Light beam deflection scanning method and an apparatus therefor

A light beam deflection scanning method including the steps of: (a) finding data for cancelling distortion of a scanning track to be made on a surface of an object by a light beam focussed on the surface by a scanning lens, the distortion being caused by the lens; (b) directing the light beam at the surface through a scanning lens and deflecting the light beam in a main scanning direction; (c) detecting the position of the light beam on the surface; and the deflecting in step (b), deflecting the light beam in a subscanning direction by an amount based on the data, to thereby cancel the distortion of the scanning track.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
The present invention generally relates to a light beam scanning method and 
an apparatus therefor, and more particularly to a light beam deflection 
scanning method and an apparatus therefor wherein a scanning light beam is 
deflected and swept on a surface of an object by a deflector member. 
A conventional light beam deflection scanning method and apparatus is 
disclosed, for example, in U.S. Pat. No. 4,084,634. Generally, a 
galvanometer mirror, a rotating monogon and polygon mirrors, etc. are 
employed as the aforementioned deflector member for producing the 
deflecting scanning in the art. Furthermore, the conventional apparatus 
includes a f.theta. lens, ftan.theta. lens, sin.sup.-1 .theta. lens or the 
like as the scanning lens between the deflector member and the surface 
being scanned. 
However, these f.theta. lens, ftan.theta. lens, and sin.sup.-1 .theta. lens 
produce an undesired bending or distortion of the scanning line tracks 
made by the light beam on the surface being scanned when the light beam 
impinges upon the lenses at an angle to a plane that is parallel to the 
optical axis thereof. Such bending does not occur when the beam is 
parallel to the optical axis. It is possible to design the beam to impinge 
upon the scanning lens exactly parallel to the optical axis. However, an 
extremely high degree of adjustment accuracy is required to do so. This 
leads to low productivity. 
FIG. 7A, in which reference mark Y denotes a main scanning direction and X 
a subscanning direction, illustrates the straight scanning line tracks 
drawn on the surface being scanned by light beams that all accurately 
impinge upon the scanning lens parallel to its optical axis. FIG. 7B 
depicts the case where two beams impinge upon the lens at an angle with 
respect to the axis and therefore make tracks having ends outwardly bent 
or distorted (according to the theory or principle described below). The 
conditions shown in FIG. 7B result in reproduction images with an 
undesirable, uneven density. 
SUMMARY OF THE INVENTION 
With a view to solving the aforementioned difficulties, it is an objective 
of the present invention to provide a novel and improved light beam 
deflection scanning method and an apparatus therefor. 
It is another objective of the invention to provide a light beam deflection 
scanning method and an apparatus therefor according to which no bent or 
distorted scanning line track is produced whatsoever. 
In order to accomplish the above objectives, there is provided a light beam 
deflection scanning method, in accordance with an aspect of the invention, 
which comprises the steps of: sweeping a focussed light beam by means of a 
scanning lens along a surface of an object while deflecting said light 
beam in a main scanning direction; determining an amount of deviation of 
said beam on said surface from what would be desired, said deviation being 
caused and specified by the characteristics of said scanning lens; and 
cancelling said amount of said deviation, by deflecting said light beam in 
a subscanning direction by an amount corresponding to said amount of said 
deviation. 
In accordance with another aspect of the invention, there is provided a 
light beam deflection scanning method comprising the steps of: (a) 
compiling data for cancelling an amount of distortion of a scanning track 
to be made on a surface of an object to be scanned by a light beam to be 
focussed on said surface by a scanning lens; (b) directing a light beam at 
said surface through said scanning lens and deflecting said light beam in 
a main scanning direction during a predetermined cycle; (c) detecting the 
position of said light beam on said surface; and (d) during said 
deflecting in step (b), deflecting said light beam in a subscanning 
direction by an amount based on said data, thereby cancelling said amount 
of distortion. 
In a preferred embodiment, the data is compiled beforehand and the data 
corresponds to the angle made by said light beam impinging on said 
scanning lens with respect to planes that are parallel to an optical axis 
of said lens. 
