drum  1  is transferred to plain paper according to a conventional electrophotographic method. 
   The polygon mirror  8  is rotated at a fast rotational speed in the direction of the arrow “A” shown in  FIG. 1 , so that the incident angle of the laser beam. L 1  on each reflecting surface of the polygon mirror  8  varies. Hence, the laser beam L 2  is deflected by the polygon mirror  8  in the main scanning direction (indicated by an arrow B in  FIG. 1 ). 
   The laser beam L 3  which is incident on the fθ reflecting lens  10  to be reflected by the reflecting mirror  14  proceeds towards the cylindrical lens  16  rather than the fθ lens  12 . As described the above, when the laser beam L 3  passes through the cylindrical lens  16 , the laser beam L 3  which proceeds towards the laser-beam detector  18  is deflected to shift substantially in parallel on a plane which is perpendicular to the rotational axis  16   a . Namely, when the laser beam L 3  passes through the cylindrical lens  16 , the laser beam L 3  which proceeds towards the laser-beam detector  18  is deflected in a direction to either delay or advance the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1 . 
   Each time the laser beam L 3  is incident on the laser-beam detector  18 , the laser-beam detector  18  outputs a pulse signal. The pulse signals output from the laser-beam detector  18  are input to a processor (not shown), and subsequently, the processor generates corresponding horizontal synchronizing pulses (HSYNC) to determine the timing of commencement of writing main scanning data; i.e. each main scanning line. 
   The horizontal synchronizing pulses are input to a clock generator so that it synchronously generates corresponding clock pulses. Subsequently the clock pulses are input to a memory for storing image data, and the stored image signals are sequentially read out of the memory in accordance with the input close pulses. The laser diode  2  outputs the laser beam L 1  which is modulated in accordance with the image signals read out of the memory. 
   The way of adjusting the angular position of the cylindrical lens  16  to deflect the incident laser beam so as to delay or advance the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1  will be hereinafter discussed. 
   First of all, the rotatable base  22  having the cylindrical lens  16  mounted thereon needs to be fitted in the circular recess  222 , with the shaft  224  being fitted into the hole  223  and with the set screw  242  being engaged with the female screw hole  243  through the circumferential slot  241 . 
   In this state, the set screw  242  is loosened and subsequently the rotatable base  22  is slightly rotated clockwise or counterclockwise about the shaft  224 , i.e., the rotational axis  16   a.    
   In the case where the cylindrical lens  16  is rotated clockwise as viewed in  FIG. 3A  from the position shown by a solid line to the position shown by a dotted line, the laser beam L 3  incident on the laser-beam detector  18  is deflected to shift to the left from the position shown by a solid line to the position shown by a two-dotted chain line in  FIG. 3A . When the polygon mirror  8  is rotated, the laser beam L 3  is scanned (moved) from right to left in  FIGS. 3A and 3B . Accordingly, the rotation of the cylindrical lens  16  as shown in  FIG. 3A  causes the laser-beam detector  18  to delay the output of a pulse signal to thereby delay the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1 . 
   On the other hand, in the case where the cylindrical lens  16  is rotated counterclockwise as viewed in  FIG. 3B  from the position shown by a solid line to the position shown by a dotted line, the laser beam L 3  incident on the laser-beam detector  18  is deflected to shift to the right from the position shown by a solid line to the position shown by a two-dotted chain line in  FIG. 3B . This makes the laser-beam detector  18  to advance the output of a pulse signal to thereby advance the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1 . 
   After the adjustment of the timing of commencement of writing each scanning line is completed, the set screw  242  is tightly fastened to fix the disc portion  221  to the circular recess  222  of the housing  26 , which completes the adjusting operation. The cylindrical lens  16 , the rotatable base  22 , the circular recess  222  and the adjusting device  24  together constitute a light beam incident position adjusting device. 
   It can be appreciated from the foregoing that the incident position of the laser beam L 3  with respect to the laser-beam detector  18  can be easily and precisely adjusted by rotating the rotatable base  22  about the rotatable axis  16   a . Hence, with the light beam incident position adjusting device, the timing of commencement of writing each scanning line with respect to the photoconductive surface of the drum  1  can be easily and precisely adjusted by rotating the rotatable base  22  about the rotatable axis  16   a.    
     FIG. 4  shows an embodiment in which, so as not to relatively move, the laser-beam detector  18 ′ (photo-detector and the laser diode  2 ′ (light-beam emitter) are supported on a common circuit substrate  100 . In this construction, since the laser-beam detector  18 ′ is fixed to the substrate  100 , the type of adjusting device that moves the photo-detector (i.e., the laser-beam detector  18 ′) cannot be used. However, in the above-described adjusting device of the present invention, the cylindrical lens  16  is rotated in order to perform adjustment; therefore, the timing of the incident laser beam L 3  on the laser-beam detector  18 ′ can be adjusted regardless of the type of photo-detector being utilized. 
