Patent Publication Number: US-7909426-B2

Title: Apparatus and method for inspecting droplet discharge characteristics of ink-jet printed head

Description:
This application is a 35 U.S.C. §371 National Stage entry of International Application No. PCT/KR2007/005666, filed on Nov. 12, 2007, and claims the benefit of Korean Application No. 10-2006-0111756, filed on Nov. 13, 2006 which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to apparatus and method for inspecting droplet discharge characteristics of an ink-jet printer head, and more particularly to apparatus and method for inspecting droplet discharge characteristics of an ink-jet printer head by photographing an overlapped droplet image for a plurality of ink droplets successively discharged from the ink-jet printer head. 
     BACKGROUND ART 
     Generally, an ink-jet printer may realize various colors with less noise due to the use of a cartridge and also ensures beautiful quality of printed text, differently from a dot printer, so the ink-jet printer is used more and more. 
     An ink-jet printer head is a means for discharging fine-droplets-of a printing ink to a desired position on a recording paper to print an image with a predetermined color. For this purpose, an ink droplet is discharged from a nozzle installed to the printer head, and the discharged ink droplet is hit to the recording paper. As intervals of hit droplets are closer, a higher quality of image may be output. 
     The printing quality using discharged ink droplets may be inspected using on characteristic values such as brightness and resolution of image, and these characteristic values depend on discharge of ink droplet of the ink-jet printer head, namely droplet discharge characteristics (or, jetting characteristics). 
     The droplet discharge characteristics of the ink-jet printer head act as an important factor to verify reliability of an ink-jet printer, and the droplet discharge characteristics of an ink-jet printer head are generally inspected before the printing process. 
     As a droplet discharge characteristic inspecting method, there was used a method of hitting an ink discharged from an ink-jet printer head to a paper, and then detecting a hit position of the ink printed on the paper. 
     However, in this method, a maximum number of ink droplets capable of being printed on an A4-sized paper without overlapping is 300,000, and the number of nozzles provided to an ink-jet printer head is about 100. Thus, in case jetting is conducted several thousand times at each nozzle, the number of droplets may exceed a capacity of one paper, so it may be impossible to determine whether jetting is conducted stably for several hours. 
     In addition, to solve this problem, there is a method of conducting the printing on a paper roll of several meters, instead of a paper sheet. However, this method has. problems of long-time consumption and much usage of paper, though position errors of hit ink droplets may be distinctly observed for each ink droplet. 
     As another method, there was proposed an ink droplet checking method that analyzes size and interval of sprayed ink droplets while scanning them according to a nozzle heat of a printer head using a digital camera. 
     This ink droplet checking method may allow to measure an abnormal jetting error caused by clogged nozzle or contamination, but it is not suitable for measuring a precise deviation required for high precision printing, namely quantitative analysis of droplet discharge characteristics according to velocity and orientation of ink droplets. 
     As another method, Japanese Laid-open Patent Publication No. 1999-227172 discloses a discharge checking device of an ink-jet printer head, including a printer head driving circuit, a camera, a camera control circuit, a stroboscope, a time delay circuit and a measuring circuit, which photographs an ink droplet discharged from a printer head several times with time intervals to measure a velocity of the ink droplet from a time difference of the ink droplet photograph. 
     This technique allows measuring an error of discharging timing since a velocity of ink droplet is measured. However, by simply measuring deviation of discharging timing, it is impossible to measure an error of discharging orientation, namely to measure a trajectory of ink droplet discharged from the printer head to a hit point, and also it is impossible to check a composite fine deviation error required for high precision printing. 
     In particular, simple image observation may allow detecting a serious jetting inferiority, but it has a limit in inspecting stability of ink jetting, required for high precision printing of a substrate that needs precision. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide apparatus and method for inspecting droplet discharge characteristics of an ink-jet printer head, which quantitatively analyzes an overlapped droplet image for a plurality of ink droplets,. photographed at the same position, thereby allowing quantitative precise inspection of inferior factors of minute ink-jetting quality. 
     Technical Solution 
