Dynamic reflective density measuring and control system for a web printing press

• US 6,058,201 A
• Filed: 05/04/1995
• Issued: 05/02/2000
• Est. Priority Date: 05/04/1995
• Status: Expired due to Fees

• Current Assignee: Web Printing Controls Co., Inc. (Forsyth Baldwin LLC)
• Original Assignee: Web Printing Controls Co., Inc. (Forsyth Baldwin LLC)
• Inventors: Gnuechtel, Herman C., Sikes, Dale R.
• Primary Examiner(s): COUSO, YON JUNG
• Application Number:
  US08/434,244
• Time in Patent Office: 1,825 Days
• Field of Search: 382/112, 250/559.05, 250/559.06, 250/559.07, 250/559.08, 358/406
• US Class Current: 382/112
• CPC Class Codes: B41F 33/0045 for automatically regulating the ink supply
• CPC Class Codes: B41P 2233/51 for colour quality control

Abstract

A system for use with a web printing press for measuring the reflective density of printed ink in real time during high speed operation. The system digitizes images of color block sets of the web, analyzes the images to determine reflective density of cyan, magenta, yellow and black. The system compensates for light scattering, light discontinuities and strobe intensity variation.

Claims

1. Apparatus for measuring the reflective density of at least a first color printed on a moving web during operation of a web printing press of the type which has an adjustable ink zone control mechanism for controlling the amount of ink of said first color that is available to be transferred to the web at spaced locations across the width of the press during a printing operation and which has a printing roller which transfers ink to the web, there being a nonprint area that extends in the lateral direction of the web resulting from attachment of printing plates to the printing roller, the press having a color block set on the web generally at each spaced location, the color block set including at least said first color, said apparatus comprising:

means for acquiring a digitized image of predetermined size of the web, said image being comprised of a plurality of individual pixels, each having an intensity value within a predetermined range;

memory means for storing said digitized images;

strobe means for illuminating at least said predetermined area of the web when said strobe means is fired;

means for calibrating said acquiring means to provide predetermined values for high and low light reflectivity levels of said digitized images;

means for measuring the presence and intensity of printed matter located in an acquired digitized image of at least a portion of one of said color block sets, and generating light scattering compensation signals for compensating for the presence of printed matter that affects the measured reflective density of at least said first color;

processing means for analyzing an acquired digitized image of a uniform surface of a standard stored in said memory means, said processing means analyzing a digitized image to determine the intensity of illumination for a plurality of locations throughout said image acquired during the firing of said strobe means, and generating light discontinuity compensation signals for compensating for uneven illumination among said locations;

said processing means being adapted to analyze the digitized images containing at least said first color and produce a reflective density value thereof, said reflective density value having been corrected by said light scattering compensation signals and said light discontinuity compensation signals.

2. Apparatus as defined in claim 1 wherein said acquiring means includes a charge coupled device (CCD) matrix sensor means that is adapted to acquire said digitized image.

3. Apparatus as defined in claim 2 further including carriage means for mounting said CCD matrix sensor means and said strobe means and for moving the same across the width of the web for acquiring digitized images of desired areas of the web.

4. Apparatus as defined in claim 2 further comprising means for measuring the intensity of illumination and color produced by said strobe means during a firing thereof and producing an intensity signal indicative of the measured intensity and color, said processing means using said strobe intensity signal to produce said reflective density that is compensated for variations in strobe firing intensity and color.

5. Apparatus as defined in claim 4 wherein said color block set includes at least one color block of said first color.

6. Apparatus as defined in claim 5 wherein said color block set comprises solid color blocks, screened blocks, combinations of solid and screened blocks, and overprinted solid blocks.

7. Apparatus as defined in claim 5 wherein said memory means includes light scattering compensation data, which if collectively plotted would define a generally asymptotic curve of light scattering compensation signals versus the high reflectivity level of pixels and distance from a color block of interest in the digital image.

