Method and apparatus for measuring the amount of discharged ink, printing apparatus, and method of measuring the amount of ink discharged in the printing apparatus

Abstract: A method of instantly measuring the amount of ink discharged from a nozzle. The method of measuring the amount of discharged ink includes: a line pattern forming step of forming a line pattern by discharging ink from an ink-jet printhead; a first image-sensing step of sensing the line pattern by an image sensing device; a second image-sensing step of sensing a portion other than the line pattern by the image sensing device; a density calculating step of calculating density of each pixel of the line pattern by dividing the luminance value of the portion sensed in said second image-sensing step by the luminance value of the line pattern sensed in said first image-sensing step and calculating a common logarithm; an integrating step of integrating the density of each pixel of the entire line pattern calculated in said density calculating step; and an ink-discharge-amount determining step of obtaining the amount of discharged ink on the basis of the integrated density calculated in said integrating step.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and an apparatus for measuring the amount of ink discharged, a printing apparatus and a method of measuring the amount of ink discharged by the printing apparatus, and more particularly, to a method and an apparatus for measuring the amount of ink discharged by single discharging operation of a printhead, such as that used in an ink-jet printer, having fine nozzles which discharge a very small amount of ink from each nozzle. For an apparatus such as the ink-jet printer which has a printhead consisting of a plurality of fine nozzles, it is quite important to control the amount of ink discharged from each nozzle to be uniform in order to stabilize printing quality. For this purpose, it is desired to correctly and instantly obtain the amount of ink discharged from each nozzle. There are known methods of measuring a small amount of colored ink droplet (e.g., color ink), namely, (1) a weighing method and (2) an absorbance method. Followings are explanation when the amount of ink discharged from a nozzle of an ink-jet printer is measured in the above two methods. (1) Weighing Method In this method, ink is discharged at a fixed interval for a predetermined time period, and the amount (weight) of used ink in the time period is measured by a chemical balance or the like (care must be taken to minimize vaporization vaporization of ink during this operation). Thereafter, by dividing the amount of ink used by the number of discharging operation performed, an average amount of ink droplet (an average discharging amount) in each discharging operation can be obtained. (2) Absorbance Method . This method uses the relationship between concentration of solution and light absorption, which is known as Lambert-Beer's law. More specifically, solution having a certain concentration is poured into a transparent container having a fixed depth, and light having intensity I o is incidented on the container from one side, then intensity I of the light transmitted through the solution in the container is measured. Since a part of the incidented light is absorbed in the solution in the container, the intensity of the light reduces while passing the solution. It is known that the intensity is lowered in proportion to the concentration of the solution. Defining A as absorbance, the relationship of this law can be expressed in the following equation: A=-Log(I 0 /I)=a·b·c where a is a slope, b is the depth of the solution and c is the concentration of the solution. By using this equation, a calibration line showing the relationship between concentration and absorbance for the ink to be used is obtained in advance. Next, ink is discharged from a nozzle toward a transparent solution of a known volume (a solution which has a light absorbance as small as possible is preferred), then absorbances by the solution containing discharged ink after each discharging operation, namely absorbances corresponding to the number of discharging operations, are measured. From the measured absorbances and the calibration line which was obtained in advance, the concentration of the solution including ink is determined. Then, the amount of ink mixed in the solution can be found by taking the amount of the original solution into consideration. The obtained amount of ink is divided by the number of discharging operations, thereby finding an average amount of ink discharged by each discharging operation. However, according to the foregoing two conventional methods, there are problems in which a certain amount of ink has to be discharged; thus, it is impossible to instantly obtain the amount of ink discharged from each nozzle. As an experiment, the applicant of the present invention measured the amount of ink discharged per nozzle by the weighing method, and found that ink corresponding to 500,000 dots were discharged, and it took 12 minutes for the measurement. Thus, it takes quite a long time to measure the amount of ink discharged per nozzle of an ink-jet printer having an ink-jet printhead consisting of 64 or 128 fine nozzles. Furthermore, in the above two methods, since the amount of ink per discharging operation is averaged, it is impossible to measure the real amount of ink discharged per nozzle in a single discharging operation. EP-A-0 747 224 describes a method according to the preamble of claim 1, and an apparatus according to the preamble of claim 10. In particular, this document describes a method of measuring the amount of ink discharged from a nozzle of an ink-jet type print head in which the density of an ink dot, or a line of dots, discharged on a glass plate is measured. The total amount of light transmitted through a window surrounding the discharged ink is compared with the total amount of light transmitted through an identical window not containing discharged ink. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above situation, and has as its object to provide a method and apparatus for instantly measuring the amount of ink discharged from a nozzle. Another object of the present invention is to provide a printing apparatus having the aforementioned measuring apparatus, and method of measuring the amount of discharged ink in the printing apparatus. According to a first aspect of the invention, there is provided a method of measuring an amount of ink discharged from a nozzle of an ink-jet type printhead in a single discharging operation, the method comprising: a discharging step of discharging an amount of ink from a nozzle of a printhead onto a predetermined material; a first image-sensing step of sensing a first image using an image sensing device, the first image being of the ink discharged on the predetermined material in said discharging step; a second image-sensing step of sensing a second image using the image sensing device, the second image being of a portion of the predetermined material away from the discharged ink; a density calculating step of calculating a density of the ink discharged in said discharging step; and an ink-discharge-amount determining step of determining said amount of ink discharged in said discharging step on the basis of the density calculated in the density calculating step, characterised by said density calculating step comprising: for each pixel sensed by the image sensing device, calculating a pixel density by calculating a logarithm of a luminance value for that pixel in the second image sensed in said second image-sensing step divided by a luminance value for that pixel in the first image sensed in said first image-sensing step; and integrating the pixel densities calculated for the pixels sensed by the image sensing device to determine said density of the discharged ink. According to a second aspect of the invention, there is provided an apparatus for measuring an amount of ink discharged from a nozzle of an ink-jet type printhead onto a receiving medium in a single discharging operation, the apparatus comprising: an image sensing device operable to sense a first image of an amount of ink discharged on the receiving medium, and a second image of a portion of the receiving medium away from said amount of discharged ink; first calculating means operable to calculate a density of said ink discharged on the receiving medium; memory means for storing a calibration line, representing a correlation between said amount of discharged ink and said density calculated by the first calculating means; and second calculating means operable to calculate said amount of discharged ink on the basis of said density calculated by the first calculating means and the calibration line, characterised in that said first calculating means is operable to calculate a pixel density for each pixel sensed by the image sensing device by calculating a logarithm of a luminance value for that pixel in the second image divided by a luminance value for that pixel in the first image, and to integrate the calculated pixel densities to determine said density of the discharged ink. Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of theinvention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, . illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention. Fig. 1 shows a configuration of an apparatus for measuring the amount of discharged ink according to the first embodiment of the present invention; Fig. 2 shows an example of a line pattern printed by using an ink-jet printer; Fig. 3 shows an example when a line pattern subjected to measurement is marked by a window having a fixed size; Fig. 4 shows how the window is divided for each pixel; Fig. 5 is a graph showing an example of a calibration line based on an experiment according to embodiments; Fig. 6 shows a printing apparatus incorporating the apparatus for measuring the amount of discharged ink; Fig. 7 illustrates a configuration of an ink-jet printhead; Fig. 8 is an explanatory view showing a method of controlling the amount of ink to be discharged by changing pulsewidths to be applied to a heater; Fig. 9 is an explanatory view showing a-method of correcting differences in amount of ink discharged from each nozzles; Fig. 10 is an explanatory view showing the method of correcting differences in amount of ink discharged from each nozzles; Fig. 11 is an explanatory view showing the method of correcting differences in amount of ink discharged from each nozzles; Fig. 12 is an explanatory view showing a method of changing printing densities; Fig. 13 is an explanatory view showing the method of changing printing densities; and Fig. 14 is an explanatory view showing the method of changing printing densities. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings. <First Embodiment> Fig. 1 shows configuration of an apparatus for measuring the amount of ink discharged according to the first embodiment of the present invention. In Fig. 1, reference numeral 1 denotes an image processing unit for measuring density of an ink dot; 2, a-personal computer (hereinafter referred to as "PC") used for con-trolling the image processing unit 1 and an XY control stage. 4; 3, an optical system for enlarging an image; 4, the XY control stage used when density of an object is continuously measured; 5, a line sensor camera for inputting an image of the object to be measured into the image processing unit 1; and 6, a light source placed under the XY control stage 4. The central part of the surface of the XY control stage 4 is made of glass, so that the object to be measured is illuminated by the light source placed underneath the glass and an object image can be inputted by the line sensor camera 5. By this, the positional relation of the line sensor camera 5 and the light source 6 is fixed in the present configuration. The PC 2 controls the XY control stage 4 through RS232C or GPIB interface, and controls the image processing unit 1. Fig. 2 is an example of a line pattern printed on a glass plate 10 by a plurality of different nozzles of an ink-jet printer. The "same nozzle direction" in Fig. 2 indicates the direction of prints of ink dots discharged from the same nozzle, and the "different nozzle direction" in Fig. 2 indicates the direction of prints of ink dots discharged from a plurality of different nozzles in a single discharging operation. Since ink and glass are incompatible with each other, a special process for mediating between the discharged ink and the glass plate 10 needs to be applied on the glass plate 10 (the glass plate is coated with polyvinyl alcohol to form an ink-absorbent layer 12 in this example). By virtue of this process, ink discharged from each nozzle during single discharging operation is uniformly absorbed by the ink-absorbent layer and forms a line pattern such as that shown in Fig. 2. As for the material of the ink-absorbent layer 12, a material which is as transparent and colorless as possible (i.e., which does not absorb light) is preferred. Under the condition where the optical system 3 focuses on the printed line pattern as shown in Fig. 2 and the magnification