Laser printing device with control of oscillation amplitude

In a laser printing device in which a laser beam is deflected in a scanning mode while its intensity is being modulated, to record a halftone image on an optical recording material, the scanning laser beam is oscillated in a direction substantially perpendicular to the scanning direction within one pitch of the scanning line formed on the optical recording material, and the oscillation of the scanning laser beam is changed in amplitude by a signal provided by a computer, whereby a halftone image is recorded with high accuracy.

BACKGROUND OF THE INVENTION 
This invention relates to laser printing device, and more particularly to a 
laser printing device which is suitable for recording or printing images 
gradated in density, or having halftones (hereinafter referred to as 
"halftone images". 
Heretofore, in order to record halftone images, a method is employed in 
which the intensity of a laser beam applied to an optical recording 
material is modified in an analog mode. However, the method is 
disadvantageous in that, when the optical recording material is employed 
which is low in the characteristic of gradation with respect to optical 
intensity, two-valued records, namely, black and white are excessively 
emphasized. This difficulty can be effectively eliminated by the 
employment of a halftone recording method in which the recording of an 
image is carried out substantially in a binary mode; that is, large dots 
are employed for black regions, and small dots are employed for gray 
regions the color of which is nearly white. In a conventional halftone 
forming system, a laser beam applied to an optical system is changed in 
position, to cause an aberration in the optical system thereby making the 
resultant light spot foggy. However, the method suffers from a difficulty 
that the relationship between the fogginess of the light spot and the 
laser beam's incident position is relatively low in linearity, and 
therefore it is difficult to record halftone images with high accuracy. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to eliminate the 
above-described difficulties accompanying a conventional method of 
recording halftone images. 
More specifically, an object of the invention is to provide a laser 
printing device which practices a halftone recording method to record 
halftone images with high accuracy. 
The foregoing object and other objects of the invention have been achieved 
by the provision of a laser printing device in which a laser beam is 
deflected in a scanning mode while the intensity of the laser beam is 
being modulated, to perform an optical recording operation with an optical 
recording material, which, according to the invention, includes: 
oscillating means for oscillating a scanning laser beam in a direction 
substantially perpendicular to a scanning direction within one pitch of a 
scanning line formed on the optical recording material, the oscillation of 
the scanning laser beam being changed in amplitude by a signal provided by 
a computer, to perform an optical recording operation. 
In the laser printing device, the gradation characteristic may be improved 
by changing the intensity of the laser beam reaching the optical recording 
material with the amplitude of oscillation of the scanning laser beam, 
because as the amplitude of oscillation increases, the intensity per 
unitary area of the laser beam applied to the optical recording material 
is decreased. 
In the laser printing device, the oscillating means may be an optical 
deflector based on an electro-optical effect. 
The nature, principle and utility of the invention will become more 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
As conducive to a full understanding of this invention, first a halftone 
recording system according to the invention will be described briefly with 
reference to FIG. 1. In the halftone recording system, a scanning line 1 
is oscillated in a direction 2 perpendicular to the scanning direction 
(the direction 2 may be called "an auxiliary scanning direction"). In this 
case, the amplitude of oscillation corresponds to the dimension of a 
halftone dot 3 or 3' in the auxiliary scanning direction. On the other 
hand, the dimension 4 of the halftone dot in the scanning direction is 
determined by the period of time for which the laser beam is modulated. 
Hence, the size of a halftone dot is recorded accurately, and accordingly 
a halftone image can be recorded with high accuracy. 
An example of a laser printing device according to this invention will be 
described with reference to FIG. 2. 
In the laser printing device, the output laser beam of a semiconductor 
laser 5 is converted into a parallel beam by a lens 6, and the laser beam 
thus processed is applied through an electro-optical element 7 adapted to 
oscillate a scanning laser beam to a rotary polygon mirror 8, where it is 
deflected in a scanning mode. The laser beam thus deflected (indicated at 
12 in FIG. 2) is applied through an F.theta. lens 9, as a light spot, onto 
a photo-sensitive drum 11; that is, the latter 11 is scanned with the 
light spot. The position of the scanning light beam is detected by a photo 
detector 10 disposed near the photo-sensitive drum 11. The output signal 
of the photo detector 10 is applied, as a synchronizing signal for 
optically recording data, to a control circuit 14. The synchronizing 
signal is as shown in FIG. 4(a). With the synchronizing signal as a 
trigger signal, a data reading clock signal is produced as shown in FIG. 