8. Apparatus as defined in claim 5 wherein said processing means analyzes substantially all of the pixels of a digitized image of the color block set to determine at least one light scattering compensation signal for the color block set, said processing means calculating said light scattering compensation signal for a block of interest of said color block set.

9. Apparatus as defined in claim 8 wherein said CCD matrix sensor means comprises at least one CCD matrix sensor having a lens and internal means for separating the light passing through the lens into three paths, each respectively containing red, green and blue light, said CCD matrix sensor means thereby having three channels, with each channel providing a digitized image of its color, said processing means analyzing substantially all of the pixels of a digitized image of each of the channels, said processing means thereby storing a light scattering compensation signal for said color block set.

10. Apparatus as defined in claim 2 wherein said CCD matrix sensor means comprises a CCD matrix sensor having a lens and an internal means for separating the light passing through the lens into three paths, each respectively containing red, green and blue light, said CCD matrix sensor means thereby having three channels, with each channel providing an image matrix of its color.

11. Apparatus as defined in claim 1 wherein said strobe means comprises first and second strobe lights located on opposite sides of said CCD matrix sensor means, said first and second strobe lights being directed toward the web to illuminate the area of the web from which the CCD matrix sensor means acquires said digitized image.

12. Apparatus as defined in claim 1 wherein said predetermined size of the web of which said digitized images are acquired are about 0.38 inches by 0.5 inches, and each of said pixels represents about 0.0006 inches of the web.

13. Apparatus as defined in claim 3 wherein said carriage means comprises a rail means extending across the width of the press and carriage structure carrying said CCD matrix sensor means and said strobe means that is moveable on said rail means, and motor means for moving said carriage structure along said rail means responsive to position control signals applied thereto.

14. Apparatus as defined in claim 1 wherein said intensity measuring means comprising at least one photodiode located adjacent said strobe means and adapted to produce a signal proportional to the intensity of the light produced by said strobe means during firing.

15. Apparatus as defined in claim 2 wherein said calibrating means comprises a first standard of predetermined size and high predetermined reflectivity from which said acquiring means can acquire a digitized image of said first standard and a second standard of predetermined size and low predetermined reflectivity from which said acquiring means can acquire a digitized image of said second standard, said processing means being adapted to analyze said digitized images of said standards and generate predetermined values for said first and second standards.

16. Apparatus as defined in claim 15 wherein said CCD matrix sensor means acquires a matrix image comprised of at least approximately 760 by 480 individual pixels, said range of intensity values being from 0 to at least 255 before said predetermined values are generated for said first and second standards.

17. Apparatus as defined in claim 16 wherein said predetermined value for said second standard is within the range of approximately 0 and 63 and said predetermined value for first standard is within the range of approximately 200 and at least 255.

18. Apparatus as defined in claim 15 wherein said processing means generates predetermined values for said first and second standards by calculating an average of the intensity values of a plurality of pixels of said digitized image of said respective first and second standards, said processing means excluding any pixels from the average calculation that have pixel values that are outside of predetermined ranges for said first and second standards.

19. Apparatus as defined in claim 16 wherein said processing means is adapted to identify the location and size of a group of pixels having values that are outside of said predetermined ranges, said group thereby being defined as a blemish, said processing means being adapted to compare the location and size of the blemishes for the images of the first and second standards, and thereby determine that particular blemishes result from contaminants being present on said CCD matrix sensor means or on said standards.

20. Apparatus as defined in claim 15 wherein each of said digitized images is comprised of a plurality of individual pixels, each of which has an intensity value which can vary over a range from near 0 to at least 255, said CCD matrix sensor means acquiring a matrix image of a surface of a highly reflective uniform standard, said apparatus storing the same in said memory means, said processing means examining the intensity value of a plurality of said individual pixels and determining a compensation factor for said pixels which have an intensity value that varies from a uniform intensity value, said compensation factor being proportional to the amount of discontinuity from said uniform intensity value, said compensation factor being stored in said memory means for use by said processing means in producing said reflective density value of said at least first color.

