Ink-jet recording system capable of recording a half-tone

An ink jet printer is able to print half-tones with greater fidelity. The printer prints an image in half-tones by creating dots having diameters which vary in correspondence to the density of the half-tone at the corresponding point in an image. The diameter of the dot is controlled by controlling the energy content of a driving pulse which creates the dot (e.g. by varying a driving pulse width). When an image point is too light to be realistically printed by the smallest diameter dot that can be made by the printer, no dot is printed. Instead, the signal representing the unprinted dot is stored and added to the signal of a subsequent image point. This way, the dot at the subsequent point is made a little darker to compensate for the non-printing of the preceding dot.

This invention relates to an ink-jet recording system capable of recording 
half-tones. 
BACKGROUND OF THE INVENTION 
An on-demand type ink-jet print head is disclosed in the U.S. Pat. No. 
3,946,398 entitled "Method and Apparatus for Recording with Writing Fluids 
and Drop Projection Means Therefore", issued to Kyser et al. There, an ink 
droplet is formed by applying a driving pulse to a piezoelectric element 
installed on a pressure chamber of the ink-jet print heat. As disclosed in 
the U.S. Pat. No. 4,281,333 entitled "Ink-On-Demand Type Ink-Jet Printer 
with Coordinated Variable Size Drops with Variable Charges", issued to the 
present inventors, the droplet size (volume) may be varied by controlling 
the energy content of a drive pulse. For example, the amplitude and/or the 
width of the driving pulse may be varied to change the size of a dot 
recorded on a recording medium. 
Therefore, in an ink-jet print head, a half-tone recording can be provided 
by controlling the energy content of a driving pulse, in response to a 
signal representing the density of an image to be recorded. However, a 
droplet-volume range is not so wide that the volume of the ink droplet can 
be adaquately varied. For example, the volumetric ratio of the 
maximum-sized ink droplet to the minimum-sized ink droplet is between 2-3, 
at the greatest. Accordingly, the half-tone reproduction range, in which 
an image recorded at a predetermined density can be reproduced by only 
varying the volume of the ink droplets, is restricted to about 0.4-1.5 in 
terms of reflection density. In particular, the half-tone reproducibility 
of the bright portions of an image is low. 
Other known half-tone recording systems include the dither method and the 
density pattern method. In these methods, the number of dots is increased 
or decreased in accordance with the density of the image to be recorded, 
to provide a half-tone reproduction thereof. In order to increase the 
number of graduations in such an imitation half-tone reproduction system, 
the number of dots is increased per unit area of the image being subjected 
to the half-tone reproduction. However, when the dot-recording density of 
a recording apparatus is constant, the resolution drops. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to provide an ink-jet 
recording system capable of recording a half-tone with a high resolution 
over a wide level range of images to be recorded. 
According to this invention, an ink-jet recording system is capable of 
recording a half-tone. When the level of a density signal for a point is 
less than a predetermined value, the ink-jet print head does not eject an 
ink droplet. Instead, the density signal for the non-printed spot is 
stored and then adding to a density signal for a subsequent point to 
provide an added density signal, as the density signal for the subsequent 
point. The added density signal is compared in level with the 
predetermined value. These operations are repeated until the level of the 
added density signal exceeds the predetermined value. When the level of 
the density signal or the added density signal is equal to or higher than 
the predetermined value, an energy content of a driving pulse applied to 
the ink-jet print head is varied in response thereto. Accordingly, a 
volume of the ink droplet ejected from the ink-jet print head is varied. 
Other features and advantages of this invention will be apparent from the 
following description of a preferred embodiment of this invention taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A principle of this invention will be described with reference to FIGS. 1 
and 2. An on-demand type ink-jet print head to be used for this invention 
has a droplet formation characteristic, as shown in FIG. 1. The abscissa 
of FIG. 1 stands for a pulse width tw of a driving pulse applied to the 
ink-jet print head. The ordinate of FIG. 1 represents a volume V of an ink 
droplet ejected from the ink-jet head. As understood from FIG. 1, the 
volume of the ejected ink droplet is proportional to the pulse width of 
the driving pulse, over the width range from t.sub.w 1 to t.sub.w 2. The 
pulse width of the driving pulse is determined in response to a density 
signal which is indicative of the density of the image to be recorded. 
FIG. 2 shows a relationship betwen the density signal D.sub.I and the pulse 
width t.sub.w, and a relationship between the pulse width t.sub.w and a 
recorded density D.sub.0 on a recording medium on which a half-tone 
recording is made at a constant dot pitch. A dot of a size corresponding 
to a level of the density signal D.sub.I can be recorded on the recording 
medium by selecting the pulse width t.sub.w corresponding to the level of 
the density signal D.sub.1. The density D.sub.0 recorded on a medium at a 
constant dot pitch is proportional to the dot size corresponding to the 
level of the density signal D.sub.I. 
According to this invention, when the level of the density signal is equal 
to or higher than the minimum density signal D.sub.I1, a driving pulse 
having a pulse width proportional to the density signal level is applied 
to the ink-jet head, to record a dot having a size proportional to the 
density signal level. When the level of the density signal is lower than 
the minimum density signal D.sub.I1, there is no ejection of an ink 
droplet. The sum of this low level density signal and of a density signal 
at a subsequent point is used as a new density signal for governing the 
dot size recording of the subsequent point. However, once a dot has been 
recorded, nothing is added to the density signal for a subsequent point. 
