Image formation apparatus

Provided is an image formation apparatus for printing pixel dots on a recording paper at pitches smaller than intervals between ink jet-out ports by using an ink jet recording head while moving the recording head in a rotation shaft direction and feeding a recording paper around the drum so as to wind the drum. The ink jet recording head has a plurality of ink jet-out ports arranged in line and is movable in a rotation shaft direction of the drum. In the case where the image formation is completed by two rounds of the drum, the recording head is shifted at the time the first drum rotation is completed, by a moving step amount determined so as to correspond to a specific feature of a human visual system, thereby suppressing image formation that is non-uniform in concentration.

BACKGROUND OF THE INVENTION
 The present invention relates to an image formation apparatus for forming
 an image by printing dots while feeding a recording medium relatively in a
 direction virtually perpendicular to a linear arrangement direction of
 recording elements of a recording head.
 As an example of the image formation apparatus of this type, conventionally
 known is an ink-jet type line printer. Such an ink-jet type line printer
 has a line recording head consisting of a plurality of ink chambers
 arranged in a main scanning direction (i.e., a line direction) and ink
 jet-out ports corresponding to the ink chambers. To improve image quality,
 it is necessary to form the ink jet-out ports in the same size and shape
 and arrange them at the same intervals (pitches). However, in practice, it
 is difficult to form all the ink jet-out ports such that they are
 identical. If the ink jet-out ports formed in a head manufacturing step
 slightly differ in size and shape, the difference in size and shape has an
 effect on amounts and directions of ink jetting out from the ink jet-out
 ports. The resultant image is non-uniform in concentration and thus
 degraded in quality .
 As shown, for example, in FIG. 9, in the case where ink is jetted out from
 individual ink jet-out ports 2 of the line recording head 1 in the same
 amount and in the same direction, ink dots 3 jetted out from the ink
 jet-out ports 2 will be identical. Consequently, pixel dots 5 will be
 printed in identical size and direction on a recording paper (recording
 medium) 4. As a whole, the formed image is uniform in concentration.
 In practice, however, the ink jet-out ports 2 of the line recording head 1
 are formed irregularly in size and shape, as shown in FIG. 10. The ink
 dots 6 jetted out from the ink-jet ports 2 therefore differ in jet out
 amount and direction. As a result, pixel dots 7 printed on the recording
 paper 4 differ in size and direction. If such dots are overlapped
 excessively, a black stripe is formed, whereas if the dots are printed at
 much larger intervals than required, a white stripe is formed. The
 resultant image is non-uniform in concentration.
 To suppress image formation from being non-uniform in concentration as much
 as possible, known is a control method shown in FIG. 11. In this method,
 assuming that a line recording head 11 consists of, for example, eight
 ink-jet ports 12, the line recording head 11 is controlled by moving the
 line recording head 11 in an arrangement direction of the ink jet-out
 ports 12 (recording elements) while a recording paper 14 is fed repeatedly
 in a direction virtually perpendicular to the arrangement direction of the
 ink jet-out ports 12, multiple times. In this case, since ink dots 13
 jetted out from ink jet-out ports 12 differ in jet-out amount and
 direction, an image is formed on the recording paper 14 (wound around a
 drum three times) by printing dots alternately up and down in a staggering
 manner while controlling movement of the line recording head 11 at every
 round.
 In other words, at a first round of the drum, first to fourth ink jet-out
 ports 12.sub.5 to 12.sub.8 from the right hand side of the line recording
 head 11 are used. More specifically, the ink jet-out ports 12.sub.5 and
 12.sub.7 are used to print dots of odd lines, whereas the ink jet-out
 ports 12.sub.6 and 12.sub.8 are used to print dots of even lines. At a
 second round of the drum, the line recording head 11 is slid rightward by
 four pitches of the ink jet-out ports. At this time, all ink jet-out ports
 12.sub.1 to 12.sub.8 of the line recording head 11 are used. More
 specifically, dots of odd lines are printed by the ink jet-out ports
 12.sub.2, 12.sub.4, 12.sub.6, 12.sub.8, whereas dots of even lines are
 printed by the ink jet-out ports 12.sub.1, 12.sub.3, 12.sub.5, 12.sub.7.
