EXPOSURE APPARATUS FIELD OF THE INVENTION The pressnt invention relates generally to optical imaging on a moving surface and more particularly to optical imaging in electrophotography. BACKGROUND OF THE INVENTION Optical imaging onto a moving substrate is well known, for example in laser printers and photocopiers, which employ a photoconductive drum for receiving the optical image. Optical information, such as an image, may be imaged or written directly onto a photoconductive drum using appropri- ate stationary optics together with a rotating polygon or hologon or a galvano-mirror to scan the drum axially. Exam- pies of such systems are illustrated and described in U. S. Patents 4,796,961; 4,547,038; 4,445,125 and 4,474,422. Alternatively, the stationary optics may include an array of light sources, such as light emitting diodes (LEDs) or miniature laser oscillators (typically, solid-state lasers), juxtaposed with the surface of the processing drum along the longitudinal dimension of the drum. Such a LED- head includes a plurality of light sources which are ar- ranged in at least one row (typically one or two rows), such that a plurality of light beams may simultaneously or sequentially illuminate the surface of the drum, writing a single line thereon. It has been found that potentially troublesome distor- tions can occur due to uneven velocity of the photoconduc- tive drum during imaging. This is particularly true in multi-color, multi-pass photocopying wherein registration misalignment may result. Additionally, image distortions during the writing of the image may be different for differ- ent color images. The varying distortions result in local areas of misregistration between images. These problems are especially severe for multi-beam systems. There are known methods for correcting some of these problems, in imaging systems using axial scanners. These methods include changing the timing of the light signals, as described in U.S. Patent 4,445,128. An improved method for

correcting misregistration is described PCT Publication No. 92/03008, published February 20, 1992 which is assigned to the same assignee as the present application. In contrast with laser axial scanners, mechanical methods for overcoming the above mentioned problems in systems using LED-heads are more limited. One of the difficulties associated with LED-head scanners involves the fact that LED-heads, which directly illuminate the photoconductor, are highly immobile compared to adjustable mediating elements which are used in axial scanners, such as mirrors or lenses. SUMMARY OF THE INVENTION It is an object of the present invention to provide improved apparatus for optical imaging onto a moving sub- strate by an array of light sources, wherein image distortion, misalignment and misregistration are avoided. In a preferred embodiment of the present invention, a displaceable array of light sources, including an array of light sources juxtaposed with a moving image-forming surface, such as a pre-charged photoreceptor surface, generates successive arrays of light beams corresponding to successive lines of an optical image. The selectively generated lines selectively discharge portions of the moving surface to form an electrostatic latent image thereon. The displaceable array of light sources is mounted on a support structure which is constrained to move only along a predetermined displacement path which is defined by a guidance and support construction. Displacing the structure shifts the line written by the array on the moving surface by a required amount, thereby compensating for misalignments caused by slight variations in the surface velocity of the moving substrate. In a preferred embodiment, the moving surface is a photoreceptor drum or belt. According to one aspect of the present invention, the guidance and support construction includes a relatively long outwardly extending support arm fixedly connected to the light source support structure and pivotably mounted on an outboard fixed pivot. Although the displacement path defined

by this construction is curved outwardly from the moving surface, the actual displacement path is substantially parallel to a plane tangent to the moving surface at the line of writing. This is because the typical displacements, which are required in order to compensate for the above mentioned slight misalignments, are very small compared to the length of the pivotable arm. In another aspect of the invention, the guidance and support construction includes a plurality of substantially parallel outwardly extending arms, one end of each of which is fixedly or pivotably connected to the light source support structure at one end of the arms and mounted, fixedly or pivotably, on a fixed mount at the other end thereof. In this arrangement, the predetermined displacement path is, again, substantially parallel to a plane tangent to the moving surface at the line of writing. In accordance with yet another aspect of the present invention, the support and guidance construction comprises a rail substantially parallel to the moving surface. In such a case, the predetermined displacement path can be constructed substantially parallel to the moving surface. In one embodiment of the present invention, the moving surface is the surface of a photoreceptor belt having a substantially planar region onto which the lines of the image are written. In this preferred embodiment, the varia- tions in distance between the LED array and the moving surface,in response to displacement of the LED array along its displacement path, may be substantially reduced. Any of the guidance and support constructions described above may be used in this embodiment. According to another aspect of the present invention, in which the moving surface is the surface of a rotating drum photoreceptor, the guidance and support apparatus includes two pivots located on the axis of rotation of the drum and two pivotable radial arms pivotably mounted on the two pivots and fixedly connected to the light source support structure. In this arrangement, again, the predetermined displacement path is substantially parallel to the moving

