Abstract:
A method for banding suppression due to vibration in an imager utilizing a light emitting diode (LED) array is disclosed. By characterizing the LED array and knowing the amplitude and frequency of the signal due to vibration which causes the banding, the LEDs&#39; driving currents are modulated in reverse direction corresponding to the banding signal to suppress the banding.

Description:
CO-REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is related to the following co-pending applications: Serial No. XXXX (Attorney Docket D/A1556Q) entitled “Banding Suppression Due to Vibration in Imager with Laser Driver” filed on the same date as the present application and commonly assigned to the present assignee, the contents of which are herein incorporated by reference. 
     
    
     
       BACKGROUND  
         [0002]    Scanned light beam imaging is employed for various purposes including electrophotographic printing. In such systems used for printing, a light beam generated, for example, by a laser is selectively modulated to vary its amplitude while it is scanned laterally across a moving photoconductor to selectively discharge the photoconductor. Such systems have been binary in nature (“on” or “off”) to accomplish printing of character information by selectively either exposing the photoconductor or leaving it unexposed. To insure complete exposure, adjacent scan lines are usually overlapped slightly. Thus, any slight misalignment of the scans or change in beam size would not be noticed.  
           [0003]    This cannot be done if the light beam modulation is other than binary because in any partial exposure, a substantial overlap would appear as a stripe of complete exposure. Alternatively, spacing the scans apart would appear as a stripe that remains unexposed. To attempt to prevent such stripes by substantially eliminating the optical and mechanical tolerances from the system would appear to be prohibitively expensive.  
           [0004]    It is therefore an object of the present invention to eliminate the perception of elongated stripes in such optical imaging systems without requiring substantial tightening of allowable optical or electromechanical tolerances.  
         SUMMARY  
         [0005]    A method for banding suppression due to vibration in an imager utilizing a light emitting diode (LED) array is disclosed. By characterizing the LED array and knowing the amplitude and frequency of the signal due to vibration which causes the banding, the LEDs&#39; deriving currents are modulated in reverse direction corresponding to the banding signal to suppress the banding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a diagrammatic view of a printing device employing a scanned light beam imaging utilizing a laser driver;  
         [0007]    [0007]FIG. 2 is a detailed diagrammatic view of the light beam modulation of FIG. 1;  
         [0008]    [0008]FIG. 3 is an example of xerographic print with halftone banding when the vibration error consists of a single frequency;  
         [0009]    [0009]FIG. 4 is a graph showing measurement of photoconductor velocity error vs. time for a prototype xerographic copier;  
         [0010]    [0010]FIG. 5 shows a graph of the fourier transform of the velocity error in FIG. 4;  
         [0011]    [0011]FIG. 6 is a graph illustrating optical power at the output of an actual LED versus drive current;  
         [0012]    [0012]FIG. 7 is a graph showing relative output power vs. current for high efficiency red LED;  
         [0013]    [0013]FIG. 8 is a graph showing rectangular irradiance profile H 1 (x)=H 0 rect(x/W);  
         [0014]    [0014]FIG. 9 shows a graph for the normalized exposure distribution due to rectangular irradiance moving, which is obtained by moving the pattern in FIG. 8 with constant velocity V 0  (i.e. with no banding) along the x-axis;  
         [0015]    [0015]FIG. 10 is a graph illustrating the exposure distribution due to sinusoidal irradiance modulation of rectangular irradiance; and  
         [0016]    [0016]FIG. 11 shows a graph resultant normalized exposure profile after modulating driving current of LED by −(1/k)m cos(2 παt). 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 illustrates a copier/printer machine employing scanned light beam imaging on an electrophotographic drum  10 . The surface of the drum comprises a photoconductor which is either coated on the surface of the drum or is in the form of a sheet material wrapped around the surface of the drum. As shown in FIG. 1, a laser  50  generates a coherent light beam  51  which is directed through an acousto-optic modulator  52 . The acousto-optic modulator selectively deflects the light beam  51  in accordance with an input signal provided at input  53 . The unmodulated, or “zero-order” beam  55  from the acousto-optic modulator is intercepted by a knife edge  56 .  
