Accurate layer registration in multiple layer electronic raster output printing employing multiple scanning beams and line interlacing

Abstract: An apparatus and method are disclosed for providing accurate layer registration for multiple layer electronic printing systems, typically multi-beam laser raster output scanners, in which accurate layer registration is critical. The method comprises determination of a time delay between page sync signal and start-of-scan signal, using this delay to determine a delay dependent offset factor called the phase registration parameter, and correctively mapping image data to proper output channels based on the phase registration parameter and the laser number. The phase registration parameter must be determined at the beginning of each scan layer and the offset factors must be re-adjusted based on the new phase delay.

Description: The present invention relates to an apparatus and method for providing accurate layer registration for multiple layer electronic printing systems. More particularly, the present invention relates to a system for reducing the positional scan-to-scan error in a multi-beam raster output scanner (ROS). Raster Output Scanners (ROS) conventionally have a reflective, multifaceted polygon which is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction while the recording medium is being advanced in an orthogonal, or "process", direction. The beam or beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample stream representing the image to be printed, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. In a four layer printing process, typical for full color applications, the image to be printed will be divided into four color layers. Each layer is representative of one of four system colors, and is exposed on a different layer (in single station applications), or on a different photoreceptor (in multi-station applications). For multiple beam printing, each beam may be intensity modulated with raster information which corresponds to one scan line of an interlaced representation of the image layer to be printed. Image layers processed for multiple-beam ROSs can be subdivided into a number of sub- images, depending upon the number of beams used in the ROS. The sub-images, stored in channels, are further subdivided into scan lines. Each scan line represents one raster of image data. The number of scan lines per image layer is dependent upon factors such as resolution, spot size and image size. The number of scan lines reflected per facet of the polygon mirror equals the number of light sources (usually laser diodes) in the ROS, and is referred to as the beam set. Each rotation of the polygon, therefore, will lay down a number of scan lines per facet (corresponding to the number of laser diodes in the beam set) separated by a multiple of the scan spacing. US-A-5 079 563 discloses an error reducing raster scan method for a raster scan device having multiple simultaneous scan lines. The method separates the multiple scan lines into sets or groups so that no two scan lines which are to be placed next to one another are in the same group, that is, the scan lines from each group are interlaced with the scan lines of the other group. This method offsets errors, for example, banding thereby providing a more acceptable appearance to an output image formed by a print head producing a plurality of print lines. The method is also applicable to multiple scanning beams. US-A-4 989 019 discloses a multi-beam scanning system which compensates for banding. The system causes multiple beams from a laser source to be separated by a distance which causes the beam to scan consecutive lines in a non-consecutive scan according to a particular beam interlacing sequence. Increasing the interlacing order provides a decrease in the banding - a number of interlacing orders being suitable for correcting print system error frequency or another frequency which has been identified. WO-A-89/12369 discloses still another multi-beam scanning system in which interlaced scanning is employed. Interlacing is the introduction of additional scan lines between scan lines already laid down by a previous facet of the polygon. The number of scan lines interspersed between any two scan lines reflected by a single facet of the rotating polygon mirror is the interlace factor and has units of scans per beam. It is because of interlacing that each channel of the image data must be selectively delayed so that it is laid down sequentially on the photoreceptor even though the beam set does not lay down adjacent scans in the photoreceptor during a single facet of the polygon. As the photoreceptor moves and the polygon rotates through a single facet, the beam set will lay down a number of scans separated by a multiple of the scan spacing. Successive facets will position successive scan lines between scan lines already laid down. If there are q equally spaced beams in the beam set, then each laser beam could be modulated with information from every qth raster line of an image, and in addition, since, in any rasterized image, there are q mutually exclusive sets of such raster lines, each laser beam could be associated with its own set of raster lines in each channel. This ensures that the superposition of the sets of raster lines associated with each laser beam will define the entire image. Typically, a start-of-scan (SOS) detector is utilized to synchronize the beams to the data rasters in the q channels, and q rasters are output for each scan. Since each scan of