Patent Publication Number: US-7907161-B2

Title: Adaptive correction system

Description:
This disclosure is directed to systems and methods for adaptive correction of deviations arising in electronic image forming devices. The adaptive correction may be performed by comparing detected values with previously known desired or expected values, for example. 
     BACKGROUND 
     Electronic image forming devices are used for many purposes, such as scanning, copying, and printing. These image forming devices may include various optical elements, such as light emitting elements, lens elements and reflection elements. The performance of these optical devices may deteriorate over time based on a number of factors including wear, e.g. aging, and influences of environmental factors, such as temperature and humidity. 
     Conventional image forming devices do not contain a detector that allows for the device to perform adaptive correction. Rather, these conventional devices require a separate detection and calibration process, which cannot be performed during regular operation. 
     SUMMARY 
     Adaptive correction techniques are disclosed that adaptively correct aberrations that may arise in electronic image forming devices, such as printers, for example. In printers, aberrations may affect print quality in image distortions vertically down a page (slow scan direction) such as wobble banding, skewing, or bowing; horizontally across the page (fast scan direction) such as scan non-linearity, end-of-line jitter, or line magnification; or vertically or horizontally such as unevenness in image formation due to intensity deviations. 
     The disclosed adaptive correction techniques may adaptively correct such deviations by monitoring system performance using calibration data interspersed between operational data such as between pages or during other convenient times, collecting system performance based on the calibration data, and changing system operating parameters while processing the operational data. Calibration data processing results may be detected by a detector that is disposed to correspond physically to the recording medium so that characteristics such as position and intensity of a light beam at the detector correspond to those of the light beam at the recording medium. In this manner, system performance may be monitored and adjusted automatically during operation. 
     For example, the disclosed adaptive correction techniques may include a correction driver that provides capability to alter fast scan position and/or intensity of the light beam to compensate for deviations such as intensity variations as well as magnification, end-of-line jitter, etc. Further, the disclosed adaptive correction techniques may include capabilities to reorder input operational data for correcting slow scan aberrations such as skewing or bowing, for example. In this way, aberrations in system performance may be adaptively corrected while the system is performing operational processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic block diagram of an exemplary embodiment of an adaptive correction system according to the disclosure; 
         FIG. 2  illustrates a perspective view of the exemplary embodiment of the adaptive correction system; 
         FIG. 3A  illustrates an exemplary embodiment of an image writing portion; 
         FIG. 3B  illustrates an enlarged portion of the exemplary embodiment of an image writing portion; 
         FIG. 4A  shows an exemplary detector; 
         FIG. 4B  shows an enlarged portion of the exemplary detector; 
         FIG. 5  show an exemplary desired spot pattern; 
         FIG. 6  show an exemplary smile pattern; 
         FIG. 7  shows exemplary detected data; 
         FIG. 8  shows an exemplary correction map; 
         FIG. 9  shows an exemplary driver circuit; 
         FIG. 10  shows an exemplary driver circuit; 
         FIG. 11  shows an exemplary scan line non-linearity pattern; 
         FIG. 12  shows exemplary detected data; 
         FIG. 13  shows an exemplary correction map; 
         FIG. 14  shows an exemplary high frequency timing circuit; and 
         FIG. 15  shows an exemplary two dimensional detector; 
         FIG. 16  shows an exemplary bowing correction; and 
         FIG. 17  shows an exemplary flow chart. 
     
    
    
     EMBODIMENTS 
       FIGS. 1 and 2  shows an exemplary adaptive correction system  100 . Adaptive correction system  100  includes a detector  140  that allows the system to detect characteristics of a light beam emitted by a light beam emitter  170  and to compare these detected values with previously known desired or expected values. Adaptive correction system  100  may also include a correction controller  110 , a system controller  120 , an input interface  130 , a memory  150 , a light beam driver  160 , optics  180  and  188 , a polygon mirror  185  having mirror facets  187 , a beam splitter  190 , a motor  193  and a photoreceptor belt  195 . While correction controller  110 , system controller  120 , input interface  130 , detector  140 , memory  150 , and driver  160  are shown to be connected using a bus architecture, other hardware architectures may be used provided that the hardware architecture allows these components in adaptive correction system  100  to communicate with each other as disclosed below. 