         [0018]    A signal representing the data to be printed is supplied at input  60  to amplitude modulation circuit  61 . The amplitude modulation signal is supplied by circuit  61  to input  53  of the acousto-optic modulator. The acousto-optic modulator deflects the light beam  51  in accordance with the presence or absence of the amplitude modulation in the direction of α. The amount a of deflection is determined by the modulation frequency.  
         [0019]    The modulation signal applied at input  53  is an amplitude modulated carrier signal. This can result in a grey-scale effect on the imaging of the light beam in that the amplitude modulation varies the duration of the carrier causing the light beam to be deflected beyond the knife edge  56 . The deflected light beam  62  is projected through beam expanding lens  63  to beam deflector  64 . Beam deflector  64  may comprise an oscillating mirror or a rotating mirror assembly having many facets about the periphery thereof. The beam deflected by deflector  64  is projected through imaging lens  66  onto the photoconductor surface  11  of drum  10 . Beam deflector  64  causes the modulated beam  62  to be scanned axially of drum  10  along line  70  across the photoconductor surface  11 .  
         [0020]    As discussed above, the amplitude modulation at input  53  to the acousto-optic modulator results in the deflection of the coherent light beam  51 , as shown by deflected beam  62 . The modulation is of a carrier signal. Therefore, amount of amplitude modulation controls the time duration that the beam  62  will be deflected beyond knife edge  56 , resulting in different exposure times of the portion of the photoconductor  11  being scanned.  
         [0021]    The beam spot size, and therefore the width of the trace  70  made by beam  62  on the photoconductor  11  may be the subject of slight variations. These variations may be from machine to machine, as variations in the provided laser or in the acousto-optic modulator, or in distance tolerances, as in the distance from lens  66  to surface  11  of drum  10 . Variations may also occur due to different operating conditions for a single machine, such as temperature causing a difference in laser output, etc. Lastly, and most importantly, periodic variations may occur within a single machine, such as vibrations within the printhead, misalignment of the rotating deflector, or facet-to-facet angular variations on the deflector itself.  
         [0022]    Any of these variations results in the appearance of interstices between adjacent scan lines  70 . For example, should the beam be slightly low on one scan, and normal or slightly high on the next adjacent scan, the two scan lines will overlap slightly. On the other hand, should the one scan be slightly high, and the next adjacent scan normal or slightly low, the adjacent scan line will be separated slightly. Either the overlap or the separation of adjacent scan lines are defined herein as interstices.  
         [0023]    By moving the light beam orthogonally with respect to the scan direction as shown by scan line  70  it breaks up any unscanned or overlapped interstices between adjacent scan lines to make the interstices appear nearly invisible. The light beam is moved orthogonally by using frequency modulation circuit  80  for supplying a frequency modulation carrier signal to carrier input  81  of amplitude modulation circuit  61 . An oscillator  82  supplies the basic carrier signal to the frequency modulation circuit  80 .  
         [0024]    As will be described, frequency modulation circuit  80  and AM modulator  61  includes source of amplitude and frequency modulation means for modulating the carrier signal in accordance with a set of governing equations. The signal supplied by frequency modulation circuit  80  to input  81 , therefore, comprises a carrier signal which varies in frequency in accordance with theses governing equations. The frequency modulated carrier is then amplitude modulated by circuit  61  and supplied at input  53  of acousto-optic modulator  52 . The resultant variations in frequency cause the amplitude modulated light beam  62  to be deflected as shown by the dither angle φ. which is about the nominal amplitude modulation α.  
         [0025]    Therefore, producing a dither deflection of beam  83  orthogonal to the scan line produces a scan line  84  variably displaced from the scan path  70  with no dither. The displacement of the scan path  84  is shown in FIG. 1 greatly exaggerated for the purpose of illustration. In reality, the dither displacement is very small, typically much less than the center-to-center displacement between adjacent scan lines.  