surface . In a preferred embodiment of the invention, the light sources in the array are light emitting diodes (LEDs) or miniature solid-state laser oscillators. Preferably, the LED array includes one row of light sources or two staggered rows of light sources. Preferably, whether the light sources expose the moving surface simultaneously or sequentially, the array generates successive arrays of light beams wherein each beam array writes a single line on the moving surface. A preferred embodiment of the present invention also includes means for measuring the movement of the movable surface and providing a surface movement reference signal. Preferably, the means for measuring surface movement is a rotary encoder and, more preferably, the surface movement reference signal includes at least one first voltage pulse for each rotation of the encoder. In accordance with a preferred embodiment of the inven- tion, the image producing means includes apparatus for generating a timing reference signal including a second voltage pulse for each line generated by the array of sources, preferably at the start of the line. The apparatus further includes, in a preferred embodi- ment, positioning apparatus for controlling the position of the array of light sources along the predetermined displacement path. The positioning apparatus responds to the relationship between the surface movement reference signal and the timing reference signal and, based on the relationship between the two reference signals, positions the support structure along the predetermined displacement path, thereby to ensure proper placement of the array of beams on the movable surface. In a preferred embodiment, the timing signal includes a second voltage pulse and the positioning apparatus provides a correction signal substantially proportional to the time difference between the first voltage pulse and the second voltage pulse. A displacement detector for determining the displace- ment of the LED array relative to a reference "zero" posi-

tion is also provided in a preferred embodiment of the invention. Preferably, the displacement detector includes a linear displacement transducer which measures the displace- ment of the LED writing head. Furthermore , the positioning apparatus preferably includes circuitry for comparing the correction signal provided by the positioning apparatus and the position measurement of the of the LED array as provided by the displacement detector.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig . 1 is a schematic illustration of apparatus for optical imaging onto a moving surface constructed and opera- tive in accordance with a preferred embodiment of the present invention; Fig. 2 is a schematic illustration of an array of light sources constructed in accordance with a preferred embodi- ment of the present invention; Fig. 3 is a schematic block diagram illustration of electronic apparatus useful in the apparatus of Fig. 1. Fig. 4A is a schematic illustration of apparatus for optical imaging onto a moving surface constructed and opera- tive in accordance with another preferred embodiment of the present invention, in which the array of light sources is supported by a single, pivotable, outwardly extending, arm; Fig. 4B is a schematic illustration of apparatus for optical imaging onto a moving surface constructed and opera- tive in accordance with a preferred embodiment of the present invention, in which the array of light sources is supported by a plurality of outwardly extending arms; Fig. 4C is a schematic illustration of apparatus for optical imaging onto a moving surface constructed and opera- tive in accordance with a preferred embodiment of the present invention, in which the array of light sources is supported by a support rail; Fig. 4D is a schematic illustration of apparatus for optical imaging onto a moving surface constructed and opera- tive in accordance with yet another preferred embodiment of the present invention, in which the array of light sources is supported by at least one radial arm; Fig. 5 is a simplified sectional illustration of electrophotographic apparatus constructed and operative in accordance with a preferred embodiment of the present inven- tion, wherein the moving surface is the surface of a drum