     Input interface  130  receives data from a data source, such as a scanner of a copier, and the data may be stored in memory  150  either directly or through system controller  120 , or the data may be processed by system controller  120  without being first stored. System controller  120  may process the data to calculate, for example, driver control signals and send the driver control signals to correction controller  110  for adaptive correction processing, generating corrected driver control signals for controlling driver  160 . 
     Light beam emitter  170  generates a light beam  175  based on the corrected driver control signals received from system controller  120 . Light beam  175  passes through optics  180 , reflects from a mirror facet  187  of polygon mirror  185 , passes through additional optics  188 , and is split by beam splitter  190  into a first or transmitted beam  175 A and a second or reflected beam  175 B. Transmitted beam  175 A writes an electrostatic charge pattern of an image by altering electrostatic charge values on photoreceptor belt  195 . Charged toner is placed onto photoreceptor belt  195  based on the electrostatic charge pattern, and the toner is placed onto an image writing portion through an electrostatic attaching process. 
     As photoreceptor belt  195  is moved in a slow scanning direction  230 , transmitted beam  175 A is scanned in a fast scan direction  240  by polygon mirror  185  across an image writing portion such as image writing portions  250 ,  260  and  290 . 
     Motor  193  rotates polygon mirror  185  while light beam  175  is reflected from one of mirror facets  187 . Thus, light beam  175  is moved in fast scan direction  240 . The rotation of polygon mirror  185  and movement of photoreceptor belt  195  are coordinated so that each mirror facet  187  of polygon mirror  185  corresponds to one line in fast scan direction  240 , and image writing portion  260  corresponds to a page, for example. 
     A sync pulse may be provided that indicates a beginning of each line on image writing portion  260 . Sync pulses may be generated based on a detector which generates the sync pulse at a specific position of each mirror facet  187  relative to light beam  175 , for example. Additionally, a physical mark on the polygon mirror  185  and a detection system for detecting the physical mark, or a detection system of some other means may provide an index pulse to indicate the location of facet one for facet tracking purposes. 
       FIG. 3A  shows an exemplary image writing portion  260 , having a length  310  and a width  330 , and  FIG. 3B  shows an enlarged portion of the exemplary image writing portion. Image writing portion  260  may be moved along slow scan direction  230  by photoreceptor belt  195  as transmitted beam  175 A sweeps across image writing portion  260 , in fast scan direction  240 , while being modulated by driver  160  to generate an image for a page to be printed. 
     Driver  160  may generate a sequence of pulses to drive light beam emitter  170 . Each generated pulse may correspond to a dot forming a pixel on photoreceptor belt  195 . As light beam  175  is swept in fast scan direction  240 , a sequence of dots is generated forming a scan line  320 . A line may be drawn across a page by firing light beam emitter  170  continuously, for example. A portion  340  of scan line  320  is expanded in  FIG. 3  to show a sequence of dots forming a line. Spacing of dots along fast scan direction  240  and spacing of lines along slow scanning direction  230  may be adjusted based on relative movement speeds of polygon mirror  185 , photoreceptor belt  195  and timing of the sequence of pulses. 
     When generating a printed page, driver  160  maybe modulated by data received from input interface  130 . For text, data may be binary 1s and 0s. Thus, driver  160  generates a sequence of pulses having amplitudes corresponding to 1s and 0s based on the data to generate the printed page. 
     As shown in  FIG. 2 , a portion  295  of photoreceptor belt  195  may be unused for printing. Portion  295  may be a portion between consecutive writing portions  250 ,  260  and  290  used for printed pages or may be unusable portions of photoreceptor belt  195 , for example, where ends of a photoreceptor strip are stitched together at a seam to form photoreceptor belt  195 . Portion  295  may be used for calibration purposes. When a portion  295  is in position, adaptive correction system  100  may feed calibration data to driver  160  to detect attributes of various components for adaptive compensation of aberrations arising in the device. 