Referring to FIG. 3, an embodiment of this invention comprises an image 
signal source 31, an adder 33, a comparator 35, a minimum density signal 
generator 37, a white-level density signal generator 39, a switch 41, 
memory means such as a memory and a register 44, a driving pulse generator 
46, a clock pulse generator 49, a read/write controller 51, and the 
ink-jet print head 48. 
The image signal source 31 operates in synchronism with a clock pulse 
appearing on a conductor 50, which is supplied from the clock pulse 
generator 49. Source 31 generates the density signal on wire 32 which is 
indicative of the image to be recorded. The density signal 32 is supplied 
to the adder 33, which is also supplied with a stored density signal 
appearing on wire 45, from the register 44, under the control of the 
read/write controller 51. The added density signal on wire 34 is supplied 
to the comparator 35, which is also supplied with a minimum density signal 
on wire 38. The minimum density signal is indicative of the minimum 
density signal level D.sub.I1 (FIG. 2) which corresponds to a minimum 
threshold level for recording a dot. The generator 37 supplies the minimum 
density signal via wire 38. 
In the comparator 35, the level of the density signal on wire 34 from the 
adder 33, is compared with the level of minimum density signal 38, to 
provide a detection signal that is fed out via wire 36. The detection 
signal on wire 36 is supplied as a switching signal to the switch 41, 
which is also supplied with the density signal on wire 34 from the adder 
33. Switch 41 also receives a white-level density signal (zero-level 
signal) via wire 40 from the generator 39. 
When the level of density signal on wire 34 is less than the level of 
minimum density signal on wire 38, the detection signal on wire 36 
controls the switch 41 so that the density signal on wire 34 is supplied 
to the register or memory means 44. The white-level density signal on wire 
40 is supplied to the driving pulse generator 46. The register 44 stores 
the density signal supplied from wire 34 and through the switch 41, under 
the control of the read/write controller 51. The density signal previously 
stored in the register 44 is replaced by the new signal. Because the 
white-level density signal (zero-level signal) on wire 40 is supplied 
through the switch 41 to the driving pulse generator 46, there is no 
ejection of the ink droplet in the ink-jet print head 48. 
Conversely, when the level of the density signal on wire 34 is equal to or 
higher than the level of the minimum density signal on wire 38, the 
detection signal on wire 36 controls the switch 41 so that the density 
signal on wire 34 is supplied to the driving pulse generator 46. The 
white-level density signal on wire 40 is supplied to the register 44. The 
white-level density signal on wire 40 is stored as a new density signal in 
the register or memory means 44. Therefore, no output is supplied from the 
register 44 to the adder 33, at a subsequent image point. On the other 
hand, the driving pulse generator 46 is supplied with the density signal 
on wire 34 to generate a driving pulse having a pulse width proportional 
to the density signal on wire 34. This driving pulse is supplied to the 
ink-jet print head 48, to record a dot having a size corresponding to the 
density signal on wire 34. The above described operation is performed in 
synchronism with the clock pulse 50. 
As described above, when the level of the density signal on wire 32 is less 
than the level of the minimum density signal 38, no dot is recorded and 
the density signal is added to the density signal for a subsequent image 
point. Accordingly, in such a low-density region, no dot is recorded until 
the sum of the density signals at several points exceeds the level of the 
minimum density signal 38. In the higher density region in which the 
density signal on wire 32 directly indicates that the level of the image 
to be recorded is higher than the level of the minimum density signal on 
wire 38, the density signal on wire 32 is supplied through the adder 33 
and the switch 41 to the driving pulse generator 46, as it is, to record a 
dot having the size corresponding to the density signal on the wire 32. 
Although the level of the density signal on wire 34 is less than the level 
of the minimum density signal on wire 38, the signal on wire 34 is stored 
in the register or memory means 44 for adding to the level of a subsequent 
point. The dot represented by the stored signal is not recorded. This 
store operation is repeated until the level of the density signal on wire 
34 exceeds the level of the minimum density signal on wire 38, in the 
above-mentioned embodiment. It is possible to replace the content of the 
register or memory means 44 (the stored density signal on wire 34) by the 
white-level density signal on wire 40 when the level of the density signal 
on wire 34 is less than the level of the minimum density signal on wire 
38, for a predetermined number of times, in succession. If the number "4" 
is selected as the predetermined succession number, in case of the 
recording density of 8 dots/mm, the lowest resolution is 2 pel/mm. When 
this number is increased, the half-tone reproduction range expands, with 
respect to the bright portions, but the resolution deteriorates. It is 
preferable to select this succession number so that a resolution of at 
least 0.5 pel/mm is obtained. 
In the above description, the pulse-width of the driving pulse is varied in 
response to the density signal which is indicative of the image to be 
recorded. It is possible to vary the pulse-width and/or the 
pulse-amplitude of the driving pulse to control the energy content of the 
driving pulse in response to the density signal. 
Those who are skilled in the art will readily perceive how to modify the 
invention. Therefore, the appended claims are to be construed to cover all 
equivalent structures which fall within the true scope and spirit of the 
invention.