Image recording apparatus

Abstract: An image recording apparatus using a plurality of recording heads (9C, 9M, 9Y, 9BK) that can prevent black lines from occurring at boundaries of each scanning of the recording heads. The total recorded ink volume at the boundaries is positively reduced so that the black lines and the overflow of ink are prevented. The apparatus includes at least a pair of a dark head (26) for discharging relatively dark ink and a light head (27) for discharging relatively light ink, and separates an input image signal into image signals applied to the dark and light heads using at least two tables (23A, 23B). The two tables, one for edge nozzles of each recording head and the other for the remaining nozzles, separate the input image signal independently so that the total ink volume discharged from the edge nozzles are limited to a relatively small amount. Further, the total volume of inks discharged from the edge nozzles of the recording heads are positively reduced by performing black abstraction processing, UCR processing and black inking processing on the basis of the image signals after the separation by the tables, which image signals are linearly correspond to the ink volume.

Description: The present invention relates to an image recording apparatus. An ink jet recording apparatus which performs printing (recording) by discharging ink onto a recording medium is known as one of the conventional image recording apparatuses. The ink jet recording apparatus has been rapidly spread recently because it is a nonimpact type recording apparatus and so it is of low noise, and color image recording can be easily implemented by using multiple inks. Fig. 1 is a schematic perspective view showing a conventional ink jet recording apparatus. In Fig. 1, a recording medium 5 wound in a roll passes transport rollers 1 and 2, is held by paper incrementing rollers 3, and is incremented in the direction of f in Fig. 1 by driving a subscanning motor 4 coupled to one of the incrementing roller 3. Across the recording medium 5, guide rails 6 and 7 are disposed in parallel, and a carriage 8 is mounted thereon. The carriage 8 moves in the lateral direction so that a recording head unit 9 mounted thereon is scanned in that direction. On the carriage 8, four color recording heads 9Y, 9M, 9C and 9Bk for yellow, magenta, cyan and black are mounted, and four ink reservoirs corresponding to those colors are disposed. Each head includes an orifice array including a plurality of ink orifices, and orifice arrays of the heads are disposed in parallel. The recording medium 5 is intermittently incremented by the amount of recording of the head unit 9, and the head unit 9 is scanned in the direction P discharging ink droplets in accordance with a binary image signal while the recording medium is stationary. With such an ink jet recording apparatus, recording medium characteristics are very important. In particular, ink running characteristics on a recording medium have strong influence on image quality. As one of the indices representing the ink running characteristics of the recording medium, a "spreading rate" is known. It indicates how many times an ink droplet ejected from an ink jet nozzle will spread after arriving on a recording medium, and is given by the following equation: spreading rate = dot diameter on the recording medium / ink droplet diameter ejected from the nozzle For example, if an ink droplet of 30 µm diameter during traveling forms a dot of 90 µm diameter on the recording medium, the spreading rate of the recording medium is 3.0. With recording media of small spreading rates, the image intensity is low, and hence, it is difficult to achieve smooth texture, high quality images. On the other hand, although recording media of a high spreading ratio can increase the image intensity, the following problems arise. A serial scanning type ink jet recording apparatus as shown in Fig. 1 sequentially records images of width d as indicated by (1), (2) and (3) in Fig. 2 by scanning in the direction A the recording head unit 9 including a plurality of ink discharge orifice arrays disposed in parallel. For example when the number of orifices per array is 256, and the recording density is 400 dots/inch (dpi), the recording width d becomes 16.256 mm (= 256 × 25.4/400). When the volume of ink landing on the recording medium is small, the actual width of the recorded image is approximately equal to the recording width d because the ink is sufficiently absorbed by the recording medium. Accordingly, when recording is performed by repeating the scanning of the recording head unit 9 in the direction A and the subscanning of the head unit 9 by the amount of d in the direction B, joint portions between two adjacent images recorded by two continuous scannings present little problem as shown in Fig. 2. The image width, however, may increase to d + Δd owing to ink running when recording high density portions where the volume of ink is great because the recording medium of a large spreading ratio cannot sufficiently absorb ink. In such a case, the subscanning of the head unit 9 by an amount of d will cause an overlap of images by the amount of Δd as shown in Fig. 2B, thereby resulting in black lines at the overlapped portions. In contrast with this, if the subscanning of the head unit 9 is set at the amount of d + Δd, white lines will appear at low density portions where the volume of ink is small. The extra amount Δd of the image width at high density portions changes depending on the spreading rate of a recording medium, or the volume of ink arriving at the recording medium, and increases consistently with the spreading rate and the ink volume. For this reason, to prevent black lines from occurring, it is necessary to use a recording medium of a small spreading rate, or to reduce the ink volume. In this case, however, the image intensity decreases as stated before, which presents a problem in that smooth texture, high quality images cannot be obtained. To solve such a problem, the applicants of the present invention proposed EP-A-0 347 257. In this proposal, if values of an image signal at the boundaries of the serial scanning exceed a predetermined amount, they are reduced before applied to recording elements involved in recording the boundaries. This makes it possible to reduce the ink volume discharged from nozzles associated with the recording elements, thereby preventing black lines from occurring at high density, boundary portions. In this method, the image signal applied to the recording elements associated with the nozzles at the edges of the discharge orifice arrays is converted by a table as shown in