Abstract:
A method of operating a digital printer comprises printing a machine-readable code and at least one test patch on at least one sheet. The photosensor detects the machine readable code on the moving sheet, and, in response to the detecting, initiates the photosensor reading the test patch on the moving sheet. The test patches can be distributed over many sheets, depending on the nature of the desired test routine and the type of sheet that has recently been output by the printer.

Description:
INCORPORATION BY REFERENCE 
   The following US Patent is hereby incorporated by reference in its entirety for the teachings therein: U.S. Pat. No. 6,639,669. 
   TECHNICAL FIELD 
   The present disclosure relates to digital printing apparatus, such as using xerographic or ink-jet technology, and carrying out image-quality tests therein. 
   BACKGROUND 
   In color printing using digital printers, it is common to use test patches for color calibration. The calibration process involves sending an image with pre-specified device signals (i.e., a “test patch”) to the printer, and making spectrophotometric measurements of the test patches with the use of a spectrophotometric scanner. The device and spectrophotometric signals together are used to build or update the calibration tables or other controls associated with the printer. 
   U.S. Pat. Nos. 6,048,117 and 6,972,867 relate to the problem of calibrating each of a large population of digital printers, using a single input scanner, as would be used, for example, with a color digital copier. The systems described in these patents use bar codes, or other kinds of machine-readable code, on the sheets on which test patches are printed, in order to identify, among other things, the printer that was the source of each test sheet. U.S. Pat. No. 6,883,892 makes a similar teaching. 
   In a high-speed, production context, it is known to provide detectors and image sensors immediately downstream of a printing apparatus for various purposes, as shown in U.S. Pat. Nos. 5,488,458; 6,324,353; 6,684,035; and 6,987,025. 
   U.S. Pat. No. 6,639,669 discloses the use of machine-readable “triggers” on sheets bearing images to be tested. 
   U.S. Pat. No. 5,148,268 gives a description of a photosensitive chip useful as an image sensor. 
   SUMMARY 
   According to one aspect, there is provided a method of operating a digital printer. A control system decides whether to print a machine readable code on a first sheet and at least one test patch on a second sheet, or to print the machine readable code and the at least one test patch on a one sheet. The sheet having the machine-readable code moves at a substantially constant velocity past the photosensor following printing. In response to the photosensor detecting the machine-readable code, the photosensor reads the test patch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified elevational view of a xerographic printer with a photosensor associated therewith. 
       FIG. 2  is a plan view of a single sheet as would emerge from the printer and move past the photosensor as in  FIG. 1 . 
       FIG. 3  is a plan view of two sheets as would emerge from the printer and move past the photosensor as in  FIG. 1 . 
       FIG. 4  is a simplified plan view of the light-sensitive portions of a photosensor. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a simplified elevational view of a printing apparatus; in this case a production-speed xerographic printer. The print engine  100  is of a type generally familiar in the art: a photoreceptor  102  rotates past imaging stations  104 , one imaging station for each CMYK primary color, and each including (not shown) a charge device, laser, and development unit. Each imaging station, controlled by digital data supplied thereto, places toner according to a color separation of a desired image on the photoreceptor  102 , and the total color image is then transferred to a print sheet at transfer station  106 . The print sheet then moves in a process direction P through a fusing station  108 , and continues, at a constant velocity, past what can be called a “photosensor”  200 . Photosensor  200 , which may include an associated light source, is positioned to read at least a strip of each sheet substantially immediately following fusing, fusing being considered in this embodiment the end of the printing process. 
     FIG. 2  is a plan view of a single sheet S as would emerge from the fuser  108  and move past the photosensor  200  as in  FIG. 1 . As shown in the Figure, the sheet S is approaching the photosensor  200  along process direction P. The photosensor  200 , in this embodiment, is of a width (perpendicular to P) of about 1 cm-3 cm, and is disposed to read a strip of comparable width on sheets S moving therepast. 
   The overall function of the disclosed system is that, when it is desired to perform a calibration, or other test routine involving one or more test patches, on a print engine  100 , the print engine is simply caused to print, on one or more sheets, a machine-readable “trigger” code, plus one or more test patches. Following printing, the sheet moves at a substantially constant velocity past the photosensor  200 . When the photosensor  200  reads and recognizes the trigger code, an associated control system, generally indicated as  300 , is programmed to expect inputs from one or more test patches. The reflected light from the test patches is received by the photosensor  200  for purposes of calibration or other control. In one embodiment, the velocity of a sheet does not appreciably change between receiving an image, such as at transfer station  106  or fuser  108 , and the reading by photosensor  200 . 