         [0026]    The circuitry of FIG. 1 is replicated in FIG. 2 with a diagrammatic illustration in more detail of the effect of the acousto-optic modulator  52  upon the incident light beam  51 . The zero-order beam  55  is the path taken by the unmodulated beam. When an input signal of high frequency is supplied to input  53  of the acousto-optic modulator, the incident beam  51  is deflected to the nominal deflection path  62 . Using a frequency of, for example, 80 MHz, as supplied by oscillator  82 , the resultant deflection angle is 13.9 milliradians. The nominal deflection is shown by α.  
         [0027]    The acousto-optic modulator  52  works on the principle that an acoustic wave generated by the signal at input  53  deflects the light beam in accordance with frequency of the input signal. The amplitude modulation provided by circuit  61  does not in itself provide grey scale, but rather supplies the drive signal from oscillator  82  at a sufficient amplitude to cause operation of the acousto-optic modulator.  
         [0028]    A different way of producing grey scale is to control the power of the laser or other light source providing the beam  51 . Thus, the presence or absence of data will be controlled by the amplitude modulation signal appearing at input  60 , but the grey scale level of that information will be controlled by the power of the incident beam  51 .  
         [0029]    In either of these grey scale reproduction techniques, any interstices between adjacent scan lines will be highly visible, as discussed above.  
         [0030]    These interstices may be broken by the application of dither through the operation of circuit  80  of FIG. 1, shown in FIG. 2 as comprising FM modulator  80  and a random source  87 . Dither has two properties which may be specified: (1) the maximum amount, or amplitude, of dither, and (2) the frequency, or rate of change, of the dither. Dither amplitude comprises the difference in amount of deflection from that comprising the nominal path at the center frequency modulation frequency  62 . The total extent comprising the maximum amplitude of dither comprises the angular difference φ. between path  90  and  91 . At the photoconductor, this total difference may be represented as the distance X. This amplitude is controlled by varying the total frequency excursion of the modulator carrier derived from circuit  82  from its nominal frequency. The frequency, or rate of change, of the dither is controlled by random source  87  in modulating the carrier at FM modulator  80  by controlling the rate of change of the modulator carrier frequency.  
         [0031]    The maximum amplitude of the dither chosen depends upon the maximum displacement error expected between adjacent scan lines. For example, the maximum displacement error, comprising the overlap or the separation between adjacent scans of the laser beam, may comprise 0.0004 inches.  
         [0032]    With the exemplary nominal carrier frequency of circuit  80  at 80 MHz, a dither maximum amplitude X of 0.4 mils, would, in an exemplary machine having a distance of approximately 25 inches between the acousto-optic modulator  52  and the photoconductor  11 , would comprise 16 microradians. Since the 80 MHz input signal at input  53  to the acousto-optic modulator produces a deflection α of 13.9 milliradians, the total frequency change, or Δf, required to produce the desired 16 microradian dither φ is 46 KHz.  
         [0033]    An example of xerographic print  100  with halftone banding when the vibration error consists of a single frequency is shown in FIG. 3. FIG. 4 shows a graph of measurement of photoconductor velocity error  102  versus time  104  for a prototype xerographic copier where the nominal velocity is 8.1 in./sec. As seen in FIG. 4, the velocity error  102  is not constant and has a modulation on the order of ±4%.  
         [0034]    [0034]FIG. 5 shows a, graph of the fourier transform  106  of the velocity error  102  in FIG. 4. FIG. 5 reveals a spectrum of four major frequencies  108  resulting from the prefuser transport (26 Hz)  110 , the cleaner (43 Hz)  112 , the photoconductor drive (46.9 Hz)  114  and the main drive motor (99.6 Hz)  116 . The zero-to-peak amplitude of each frequency component is between {fraction (1/2)} and 2% of normal velocity. Vibrations such as these, when present in a printer, can result in serious print defects including banding in halftones.  