Fig. 3: if the input image signal to the table exceeds a predetermined value T, the output of the table fed to the recording elements is restricted at a fixed value F. Thus, in the case where the density of the image signal applied to the recording elements associated with edge nozzles is rather high, the volume of ink discharged from the edge nozzles is restricted, thereby circumventing the occurrence of black lines at the joint portions. On the other hand, a recording apparatus has been proposed which includes a plurality of recording heads discharging inks of different concentration to improve gradation. For example, a color image recording apparatus has been proposed which uses a several sets of inks, each set consisting of inks belonging to the same color family and having different concentrations, such as light black ink and dark black ink, light cyan ink and dark cyan ink, light magenta ink and dark magenta ink, and light yellow ink and dark yellow ink, thereby improving graininess at highlighted portions. Using this head unit makes it possible to ameliorate the problem in that dots appear unnaturally in highlighted portions in an image recorded by a binary recording type color image recording apparatus. Fig. 4 shows the schematic arrangement of a conventional color ink jet recording apparatus of this type. This recording apparatus is different from the recording apparatus of Fig. 1 in the construction of the recording head unit 9. The recording head unit 9 comprises eight recording heads 9-1BK for light black, 9-2BK for dark black, 9-1C for light cyan, 9-2C for dark cyan, 9-1M for light magenta, 9-2M for dark magenta, 9-1Y for light yellow, and 9-2Y for dark yellow, and four sets of color ink reservoirs, each of which comprises two inks of the same color family with different concentrations. The recording medium 5 is intermittently incremented by the amount of a recording by the recording head unit 9, and the recording is carried out while the recording medium 5 is stationary. More specifically, the recording head unit 9 is scanned in the direction of the arrow P along the guide rails 6 and 7 discharging ink droplets in accordance with an image signal, thereby forming an color image consisting of dot matrices on the recording medium 5. Generally, the signal processing system of the color image recording apparatus of this type is arranged as shown in Fig. 5. Since the signal processing system has a similar construction for each of the four color families of black, cyan, magenta and yellow, the system for one color family is described. In Fig. 5, an input image signal S0 is divided in accordance with its intensity by a dark/light separating circuit 11. More specifically, the input image signal S0 is divided into a light black image signal S1 and a dark black image signal S2 by a dark/light separating table having input and output characteristics as illustrated in Fig. 6, and the image signals S1 and S2 are sent to a multiple-to-binary converter 12 which converts the multi-level image signals S1 and S2 into binary signals, and sends them to corresponding head drivers 13-1 and 13-2. The head drivers drive corresponding heads 9-1 and 9-2, thus recording an image. Here, the head driver 13-1 and the recording head 9-1 are for light ink, and the head driver 13-2 and the recording head 9-2 are for dark ink. Fig. 6 illustrates the driving duty of the dark and light heads for the input image signal S0. When the input image signal S0 is smaller than P (P = 127 for an 8-bit signal, for example), only the light head 9-1 is used. On the other hand, when the input image signal S0 is greater than P, the recording duty of the light head 9-1 is gradually reduced whereas the recording duty of the dark head 9-2 is gradually increased. For example, consider one pixel comprising four squares, onto each of which one ink drop can be discharged. One pixel can be formed by discharging ink drops up to twice from two nozzles of the dark head and the corresponding two nozzles of the light head, the nozzles being arranged in the vertical direction of each head. For instance, let us assume that, at the first discharge, only light ink droplets are discharged from the two nozzles of the light head, and at the second discharge, a light ink droplet is discharged from the upper nozzle of the light head and a dark ink droplet is discharged from the lower nozzle of the dark head. Thus, the duty of the light head is 75% and that of the dark head is 25% in forming this pixel. By driving two recording heads in accordance with the image signal as described above, the concentration of images can be controlled as shown in Fig. 7. Although the recorded image density characteristics as shown in Fig. 7 can be achieved by using only the dark head 9-2, using both the dark and light heads is advantageous in that the rough feeling of dots at low density portions is reduced, and a smooth gradation image can be achieved. However, the recording by using the dark and light heads cannot sometimes accomplish sufficient effects even if the image signal correction for the edge nozzles is performed as stated before. More specifically, when the image signal correction is carried out by using the dark/light separation table whose characteristics are shown in Fig. 6, the total ink volume (the sum of the light ink volume and the dark ink volume) discharged from the heads reaches the fixed maximum after the input image signal S0 exceeds the value P as shown in Fig. 8. In the region where the image signal S0 is greater than P, both the dark and light heads are used as shown in Fig. 6. In this case, the ink volume discharged from the light head decreases as the ink volume discharged from the dark head increases, and hence, the total discharged ink volume is maintained at the fixed value. Consequently, if the maximum total ink volume is discharged from the edge discharging nozzles of the dark and light recording heads, the problem of ink running and the black lines may occur. To avoid this, the total ink volume discharged from the edge nozzles must be restricted if the input image signal exceeds a predetermined value Q smaller than P. To achieve this, a table conversion as shown in Fig. 9 must be performed on the image signal S0 inputted to the edge nozzles. Here, the value Q is determined experimentally by practically printing an image on a recording medium so as to obtain optimum conditions, and the value S in Fig. 9 corresponds to the level Q of the input image signal S0 as shown in Figs. 6. This conversion, however, is sometimes ineffective to restrict the ink discharge volume for the following reason. As will be seen from Fig. 6, when the image