Optical beam scanning unit with slit for producing horizontal synchronizing signal

A laser generation unit 20 emits a laser beam. A polygon mirror 27 deflects the laser beam to irradiate an image forming surface in horizontal scans. A beam detection mirror 25 reflects the laser beam at a start of each horizontal scan. A slit body 36 has a slit 44 oriented so the laser beam passes therethrough after being reflected off the mirror. A sensor 26 is disposed so that the laser beam falls incident thereon after passing through the slit 44. Light-obstructing walls 35a and 35b of the slit body 36 are slanted an appropriate angle with regards to an optical axis of the laser beam from the beam detection mirror 25. When reflected at the surfaces of the light-obstructing walls, the laser beam will not directly return to the beam detection mirror 25 and then to the polygon mirror 27. Any ghost images will not be formed on the image forming surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical beam scanning device and more 
particularly to the configuration of a slit mechanism for receiving a 
scanning optical beam and for producing a horizontal synchronizing signal. 
2. Description of the Related Art 
Japanese Patent Application Publication Kokai No. HEI-5-19194 describes an 
optical beam scanning unit (optical beam scanning device) for use in an 
electrophotographic image forming apparatus such as a laser printer, a 
copier, or a facsimile machine. The optical beam scanning unit includes: a 
laser unit for emitting a laser beam; a rotatable polygon mirror such as a 
hexagonal mirror for deflecting the laser beam; and a lens system for 
focusing the laser beam into a beam spot onto a subject medium thereby 
scanning the beam spot over the subject medium. These components are all 
encased in a box-shaped housing case. 
The laser unit emits a laser beam modulated according to image data. The 
laser beam irradiates and reflects off the rotating polygon mirror. The 
reflected laser beam passes through the lens system and passes through a 
long and narrow window formed in one side of the housing case and 
converges on the surface of the medium to be scanned. The incident laser 
beam scanningly irradiates the medium in one scanning line by one surface 
of the polygon mirror. 
A write start beam detecting unit is provided to the optical beam scanning 
unit for detecting a write start timing at which writing with the laser 
beam of one scanning line starts on the subject medium. The write start 
beam detecting unit includes: a write start beam detection mirror; a slit 
body; an optical fiber; and a photosenser. While being deflected by one 
surface of the polygon mirror, the laser beam first reaches the write 
start beam detection mirror and then starts forming a scanning line on the 
subject medium. When reaching the write start beam detection mirror, the 
laser beam reflects off the write start beam detection mirror and then 
passes through a thin slit formed in the slit body to fall incident on the 
optical fiber. The laser beam is guided by the optical fiber to the 
photosensor which is positioned at the opposite end of the optical fiber. 
Upon receiving the laser beam, the photosensor produces a write start beam 
detection signal. The laser unit will be modulated by the image data in 
synchronization with the write start beam detection signal. Thus, the 
write start beam detection signal is used as a horizontal synchronization 
signal. 
The slit body is constructed from a pair of light-obstructing walls that 
form the left and right sides of the slit. The light-obstructing walls 
have planer surfaces orthogonal to the optical axis of the laser beam 
reflected from the write start beam detection mirror. Light directed 
toward and incident on the slit body partly reflects off the planer 
surfaces of the light-obstructing walls back to the polygon mirror via the 
write start beam detection mirror. After reflecting off of and being 
deflected by the polygon mirror, the laser beam will travel through the 
lens system to irradiate the surface of the medium to be scanned. Because 
this light converges on the surface of the medium, undesired ghost images 
such as black lines are formed in the image forming region of the subject 
medium. 
To prevent interference caused when light reflected off of the write start 
beam detection mirror toward the photosensor intersects in the housing 
case with scanning light for forming an image on the surface of the 
subject medium, or when the scanning light reflects off the slit body or 
the photosensor, the housing case must be made large to secure space for 
the write start beam detection mirror and the photosensor. 