 At a final round (third round), the line recording head 11 is slid further
 rightward by four pitches of ink jet-out ports. In this case, first to
 fourth ink jet-out ports 12.sub.1 -12.sub.4 from the lefthand side are
 used. That is, the ink jet-out ports 12.sub.1 and 12.sub.3 are used for
 printing dots of odd lines, whereas the ink jet out-ports 12.sub.2 and
 12.sub.4 are used to print dots of even lines.
 In the recording method of this type, if the ink jet-out ports 12.sub.1
 -12.sub.8 are irregularly formed in size and shape, the effect of the
 irregularity in size and shape on the image formed on the recording paper
 14 is reduced in half. As a result, the black and white stripes as shown
 in FIG. 10 are not clearly visible, so that image formation that is
 non-uniform in concentration can be suppressed.
 Furthermore, another control method is known which is concerned with
 increasing resolution. In the control method, the line recording head 11
 is slid in the arrangement direction of the ink jet-out ports 12 by L/K (L
 is an arrangement interval of the ink jet-out ports 12), while the
 recording paper 14 is repeatedly fed in the direction virtually
 perpendicular to the arrangement direction of the ink jet-out ports 12,
 multiple times. In the case where the recording paper 14 is wound around
 the drum twice, pixel dots 15 (indicated by hatched lines declining
 rightward in the figure) are printed on the recording paper 14 at a first
 round of the drum, as shown in FIG. 12. At the second round, the line
 recording head 11 is slid rightward by L/2 (L is an arrangement interval
 of the ink jet-out ports 12), and then, recording pixel dots 16 (indicated
 by hatched lines declining leftward in the figure) are printed on the
 recording paper 14. In this manner, the resolution of the formed image is
 improved twice as high as that of the line recording head 11.
 As described, in the image formation method for improving the resolution of
 the image by controlling the movement of the line recording head 11, if
 the ink dots jetted out from the ink jet-out ports 12 of the line
 recording head 11 differ in jet-out amount and direction, the black stripe
 (which is formed by excessively overlapping dots than required) and the
 white stripe (which is formed by printing dots at intervals which are
 excessively far from each other than required) emerge, as shown in FIG.
 13. As a result, the obtained image is non-uniform in concentration as a
 whole.
 To overcome this problem, it is considered that the image is formed by two
 control methods in combination, as shown in FIG. 14. One is a control
 method in which the line recording head 11 is slid by L/2 (L is an
 arrangement interval of the ink jet-out ports 12) and the other is a
 control method in which dots are printed on the recording paper 14
 alternately up and down in a staggering manner. To be more specific, at a
 first round of the drum, pixel dots 17 (indicated by a lattice pattern in
 the figure) are printed on the recording paper 14. At a second round of
 the drum, the line recording head 11 is slid rightward by L/2, and then,
 pixel dots 18 (indicated by hatched line declining rightward) are printed
 on the recording paper 14. At a third round of the drum, the line
 recording head 11 is slid further rightward by L/2, and then, pixel dots
 19 (indicated by hatched lines declining leftward) are printed. At a
 fourth round of the drum, the line recording head 11 is further slid
 rightward by L/2 and then pixel dots 20 (indicated by a white circle in
 the figure) are printed on the recording paper 14. In this manner, the
 resolution of the image is improved twice as high as that of the line
 recording head 11. At the same time, image formation that is non-uniform
 in concentration due to irregular arrangement of the ink jet-out ports can
 be successively suppressed.
 However, the aforementioned control technique has a problem in that it
 takes too much time to form the image since the time required for image
 formation is doubled as compared to the control method which only
 increases the resolution.
 BRIEF SUMMARY OF THE INVENTION
 An object of the present invention is to provide an image formation
 apparatus capable of improving image quality by suppressing image
 formation that is non-uniform in concentration due to irregularity in size
 and shape of ink jet-out ports, and capable of reducing the time required
 for image formation.