     Returning to  FIG. 1 , reflected beam  175 B may be directed toward detector  140 . Detector  140  and photoreceptor belt  195  may be disposed so that positions of transmitted beam  175 A on the photoreceptor belt  195  map consistently with positions of reflected beam  175 B on detector  140 . Thus, attributes of reflected beam  175 B on the detector  140 , such as intensity and position, should correspond to attributes of transmitted beam  175 A on the photoreceptor. 
       FIG. 4A  shows an exemplary embodiment of detector  140  that may include sensors  140 . 1 - 140 .p (where p is an arbitrary positive integer).  FIG. 4B  shows sensors  140 . 1 - 140 .x which maybe a small portion of detector  140 . Sensors may be grouped into groups such as groups  410  and  420  where each group of sensors corresponds to a single dot (or patch of dots) formed on photoreceptor belt  195 . A number of sensors in each group may be set as desired based on detection goals. Each of sensors  140 . 1 - 140 .p may detect an intensity of reflected beam  175 B at the particular position of each of the respective sensors. Thus, detector  140  may detect an actual position of reflected beam  175 B which corresponds to an actual position of transmitted beam  175 A on photoreceptor belt  195 . 
     The sensors may have parameters such as a height h 1 , a width w 1 , and a pitch (separation distance) p 1 . These parameters may be set based on detection needs. For example, height h 1  may be set for detecting a range of possible light dot vertical positions and/or light dots generated by a vertical stack of multiple light beams that are swept across the scan line direction. Width w 1  and pitch p 1  may be set based on a desired scan line and light dot detection resolutions, for example. 
     Detector  140  may also contain sensors for detecting aberrant conditions. For example, sensors may be provided beyond the starting and ending positions of a scan line so that a deviation from a desired scan line starting and ending positions may be detected. Also detector  140  may include multiple lines of sensors forming a two-dimensional detector  140  for detecting aberrations in the slow scan direction  230 , as shown in  FIG. 15  and discussed in more detail below. 
     A complete scan line may include thousands of dots. While every dot may be detected and processed to detect possible aberrations that may occur, samples of the scan line may be used instead of the complete scan line for detection and correction processing.  FIGS. 5 and 6  show 6 such samples. Also, while in most cases the samples should be evenly distributed across the scan line, non-uniform distributions may be desirable if particular portions of the scan line require higher detection resolution than other portions based on specific circumstances. The 6 samples shown in  FIG. 6  are assumed to be uniformly spaced across the scan line. 
       FIG. 5  shows an example of dots  502 - 512  detected by sensor groups  530 - 540  in a dot pattern  500  where each of dots  502 - 512  appears centered on each respective sensor group  530 - 540 , and has a same desired intensity. This is a desired condition.  FIG. 6  shows sample dots  614 - 624  detected by sensor groups  630 - 640  that have intensity aberrations. 
       FIG. 6  shows a pattern  600  where dots  618  and  620  have intensities less than desired while dots  614 ,  616 ,  622  and  624  have desired intensities. Pattern  600  illustrates an exposure non-uniformity aberration, sometimes referred to as smile. In particular, a smile may result when intensity of the light beam  175  received on photoreceptor  195  increases and then decreases, or decreases and then increases. A smile aberration may be a result of environmental effects such as temperature changes that affect reflectivity of a mirror facet  187  of polygon mirror  185  differently at its center than near its edges, for example. In addition, the optical prescription of the optics  180  and  188  in the system can also vary with these environmental effects. 
       FIG. 7  shows detected data  700  arranged in a two dimensional table, where one dimension corresponds to facets F 1 -Fn of polygon mirror  185 , and the other dimension corresponds to positions BP 1 -BPm of detector  140  along fast scan direction  240 . Positions BP 1 -BPm may correspond to sensor groups, such as sensor groups  530 - 540 , for example. The number n may be a total number of mirror facets  187  and m may be a total number of detection positions. Although a two dimensional table is shown in  FIG. 7 , detected data  700  may be arranged in any number of dimensions. For example, if variation in time is important, then a dimension corresponding to time may be added, or if all facets F 1 -Fn behave substantially the same, a one dimensional table that contains an average value for all facets F 1 -Fn may be arranged. 
