Light beam recorder having beam duration and intensity controlled in accordance with scanning speed

A laser beam recorder arranged to record data with dots of a constant size, constant pitch and constant density, irrespective of possible differences in scanning speed between the middle and peripheral portions of a recording medium. The timing of irradiating the recording medium with the light beam is precipitated and the duration of the irradiation with the light beam is shortened as the scanning speed increases, whereby the dot size and dot pitch can be made constant irrespective of variation in the scanning speed. Further, the light beam is intensified as the scanning speed increases. Though the duration of the irradiation with the light beam is reduced with the increasing scanning speed, the amount of energy for recording one dot is maintained constant by intensifying the light beam as described, whereby the dot density can be maintained constant.

FIELD OF INDUSTRIAL APPLICATION 
The present invention relates in general to light beam recorders, and in 
particular to light beam recorders of the type using a light beam to 
record information, represented by characters or the like, in the form of 
a row or rows of dots on a recording medium. 
PRIOR ART 
An apparatus using a light beam for recording information, represented by 
characters or the like, is known, for example, from Japanese Laid-open 
Patent Application No. 67722/1980 which discloses a Laser Computer Output 
Microfilmer (LASERCOM) in which a laser beam is scanned in accordance with 
computer-output information to directly record information of characters 
or the like on a recording medium such as a microfilm. The LASERCOM 
comprises: an argon laser for generating a laser beam; a light modulator 
for modulating the laser beam in accordance with character information; a 
rotating polygon mirror for deflecting the laser beam modulated by the 
light modulator, in such a manner that the beam is swept in a principal 
scanning direction; and a galvanometer equipped with a deflecting mirror 
for deflecting light reflected from the rotating polygon mirror in such a 
manner that the reflected light is swept in a subsidiary scanning 
direction The arrangement is such that information such as characters or 
the like are recorded on a recording medium by two-dimensionally scanning 
the laser beam from the light modulator through a scanning lens over the 
recording medium using the rotating polygon mirror and the galvanometer. 
The rotating polygon mirror is driven by a motor so as to rotate at a 
constant speed in order to deflect the laser beam in the principal 
scanning direction; it is very difficult to make the polygon mirror rotate 
at a constant speed and in fact the rotating speed will vary to a some 
degree resulting in a distorted recording pattern having a deviation in 
the position of dots or scanned points from the ideal pattern. On the 
other hand, where a polygon mirror is rotated at a constant speed to 
deflect a light beam for recording characters on a planar recording 
medium, there is a problem in that the pitch of the dots is not constant 
because marginal portions of the recording medium are more distant from 
the rotating polygon mirror than the middle portion of the recording 
medium. As a result the scanning speed is faster at the marginal portions 
than at the middle portion. This makes the spacing between consecutive 
dots wide at the marginal portions. However, they become narrower toward 
the middle portion. 
A laser recorder is known from U.S. Pat. No. 3389403, which uses a reading 
laser beam in addition to a recording laser beam. The reading laser beam 
is deflected by a rotating polygon mirror so as to scan a linear encoder 
having a plurality of slits juxtaposed with a fixed pitch aligned in the 
direction of deflection. A photoelectric pulse signal obtained by the 
linear encoder is input into a character generator such as a video clock 
signal for generating a video signal in accordance with the scanning 
position of the rotating polygon mirror, whereby dots can be recorded in 
correct positions. 
PROBLEMS WHICH THE INVENTION ATTEMPTS TO SOLVE 
However, there is a problem in that, though a constant pitch of the dots 
may be achieved, the density of the dot is not constant, i.e. dots in the 
middle portion of the recording medium are more dense than those in the 
peripheral portions, since, as stated previously, the scanning speed is 
slower at the middle portion of the recording medium than at the 
peripheral portions and as a result the amount of energy of light used to 
record one dot is larger at the middle portion than at the peripheral 
portions. Similarly, in the case of a resonant scanner using a 
sinusoidally oscillating deflecting mirror in place of a rotating polygon 
mirror, a constant dot pitch may be achieved but dots are more dense in 
the middle portion than in the peripheral portions since the scanning 
speed is higher in the middle portion than in the peripheral portions. 
When the resonant scanner is used, with the scanning efficiency being 
assumed to be 70%, there occurs a distortion of about 10% between the 
middle and peripheral portions of the image recording area; it is 
difficult to make the dot pitch constant by correcting distortion of the 
lens system, since the lens system is capable of correcting distortion of 
about 2-3%. 
