Patent Publication Number: US-2011075162-A1

Title: Exposure averaging

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. ______ (Docket 95645) filed Sep. 29, 2009 by John Saettel, entitled “Color To Color Registration Target”, commonly assigned U.S. patent application Ser. No. ______ (Docket 94958) filed Sep. 29, 2009 by John Saettel, entitled “A Calibration System for Multi-Printhead Ink Systems”, and commonly assigned U.S. patent application Ser. No. ______ (Docket 95644) filed Sep. 29, 2009 by John Saettel, entitled “Automated Time of Flight Speed Compensation”, the disclosures of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to high-speed, multi-color printing systems in which test marks are imaged for calibration and the like and, more particularly to, capturing multiple lines of a test pattern in one image capture by flashing the strobe when each line of the test pattern passes the same location while holding the camera exposure open during the entire image capture time period. 
     BACKGROUND OF THE INVENTION 
     High-speed, multi-color printing systems print test patterns that are subsequently captured for use in calibration and the like. U.S. Pat. No. 4,794,453 discloses one such high-speed printing system that prints and captures test patterns. In this disclosure, a test pattern is created by printheads, and a camera and strobe are synchronized so that a test pattern is captured. In order to minimize interference, the strobe is discharged a predetermined time before image capture. To capture multiple lines of a test pattern, the above-described image capture process must be repeated for each line of the test pattern. 
     Although the above-described method is satisfactory, improvements are always desired. One such improvement is to capture multiple lines in a single image capture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, the invention resides in a method for capturing an image in a multi-color printing system, the method comprising the steps of providing a test pattern on a print media having a plurality of test patterns aligned in a first direction in a repeating pattern; providing an image capture device having a shutter that controls a time interval during which a single image is captured; and flashing a strobe light multiple times within the time interval in which a single image is captured; wherein each strobe light flash is synchronized with motion of the test pattern past the image capture device so that each test pattern is illuminated by a single strobe-light flash at substantially the same location within the captured image. 
     It is an object of the present invention to capture multiple test pattern lines in a single image capture. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The present invention has the advantage of reducing the maximum instantaneous lighting requirement by a factor of five. This allows fewer and lower brightness LED&#39;s to be used which reduces cost. It also allows a shorter duration strobe pulse to be used which reduces motion blur to yield a better image. It also filters out some of the noise in the measured position of the registration marks introduced by random artifacts on the substrate that can have the same spectral response as the printed marks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
       While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of the calibration system of a multi-printhead printing system of the present invention; 
         FIG. 2  is a side view of an image capture device of the present invention used in  FIG. 1 ; 
         FIG. 3  is a bottom view of  FIG. 2 ; and 
         FIG. 4  is a diagram illustrating test patterns printed along the print media with strobe and exposure timing pulses to capture images of the test patterns according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , there is shown a block diagram of the printing system  10  of the present invention. The printing system  10  includes a transport for transporting the print media  20  through various stages of the printing process. Four printheads (T 1 , T 2 , T 3  and T 4 ) span over the print media  20  each for dispensing ink of a different color on the print media  20  as the media  20  moves relative to the printheads T 1 -T 4 . In the preferred embodiment, each printhead T 1 -T 4  prints a series of test marks so that, after printing by the last printhead T 4 , a series of four test mark test patterns are printed as shown in  FIG. 4 . Referring back to  FIG. 1 , four ink holding receptacles  44 , each of a different color, are respectively attached to each printhead T 1 -T 4  for supplying ink thereto. Three image capture devices  50   a ,  50   b  and  50   c  are respectively disposed immediately downstream and in close proximity of each of the last three printhead T 2 -T 4  but not the first printhead T 1 . Each image capture device  50   a ,  50   b  and  50   c  includes a digital camera and a light source. Typically the light sources are strobe lights for producing a plurality of short bright flashes of light to allow an image to be captured without motion blur. Typically the strobe lights consist of a plurality of Light Emitting Diodes (LEDs), commonly of red, green and blue LEDs that are the color compliments of cyan, magenta, and yellow inks, respectively, that are printed by the printheads. By using LEDs that are the color complement of the color inks, image contrast is enhanced. For example, a yellow mark on the print media will appear as a high contrast dark mark when illuminated only with a blue LED. Black ink which absorbs all colors shows up in high contrast with any visible light LED so a separate LED is not needed for the black ink. Each image capture device  50   a - 50   c  captures an image of the media  20  after the printhead T 2 -T 4  prints its respective ink on the media  20  for providing feedback as to whether calibration of the printing system is needed and, if so, the degree of calibration to be preformed, as described in commonly-assigned and co-pending U.S. patent application Ser. No. ______ (D-94958). A drive motor (not shown) connected to a drive roller  60  exerts force on the print media for moving it through the printing system  10 . 
