Recording method for gradation recording with light- and dark-colored inks and apparatus therefor

Abstract: In forming an image with plural recording heads corresponding to plural inks of different densities, the image data are so distributed that the image is formed with the ink of low density until the image density reaches a predetermined density level, but is formed with the inks of low and high densities when the image density exceeds the predetermined density level, and the distributed data are corrected according to the detection of temperature relating to the recording heads, whereby a desired recorded image can be obtained even in the presence of a variation in the head temperature. Also the image data are distributed in such a manner that, when the remaining amount of one of the plural inks becomes low, the recording is conducted with the remaining ink, whereby a desired image can be obtained even when the remaining amount of one of the plural inks with different densities becomes low.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ink jet recording method for recording an image on a recording medium using recording means arranged to discharge a plurality of inks of the same color but different densities, to an apparatus for carrying out such a method. Related Background Art In a conventional ink jet recording method, the recording is formed by depositing ink droplets onto a recording medium such as paper by discharging ink from plural ink discharge openings formed on a recording head in accordance with data signals. Such recording method is utilized for example in a printer, a facsimile apparatus or a copying apparatus. In such apparatus the use of a heat-generating element (electrothermal energy converting member) which is supplied with an electric signal to heat the ink locally to induce a variation in the pressure thereof to cause discharge of ink from a discharge opening, and the use of an electromechanical converting member such as a piezoelectric element to cause ink discharge are known. In such recording methods, half tone recording is achieved by dot density control in which the half tone is represented by the number of recording dots per unit area, with the recording dots being maintained at a constant size, or by dot diameter control in which the half tone is represented by the control of the size of the recording dots. In general dot density control is more popular because dot diameter control is limited by the complex control method required to vary precisely the recording dot size. Also where the ink discharge means consists of an electrothermal energy converting member which can achieve a high resolution because it enables easier manufacture and can have a high density of discharge openings, the above-mentioned dot density control is generally employed since the amount of variation in ink discharge pressure is difficult to control, so that it is difficult to modulate the diameter of the recording dot over a wide range. One of the representative binarization methods employed in said dot density control for continuous tone representation is the systematic dither method, but said method is associated with the drawback that the number of gradation levels is limited by the matrix size. More specifically, a larger number of gradation levels requires a larger matrix size, which leads to a larger pixel composed of said matrix in the recorded image, whereby the resolving power becomes deteriorated. Another representative binarization method is the conditionally determined dither method, such as the error diffusion method, in which a threshold value is varied in consideration of the pixels surrounding the input pixel, in contrast to the above-mentioned systematic dither method which is an independently determined dither method wherein the threshold value for binarization is determined independently from the input pixel. Conditionally determined dither methods, represented by the error diffusion method, have certain advantages such as the compatibility of the gradation and the resolving power, and extremely little generation of moire patterns in the recorded image where the original image is a printed image, but are also associated with the drawback of granularity in lighter areas of an image, leading to a tendency that the image quality is evaluated as being low. This drawback is particularly conspicuous in a recording apparatus with a low recording density. To overcome such granularity, there has been proposed an ink jet recording method which uses two recording heads for respectively discharging light-colored ink and dark-colored ink, and which forms the recording dots using the light-colored ink for low-density (light) level to intermediate density level areas of the image, and uses the dark-colored ink for the intermediate density level to high-density (dark) level areas of the image. Fig. 4 shows a principal part of a conventional color ink jet recording apparatus of the serial printing type, employing such light and dark inks. On a carriage 401 there are provided, at a predetermined mutual distance, a recording head Bkk for discharging dark black-colored ink, a head Bku for discharging light black-colored ink, a head Ck for discharging dark cyan-colored ink, a head Cu for discharging light cyan-colored ink, a head Mk for discharging dark magenta-colored ink, a head Mu for discharging light magenta-colored ink, a head Yk for discharging dark yellow-colored ink, and a head Yu for discharging light yellow-colored ink. A recording material, consisting for example of paper of a thin plastic sheet, is supplied between transport rollers (not shown), is pinched between discharge rollers 402, and is advanced in the illustrated direction by a transport motor (not shown). The carriage is guided and supported by a guide shaft 403 and an encoder. Said carriage reciprocates along said guide shaft, by means of a driving belt 404, driven by a carriage motor 405. In the interior (liquid path) of each ink discharge opening of the above-mentioned recording heads, there is provided a heat-generating element (electrothermal converting member) for generating the thermal energy for ink discharge. An image can be formed by driving said heat-generating elements based on recording signals, according to the timings read by said encoder, and discharging and depositing the ink droplets onto the recording material, in the order of dark black, light black, dark cyan, light cyan, dark magenta, light magenta, dark yellow and light yellow. In a home position of the carriage, selected to be outside the recording area, there is provided a recovery unit having a capping portion 406, for maintaining stability of the ink discharge. The above-explained light-dark multi-density recording method employing plural inks of different densities for the same color can improve the gradation particularly in the highlight area, and can reduce the dot granularity, thereby attaining higher image quality even by a simple increase of the density levels from two to three. This is because the noise associated with single dots in a highlight area can be reduced by the deposition of lighter ink (with lower density) in such a highlight area. However, in ink jet recording apparatus employing such light and dark inks, stable ink discharge may become difficult, depending on the situation of use. If, amongst plural inks of different densities of the same color, at least one ink cannot be discharged in stable manner, the desired image can no longer be obtained. In such ink jet recording apparatus utilizing such dark and light inks, the multi-value luminance signals of R, G and B colors supplied from a host equipment are converted in a color processing unit into multi-value density signals of Y, M, C and Bk colors, which are then divided, by a distribution table of a dot developing unit, into those to be recorded with the dark inks and those to be recorded with the light inks, and the thus-divided signals are binarized for recording by the recording heads. Consequently, if either of the dark and light inks is exhausted or becomes low in the remaining amount, the desired image can no longer be obtained even though the colors required for image formation are still present. JP-A-3 005 157 solves the problem of darker ink not being available by using the lighter ink to compensate using a suitable table. This corresponds to the preamble of claims 1 and 21. Also the stability of the ink discharge amount is significantly influenced by the temperature of ink or recording head and that of the atmosphere. In general the discharge amount is low when the ink temperature is low, and becomes larger as the ink is heated. As a result there has been encountered a drawback that the density of the recorded image varies depending on the temperature of the recording inks. In consideration of the foregoing, the present invention is concerned with providing an ink jet recording method capable of providing a desired image even when the stable ink discharge becomes difficult in at least one among plural inks of different densities of the same color, and an apparatus therefor. EP-A-0150119 discloses an ink jet recording apparatus wherein, below a threshold image density, a recording head using a weaker ink and/or a smaller nozzle is used while above that threshold, a head using a denser ink and/or a larger nozzle is used. EP-A-0388978 describes a recording apparatus wherein a light/dark ink image data distribution table is used to enable gradation recording to be reliably performed. In one aspect, the present invention provides an ink jet recording apparatus as claimed in claim 1. In another aspect, the present invention provides a method as claimed in claim 21. The recording means is preferably a head for discharging ink utilizing thermal energy, provided with a thermal energy converting member for supplying thermal energy to the ink to induce a state change in the ink, to cause discharge of ink from the discharge opening. An embodiment of the present invention provides an ink jet recording apparatus capable of providing a desired image recording density even when the temperature of the recording head varies, thereby enabling recording with excellent gradation. An embodiment of the present invention provides an ink jet recording apparatus for forming an image with plural recording heads corresponding to plural inks of different densities, comprising temperature detection means for detecting the temperature relative to said recording heads; distribution means for distributing the image data into data respectively corresponding to said plural inks; and correction means for correcting the data distributed by said distribution means, based on the temperature data detected by said temperature detection means. An embodiment of the present invention also provides a recording apparatus for forming an image with plural recording heads corresponding to plural inks of different densities, comprising temperature detection means for detecting the temperature relative to said recording heads; and distribution means for distributing the image data into plural data respectively corresponding to said plural inks and also according to the temperature data detected by said temperature detection means. An embodiment of the present invention furthermore provides a recording method for forming an image with plural recording heads corresponding to plural inks of different densities, comprising steps of detecting the temperature relative to said recording heads, distributing the image data into plural data respectively corresponding to said plural inks and also corresponding to said detected temperature data, and forming an image on a recording medium with said recording heads, based on said distributed data. In an embodiment of the present invention, the temperature relative to the recording heads is detected, then the image data are distributed into plural data respectively corresponding to the plural inks and also corresponding to the detected temperature data, and an image is formed on a recording medium with said recording heads, based on said distributed data. Thus a desired image recording density can be obtained even when the temperature of the recording heads varies, and the tonal reproducing ability can be significantly improved. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a view showing the flow of image signal processing; Fig. 2 is a flow chart showing the sequence of selecting tables for distributing the density signals of different colors into signals of different densities corresponding to the inks of different densities; Figs. 3A to 3C are views showing plural distribution tables for converting the input density signal of each color into light and dark output density signals; Fig. 4 is a view showing the principal part of a conventional color ink jet recording apparatus of the serial printing type, employing light and dark inks; Fig. 5 is a perspective view showing the principal part of a color ink jet recording apparatus embodying the present invention; Fig. 6 is a view showing the structure of an ink jet unit of said embodiment; Fig. 7 is a view of the grooved ceiling board 1300 of the ink jet unit of said embodiment, seen from the side of the heater board 100; Fig. 8 is a block diagram showing an example of the control system of the apparatus; Fig. 9 is a chart showing the relationship between the remaining ink amount and the resistance between the electrodes for detecting the remaining ink amount; Fig. 10 is a view showing the state of detection of the remaining ink amount; Figs. 11A and 11B are views showing the detection of the remaining ink amount in an embodiment; Fig. 12 is a view showing an example of the detection of the remaining ink amount; Fig. 13 is a chart showing the variation in the internal pressure in the ink supply unit; Fig. 14 is a view of an ink tank for forming light and dark ink by ink mixing in the head; Fig. 15 is a cross-sectional view of a state in which a dark ink cartridge and a light ink cartridge are connected; Fig. 16 is a schematic view of an ink jet recording apparatus; Fig. 17 is a schematic perspective view of recording heads; Fig. 18 is a schematic perspective view showing the structure of the ink discharge portion of the recording head; Fig. 19 is a block diagram showing the structure of an ink jet recording apparatus; Fig. 20 is a block diagram of an image signal processing unit; Fig. 21 is a chart showing an example of the conversion graph of the light-dark distribution table; Fig. 22 is a chart showing an example of the conversion graph for data correction; Fig. 23 is a schematic view of an ink jet recording apparatus in which the present invention is applicable; Fig. 24 is a schematic perspective view of recording heads; Fig. 25 is a schematic view showing the structure of a recording head; Fig. 26 is a view showing the structure of the grooved ceiling plate; Fig. 27 is a view showing the structure of an ink jet cartridge (IJC) in which four heads are integrated; Fig. 28 is a view showing the loading operation of the ink jet cartridge and the ink tank on the carriage; Fig. 29 is a block diagram of an image signal processing unit; Figs. 30A and 30B are charts showing an example of the conversion graph of the light-dark distribution table; Fig. 31 is a chart for explaining the driving method by divided pulse width modulation; Fig. 32 is a chart showing the dependence of the ink discharge amount on the prepulses; Fig. 33 is a chart showing the dependence of the ink discharge amount on the temperature; Fig. 34 is a chart relating to the discharge amount control; Fig. 35 is a block diagram showing the schematic structure when the present invention is applied to an information processing apparatus; Fig. 36 is an external view of the information processing apparatus; and Fig. 37 is an external view showing another example of the information processing apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 5 is a perspective view showing the principal part of a color ink jet recording apparatus embodying the present invention. On a carriage 501 there are provided, with a predetermined distance, ink jet units respectively having an array of discharge openings for discharging light black-colored ink, an array for light cyan-colored ink, an array for light magenta-colored ink, and an array for light yellow-colored ink, and ink jet units respectively having an array of discharge openings for discharging dark black-colored ink, an array for dark cyan-colored ink, an array for dark magenta-colored ink and an array for dark yellow-colored ink. A recording material, such as paper or a thin plastic sheet, is supplied by transport rollers (not shown), and is pinched by discharge rollers 502 for advancement in the direction of an arrow, by means of an unrepresented transport motor. Said carriage is guided and supported by a guide shaft 503 and an encoder. Said carriage reciprocates along said guide shaft, by means of a driving belt 504 driven by a carriage motor 505. In the interior (liquid path) of each ink discharge opening of the above-mentioned ink jet unit, there is provided a heat-generating element (electrothermal energy converting member) for generating the thermal energy for ink discharge. An image can be formed by driving said heat-generating elements based on the recording signal and according to the timings read by said encoder, and discharging and depositing ink droplets onto said recording material in the order of the dark-colored inks and then the light-colored inks. In a home position of the carriage, selected outside the recording area, there is provided a recovery unit having a capping unit. When the recording operation is not conducted, the carriage is moved to said home position and the caps of said capping unit tightly cover the faces, having the ink discharge openings, of the corresponding ink jet units, thereby preventing the clogging of the discharge openings, resulting from the ink adhesion caused by evaporation of the ink solvent or from the deposition of dusts. The capping function of said capping unit is also utilized for the idle discharge mode in which the ink is discharged from the discharge openings to said cap in a spaced state, in order to resolve discharge deficiency or clogging in the ink discharge openings of lower recording frequency, and utilized also for the recovery of the ink discharge openings showing deficient discharge, by activating an unrepresented motor in the capped state of said discharge openings thereby sucking ink forcedly from said discharge openings. Also adjacent to said capping unit, there may be provided a blade or a wiping member, for cleaning the faces, having the ink discharge openings, of the ink jet units. Fig. 6 is an exploded perspective view showing the structure of the ink jet unit to be employed in the present embodiment, wherein a wiring board 200 is connected, at an end thereof, with the wirings of a heater board 100, and is provided, at the other end, with plural pads for receiving electrical signals from the main body of the apparatus, corresponding respectively to the electrothermal energy conversion members. Consequently the electrical signals from said main body can be supplied to the respective electrothermal energy conversion members. A metal support member 300, for supporting the rear surface of the wiring board 200 by planar contact, constitutes a bottom plate of the ink jet unit. A pressure spring 500 is provided, for applying an elastic pressure on a linear area in the vicinity of the ink discharge openings of a grooved ceiling plate 1300, with a portion formed in a substantially U-shaped cross section, claws for engaging with holes formed in the base plate, and a pair of rear legs for receiving the force, applied to the spring, by the base plate. The force of said spring maintains the wiring board 200 and the grooved ceiling plate 1300 in pressurized contact. The wiring board 200 is adhered to the support member for example by adhesive material. At an end of an ink supply tube 2200, there is provided a filter 700. An ink supply member 600 is prepared by molding, and the grooved ceiling plate 1300 is provided with liquid paths communicating with the respective ink discharge openings. The ink supply member 600 is fixed to the support member 300 in simple manner by inserting two rear pins (not shown) of the ink supply member 600 into holes 1901, 1902 of the support member and thermally fusing the protruding ends of said pins. In this state, there is formed a uniform gap between an orifice plate 410 and the ink supply member 600. Sealing agent is poured from an inlet formed on the upper part of said ink supply member, thereby sealing the bonding wires and the above-mentioned gap between the orifice plate 410 and the ink supply member 600, and is further guided through a groove 310 formed on the support member 300 thereby completely sealing the gap between the orifice plate 410 and the front end of the support member 300. Fig. 7 is a perspective view of the grooved ceiling plate 1300 to be employed in the present embodiment, seen from the side of the heater board 100. There are provided plural liquid chambers, which are mutually separated by partition walls 10a - 10c and are respectively provided with ink supply openings 20a - 20d. Said partition walls 10a - 10c are provided, on contact faces with the heater board 100, with grooves 30a - 30c, which communicate with the external peripheral part of the grooved ceiling plate 1300. After said plate 1300 is maintained in contact with the heater board, said external peripheral part is sealed with the sealing agent as explained above. In this operation, the sealing agent enters along said grooves, thereby filling the gap between said board and the heater board. In this manner, the liquid chambers can be completely separated by the steps known in the conventional head. The structure of said grooves varies according to the physical properties of the sealing agent and has to be shaped corresponding to the sealing agent to be employed. The above-explained structure of separate plural liquid chambers allows to supply different inks to the ink discharge openings. In the following there will be explained the detection of the remaining ink amount. Fig. 8 shows an example of the control system in the present embodiment, wherein a control unit 200 in the form of a microcomputer incorporating an A/D converter includes distribution means for determining the proportion of use of each of plural inks in the present invention, and variation means for varying the proportion determined by said distribution means. There are also shown a recording head unit 10, a voltage conversion circuit 300, an alarm device 400 composed of a display unit such as an LED, or an acoustic alarm device such as a buzzer, or a combination thereof, a main scanning mechanism 500 including a motor etc. for causing a scanning motion of the carriage HC in the recording operation, and a sub scanning mechanism 600 including a motor for transporting the recording medium etc. There is also shown a remaining amount detection signal V from a tank. In the present embodiment, a constant current is given between two electrodes provided in an ink chamber b, and the remaining ink amount therein is detected by the resistance between said two electrodes. The remaining ink amount and the resistance between the electrodes are correlated as shown in Fig. 