The present invention has been made in an attempt to solve the 
above-mentioned problems, and its one object is to provide a light beam 
recorder which can record information with dots having an identical 
density and a constant pitch. 
MEASURES TO SOLVE THE PROBLEMS 
To solve the problems, the present invention includes: a light source for 
generating a light beam; a means for scanning the light beam generated by 
said light source over a recording medium, and a control means for 
establishing the timing of irradiating the recording medium with said 
light beam and shortening the duration of irradiation with the light beam 
in accordance with recorded information as the scanning speed is 
increased, and for intensifying said light beam as the scanning speed 
increases. 
OPERATION 
The scanning means according to the present invention will deflect a light 
beam generated by a light source in a principal scanning direction of a 
recording medium in order to sweep the light beam in the principal 
scanning direction for scanning The control means will increasingly 
initiate the timing of irradiation of the recording medium with the light 
beam and shorten the duration of the irradiation with the light beam in 
accordance with information to be recorded in the recording medium. Thus, 
by increasing the imitation of the irradiation timing and shortening the 
irradiation timing interval as the scanning speed is increased, it is 
possible to make constant the product of the length of time during which 
the light beam moves and the scanning speed, i.e. the displacement of the 
light beam, whereby the pitch between dots successively recorded in a 
recording medium, can be made constant. Also, since the duration of 
irradiation with the light beam is increasingly shortened as the scanning 
speed increases, the displacement of the light beam required to record a 
single dot can be made constant Also the size of dots can be made 
constant. Further, the control means will increasingly intensify the light 
beam as the scanning speed increases. By intensifying the light beam as 
the scanning speed increases, the product of the light beam intensity and 
the irradiation duration, i.e. the amount of energy required to record a 
single dot, can be made constant and thereby the dot density can be made 
constant, even if the irradiation duration is varied by the control means. 
EFFECTS OF THE INVENTION 
In accordance with the present invention, the duration of irradiation of 
the light beam is shortened and the light beam is intensified as the 
scanning speed is increased. The product of the irradiation duration and 
the intensity, i.e. the amount of light energy required to record a single 
dot, can thereby be advantageously made constant. Further, the timing of 
light beam irradiation and the irradiation duration are controlled 
depending on the scanning speed The pitch and size of dots can thereby be 
advantageously made constant and hence recording can be performed with 
dots having a constant pitch therebetween, a constant size, and a constant 
density, irrespective of changes in the scanning speed.

EMBODIMENTS 
Embodiments of the present invention will now be described in detail with 
reference to the drawings. FIG. 1 is a block diagram showing an optical 
system and a control system of a first embodiment of a light beam recorder 
according to the present invention which are used for recording character 
information or the like on microfilm. The optical system of the light beam 
recorder is used for scanning a recording laser beam in a principal 
scanning direction and for imaging on a recording medium. The control 
system is for controlling the optical system and at the same time 
controlling the carrying of the recording medium at a predetermined speed 
in a subsidiary scanning direction. 
First, the optical system will be described, with reference to FIGS. 1 and 
2. The optical system has a semiconductor laser 10 for generating a laser 
beam, which can be turned on and off. 
A collimator 12 is provided on the laser-beam-projecting side of the 
semiconductor laser 10 so as to collimate the laser beam from the 
semiconductor laser 10 into a parallel light beam. To deflect the laser 
beam emitted from the collimator 12 for sweeping in the principal scanning 
direction, a resonant scanner 14 is placed on the laser-beam-projecting 
side of the collimator 12 for causing a deflecting mirror 16 to oscillate 
in a resonating manner in accordance with a sinusoidal oscillation 
represented by the following equation (1): 
EQU .phi.=.phi..sub.0 sin .omega.t . . . (1), 
where .phi. represents the angle of rotation of the scanner 14, .phi..sub.0 
represents the amplitude of the resonant scanner 14, .omega. represents an 
angular frequency and t represents time. 