signal S0 is greater than Q, the image signals S1 and S2 applied to the dark and light recording heads will exceed the value S in the regions A and C of Fig. 6, respectively. Accordingly, the image signals S1 and S2 are restricted as shown in Fig. 9 in the regions A and C. On the other hand, the image signals S1 and S2 do not exceed S in the region B of Fig. 6 even if the input image signal S0 is greater than P. Thus, the image signals S1 and S2 are not restricted in the region B. As a result, the total ink volume is not restricted, and the black lines are likely to occur. The characteristic lines of Fig. 6 will now be explained in more detail. When the image signal S0 takes a value from 0 to 127 in the 8-bit representation, only the image signal S1 applied to the recording head 9-1 for light ink is outputted. This means that the optical density is achieved by recording a number of light ink dots on the recording medium so that the degradation of an image due to distinct individual dots can be prevented. On the other hand, when the image signal S0 ranges from 128 to 255 in the 8-bit representation, the value of the image signal S2 applied to the recording head 9-2 for the dark ink after the separation by the table increases linearly with the image signal S0, thus increasing the discharge volume of the dark ink. In contrast, the image signal S1 applied to the recording head 9-1 for the light ink decreases linearly with the image signal S0 so as to restrict the total ink discharge volume. If the discharge volume of the light ink is not decreased, the total ink discharge volume will exceeds the allowable ink volume of a recording medium when a plurality of color inks are superimposed, thereby causing the overflow of the ink. To avoid the overflow of ink, UCR (under color removal) processing and inking processing of black ink are carried out so that the total ink discharge volume is reduced. More specifically, at portions where cyan, magenta and yellow inks are superimposed to represent black, the volumes of these inks are reduced, and black ink is used in place of theses inks, thus reducing the total ink discharge volume. The conventional method, however, employs the image signal S0 before the dark/light separation to perform the UCR and inking processings. This presents a problem in that the total ink volume cannot be effectively reduced by the following reasons. Fig. 10 illustrates the relationship between the input image signal S0 and the recorded ink volume Vink per color (the sum of light ink and dark ink) when the image signal S0 is separated by the table as shown in Fig. 6. As will be seen from Fig. 10, the recorded ink volume of each color is maintained at a fixed value even if the image signal S0 decreases in the range from 128 to 255 in the 8-bit representation. Thus, the reduction of the recorded ink volume cannot be expected by the UCR processing. As a result, there still remains a problem that the total recorded ink volume cannot be effectively reduced. Moreover, when the three inks of cyan, magenta and yellow (that is, the light cyan and dark cyan, the light magenta and dark magenta, and the light yellow and dark yellow) are superimposed, the total ink volume may exceed the allowable ink volume of a common recording medium, thereby causing the overflow of inks on the recording medium, even if the image signal S1 is decreased as the image signal S0 increases in the range of 128 - 256 in the 8-bit representation. JP-A-1-55248 discloses an ink jet recording apparatus which uses either a dark ink with a dye having good colour tone or a light ink with a dye having good light resisting properties. The present invention provides an image recording apparatus as set out in claim 1. An embodiment of the present invention provides an image recording apparatus which can prevent black lines at the boundaries corresponding to the edge nozzles of the recording heads when recording is performed using a plurality of heads of different ink concentrations. An embodiment of the present invention provides an image recording apparatus which can positively reduce the total ink volume so that the overflow of inks is prevented and a high quality image can be formed. The driving duty may be the number of actual discharges to a unit area per number of possible discharges to the unit area. The separating means may select one of the tables which reduces the ink discharge volumes at the edge nozzles to a smaller amount than that at the remaining nozzles if the image signals are larger than a predetermined value. The separating means may select one of the tables on the basis of the image signals associated with a pixel to be recorded and its neighbouring pixels. The separating means may further comprise operation means for performing a weighted summation on image signals of respective colours, and may select one of the tables in accordance with the output of the operation means and the image signals. The image recording apparatus may further comprise temperature detecting means for detecting the temperature of each of the recording heads, and the separating means may select one of the tables in accordance with the locations of the nozzles and the temperature detected by the temperature detecting means. The plurality of recording heads may comprise recording heads for cyan, magenta, yellow and black. Each of the recording heads may cause state transition in ink by utilizing thermal energy, and discharge ink on the basis of the state transition. According to the present invention, the duty of the recording heads are relatively reduced when the image signal is associated with the edge nozzles of the nozzle array of each recording head. This will limit the total volume of ink discharged from the edge nozzles of the recording heads in accordance with the image signal, and thus, prevent the occurrence of black lines at boundaries of each scanning, which might be formed by the discharge of ink from the edge nozzles. In addition, a high quality image with little black lines can be achieved by a serial scanning recording apparatus of a simple construction in hardware as well as in software. Moreover, since the present invention performs the black abstraction processing, UCR processing and black inking processing on the basis of the image signals after the dark/light separation, the volume of recorded inks linearly corresponds to the image signals. This is because it is not before the dark/light separation but only after the separation that the actually discharged ink volume can be predicted. This makes it possible to positively reduce the volume of ink, thereby providing high quality images without the overflow of