     In order to accomplish the above object, there is provided an apparatus for inspecting droplet discharge characteristics of an ink-jet printer head that subsequently discharges ink droplets, which includes a photographing means for operating a digital camera to generate an overlapped droplet image for a plurality of ink droplets at a predetermined photographing point before each of the subsequently discharged ink droplets hits a target point; a light emitting means for operating a speed light to give a light to an ink droplet that passes the photographing point after a predetermined delay time from a discharging time point of each ink droplet; and a droplet discharge characteristic inspecting means for calculating a signal-to-noise ratio distribution of each pixel of the overlapped droplet image, calculating an overlap rate of droplet images of each pixel from the calculated signal-to-noise ratio distribution to calculate a center coordinate of a predetermined number of droplet images, and quantitatively calculating and outputting deviation to droplet jetting velocity and direction using the calculated center coordinate of droplet image, a droplet jetting coordinate and the delay time. 
     Preferably, the droplet discharge characteristic inspecting means includes an image receiving module for receiving the overlapped droplet image from the photographing means; a signal-to-noise ratio calculating module for calculating a signal-to-noise ratio of each pixel of the overlapped droplet image; an overlap rate calculating module for calculating a droplet image overlap rate of each pixel from the signal-to-noise ratio of each pixel; a droplet position calculating module for calculating a center coordinate of a predetermined number of droplet images by statistically analyzing droplet size, position distribution of pixels with the same overlap rate of each pixel, and the number of photographed droplets; and a droplet discharge characteristic value calculating module for quantitatively calculating deviation for droplet jetting velocity and direction from the center coordinate of the droplet image, a droplet jetting coordinate and the delay time. 
     Preferably, the deviation is a standard deviation of the jetting velocity and direction with respect to a predetermined number of droplets. 
     The apparatus for inspecting droplet discharge characteristics of an ink-jet printer head according to the present invention may further include a jetting stability inspecting module for determining stability of the droplet discharge characteristics depending on whether the standard deviation exceeds a criterion value, and then outputting the determination result through an external device. 
     Selectively, the signal-to-noise calculating module may output the signal-to-noise ratio distribution of each pixel of the overlapped droplet image through an external device. 
     In another aspect of the present invention, there is also provided an apparatus for inspecting droplet discharge characteristics of an ink-jet printer head that subsequently discharges ink droplets, which includes a photographing means for operating a digital camera to generate an overlapped droplet image for a plurality of ink droplets at a predetermined photographing point before each of the subsequently discharged ink droplets hits a target point; a light emitting means for operating a speed light to give a light to an ink droplet that passes the photographing point after a predetermined delay time from a discharging time point of each ink droplet; and a droplet discharge characteristic inspecting means for presuming a center coordinate of a pixel with a predetermined gray level in a long or short axis direction of the overlapped droplet image and a center coordinate of the overlapped droplet image as a center coordinate of a droplet whose discharge characteristics will be inspected, and quantitatively calculating and outputting deviation of droplet jetting velocity and direction using the presumed center coordinate of droplet, a droplet jetting coordinate and the delay time. 
     Preferably, the droplet discharge characteristic inspecting means calculates a histogram distribution according to the gray level in a long or short axis direction of the overlapped droplet image, and then presuming a coordinate of a pixel corresponding to kσ (k is a constant, σ is a standard deviation) on the histogram distribution and a center coordinate of the overlapped droplet image as a center coordinate of a droplet whose discharge characteristics will be inspected. 
     Preferably, the deviation of the droplet jetting velocity and direction is a standard deviation of jetting velocity and direction of the droplet whose center coordinate is presumed. 
     Selectively, the droplet discharge characteristic inspecting means may determine stability of discharge characteristics depending on whether the standard deviation exceeds a criterion value, and then output the determination result through an external device. 
     In still another aspect of the present invention, there is also provided a method for inspecting droplet discharge characteristics of an ink-jet printer head, which includes obtaining an overlapped droplet image by photographing a plurality of ink droplets successively discharged from the ink-jet printer head at the same photographing point; calculating a signal-to-noise ratio distribution of each pixel of the overlapped droplet image; calculating a center coordinate of a predetermined number of droplet images by calculating a droplet image overlap rate of each pixel from the calculated signal-to-noise ratio distribution; and quantitatively calculating and outputting deviation of jetting velocity and direction of the droplet by using the calculated center coordinate of droplet, a droplet jetting coordinate, and a delay time between a droplet jetting time point and a light giving time point for photographing the droplet. 
