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Claims  |
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What is claimed is:
1. An apparatus for sensing and subsampling luminance and chrominance of
features on an object being scanned in a fast scan direction and a slow
scan direction, comprising:
an array of n subsampling sensor sets spanning a fast scan direction width
of n times 2 pixels, each of said sensor sets including, aligned in a slow
scan direction column,
a set of 2 pixel-sized green sensors aligned in a fast scan direction line
for determining pixel-by-pixel green color and luminance,
a red sensor extending 2 pixels wide in said fast scan direction and m
pixels long in said slow scan direction for detecting red color, and
a blue sensor extending m pixels wide in said fast scan direction and 2
pixels long in said slow scan direction for detecting blue color; and
a main processor for determining a high resolution luminance according to a
luminance output of said green sensors whereby said luminance outputs are
transformed into Y tristimulus values and thereafter to high resolution L*
values corresponding to the features of the object and a low resolution
subsampled chrominance output according to said red sensor and said blue
sensor color determinations whereby said luminance and chrominance outputs
are transformed into X, Y and Z tristimulus values and thereafter to color
corrected L* a* and b* values corresponding to the features of the object,
wherein said red sensor and said blue sensor outputs are multiplexed for
transmission on a chrominance channel prior to transformation into the X,
Y and Z tristimulus values, wherein said green sensor outputs are
transmitted On a luminance channel and wherein said main processor
comprises a first, pretransformation dual output analog to digital
converter for receiving analog voltage inputs from the chrominance
channel, and a second pretransformation dual output analog to digital
converter for receiving analog voltage inputs from the luminance channel,
whereby each converter provides digital outputs according to a first ratio
of corresponding analog inputs up to a preselected voltage input value and
whereby each converter provides the digital outputs according to a second
ratio of corresponding analog inputs above the preselected voltage input
value.
2. Am apparatus for sensing and subsampling luminance and chrominance of
features on an object being scanned in a fast scan direction and a slow
scan direction, comprising:
an array of n subsampling sensor sets spanning a fast scan direction width
of n times m pixels, each of said sensor sets including, aligned in a slow
scan direction column,
a set of m pixel-sized green sensors aligned in a fast scan direction line
for determining pixel-by-pixel green color and luminance,
a red sensor extending m pixels wide in said fast scan direction and m
pixels long in said slow scan direction for detecting red color, and
a blue sensor extending m pixels wide in said fast scan direction and m
pixels long in said slow scan direction for detecting blue color; and
a main processor for determining a high resolution luminance according to a
luminance output of said green sensors whereby said luminance outputs are
transformed into Y tristimulus values and thereafter to high resolution L*
values corresponding to the features of the object and a low resolution
subsampled chrominance output according to said red sensor and said blue
sensor color determinations, whereby said luminance and chrominance
outputs are transformed into X, Y and Z tristimulus values and thereafter
to color corrected L*, a* and b* values corresponding to the features of
the object, wherein the main processor comprises a pretransformation
chrominance channel summing processor for increasing a chrominance output
precision, and a pretransformation luminance channel summing processor for
increasing a luminance output precision and wherein said chrominance
channel summing processor sums a quantity m of red color detections from
said red sensors and m blue color detections from said blue sensors, and
wherein said luminance channel summing processor sums 2 times m green
sensor detections of luminance from said green sensors.
3. An apparatus for sensing and subsampling luminance and chrominance of
features on an object being scanned in a fast scan direction and a slow
scan direction by an array having green, red and blue sensors, comprising
a main processor for determining a high resolution luminance according to
a luminance output of said green sensors and a low resolution subsampled
chrominance output according to a summed green sensor output of n ,green
sensors, a summed red sensor output of n red sensors and a summed blue
sensor output of n blue sensors, whereby said luminance and chrominance
outputs are transformed by said main processor into X, Y and Z tristimulus
values and thereafter to L*, a* and b* values corresponding to said
features of the object, wherein said red sensor outputs are transmitted on
a red channel, said blue sensor outputs are transmitted on a blue channel
and said green sensor outputs are transmitted on a green channel prior to
transformation into the X, Y and Z tristimulus values and wherein said
main processor comprises a pretransformation red channel dual output
analog to digital converter, a pretransformation blue channel dual output
analog to digital converter and a pretransformation green channel dual
output analog to digital converter, wherein each red, blue and green
channel dual output analog to digital converter provides said digital
outputs according to a first ratio of corresponding analog inputs up to a
preselected voltage input Value and whereby and whereby each red, blue and
green channel dual output analog to digital converter provides said
digital outputs according to a second ratio of corresponding analog inputs
above said preselected voltage input value.