In order to adjust the write start timing, the slit body must be moved in a 
direction normal to the optical axis of the laser beam reflected from the 
write start beam detection mirror, i.e., in a direction parallel to the 
light-obstructing wall surface. However, thus moving the slit body in a 
direction normal to the optical axis of laser light requires a large 
housing case with space enough for such movement. A large housing case, 
however, is undesirable. 
In order to move the slit body in the direction orthogonal to the optical 
axis of laser light from the write start beam detection mirror, a 
contrivance such as forming a guide groove parallel to the direction of 
movement is necessary. Even when such a contrivance is provided, minute 
adjustments are still difficult. 
SUMMARY OF THE INVENTION 
It is an objective of the present invention to provide a compact optical 
beam scanning device capable of forming images on a subject medium without 
generation of ghost images and wherein adjustment of the write start 
position is simple. 
In order to achieve the above objective and other objectives, the present 
invention provides an optical beam scanning device comprising: a light 
source emitting a laser beam; a deflection unit deflecting the laser beam 
to irradiate an image forming surface in horizontal scans; a mirror 
reflecting the laser beam at a start of each horizontal scan; a slit body 
having a light-obstructing wall for defining a slit oriented so the laser 
beam passes therethrough after being reflected off the mirror, the 
light-obstructing wall being slanted with regards to an optical axis of 
the laser beam reflected from the mirror; and a sensor disposed so that 
the laser beam falls incident thereon after passing through the slit, the 
sensor producing a horizontal scan start timing signal upon receiving the 
laser beam. 
When the deflection unit, the slit body, and the sensor are encased in a 
housing, a partition wall may be provided for separating an optical path 
from the deflection unit to the mirror and an optical path from the mirror 
to the slit body. 
It is preferable that the slit body may be rotatably mounted for adjusting 
the position of the slit in a direction perpendicular to the optical axis 
of the laser beam. The slit body may be rotatably mounted with its center 
of rotation for selectively cause the laser beam travelling along its 
optical axis adjacent to the center of rotation to fall incident on the 
slit. 
When the light-obstructing wall extends toward an inner surface of an 
encasing wall of the housing case, a protrusion may be provided to the 
inner surface of the encasing wall so as to obstruct light passing between 
the light-obstructing wall and the encasing wall. 
According to another aspect, the present invention provides an optical beam 
scanning device comprising: a light source emitting a laser beam; a 
deflection unit deflecting the laser beam to irradiate an image forming 
surface in horizontal scans; a mirror reflecting the laser beam at a start 
of each horizontal scan; a slit body having a light-obstructing wall for 
defining a slit oriented so the laser beam passes therethrough after being 
reflected off the mirror, the slit body being rotatably mounted for 
adjusting the position of the slit in a direction perpendicular to the 
optical axis of the laser beam from the mirror; and a sensor disposed so 
that the laser beam falls incident thereon after passing through the slit, 
the sensor producing a horizontal scan start timing signal upon receiving 
the laser beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An optical beam scanning device according to a preferred embodiment of the 
present invention will be described while referring to the accompanying 
drawings wherein like parts and components are designated by the same 
reference numerals to avoid duplicating description. 
First, a laser beam printer 1 employing an optical beam scanning device 
according to the present embodiment will be described while referring to 
FIGS. 1 through 3. The printer 1 has a detachable sheet supply cassette 3 
mounted at the upper side of the case 2. Sheets P serving as a recording 
medium are stacked in the sheet supply cassette 3. A sheet feed roller 4 
and a separation pad 5 are provided for separating one sheet P at a time 
from the stack in the sheet supply cassette 3. A pair of rollers 6 takes 
the separated sheet P and supplies it to an image forming unit 9. 
The image forming unit 9 is housed in the case as a single unit and 
includes a rotatable photosensitive drum 7 serving as a photosensitive 
body; a transfer roller 8 serving as a transfer means; a developing unit 
12 having a developing roller 11 and a toner cartridge 10 positioned near 
the sheet supply cassette 3; a charge unit 13 disposed below the 
photosensitive drum 7; and a cleaning unit 14 disposed to the side of the 
photosensitive drum 7. 