of the optical system 3 and the intensity of the light source 6 are properly adjusted, an image thereof is inputted by the line sensor camera 5. Although the magnification ratio is five in the present embodiment, the present invention is not limited to this magnification. The line sensor camera 5 is a monochrome camera. An image inputted by the line sensor camera 5 is formed with minimal pixel units to which the image processing unit 1 can resolve (the present apparatus utilizes an A/D converter having 8 bits), and each minimal pixel can express luminance level in 256 tones, between 0 and 255 tone, in accordance with the intensity of the transmitted light through each pixel. Next, a method of measuring density of a line pattern (corresponding to the amount of ink discharged from each nozzle) will be explained. In the present embodiment, density is expressed by the following equation: pixel density = Log(pixel reference luminance/pixel luminance) and is hereinafter described in detail. In the present embodiment, the density of a line pattern is determined by how much of the incident light is absorbed while transmitting through the line pattern with color (density) which is subjected to measurement. The higher the density of the line pattern subjected to measurement, the more light is absorbed, and the intensity of the transmitted light decreases. Therefore, the luminance level of the minimum pixel in the area of the line pattern, subjected to measurement, decreases. On the contrary, if the density of the line pattern is low, the luminance level of the minimum pixel must be high. The present embodiment focuses upon this fact, and the density is replaced by light absorbances (although what the image processing unit 1 actually measures is a luminance level). Then, an image of a line pattern subjected to measurement, inputted by the line sensor camera as described above, is marked by a frame having a fixed size (hereinafter referred to as a "window") as shown in Fig. 3. The window consists of n x m pixels as shown in Fig. 4 (since the line sensor camera is used, the image as shown in Fig. 4 is obtained by scanning in the widthwise direction of the line pattern for the amount of m pixels). Herein, a luminance level of each pixel (pixel luminance: Qnm) is obtained. Furthermore, an image of a portion having no line patterns (the portion of plain glass) is marked by the window having the same size, and the luminance level (pixel reference luminance: Inm) of each pixel is obtained. The pixel reference luminance is shown in Fig. 4 where a window is drawn by broken lines. Then, pixel density with respect to each of the corresponding pixels is calculated by the following equation based on the aforementioned equation (1): pixel density Dnm = Log(pixel reference luminance Inm/pixel luminance Qnm) By using equation (2), density for each pixel is obtained. All of the pixel density Dnm, obtained with respect to n=1~n, m=1~m (i.e., density of all pixels in the window) are summed up-to obtain density of the entire window. The window size may be arbitrarily determined in consideration with the size of the line pattern to be measured (it should be noted that the window size must be at least large enough to include the entire line pattern). If the window size is set too large for the line pattern, a pixel around the line pattern, where density is low, will have pixel density Inm≒Qnm. Substituting this into equation (2), the following equation is obtained: pixel density Dnm = Log(Inm/Qnm)≒ Log1 = 0 More specifically, if the window too large in size is marked on the line pattern, pixels around the line pattern where density is low will have pixel density≒0. Thus, even if the pixel density≒0 is added, the sum of the pixel density in the window does not change much. In other words, if the window too large in size is marked on the line pattern, the obtained density will substantially represent only the portion of the line pattern. Therefore, according to the method of the present embodiment, correct density of the line pattern is calculated regardless of the window size. The XY control stage 4 is controlled by the PC 2 to serially read the line pattern. Then, by marking the line pattern with the window, density of the entire line pattern can be obtained. On the basis of the density of the line pattern obtained in the foregoing manner, the amount of ink discharged by single discharging operation of a nozzle, which has printed the line pattern, is obtained by utilizing a calibration line which will be described later. Note that assuming that the line pattern is formed by ink discharged for fifty times, the density of the line pattern obtained above is the sum of density of ink dots corresponding to the fifty times of ink discharged. Therefore, to find an amount of ink discharged by single discharging operation based on the calibration line, the density of the line pattern is divided by the number of times of ink discharged to form the line pattern. Next, description will be provided on a method of finding a calibration line which is to be used as a reference for measuring the amount of ink discharged by an arbitrary nozzle of an ink-jet printhead in a single discharging operation under arbitrary conditions. In the description below, the amount of ink discharged by single discharging operation usually is an ink droplet. However, since there are cases in which the ink does not form a droplet, an expression, "the amount of ink discharged by single discharging operation" is used instead of "an ink droplet." As the first process, amounts of ink discharged from at least two different nozzles among a plurality of nozzles of the ink-jet printhead, subjected to measurement of the ink discharging amount, are measured by using the weighing method or the absorbance method which have been described as the prior art. Herein, it is preferable that each of the at least two different nozzles has as much different discharging amount as possible under a fixed condition. In the present embodiment, the amounts of ink discharged by single discharging operation from four different nozzles, each having different discharging amount under a fixed condition, are measured in advance by using the weighing method. Next, ink is discharged on the glass plate 10 from the four nozzles, whose ink discharging amounts in a single discharging operation are obtained as above, under the same condition as that described above. The density of ink dots (may be a line pattern) formed on the glass plate 10 is measured by the method as described above. By performing the above