Abstract:
An inkjet printer includes a printer carriage positioned on a first side of a platen and that moves across at least a portion of the platen; a light source positioned on a second side of the platen which second side is different from the first side; a sensor positioned on the printer carriage that detects an amount of light illuminated from the light source; an electronic device that receives data indicating the amount of light transmitted through a media patch with known characteristics; wherein the electronic device compares the amount of transmitted light to stored target values to determine a variation of the sensor response for forming a correction factor; wherein the electronic device uses the correction factor to calibrate at least a first signal of the inkjet printer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly assigned U.S. patent application Ser. No. ______ (Docket #K000359) filed concurrently herewith by Thomas D. Pawlik et al., entitled “A Method For Adjusting A Sensor Response”, and commonly assigned U.S. patent application Ser. No. ______ (Docket #96541) filed concurrently herewith by Thomas D. Pawlik et al., entitled “Method For Determining Variance Of Inkjet Sensor”, the disclosures of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to inkjet printers having a sensor that illuminates a print media and receives transmitted light for determining print media type, and more particularly an apparatus for obtaining calibration data, if needed, for the sensor due to light intensity variations and calibration data for varying the light intensity due to the type of detected paper. 
       BACKGROUND OF THE INVENTION 
       [0003]    An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead. 
         [0004]    A common type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced; the carriage direction of motion is reversed, and the image is formed swath by swath. 
         [0005]    The ink supply on a carriage printer can be mounted on the carriage or off the carriage. For the case of ink supplies being mounted on the carriage, the ink tank can be permanently integrated with the printhead as a print cartridge, so that the printhead needs to be replaced when the ink is depleted, or the ink tank can be detachably mounted to the printhead so that only the ink tank itself needs to be replaced when the ink tank is depleted. Carriage mounted ink supplies typically contain only enough ink for up to about several hundred prints. This is because the total mass of the carriage needs be limited so that accelerations of the carriage at each end of the travel do not result in large forces that can shake the printer back and forth. 
         [0006]    Pickup rollers are used to advance the media from its holding tray along a transport path towards a print zone beneath the carriage printer where the ink is projected onto the media. In the print zone, ink droplets are ejected onto the media according to corresponding printing data. 
         [0007]    It is noted that consumers use a plurality of different types of media for printing in inkjet printers. Commonly assigned and pending U.S. patent application Ser. No. 12/959,461 uses a sensor having a light source and detector for detecting the type of media being used for printing. As with any light source, light intensity may vary slightly over time causing the resulting signal used for detecting the media type to correspondingly vary. 
         [0008]    Although the currently used apparatuses and methods for detecting the media type are sufficient, there exists a need to detect such light variations using transmissive optics and to calibrate the photo-detector signal accordingly for permitting accurate detection of media type. Consequently, the present invention provides a method for detecting the light variation and providing a calibration signal. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, the invention resides in an inkjet printer comprising (a) a printer carriage positioned on a first side of a platen and that moves across at least a portion of the platen; (b) a light source positioned on a second side of the platen which second side is different from the first side; (c) a sensor positioned on the printer carriage that detects an amount of light illuminated from the light source; (d) an electronic device that receives data indicating the amount of light transmitted through a media patch with known characteristics; wherein the electronic device compares the amount of transmitted light to stored target values to determine a variation of the sensor response for forming a correction factor; wherein the electronic device uses the correction factor to calibrate at least a first signal of the inkjet printer. 