9. When the remaining ink amount reaches a predetermined amount or becomes less, said resistance becomes higher to indicate that the remaining ink amount has reached said predetermined amount. Fig. 10 illustrates an ink cartridge employed in the present embodiment, and shows the detecting operation for the remaining ink amount. The interior of the ink cartridge body 1, defined by the tank walls 101, is divided by a wall 102 into two ink chambers a, b which mutually communicate at the bottom. An ink chamber a is filled with a compressed absorbent member 103 with regulated capillary force. The ink cartridge is further provided with an ink supply part 2 for connection with the ink jet recording head, an air communicating part 3, pins (electrodes) 4 for detecting the remaining ink amount, and an ink filling aperture 5, which is closed with a stopper 51 made for example of rubber. The ink chamber b is maintained at a reduced pressure. The positional relationship of the air communicating part 3, the ink supply part 2, the remaining amount detecting pins 4 and the ink filling aperture 5 is not limited to the illustrated one. When the ink surface in the ink chamber b becomes lower than the upper one of two electrodes 4, the resistance therebetween is abruptly elevated as shown in Fig. 10, whereby a corresponding voltage is generated therebetween. Said voltage is supplied either directly or through a voltage conversion circuit 300 to an A/D converter of the control unit for A/D conversion. When thus obtained detection value becomes larger than a predetermined value Rth, a signal for switching the distribution table is sent to a table selection circuit in the control unit 200. In the present embodiment, the remaining ink amount is detected by the resistance between the electrodes provided in the ink tank, but such detection is not limited to the above-mentioned method and may also be achieved for example by a mechanical method or an optical method. Fig. 15 is a cross-sectional view of a state in which the cartridge for dark ink and that for light ink are mutually connected, wherein the ink filling apertures 5 and the stoppers 51 are omitted. The upper and lower ink chambers b are both maintained at a reduced pressure. (Embodiment 1) Fig. 1 is a view best representing the feature of the present invention, showing the process flow in case of image printing with dark and light inks by table selection and also in case either of the dark and light inks of a same color is exhausted. In Fig. 1, a color processing unit converts the red image luminance signal R, the green image luminance signal G and the blue image luminance signal B, by an input correction circuit into a cyan image density signal C, a magenta image density signal M and a yellow image density signal Y. These signals are subjected to color processing and conversion into new image density signals C, M, Y, Bk of cyan, magenta, yellow and black colors in a color correction (masking) circuit and a UCR (undercolor removal)/black generation circuit, and is then subjected to gamma conversion into image density signals C, M, Y, Bk of cyan, magenta, yellow and black colors by an output correction circuit. A dot developing unit positioned next selects the distribution table for distributing said density signals C, M, Y, Bk into those for recording heads for printing the inks of higher density and those for recording heads for printing the inks of lower density. According to thus selected distribution table, the density signals C, M, Y, Bk generated in the color processing unit are distributed into image density signals Ck, Mk, Yk, Bkk of dark cyan, dark magenta, dark yellow and dark black colors with higher dye densities and image density signals Cu, Mu, Yu, Bku of light cyan, light magenta, light yellow and light black colors of lower dye densities. Thus distributed signals are respectively binarized, and inks are discharged from the arrays of ink discharge openings of the ink jet units, according to said signals, thereby forming a color image. In Fig. 1, a distribution table A is used for distributing the printing data into those for dark inks and those for light inks so as to attain characteristics as shown in Fig. 2. A distribution table B is used in case of printing with the inks of dark colors only, without using those of light colors. A distribution table C is used in case of printing with the inks of light colors only, without using those of dark colors. In this case there is shown a printing operation of two images by two main scannings with the light-colored ink. In practice, an image density comparable to that printed with the dark-colored ink may be obtained with the light-colored ink only, by recording plural images in superposed manner in plural main scanning operations. The contents of the distribution tables shown in Fig. 1 are shown in Figs. 3A to 3C. The distribution table A shown in Fig. 3A is used for distributing the printing data of each color, generated in the color processing unit, to the data for the dark-colored ink and those for the light-colored ink, and there are given the output data for the light cyan ink and the dark cyan ink, respectively by a solid line and a chain line, as a function of the input data. As will be apparent from the chart, the light-colored ink only is used until the input data reach a level a , and the light-colored ink and the dark-colored ink are combinedly used beyond said level a . The distribution table B, shown in Fig. 3B, distributes the data of each color, generated in the color processing unit, to the dark-colored ink only, wherein shown are the output data for the dark-cyan ink, by a chain line, as a function of the input data. The distribution table C, shown in Fig. 3C, distributes the data of each color, generated by the color processing unit, only to the light-colored ink, wherein shown are the output data for the dark-cyan ink for a first image, by a solid line, and the output data for the dark-cyan ink for a second image, by a double-dotted chain line, both as a function of the input data. As will be apparent from this chart, the first image alone is printed until the input data reach a level a , and the first and second images are superposedly printed beyond said level a to form the image of the high density area. The table selection shown in Fig. 1 automatically selects the tables A to C by a sequence shown in Fig. 2, to be executed by a control unit 200 shown in Fig. 8. In the sequence shown in Fig. 2, the detection of the remaining ink amount is executed for all the colors, for example at the completion of printing of a page. For example, if the light-cyan ink is less than a predetermined remaining amount, there is selected, only for the cyan color, a distribution table for printing with the dark-colored ink only without the light-colored ink, thereby preventing the lack of printing in low cyan density areas and thus obtaining a satisfactory image. Also if the dark-cyan ink is less than the predetermined remaining amount, there is selected, only for the cyan color, the distribution table for printing with the light-colored ink only without the dark-colored ink and there is also adopted the superposed recording methods by plural main scannings, thereby preventing the failure of printing of the high density areas and thus obtaining a satisfactory image. In the above-explained example, in case either of the dark and light cyan inks is exhausted, the distribution table for using the other remaining ink only is selected for the cyan color only, but there is preferably selected the same distribution table for all the colors. Also each of the distribution tables A to C may be composed of a single table for distributing the dark-and light-colored inks in the same manner for all the colors, or may be composed of plural different tables respectively corresponding to the colors and providing optimum distributions thereto. Also in the present embodiment the distribution tables are automatically selected according to the detection of the ink remaining amounts, but there may be provided a switch for selecting the distribution tables for arbitrary selection by the user. Also the selection of the distribution tables may be changed not necessarily at the completion of printing of a page but at any predetermined timing such as at the completion of printing of a line. Furthermore, the distribution table explained above is designed for printing with two inks of higher and lower densities for each color, but it may also be designed for printing with three inks of high, medium and low densities or with an even larger number of kinds of ink. Also in the present embodiment, the failure in the stable ink discharge in at least one among plural inks is judged from the low remaining ink amount, but it may also be detected by another factor not related to the low remaining amount, such as the failure in the ink discharge resulting from the clogging of the ink discharge openings. As explained in the foregoing, the ink jet recording apparatus of the present invention for printing with inks of higher and lower densities for each color comprises, in addition to the distribution table for distributing the density signals of C, M, Y, Bk colors to the density signals for printing with the inks of higher density and those for printing with the inks of lower density, a distribution table for distributing said density signals only to the inks of higher density, a distribution table for distributing said density signals only to the inks of lower density, and means for selecting said distribution tables, thereby enabling to obtain a normal image even when the ink of higher or lower density is exhausted or becomes not discharged properly. (Example 1 included for illustrative purposes and not falling within the scope of the invention claimed) In the following there will be explained an illustrative example, in which the distribution tables are selected not by the stability of ink discharge but according to the density distribution of the image. The output image of a page is scanned in advance for detecting the density distribution of the image data, and, if the density distribution is concentrated in a high density region, the aforementioned distribution table B (Fig. 3B) is selected to effect the printing with the dark-colored inks only. On the other hand, if the density distribution is concentrated in a low density region, the aforementioned distribution table C (Fig. 3C) is selected to effect the printing with the light-colored inks only. The density distribution may be detected among the developed image data of a predetermined amount, or, in case of a copying apparatus or the like, by scanning of the original image. Utilizing the detection of the density distribution of the image data, allows to obtain a uniform image without the granularity resulting from the printing with the dark-colored ink in the vicinity of a density level where the dark- and light-colored inks are switched. (Embodiment 2) In the following there will be explained another embodiment of the present invention, in which, as shown in Figs. 11A and 11B, the tank walls 101 are composed of a transparent or semitransparent material whereby optical detection of the remaining ink amount is rendered possible. In this case, there are provided, on a wall in the ink chamber b, a reflecting plate 42 such as a mirror, and, outside the tank, a photosensor consisting of a light-emitting element 43 and a photosensor 44. Said light-emitting element 43 and photosensor 44 may be provided on the carriage or in the home position where the recovery system is provided. In the configuration shown in Figs. 11A and 11B, the light-emitting element 43 emits the light at a certain