Abstract:
A spot detector including a light source with a lens arranged in the path of the light from the source and a lens disposed in the path of the light reflected from the medium surface. An opaque mask is provided on the far side of the receiving lens. The opaque mask is configured with at least one slit such that the size and location in two dimensions of a single spot may be detected. A photodetector generates a voltage proportional to the amount of light impinging on its photosensitive surface, which depends upon whether or not a spot is present. According to one embodiment of he present invention, a layered mask is provided wherein the optical working distance to a given mask opening is staggered by placing it on different mask substrate layers. Accordingly, different portions of the depth of field can be resolved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical device for detecting the size and/or location of a spot on a surface. 
     2. Description of the Related Art 
     The present invention relates to an optical device, comprising an optical emitter and an optical detector, for illuminating a spot on a surface of an object, detecting the light reflected from the surface and determining the size and/or location of the spot on the surface. Although this invention has a wide variety of applications, the preferred embodiment is described in connection with an ink jet printer. 
     In ink jet printing, droplets of ink are selectively propelled from a plurality of drop ejectors in a print head, in accordance with digital instructions, to create a desired image on a copy surface. The print head typically includes a linear array of ejectors for conveying the ink to the copy medium, such as paper, overhead transparencies, and the like. The print head may move back and forth relative to a surface, for example, to print characters, or the linear array may extend across the entire width of a copy sheet moving relative to the print head. The ejectors typically include capillary channels or other ink passageways which are connected to one or more ink supply manifolds. Ink from the manifolds is retained within each channel until, in response to an appropriate signal, the ink in the channels is rapidly heated and vaporized by a heating element, such as a thermal resistor, disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle onto the copy sheet. When a quantity of ink in the form of a droplet is ejected from the ejector to a copy surface, the resulting spot becomes part of a desired image. 
     Selection of spot location for a large number of droplets is crucial to image quality in ink jet printing. If the locations of droplets ejected from the print head over the course of producing a single document vary significantly, the lack of uniformity will have noticeable effect on the quality of the image. This is particularly important in color printing, since different colored images must be overlaid on one another. A misalignment of the images can result in a defocused, staggered appearance. 
     In order to ensure proper alignment of the ejectors and to ensure that all the ejectors are firing properly, test patterns made up of a plurality of spots may be printed onto the copy surface in an unobtrusive location and then illuminated by an LED. A reflected signal is detected by a photodetector. If a spot is present, a different signal results from when the spot is absent. It is known to provide a mask between the copy medium and the photodetector to detect individual spots of the pattern. For example, the system disclosed in U.S. Pat. No. 4,907,013 provides a linear plurality of spots which are detected with the help of a mask having a slit approximately the same width as the spots. The spots are at an angle from the direction of motion such that detection of an X and Y location of each spot may be determined. However, the system described therein requires a plurality of spots that are visible to the naked eye and a mask slit which is approximately the same width as each spot. The longitudinal direction can only be determined by the angled disposition of the spots, and the spot size cannot be determined. 
     In addition, such known spot detection systems are deficient in that the effective image produced degrades over a large depth of field. This problem can be overcome by changing the operating distance between the mask and the receiver optics as the operating distance to the paper is varied. However, this solution is mechanically complex and expensive. 
     Accordingly, there is a need for a spot detector which is capable of detecting a single spot and which is further capable of determining both the spot size and transverse and longitudinal location. There is a further need for an improved method for a spot detector having reduced effective image degradation over a large depth of field. 