measurement, the amounts of ink discharged from the four nozzles and the density of the ink dots formed with the discharged ink are obtained in one-to-one relationship. Note that the density data of the ink dots, formed by the four nozzles, is obtained based on an average value of density of 50 samples of printed dots. Standard deviation of the density data in the aforesaid measurement is within 5% with respect to the average value. Fig. 5 is a graph showing the relation between a discharging amount of ink in a single'discharging operation and density of an ink dot formed by the discharged ink on the glass plate 10. In Fig. 5, small black points indicate the amounts of ink discharged by the four nozzles and density of the ink dots. It is seen from Fig. 5 that the four points are positioned in substantially a straight line. Therefore, by drawing a straight line passing through the four points, density of an ink dot corresponding to an arbitrary amount of discharged ink can be obtained on the straight line, in one-to-one relationship. This straight line will be called a "calibration line." Since the calibration line is expressed by a straight line, at least two points plotted on the graph are necessary to obtain the calibration line. Therefore, it is possible to obtain a calibration line by using a minimum of two nozzles, instead of using the four nozzles as described above. However, since data indicative of the amount of discharged ink, measured by the weighing method or the absorbance method, is used to obtain a calibration line, the accuracy of said method of measurement directly affects the accuracy of measurement of the amount of discharged ink. Therefore, it is considered preferable to use more than two nozzles to obtain the calibration line. Further, a calibration line needs to be independently measured each time ink to be used is changed. By measuring the density of the line pattern formed with ink discharged from an arbitrary nozzle under an arbitrary condition in the aforementioned method, it is possible to obtain the amount of ink discharged from each nozzle by referring to the calibration line. Note that if the data indicative of the obtained calibration line is stored in a memory of the PC 2, it is possible to immediately convert the density measurement data to discharging amount data by using the PC 2. Next, description will be provided on a printing apparatus having the above-described function for measuring the amount of discharged ink. Fig. 6 shows a printing apparatus incorporating the apparatus for measuring the amount of discharged ink. In Fig. 6, reference numeral 51 denotes a personal computer (hereinafter referred to as PC) having image processing function for controlling the printing apparatus and the apparatus for measuring the amount of discharged ink; 52, a main body of the printer; 53, a printer stage where a printing medium is set; and 54, an ink-jet type printhead which performs printing by moving left to right. Further, reference numeral 55 denotes a printing medium such as a sheet of paper; 56, a line sensor camera; 57, a microscope for magnifying a printed line pattern; 58, a stage for the microscope (being hollow so that it can utilize a light source);- 59, a light source; 60, a transparent plate such as a glass plate; and 61, a roller used for moving the transparent plate 60 on the stage 58 for the microscope 57. In the foregoing apparatus, the printhead 54 prints on the printing medium 55 while the printhead 54 moves back and forth in the right and left directions. After the printhead 54 has printed for a predetermined time period or the predetermined number of lines, the printhead 54 moves to the transparent plate 60 upon which the printhead 54 prints a line pattern by using the nozzles currently being used. The printed line pattern on the transparent plate 60 is moved under the microscope 57, and the density of the line pattern is measured in accordance with the aforementioned method by using the light source 59 and line sensor camera 56. Next, the PC 51 instantly converts each measured density into the amount of discharged ink, by referring to the calibration line obtained in advance. If the amount of discharged ink is outside of a predetermined range, for instance, a pulsewidth or the like, to be applied to the nozzle of the printhead is changed so as to properly control the amount of ink discharged from the nozzle. In this case, from the time at which the printhead 54 has completed printing a line pattern on the transparent plate 60 until the time the amount of discharged ink is calculated, it is not necessary for the printer to suspend the printing operation, i.e. calculation of the amount of discharged ink and printing operation can be performed in parallel. Further, by developing a print pattern of several lines on an image memory of the apparatus, the PC 51 can predict which nozzle is used continuously for how long time. The PC 51 decides the timing to measure amount of discharged ink based on the prediction. Therefore, there would be a case where the amount of discharge ink is not measured at all during the printing operation, depending on a printing pattern. This series of control can be arbitrarily changed by changing a control program stored in the PC 51. Hereinafter, description will be provided on a configuration of an ink-jet printhead and a method of controlling the amount of ink discharged from the ink-jet printhead. Fig. 7 is an illustration showing the configuration of an ink-jet printhead IJH. Referring to Fig. 7, the ink-jet printhead IJH mainly comprises a heater board 104 as a board on which a plurality of heaters 102 for heating ink are formed, and a ceiling plate 106 mounted on the heater board 104. A plurality of discharging openings (nozzles) 108 are formed in the ceiling plate 106. Tunnel-like fluid passages 110 communicating with the discharging openings 108 are formed therebehind. The respective fluid passages 110 are isolated from the adjacent fluid passages by partition walls 112. The respective fluid passages 110 are commonly connected to one ink chamber 114 at the rear end of the fluid passages. Ink is supplied to the ink chamber 114 via an ink inlet 116, then supplied from the ink chamber 114 to each fluid passage 110. The heater board 104 and the ceiling plate 106 are positioned such that the position of each heater 102 coincides with that of a corresponding fluid passage 110, and are assembled into the state shown in Fig. 7. Although Fig. 7 shows only two heaters 102, the heater 102 is actually arranged in correspondence with each fluid passage 