     In further another aspect of the present invention, there is also provided a method for inspecting droplet discharge characteristics of an ink-jet printer head, which includes obtaining an overlapped droplet image by photographing a plurality of ink droplets successively discharged from the ink-jet printer head at the same photographing point; presuming a center coordinate of a pixel with a predetermined gray level in a long or short axis direction of the overlapped droplet image and a center coordinate of the overlapped droplet image as a center coordinate of a droplet whose discharge characteristics will be inspected; and quantitatively calculating and outputting deviation of droplet jetting velocity and direction using the presumed center coordinate of droplet, a droplet jetting coordinate and a delay time between a droplet jetting time point and a light giving time point for photographing the droplet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawing in which: 
         FIG. 1  is a block diagram schematically showing an apparatus for inspecting droplet discharge characteristics of an ink-jet printer head according to a preferred embodiment of the present invention; 
         FIG. 2  is a time chart illustrating applying time points of a driving signal, a photographing control signal and a light emission control signal according to a preferred embodiment of the present invention; 
         FIG. 3  is a block diagram schematically showing a droplet discharge characteristic inspecting means according to a preferred embodiment of the present invention; 
         FIGS. 4 to 6  are schematic views showing the phenomenon that a signal-to-noise ratio is increased according to an overlap rate of droplet images in an overlapped droplet image; 
         FIG. 7  is a flowchart illustrating the process of generating an overlapped droplet image according to a preferred embodiment of the present invention; and 
         FIG. 8  is a flowchart subsequently illustrating the process of inspecting droplet discharge characteristics of an ink-jet printer head according to a preferred embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, preferred embodiments of the present invention will be described in detailed with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention. 
     In the embodiment of the present invention, the term ‘jetting stability’ of an ink-jet printer head is defined as corresponding to ‘hit accuracy’ of an ink droplet discharged from a nozzle provided to the ink-jet printer head. 
       FIG. 1  is a block diagram schematically showing an apparatus for inspecting droplet discharge characteristics of an ink-jet printer head according to a preferred embodiment of the present invention. 
     Referring to  FIG. 1 , the apparatus for inspecting droplet discharging characteristics of an ink-jet printer head according to the present invention includes an operating module  100  for controlling operation of an ink-jet printer head  101  from which an ink droplet A is discharged, a light emitting module  200  for controlling light emission of a speed light  201  that irradiates a light toward the discharged ink droplet A, a photographing module  300  for controlling operation of a digital camera  301  that photographs a plurality of ink droplets A passing a predetermined photographing point, a droplet discharge characteristic inspecting means  400  for inspecting droplet discharge characteristics of an ink droplet using an overlapped droplet image of the plurality of ink droplets A, photographed by the digital camera  301 . 
     The operating module  100 , the light emitting module  200  and the photographing module  300  are configured as a circuit for transmitting/receiving control signals of a control module  500 . As an example, the operating module  100 , the light emitting module  200 , the photographing module  300  and the control module  500  may be provided as a PCB (Printed Circuit Board), but not limitedly. The control module  500  applies an operating signal to the operating module  100 , and also applies a light emission control signal and a photographing control signal, synchronized with the operating signal, to the light emitting module  200  and the photographing module  300 , respectively. 
     Here, an applying time point of the operating signal is substantially identical to the time point that the ink droplet A is discharged from the ink-jet printer head  101 . An applying time point of the light emission control signal is a predetermined time later than the applying time point of the operating signal. A time difference between the light emission control signal applying time point and the operating signal applying time point is corresponding to the time consumed during which the ink droplet A passes the droplet photographing point of the digital camera  301  after being discharged from the ink-jet printer head  101 . An applying time point of the photographing control signal is synchronized with the applying time point of the operating signal, but it is ahead of the applying time point of the light emission control signal. In addition, an applying period of the photographing control signal is longer than an applying period of the light emission control signal. Thus, while the photographing control signal is applied one time, a plurality of light emission control signals are applied. The operating module  100  operates the ink-jet printer head  101  whenever receiving an operating signal from the control module  500  such that an ink droplet A is discharged from a nozzle (not shown) and then hit to a targeted point of a target B. Here, the target B is preferably a substrate used for making an electronic circuit or a display device, but not limitedly. 