     SUMMARY OF THE INVENTION 
     These and other problems in the prior art are overcome in large part by a spot detector according to the present invention. A spot detector according to an embodiment of the present invention includes a light source with a lens arranged in the path of the light from the source and a lens disposed in the path of the light reflected from the medium surface and received by a photodetector. An opaque mask is provided and configured with at least one slit. The mask is used as a position and size detector. Depending on the size of the spot and the path of the spot relative to the mask, the photodetector is able to generate voltage pulse patterns as a function of the size and location of the spot. According to one embodiment of the present invention, a layered mask is provided wherein the optical working distance to a given mask opening is staggered by placing it on different mask substrate layers. Accordingly, different portions of the depth of field can be resolved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention is obtained when the following detailed description is considered in conjunction with the following drawings in which: 
     FIG. 1 is a diagram illustrating an exemplary spot detector according to the present invention; 
     FIG. 2 illustrates an exemplary optical mask pattern according to an embodiment of the present invention; 
     FIG. 3 illustrates signal acquisition and processing logic according to an embodiment of the invention; 
     FIG. 4 illustrates signal acquisition and processing logic according to an alternate embodiment of the invention; 
     FIG. 5 illustrates various signals of use according to the present invention; 
     FIGS. 6A-6B illustrate relative signal responses for various gaussian and pill spots; 
     FIG. 7 illustrates pulse width and rise time versus normalized spot width; 
     FIG. 8 is a comparison of pulse width for gaussian and pill spots; 
     FIGS. 9A-9B illustrate peak signals for elliptical spots; 
     FIGS. 10A and 10B illustrate multi-layer masks according to embodiments of the present invention; 
     FIGS. 11A-11C illustrate exemplary optical configurations for spot detectors according to the present invention; and 
     FIGS. 12A-12C illustrate an exemplary optical system for use according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention will be described in terms of a color ink jet printer that has multiple colors of ink, for example 3 colors. A microprocessor in the printer determines where to place spots of each color on a print medium, such as a piece of paper. In the preferred embodiment, all spots of a first color in the entire image will be printed before moving on to subsequent colors. For each spot to be printed, a microprocessor in the printer will calculate X and Y coordinates for the spot. The Y coordinate will be used to control a mechanism that moves the paper in the Y direction, while the X coordinate will be used to move a print carriage in the X direction. For a given set of X, Y coordinates, the paper will be moved so that the appropriate Y location on the paper is under the print carriage and the print carriage will be moved so that a print head is above the appropriate X location on the paper. When the print head is above the correct X, Y position on the paper, a spot of ink is generated by the print head. A first pass is made over the paper printing all spots of the first color, followed by subsequent passes to print spots in the other colors. During subsequent passes, spots printed in previous passes will be located prior to or during the subsequent passes to verify the precise location of the print head relative to the paper. There may be a number of spots throughout the page of paper that are detected in this manner. And these spots could be a part of the image, or they could simply be reference spots added just for alignment purposes. The reference spots can be made small enough to not be noticeable to a person looking at the image. This alignment function is important because there may be slippage of the paper during and in between passes. If this slippage is not detected and adjusted for, the multiple colors will be offset from one another, causing a defocused, staggered appearance. 
     When detecting a reference spot, the paper is moved so that the appropriate Y location on the paper is placed underneath the print carriage. In the preferred embodiment, the spot sensor is mounted to and moves with the print carriage. Thus, the appropriate Y location on the paper is also under the spot sensor. The print carriage is then moved in the X direction so that the ink spot passes under the spot sensor. Signal information from the spot sensor is correlated with information regarding the Y positioning of the paper, the X positioning of the carriage, and the velocity of the carriage to determine the precise position of the print head relative to the spot on the paper. If the velocity of the carriage is not provided by the microprocessor, then it can be calculated from multiple values for the X positioning of the carriage and the elapsed time between the multiple values. The microprocessor in the printer can then make adjustments for any discrepancy between the anticipated location of the spot and the actual location. A person of skill in the art will appreciate that various aspects of the invention can be adjusted depending on the precision required for the printing and the precision of the mechanisms that move the paper and the print carriage. For example, if the moving mechanisms are relatively less precise or the printing must be more precise, than the number of reference spots may be increased, the area detected by the spot sensor may be increased, etc. 
     Turning now to the drawings, and with particular attention to FIG. 1, a diagram illustrating a spot detector  500  for use in an ink jet printer according to an embodiment of the present invention is shown. The spot detector  500  is configured to detect a size and location of a spot  514  on the surface of a copy medium  512  as the copy medium  512  and the spot detector  500  move relative to one another within the printer. Typically, the copy medium  512  may be paper, transparencies, envelopes, labels or virtually any medium that can be printed on by an ink jet powder. It is noted that, while described in the context of ink jet printing, the present spot detector may be employed in a variety of other applications, such as printed circuit fabrication. Thus, the figures are exemplary only. 