4(b). The data reading clock signal thus produced is used to read data as 
shown, for instance, in FIG. 4(d) which is to be optically recorded. The 
data thus read is applied to the semiconductor laser 5 shown in FIG. 2, to 
binary-modulate the output laser beam of the semiconductor laser 5. Thus, 
the data is optically recorded on the photo-sensitive drum 11. In 
synchronization with the production of the data reading clock signal, a 
gradation clock signal is produced as shown in FIG. 4(c) the frequency of 
which is an integer factor of the data reading clock signal. The data to 
be recorded includes a binary signal as shown in FIG. 4(d) and a signal 
for gradation as shown in FIG. 4(e). In FIG. 4(e), the signal smaller in 
amplitude indicates data of gray which is nearly white, and the signal 
larger in amplitude indicates data of a color which is nearly black. The 
signal for gradation (FIG. 4(e)) (hereinafter referred to as "a gradation 
signal (e)" and the gradation clock signal (FIG. 4(c)) (hereinafter 
referred to as "a gradation clock signal (c)" are used to form a signal 
for driving the electro-optical element 7 which is shown in FIG. 4(f) 
(herein after referred to as "a drive signal (f)". 
The signal for driving the electro-optical element 7 (FIG. 4(f)) is formed 
by a circuit shown in FIG. 5. The circuit comprises a mixer 17 which 
receives the gradation signal (e) and the gradation clock signal (c). The 
gradation clock signal (c) is shown substantially as a sinusoidal wave 
signal (c) in FIG. 5, being considerably high in frequency. In the mixer 
17, the gradation clock signal (e) is amplitude-modulated with the 
gradation signal (c), to provide the aforementioned drive signal (f). 
The electro-optical element 7 is so positioned that, when the drive signal 
(f) is applied thereto from the control circuit 14 through an amplifier 
13, the scanning laser beam is oscillated in a direction perpendicular to 
the scanning direction. Hence, as indicated at 18 in FIG. 6, the data are 
recorded as area-modulated halftones on the photo-sensitive drum. 
One example of the electro-optical element 7 is shown in FIG. 3, and 
includes an electro-optical crystal 7-2 and two electrodes 7-1 and 7-3 
formed on the upper and lower surfaces of the crystal 7-2 by vacuum 
deposition. Upon application of a voltage between the electrodes 7-1 and 
7-3, the refractive index of the electro-optical crystal 7-2 is changed, 
so that the emergent laser beam 16 is deflected as indicated by the arrow. 
It is preferable that the electro-optical element is of an optical 
waveguide type, because it can be driven with a low voltage. 
FIG. 4(d') is for a description of a halftone recording operation which is 
carried out in the scanning direction. In FIG. 4(b), the data reading 
clock pulse interval corresponds to one picture element. In the case of 
FIG. 4(d'), the semiconductor laser is driven by using the gradation clock 
signal shown in FIG. 4(c) in such a manner that a pulse short in duration 
is applied for a picture element which is nearly white while a pulse long 
in duration is applied for a picture element which is nearly black. The 
addition of the above-described function makes it possible to record 
halftones both in a direction perpendicular to the scanning direction and 
in a direction parallel with the scanning direction, with the result that 
images can be recorded with high accuracy. 
Thus, the laser printing device of the invention can record halftone images 
with high accuracy according to the halftone recording method. 
While there has been described in connection with the preferred embodiment 
of this invention, it will be obvious to those skilled in the art that 
various changes and modifications may be made therein without departing 
from the invention, and it is aimed, therefore, to cover in the appended 
claims all such changes and modifications as fall within the true spirit 
and scope of the invention.