     The light emitting module  200  operates the speed light  201  whenever receiving a light emission control signal from the control module  500  such that a light for photographing an ink droplet A is provided. 
     The photographing module  300  operates the digital camera  301  whenever receiving a photographing control signal from the control module  500  such that a plurality of ink droplets A passing a predetermined photographing point is photographed as one frame image. Since a plurality of light emission control signals are applied while one photographing control signal is applied, the digital camera  301  generates an overlapped droplet image for the plurality of ink droplets A passing the photographing point. That is to say, since the digital camera  301  keeps its exposed state while the plurality of ink droplets A are passing, the digital camera  301  photographs images of the ink droplets A as much as the number of light emission control signals applied-while the exposed state is kept, thereby generating an overlapped droplet image. The overlapped droplet image has a pattern that a plurality of droplet images are overlapped. If the overlapped droplet image is generated, the photographing module  300  outputs it to the droplet discharge characteristic inspecting means  400 . 
     Here, the speed light  201  and the digital camera  301  are preferably installed to face each other based on the photographing point. The digital camera  301  is preferably an image sensor camera having CCD or CMOS, and the speed light  201  is preferably a Strobo light that generates a streamer instantly according to the application of a light emission control signal. 
     The droplet discharge characteristic inspecting means  400  calculates a signal-to-noise ratio distribution of the overlapped droplet image photographed by the digital camera  301  according to the control of the control module  500 , calculates a droplet image overlap rate of each pixel from the calculated signal-to-noise ratio distribution, and quantitatively inspects and outputs characteristics of jetting velocity and jetting direction of the ink droplet. 
       FIG. 2  is a time chart illustrating applying time points of an operating signal, a light emission control signal and a photographing control signal while droplet discharge characteristics of the ink-jet printer head  101  are inspected. 
     Referring to  FIGS. 1 and 2 , the control module  500  applies operating signals (a) to the operating module  100  at time points T 1 , T 2 , T 3 , . . . such that ink droplets A are discharged from the ink-jet printer head  101 . The control module  500  applies a photographing control signal (β) to the photographing module  300  after a predetermined time (Δt 1 ) passes from the time point T 1 , thereby operating a shutter of the digital camera  301 . The digital camera  301  keeps an exposed state for a predetermined time from the time point that the photographing control signal (β) is applied thereto. In this state, the control module  500  applies a light emission control signal (γ) to the light emitting module  200  after a predetermined time (Δt 2 ) passes from the time point T 1 . The light emission control signal (γ) is synchronized with the time point that the ink droplet A passes the photographing point, and it is applied several times while the digital camera  301  keeps its exposed state. Thus, the digital camera  301  photographs an overlapped droplet image for a plurality of ink droplets A by just one exposure. 
       FIG. 3  is a block diagram schematically showing the droplet discharge characteristic inspecting means  400  according to the present invention. 
     Referring to  FIG. 3 , the droplet discharge characteristic inspecting means  400  quantitatively inspects droplet discharge characteristics of the ink-jet printer head  101  according to the control of the control module  500  and then outputs the inspection result. 
     The droplet discharge characteristic inspecting means  400  includes an image receiving module  410 , a signal-to-noise ratio calculating module  420 , an overlap rate calculating module  430 , a droplet position calculating module  440 , a droplet discharge characteristic calculating module  450 , and a jetting stability inspecting module  460 . 
     The image receiving module  410  receives an overlapped droplet image from the photographing module  300  and then outputs it to the signal-to-noise ratio calculating module  420 . 
     The signal-to-noise ratio calculating module  420  calculates a signal-to-noise ratio of each pixel of the overlapped droplet image. The signal-to-noise ratio is calculated based on Gray level analysis. However, the present invention is not limited thereto. 
     The signal-to-noise ratio of each pixel of the overlapped droplet image is increased in proportion to the number of overlapping of the droplet images. It may be explained using the ensemble average theory. That is to say, when n number of droplet images are overlapped, the sum of signals of each pixel included in the overlapped droplet image is n times of the corresponding pixel signal S x  of an individual droplet image, and noise of each pixel is
 