On the laser-beam-projecting side of the resonant scanner 14, there is 
placed a scanning lens 18 having a focal plane position on a recording 
medium 20. Between the scanning lens 18 and the recording medium 20, a 
photoelectric converter 22 is provided 
As shown in FIG. 2, the resonant scanner 14 includes a frame comprising an 
L-shaped member 50 having a flat spring 52 fixed thereto at one end 
thereof. A deflecting mirror 16 is positioned between the bottom surface 
of the L-shaped member 50 and the flat spring 52. The deflecting mirror 16 
has a top side connected to the flat spring 52 through a torsion bar 54, 
and a bottom side connected to the bottom surface of the L-shaped member 
50 through another torsion bar 56. An armature coil 58 is fixed to the 
rear surface of the deflecting mirror 16. A ring-shaped iron core 60 
extends through the armature coil 58. A speed detecting coil 64 for 
detecting the rotating speed of the deflecting mirror 16, together with a 
drive coil 62 for moving the armature coil 58, thereby resonating the 
deflecting mirror 16 for oscillation, are wound around the iron core 60. 
Sixty-six 66 designates a permanent magnet, and 68 designates a turning 
tab In the present scanner, a pulsed electric current supplied to the 
drive coil 62 will move the armature coil 58 in a predetermined direction. 
An interruption of such electric current causes the deflecting mirror 16 
to return to its original position at this time the torsion bars 54 and 56 
exert a torsional force causing the deflecting mirror 16 to oscillate in 
an resonating manner. A electric current which is generated by the 
movement of the armature coil 58 is detected by the speed detecting coil 
64. 
Next, the control system of the above-mentioned laser beam recorder will be 
described. The control system includes a drive circuit 32 for supplying a 
driving signal A to the drive coil 62 of the resonant scanner 14' and a 
zero crossing direction detecting circuit 26 connected to the 
photoelectric converter 22. The zero crossing direction detecting circuit 
26 is connected to an absolute value circuit 28 which in turn is connected 
to receive a speed signal B from the speed detecting coil 64 of the 
resonant scanner 14. The absolute value circuit 28 is connected, via an 
amplifying and biasing circuit 30 for amplifying the output from the 
absolute value circuit 28 and adding a predetermined bias to the amplified 
output, to a VCO 34 for generating a signal having a frequency which is 
proportional to the voltage, and to a V/I converting circuit 42 for 
converting a voltage V into an electric current. The VCO 34 is connected 
to a phase synchronizing circuit 36 to which said zero crossing direction 
detecting circuit 26 is connected. The phase synchronizing circuit 36 is 
connected to the drive circuit 44 through a horizontal timing generating 
circuit 38 and through a P/S converting circuit 40 which converts parallel 
signals P into a serial signal S. Picture element data is input into the 
P/S converting circuit 40. The V/I converting circuit 42 is likewise 
connected to the drive circuit 44 connected to the semiconductor laser 10. 
As shown in FIG. 3, the photoelectric converter 22 comprises a pair of 
juxtaposed photo detectors 22A and 22B, of which the photo detector 22A is 
connected via an amplifier 70 to a subtracter 76 and to a direction 
detecting circuit 74. On the other hand, the other photo detector 22B is 
connected via another amplifier 72 to subtracter 76 and to direction 
detecting circuit 74. The subtracter 76 will provide an output, which is 
the output from the amplifier 70 minus the output from the amplifier 72, 
to the zero crossing detecting circuit 78. Because the output from the 
subtracter 76 will vary generally sinusoidally as the laser beam traverses 
the photoelectric detector 22, it is possible to detect the time point at 
which the output from the amplifiers 70 and 72 become equal to each other, 
i.e. the time point at which the laser beam passes midway between the two 
photo detectors 22A and 22B, by detecting zero crossing points of the 
sinusoid. The zero crossing detecting circuit 78 is connected to the 
absolute value circuit 28 and to an AND gate 80. The direction detecting 
circuit 74 will determine whether the light beam has passed in the 
direction from the photo detector 22A to the other photo detector 22B, or 
in the reversed direction, depending on the outputs from the amplifiers 70 
and 72. The output from the direction detecting circuit 74 is supplied to 
the AND gate 80 whose output is supplied to the phase synchronizing 
circuit 36. By detecting the direction in which the light beam has passed 
by means of the direction detecting circuit 74 and delivering a signal, 
selection can be made between a principal scanning in a single direction 
and a reciprocating principal scanning. 
FIG. 4 shows the details of the driver circuit 44 as well as the connection 
of the drive circuit 44 and the semiconductor laser 10, wherein the output 
of the P/S converting circuit 40 is connected via an inverter 86 to a 
differential current switch 84, and further to another differential 
current switch 82. An electric current I for determining the intensity of 
light is supplied from the V/I converting circuit 42 to the differential 
current switches 82 and 84. The differential current switch 82 is 
connected to ground via a dummy diode 88, while the other differential 
current switch 84 is connected to ground via the semiconductor laser 10. A 
series circuit, comprising a photodiode 90 for detecting light emitted 
from the semiconductor laser 10 and an automatic light quantity 
controlling circuit 92, is connected in parallel with the semiconductor 
laser 10, which is arranged in such a way as to automatically increase the 
intensity of the electric current, thereby increasing the light quantity 
to a constant value corresponding to the electric current I from the V/I 
converting circuit 42 when the photodiode 90 receives small quantities of 
light. 