angle, and the photosensor 44 receives the light reflected by the reflecting plate. For example, the light-emitting element 43 is composed of an LED, and the photosensor 44 is composed of a phototransistor. Fig. 11A shows a state in which the ink is fully present, and the light from the light-emitting element 43 is intercepted by the ink and scarcely reaches the photosensor 44 so that the detection output thereof is small. However, when the ink is consumed to a state shown in Fig. 11B, the light from the light-emitting element 43 is scarcely intercepted so that the detection output of the photosensor becomes higher. Fig. 12 shows a variation in which the light-emitting element and the photosensor are positioned mutually opposite across the ink tank. Also in this case the ink chamber b is composed of a transparent or semitransparent material. This configuration can dispense with the reflecting plate, and also can improve the detection sensitivity since the light is directly received. The above-mentioned configurations of the detection means for the remaining ink amount allow to obtain the detection output from the photosensor 44 in the form of an analog signal. Consequently, in a color ink jet recording apparatus employing two inks of higher and lower densities for each of C, M, Y and Bk colors, it is rendered possible to use up the inks of higher and lower densities almost at the same time, by providing plural distribution tables corresponding to the differences between the detection outputs of the inks of higher and lower densities of each color and suitably selecting said distribution tables. In the structures shown in Fig. 11A, 11B and 12, the ink chamber b is maintained at a reduced pressure. (Embodiment 3) In the following there will be explained another embodiment of the present invention. Fig. 13 shows the variation in the internal pressure in the ink supply unit, according to the ink supply amount in an ink tank as shown in Fig. 1. In an initial state of recording, a small amount of ink is present in the ink chamber a, and a negative pressure is generated by the capillary force of the compressed ink absorbent member. As the ink amount in the ink chamber a becomes less by the ink supply, the negative pressure in said ink chamber a, generated by the capillary force determined by the compression rate (pore size distribution) of the compressed absorbent member, gradually increases. As the ink is consumed more, the ink distribution in the ink chamber a becomes stabilized, and the internal pressure remains thereafter substantially constant, by the introduction of air into the ink chamber b (stable ink supply period). When the ink in the ink chamber b is exhausted by further ink consumption, the ink in the chamber a starts to be consumed again, so that the negative pressure in the chamber a is changed. The stable ink discharge becomes no longer obtained when the negative pressure in the ink chamber a exceeds a certain value. It is therefore possible to detect the remaining ink amount by the negative pressure in the ink chamber, and to switch the distribution table upon detection of a state where the stable ink discharge becomes no longer possible, thereby preventing the deterioration of the image resulting from the unstable ink discharge. The present invention is not limited to the foregoing embodiments, but is widely applicable to the ink jet recording apparatus for recording with plural inks of different densities. In the following there will be explained an embodiment in which the density is regulated by employing high-density ink and clear ink. Fig. 14 is a schematic exploded perspective view of an ink tank with improved efficiency of ink use, wherein the ink tank is divided by a partition 13c into an ink chamber 13a for dark ink and an ink chamber 13b for clear ink (dye-free ink consisting solely of solvent), and the color density is regulated according to the area ratio of ink supply apertures 15a, 15b. Said ink chambers 13a, 13b for the dark ink and the clear ink are used as negative pressure generating chambers, and there are further provided ink containers 13e, 13f which are separated from said negative pressure generating chambers by partitions 13g, 13h having apertures 13i at the bottom. After the inks in negative pressure generating members 14a, 14b are used, the inks are supplied from the ink containers 13e, 13f, whereby the ink supply pressure is maintained at a constant head pressure, and uniform ink supply thus made possible. The ink chambers are provided with air communicating holes 13d. In the ink tank explained above, the area ratio of the supply apertures of the ink tank is regulated respectively according to the darker and lighter colors of the recording heads by the partition provided in the tank, and the divided chambers are respectively filled with the dense-colored ink and the clear ink. When said ink tank is connected to the recording head, said inks flow into said recording head with a ratio determined by said area ratio of the apertures and are mixed in the recording head, thereby providing the ink of an intermediate density. When the clear ink is exhausted, the ink of the lower density becomes no longer discharged, so that the proper image can no longer be obtained. Consequently the remaining amount of the clear ink is detected, and the ink distribution table is switched when the clear ink is exhausted, whereby a proper image can be obtained until the ink of each color is used up. In the foregoing explanation, the two chambers are respectively filled with the ink of each color and the clear ink, but they may be replaced by the inks of different densities of a same color. In such case the remaining ink amount is detected for both inks, and the ink distribution table is switched according to the result of said detection, whereby all the inks in the ink tank can be used up while the proper image recording is maintained all the time. As detailedly explained in the foregoing, the present invention, in recording on a recording medium by ink discharge from the discharge openings of a recording head employing plural inks of different densities of a same color, allows to obtain a proper image even when the stable ink discharge becomes difficult in at least one of said plural inks, by effecting the recording operation by the remaining ink. (Illustrative Example 2 not falling within the scope of the invention claimed) Fig. 16 is a perspective view of the principal part of a color ink jet recording apparatus. On a carriage 623, there are provided, in parallel manner, an ink jet unit 611A having an array of discharge openings for the ink of dark color, and an ink jet unit 611B having an array of discharge openings for the ink of light color. A recording material P such as paper or a thin plastic sheet, advanced by transport rollers (not shown), is pinched by discharge rollers 621 and is advanced as indicated by an arrow, by means of an unrepresented transport motor. The carriage 623 is guided and supported by a guide shaft 622 and an encoder (not shown). The carriage 623 reciprocates along the guide shaft 622, by means of a driving belt 624 driven by a carriage motor 625. In the interior (liquid path) of each ink discharge opening of said ink jet unit, there is provided a heat-generating element (electrothermal energy converting member) for generating thermal energy for ink discharge. An image is formed by driving said heat-generating elements based on the recording signal, according to the timings read by the encoder (not shown), thereby discharging and depositing ink droplets onto said recording material P in the order of the dark-colored ink and the light-colored ink. In a home position (HP) of the carriage selected outside the recording area, there is provided a recovery unit having a capping unit 626. When the recording operation is not executed, the carriage 623 is moved to the home position (HP) and the caps of the capping unit 626 tightly close the faces, having the discharge openings, of the corresponding ink jet units, thereby preventing the clogging of the ink discharge openings, resulting from the ink deposition caused by evaporation of the ink solvent or from the dust deposition. The capping function of said capping unit 626 is also utilized in the preliminary discharge mode, in which the ink discharge is executed from the ink discharge openings to the capping unit 626 in a separated state, in order to resolve the discharge failure or clogging in the discharge openings of lower frequency of recording, and in the ink discharge recovery operation in which an unrepresented pump is activated in the capped state to suck the ink forcedly from the ink discharge openings thereby restoring the discharge openings with discharge failure. Also adjacent to the capping unit 626, there may be provided a blade or a wiping member for cleaning the face, having the ink discharge openings, of the ink jet unit. Fig. 17 is a schematic perspective view of the arrays of the ink discharge openings of the recording heads 612, seen from the side of the recording material, and Fig. 18 is a partial perspective view, schematically showing the structure of the ink discharge part. Referring to Figs. 17 and 18, a head 612A for the dark-colored ink and a head 612B for the light-colored ink are provided in parallel manner, and each head is provided with an orifice face 601 containing plural discharge openings 602. In a liquid path 602 communicating with each discharge opening 602, there is provided a discharge energy generating element 604 for generating energy required for the ink discharge. An arrow y in Fig. 17 indicates the scanning direction of the carriage 623. In Fig. 18, there is provided a sensor 605 for detecting the temperature of the recording head. Said sensor 605 is composed of a diode sensor, and is provided at each end of the array of the discharge openings. The kind of the temperature detecting means is not limitative. The temperature may be detected by other sensors such as thermistor, or may be estimated by calculation, based on the duty ratio of the printed dots, in the counting of the number of the printed dots. Fig. 19 is a block diagram showing the basic configuration of the color ink jet recording apparatus. In Fig. 19, there are shown an image input unit 641 for optically reading the original image for example with a CCD, or for entering image luminance signals (R, G, B signals) for example from a host computer or a video equipment; an operation unit 642 provided with various keys for setting various parameters and for instructing the start of printing operation; and a CPU 643 for controlling the entire recording apparatus according to various programs stored in a ROM. A ROM 644 stores various programs for operating this recording apparatus, such as a control program and an error process program. More detailedly, said ROM includes an input gamma conversion table 644a to be referred to in the process of an input gamma conversion circuit; masking coefficients 644b to be referred to in the process of a color correction (masking) circuit; a black generation/UCR table 644c to be referred to in the process of a black generation/UCR circuit; a density distribution table 644d to be referred to in the process of a density distribution circuit to be explained later; and programs 644e mentioned above. There are further shown a RAM 645 to be used as a work area for the programs stored in the ROM and a temporary diversion area in the error processing; a processing unit 646 for the image signal processing to be explained later; a printer unit 647 for forming a dot image, in the recording operation, based on the image signal processed by said signal processing unit; and a bus line 648 