21. Apparatus as defined in claim 20 wherein said CCD matrix sensor means acquires said matrix image of said white standard, said processing means examining each pixel of said image and determining a compensation factor therefor.

22. Apparatus as defined in claim 13 wherein said motor means comprises a stepping motor which moves said carriage structure along said rail means, said processing means providing said position control signals to control said carriage structure so that said CCD matrix sensor means is positioned to acquire an image of the web.

23. Apparatus as defined in claim 2 which is adapted to locate the edge of the moving web during operation, whereby said processing means causes said carriage means to move said CCD matrix sensor means to move to a location on the web near an edge and acquire a matrix image of a portion of the web and activate said acquiring means in conjunction with firing said strobe means, the image being comprised of a matrix of individual pixels each having a low standard deviation of intensity values below a predetermined limit, with the pixels of an image of unprinted web having a low standard deviation of intensity values, the pixels of an image of the web support roller having a high standard deviation of intensity values, and the pixels of a printed web portion having a standard deviation of intensity values generally comparable to the standard deviation of intensity values of the web support roller;

said processing means analyzing said image to determine the existence of low standard deviation of intensity values of the pixels and determining an edge of the web by moving in a direction toward one of said edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web;

said processing means causing said carriage means to move said CCD matrix sensor means to move to a location where said acquiring means acquires one or more successive new digitized images in the longitudinal direction of the web in the event low standard deviation of intensity values were determined not to exist in each previous image and analyzing the pixels of the new digitized image generally in the longitudinal direction of the web until low standard deviation of intensity values of the pixels are determined to exist, and thereafter determining the edge of the web by moving in a direction toward one of said edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web.

24. Apparatus as defined in claim 2 wherein the web has a plurality of color block sets printed thereon in the lateral direction of the web at predetermined locations, said plurality of color block sets also having been printed at successive locations in the longitudinal direction of the web, said apparatus being adapted to locate the edge of the moving web during operation, whereby said processing means causes said carriage means to move said CCD matrix sensor means to move to a location on the web near an edge and said acquiring means acquires a digitized image of a portion of the web containing a color block set by moving said CCD matrix sensor means to a location on the web where a color block set is located and actuating said CCD matrix sensor means in conjunction with firing a strobe means for illuminating the web during the image acquiring step, the digitized image being comprised of a matrix of individual pixels, each having an intensity value within a predetermined large range, with the pixels of an image of unprinted web having a low standard deviation of intensity values, the pixels of an image of the web support roller having a high standard deviation of intensity values, and the pixels of a printed web portion having a standard deviation of intensity values generally comparable to the standard deviation of intensity values of the web support roller

said processing means analyzing said digitized image to determine the location of a color block set;

said processing means causing said carriage means to move said CCD matrix sensor means to a location longitudinally displaced said predetermined longitudinal distance relative to the prior image containing the color block set and causing said acquiring means to acquire a new digitized image, said processing means analyzing the pixels of the new digitized image by moving in a direction toward one of the edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web.

25. Apparatus as defined in claim 23 wherein said processing means stores the location of said detected edge, and causes said carriage means to move in the opposite direction and cause said acquiring means to acquire successive digitized images to detect the location of the opposite edge and calculating the distance between said detected edge locations to determine the width of the web.

26. Apparatus as defined in claim 24 wherein said processing means stores the location of said detected edge, and causes said carriage means to move in the opposite direction and cause said acquiring means to acquire successive digitized images to detect the location of the opposite edge and calculating the distance between said detected locations to determine the width of the web.

27. Apparatus as defined in claim 10 wherein said digitized images of each channel are stored in said memory means, said processing means being adapted to analyze each of said digitized images and produce a reflective density value for each of said colors.

