This invention relates generally to imaging devices and more particularly to imaging devices with a plurality of imagers that provide sequential images to form a composite image. 
Imaging devices often utilize a first color to produce an image portions of which are desired to be highlighted using a second color.  In order to produce the desired results the imaging device must precisely register the highlight color with the first image.  Highlight color image registration is often challenging.  It is often the case that a highlight printer is designed as a retrofit of a monochromatic engine in which the quality of the motion of the photoreceptor is only good enough to limit the banding to a tolerable level.  The monochromatic image is typically laid down at a constant rate of lines per unit time.  If the second imager is also caused to write at a constant rate, serious errors in color to color registration may occur. 
In single pass electrophotographic printers having more than one process station which provide sequential images to form a composite image, critical control of the registration of each of the sequenced images is required.  This is also true in multiple pass color printers, which produce sequential developed images superimposed onto a photoreceptor belt for charging with toner to form a multi-color image.  Failure to achieve registration of the images yields printed copies in which the color separations forming the images are misaligned.  This condition is generally obvious upon viewing of the copy; as such copies usually exhibit fuzzy color separation between color patches, bleeding and/or other errors which make such copies unsuitable for intended uses. 
A typical highlight color reproduction machine records successive electrostatic latent images on the photoconductive surface.  When combined, these electrostatic latent images form a latent image corresponding to the entire original document being printed.  One latent image is usually developed with black toner.  The other latent image is developed with color highlighting toner, e.g. red toner.  These developed toner powder images are transferred sequentially to a sheet to form a color highlighted document.  Such color highlighting reproduction machine can be of the so-called single-pass variety, where the color separations are generated sequentially by separate imaging and toning stations, or of the so-called multiple-pass variety, where the separations are generated by a single imaging station in subsequent passes of the photoreceptor and are alternatively toned by appropriate toning stations.  A particular variety of single-pass highlight color reproduction machines using tri-level printing has also been developed.  Tri-level electro-statographic printing is described in greater detail in U.S. Pat.  No.  4,078,929.  As described in this patent, the latent image is developed with toner particles of first and second colors simultaneously.  The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. 
Another type of color reproduction machine which may produce highlight color copies initially charges the photoconductive member.  Thereafter, the charged portion of the photoconductive member is discharged to form an electrostatic latent image thereon.  The latent image is subsequently developed with black toner particles.  The photoconductive member is then recharged and image wise exposed to record the highlight color portions of the latent image thereon.  A highlight latent image is then developed with toner particles of a color other than black, e.g. red, and then developed to form the highlight latent image.  Thereafter, both toner powder images are transferred to a sheet and subsequently fused thereto to form a highlight color document. 
The operation of highlight and color printers is well known and is described in greater detail in U.S. Pat.  Nos.  5,113,202; 5,208,636; 5,281,999; and 5,394,223, the disclosures of which are hereby incorporated herein by this reference. 
A simple, relatively inexpensive, and accurate approach to register latent images superimposed in such printing systems has been a goal in the design, manufacture and use of electrophotographic printers.  This need has been particularly recognized in the color and highlight color portion of electro-photography.  The need to provide accurate and inexpensive registration has become more acute, as the demand for high quality, relatively inexpensive color images has increased. 
The disclosed imaging device utilizes a second imager for forming the highlight latent image at a time following the forming of the first latent image that accounts for irregularities in the movement of the photoreceptor belt between the first imager and the second imager.  If the second imager is an LED bar as disclosed herein, one can take advantage of its ability to fire a line of data whenever it is most appropriate for color registration. 