√{square root over (n)}
 
time of noise N x  of the corresponding pixel of an individual droplet image due to the random characteristic. Thus, a signal-to-noise ratio of each pixel of the overlapped droplet image is equal to a value obtained by multiplying a square root
 
√{square root over (n)}
 
of the overlap number n by the signal-to-noise ratio of the individual droplet image as seen in the following equation 1.
 
     
       
         
           
             
               
                 
                   
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     In the equation 1, S/N is a signal-to-noise ratio of each pixel of the overlapped droplet image, and S x /N x  is a signal-to-noise ratio of each pixel of the individual droplet image. 
       FIGS. 4 to 6  are schematic views showing the concept that a signal-to-noise ratio is increased according to the number of overlap of an individual droplet image in the overlapped droplet image. 
       FIG. 4  shows the case that a direction deviation of ink jetting is generated, and thus droplet pass different positions in a horizontal direction when the speed light is operated according to the application of a light emission control signal. If an overlapped droplet image is generated as shown in  FIG. 4 , three droplet images are overlapped in a pixel corresponding to A 1  region, two droplet images are overlapped in a pixel corresponding to A 2  region, and no overlap of droplet image occurs in a pixel corresponding to A 3  region. Thus, image flowing is decreased and a signal-to-noise ratio is increased from the A 3  region to the A 1  region. 
       FIG. 5  shows the case that a velocity deviation of ink jetting is generated, and thus droplets pass different positions in a vertical direction when the speed light is operated according to the application of a light emission control signal. If an overlapped droplet image is generated as shown in  FIG. 5 , three droplet images are overlapped in a pixel corresponding to A 1  region, two droplet images are overlapped in a pixel corresponding to A 2  region, and no overlap of droplet image occurs in a pixel corresponding to A 3  region. Thus, image flowing is decreased and a signal-to-noise ratio is increased from the A 3  region to the A 1  region. 
       FIG. 6  shows the case that a velocity deviation and a direction deviation of ink jetting are generated at the same time, and thus droplets pass different positions in a diagonal direction when the speed light is operated according to the application of a light emission control signal. If an overlapped droplet image is generated as shown in  FIG. 6 , three droplet images are overlapped in a pixel corresponding to A 1  region, two droplet images are overlapped in a pixel corresponding to A 2  region, and no overlap of droplet image occurs in a pixel corresponding to A 3  region. Thus, image flowing is decreased and a signal-to-noise ratio is increased from the A 3  region to the A 1  region. 
     The signal-to-noise ratio calculating module  420  may generate a signal-to-noise ratio distribution by giving the same color to pixels with the same signal-to-noise ratio in the overlapped droplet image and giving more shading as the signal-to-noise ratio is greater, and then output the signal-to-noise ratio distribution to an external device. Here, the external device may be a printing deice or a display device, well known in the art. 
     As shown in  FIGS. 4 to 6 , the signal-to-noise ratio distribution of the overlapped droplet image has an inherent pattern as deviation is generated in jetting velocity and/or direction. If deviation of jetting velocity or direction is not generated, all droplet images are photographed at the same position, so there would be no orientation in droplet image arrangement. Thus, by analyzing the pattern of the signal-to-noise ratio distribution, it is possible to easily check whether a cause of inferiority is in velocity deviation of ink jetting, direction deviation of ink jetting, or both of them. 
     The overlap rate calculating module  430  calculates a droplet image overlap rate of each pixel from the signal-to-noise ratio of each pixel. Here, the droplet image overlap rate is a quantitative factor indicating how much droplet images are overlapped in the corresponding pixel. If the droplet image overlap rate is great, it means a lot of droplet images are overlapped as much. The droplet image overlap rate of each pixel may be calculated using the following equation 2, but not limitedly. 
     
       
         
           
             
               
                 
                   
                     