photoreceptor; and Fig . 6 is a simpli fied sectional illustration of electrophotographic apparatus constructed and operative in accordance with a preferred embodiment of the present inven- tion, wherein the moving surface is the surface of a belt photoreceptor. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig. 1, which generally illus- trates apparatus for optical imaging onto a moving substrate constructed and operative in accordance with preferred embodiments of the present invention. The apparatus com- prises a conventional photoconductor drum 8 which is associ- ated with a rotary encoder 9 such as a Model 8335 Encoder manufactured by Teledyne Gurley, Troy, NY. There is provided an optical image source support structure 10 including an array 12 of light sources 14, which are typically light emitting diodes (LEDs) or solid state laser oscillators. Support structure 10 and light source array 12 are sometimes hereinafter referred to as "LED-head" and "LED array," respectively, for convenience only and not in order to limit the scope of the present invention. LED array 12 may comprise a single row of adjacent LEDs, whose centers all lie on a common straight line. Alternatively, as can be clearly seen in Fig. 2, array 12 may comprise a number of staggered rows (typically two rows), such that each of the LEDs in one row is staggered with respect to the LEDs in another row. Such a staggered arrangement increases the number of pixels which can be written onto a single line of the image. In a preferred embodiment of the invention, array 12 is juxtaposed with the surface of drum 8, and the longitudinal dimension of the array (i.e. parallel to the rows) is paral- lei to the longitudinal axis of the drum. Thus, when array 12 generates an array of light beams, the beams directly illuminate pixels along a line on the surface of drum 8 parallel to the longitudinal axis of the drum. As is well known in the art, array 12 is driven by a driver system (not shown) that is configured to supply timed

pulses to the array of sources such that a desired latent image is formed on the photoreceptor. In accordance with preferred embodiments of the invention, LED-head 10 may be moved in a constrained manner such that array 12 is displaced to a desired position, with respect to a reference "zero" position 21, along a prese- lected displacement path 14 . Path 14 may be straight or curved, depending on the specific embodiment, as long as the distance between array 12 and surface 8 is maintained sub- stantially constant. A mechanical support and guidance structure 16, which guides LED-head 10 in a constrained manner, ensures that array 12 will not change its position other than by controlled displacement along path 14. Support construction 16 may be any mechanical construction capable of restricting movement of the LED-head such that only controlled displacements along path 14 will be possible. Specific embodiments of apparatus 16 are described below with reference to Figs. 4A-4D. Further, in a preferred embodiment, LED-head 10 is displaced by a linear gear 17 which is driven by a servo motor 18. Preferably, gear 17 is pivotably connected to LED-head 10, thereby constrained movement of LED-head 10 is allowed even when gear 17 is restricted to move along a path not parallel to path 14. When servo-motor 18 rotates, linear gear 17 moves back or forth (depending on the direction of rotation of motor 18), preferably along a straight path roughly parallel to path 14, causing corresponding movement of LED-head 10 and, therefore, displacement of array 12 along path 14. The displacement of array 12 changes the circumferential location of the lines which are written on the surface of drum 8. When motor 18 does not rotate, gear 17 is unable move and, therefore, array 12, which is now also prevented from moving along path 14, maintains a fixed position. Although motor 18 and gear 17 are preferred, it should be appreciated that any other suitable means for displacing the LED-head may be used, such as a linear displacement motor or a motor-driven chain or belt. Motor 18 is controlled by a position control

1 circuitry 20 that is preferably responsive, inter alia, to

2 the output of a linear voltage displacement transducer

3 ( LVDT ) 22 and to control s ignal s received f rom

4 control electronics 24 . Linear transducer ( LVDT ) 22

5 functions as a position detector for preci sely

6 indicating the actual displacement of LED-head 10 with

7 respect to a "zero" reference 21. Control electronics

8 24 receives inputs from encoder 9 , preferably in the

9 form of encoder pulses , and electrical line generation

10 ( LG) signal pulses from LED-head 10. Control electronics

11 24 also communicates with LED-head 10 to supply data

12 and generation control signals for LED array 12.