In another preferred embodiment, the method further comprises the step of 
reciprocating said light beam in said subscanning direction a 
predetermined number of times within said cycle during said step (b). 
In accordance with still another aspect of the invention, there is provided 
a light beam deflection scanning apparatus comprising: a light source for 
emitting a scanning light beam; a scanning lens for focussing said light 
beam on a surface of an object to be scanned; means for deflecting said 
light beam in a main scanning direction; and means for deflecting said 
light beam in a subcanning direction and cancelling an amount of 
distortion of a scanning track made on said surface by said focussed light 
beam, said distortion being caused and specified by the characteristic of 
said scanning lens. 
In a preferred embodiment, said deflecting and cancelling means comprises 
means for determining said amount of said distortion. 
In another preferred embodiment, said determining means comprises means for 
detecting a falling position of said focussed light beam on said surface 
and outputting data for cancelling distortion corresponding to said 
detected position. 
In still another preferred embodiment, said detecting and outputting means 
comprises a grating sensor member. 
In a further preferred embodiment, said detecting and outputting means 
comprises a lookup table member for storing therein data (previously 
determined) for cancelling distortion. Said data corresponding to falling 
positions of said focussed light beam on said surface. 
In a yet further preferred embodiment, said deflecting and cancelling means 
comprises an acoustooptical modulation member for deflecting said light 
beam in said subscanning direction by means of an ultrasonic wave. 
Hence, since even the light beam that fails to impinge upon the scanning 
lens parallel to the optical axis thereof is capable of drawing the 
straight scanning line tracks free of any bending or distortion 
whatsoever. Thus, the original image can be reproduced with an even 
density.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Described hereunder is a theory or principle explaining why bent or 
distorted scanning line tracks are made on the surface being scanned based 
on the characteristics of the scanning lens being utilized when the light 
beam impinges upon the lens at an angle with respect to planes that are 
parallel to the optical axis of the lens. 
FIG. 6 is a schematic view illustrating the light beam sweeping system 
utilizing a galvanometer mirror 1 as the deflector member. 
A light beam l.sub.1, emitted from a light source not shown, impinges upon 
the mirror 1 as the mirror rotates on an axis perpendicular to the plane 
of the drawing, and then is reflected by the mirror 1 at point O, so that 
it is focused onto a surface being scanned 3 through a scanning lens 2. 
Thus, the beam is swept on the surface 3 as the mirror 1 is rotated. 
When the f.theta. lens, which is most commonly employed in the art, is 
applied as scanning lens 2, it exhibits the characteristics according to 
the following equation: 
EQU y'=f.theta. 
where f represents a focal length of the lens 2; .theta. an angle made by 
the beam l.sub.1 deflected at deflection point O with optical axis l.sub.0 
of the lens 2; and y' a distance between falling point 4 of the beam 
l.sub.1 on the surface 3 and the optical axis l.sub.0 along the surface 3. 
Light beams which do not impinge upon the scanning lens parallel to the 
optical axis thereof will not draw straight scanning line tracks on the 
surface 3 as demonstrated below. 
FIG. 8 is a schematic perspective view depicting optical paths of the light 
beam sweeping system, by means of an X-Y-Z rectangular coordinate system. 
The galvanometer mirror not shown is rotated on the X-axis, and the 
scanning lens is disposed such that its optical axis coincides with 
Z-axis. A quadrangle "ABCD" represents part of the principal plane of the 
lens, while another quadrangle "A'B'C'D'" represents part of the surface 
being scanned. The two quadrangles are both perpendicular to the optical 
axis, i.e. Z-axis, and are also analogous to each other. 
The main scanning direction of the scanning system is in the Y-axis 
direction, and points O, A, A', D, D', E, F', and H are all on the Y-Z 
plane. 
A first light beam defined by the points "HODE" impinges upon the 
galvanometer mirror, and is reflected at the point O on the mirror and 
deflected in the Y-Z plane, and finally falls on the point E on the 
surface being scanned. A second beam represented by the points "GOCF" 
upwardly impinges upon the mirror at an angle of .alpha. radian with 
respect to the Y-Z plane, and is reflected at the point O and falls on the 
point F. Point F' is an end of a normal vertically extending from the 
point F to line A'D'. 