110. When a predetermined driving pulse is supplied to the heater 102 in the assembled state shown in Fig. 7, ink above the heater 102 is led to film boiling and a bubble is produced, and the ink is pushed and discharged from the discharging opening 108 upon volume expansion of the bubble. Therefore, the size of a bubble can be adjusted by controlling the driving pulse applied to the heater 102, e.g., controlling the magnitude of electric power. That is, the volume of the ink discharged from each discharging opening can be controlled as desired. Fig. 8 is a timing chart for explaining a method of controlling the amount of ink discharged by changing electric power supplied to each heater in the aforesaid manner. In this embodiment, two types of constant-voltage pulses are applied to each heater 102 to adjust the amount of discharged ink. The two pulses are a preheat pulse and a main heat pulse (which is simply referred to as a heat pulse hereinafter) as shown in'Fig. 8. The preheat pulse is a pulse for heating the ink to a predetermined temperature before the ink is actually discharged. The pulsewidth of this pulse is set to be smaller than a minimum pulsewidth t5 required to discharge the ink. Therefore, the ink is not discharged by this preheat pulse. The preheat pulse is applied to each heater 102 to increase the initial temperature of the ink to a predetermined temperature in advance so as to always make the amount of discharged ink constant when a constant heat pulse is applied to the heater 102 afterward. In contrast to this, the temperature of the ink may be adjusted in advance by adjusting the width of a preheat pulse. In this case, for the same heat pulse, the amount of ink discharged can be changed. In addition, by heating ink before application of a heat pulse, the preparation time required to discharge the ink upon application of the heat pulse can be shortened, which improves the responsiveness of the printhead to the heat pulse. The heat pulse is a pulse for actually discharging the ink. The pulsewidth of the heat pulse is set to be larger than the minimum pulsewidth t5 required to discharge the ink. Energy generated by each heater 102 is proportional to the width (application time) of a heat pulse. Therefore, variations in the characteristics of the heaters 102 can be adjusted by adjusting the width of each heat pulse. Note that the amount of discharged ink can be also adjusted by adjusting the interval between a preheat pulse and a heat pulse to control the dispersing state of heat upon application of the preheat pulse. As is apparent from the above description, the amount of ink discharged can be controlled both by adjusting the application time of a preheat pulse or a heat pulse and by adjusting the interval between application of a preheat pulse and that of a heat pulse. Therefore, by adjusting the application time of a preheat pulse and a heat pulse or the interval between application of a preheat pulse and that of a heat pulse as needed, the amount of ink discharged or the responsiveness of the printhead discharging the ink to an applied pulse can be adjusted as desired. Such adjustment of the amount of ink discharged will be described in detail next. Assume that the ink is discharged in different amounts from the discharging openings (nozzles) 108a, 108b, and 108c upon application of the same energy, as shown in Fig. 8. More specifically, assume that when predetermined energy is applied at a predetermined temperature, the amount of ink discharged from the nozzle 108a is 36 pl (pico-liters); the amount of ink discharged from the nozzle 108b, 40 pl; and the amount of ink discharged from the nozzle 108c, 40 pl, and the resistance of heaters 102a and 102b respectively corresponding to the nozzles 108a and 108b is 200 Ω, and the resistance of a heater 102c corresponding to the nozzle 108c is 210 Ω. Further, assume that the amounts of ink discharged from the nozzles 108a, 108b, and 108c are to be adjusted to 40 pl. The widths of a preheat pulse and a heat pulse may be adjusted to adjust the amounts of ink discharged from the nozzles 108a, 108b, and 108c to the same amount. Various combinations of the widths of preheat pulses and heat pulses are conceivable. In this embodiment, the amounts of energy generated by heat pulses are made equal for the three nozzles, and the amounts of ink discharged are adjusted by adjusting the widths of preheat pulses. Since the heaters 102a and 102b for the nozzles 108a and 108b have the same resistance, i.e., -200 Ω, the amounts of energy generated by heat pulses can be made equal by applying voltage pulses having the same width to the heaters 102a and 102b. In this embodiment, the width of each voltage pulse is set to be t3 which is longer than the aforementioned width t5. However, the ink is discharged in different amounts, i.e., 36 pl and 40 pl, from the nozzles 108a and 108b upon application of identical energy. In order to increase the amount of ink discharged from the nozzle 108a, a preheat pulse having a width t2, which is longer than a width t1 of a preheat pulse applied to the heater 102b, is applied to the heater 102a. With this operation, the amounts of ink discharged from the heaters 108a and 108b can be adjusted to 40 pl. The heater 102c for the nozzle 108c has a resistance of 210 Ω, which is higher than the resistance of the two other heaters 102a and 102b. For this reason, in order to cause the heater 102c to generate the same amount of energy as that generated by the two other heaters, the width of a heat pulse must be set to be longer than that of the above heat pulse. In this embodiment, therefore, the width of the heat pulse is set to be t4 which is longer than the aforementioned width t3. Since the amounts of ink discharged from the nozzles 108b and 108c upon application of the same amount of energy are the same, the width of a preheat pulse required is equal to that of a preheat pulse applied to the heater 102b. That is, a preheat pulse having the width t1 is applied to the heater 102c. In the above manner, the same amount of ink can be discharged from the nozzles 108a, 108b, and 108c which discharge ink in different amounts upon application of a predetermined energy to corresponding heaters having different resistance from each other. In addition, the amounts of discharged ink may be intentionally made different from each other. Note that preheat pulses are used to reduce variations in the amount of ink discharged from each nozzle. Next, two typical methods for reducing unevenness in printing by an ink-jet printhead will be described. Figs. 