Image forming apparatus and method

This is an image forming apparatus provided with a recording head obtained by overlapping and disposing a plurality of short recording heads with a nozzle array composed of ink-jet nozzles arrayed in one direction, for jetting ink from the ink-jet nozzle, based on an image signal value and forming an image. The image forming apparatus comprises an overlap correction control unit for controlling drive of the ink-jet nozzles, according to a phase difference in a ink-jet nozzle position between the overlapped short recording heads to correct optical density of an image formed on an overlapped part of the recording heads and an optical density characteristic correction control unit for controlling drive of the ink-jet nozzles, according to a recording optical density characteristic of a nozzle array composed of the ink-jet nozzles independently of the overlap correction control unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-180857 filed Jun. 21, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus provided with a recording head with a plurality of nozzle arrays, for forming an image on an image-formed material, and more particularly, relates to an image forming apparatus provided with at least one long recording head composed of a plurality of short recording heads.

2. Description of the Related Art

For example, the recording head of the ink-jet type image forming apparatus has a tendency to multiply or lengthen a recording device (ink-jet nozzle) in order to meet the requirements of high-speed image formation (image recording).

As such an image forming apparatus, a structure provided with a so-called line head for disposing (forming) a recording device (ink-jet nozzle) across one side (width direction) of an image-formed material (recording medium) is known.

An image forming apparatus provide with a line head can form an image all over this image-formed material by relatively moving the image-formed material in the direction orthogonal to the ink-jet nozzle array direction of a line head (sub scan direction) and then jetting ink on the image-formed material from the ink-jet nozzle. Since the image forming apparatus provide with a line head needs neither the movement of a carriage nor the intermittent conveyance of the image-formed material, it can rapidly and easily form an image.

However, the line head has disadvantages that its cost is high compared with a short recording head, that its quality yield is bad, that its reliability is low and the like.

An image forming apparatus utilizing the advantages in cost of a short recording head, in its quality yield, in its reliability and the like by disposing a plurality of short recording heads in each of which a plurality of ink-jet nozzles is arrayed in one direction (main scan direction) in order to solve these problems is also known.

However, in the line head composed of such short recording heads, if there is a phase difference in pitch between nozzle arrays (improper pitch), striped optical density unevenness, white-wiping and the like are sometimes formed.

Patent reference 1 (Japanese Patent Application Publication No. 2002-144542) discloses am image recording method capable of recording a high-quality image without color/optical density unevenness or the like in an image recording apparatus in which one recording head (line head) is obtained by disposing a plurality of short (recording) heads.

The image recording method disclosed by patent reference 1 is described below with reference toFIGS. 1A˜1C.

The recording head10shown inFIG. 1Acomprises a plurality of short heads12A and12B. In this recording head10, a plurality of adjacent ink-jet nozzles11A of the short head12A and a plurality of adjacent ink-jet nozzles11B of the short head12B are disposed in such a way that a part of them overlaps when viewed from the sub scan direction. This joint area (overlapped area) corresponds to ink-jet nozzles11A-1˜11A-3on the short head side12A and ink-jet nozzles11B-1˜11B-3on the short head side12B.

In image record by the recording head10with such a structure, for example, as shown in the area “a” ofFIG. 1B, a high-optical density recording area occurs along the sub scan direction in the joint area of the ink-jet nozzles11A-1˜11A-3of the short head12A and the ink-jet nozzles11B-1˜11B-3of the short head12B, and a high-quality image cannot be recorded.

In such a case, a record control unit for controlling the recording head10to record an image, which is not shown inFIG. 1, determines the ink-jet nozzle11A-2of the short head12A and the ink-jet nozzle11B-2of the short head12B, which is shown by an one-dot chain line as one example, as a joint point and the use of the ink-jet nozzles11A-1and11B-1located further on the top end is stopped.

Since the space between the ink-jet nozzles11A-2and11B-2, which are the joint point is narrower than a proper pitch, the o optical density in the joint point becomes higher than a proper value, as shown in the area “a” ofFIG. 1A. In order to correct this, the record control unit stops the drive of one of the ink-jet nozzles11on every another line in the sub scan direction to record an image with proper optical density. In an example of an area “b” ofFIG. 1B, the drive of the ink-jet nozzle11B-2of the short head12B is stopped on every another line in the sub scan direction.

The image recording method of patent reference 1 can record a high-quality image without color/optical density unevenness and the like, by performing such control.

In this case, in an ink-jet recording head using a piezoelectric device (PZT), generally the amount of ink jetted from the ink-jet nozzle at the head end increases or decreases compared with that in an area other than the end, that is, a non-end area. In the case of an image recording apparatus with one recording head, even when there is a little change in optical density due to the change of the amount of ink jetted from a specific number of ink-jet nozzles on the end in such a phenomenon, a part whose optical density has changed becomes the end of an image recording area. Therefore, the optical density unevenness of a recorded image is not remarkable. However, in the case of an image recording apparatus with a line head obtained by adjacently disposing a plurality of recording heads, the joint part of adjacent recording heads becomes inner than the end of the image recording area. Therefore, when optical density unevenness occurs in this part, striped optical density unevenness, white-wiping and the like becomes remarkable in a recorded image.

Patent reference 2 (Japanese Patent Application Publication No. 2003-320647) discloses a method for visually reducing optical density unevenness due to the fluctuations of the jet amount of ink (ink jet volume) at the end of such a recording head.

The method of this patent reference 2 determines which an input image signal is, the end area signal or non-end area signal of a corresponding recording head. If it is determined that that it is the end area signal, an end area correction process is performed. If it is determined that that it is the non-end area signal, a non-end area correction process is performed.

In the end area correction process, the end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas. In the non-end area correction process, the non-end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas, and it is also corrected in such a way that a optical density value gradually decreases from the end toward the center. The method of patent reference 2 corrects the end and non-end areas and also reduces optical density unevenness.

However, although patent reference 1 discloses a method for improving the optical density unevenness of a nozzle array overlapped part between adjacent short heads, it does not disclose a method for improving unevenness due to a non-uniform recording characteristic between nozzles arrays. Therefore, only the method disclosed by patent reference 1 cannot improve unevenness due to this non-uniform recording characteristic.