Then, the following equations established: 
##EQU1## 
where .beta. is an angle made by the first light beam with respect to the 
optical axis; .theta. an angle made by the second light beam with respect 
to the optical axis; .gamma. an angle made by diagonals of the quadrangles 
"ABCD" and "A'B'C'D'" with respect to the Y-Z plane; m a distance between 
the points A and D; n a distance between the points C and D; M a distance 
between the points A' and F'; N a distance between the points F and F'; 
and a a distance between the points O and A. 
When the f.theta. lens is applied as the scanning lens, the following 
equations are established based on its characteristic as aforesaid: 
EQU M=f.theta. cos (5) 
EQU N=f.theta. sin (6) 
Substituting for n and m in Eqs. (1) and (2) from Eqs. (3) and (4): 
##EQU2## 
The following tables illustrate the calculation results when values are 
actually applied in the above equations: 
TABLE I 
______________________________________ 
When f = 800 mm, and .alpha. = 3.27" 
.beta..degree. 
M(mm) M-f (um) N(um) N-No(um) 
______________________________________ 
0 0 0 12.683 0 
5 69.8 -1.2 .times. 10.sup.-5 
12.699 0.016 
8 111.7 -1.9 .times. 10.sup.-5 
12.724 0.041 
10 139.6 -2.4 .times. 10.sup.-5 
12.747 0.065 
12 167.6 -2.9 .times. 10.sup.-5 
12.776 0.093 
15 209.4 -3.6 .times. 10.sup.-5 
12.829 0.146 
______________________________________ 
TABLE II 
______________________________________ 
When f = 800 mm, and .alpha. = 6.55" 
.beta..degree. 
M(mm) M-f (um) N(um) N-No(um) 
______________________________________ 
0 0 0 25.404 0 
5 69.8 -2.3 .times. 10.sup.-5 
25.437 0.032 
8 111.7 -3.8 .times. 10.sup.-5 
25.484 0.083 
10 139.6 -4.7 .times. 10.sup.-5 
25.534 0.129 
12 167.6 -5.7 .times. 10.sup.-5 
25.591 0.187 
15 209.4 -7.1 .times. 10.sup.-5 
25.697 0.293 
______________________________________ 
TABLE III 
______________________________________ 
When f = 800 mm, and .alpha. = 122.47" 
.beta..degree. 
M(mm) M-f (um) N(um) N-No(um) 
______________________________________ 
0 0 0 475.00 0 
5 69.8 -8.2 .times. 10.sup.-3 
475.60 0.60 
8 111.7 -1.3 .times. 10.sup.-2 
476.54 1.55 
10 139.6 -1.7 .times. 10.sup.-2 
477.42 2.42 
12 167.6 -2.0 .times. 10.sup.-2 
478.49 3.49 
15 209.4 -2.5 .times. 10.sup.-2 
480.47 5.47 
______________________________________ 
where No is a value of N when .beta. = 0 in all the above tables. 
As explicit from the above tables, it is appreciated at angles with respect 
to planes parallel to the at angles of some degrees with planes parallel 
with the optical axis thereof, fail to draw the straight scanning line 
tracks even when the generally accepted f.theta. lens is employed as the 
scanning lens. This will hold true when the number of impinging light 
beams is small and even when each light beam is so large in radius that 
the angle .alpha. defined as above is considerably large and hence the 
value N-No is rather large, not to mention the case where the number of 
beams is large. 
When the same study is made on applications of the ftan.theta. and 
sin.sup.-1 .theta. lenses, it is demonstrated that bending or distortion 
of the scanning line tracks also takes place. For example, when the 
ftan.theta. lens is applied as the scanning lens 2 in the light beam 
sweeping system shown in FIG. 6, it exhibits the characteristics expressed 
below: 
EQU y'=ftan.theta. 
Further, the following equations are established accordingly: 
##EQU3## 
Where No is a value of N when .beta.=0, and .alpha.o a value of .alpha. 
when .beta.=0. 
The following table illustrates calculation results based on the above 
equations. 