     Entries in detected data  700  may contain values detected at each of the corresponding detector positions BP 1 -BPm. These values may be any type of detectable values such as horizontal or vertical dot positions or dot intensity, etc. A new dimension may be added to accommodate each additional type of detected value, for example. 
     Assuming that the values shown in  FIG. 7  are intensity values, and an intensity value of 250 microwatts is desired, then detected values corresponding to facet F 2  indicate a smile aberration due to lower than desired intensities of 230 microwatts at BP 3  and BP 4  positions while detected values at other positions BP 1 , BP 2 , BP 5  and BP 6  for facet F 2  and for all other facets are at desired values of 250 microwatts. Correction controller  110  may process detected data  700  to identify and correct errors such as the smile aberration by generating or updating a correction map. 
     As noted above, positions BP 1 -BPm maybe distributed uniformly across a scan line. Thus, while adjacent in detected data  600 , positions BP 3  and BP 4  may be physically separated by many pixel positions. Low intensity values of facet F 2  between BP 3  and BP 4  may indicate that transmitted and reflected beams  175 A and  175 B have low intensities for all pixels between positions BP 3  and BP 4 . Assuming that these low intensity values are substantially uniform between BP 3  and BP 4 , these pixels may be corrected by increasing driving current of light beam emitter  170  based on the current-intensity characteristics of light beam emitter  170 . The amount of increase may be stored in a correction map  800  as shown in  FIG. 8 , for example. 
       FIG. 8  shows a two-dimensional correction map  800  where the vertical dimension corresponds to facets F 1 -Fn of polygon mirror  185 , and the horizontal dimension corresponds to correction positions CP 1 -CPk of the transmitted beam  175 A along the fast scan direction  240 . While correction map  800  may provide correction of every pixel for every facet of F 1 -Fn, in a particular situation such as a smile aberration of detected data  700 , less than all of the pixel values require correction. Thus, actual stored values of correction map  800  may be a smaller number than shown. Although two dimensions are shown in  FIG. 8 , correction map  800  may have any number of dimensions as may be required for correction. 
     Correction map  800  makes corrections based on detected data, such as detected data  700 , to correct aberrations detected along facet F 2 . Thus, all of the entries in rows other than the row corresponding to facet F 2  contain “0”, because F 2  is the only facet showing an aberration to be corrected (a smile, in this case). In facet F 2 , pixels between CP 3  and CP 4  may benefit from an intensity boost. 
     While correction map  800  assumes uniform correction between CP 3  and CP 4 , various other assumptions can be made resulting in non-uniform corrections. In particular, adaptive correction system  100  may analyze detected data and perform curve fitting operations to model the detected data. For example, facets F 1 -Fn may have been well characterized. Thus, actual effects of a dependency of facet reflectivity on environmental parameters such as temperature or humidity may be determined based on detected data  700 . 
     For example, the detected intensities for facet F 2  may be sufficient to determine transmitted beam intensities for all pixels of scan lines generated based on facet F 2 . Adaptive correction system  100  may generate correction map  800  for facet F 2  based on curve fitting techniques or interpolation techniques, etc., to generate proper correction values for obtaining desired transmitted beam intensities for facet F 2 . This correction process may be executed on a continuous basis as device operation allows so that environmental effects on characteristics of transmitted beam  175  may be adaptively corrected as the environment changes. 
     Adaptive correction system  100  may correct various aberrations based on a correction map, such as correction map  800 , by controlling driver  160 , for example, to correct the smile aberration discussed above. In particular, the intensity of light beam  175  may be adjusted by adjusting a current value that drives light beam emitter  170 . 
       FIG. 9  shows an exemplary driver circuit  900  that may be included in driver  160  for driving light beam emitter  170  based on correction parameters derived from correction data  800 . Driver circuit  900  may include a FIFO  905 , a digital-to-analog converter (DAC)  920 , a variable resistor R 1 , a resistor R 2 , and an amplifier  965 . Amplifier  965 , variable resistor R 1  (which may be implemented using a transistor  930 , for example), and resistor R 2  may collectively form an adjustable gain amplifier  980  that has a gain of R 2 /R 1 . Thus, the gain may be adjusted by adjusting the value of variable resistor R 1 . 