         [0010]    These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
           [0012]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a schematic representation of an inkjet printer system; 
           [0014]      FIG. 2  is a perspective view of a portion of a printhead; 
           [0015]      FIG. 3  is a perspective view of a portion of a carriage printer; 
           [0016]      FIG. 4  is a schematic side view of a media path in a carriage printer of the present invention; 
           [0017]      FIG. 5  is a block diagram illustrating the components of the print side transmittance sensor; 
           [0018]      FIG. 6  shows a simulated trace of the time-varying intensity values of the illumination sources; 
           [0019]      FIG. 7  is also a block diagram illustrating a second embodiment of  FIG. 5 ; 
           [0020]      FIG. 8  shows a simulated trace from the sensor in  FIG. 6  including the phases of transmittance measurement on a media patch and barcode scan on the print side of the media; 
           [0021]      FIG. 9  shows a second embodiment of  FIG. 8  where the transmittance measurement and barcode scan are both performed on the print side of the media; 
           [0022]      FIG. 10  shows a third embodiment of  FIG. 8  where the transmittance measurement and barcode scan are both performed on the print side of the media and the sensor performance is attenuated or amplified according to the result of the transmittance measurement; 
           [0023]      FIG. 11  shows a fourth embodiment of  FIG. 8  where the transmittance measurement is performed on both the media patch and the print side of the media; and 
           [0024]      FIG. 12  is an alternative embodiment of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Before discussing the present invention, it is useful to have a clear understanding of the terms used herein. As used herein, high and low intensity light pulses are defined as being on the high and low intensity side of a nominal light intensity In and given by the formula (In+ΔIn) for the high intensity light pulse and (In−ΔIn) for the low intensity light pulse, where ΔIn is preferably 0.1-10 percent although other ΔIn may also be used. It should be noted that although the term light is used herein, it is meant to also include electromagnetic radiation outside the visible spectrum. 
         [0026]    Referring to  FIG. 1 , a schematic representation of an inkjet printer system  10  is shown for its usefulness with the present invention and is fully described in U.S. Pat. No. 7,350,902, which is incorporated by reference herein in its entirety. Inkjet printer system  10  includes an image data source  12 , which provides data signals that are interpreted by a controller  14  as being commands to eject drops. Controller  14  includes an image processing unit  15  for rendering images for printing, and the controller  14  outputs signals to an electrical pulse source  16  of electrical energy pulses that are inputted to an inkjet printhead  99 , which includes at least one inkjet printhead die  110 . A look-up table  17  includes bi-directional communication with the controller  14  that is used in determining media type as described in U.S. Pat. No. 7,635,853 and will not be further discussed herein. 
         [0027]    In the example shown in  FIG. 1 , there are two nozzle arrays. Nozzles  121  in the first nozzle array  120  have a larger opening area than nozzles  131  in the second nozzle array  130 . In this example, each of the two nozzle arrays has two staggered rows of nozzles, each row having a nozzle density of 600 per inch. The effective nozzle density then in each array is 1200 per inch (i.e. d= 1/1200 inch in  FIG. 1 ). If pixels on the recording medium  20  were sequentially numbered along the media advance direction, the nozzles from one row of an array would print the odd numbered pixels, and the nozzles from the other row of the array would print the even numbered pixels. 
         [0028]    In fluid communication with each nozzle array is a corresponding ink delivery pathway. Ink delivery pathway  122  is in fluid communication with the first nozzle array  120 , and ink delivery pathway  132  is in fluid communication with the second nozzle array  130 . Portions of ink delivery pathways  122  and  132  are shown in  FIG. 1  as openings through printhead die substrate  111 . One or more inkjet printhead die  110  will be included in inkjet printhead  99 , but for greater clarity only one inkjet printhead die  110  is shown in  FIG. 1 . The printhead die are arranged on a support member as discussed below relative to  FIG. 2 . In  FIG. 1 , first ink source  18  supplies ink to first nozzle array  120  via ink delivery pathway  122 , and second ink source  19  supplies ink to second nozzle array  130  via ink delivery pathway  132 . Although distinct ink sources  18  and  19  are shown, in some applications it may be beneficial to have a single ink source supplying ink to both the first nozzle array  120  and the second nozzle array  130  via ink delivery pathways  122  and  132  respectively. Also, in some embodiments, fewer than two or more than two nozzle arrays can be included on printhead die  110 . In some embodiments, all nozzles on inkjet printhead die  110  can be the same size, rather than having multiple sized nozzles on inkjet printhead die  110 . 