     The spot detector  500  includes an illuminator  501  and a receiver  503 . The illuminator  501  is configured to project a beam of light onto the surface of the copy medium  512 . The reflected light is detected by the receiver  503 . Since the intensity of the reflected light is dependent upon the uniformity of the surface of the copy medium  512 , the receiver can identify the size and location of any spots  514  on the surface of the copy medium  512 , as will be described in greater detail below. 
     Briefly, the spot detector  500  may include a barrel mount bracket  550  for mounting the illuminator  501  and the receiver  503  at predetermined angles relative to the copy medium. The barrel mount bracket  550  is adapted to carry an illuminator barrel  552 , for example, at an angle (for example, 23 degrees) from the receiver. The receiver  503  is carried by a receiver barrel  558  which, in turn, is carried by the barrel mount bracket. The receiver barrel is adapted to carry the receiver components so that the receive optical path is at a predetermined angle relative to the copy medium, for example 90 degrees. 
     An illuminator lens  508  is mounted to the illuminator barrel  552  by an illuminator lens cell  554 . As illustrated, the illuminator lens  508  is mounted at an end of the illuminator barrel proximate to the surface of the copy medium  512 . However, other embodiments may have the illuminator lens  508  mounted in different positions in the illuminator barrel  552 . In addition, it is noted that, while illustrated as a single lens, the illuminator lens  508  may include a multiple lens system, including one or more cemented lenses. For example, the illuminator lens  508  may be a doublet having four surfaces and transmission of 0.8. Alternatively, an aspheric lens having an F number of about 2.0 may be employed. 
     An illuminator light emitting diode (LED)  504  is positioned within a die cup  502  and mounted on the illuminator barrel  552  at an opposite end of the illuminator barrel  552 . The die cup  502  provides improved coupling efficiency by concentrating the LED energy into a decreased cone angle. In one embodiment, the illuminator LED  504  is a  660  nanometer red LED having 10 milliwatts of total power. The die cup  502  may have a diameter of approximately 1 mm. The diode system is such that after epoxy potting, 6.5 milliwatts of available output are available. It is noted that the positioning of the illuminator LED  504  may vary relative to the illuminator lens  508 , but in one embodiment, the illuminator LED  504  is placed at twice the focal length of the illuminator lens  508  to allow 1:1 re-imaging onto the surface of the copy medium  512 . Thus, the energy collected is imaged onto a spot approximately the same size as the die cup  502 . It is noted that performance may be varied through changing the diameter and depth of the die cup  502 , as well as providing different lenses (for example, an aspheric lens having 2:1 demagnification, or varying the F number of the lens). 
     A fiber optic integrator  506  may be provided interposed between the illuminator LED  504  and the illuminator lens  508 . The fiber optic integrator  506  may have a diameter of approximately 1 mm and an F number of approximately 0.8. The fiber optic integrator is provided to homogenize the projected spot. The fiber optic integrator  506  thus creates a more uniform source which is re-imaged onto the surface of the copy medium  512 . It is noted that other embodiments may not employ a fiber optic integrator, and have less uniformity but less path loss 
     For color operation (i.e., detection of color spots), the illuminator can include two or three LEDs of different wavelengths to discriminate the color of the printed spot. If the color of the printed spot is known before scanning, the appropriate LED to discriminate the ink color can be selected. 
     The receiver  503  includes a receiver barrel  558  for mounting the receiver components. The receiver barrel  558  may be positioned roughly perpendicularly to the surface of the copy medium  512 . In particular, a receiver lens  510  may be mounted on the receiver barrel  558  by a receiver lens cell  556 . As illustrated, the receiver lens  510  is mounted at an end of the receiver barrel  558  proximate to the surface of the copy medium  512 . However, other embodiments may have the receiver lens  510  mounted in different positions in the receiver barrel  558 . In addition, it is noted that, while illustrated as a single lens, the receiver lens  510  may include a multiple lens system, including one or more cemented lenses. For example, the receiver lens  510  may be a doublet having four surfaces, a transmission of about 0.8 and an F number of about 5.0. A lens with a relatively high F number is employed to ensure adequate image quality over a broad depth of field. 