Although patent reference 2 discloses a method for improving unevenness due to non-uniform ink jet volume from the ink-jet nozzles, the relative positions of short heads are adjusted and disposed in such a way that one ink-jet nozzle in the joint part of adjacent short heads can be matched with the other ink-jet nozzle in the joint part when viewed from the sub scan direction.

The recording devices (ink-jet nozzle) are formed at very fine intervals. For example, if its resolution is 300 dpi, the interval becomes 85 μm. In the method disclosed by patent reference 2, for example, a locating mechanism for locating a ink-jet nozzle of one short head and the ink-jet nozzle of the other adjacent short head at intervals of 85 μm with no error is needed, which incurs the cost-up of the apparatus.

The generation factor and degree of optical density unevenness due to the nozzle array overlapped part of this short head and those of optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles are different.

FIG. 2Ashows the recording optical density characteristic of a recording head (short head) in which the ink-jet nozzle at the nozzle array end is higher than that at non-end.

FIG. 2Bshows the recording optical density characteristic of a recording head (line head) in which the two short heads with the recording optical density characteristic shown inFIG. 2Aare adjacently disposed in such a way that a part of their ink-jet nozzles are overlapped when viewed from the above-described sub scan direction.

Since as shown inFIG. 2A, the optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles is due to the little structural or manufacturing error in the jet amount of ink (ink jet volume) between recording heads, its optical density change value becomes small and gentle.

Since as shown inFIG. 2B, the optical density unevenness in the nozzle overlapped part of a recording head is due to a optical density change according to the phase difference in a ink-jet nozzle position between nozzle arrays of adjacent recording heads, its optical density change value becomes large and steep.

Therefore, in order to correct optical density unevenness due to these two factors, a sufficient correction effect cannot be obtained only by simply combining two methods (technologies) disclosed by patent references 1 and 2, and accordingly no high-quality image can be obtained.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image forming apparatus capable of making striped optical density unevenness, white-wiping and the like, due to the joint part of adjacent recording heads unremarkable and also correcting optical density unevenness due to the non-uniform recording optical density characteristic between short recording heads, without disposing short recording heads according to strict position adjustment in an image forming apparatus with one long recording head obtained by disposing a plurality of short recording heads.

The present invention presumes an image forming apparatus provided with a recording head obtained by overlapping and disposing a plurality of short recording heads each with ink-jet nozzles arrayed in one direction, for and jetting ink from the ink-jet nozzles, based on an image signal value and forming an image, which comprises an overlap correction control unit and a optical density characteristic correction control unit.

The overlap correction control unit controls the drive of ink-jet nozzles, according to the phase difference in a ink-jet nozzle position between overlapped short recording heads to correct the optical density of an image formed by the overlapped part of recording heads.

The optical density characteristic correction control unit operates independently of the overlap correction control unit and controls the drive of ink-jet nozzles, according to the recording optical density characteristic of the nozzle array of the ink-jet nozzles.

The present invention also includes the image formation method of the image forming apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3shows an example of the conceptual configuration of the image forming apparatus of the preferred embodiment.

FIG. 3show examples of both the configurations of the image forming apparatus30in the case where it is of a serial type and of a full line type in one drawing.

The serial type image forming apparatus30shown inFIG. 3comprises a mounting table32provided with a moving mechanism for mounting and holding an image-formed material31or a moving mechanism for holding in such a way as to move the image-formed material31in the sub scan direction. In the upper section of the mounting table32, at least one of recording units34-1˜34-m (m=an integer of 2 or more), each with a plurality of nozzle arrays (short recording heads) and each with a plurality of ink-jet nozzles (ink jet outlets) formed, are disposed on a moving object33. The moving object33is movably supported by the moving mechanism, which is not shown inFIG. 3, and is moved in the main scan direction (Hx0˜Hxn) and sub scan direction (Hy0˜Hyn) or only in the main scan direction (Hx0˜Hxn). In the case of this serial type image forming apparatus, an image is formed on the image-formed material31mounted on the mounting table32or held in such a way as to move in the sub scan direction while at least one of the recording units34-1˜34-m, disposed on the moving object33relatively, move only in the main scan direction or in both the main scan and sub scan directions.

The full line type image forming apparatus30shown inFIG. 3comprises a moving mechanism for mounting the image-formed material31and moving and carrying it in the sub scan direction, which is not shown inFIG. 3. In the upper section of this moving mechanism, which is not shown inFIG. 3, carriages35in which at least one of the recording units36-1˜36-m (m=integer of two or more) each with a plurality of nozzle arrays each composed of a plurality of ink-jet nozzles are oppositely deposited. In the case of this full line type image forming apparatus, an image is formed on the image-formed material31while the moving mechanism, which is not shown inFIG. 3, relatively moves the image-formed material31held in such a way as to move in the sub scan direction against at least one of the recording units36-1˜36-m.

In the carriage35indicated by a two-dot chain line inFIG. 3, a plurality of ink-jet nozzles of at least one of the recording units36-1˜36-m are disposed as a plurality of nozzle arrays (short recording heads) at least across the length of over the width in the main scan direction (Hx) of the image-formed material31.

In the case of this full line type image forming apparatus, an image is formed by moving the top end31ain the sub scan direction (Hy) of the image-formed material31to My0˜Myn by the moving mechanism, which is not shown inFIG. 3, and further jetting ink on the image-formed material31by at least one of the recording units36-1˜36-m while moving the back end31bin the sub scan direction (Hy) of the image-formed material31to My0˜Myn inFIG. 3.

As described above, in each image forming apparatus with a different configuration, the serial type image forming apparatus adjusts an image formation point (ink jet point) on the image-formed material31by changing jet timing for each of a plurality of nozzle arrays in at least one of the recording units34-1˜34-m while a control unit37controls to move the moving object33with at least one of the recording units34-1˜34-m in the main scan direction.

The full line type image forming apparatus adjusts an image formation point (ink jet point) on the image-formed material31by changing jet timing for each of a plurality of nozzle arrays in at least one of the recording units36-1˜36-m while a control unit37controls to move the image-formed material31in the sub scan direction by the moving mechanism, which is not shown inFIG. 3.

In the image forming apparatus30of the preferred embodiment, the control unit37comprises a optical density characteristic correction control unit and an overlap correction control unit, which are described later. When forming an image, the image forming apparatus30corrects an input image signal41, using an optical density characteristic correction control unit46and an overlap correction control unit47, based on an image formation mode set from a mode setting unit38by an operator or a correction value recorded in the control unit37as an already known value.

