Patent Publication Number: US-6659578-B2

Title: Tuning system for a compact optical sensor

Description:
INTRODUCTION 
     The present invention relates generally to optical sensing systems, such as those which are used in hardcopy devices for scanning and/or printing images on print media, for example, using inkjet printing technology. 
     Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as “ink,” onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are described and shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company of Palo Alto, Calif. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). 
     To clean and protect the printhead, typically a “service station” mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as “spitting.” The waste ink is collected at a spitting reservoir portion of the service station, known as a “spittoon.” After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. 
     Optical sensors have been incorporated into various inkjet printing mechanisms, such as printers and plotters, for the past several years. These optical sensors illuminated the media using one to twelve light emitting diodes (“LEDs”). In U.S. Pat. No. 6,036,298, currently assigned to the present assignee, the Hewlett-Packard Company, a single monochromatic, or “quasimonochromatic” LED was proposed using a blue LED. This patent also has a detailed description of several prior art optical sensors, including those using the red and green LEDs. A single LED optical sensor emitting a blue-violet light was first introduced in the DeskJet® 990C model color inkjet printer last year. The single blue-violet LED illuminated the media, while two sensors received light reflected from the media, with one receiving diffuse light beams, and the other receiving specular light beams. Incoming light was restricted by two different stops, two rectangular windows having longitudinal axes which were perpendicular to one another. From information gathered by the sensor, the printer controller determined which type of media was entering the printzone and then adjusted the printing routines to provide an optimal image on the particular media used. 
     Unfortunately, all of these earlier optical sensors employed in inkjet printing mechanisms used bulky, commercial LEDs, which caused the sensors to occupy a large amount of space within the printing mechanism. It is believed that earlier this year, plotter designers for the Hewlett-Packard Company introduced a three LED optical sensor, using LEDs of the colors blue, green, and amber in the Designet® 10 ps, 20 ps and 50 ps models of color inkjet plotters. While the amount of space consumed by a sensor in a large floor mounted plotter has little impact on the overall desirability of the unit, in the desktop printing market, many consumers prefer a compact printing unit which occupies very little desk space, known in the art as having a small “footprint.” Thus, in the desktop printer market, use of a wide bulky sensor mounted on the printhead scanning carriage increased the overall width of the printer by up to an inch (2.54 cm). While plotter designers were able to use optical sensors having multiple LEDs without impacting the overall plotter design, designers of desktop printers strived to find ways to use a single LED, for instance as described above in U.S. Pat. No. 6,036,298 and as sold in the DeskJet® 990C model color inkjet printer, mentioned above. Use of two or more LEDs in the desktop printer market was unthinkable, due to the adverse impact such a multiple LED sensor would have on a printer&#39;s footprint, theoretically making a printer up to two inches (5.08 cm) wider. Such an additional width in a desktop printer could well make consumers turn away from the printer, and buy a more compact printer produced by a competitor, even at the expense of sacrificing the print quality benefits achieved by printers employing an optical sensor system. Furthermore, while these earlier optical sensor systems may have had some calibration at the factory, none are known to have had any way of automatically calibrating the sensors after the printing units left the factory. 
     One hand held color scanner has been developed by Color Savvy, of Springboro, Ohio, as described in the paper entitled “An LED Based Spectrophotometric Instrument, ” by Michael J. Vrhel, published as a part of the IS&amp;T/SPIE Conference on Color Imaging: Device-Independent Color, Color Hardcopy , and Graphic Arts IV, San Jose, Calif., January 1999 (SPIE Vol. 3648, No. 0277-786X/98), as well as the system described in Color Savvy&#39;s International Patent Application No. PCT/US97/16009, published Mar. 19, 1998, International Application No. WO 98/11410. Indeed, Color Savvy even advertises a scanning adapter that may be attached to the printhead scanning carriage of some inkjet printers, allowing the system to scan previously printed images. These devices made by Color Savvy are designed to “see” an infinite variety of different colors, shades and hues, and to accomplish this objective in a satisfactory manner, Color Savvy needs eight to sixteen different colored LEDs to illuminate the image. As mentioned above, such a bulky sensor having multiple LEDs will be too cumbersome for use in typical inkjet printers. Note that the Color Savvy adapter, when placed in an inkjet printer, rendered the unit unusable for printing. 
    