A fixing unit 17 having a pressure roller 15 and a thermal roller 16 is 
disposed at the sheet-discharge side of the image forming unit 9. 
A scanning unit 18 is disposed below the image forming unit 9. The scanning 
unit 18 is an optical beam scanning device of the present embodiment for 
scanning the undersurface of the photosensitive drum 7 with laser light in 
a line parallel to the axis of the photosensitive drum 17 (i.e., in a 
horizontal scanning line). A housing case 30 of the scanning unit 18 is 
formed into a box shape from plastic reinforced with glass fibers. The 
housing case 30 has an upper panel 30a. A downward facing opening is 
formed in the housing case 30. The housing case 30 is mounted on the upper 
surface of a panel portion 2b of a main frame 2a in the case 2 by resting 
on a seal material 42 such as sponge rubber. Reinforcing plates 40 and 41 
detachably fix the upper panel 30a of the housing case 30 to the left and 
right sides of the main frame 2a. 
In a manner to be described later, the scanning unit 18 serves to emit a 
laser beam that converges on the surface of the photosensitive drum 7. 
Because a uniform charge has been formed on the surface of the 
photosensitive drum 7 by the charge unit 13, the laser beam, which has 
been modulated according to image data transmitted from an external 
device, such as an external control device which is not shown in the 
drawings, forms a latent static image on the surface of the photosensitive 
drum 7. The latent static image is developed into a visible toner image by 
charged toner particles supplied by rotation of the developing roller 11. 
Afterward, the toner image is transferred onto the recording sheet P 
supplied to the transfer position between the photosensitive drum 7 and 
the transfer roller 8. Next, the sheet P with the toner image formed 
thereon is transported between the heat roller 16 and the pressure roller 
15 of the fixing unit 17. Pressure and heat applied to the toner image 
fixes the toner image to the sheet P. The sheet P is then discharged onto 
a discharge tray 63 by a pair of discharge rollers 62. 
A control board 60 and a high-voltage board 61 are disposed beneath the 
main frame 2a. The high-voltage board 61 is for applying high voltage to 
the transfer roller 8 and the charge unit 13. 
Next, the scanning unit 18 will be described while referring to FIG. 2 
through 8. The scanning unit 18 includes: a laser generation unit 20; a 
rotatable polygon mirror 27; a f.theta. lens 21; a cylindrical lens 22; a 
reflect-back mirror 23; a write start beam detection mirror 25; a slit 
body 35; a condenser lens 36; and a write start beam detection sensor 26 
such as a photosensor. These components are all mounted on the 
undersurface of the upper panel 30a in the housing case 30. 
The laser generation unit 20 serving as a light source is constructed from 
a semiconductor laser and a collimator lens, neither of which are shown in 
the drawings, housed in a block 28. The laser generation unit 20 is 
positioned at one corner of the housing case 30 far from the 
photosensitive drum 7 and fixed to the lower surface of the upper panel 
30a by screws. A printed circuit board 29 is attached to the back surface 
of the block 28. The printed circuit board 29 is connected to the external 
control device for receiving the image data from the external control 
device. 
A drive motor 31 for rotating the polygon mirror 27 in the direction 
indicated by the arrow A in FIG. 4 is fixed to the interior surface of the 
upper panel 30a. The laser generation unit 20, the polygon mirror 27, and 
the photosensitive drum 7 are oriented so that the laser beam falling 
incident on the polygon mirror 27 from the laser generation unit 20 has an 
optical axis substantially parallel to the axis of rotation of the 
photosensitive drum 7. The f.theta. lens 21 and the cylindrical lens 22 
are disposed in the optical path of the laser beam L deflected by the 
mirror surfaces of the polygon mirror 27. The f.theta. lens 21 is fixed at 
either ends to the upper panel 30a by fasteners 32a and 32b made from 
plate springs. The cylindrical lens 22 is also fixed at either ends to the 
upper panel 30a by fasteners 33a and 33b also made from plate springs. The 
reflect-back mirror 23 is disposed to reflect the laser beam L after 
exiting from the cylindrical lens 22 upwardly toward the lower surface of 
the photosensitive drum 7. A slit-shaped opening 34 is formed in the upper 
panel 30a of the housing case 30 running parallel to the axis of rotation 
of the photosensitive drum 7 at a position in close proximity to the lower 
surface of the photosensitive drum 7. A cover plate 24 formed from a 
light-transmitting material such as glass covers the slit-shaped opening 
34. The laser beam L deflected by the polygon mirror 27 passes through the 
cover plate 24 and scans, following the axial direction of the 
photosensitive drum 7, across the lower surface of the photosensitive drum 
7. Thus, the laser beam L performs horizontal scans on the photosensitive 
drum 7. 
The f.theta. lens 21 functions to converge an incident laser beam into a 
spot on the surface of the photosensitive drum 7 at a position away from 
the optical axis of the f.theta. lens 21 a distance that is proportional 
to the deflection angle .theta. defined between the optical axis of the 
f.theta. lens 21 and the laser beam reflected off of the polygon mirror 
27. 
The beam detector mirror 25 is positioned proximate to one end in the 
lengthwise direction of the reflect-back mirror 23. As the polygon mirror 
27 rotates in the direction A of FIG. 3, each mirror surface of the 
polygon mirror 27 deflects the laser beam first toward the beam detector 
mirror 25 and then toward the reflect-back mirror 23. Accordingly, the 
laser beam falls incident on the beam detection mirror 25 immediately 
before starting each scan on the photosensitive drum 7. 
The beam detection mirror 25 is fixed in that position by plate springs 
(not shown in the drawings). The beam detector mirror 25 is for reflecting 
off the incident laser beam toward the slit body 35. As shown in FIG. 3, 
the beam detection mirror 25, the slit body 35, the condenser lens 36 and 
the beam detection sensor 26 are arranged substantially in line. The 
mirror 25, the slit body 35, and the condenser lens 36 are positioned 
substantially on the optical axis of the condenser lens 36. With this 
positioning, when the laser beam passes through the slit body 35, the 
laser beam travels through the condenser lens 36 and falls incident on the 
beam detector sensor 26. 
A partition wall 43 is provided for separating the optical path from the 
polygon mirror 27 to the beam detection mirror 25 and the optical path of 
light reflected from the beam detector mirror 25, transmitted through the 
slit body 35 and the condenser lens 36, and incident on the beam detector 
sensor 26. The partition wall 43 ensures that only the laser beam L 
reflected off of the beam detector mirror 25 is directed toward the slit 
body 35. The partition wall 43 prevents the laser beam L deflected by the 
polygon mirror 27 from being scattered by the interior surface of the 
encasing wall 30b to fall incident on the slit body 35, the condenser lens 
36, or the beam detector sensor 26. In the present embodiment as depicted 
in the drawings, the partition wall 43 extends substantially parallel to 
the encasing wall 30b at one side of the housing case 30 (i.e., the right 
side of the housing case 30 in FIGS. 3 and 4). 
As shown in FIG. 5, the slit body 35 is formed from a composite resin 
material, for example, and is mounted to the inter surface of the upper 
panel 30a of the housing case 30. The slit body 35 includes a base 35c 
following the inner surface of the upper panel 30a. A boss 35d formed on 
the base 34c is fitted in a mounting depression 45 formed at the inner 
surface of the upper panel 30a. A light-obstructing wall of substantially 
L-shaped cross-section is provided to the base 35c. The light-obstructing 
wall extends substantially perpendicularly to the base 35c. The 
light-obstructing wall is constructed from a right and left side walls 35a 
and 35b, between which a narrow slit 44 is formed to extend also 
substantially perpendicularly to the base 35c. 
The mounting depression 45 is provided to the inter surface of the upper 
panel 30a, at a position substantially on an optical axis of the condenser 
lens 36 that extends from the condenser lens 36 to the beam detection 
mirror 25. The slit body 35 is oriented so that the slit 44 is also 
positioned substantially on the optical axis of the condenser lens 36. 
With this positioning, the slit body 35 has the slit 44 at a position away 
from the cylindrical lens 22 by a distance equal to the distance between 
the cylindrical lens 22 and the surface of the photosensitive drum 7. 
Thus, the laser beam L from the beam detection mirror 25 is properly 
focused on the slit 44, whereby the laser beam is efficiently introduced 
to the condenser lens 36. 
The photosensor 26 is positioned on the back side of the condenser lens 36. 
Laser light collected by the condenser lens 36 is introduced to the 
photosensor 26. The photosensor 26 produces a write start beam detection 
signal, upon receiving laser light of a predetermined amount or more. 
With the above-described structure, the laser beam L emitted from the laser 
generation unit 20 reflects off the polygon mirror 27 and passes through 
the f.theta. lens 21 and the cylindrical lens 22. When the laser beam L is 
deflected by one surface of the polygon mirror 27 which is rotating in the 
direction A of FIG. 4, the laser beam first falls incident on the beam 
detector mirror 25 and then on the reflection-back mirror 23. While being 
incident on the beam detector mirror 25, the laser beam L reflected by the 
beam detection mirror 25 scans in the direction B of FIG. 5 from near the 
encasing wall 30b toward the partition wall 43 by a small angle .theta.1. 
When the laser beam L enters the boundaries of the slit 44 and falls 
incident on the slit 44 in focus, the laser beam L is efficiently 
introduced to the condenser lens 36. The condenser lens 36 collectively 
introduces the laser light to the photosensor 26. Because at this 
instance, laser light of an energy equal to or greater than the 
predetermined level falls incident on the beam detector sensor 26, the 
beam detector sensor 26 produces a write start beam detection signal. 
According to the present invention, the orientation of the slit body 35 
allows the surfaces of the left and right light-obstructing walls 35a and 
35b to be at a slant to the optical axis of the laser beam L that scans by 
the angle .theta.1 in the direction B in FIG. 5. In other words, the walls 
35a and 35b are oriented so that any lines extending perpendicularly from 
the walls 35a and 35b are not directed toward the mirror 25 but are 
directed toward the walls 43 and 30b, respectively. In addition, the walls 
35a and 35b are oriented so that the edge surfaces of the walls forming 
the slit 44 are not oriented to reflect a too large amount of the laser 
beam L. 
Though the orientation of the wall 35a is determined dependently on the 
shape of the separation wall 43, a line extending perpendicularly from the 
wall 35a is preferably shifted from a line extending from the wall 35a to 
the mirror 25 by an angle falling in a range of 5.degree. and 60.degree. . 
More preferably, the angle should fall in a range of 12.2.degree. and 
34.4.degree.. 
According to this structure of the slit body 35, the laser beam L reflected 
off of the surfaces of the walls 35a and 35b will not directly return to 
the beam detector mirror 25 and so will not be reflected off by the 
polygon mirror 27 again. Therefore, ghost images will not be formed on the 
image forming region at the surface of the photosensitive drum 7. 
Even if the laser beam L returns from the walls 35a and 35b to the beam 
detector mirror 25, it will be after repeatedly scattered at the inner 
walls of the partition wall 43 and the housing case 30. This scattering 
attenuates the light energy about to 50% or less of its original energy. 
This residual energy is insufficient to form a latent image on the surface 
of the photosensitive drum 7 so that ghost images will not be produced so 
that good quality images can be obtained. 
By thus forming the slit 44 in the light-obstructing walls 35a and 35b 
slanted at the appropriate angle to the optical axis of the incident laser 
beam L, the incident light will partly hit the edge corner portion of the 
slit 44 so that the rising edge of the beam detection signal will become 
sharper. A detection signal can be more accurately and stably obtained. 