FIG. 4shows an example of the configuration of the image forming apparatus of the preferred embodiment.

The image forming apparatus of the preferred embodiment shown inFIG. 4comprises a moving mechanism43including the above-described the mounting table32for supporting the moving object33in such a way as to move in a prescribed direction in addition to the moving object33, at least one the recording units34-1˜34-m, at least one the recording units36-1˜36-m, the control unit37and the mode setting unit38, which are shown inFIG. 3.

The at least one of the recording units34-1˜34-m or36-1˜36-m comprise at least one of the nozzle array driver circuits50-1˜50-m (m=integer of two or more) and a nozzle array units51-1˜51-m (m=integer of two or more). Then, at least one of the nozzle array units51-1˜51-m comprises a plurality of nozzle arrays (short recording heads). The control unit37comprises a plain memory44, a timing generation unit45, the optical density characteristic correction control unit46, and the overlap correction control unit47. The moving mechanism43comprises a moving mechanism driver unit48and a moving position information generating unit49. In the case of the serial type image forming apparatus, the moving mechanism further comprises the mounting table32.

The control unit37is composed of a CPU and like, and supervises and controls this entire image forming apparatus30. The control unit37controls the moving mechanism driver unit48of the moving mechanism43, processes a signal indicating the location of the moving object33notified by the moving position information generating unit49, instructs to control the timing generation unit45, stores an externally inputted the input image signal41in the plain memory44and so on. The control unit37also corrects the input image signal41by the optical density characteristic correction control unit46and the overlap correction control unit47. The optical density characteristic correction control unit46controls the drive of each ink-jet nozzle to determine the jet amount of ink, according to the recording optical density characteristic of nozzle arrays (short recording heads) constituting the nozzle array units51-1˜51-m. The overlap correction control unit47controls the drive of each ink-jet nozzle to determine the jet amount of ink, according to the phase difference in a ink-jet nozzle position between nozzle arrays (short recording heads). This detailed correction process is described later.

The moving mechanism driver unit48of the moving mechanism43is composed of a motor and the like. The moving mechanism driver unit48moves the moving object33only in the main scan direction, only in the sub scan direction or both in the main and sub scan directions to a prescribed position, based on the control/instruction of the control unit37. The moving position information generating unit49is composed of a rotary encoder and the like. The moving position information generating unit49notifies the control unit37of the amount of movement of the moving object33and its location as signals. The moving position information generating unit49can also comprise a high frequency generation circuit for multiplying by an integer the frequency of a pulse signal generated by the rotary encoder as requested.

The mode setting unit38is provided as a part of an input operation panel for the entire image forming apparatus30. Alternatively, it is provided as an individual input operation panel. By operating and instructing the mode setting unit38, the operator sets image formation modes, such as the correction function stoppage of the optical density characteristic correction control unit46, the correction function stoppage of the overlap correction control unit47and the like, in the image forming apparatus30. When a correction mode selected by the type of the image-formed material31or the like, the optical density characteristic correction control unit46and the overlap correction control unit47, which are described later, correct the input image signal41, based on this correction mode.

An imaging apparatus42shown inFIG. 4is an external apparatus provided with a function to detect the optical density of an image formed on the image-formed material31, such as a scanner connected to the image forming apparatus30or the like. The imaging apparatus42, for example based on the instruction from the control unit37, detects the optical density of the image formed on the image-formed material set in the imaging apparatus42and notifies the control unit37of the optical density.

Although in the preferred embodiment shown inFIG. 4, the image forming apparatus30and the imaging apparatus42are separated, the present invention is not limited to this configuration. The imaging apparatus42can also be provided inside the image forming apparatus30and adjustment information about image optical density unevenness and the like can also be calculated based on the detection result of the imaging apparatus42.

The optical density characteristic correction control unit46controls the drive of each ink-jet nozzle, based on a detection value notified by the imaging apparatus42, which is the external apparatus of the image forming apparatus30, according to a nozzle array optical density characteristic of each nozzle array (short recording head) to correct image optical density unevenness. The overlap correction control unit47controls the drive of each ink-jet nozzle, based on a detection value notified by the imaging apparatus42, according to the phase difference in the overlapped part of nozzle arrays (short recording heads) (relative position between the ink-jet nozzle in one nozzle array and the ink-jet nozzle in the other nozzle array) to correct image optical density unevenness.

These optical density characteristic correction control unit46and the overlap correction control unit47can be realized by dedicated hardware. Alternatively, they can be realized by a software method of executing a program by the CPU of the control unit37. If they are configured by hardware, it is preferable to configure each of them by different hardware in order to prevent the reciprocal influences of their correction processes.

The timing generation unit45determines each jet timing of each nozzle array (short recording head) of at least one of the nozzle array units51-1˜51-m, based on a position adjustment parameter predetermined according to the position information in the sub scan direction of each nozzle array to control/instruct the jet position adjustment of at least one of the recording units34-1˜34-m or36-1˜36-m. The recording units34-1˜34-m or36-1˜36-m forms an image for one line according to the input image signal41notified by the control unit37by forming an image by each nozzle array (short recording head) of the at least one of the nozzle array units51-1˜51-m in this jet timing.

Next, an example of the nozzle array unit array in the case of the full line type image forming apparatus of the preferred embodiment is described.

As shown inFIG. 5, in the carriage35fixed and deposited opposed to the moving mechanism43, the nozzle array units51-1˜51-m in which each color is made by jointing two short recording heads (inFIG. 5, m=4; four nozzle array units51of black (K) the nozzle array unit51-1, cyan (C) the nozzle array unit51-2, magenta (M) the nozzle array unit51-3and yellow (Y) the nozzle array unit51-4) are disposed in the sub scan direction from the upper stream conveyance direction of the image-formed material31at prescribed intervals. In the nozzle array units51-1˜51-m, a plurality of short recording heads51A-1,51B-1˜51A-m,51B-m (inFIG. 5, m=4; (K) short recording heads51A-1and51B-1, (C) short recording heads51A-2and51B-2, (M) short recording heads51A-3and51B-3and (Y) short recording heads51A-4and51B-4) are disposed corresponding to the nozzle array units51-1˜51-m. Each pair of short recording heads51A-1and51B-1˜51A-4and51B-4comprises one to a plurality of nozzle arrays for each short recording head.