28. Apparatus as defined in claim 27 wherein the color block set printed on the web includes a color block of at least each of said three colors, said processing means examining the pixels of the digitized image to locate each of said color blocks and analyze the pixels of each color block to produce a reflective density value for each color.

29. Apparatus as defined in claim 28 wherein said processing means analyzes each digitized image from said three channels, each channel being adapted to analyze a specified one of said colors, said processing means locating a color block having the same color as the specified channel color by detecting the intensity value thereof, the intensity value of a color block having the same color as the channel color being low relative to a color block that is different from the channel color.

30. Apparatus as defined in claim 29 wherein said processing means analyzes said digitized image of said color block set to locate said color block, said processing means examining the pixels of said image to locate a group of pixels having said low intensity value, examining pixels in each direction vertically and horizontally until low intensity pixels cease to exist, thereby defining the coordinates of the outer periphery of one of said color blocks, examining each of a predetermined number of pixels from the interior of said color block to determine if the intensity value thereof is within predefined limits and averaging the intensity values for all of said pixels that are within said limits, to thereby produce an uncompensated reflective density value.

31. Apparatus as defined in claim 30 wherein said processing means discards said uncompensated reflective density value when the total number of pixels included in the averaging is below a predetermined limit.

32. Apparatus as defined in claim 31 wherein said predetermined number of pixels that are examined is approximately 2800 and said predetermined limit of total number of pixels averaged is approximately 2000.

33. Apparatus as defined in claim 30 wherein said processing means calculates the size of the color block and determines that it is within acceptable limits, and terminates the analysis of the color block if it is outside of acceptable limits.

34. Apparatus as defined in claim 1 wherein said reflective density value is stored in said memory means and is adapted to be read out together with other data relating to a printing job by the press.

35. Apparatus as defined in claim 34 wherein said other data comprises the date, press identification data, job identification data, and the number of the press run for which the reflective density values were produced.

36. Apparatus as defined in claim 2 wherein said memory means is adapted to simultaneously store at least two digitized images, said processing means controlling said CCD matrix sensor means, strobe means and carriage means so that a successive image is acquired by said acquiring means and is stored in said memory means while said processing means is analyzing a prior acquired digitized image to produce said reflective density value for said first color.

37. Apparatus as defined in claim 36 wherein said processing means controls said carriage means so that it is positioned laterally away from one of the edges of said web and is accelerated to a predetermined speed before it reaches said edge so that it traverses the width of the web while said CCD matrix sensor means is acquiring a matrix image of each of the color block sets at each location on successive revolutions of said printing roller.

38. Apparatus as defined in claim 37 wherein each reflective density analysis requires about 70 milliseconds.

39. Apparatus as defined in claim 10 further including a generally sealed housing means mounted to said carriage means for containing said CCD matrix sensor means, said housing means including a nozzle means having a light admitting opening aligned with said lens of said CCD matrix sensor means, said nozzle means including means for admitting air near said lens and directing the air toward the opening for preventing dust and the like from entering the opening.

40. Apparatus as defined in claim 39 further including connecting means for transmitting air from a source of positive air pressure to said nozzle means.

41. Apparatus as defined in claim 1 wherein said color block set includes color blocks of cyan, yellow, magenta and black located adjacent one another.

54. Apparatus as defined in claim 36 wherein said memory means is adapted to store an image of an area other than the area that contains the color block sets, said apparatus further including monitor means for displaying selected images for an operator.

