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Self-indicating test page for use in setting density level and color

1. bias voltage are represented by changes in a density setting In general as the setting increases the laser power increases and the developer AC bias increases The graph clearly shows that as the dpi or Lpi increases over the range from 26 dpi to 300 Lpi the effect of a change in density setting on the optical density also increases At 26 dpi the optical density varies from approximately 0 24 to 0 29 for a change in density setting from 2 to 2 At 300 Lpi the optical density varies from approximately 0 13 to 0 50 for the same change in density setting Since the fine dots are much more sensitive to the oper ating conditions than the coarse dots the optical density of the fine dots is affected much more than the optical density of the coarse dots by a change in the operating conditions Therefore a change in operating conditions results in a shift ofthe area along the fine dot gradient 20 where the fine dots appear to blend into the coarse dot background The coarse dot pattern 15 acts as a stable reference compared to the fine dot gradient 20 thus the test page is self indicating and no external reference is needed Since the fine dot gradient 20 is affected more than the coarse dot pattern 15 by changes in operating conditions the coarse dot pattern 15 has a stable optical density relative to the optical density of the fine dot gradient 20 for all acceptable operating conditions Acceptable operating con ditions are those o
2. is a flow chart showing a preferred method of the present invention FIG 4 is a preferred embodiment of a gray balance test pattern DETAILED DESCRIPTION OF THE INVENTION FIG 1 is a preferred embodiment of a dot gain test pattern 10 produced by a laser printer according to principles of the present invention for testing the optical density of a color of the laser printer The test pattern 10 is typically printed on a test page by the laser printer The dot gain test pattern 10 comprises a portion that is a uniform coarse dot pattern 15 and a portion that is a fine dot gradient 20 In this embodiment arrows 20 disposed across the page represent the fine dot gradient 20 Obviously however other patterns could equally be used The coarse dot pattern 15 is uniform in optical density In a preferred embodiment the coarse dot pattern 15 is 26 dots per inch dpi The fine dot gradient 20 varies in optical density producing a spectrum ranging from high to low optical density In the figure high to low density is portrayed from left to right varying from arrow 20 to arrow 20 across the page each arrow 20 having a uniform density The optical density of the fine dot gradient 20 is varied by pulse width modulation of the laser beam to produce the spectrum of optical densities In a preferred embodiment the fine dot gradient 20 is 150 lines per inch Lpi In a laser printer with a correct density level at some point along the fine dot gradi
3. the pattern to the top and magenta varies from low to high in the direction from left to right The center circle 210 of the gray balance test should match the background 215 If the center circle 210 does not match the background 215 a change in color density is indicated If a circle other than the center circle 210 matches the background 215 the test pattern indicates how the density levels should be changed in order to obtain a correct color balance For example if the upper right circle 220 matches the background 215 the magenta and yellow color density levels are both too low Since the matching circle 220 is two circles away from the center circle 205 in both the horizontal and vertical directions both magenta and yellow should be increased by two density levels In summary what has been described above are the preferred embodiments for a system and method for setting the color density level of a laser printer and a test page for use in setting the color density level of a laser printer While the present invention has been described by reference to specific embodiments it will be obvious that other alterna tive embodiments and methods of implementation or modi fication may be employed without departing from the true spirit and scope of the invention What is claimed is 1 A test page produced by a laser printer the laser printer having at least one primary color the test page for setting a density level of the laser prin
4. the point at which the fine dot gradient 20 blends into the coarse dot pattern 15 is located to the right of the mid spectrum area 25 then a need for a decrease in color density is indicated The fine dots which make up the fine dot gradient 20 are actually fractions of a full dot As such the fine dots do not receive full exposure to the drum The fine dots are very sensitive to changes in the operating conditions of the printer because of their less than full exposure to the drum In contrast the coarse dots are actually clusters of full dots Each full dot receives full exposure to the drum Accordingly the coarse dots are not very sensitive to changes in the operating conditions of the printer Changes in the operating conditions which may affect the color density produced by the laser printer include but are not limited to humidity temperature the discharge voltage of the photoconductor laser power developer AC bias toner charge and frequency of use Differences in sensitivity to the operating conditions can be demonstrated by showing how the different dot sizes react to a change in laser power and or developer AC bias voltage FIG 2 graphically shows the difference in sensitivity between the fine dots and the coarse dots The graph shows the relationship between the dpi or Lpi and the change in optical density caused by a change in laser power and or developer AC bias voltage These changes in laser power and or developer AC