For example, a plurality of ink drops with the same volume is jetted from each ink jet outlet of the nozzle array units51-1˜51-4. Thus, the control unit37controls the number of ink drops jetted from the nozzle array units51-1˜51-4by the nozzle array driver circuits50-1˜50-4to adjust optical density gradation, that is, by a multi-drop method.

For example, if each jet outlet of the nozzle array units51-1˜51-4can jet seven ink drops at the maximum, zero to seven ink drops including the case where no ink is jetted are jetted on the image-formed material31and a dot corresponding to each number of ink drops is formed. Thus, the control unit37can control optical density gradation of eight grades including zero grade.

Each of the color the nozzle array units51-1˜51-4comprises a prescribed number of fluid jet outlets (ink-jet nozzles) across over the width in the main scan direction of the carried image-formed material31. For example, in (K) the nozzle array unit51-1made by jointing two short recording heads, a plurality of ink-jet nozzles at the end of the short recording head51A-1and a plurality of ink-jet nozzles at the end of the short recording head51B-1are jointed in such a way as to overlap when viewed from the sub scan direction (the nozzle array units51-2˜51-4also have the same structure as (K) the nozzle array unit51-1).

As described above, if both ends of each nozzle array of each pair of short recording heads51A and51B are overlapped and disposed, the control unit37can selectively the input image signal41to each short recording head disposed in the carriage35to form an image. Therefore, the strict position adjustment in the nozzle array direction (main scan direction) of each short recording head can be omitted, thereby realizing an inexpensive position adjustment mechanism.

As a position adjustment mechanism for each of the nozzle array units51-1˜51-4, for example, a mechanical adjustment mechanism for finely rotating the nozzle array units51-1˜51-4can also be provided.

Next, an example of the configuration of the full line image forming apparatus30of the preferred embodiment is described.

As shown inFIG. 5, the moving mechanism43disposed opposed to the carriage35is disposed in the lower stream than an edge sensor62for detecting at least one end of the image-formed material31in the conveyance route of the image-formed material31.

An endless belt for forming a plurality of holes in the moving object33shown inFIG. 3is installed one roller63aand the other roller63bto rotate the other roller63bby connecting it to the motor of the moving mechanism driver unit48and connect the rotary encoder of the moving position information generating unit49to the one roller63a. In the lower section of this endless belt, for example, an absorbing fan, which is not shown inFIG. 5, to absorb the image-formed material31.

The control unit37shown inFIG. 3places the image-formed material31carried from the upper stream of the moving mechanism43on the endless belt of the moving mechanism43and absorbs it. Then, the control unit37forms an image while moving the lower section of the short recording heads51A-1,51B-1˜51A-4,51B-4corresponding to color the nozzle array units51-1˜51-4disposed in the carriage35.

The control unit37converts the distance between the edge sensor62and the nozzle array of the short recording head51A-1corresponding to the color the nozzle array units51-1˜51-4, between the edge sensor62, the distance between the edge sensor62and the nozzle of the short recording head51B-1, the distance between the edge sensor62and the nozzle array of the short recording head51A-2, the distance between the edge sensor62and the nozzle array of the short recording head51B-2, the distance between the edge sensor62and the nozzle array of the short recording head51A-3, the distance between the edge sensor62and the nozzle array of the short recording head51B-3, the distance between the edge sensor62and the nozzle array of the short recording head51A-4and the distance between the edge sensor62and the nozzle array of the short recording head51B-4into the accumulated number of pulses of the rotary encoder and stores it in the non-volatile memory of the control unit37in advance.

Thus, the control unit37starts counting the pulse signals of the rotary encoder in the moving position information generating unit49, using the signal which the edge sensor has detected, for example, the top end of the image-formed material31carried on the conveyance route as trigger information. Then, the control unit37forms an image in each ink jet timing the counted number of the pulse signals of this rotary encoder and the accumulated number of pulses of the rotary encoder, based on the input image signal41, stored in advance in accordance with the nozzle array position of each of the short recording heads51A-1,51B-1˜51A-4,51B-4coincide with each other.

Next, the control method of the control unit of the image forming apparatus of the preferred embodiment is described in detail.

The input image signal41inputted from outside the image forming apparatus30is stored in the plain memory44as a 1˜n-lines of image signal (n=integer of two or more) under the control of the control unit37. In the case of the full line type image forming apparatus30of the preferred embodiment, this 1˜n lines of image signal corresponds to one line in the width direction of the recording area of the image-formed material31.

If the image forming apparatus30of the preferred embodiment comprises, for example, the color the nozzle array units51-1˜51-4, the plain memory44shown inFIG. 5, divides the 1˜n lines of image signal in relation with each nozzle array of the short recording heads51A-1,51B-1˜51A-4,51B-4and stores them.

The control unit37reads the 1˜n lines of the input image signal41stored in the plain memory44in synchronization with the count value of the moving position information generating unit49, generated by carrying the image-formed material31after for example, the edge sensor62has detected its top end, and inputs it to the optical density characteristic correction control unit46.

The optical density characteristic correction control unit46calculates a optical density correction signal by a process, which is described later, based on a optical density correction co-efficient set in advance according to the recording optical density characteristic of each ink-jet nozzle and the image signal value of each pixel formed on the image-formed material31by each ink-jet nozzle in the non-overlap part in which the ink-jet nozzles of the short recording heads do not overlap and which is jointed when viewed from the sub scan direction.

This optical density correction signal is a signal level quantization optical density correcting signal which can be inputted to each short recording head, and in each ink-jet nozzle of the nozzle array units51-1˜51-4of the preferred embodiment, it converts its optical density into eight gradation values of zero to seven grades.

It is preferable to diffuse a quantizing error caused by quantization conversion among peripheral pixels by an error diffusion method or the like.

If the image forming apparatus30of the preferred embodiment comprises, for example, the color the nozzle array units51-1˜51-4shown inFIG. 5, the image signal of each of 1˜n lines converted into a quantization optical density correction signal by the optical density characteristic correction control unit46is divided into image signals for each nozzle array and is inputted to the overlap correction control unit47.

Next, nozzle array overlap correction is described with reference toFIGS. 6,7and8.