TABLE IV 
______________________________________ 
When f = 800 mm, and .alpha.o = 6.45" 
.beta..degree. 
0 3 6 9 12 15 
______________________________________ 
N(mm) 25.0 25.03 25.14 
25.31 25.56 
25.88 
N-No(mm) 0 0.03 0.14 0.31 0.56 0.88 
______________________________________ 
Now, referring to the drawings, preferred embodiments of the invention are 
described below. 
FIRST EMBODIMENT 
FIG. 1A is a schematic view depicting a multiple light beam deflection 
exposing scanning system for recording a reproduced image on a 
photosensitive material according to a first embodiment of the invention, 
which system is provided with a multiple-light beams producing unit which 
utilizes an acoustooptical modulator (hereinafter, "AOM") which is 
actuated by ultrasonic wave signals of different frequencies. 
A light beam is emitted by a laser beam source 11, and reduced in radius by 
a beam condenser 12, and thereafter impinges upon the AOM 13, to which are 
applied three ultrasonic waves each with a distinct frequency, so that the 
beam is deflected and caused to travel along three divided optical paths 
by the AOM in a manner to be described below. 
The lens 14 is disposed at a distance equal to its focal length from the 
center of the AOM 13, and the light beams in the paths are focussed at 
back focuses Q.sub.1 of the lens 14. In FIG. 1, the optical paths thus 
formed are distinguishably denoted in solid lines, chain dot lines, and 
phantom. 
A zero-order light beam, as depicted in broken lines in FIG. 1, is shielded 
by a suitable shutter member 15, whereas three first-order light beams 
proceed to impinge upon a collimator lens 16 from the positions Q.sub.1 
and converge upon a galvanometer mirror 17, which rotates upon an axis 
extending parallel to the plane of the drawing in order to periodically 
deflect the beams in a direction perpendicular to the plane of the 
drawing. Thereafter, the light beams impinge upon a scanning lens 18 so 
that light spots in number equal to the number of the beams are focussed 
on a photosensitive surface being scanned 19 which is moved in a 
subscanning direction denoted by an arrow X in FIG. 1A by a moving member 
not shown. Thus, the beams scan and thereby expose the surface 19 in the 
direction vertical to the plane of the drawing as the galvanometer mirror 
17 is rotated. 
Furthermore, a half mirror 20 is provided before the AOM 13 for branching 
off a subsidiary light beam for the production of timing pulses. The 
subsidiary light beam is caused to impinge upon the galvanometer mirror 17 
(but follows an optical path different from that of the exposing light 
beams) via mirrors 21, 22, and 23. The subsidiary beam is then reflected 
by a mirror 24 toward a grating sensor 25 located at a suitable position, 
thereby yielding timing pulse signals synchronously with the advancing of 
the main scanning beams. 
The grating sensor 25 is formed of a grating and a photoelectric detector 
equipped therebehind. The grating is assembled of numerous light 
transmitting and excluding parts alternately disposed at a specified 
pitch, as shown in FIG. 1B. As the subsidiary light beam runs on the 
grating sensor 25 in a lengthwise direction synchronously with the main 
scanning of the exposing light beams, the photoelectric detector outputs 
pulse signals for every light transmitting part. 
As described and demonstrated above, there arises a problem in that bent or 
distorted scanning line tracks are formed by light beams that impinge upon 
the scanning lens 18 at angles with respect to the optical axis of the 
lens when the f.theta. lens, ftan.theta. lens, sin.sup.-1 .theta. lens, or 
the like is applied as the scanning lens 18. Accordingly, the system 
illustrated in FIG. 1A is designed to eliminate such problem by 
controlling the amount of the deflection for each exposing light beam in 
the AOM 13, depending upon the falling position of the beam on the surface 
being scanned 19. 
FIG. 2 is a block diagram schematically explaining the mode or manner in 
which the single light beam emitted from the laser source 11 is caused to 
travel along divided optical paths by applying plural types of frequencies 
to the AOM and at the same time a deflection angle of each of multiple 
light beams is controlled depending on the main scanning positions of the 
focussed light spots of the beams on the surface being scanned 19. 