         [0035]    The signal in FIG. 5 is characterized as having an amplitude and frequency as follows:  
           V=V   0   +a  cos(2  πft )   (1)  
         [0036]    where V 0  is the nominal velocity;  
         [0037]    a is the amplitude of error velocity due to vibration; and  
         [0038]    f is temporal frequency due to vibration.  
         [0039]    As the laser imager exposes photoconductor  11 , any vibration by the photoconductor  11  in the process direction (perpendicular to the raster lines) will cause undesirable variation in lines and patterns running across the process (parallel to the raster lines). The exposure received by a line on the photoconductor  11  at the spatial coordinate x′ is given by:  
               E        (     x   ′     )       =       ∫     -   ∞     ∞              H   1          (   x   )                H   2          (   t   )       /     v        (   t   )                            x                 (   2   )                               
 
         [0040]    where H 1 (x) is the spatial variation of the image irradiance profile in the process direction (x), and H 2 (t) describes the time variation of the desired irradiance pattern (eg. on at time zero, off at time t 1 , with constant irradiance). H 2 (t) is the “on/off” bit pattern of a single writing element (pixel) as it varies in time. The velocity of the photoconductor  11 , with an unwanted vibration, is given by:  
           v ( t )= v   0   +a  cos(2  πft )   (3)  
         [0041]    where v 0  is the constant nominal velocity and a is the amplitude of the velocity modulation of temporal frequency f. The relationship between time and position on the photoconductor  11  is given by:  
           t =( x - x ′)/ v   0   −a  sin(2  πft )/2  πfv   0    (4)  
         [0042]    where x′ is an arbitrary initial position on the photoconductor  11 .  
         [0043]    To compensate for the velocity error and eliminate the line variation, correction must be made to the on/off timing of the image bar (FM) and the time dependent radiance (AM) of the image.  
         [0044]    To illustrate the principle of FM correction analytically, consider exposing a single line of desired width W 0  and exposure level E 0 . If the photoconductor  11  is exposed for a nominal time t 0 =W 0 /v 0 , by an ideal infinitely narrow irradiance distribution, H 2 (x)=δ(x), the actual width W 1  of the exposure profile (5) will be given by:  
                 W   1          (     t   0     )       =         ∫   0   t0            v        (   t   )                          t         =       ∫   0   t0          [         v   0     +       acos        (     2      π                 f                 t     )               t         =           v   0          t   0       +         asin        (     2      π                 f                   t   0       )       /   2        π                 f       =       W   0     +     e                   sin        (     2      π                 f                   t   0       )                             (   5   )                               
 
         [0045]    which has an error of magnitude e=a/2πf. To eliminate this line-width error, the on-time must be adjusted to a value t 1  such that W 1 (t 1 )=W 0 . That is,  
           W   1 ( t   1 )= v   0   t   1   +e  sin(2  πft   1 )= W   0   =v   0   t   0    (6)  
         [0046]    or  
           t   1   +e  sin(2  πft   1 )/ v   0   =t   0 ,   (7)  
         [0047]    This transcendental equation can be solved for the required on-time t 1  in terms of the nominal on-time t 0 , e, v 0  and f.  
         [0048]    The FM corrected line exposure pattern is now given by:  
               E        (     x   ′     )       =         ∫     -   ∞     ∞              H   1          (   x   )                H   2          (   t   )       /     v        (   t   )                            x         =       ∫     -   ∞     ∞              H   1          (   x   )              rect        (     t   /     t   1       )       /     v        (   t   )                            x                   (   8   )                               
 
         [0049]    where H 2  is a rectangle of the correct width W 0 , i.e. the on-time has been corrected. Thus, the geometrical properties of the image by FM correction are made properly. However, the line exposure still depends on the variable velocity v(t) and will not equal the desired exposure E 0 . Thus FM correction is only partially complete. This approach may be applied to any pattern of lines and solid areas.  