In the description of this nozzle array overlap correction, it is assumed that there is a phase difference between the ink-jet nozzle of one nozzle array in the overlapped part and the ink-jet nozzle on the other nozzle array when the one nozzle array and the other nozzle array are overlapped and adjacently disposed.

For example, when resolution is 300 dpi, the distance “a” between ink-jet nozzles of a recording head shown inFIG. 6is approximately 85 μm. The distance δ shown inFIG. 6indicates the phase difference in the main scan direction between the ink-jet nozzle of a recording head71and the ink-jet nozzle of a recording head72. InFIG. 6A, this distance δ in a phase difference is the distance between the ink-jet nozzle73of the recording head71and the ink-jet nozzle74of the recording head72. InFIG. 6B, this distance δ in a phase difference is the distance between the ink-jet nozzle75of the recording head71and the ink-jet nozzle76of the recording head72.

An image signal is divided and inputted to the slashed ink-jet nozzles of the recording heads71and72shown inFIGS. 6A and 6B, and an image is formed by output dots formed by ink that is jetted from each ink-jet nozzle of the recording heads71and72(dot image).

No ink is jetted from each ink-jet nozzle not slashed inFIG. 6.

If the recording heads71and72are disposed in the position relationship shown inFIG. 6A, the output dot formed on the image-formed material31is formed in the position relationship shown inFIG. 7A. The main scan direction phase difference δ in this case has the relationship of δ>a, and the distance between the output dot81of the ink-jet nozzle73and the output dot82of the ink-jet nozzle74increases.

Thus, in the nozzle array overlapped part, the optical density of the output dot formed on the image-formed material31decreases. Therefore, the optical density of the nozzle array overlapped part becomes lower than the optical density of the output dot of the ink-jet nozzle adjacent to the nozzle array overlapped part, and as a result, striped optical density unevenness occurs.

If the recording heads71and72are disposed in the position relationship shown inFIG. 6B, the output dot formed on the image-formed material31is formed in the position relationship shown inFIG. 7B. The main scan direction phase difference δ in this case has the relationship of δ<a, and the distance between the output dot81of the ink-jet nozzle73and the output dot82of the ink-jet nozzle74decreases.

Thus, in the nozzle array overlapped part, the optical density of the output dot formed on the image-formed material31increases. Therefore, the optical density of the nozzle array overlapped part becomes higher than the optical density of the output dot of the ink-jet nozzle adjacent to the nozzle array overlapped part, and as a result, striped optical density unevenness occurs.

In this preferred embodiment, if one nozzle array and the other nozzle array are overlapped and adjacently disposed, optical density unevenness caused according to the phase difference between the ink-jet nozzle of the one nozzle array in the overlapped part and the ink-jet nozzle of the other nozzle array is corrected by controlling the jet of each ink-jet nozzle and changing the output dot formed on the image-formed material31by the overlap correction control unit47.

The change of an output dot includes at least one of the modification of a target ink-jet nozzle jetting ink and the modification of the diameter of a dot formed by ink jetted by the ink-jet nozzle.

FIG. 8Ashows an output dot formed on the image-formed material31after the nozzle array overlap correction when the main scan direction phase difference δ has the relationship of δ>a, as shown inFIG. 7A.

The output dot86shown inFIG. 8Ais formed on the image-formed material31by the ink-jet nozzle77of the recording head72shown inFIG. 6A. The amount of ink jetted when forming the output dot86is determined by multiplying the input image signal41by a optical density correction co-efficient according to the above-described main scan direction phase difference δ and quantizing this multiplied value into eight gradation values by the overlap correction control unit47.

In this way, in this preferred embodiment, the optical density unevenness of the nozzle array overlapped part can be corrected by adding the output dot86shown inFIG. 8Aby the nozzle array overlap correction of the overlap correction control unit47.

FIG. 8Bshows an output dot formed on the image-formed material31after the nozzle array overlap correction when the main scan direction phase difference δ has the relationship of δ<a, as shown inFIG. 7B.

The output dot86shown inFIG. 8Bis formed on the image-formed material31by the ink-jet nozzle76of the recording head72shown inFIG. 6B. The amount of ink jetted when forming the output dot84is determined by multiplying the input image signal41by a optical density correction co-efficient according to the above-described main scan direction phase difference δ and quantizing this multiplied value into eight gradation values by the overlap correction control unit47.

In this way, in this preferred embodiment, the optical density unevenness of the nozzle array overlapped part can be corrected by changing (reducing) the dot diameter of the output dot84′ shown inFIG. 8Bby the nozzle array overlap correction of the overlap correction control unit47.

The above-described nozzle array overlap correction of the overlap correction control unit47is applied to several lines of image signals of a plurality of line image signals in the input image signal41(the above-described 1˜n lines of image signals). In the course of this nozzle array overlap correction, a quantizing error caused when quantizing the input image signal into eight gradation values is diffused into one subsequent line of image signal.

FIG. 9Ashows an output dot formed on the image-formed material31by a plurality of lines of image signals after the nozzle array overlap correction.

As shown inFIG. 9A, the joint part of the output dots formed on this image-formed material31is formed linearly when viewed from the sub scan direction.

As shown inFIG. 9B, the optical density unevenness of the joint part of the output dots formed on this image-formed material31can be made unremarkable by moving the joint part, for example, in the main scan direction at random.

This joint part of output dots can be at random when viewed from the sub scan direction, by moving the switch position between an image signal divided into one nozzle array of the overlap part and an image signal divided into the other nozzle array.

In the nozzle array overlap correction of this preferred embodiment, the optical density correction of the nozzle overlapped part is applied only to the one output dot corresponding to the joint part, and no quantizing error caused by this nozzle array overlap correction is diffused into an output dot adjacent to the output dot of the joint part.

In the optical density correction process of this preferred embodiment, the quantizing error caused by the nozzle array overlap correction is prevented from diffusing into outside the nozzle array overlap part by providing the optical density characteristic correction control unit46and the overlap correction control unit47in the former and later stages, respectively.

Thus, in the optical density correction process of this preferred embodiment, both optical density unevenness due to a nozzle array overlapped part, the change inclination of whose optical density value is large and steep and optical density unevenness due to a non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to the nozzle overlapped part, the change of whose optical density value is small and gentle can be corrected.

Although as described above, the optical density correction of a nozzle array overlapped part of this preferred embodiment is applied only to one output dot of a ink-jet nozzle corresponding to the above-described joint part, the application of the optical density correction is not limited to this ink-jet nozzle and can also be applied to a plurality of ink-jet nozzles.