     The receiver lens  510  projects the image of the illuminated region onto a photodetector  516 , which is mounted on a mask and detector assembly  557 . The mask and detector assembly  557  is mounted on the receiver barrel  558  via pins  560   a ,  560   b  at an end opposite to the receiver lens  510 . Again, the photodetector  516  and the receiver lens  510  may be positioned differently relative to the receiver barrel  558 . A mask  518  interposed between the photodetector  516  and the receiver lens  510  is patterned to convert the image on the detector surface into variations in optical signal strength. As will be discussed in greater detail below, timing and changes in signal intensity reflect both the location and size of the scanned spot. The photodetector  516 , which may be a photodiode, converts the optical signal into an electric current. As will be discussed in greater detail below, the behavior of the modulated current is dependent upon the Y-axis position of the spot, the scan velocity of the printer head, the intensity uniformity of the illumination on the copy medium and the pattern on the mask. 
     The photodetector  516  is coupled to a processing unit  564 , which will be discussed in greater detail below. 
     As discussed above, the mask  518  is provided to convert the spatial intensity variations of the moving spot image into a modulation of optical signal strength. A pattern is provided on the mask  518  to encode a specific output signal signature as the spot  514  moves through the pattern. Different signal patterns are developed, depending upon the size of the spot  514  and the path through which the spot  514  moves. 
     An exemplary mask pattern is illustrated in FIG.  2 . As illustrated, the mask  518   a  includes a pair of apertures,  120   a ,  122   a  defining active regions (i.e., signal strength changes as the spots move through the regions). Exemplary spots  124   a ,  126   a  move from right to left across the mask  518   a . As illustrated, the mask apertures  120   a ,  122   a  have a length of about 10 mils in the vertical (Y) direction, although different sized apertures may be employed. The aperture  122   a  is configured to be roughly perpendicular to the direction of spot movement. The aperture  120   a  is angled from the direction of movement. 
     Turning briefly to FIG. 5, the spot detector  500 , and more particularly the photodetector  516 , generates an electronic signal that is generally similar to the signal  5002  illustrated in FIG. 5, when the reflected image of the ink spot passes through the mask  518 . The first pulse in the signal  5002  results from the image of the spot passing through the mask aperture  122   a  and the second pulse in the signal  5002  results from the image passing through the mask aperture  120   a . The actual X position of the spot relative to the expected X position is determined by comparing the timing of the first pulse in the signal  5002  with the encoded X position values received from a head carriage encoder. Interpolation may be used between multiple X position values to determine a more precise head carriage location, if desired. The actual Y position of the spot relative to the expected Y position is determined by evaluating the time span between the two pulses in the signal  5002 . For a given head carriage speed, the time span between the pulses will be larger if the spot is further up the Y axis illustrated in FIG.  2 . 
     Turning now to FIG. 3, a diagram illustrating a data acquisition and processing system  3000 , such as the processing unit  564 , according to an embodiment of the present invention is illustrated. The process shown is based on sensor signal digitization followed by digital processing. The use of the digitized signal allows the extraction of peak signal values in addition to the crossing times, to allow the extraction of shape and orientation information on the spot. Signal crossing points are processed in order to calculate spot position and size as will be discussed in greater detail below. In particular, inputs are received from a known head tractor encoder and head carriage encoder (not shown) to a data encoding and storage unit  3002 . The head tractor and the head carriage information is used, for example, to determine carriage speed and position. In addition, the mask detector signal is provided to an A/D converter  3001  prior to being provided to the data encoding and storage unit  3002 . The output from the data encoding and storage unit  3002  is provided to a signal position table  3004 . The signal position table  3004  contains the signal strength record correlated with the head position based on the tractor and carriage encoder outputs. The outputs from the signal position table  3004  are then provided to a signal parameter extraction unit  3006 . The signal parameter extraction unit  3006  uses the stored data after the sensor has swept over the ink spot. In particular, the signal parameter extraction unit  3006  performs a maximum value calculation  3006   a , determines a mean value and correlated position at mean crossings  3006   b , and determines rise/fall pulse width calculations  3006   c . From these calculations, the X and Y positions of the ink spot may be determined by calculation unit  3008 . Finally, the spot size may be estimated by unit  3010 , as will be discussed in greater detail below. 