         [0029]    The drop forming mechanisms associated with the nozzles are not shown in  FIG. 1 . Drop forming mechanisms can be of a variety of types, some of which include a heating element to vaporize a portion of ink and thereby cause ejection of a droplet, or a piezoelectric transducer to constrict the volume of a fluid chamber and thereby cause ejection, or an actuator which is made to move (for example, by heating a bi-layer element) and thereby cause ejection. In any case, electrical pulses from electrical pulse source  16  are sent to the various drop ejectors according to the desired deposition pattern. In the example of  FIG. 1 , droplets  181  ejected from the first nozzle array  120  are larger than droplets  182  ejected from the second nozzle array  130 , due to the larger nozzle opening area. Typically other aspects of the drop forming mechanisms (not shown) associated respectively with nozzle arrays  120  and  130  are also sized differently in order to optimize the drop ejection process for the different sized drops. During operation, droplets of ink are deposited on a recording medium  20 . 
         [0030]      FIG. 2  shows a perspective view of the inkjet printhead  99  plus ink sources  18  and  19 . Inkjet printhead  99  includes two printhead die  251  (similar to printhead die  110  in  FIG. 1 ) that are affixed to mounting substrate  255 . Each printhead die  251  contains two nozzle arrays  253  so that inkjet printhead  99  contains four nozzle arrays  253  altogether. The four nozzle arrays  253  in this example are each connected to ink sources (not shown in  FIG. 2 ), such as cyan, magenta, yellow, and black. Each of the four nozzle arrays  253  is disposed along nozzle array direction  254 , and the length of each nozzle array along the nozzle array direction  254  is typically on the order of 1 inch or less. Typical lengths of recording media are 6 inches for photographic prints (4 inches by 6 inches) or 11 inches for plain paper (8.5 by 11 inches). Thus, in order to print a full image, a number of swaths are successively printed while moving inkjet printhead  99  across the recording medium  20 . Following the printing of a swath, the recording medium  20  is advanced along a media advance direction that is substantially parallel to nozzle array direction  254 . 
         [0031]    Also shown in  FIG. 2  is a flex circuit  257  to which the printhead die  251  are electrically interconnected, for example, by wire bonding or TAB bonding. 
         [0032]    The interconnections are covered by an encapsulant  256  to protect them. Flex circuit  257  bends around the side of inkjet printhead  99  and connects to connector board  258  on rear wall  275 . A lip  259  on rear wall  275  serves as a catch for latching inkjet printhead  99  into the carriage  200 . When inkjet printhead  99  is mounted into the printhead carriage  200  (see  FIG. 3 ), connector board  258  is electrically connected to a connector on the carriage  200  so that electrical signals can be transmitted to the printhead die  251 . Inkjet printhead  99  also includes two devices  266  mounted on rear wall  275 . When inkjet printhead  99  is properly installed into the carriage of a printhead carriage  200 , electrical contacts  267  will make contact with an electrical connector on the carriage. 
         [0033]      FIG. 3  shows a portion of a desktop carriage printer. Some of the parts of the printer have been hidden in the view shown in  FIG. 3  so that other parts can be more clearly seen. Printer chassis  300  has a print region  303  across which printhead carriage  200  is moved back and forth in carriage scan direction  305  between the right side  306  and the left side  307  of printer chassis  300 , while drops are ejected from printhead die  251  (not shown in  FIG. 3 ) on inkjet printhead  99  that is mounted on carriage  200 . Carriage motor  380  moves belt  384  to move printhead carriage  200  along carriage guide rail  382 . 
         [0034]    The mounting orientation of inkjet printhead  99  is rotated relative to the view in  FIG. 2 , so that the printhead die  251  are located at the bottom side of inkjet printhead  99 , the droplets of ink being ejected downward onto the recording medium in print region  303  in the view of  FIG. 3 . Cyan, magenta, yellow and black ink sources  262  are integrated into inkjet printhead  99 . Paper or other recording medium (sometimes generically referred to as paper or media herein) is loaded along paper load entry direction  302  toward the front of printer chassis  308 . 
         [0035]    A variety of rollers are used to advance the medium through the media transport path  345  (indicated by the dot dash lines) of the printer as shown schematically in the side view of  FIG. 4 . It is noted that  FIG. 4  illustrates a L-shaped configuration for paper entry, although other configurations are also usable with the present invention. A stack of media  370  is disposed in a media tray  346  for providing a print media. In this example, a pick-up roller  320  moves the top sheet of the media  371  (referred to as recording medium  20  in  FIG. 1 ) in the direction of arrow, media load entry direction  302 . A turn roller  322  acts to move the media around an angled path so that the media  371  continues to advance along media advance direction  304  from the rear  309  of the printer chassis (with reference also to  FIG. 3 ). The media  371  is then moved by feed roller  312  and idler roller(s)  323  to advance across print region  303 . From there, the media  371  advances to a discharge roller  324  and star wheel(s)  325  so that printed media exits along media advance direction  304 . 