42. Apparatus for measuring the reflective density of at least a first color printed on a moving web during operation of a web printing press of the type which has an adjustable ink zone control mechanism for controlling the amount of ink of said first color that is available to be transferred to the web at spaced locations across the width of the press during a printing operation and which has a printing roller which transfers ink to the web, there being a nonprint area that extends in the lateral direction of the web, the nonprint area resulting from attachment of printing plates to the printing roller, the press having a color block set on the web generally at each spaced location, the color block set including at least said first color, said apparatus comprising:

a multiple CCD matrix sensor for acquiring a matrix image of the web of predetermined size;

means for digitizing said image so that it is comprised of a large number of individual pixels, each of which can have an intensity value within a predetermined range, memory means for storing at least one digitized image;

strobe means for illuminating the web when said strobe means is fired in response to strobe control signals being applied thereto;

carriage means for mounting said CCD matrix sensor and said strobe means and for moving the same across the width of the web for acquiring digitized images of desired areas of the web in response to position control signals being applied thereto;

means for calibrating said CCD matrix sensor means and said digitizing means to provide respective predetermined values for high and low light reflectivity levels of said digitized images;

means for measuring the presence and intensity of printed matter located in an acquired digitized image of a color block set, and generating a light scattering compensation signal for compensating for the presence of printed matter that affects the measured reflective density of at least said one color;

processing means for analyzing said digitized images acquired by said CCD matrix sensor means and stored in said memory means;

said processing means analyzing a digitized image of a uniform highly reflective surface produced by the firing of said strobe means to determine the intensity and color of illumination at a plurality of locations throughout said image of predetermined size and generating light discontinuity compensation signals for compensating for discontinuities of illumination at said locations where said intensity is determined;

said processing means being adapted to analyze the digitized images of said color block sets and utilizing said light scattering compensation signals and said light discontinuity compensation signals, produce a reflective density value of said at least first color that is compensated for light scattering and light discontinuity.

43. Apparatus as defined in claim 42 further including means for measuring the intensity and color of illumination produced by said strobe means during a firing thereof and producing an intensity signal indicative of the measured intensity and color, said processing means utilizing said intensity signal to produce a reflective density value that is also compensated for variations of strobe light intensity and color.

44. Apparatus for measuring the reflective density of at least one color block printed on a moving web having a predetermined width during operation of a web printing press of the type which has an adjustable ink zone control mechanism for controlling the amount of ink of said first color that is available to be transferred to the web at spaced locations across the width of the press during a printing operation and which has a printing roller which applies ink on the web, said press having printed a color block set on the web at predetermined locations relative to each ink zone control mechanism spaced location, the color block set at least including the color block of said first color, said apparatus comprising:

a multiple CCD matrix sensor for acquiring a matrix image of the web of predetermined size;

means for digitizing said image so that it is comprised of a large number of individual pixels, each of which can have an intensity value within a predetermined range;

memory means for storing at least one digitized image;

strobe means for illuminating the web when said strobe means is fired in response to strobe control signals being applied thereto;

carriage means for mounting said CCD matrix sensor and said strobe means and for moving the same across the width of the web for acquiring digitized images of desired areas of the web in response to position control signals being applied thereto;

means for calibrating said CCD matrix sensor means and said digitizing means to provide predetermined values for high and low light reflectivity levels of said digitized images;

means for measuring the presence and intensity of printed matter located in an acquired digitized image of a color block set, and generating a light scattering compensation signal for compensating for the presence of printed matter that affects the measured reflective density of at least said first color;

processing means for analyzing said digitized images and stored in said memory means;

said processing means being adapted to analyze the digitized images of said color block sets and utilize said light scattering compensation signals to produce a reflective density value of said at least first color that is compensated for light scattering.

45. Apparatus as defined in claim 44 wherein said processing means analyzes a digitized image of a uniform surface to determine the intensity and color of illumination at a plurality of locations throughout said image produced during the firing of said strobe means and generating light discontinuity compensation signals for compensating for discontinuities of illumination at said locations where said intensity is determined, said processing means utilizing said light discontinuity compensation signals to produce a reflective density value that is compensated for light discontinuities.

46. A method for determining the reflective density value of ink of at least a color block of a first color printed on a moving web having a predetermined width during operation of a web printing press of the type which has an adjustable ink zone control mechanism for controlling the amount of ink of said first color that is available to be transferred to the web at spaced locations across the width of the press during a printing operation and which has a printing roller which applies ink on the web, there being a nonprint area that extends in the lateral direction of the web resulting from attachment of printing plates to the printing roller, said press having printed a color block set on the web at predetermined locations relative to each ink zone control mechanism spaced location, the color block set including the color block of said first color, said method comprising:

acquiring a matrix image of the color block set by activating a CCD matrix sensor and a strobe means at a portion of the web containing the color block set;

digitizing said matrix image to produce a digitized image, said digitized image being comprised of a large plurality of individual pixels, each pixel having an intensity value within a predetermined range;

identifying the pixels of said color block and analyzing a predetermined number of said color block pixels to determine valid pixels, said pixels being valid when their associated intensity values are below a predetermined threshold;

adjusting the intensity values of each of said valid pixels to compensate for discontinuities in the total amount of light produced by the strobe means during the acquiring of the image;

adjusting the intensity values of each of said valid pixels to compensate for light scattering effects that are influenced by the existence and nature of printed matter within the image adjacent to the color block set; and

,

averaging the adjusted intensity values of said predetermined number of valid pixels to produce said reflective density value.

47. A method as defined in claim 46 wherein said large plurality of pixels comprises approximately 480 rows of 780 pixels, substantially all of said pixels having an intensity value within the predetermined range of about 0 to 255.

48. A method as defined in claim 46 wherein said predetermined number is approximately 2000.

49. A method as defined in claim 46 wherein said predetermined threshold is in the lower half of said predetermined range.

50. A method for determining the edge of a moving web during operation of a web printing press, said press being of the type which has a printing roller which transfers ink to the web, there being a nonprint area in printing that extends in the lateral direction of the web because of the attachment of a printing plate to the roller, said method comprising the steps of:

acquiring a digitized image of a portion of the web by moving a CCD matrix sensor means to a location on the web near an edge and activating the CCD matrix sensor means in conjunction with firing a strobe means for illuminating the web during the image acquiring step, the image being comprised of a matrix of individual pixels each having an intensity value within a predetermined large range, with the pixels of an image of unprinted web having a low standard deviation of intensity values, the pixels of an image of the web support roller having a high standard deviation of intensity values, and the pixels of a printed web portion having a standard deviation of intensity values generally comparable to the standard deviation of intensity values of the web support roller;

analyzing said image to determine the existence of low standard deviation of intensity value of the pixels and determining the edge of the web by moving in a direction toward one of the edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web,

acquiring one or more successive new images in the longitudinal direction of the web in the event low standard deviation of intensity value were determined not to exist in each previous image and analyzing the pixels of the new image generally in the longitudinal direction of the web until low standard deviation of intensity value of the pixels are determined to exist, and thereafter determining the edge of the web by moving in a direction toward one of the edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web.

51. A method for determining the edge of a moving web during operation of a web printing press, wherein the web has a plurality of color block sets printed thereon in the lateral direction of the web at predetermined locations, said plurality of color block sets also having been printed at successive locations in the longitudinal direction of the web, the color block sets including a color block of at least a first color, the press being of the type which has a printing roller which transfers ink to the web, there being a nonprint area in printing that extends in the lateral direction of the web because of the attachment of a printing plate to the printing roller, the nonprint area being a predetermined longitudinal distance from said plurality of predetermined color block set locations, said method comprising the steps of:

acquiring a digitized image of a portion of the web containing a color block set by moving a CCD matrix sensor means to a location on the web where a color block set is located and activating the CCD matrix sensor means in conjunction with firing a strobe means for illuminating the web during the image acquiring step, the image being comprised of a matrix of individual pixels each having a standard deviation of intensity values within a predetermined large range, with the pixels of an image of unprinted web having a low standard deviation of intensity values, the pixels of an image of the web support roller having a standard deviation of intensity values less than that of the unprinted web, and the pixels of a printed web portion having a standard deviation of intensity values generally comparable to the standard deviation of intensity value of the web support roller;

analyzing said image to determine the location of a color block set;

acquiring a new image longitudinally displaced said predetermined longitudinal distance relative to the prior image containing the color block set and analyzing the pixels of the new image by moving in a direction toward one of the edges and detecting the location of where the pixels cease having said low standard deviation of intensity values, thereby locating the edge of the web.