    
     DRAWING FIGURES 
     FIG. 1 is a perspective view of one form of a hardcopy device, here shown as an ink printing mechanism, and in particular, a desktop inkjet printer incorporating one form of a compact optical sensing system of the present invention. 
     FIG. 2 is a bottom perspective view of one form of a compact optical sensor used in the sensing system of FIG.  1 . 
     FIG. 3 is a side elevational sectional view of the compact optical sensor of FIG. 2, shown monitoring a portion of a sheet of print media, such as paper. 
     FIG. 4 is an exploded view of the compact optical sensor of FIG.  2 . 
     FIG. 5 is a graph showing the relative specular reflectances and specular absorbances versus illumination wave length for cyan, yellow, magenta and black inks, and for blue, green, soft-orange and red illuminating LEDs used by the optical sensor of FIG. 2 when monitoring images printed on white media, such as plain paper. 
     FIG. 6 is a perspective view of an alternate hardcopy device, here showing several components of a printing system which may be used in variety stores, drug stores, the like, to print photographic-quality pictures taken on film or digitally, including one form of a calibrating system for use with a compact optical sensor, such as shown above in FIG.  2 . 
     FIG. 7 is a perspective view of one form of a printhead service station, including the calibrating system of FIG.  6 . 
     FIG. 8 is an enlarged, partially fragmented, top plan view of the calibrating system of FIG.  6 . 
     FIG. 9 is a side elevational, sectional view taken along lines  9 — 9  of FIG.  8 . 
     FIG. 10 is a top plan view of the calibrating system of FIG. 6, shown in a printing position. 
     FIG. 11 is a top plan view of the calibrating system of FIG. 6, shown in a calibrating position. 
     FIG. 12 is a top plan view of the calibrating system of FIG. 6, shown in a storage position during a period of printing inactivity. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an embodiment of a hardcopy device  20  having a reciprocating head, which may be constructed in accordance with the present invention such as a scanner, an inkjet printing mechanism, or multi-function hardcopy device having both scanning and printing capabilities. Initially, for the purposes of illustration, the hardcopy device  20  is described as an inkjet printing mechanism, here shown as an “off-axis” inkjet printer  20 , constructed in accordance with the present invention, which may be used for printing business reports, correspondence, desktop publishing, and the like, in an industrial, office, home or other environment. A variety of inkjet printing mechanisms are commercially available. For instance, some of the printing mechanisms that may embody the present invention include plotters, portable printing units, copiers, cameras, video printers, and facsimile machines, to name a few, as well as various combination devices, such as a combination facsimile/printer which has both scanning and printing capabilities. For convenience the concepts of the present invention are illustrated first in the environment of an inkjet printer  20 . 
     While it is apparent that the printer components may vary from model to model, one typical inkjet printer  20  includes a chassis  22  surrounded by a housing or casing enclosure  24 , the majority of which has been omitted for clarity and viewing the internal components. Sheets of print media are fed through a printzone  25  by a print media handling system  26 . The print media may be any type of suitable sheet material, such as paper, card stock, envelopes, fabric, transparencies, mylar, and the like, but for convenience, the illustrated embodiment is described using plain paper as the print medium. The print media handling system  26  has a media input, such as a supply or feed tray  28  into which a supply of media is loaded and stored before printing. A series of conventional media advance or drive rollers (not shown) powered by a conventional motor and gear assembly (not shown) may be used to move the print media from the supply tray  28  into the printzone  25  for printing, and then into the output tray  30  for drying. Some inkjet printers employ a series of retractable and/or extendable wings (not shown) upon which a freshly printed sheet momentarily dries before being dropped into the output tray, to prevent smearing of a previously printed sheet lying below in the output tray  30 . The media handling system  26  may include a series of adjustment mechanisms for accommodating different sizes of print media, including letter, legal, A4, envelopes, photo media, and the like. To secure the generally rectangular media sheets in the input tray, a sliding width adjustment lever  32  and a sliding length adjustment lever  34  may be used. 
     The printer  20  may receive inputs from a variety of different mechanisms, such as through a keypad  36 . In the illustrated embodiment, the chassis  22  supports a guide rod  38  which in turn, slidably supports a printhead carriage  40 . The carriage  40  moves back and forth reciprocally over a printzone  25 , and into a servicing region  42 . The carriage  40  may be driven by a conventional carriage propulsion system, such as via an endless belt and drive motor (not shown). The carriage propulsion system also has a positional feedback system, such as a conventional optical encoder system including an encoder strip  44  and an encoder strip reader (not shown) mounted on the carriage  40 . Signals regarding the carriage position are then fed to a controller portion  45  of the printer. The controller  45  also controls media movement through the printzone, ink ejection for printing, and various servicing routines. The various electrical conductors and wiring for coupling the controller to these different subsystems of printer  20  have been omitted for clarity. As used herein the printer controller  45  is illustrated schematically as a microprocessor, that receives instructions from a host device, typically a computer, such as a personal computer (not shown) indeed, many of the printer controller functions may be performed by the host computer, by electronics on board the printer, or by interactions therebetween. As used herein, “printer controller 45” encompasses these functions, whether performed by the host computer, the printer, an intermediary device therebetween, or by a combined interaction of such elements. A monitor coupled to the host computer may be used to display visual information to an operator, such as the printer status or a particular program being run on the host computer. Personal computers, their input devices, such as keyboard and/or a mouse device, touch pads, and monitors are all well known to those skilled in the art. 