the polygon will produce q raster lines, the recording medium will be advanced a distance corresponding to q times the reciprocal of the raster density, d, between each start of scan signal. In order to synchronize the recording medium position with the beginning of the image rasters, a page-sync signal may be produced by a suitable means to signify a particular position of the recording medium (usually the top of page). After that time, but synchronized with SOS, raster information can begin to flow. Since there is generally an asynchronous relationship between page-sync and SOS, the initial image rasters will have a positional error in the process direction corresponding to the distance between q rasters on the photosensitive recording medium. For single layer printing, such as black and white, this may not be a problem, because an image registration error in the process direction of only q rasters may be unnoticeable. However, for multiple layer printing such as color printing, where layer registration is important, a positional error of q/d between layers is unacceptable. In multiple layer printing applications, the top edge (i.e. top of page or page-sync) signal can occur when a laser diode is coincident with a scan placed by another laser diode in the previous layer. The need to divert data from the same memory channel that drove the laser on the previous layer into this new laser diode requires the channel-to-laser assignments to be switched, and the appropriate delays be generated. The situation is further complicated by the fact that the information, defining which laser diode is coincident with which channel is not available until the top edge synchronization (page sync) signal occurs, so there is little time to effect the change. Suitable means must be provided to measure the laser to channel error, redefine the laser to channel assignment and generate appropriate delays for each layer to be printed. Therefore, it is understood that the raster positioning precision that is subject to a positioning error of q/d rasters for each layer scanned is unacceptable in multi-layer multibeam ROS printing. This error can be reduced to one raster by providing a system that allows for corrective mapping of memory locations and channel assignments of image data, with respect to the delay between the SOS and the page-sync for each layer to be printed. It is an object of this invention to provide a multi-beam ROS printer with a means for mapping rasters into channels based on the delay between the SOS and the page sync ensuring accurate layer registration in multi-layer ROS printer systems. In accordance with one aspect of the present invention, there is provided an apparatus as defined in claim 1, for recording image data on a recording surface using a plurality of light beams, the apparatus comprising:- beam generating means for generating a plurality of light beams; modulating means for independently modulating the light beams in accordance with the image data, the image data comprising a plurality of image layers which are individually recorded on the recording surface, the modulating means comprising image generator means for storing the image data in the form of raster scan lines and assigning the data to a plurality of output channels in accordance with a predetermined raster scanning pattern, and programmable delay means for synchronising the output channels from the image generator means with the light beams generated by the beam generating means; and optical means for directing and focusing the modulated light beams onto the recording surface; characterised in that the apparatus further comprises detecting means for detecting start of scan (SOS) signal and page-sync signal; delay determining means for determining a data signal corresponding to the delay between SOS and page-sync signals; and switching means for assigning the output channels from the image generator means as channel-to-laser paths for the beam generating means in accordance with the data signal and for adjusting the assignment of the channel-to-laser paths in real time to provide accurate registration of subsequent image layers on the recording surface. In accordance with another aspect of the present invention, there is provided a method as defined in claim 6, for recording image data on a recording surface using a plurality of light beams, the method comprising the steps of:- generating a plurality of light beams; arranging the image data into layers based on the number of layers to be recorded; storing the image data in the form of raster scan lines; assigning the image data to a plurality of output channels in accordance with a predetermined raster scanning pattern; synchronising the output channels with the light beams; modulating the light beams in accordance with the image data; and directing and focusing the modulated light beams onto the recording surface; characterised in that the method further comprises the steps detecting a start of scan (SOS) signal and a page-sync signal; determining the delay between SOS and page-sync signals; assigning the output channels as channel-to-laser paths in accordance with the data signal; and adjusting the assignment of channel-to-laser paths in real time to provide accurate registration of subsequent image layers on the recording surface. In order to reduce raster positioning error, a method in accordance with the invention comprises expressing memory raster numbers and channel assignments as a function of laser diode number and phase registration