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     In the equation 2, S n-i /N n  is a signal-to-noise ratio of each pixel of the overlapped droplet image photographed according to the present invention, S n /N n  is a signal -to-noise ratio of each pixel under assumption that all droplet images are completely overlapped, and n and i are a total number of droplet images and a number of non-overlapped droplet images, respectively. A droplet image overlap rate of each pixel is calculated as ratios of S n-i /N n  and S n /N n . 
     Meanwhile, the overlap rate calculating module  430  may obtain a signal-to-noise ratio according to an overlap number of droplet images, then configure the overlap number of droplet images and the signal-to-noise ratio into a lookup table format, and then calculate an overlap rate of each pixel of the overlapped droplet image with reference to the lookup table. 
     The overlap rate calculating module  430  calculates an overlap rate of each pixel and then outputs it to the droplet position calculating module  440 . Then, the droplet position calculating module  440  statistically analyzes droplet size, distribution of pixels with the same overlap rate as each pixel of the overlapped droplet image, and the number of photographed droplets, and then calculates a center coordinate of each droplet image of the overlapped droplet image. That is to say, a center coordinate of each droplet image of the overlapped droplet image shown in  FIGS. 4 to 6  as examples is calculated. 
     As an alternative, the droplet position calculating module  440  may presume a coordinate of a pixel with a predetermined gray level in a long or short axis direction of the overlapped droplet image and a center coordinate of the overlapped droplet image as a center coordinate of a droplet image whose droplet discharge characteristics will be analyzed. In this case, the droplet position calculating module  440  is operated as follows. For reference, it is presumed that the number of droplets whose center coordinates will be estimated is presumed as three. However, the present invention is not limited thereto. First, circularity of a droplet image and length and direction of longest axis and width (or, a maximum width measured in a direction perpendicular to the longest axis) are checked. After that, a gray level data of each pixel is extracted along a long or short axis. Subsequently, a histogram of gray level according to position of each pixel, which configures the long or short axis, is obtained. The obtained histogram has normal distribution, so an average value and a standard deviation are obtained in a statistical method from the distribution. After that, coordinates of two pixels corresponding to the standard deviations 1σ and 2σ and a coordinate of a center pixel of the overlapped droplet image are obtained. The obtained three coordinates will be center coordinates of droplets to be presumed. If the number of standard deviations is increased, the number of ink droplets whose center coordinates will be presumed is also increased, as apparent to those having ordinary skill in the art. 
     The droplet position calculating module  440  receives the center coordinate of individual droplet image, and then outputs it to the droplet discharge characteristic calculating module  450 . The droplet discharge characteristic calculating module  450  calculates droplet discharge characteristics of droplets whose center coordinates are already obtained. The droplet discharge characteristics include jetting velocity and direction of each droplet whose center coordinate is obtained, and their average value a standard deviation. The jetting velocity of a droplet is calculated using a distance between the center coordinate of the droplet image and a coordinate assigned to an end of the ink jetting nozzle and a delay time between the operating signal for ink droplet jetting and the photographing control signal. The jetting direction of a droplet is calculated using a vector between the center coordinate of the droplet image and the coordinate assigned to the end of the ink jetting nozzle. After calculating the jetting velocity and direction of each droplet, the droplet discharge characteristic calculating module  450  calculates average value and standard deviation for the jetting velocity and direction. The droplet discharge characteristic calculating module  450  may receive presumed center coordinates of a predetermined number of droplets from the droplet position calculating module  440 . In this case, jetting velocity and direction of droplet and their average value and standard deviation are calculated using the presumed center coordinates. 
     The droplet discharge characteristic calculating module  450  may output the calculated droplet discharge characteristics through an external device. Here, the external device may be a printing device or a display device, well known in the art. The information output as mentioned above may be utilized to correct droplet discharge characteristics of the ink-jet printer head. The droplet discharge characteristic calculating module  450  may output the calculated droplet discharge characteristics to the jetting stability inspecting module  460 . 
     The jetting stability inspecting module  460  compares the magnitude of standard deviation for the jetting velocity and direction with a predetermined criterion, and then determines jetting stability based on whether they exceed the criterion value. That is to say, if the standard deviation of jetting velocity and direction does not exceed the criterion, the jetting stability inspecting module  460  may determined that there is no problem in jetting stability, and then output the determination result through an external device. Meanwhile, if the standard deviation of jetting velocity and direction exceeds the criterion, the jetting stability inspecting module  460  may determine that there is a problem in jetting stability, and then output the determination result through an external device. At this time, the output information preferably includes the kind of droplet discharge characteristic (velocity and/or direction) having a problem in stability, and an exceeding rate beyond the criterion. 
     Now, operations of the droplet discharge characteristic inspecting apparatus of an ink-jet printer head according to the present invention will be explained in detail with reference to  7  and  8 . 