         [0036]    The motor that powers the media advance rollers is not shown in  FIG. 3 , but the hole  310  at the printer chassis right-side  306  is where the motor gear (not shown) protrudes through in order to engage feed roller gear  311 , as well as the gear for the discharge roller (not shown). For normal media pick-up and feeding, it is desired that all rollers rotate in forward rotation direction  313 . Toward the printer chassis left-side  307 , in the example of  FIG. 3 , is the maintenance station  330 . 
         [0037]    Toward the printer chassis rear  309 , in this example, there is located the electronics board  390 , which includes cable connectors  392  for communicating via cables (not shown) to the printhead carriage  200  and from there to the inkjet printhead  99 . Also on the electronics board are typically mounted motor controllers for the carriage motor  380  and for the media advance motor, a processor and/or other control electronics (shown schematically as controller  14  and image processing unit  15  in  FIG. 1 ) for controlling the printing process, and an optional connector for a cable to a host computer. 
         [0038]    Referring back to  FIG. 4 , the printhead carriage  200  includes a transmittance sensor  97  having an aperture and a photo-detector, all of which are discussed hereinbelow. Movement of the printhead carriage  200  by the carriage motor  300  and belt  304  simultaneously moves the attached transmittance sensor  97  in a direction perpendicular to the media feed direction  304 . A light source  100  is positioned opposing the printhead carriage  200  such that the light source illuminates the non-print dise of the media  374  and an optional media patch  98 , and transmitted light is captured by the transmittance sensor  97 . Preferably, the light source emits infrared radiation although other wavelengths in the visible and ultraviolet range may be used. 
         [0039]    The transmittance sensor  97  identifies the particular type of media  371  currently being used for printing by detecting a barcode  372  that is printed on the non-print side of the media. The sensor  97  detects the lines of the barcode  372  as an attenuation of light transmitted through the media emitted from a light source  100 . It is noted that the printer  10  uses any of a plurality of media types for printing (matte, plain or glossy), and the printer  10  identifies the particular type of media being used so that corresponding printing adjustments can be made. 
         [0040]    The optical components of the transmittance sensor  97  and light source  100  are subject to manufacturing tolerances, which necessitates an initial calibration. In addition, over time the light source  100  or photodetector may become degraded so that the corresponding signal from the transmittance sensor  97  varies from the signal present when the sensor was initially configured. The degradation can be due to aging of the optoelectronic components or deposition of ink spray. In addition to identifying the media type, the transmittance sensor  97  of the present invention is used to detect variations in the signal from the light source  100  and photo-detector system that may occur over time. 
         [0041]    An optional media patch  98  of known characteristics (typically either matte or glossy) is placed in a location suitable for the light source  100  to optically illuminate the media patch  98  and for the transmittance sensor  97  to capture the transmitted light. For example, the transmittance sensor  97  may be located to the side of the printhead carriage  200  and the media patch  98  may be located in the print region  303  at a position slightly below the media plane such that it can be illuminated by the light source  100  prior to media pick-up and feeding to the print region  303  as shown in  FIG. 4 . Alternatively, the media patch  98  can be located in plane with the media but to either side of the print region  303 , i.e., outside of the footprint of the media. This media patch  98  is used in certain embodiments to determine whether there is degradation of the transmittance sensor  97  as described herein below. 