     In the printzone  25  the media receives ink from an inkjet cartridge, or here in the illustrated embodiment from six inkjet cartridges  50 ,  51 ,  52 ,  53 ,  54  and  55  carrying (1) light cyan, (2) cyan, (3) black, (4) magenta, (5) light magenta and (6) yellow colors of ink, respectively. The illustrated inkjet printer  20  is known as an “off-axis” inkjet printer, because the carriage mounted cartridges  50 - 55  carry only a small supply of ink, which is replenished through a series of flexible ink tubes  56  from a stationary main reservoir portion  58  of the printer. In the illustrated embodiment, the main reservoir portion  58  houses six separate ink reservoirs  60 ,  61 ,  62 ,  63 ,  64 , and  65  which supply ink to the respective inkjet cartridges  50 ,  51 ,  52 ,  53 ,  54 , and  55 . In contrast to the off-axis ink delivery system shown in FIG. 1, a suitable substitution may be an inkjet printer having replaceable cartridges, which carry the entire ink supply within the carriage  40  as it reciprocates over the printzone  25 . Hence, a replaceable cartridge system may be considered as an “on-axis” system because the entire ink supply is carried along a scanning axis  66 , which is defined by the guide rod  38 . While one form of an on-axis system carries replaceable cartridges where both the ink ejecting printhead and the ink reservoir are supplied as a unit and replaced when the cartridge is empty, another on-axis system is known in the industry as a “snapper.” In a snapper system, the printheads are permanently or semi-permanently mounted to the printhead carriage, and the ink supply is a separate unit which is snapped onto the printhead. 
     A variety of different types of inkjet printheads may be employed, such as thermal printheads, piezo-electric printheads, and silicon electrostatic actuator (“SEA”) printheads, as well as other types of printhead technology known to those skilled in the art. One example of SEA inkjet technology is disclosed in U.S. Pat. No. 5,739,831 to Nakamura (assigned to the Seiko Epson Corporation). The illustrated embodiment presumes that thermal inkjet printheads are used where a firing resistor is associated with each one of the ink ejecting nozzles. Upon energizing a selected resistor, a bubble of gas is formed which ejects a droplet of ink from the nozzle and onto a sheet of paper in the printzone  25  under the nozzle. The printhead resistors are selectively energized in response to firing command control signals received by the carriage  40  from the controller  45 , with the carriage  40  delivering these firing signals to the printheads of each of the cartridges  50 - 55 . 
     Compact Optical Sensing System 
     Also shown in FIG. 1, and in greater detail in FIGS. 2 through 4, is a compact optical sensor system  100 , constructed in accordance with the present invention. In FIG. 1, we see the sensor  100  being mounted on an outboard side of the carriage  40 . As used herein, the term “inboard” refers to components facing toward the printzone  25 , that is, in the positive X-axis direction, whereas the term “outboard” refers to components facing toward the servicing region  42 , that is, in the negative X-axis direction. The optical sensor  100  includes a housing or frame  102  shown in FIG. 4 as defining one or more mounting fixtures, such as mounting hole  104  for attaching the sensor  100  to carriage  40 . Alternatively, it is apparent that the sensor housing  102  and other external components may be formed as an integral part of carriage  40  in some implementations. 
     The sensor  100  also includes a printed circuit assembly (“PCA”)  105 , which was instrumental in creating the illustrated embodiment of the compact sensor system  100 . The PCA  105  has a connector receptacle  106  that communicates with controller  45 , via, for instance, conventional flexible cables (not shown) which connect the controller  45  with carriage  40  to deliver firing signals to the printheads of the inkjet cartridges  50 - 55 . The PCA  105  includes two light-to-voltage converters, or photodiodes  108 ,  110  for receiving diffuse and specular reflected light, respectively. Note that the specular portion of the sensor  100  is only needed presently for media type sensing, so if only information about color matching and the inks being laid down by the printer  20  is desired, then the specular photodiode  110  and related specular components may be omitted. Preferably, each of the photodiode light-to-voltage converters  108 ,  110  are identical in construction to provide ease of manufacturing and a more economical, compact optical sensor  100 . The illustrated output voltage is an analog signal which is passed through an amplifier with a specified gain, for instance, a three times gain. This amplified signal is then passed to an analog-to-digital (“A/D”) converter which may be a portion of the printed circuit assembly  105 , a portion of the electronics onboard carriage  40 , or a portion of the controller  45 . 
     The PCA board  105  is constructed such that the specular and diffuse photodiodes  108 ,  110  receive light through incoming light passages  112 ,  114  defined by the housing  102 . To align the photodiodes  108 ,  110  with the light passages  1124 ,  114 , the housing  102  includes a support surface  115 , which preferably has a lip, shown to the right of photodiode  110  in FIG. 3, under which the PCA board  105  is received. In the illustrated embodiment, the PCA board  105  defines an alignment hole  116  therethrough, which when assembled is received upon an alignment post  118  extending upwardly from the housing support surface  115 , as shown in FIG.  3 . 
     The PCA board  105  includes four light emitting diodes (LEDs)  120 ,  122 ,  124  and  126  which, in the illustrated embodiment are the colors, blue, green, red and soft-orange, respectively. The construction of the printed circuit assembly  105  advantageously uses a chip-on-board (“COB”) process where the bare silicon die for each component is wire bonded directly to the printed circuit board assembly. Thus, in the illustrated embodiment, the LEDs  120 - 126  may be closely grouped together, in a space smaller than that occupied by a factory-made, single-packaged LED, such as that disclosed in U.S. Pat. No. 6,036,298, as well as that commercially sold in the DeskJet® 990C model color inkjet printer. Note that the LEDs  120 - 126  and photodiodes  108 ,  110  have been drawn with some artistic license in FIG. 4 to be about twice their normal size to better illustrate the concepts introduced herein. By clustering the LEDs  120 - 126  so closely, a single outgoing optical light path  128  defined by the housing  102  may accommodate light generated by all of these LEDs. While the chip-on-board process has been used in other implementations, the inventors believe this to be the first such use of the process in manufacturing an optical sensor, such as sensor  100 , for monitoring various processes associated with inkjet printing, including: (1) closed-loop color calibration, (2) automatic printhead alignment, (3) media type sensing, (4) swath height error correction, and (5) linefeed calibration. 