     An alternate embodiment of a data acquisition and processing system  4000  is illustrated in FIG.  4 . The embodiment of FIG. 4 processes the signal data in the sensor head to extract the signal crossing times which are subsequently passed to the printer controller. The embodiment of FIG. 4 is suitable for applications where a dedicated A/D converter is not appropriate. Again, however, signal crossing points are processed to calculate spot position and size. Briefly, the embodiment of FIG. 4 may employ a custom processing ASIC (application specific integrated circuit) to eliminate the need for a dedicated A/D converter to repetitively sample the signal. The ASIC stores threshold crossing times of the pair of signal pulses and passes these times to the printer during or after the spot scan. Analog processing samples the peak value of each pulse to establish a maximum reference value which is scaled to establish a series of thresholds relative to a delayed signal waveform. The embodiment uses a state machine which simply successively steps through the threshold levels rather than having multiple comparators at each crossing level. 
     The timing diagram shown in FIG. 5 illustrates a representative waveform resulting from the ink spot moving through the mask. Turning back to FIG. 4, the input signal is filtered in a signal filtering unit  4016  to limit the detection bandwidth to meet the fastest rate of change expected during the spot scan. The output of the signal filtering unit  4016  is the signal  5002 . Following the input filter, the peak detector  4014  samples the maximum value of each pulse and holds it through the duration of the signal pulse. At the completion of each pulse, the peak detector is reset, allowing it to sample the next mask opening spot crossing. The peak detector output is shown as signal  5004 . 
     The maximum value peak detector output is used to establish a series of comparison reference levels which are progressively scaled by the factors 0.2, 0.5 and 0.8 to set the desired 20%, 50% and 80% threshold crossing levels by threshold weighting unit  4004 . In parallel with the peak detector, a network delay  4012  delays the incoming detector signal by half the maximum pulse width duration to allow the sampling of the signal peak prior to the leading edge of the delayed signal crossing the first 20% level. The delay can be provided by conventional active filter-based all pass network or through the use of a switched capacitor network. The output from the delay network is signal  5006 . As the signal crosses each threshold level, a positive going edge clock samples the value of the system clock timer. This timer value is processed in the printer controller with the position feedback that the scanner had at the time of the crossings. Adjustments for signal propagation delays may be desired. 
     The X and Y position determination is based on the estimation of the position where the mask opening overlays the ink spot in relation to information regarding the position of the print head. Assuming a constant head velocity over the region where the spot is located, the maximum signal will occur when the spot is centered within the mask opening and the peak will be centered between the 50% crossing points on the leading and falling edges. While half maximum detection on one edge may be used to resolve both the X and Y components in the ink spot in the determination of the X axis location, the use of half maximum detection on only the rising edge introduces an estimation bias dependent on the size and radial distribution of the spot. However, using both edges eliminates this bias error and improves estimation uncertainty by a {square root over (2)} factor by averaging the position uncertainty of two edges rather than one. Since the Y position is based on the relative difference between crossings rather than a single point, this bias error does not occur for the Y position determination. However, the Y axis spot position estimation also benefits from the estimation accuracy improvement resulting from using two pulse edges in the position calculation. 
     Determination of spot size is more difficult than spot position. Ideally, the peak value of the reflected signal should be correlated to the size of the spot and the contrast between the paper ink absorption. Unfortunately, the shape of the spot may not be uniform and the reflective characteristics of the paper may not be known so other information must be gained from the scanning data. In addition to the crossing time of the spot in the mask opening, information may also be obtained from the rise and fall times, pulse width and peak signal from the vertical and inclined portions of the mask. In the case of a circular ink spot, the signal response to the vertical and inclined portions of the mask are correlated. As long as the spot passes through the slot away from the edges of the mask opening, the spot passing through the inclined portion will appear to travel slower through the slit than with the vertical portion. This produces an increased rise/fall time and pulse width proportional to the cosine of the slit angle relative to vertical, for all spot sizes. 