delay. These expressions are then used to provide real time switching of the channel assignments and memory raster numbers to ensure accurate registration by providing the raster number that is positionally best suited for output depending on the photoreceptor position when SOS occurs. By way of example only, an apparatus and method in accordance with the invention will be described with reference to the attached drawings, in which: Figure 1 is a simplified schematic diagram of a laser printer having a multiple spot scanner; Figure 2 is a block diagram illustrating the raster data memory and channel assignment mapping and time delay elements of the printer; and Figure 3 is a simplified schematic diagram showing a scanning pattern having an interlace factor of 3. In Fig. 1, there is shown a conventional multibeam ROS printer (shown only in relevant part) having a reflective multifaceted polygon 24 which is rotated about its central axis 50, by a motor to repeatedly scan multiple incident, intensity modulated light beams 10, 12, 14, 16 across a photosensitive recording medium 30, in a line scanning direction which is parallel to the central axis of the photosensitive medium 30 denoted by the line 60. While being scanned by the light beams 10, 12, 14, 16, the recording medium 30 is advanced (by a means not shown) in an orthogonal direction denoted by the arrow 62, so the light beams 10, 12, 14, 16 expose it in accordance with a raster output scanning pattern representing the image to be printed. As a general rule, the light beams 10, 12, 14, 16 are supplied by laser diodes housed in a beam generating diode array 74 and are intensity modulated in accordance with data samples representing the image to be printed. The beams 10, 12, 14, 16 generated by the laser diode array 74 pass through an aperture 26 that truncates the beams to their mutually overlapping widths. The beams then pass through a collimator 20 and a cylinder lens 22 that focuses the beams 10, 12, 14, 16 on the polygon mirror 24. The beams 10, 12, 14, 16 are then deflected onto an optical path toward the photosensitive recording medium 30 by the rotating polygon mirror 24. The beams 10, 12, 14, 16 then pass through a corrective optical system 32, which compensates for various beam errors. Prior to printing, the image to be printed is represented by data in the form of rasters. In Figure 2, the raster data 124 is sent to the image generator 80, of the type commonly used and well-known in the art. The raster data 124 is assigned to memory locations and then to channels 96, 98, 100, 102 in accordance with a predetermined raster scanning pattern representing the image to be printed. Each channel is independent of the others, and separate raster pointers are utilized in each channel to specify which raster line is to be output by each channel. Initially the pointers are all set to zero. After the image generator 80 receives the raster data 124, it receives a delayed line sync signal 112, 114, 116, 118 from the programmable delay 82. Each line sync signal causes each channel that receives a line sync signal to output one raster line, and increment its raster pointer The programmable delay 82 uses a global line sync signal 120 from a counter 84, which generates one line sync signal from each start of scan (SOS) signal 92 after receiving the page-sync signal 94. The programmable delay 82 also uses the phase counter 86, which counts the number of scans (i e. in increments of 1/d) between the SOS signal 92 and page- sync signal 94, to determine the number of line sync signals to delay each individual channel so that all rasters are placed in sequential order on the photoreceptor even though the rasters are interlaced. The delay generated by the programmable delay 82 is in integer units of line sync signals, and is indicative of the number of scan lines that the recording medium 30 has advanced in the process direction, in the time between page-sync signal and SOS signal. The rasters in channels 96, 98, 100, 102 are output by the image generator 80 to the programmable channel assignment switch 88, which assigns the channels 96, 98, 100, 102 to the appropriate laser diodes via signals 104, 106, 108, 110 The switch 88 is a logic device, such as, for example, a programmable logic array. The switch 88 uses data 90 from the phase counter 86 to assign the channel-to-laser paths in real time. Each layer can, therefore, be scanned with a maximum positional error of one raster line in the process direction. Figure 3 shows an example of interlaced printing having an interlace factor of 3. The beams 10, 12, 14, 16 reflected off the polygon 24 pass through the corrective optical system 32 and expose the photosensitive recording medium 30. As the recording medium 30 advances in the process direction, the beams 10, 12, 14, 16 expose the medium 30 with every third raster of the image to be printed. Subsequent rasters in the process direction are then placed in the space between the previously laid down scan lines. Interlacing is used to increase speed and to enhance the resolution of the images to be printed. In the preferred embodiment, the image generator 80 used to drive such a multiple beam system will typically have q channels which correspond to the raster information required by each of the q lasers. Using known parameters explained below, such as laser diode assignment number, phase registration delay and interlace factor, expressions for raster assignment numbers and channel assignment numbers can be derived. Once derived, these