     The illustrated embodiment includes two optional filter elements, one a diffuse filter element  130 , and the other a specular filter element  132 , preferably of colors selected to block long, infrared wavelengths, although in some implementations, other filters may be used to either filter or pass through more specific wavelength bands. In the illustrated embodiment, the filter elements  130 ,  132  are infrared wavelength blocking filters, such as those designed to block infrared wavelengths between 700 and 1000 nm (nanometers). Each of the filter elements  130 ,  132  are received within a recessed shelf portion  134 ,  136  defined by the housing  102 . The filter elements  130 ,  132  serve to limit the incoming light to the diffuse and specular photodiodes  108 ,  110  to light within the regions of the visible spectrum. In the preferred embodiment, an upper portion of the incoming light passages  112 ,  114  is molded with a square diffuse stop, and a rectangular specular stop, with the longitudinal axis of the specular stop running perpendicular to the longitudinal axis of the housing  102 , that is, parallel with the X-axis. Use of such a specular stop was made in the DeskJet® 990C model color inkjet printer. Again, the term “stops” refers to a window through which incoming light passes before it is received by in this case, the specular photodiode  110 . 
     The compact optical sensor  100  also includes a lens assembly  140 , which is received by a pair of lower extremities  142  of the housing  102  preferably via a pair of snap fitments, such as the snap fitment  144 . In this manner, the filter elements  130 ,  132  are held in place within recesses  134 ,  136  by the lens assembly  140 . The lens assembly  140  includes an outgoing LED lens  145 , and two incoming lenses, here, a diffuse lens  146  and a specular lens  148 . The lens elements  145 ,  146  and  148  are preferably selected to better focus and direct the light beams to follow the paths shown in FIG. 3, and as discussed further below after the remaining components of the optical sensor  100  have been introduced. 
     Preferably the sensor  100  includes an ambient light shield member  150 . The ambient light shield  150  slides over the lens assembly  140  and is attached to the housing  102 , for instance using various snap fitments, bonding elements, such as adhesives, fasteners or the like (not shown). The ambient light shield  150  has a pair of opposing slots  152  and  154  which are located to receive and secure a clear aerosol shield member  155 . The aerosol shield  155  in the illustrated embodiment is inserted through slot  152  then through slot  154 , with the forward insertion being limited by a stop  156  encountering a portion of the body of the ambient light shield  150  (see FIG.  2 ). A snap fitment member  158  flexes upwardly during insertion of the aerosol shield  155 , then latches down over a lower portion of the slot  154  (see FIG. 2) to hold the aerosol shield  155  in place within the ambient light shield  150 . Preferably, the aerosol shield  155  has an anti-reflection coating or property which allows light beams to pass therethrough without undue interference from the aerosol shield  155 . 
     The term “aerosol” refers to tiny ink droplets which are emitted by the ink ejecting printhead nozzles in addition to the main droplet which is intended to hit the print media and create an image. These ink aerosol satellites randomly float throughout some models of inkjet printers, and eventually some land on internal components of the printer mechanism. To prevent these floating ink aerosol satellites from landing on the lens assembly  140 , and fouling or otherwise permanently altering the incoming light received by the photodiodes  108 ,  110 , the aerosol shield  155  serves to collect a majority of these mischievous aerosol satellites. Use of the snap fitment  158  allows the aerosol shield  155  to be removed from the ambient light shield  150  and cleaned or replaced periodically during the lifetime of the printing mechanism  20 . Preferably, the thickness of the aerosol shield  155  is only slightly less than the depth of slots  152  and  154 , so the aerosol shield  155  serves to isolate the interior of the ambient light shield  150  from contamination by these ink aerosol satellites. 
     Now the components of the optical sensor are understood, we will turn to the operation of the compact optical sensor  100 , as shown in the cross-sectional view of FIG.  3 . In FIG. 3, we see the LEDs  120 ,  122 ,  124 , and  126  emitting light beams through the outgoing passageway  128 , through the outgoing lens  145 , and emerging as light beams  160 ,  162 ,  164 , and  166 , respectively exiting through a light entrance/exit chamber portion  168  of the ambient light shield  150 . The emerging light beams  160 - 166  impact an upper exposed print surface of a sheet of print media  169 , here, a sheet of plain paper in the illustrated embodiment. Light beams i  60 ,  162 ,  164 , and  166  are reflected directly off the media  169  as upwardly directed diffuse light beams  170 ,  172 ,  174 , and  176 , respectively. For those who may be unfamiliar with the science of optics, the term “diffuse” refers to light which is scattered (at any angle) when reflected from a surface. The portion of the diffuse light which is used in the illustrated embodiment are the perpendicular beams reflected off of the media  169 , as shown for the diffuse light beams.  170 - 176  in FIG.  3 . The incoming diffuse light beams  170 - 176  pass through lens  146 , through filter  130 , and through the incoming light chamber  112  and through a rectangular stop or window  178  where they are received by the diffuse photodiode  108 . The photodiode  108  is a light-to-voltage converter, as mentioned above, which interprets these incoming diffuse light beams  170 - 176  and produces a voltage signal proportionate to the intensity of these incoming light beams. This voltage signal is sent via receptical  106  and cable  107 , through the carriage  40  to controller  45 , where this information is then used by the controller to adjust various printing parameters, as mentioned above. 
     Besides forming diffuse light beams  170 - 176 , the incoming light beams  160 ,  162 ,  164  and  166  reflect off of the media  169  to form incoming specular light beams  180 ,  182 ,  184  and  186 , respectively. To those familiar with the science of optics, it will be apparent that the specular light beams  180 - 186  are reflected off of the media  169  at the same angle A as the incoming light beams  160 - 166  impacted the media  169 , in a principle known as “angle of incidence equals angle of reflection.” In the illustrated embodiment, preferably the irradiance from each illuminating LED  120 - 126  strikes the print surface plane of the sheet of media  169  at an angle of about 45-65°, or more preferably at an angle of 45°, referenced from the print surface of the media  169 . 
     The specular reflectance light beams  180 - 186  pass through the light chamber  168  of the ambient light shield  150 , through the aerosol shield  155 , through the incoming specular lens  148 , through the specular filter element  132 , through the incoming light passageway  114 , then through a specular stop window  187 , after which they are received by the specular photodiode  110 . The photodiode  110 , which is a light-to-voltage converter, interprets the incoming light beams  180 - 186  and sends a signal to the controller  45 , preferably in the same manner as described previously for signals provided by the diffuse photodiode  108 . Additionally, in the embodiment of FIG. 3, the media sheet  169  is shown as being supported in printzone  25  by a media support surface  188 , which may take the form of a platen, pivot, or other type of conventional printzone media support system. Besides just print media  169 , other components within the printer  20  may be monitored by the optical sensor  100 , such as a reference target, discussed further below, or other objects within the print engine, such as black or white target references, or various structures of the media support surface  188 , particularly, when a transparent sheet of media is to be printed upon. 