         [0042]    Referring to  FIG. 5 , there is shown an embodiment of the transmittance sensor  97 . As the printhead carriage  200  is maintained in a stationary position, the illumination source  100 , or optionally a plurality of illumination sources  100 ,  100   a ,  100   b , emit a sequence of light pulses onto the non-print side of the media  101   a , or alternatively onto the media patch  98 . The detector  103  faces the print side of the media  101   b  and the light source  100 , or light sources  100 ,  100   a  and  100   b , faces the non-print side of the media  101   a . Preferably a low intensity light pulse (I 0 −ΔI 0 ) is emitted first, immediately followed by a high intensity light pulse (I 0 +ΔI 0 ). This sequence is preferably repeated a number of times so that sufficient data points are collected although one sequence may also be used for time efficiency. The transmitted light  105  passes through an aperture  104  and is received by the photodetector  103 . It is noted that the repeat frequency is chosen high enough such that the time variant signal is amplified by the AC-coupled amplifier. Preferably the repeat frequency is at or above the −3 dB point of the high pass filter circuit of the AC coupled amplifier. Although the present invention uses a low intensity light pulse followed by a high intensity light pulse, a high intensity pulse may be emitted first followed by a low intensity light pulse. 
         [0043]    Referring to  FIG. 6 , each pulse sequence consists of alternating intensities of (I 0 +ΔI 0 ) and (I 0 −ΔI 0 ) for illumination source  100 . These light pulses are detected by the photodetector  103 . It should be obvious to a person skilled in the art that a light source intensity can be regulated by changing the current, or by changing the duty cycle using high frequency pulse width modulation. Although not preferred in this invention, light intensity modulation by a mechanical or photoelectric modulator is also possible. 
         [0044]    A fraction of the illumination light that is transmitted through the media then passes through an aperture  104  (see  FIG. 5 ). The photo-detector  103  detects light passing through the aperture  104 . Photodetector  103  and aperture  104  are mechanically coupled to the printhead carriage  200 . The signal from detector  103  is then used by the controller  14  to determine transmittance of the print media  101 , or alternatively the media patch  98 . 
         [0045]    Following the detection of the light pulses, the illumination source  100  is set to emit constant light of the intensity I 0 ′ and the printer carriage  200  is moved across the media in the direction perpendicular to the media advance direction  304 . During the printer carriage motion, the signal from the photodetector  103  is recorded by the controller  14 . 
         [0046]    Referring to  FIG. 7 , there is shown an alternative embodiment of the present invention. In this embodiment, the photodetector  103  is positioned such that it faces the non-print side of the media  101   a  or the media patch  98  and the light source  100  and aperture  104  are facing the print side of the media  101   b  or the opposite side of the media patch  98  and mechanically coupled to the printhead carriage  200 . The illuminating light is confined by the aperture  104  and is incident on the print side of the media  101   b  or the media patch  98 . The portion of the light that is transmitted through the media  371  is captured by the photodetector  103 . As the printer carriage  200  is maintained in a stationary position, the illumination source emits a sequence of high and low light pulses onto the print side of the media  101   b  or media patch  98 . Following the detection of the light pulses, the illumination source  100  emits a constant light of the intensity I 0 ′ while the printhead is simultaneously moved at a constant velocity across the media in the direction perpendicular to the media advance direction  304 . During the printhead motion, the signal from the photodetector  103  is recorded by the controller  14 . 
         [0047]    Both sensor configurations in  FIGS. 5 and 7  are able to measure transmittance of the media  101  or media patch  98  during the phase in which the illumination intensity is modulated and the printhead carriage  200  is not moving. They are further able to detect the lines of the barcode  372  that are printed on the non-print side of the media  101   a  as a time variant attenuation of the transmittance signal as the carriage is moved across the media surface at constant velocity. 
         [0048]    The following  FIGS. 8 through 10  describe how this data collected by the photodetector  103  is used to improve robustness of media detection. 
         [0049]    Referring to  FIG. 8 , there is shown simulated data from the photodetector  103  of transmittance sensor  97  described in  FIG. 5  using the media patch  98 . The signals from the photodetector  103  are processed through an analog to digital converter for producing a digital signal which is a more suitable form for analysis. While the printhead carriage  200  is stationary in phase  604 , the signal is monitored and it produces a first distinct segment of data: region  601  is from modulated light transmitted through the media patch  98 . The amplitude  607  of the transmittance signal ( 601 ) is compared by the controller  14  to stored target values for the media type identical to the media patch  98  which are stored in look-up table  17  (see  FIG. 1 ). If the signal varies from the original signal target value, this indicates a degradation of the transmittance sensor  97 , and the signal for identifying media type is then amplified or attenuated by the percent of the detected variance increase. If no difference is detected, the actual signal is used without any amplification or attenuation. Amplification or attenuation can be achieved by several methods. These include modification of the AC amplifier gain, adjustment of the light source intensity, mathematical processing of the digitized sensor signal or processing of the parameters derived from it by multiplication with a calibration factor. The result is a sensor signal that is compensated for degradation effects and represents a normalized sensor response. 