   As shown in  FIG. 2 , a sheet S that is printed for purposes of calibration bears an image created by the print engine  100 , the image including a portion of machine-readable trigger code indicated as C, placed on sheet S to be viewable by photosensor  200  as the sheet S moves therepast. Immediately adjacent the machine readable code C is what is here a white space (with no marking material), but what can be broadly considered a control or “white balance” area for establishing a control reflection from the light source associated with the photosensor  200 . Following the control area, as sheet S moves past photosensor  200 , a series of test patches T is then positioned sequentially past the photosensor  200  for recording according to a predetermined image-quality procedure. 
   With a high-speed print engine  100 , or a test routine involving a large number of necessary test patches, the test patches associated with a test routine initiated by a trigger code C may have to be spread among a plurality of sheets S.  FIG. 3  is a plan view of two sheets, S 1  and S 2 , as would emerge from the fuser  108  and move past the photosensor  200  as in  FIG. 1 . The first sheet to emerge from the print engine  100 , indicated as S 1 , includes the machine-readable code and a predetermined length of control area; the second sheet S 2  includes the test patches T. In various embodiments, the length along the process direction P of the control area of each test patch will be determined by the overall process speed of sheets emerging from the print engine  100 , as well as the responsivity of the control systems associated with photosensor  200  (each of which affects a “dwell time” each patch must remain under the photosensor  200 ). 
   A control system such as  300  is programmed to fit test patches on whatever size sheets happen to be emerging from the machine at a given time: whereas the sheet S in  FIG. 2  is on a relatively large, short-edge-fed stock, the sheets S 1 , S 2  are on smaller, long-edge-fed stock. Since the smaller sheets cannot fit all the necessary test patches on each sheet, multiple sheets are printed. Control system  300  configures test patches on one or multiple sheets depending, for instance, on the size of the stock on which the last previous set of images was printed (such as by taking information relating to a recent print job). In this way, the printer would not have to momentarily switch, such as in the middle of a long print run, to a different input tray just to output one or two sheets for the test routine. Another parameter control system  300  takes into account is the number of individual test patches required for a particular test triggered by the immediate machine-readable code C. 
   The machine-readable code C must be of a configuration that is recognizable by photosensor  200  at process speeds: familiar bar-code technology can be used, but less sophisticated machine-recognizable images can be used as well. A control system can be provided that associates different machine-readable codes with performing different types of test routines, causing the photosensor  200  to “expect” different types of test patches depending on the specific machine-readable trigger code. 
   In one embodiment, the printing of the test sheet such as S, having the machine-readable trigger code thereon, is the primary channel for initiating image-quality test routines. In other words, in a practical application, an overall control system governing the engine  100  initiates each of various possible test routines by directly influencing the job queue or other source of image data controlling the engine  100 , causing the engine to output one or more images including the desired trigger machine-readable code and related test patches. The particular test routine is initiated only when the photosensor  200  detects the trigger code, and generally not through any other channel to the control system  300 . 
   When the photosensor  200  is used as the primary channel for initiating test routines, test routines can be scheduled through the same channels through which print jobs are scheduled, such as shown in  FIG. 1  as computer  400 . (In a practical application, computer  400  can be one of any number of stand-alone computers that are capable of sending print jobs to engine  100 .) In one strategy, a computer acting as a source of print jobs can schedule various desired tests as print jobs, scheduling the test routines between print jobs as needed, e.g., at the beginning of a shift, or following a set of jobs adding up to a total number of prints within a predetermined range. In another strategy, for instance, a test routine is scheduled following every 10,000 prints: the counting of prints can be carried out via the print-queue software, and, in response to hitting the 10,000 count, an “interrupt job” including the test sheets is scheduled as an interrupt to whatever job is being printed at the time; but even in this case, the test routines are scheduled through the print-queue software. (Interrupt jobs are a familiar concept and utility in many types of job scheduling software.) The feedback loop controlling the engine  100  based on test patch data collected from photosensor  200  thus operates largely independently of software sending image data to the engine  100 . 
     FIG. 4  is a simplified plan view of the light-sensitive portions of a photosensor  200 . In one embodiment, the photosensor is capable of distinguishing images suitable for recognizing machine-readable code such as a bar code, and also of accurately measuring reflected colors from the test patches, in effect acting as a spectrophotometer. As shown in the Figure, one embodiment of such a photosensor includes three linear arrays  202 R,  202 G, and  202 B of photosites, each linear array including one RGB primary-color filter. U.S. Pat. No. 5,148,268 (and patents referenced thereby) gives a description of a photosensitive chip suitable for such a purpose. Alternatively, a photosensor arrangement such as shown and described in FIGS. 1 and 2 of U.S. Pat. No. 6,639,669 can be used. Although the area viewed by the photosensor  200  in the Figures is relatively narrow compared to the width of sheets being printed upon, it may be desirable in some circumstances to provide a photosensor that extends the full width of the sheet path. 
   Although engine  100  is shown as a color xerographic printing machine, the above-described embodiment can be readily adapted for any kind of printing technology, such as monochrome xerography, or ink-jet or offset printing. The described system can be embodied in hardware, software, or a combination thereof. 
   The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.