     By constructing the printed circuit assembly  105  using the chip-on-board process, where the semiconductor dies for the LEDs  120 - 126  and the photodiodes  108 ,  110  (light-to-voltage converters) are wire bonded or soldered directly to the printed circuit board, the resulting optical sensor  100  is far more compact than those previously achieved in the inkjet printing arts. For example, the blue-violet optical sensor used in the DeskJet® 990C model color inkjet printer, was nearly three times the height of the illustrated compact optical sensor  100 , and this earlier sensor was only capable of carrying a single blue-violet light emitting diode. Furthermore, the addition of the ambient light shield  150  isolates the photodiodes  108 ,  110  from signal corruption caused by external light sources. Use of the aerosol shield  155  advantageously protects the lens assembly  140  from being occluded by floating ink aerosol satellites generated during the printing process. Moreover, by having the aerosol shield  155  be removable and cleanable, the integrity of the optical sensor  100  is preserved over the lifetime of the printing unit  20 . 
     Furthermore, use of the chip-on-board process to assemble the printed circuit assembly  105  allows the four light emitting diodes  120 - 126  to use a single common optical path  128  for all four emitters, creating a compact optical sensor  100  in a fashion which, to the best knowledge of the inventors, has never been used in the inkjet printing arts. Additionally, by using four different colors of light emitting diodes  120 - 126 , the single compact optical sensor  100  is capable of media type sensing, color calibration (specifically, color, hue and intensity compensation), automatic pen alignment and swath height error/linefeed calibration, four features which have never before been accomplished using a single sensor element in the inkjet printing arts. Thus, the compact optical sensor  100  is more economical, saves space, and is capable of far more functions than previous optical sensors employed in inkjet printing. 
     Moreover, use of the ambient light shield  150  and the aerosol shield  155  make the sensor  100  very robust in operation over a wide range of printing environments, providing a low maintenance, long lifetime sensor for achieving optimal high quality printed images. Additionally, use of the chip-on-board technology for forming the printed circuit assembly  105  allows four different colored LEDs  120 - 126  to be employed in the same width package as that employed for the monochromatic optical sensing system of U.S. Pat. No. 6,036,298, mentioned above. 
     In the illustrated embodiment, the diffuse reflectance beams  170 - 176  detect the presence of the primary inks used in inkjet printers, such as, cyan, light cyan, magenta, light magenta, yellow and black. The specular light beams  180 - 186  are used to determine the reflective and other surface properties of the media  169 , from which the type of media being fed into the printzone  25  may be determined, and the print routines then adjusted to match the type of media, for instance in the manner used in the DeskJet® 990C model color inkjet printer. Indeed, use of the four different colored LEDs  120 - 126  allows the compact optical sensor  100  to collect data which the controller  45  then may map to a three-dimensional color space which correlates to human perception of color. Moreover, while four light emitting diodes  120 - 126  are illustrated, it is apparent that other implementations may cluster additional LEDs above the outgoing light chamber  128 , or another cluster of LEDs may be provided in the region of the specular photodiode  110  on the printed circuit assembly  105 , foregoing media type determination in favor of additional color sensing capability. 
     Another particular advantage made use of in the optical sensor  100  is the arrangement of the colors of the LEDs  120 - 126 . In the illustrated embodiment, it is preferred to have LED  120  to be a blue color, LED  122  to be a green color, LED  124  to be a red color and LED  126  to be a soft-orange color, with LEDs  120  and  124  being furthest away from the diffuse photodiode  108 , and LEDs  122  and  126  being closer to the diffuse photodiode  108 . In the illustrated embodiment, using the particular types of LEDs  120 - 126  and lens  145  selected, this physical arrangement yielded the most economical and highest performance sensor  100  for consumers. 
     Tuning System 
     FIG. 5 shows a graph  200  illustrating the manner in which the colors for the LEDs  120 - 126  were selected, here based upon the colors of ink and their specular responses used in the printer  20 . In FIG. 5, we see the various wavelengths and percentage of reflectance and percentage of absorbance shown for the four primary colors ejected by the printing unit  20  and for the four LEDs  120 - 126  of sensor  100 . For the inks, graph  200  shows a cyan colored ink trace  202 , a magenta colored ink trace  204 , a yellow colored ink trace  206  and a black color ink trace  208 . In the illustrated embodiment, graph  200  shows a blue LED ink trace  210  which is emitted by LED  120 , a green LED trace  212  which is emitted by LED  122 , a red LED ink trace  216  which is emitted by LED  124 , and a soft-orange LED ink trace  214  which is emitted by LED  126 . 
     As used herein, the definitions of a few terms may be helpful: 
     “Reflectance” is the ratio of the reflected light divided by the incident light, expressed in percent. 
     “Absorbance” is the converse of reflectance, that is, the amount of light which is not reflected but instead absorbed by the object, expressed in percent as a ratio of the difference of the incident light minus the reflected light divided by the incident light. 
     “Diffuse reflection” is that portion of the incident light that is scattered off the surface of the media  169  at a more or less equal intensity with respect to the viewing angle, as opposed to the specular reflectance which has the greatest intensity only at the angle of reflectance. 
     “Specular reflection” is that portion of the incident light that reflects off the media at an angle equal to the angle at which the light struck the media, the angle of incidence. 
     The four FEDs  120 - 126  preferably each have a centroid wavelength, which is the center wavelength where half of the total emitted energy is on each side of the wavelength, as shown in the following table: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CENTROID WAVELENGTH OF THE DIFFERENT LEDs 
               