In the optical density correction of a nozzle array overlapped part of this preferred embodiment, for example, as shown inFIG. 10, the optical density unevenness can also be corrected by multiplying the input image signal41inputted to a plurality of ink-jet nozzles corresponding to the nozzle array overlapped part by a optical density correction co-efficient and controlling it.

FIG. 10Ashows a optical density correction co-efficient multiplied to each ink-jet nozzle of one nozzle array (left side) in an overlapped part when the one nozzle array and the other nozzle array are overlapped and adjacently disposed.FIG. 10Bshows a optical density correction co-efficient multiplied to each ink-jet nozzle of the other nozzle array (right side) in an overlapped part when the one nozzle array and the other nozzle array are overlapped and adjacently disposed.

As shown inFIG. 10C, in the optical density correction of a nozzle array overlapped part of this preferred embodiment, the optical density unevenness can be corrected as a result, by forming an image after multiplying the input image signal41inputted to each ink-jet nozzle of one nozzle array (left side) in an overlapped part by the optical density correction co-efficient shown inFIG. 10Aand multiplying the input image signal41inputted to each ink-jet nozzle of one nozzle array (right side) in the overlapped part by the optical density correction co-efficient shown inFIG. 10B.

FIG. 1Cshows a nozzle array overlap correction area93to which nozzle array overlap correction is applied, by dotted lines91and92. In the course of this nozzle array overlap correction, a quantizing error caused when quantizing the optical density into eight gradation values is diffused into the input image signal41corresponding to this nozzle array overlap correction area93.

For example, as shown inFIG. 11, in the optical density correction of a nozzle array overlap part of this preferred embodiment, a plurality of error conversion/distribution co-efficients for diffusing a quantizing error from a processed pixel to a peripheral pixel is provided and used for separate purposes.

FIG. 11shows a pixel X whose caused quantizing error is processed and an error conversion/distribution co-efficient (FIG. 16) into its peripheral pixels. For example, a quantizing error caused by quantization is diffused into the pixel in which “7” is indicated at the ratio of 7/16.

The error conversion/distribution co-efficient K1shown inFIG. 11Ais a normal diffusion co-efficient and is applied to a nozzle array overlap correction area93other than the area enclosed by dotted lines91and92shown inFIG. 10C.

The error conversion/distribution co-efficient K2shown inFIG. 11Bis designed to diffuse no error into the right side of the pixel X and is applied to a pixel on the right boundary of the nozzle array overlap correction area93enclosed by the dotted line92shown inFIG. 10C. The error conversion/distribution co-efficient K3shown inFIG. 11Cis designed to diffuse no error into the left side of the pixel X and is applied to a pixel on the left boundary of the nozzle array overlap correction area93enclosed by the dotted line91shown inFIG. 10C.

As described above, in the optical density correction of a nozzle array overlapped part of this preferred embodiment, a quantizing error caused when quantizing the optical density into eight gradation values in its correction course can be prevented from diffusing into an area adjacent to the nozzle array overlap correction area93by restricting the diffusion range of a quantizing error.

Therefore, in the optical density correction process of this preferred embodiment, both optical density unevenness due to a nozzle array overlapped part, the change inclination of whose optical density value is large and steep and optical density unevenness due to a non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to the nozzle overlapped part, the change of whose optical density value is small and gentle can be corrected.

Next, the determination procedure of a optical density correction parameter composed of a optical density correction co-efficient and the like, which is set in advance in the optical density characteristic correction control unit46and the overlap correction control unit47in the optical density correction process of this preferred embodiment is described.

This optical density correction parameter is determined at the time of plant shipment, re-adjustment after exchanging the recording head of a nozzle array unit, of the image forming apparatus30.

FIG. 12is a flowchart showing the determination procedure of the optical density correction parameter.

The image forming apparatus30reads a test image formed on the image-formed material31by the imaging apparatus42connected to the image forming apparatus30and determines this optical density correction parameter, based on its result.

The optical density correction process of this preferred embodiment is performed by setting this determined optical density correction parameter in the optical density characteristic correction control unit46and the overlap correction control unit47of the image forming apparatus30.

It is preferable to set this optical density correction parameter after setting basic image formation parameters, such as the main and sub scan direction positions of the test image formed on the image-formed material31, color overlap position information for piling several pieces of color ink on the same position of the image-formed material31in a prescribed pattern as a test image or the like.

In the optical density correction determining procedure shown inFIG. 12, firstly in step S1, the nozzle array optical density correction of the correction function by the optical density characteristic correction control unit46and the nozzle array overlap correction of the correction function by the overlap correction control unit47are stopped.

In the optical density correction parameter determining procedure of this preferred embodiment, the stoppage (off)/execution (on) of this correction function can also be switched by an operator operating the operation panel of the image forming apparatus30or the like. Alternatively, the image forming apparatus30can automatically switch between the stoppage (off)/execution (on) of this correction function when a optical density correction parameter determination mode is started.

Then, in step S2, a test image1for determining a nozzle array overlap correction parameter in the image forming apparatus30is formed on the image-formed material31. As described above, it is preferable for the test image1formed in step S2to be suited to detect the phase difference in a jet position between overlapped nozzle arrays of a nozzle array unit disposed in such a way that the respective ends of nozzles arrays of a plurality of recording heads overlap when viewed from the sub scan direction.

Therefore, as the test image, an image in which a reference distance between ink-jet nozzles can be measured or an image in which a phase difference can be estimated from optical density unevenness caused by the phase difference is used.

Since this test image1is used as a correction reference, an image is formed after the correction is stopped in step S1.

FIG. 13Ashows an example of a recording head with nozzle arrays having non-uniform forming characteristic (recording characteristic), in which images with different optical density are formed on the image-formed material31depending on the ink-jet nozzle position of a nozzle array.

FIG. 13Bshows the optical density of an image which the recording heads with non-uniform forming characteristic (recording characteristic) shown in shown13A form on the image-formed material31by nozzle array units disposed with an overlapped part while the optical density correction of this preferred embodiment is stopped.

The optical density value of the nozzle array overlapped part shown inFIG. 13Bis large and its change inclination is steep. The test image1is used to determine a nozzle array overlap parameter for correcting the optical density of this nozzle array overlapped part.