52. A method for measuring the reflective density of at least a first color printed on a moving web during operation of a web printing press, said press having printed a color block set on the web, the color block set including a sample of said first color, said method comprising the steps of:

acquiring a digitized image of a highly reflective standard by moving a CCD matrix sensor means to a location where said highly reflective standard is located, acquiring a matrix image thereof, digitizing the matrix image and generating a predetermined reflectivity value therefor;

acquiring a digitized image of a highly nonreflective standard by moving a CCD matrix sensor means to a location where said highly nonreflective standard is located, acquiring a matrix image thereof, digitizing the matrix image and generating a predetermined reflectivity value therefor;

analyzing said acquired digitized image of said reflective standard by measuring the pixel values at a plurality of locations throughout said acquired image and generating light discontinuity compensation signals for compensating for discontinuities of illumination at said locations where said values are measured;

acquiring a digitized image of a portion of the web by moving a CCD matrix sensor means to a location on the web where the color block set is printed and activating the CCD matrix sensor means in conjunction with firing a strobe means for illuminating the color block set during the image acquiring step, acquiring a matrix image thereof, digitizing the matrix image;

storing the digitized images of said color block set in a memory means;

measuring the intensity of illumination produced by said strobe means during a firing thereof and producing an intensity signal indicative of the intensity and color measured;

measuring the presence and intensity of printed matter located in an acquired digitized image having a color block set, and generating a light scattering compensation signal for compensating for any light scattering effects that are influenced by the presence and nature of printed matter in the image that affects the measured reflective density of at least said first color in the image;

analyzing the stored digitized images of said color block sets and producing a reflective density value of at least said first color, said reflective density value having been compensated for by said strobe intensity and color signals, said light scattering compensation signals and said light discontinuity compensation signals.

53. Apparatus for measuring the reflective density of at least a first color of a color block set printed on a moving web during operation of a web printing press, said apparatus comprising:

means for acquiring a digitized image of predetermined size of the web, said image being comprised of a plurality of individual pixels;

memory means for storing said digitized images;

strobe means for illuminating at least said predetermined area of the web when said strobe means is fired;

means for calibrating said acquiring means to provide predetermined values for high and low light reflectivity levels of said digitized images;

processing means for analyzing said digitized images containing at least said first color and produce a reflective density value thereof.

55. A printed target pattern for printing on a test print area of an elongated web by a web printing press, said pattern being adapted for use in determining a print contrast value of ink printed on the web, said target pattern comprising:

a solid print area;

a nonsolid generally uniformly screened print area having ink printed thereon which covers a percentage of said nonsolid print area, said percentage being within a predetermined range;

wherein said solid print area and said nonsolid print area are positioned adjacent one another substantially in the longitudinal direction of the web.

56. A pattern as defined in claim 55 wherein said predetermined range is approximately 70 to 80 percent.

57. Apparatus for measuring the reflective density of a block of predetermined size and color printed on a moving web of a web printing press, said apparatus comprising:

means for acquiring a digitized matrix image of a predetermined size of the moving web including said block, said digitized image being comprised of a plurality of individual pixels having an intensity value within a first predetermined range;

means for analyzing said image to determine that said image contains a first predetermined number of pixels of said block to comprise a valid block, said analyzing means analyzing said pixels of said valid block to determine the reflective intensity thereof.

58. Apparatus as defined in claim 57 wherein a said valid block comprises a first predetermined number of pixels having intensity values within a second predetermined range within said first predetermined range.

59. Apparatus as defined in claim 57 wherein said analyzing means analyzes said pixels of said block to determine the size and shape of said block to determine that said block is valid.