     If the spot is not uniform, i.e., deviates from circular, and it is comparable in size to the slot width, the peak signal will be different between the slot crossing signals. The rise and fall time, and to a lesser extent pulse width, are correlated to the radial distribution of spot intensity at the mask plane. When the spot is smaller than the slit, the half width pulse width remains constant at the slit width. When the pulse diameter is comparable or larger than the slot width, the normalized pulse width increases. The rise time generally follows the spot diameter until the spot diameter is larger than the slit opening. At larger spot diameters, the rise time response rolls off. 
     FIGS. 6A and 6B illustrate relative signal response for various gaussian and pill spots, respectively. As can be seen, the responses for similarly-sized spots are similar. FIG. 7 illustrates a graph of pulse width and rise time versus normalized spot width for gaussian spot, i.e., a circular spot with a gaussian radial distribution. The simulation occurs against a fixed slit opening. Four spots with increasing diameters are passed across the opening and the rise time and pulse width are calculated from the normalized pulse response. All parameters are normalized to the slit width. As shown in FIG. 7, when the spot is smaller than the slit the half-width (50% points) pulse width remains constant at the slit width. When the spot diameter is comparable or larger than the slot width, the normalized pulse width increases. The rise time generally follows the spot diameter until the spot diameter is larger than the slit opening. At large spot diameters, the rise time response falls off. As shown in FIG. 8, both gaussian and pill function radial spot distributions result in generally similar signal outputs. In particular, FIG. 8 shows the results taking the product of the rise time and pulse width and plotting the result versus a spot diameter. 
     When a circular spot passes through a second inclined slit, the pulse width and rise time are proportional to the responses of the first slit by a one over cosine theta factor where theta is the angle of offset. The peak signal response is equal to the first response. 
     In the case of an elliptical pulse shape, the response of the second slit is no longer proportional to the first. The greater the degree of asymmetry, the larger the discrepancy. The orientation of the spot also affects the response. The effect on the peak signal difference is shown in FIGS. 9A-9B for an elliptical spot. As the spot width approaches the width of the slit, the response of the second, i.e., angled slit increases at a greater rate than the first. In the case of an angled elliptical pulse, as the spot width approaches the width of the slit, the response of the second slit increases more than when the pulse is not angled. 
     Thus, the product of the rise time and the pulse width can be used to approximate the spot size. In addition, the response of the second slit can be compared to the response of the first slit to determine whether the spot is more circular or more elliptical. If the responses are not proportional, then the spot is more elliptical and the spot size approximation may not be accurate. This condition can be reported to the printer microprocessor as a possible problem with the ink jet mechanism. 
     Turning now to FIGS. 10A and 10B, alternative embodiments of the mask are illustrated. Both the operating wavelengths and variations in the copy medium to lens operating distance cause movement of the point of best focus along the Z axis (perpendicular to the surface of the copy medium) at the mask image plane. This variation in focus leads to defocusing of the image and increasing the blur size. This effective image degradation of the depth of field can be reduced by changing the operating distance between the mask and the receiver lens as the operating distance to the surface of the copy medium is varied. As can be appreciated, this solution can be mechanically expensive and complex. Accordingly, according to an embodiment of the present invention, the optical working distance to a given mask opening is staggered by placing it on different (i.e., non-coplanar) mask substrate layers, such that different portions of the depth of field can be optimally resolved. FIG. 10A illustrates an exemplary multiple layer mask. Each opening  320   a ,  322   a ,  322   b  is at a different layer in the substrate. Aperture  320   a  is focused in the center of the depth of field; aperture  322   a  is focused at the near side of the depth of field; and aperture  322   b  is focused at the far side of the depth of field. 
     The layered mask effectively forms a series of spatial filters matched to regions over the depths of field. As the spot moves across the field, covered by the mask openings, a series of pulses are generated. If the spot is sufficiently small to be sensitive to degraded image quality, the pulse with the sharpest rise and fall transitions will indicate the mask openings placed at the point of best focus. Larger spots will yield relatively the same response at different openings. 
     In addition, color correction can be provided by using a modified staggered mask. The basic modified chevron pattern may be replicated on different mask layers, such that the variation in distance of the given layer reflects the effect of focal distance of the lens at a given wavelength. Thus, as illustrated in FIG. 10B, chevron pattern  350 ,  352 ,  354  each has a corresponding illuminated region of different wavelengths. Thus, the different LED outputs may be focused on different spots. 
     More particularly, turning now to FIGS. 11A-11C, various optical configurations for color spot illumination are illustrated. Turning now to FIG. 11A, a system  400   a  is shown in which the illuminated regions  420   a  for each wavelength are coaligned. More particularly, different wavelength (i.e., color) LEDs  406   a ,  408   a  are provided, with corresponding illuminator lenses  411   a ,  412   a , respectively. To ensure a common area of illumination, the illuminator systems are mounted roughly symmetrically about the receiver system. A receiver including a photodetector  402   a , a mask  404   a  and a receiver lens  410   a  are provided to receive the reflected signals. 
     FIG. 11B, on the other hand, illustrates a system in which separate illuminated regions are provided for each wavelength or color. Again, the receiver includes a photodetector  402   b , a mask  404   b , and a receiver lens  410   b . The illuminator system, however, includes different wavelength LEDs  406   b ,  408   b  mounted side-by-side, with a single illuminator lens interposed between the LEDs  406   b ,  408   b  and the separate illumination regions  420   b  on the surface of the copy medium. In this embodiment, although there are separate illuminated regions for each color, only one is activated at a time. As the spot moves across each field, successive illumination zones are activated to prevent spots from outside the desired registration region from influencing the modulation signal resulting from the desired spot movement. 
     An alternate configuration is illustrated in FIG.  11 C. As illustrated in FIG. 11C, the illuminator and detector systems are mounted coaxially. The illuminator LEDs  406   c ,  408   c  are mounted to project onto illuminator optics  412   c ,  410   c  and thereafter onto the spot on the surface of the copy medium. A detector  402   c  and associated support circuitry is mounted between the illuminator LEDs  406   c ,  408   c  and the surface of the copy medium. A receiver lens  410   a  is further interposed between the detector  402   c  and the surface of the copy medium. The receiver lens  410   a  receives the reflected illumination from the surface of the copy medium and projects it onto the detector. The configuration improves the intensity uniformity and the circularity of the illuminated region. The optics used for illumination employ a pair of spherical lenses with F numbers at approximately 1.0 to optimize throughput. The focal length of the lens is slightly longer than twice the receiver lens&#39;s F number. The LED die cup and support board are placed behind the detector IC. 
     An exemplary lens  600  is shown in FIGS. 12A-12C. The lens includes a detector lens  620  and an illuminator lens  622 . The lens  600  is mounted between a circuit board  602  and a window  608  which may include a mask as described above. A spot  514  is positioned on the medium  606  which is detected by the spot detector as described. 
     The circuit board  602  includes a plurality of LEDs  604   a - 604   d  and a detector  606 . In one embodiment, the LED  604   a  is a green LED positioned in a die cup; LED  604   b  is a red LED; LED  604   c  is a blue LED positioned in a die cup; and LED  604   d  is an IR (infrared) LED. The LEDs  604   a - 604   d  project light through lens  600  onto the spot  514 . The lens  600  has a faceted detector lens  620  which is configured to transmit the light from LEDs of particular wavelengths. In particular, transmit lens  622  includes a green facet  630   a , a red facet  630   b , a blue facet  630   c , and an IR facet  630   d . Each facet  630   a - 630   d  receives and transmits the light from the respective LED  604   a - 604   d  onto the spot  514 . 
     Corresponding light is reflected back onto the lens  600 . In particular, the light is provided to the receiver lens  620  and from there to the detector  606 . The detector  606  in turn provides signals corresponding to the impinging light to a detector processor. 
     The invention described in the above detailed description is not intended to be limited to the specific form set forth herein but, on the contrary, it is intended to cover such alternatives, modifications and equivalents as reasonably can be included within the spirit and the scope of the appended claims.