expressions can be evaluated in real time by devices such as, for example, a programmed logic array (i.e., for example, the programmable channel assignment switch 88) that will perform the appropriate switching functions to assign channels to ensure that a particular laser diode is providing the raster number that is best suited for output depending on the photoreceptor 30 position when the SOS signal occurs. The output channel assignment number, ch, is an integer ranging from 0 to q-1. An integer, m, represents the sequential memory addresses of the individual rasters in a block of memory storing a particular set of raster information corresponding to q raster lines of information. L, the laser assignment number, designates which of q laser beams is being addressed. The laser assignment number L ranges from 0 to q-1. The phase registration parameter, defined in more detail below, is representative of the delay between the page sync signal and the start of scan signal and has units of scans. The phase registration parameter p ranges from 0 to q-1. l, the interlace factor, is the directed distance, measured in rasters, spanned by any two adjacent light beams of the beam set. r is a constant bias offset for the calculation of R, a variable representing the raster assignment number, both integers. Finally, S is an integer representing the scan number. A numbering system can be defined that maps rasters into channels and memory locations. Typically, there will be one channel for each laser diode, and each channel will contain enough memory or processing power to provide one of the q mutually exclusive sets of rasters. Channel ⊘ contains raster ⊘, channel 1 contains raster 1,..., and channel (q-1) contains raster (q-1). All these rasters reside in memory location m = ⊘ of the separate channel hardware. To continue with this mapping scheme, it is convenient to have raster R reside in a channel defined by ch = R mod q, and memory location m = R div q of that channel (i.e., take R and continuously subtract q from R until no more subtractions may take place without the answer going negative, m is the number of subtractions made and ch is the remainder). Now scans may be mapped into these hardware related parameters. As shown before, each scan will provide q rasters to q lasers spaced by interlace factor l. If raster r is assigned to laser 0, raster 1xl + r would be assigned to laser 1, raster 2xl + r would be assigned to laser 2, and, in general, raster [Lxl + r] should be assigned to laser number L to properly interlace. Scan ⊘ can now be defined as the scan that delivers raster r, which will be fixed more precisely later. For subsequent scans, the memory locations for all rasters may be calculated directly by adding S, the scan number, to the values obtained at scan ⊘. So the mapping of the channel assignment numbers ch and memory locations into hardware is given by: ch = [Lxl + r] mod q m = S + [Lxl + r] div q The phase registration parameter, p, may now be used to further qualify these relationships. The phase registration parameter is determined by breaking up the time interval between page sync and SOS signals based on the number of scans that would be laid down during that period of time. This is accomplished by dividing a time interval equal in length to one SOS-to-SOS time interval (and immediately following the page sync-signal) into q equally spaced sub-intervals, and designating each in ascending time, the designation of p = 0,1,2,..., (q-1). By definition, the SOS signal must occur within one of these sub-intervals. The phase registration parameter p is defined by which one of these sub-intervals the SOS occurs in after the occurrence of the page sync signal. In other words, p is the time delay, measured in scans, between page sync and the SOS signals. When the page sync signal occurs, a given laser will be poised to output a particular line of raster data. While waiting for the SOS signal, the recording medium will move a number of raster lines p (as indicated above). Adding p to r will allow the calculated value of R to track the moving recording medium to give one raster registration resolution. Scan ⊘ is now redefined to be the scan that delivers raster r + p Substituting r + p for r in equations (1) and (2) the equations for the channel assignment and memory location are: ch = [Lxl + r + p] mod q m = S + [Lxl + r + p] div q Now these equations must be evaluated in real time since p is defined at page-sync time for each scan layer. Once evaluated, a programmable channel assignment switch, such as, for example a programmed logic array, that performs the appropriate channel switching and memory assignments, as required, by equations (3) and (4), can be provided. Note that the parameter r may be assigned in advance to allow for appropriate printing of the top margin, and to ensure that no image rasters are skipped due to interlacing. Assuming that memory locations with a negative raster number are non-printing, r can be selected as a negative number to ensure that the raster numbers are negative until printing of the actual image is desired. To summarise, a means and method have been described for reducing the positional scan-to-scan error in multiple layer printing systems, thereby providing increased raster positioning precision, in multiple layers, by permitting channel assignments and memory locations for image rasters as a function of laser assignment number and a phase registration parameter in real time according to the mapping scheme and hardware provided.