9 to 11 show a method of correcting differences between amounts of ink discharged from a plurality of nozzles of the ink-jet printhead IJH (called "bit correction", hereinafter). First, as shown in Fig. 9, three nozzles, a nozzle 1, a nozzle 2 and a nozzle 3, for example, of the ink-jet printhead IJH discharge ink onto a predetermined plate P. Then, the sizes of ink dots made of the ink discharged from respective nozzles 1, 2 and 3 on the plate P are measured, and the amount of ink discharged from each nozzle is found. Upon measuring the amount of ink discharged, heat pulses (referring to Fig. 8) applied to heaters of the nozzles are first set to a fixed pulsewidth, and the preheat pulsewidths (referring to Fig. 8) are changed as has been described above. As a result, as shown in Fig. 10, curves showing relationship between preheat pulsewidths (referred to as "heating time period" in Fig. 10) and the amounts of ink discharged are obtained. If it is desired that the amount of ink discharged from each nozzle is to be fixed to 20 ng, then it can be found from the graph shown in Fig. 10 that the pulsewidth to be applied to the nozzle 1 is 1.0 µs, to the nozzle 2, 0.5 µs, and to the nozzle 3, 0.75 µs. Therefore, by applying the preheat pulses having above pulsewidths to the heaters of the nozzles, it is possible to adjust the amount of ink discharged from each nozzle to a fixed amount of 20 ng. To correct the amount of ink discharged from each nozzle as described above is called bit correction. In this embodiment, the pulsewidth of the preheat pulse is changed in four stages, which achieves correction range of the amount of ink discharged by about 30% of the amount. Further, the resolution of the correction is between 2% and 3%. Next, Figs. 12 to 14 show a method of correcting unevenness in printing in the scanning direction of the ink-jet printhead (called "shading correction", hereinafter) by adjusting density of dots (i.e., the number of dots printed in a unit area) printed by ink-discharging nozzles. As shown in Fig. 12, for example, when the amount of ink discharged from the nozzle 3 of the ink-jet printhead is defined as reference, assume that the amount of ink discharged from the nozzle 1 is about 10% less than the reference, and the amount of ink discharged from the nozzle 2 is about 20% more than the reference. Under this condition, while the ink-jet printhead IJH scans, heat pulses are applied to the heater of the nozzle 1 once every nine reference clocks; to the heater of the nozzle 2, once every 12 reference clocks; and to the heater of the nozzle 3, once every 10 reference clocks as shown in Fig. 13. Thus, the number of discharging operations in the scanning direction can be adjusted for each nozzle, thereby it is possible to set density of printed ink dots in the scanning direction to a uniform density as shown in Fig. 14, thus preventing unevenness in printing. To correct density of printed ink dots in the scanning direction as described above is called "shading correction". In this embodiment, this correction achieves correction range of the density of ink discharged by about 40% of the density. Further, it is possible to control the distance between each dots to be infinitely short theoretically <i.e., to increase resolution). However, if doing so, the amount- of data greatly increases, which makes the processing speed slower. Therefore, about 10% increase in resolution is the substantial limitation. Upon measuring the amount of ink discharged by using the measuring apparatus according to the first embodiment and actually performing printing operation by a printing apparatus in accordance with the measured results, it is possible to correct unevenness in density, caused by difference in the amount of ink discharged from each nozzle, by properly adjusting the amount of ink discharged by using the method of the changing pulsewidth of a preheat pulse or the method of changing density of printed dots. <Second Embodiment> In the first embodiment, a line sensor camera is used to read a line pattern image. Besides the line sensor camera, a CCD camera or other area sensor may be used. Increasing tones of luminance levels (256 tones in the first embodiment, since an A/D converter having 8 bits is used) to e.g., 16 bits, may further increase accuracy of measured data. Furthermore, the image pattern to be printed on a glass plate is not limited to a line pattern, but may be a dot pattern. In this case, printing must be performed under the same condition of driving, and a size of the window to be marked on the printed image which has been inputted in the image processing unit must be large enough to include all the dots. Each of the embodiments described above has exemplified a printer, which comprises means (e.g., an electrothermal transducer, laser beam generator, and the like) for generating heat energy as energy utilized upon execution of ink discharge, and causes a change in the state of ink by the heat energy, among the ink-jet printers. According to this ink-jet printer and printing method, a high-density, high-precision printing operation can be attained. As the typical arrangement and principle of the ink-jet printing system, one practiced by use of the basic principle disclosed in, for example, U.S. Patent Nos. 4,723,129 and 4,740,796 is preferable. The above system is applicable to either one of so-called an on-demand type and a continuous type. Particularly, in the case of the on-demand type, the system is effective because, by applying at least one driving signal, which corresponds to printing information and gives a rapid temperature rise exceeding film boiling, to each of electrothermal transducers arranged in correspondence with a sheet or liquid channels holding a liquid (ink), heat energy is generated by the electrothermal transducer to effect film boiling on the heat acting surface of the printhead, and consequently, a bubble can be formed in the liquid (ink) in one-to-one correspondence with the driving signal. By discharging the liquid (ink) through a discharge opening by growth and shrinkage of the bubble, at least one droplet is formed. If the driving signal is applied as a pulse signal, the growth and shrinkage of the bubble can be attained instantly and adequately to achieve discharge of the liquid (ink) with the particularly high response characteristics. As the pulse-form driving signal, signals disclosed in U.S. Patent Nos. 4,463,359 and 4,345,262 are suitable. Note that further excellent printing can be performed by using the conditions described in U.S. Patent No. 4,313,124 of the