   The device for adjusting the rotational position of the cylindrical lens  16  (and fixing the cylindrical lens  16  to the housing  26 ) is not limited solely to the particular aforementioned device (i.e., the adjusting device  24 ) but can be any other device as long as it bears a similar function.  FIG. 5  shows another embodiment of the adjusting device for adjusting the rotational position of the cylindrical lens  16 . In this embodiment the housing  26  is provided on a bottom surface thereof with a circular recess  222 ′ which corresponds to the circular recess  222  of the previous embodiment. A circular through hole  225  through which the cylindrical lens  16  can be inserted in the housing  26  is formed at the center of the bottom of the circular recess  222 ′. A rotatable base  22 ′, which corresponds to the rotatable base  22  of the previous embodiment, is not provided with a shaft which corresponds to the shaft  224  of the rotatable base  22 . When the rotatable base  22 ′ is set on the housing  26 , the disc portion  221  of the rotatable base  22 ′ is rotatably fitted in the circular recess  222 ′ with the cylindrical lens  16  being inserted into the housing  26  through the through hole  225 . With such a adjusting (fixing) device, the cylindrical lens  16  can be fixed to the housing  26  in place from outside the housing  26 , which makes it easier to set the cylindrical lens  16  on the housing  26 . 
   In the aforementioned embodiments, the rotatable base  22  (or  22 ′) is fixed to the housing  26  using only one set screw  242 . However, the rotatable base  22  (or  22 ′) can be fixed to the housing using more than one set screw.  FIG. 6  shows another embodiment using two set screws  242  to fix the disc portion  221  of the rotatable base  22  to the housing  26 .  FIG. 7  shows yet another embodiment using three set screws  242  to fix the disc portion  221  of the rotatable base  22  to the housing  26 . In  FIG. 6  the two set screws  242  are positioned on respective sides with respect to the path of the laser beam L 3  so as to face respective ends (right and left ends as viewed in  FIG. 6 ) of the cylindrical lens  16 . In  FIG. 7  the three set screws  242  are positioned at regular intervals in a circumferential direction of the disc portion  221 . 
     FIG. 8  shows another embodiment of the adjusting device for adjusting the rotational position of the cylindrical lens  16 . In this embodiment the disc portion  221  is fixed to the housing  26  by a adjusting device  30  which is composed of a leaf spring  302  and two set screws  303  for securing the leaf spring  302  to the housing  26 . The leaf spring  302  has a substantially rectangular shape and is provided at a center thereof with a circular hole  301  in which the cylindrical lens  16  is positioned. The longitudinal length of the leaf spring  302  is larger than the diameter of the disc portion  221  so as to press the same against the housing  26 . The leaf spring  302  is provided, on a surface thereof facing the disc portion  221 , with two projections  304  which are positioned on respective sides with respect to the cylindrical lens  16  to be aligned along the path of the laser beam L 3 , as can be seen in  FIG. 8 . The leaf spring  302  is further provided at respective ends thereof with two slits through which the two set screws are respectively inserted to be screwed into the housing  26 . In a state where the leaf spring  302  is tightly secured to the housing  26  by the set screws  303 , the two projections  304  of the leaf spring  302  come into pressing contact with the disc portion  221 , so that the disc portion  221  is tightly held between the leaf spring  302  and the housing  26 , so that the disc portion  221  is fixed to the housing  26 . 
     FIG. 9  shows yet another embodiment of the adjusting device for adjusting the rotational position of the cylindrical lens  16 . In this embodiment the disc portion  221  is fixed to the housing  26  by a adjusting device  40  which includes a leaf spring  402  and a set screw  403  for securing the leaf spring  402  to the housing  26 . The leaf spring  402  has a substantially U-shape and is provided with two parallel projecting portions  401  between which the cylindrical lens  16  is positioned. The projecting portions  401  are positioned on respective sides relative to the path of the laser beam L 3 , as can be seen in  FIG. 9 . Each projecting portion  401  is provided, at its tip on a surface thereof facing the disc portion  221 , with a projection  404 . In a state where the leaf spring  402  is tightly secured to the housing  26  by the set screw  403 , the two projections  404  of the leaf spring  402  come into pressing contact with the disc portion  221 , so that the disc portion  221  is tightly held between the leaf spring  402  and the housing  26 , so that the disc portion  221  is fixed to the housing  26 . 
     FIG. 10  shows an embodiment of device for rotating the cylindrical lens  16 . In this embodiment, the cylindrical lens  16  is positioned in place by inserting the same into the housing  26  from outside the housing  26 , and the operation of rotating the cylindrical lens  16  can be carried out from outside the housing  26 . 