A plurality of (three in this embodiment) oscillators 26a, 26b, and 26c are 
provided for yielding ultrasonic wave signals fa, fb, and fc each having a 
distinct frequency. The signals fa, fb, and fc are transmitted to 
corresponding modulation circuits 27a, 27b, and 27c, respectively, and 
undergo on-off control and amplitude modulation based on image signals Sp 
that are applied to the circuits 27a, 27b, and 27c, and then inputted 
parallel to a mixer circuit 28. The circuit 28 synthesizes these signals 
fa, fb, and fc into a high frequency signal having plural frequency 
components, and applies the signal to a transducer 29 of the AOM 13. Thus, 
in the AOM compressed standing waves are formed with pitches corresponding 
to the frequency components of the ultrasonic wave signal. 
If the light beam L is emitted by the laser beam source 11 to the AOM 13, 
when the AOM 13 has the compressed standing waves produced therein, the 
beam L is split into a 0-order light beam Lo not diffracted and three 
first-order light beams L.sub.1, L.sub.2, and L.sub.3 diffracted at angles 
corresponding to the respective component frequencies. The separation of 
the light beam into plural beams by the application of plural ultrasonic 
wave signals to the single AOM is conducted in the manner conventionally 
known to the art. 
Arrangements are provided for altering and adjusting the frequencies 
yielded by the oscillators 26a through 26c. Specifically, there is 
provided a light beam position detection circuit 30, a look-up table 31 
(hereinafter referred to as "LUT"), and frequency control circuits 32a 
through 32c respectively corresponding to the oscillators 26a through 26c. 
The light beam position detection circuits 30 is designed to detect the 
falling positions of the individual light beams on the surface being 
scanned 19 on the basis of timing pulses having a suitable pitch that are 
made by the multiplication of, or the frequency division of, the pulse 
signals yielded by the grating sensor 25, and to input data concerning 
such positions to the LUT 31. 
In the LUT 31, there is stored modulation data regarding the frequencies to 
be applied to the AOM 13, which data will be used to correct the amount of 
deviation (expressed as N-No as aforementioned) of each light beam that 
does not impinge upon the scanning lens in parallel to the optical axis of 
the lens, in dependence on the falling position of the beam upon the 
surface being scanned 19. Such data is predetermined for angles (expressed 
as .alpha. in the foregoing) made by an impinging light beam upon the 
scanning lens with respect to planes that are parallel to the optical axis 
of the lens. These angles can be calculated based on the deflection angles 
of the beams determined in the AOM 13. Consequently, the LUT 31 outputs 
scanning line distortion correction data concerning exposing light beams 
to the frequency control circuits 32a through 32c upon receiving the above 
position data from the detection circuit 30. 
Based on the correction data, the control circuits 32a through 32c 
modulate the frequencies produced by the corresponding oscillators 26a 
through 26c in order to adjust the deflection angles of the exposing light 
beams determined by the AOM 13 so that the desired straight scanning line 
tracks are duly formed by the beams on the surface being scanned 19. 
The intervals among the three light beams at the back focuses Q.sub.1 of 
the lens 14 depend upon the deflection angles of the beams created by the 
AOM 13 as shown in FIG. 1. Further, these deflection angles are governed 
by the frequencies of the ultrasonic wave signals given to the AOM 13. 
Suitable selection of the fundamental frequencies of the oscillators 26a 
through 26c makes it possible to set, as desired, values for the pitches 
of the beams at the back focuses Q.sub.1. Hence, once can determine at 
will the values of the pitches of scanning lines on the surface being 
scanned 19. The last mentioned values are governed by the values of the 
pitches of the beams at the back focuses Q.sub.1 and the diameters of the 
beam spots on the surface 19. Furthermore, properly modulating the 
frequencies produced by the oscillators 26a through 26c in dependence on 
the falling positions of the beams on the surface being scanned enables 
one to correct otherwise bent or distorted scanning line tracks drawn by 
the beams. Thus whole scanning lines can be made parallel and straight. 
The following table illustrates an example of deflection angle adjustment 
data for cancelling deviation expressed as N-No to correct scanning line 
tracks made by a light beam into straight tracks when the beam impinges 
upon the ftan.theta. lens at an angle of .alpha.o=6.45" on condition that 
f=800 mm: 
TABLE V 
______________________________________ 
.beta..degree. 