Next, the operation of the embodiment of the invention will be described, 
with reference to FIGS. 1 and 5. The drive circuit 32 supplies a pulsed 
drive signal A, so that the deflecting mirror 16 is resonated in 
accordance with the sinusoidal wave expressed in the equation (1) above. 
Consequently, the deflecting mirror 16 is resonated in accordance with the 
waveform shown at (1) in FIG. 5, and a speed signal B shown at (2) in FIG. 
5 is obtained from the speed detecting coil 64 of the resonant scanner 14. 
At this time, if the semiconductor laser 10 is emitting a laser beam, then 
a parallel light beam is thrown via the collimator 12 to the resonant 
scanner 14, and the parallel light beam reflected from the resonant 
scanner 14 is directed onto and swept over the recording medium When the 
photoelectric converter 22, that is positioned between the scanning lens 
18 and the recording medium 20, has detected the passing laser beam, the 
zero crossing direction detecting circuit 26 will provide a signal 
indicative of the passing of the laser beam to the absolute value circuit 
28, and another signal indicative of its direction to the phase 
synchronizing circuit 36. In response to the signal indicative of the 
passing of the laser beam from the zero crossing direction detecting 
circuit 26, the absolute value circuit 28 will derive the absolute value 
of the speed signal B and will provide this as its output C shown at (3) 
in FIG. 5 to the amplifying and biasing circuit 30. The amplifying and 
biasing circuit 30 will amplify the output C from the absolute value 
circuit 28 by a gain a and will add a predetermined bias b to the 
amplified output, and will provide it to the VCO 34 and to the V/I 
converting circuit 42 as its output D shown at (4) in FIG. 5. Thus, the 
V/I converting circuit 42 will provide to the drive circuit 44 a current I 
which is proportional to the output D from the amplifying and biasing 
circuit 30 i.e. a current I having a waveform similar to that which is 
shown at (4) in FIG. 5 and which varies along the convex portions of a 
cosine curve as the sweeping speed increases. 
On the other hand, the VCO 34 will output a signal E having a frequency 
which is proportional to the output D from the amplifying and biasing 
circuit 30, i.e. a signal E consisting of a pulse train whose pulse width 
becomes maximum (i.e. whose frequency becomes minimum) at a point where 
the speed signal B is zero, and whose pulse width becomes minimum (i.e. 
whose frequency becomes maximum) at a point where the amplitude of the 
speed signal B is maximum. The output from the VCO 34 is phase corrected 
by the phase synchronizing circuit 36 for coordinating the phase within 
one principal scanning cycle in accordance with the signal indicative of 
the laser beam direction from the zero crossing direction detecting 
circuit 26, and is provided to the P/S converting circuit 40 by the 
horizontal timing generating circuit 38 at a recording starting point. 
Picture element data represented by parallel signals is converted into a 
serial signal by the P/S converting circuit 40 in accordance with the 
output from the horizontal timing generating circuit 38, of which the 
converted signal is supplied to the drive circuit 44. FIG. 6(1) shows an 
image recording area 21 of the recording medium. The drive circuit 44, 
receiving the current I from the V/1 converting circuit 42, provides an 
output, i.e. a video signal F which as shown in FIG. 6(2) has smaller 
amplitudes and larger pulse widths at peripheral portions of the image 
recording area 21 shown in FIG. 6 (1) and has larger amplitudes and 
smaller pulse widths at the middle portion of the image recording area. 
Thus, the product of amplitude or power P1 and pulse width W1 is equal to 
the product of amplitude P2 and pulse width W2, and the energy for 
recording one dot is the same throughout the image area. 
Next, the quantities corrected due to scanning speed variations will be 
described. 
The scanning speed v of the laser beam on the recording medium surface may 
be expressed by the following equation (2) which is derived by 
differentiating the above-noted expression (1): 
##EQU1## 
The varying waveforms of the angle of rotation .phi. and the speed v are 
shown at (1) and (2) in FIG. 7. 