   In this embodiment, similar to the embodiment shown in  FIG. 5 , the housing  26  is provided on a bottom surface thereof with a circular recess  222 ′. A circular through hole  225  through which the cylindrical lens  16  can be inserted in the housing  26  is formed at the center of the bottom of the circular recess  222 ′. The disc portion  221  of this embodiment is provided with two circumferential slots  241  for fixing the disc portion  221  to the mousing  26  by two set screws  242  respectively inserted into the two circumferential slots  241 . The disc portion  221  is further provided with a radial slot  601  which extends in a radial direction of the disc portion  221 . The disc portion  221  is rotatably fitted in the circular recess  222 ′ with the cylindrical lens  16  being inserted into the housing  26  through the through hole  225 . A tool  603  is used to rotate the cylindrical lens  16 . The tool  603  is provided at the tip thereof with an engaging pin  602  which can be inserted into the radial slot  601 . The axis of the engaging pin  602  extends parallel with, but deviates from, the rotational axis of the tool  603 , so that the disc portion  221  is rotated when the tool  603  rotates about its rotational axis with the engaging pin  602  being inserted into the radial slot  601 . Each set screw  242  needs to be loosened in advance when the disc portion  221  is rotated by the tool  603 . The slot  601  and the tool  603  together constitute a device  60  for externally rotating the cylindrical lens  16 . 
   In a state where the engaging pin  602  is engaged with the radial slot  601 , rotating the tool  603  without moving the same in a radial direction thereof causes the disc portion  221  (the cylindrical lens  16 ) to rotate clockwise or counterclockwise in a direction shown by an arrow in  FIG. 10 . Hence, with the use of the device  60 , the incident position of the laser beam L 3  on the laser-beam detector  18  can be finely and easily adjusted even from outside the housing  26  of the scanner. After the adjusting operation (i.e., the rotation of the cylindrical lens  16 ) is completed, the tool  603  is disengaged from the disc portion  221  and subsequently each set screw  242  is tightly fastened to fix the disc portion  221  to the circular recess  222 ′ of the housing  26 , which completes the adjusting operation. 
     FIG. 11  shows another embodiment of a device for rotating the cylindrical lens  16 . In this embodiment, similar to the previous embodiment shown in  FIG. 10 , the cylindrical lens  16  is positioned in place by inserting the same into the housing  26  from outside the housing  26 , and the operation of rotating the cylindrical lens  16  can be carried out from outside the housing  26 . The housing  26  is provided on a bottom surface thereof with a circular recess  222 ′. A circular through hole  225  through which the cylindrical lens  16  can be inserted in the housing  26  is formed at the center of the bottom of the circular recess  222 ′. The disc portion  221  of this embodiment is provided with two circumferential slots  241  for fixing the disc portion  221  to the housing  26  by two set screws  242  respectively inserted into the two circumferential slots  241 . The disc portion  221  is further provided on an outer peripheral surface thereof with a circumferential gear  701 . The disc portion  221  is rotatably fitted in the circular recess  222 ′ with the cylindrical lens  16  being inserted into the housing  26  through the through hole  225 . The housing  26  is provided with a small circular recess  226  which is connected with the circular recess  222 ′. In this embodiment a tool  703  is used to rotate the cylindrical lens  16 . The tool  703  is provided at the tip thereof with a pinion gear  702  which can be fitted in the small circular recess  226 . The pinion gear  702  meshes with the circumferential gear  701  of the disc portion  221  when the pinion gear  702  is fitted in the small circular recess  226 . Each set screw  242  needs to be loosened in advance when the disc portion  221  is rotated by the tool  703 . The circumferential gear  701 , the tool  703  and the small circular recess  226  together constitute a device  70  for rotating the cylindrical lens  16 . 
   The pinion gear  702  is engaged with the circumferential gear  701  by inserting the pinion gear  702  into the small circular recess  226  when the cylindrical lens  16  needs to be rotated. In a state where the pinion gear  702  is engaged with the circumferential gear  701 , rotating the tool  703  causes the disc portion  221  (the cylindrical lens  16 ) to rotate clockwise or counterclockwise in a direction shown by an arrow in  FIG. 11 . Hence, with the use of the device  70 , the incident position of the laser beam L 3  on the laser-beam detector  18  can be finely and easily adjusted even from outside the housing  26  of the scanner. After the adjusting operation (i.e., rotation of the cylindrical lens  16 ) is completed, the tool  703  is taken out of the small circular recess  226  of the housing  26  and subsequently each set screw  242  is tightly fastened to fix the disc portion  221  to the circular recess  222 ′ of the housing  26 , which completes the adjusting operation. 
   In each of the aforementioned embodiments, although the cylindrical lens  16  as an optical member is fixed to the disc portion  221 , the cylindrical lens  16  can be replaced by a plane-parallel plate to attain a similar effect.  FIGS. 3A and 3B  show a sectional portion of the cylindrical lens  16 ; the sectional portion of the cylindrical lens  16  does not have any power in scanning (beam shifting) direction (right to left in  FIGS. 3A and 3B ) with respect to the laser-beam detector  18 . In view of this aspect, if this sectional portion is replaced by an equivalent plane-parallel plate that does not have any power in the scanning direction, a similar beam-shifting effect as shown in  FIGS. 3A and 3B  is carried out by rotating the plane-parallel plate. However, a cylindrical lens  16  is used in the above-described embodiment as the cylindrical lens facilitates collection of the laser beam L 3  onto the laser-beam detector  18 . 
   Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.