5. US006034711A United States Patent 5 11 Patent Number 6 034 711 Trask et al 4 Date of Patent Mar 7 2000 54 SELF INDICATING TEST PAGE FOR USE IN 3 600272 3 1991 Japan em HO4N 1 46 SETTING DENSITY LEVEL AND COLOR 4 170268 6 1992 Japan nennen HO4N 1 40 BALANCE IN A COLOR LASER PRINTER 75 Inventors Jeffrey L Trask Brian Hoffmann both of Boise Id 73 Assignee Hewlett Packard Company Palo Alto Calif 21 Appl No 08 611 892 22 Filed Mar 6 1996 51 Jt E se ee ea G03G 15 01 52 U S CL scannen 347 240 347 251 347 254 399 39 399 49 399 72 58 Field of Search 1 347 240 254 347 251 399 39 49 72 181 56 References Cited U S PATENT DOCUMENTS 4 310 248 1 1982 Meredith sss 356 402 5 604 567 2 1997 Dundas et al sss 399 39 FOREIGN PATENT DOCUMENTS 0518525 12 1992 European Pat Off HO4N 1 46 0658962 12 1995 European Pat Off HO4N 1 60 62 251628 11 1987 Japan e GO1J 3 52 Primary Examiner N Le Assistant Examiner Hai C Pham 57 ABSTRACT A test page system and method are used to set a color density level of a laser printer by producing a dot gain test pattern The laser printer has at least one primary color The dot gain test pattern is self indicating and thus requires no external reference in order to determine a correct color density level A preferred embodiment of the dot gain test pattern includes a a
6. ary color the system comprising a means for producing a stable pattern having a color density which is stable with respect to changes in operating conditions of the laser printer b means for producing a sensitive pattern having a color density which is sensitive to changes in the operating conditions of the laser printer and wherein the sensitive pattern in reference to the stable pattern indicates without reference to any other source the density level of the laser printer for the at least one primary color 7 The system of claim 6 wherein the stable pattern includes a uniform coarse dot pattern and wherein the sensitive pattern includes a gradient of fine dot densities the gradient printed adjacent to the coarse dot pattern wherein a correct color density level of the laser printer for the at least one primary color is indicated when a mid spectrum area of the fine dot gradient appears to blend into the coarse dot pattern 8 A method for setting a color density level of a laser printer the laser printer having at least one primary color the method comprising a printing a stable pattern having a color density which is stable with respect to changes in operating conditions of the laser printer b printing a sensitive pattern having a color density which is sensitive to changes in the operating condi tions of the laser printer wherein the sensitive pattern in reference to the stable pattern indicates without refere
7. ce test may be printed on the test page A gray balance test is also referred to as a neutral gray test A gray balance test is employed in order to balance the color densities of the primary colors typically CMY The gray balance test is also a more visible way to check for the correct amount of yellow color density than the dot gain test pattern 10 In order to balance the color densities of the primary colors a light neutral gray area is produced using only the primary colors CMY Another light neutral gray area is produced using only the color black The two areas of light neutral gray are compared If the balance of CMY colors is not correct the neutral gray produced with only CMY will have a noticeable hue compared to the same neutral gray produced with only black In a preferred embodiment the gray balance test pattern is composed of 25 circles in a grid as shown in FIG 4 The grid background 215 is a light neutral gray color composed of black only The circles 205 are also a light neutral gray color but composed of CMY only Each circle has a slightly 6 034 711 5 different hue representative of a high or low density of one or two of the primary colors The circles are arranged so that they constitute a two directional spectrum ranging from low to high density for each of two primary colors in each of the two directions In this embodiment the density of yellow varies from low to high in the direction from the bottom of
8. d difficult because it requires from the user the ability to accurately compare the test page with the reference page In order to maintain consistent color densities in offset printing a dot gain test is used In the color printing industry offset printing is the standard for producing high quality color results A dot gain test compares the appearance of fine dots with the appearance of much coarser dots The high perimeter area ratio of the fine dots makes them much more sensitive to factors that cause more or less ink to be deposited A gradient of fine dot densities is printed in a uniform coarse dot background When viewed from a dis tance where the coarse dots cannot be independently distinguished the mid spectrum area of the fine dot gradient blends into the coarse dot background if the color density is correct Accordingly given the forgoing backgrounds relating to offset printing color laser printers and color density levels objects of the present invention are to provide a new test page system and method for setting the color density level of a laser printer SUMMARY OF THE INVENTION According to principles of the present invention in a preferred embodiment a test page system and method are used to set a color density level of a laser printer by providing a dot gain test pattern The laser printer has at least one primary color The dot gain test pattern is self indicating and thus requires no external reference in
9. ent 20 across the page the optical density of the fine dots will essentially match the optical density of the uniform coarse dot background 15 A laser printer with a correct density level will produce pat terns with an optical density of essentially 0 27 0 05 At the point where the optical densities essentially match the fine dot gradient 20 will appear to blend in with the coarse dot background 15 Although the test pattern 10 1s shown using only the color black the test pattern 10 will work equally well for any of the primary colors of a laser printer For a typical laser printer the only primary color is black K For a color laser printer the additional primary colors are typically cyan magenta and yellow CMY In a preferred embodiment for a correct color density the point at which the fine dot gradient 20 blends into the coarse 6 034 711 3 dot pattern 15 is located in the area 25 mid spectrum across the gradient 20 A correct color density indicates that no change in color density is needed If the point at which the fine dot gradient 20 blends into the coarse dot pattern 15 is not located in the area 25 mid spectrum across the gradient 20 then a need for a change in color density is indicated In this embodiment if the point at which the fine dot gradient 20 blends into the coarse dot pattern 15 is located to the left of the mid spectrum area 25 then a need for an increase in color density is indicated Conversely if
10. ill be shifted by a change in the developer AC bias voltage or the laser power This shift is due to the difference in how the fine dot gradient 20 and the coarse dot pattern 15 are affected by the change in the developer AC bias voltage and the laser power The density setting is adjusted in reference to the dot gain test pattern 10 which indicates whether the color density is too low or too high FIG 3 is a flow chart showing a preferred method of the present invention Referring to FIGS 1 and 3 the first step is to print 110 a dot gain test pattern 10 in a primary color Next the test pattern 10 is examined by a user 115 The test pattern 10 indicates whether the color density needs to be adjusted and whether the color density needs to be increased or decreased If the color density does need to be adjusted the user adjusts the color density 120 as indicated for the primary color in which the test pattern 10 was printed Another dot gain test pattern 10 is then printed 110 in the same color as the first dot gain test pattern 10 If the test pattern 10 indicates that another adjustment in color density is necessary the process is repeated until the color density for the tested color is correct Although reference has been made only to one dot gain test pattern 10 on the test page it is noted that multiple dot gain patterns 10 each for a different primary color may be printed on each test page In addition to a dot gain test a gray balan
11. invention relates in general to image transfer tech nology and more specifically to a self indicating test page for use in setting color density level in a laser printer BACKGROUND OF THE INVENTION In laser printers both the engine electro photographic EP process control tables and the formatter pulse width PW control tables are based on print quality characteriza tion done during life testing However factors such as the usage rate EP process components and printing environ ment will vary over time It is not possible to anticipate how these factors will vary from life test conditions Changes in these factors cause noticeable and typically objectionable color density differences between printers over time These color differences are a significant cause of service calls on color laser printers which usually result in replace ment of the drum cartridge and or replacement of the color developer Prior methods of correcting the color differences without a service call include installing optical density detectors inside the printer The optical density detectors provide feedback to the printer which in turn adjusts the color density levels This approach adds cost and complexity to the printer Another approach is to print out a test page and to compare the test page with a reference page typically included in the user s manual This approach is inconvenient because it requires the user to keep track of the reference page an
12. nce to any other source the density level of the laser printer for the at least one primary color of the laser printer and c adjusting the color density level of the laser printer for the at least one primary color based on visual obser vation of the sensitive pattern in reference to the stable pattern 9 The method of claim 8 wherein the stable pattern includes a uniform coarse dot pattern and wherein the sensitive pattern includes a gradient of fine dot densities the gradient printed adjacent to the coarse dot pattern wherein a correct color density level of the laser printer for the at least one primary color is indicated when a mid spectrum area of the fine dot gradient appears to blend into the coarse dot pattern 10 The method of claim 8 further including repeating steps a and b until the color density level is correct 11 The method of claim 8 further including a printing a gray balance test pattern and b further adjusting the color density level of the laser printer for the at least one primary color based on visual observation of the gray balance test pattern
13. order to determine a correct color density level A preferred embodiment of the dot gain test pattern includes a a stable pattern having a color density which is stable with respect to changes in the laser printer operating conditions and b a sensitive pattern having a color density 10 15 20 25 30 35 40 45 50 55 60 65 2 which is sensitive to changes in the operating conditions wherein the sensitive pattern in reference to the stable pattern indicates the density level of the laser printer for the at least one primary color According to further principles of the present invention in a preferred embodiment the stable pattern includes a uni form coarse dot pattern and the sensitive pattern includes a gradient of fine dot densities the gradient printed adjacent to the coarse dot pattern wherein a correct density level of the laser printer for the at least one primary color is indicated when a mid spectrum area of the fine dot gradient appears to blend into the coarse dot pattern Other objects advantages and capabilities of the present invention will become more apparent as the description proceeds DESCRIPTION OF THE DRAWINGS FIG 1 is a dot gain test pattern produced by a laser printer using a preferred embodiment of the present invention FIG 2 is a graph showing the dot sensitivity to changes in developer AC bias voltage and or laser power versus dots per inch or lines per inch FIG 3
14. perating conditions which allow the laser printer to function properly as a laser printer The dot gain test pattern 10 is self indicating because no external reference is needed in order to determine if the color density is correct Other types of test patterns for laser 10 15 20 25 40 45 50 55 60 65 4 printers are not self indicating because they rely on a comparison between the test pattern and some external reference Typically the external reference is included in the user s manual The external reference could also be the memory of the user If a user has to look at a test pattern and use judgement to determine if the color density appears correct the user s memory of what a correct color density looks like is an external reference In order to remedy an incorrect color density compensa tion is made to account for the change in operating condi tions from life test conditions The process control engine for the laser printer has the ability to modify the developer AC bias voltage and the laser power In a preferred embodiment adjustments in the laser power and or devel oper AC bias voltage from the normal electro photographic EP process tables are represented by a change in a density setting As discussed above typically as the setting increases the laser power increases and the developer AC bias increases The area along the fine dot gradient 20 that blends in with the coarse dot pattern 15 w
15. stable pattern having a color density which is stable with respect to changes in the laser printer operating conditions and b a sensitive pattern having a color density which is sensitive to changes in the operating conditions wherein the sensitive pattern in reference to the stable pattern indicates the density level of the laser printer for the at least one primary color Additionally in a preferred embodiment the stable pattern includes a uniform coarse dot pattern and the sensitive pattern includes a gradient of fine dot densities the gradient printed adjacent to the coarse dot pattern wherein a correct density level of the laser printer for the at least one primary color is indicated when a mid spectrum area of the fine dot gradient appears to blend into the coarse dot pattern 11 Claims 4 Drawing Sheets 15 20 6 034 711 Sheet 1 of 4 Mar 7 2000 U S Patent 01 L Sis Ge U S OPTICAL DENSITY Patent Mar 7 2000 Sheet 2 of 4 6 034 711 71 o 1 DENSITY SETTING 26dpi e 150lpi a 300Ipi SENSITIVITY VS DOTS LINES PER INCH FIG 2 U S Patent Mar 7 2000 Sheet 3 of 4 6 034 711 START PRINT DOT GAIN TEST PATTERN ADJUST COLOR DENSITY LEVEL AS INDICATED BY TEST PATTERN FIG 3 U S Patent 6 034 711 1 SELF INDICATING TEST PAGE FOR USE IN SETTING DENSITY LEVEL AND COLOR BALANCE IN A COLOR LASER PRINTER FIELD OF THE INVENTION This
16. ter for the at least one primary color the test page comprising a a stable pattern having a color density which is stable with respect to changes in operating conditions of the laser printer b a sensitive pattern having a color density which is sensitive to changes in the operating conditions of the laser printer and wherein the sensitive pattern in reference to the stable pattern indicates without reference to any other source the density level of the laser printer for the at least one primary color 2 The test page of claim 1 wherein the stable pattern includes a uniform coarse dot pattern and wherein the sensitive pattern includes a gradient of fine dot densities the gradient printed adjacent to the coarse dot pattern wherein a correct density level of the laser printer for the at least one primary color is indicated when a mid spectrum area of the fine dot gradient appears to blend into the coarse dot pattern 3 The test page of claim 1 wherein at least one primary color is selected from the group consisting of black cyan magenta and yellow 4 The test page of claim 1 further including a gray balance test pattern 10 15 20 30 35 40 45 50 55 6 5 The test page of claim 4 wherein at least one primary color is selected from the group consisting of black cyan magenta and yellow 6 A system for setting a color density level of a laser printer the laser printer having at least one prim

Self-indicating test page for use in setting density level and color

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