Assuming that the scanning efficiency .eta. is k, and the effective angle 
of rotation of the deflecting mirror is within the range of -k.phi..sub.0 
.ltoreq..phi..ltoreq.k.phi..sub.0, the range of speed variations may be 
expressed as follows: 
EQU .phi..sub.0 .multidot..omega..multidot.cos (sin.sup.-1 k)&lt;.nu.&lt;.phi..sub.0 
.multidot..omega. . . . (3) 
Assuming that the scanning efficiency n=70%, 
EQU cos (sin.sup.-1 0.7).apprxeq.0.71 
which means that the speed must be varied by 71% from the image center 
along the cosine curve. 
The quantities which must be actually corrected will be calculated. 
Assuming that the maximum value of the period to be corrected is T.sub.1, 
and since the correction is made along the cosine curve, the period 
summation of corrected intervals is expressed as follows: 
##EQU2## 
Here, for example, f represents the frequency and m represents the dot 
rate which amounts to 3,360 dots/ 7.2 mm. 
By substituting the following expression (5) for the expression (4): 
##EQU3## 
and assuming that x=0, n.ident.m/2, .theta..ident.2sin.sup.-1 k/m, and 
r-1.ident.r, then the following expression can be obtained: 
##EQU4## 
Thus, the following expression can be obtained: 
##EQU5## 
Thus, in this example, the period is 46.15 (nsec) i.e. the frequency is 
21.67 MHz, in the peripheral portions. For the middle portion, 
##EQU6## 
so that 
EQU k.sub.2 =cos (sin.sup.-1 0.7)=0.71 
and thus 
EQU k.sub.2 T.sub.1 =0.71.times.46.15=32.31 (nsec). 
Thus, in this example, the period is 32.31 (nsec), i.e. the frequency is 
30.95 MHz, in the middle portion 
A second embodiment of the present invention will next be described, with 
reference to FIG. 8. In FIG. 8, parts having corresponding elements in 
FIG. 1 are designated by corresponding marks and will not be described 
again. A semiconductor laser 11 for generating a video clock signal is 
placed such that the p-n junction plane is orthogonal to the p-n junction 
plane of another semiconductor laser 10 for emitting a writing laser beam. 
Between a collimator 12 and a resonant scanner 14, there is placed a 
polarizing beam splitter 15, and between the polarizing beam splitter 15 
and the semiconductor laser 11 there is placed another collimator 13. 
Between the resonant scanner 14 and a scanning lens 18, there is placed a 
polarizing beam splitter 100. The laser beam reflected from the polarizing 
beam splitter 100 is directed toward a linear encoder 104. The linear 
encoder 104 comprises an opaque plate having a large number of elongated 
transparent portions of the same width formed therein with a constant 
pitch therebetween. When the linear encoder 104 is horizontally scanned, a 
photoelectric converter 106 will output a photoelectric pulse signal M 
which is amplified by an amplifier 108 to a predetermined level. The 
photoelectric pulse signal M is shown in FIG. 10. The photoelectric pulse 
signal M has blanking intervals of duration 2a which correspond to the 
light beam scanning opaque portions of the linear encoder 104 adjacent to 
the opposite ends thereof; the photoelectric pulses between one blanking 
interval and the next blanking interval is generated by one horizontal 
scanning cycle of the deflecting mirror 16. The period .tau. will vary 
with the scanning speed determined by the deflecting mirror 16, i.e. the 
period becomes shorter as the scanning speed increases and becomes longer 
as the scanning speed decreases. 
The photoelectric pulse signal M is supplied to a video clock signal 
generating circuit 110 via an amplifier 108. The video clock signal 
generating circuit 110 will be described, with reference to FIG. 9. 
Waveforms occurring at various parts of FIG. 9 are shown in FIG. 10. A PLL 
circuit, consisting of a phase comparator 122, a low-pass filter 124, a 
gain/bias adjusting circuit 126, a VCO (voltage-controlled oscillator) 
128, and a frequency divider 132, will multiply the photoelectric pulse 
signal M by a predetermined factor. That is, the VCO 128 will output a 
clock signal G which is divided by the frequency divider 132 into a 
fraction of 1/N. The divided comparison pulse signal H is input into the 
phase comparator 122 which compares its phase with that of the 
photoelectric pulse signal M. If the phases do not correspond to each 
other, then a signal is delivered which has a pulse width varying 
depending on the direction and magnitude of the lag. This pulse signal is 
converted into a direct current by the low-pass filter 124 and provided to 
the gain/bias adjusting circuit 126. The gain/bias adjusting circuit 126 
will output a d.c. voltage whose level varies such that the phase lag is 
corrected, and such d.c. current is applied to the VCO 12B. Thus, the PLL 
circuit is feedback controlled in such a way that the phase of the 
photoelectric pulse signal M will correspond to that of the comparison 
pulse signal H; at the moment the phases come to correspond together, 
phase locking control is effected and the VCO 128 will output a clock 
signal G whose timing corresponds to the photoelectric pulse signal M and 
whose frequency is that of the photoelectric pulse signal M multiplied by 
a factor of N. 