         [0050]    In order to completely eliminate the unwanted effects of vibration, AM correction must be employed wherein the irradiance of the image bar is corrected in real time to provide the correct level in the presence of vibrations. This is achieved by multiplying the irradiance function, H 2 (t), by an amplitude function H 3 (t), which controls the level of irradiance, with the property that:  
           H   3 ( t )= v ( t )/ v   0    (9)  
         [0051]    Then the exposure pattern will be given by  
               E        (     x   ′     )       =         ∫     -   ∞     ∞              H   1          (   x   )              H   2          (   t   )                H   3          (   t   )       /     v        (   t   )                            x         =       ∫     -   ∞     ∞              H   1          (   x   )                H   2          (   t   )       /     v   0               x                   (   10   )                               
 
         [0052]    which has been corrected for the time-varying photoconductor velocity. However AM correction alone is not complete either since the on-time (write command) is not corrected to account for the vibration. This can be seen by substituting for the time variable in H 2  where:  
           t =( x - x ′)/ v   0   −e  sin(2  ft )/ v   0    (11)  
         [0053]    The lowest order solution of this equation is given by:  
           t =( x - x ′)/ v   0   −e  sin[2  πf ( x - x ′)/ v   0   ]/v   0    (12)  
         [0054]    which yields  
               E        (     x   ′     )       =       1   /     v   0              ∫     -   ∞     ∞              H   1          (   x   )              H   2          [         (     x   -     x   ′       )     /     v   0       -     e                   sin        [     2      π                     f        (     x   -     x   ′       )       /     v   0         ]           ]                          x                   (   13   )                               
 
         [0055]    It is clear that this resulting exposure is not exactly equal to the exposure in the absence of vibrations, which is given by:  
               E        (     x   ′     )       =       1   /     v   0              ∫     -   ∞     ∞              H   1          (   x   )              H   2          [       (     x   -     x   ′       )     /     v   0       ]               x                   (   14   )                               
 
         [0056]    Thus AM correction likewise only addresses part of the problem. Both AM and FM corrections are needed. The acousto-optic modulator  52  works on the principle that an acoustic wave generated by the signal at input  53  deflects the light beam in accordance with frequency of the input signal. The amplitude modulation provided by circuit  61 . In order to suppress banding, AM signal (which is governed by equation (9) (i.e H 3 (t)=v(t)/v0, produced by AM module  68  in FIG. 2)) should modulate block  61  and FM signal which is governed by Equation (7) (i.e. t 1 +e sin(2πft 1 )/v 0 =t 0 ) should modulate block  80 .  
         [0057]    In another embodiment, if a light emitting diode or LED array (not shown) is used in place of the laser source described above for pixel placement another set of governing equations is derived based on the electrical characteristics for a diode. The LED output optical power P 0  is related to output photon flux. Each photon has energy hν, so that:  
           P   0   =hνφ   0 =η ex   hνI/e    (15)  
         [0058]    Where P 0  is the output optical power;  
         [0059]    h is Plank&#39;s constant=6.6262×10 −34 J−s;  
         [0060]    ν is photon frequency;  
         [0061]    Φ 0  is the output photon flux;  
         [0062]    e is Electron charge=1.6022×10 −19  C.;  
         [0063]    η ex  is the external quantum efficiency;  
         [0064]    I is the LED drive current;  
         [0065]    In accordance with Equation (15), the LED output power P 0  should be proportional to the driving current I. In practice, however, this relationship is valid only over a restricted range. For the particular device whose light-current characteristic is shown in FIG. 6, the emitted optical power  118  is proportional to the drive current  120  when the latter is less than about 75 mA.  