         [0050]    The next region of the chart,  603 , is the signal while the printhead encounters the leading edge of the media (phase  606   a ), moves across the media surface (phase  605 ) and eventually encounters the edge of the media in phase  606   b . During the path of the printhead across the media the sensor  97  encounters several positions where the barcode lines  372  attenuate the detector signal. These lines are evident in the photodetector signal  603  as deviations from the mean photodetector signal. Image representative of a barcode pattern is shown as  608 . Because of the AC-coupling of the amplifier, the typical line shape is a negative peak when the photodetector  103  moves onto the barcode line, immediately followed by a positive peak when the photodetector moves off the barcode line. The microcontroller  14  analyzes the recorded transmittance photodetector signal  603  after normalization and determines the position and strength of the barcode lines. By comparing these parameters with a matrix of stored values for the barcode properties of various media, the controller  14  can identify the media. 
         [0051]    Referring to  FIG. 9 , there is shown simulated data from the detector described hereinabove in  FIG. 5  using the print side of the media  101   a . This data includes all the same descriptions as for  FIG. 8 , but it is noted that the transmittance signal  611  is obtained with the transmittance sensor  97  facing the print side of the media  101   a . The photodetector signal  611  results from modulated light transmitted through the media  101 . With the printhead carriage  200  stationary  614 , the media loaded in the printer is plain paper. Because plain paper is more translucent than the thicker photo paper, proportionally more light reaches the photodetector  103 . This is evident in the larger amplitude  607  of the photodetector signal  611 . Subsequently the transmittance sensor  97  moves across the media surface (phase  605 ) and eventually encounters the edge of the media in phase  606   b . Because more light reaches the photodetector  103 , the signal in phase  605  also contains more noise. The noise originates mainly from the paper fiber microstructure in the media. This poses a problem for the barcode detection because the noise can be interpreted as barcode lines and consequently plain paper can be misidentified as barcoded photo media. 
         [0052]    Referring now to  FIG. 10 , there is shown how this problem can be avoided using the present invention.  FIG. 10  includes all the descriptions as in  FIG. 8 . In this figure, the amplitude  607  of the transmittance signal ( 611 ) is compared by the controller  14  to stored target values for a typical photo paper which are stored in look-up table  17  (see  FIG. 1 ). In this case the measured amplitude  607  of signal  611  is substantially higher than expected for photo paper. From the deviation, a calibration factor is obtained to compensate for the amount of light transmitted for the particular paper type detected (plain or photo paper) and it is used to normalize sensor response. For example, if the quantity of detected light is put on a scale of 1 to 10, the calibration factor correspondingly varies the light intensity in ten increments so that a first paper type has a first intensity and a second paper type has a second intensity different from the first intensity. It is noted that a specific type of paper may have a variation in light transmission due to manufacturing tolerances and that this calibration factor will also vary to compensate for this variation. The following scan across the media surface  605  is conducted using the attenuated or amplified sensor response. As a consequence of the calibration, the noise amplitudes are substantially lower and the signal is not misidentified as barcode lines. The loaded media can be identified reliably as plain paper because no barcode lines are found and the magnitude of the calibration factor indicates a more translucent media than photo media. This scheme is also beneficial to normalize the sensor response for photo media of different thicknesses. The controller  14  selects an optimal print mode for the determined media type. 