ink. The above and other effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. Fig. 1 is a schematic perspective view showing an example of an ink jet recording apparatus; Figs. 2A and 2B are diagrams illustrating boundaries in a conventional serial type ink jet recording apparatus; Fig. 3 is a diagram illustrating a conventional correction table for the edge nozzles of recording heads; Fig. 4 is a schematic perspective view showing an example of a color image recording apparatus; Fig. 5 is a block diagram showing an arrangement of the signal processing system of the conventional color image recording apparatus; Fig. 6 is a diagram illustrating a conventional dark/light separation table; Fig. 7 is a diagram illustrating the relationship between an image signal and recorded image densities; Fig. 8 is a diagram illustrating the relationship between the image signal S0 and the total discharged ink volume in the conventional recording apparatus; Fig. 9 is a diagram illustrating the conversion of an image signal to restrict the total ink discharge volume; Fig. 10 is a diagram illustrating the relationship between the image signal S0 before the dark/light separation and the recorded ink volume per color in the conventional ink jet recording apparatus as shown in Fig. 4; Fig. 11 is a block diagram showing the arrangement of a control system of a first embodiment of an image processing apparatus according to the present invention; Fig. 12A is a diagram illustrating a dark/light separating table for the image signal S0 corresponding to nozzles other than the edge nozzles of a recording head; Fig. 12B is a diagram illustrating a dark/light separating table for the image signal S0 corresponding to the edge nozzles of a recording head; Fig. 13 is a diagram illustrating the relationship between the image signal S0 and the total ink discharge volume; Figs. 14A and 14B are diagrams illustrating the principle of a second embodiment of an image processing apparatus according to the present invention; Fig. 15 is a block diagram showing the arrangement of a controlling system of the second embodiment of the image processing apparatus according to the present invention; Figs. 16A - 16F are diagrams illustrating dark/light separation tables of the second embodiment; Fig. 17 is a block diagram showing the arrangement of a controlling system of a third embodiment of the image processing apparatus according to the present invention; Fig. 18 is a block diagram showing the arrangement of a controlling system of a fourth embodiment of the image processing apparatus according to the present invention; Figs. 19A - 19G are diagrams illustrating dark/light separation tables of the fourth embodiment; Fig. 20 is a block diagram showing the arrangement of a controlling system of a fifth embodiment of the image processing apparatus according to the present invention; Figs. 21A, 21B-1, 21B-2, 21B-3, 21C-1, 21C-2, 21C-3, 21D-1, 21D-2 and 21D-3 are diagrams illustrating dark/light separation tables of the fifth embodiment; Fig. 22 is a block diagram showing the arrangement of a controlling system of a sixth embodiment of the image processing apparatus according to the present invention; The invention will now be described with reference to the accompanying drawings. EMBODIMENT 1 Fig. 11 is a block diagram showing the image signal processing portion of a first embodiment of an ink jet recording apparatus in accordance with the present invention. In the following embodiments, it is assumed that a recording apparatus wherein ordered dithering or error diffusion technique is employed to convert the input signal into a binary signal, and to control the intensity by the number of dots in superpixels. With regard to these techniques, the description on pages 350-352 of "Output Hardcopy Device", 1988, edited by Robert C. Durbeck, et al., Academic Press, Inc. is incorporated here by reference. An image signal 21 inputted to this apparatus undergoes processing such as a gamma conversion in an image processor 22, and is supplied to a dark/light separating table 23 as an input image signal S0. The dark/light separating table 23 comprises an ROM storing two types of dark/light separating tables 23A and 23B having separating characteristics as illustrated in Figs. 12A and 12B, respectively. The two tables 23A and 23B are switched by the control signal CT fed from a switch controller 28. The switch controller 28 comprises a microcomputer or a logic circuit which receives the clock signal of the input image signal 21, and decides whether or not the input image signal 21 corresponds to edge nozzles of recording heads. If the image signal 21 corresponds to the edge nozzles, the switch controller 28 outputs the control signal CT of "1", otherwise, the control signal CT of "0", and send the control signal CT to the dark/light separating table 23. The table 23 employs the table 23A as shown in Fig. 12A when the control signal CT is "0", and the table 23B as shown in Fig. 12B when the control signal CT is "1". Using the separating table 23A gives the total discharged ink volume as indicated by (a) in Fig. 13: The total discharged ink volume takes a fixed value Va in the range where the input image signal S0 is greater than P as stated before with regard to Fig. 8. In contrast, using the separating table 23B gives the total discharged ink volume as indicated by (b) in Fig. 13: The total discharged ink volume takes a fixed value Vb in the range where the input image signal S0 is greater than Q. The fixed value Vb is smaller than Va because the table 23B has separating characteristics as indicated by solid lines in Fig. 12B, which is different from the characteristics of the normal table 23A whose characteristics are indicated by dotted lines. Thus, the ink volume discharged from the edge nozzles of the recording heads are reduced to smaller values than in the normal case, thereby preventing black lines from occurring. Here, the values P and Q can be determined as follows: First, the value P is determined depending on (1) the concentrations of the dark and light inks, and (2) the color development characteristics of each recording medium. Although the value P is set at 127 or half the maximum value of the image signal in this embodiment, it is not limited thereto. On the other hand, the value Q is determined depending on the running characteristics of a recording medium, and is experimentally determined by printing to achieve optimum results. Furthermore, the figure of Fig. 12B is not restricted to a trapezoid: a trapezium whose top side slopes can also be employed. The edge nozzles are specified in accordance with an amount of ink running. Thus, the