Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION OF THE INVENTION
The present inven ion relates to an optical printer
capable of changing a character magnification ratio.
It is an object of an aspect of this invention to
provide a scanning mechanism with a means for changing the
magnification ratio of characters without changing the speed
of the scanning mechanism.
It is an object of an aspect of this invention to
change the magnification ratio of characters without changing
the speed of the scanning mechanism while maintaining the
optical density of the various size characters constant.
In accordance with this invention there is provided
an optical printer comprising: an illumination source
generating a beam of light, a recording medium, a main scanning
member in the path of said beam for scanning said beam at a
fixed speed in a main direction across said recording medium,
an acousto-optic deflector located in the path of said beam
between said main scanning member and said illumination source,
said deflector scanning said beam onto said main scanning
member in a direc:tion transverse to said main direction, a
deflection signa] generator operatively connected to said
deflector for adjusting the stroke length of auxiliary scanning
in accordance with the magnification ratio of a character to
be printed and for simultaneously adjusting the frequency of
auxiliary scanning by the reciprocal of the magnification ratio
of the character to be printed.
This invention will become apparent from the
following description with reference to the drawings wherein:
Figure 1 illustrates a conventional optical printer;
Figure 2 illustrates a character composed of a dot
matrix;
Figure 3 is a waveform chart showing the timing of
main and auxiliary scanning in the printer of Figure l;
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Figure 4 is a waveform chart showing the timing of
~uxiliary scanning and light modulation in the printer of
. Figure l;
' Figure 5 is a schematic illustration of an optical
printer embodying the present inven~ion;
Figure 6 illustrates a printing pattern obtained by
' the printer of Figure 5; and
Figure 7 is a waveform chart showing the timing of
' each operation in the printer of Figure 5.
With the recent trend toward attainment of high-speed
' printers for use in computer output units and other similar units,
the optical printer based on the combination of an optical charac-
~, ter generator and electrophotography has come to be employed.
Generally,,the optical printer is composed in such a m~ner as shown
in Figure 1. A light beam emitted from a laser light source (1)
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is modulated by a light modulator (2) in accordance with infor-
mation. The light modulator (2) consists of, for example, a
pockel-type cell which is made of potassium dehydrogen phosphate
and serves to convert periodically a refractive index in propor-
tion to the level of an applied voltage representing an electricsignal. After modulation, scanning in the column or vertical
printing direction (hereinafter referred to as auxiliary scanning)
is performed by a light deflector (3) consisting of an acousto- ;
optic element, and simultaneously scanning in the row or hori-
zontal printing direction (hereinafter referred to as main
scanning) is performed by a mechanical deflector (4) consisting
of a rotary mirror or galvano-mirror. The light beam thus
deflected is projected through a suitable optical system (5)
onto a sensitive member (6), such as sensitive drum or photo-
15 graphic plate of a printing mechanism based on electrostatic ~ `
photography, so that a character pattern is recorded. In the
above-mentioned printer, when the height of auxiliary scanning
is set to the height of a character to be printed, an optical
image covering characters of one line is formed by one main
2Q scanning. For example, in case a character pattern consists of
a K-row, L-column dot matrix as illustrated in Figure 2, when
printing N characters per line, auxiliary scanning is executed
L x N times during one main scanning. The relationship between
main scanning and auxiliary scanning is shown in Figure 3.
In the meanwhile, the waveform modulated by the light
modulator (2) is synchronized with the auxiliary scanning as
plotted in Figure 4. With regard to a video clock signal serving
as reference timing for modulation of the light beam, K pulses,
equal in number to the horizontal rows of the matrix constituting
3a one character, are generated per auxiliary scanning so that the
light beam is modulated dot by dot synchronously with the video
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clock pulses, and one vertical column of the character is
printed by the light beam modulation corresponding to K dots.
One character is printed by repeating this procedure L times
(corresponding to the number of columns in the character matrix),
and one-line printing is achieved by repeating the one-character
printing N times. In the aforementioned optical printer, how-
ever, it is extremely difficult to change the character size
halfway through the line during printing for the following
reasons. That is, the size of a character to be printed is
changed generally by changing the main and auxiliary scanning
speeds. However, changing the character size in this manner
presents technica] difficulties in making a substantial change
in the main scanning speed due to the inertia of the mechanical
deflector.