for transmitting the address signals, data, control signals etc. within the present apparatus. In the following there will be given an explanation on the image signal processing unit. Fig. 20 shows the block diagram of the image signal processing system. An image processing circuit 651 effects masking, UCR (undercolor removal) etc., and any general image processing flow is applicable therein. The single-color data after color processing are fetched by a density distribution unit 652, which distributes the input data to the data for light-colored ink and those for dark-colored ink, according to the density distribution table 644d in the ROM 644 shown in Fig. 19. Fig. 21 shows an example of the conversion graph of the density distribution table, wherein a solid line and a chain line respectively correspond to the data for light-colored ink and those for dark-colored ink. If the 8-bit single-color data are within a range from 0 to 128, there are released the data for dark-colored ink at "0", and those for light-colored ink within a range from "0" to "255". If the single-color data are within a range from 128 to 255, the released data for dark-colored ink is varied from "0" to "255" while those for light-colored ink is varied from "255" to "0". In short, the recording operation is conducted principally with the ink of lower dye concentration (light-colored ink) if the input data are low (highlight side), and principally with the ink of higher dye concentration (dark-colored ink) if the input data are high. However, as explained before, the ink discharge amount in the ink jet recording apparatus is significantly influenced by the temperature of the recording head. The ink discharge amount becomes larger as the ink temperature becomes higher, so that the density of the recorded image varies depending on the temperature of the recording head. Consequently, a data correction circuit 653 corrects the data for light-colored ink and those for dark-colored ink distributed by the density distribution table, based on the temperature data obtained from the temperature sensor 656 of the light-colored ink head and the temperature sensor 657 of the dark-colored ink head. Fig. 22 shows an example of the conversion graph for data correction, to be executed by said data correction circuit 653. If the head temperature is low, the graph is shifted to a side A to increase the output data, thereby increasing the density. Also if the head temperature is high, the graph is shifted to a side B to decrease the output data, thereby reducing the density. The data for the dark- and light-colored inks, thus corrected according to the head temperature, are respectively binarized in the binarizing circuits 654, 655 shown in Fig. 20, and are supplied in the form of on/off data (1-bit signal) to the respective heads. The foregoing refers to a single-color ink, but is also applicable to a color recording apparatus utilizing inks of higher and lower densities in plural colors such as cyan, magenta, yellow and black. Also the dye concentration of the ink is not limited to two levels of high and low, but may also be selected at three or more levels. This is because, if the dot density is different significantly between the inks of high and lower densities, the tonal rendition does not become linear in a portion where said inks of higher and lower densities are switched and a pseudo contour tends to appear in such portion. Also in such ink switching portion, there may result a variation in the granularity or the hue of the recorded image, thereby giving unnatural impression. For this reason it is preferable to conduct the recording operation with an increased number of ink densities, such as inks of higher, medium and lower densities. The configuration explained above allows to obtain desired density in the recorded image even when the temperature of the recording head varies, thereby significantly improving the tonal reproducibility. (Embodiment 4) In the following there will be explained an embodiment of the ink jet recording apparatus of the present invention, wherein the ink jet recording head is provided with plural liquid chambers in which the inks of different densities of a same color are supplied for improving the tonal reproducibility. Fig. 23 is a perspective view of the principal part of the color ink jet recording apparatus of an embodiment 6, wherein the functions are basically same as those described in the illustrative Example 2. Fig. 24 is a schematic perspective view of the arrays of the ink discharge openings on the recording heads in said recording apparatus, seen from the side of the recording material. The ink jet recording apparatus illustrated in Fig. 23 has the recording heads for the inks of four colors of C (cyan), M (magenta), Y (yellow) and K (black), wherein each of the recording heads, arranged in parallel manner on a carriage 623, is provided with an array of the discharge openings for discharging the ink of dark color, and an array of the discharge openings for discharging the ink of light color. In the recording operation with the inks of dark and light colors, the aberration in the deposited positions of the dots of the dark and light colors is also important, and the density may vary by such aberration in the dot position. In the present embodiment, the arrays of the discharge openings for the inks of different densities are provided on a single recording head, so that said arrays are relieved from the vertical and horizontal positional aberrations, encountered in case said arrays are provided on different recording heads and the aberration in density scale, resulting from the deposited position can therefore be reduced. Fig. 25 is an exploded perspective view of the ink jet recording head to be employed in the present embodiment. A wiring board 200 is connected, at an end thereof, with the wirings of a heater board 100, and is provided, at the other end, with unrepresented plural pads serving to receive the electrical signals from the main body of the apparatus and respectively corresponding to the electrothermal converting members, whereby said electrical signals from said main body are transmitted to the respective electrothermal converting members. A metal support member 300, for supporting the rear surface of the wiring board 200 by planar contact, constitutes a base plate of the ink jet unit. A pressure spring 500 is provided, for applying an elastic pressure on a linear area in the vicinity of the ink discharge openings of a grooved ceiling plate 1300, with a portion formed in a substantially U-shaped cross section, claws for engaging with holes formed in the base plate, and a pair of rear legs for receiving the force, applied to the spring, by the base plate. The wiring board 200 and the grooved ceiling plate 1300 are maintained in pressure contact by the force of said spring. The wiring board 200 is adhered to the support member for example by adhesive material. At an end of an ink supply tube 2200, there is provided a filter 700. An ink supply member 600 is prepared by molding, and the grooved ceiling plate 1300 is integrally provided with an orifice plate 1201 and liquid path 1500 leading to respective ink supply apertures. The ink supply member 600 is fixed to the support member 300 in simple manner by inserting two rear pins (not shown) of the ink supply member 600 into holes 1901, 1902 of the support member and thermally fusing the protruding ends of said pins. In this state, there is formed a uniform gap between the orifice plate 1201 and the ink supply member 600. Sealing agent is poured from an inlet formed on the upper part of said ink supply member, thereby sealing the bonding wires and the above-mentioned gap between the orifice plate 1201 and the ink supply member 600, and also sealing the gap to the front end of the support member 300. Fig. 26 is a perspective view of the grooved ceiling plate 1300 of the recording head to be employed in the present embodiment, seen from the side of the heater board 100. There are provided plural liquid chambers, which are mutually separated by partition walls 10, and are respectively provided with ink supply openings 20a, 20b. Said partition walls 10 are provided, on contact faces with the heater board 100, with grooves 30, which communicate with the external peripheral part of the grooved ceiling plate 1300. After said plate 1300 is maintained in contact with the heater board, said external peripheral part is sealed with the sealing agent as explained above. In this operation, the sealing agent enters along said grooves, thereby filling the gap between said board and the heater board. In this manner, the liquid chambers can be completely separated by the steps known in the conventional head. The structure of said grooves varies according to the physical properties of the sealing agent and has to be shaped corresponding to the sealing agent to be employed. The above-explained structure of separate plural liquid chambers allow to supply different inks to the ink discharge openings. Fig. 27 shows the structure of an integral 4-head ink jet cartridge (IJC) in which the recording heads of four colors C, M, Y, K are integrally assembled by a frame 3000. Inside said frame 3000, the four recording heads are mounted with a predetermined mutual distance, and are fixed with the arrays of the ink discharge openings adjusted in the vertical and horizontal positions. There are also shown a frame cover 3100, and a connector 3200 for supplying the pads, provided on the wiring boards 200 of the four recording heads with the electrical signals from the main body of the apparatus. Fig. 28 shows the state of the integral 4-head ink jet cartridge mounted on the carriage 623. Each ink tank is divided into upper and lower chambers, which are respectively filled with the inks of higher and lower densities. On said carriage, the ink jet cartridge (IJC) and the four ink tanks (IT) of C, M, Y and K colors are coupled by pressurized contact, whereby each ink is supplied from the corresponding ink tank to the recording head. Also in this embodiment, a diode sensor is provided on the heater board 100 as temperature detection means for the recording heads. As explained in the foregoing, the temperature detection means is not particularly limited to such form, and the temperature may be detected by other detectors such as thermistor, or may be estimated by calculation from the duty ratio of the printed dots. Fig. 29 is a block diagram of the image signal processing system, wherein an image processing circuit 651 includes masking and UCR (undercolor removal), and any commonly used image processing flow is applicable thereto. The C, M, Y and K data after image processing are fetched by a succeeding density distribution unit 652. In the present embodiment, according to the temperature data from the temperature sensor 658 of the recording head, an optimum one is selected from the plural density distribution tables prepared in advance, and the input data are distributed to the data for the light-colored ink and those for the dark-colored ink, according to thus selected distribution table. Fig. 30A shows an example of the distribution table for a low head temperature, while Fig. 30B shows an example of the distribution table for a high head temperature. These examples will be explained in the following, in comparison with the distribution table shown in Fig. 21. As the discharge amount is low for a low head temperature, the output data for the dark- and light-colored inks are increased for a given input data as shown in Fig. 30A thereby elevating the image density. Since the output data for the dark-colored ink are already saturated in a high level area (around 255) of the