0 3 6 9 12 15 
.alpha.' 
0.9167 0.9154 0.9116 0.9054 0.8966 0.8854 
______________________________________ 
where .alpha.' = tan.sup.-1 (1.6 .times. 10.sup.-2 cos.beta.). 
Moreover, the following table shows an example of data concerning the 
frequencies of the ultrasonic wave signal to be applied to the AOM which 
assures a properly adjusted deflection angle of a beam that impinges upon 
an ftan.theta. lens at an angle of .alpha..sigma.=6.45" on condition that 
f=800 mm: 
TABLE VI 
______________________________________ 
.beta..degree. 
0 5 8 10 12 15 
frequency 
240 239.4 238.4 
237.6 236.5 
234.5 
(MHz) 
______________________________________ 
Needless to say, the number of the exposing light beams is not limited to 
that described in this embodiment, and can be freely chosen. 
EMBODIMENT II 
Although in the first embodiment a plurality of parallel light beams are 
arranged to simultaneously impinge upon the deflector member such as the 
galvanometer mirror, the present invention is not limited thereto. For 
example, it is also possible to apply the present invention to so called 
wobble scanning wherein a light spot of a single scanning light beam moves 
on the surface being scanned in a subscanning direction in a higher cycle 
than a cycle in which it moves in a main scanning direction. In wobble 
scanning, a plurality of scanning lines can be scanned in one main 
scanning period. FIG. 3 schematically illustrates a second embodiment 
embodying this concept. 
A light beam is emitted by a laser beam source 81 to an AOM 84 through 
lenses 82 and 83, and a first image 85 thereof is focussed within the AOM 
84. Between the lenses 82 and 83, there is disposed a half mirror 86 for 
branching a subsidiary beam off from the main beam for timing control. 
The main light beam is deflected in the AOM 84 at an angle in accordance 
with the frequency of the ultrasonic wave signal applied to the AOM 84, 
and directed along the optical axis L.sub.4 of a lens 87 positioned at a 
distance equal to its focal length from the deflection point so as to 
focus a second image 88 after the beam passes through the lens 87. The 
beam is then reflected by a mirror 89 and transformed into a parallel 
luminous flux by a lens 90. Thereafter, the beam is reflected by a mirror 
91 and a third image thereof 93 is focussed by a lens 92. 
The light beam travelling from the point at which the third image 93 is 
focussed is again formed into a parallel luminous flux by a collimator 
lens 94, and impinges upon a galvanometer mirror 95. As in the first 
embodiment, the galvanometer mirror 95 is rotated on an axis extending 
parallel to the plane of the drawing in order to periodically deflect the 
impinging beam in a direction normal to the plane of the drawing. A light 
spot of the deflected light beam is focussed by a scanning lens 96 on a 
photosensitive surface 97 which is moved in the subscanning direction X by 
a moving member not illustrated, and thus the light spot runs on the 
surface 97 as the galvanometer mirror 95 is rotated. 
The branched subsidiary light beam is reflected by a mirror 98 and impinges 
upon the galvanometer mirror 95 through the collimator lens 94 together 
with the exposing main light beam. The subsidiary beam is then deflected 
by the mirror 95 synchronously with the deflection of the exposing beam so 
as to scan a grating sensor 100 via the scanning lens 96 and a mirror 99, 
thus yielding timing pulses as in the first embodiment. 
From FIG. 3, it is appreciated that the exposing light beam of parallel 
luminous flux impinges upon the galvanometer mirror 95 at a slight slant 
angle with respect to optical axis L.sub.4 denoted in chain dot lines, and 
that the amount of the slant angle depends on the distance of the light 
beam incident on the lens 90 from the optical axis L.sub.4 and further 
that the incident point of the beam on the lens 90 is governed by the 
deflection angle determined by the AOM 84. 