13 Reference is now made to Fig . 3 , which illustrates in

14 simplif ied block diagram form , electronic circuitry 29

15 useful in the apparatus of Fig . 1 . This circuitry

16 incorporates displacement control 20 , LVDT 22 , servo-motor

17 18 and control electronics 24 , as indicated by the dashed

18 lines in Fig . 2. Generally speaking , the position of LED

19 array 12 along path 14 is governed by a servo loop 30

20 including servo-motor 18 and LVDT 22 . Pos it ion error

21 detection circuitry 32 supplies an input to servo loop 30

22 and is operative to provide a measure of the time difference

23 between the encoder pulses and the LG signal pulses . The

24 feedback signal in loop 30 is the actual position of the LED

25 array ( or array ) measured by LVDT 22. The output of LVDT 22

26 will be referred to as the actual LED array position signal .

27 It is noted that the circuitry of Fig . 3 does not

28 change the velocity of drum 8 or the writing rate of the

29 LED-head , but affects the position of LED array 12 along

30 path 14 , and thereby affects the circumferential location of 1 the lines written on the surface of drum 8. 2 Position error detection circuitry 32 employs the LG 3 and encoder signals to advance two counters 34 and 36 re- 4 spectively . Counters 34 and 36 also receive LG sync and 5 encoder sync signals respectively, which originate in con-

36 trol electronics 24. The difference between the counts in 7 counters 34 and 36 is determined by a subtracter 38 and

38 supplied to a digital to analog converter 40. The analog

output of digital to analog converter 40 is supplied via a time average filter 42 to feedback loop 30. The output of the time average filter is proportional to the time differ- ence between corresponding counts of counters 34 and 36 and represents the misalignment between the actual position of the written line on the surface of the drum and the desired position of the line on the surface. The output of time average filter 42 thus provides a correction signal propor- tional to the LED-head displacement correction required to return the written line to the proper position on the drum. It will be referred herein as the desired LED array position signal. In feedback loop 30, a summation circuit 44 subtracts actual LED array position signal from the desired LED array position signal, to produce an error signal, which is sup- plied via an amplifier 46 to servo-motor 18 for control of the operation thereof. Once the LED array reaches the de- sired position, i.e. when the written line is correctly positioned on the surface of drum 8, the actual and desired LED array position signals are identical and the error signal goes to zero, and servo-motor 18 stops. As the scanning of the image continues, i.e. as the drum rotates, any variation from correct write-line posi- tioning on the surface of drum 8, caused for example by speed variation of the drum, will cause the error signal to be different from zero, and servo-motor 18 will be activated to change the position of LED array 12, such that the line written on drum 8 will be correctly positioned. The apparatus and method described herein can provide position correction which is greater than the write-line spacing. In practice, if a substantial deviation between write-line rate and encoder rate exists, the circumferential deviation by the end of a page can be substantial. In ex- treme cases the correction can be as high as several times the width of the written line over the length of a page. In order to prevent the LED-head from exceeding its allowed range of displacement for multi-page printing, the system is reset at the end of each page.