Claims: Apparatus for recording image data (124) on a recording surface (30) using a plurality of light beams (10, 12, 14, 16), the apparatus comprising:- beam generating means (74) for simultaneously generating a plurality (q) of light beams (10,12, 14,16); modulating means (80, 82) having a respective plurality of output channels each coupled to a respective input of said beam generating means, for independently modulating the light beams (10, 12, 14, 16) in accordance with the image data (124), the image data (124) comprising a plurality of image layers which are to be individually recorded on the recording surface (30), the modulating means (80, 82, 88) comprising image generator means (80) for storing the image data (124) of an image layer in the form of raster scan lines and assigning the stored data to said plurality of output channels (96, 98, 100, 102) in accordance with a predetermined interlaced raster scanning pattern, and programmable delay means (82) for synchronising the signals from the output channels (96, 98, 100, 102) from the image generator means (80) with a process direction position of said light beams (10, 12, 14, 16) generated by the beam generating means (74); and optical means (20, 22, 24, 26, 32) for directing and focusing the modulated light beams (10, 12, 14, 16) onto the recording surface (30), and means for scanning said surface by said light beams along a scanning direction substantially transverse to a process direction of motion of said surface in relation to said light beams. the apparatus further comprising detecting means for detecting a start of scan (SOS) signal (92) and a page-sync signal (94); delay determining means (86) for determining a data signal (90) corresponding to the delay between said start of scan and page-sync signals (92, 94); and switching means (88) for assigning the output channels (96, 98, 100, 102) from the image generator means (80) as channel-to-laser paths (104, 106, 108, 110) for the beam generating means (74) in accordance with the data signal (90) for adjusting the assignment of the channel-to-laser paths (104,106,108, 110) in real time to provide accurate registration of subsequent image layers on the recording surface (30). Apparatus as claimed in claim 1, wherein the beam generating means (74) comprises a plurality of laser diodes. Apparatus as claimed in claim 1 or claim 2, wherein the recording surface (30) comprises a photoreceptive surface. Apparatus as claimed in any one of the preceding claims, wherein the optical means (20, 22, 24, 26, 32) includes a polygon mirror (24) rotatably mounted on a central axis (50) for deflecting the light beams (10, 12, 14, 16) onto the recording surface (30). Apparatus as claimed in any one of the preceding claims, wherein the switching means (88) comprises a programmed logic array device. A method of recording image data (124) on a recording surface (30) using a plurality of light beams (10, 12, 14, 16), the method comprising the steps of:- generating a plurality (q) of light beams (10, 12, 14, 16) simultaneously; arranging the image data (124) into layers based on the number of layers to be recorded; storing the image data (124) of an image layer in the form of raster scan lines; assigning the stored image date (124) to a plurality of output channels (96, 98, 100, 102) in accordance with a predetermined interlaced raster scanning pattern each output channel of said plurality of output channels being assigned to a respective one of said light beams; synchronising the signals from the output channels (96, 98, 100, 102) with a start of scan position of said light beams (10, 12, 14, 16); modulating the light beams (10,12,14,16) independently in accordance with the image data (124) assigned to said output channels; and directing and focusing the modulated light beams (10, 12, 14, 16) onto the recording surface (30); scanning said surface by said light beams along a scanning direction substantially transverse to a process direction of motion of said surface in relation to said light beams; the method further comprising the steps of detecting a start of scan (SOS) signal (92) and a page-sync signal (94); determining the delay between said start of scan and page-sync signals (92, 94) and providing a corresponding data signal (90); assigning the output channels (96, 98, 100, 102) as channel-to-laser paths (104, 106, 108, 110) in accordance with the data signal (90) for adjusting the assignment of channel-to-laser paths (104, 106, 108, 110) in real time to provide accurate registration of subsequent image layers on the recording surface (30). A method as claimed in claim 6, wherein the step of adjusting the assignment of channel-to-laser paths (104, 106, 108, 110) is made in accordance with the relations: m = S + (L x l + r + p) div q ch = (L x l + r + p) mod q wherein: m is a memory location of a set of output scan line data stored in an image generator (80), the set corresponding to q raster lines of information; ch is an output channel assignment for scan line data; S is an index number of a scan ; I is the interlace factor ; q is the number of light beams ; p is a phase registration parameter representing the delay between the page sync and start of scan signals ; L is a light beam index number; and r is a constant offset bias for the calculation of R, a raster assignment number the method further including the steps of:- determining a new phase registration parameter, p, for each image layer to be recorded, at page sync time; and re-evaluating the assignment of channel-to-laser paths (104, 106, 108, 110) in real time for each layer to be recorded using the new phase registration parameter, p.

Inventors: CURRY DOUGLAS N, CURRY, DOUGLAS N.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: H04N1
IPC codes: G06K15, H04N1, B41J2, G02B26