     The printing system  10  includes various components that perform process control and analysis. In this regard, an image system analyzer  70  receives the images captured by the image capture devices  50   a - 50   c  respectively located downstream of each printhead T 2 -T 4  to determine whether the ink marks printed by the respective printheads T 1 -T 4  are aligned relative to each other as expected if aligned properly. In general, the image system analyzer  70  converts the images into bit maps, identifies each of the test marks, and determines their locations within the image, and calculates their alignment relative to each other in both the x and y directions, if any. Based on the result, the image system analyzer  70  sends a signal to the process controller  80 . The printing system also includes a clock  75  that creates a clock pulse train. The clock  75  communicates with the process controller  80 , which uses the clock pulse train to create a frequency-shifted pulse train for each of the printheads T 2 , T 3 , and T 4 . It is noted that, in a four ink system, three images are captured with the initial ink mark not being imaged alone as there is no relative relationship by which the initial mark may be analyzed for correctness. 
     An encoder  90  is used to monitor the motion (in the direction of the arrow) of the print media  20  through the printing system  10 . Typically the encoder  90  is in the form of a rotary encoder that creates a defined number of pulses per revolution. The rotary encoder is connected to a roller or wheel (not shown) that is rotated by the moving paper. The circumference of the wheel or roller, in combination with the defined number of pulses per revolution of the rotary encoder  90 , determines the number of encoder pulses per centimeter or inch of paper travel. The output of the encoder  90 , in the form of an encoder pulse train is used by the process controller  80  for controlling the placement of the print media  20  along the direction of print media travel. Typically the spacing of pixels in the in-track direction (along the direction of paper motion) corresponds to N times the spacing between encoder pulses, where N is a small (&lt;10) integer. To properly print a multi-color document, the print data sent to each printhead T 2 -T 4  downstream of the first printhead T 1  must be delayed by increasing amounts relative to the data of the first printhead. These delays are normally defined in terms of a delay count or the number of the encoder pulses that correspond to the spacing along the paper path of the printheads T 2 -T 4  from the first printhead T 1 . For example, if the second printhead T 2  is located 8.5 inches downstream of the first printhead T 1  and the encoder  90  produces 600 pulses per inch, the print data to the second printhead T 2  would be delayed by 5100 pulses relative to the data to the first printhead T 1 . 
     The print media  20  passes under and in the optical path of the digital camera  100  in order to capture the printed test marks from the printheads T 1 -T 4 . Various digital cameras  100  can be employed provided they have sufficient optical resolution and light sensitivity to capture images of the test marks. One such useful camera is the IMP-VGA210-L from Imperx. This is a black and white camera with a 640×480 pixel resolution. It is able to output images at a rate of 210 complete frames per second through a CameraLink™ interface to an image processing system. This camera also has an external trigger and an externally controllable electronic shutter so that acquisition of images and the shutter time for acquiring an image can be controlled by the process controller  80 . This camera also allows a portion of the active pixels in the captured image frame to be defined as an area of interest. The camera sensor then uses only that portion of its active pixels for image capture, and only transfers the image data corresponding to that area of interest to the image system analyzer  70 . By so doing, the camera is able to capture and transfer partial frame images at higher frame rates than its complete frame rate. An infinite conjugate micro-video lens from Edmund Optics, #56776, with a 25 mm focal length and a 1:1 magnification is an effective lens for use with this camera. In one embodiment, the strobe lights  110  are light emitting diodes, two LED&#39;s each of red, green and blue, arranged circular around the lens of the camera. Light emitting diodes from Luxeon, such as LXHL-PH09, LXHL-PM09, and LXHL-PRO09, are examples of usable LED&#39;s. The image capture device may be mounted on a carriage downstream of each printhead so that the image capture device is adjustable in position in a cross-track direction. Alternatively, the image capture device may be mounted directly to downstream side of the printhead so that it can capture the image of the test marks printed by that printhead and the first printhead. 
     During the printing process however, it is possible for the effective spacing between the printheads T 1 -T 4  to vary, due, for instance, to stretching of the print media  20 , resulting in misregistration of the images from the various printheads T 1 -T 4 . If by means of the image capture device and the image processing unit such a registration error is detected, the process controller  80  can modify the operation of the printing system  10  to correct for this misregistration as described in commonly-assigned and co-pending U.S. patent application Ser. No. ______ (D-94958). 
     It is noted that, while the description above describes the printer in terms of four printheads each printing a separate color, the invention is not limited to printing systems having exactly four printheads. 
     Referring to  FIGS. 2 and 3 , there is shown an exemplary image capture device  50   a - 50   c  of the present invention. The image capture device  50   a - 50   c  includes a digital camera  100  having a plurality of strobe lights  110 . A lens  120  is disposed in the optical path of the digital camera  100  for providing optical focus for the digital camera  100 . An image sensor  105  is disposed in the camera  100  for converting incident light into charge packets, as is well known in the art. According to design preference, the image sensor  105  may use only a portion of its pixels  108  for image capture (i.e., charge collection) as described hereinabove. For illustration purposes, the image sensor  105  is oriented so as to illustrate the pixels  108 , but in actual practice, the pixels  108  are physically oriented so as to capture the incident light passed through the lens  120 . Optionally, the image capture device  50   a - 50   c  includes a light shield  115  for shielding the camera  100  from ambient light. 