At the end of every page, preferably determined by a preselected encoder pulse, a reset command is provided by means of the LG and encoder sync signals. The reset command is operative to set the outputs of counters 34 and 36 to zero and thus to set the desired LED array position signal to zero momentarily. The first LG signal pulse after this reset is the new reference LG signal pulse both for LED array displacement and for data transmission to LED-head 10. The coordinated reset of the desired position signal to zero, together with the resetting of the LG reference count removes the accumulated position correction and realigns the system. Within a few counts the desired position signal returns to a proper value, and LED array 12 returns to a small deviation from its zero-reference position. The con- current resetting of the LG reference count assures that the desired position signal corresponds to the actual require- ment for the scan. _v After reset, the position of LED array 12 is dynamical- ly readjusted as described above to substantially eliminate scan misalignment. At a given encoder count, corresponding to the start of the print location, the scan is enabled and printing begins. The zero-reference position of LED array 12 can be adjusted by supplying an off-set voltage either at the input to loop 30, or alternatively within the loop, as for example by a non-zero off-set voltage for amplifier 46. Alternative- ly the reference position can be adjusted by delaying the encoder or LG pulses. It is a feature of an embodiment of the present inven- tion that the frequency of the output of encoder 9 is se- lected to be the same as that of the LG signals, thereby permitting a relatively easy phase difference analysis of the signals to be carried out. Reference is now made to Fig. 4A, which illustrate a preferred embodiment of guidance and support construction 16. In this embodiment, construction 16 includes a support arm 60 which is fixedly mounted at one end to the radially outward side (i.e away from the drum) of LED-head 10 and

which extends radially outwards from the LED-head. The outer end of arm 60 is pivotably mounted on a fixed pivot 62 which, in a preferred embodiment, is located far away from LED-head 10. When linear gear 17 moves LED-head 10, as described above, arm 60 pivots with respect to fixed pivot 62 and, therefore, LED-array 12 moves along a well defined, outwardly curved, displacement path 64. In a preferred embodiment, arm 60 is very long compared to the circumferential displacements which are typically required of array 12 and, therefore, displacement path 64 is substantially straight. Since the displacements of LED-head 10 are generally small, the distance between array 12 and the surface of drum 8 varies only slightly in response displacements of array 12 along path 64. Reference is now made to Fig. 4B, which illustrates another preferred embodiment of guidance and support con- struction 16. In this embodiment, construction 16 includes a plurality of pivoting arms 70, pivotably mounted on a plu- rality of respective mounts 72 which are located on a fixed radially outboard plane. At their radially inward end, pivoting arms 70 are pivotably connected to respective head- pivots 75 on the outward side of LED-head 10. Alternatively, arms 70 may be fixedly mounted (not shown in Fig. 4B) on mounts 72 and 75 (or preferably to the sides of head 10) and, in such a case, arms 70 are made of a slightly elastic material. Pivoting arms 70, mounts 72 and pivots 75 are appropriately arranged, such that arms 70 are substantially parallel and sufficiently spaced apart along the width of LED head-10 (i.e. perpendicular to the rows of LEDs). When linear gear 17 moves LED-head 10, arms 70 pivot with respect to mounts 72 and LED-array 12 moves along a somewhat outwardly curved displacement path 74. Alternatively, when arms 70 are fixedly mounted on mounts 72 and 75, arms 70 elastically bend and, again, array 12 moves along path 74. At their inward end, arms 70 pivot around head-pivots 75 and, therefore, the LED-head faces a constant direction independent of the displacement of array 12 along path 74. This is in contrast to the embodiment of Fig. 4A,

wherein arm 60 is fixedly connected to LED-head 10. More specifically, in the embodiment of Fig. 4B, array 12 faces a direction perpendicular to a plane tangent to path 74 at the reference line of writing (i.e. the line of writing at zero displacement of array 12). Although path 74 is not parallel to the surface of drum 8, the difference between the two is slight due to the relatively small displacement of array 12. Reference is now made to Fig. 4C, which illustrates yet another preferred embodiment of guidance and support construction 16. The outward side of LED-head 10 is slidably mounted on rails 80, which guide LED-head 10 such that LED array 12 moves along a circular displacement path 84 in response to the movement .of gear 17. In this embodiment, path 84 is parallel to the surface of drum 8 and, therefore, the distance between array 12 and drum 8 is constant, regardless of the displacement of array 12 along path 84. ' Referring to Fig. 4D, which illustrates a further, preferred, embodiment of the invention. Radial arms 90 are pivotably mounted on respective axial pivots 92, which lie on the axis of rotation of drum 8. Axial pivots 92 may be axial extensions of the axis of drum 8. The outer ends of arms 90 are respectively connected to the longitudinal ends of LED-head 10. As in the previous embodiment, array 12 is displaced along a displacement path 94 which is substantial- ly parallel to the surface of drum 8 and, therefore, the writing distance (i.e. the distance between array 12 and drum 8) is constant. Reference is now made to Fig. 5, which illustrates electrophotographic imaging apparatus constructed and opera- tive in accordance with a preferred embodiment of the present invention. This and other embodiments of the inven- tion are described in the context of liquid developer sys- tems with negatively charged toner particles and positively charged photoreceptors. Such systems operate in a "write- white" mode, for which areas which are not to be toned are exposed to light. The invention is also useful for other combinations of toner charge, photoreceptor charge as well