invention which relates to the temperature rise rate of the heat acting surface. As an arrangement of the printhead, in addition to _ the arrangement as a combination of discharge nozzles, liquid channels, and electrothermal transducers (linear liquid channels or right angle liquid channels) as disclosed in the above specifications, the arrangement using U.S. Patent Nos. 4,558,333 and 4,459,600, which disclose the arrangement having a heat acting portion arranged in a flexed region is also included in the present invention. In addition, the present invention can be effectively applied to an arrangement based on Japanese Patent Laid-Open No. 59-123670 which discloses the arrangement using a slot common to a plurality of electrothermal transducers as a discharge portion of the electrothermal transducers, or Japanese Patent Laid-Open No. 59-138461 which discloses the arrangement having an opening for absorbing a pressure wave of heat energy in correspondence with a discharge portion. Furthermore, as a full line type printhead having a length corresponding to the width of a maximum printing medium which can be printed by the printer, either the arrangement which satisfies the full-line length by combining a plurality of printheads as disclosed in the above specification or the arrangement as a single printhead obtained by forming printheads integrally can be used. In addition, not only an exchangeable chip type printhead, as described in the above embodiment, which can be electrically connected to the apparatus main unit and can receive ink from the apparatus main unit upon being mounted on the apparatus main unit but also a cartridge type printhead in which an ink tank is integrally arranged on the printhead itself can be applicable to the present invention. It is preferable to add recovery means for the printhead, preliminary auxiliary means, and the like provided as an arrangement of the printer of the present invention since the printing operation can be further stabilized. Examples of such means include, for the printhead, capping means, cleaning means, pressurization or suction means, and preliminary heating means using electrothermal transducers, another heating element, or a combination thereof. It is also effective for stable printing to provide a preliminary discharge mode which performs discharge independently of printing. Moreover, in each of the above-mentioned embodiments of the present invention, it is assumed that the ink is a liquid. Alternatively, the present invention may employ ink which is solid at room temperature or less, or ink which softens or liquefies at room temperature, or ink which liquefies upon application of a printing signal, since it is a general practice to perform temperature control of the ink itself within a range from 30°C to 70°C in the ink-jet system, so that the ink viscosity can fall within a stable discharge range. In addition, in order to prevent a temperature rise caused by heat energy by positively utilizing it as energy for causing a change in state of the ink from a solid state to a liquid state, or to prevent evaporation of the ink, ink which is solid in a non-use state and liquefies upon heating may be used. In any case, ink which liquefies upon application of heat energy according to a printing signal and is discharged in a liquid state, ink which begins to solidify when it reaches a printing medium, or the like, is applicable to the present invention. In this case, ink may be situated opposite to electrothermal transducers while being held in a liquid or solid state in recess portions of a porous sheet or through holes, as described in Japanese Patent Laid-Open No. 54-56847 or 60-71260. In the present'invention, the above-mentioned film boiling system is most effective for the above-mentioned inks. As has been set forth above, according to the present invention, by measuring density of an ink dot or a line of a line pattern formed with ink discharged from a printhead and finding the amount of ink discharged on the basis of the measured density, the amount of ink discharged can be obtained instantly for each discharging operation. Moreover, by calculating density of each pixel in the line pattern and integrating the density of each pixel for the entire line pattern, even in a case where the density of the line pattern is not uniform in the same line pattern, it is possible to obtain correct density of the entire line, thus, obtaining a correct amount of discharged ink. Further, by finding correlation between the amount of discharged ink and the density of an ink dot formed by the discharged ink in advance, the amount of ink discharged from an arbitrary nozzle can be easily found by referring to the correlation and the measured density of the ink dot or the line pattern formed by the ink discharged from the arbitrary nozzle. Furthermore, it is possible to instantly obtain the density of the ink dot or the line pattern by forming ink dots or a line pattern on a transparent plate, inputting an image of them by using a camera while illuminating the ink dots or the line pattern with light, and applying image processing to the input image. Further, by using at least two nozzles whose discharging amounts are known, and finding a calibration line showing relationship between the density of ink dot formed by ink discharged by the nozzles and the amount of ink discharged in advance, the amount of ink discharged from an arbitrary nozzle under an arbitrary condition can be determined from the calibration line and the density of an ink dot or a line of a line pattern formed with the ink discharged from the arbitrary nozzle. Moreover, in order to determine the amount of ink discharged from a nozzle in advance, the weighing method or the absorbance method can be used, thereby the amount of ink discharged from the nozzle can be correctly determined. Furthermore, it is possible to measure density of a plurality of dots or lines of a line pattern continuously by providing the XY stage or the X stage capable of moving with respect to a camera which performs image processing on an ink absorbent medium. Accordingly, amounts of ink discharged from a plurality of. nozzles of a printhead can be measured continuously. Furthermore, by incorporating the apparatus for measuring the amount of ink into a printing apparatus, it becomes possible to feed back data on the discharged amount of ink obtained in the measuring apparatus to the printing apparatus so that the data is used to control the amount of ink to be discharged by a uniform amount. Accordingly, printing quality can be improved.