            
           
           
               
               
               
            
               
                 ITEM 
                 LED 
                 CENTROID 
               
               
                 NO. 
                 COLOR 
                 WAVELENGTH 
               
               
                   
               
               
                 120 
                 Blue 
                 469 
               
               
                 122 
                 Green 
                 530 
               
               
                 124 
                 Red 
                 645 
               
               
                 126 
                 Soft 
                 607 
               
               
                   
                 Orange 
               
               
                   
               
            
           
         
       
     
     In Table 1, each of the centroid wavelengths has a tolerance of plus or minus ten nanometers (+/−10 nm) in the illustrated embodiment. 
     Indeed, one of the primary objectives in designing a commercial embodiment of the compact optical sensor  100  was to use LEDs  120 - 126  which were commercially available. For example, a better selection for the green LED  122  would have been an LED having a centroid of approximately 530 nm, shifting the green LED trace  212  slightly to the right from the position shown in FIG.  5 . Unfortunately, a green LED having a centroid of 530 nm was not commercially available, and the best available compromise was an LED having a centroid of 515-525 nm, or nominally an LED having a centroid of 521 nm, as illustrated in FIG.  5 . 
     In the Introduction section above, a hand held scanning unit made by Color Savvy was described, with an article and a U.S. Patent to Color Savvy being mentioned specifically. This Color Savvy device required eight to sixteen different colored LEDs to illuminate a target area, which if employed in the context of an inkjet printer, may unnecessarily increase the overall cost, and size or footprint of the product. Rather than requiring a eight to sixteen different colored LEDs, the optical sensor system  100  advantageously made use of two separate realizations. The first realization was that for each output color of a printed image, there is only one particular combination of the four colors of ink, cyan, magenta, yellow and black, which are used to arrive at a particular given color of an image. The second realization was that for proper color balance, tuning and calibration, out of millions of colors which may be obtained using the cyan, magenta, yellow and black inks, only a select group of four hundred colors needed to be analyzed. 
     Of this four hundred colors, the first one hundred colors consisted of different intensities of each of the basic colors, cyan, magenta, yellow and black. Different inkjet cartridges, installed in the carriage  40  may have slightly different characteristics, resulting in ink droplets having different drop weights being ejected by different pens. Drop weight affects the intensity of the resulting color, with bigger droplets forming darker or more intense colors in the printed image. One way to compensate for these different drop weight variations from pen-to-pen is to eject more ink droplets to darken the shade, or fewer ink droplets to lighten the shade. Thus, by measuring the color intensity produced over a specified range, for instance by printing a pattern where each progressive color sample has an increased number of droplets which should ideally produce increasingly darker shades of a color, the printer controller  45  may reference readings received from the optical sensor  100  and compare them to known values, and in turn then vary the number of droplets printed by a particular pen, or nozzles of the pen to achieve a desired shade, consistency or intensity of the resulting image. 
     These considerations resulted in the selection of a total of about one hundred different shade or intensity patterns for the color samples where only one color of ink is employed. The remaining about three hundred colors of the selected group of about four hundred for color calibration were based on a grid of varying shades of gray spanning the range from black to white, with some samples tinted with colors, such as pinks, greens and purples, as specified by color imaging designers. Given this group of four hundred different colors to detect, rather than millions of colors, designers of the illustrated sensor  100  then arrived at the four different colored LEDs having traces  210 - 216  shown in FIG.  5 . 
     Arriving at this selection of four LED colors was accomplished by an intensive study evaluating reflections from the interaction of a variety of different illuminating colors with each of the test colors. These interactions were either found through laboratory measurements, or by graphical or mathematical comparisons of the spectral responses of the inks versus the illumination data provided by the manufacturers of the variety of LEDs available. After this preliminary evaluation, different groups or subsets of LEDs were selected for further more intensive study and reevaluation, first studying subsets of three LEDs, then later by studying subsets of four LEDs. Each subset of LEDs selected was capable together of allowing identification and distinction between each test color of the selected group. During this process, a test patch sample of the test colors was printed and measured with a reference measurement device which generated a set of reference reflection data for the different colors of the patch sample. These actual color measurements may be made using a reference measurement device, such as an expensive laboratory piece of equipment, for instance a spectrophotometer. The patch sample was then illuminated with the LEDs of each subset and a measured set of reflection data was accumulated, then compared with the reference reflection data. The subset of LEDs having the lowest error values were then selected, for instance, based on selected printing product criteria, such as which shades are preferred, a particular printer model, or a particular set of inkjet inks. For example, the criteria may be based on the desired image output, such as whether particular colors, shading or grays are preferred. These colors may also be affected by other selected printing product considerations beyond the ink and printer model selections, such as pre-printing or post-printing treatments of the media, such as an overcoating or laminating process. 
     When measuring any particular color sample of the select group of 400 different shades, each of the four LEDs  120 - 126  is illuminated in sequence, with the resulting diffuse light beams  170 - 176  then being interpreted by the diffuse light-to-voltage converter  108  to find the percentage of reflectance and/or absorbance. By comparing the reflectance values received when illuminated by the different LEDs  120 - 126 , the various shades are distinguished by controller  45 . For instance, turning to FIG. 5, the cyan ink curve  202  may be distinguished from the other ink curves because the blue LED generates maximum reflectance, the green LED a medium reflectance, and the soft orange and red LEDs generate minimal reflectances. For the magenta ink curve  204 , the blue LED generates a small reflectance, the green LED generates a minimal reflectance, the orange LED generates a medium reflectance, while the red LED generates a high reflectance. Table 2 illustrates the various reflectances for each color ink and each LED. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 REFLECTANCES FOR INKS BY ILLUMINATION COLOR 
               
            
           
           
               
               
               
               
               
            
               
                 INK 
                 BLUE 
                 GREEN 
                 ORANGE 
                 RED 
               
               
                 COLOR 
                 LED 
                 LED 
                 LED 
                 LED 
               
               
                   
               
               
                 Cyan 
                 High 
                 Moderate 
                 Low 
                 Low 
               
               
                 Magenta 
                 Low 
                 Minimal 
                 Moderate 
                 High 
               
               
                 Yellow 
                 Low 
                 Moderate 
                 High 
                 High 
               
               
                 Black 
                 Minimal 
                 Minimal 
                 Minimal 
                 Low 
               
               
                   
               
            
           
         
       
     