1 as for other writing systems, such as "write-black" systems.

2 It is also equally useful for powder toner systems.

3 As in conventional electrophotographic systems , the

4 apparatus of Fig. 5 comprises drum 8 arranged for rotation

5 about an axle 11 in a direction generally indicated by arrow

6 13. Drum 8 is formed with a cylindrical photoreceptor

7 surface 15.

8 A corona charging device 19 is operative to generally

9 uniformly charge photoreceptor surface 15 with a positive

10 charge. Continued rotation of drum 8 brings charged photo-

11 receptor surface 15 into image receiving relationship with

12 an exposure unit including LED array 12 (Fig. 3). LED array

13 12, as described hereinabove in detail, in accordance with a

14 preferred embodiment of the invention, generates a desired

15 sequence of optical image lines, which together form a

16 desired optical image, onto charged photoreceptor surface

17 15, selectively discharging the photoreceptor surface, thus

18 producing an electrostatic latent image thereon.

19 Continued rotation of drum 8 brings charged

20 photoreceptor surface 15 bearing the electrostatic latent

21 image into operative association with a development unit 23

22 that is operative to apply a liquid developer to develop the

23 electrostatic latent image. Developing unit 23 preferably

24 comprises a developing plate or a developing roller or a

25 number of rollers for applying toner onto the surface of the

26 drum. The direction of rotation of the developing rollers,

27 if such are used, may be the same as or opposite to the

28 direction of rotation of drum 8, depending on the specific 9 system. For multicolor copying or printing, the development 0 unit 23 can, for example, comprise a plurality of 1 developers, one for each color, which are selectively 2 engaged with the photoreceptor, as described, for example in 3 U.S. Patent 4,690,539, which is incorporated herein by 4 reference, or a single development station where the liquid 5 toner is changed between colors, or any other suitable 6 liquid or powder toner system. 7 In accordance with a preferred embodiment of the 8 invention, following application of liquid developer

thereto, photoreceptor surface 15 passes a typically positively charged rotating roller 25, preferably rotating in a direction indicated by an arrow 27. Roller 25 functions as a metering roller and reduces the thickness of liquid on photoreceptor surface 15. Typically the spatial separation of roller 25 from photoreceptor surface 15 is about 50 microns. It is appreciated that when a reverse roller (or rollers) are used to develop the surface, as described above, roller 25 is generally not required. The image which passes roller 25 should be relatively free of pigmented particles except in the region of the latent image. Downstream of roller 25 there is. preferably provided a rigidizing roller 29. Rigidizing roller 29 is preferably formed of a resilient polymeric material, such as conductive resilient polymeric material as described in either or both of U.S. Patents 3,959,574 and 3,863,603. Roller 29 is pref- erably resiliently urged against photoconductive surface 15. Such a rigidizing roller is described in applicant's U.S. Patent 5,028,964. Downstream of rigidizing roller 29 there is provided an intermediate transfer member 31, which rotates in a direc- tion opposite to that of photoreceptor surface 15, as shown by arrow 41, providing zero relative motion between their respective surfaces at the point of propinquity. Intermedi- ate transfer member 31 is operative to receive the toner image from photoreceptor surface 15 and to transfer the toner image to a receiving substrate 43, such as paper. A roller 44 may be provided to press substrate 43 against intermediate transfer member 31 to improve transfer. Dis- posed internally of intermediate transfer member 31 there may be provided a heater 45, to heat intermediate transfer member 31. Various types of intermediate transfer members are known and are described , f or example , in U . S . Patent 4 , 684 , 238 and in as signee ' s copending U . S . Patent application entitled METHOD AND APPARATUS FOR IMAGING USING AN INTERMEDIATE TRANSFER MEMBER filed January 4 , 1989 , and