Claims: A method of measuring an amount of ink discharged from a nozzle (108) of an ink-jet type printhead (54) in a single discharging operation, the method comprising: a discharging step of discharging an amount of ink from a nozzle (108) of a printhead (54) onto a predetermined material (10); a first image-sensing step of sensing a first image using an image sensing device.(5), the first image being of the ink discharged on the predetermined material (10) in said discharging step; a second image-sensing step of sensing a second image using the image sensing device (5), the second image being of a portion of the predetermined material (10) away from the discharged ink; a density calculating step of calculating a density of the ink discharged in said discharging step; and an ink-discharge-amount determining step of determining said amount of ink discharged in said discharging step on the basis of the density calculated in the density calculating step, characterised by said density calculating step comprising: for each pixel sensed by the image sensing device (5), calculating a pixel density by calculating a logarithm of a luminance value for that pixel in the second image sensed in said second image-sensing step divided by a luminance value for that pixel in the first image sensed in said first image-sensing step; and integrating the pixel densities calculated for the pixels sensed by the image sensing device to determine said density of the discharged ink. The method of measuring an amount of discharged ink according to claim 1, wherein the discharging step comprises forming a line pattern on the predetermined material. The method of measuring the amount of discharged ink according to either claim 1 or claim 2, further comprising a preliminary measuring step of preliminarily obtaining a correlation between the amount of ink discharged by a single discharging operation and a density of an ink dot formed by the ink, before said ink-discharge-amount determining step. The method of measuring the amount of discharged ink according to any preceding claim, wherein the predetermined material is a transparent plate (10), and wherein said first and second image-sensing steps comprise emitting light from a light source placed behind the transparent plate and sensing light transmitted through the transparent plate. The method of measuring the amount of discharged ink according to any preceding claim, further comprising a discharge-amount adjusting step of adjusting the amount of discharged ink to a desired amount on the basis of the amount of discharged ink determined in said determining step, after said ink-discharge-amount determining step. The method of measuring the amount of discharged ink according to claim 5, further comprising a discharging step of discharging ink to a print medium (55) after said discharge-amount-adjusting step. The method of measuring the amount of discharged ink according to claim 1 or claim 2, further comprising: a preliminary measuring step of preliminarily measuring the amount of ink discharged by a single discharging operation from each of a plurality of ink discharging nozzles (108) of the printhead (54) under a predetermined condition; a first density measuring step of discharging ink onto a predetermined material from at least two nozzles, whose discharging amounts are different from one another, of the plurality of ink discharging nozzles under the predetermined condition, and measuring the density of ink dots formed with ink discharged by said two nozzles (108); and a calibration-line generating step of obtaining a calibration line, representing a correlation between the amount of ink discharged by single discharging operation and density of an ink dot, based on the data indicative of the density of the ink dots formed with ink discharged by the two nozzles (108) obtained in said first density measuring step and the data indicative of the amount of ink obtained in said preliminary measuring step with respect to said at least two nozzles (108). The method of measuring the amount of discharged ink according to claim 7, wherein in said preliminary measuring step, the amount of ink discharged from each of the plurality of ink discharging nozzles (108) is measured by a weighing method or an absorbance method. The method of measuring the amount of discharged ink according to any preceding claim, wherein the method is performed by a printing apparatus (52). An apparatus for measuring an amount of ink discharged from a nozzle of an ink-jet type printhead onto a receiving medium (10) in a single discharging operation, the apparatus comprising: an image sensing device (5) operable to sense a first image of an amount of ink discharged on the receiving medium (10), and a second image of a portion of the receiving medium (10) away from said amount of discharged ink; first calculating means (1) operable to calculate a density of said ink discharged on the receiving medium (10); memory means (1) for storing a calibration line, representing a correlation between said amount of discharged ink and said density calculated by the first calculating means (1); and second calculating means (1) operable to calculate said amount of discharged ink on the basis of said density calculated by the first calculating means (1) and the calibration line, characterised in that said first calculating means (1) is operable to calculate a pixel density for each pixel sensed by the image sensing device (5) by calculating a logarithm of a luminance value for that pixel in the second image divided by a luminance value for that pixel in the first image, and to integrate the calculated pixel densities to determine said density of the discharged ink. The apparatus for measuring an amount of discharged ink according to claim 10, wherein the image sensing device (5) is operable to sense a line pattern formed on the receiving medium (10) by said discharged ink. The apparatus for measuring an amount of discharged ink according to claim 10 or claim 11, wherein said receiving medium (10) is a transparent plate, and said apparatus further comprises a light source (6) for emitting light on the discharged ink from behind the receiving medium (10). The apparatus for measuring an amount of discharged ink according to any of claims 10 to 12, further comprising an XY stage (4) for moving said receiving medium relative to said image sensing device (5), automatically and consecutively measuring integrated density of a plurality of patterns of discharged ink formed on said receiving medium (10), and consecutively obtaining each amount of discharged ink which has formed the plurality of patterns. The apparatus for measuring an amount of discharged ink according to any of claims 10 to 13, further comprising adjusting means (51) for adjusting the amount of ink discharged from the printhead on the basis of the amount of discharged ink calculated by said second calculating means (1). A printing apparatus for performing printing by utilizing an ink-jet printhead (54), the printing apparatus comprising an apparatus for measuring an amount of ink discharged from the ink jet printhead as claimed in any of claims 10 to 14.

Inventors: AKAHIRA MAKOTO, SAITO TADAO, WADA SATOSHI, AKAHIRA, MAKOTO, SAITO, TADAO, WADA, SATOSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: B41J2
IPC codes: B41J2