Since the distance of the falling position of the light beam on the surface 
being scanned 97 from the optical axis L.sub.4 along the surface 97 is 
varied by the amount of the slant angle at which the light beam impinges 
upon the scanning lens 96 with the optical axis L.sub.4 thereof, it is 
possible to control the focussed point of the light beam on the surface 97 
with respect to a direction intersecting the main scanning direction at a 
right angle (in a direction along the plane of the drawing) by 
appropriately controlling the deflection angle of the beam in the AOM 84. 
When scanning by deflecting a single beam, the light beam scans the surface 
to be scanned along only one scanning line in the main scanning direction, 
as depicted in FIG. 4A. However, according to the conventional wobble 
scanning technique, altering the deflection angle control signal supplied 
to the AOM 84 in FIG. 3 to a certain extent permits the light beam to be 
reciprocally deflected in the subscanning direction in a higher cycle than 
that in which the galvanometer mirror 95 is rotated. Thus, it is possible 
for the beam to cover plural main scanning lines during one rotation of 
the galvanometer mirror 95, as illustrated in FIG. 4B. Furthermore, to 
attain a similar end, there is provided another AOM in addition to the AOM 
84. With the additional AOM, the exposing light beam draws scanning line 
tracks as shown in FIG. 4C when the AOM 84 is inclined at a certain angle 
and the additional AOM is disposed at an angle perpendicular to the 
deflection angle of the light beam specified by the AOM 84. At the same 
time, distortion of the plural scanning lines caused by the scanning lens 
can be corrected by adding to the deflection angle control signal that is 
supplied to the AOM 84 the amount needed to correct and cancel the 
distortion. 
FIG. 5 is a block diagram depicting an embodiment of a system for 
correcting the distortions of the scanning line tracks on the surface 
being scanned, which system is applicable in the arrangement shown in FIG. 
3. 
A light beam position detection circuit 101 detects the falling position of 
the focussed light beam on the surface being scanned 97 based on pulse 
signals produced by a grating sensor 100, and then delivers data 
concerning such position to an LUT 102, in the same manner as in the first 
embodiment. 
As in the first embodiment, data is stored in the LUT 102 for use in 
adjusting the deflection angle of the light beam in the AOM 84 in order to 
correct the falling position of the beam on the surface 97. The deflection 
angle is peculiar to the characteristics of the scanning lens being 
employed. Upon receiving the position data delivered from the detection 
circuit 101, the LUT 102 outputs distortion correction data respectively 
corresponding to (1) the falling position of the beam and (2) the angle at 
which the beam impinges upon the scanning lens with the planes that are 
parallel to the optical axis of the lens. 
The distortion correction data are transmitted to an adder 103 at a stage 
subsequent to the LUT 102. The adder 103 adds light beam deflection angle 
control signals to be supplied to the AOM 84. The deflection angle control 
signals including the proper distortion correction data components are 
then converted into analog signals by a digital-to-analog converter 104, 
and then applied to the AOM 84 through an AOM driving circuit 105. 
Thus, a single light beam which has undergone modulation by the AOM 84 so 
given such control signal is capable of drawing on the surface being 
scanned scanning line tracks all of which are free of any bending or 
distortion whatsoever. 
Unlike the first embodiment, this embodiment can be applied in scanning the 
image to be reproduced and obtaining image signals thereof as well as in 
exposing the reproduced image on the photosensitive material. 
Specifically, since the single light beam is moved at a high speed in the 
subscanning direction so as to scan a plurality of main scanning lines in 
one main scanning period and since there is only one scanning light spot, 
it is possible for the beam to pick up image information of only one pixel 
at a time in scanning an original image. This permits the application of 
original image input scanning. In original image input scanning, the 
original image is placed at a position of the surface 97 of the 
photosensitive material and light reflected by or penetrating the original 
image is caused to fall on a photoelectric detector in order to obtain 
image signals thereof. Thus, the system shown in FIG. 3 can be used for 
original image input scanning. 
Although the galvanometer mirror is utilized for the deflector member in 
the foregoing embodiments, other devices such as a polygon or monogon 
mirror can be used as the deflector member. 
While the preferred embodiments of the present invention have been 
described above, it should be understood that various modifications may be 
made herein without departing from the spirit of the invention and the 
scope of the appended claims.