edge nozzles are defined as each one nozzle or a few nozzles at the top and bottom of the nozzle array in accordance with pixel density and ink absorption ratio of a recording medium. EMBODIMENT 2 In the first embodiment, the total ink discharge volume is restricted if the image signal supplied to the edge nozzles indicates high intensity. However, if the image signal associated with the boundaries of a recording indicates high intensity in isolation, the spread of the image width due to running will be little. In such a case, the first embodiment may present a problem because it reduces the discharge volume from the edge nozzles regardless of this fact. In contrast, large volume of ink discharged from the nozzles near the edges will cause a running, and hence, the discharge volume from the edge nozzles may have to be further reduced to achieve sufficient effect. The second embodiment according to the present invention is implemented in view of this problem. Fig. 14A schematically illustrates the nozzle array of the recording head 9 as shown in Fig. 1. Reference numeral 51 designates nozzles. Fig. 14B illustrates a matrix 52 of image data where the row i indicates the image data supplied to the bottom nozzle of the recording head 9 during the previous scanning of the head, the row j represents the image data supplied to the top nozzle of the recording head 9 during the current scanning of the head, and the row k indicates the image data applied to the immediately below the top nozzle, that is, the second top nozzle. In addition, the m column indicates the image data associated with the current discharge, the l column represents the image data associated with the previous discharge, and the n column indicates the image data associated with the next discharge. In this embodiment, a 3 × 3 pixel matrix is used which is arranged around the central pixel (j,m) recorded by the discharge from the edge nozzle so that the volume of ink discharged from the edge nozzle is controlled by the sum of the image data in the matrix. More specifically, each image data in the matrix is weighted in accordance with the location in the matrix by multiplying a weighting coefficient, and the total sum of the weighted image data is calculated. If the total sum exceeds a predetermined value, the value of the image data applied to the edge nozzle is limited. Fig. 15 is a block diagram showing the arrangement of a controlling portion performing the processing described above. In Fig. 15, reference numerals 60a - 60i denotes buffers for temporarily storing the image data as shown in Fig. 14B. The image data outputted from the buffers 60a - 60i are supplied to multipliers 61a - 61i. The multipliers 61a - 61i multiply weighting coefficients α 1 - α 9 to the image data, respectively, and provides the results for an adder 62. The adder 62 adds all the outputs of the multipliers 61a - 61i, and supplies the resultant sum SM to the dark/light separating table 23 including an ROM. The dark/light separating table 23 stores six types of dark/light separating tables as shown in Figs. 16A - 16F. These six table are different in the degree of restricting the ink volume, and are switched by the control signal CT sent from the switch controller 28 and the sum SM fed from the adder 62. More specifically, when the control signal CT is "0" the separating table as shown in Fig. 16A is selected. On the other hand, when the control signal CT is "1", one of the five tables as shown in Figs. 16B - 16F is selected in accordance with the sum SM fed from the adder 62. For example, if the sum SM is small, the table as shown in Fig. 16B is selected whose degree of restriction is small. As the sum SM increases, the degree of restriction is raised as illustrated in Figs 16C - 16F. According to the second embodiment, the ink volume discharged from the edge nozzles is restricted taking into account not only the image data associated with the edge nozzles but also with the nozzles near the edge nozzles, thus effectively preventing black lines from occurring. EMBODIMENT 3 The present invention can be applied to a color recording apparatus as in a third embodiment. Since the color recording uses recording heads for a plurality of colors, the ink volume exceeds that of a monochromatic recording. Since the total ink volume for one pixel is the sum of all colors, independent application of the present invention to each color cannot achieve sufficient effect. The present embodiment improves this. Fig. 17 is a block diagram of a controlling system in accordance with the third embodiment. Reference characters 31BK, 31M, 31Y and 31C denote input image signals for black, magenta, yellow and cyan, respectively. These signals are inputted to a weighting circuit 32 which performs weighted summation on respective image signals. Assuming that the values of the cyan, magenta, yellow and black image signals are C, M, Y and BK, the sum SM is given by SM = α 1 C + α 2 M + α 3 Y + α 4 BK where α 1 - α 4 are weighting coefficients which can be obtained experimentally from the spread ratio of each color ink. If the simple summing achieves enough effect, they can be determined as α 1 = α 2 = α 3 = α 4 = 1. The sum SM is inputted to the dark/light separating table 23 which stores the six types of tables as shown in Figs. 16A - 16F, and selects one of them most suitable for separating the cyan image signal in accordance with the control signal CT and the sum signal SM. Thus, the ink restriction at the edges of the recording heads is carried out taking account of the total volume of all the color inks, thereby preventing black lines from occurring. In the three embodiments described above, although a pair of dark and light heads for each color are employed, the present invention can also be applied to the case where a set of three or more heads are used for each color. EMBODIMENT 4 In the embodiments described above, the occurrence of black lines at boundaries is prevented by using different separating tables for edge nozzles and the other nozzles. The ink discharge volume, however, changes depending on the temperature of the recording heads. This is because the viscosity of ink reduces, and hence, the ink discharge volume increases as the temperature rises. Accordingly, when the recording heads continue recording to reach a high temperature, or are used in a high ambient temperature, the ink discharge volume increases, thereby causing more distinct black lines. The fourth embodiment controls the selection of the separating tables in view of this fact. Fig. 18 is a block diagram showing the arrangement of a controlling system associated with the fourth embodiment of the present