     Referring to  FIG. 4 , there is shown the media having the test marks  125 - 1  through  125 - 4  printed thereon by the printheads T 1 -T 4 , respectively. A set of test marks from the various printheads whose intended positions on the print media have a defined relationship form a test pattern. In this example, the test pattern  130  consists of the test marks  125 - 1  through  125 - 4  within the phantom box. The test marks in this pattern have a defined spacing relative to each other. While the test marks in this example test pattern  130  are evenly spaced along a line, other spatial relationships between the test marks are also possible. In this illustration, it is noted that the test pattern consists of marks from each printhead T 1 -T 4 ; however, the present invention is also applicable when the test pattern consists of test marks from at least two printheads. These test patterns are regularly spaced along the direction of print media motion, denoted by the arrow. In one preferred embodiment, the test patterns are spaced apart from each other by a fixed distance that is within the range of 0.040 to 0.15 inches 
     As the print media moves through the printing system, the test patterns that are regularly spaced along the print media move in succession through the field of view of an image capture device  50  at uniform time intervals. Based on the output from the encoder  90 , the process controller  80  creates strobe firing pulses  135  to synchronize the flash of the strobe lights  110  with the motion of the test pattern  130 . In this manner, each test pattern is illuminated by a single strobe-light flash when they are each at substantially the same location within the field of view of the camera. 
     The process controller also creates camera capture pulses  140  that controls the shutter time interval over which the digital camera  100  acquires an image. As shown, the time duration of the camera capture pulse  140  is long compared to a strobe pulse, allowing a plurality of strobe pulses, five strobe pulses in this embodiment, to occur during a single image capture. As the amount of ambient light reflected from the print media while it is in the field of view of the digital camera is small compared to the amount of strobe light reflected from the print media, the acquired image is essentially a superposition of five strobed test pattern images. Capturing an image with such a superposition of strobed images, enables the acceptable images to be acquired while reducing significantly the illumination intensity requirements of the strobe. This allows fewer and lower brightness LEDs to be used, which reduces cost. It also allows a shorter duration strobe pulse to be used which reduces motion blur to yield a better image. 
     The superposition of multiple strobe images to form a single captured image also serves to produce an exposure averaged image from multiple test patterns. This is beneficial as it averages away image artifacts that may be associated with a single test pattern. It is known that there can be some variation in the appearance of individual test patterns. These can be produced, for example, by wicking of ink along individual fibers in the print media, by variations in the uniformity of various print media coatings, or by the presence of ink splash droplets around the printed test marks. This exposure averaging process by combining multiple test pattern images into a single captured image makes the measurement of test mark location less sensitive to the presence of print artifacts associated with a single test mark. 
     While  FIG. 4  illustrated the use of five strobe flashes within the time interval during which a single image is captured, other numbers of strobe flashes may be employed. In one embodiment, number of strobe flashes within the time interval during which a single image is captured depends on speed of the print media through the multi-color printing system. In another embodiment, the number of strobe flashes within the time interval depends on either or both brightness and contrast of a previously captured image. For example, if the previously captured image had low brightness and therefore low contrast, the process controller causes the number of strobe pulses to be increased to improve the brightness of acquired images. 
     In another embodiment, the contrast of the captured test pattern images is enhanced by using different number of strobe flashes for the various LEDs  110  that surround the digital camera  100 . By way of example, the red LED may be flashed fewer times than the green or blue LEDs during the shutter time of the camera to acquiring a single image. Such a variation in the number of strobe flashes for the various LEDs may be desirable based on differences in the brightness of the different color LEDs, differences in the absorption of the inks at the wavelengths of the LEDs, or the color of the print media. As the registration of the image plane printed by each printhead T 2 -T 4  is made relative to the image plane printed by printhead T 1 , the image capture device associated with a printhead only needs to capture high contrast images of the test mark printed by the associated printhead and the test mark printhead by printhead T 1 . For example, if T 1  prints cyan ink and T 4  prints yellow ink, image capture device  50   c  should yield high contrast images of the cyan and yellow test marks. As red is the color complement of cyan and blue the color complement of yellow, the red and blue LEDs of image capture device  50   c  are flashed more times than the green LED during the capture of a single image by the camera. 
     Preferably, each test pattern is spaced apart from each other at substantially a range of 0.040 to 0.15 inches and a frame rate of the camera  100  is substantially between a range of 100 to 300 frames per second. This enables the printing system to capture multiple images within a single document, and color registration corrections to be made multiple times within a document. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
         T 1 -T 4  printheads 
           10  printing system 
           20  print media 
           44  holding receptacles 
           50   a - 50   c  image capture devices 
           60  drive roller 
           70  image system analyzer 
           75  clock 
           80  process controller 
           90  encoder 
           100  digital camera 
           105  image sensor 
           108  pixels 
           110  strobe light 
           115  light shield 
           120  lens 
           125  test marks 
           130  test pattern 
           135  strobe firing pulse 
           140  camera capture pulse