bearing serial number 7/293 , 456 ( ALSO PUBLISHED AS WO 90/02977 ) , the disclosures of both of which are incorporated herein by reference. In an alternate preferred embodiment of the invention the developed image may be transferred directly to the final substrate and intermediate trans fer member 31 may be omitted . Alternatively, a plurality of tandem intermediate transfer members may be used to transfer the developed image a number of times before f inal transfer to the f inal substrate. Following the trans fer of the toner image to intermediate transfer member 31 , photoreceptor surface 15 engages a cleaning station 49 . This station may be any conventional cleaning station, comprising a cleaning roller 50 which may comprise a suitable resilient material such as foam polyethylene or neoprene . Cleaning roller 50 may be wetted by clean lubricating cleaning liquid , which may comprise liquid developer from which all or nearly all of the toner particles have been removed . Cleaning roller 50 is preferably driven so that its surface moves opposite to surface 15 at their nip, to provide scrubbing action for removal of residual particles and carrier liquid from photoconductor surface 15 . A scraper 56 completes the removal of any residual toner which may not have been removed by cleaning station 49. A lamp 58 completes the cycle by removing any residual charge, characteristic of the previous image, from photocon- ductor surface 15. In printing a full color image, the above process is repeated for successive colors , and successively developed images are successively transferred to intermediate transfer member 31 . The images may then be successively transferred to substrate 43 , in mutual al ignment , or they may be transferred as a group from intermediate transfer member 31 to substrate 43. Reference is now made to Fig. 6 , which illustrates an alternative imaging apparatus constructed and operative in accordance with a preferred embodiment of the present inven-

tion. In this embodiment, a photoconductor belt 6 is used instead of drum 8. Belt 6 moves along a predefined path, passing by the different stations described above with reference to Fig. 5. Belt 6 is arranged such that a substan- tially planar region 7 of the belt is juxtaposed with LED array 12. Such an arrangement of belt 6 may be achieved, for example, by tightly mounting the belt on a plurality of adjacent roller-guides 4, of which at least two rollers 5 are sufficiently spaced apart. In this arrangement, planar region 7 is the outer surface of the portion of belt 6 which is stretched between rollers 5, and the size of region 7 depends on the separation between rollers 5. It should be appreciated that any other suitable arrangement of belt 6, in which the surface of the belt includes a substantially planar region such as planar region 7, may be used. It should be appreciated that any suitable support and guidance structure 16, such as the structures described herein with reference to Figs. 4A-4C, may be used to guide and support LED-head 10 in the embodiment of Fig. 6. If a pivoting arm guidance structure is used, such as the struc- tures of Figs. 4A or 4B, the substantially straight dis- placement path 14 of array 12 may be substantially parallel to region 7, thereby a substantially constant distance between array 12 and surface 7 may be maintained for any displacement of array 12. If a guiding rail structure is used, such as the structure of Fig. 4C, straight rails may be used to guide array 12 along a straight path substantial- ly parallel to region 7, thereby maintaining a constant separation between array 12 and region 7. Furthermore a control system, similar to that shown in Figs. 1 and 3 is used to determine and control the optimal position of head 10. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been shown and described herein. Rather the scope of the present invention is defined only by the following claims: