Optical scanning apparatus and recording apparatus using the same

There is disclosed an optical scanning system in which axis alignment in a subscanning direction and adjustment of a spot is easily performed, the spot diameter is variable and a change in efficiency of an optical path due to these changes is small. The optical scanning system comprises two flat plate glasses which are provided between a cylindrical lens for converging an optical beam in a subscanning direction and a polygon mirror and are rotatable around an axis perpendicular to the subscanning direction and may be fixed after an adjustment. Axis alignment in a subscanning direction is obtainable by adjustment of the angle of one flat plate glass, and change in the spot diameter is obtainable by rotating two flat plate glasses in opposite directions while change of the axis in the subscanning direction and change in efficiency of the optical path are kept small.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical scanning apparatus for scanning 
a surface with a deflected optical beam, such as a laser light, and to a 
recording apparatus using the optical scanning apparatus. 
2. Description of the Related Arts 
Optical scanners for scanning a surface in two dimensional directions with 
a spot of light formed on the surface from a repetitively deflected 
optical beam such as a laser have been widely used in printers such as 
laser printers. 
A polygon mirror or a galvanomirror is usually used as deflecting means of 
the optical scanner. In the deflecting means, an error of the position of 
a scanning line readily occurs due to a leaning error of the reflecting 
surface. In order to correct such an error, a cylindrical lens for 
converging an incident beam once in a subscanning direction to focus in 
the vicinity of the reflecting surface of the deflecting means is provided 
to give a conjugate relationship between the reflecting surface and the 
scanned surface (i.e. a recording surface). 
The optical system of such an optical scanning apparatus should be capable 
of moving the cylindrical lens in an optical axis direction in order to 
absorb manufacturing errors of the lenses and the housing so as to 
converge the optical beam to form the spot precisely on the scanned 
surface, to absorb the variations in astigmatism in case where a 
semiconductor laser is used, or to adjust the spot diameter in the 
subscanning direction. 
An approach to move the cylindrical lens in the subscanning direction has 
been known for adjusting the axes of the cylindrical lens and the optical 
system in the subscanning direction. 
It has been demanded that the recording apparatus using the optical 
scanning apparatus has a capability of changing the recording density at 
need. It is necessary that the effective diameter of a spot of the optical 
beam formed on a recording surface can be changed to an optimum value for 
video recording in order to maintain the best quality of the recorded 
video image without spaces between recording spots when the recording 
density is changed. It is possible to adjust the recording spot size in 
the main scanning direction by changing the modulation (switching) time of 
the optical beam. Accordingly, it is common that the optical system is 
arranged so that only the diameter of the spot in the subscanning 
direction is variable. An approach of changing the spot diameter in the 
subscanning direction may be an approach of changing power of the 
cylindrical lens, an approach of adjusting an opening diameter of an 
aperture to change the spot diameter of the optical beam incident upon the 
cylindrical lens, and an approach of intentionally defocusing the spot in 
the subscanning direction. An arrangement for adjusting a cylindrical lens 
as the third approach is disclosed in JP-A-57-144517. 
Such an optical system is a so-called toric lens system in which the powers 
in the main scanning direction and in the subscanning direction are 
different from each other. Therefore, the axes of a plurality of lenses 
should be aligned with a high precision. It is generally necessary to 
carry out rotation of the lenses around the optical axis with a high 
precision. Particularly, since the cylindrical lenses are small in size 
and a space for mounting the lenses is narrow, the precision required for 
mounting them is considerably high. In other words, an extremely fine 
alignment around the optical axis is required for the cylindrical lenses. 
Therefore, an adjusting and holding mechanism with a high precision is 
needed for adjusting the cylindrical lenses in the optical axis to change 
the spot diameter or for adjusting the cylindrical lenses to align the 
axes in the scanning directions. 
The approach to change power of cylindrical lenses requires lens exchange 
or cylindrical lenses as zoom lenses which the zooming in one direction. 
The former approach to exchange the lenses requires readjustment on 
exchange and the latter requires an expensive zoom lens mechanism. 
Although the approach to adjust the diameter of the incident optical beam 
has an advantage that the geometrical shape of the image is not changed, 
the approach has a defect that the efficiency of the optical path changes 
according to change in diameter of the beam. In case a semiconductor laser 
is used as the light source, there is a problem that the amount of change 
in the efficiency of the optical path or in the spot diameter is not 
constant due to variations of the emission angle of the semiconductor 
laser. 
Takaoka et al. disclose in JP-A-52-84748 a method of finely adjusting 
position of convergence of a laser light by controlling an effective 
thickness of flat plate glasses combined with wedge type prisms. This 
method cannot adjust the optical axis. 
Caviglia et al. and in U.S. Pat. No. 4,900,120 disclose a coupling device 
for adjusting direction of an optical axis of a collimated optical beam by 
rotation of flat plate glasses but they do not consider the adjustment of 
the focal position. 
Morimoto et al. in U.S. Pat. No. 4,850,686 disclose a method of adjusting 
the axis of light rays by the combination of prisms rather than flat plate 
glasses. The spot size cannot be adjusted by Morimoto method. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an optical 
scanning apparatus having a simple structure including an optical system 
in which axis alignment in the subscanning direction and spot adjustment 
are easy and in which the spot diameter is variable and a change in 
efficiency of the optical path due to these changes is considerably low. 
The optical scanning apparatus of the present invention having a 
cylindrical lens and deflecting means is characterized in that a plurality 
of light transmitting flat plates are disposed between the cylindrical 
lens and the deflecting means and the flat plates are supported so that 
the inclination of the flat plates relative to an optical beam is 
changeable in a subscanning direction in which the cylindrical lens has a 
power. 
The flat plates in the optical scanning apparatus of the present invention 
will give no influence to a collimated optical beam emitted from a light 
source in the main scanning direction in which the cylindrical lens has no 
power. 
The system functions in the subscanning direction as follows: a main light 
ray shifts in parallel by a distance .delta.1 by a first flat plate which 
is inclined by an angle .theta.1 relative to a perpendicular plane to the 
optical axis and then shifts by a distance .delta.2 in the opposite 
direction by a second flat plate which is inclined by an angle .theta.2 to 
the perpendicular plane in a direction opposite to the first flat plate. 
Accordingly, rotating either of the flat plates or rotating both of the 
flat plates by .theta.1 and .theta.2 in opposite direction provides fine 
adjustment or alignment of the optical axis. In case where no alignment of 
axes is necessary, rotation of both flat plates by equal angles in 
opposite directions gives the optical axis no shift during an adjustment 
which will be described hereinafter. 
The parallel movement of an incident light ray when it transmits through a 
flat plate glass increases as its incident angle increases. Since the 
optical beam is a converged light having distribution of incident angles, 
the imaged spot has a distribution and focuses far away by a distance 
.delta.z in the axial direction by the effect of the flat plate glasses. 
The distance .delta.z increases as the inclinations .theta.1 and .theta.2 
of the flat plates increase. This causes the imaging point of a scanning 
lens to move backward by .delta.z'. The position of the cylindrical lens 
is preliminarily set so that the imaging point is slightly frontward from 
the former position. Thus, the flat plates may be inclined by .theta.1 and 
.theta.2 to perform focussing in the subscanning direction. If the amount 
of movement of the imaging point .delta.z' is increased by further 
inclinations .theta.1 and .theta.2, the spot diameter in the subscanning 
direction is resultantly enlarged. The shift of the light beam changes in 
a substantially linear relation to the inclination of the flat plate while 
the inclination is small. Accordingly, if adjustment of the spot and 
change of the spot diameter are performed by inclining both flat plates 
after the axis alignment has been achieved by inclining either one of the 
flat plates, the shift of the optical axis in the subscanning direction at 
the imaging point of the cylindrical lens is small. 
Adjustment and change of the spot diameter will be described with reference 
to FIGS. 1 and 2. In an optical system shown in FIG. 1, an optical beam 
which is a collimated laser light is converged in a subscanning direction 
(the vertical direction in parallel to the paper in FIG. 1) by a 
cylindrical lens 103 and is imaged with a distribution in a line in the 
vicinity of reflection surfaces of a polygon mirror (not shown). An 
imaging lens system comprising first and second lenses 131 and 132 
converges the optical beam reflected on the polygon mirror and incident 
upon a scanned surface so as to form a small spot on the surface and 
converts a constant angular velocity scanning achieved by the polygon 
mirror into a constant speed scanning. Parallel flat plate glasses 110a 
and 110b each having opposite sides applied with anti-reflection coating 
are disposed between the cylindrical lens 103 and the polygon mirror for 
shifting the optical axis while reducing the loss of the optical beam 
transmitting therethrough. 
In such an optical system, the flat plates in the optical scanning 
apparatus of the present invention will give no influence to a collimated 
optical beam emitted from a light source in the main scanning direction 
(the perpendicular direction to the paper in FIG. 1) in which the 
cylindrical lens has no power. 
The system functions in the subscanning direction as follows. A main light 
ray will now be considered. The main light ray is shifted in parallel by 
.delta.1 by the first flat plate 110a which is inclined by .theta.1 
relative to a perpendicular plane to the optical axis and then shifted by 
.delta.2 in the opposite direction by the second flat plate 110b which is 
inclined by .theta.2 in a direction opposite to the first flat plate 110a 
relative to the perpendicular plane. Accordingly, by rotating either of 
the flat plates 110a and 110b or rotating the flat plates by .theta.1 and 
.theta.2 in opposite direction, fine adjustment or alignment of the 
optical axis is possible. In case where no alignment of axes is necessary, 
rotation of both flat plates 110a and 110b by equal angles keeps the 
optical axis with no shift during an adjustment which will be described 
hereinafter. 
Since the optical beam is a converged light, the imaging point is moved 
backward by .delta.z due to the inclinations .theta.1 and .theta.2 of the 
flat plate glasses 110a and 110b. This causes the imaging point of a 
imaging lens to move backward by .delta.z'. The position of the 
cylindrical lens 103 is preliminarily set so that the imaging point is 
slightly frontward to the former position. Thus, the flat plates 110a and 
110b may be inclined by .theta.1 and .theta.2 to perform moving the 
imaging point in the subscanning direction. If the amount of movement of 
the imaging point .delta.z' is increased by further inclinations .theta.1 
and .theta.2, the spot diameter in the subscanning direction is 
resultantly enlarged. The relation between the shift of the light beam and 
the inclination of the flat plate is substantially linear while the 
inclination is small as shown in FIG. 2. Accordingly, if the adjustment of 
the spot and change of the spot diameter are performed by inclining both 
flat plates 110a and 110b by the same angles after the axis alignment has 
been achieved by inclining either one of the flat plates, the shift of the 
optical axis in the subscanning direction at the imaging point of the 
cylindrical lens is kept in small while the inclination is small.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described with reference to 
the attached drawings. 
FIGS. 3, 4 and 5 show a first embodiment of the present invention. FIG. 3 
shows a plan of an optical scanning apparatus, FIG. 4 shows a sectional 
view taken along the line IV--IV in FIG. 3, and FIG. 5 shows the detail of 
a section from a cylindrical lens to a polygon mirror. 
An optical beam which is a collimated laser light generated from a laser 
unit 200 is converged in a subscanning direction (a vertical direction in 
parallel with the paper plane in FIG. 4) by a cylindrical lens 103 and is 
imaged in a line in the vicinity of a reflection surface of a polygon 
mirror 101. The polygon mirror 101 is mounted on a rotary shaft of a 
scanner motor 104 and is rotated at a constant speed in a direction of an 
arrow A for deflecting the optical beam. An imaging lens system comprises 
first and second lenses 131 and 132 and converges the optical beam so that 
a small spot is formed on a surface 140 to be scanned and converts a 
constant angular speed scanning performed by the polygon mirror 101 into a 
constant speed scanning. Flat plate glasses 110a and 110b which are 
mounted on glass holding rods 106 and 107, respectively, are disposed 
between the cylindrical lens 103 and the polygon mirror 101. An 
anti-reflection coating is applied upon both sides of the flat plate 
glasses 110a and 110b. The respective components are mounted on a housing 
100 with a predetermined positional relation and necessary precision. 
A semiconductor laser 202 is mounted on a laser mounting plate 201 screwed 
on a lens holder 204 in the laser unit 200. A collimator 203 is held on 
and secured to the lens holder 204 after being adjusted so that an emitted 
light is collimated. The lens holder 204 is provided on the outer side 
thereof with a protection cover 205 which also serves as an electrical 
insulator. The cover 205 holds an aperture 206 with its opening coaxial 
with the collimator 203 so that the diameter of the optical beam which is 
incident upon the cylindrical lens 103 is maintained constant. The 
cylindrical lens 103 is mounted on a holder 102, which is in turn mounted 
on the housing 100. A drive circuit 210 is connected with the 
semiconductor laser 202. 
As illustrated in FIG. 5 in detail, the holder 102 which holds the 
cylindrical lens 103 includes a flange having a notch 102a on a part 
thereof and is rotatable around an optical path while the position of the 
holder 102 in a direction of the optical axis is held constant by fitting 
the flange into a recess 100a formed on a part of the housing 100. The 
flange is positively secured to the housing in a downward direction by a 
press plate 120 and set screws 121a and 121b therethrough. Adjustment of 
the rotational angle of the cylindrical lens 103 around the optical axis 
is carried out by applying a rotational force upon the notch 102a while 
the set screws 121a and 121b are loosely screwed. Although the rotational 
angle of the cylindrical lens 103 around the optical axis is adjusted 
according to the present embodiment, adjustment of the angle may be 
eliminated due to a careful design of lenses, mounting structure or 
overall optical system. 
A lever 111 is secured to one end of the glass holder 106. A lever 112 is 
secured to one end of the glass holder 107 via a fitting 113. The lever 
112 is rotatable around a pin 116 of the fitting 113 so that the linking 
angle between the lever 112 and the fitting 113 is adjustable within a 
range of an elongated hole 112a formed on the lever 112. The lever 112 and 
the fitting 113 are secured by a screw 115 after completion of adjustment. 
The levers 111 and 112 are provided with elongated holes 112b and are 
engaged with a pin 118b provided on an adjusting plate 118. The adjusting 
plate 118 is secured to the housing 100 by a screw 119 which passes 
through the elongated hole 118a so that the plate 118 can move in upward 
and downward directions. When the pin 118b is moved downward, the levers 
111 and 112 are pulled downward and the glass holders 106 and 107 are 
rotated around respective axes in opposite directions. 
Adjustment is performed in the present embodiment as follows: the flat 
plate glasses 110a and 110b are disposed so that they are perpendicular to 
the optical axis when the adjusting plate 118 is positioned in the 
uppermost position; alignment of axes in the subscanning direction is 
performed by rotating the glass holder 107 while the adjusting plate 118 
is in the uppermost position and the lever 112 is secured to the fitting 
113 by a screw 115; and then, adjustment of the spot diameter in the 
subscanning direction on the scanned surface 140 is performed by finely 
moving the adjusting plate 118 in a downward direction. This completes the 
adjustment to provide the minimum spot diameter. In case where the spot 
diameter is increased to change the recording density, an operator may 
unscrew the screw 119, move the adjusting plate 118 to a lower appropriate 
position and fasten the plate 118 again by the screw 119. 
In the present embodiment, axis alignment in the subscanning direction may 
be performed in an independent and easy manner. Adjustment of changing 
(increasing or decreasing) the spot in size can be performed only by a 
simple operation such as rotation of two flat plate glasses 110a and 110b. 