         [0053]    Referring to  FIG. 11 , there is shown a combination of the detection schemes of  FIGS. 8 and 10 . It is noted that items  600 ,  606   a , and  606   b  are the same as previously described. During the time period when the printer carriage  200  is stationary  604  and the sensor is facing a surface of known transmittance such as media patch  98 , light source  100  is pulsed using high and low intensity light pulses which creates transmittance signal  601 . This signal is compared to stored values for the target of known transmittance. The variance is used to amplify or attenuate sensor response according to the process described in  FIG. 8 . This creates a calibrated sensor response. Subsequently, the printhead carriage  200  is moved  605  to a position where the transmittance sensor  97  faces the print side of the media  101   a . During another stationary phase  614 , the light source  100  is pulsed using high and low intensity light pulses which creates transmittance signal  611 . The normalized sensor signal during phase  611  is compared to predicted values for glossy photopaper, matte photopaper and plain paper. This comparison yields a predicted first media type from the transmittance measurement. If signal  611  deviates from a predetermined value for photo media, the sensor response is attenuated or amplified accordingly for the subsequent barcode scan. In phase  605 , the sensor is moved across the media surface and the sensor signal is recorded by the microcontroller  14 . The second calibration ensures that the barcode scan is conducted with an optimized sensor response such that barcode lines  372  can be reliably identified. Like in  FIG. 10 , the absence of detected barcode lines and a large positive deviation of signal  611  from the predetermined value for photo paper are indicative of plain paper. The added benefit of the scheme in  FIG. 11  is that degradation of the sensor can be compensated via the transmittance measurement of the media patch  98 . 
         [0054]      FIG. 12  is an alternative embodiment of  FIG. 4  having the roller  322  omitted and is generally referred to as a flat paper entry. 
         [0055]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10  Inkjet printer system 
           12  Image data source 
           14  Controller 
           15  Image processing unit 
           16  Electrical pulse source 
           17  Look-up table 
           18  First ink source 
           19  Second ink source 
           20  Recording medium 
           97  Transmittance sensor 
           98  Media patch 
           99  Inkjet printhead 
           100  Illumination source 
           100   a  Illumination source 
           100   b  Illumination source 
           101  Media 
           101   a  Media, non-print side 
           101   b  Media, print side 
           103  Photodetector 
           104  Aperture 
           105  Transmitted radiation 
           110  Inkjet printhead die 
           111  Substrate 
           120  First nozzle array 
           121  Nozzle(s) 
           122  Ink delivery pathway (for first nozzle array) 
           130  Second nozzle array 
           131  Nozzle(s) 
           132  Ink delivery pathway (for second nozzle array) 
           181  Droplet(s) (ejected from first nozzle array) 
           182  Droplet(s) (ejected from second nozzle array) 
           200  Carriage 
           251  Printhead die 
           253  Nozzle array 
           254  Nozzle array direction 
           255  Mounting substrate 
           256  Encapsulant 
           257  Flex circuit 
           258  Connector board 
           259  Lip 
           262  Ink sources 
           266  Device 
           267  Electrical contact 
           275  Rear Wall 
           300  Printer chassis 
           302  Media load entry direction 
           303  Print region 
           304  Media advance direction 
           305  Carriage scan direction 
           306  Right side of printer chassis 
           307  Left side of printer chassis 
           308  Front of printer chassis 
           309  Rear of printer chassis 
           310  Hole (for media advance motor drive gear) 
           311  Feed roller gear 
           312  Feed roller 
           313  Forward rotation direction (of feed roller) 
           320  Pick-up roller 
           322  Turn roller 
           323  Idler roller 
           324  Discharge roller 
           325  Star wheel(s) 
           330  Maintenance station 
           345  Media transport path 
           346  Media tray 
           370  Stack of media 
           371  Media 
           372  Barcode 
           380  Carriage motor 
           382  Carriage guide rail 
           384  Belt 
           390  Printer electronics board 
           392  Cable connectors 
           601  LED  100  is modulated between two brightness levels (I 0 −ΔI 0           I 0 +ΔI 0 ) for n periods. Sensor  97  is facing a target of known transmittance  98   
           603  LED  100  is set at brightness I 0 ′ 
           604  Sensor is at a position facing a target of known transmittance  98  and not moving 
           605  Sensor is moving across the front side of the media at a constant velocity using carriage motion 
           606   a  Sensor in front of the media edge 
           606   b  Sensor is past the media edge 
           607  Amplitude of the sensor response to the modulation scheme  601   
           608  Image representative of a barcode pattern 
           611  LED  100  is modulated between two brightness levels (I 0 −ΔI 0           I 0 +ΔI 0 ) for n periods. Sensor  97  is facing the print side of the media  101   
           614  Sensor is at a position facing the print side of the media  101  and not moving