When mounting the slit body 35 onto the upper panel 30a, the boss 35d is 
first inserted into the mounting depression 45. Then, the orientation of 
the slit body 35 is adjusted by rotating the slit body 35 while pressing a 
jig against the base 35c. The slit body 35 is rotated about the boss 45 
within a predetermined angle that the distance of the slit 44 from the 
cylindrical lens 22 falls in a so-called focal depth range that the laser 
beam L is focused on the slit 44. The slit 44 can therefore be positioned 
to receive a laser beam that travels along its optical axis that extends 
on or adjacent to the approximate center of the boss 35d. In other words, 
the rotational adjustment allows the slit 44 to be positioned to receive a 
laser beam travelling along its optical axis that is substantially aligned 
with, or displaced by a slight distance, but still in approximately 
parallel with the optical axis of the condenser lens 36. 
Thus, this rotation of the slit body 35 adjusts the slit 44 in a direction 
substantially orthogonal to the optical axis of the laser beam L, thereby 
adjusting the timing at which the laser beam L falls incident on the 
photosensor 26 and the photosensor 26 produces the detection signal. The 
above-described rotating adjustment can move the slit 44 across a broad 
range in the direction orthogonal to the optical axis of the incident 
laser beam, through rotating the slit body 35 by a small rotation angle. A 
small space is sufficient for rotating the slit body 35, in comparison 
with moving the slit body 35 entirely in the direction orthogonal to the 
optical axis of the incident laser beam. Because thus rotating the slit 
body 35 by the small rotation angle can prevent the slit 44 from shifting 
from the focal depth region, it is possible to efficiently introduce the 
laser beam L through the slit 44 to the condenser lens 36. 
After the slit body 35 is thus properly oriented, the boss 35d or the base 
35c of the slit body 35 can be fixed to the inner surface of the upper 
panel 30a using adhesive. 
In the drawings, the slit body 35 is oriented so that the slit 44 is 
positioned upstream from the boss 35d, in terms of the incident laser beam 
L. However, the slit body 35 can be oriented so that the slit 44 is 
positioned downstream from the boss 35d, as long as the slit 44 is 
positioned in the focal depth range of the cylindrical lens 22. 
According to the present embodiment, as shown in FIG. 5, a protrusion 46 is 
provided to the inner surface of the encasing wall 30b near the 
light-obstructing wall 35b of the slit body 35. The protrusion 46 extends 
in a direction parallel to the direction in which the slit 44 extends. The 
protrusion 46 will scatter the laser beam L from the beam detector mirror 
25 that approaches the inner wall of the encasing wall 30b and that passes 
through by the slit body 35 unobstructed by the light-obstructing wall 
35b. Therefore, light of the laser beam L that passes by the slit body 35 
in this manner will not fall directly incident on the condenser lens 36 
and so will not be mistakenly detected as a write start beam detection 
signal. 
In a modification shown in FIG. 9, the light receiving surface of the beam 
detector sensor 26 is oriented at a slant of an appropriate angle to the 
optical axis of the laser beam L. In this way, light reflected off the 
light reception surface of the beam detector sensor 26 can be prevented 
from returning directly to the beam detector mirror 25. 
When the sensor 26 produces an electric beam detection signal as described 
above, the laser drive board 37 receives and processes the electric signal 
as a horizontal synchronization signal (horizontal scan start timing 
signal), and outputs the electric signal to the external control device 
(not shown). The external control device controls the laser drive unit 20 
in accordance with the horizontal synchronizing signal via the printed 
circuit board 29. 
The laser drive board 37 is fixed by a screw 38 in abutment contact with 
the edge of a rib protruding downward from the inner surface of the upper 
panel 30a. As shown in FIGS. 3, 7, and 8, a pair of attachment grooves 47a 
and 47b are formed to the interior of the upper panel 30a and the 
partition wall 43 respectively substantially in a U-shape when viewed from 
below. The condenser lens 36 is mounted tightly to attachment grooves 47a 
and 47b and prevented from falling off the laser drive board 37. Legs or 
other portions of the beam detection sensor 26 can be fixed directly to 
the laser drive board 37 by soldering or other method. This makes 
attaching the beam detector sensor 26 easier than if the beam detector 
sensor 26 were mounted independently. 