invention. In this figure, the input image signal S0 is inputted to the dark/light separating table 23 including an ROM storing dark/light separating tables as shown in Figs. 19A - 19G, and is separated into the dark signal S2 and the light signal S1. The dark signal S2 and the light signal S1 are inputted to a dark head portion 26 and a light head portion 27, respectively, to form an image by discharging dark ink and light ink. Temperature sensors 26A and 27A are attached to these head portions 26 and 27, respectively. The temperature sensors 26A and 27A detect the temperature of the head portions 26 and 27, and supplied the dark/light separating portion 23 with temperature signals T2 and T1. The control signal CT takes a value "1" if the image signal S0 fed to the dark/light separating table 23 corresponds to the edge nozzles, and otherwise takes a value "0". When the control signal CT is "0", a separating table as shown in Fig. 19A is used. When the control signal CT is "1" and the temperatures of all the recording heads are kept within a predetermined range (below 45 °C, for example), a separating table as shown in Fig. 19B is employed so that the maximum ink volumes discharges from the dark and light heads are restricted, thereby preventing black lines from occurring at the boundaries. When the temperature of the light head rises, the maximum value of the light signal S1 is further limited in accordance with the degree of the temperature rise as shown in Figs. 19C and 19D. On the other hand, if the temperature of the dark head rises, the maximum value of the dark signal S2 is further limited in accordance with the degree of the temperature rise as shown in Figs. 19E and 19F. Moreover, if the temperatures of both the dark and light heads rise, the maximum values of both the dark and light signals S1 and S2 are limited by a table as shown in Fig. 19G. According to this embodiment, one of the dark/light separation tables as shown in Figs. 19A - 19G is selected in accordance with the temperatures of the dark and light heads so that the occurrence of black lines is prevented independently of the temperature. EMBODIMENT 5 The weighted summation on the image data using the neighboring pixels as in the second embodiment can be applied to the method of the fourth embodiment which selects one of the separating tables taking account of the temperatures of the recording heads. Fig. 20 is a block diagram of an example of a control system for such processing. In this figure, like reference numerals denote like portions in Fig. 15, and hence, the description thereof is omitted here. The dark/light separating table 23 comprises a plurality of tables as shown in Figs. 21A to 21D, and selects one of them in response to the sum SM, the temperature T2 of the dark head, the temperature T1 of the light head, and the control signal CT. Figs. 21A - 21D illustrate separating tables employed in this embodiment. When the control signal CT is "0", that is, when the input image signal S0 does not correspond to the edge nozzles, the separating table as shown in Fig. 21A is used. On the other hand, when the control signal is "1", that is, when the input image signal S0 corresponds to the edge nozzles, the separating table is switched from the table of Fig. 21B-1, to that of Fig. 21C-1, and then to that of Fig. 21D-1, as the sum SM increases. For example, if the temperatures of the recording heads are maintained within a predetermined temperature range (below 45 °C, for example) and the sum SM is also within a predetermined range, the table as shown in Fig. 21B-1 is used. If the temperatures of the recording heads are in the predetermined temperature range and the sum SM increases beyond the predetermined range, one of the tables as shown in Figs. 21C-1 and 21D-1 is selected in accordance with the value of the sum SM. If the temperatures of the recording heads exceeds the predetermined temperature range (45 °C, for example), one of the tables other than these primary tables is selected. For example, if the temperature of the light head exceeds the predetermined temperature range while the primary table as shown in Fig. 21C-1 is selected in accordance with the sum SM, the table as shown in Figs. 21C-2 is selected. Further, if the temperature of the dark head exceed the predetermined range (45 °C, for example), the table as shown in Fig. 21C-3 is selected. In summary, the table is selected according to the rules as shown in Table 1. Control Signal (CT) SM T1 T2 Table to be selected 0 - - - 21A 1 small low low 21B-1 1 small high low 21B-2 1 small low high 21B-3 1 medium low low 21C-1 1 medium high low 21C-2 1 medium low high 21C-3 1 large low low 21D-1 1 large high low 21D-2 1 large low high 21D-3 If both temperature signals exceed the predetermined temperature range, the table that restrict both maximum values for the dark and light heads should be selected. According to the fifth embodiment, the occurrence of black lines can be prevented taking into account the ink volumes of neighboring pixels. EMBODIMENT 6 The temperatures of the recording heads can be taken into consideration in the color recording as in the third embodiment. Fig. 22 is a block diagram showing the arrangement of a control system associated with a sixth embodiment of the present invention. The weighted sum SM of respective colors is inputted to the dark/light separating table 23. The separating table 23 stores ten types of tables as shown in Figs. 21A - 21D, and selects the optimum separating table in accordance with the control signal CT, the sum SM and the head temperatures T1 and T2. Thus, the volume of ink discharged from the edge nozzles can be limited in accordance with the temperatures of the recording heads taking into account the ink volume of each color, thereby preventing the occurrence of the black lines boundaries. Although the multi-level image signal is used to drive the recording heads in the embodiments described above, a binary image signal consisting of only two levels of "0" and "1" can also be used to obtain similar effect by subsampling "1" signal at a certain probability instead of restricting the dark level. (SUPPLEMENT) The present invention achieves distinct effect when applied to a recording head or a recording apparatus which has means for generating thermal energy such as electrothermal transducers or laser light, and which causes changes in ink by the thermal energy so as to eject ink. This is because such a system can achieve a high density and high resolution recording. A typical structure and operational principle thereof is disclosed in U.S. patent Nos. 4,723,129 and 4,740,796, and it is preferable to use this basic