4. An electrophotographic printing machine having an apparatus for sensing
and subsampling luminance and chrominance of a sheet having multicolored
indicia thereon, and means responsive to L* a* and b* values for
reproducing a copy of the sheet being scanned in a fast scan direction and
a slow scan directing, said apparatus comprising:
an array of n subsampling sensor sets spanning a fast scan direction width
of n times m pixels, each of said sensor sets including, aligned in a slow
scan direction column,
a set of 2 pixel-sized green sensors aligned in a fast scan direction line
for determining pixel-by-pixel green color and luminance,
a red sensor extending 2 pixels wide in said fast scan direction and 2
pixels long in said slow scan direction for detecting red color, and
a blue sensor extending 2 pixels wide in said fast scan direction and 2
pixels long in said slow scan direction for detecting blue color; and
a main processor for determining a high resolution luminance according to a
luminance output of said green sensors whereby said luminance outputs are
transformed into Y tristimulus values and thereafter to high resolution L*
values corresponding to the multicolored indicia on the sheet and a low
resolution subsampled chrominance output according to said red sensor and
said blue Sensor color determinations whereby said luminance and
chrominance outputs are transformed into X, Y and Z tristimulus values and
thereafter to color corrected L* a* and b* values corresponding to the
multicolored indicia on the sheet, wherein said red sensor and said blue
sensor outputs are multiplexed for transmission on a chrominance channel
prior to transformation into the x, Y and Z tristimulus values and wherein
said green sensor outputs are transmitted on a luminance channel and
wherein said main processor comprises a first pretransformation dual
output analog to digital converter for receiving analog voltage inputs
from the chrominance channel, and a second pretransformation dual output
analog to digital converter for receiving analog voltage inputs from the
luminance channel, whereby each converter provides digital outputs
according to a first ratio of corresponding analog inputs up to a
preselected voltage input value and whereby each converter provides the
digital outputs according to a second ratio of corresponding analog inputs
above the preselected voltage input value.
5. An electrophotographic printing machine having an apparatus for sensing
and subsampling luminance and chrominance of a sheet having multicolored
indicia thereon, and means responsive to L* a* and b* values for
reproducing a copy of the sheet being scanned in a fast scan direction and
a slow scan direction, said apparatus comprising:
an array of n subsampling sensor sets spanning a fast scan direction width
of n times m pixels, each of said sensor sets including, aligned in a slow
scan direction column,
a set of 2 pixel-sized green sensors aligned in a fast scan direction line
for determining pixel-by-pixel green color and luminance,
a red sensor extending 2 pixels wide in said fast scan direction and 2
pixels long in said slow scan direction for detecting red color, and
a blue sensor extending 2 pixels wide in said fast scan direction and 2
pixels long in said slow scan direction for detecting blue color, and
a main processor for determining a high resolution luminance according to a
luminance output of said green sensors whereby said luminance outputs are
transformed into Y tristimulus values and thereafter to high resolution L*
values corresponding to the multicolored indicia on the sheet and a low
resolution subsampled chrominance output according to said red sensor and
said blue sensor color determinations whereby said luminance and
chrominance outputs are transformed into X, Y and Z tristimulus values and
thereafter to color corrected L* a* and b* values corresponding to the
multicolored indicia on the sheet, wherein the main processor comprises a
pretransformation chrominance channel summing processor for increasing a
chrominance output precision, and a pretransformation luminance channel
summing processor for increasing a luminance output precision and wherein
said chrominance channel summing processor sums a quantity 2 of red color
detections from said red sensors and 2 blue color detections from said
blue sensors, and wherein said luminance channel summing processor sums 2
times 2 green sensor detections of luminance from said green sensors. |
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Claims |
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Description |
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The present invention is directed to an apparatus for subsampling chroma
data in scanned images. More particularly, the present invention is
directed to a chrominance subsampling processor and sensor array for use
in scanning features of a sheet or object for printing, processing,
storage and/or transmission.
Chroma sensing and sampling involves the detection of areas (often referred
to as pixels) of color using CCD full width or other arrays of sensors.
The color data from scanning arrays of sensors is generally collected by
overlaying photosensitive sensors with filters so as to detect red, green
and blue (RGB) intensity. Subsequently, color conversion is performed such
that the RGB values of a color are converted to the corresponding CIE
tristimulus values x, y and z. Thereafter, these tristimulus values are
transformed to the L*a*b* color space. The resultant hue, chroma and
lightness values can thereafter be transformed back to RGB values or the
CMYK values for color output to a printer, to memory or to another device.
With the new developments in semiconductor and digital signal processing
techniques, chroma subsampling is now being employed in cameras and video
camcorders. Progress has been made in overcoming a number of difficulties
encountered with certain subsampling implementations, such as in U.S. Pat.
No. 4,605,956 to Cok, which teaches a digital signal processing technique
was used to correct for color fringing effect of sharp edges due to chroma
subsampling. While chroma subsampling has been employed in tile NTSC
television broadcast standard, color subsampling is generally not used by
most analog color TV cameras and monitors. The complexity of converting
back and forth between subsampled and non-subsampled signals for gamma and
color correction between cameras and monitors in the analog domain are
highly complex and can require expensive hardware and software to
implement.
