Method of producing a halftone picture by vibrating light source

A spot of light is formed upon a photosensitive material which is moved in a scanning direction, and the spot is vibrated to and fro along a vibration direction at a considerable angle to the scanning direction, and the amplitude of vibration, and the center of the vibration path, are controlled so that the spot exposes areas on the photosensitive material which form half-tone dots.

This invention relates to a method for producing a halftone picture by 
scanning, in which halftone dots produced are aligned on inclined lines at 
a certain screen angle to the scanning direction. 
Many methods and means for reproducing a halftone picture by a picture 
scanning machine, such as a scanner for plate-making, have heretofore been 
developed. 
For example, a method for reproducing a halftone picture has been invented 
by the same inventors as the present invention, and has been described in 
Japanese Patent Laying-open Publication No. 53-49501. The present 
invention is an improvement of this prior invention. 
In this prior invention, as shown in FIG. 1, a light ray generated from a 
light source such as a laser tube, which produces a light spot "a" which 
scans a photo-sensitive material attached to a rotating recording 
cylinder, is oscillated in the direction of the cylinder's axis by an 
acoustic-optical inclinator element of well-known construction, so as to 
construct halftone dots, as shown by dotted lines. Further, in order to 
remove the defect of over-exposure of the photosensitive material during 
production of the front and the end portions of the dot, and 
under-exposure of the photosensitive material during production of the 
middle portion of the dot, which may occur because the laser ray moves 
slowly at the front and end portions because of its short stroke and 
quickly in the middle portion because of its long stroke, the brightness 
of the light spot is reduced during production of the front and the end 
portions of the dot, and is increased during production of the middle 
portion, by an optical modulator element of a well-known type, which may 
be either an acoustic-optical or an electric-optical modulator element. 
This method is useful, but it has a defect. A halftone picture in which the 
halftone dots are aligned on inclined lines having a certain non-zero 
screen angle relative to the scanning direction cannot be produced. The 
half-tone dots produced by this prior art are aligned on lines having no 
screen angle. 
As is well-known in the art, when several different halftone pictures are 
to be used for color separations, it is necessary that the pictures should 
have different screen angles, in order to avoid undesirable moire 
patterns. Therefore the abovementioned defect of the prior art is quite 
serious. 
Therefore it is an object of the present invention to provide a method for 
reproducing a halftone picture in which the halftone dots produced are 
aligned on inclined lines having a certain screen angle relative to the 
scanning direction, which has a simple operation, and which is reliable. 
According to the present invention, this object is accomplished by the 
provision of a method for producing a halftone picture on a photosensitive 
material, comprising the steps of forming a spot of light on the surface 
of the material, causing motion of the material relative to the spot in a 
scanning direction, vibrating the spot to and fro along a vibration 
direction which is at a substantial angle to the scanning direction 
through a certain vibration width about a central point, the vibration 
center, and controlling the amplitude of the vibration width and the 
position of the vibration center so that the spot exposes areas on the 
photosensitive material which form half-tone dots, as the material moves 
relative to the spot in the scanning direction.

In FIG. 2 there is shown one embodiment of a machine for performing a 
method according to the present invention. 
A laser light ray generated from a light source 1 such as a laser tube is 
projected onto a photosensitive material 7 attached to a recording 
cylinder 6 via an optical modulator element 2, an optical inclinator 
element 3, a condenser lens 4, and a focusing lens 5. While the brightness 
of the light ray is controlled by the optical modulator element 2, the 
light ray, before it impinges upon the photosensitive material 2, is 
inclined by the optical inclinator element 3, by an amount depending upon 
the frequency of the signal input to the optical inclinator element 3. 