         [0066]    Referring to FIG. 7, there is shown a graph of relative output power  122  vs. current  124  for high efficiency red LED, normalized at 10 mA. FIG. 7 is a typical representation of LED, which is used in LED bar for most printer applications (the drive current varying from 1.5 mA to 4.5 mA). Considering that the vibration velocity modulation, m, is less than 4%, the deviation in derive current is less than 0.18 mA, to first order approximation it may be assumed that the optical power  122  vs. drive current  124  of each LED is linear.  
         [0067]    In accordance with another embodiment for applying electronic banding suppression for an imager in a printer with an LED array (not shown) the equations for modulating the driving current for each LED will be derived. Starting with rectangular irradiance distribution, H 1 (x), of width W modulated in time with a simple harmonic, H 2 (t), with amplitude m &amp; temporal frequency a, we have the expression for H 1 (x) is:  
           H ( x )= H   0   rect ( x/W )   (16)  
         [0068]    Referring now to FIG. 8, the rectangular irradiance profile H 1 (x)=H 0 rect(x/W)  126  is shown in a graphical representation. FIG. 9 shows the normalized exposure distribution  128  due to rectangular irradiance moving, which is obtained by moving the pattern in FIG. 8 with constant velocity V 0  (i.e. with no banding) along the x-axis. The equation for H 2 (t) with simple harmonic time dependence is:  
           H   2 ( t )=1 +m  cos(2  παt )   (17)  
         [0069]    where (α) is the temporal frequency and (m) is the modulation. Then the Fourier transform of the temporal part of the irradiance is:  
           {tilde over (H)}   2 ( V   0   u )=δ( u )+ m [δ( u−α/V   0 )+δ( u+α/V   0 )]/2   (18)  
         [0070]    where δ is the “Dirac delta function.” The Fourier transform of the exposure for this example is:  
         {tilde over ( E )}( u )= {tilde over (H)}   1 (− u )· {tilde over (H)}   2 ( V   0   u )   (19)  
         [0071]    Substituting for {tilde over (H)} 2 (V 0 u) in Equation (19) gives:  
           {tilde over (E)} ( u )= {tilde over (H)}   1 (− u )δ( u )/ V   0   +m{tilde over (H)}   1 (− u )[δ( u−α/V   0 )+δ( u+α/V   0 )]/2 V   0    (20)  
         [0072]    Now the first term in Equation (7) gives the spatially constant (average) exposure, while the second term gives the exposure modulation due to the time varying intensity. This can be seen by the inverse Fourier transformation of Equation (20):  
           E ( x )=(1 /V   0 ) {tilde over (H)}   1 (0)+ m{tilde over (H)}   1 (−α/ V   0 )exp(+ i 2  παx/V   0 )/2 V   0 )/2 V   0   +m{tilde over (H)}   1 (+α/ V   0 )exp(− i 2  παx/V   0 )/2 V   0    (21)  
         [0073]    The spatially constant term is (1/V 0 ){tilde over (H)} 1 (0), while the Fourier transform component due to the oscillation, {tilde over (H)} 1 (±α/V 0 ), gives the spatial variation of exposure.  