Then, in step S3, the imaging apparatus42reads the test image1formed in step S2.

Then, in step S4, the phase difference in a ink-jet nozzle position between overlapped nozzle arrays is detected based on the reading result in step S3and a nozzle array overlap parameter, such as a optical density correction co-efficient or the like, is determined based on the phase difference. Then, the determined nozzle array overlap correction parameter is stored in the non-volatile memory of the overlap correction control unit47.

In this determination of the nozzle array overlap correction parameter in step S4, it is preferable to eliminate optical density unevenness due to the non-uniform forming characteristic (recording characteristic) for each nozzle array, in which the change of its optical density value is small and gentle in order to determine parameters for correcting only optical density unevenness due to the phase difference in a ink-jet nozzle position between overlapped nozzle arrays and to improve the accuracy of this nozzle array overlap correction parameter.

Then, in step S5, nozzle array overlap correction by the overlap correction control unit47is executed (made on) and nozzle array optical density correction by the optical density characteristic correction control unit46is stopped (made off).

Then, in step S6, a test image2for determining a nozzle array optical density correction parameter is formed on the image-formed material31. It is preferable for this test image2to be an image in which optical density unevenness due to a non-uniform forming characteristic (recording characteristic) for each nozzle array, such as a half-tone image formed by dots of one to seven grades or the like.

The test image2formed in step S6becomes the image whose optical density unevenness in a nozzle array overlapped part is corrected by the nozzle array overlap correction.

Then, in step S7, the imaging apparatus42reads the test image2formed in step S6.

Then, in step S8, a nozzle array optical density correction parameter is determined by the reading result in step S7. Then, the determined nozzle array optical density correction parameter is stored in the non-volatile memory of the optical density characteristic correction control unit46.

This nozzle array optical density correction parameter determined in step S8can improve its accuracy since it determines the parameter value based on an image whose optical density unevenness in a nozzle array overlapped part is corrected by the nozzle array overlap correction.

Next, the mode setting operation in the case where an operator forms an image in the optical density correction process of the image forming apparatus30in this preferred embodiment and the optical density correcting operation based on this mode setting process are described.

In the image forming apparatus30of this preferred embodiment, an operator can set an image forming mode suitable for a difference in the data of an image formed on the image-formed material31(the input image signal41) or a difference in the material quality or the like of the image-formed material31in the mode setting unit38.

The image forming apparatus30of this preferred embodiment determines the respective optical density correction methods of the optical density characteristic control unit46and the overlap correction control unit47, based on the selected image forming mode to form an image.

If the operator does not set (select) the image forming mode, priority is given to a optical density correction process set as a default, which is described later, to correct a formed image.

Next, an operator-settable image forming mode is described.

The image forming apparatus30of this preferred embodiment has two types of modes; for example, a linear priority mode in which image data is character information, graphics or the like and a hue priority (optical density priority) mode in which image data is a picture or the like.

Since in the linear priority mode, priority is given to character information or the linearity of each line in graphics, no optical density correction process is performed.

Since in the hue priority (optical density priority) mode, priority is given to the reproducibility of hue for each ink color of, for example, black (K), cyan (C), magenta (M) and yellow (Y) and the hue of an overlapped color in the case where dots of each ink color is formed in the same position of the image-formed material31, a optical density correction process is performed.

The optical density correction processes in the case of forming an image are the same in improving the uniformity of image optical density. However, the optical density of the entire formed image has a tendency to decrease.

This is because a general optical density correction process reduces the output dots in a high-optical density part in such a way as to match image optical density in the high-optical density part of a formed image on the image-formed material31to that of the low-optical density part of the formed image on the image-formed material31, that is, image optical density in a part in which the diameter of an output dot jetted from a recording head is small and to make the image optical density uniform.

The reduced degree of optical density in this entire image varies depending on the difference in the material quality of the image-formed material31. In the image formation on a piece of ink-jet dedicated coated paper, since image optical density is kept high by the respective characteristics of this piece of ink-jet dedicated coated paper and ink, the image optical density is little influenced by the image optical density reduction in the optical density correction process. However, in the image formation on a piece of corrugated paper and a piece of cardboard, since the surface of a piece of corrugated paper and a piece of cardboard which is not especially coated for ink jet is applied and the soaked amount of ink increases, the image optical density is greatly influenced by the image optical density reduction in the optical density correction process.

However, since image data on a piece of corrugated paper and a piece of cardboard is often fairly large character data, little optical density unevenness is no problem.

In image formation on a piece of regular paper which is not especially coated for ink jet, the regular paper has an intermediate characteristic between a piece of ink-jet dedicated coated paper and a piece of corrugated paper/cardboard.

Therefore, in the image forming apparatus30of this preferred embodiment, an operator can set an image forming mode suitable for a difference in the data of an image formed on the image-formed material31(the input image signal41) or a difference in the material quality or the like of the image-formed material31in the mode setting unit38.

Next, the optical density distribution (optical density unevenness) status after the image forming apparatus30of this preferred embodiment performs the respective optical density correction processes of the optical density characteristic correction control unit46and the overlap correction control unit47, based on the selected hue priority (optical density priority) mode is described with reference toFIG. 14.

FIG. 14Ashows the optical density distribution (optical density unevenness) in the case where an image is formed on the image-formed material31after applying only nozzle array overlap correction by the overlap correction control unit47to nozzle array units disposed in such a way that the respective ends of the nozzle arrays of a plurality of recording heads overlap when viewed from the sub scan direction.

FIG. 14Bshows an example of a optical density correction value (optical density correction parameter) added by the nozzle array optical density correction by the optical density characteristic correction control unit46.

FIG. 14Cshows the optical density distribution (optical density unevenness) in the case where an image is formed after the nozzle array optical density correction is applied using the optical density correction value shown inFIG. 14B.

Of three types of optical density correction values shown inFIG. 14B, one indicated by a solid line101is the inverse function of the optical density distribution (optical density unevenness) shown inFIG. 14A, and by correcting the input image signal41according to this inverse function, the optical density distribution (optical density unevenness) indicated by a solid line104inFIG. 14Ccan be corrected.

Since there occurs almost no optical density unevenness in the optical density correction process indicated by this solid line104, it is suited to form a picture or the like on a piece of ink-jet dedicated coated paper.