     Of course, the percent reflectance shown in FIG. 5 varies with the amount of ink which is laid down upon a sheet of media, but during such a calibration sequence, the controller  45  generates firing signals which command the light cyan, cyan, black, magenta, light magenta and yellow ink cartridges  50 - 55  eject a known drop count or number of droplets for each sample measured. 
     In arriving at the particular colors of LEDs  120 - 126  which are shown in FIG. 5, a series of simulated and physical experiments were run. In developing the illustrated sensor  100 , following the realization that only four hundred colors need to be detected given the particular inks employed and the knowledge of which combinations of these inks produced a desired color, the sensor designers named herein worked to find an optimal group of LEDs which, using the chip-on-board process, were capable of being assembled into the compact optical sensor  100 . During the early development stages, a three LED sensor was proposed, having only red, green and blue LEDs. 
     In this early prototype three LED color set, there were some noticeable errors. For instance, since the viewing audience of the ultimate images produced by printer  20  are humans, selections were based on human perception. One mathematical model for determining variation in color, such as varying shades of pink or gray, is referred to as “Delta E.” A Delta E value of one refers to different shades which are barely distinguishable from one another, while a Delta E of two refers to shades which are certainly different. Using only blue, green and red LEDs, errors were found on the order of a Delta E of two, meaning that the shades were noticeably different to most people. This result was not satisfactory to the inventors herein, and the search continued for a way to bring down the Delta E value. This continuing quest resulted in the selection of the soft-orange LED  126  which produces curve  214  in FIG.  5 . The addition of the fourth LED, here the soft-orange LED  126 , yielded half the error value, dropping the Delta E value from two to a value of one. Thus, by using the four LEDs having the waveforms  210 - 216  shown in FIG. 5 (although a better green would have a centroid of 530 nm rather than the 521 nm shown for the commercially available green LED curve  212 ) yielded results which the inventors found acceptable while still allowing the sensor  100  to be an economical unit for incorporation into inkjet printing mechanisms. 
     Given this knowledge of the illustrated the compact optical sensor  100 , as well as how the four LEDs  120 - 126  were selected, and based on the realization that only four hundred test colors need to be monitored using the specific inks for which the printer  20  is designed, the manner in which this information may be used to provide optimal quality images for human viewers will be illustrated. The resulting image appearing on a sheet of media  169  may vary due to a myriad of different conditions (e.g., environmental conditions, including altitude, temperature and/or humidity), or due to the particular printhead which is ejecting the colors (different pens eject different drop weights in response to a given firing signal, resulting in different color intensities). Other factors may influence the resulting image, including the type of media upon which an image is being printed (plain paper, glossy media, photo media, transparency media, various colors of media such as pink, green, orange, blue, and even brown paper lunch sacks or fabrics). Because of these varying conditions, the resulting printed color often does not match the desired color. 
     At least two methods may be used to determine how to adjust the commanded color in a print mechanism, such as printer  20 , to obtain the desired color. First, by measuring the actual color produced from a composite of colorants (light cyan, cyan, black, magenta, light magenta, yellow) as well as knowing the desired color, it is possible to compensate for the difference between the actual and desired values by modifying the commanded color to make the actual and desired values agree. Second, it is possible to determine the actual amount of a single colorant deposited in a test region, then knowing the desired amount and reading the resulting appearance, the amount deposited for printing the image may be compensated by accounting for this difference to make the resulting image the one which is desired. Specifically, desired composite colors may then be obtained by using an a-priori knowledge of the colors resulting from specific mixtures of colorants (light cyan, cyan, black, magenta, light magenta, yellow). This a-priori knowledge found by printing a test sample, then takes into account not only the ink-to-ink interactions, but also the ink-to-media interactions. For instance, a brown paper sack may have more absorbance of the inks than a piece of plain paper, and a transparency may have less absorbance than plain paper or glossy photo paper. Knowledge of the absorbance of the ink into the media (to be distinguished from reflectance/absorbance shown in FIG. 5) may allow the controller  45  to deposit fewer droplets upon the less absorbent media to yield a clearer, crisper image. 
     Implementing either of these two methods requires the measurement of a printed color sample, and the comparing of this measurement with known values for producing desired colors. In the illustrated embodiment, the selection of the blue, green, soft-orange and red LEDs provide information about the amounts of each colorant in a composite color sample, for instance a green or purple sample, the controller  45  may then compute the resulting color quite accurately. Once the resulting color, given standard ink ejection parameters, is known these ink ejection parameters may be adjusted to obtain the desired color in the resulting image. 
     While variations in the ink ejecting printheads of cartridges  50 - 55  have been mentioned, it is apparent that the LEDs  120 - 126  may each vary from sensor to sensor so that one particular manufacturing lot of LEDs may be slightly different in emission wavelength from another lot. By calibrating each manufactured sensor  100  on test targets in the factory, using the same ink colorants, a customized curved fit may be made to compensate for such LED variations. Thus, at the factory compensation for LED variations may be made without requiring the use of specially selected and expensive LEDs for use in sensor  100 , again, resulting in a more economical compact optical sensor  100  for use in the printing unit  20 . 
     In the past, color sensors employed in the inkjet printing arts have either had to be designed with very accurate, and thus very expensive components, or they have used generic color standards to calibrate less accurate components. However, when building a color sensor capable of accurately determining the perceived color for a patch of arbitrary spectral characteristics, the resulting product was more expensive than tailoring a sensor design to work with a more limited set of color samples. As illustrated herein, the compact optical sensor  100  provides accurate color measurements while using inexpensive components, including LEDs  120 - 126  and photodiodes  108 ,  110 , by optimizing for a limited specific set of colors, such as the set of four hundred colors mentioned above, and with each sensor  100  being factory calibrated to compensate for component variation found when viewing a standard color set. 
     Calibrating System 
     FIG. 6 shows one form of a calibrating or target system  300 , constructed in accordance with the present invention for use with an optical sensor, such as the compact optical sensor  100  when employed in an alternate form of an inkjet printing mechanism, here shown as a photographic printer  302 . The photographic printer  302  is shown in a rudimentary format, including several internal working components that reside in a casing or housing (not shown) surrounding these mechanisms. The photo printer  302  may be constructed for use in a home, office or other environment, such as within a supermarket or variety store where one portion of the mechanism develops chemical-based film taken by a conventional camera, or processes digital images taken by a digital camera, and then prints these images on high quality media  304 , such as photographic media. 