input data, the data for the light-colored ink are further added for elevating the image density. As the discharge amount becomes higher at a high head temperature, the output data are made lower, than in the standard state shown in Fig. 21, for a given input level in order to suppress the image density as shown in Fig. 30B. The switching of the distribution table in this manner according to the temperature of the recording head allows to always obtain a constant image density even in the presence of a temperature-dependent variation in the ink discharge amount, thereby enabling to realize an ink jet recording apparatus with satisfactory tonal rendition. The method of distribution of the input data into the data of the light-colored ink and those of the dark-colored ink based on the distribution table is same as in the embodiment 5. Based on the distribution table selected according to the temperature of the recording head, the input C, M, Y, K data are distributed into the data for the light-colored inks (C', M', Y', K') and those for the dark-colored inks (C", M", Y", K"), which are respectively binarized by binarizing circuits 654, 655 shown in Fig. 29 and are supplied, in the form of on/off data (1-bit signals) to the respective recording heads. The present embodiment also enables to always reproduce a constant tonal density rendition regardless of the temperature of the recording heads, and can simplify the process by averaging the temperatures of the recording heads for the light- and dark-colored inks. However, in case the inks of different densities are dividedly used within a recording head, as in the present embodiment, the temperatures of the heads for the light- and dark-colored inks may become not considerable as an average when the number of the discharge openings increases. In such case, the distributed data for each ink may be corrected according to the temperature detected independently in each ink area. Also the recording head, which is internally divided into plural ink areas as in the present embodiment, may be combined with the image signal processing system employed in the embodiment 5, in which the data for the dark- and light-colored inks distributed by a distribution table are corrected according to the head temperature. The configuration containing the arrays of the discharge openings for the inks of different densities within a single head enables to limit the number of recording heads, thereby achieving compactization of the apparatus. The foregoing embodiment has been explained by a 4-color ink jet recording apparatus employing cyan, magenta, yellow and black colors, but the number of colors is not limited to such embodiment and the present invention is likewise applicable to a recording apparatus employing the inks of different densities of a single color or plural colors of a difference number. Also the dye concentration of the ink is not limited to two levels of high and low, but may be selected in three or more levels. If the dot density is significantly different between the inks of high and low densities, the tonal reproduction does not become linear in a portion where the inks of high and low densities are switched, and there may easily result a pseudo contour. Also in such ink switching portion, there may result a variation in the granularity or in the hue of the recorded image, thus giving an unnatural impression. It is therefore preferable to effect the recording operation with an increased number of densities of the ink, for example low, medium and high densities. (Other embodiments) For controlling the ink discharge amount of the ink jet recording head, there has been proposed the PWM (pulse width modulation) control. Said PWM control method will be explained in detail in the following, with reference to the attached drawings. Fig. 31 is a view for explaining the divided pulses, wherein Vop indicates a driving voltage; P1 indicates the pulse width of an initial pulse (hereinafter called pre-pulse) of divided plural heating pulses; P2 is an interval time; and P3 is the pulse width of a second pulse (hereinafter called main pulse). T1, T2 or T3 indicates the time for defining P1, P2 or P3. The driving voltage Vop is a factor determining the electric energy required for generating the thermal energy by the electrothermal converting element provided in the ink path, and is determined by the area, resistance and film structure of said electrothermal converting element and by the structure of the ink flow path. There stand relationships P1 = T1, P2 = T2 - T1 and P3 = T3 - T2. The PWM control of the present embodiment may also be named as pre-pulse width modulation control, in which, for the discharge of an ink droplet, the pulses are given in succession with widths P1, P2 and P3, and the width of the pre-pulse is modulated according to the temperature of the recording head or of the ink. Said pre-pulse is used principally for controlling the ink temperature in the liquid path, and has an important role in the discharge amount control. The width of said pre-pulse is preferably selected at such level as not to form a bubble in the ink, by the energy generated by the electrothermal converting element in response to said pre-pulse. The interval time is to secure a time for transmission of the energy, generated by said pre-pulse, to the ink in the liquid path. The main pulse serves to generate a bubble in the ink present in the liquid path, thereby discharging the ink from the discharge opening, and its width is preferably determined according to the area, resistance and film structure of the electrothermal converting member and the structure of the liquid path. Fig. 32 is a chart showing the dependence of the discharge amount on the width of the pre-pulse, wherein V0 indicates the discharge amount at P1 = 0, which is determined by the head structure. As indicated by a curve a in Fig. 32, in response to the increase of the width P1 of the pre-pulse, the discharge amount Vd increases linearly within a range of said width P1 from 0 to P1LMT. Beyond said level P1LMT, the discharge amount loses linearity in the increase, and becomes saturated at a pulse width P1MAX. The range up to the pulse width P1LMT, in which the discharge amount Vd varies linearly in response to the variation of the pulse width P1, is effective for the discharge amount control by the variation of the pulse width P1. If the pulse width becomes larger than P1MAX, the discharge amount Vd becomes smaller than the maximum value VMA. This is due to a phenomenon that, by the application of a pre-pulse of a width in the above-mentioned range, small bubbles are formed on the electrothermal converting member and, the next main pulse being applied before said small bubbles vanish, said small bubbles disturb the bubble generation by said main pulse, thereby suppressing the discharge amount. Such range is called pre-bubbling range, in which the discharge amount control by the pre-pulse width becomes difficult. In the range P1 = 0 to P1LMT in Fig. 32, the slope of the linear portion is defined as the pre-pulse dependence coefficient which is represented by: Kp = ΔVop/ΔP1 which is not dependent on the temperature but is determined by the head structure, driving conditions, physical properties of the ink etc. In Fig. 32, curves b, c show the variations in other recording heads, indicating that the discharge characteristics are specific to the recording head. On the other hand, another factor determining the discharge amount of the ink jet recording head is the ink temperature of the discharge part (said temperature being sometimes replaceable by the temperature of the recording head). Fig. 33 shows the temperature dependence of the discharge amount. As indicated by a curve a therein, the discharge amount Vd linearly increases as a function of the temperature TH of the recording head. The slope of said line is defined as the temperature dependence coefficient, which is given by: KT = ΔVdt/ΔTH. This coefficient is independent from the driving conditions but is determined by the head structure, physical properties of the ink etc. Also in Fig. 33, curves b, c indicate the behaviors of other recording heads. Fig. 34 shows an actual control chart, incorporating the relations shown in Figs. 32 and 33. In Fig. 34, T0 indicates the maintained temperature of the recording head. If the ink temperature at the discharge portion is lower than T0, the recording head is heated with a sub heater (not shown). Consequently the discharge amount control according to the ink temperature is conducted above said temperature T0. In Fig. 34, the discharge amount can be stabilized within a temperature range indicated as the PWM control range, and there is shown the relation between the discharge amount and the ink temperature at the discharge portion when the pre-pulse width is varied in 11 steps. Even in the presence of a variation in the ink temperature at the discharge portion, the discharge amount can be controlled within a width ΔV around the target discharge amount Vd0, by varying the width of the pre-pulse for each temperature step of ΔT of the ink temperature. However, even with this PWM control, the discharge amount cannot be stabilized in a temperature range beyond the upper limit temperature TL of the PWM control. In the present embodiment, therefore, the discharge amount is regulated by heating with the sub heater, if the temperature of the recording head is lower than T0. Then, within a temperature range from T0 to T1, the discharge amount is regulated by the PWM control. In a temperature range beyond TL wherein the discharge control by such PWM control becomes impossible, the distributed data for the inks of different densities are corrected as explained in the foregoing embodiments 1 and 2. In this manner the tonal reproduction of the recorded image is improved, whereby an image without unevenness in the density can be obtained. As explained in the foregoing, the present invention, through the combination with the PWM discharge amount control, can improve the tonal rendition over a wider temperature range. Also in case of preparing plural distribution table, the present embodiment can reduce the number thereof in comparison with the embodiment 1 or 2, because a situation of low temperature need not be considered. As detailedly explained in the foregoing, the present invention is to detect the temperature relative to the recording head, to distribute the image data to plural sets of data respectively corresponding to plural kinds of inks, according to the detected temperature data, and to form an image on a recording medium by means of said recording head, based on thus distributed data, thereby being capable of providing the desired recorded image density even in the presence of a variation in the temperature of the recording head, and significantly improving the tonal reproducibility. The foregoing embodiments employ an ink jet recording head for recording, but such embodiments are not limitative and the present invention is applicable also, for example, to a thermal head or a thermal transfer head. Among various ink jet recording methods, the present invention brings about a particular effect when applied to a recording head of a system utilizing thermal energy for forming a flying liquid droplet for recording, and a recording apparatus utilizing such recording head. The principle and representative configuration of said system are disclosed, for example, in the U.S. Patent Nos. 4,723,129 and 4,740,796. This system is applicable to so-called on-demand recording or continuous recording, but is particularly effective in the on-demand recording because, in response to the application of at least a drive signal representing