Of the three types of optical density correction values shown inFIG. 14B, one indicated by a dotted line103is a optical density correction value in the case where no nozzle array optical density correction is performed, and as shown by a dotted line106inFIG. 14C, the input image signal41is formed without applying any process to it. The optical density correction process indicated by this dotted line106is greatly influenced by the image optical density reduction in the optical density correction process. Therefore, it is suited to form an image on a piece of corrugated paper, a piece of cardboard and the like.

Of the three types of optical density correction values shown inFIG. 14B, one indicated by the dotted line102is the intermediate optical density correction value between the optical density correction value indicated by the solid line101and the optical density correction value indicated by the dotted line103, and an almost sufficient optical density correction effect can be obtained by little image optical density reduction in the optical density correction process. Therefore, it is suited to form an image on a piece of regular paper which is not especially coated for ink jet is applied.

As shown by the dotted line inFIG. 14B, this intermediate optical density correction value can be obtained by the inverse function of the optical density distribution obtained by reading an image formed on the image-formed material31by the imaging apparatus42by a co-efficient with a specific size.

The image forming apparatus30of this preferred embodiment stores a plurality of optical density correction values, such as three types of optical density correction values indicated by the solid line101and dotted lines102and103in advance.

An operator can select a hue priority (optical density priority) mode corresponding to this selected contents and form an image to which proper nozzle array optical density correction is applied, by selecting and setting conditions, such as the type of image data (the input image signal41), the type of the image-formed material31and the like, in the mode setting unit38of the image forming apparatus30, for selection/setting excluding the linear priority mode.

For the hue priority (optical density priority) mode selected by the operator, image quality, specifically, a correction value for the optical density correction process can also be selected when performing the optical density correction process. Alternatively, it can be selected which the optical density of an image to be corrected is, high or low.

The optical density correction by the image forming apparatus30of this preferred embodiment can also be realized not only in the case of the full line type image forming apparatus30but also the serial type image forming apparatus30.

FIG. 15shows the simplified configuration of the serial type image forming apparatus and shows the simplified configuration of the serial type image forming apparatus in which the image-formed material31is not moved in the sub scan direction.

FIG. 15also roughly shows a carriage33B by a two-dot chain line, in which the disposition form of color nozzle array units are different from that of a carriage33A, in addition to the carriage33A in which color nozzle array units are disposed in the main scan direction in parallel and at prescribed intervals in the moving object33.

In the serial type image forming apparatus30shown inFIG. 15, the carriages33A or33B are supported by a support member118in such a way to move back and forth in the upper section of the fixed and mounted image-formed material31. At one end of this support member118, one the moving mechanism43is constituted by the motor of one the moving mechanism driver unit48for rotating a pulley115and a rotary encoder of one the moving position information generating unit49. At the other end of the support member118, a pulley116is disposed opposed to the pulley115in the main scan direction. The pulleys115and116are installed in such a way as to move back and forth in the main scan direction, and are connected to the carriage33A or33B by a connecting unit117a.

Thus, the carriage33A or33B is moved back and forth in the main scan direction by the rotation of the motor of one the moving mechanism driver unit48, and also the moving position of the carriage33A or33B is notified to the control unit37of the image forming apparatus30.

In the vicinity of the other end of the support member118, a pulley122is disposed. In the position opposed to this pulley122in the sub scan direction, other the moving mechanism43is constituted by the motor of other the moving mechanism driver unit48for rotating a pulley121and the rotary encoder of other the moving position information generating unit49provided for the rotation shaft of this motor. In the pulleys121and122, an endless belt123is installed in such a way as to move back and forth in the sub scan direction and is connected to the other end of the support member118by a connecting unit123a.

Thus, the support member118is moved back and forth in the sub scan direction by the rotation of the motor of other the moving mechanism driver unit48, and also the rotary encoder of other the moving position information generating unit49notifies the control unit37of the image forming apparatus30of the moving position of the support member118.

If the image forming apparatus of the preferred embodiment is of a serial type, by adopting such a configuration, the serial type image forming apparatus30of the preferred embodiment can form an image by scanning the carriage33A or33B on the image-formed material31along a scan trace125while informing the control unit37of a moving position in the case where the carriage33A or33B is moved in the main and sub scan directions in the moving object33.

FIG. 16shows an example of the configuration of a carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage.

In the carriage33A shown inFIG. 16, a (K) nozzle array unit51-1, a (C) nozzle array unit51-2, a (M) nozzle array unit51-3and a (Y) nozzle array unit51-4are disposed as color-corresponding nozzle array units in parallel in the main scan direction at prescribed intervals. The color the nozzle array units51-1˜51-4are composed of a pair of short recording heads51A-1and51B-1˜51A-4and51B-4, respectively.

FIG. 17shows an example of the configuration of each carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage.

In the carriage33B shown inFIG. 16, (K), (C), (M) and (Y) nozzle array units51-1,51-2,51-3and51-4(51-3and51-4are not shown inFIG. 17), are disposed as color-corresponding nozzle array units in the sub scan direction at prescribed intervals or at no intervals in parallel with the sub scan direction. The color the nozzle array units51-1˜51-4are composed of a pair of short recording heads51A-1and51B-1˜51A-4and51B-4, respectively.

As described so far, according to the image forming apparatus of this preferred embodiment, in the image forming apparatus comprising one long recording head by disposing a plurality of short recording heads, striped optical density unevenness due to the joint part of adjacent short recording heads, white wiping and the like can be made not remarkable and optical density unevenness due to the non-uniform recording optical density characteristic of the short recording heads can also be corrected when forming an image, without strictly adjusting the positions of the short recording heads.

According to the image forming apparatus of this preferred embodiment, since an quantizing error caused by nozzle array overlap correction is not diffused into outside the nozzle array overlapped part, both optical density unevenness due to the nozzle array overlapped part, whose optical density value is large and, the change inclination of whose optical density value is steep and optical density unevenness due to the non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to a nozzle array overlapped part, the change of whose optical density value is small and gentle can be corrected.

Although the nozzle array unit of this preferred embodiment comprises two nozzles arrays composed of a plurality of nozzles, it can also comprise three or more such array units.

The nozzle array unit of this preferred embodiment can also similarly comprise short recording heads obtained by jointing a plurality of recording heads with one or more nozzle arrays and a long recording head can be obtained by disposing a plurality of these short recording heads in one direction.