principle to implement such a system. Although this system can be applied either to on-demand type or continuous type ink jet recording systems, it is particularly suitable for the on-demand type apparatus. This is because the on-demand type apparatus has electrothermal transducers, each disposed on a sheet or liquid passage that retains liquid (ink), and operates as follows: first, one or more drive signals are applied to the electrothermal transducers to cause thermal energy corresponding to recording information; second, the thermal energy induces sudden temperature rise that exceeds the nucleate boiling so as to cause the film boiling on heating portions of the recording head; and third, bubbles are grown in the liquid (ink) corresponding to the drive signals. By using the growth and collapse of the bubbles, the ink is expelled from at least one of the ink ejection orifices of the head to form one or more ink drops. The drive signal in the form of a pulse is preferable because the growth and collapse of the bubbles can be achieved instantaneously and suitably by this form of drive signal. As a drive signal in the form of a pulse, those described in U.S. patent Nos. 4,463,359 and 4,345,262 are preferable. In addition, it is preferable that the rate of temperature rise of the heating portions described in U.S. patent No. 4,313,124 be adopted to achieve better recording. U.S. patent Nos. 4,558,333 and 4,459,600 disclose the following structure of a recording head, which is incorporated to the present invention: this structure includes heating portions disposed on bent portions in addition to a combination of the ejection orifices, liquid passages and the electrothermal transducers disclosed in the above patents. Moreover, the present invention can be applied to structures disclosed in Japanese Patent Application Laying-open Nos. 123670/1984 and 138461/1984 in order to achieve similar effects. The former discloses a structure in which a slit common to all the electrothermal transducers is used as ejection orifices of the electrothermal transducers, and the latter discloses a structure in which openings for absorbing pressure waves caused by thermal energy are formed corresponding to the ejection orifices. Thus, irrespective of the type of the recording head, the present invention can achieve recording positively and effectively. The present invention can be also applied to a so-called full-line type recording head whose length equals the maximum length across a recording medium. Such a recording head may consists of a plurality of recording heads combined together, or one integrally arranged recording head. In addition, the present invention can be applied to various serial type recording heads: a recording head fixed to the main assembly of a recording apparatus; a conveniently replaceable chip type recording head which, when loaded on the main assembly of a recording apparatus, is electrically connected to the main assembly, and is supplied with ink therefrom; and a cartridge type recording head integrally including an ink reservoir. It is further preferable to add a recovery system, or a preliminary auxiliary system for a recording head as a constituent of the recording apparatus because they serve to make the effect of the present invention more reliable. As examples of the recovery system, are a capping means and a cleaning means for the recording head, and a pressure or suction means for the recording head. As examples of the preliminary auxiliary system, are a preliminary heating means utilizing electrothermal transducers or a combination of other heater elements and the electrothermal transducers, and a means for carrying out preliminary ejection of ink independently of the ejection for recording. These systems are effective for reliable recording. The number and type of recording heads to be mounted on a recording apparatus can be also changed. For example, only one recording head corresponding to a single color ink, or a plurality of recording heads corresponding to a plurality of inks different in color or concentration can be used. In other words, the present invention can be effectively applied to an apparatus having at least one of the monochromatic, multi-color and full-color modes. Here, the monochromatic mode performs recording by using only one major color such as black. The multi-color mode carries out recording by using different color inks, and the full-color mode performs recording by color mixing. Furthermore, although the above-described embodiments use liquid ink, inks that are liquid when the recording signal is applied can be used: for example, inks can be employed that solidify at a temperature lower than the room temperature and are softened or liquefied in the room temperature. This is because in the ink jet system, the ink is generally temperature adjusted in a range of 30°C -70°C so that the viscosity of the ink is maintained at such a value that the ink can be ejected reliably. In addition, the present invention can be applied to such apparatus where the ink is liquefied just before the ejection by the thermal energy as follows so that the ink is expelled from the orifices in the liquid state, and then begins to solidify on hitting the recording medium, thereby preventing the ink evaporation: the ink is transformed from solid to liquid state by positively utilizing the thermal energy which would otherwise cause the temperature rise; or the ink, which is dry when left in air, is liquefied in response to the thermal energy of the recording signal. In such cases, the ink may be retained in recesses or through holes formed in a porous sheet as liquid or solid substances so that the ink faces the electrothermal transducers as described in Japanese Patent Application Laying-open Nos. 56847/1979 or 71260/1985. The present invention is most effective when it uses the film boiling phenomenon to expel the ink. Furthermore, the ink jet recording apparatus of the present invention can be employed not only as an image output terminal of an information processing device such as a computer, but also as an output device of a copying machine including a reader, and as an output device of a facsimile apparatus having a transmission and receiving function. Although specific embodiments of an image recording apparatus constructed in accordance with the present invention have been disclosed, it is not intended that the invention be restricted to either the specific configurations or the uses disclosed herein. Modifications may be made in a manner obvious to those skilled in the art. Accordingly, it is intended that the invention be limited only by the scope of the appended claims.