A change in transmission factor due to the change in an incident angle of 
the light beam upon flat plate glasses 110a and 110b can be suppressed to 
the minimum value. 
FIG. 6 shows another embodiment which is substantially identical with the 
first embodiment except a mechanism for achieving rotation of the flat 
plates glasses 110a and 110b. Description of the identical components is 
omitted herein. 
Operation for changing the spot diameter is performed by an electromagnetic 
solenoid in the present embodiment. A movable plate 302 is mounted on a 
plunger 301a of the electromagnetic solenoid 301 and is movable in an 
upward and a downward direction. The plunger 301a is extended by a spring 
310. A coupling plate 303 is used in lieu of the adjusting plate 118 in 
the above mentioned first embodiment. A pin 304 which is provided on the 
coupling plate 303 is engaged with elongated holes 112b of the levers 111 
and 112 and the coupling plate 303 is secured to the movable plate 302 by 
an adjusting screw 305 passing through an elongated hole 303a. 
Axis alignment in the subscanning direction in the present embodiment is 
identical with that of the above mentioned first embodiment. Adjustment of 
the spot diameter in the subscanning direction is performed by unscrewing 
the screw 305 to bring the screw into semisecured state for adjusting the 
relative position of the coupling plate 303 with respect to the movable 
plate 302. 
The spot diameter is changed by switching the drive current of the 
electromagnetic solenoid 301. When the electromagnetic solenoid 301 is 
deenergized, the plunger 301a is biased downward by a spring 310 to the 
lowermost position. When the solenoid 301 is energized, the plunger 301a 
is biased upward against the spring 310 to the uppermost position. This 
causes the lever 111 and the flat glass plate 110a to rotate 
counterclockwise and the lever 112 and the flat plate glass 110b to rotate 
clockwise so that the flat plate glasses 110a and 110b are inclined in 
directions opposite to those in the first embodiment. 
In the present embodiment using the electromagnetic solenoid 301, the spot 
diameter is selected between a large and a small diameter. It is possible 
to change the diameter in response to an electrical signal. A system for 
detecting the angle of the flat plate glass may be omitted by 
preliminarily setting a required moving range. The spot diameter can be 
easily changed while an optical scanning apparatus is incorporated in an 
instrument such as a printer. 
FIG. 7 shows a further embodiment in which a motor 351 rotates the flat 
plate glasses. Description of components common to those in the above 
mentioned embodiments will be omitted herein. Worm wheels 353 and 354 are 
secured to the ends of the shafts of the glass holders 106 and 107, 
respectively. A worm 352 mounted on a rotary shaft of the motor 351 is 
meshed with the worm wheels 353 and 354. A reference point for the 
absolute angle of the flat plate glass secured to the glass holder 106 is 
detected by the actuation of a limit switch 355 caused by an arm 358 
mounted on the worm wheel 353. The reference point is adjusted by 
unscrewing a screw 359 to adjust the fixing position in which the limit 
switch 355 is secured by a screw 359 passing through an elongated hole of 
a mounting plate. 
Securing of the worm wheel 354 to the shaft end of the glass holder 107 is 
carried out by a setscrew 357 after the relative angle of the flat plate 
glass 110a is adjusted to the flat plate glass 110b by adjusting the 
relative angle of the worm wheel 354 to the glass holder 107. The axial 
alignment in the subscanning direction is carried out in such a manner. 
Since it will suffice to operate the motor 351 to adjust the spot or to 
change the spot diameter in the present embodiment, the control will be 
obtained easier. Since the spot diameter can be continuously changed, the 
applicability is enhanced. 
FIG. 8 shows a further embodiment. In this embodiment, worm wheels 365 and 
366 are secured to the glass holders 106 and 107, respectively, and are 
meshed with worms 363 and 364 mounted on rotary shafts of motors 361 and 
362, respectively. Arms 370 and 380 secured to the worm wheels 365 and 366 
respectively act on photointerrupters 371 and 381 for detecting the 
inclination angles of the flat plate glasses 110a and 110b, respectively. 