The present invention accomplishes changing the cha-
racter size by setting the main scanning speed of a mechanical
deflector to a constant predetermined value and changing a
printing magnification ratio through changes in the auxiliary
scanning period and the auxiliary scanning stroke length, there-
by providing an improved printer which is capable of changing
the printing magnification ratio without undergoing any undesir-
able effect from inertia of the mechanical deflector.
The present invention is described in detail with
reference to an exemplary embodiment as shown in Figures 5
through 7. When characters to be printed and a printing magni-
fication ratio are specified by a computer (not shown), a video
clock generating circuit (7) generates video clock pulses of a
period proportional to the magnification ratio in the auxiliary
scanning direction for the characters to be printed. In Figure
6, the time from tl to t2 denotes normal-size printing; the time
from t2 to t3 is a space period between characters for adjustment
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of a printing interval; and the time from t3 to t4 shows the
auxiliary scan length doubled wherein a character is magnified
twice both vertically and horizontally as compared with the
normal-size printing. In this case, the video clock period is
; 5 increased twofold in accordance with the printing magnification
ratio for the character, which is doubled both vertically and
horizontally. The main scanning speed of a mechanical deflec-
tor (4) is maintained at a constant speed, and the auxiliary
scanning speed of an acousto-optic deflector (3) also remains
constant. Thus, the dot-to-dot interval in the auxiliary
scanning direction (vertical) is increased to double the time
required for the normal-size auxiliary scanning, thereby doubl-
ing also the dot-to-dot interval in the main scanning direction
(horizontal) to effect printing of a character, which is magni-
fied twice both vertically and horizontally. The video clockpulses produced from the video clock generator (7) are applied
to a dot matrix character information signal generator (8), and
a modulator driving circuit (9) operates synchronously with the
video clock pulses during a period of the presence of character
information signals in accordance with the corresponding print-
ing pattern so that the light beam emitted from the laser light
source ~1) is modulated by the light modulator (2) connected to
the modulator driving circuit (9).
In the meantime, the video clock pulses are applied
also to a deflection signal generator (11), and the deflection
signal therefrom is converted into a suitable frequency by a
voltage-to-frequency converter (10) and then is applied to the
acousto-optic deflector (3) so as to deflect the light beam in
the auxiliary scanning direction (vertical).
When the printing magnification ratio is increased
twice both vertically and horizontally, the character area
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multiplies substantially four times, but the optical density of
the character is reduced to one half because the printing time
per dot, which is proportional to the extended time in the
column direction of one character, increases only twice. Even
though a fixed optical density is retained in the individual
dots printed, it is not desirable that the density of the charac-
ter as a whole change with a change in the printing magnification
ratio. Therefore, an auxiliary gate pulse generator (12) (see
Figure 5) provides auxiliary gate pulses proportional to the -
square of the relative character size (see Figure 7). A light
modulation signal is projected according to the AND of the
auxiliary gate pulse and the character information signal, and
light modulation is executed in accordance with the light modu-
lator signal thereby maintaining the apparent character density
at a fixed value.
As described in detail above, the feature of changing
a printing magnification ratio while holding the main scanning
speed of the mechanical deflector constant renders the present
invention capable of achieving a change of the character size
2Q anywhere on a line, which has been difficult heretofore due to
overcoming the inertia of the mechanical deflector. Thus, the
provision of, for example, an eight-point character font in the
printer renders possible the printing of any desired character
size such as 14-point, 16-point, etc.
Accordingly, the present invention facilitates reading
of different character fonts and, furthermore, ensures high
reliability of a printer since the component parts of a mechan-
ical deflector including the rotary mirror are actuated at a
fixed speed without being affected by jitter due to varying
speeds.
It is to be understood that although the light modula-
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tor and the acousto-optic deflector employed in the foregoing
embodiment are individual and independent of each other, the
acousto-optic deflector may be so composed as to combine the
function of the light modulator, and the light beam is not
limited to laser light alone but may be any visible light as
long as it is of a fixed wavelength.
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