Claims: An image recording apparatus for performing image recording using a plurality of recording heads, each of said recording heads having a plurality of nozzles for discharging ink, and at least two of said recording heads being arranged to discharge inks of the same colour family but with different concentrations, characterized in that the apparatus comprises: separating means (23) having at least two different types of tables each for controlling separation of a single colour family input image signal into respective separate output image signals determining the driving duty of said at least two different concentration ink discharging recording heads in accordance with the intensity of the input image signal to be recorded, the two different types of tables being arranged so that the maximum total volume of ink discharged by said at least two different concentration ink discharging recording heads varies in accordance with the table used to produce said separate output image signals; separation control means (28) for determining whether the nozzles in each of said at least two recording heads from which ink is to be discharged in accordance with the image signal are nozzles at the edge of the recording head and for selecting the one of said at least two different types of tables not providing the greatest maximum total ink discharge volume when the nozzles from which ink is to be discharged in accordance with the image signal are determined to be edge nozzles; and head driving means for driving each of said recording heads in accordance with the head driving duty determined by said separating means (23). An image recording apparatus according to claim 1, wherein the driving duty is the number of actual discharges to a unit area per number of possible discharges to the unit area. An image recording apparatus according to claim 1 or claim 2, wherein the plurality of nozzles are arranged in a line in each recording head to form a nozzle array and the separation control means is arranged to select one of the tables representing an ink discharge volume of the edge nozzle at edges in the nozzle array for which the discharge volume is smaller than that of the remaining nozzles if values representative of concentrations of the input image signals are larger than a predetermined value. An image recording apparatus according to claim 1, wherein said separation control means (28) is arranged to select one of said tables on the basis of the image signals associated with a pixel to be recorded and its neighbouring pixels. An image recording apparatus according to claim 1, wherein said separating means (23) further comprises operation means (32) for performing a weighted summation on image signals of respective colours, and is arranged to select one of the tables in accordance with the output of said operation means and the image signals. An image recording apparatus according to any one of the preceding claims, further comprising temperature detecting means (26A, 27A) for detecting the temperature of each of said recording heads, wherein said separating means (23) is arranged to select the predetermined table from the plurality of tables in accordance with the locations of the nozzles and the temperature detected by said temperature detecting means (26A, 27A). An image recording apparatus according to any one of the preceding claims, wherein the plurality of recording heads comprise recording heads for cyan, magenta, yellow and black. An image recording apparatus according to any one of the preceding claims, wherein each of the recording heads is arranged to discharge ink by causing a state transition by utilising thermal energy. An image recording apparatus according to claim 4 or claim 5, wherein the nozzles are located at a plurality of locations, said apparatus further comprising temperature detecting means (26A, 27A) for detecting a temperature of each of said recording heads, wherein said separating means (23) is arranged to select the predetermined table from the plurality of tables in accordance with the locations of the nozzles and the temperature detected by said temperature detecting means (26A, 27A). An image recording apparatus according to claim 4, wherein the neighbouring pixels are in a direction of arrangement of nozzles of the recording heads. An image recording apparatus according to any one of the preceding claims, wherein said separation control means (28) is arranged to select the table on the basis of a recording medium used for recording. An image recording apparatus according to any one of the preceding claims, wherein the plurality of recording heads are associated with different colour families and the separation control means (28) is arranged to perform selection of the table only for recording heads associated with black ink. An image recording apparatus according to claim 12 when dependent upon claim 5, wherein said separation control means (28) is arranged to vary weighting of pixels recorded by the recording heads associated with black ink in accordance with a type of recording medium. An image recording apparatus according to claim 12, wherein the plurality of colours are black, cyan, magenta, and yellow. An image recording apparatus according to claim 14, wherein said separation control means (28) is arranged to generate a signal supplied to said head driving means in such a manner that the signal associated with the black ink is outputted second or later in order in the plurality of colour inks.

Inventors: DANZUKA TOSHIMITSU, FUKUSHIMA HISASHI, MIURA YASUSHI, SUZUKI AKIO, DANZUKA, TOSHIMITSU, FUKUSHIMA, HISASHI, MIURA, YASUSHI, SUZUKI, AKIO, DANZUKA, TOSHIMITSU, C/O CANON KABUSHIKI KAISHA, FUKUSHIMA, HISASHI, C/O CANON KABUSHIKI KAISHA, MIURA, YASUSHI, C/O CANON KABUSHIKI KAISHA, SUZUKI, AKIO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: B41J2
IPC codes: B41J2, H04N1