This inclination is, in this embodiment, arranged to be in a vibration 
direction perpendicular to the scanning direction, which is the 
circumferential direction of the rotating cylinder 6. During operation the 
frequency of the signal input to the optical inclinator element 3 changes 
up and down very rapidly compared to the scanning speed, so that the spot 
of light formed by the light ray upon the photosensitive material 7 is 
moved to and fro very quickly compared to the scanning speed. Thus it is 
seen that the optical inclinator element 3 is frequency-controlled, while, 
by contrast, the optical modulator element 2 is controlled by the 
amplitude of the signal fed into it. 
By suitably controlling the position of the center of the vibration path of 
the light spot, and the width of the vibration, it is possible, for 
instance, to cause the light spot to move so as to expose areas of the 
photosensitive material 7 which form halftone dot areas as required, 
according to the picture signals obtained by scanning an original picture 
and performing color separations on it. The method of controlling the 
light spot will now be described. 
A pulse generator 8 is arranged coaxially to the recording cylinder 6 and 
generates pulse signals which serve for synchronization of the scanning 
with the rotation of the cylinder, and which are sent to a halftone dot 
signal generator 10. In this embodiment, picture signals picked up by 
scanning an original picture (not shown) are also sent to the halftone dot 
signal generator 10. However, it is quite within the scope of the present 
invention that the picture signals may be obtained from another kind of 
source, for instance from a computer memory. In any event, the halftone 
dot signal generator 10 receives input picture signals and synchronization 
signals and produces output width signals which represent the width 
required for the vibration path of the light spot, and position signals, 
which represent the position required for the center of the vibration path 
of the light spot. 
The halftone dot signal generator, as shown in FIG. 3, comprises a first 
memory 102 for the width signals, a second memory 103 for the position 
signals, a phased lock loop circuit 101 which converts the frequency of 
the pulse signals coming from the pulse generator 8 and generates reading 
clock pulses for reading the position signals and the width signals stored 
in the first and the second memories 102 and 103 corresponding to the 
sampling pitches of the picture signals. 
In the first and the second memories 102 and 103 there are stored a series 
of digital values of the width signals and the position signals for the 
halftone dots, a different set of which is provided for each halftone dot 
area rate to correspond to a certain level of picture signal. Further, a 
different set of values for the width signals and position signals for 
each dot area rate is provided for each desired screen angle. By the 
nature of the process of the present invention, halftone dot pictures can 
only be produced which have a screen angle, relative to the scanning 
direction, which is the arc cotangent of an integer. In FIG. 6, for 
example, where the scanning direction is up and down, the screen angle is 
the arc cotangent of 4. This means that on each scan line the dots whose 
centers are within that scan line occur in an exactly repeating pattern. 
In accordance with this, for each halftone dot area rate required, and for 
each desired screen angle, width and position signals such as shown, for 
example, in FIG. 4 are stored in the memory. The phased lock loop circuit 
101 produces an output frequency which is a multiple of the input 
frequency supplied by the pulse generator 8 and which reads values of the 
width and position signals from the memories 102 and 103 in succession 
until the pattern required for these signals repeats according to the 
repeating required on the scan line, as above explained. Then the reading 
addresses from the memories are set back again to the beginning, and the 
process is repeated. 
The width and position signals required to produce halftone dots 
corresponding to different halftone dot area rates must of course all be 
stored in the memories together, since the color values of the picture may 
vary from moment to moment across it. However, the width and position 
signals required for different screen angles need not all be stored in the 
memory together, but when the machine of FIG. 2 is being reset for a 
different screen angle the memory may be reloaded in some appropriate 
fashion with the new halftone dot signals required for the new screen 
angle. Or, alternatively, the signals required for a variety of screen 
angles may all be held in the memory together and a suitable means may be 
provided to access one row or another of them, depending upon the actual 
screen angle required. In any event, this is a matter of conventional 
technology, well known to those skilled in the art, and is not germane to 
the concept of the present invention. 
In FIG. 4 is shown an example of a width signal and a position signal 
stored in the two memories 102 and 103. In this figure (a) represents the 
outline of the halftone dot that is to be generated; (b) represents the 
width signal that is to determine the width of the vibration of the light 
spot; and (c) represents the position signal that is to determine the 
position of the center of the vibration of the light spot. The halftone 
dot generated has a screen angle .theta. with respect to the scanning 
direction, which in this figure is left to right. 