     The gain of adjustable gain amplifier  980  may be controlled by digital values stored in FIFO  905 . As an example, FIFO  905  may include entries for all dots generated using every facet F 1 -Fn of polygonal mirror  185 . Thus, for an 8 inch scan line having a resolution of 1200 dots per inch and 6 facets, for example, FIFO  905  may include 1200×6=72000 entries where each entry specifies a gain value for each dot generated using each of the facets F 1 -Fn. In this way, the intensity of every dot generated using every facet F 1 -Fn may be controlled in a high bandwidth system. 
     FIFO  905  may be clocked by an input data clock (that is synchronized with a video clock of adaptive correction system  100 ), so that a new control signal may be output from FIFO  905  for each input data point. Thus, as data received from input interface  130  is streamed to adjustable gain amplifier  980  in synchronization with input port or data port  990 , a unique gain value may be applied to each data point, and the intensity of each dot generated by light beam emitter  170  may be individually adjusted. 
     As shown in  FIG. 9 , output of FIFO  905  is converted to an analog value by DAC  920 , which in turn controls the gain via variable resistor R 1 . Input data port  990  may be in the form of binary values of 0s or 1s. When a 1 is received, output of adjustable gain amplifier  980  is converted to a current value by transistor  960  acting as a voltage-to-current converter that results in an adjusted intensity being output by light beam emitter  170 . When a 0 is received, the output of adjustable gain amplifier  980  is set to a value that results in a substantially 0 intensity being output from light beam emitter  170 . 
     For the smile aberration example discussed above, only the intensity values of facet F 2  need correction. Assuming that only pixels between CP 3  and CP 4  inclusively need to be corrected by a fixed multiplier value (determined to be some value greater than 1 for a smile, for example), entries of FIFO  905  corresponding to CP 3  and CP 4  are set to this calculated value while all other entries are set to 1. In this way, intensity of transmitted beam  175 A will remain at the desired constant value for all pixels of a printed page. 
     Detected data  700 , correction map  800  and/or FIFO  905  may be optimized, and simple structures may be used, in situations where a number of aberrations are expected to be low. For example, detected data  700  and correction data  700  may store only values that correspond to aberrations, and FIFO  905  may be implemented using a few registers and counters so that consecutive values of 1 may be simply applied while counting a number of video clock pulses instead of using dedicated memory for storing the large number of 1s, for example. 
     Additionally, while adjustable gain amplifier  980  is discussed as an example, other circuit structures may be used. For example, instead of variable resistor R 1 , a bank of resistors R 1   a  may be provided. One of the resistors in R 1   a  may input the input data stream while the other resistors in R 1   a  may be connected between a reference voltage via individual switches and positive input of amplifier  965 . Data from FIFO  905  may control the switches to be on or off. In this configuration, amplifier  965 , resistor R 2  and resistor bank R 1   a  form a summer so that the intensity of each dot may be adjusted by adding a correction value. 
     For example, if nine resistors R 11 -R 19  are provided in a binary weighted resistor array or resistor bank R 1   a,  as shown in  FIG. 10 , input data port  990  may connected through R 19  and R 11 -R 18  may be set to binary weighted values so that an 8 bit correction value from FIFO  905  may set corresponding switches to increase the intensity of a dot by adding the 8 bit correction value. For the above example, 20 microwatts would be added for pixel values between BP 3  and BP 4  positions. 
       FIG. 11  shows a pattern  1100  that illustrates a scan line non-linearity aberration. Scan non-linearity aberration may be caused by the angular changes of the reflected beam of the polygon. In an uncorrected system, the placement of the spots in the image as the polygon rotates would be a function of the tangent of the scan angle. Post-polygon elements, called F-theta lenses, correct for much of the optical scan linearity, but some residual non-linearity may remain. The remaining error may be fixed by adding electronic modulation of the pixel clock frequency during the scan line time. See, for example, U.S. Pat. No. 4,860,237 to Curry. 