 According to one aspect of the present invention, the image formation
 apparatus has a recording head having a plurality of recording elements
 arranged in line. Using the recording head, an image is formed by printing
 dots on a recording medium while the recording medium is fed relatively in
 a direction virtually perpendicular to an arrangement direction of the
 recording elements of the recording head. The present invention is
 characterized in that dots are printed at pitches smaller than arrangement
 intervals of the recording elements by feeding the recording medium to the
 recording head repeatedly multiple times while the recording head is being
 moved in the arrangement direction of recording elements, and in that the
 recording head is slid by a moving amount determined so as to correspond
 to a specific feature of a human visual system every time the feed of the
 recording medium is repeated to the recording head.
 According to the aspect of the present invention, quality of the image can
 be improved by successively suppressing image formation that is
 non-uniform in concentration due to the irregularity of ink jet-out ports
 in size and shape. Furthermore, the time required for image formation can
 be reduced.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Now, an embodiment of the present invention will be explained with
 reference to FIGS. 1 to 7. In this embodiment, the present invention is
 applied to an ink-jet type line printer.
 FIG. 1 is a view showing a main portion of an entire structure of a
 printer. The printer has a main case 21 housing a drum 22. The drum is
 rotated at a constant speed in the direction indicated by an arrow in the
 figure. A recording paper 25 (recording medium) to be fed through paper
 feed rollers 23, 24 is wound around the drum 22.
 A paper feed cassette 26 is fitted to the bottom portion of the main case
 21. Recording paper sheets 25 mounted on the mounting plate 27 of the
 paper feed cassette 26 is taken out one by one and sent to the paper feed
 rollers 23, 24. On the other hand, the recording paper sheets 25 can be
 manually fed through a manual feeding tray 29 which is fitted to an
 openable side portion of the main case 21. The manually fed recording
 paper sheet is also sent to the paper feed rollers 23, 24 by a paper feed
 roller 30. Note that a paper-feed switching means 31 is responsible for
 switching the paper feed routes to be controlled by the paper feed roller
 28 and by the paper feed roller 30.
 A charging roller 32 is arranged so as to face the drum 22. The charged
 roller 32 is responsible for adsorbing the recording paper sheet 25 fed
 through the paper feed rollers 23, 24, on the drum surface. Furthermore, a
 printing mechanism 34 is arranged so as to face the drum 22. The printing
 mechanism 34 has an ink-jet recording head 33 having a plurality of
 recording elements arranged in lines. The printing mechanism 34 is movable
 in a recording element arrangement direction (i.e., along a rotation shaft
 of the drum 22). Therefore, the recording paper 25 is fed in a direction
 perpendicular to the recording element arrangement direction of the
 ink-jet recording head 33 by rotation of the drum 22.
 The ink-jet recording head 33 has a plurality of ink jet ports 33a (serving
 as recording elements) arranged at intervals (pitches) L. Ink chambers are
 formed corresponding to ink jet-out ports 13a.
 The printing mechanism 34 comprises a reciprocal moving mechanism 35 having
 the ink jet recording head 33 mounted thereon, a motor unit 36 having a
 reciprocal moving rod and a linear motor, and a back-and-forth moving
 means 37. The ink jet recording head 33 is moved back and forth by the
 back-and-forth moving means 37 with respect to the drum 22; at the same
 time, the ink jet recording head 33 is moved reciprocally in the rotation
 shaft direction of the drum 22. The reciprocal movement of the ink-jet
 recording head 33 is controlled by the motor unit 36, which moves the
 reciprocal moving mechanism 35 in the rotation-shaft direction of the drum
 22.
 The drum 22 has a recording paper detachment means having a detachment nail
 38, which is capable of being inserted between the drum surface and the
 recording paper sheet 25. The detachment nail 38 removes the recording
 paper sheet 25 and discharges it into a recording paper sheet discharge
 means 39. The recording paper discharge means 39 comprises a belt conveyer
 40 which is in contact with a non-recording surface of the recording paper
 25 and a pressing means 41 for pressing the recording paper sheet 25
 against the belt conveyer 40.