The photoelectric pulse signal M is input into a first retriggerable 
multivibrator 136. The first multivibrator 136 has a time constant which 
is set to be longer than the period of the photoelectric pulse signal M. 
The first multivibrator 136 has a low level output after the completion of 
the immediately preceding horizontal scanning cycle, and is turned high 
when triggered by the first pulse of the photoelectric pulse signal M 
occurring in the next horizontal scanning cycle The output signal J from 
the first multivibrator 136 is supplied to reset terminals of a pre-set 
counter 116 and of a flip-flop 118, resetting these with its rising edge. 
The pre-set counter 116 counts a predetermined number of pulses in order to 
invalidate the pulses of the photoelectric pulse signal M occurring while 
the PLL circuit is locked up. When it has counted the predetermined number 
of pulses, it will reset the flip-flop 118. The output signal K from the 
flip-flop 118 is input into a gate circuit 120 and opens this gate. Since 
the gate circuit 120 also receives the clock signal G from the VCO 128, 
the gate circuit 120 is opened when the flip-flop 120 is set, whereby the 
clock signal G is output therefrom as the video clock signal. 
Thus, the locking up time is converted into a corresponding number of 
pulses of the photoelectric pulse signal; the number of pulses is counted 
by the counter 116; when the predetermined number has been reached and the 
phase locking control has been effected, the flip-flop 118 is set to open 
the gate circuit 120, whereby the clock signal G can be taken out as the 
video clock signal. A stable video clock signal can thus be obtained whose 
timing corresponds to that of the photoelectric pulse signal M and whose 
frequency is that of the photoelectric pulse signal M multiplied by a 
factor of N. Since a longer duration of the locking up will make the 
effective scanning line length shorter and will reduce the solution, the 
duration of the locking up is determined depending on the difference in 
phase and frequency between the photoelectric pulse signal M and the 
comparison pulse signal H. Thus, the amount of phase correction is small 
and the locking up is quickened if such differences are small. In view of 
this, the output signal J from the first multivibrator 136 is input onto a 
second multivibrator 134 having a very short period; the frequency divider 
132 is set by the pulse signal L from the second multivibrator 134; these 
two are forcibly synchronized at the beginning point of the first pulse of 
the photoelectric pulse signal; whereby the amount of phase correction is 
reduced and the duration of the locking up is shortened. Between the first 
multivibrator 136 and the gain/bias adjusting circuit 126 there is 
connected an amplifier 130 which, while the phase locking control is 
effected, amplifies the output signal from the first multivibrator 136 in 
such a manner that a d.c..current of the same level as the d.c. current 
output from the gain/bias adjusting circuit 126 will also be output during 
blanking periods. 
As in the first embodiment, the video clock signal from the video clock 
signal generating circuit 110 is supplied to the drive circuit 44 via the 
horizontal timing generating circuit 38 and via the P/S converting circuit 
40. Meanwhile, an F/V converting circuit 112 is connected to the video 
clock signal generating circuit 110; the circuit 112 provides a voltage V 
which is proportional to the frequency F of the video clock signal. The 
output from the F/V converting circuit 112 is connected to the drive 
circuit 44 via the V/1 converting circuit 42. Therefore, a video signal 
can be obtained which is similar to the one shown at (2) in FIG. 6. 
The above-described embodiments use a resonant scanner to perform the 
principal scanning operation. The present invention however is also 
applicable to arrangements where the principal scanning operation is 
performed using a rotating polygon mirror or a galvanometer. Further, 
though the described embodiments use a semiconductor laser as the light 
source, and argon laser or a light modulator can also be used as in the 
case of the LASERCOM. Further, the horizontal timing circuit may be 
provided as required; a phase-shifting circuit for shifting the phase by a 
predetermined amount may be provided at the output of the amplifying and 
biasing circuit.