       FIG. 11  could also represent another aberration, scan line magnification, which could have a similar appearance to scan line non-linearity, but represents a distinct phenomenon. In particular, although both scan line non-linearity and scan line magnification may result in spots, which are intended to be evenly spaced across the scan line, appearing more and more in adjacent (overlapping) sensor groups, as seen in  FIG. 11 , magnification also results in the scan line expanding beyond, or shrinking beneath, its intended length. That is, in a scan line magnification aberration a length of a scan line appears magnified so that a scan line becomes longer (positive magnification) or shorter (negative magnification) than a desired scan line. For example, pattern  1100  shows a sequence of light pulses in which dots of pixels are increasingly disposed in locations of adjacent pixels. As noted above, sensor groups may include several sensors such as groups  410  and  420  shown in  FIG. 4 . 
     Scan line magnification aberration may be caused by motor  193  driving polygonal mirror  185  either too fast or too slow relative to the pulse timings of light beam  175  for example. If this is the case, then the scan line magnification aberration would appear for all facts F 1 -Fn. Other variations such as physical deformation of polygonal mirror  185  so that one of facets F 1 -Fn becomes curved, for example, may result in magnification aberration only in a single facet F 1 -Fn. Correction for the scan line magnification aberration may be determined by sensing the start and end points of the scanned line and calculating the difference between the actual scan line and the desired scan line, for example. 
     Returning to the discussion of scan line non-linearity, as the polygon facets deform due to the high rotational speeds, thereby changing the reflected angle of the beam for a given rotational position, and also as both the polygon and the post-polygon optics are affected by environmental effects, electronic correction of scan line non-linearity may benefit from re-optimization. The following discussion assumes that all the facets F 1 -Fn exhibit substantially the same scan non-linearity aberration. However, the discussion may be applicable to any type of scan non-linearity aberration. 
     Sensors of detector  140  may be disposed so that the middle two sensors detect a dot when properly positioned and the sensors on either side serve as guard sensors and are activated only when the dot is out of its proper position. For dot position detection, an intensity threshold may be determined so that a sensor output is a 1 if a detected intensity exceeds a threshold and a 0 if the detected intensity is below the threshold. Thus, each sensor group may be represented by a 4 bit pattern. For example, 0110 may represent a properly positioned dot. 
       FIG. 12  shows detected data  1200  based on pattern 1100 occurring in facets F 1 -Fn. Entries of all facets F 1 -Fn at dot positions BP 3  and BP 4  include sensor patterns 0110 showing that each corresponding dot is located in the desired position. However, at BP 1 , BP 2  and BPm dot positions, aberrations are detected. For example, at BP 1 , the detected pattern is 1000 indicating that the dot at this position has shifted to the left and only the right edge is detected by the left guard sensor. 
       FIG. 13  shows a possible correction map  1300  for correcting scan line non-linearity  1100 . Each of CP 1 , CP 2  and CPk has entry values of −2, −1 and 2, respectively, indicating that the dots at CP 1  and CP 2  should be moved 2 and 1 position increments to the right, respectively, and dot position at CPk should be moved 2 position increments to the left. 
     As noted above, a sync pulse may be provided that corresponds to each facet F 1 -Fn, and a sequence of dots of a scan line may correspond to a sequence of pulses that drives light beam emitter  170  via driver  160 . Accordingly, increasing or decreasing the time that each pulse is applied by changing the frequency of the pixel clock to light beam emitter  170  changes the position of the corresponding dot to the right or left, respectively, in the scan line. 
     As noted in connection with  FIG. 7 , entries in detected data  1200  at BP 1 -BPm positions represent a relatively small portion of the total number of dots of a scan line. Thus, there may be many dots in between each of the positions BP 1 -BPm, that may also need to be repositioned to correct the scan non-linearity aberration. If it is assumed that dot position aberrations are distributed in a known manner based on accurate characterization of the polygonal mirror  185 , for example, then the scan non-linearity aberration may be corrected for all the dots by appropriately modifying the positions of associated variable pulses based on known properties of polygonal mirror  185  and correction map  1300 , for example. 