 The recording paper sheet 25 is conveyed by the belt conveyer 40 to a
 terminal end of the belt conveyer 40 and then discharged outward through a
 side face of the main case 21.
 The resolution of the image formed by the apparatus is set at K-fold (K is
 an integer of 2 or more) of the resolution of the interval (pitch) L of
 ink jet-out ports 33a of the ink jet recording head 33. The ink jet
 recording head 33 moves by T.times.L/K (T is a positive integer
 representing a head moving step amount) in the rotation-shaft direction of
 the drum 22 when the drum 22 rotates once. The head moving step amount T
 is determined in consideration of a specific feature of human
 visualization. To be more specific, a human eye is characterized by a band
 pass filter which passes only light having frequencies within a certain
 band zone. Since the eye acts as the band filter as mentioned, the image
 captured by the human eye slightly differs from the image detected by
 retina and transmitted to the brain.
 For example, the image formed in FIG. 3 has a resolution twice as high as
 that of the interval (pitch) L of the ink jet-out ports 33a of the ink jet
 recording head 33. In this case, the ink jet recording head 33 moves by
 3.times.L/2 in the rotation-shaft direction of the drum 22 when the drum
 22 rotates once. In other words, the head moving step amount T is set at
 3.
 In this case, at the first rotation of the drum 22, pixel dots 51
 (indicated by an open circle) are printed by jetting out ink from ink
 jet-out ports 33a of the ink jet recording head 33. At a second rotation
 of the drum 22, pixel dots 52 (indicated by hatched lines declining
 leftward) are printed by jetting out ink from ink jet-out ports 33a of the
 ink jet recording head 33.
 If the image formation is formed by printing pixel dots as mentioned above,
 it is possible to form an image improved in resolution and in quality
 which is rated as the most suitable as viewed from human eyes. More
 specifically, the drum 22 is required to rotate only for improving the
 resolution but not for suppressing the black and white stripes. It is
 therefore possible to enhance the resolution and to suppress image
 formation that is non-uniform in concentration due to irregularity of the
 ink jet-out ports 33a in size and shape by rotating the drum few number of
 times, with the result that the image quality is improved and the time
 required for image formation can be reduced.
 Now, we will describe how to determine the head moving step amount T.
 In general, a figure showing concentration distribution of pixel dots
 printed by the ink-jet printer can be approximated by means of "Gauss
 distribution", which is often used in analysis of natural phenomena. In
 the case where the coordinate in the arrangement direction of ink jet-out
 ports 33a of the ink jet recording head 33 is represented by x, and the
 coordinate in the feed direction of the recording paper sheet 25 is
 represented by y, the concentration distribution function of image data
 can be given by the following equation (1):
 ##EQU1##
 Where Dmax is the maximum concentration value, x0 and y0 are off-set
 positions of a pixel dot; a, b are radii at 1/e2 of a spot in the x
 direction and the y direction, respectively. In this case, if x and y are
 present within the range of one pixel, the concentration distribution
 function represented by the aforementioned equation (1) is regarded as a
 minimum unit for forming an image. Hence, the entire image is formed by
 synthesizing the concentration distributions of the minimum unit. That is,
 the minimum unit may be regarded as a basic distribution function for
 image formation.
 For example, in the case where an area-modification ink-jet printer is
 employed as the ink-jet printer, the basic concentration function depends
 on Dmax, a and b (a and b are variable depending upon respective tones).
 However, since the dot concentration figure actually formed by the ink jet
 printer slightly differs from that of the Gauss distribution mentioned
 above, the Equation (2) modified from the Equation (1) may be used. FIG. 4
 shows a curve g1 indicating the Gauss distribution function in comparison
 with a curve g2 indicating the basic concentration distribution function
 for use in simulation in practice. The curve indicates a one-dimensional
 function in the x direction.