In FIGS. 5, (a) and (d), represent examples width and position signals 
corresponding to a halftone dot area of 50%, as stored in the memories. If 
the picture signal input to the halftone dot generator 10 requires 
halftone dots with 50% dot area, then the generator 10 generates these 
signals, as in FIG. 5. Referring to FIG. 2, the width signal, such as 
shown in FIG. 5(a), is sent to a mixer 19 which also receives a carrier 
signal as shown in FIG. 5(b) produced by a carrier frequency generator 18. 
The mixer 19 amplitude-modulates the carrier signal according to the width 
information, and outputs an amplitude-modulated signal, shown in FIG. 
5(c). 
The frequency of the carrier signal determines the number of vibrations to 
and fro on the photosensitive material 7 performed by the light spot, as 
caused by the optical inclinator element 3. This frequency is usually set 
to a tenth of the frequency (several MHz) of a radio frequency signal 
generated from a radio frequency signal generator 11. 
This radio frequency signal from the radio frequency signal generator 11 is 
sent to a mixer 12 and is amplitude-modulated there by the width signal 
fed from the halftone dot signal generator 10 in the same manner as in the 
mixer 19. The modulated signal is sent to a power amplifier 17 and then to 
the optical modulation element 2 so as to control the brightness of the 
light ray fed from the light source 1, in the same way as in the 
above-mentioned prior art. 
The carrier signal as modulated in the mixer 19 is then sent to an adder 13 
and to it there is added the position signal shown in FIG. 5(d) sent from 
the halftone dot signal generator 10, to obtain a halftone dot form signal 
as shown in FIG. 5(e) which is adapted to produce halftone dots at a 
certain screen angle to the scanning direction. 
This thus-obtained halftone dot form signal is then sent to a 
voltage-frequency converter 15 via a voltage amplifier 14 and is converted 
there into a frequency-modulated signal which is fed to the optical 
inclinator element 3 via a power amplifier 16 and which inclines the light 
beam fed from the optical modulator element 2 to and fro so as to cause it 
to form halftone dots having a certain screen angle with respect to the 
scanning direction on the photosensitive material 7. The outline of the 
general form of these dots is shown in FIG. 5(e) by a one-dotted line. 
The above describes the basic method of forming halftone dots according to 
the present invention. FIG. 6, in which the halftone dot area is 50%, 
further illustrates the process. 
During each scanning line the halftone dots whose centers are included in 
that scanning line are produced. Thus by the first scanning S.sub.1 the 
halftone dots D.sub.11, D.sub.12, and D.sub.13 are produced. Then the 
second scanning S.sub.2 produces the halftone dots D.sub.21 and D.sub.22, 
the third scanning S.sub.3 produces the halftone dots D.sub.31 and 
D.sub.32, the fourth scanning S.sub.4 produces the dots D.sub.41 and 
D.sub.42, the fifth scanning S.sub.5 produces the dots D.sub.51, D.sub.52, 
and D.sub.53, and so on. 
In this case it is necessary to determine in advance whether the halftone 
dots in question to be recorded are included in the scanning line being 
performed. 
Since the centers of the halftone dots are determined once the number of 
lines per unit length of the screen, and the screen angle, are determined, 
these locations of the centers of the halftone dots can be stored in the 
memories in advance. Thus they can be read in sequence and a 
discrimination can be made as to whether the halftone dots lie in the 
scanning line being recorded or not. However, in this case the memories 
may have to have a very large capacity. Another way is to calculate the 
centers of the halftone dots to be recorded for each scanning line and to 
control the optical modulator element by the calculation results. This is 
more practicable. The details will be obvious to one skilled in the art. 