The mounting plates 372 and 382 of the photointerrupter 371 and 381 are 
provided with elongated holes. The mounting plates 372 and 382 are mounted 
by setscrews 373 and 383, respectively. This makes it possible to set 
references of the inclination angles of the flat plate glasses 110a and 
110b and to adjust axis alignment. Although two motors are necessary in 
the present embodiment, the inclination angle of each of the first plate 
glasses 110a and 110b can be independently changed for adjusting or 
changing the spot diameter as mentioned above. 
Means for detecting the flat plate glasses 110a and 110b may comprise known 
electrical, magnetic or mechanical sensors other than those which have 
been described with reference to FIGS. 7 and 8. Use of stepping motors as 
the motors 351, 361 and 362 in the embodiments shown in FIGS. 7 and 8 
makes it possible to form a control apparatus comparatively easily. 
Although the above mentioned embodiments are explained to perform both of 
the rotation adjustment of the cylindrical lens around the optical axis 
and the axis alignment in the subscanning direction, they are similarly 
applicable to a structure in which either or both of the adjustment and 
the axis alignment is unnecessary. In case where the axis alignment in the 
subscanning direction is unnecessary, it suffices to omit the lever 112 
and fitting 113 but to use a component having a shape similar to that of 
the lever 111. This is the same in embodiments which will be described 
hereinafter. This is the same in cases where a component such as a mirror 
having a similar function is used in lieu of the cylindrical lens and 
where the configuration of the imaging optical system in the apparatus is 
changed. Although the embodiments in which the axis of the rotation of the 
flat plate glass is perpendicular to the subscanning direction have been 
described, it is apparent that similar effects are obtained irrespective 
of the direction of the axis of the rotation if the angular component may 
change in the subscanning direction. 
Though it suffices to secure the flat plate glasses to the glass holders, 
the flat plate portions, support portions and axial portions may be formed 
in a single body of optical plastics to further simplify the structure. 
Two flat plate glasses may be different therebetween in thickness or 
refractive index. In this case, an approximate effect can be obtained by 
changing the length of the lever to provide different rotational angles or 
by changing the rotation of the motors. 
An embodiment of a recording apparatus to which the optical scanning 
apparatus of the present invention is applied will be described. The 
recording apparatus is a laser printer in which the recording density can 
be switched to select one from two densities such as normal and high 
densities. Its block diagram is shown in FIG. 9. 
A photosensitive drum 552 is driven to rotate via a belt 551 by a main 
motor 550 and is exposed to images by the above mentioned optical scanning 
apparatus for performing video image recording. In the laser printer using 
the xerography, the photosensitive drum 552 is uniformly charged by a 
charging device and is exposed to an image to form an electrostatic latent 
image by the optical scanning apparatus. The electrostatic latent image is 
developed to provide a toner image, which is then transferred and fixed to 
a recording paper and the paper which bear the toner image is discharged. 
A semiconductor laser within the laser unit 200 is controlled by a laser 
control circuit 210 to modulate the quantity of light or to switch the 
light. The laser control circuit 210 comprises an APC circuit (a light 
quantity control circuit) 507 for controlling the quantity of light and a 
driver circuit 508 for performing switching modulation. The optical beam 
having a given quantity of light is switching modulated under control of 
the main controller 501 in accordance with data to be recorded. 
A main motor controller 519 which controls the rotation of the main motor 
550 comprises two oscillators 520 and 521 which oscillate at different 
frequencies. One of the outputs from the oscillators 520 and 521 is 
selected and is supplied to a drive circuit 523 as a reference signal. The 
main controller 501 sends control signals for controlling the selector 522 
and the drive circuit 523 so as to preset the rotational speed and perform 
the operation and termination of the apparatus and monitoring of 
abnormalities. 
A scanner controller 514 which controls a scanner motor 104 for rotating a 
polygon mirror 101 comprises two oscillators 515, 516, a selector 517 and 
a drive circuit 518 similarly to the main motor controller 519. Presetting 
of the rotational speed, operation, stopping, monitoring of abnormalities 
is controlled by the main controller 501. 