the recording information to an electrothermal converter element positions corresponding to a liquid channel or a sheet containing liquid (ink) therein, said element generates thermal energy capable of causing a rapid temperature increase exceeding the nucleus boiling point, thereby inducing film boiling on a heat action surface of the recording head and thus forming a bubble in said liquid (ink), in one-to-one correspondence with said drive signal. Said liquid (ink) is discharged through a discharge opening by the growth and contraction of said bubble, thereby forming at least a liquid droplet. Said drive signal is preferably formed as a pulse, as it realizes instantaneous growth and contraction of the bubble, thereby attaining highly responsive discharge of the liquid (ink). Such pulse-shaped drive signal is preferably that disclosed in the U.S. Patent Nos. 4,463,359 and 4,345,262. Also the conditions described in the U.S. Patent No. 4,313,124 relative to the temperature increase rate of said heat action surface allows to obtain further improved recording. The configuration of the recording head is given by the combinations of the liquid discharge openings, liquid channels and electrothermal converter elements with linear or rectangular liquid channels, disclosed in the above-mentioned patents, but a configuration disclosed in the U.S. Patent No. 4,558,333 in which the heat action part is positioned in a flexed area, and a configuration disclosed in the U.S. Patent No. 4,459,600 also belong to the present invention. Furthermore the present invention is effective in a structure disclosed in the Japanese Patent Laid-open Application No. 59-123670, having a slit common to plural electrothermal converter elements as a discharge opening therefor, or in a structure disclosed in the Japanese Patent Laid-open Application No. 59-138461, having an aperture for absorbing the pressure wave of thermal energy, in correspondence with each discharge opening. A full-line type recording head, capable of simultaneous recording over the entire width of the recording sheet, may be obtained by plural recording heads so combined as to provide the required length as disclosed in the above-mentioned patents, or may be constructed as a single integrated recording head, and the present invention can more effectively exhibit its advantages in such recording head. The present invention is furthermore effective in a recording head of interchangeable chip type, which can receive ink supply from the main apparatus and can be electrically connected therewith upon mounting on said main apparatus, or a recording head of cartridge type in which an ink cartridge is integrally constructed with the recording head. Also the recording apparatus is preferably provided with the discharge recovery means and other auxiliary means for the recording head, since the effects of the recording head of the present invention can be stabilized further. Examples of such means for the recording head include capping means, cleaning means, pressurizing or suction means preliminary heating means composed of electrothermal converter element and/or another heating device, and means for effecting an idle ink discharge independent from the recording operation, all of which are effective for achieving stable recording operation. Furthermore, the present invention is not limited to a recording mode for recording a single main color such as black, but is extremely effective also to the recording head for recording plural different colors or full color by color mixing, wherein the recording head is either integrally constructed or is composed of plural units. In the foregoing embodiments of the present invention, the ink is assumed to be liquidous, but the present invention is also applicable to the ink which is solid below room temperature but softens or liquefies at room temperature, or which softens or liquefies within a temperature control range from 30°C to 70°C, which is ordinarily adopted in the ink jet recording. Thus the ink only needs to be liquidous when the recording signal is given. Furthermore, the present invention is applicable to ink liquefied by thermal energy provided corresponding to the recording signal, such as the ink in which the temperature increase by thermal energy is intentionally absorbed by the state change from solid to liquid, or the ink which remains solid in the unused state for the purpose of prevention of ink evaporation, or the ink which starts to solidify upon reaching the recording sheet. In these cases the ink may be supported as solid or liquid in recesses or holes of a porous sheet, as described in the Japanese Patent Laid-open Application Nos. 54-56847 and 60-71260, and placed in an opposed state to the electrothermal converter element. The present invention is most effective when the above-mentioned film boiling is induced in the ink of the above-mentioned forms. Furthermore, the recording apparatus of the present invention may be constructed, not only as an integral or separate image output terminal for an information processing equipment such as a word processor or a computer, but also as a copying apparatus combined with an image reader or a facsimile apparatus having the function of transmission and reception. Fig. 35 is a block diagram showing the schematic configuration, when the ink jet recording apparatus of the present invention is applied to an information processing apparatus having the functions of word processor, personal computer, facsimile apparatus and copying apparatus, wherein provided are a control unit 1201 for controlling the entire apparatus, provided with a CPU such as a microprocessor and various I/O ports and effecting the control by sending control signals and data signals to various units and receiving control signals and data signals therefrom; a display unit 1202 for displaying various menus, document information and image data read by an image reader 1207; and a transparent pressure-sensitive touch panel 1203 provided on said display unit 1202 and serving for the input of an item displayed on the display unit 1202 or the input of coordinates, by a finger pressure applied thereon. An FM (frequency modulation) sound source unit 1204 reads the sound information, prepared for example by a music editor and stored in a memory 1210 or an external memory 1212 in the form of digital data, and effect frequency modulation. The electrical signal from said FM sound source is converted into audible sound by a loudspeaker 1205. A printer unit 1206 embodies the recording apparatus of the present invention, as an output terminal for the word processor, personal computer, facsimile and copying apparatus. An image reader 1207, for photoelectrically the original data, is positioned in the transport path of the originals, and reads the originals for facsimile transmission, for copying and for other purposes. A facsimile transmission/reception unit 1208 for effecting facsimile transmission of the original data read by the image reader 1207, and reception and decoding of the received facsimile signal, has the function of interfacing with the external line. A telephone unit 1209 has various telephone functions including the ordinary telephone functions and the message telephone function. A memory unit 1210 includes a ROM for storing a system program, a managing program and other application programs, a character font and a dictionary, a RAM for storing application programs loaded from the external memory 1212 and character information, and a video RAM. A keyboard unit 1211 serves for entering document information and various commands. An external memory 121, employing a floppy disk or a rigid disk as the memory medium, is used for storing character information, music or sound information, application programs of the user etc. Fig. 36 is an external view of the information processing apparatus shown in Fig. 35. A flat panel display 1301 is used for displaying various menus, pattern information and document information. On said display 1301 there is provided a touch panel, for entering the coordinates or various items, by pressing for example with a finger. A handset 1302 is used when the apparatus functions as a telephone set. A keyboard 1303, provided with various function keys 1304, is detachably connected to the main body through a cord, and serves for entering various character information and various data. An insertion slot 1305 for the floppy disk is provided. A sheet stacker 1307 is used for stacking the originals to be read by the image reader 1207, and the originals after image reading are discharged from the rear part of the apparatus. An ink jet printer 1306 is provided for image recording for example in the facsimile reception. The above-mentioned display 1301 may be composed of a cathode ray tube, but is preferably composed of a flat panel display such as liquid crystal display utilizing ferroelectric liquid crystal, because such flat panel display can achieve compactization, a smaller thickness and a lighter weight. In the use of the above-explained information processing apparatus as a personal computer or a word processor, the information entered from the keyboard 1211 shown in Fig. 35 is processed in the control unit 1201 according to a predetermined program, and the result is released as an image from the printer unit 1206. Also in the function as a facsimile reception unit, the facsimile information supplied through a communication channel to the facsimile transmission/reception unit 1208 is processed in the control unit 1201 according to a predetermined program, and the result is released as a received image from the printer unit 1206. Also in the function as a copying apparatus, the original is read by the image reader 1207, and the read original data are supplied through the control unit 1201 and released as a copy image from the printer unit 1206. Also in the function as a facsimile transmitter, the original data read by the image reader 1207 are subjected to a transmission process by the control unit 1201 according to a predetermined program, and the result is transmitted to the communication channel through the facsimile transmission/reception unit 1208. The above-explained information processing apparatus may also be constructed as an integral unit incorporating the ink jet printer as shown in Fig. 37, for achieving improved portability. In Fig. 37, components equivalent to those in Fig. 36 are represented by same numbers. The application of the recording apparatus of the present invention to the multi-function information processing apparatus explained above allows to obtain a recorded image of higher quality, whereby the functions of said information processing apparatus can be further improved. As detailedly explained in the foregoing, in recording on a recording medium by discharging ink from discharge openings of a recording head utilizing plural inks of different densities of a same color, if stable discharge becomes difficult in at least one of said plural inks, switches a distribution table so as to effect the recording operation with the ink of the remaining kind, thereby providing a proper image. Also as described above, the recording apparatus may be arranged to detect the temperature relative to a recording head, distribute the image data to plural sets of data, respectively corresponding to the plural inks and also corresponding to the detected temperature data, and form an image on a recording medium with said recording head, based on thus distributed data. It is therefore rendered possible to obtain a desired recorded image density even in the presence of a variation in the temperature of the recording head, thereby significantly improving the total reproducibility.