 ##EQU2##
 The concentration distribution of the synthesized image is determined by
 synthesizing the basic distribution functions and thus represented by the
 following equation (3):
 ##EQU3##
 where m is the number of pixels in the arrangement direction of the ink-jet
 ports 33a; n is the number of pixels in the feed direction of the
 recording paper sheet 25, and M and N are the sampling numbers
 (corresponding to dividing numbers of each pixel) in the x direction and y
 direction, respectively.
 However, the printing positions of the pixel dots are shifted from right
 positions in most cases by the irregularity of the ink-jet ports 33a in
 size and shape which are formed by causes in the head manufacturing step.
 Therefore, in consideration of error equations g(x.sub.j) and h(y.sub.j)
 of the pixel dot recording portion in the x direction and Y direction,
 respectively, the pixel dot recording position is represented by the
 following equation (4):
 ##EQU4##
 In the case of analyzing image signals in a frequency space, a
 two-dimensional frequency analysis is generally employed. If the image
 signal is treated as a one-dimensional signal for brevity sake, the basic
 concentration distribution function of an image is represented by the
 following equation (5):
 ##EQU5##
 Furthermore, taking an error of the pixel dot recording position into
 consideration, the concentration distribution of the synthesized image is
 represented by the following equation (6):
 ##EQU6##
 Since the human visual system can be regarded as a filter passing
 frequencies within a certain band zone, convolution of the spatial
 frequency characteristics of the image and of the human visual system must
 be carried out. The convolution made in the frequency space results in a
 product of two functions as shown in the following equation (7):
EQU G(.mu.)=E(.mu.)H(.mu.) (7)
 .mu.: frequency
 H(.mu.): spatial frequency characteristics of visualization
 E(.mu.): spatial frequency characteristics of image
 (Fourier transform of e.sub.2 (x))
 A final evaluation amount is an integration value of G(.mu.) in an
 effective frequency region as given by
 ##EQU7##
 The larger the evaluation amount represented by the equation (8), the more
 sensitive a human eye to the white and black stripes formed by the
 irregularity of ink jet-out port 33a in size and shape. In other words, if
 the head movement step amount T indicating a low evaluation amount, is
 selected, the most preferable image as viewed from human eyes can be
 printed.
 Now, we will explain the evaluation when an image is actually formed with a
 resolution of 300 dpi.
 FIG. 5 is a graph plotting evaluation amount by simulation at each of head
 moving step amounts T=1, 3, 5, and 7 when the image is formed with a
 resolution of 300 dpi by using a recording head having the ink jet-out
 ports 33a arranged at intervals of 150 dpi. Curves g3, g4, g5 of the graph
 in the figure respectively show evaluation amounts in the cases three
 types of ink jet-out head different in size and shape of jet out ports are
 used.
 The evaluation amount is the lowest at the head moving step amount T=3 in
 any head of g3, g4, and g5. In other words, an image can be obtained in
 the most satisfactory quality at the head moving step amount T=3 when the
 image is formed with a resolution of 300 dpi.
 To prove it, a subjective evaluation is further made. As the subjective
 evaluation, one-to-one comparison (alternative evaluation of yes and no)
 is made by a plurality of panels with respect to a pair of objects.
 Generally, all combinations: .sub.n C.sub.2 :n(n-1)/2 , are checked by
 changing the order or positions.
 The results are shown in the graph of FIG. 6. The graphs g6, g7, g8
 respectively show the evaluations when three types of ink jet-out heads
 having different sizes and shapes of the jet-out ports. Evaluation is made
 in such a scoring method that two points are given to the image superior
 in quality and a zero point is given to the image inferior in quality.
 Accordingly, the image earning a higher score means high in quality. Since
 the highest evaluation score is given at the head moving step amount T=3
 in the subjective evaluation, the highest image quality is obtained at
 T=3.
 As described above, the subjective evaluation results made by people agree
 with the objective evaluation results calculated by simulation. It is
 therefore successively proved that the head moving step amount T in
 accordance with the specific feature of human visualization can be
 effectively determined.
 FIG. 7 shows the head moving step amount T versus the evaluation amount for
 two types of images with resolutions of 300 dpi and 600 dpi, respectively.