Alternatively, the beam detector sensor 26 or the laser generation unit 20 
could be fixed to the outer side of one of the encasing side walls of the 
housing case 30. Also, the present invention could be applied to optical 
devices used in image forming processes employed in facsimile machines, 
copiers, and other electrophotographic image forming devices. 
As described above, in the optical beam scanning device of the present 
invention, the light source emits a laser beam. The deflection unit 
deflects the laser beam to irradiate the image forming surface in 
horizontal scans. The beam detection mirror reflects the laser beam at a 
start of each horizontal scan. The slit body has a slit oriented so the 
laser beam passes therethrough after being reflected off the mirror. The 
sensor is disposed so that the laser beam falls incident thereon after 
passing through the slit. According to the present invention, a 
light-obstructing wall of the slit body is slanted an appropriate angle 
with regards to an optical axis of the laser beam from the beam detection 
mirror. When reflected at the surface of the light-obstructing wall, the 
laser beam will not directly return to the beam detection mirror and then 
to the deflection unit. Accordingly, any ghost images will not be formed 
on the image forming surface. A high quality image will be formed on the 
image forming surface. 
The deflection unit, the slit body, and the sensor are entirely mounted in 
the housing case. According to the present invention, the partition wall 
is provided for separating the optical path from the deflection unit to 
the beam detection mirror and the optical path from the beam detection 
mirror to the slit body. Therefore, even if the optical path of the laser 
beam emitted toward the image forming surface in the vicinity of the beam 
detection mirror is substantially parallel to the optical path of the 
laser beam emitted toward the beam detection mirror, the laser beam 
emitted to fall incident on the image forming surface is blocked by the 
partition wall so that the write start timing is stably detected even if 
the laser beam is scattered by the inner surface of the housing case. 
Providing the partition wall allows positioning the beam detection mirror 
and the sensor where necessary to that the optical path of light from the 
deflection unit to the image forming surface in the vicinity of the beam 
detection mirror can be set substantially parallel to the optical path 
from the mirror to the slit. This allows forming the housing case into a 
compact size. 
The slit body is rotatably mounted so that position of the slit can be 
adjusted in a direction perpendicular to the optical axis of the laser 
beam from the beam detection mirror. Therefore, adjustment operations 
become extremely easy because the position can be adjusted by merely 
rotating the slit body around its center of rotation. There is not need to 
secure a large space in the housing case, because the slit body is rotated 
around its axis (i.e., center of rotation), which is away from the slit 
position, and not by moving the entire slit body. 
In order to move the position of the slit in the slit body substantially 
normal to the optical axis of the incident laser beam, the slit body is 
rotated around its rotation axis within a range that allows only the 
optical beam that travels along an optical axis positioned adjacent to the 
rotational axis to fall incident on the slit. Accordingly, it is possible 
to reduce the amount with which the position of the slit is mistakenly 
shifted downstream or upstream in regards to the optical axis of the laser 
beam, thereby preventing light from being incident on the sensor out of 
focus. With this rotating adjustment, it is possible to position the slit 
body at a position that selectively allows the slit to receive the optical 
beam that travels along its optical axis that is substantially aligned 
with, or displaced by a slight distance, but still in parallel with a line 
connecting the slit and the center of rotation, i.e., the approximate 
center of the boss. 
The protrusion is provided to the inner surface of the encasing wall so as 
to obstruct light which has passed between the light-obstructing wall of 
the slit body and the encasing wall. Therefore, even when the laser beam 
passes through between the slit body and the encasing wall, the laser beam 
is prevented by the protrusion from falling incident on the sensor. The 
sensor will not erroneously produce the detection signal at a timing 
different from the desired write start timing. 
While the invention has been described in detail with reference to the 
specific embodiment thereof, it would be apparent to those skilled in the 
art that various changes and modifications may be made therein without 
departing from the spirit of the invention, the scope of which is defined 
by the attached claims.