     In the illustrated embodiment, the media  304  is fed from a supply roll  306 , which is supported by a roller assembly  308 , in a fashion similar to that employed in many inkjet plotters, with a conventional cutting mechanism used to separate such photographs being omitted from the view of FIG.  6 . The photo printer  302  may be constructed with an off-axis ink supply system as shown in FIG. 1, or with a set of replaceable cartridges  310 ,  311 ,  312 ,  313 ,  314  and  315 , which preferably carry inks of the colors light cyan, cyan, black, magenta, light magenta, and yellow, respectively. The pens  310 - 315  may purge or spit ink to clear their ink ejecting nozzles into a spittoon  316  when moved over a servicing region  318  by a carriage  320  in which all of the pens  310 - 315  are nestled. The carriage  320  moves along a guide rod  322  which defines a scanning axis  324 , allowing the carriage to move not only into the servicing region  318 , but into a printzone  25 ′. In the printzone  25 ′, the pens  310 - 315  selectively eject ink to form an image on the media  304 , preferably in response to signals received from a controller, such as controller  45  shown in FIG.  1 . 
     FIG. 6 also illustrates a service station  325  as having a base  326 , a bonnet  328 , and a pallet  330  which holds various printhead servicing components. In the illustrated embodiment, the pallet  330  moves back and forth in forward and rearward directions as indicated by the double headed arrow  332 , when driven by a motor  334  linked to a gear assembly (not shown). The pallet  330  may carry various printhead servicing features, such as wipers, primers, or the illustrated cap assembly  336 . In the illustrated embodiment, the service station base  326  and/or bonnet  328  may define a mounting shelf  338  upon which the calibrating or target system  300  is supported. 
     FIG. 7 shows the service station  325  in greater detail. Here we see the capping assembly  336  as including six printhead caps  340 ,  341 ,  342 ,  343 ,  344  and  345  which selectively seal the printheads of pens  310 ,  311 ,  312 ,  313 ,  314  and  315 , respectively. Also shown in greater detail in FIG. 7 is the calibrating system  300 , which includes a spring biased cover arm or door  350 , which is pivotally attached to the support shelf  338  by a pivot post  352  extending upwardly therefrom. A biasing member, such as a torsion or coil spring  354  is used to bias the cover door  350  into a printing position as shown in FIG.  7 . The spring  354  has first and second ends  356  and  358 , which are secured in place by spring holders  360  and  362 , respectively, projecting upwardly from the service station mounting shelf  338 . The cover door  350  also has a spring holder portion  364  which assists in keeping the biasing spring  354  in place. To assist in holding the cover door  350  in place, the shelf  338  defines a curved or arced guide track  366  within which a guide foot  368  projecting downwardly from the cover arm  350  is engaged, as shown in FIG.  8 . 
     FIGS. 8 and 9 show a replaceable target member  370  which forms a portion of the target system  300 . In the illustrated embodiment, the shelf  338  defines a target base  372  over which the target  370  is laid and then covered by a target cover member  374 . The target cover  374  defines a cover window  375  through which a portion of the target  370  is visible. Preferably, the target  370  is formed of a replaceable and duplicatable color of die-cut plastic film, such as one having the color of Hewlett-Packard Company&#39;s Bright White® brand inkjet media. A central post  376  projecting upwardly from the base  372  intersects holes defined by both the target  370  and the cover  374  to align the target, cover and base. The target cover and base  374 ,  372  together define a pair of target attachment assemblies  377 , as shown in greater detail in FIG.  9 . The target base  372  defines a pair of slots  378  therethrough, which each receive a pair of snap fitment finger members  380 , projecting downwardly from the target cover  374 . The target base  372  has a pair of ramp features  382  over which the finger members  380  of the target cover  374  slide and snap in place to secure the cover  374  and target  370  to the base  372 . 
     FIGS. 10,  11  and  12  show different stages of operation of the cover door  350 , with FIG. 10 showing the position of the door  350  for printing, as also shown in FIGS. 6 and 7, FIG. 11 showing a target reading position, and FIG. 12 showing a storage position where the printheads  310 - 315  are sealed by caps  340 - 345 , respectively. In FIG. 10 we see the cover door  350  as defining a door window  390 , which is preferably of approximately the same size as the cover window  375 . 
     In FIG. 10 we see the carriage  40  and sensor  100  entering the servicing region  318 , as indicated by arrow  392 . As shown in FIG. 11, the sensor  100  includes an outer impact or opening wall  394  which comes in contact with and pushes upon a door opener feature  395  on the cover door  350 . FIG. 11 shows the cover door moved from the printing position of FIG. 10 into a target reading position, where the door window  390  and the cover window  375  are aligned to expose the target  370  for viewing by the optical sensor  100 . In FIG. 12, the printhead carriage  40  has moved further in the direction of arrow  392  to move the cover door  350  into a storage position, where the target  370  is again covered by door  350 , preventing aerosol contamination during storage, as well as during printing as shown in FIGS. 6,  7  and  10 . 
     In operation, the target or calibrating system  300  is used to recalibrate for any defects in sensor  100  before beginning to print a sheet. These defects, are not truly defects, but merely refer to sensor aging or drift, that is, aging of the LEDs  120 - 126  and the drift in the output value of the photodiodes  108 ,  110  which is expected over time for such electrical components. Use of the calibrating target  370  may also compensate for aging and contamination build-up on the optical path components, such as those caused by aerosol and dust accumulation. Use of the target  370  allows the printer controller, such as controller  45 , to detect and measure these aging results and electronic drift of these components, then to allow the system to perform an internal calibration before printing a sheet. 
     Use of the cover door  350  advantageously prevents the target  370  from becoming contaminated with inkjet aerosol, dust, debris and other contaminants, by only allowing the target  370  to be viewable during a reading, and otherwise being covered during printing as well as during periods of printer inactivity when the printheads  310 - 315  are sealed by caps  340 - 345 . Thus, by keeping the target  370  in a pristine, clean state, a reference system is available for the sensor  100 , which does not degrade over time. However, in some implementations it may desirable to change out the target surface  370 , which is easily accomplished by unsnapping the target cover  374  from the target base  372  and either rotating the target  370  so a fresh quadrant of the target is available, or replacing the dirty target  370  with a fresh one. The cover door  350  then acts as a shutter for the white calibrating reference target  370 , so that the target is only exposed for small periods of time during which optical sensor readings are taken. Indeed, covering of the target  370  with door  350  is necessary due to the amounts of ink aerosol generated during purging or spitting of the printheads into the spittoon  316 , which is accessible to the pens  310 - 315  when the pallet  330  is moved into a retracted position by motor  334 . By having the cover door  350  only briefly open when the sensor  100  is in alignment with target  370 , the exposure of the target  370  to ink aerosol, dust particles, paper fibers and other contaminants is minimal. 
     While other products like scanners and hand held colorimeters have used white reference targets, they were not concerned with exposure to ink aerosol contaminants, as encountered in the inkjet printing environment, and thus had no need for a protective door  350 . Use of the cover door  350  and target  370  enables the sensor  100  to provide a high-precision calibration process which occurs robustly over time in the relatively dirty environmental of an inkjet printer. Furthermore, use of the spring biased cover door  350  is simple and economical to implement, although motor or solenoid actuated shutter systems may also be useful in higher end, more expensive products if desired.