In FIG. 7, as another example, are shown the patterns of halftone dots 
having halftone dot area rates of 12.5%, 50% and 87.5%. In each case the 
dots in each successive scan line are recorded according to whether or not 
their centers lie in that scan line. It will be seen that when the 
halftone dot area rate is greater than 50% the dots overlap. In general 
according to the present invention when a dot whose center lies in a scan 
line is being drawn the position of the light spot may move out far beyond 
that scan line and encroach on the neighboring scan lines. 
In FIG. 8 there is shown another embodiment of a machine for performing a 
method according to the present invention, in which elements which 
correspond to numbered elements in FIG. 2 are designated by the same 
reference numerals. 
In this case the radio frequency signal generator 11 acts also as the 
carrier frequency generator 18 in FIG. 2, and it is a simpler 
construction, when the optical inclinator element 2 is adapted to respond 
quickly enough to the signal input to it, which has a frequency of several 
tens of MHz. 
It will be appreciated that if the optical inclinator element 3 is also 
provided with the function of an optical modulator element the separate 
optical modulator element can be omitted. Such has been done in another 
embodiment. In this case the frequency-modulated signals shown in FIG. 
9(a), which are output from the voltage-frequency converter 15 in FIG. 2, 
are again amplitude-modulated by the width signals from the halftone dot 
signal generator 10 to obtain the amplitude-and-frequency-modulated 
signals shown in FIG. 9(b), which are sent to the optical 
inclinator/modulator element 3 for controlling the light ray fed thereto. 
In this embodiment an additional advantage is obtained since only one 
optical inclinator/modulator element is used and there is no need to 
adjust the setting of the optical modulator element with respect to the 
optical inclinator element. This is more convenient. 
In the embodiments described above the vibration direction of the light 
spot, that is to say, the direction in which it is moved by the optical 
inclinator element, is perpendicular to the scanning direction. However, 
this is not a necessary limitation. In various embodiments this angle 
could be different, by making the setting angle of the optical modulator 
different, and it would merely be necessary to store different contents in 
the memories. Another particular embodiment can be obtained when the angle 
between the scanning direction and the vibration direction, added to the 
screen angle with respect to the scanning direction of the halftone dots 
produced, is 90.degree.; that is to say, when the vibration lines are 
diagonally across the halftone dots generated, as shown in FIG. 10(e), 
wherein the scanning direction is from left to right. 
In this case the required screen angle is obtained by inclining the optical 
inclinator element with respect to the axis of the recording cylinder by 
the screen angle. The width signals and the position signals stored in the 
halftone dot signal generator's memories have simpler forms, as shown in 
FIGS. 10(a) and (d), in which the halftone dot area is 50%. With respect 
to the embodiment shown in FIG. 2, when the picture signal is input to the 
halftone dot signal generator 10 the width signal shown in FIG. 10(a) and 
the position signal shown in FIG. 10(d) are output. The width signal is 
sent to the mixer 19. The carrier signal shown in FIG. 10(b) generated by 
the carrier signal frequency generator 18 is fed to the mixer 19 and is 
amplitude modulated there, obtaining the amplitude modulated signal shown 
in FIG. 10(c). The carrier signal modulated in the mixer 19 is then sent 
to the adder 13 and is added there to the position signal shown in FIG. 
10(d) to obtain the halftone dot form signal shown in FIG. 10(e) having 
the desired screen angle. Thus halftone dots of the required form are 
recorded, as described above. 
Although the present invention has been shown and described with reference 
to preferred embodiments thereof, it will be readily understood that 
various changes and modifications can be made to the detail thereof by a 
person skilled in the art without departing from the scope of the 
invention. For example, it would be possible to memorize in the memories 
only a width signal and a position signal corresponding to the halftone 
dot area rate of 100%, and to calculate the width and position signals 
corresponding to other halftone dot area rates from them on a continuous 
basis. Other alterations can be easily thought of. Therefore it is desired 
that the scope of protection granted should not be limited by the details 
of any of the embodiments described, or of the illustrations, both of 
which were given for purposes of explanation only, but only by the 
appended claims.