     An exemplary circuit  1400  shown in  FIG. 14  may be included in driver  160 , for example, to perform the above described pulse start time correction to correct a magnification aberration. Circuit  1400  may include a comparator  1402  that compares a value of a clock counter  1404  against corrected pulse start times output from a FIFO  1406 . Counter  1404  counts a number of clock pulses output from system clock  1408  after the sync pulse received on start line  1410 . When a count value of counter  1404  matches the output value of FIFO  1406 , a comparator output  1412  of comparator  1402  becomes active and triggers driver  160  to output a next pulse that results in light beam emitter  170  generating a dot. FIFO  1406  also receives comparator output  1412  and increments its pointer to output a value corresponding to a next pulse start time. In this way, magnification aberration may be corrected for every dot of a scan line. 
     While the above describes correcting pulse start times, circuit  1400  may also be used to correct pulse widths. Scan non-linearity aberration may also result in pulse widths becoming either too long or too short. As discussed above, dots may be repositioned by correcting pulse start positions. However, the repositioned pulses may be either too wide or too narrow and may result in unacceptable image outputs. Therefore, the clock frequency is not only frequency modulated to provide new positional placement of the spot, but it is also frequency modulated to provide a correction for the tangential (or fast scan) spot size within a given region of the polygon. 
     The above described circuit  1400  may correct any types of non-uniform positioning of dots along a scan line. Depending on particular irregularities of facets F 1 -Fn, dots may be positioned too closely or too far apart in regular or irregular patterns. Detector  140  may be provided with sufficient number of sensors and sensor groups to detect such aberrations. Additionally, pulse widths may be too wide or too narrow, resulting in width aberrations, in which light spots that are too large or too small. All of these aberrations may be corrected by comparing the detected positions and widths with previously known desired or expected positions and widths, and repositioning the start and stop positions of light spots by correcting pulse start and stop times, as discussed above. 
     In addition to correcting positional aberrations using circuit  1400  to modify the start times of pulses, positional aberrations may also be corrected by slowing or accelerating the speed of polygon mirror  185 . Thus, adaptive correction system  100  may first detect positional aberrations, then calculate the appropri ate acceleration or deceleration of polygon mirror  185  to generate a correction map (including, for example, curve fitting detected data), and then use the calculated values in the correction map to accelerate or decelerate polygon mirror  185  to correct the positional aberration. 
     Aberrations causing the non-uniform dot positions discussed above occur in the fast scan direction. Similar aberrations may occur in the slow scan direction, such as bowing aberration, for example. To detect bowing aberration, detector  140  maybe modified into detector  1500  that may include two dimensional sensors and sensor groups, as shown in  FIG. 15 . 
       FIG. 15  shows a two dimensional detector  1500  for detecting reflected beam  175 B along two substantially different (horizontal and vertical dimensions). In particular, detector  1500  contains sensor groups such as group  1505 , which contains three middle row sensors  1510 , three top row sensors  1520 , and three bottom row sensors  1530 . Detector  1500  contains other sensors groups, extending in both the horizontal and vertical directions. For example, a sensor group to the right of group  1505  contains a sensor  1515  in the same middle row as sensors  1510 , and a sensor group even farther to the right contains sensors  1525  and  1535 , in the same top and bottom rows as sensors  1520  and  1530 , respectively. 
     Detector  1500  may detect a desired light beam, such as reflected beam  175 B, across the middle row in any series of horizontally contiguous sensors. In particular, desired light spots may impinge upon the sensor, such as the middle sensor among sensors  1510 , in the center of each sensor group. Furthermore, detector  1500 , like detector  140 , may also only detect a sample of reflected beam  175 B as the beam sweeps across the scan line direction. 
     The sensors in the top and bottom rows of corresponding sensor groups, such as sensors  1520  and  1530  in group  1505 , allow detector  1500  to detect aberrations in the vertical dimension, such as bowing. For example, although a desired reflected beam  175 B may sweep across only sensors in the middle row, such as sensors  1510 , a bow may occur when transmitted beam  175 A, and therefore also reflected beam  175 B, deviates in the vertical direction, to sweep across any of the sensors in the top and bottoms rows, such as sensors  1520  and  1530 . 