In the present embodiment, operation for changing the inclination angles of 
the flat plate glasses 110a and 110b is carried out by the two stepping 
motors 361 and 362, respectively, as is described with reference to FIG. 
8. The stepping motors 361 and 362 are controlled by the motor drivers 509 
and 510, respectively. An output signal from a sensor which detects the 
reference position is supplied to the main controller 501. 
A video clock oscillating circuit 502 which supplies the main controller 
501 with video clock signals or reference signals for timing the operation 
of video imaging or recording comprises two oscillators 503 and 504 which 
oscillate at different frequencies and a selector 505 and is controlled to 
select one of the frequencies in response to a control signal from the 
main controller 501 for outputting a clock having the selected frequency. 
An interface controller 524 which is connected with the main controller 501 
controls communication with a host apparatus and flow of recording 
information. 
Low density recording is carried out in normal mode in the present 
embodiment and high density recording is selectable when necessary. Since 
the frequency of the video clock increases in proportion to the square of 
recording density, the uppermost of the video clock is often limited by 
the operation speed of circuit elements or of the host apparatus. 
Therefore, depending upon the kind of the recording information and the 
purpose of recording, the recording speed or density is changed. Low 
density recordings are performed if high speed recordings are needed, and 
vice versa. It is necessary to lower the quantity of light of an optical 
beam (optical output of the semiconductor laser) in case of low speed 
recording than that in case of high speed recording. Although the spot 
diameter changes in two steps in the present embodiment, the apparatus may 
be constructed to perform recording with spots of a changeable diameter 
while the recording speed and video clock are kept constant. 
FIG. 10 is a flow chart for a control processing which is executed by the 
main controller 501 for controlling change of the recording density. 
In step 601, the stepping motors 361 and 362 are rotated for presetting the 
flat plate glasses 110a and 110b at the reference positions and thus the 
inclination angles of the flat plate glasses 110a and 110b are preset in 
positions in which detection signals are obtained from the sensors 513 
(e.i. photointerruptors 371 and 381). 
Selection of either one of high and low recording densities is then made in 
step 602. In case the high density recording is selected, the program step 
will proceed to step 603, in which the stepping motors 361 and 362 are 
rotated to reduce the spot diameter Thereafter, selection signal of the 
selector 522 is controlled to lower the rotational speed of the main motor 
550 (step 604). Selection signal of the selector 517 is controlled to 
lower the rotational speed of the scanner motor 104 (step 605). An APC 
circuit 507 is controlled to decrease the quantity of laser light (step 
606). The selector 505 is controlled to select the video clock having the 
low frequency adapting to the selected recording density (step 607). 
Operation for changing the recording density is completed. 
After completion of such a processing for changing the recording density, 
switching control of the laser beam is executed in accordance with the 
video signal similarly to conventional printer. 
By changing the structure of the actuator for rotating the flat plate 
glasses 110a and 110b, and/or increasing the number of the oscillators to 
increase the number of frequencies of the clock signal, the optical 
scanning apparatus may adapt to three or more recording densities and/or 
recording speeds. By combination of recording speed with clock frequency, 
it is possible to change the recording density in a main scanning 
direction (optical scanning direction) independently of that in a 
subscanning direction (in a direction to which the photosensitive drum is 
moved). Thus it is possible to easily preset the appropriate spot diameter 
under each condition. 
In accordance with the present invention, axis alignment in a subscanning 
direction and adjustment of spot are easy and the spot diameter is 
variable as mentioned above. An optical scanning system in which a change 
in efficiency of optical path due to the change in spot diameter and the 
alignment is remarkably low can be embodied. Since light transmitting flat 
plates are used, sensitivity of adjustment is low and the sensitivity can 
be selected by changing the thickness and/or refraction index of the flat 
plates. Since high precision is not required for the arrangement of the 
flat plates, production efficiency is enhanced.