According to one aspect of the disclosure, an imaging device for producing multicolor images from image data containing data representing an image of a first color and an image of a second color to be registered relative to the image of the first color onto a substrate by transferring toner of the first and second colors to the substrate is provided.  The imaging device comprises a first imager, a second imager, a photoreceptor belt, a plurality of rollers, an angular position sensor, a first index sensor, a second index sensor, an image data source and a controller.  The first imager is configured to generate an optical output corresponding to the image of the first color at a first exposure station.  The second imager is configured to generate an optical output corresponding to the image of the second color at a second exposure station.  The photoreceptor belt is configured to have a charge placed thereon for modification by the optical output of the first imager to be receptive to a charged toner of the first color and for modification by the optical output of the second imager to be receptive to a charged toner of the second color, the photoreceptor belt being configured to include an index.  The plurality of rollers are mounted to a frame of the imaging device for defining a process path along which the photoreceptor belt is driven in a process direction.  The plurality of rollers comprises a drive roller and a tensioning roller.  The drive roller has a longitudinal axis about which it is mounted to rotate and a drive surface formed generally concentrically about the longitudinal axis for which eccentricity versus phase angle from a reference point data is known.  The drive surface has a nominal circumference and is configured to drive the photoreceptor belt.  The tensioning roller provides tension to the photoreceptor belt as it is driven about the process path.  The angular position sensor detects the phase angle of the drive roller from the reference point.  The first index sensor is mounted along the process path for sensing the passage of the index on the belt.  The second index sensor is mounted along the process path for sensing the passage of the index on the belt.  The image data source generates image data for generating an image including graphics of the first color and graphics of the second color.  The image data includes a line to be printed in the first color and in the second color.  The controller is coupled to receive signals from the first index sensor, second index sensor, angular position sensor, first imager and image data source and is configured to drive the second imager to generate an optical output.  The controller includes memory and a processor.  The memory stores the eccentricity versus phase angle from a reference point data, the time at which the first index sensor senses the passage of the index on the belt, and the time at which the second index sensor senses the passage of the index on the belt.  The processor calculates an appropriate time delay for starting the generation of the optical output of the second imager for printing the line to be printed in the first color and in the second color based on the time of the starting of the generation of the optical output by the first imager to print the line to be generated in the first color and in the second color, the signal received from the first index sensor, the signal received from the second index sensor, the signal received from the angular position sensor, and the eccentricity versus phase angle from a reference point data. 
According to another aspect of the disclosure, an imaging device for producing multicolor images from image data containing data representing an image of a first color and an image of a second color to be registered relative to the image of the first color onto a substrate by transferring toner of the first and second colors to the substrate is provided.  The imaging device comprises a raster output scanner (“ROS”) imager, a light emitting diode (“LED”) imager, a photoreceptor belt, a plurality of rollers mounted to a frame of the imaging device, an angular position sensor, an image data source and a controller.  The ROS imager is configured to generate an optical output corresponding to the image of the first color at a first exposure station.  The LED imager is configured to generate an optical output corresponding to the image of the second color at a second exposure station.  The photoreceptor belt is configured to have a charge placed thereon for modification by the optical output of the ROS imager to be receptive to a charged toner of the first color and for modification by the optical output of the LED imager to be receptive to a charged toner of the second color.  The plurality of rollers mounted to a frame of the imaging device define a process path along which the photoreceptor belt is driven past the ROS and LED imagers in a process direction.  The plurality of rollers comprises a drive roller having a longitudinal axis about which it is mounted to rotate and a drive surface formed generally concentrically about the longitudinal axis.  The drive roller exhibits an eccentricity for which a formula relating angular position as a function of the phase angle of the drive roller to eccentricity is known.  The drive surface has a nominal circumference and is configured to drive the photoreceptor belt.  The angular position sensor detects the phase angle of the drive roller.  The image data source generates image data for generating an image including graphics of the first color and graphics of the second color.  The image data includes a line to be printed in the first color and in the second color.  The controller is coupled to receive signals from the angular position sensor, ROS imager and image data source and is configured to drive the LED imager to generate an optical output.  The controller includes memory and a processor.  The memory stores the formula relating angular position as a function of the phase angle of the drive roller to eccentricity.  The processor calculates an appropriate time delay for starting the generation of the optical output of the LED imager for printing the line to be printed in the first color and in the second color based on the time of the starting of the generation of the optical output by the ROS imager to print the line to be generated in the first color and in the second color, the signal received from the angular position sensor, and the formula relating angular position as a function of the phase angle of the drive roller to eccentricity. 
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived. 
These figures merely illustrate the disclosed methods and apparatus and are not intended to exactly indicate relative size and dimensions of the device or components thereof.