 More specifically, a graph g9 shows an image with a resolution of 300 dpi.
 A graph g10 shows an image with a resolution of 600 dpi. The lowest
 evaluation is given at the head moving step amount T of 3 in the case of
 300 dpi resolution, and at the head moving step amount T of 5 in the case
 of 600 dpi.
 As described, it is found that the most suitable head moving step amount T
 varies depending upon the resolution of the formed image. Therefore, if
 the head moving step amount T is controlled in accordance with the
 resolution of the printer to be used, the image can be formed in the most
 preferable quality in consideration of the specific feature of human
 visualization.
 In the foregoing, an example of the ink jet printer was described which has
 the printing mechanism 34 equipped with an ink jet recording head 33
 having a plurality of recording elements arranged in line and being
 movable in the rotation shaft direction of the drum 22. However, the ink
 jet printer is not limited thereto. The present invention can be further
 applied to an ink jet color printer having a printing mechanism 341
 equipped with four ink jet recording heads 331, 332, 333, 334 having a
 plurality of recording elements arranged in line and being movably in the
 rotation shaft direction of the drum 22, as shown in FIG. 8. In this case,
 the ink jet recording head 331 is set at "yellow", the ink jet recording
 head 332 is set at "Cyan", the ink jet recording head 333 is set at
 "magenta", and the ink jet recording head 334 is set at black, and then,
 these recording heads may be driven on the basis of the head moving step
 amount T which is determined in correspondence to the specific feature of
 a human visual system, in the same way as in the case of the ink-jet
 recording head 33. In this case, if the four ink jet recording heads
 331-334 are mounted on the reciprocal moving mechanism 35, they can be
 controlled simultaneously.
 In the aforementioned embodiments, we explained the case where the image is
 formed with resolutions of 300 dpi or 600 dpi by using the recording head
 having the ink jet out ports arranged at intervals of 150 dpi. The same
 explanation can be adopted to the cases where the image is formed with a
 resolution of 2A dpi or 4A dpi by the use of the recording head having ink
 jet out ports 33a arranged at intervals of A dpi.
 Furthermore, an image having a resolution of 400 dpi can be formed by using
 a recording head having a plurality of recording elements arranged at
 intervals of 200 dpi if the recording head is moved by the predetermined
 number of head moving steps which is defined by the method according to
 the present invention.
 Similarly, an image having a resolution of 800 dpi can be formed by using a
 recording head having a plurality of recording elements arranged at
 intervals of 200 dpi if the recording head is moved by the predetermined
 number of head moving steps which is defined by the method according to
 the present invention.
 Similarly, an image having a resolution of 360 dpi can be formed by using a
 recording head having a plurality of recording elements arranged at
 intervals of 180 dpi if the recording head is moved by the predetermined
 number of heat moving steps which is defined by the method according to
 the present invention.
 Still similarly, an image having a resolution of 720 dpi can be formed by
 using a recording head having a plurality of recording elements arranged
 at intervals of 180 dpi if the recording head is moved by the
 predetermined number of heat moving steps which is defined by the method
 according to the present invention.
 In the aforementioned embodiments, we described the case in which the
 recording paper is fed in the direction virtually perpendicular to the
 arrangement direction of the ink jet-out ports of the ink jet recording
 head while the recording paper is wound around the drum. However, the
 present invention is not limited thereto. For example, the recording paper
 may be fed by being mounted on a belt conveyer etc. Alternatively, the
 recording head may be fed relative to the ink jet recording head by moving
 the ink jet recording head with respect to a recording paper immobilized.
 In the embodiments mentioned above, we described the case in which the
 present invention is applied to the ink jet printer. However, the present
 invention is not limited thereto. The present invention may be applied to
 a printer having an LED recording head having a plurality of LED recording
 elements arranged in line, a printer having a thermal recording head
 having a plurality of thermal recording elements arranged in line, and a
 printer having an edge-emitter recording head having a plurality of EL
 recording elements arranged in line.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.