     Bowing aberrations may be corrected by many methods. If light beam emitter  170  includes multiple emitters positioned so that respective dots are separated in the slow scan direction, then small bowing aberrations may be corrected by selecting a different light emitter. If large bowing aberration is detected, then operational data remapping may be beneficial. 
     For example, suppose that some of the dots are desired to appear on line L, but instead appear on line L+1 (one line after line L) because of bowing. Then software and/or hardware may alter the data generating line L−1 (one line preceding line L), so that line L−1 includes data for generating the dots intended for line L which had previously been drawn incorrectly as line L+1 because of the bowing. In that case, the bowing in adaptive correction system  100  will cause the dots in the data for line L−1 to appear in line L, as desired. 
       FIG. 16  shows an example of correcting bowing aberration, as described above. Data portion  1601  contains the order numbers for sending data to driver  160 . For example, the first position in line L−1 of data portion  1601  contains a “1” for writing the intended spot. Similarly, the first position in line L contains “M+1”, because that location corresponds to writing the next spot after all of the first through M spots for line L−1 have been written. For purposes of discussion, data corresponding to line L−1 are labeled as A, data corresponding to line L are labeled as B, and data corresponding to line L+1 are labeled as C. 
     For a bowing aberration, data portion  1601  may be written onto photoreceptor belt  195  as shown in output portion  1602 . Thus, some of the data of line L−1 are written to line L, for example. Similarly, some of the data of line L are instead written to line L+1. 
     Bowing aberrations may be corrected by reordering the data as shown in data portion  1603 . In this way, when line L−1 is written, the data corresponding to line L will be written onto proper positions. Thus, bowing aberrations may be corrected by processing the operational data in a compensating order. Operational data reordering and emitter selection may be used together to compensate for bowing aberrations. 
       FIG. 17  shows a flow chart  1700  for an exemplary process that performs adaptive correction for a device. In particular,  FIG. 17  shows a process which may correct both intensity aberrations, such as those shown in  FIG. 5 , as well as position aberrations, such as scan line magnification and bowing using the adaptive correction system and techniques discussed above. Although the process may correct both position and intensity aberrations, one may also just correct one type of aberration, or more than two aberrations, including other types of aberrations than intensity and position. 
     The process begins at step  1705  and goes to step  1710 . In step  1710 , a determination is made whether the device is currently in a state in which adaptive correction is preferably performed, such as a state in which more resources are available for performing adaptive correction. For example, correction may be performed when the device is (i) in between print jobs, (ii) in between pages of a print job, and (iii) in a wait state, for example. If the device is not currently in a preferred state for performing adaptive correction, the process returns to step  1755 . 
     However, if adaptive correction can be performed, the process goes to step  1715 . In step  1715 , the process writes calibration data to a photoreceptor, for example, and goes to step  1720 . In step  1720 , the process determines whether a slow scan aberration, such as bowing, exists. If bowing is detected, the process goes to step  1725 ; otherwise, the process goes to step  1730 . 
     In step  1725 , the process corrects the slow scan position aberrations by either selecting an appropriate light emitting element if available, or remapping operational data, or both, and goes to step  1730 . 
     At step  1730 , the process determines whether a fast scan aberration, such as magnification or scan non-linearity, exists. If aberration in the fast scan direction is detected, the process goes to step  1735 , otherwise the process goes to step  1740 . 
     In step  1735 , the process corrects the fast scan aberration by changing pulse timing of light beam emitter  170 , for example, and the process goes to step  1740 . 
     At step  1740 , the process determines whether an intensity aberration exists. If an intensity aberration is detected, then the process goes to step  1745 ; otherwise, the process goes to step  1750 . In step  1745 , the process corrects the intensity aberration and goes to step  1750 . 
     At step  1750 , the process applies the corrected values generated in steps  1725 ,  1735 , and/or  1745 , for example, to drive the light emitting elements to correct the detected aberrations and goes to step  1755 . If the device is turned off, then the process goes to step  1760  and ends; otherwise, the process returns to step  1710 . 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.