         [0074]    The exposure distribution may be written in a form that is explicitly real by using the fact that, for real H 1 (x), the transform {tilde over (H)} 1 (−u) is the complex conjugate of {tilde over (H)} 1 (+u). Thus Equation (21) becomes:  
           E ( x )=(1/ V   0 ) {tilde over (H)}   1 (0)+( m/V   0 )[ Re[{tilde over (H)}   1 (α/ V   0 )] cos(2  παx/V   0 )+ Im[{tilde over (H)}   1 (+α/ V   0 )] sin(2 παx/V   0 )]  (22)  
         [0075]    where Re[ ] and Im[ ] denote the real and imaginary parts of {tilde over (H)} 1 (α/V 0 ). By differentiation of Equation (22) the maximum exposure values are defined as:  
           E   max· =(1 /V   0 ) {tilde over (H)}   1 (0)+( m/V   0 )| {tilde over (H)}   1 (α/ V   0 )| 
           E   min· =(1 /V   0 ) {tilde over (H)}   1 (0)−( m/V   0 )| {tilde over (H)}   1 (α/ V   0 )| 
         [0076]    where  
         [0077]    |  {tilde over (H)}   1 (α/ V   0 )|=[ Re   2   [{tilde over (H)}   1 (α/ V   0 )]+ Im   2   [{tilde over (H)}   1 (+α/ V   0 )]] 1/2    
         [0078]    The exposure modulation  
           M= ( E   max·   −E   min· )/( E   max·   +E   min· )   (23)  
         [0079]    is then given by  
           M= ( m/{tilde over (H)}   1 (0))|  {tilde over (H)}   1 (α/ V   0 )|  (24)  
         [0080]    Thus the exposure modulation is proportional to the irradiance modulation, m, and to the magnitude of the Fourier transform of the spatial part of the irradiance profile, divided by its average value. Note that the exposure modulation can be obtained directly from the transformed exposure, Equation (20), by adding the positive and negative frequency components and dividing by the average exposure as follows:  
           M=[ |{tilde over (E)} (α/ V   0 )|+|  {tilde over (E)} (−α/ V   0 )/ {tilde over (E)} (0)   (25)  
         [0081]    The formulas are easy to understand in the case of an arbitrary symmetric irradiance profile, H 1 (x). Then {tilde over (H)} 1 (α/ V   0 )={tilde over (H)} 1 (−α/V 0 ) and in this case Eq. (8) reduces to:  
           E ( x )=(1 /V   0 ) {tilde over (H)}   1 (0)+( m/V   0 ) {tilde over (H)}   1 (α/ V   ) )] cos(2 παx/V   0 )   (26)  
         [0082]    The exposure distribution is a harmonic function with spatial frequency  
           u=α/V   0    (27)  
         [0083]    and modulation  
           M= ( m/{tilde over (H)}   1 (0)))|  {tilde over (H)}   1 (α/V 0 ) |  (28)  
         [0084]    where {tilde over (H)} 1 (α/V 0 ) is real in the case of symmetric H 1 (x). The transform of the rectangular case of Equation (26) (FIG. 8) is:  
           {tilde over (H)}   1 ( u )= H   0   W  sin  c  ( Wu )  
         [0085]    where sin c(z)=(sin (πz))/(πz). In this case the exposure distribution is given by:  
           E ( x )=( H   0   W/V   0 )[1+m sin  c ( Wα/V   0 )cos(2 πα/V   0 )]  (29)  
         [0086]    Referring now to FIG. 10, the exposure distribution due to sinusoidal irradiance modulation of rectangular irradiance, Equation (29) is shown. The exposure modulation is then:  
           M=m  sin  c ( Wα/V   0 )   (30)  
         [0087]    Referring once again to FIG. 7 is a typical representation of LED I-V characteristics which is used in LED bar. Considering that the vibration velocity modulation, m, is less than 4% , a first order approximation assumes that the optical power vs. drive current of each LED is linear. In order to eliminate banding due to vibration (m cos(2 παt)), the driving current is modulated of each LED by −(1/k)m cos(2 παt). Where k is the slope of the optical output power of the LED vs. drive current, which is obtained using FIG. 7. Modulating driving current of LED by −(1/k)m cos(2 παt) will produce—(H 0 W/V 0 )m sin c(Wπ/V 0 )cos(2 πα/V 0 ) extra exposure power profile ( because of the linearity). When this added to Equation (29) a constant exposure distribution of H 0 W/V 0  is derived. The resultant normalized exposure is shown in FIG. 11 wherein the resultant normalized exposure profile after modulating driving current of LED by −(1/k)m cos(2 παt) is shown. Though rectangular irradiance distribution, H 1 (x), is used the method is so general that any irradiance distribution (i.e. Gaussian, etc.) could be used for H 1 (x). This method may also be extended to multi-temporal vibration frequency &amp; could also correct banding due to irradiance modulation.  
         [0088]    While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.