Claims: A recording apparatus for recording an image on a recording medium using recording means (10) arranged to discharge a plurality of different density inks onto the recording medium, comprising: distribution means (200) for distributing image data to be recorded between said plurality of different density inks by using a distribution table defining the relative proportions of said different density inks to be used, detection means (4; 43,44; 605) for detecting a condition affecting the ability of the recording means to discharge ink, the distribution means being arranged to select the distribution table from a plurality of distribution tables each defining different relative proportions of the different density inks in accordance with both the image density indicated by the image data and any such condition detected by the detection means for enabling recording at a similar gradation to that which would have been achieved if no such condition had been detected; characterised in that ; if recording by ink having a relatively high density cannot be performed, said distribution means is arranged to select a distribution table for causing recording to be performed by utilizing ink having a relatively low density if the image density indicated by the image data is less than a predetermined density and to select as said table a table for causing recording to be performed in a plurality of recording steps so as to overlap a plurality of images if the image density indicated by the image data is equal to, or more than, said predetermined density. A recording apparatus according to claim 1, where said distribution means is arranged to select as said table for causing recording to be performed using relatively low density ink if recording cannot be performed with relatively high density ink a distribution table for causing the image data to be recorded in a single recording step when the image density indicated by the image data is lower than a predetermined density, and for causing the image data to be recorded in a plurality of recording steps such that the images recorded in the different recording steps overlap where the image data is higher than a predetermined density indicated by the image. A recording apparatus according to claims 1 and 2, wherein said detection means (4; 43,44) is adapted to detect the ability of the recording means to discharge ink by detecting the amount of ink remaining for that head. A recording apparatus according to claim 3, wherein said remaining amount detection means (4) is adapted to detect the amount of ink remaining by detecting the resistance between electrodes (4) provided in an ink tank for storing the ink. A recording apparatus according to claim 3, wherein said remaining amount detection means (43,44) is adapted to detect the amount of ink remaining by optically detecting the ink in an ink tank for storing the ink. A recording apparatus according to claim 3, wherein said remaining amount detection means is adapted to detect a negative pressure in an ink tank for storing the ink. A recording apparatus according to claim 1, wherein said detection means is adapted to detect clogging of an ink outlet nozzle of the recording means. A recording apparatus according to claim 1, wherein said detection means (605) is adapted to detect the condition affecting the ability of the recording means to discharge ink by detecting the temperature of said recording means. Apparatus according to claim 8, wherein said distribution means (200) is arranged to select the distribution table from plural distribution tables including a first table so prepared as to effect the recording with a relatively high image density, and a second table so prepared as to effect recording with a relatively low image density. A recording apparatus according to claim 9, wherein said distribution means (200) is adapted to select said first table when the temperature of said recording head is low, or said second table when the temperature of said recording head is high, according to the output of said detection means. A recording apparatus according to any one of claims 1 to 8, wherein said plural inks of different densities consist of a first ink of a high density and a second ink of a low density. A recording apparatus according to claim 11, wherein said distribution means (200) is arranged to select between a first table for recording with a first high density ink and a second low density ink, a second table for recording with said first ink only, and a third table for recording with said second ink only. A recording apparatus according to claim 11, wherein said first table is so prepared that the recording is conducted with said second ink until the image density indicated by the image data reaches a predetermined density level, and is conducted with said first and second inks when the image density indicated by the image data exceeds said predetermined density level. A recording apparatus according to claim 12 or 13, wherein said third table is so prepared that the recording is conducted with said second ink until the image density indicated by the image data reaches a predetermined density level, and the recording is conducted plural times in superposed manner with said second ink when the image density indicated by the image data exceeds said predetermined density level. A recording apparatus according to any one of the preceding claims, comprising recording means (401) comprising a separate recording head for each ink. A recording apparatus according to any one of claims 1 to 14, comprising recording means comprising an integral head for inks of the same color but different density. A recording apparatus according to any one of claims 1 to 14, comprising recording means for recording with a plurality of different color inks. A recording apparatus according to claim 15, 16 or 17, wherein the recording means comprise thermal energy generating means for generating thermal energy to cause ink discharge. A recording apparatus according to claim 18, wherein the thermal energy generating means are arranged to generate thermal energy to induce a change of state in the ink so as to cause ink discharge. A recording apparatus according to claim 18 or 19, comprising drive means for driving said recording means for causing ink discharge by supplying to the recording means a first pulse providing insufficient to cause ink discharge followed after an interval by a second pulse for causing ink discharge. A method for recording an image on a recording medium using recording means arranged to discharge a plurality of different density inks onto the recording medium, comprising: distributing image data to be recorded between said plurality of different density inks by using a distribution table defining the relative proportions of said different density inks to be used, detecting using detecting means a condition affecting the ability of the recording means to discharge ink, and selecting the distribution table from a plurality of distribution tables each defining different relative proportions of the different density inks in accordance with both the image density indicated by the image data and any such detected condition for enabling recording at a similar gradation to that which would have been achieved if no such condition had been detected; characterised by ; if recording by ink having a relatively high density cannot be performed selecting a distribution table for causing recording to be performed by utilizing ink having a relatively low density if the image density indicated by the image data is less than a predetermined density, and selecting as said table a table causing recording to be performed in a plurality of recording steps so as to overlap a plurality of images if the image density indicated by the image data is equal to, or more than, said predetermined density. A method according to claim 21, which comprises selecting as said table for causing recording to be performed using relatively low density ink if recording cannot be performed with relatively high density ink a distribution table for causing the image data to be recorded in a single recording step when the image density indicated by the image data is lower than a predetermined density, and for causing the image data to be recorded in a plurality of recording steps such that the images recorded in the different recording steps overlap where the image data is higher than a predetermined density indicated by the image. A method according to claim 22, which comprises detecting the ability of recording means to discharge ink by detecting the amount of ink remaining. A method according to claim 23, which comprises detecting the amount of ink remaining by detecting the resistance between electrodes in an ink tank. A method according to claim 23, which comprises detecting the amount of ink remaining by detecting the amount of light which passes through at least a portion of an ink tank. A method according to claim 23, which comprises detecting the amount of ink remaining by detecting a negative pressure in an ink tank for storing the ink. A method according to claim 22, which comprises detecting the ability of the recording means to discharge ink by detecting clogging of an outlet of an ink outlet nozzle of the recording means. A recording method according to claim 22, which comprises detecting the ability of the recording means to discharge ink by detecting the temperature of said recording means. A method according to claim 28, which comprises selecting the distribution table from plural distribution tables including a first table so prepared as to effect the recording with a relatively high image density, and a second table so prepared as to effect recording with a relatively low image density. Apparatus according to claim 29, which comprises selecting said first table when the temperature of said recording means is low, or said second table when the temperature of said recording means is high. A method according to claim 21, which comprises detecting the ability of the recording means to discharge ink by detecting whether stable ink discharge has become difficult for at least one of said different density inks and selecting a distribution table which does not use that ink. A method according to any one of claims 21 to 27, wherein said plural inks of different densities consist of first ink of a high density and second ink of a low density. A method according to claim 32, wherein said plural distribution tables include a first table for recording with said first and second inks, a second table for recording with said first ink only, and a third table for recording with said second ink only. A method according to claim 33, wherein said first table is so prepared that the recording is conducted with said second ink until the image density indicated by the image data reaches a predetermined density level, and is conducted with said first and second inks when the image density indicated by the image data exceeds said predetermined density level. A method according to claim 33 or 34, wherein said third table is so prepared that the recording is conducted with said second ink until the image density indicated by the image data reaches a predetermined density level, and the recording is conducted plural times in superposed manner with said second ink when the image density indicated by the image data exceeds said predetermined density level. A method according to claim 33, which further comprises selecting said second table when the remaining amount of said second ink becomes less than a predetermined amount, and selecting said third table when the remaining amount of said first ink becomes less than a predetermined amount. A recording method according to any one of claims 21 to 36, which further comprises effecting recording with plural inks of different densities in each of plural colors. A method according to any one of claims 21 to 37, which comprises using recording means provided with a thermal energy generating means for generating thermal energy for heating the ink. A method according to claim 38, which comprises discharging ink by inducing a state change in the ink by applying thermal energy to the ink using said thermal energy conversion member. A signal carrying processor implementable instructions for controlling a processor to implement the method of any one of claims 21 to 29, and claims 31 to 39. A storage medium storing processor implementable instructions for controlling a processor to implement the method of any one of claims 21 to 29 and claims 31 to 39. A storage medium as claimed in claim 41 wherein said storage medium comprises a ROM (644).

Inventors: AKIYAMA YUJI, GOTOH FUMIHIRO, HIRABAYASHI HIROMITSU, KANDA HIDEHIKO, KASHINO TOSHIO, MATSUBARA MIYUKI, SUGIMOTO HITOSHI, AKIYAMA, YUJI, GOTOH, FUMIHIRO, HIRABAYASHI, HIROMITSU, KANDA, HIDEHIKO, KASHINO, TOSHIO, MATSUBARA, MIYUKI, SUGIMOTO, HITOSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H04N1
IPC codes: B41J2, H04N1