fig1 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 . fig2 is a plan view of a single sheet s as would emerge from the fuser 108 and move past the photosensor 200 as in fig1 . 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 fig2 , 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 . fig3 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 fig1 . 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 fig2 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 fig1 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 . fig4 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 fig1 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 .