     First, a process of generating an overlapped droplet image is explained with reference to  FIGS. 1 and 7 . If the control module  500  outputs an operating signal to the operating module  100 , the operating module  100  operates the ink-jet printer head  101  according to the operating signal such that an ink droplet is discharged from the nozzle of the ink-jet printer head  101  (S 110  to S 130 ). 
     Subsequently, the control module  500  applies a photographing control signal and a light emission control signal to the photographing module  300  and the light emitting module  200 , respectively (S 140 , S 150 ). Here, applying time point and period of the photographing control signal and the light emission control signal are already explained in detail above with reference to  FIG. 2 . Then, the light emitting module  200  operates the speed light  201  to give a light whenever an ink droplet A passes a photographing point (S 160 ), and the photographing module  300  keeps an exposed state of the digital camera  301  for a predetermined time, thereby photographing a plurality of ink droplets A passing the photographing point to generate an overlapped droplet image (S 170 , S 180 , S 190 ). After that, the photographing module  300  sends the generated overlapped droplet image to the droplet discharge characteristic inspecting means  400  (S 200 ). 
     Now, a process of quantitatively inspecting droplet discharge characteristics of the ink-jet printer head  101  will be explained with reference to  FIGS. 1 and 8 . The droplet discharge characteristic inspecting means  400  receives the overlapped droplet image from the photographing module  300  under the control of the control module  500  (S 210 ). 
     Subsequently, the droplet discharge characteristic inspecting means  400  calculates a signal-to-noise ratio of each pixel of the overlapped droplet image (S 220 ). 
     And then, the droplet discharge characteristic inspecting means  400  calculates an overlap rate of each pixel from the signal-to-noise ratio of the overlapped droplet image (S 230 ). 
     After that, the droplet discharge characteristic inspecting means  400  statistically analyzes droplet size, distribution of pixels with the same overlap rate as each pixel of the overlapped droplet image, and the number of photographed droplets to calculate a center coordinate of each individual droplet image of the overlapped droplet image (S 240 ). As an alternative, the droplet discharge characteristic inspecting means  400  may presume center coordinates of a limited number of droplet images by means of statistical analysis. Thos presumption method of center coordinate is already explained above. 
     Then, the droplet discharge characteristic inspecting means  400  calculates droplet discharge characteristics (S 250 ). Here, the droplet discharge characteristics include velocity and direction of droplets whose center coordinates are obtained, and their average value and standard deviation. A calculation method of each droplet discharge characteristic is already explained above. 
     Finally, the droplet discharge characteristic inspecting means  400  compares standard deviation for the jetting velocity and direction among the droplet discharge characteristics calculated in the step S 250  with a predetermined criterion, and then determines jetting stability based on whether they exceed the criterion value and outputs the determination result to an external device (S 260 ). 
     Meanwhile, the droplet discharge characteristic inspecting means  400  may output the signal-to-noise ratio distribution of each pixel of the overlapped droplet image, calculated in the step S 220 , and the droplet discharge characteristic values calculated in the step S 250  through an external device. Then, the droplet discharge characteristics may be qualitatively analyzed using the pattern of signal-to-noise ratio distribution, and the droplet discharge characteristics of the ink-jet printer head may be quantitatively analyzed using the droplet discharge characteristic values. 
     Operations of the above droplet discharge characteristic inspecting means  400  may be coded as a program algorithm executable by a computer and then loaded to a general computer. In this case, it is apparent that unit modules of the droplet discharge characteristic inspecting means  400  may be understood as functional logic blocks of a program. In addition, the overlapped droplet image may be transmitted to the droplet discharge characteristic inspecting means  400  through an I/O interface of a general computer. In case the droplet discharge characteristic inspecting means  400  is implemented as a program, the program may be recorded in a medium readable by a computer. The computer-readable medium may include program instructions, data files and data structures in single or in combination. The program instructions recorded in the medium may be specially designed or configured for the present invention or already well known to the persons having ordinary skill in the computer program fields. The computer-readable medium includes magnetic media such as hard disk, floppy disk and magnetic tape; magneto-optical media such as floptical disk; and hardware devices specially configured to store and execute program instructions such as ROM, RAM and flash memory. The medium may be a transmission medium such as optical or metallic wire and waveguide, including a carrier wave, which sends a signal designating program instructions and data structure. The program instruction may be machine codes made by a compiler or the like, or high-level language codes executed by a computer using an interpreter or the like. The hardware device may be configured as at least one software module to execute the operations of the present invention, or vice versa. 
     The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, droplet discharge characteristics are quantitatively inspected using an overlapped droplet image, so a complex fine deviation error, required in high precision printing, may be easily inspected. 
     In addition, velocity and direction of an ink droplet discharged from an ink-jet printer head to a hitting point may be quantitatively analyzed. 
     Moreover, jetting stability of the ink-jet printer head may be precisely inspected, thereby improving stability and efficiency of the ink-jet process used for making an electronic circuit or a display device.