It is desirable in may applications to employ chroma subsampling schemes in
scanners in digital copiers or other devices used to reproduce, store or
process color documents. Chroma subsampling may therefore be usefully
employed in a number of color scanning situations due to a number of
factors. One such factor stems from the reduced spatial sensitivity of the
cone sensors of the human eye (as opposed to the rods which sense black
and white components). Rather than requiring additional processors to
perform color correction on scanned data, a subsampling sensor array can
therefore be employed in document scanners as taught by the present
invention to eliminate aspects of otherwise necessary hardware and
software, while providing desirable subsampled/corrected color data
output. In color document scanning applications as well as other
situations, the amount or quantity of data that must be processed, stored
and transmitted is of major concern in determining the feasibility and
cost of subsampling chrominance. The hardware and software requirements
for handling this task using previous subsampling methods and apparatuses
can be prohibitive.
In the past a variety of arrangements have been used to sample chrominance
data and otherwise employ color sensing and scanning, to include those
taught by the following disclosures that may be relevant:
U.S. Pat. No. 5,119,181
Issued: Jun. 2, 1992
Patentee: Peregaux et al.
U.S. Pat. No. 5,077,810
Issued: Dec. 31, 1991
Patentee: D'Luna
U.S. Pat. No. 5,067,010
Issued: Nov. 19, 1991
Patentee: Ishii et al.
U.S. Pat. No. 4,969,204
Issued Nov. 6, 1990
Patentee: Melnychuck et al
U.S. Pat. No. 4,656,515
Issued Apr. 7, 1987
Patentee: Christopher
U.S. Pat. No. 4,652,908
Issued: Mar. 24, 1987
Patentee: Fling et al.
U.S. Pat. No. 4,633,300
Issued: Dec. 30, 1986
Patentee: Sakai
U.S. Pat. No. 4,605,956
Issued: Aug. 12, 1986
Patentee: Cok
U.S. Pat. No. 5,119,181 to Peregaux et al. discloses a color chip
construction adapted for use in fabricating full width arrays in which the
individual chip photosites consisting of a blue, green and red photodiode
shaped and positioned to provide a rectangular photosite with square sides
that enhance butting of the color chip with other like color chips to form
full width color arrays.
U.S. Pat. No. 5,077,810 to D'Luna discloses a digital processing
architecture for a high resolution image sensor uses a plurality of like
digital processors for time-divided processing of the output of the
sensor. Each processor is operational according to start and stop signals
from a programmable sequencer. In a preferred embodiment, two sets of
processors handle a line resolution of 1024 pixels, one set doing the
first half of each line and the other set doing the second half. This is
of particular utility where vertical processing is required, and the full
line delays needed are divided into partial resettable delays resident in
each of the processors.
U.S. Pat. No. 5,067,010 to Ishii et al. discloses a color video signal
processing device in which pixels are thinned out for a whole picture
plane with respect to each of two kinds of digital color difference
signals in accordance with a predetermined role. The encoding is executed
on a unit basis of a block consisting of (n.times.m) samples where (n and
m are integers no less than 2) which are formed with respect to each of
the two kinds of color difference signals whose pixels have been thinned
out or a block consisting of (n.times.m) samples formed so as to include
both of the two kinds of color difference signals whose pixels had been
thinned out. The data compression is executed on a block unit basis.
U.S. Pat. No. 4,969,204 to Melnychuck et al. discloses an image processing
method for hierarchical storage and display of high resolution digital
images. Reduced resolution versions of the image are available for quick
display on a monitor, while the high resolution image may be be accessed
as a photographic quality hard copy. A hybrid coding scheme based on
residuals is used to store the data.
U.S. Pat. No. 4,656,515 to Christopher discloses a television display
including circuitry for reducing the amount of memory needed to hold one
field of the reduced size image. In the display apparatus, digital samples
representing the large and small picture signals are developed at
substantially equal rates by separate circuitry. (This requirement for
additional/separate subsampling processing capability is also common to
other known subsampling applications such as color correction.)
Subsampling circuitry stores One out of every five of the samples
representing a horizontal line Of the small picture. These samples are
displayed, synchronous with the large picture at a rate three-fifths times
the display rate of the large picture samples to produce an apparent size
reduction of one-third in the horizontal direction.
U.S. Pat. No. 4,652,908 to Fling et al. discloses a display including a
filtering system for processing the video signals which produce the
reduced-sized image. The filtering system includes an anti-aliasing filter
which reduces the amplitude of the components of the video signals which
may cause aliasing distortion when the image is subsampled. However, the
filter passes substantial amounts of these components. The filtered video
signal is subsampled and applied to a peaking filter which amplifies the
band of frequencies containing the aliasing components relative to lower
frequency bands to improve thee appearance of detailed portions of the
reproduced image.
U.S. Pat. No. 4,633,300 to Sakai discloses a color information detecting
device is constructed of detectors each for detecting a one of a number of
different colors and each having a number of light receiving faces. The
detectors are arranged on the same plane independently of each other. The
light receiving faces of each detector are electrically connected and have
their center of sensitivity distribution located at about the same point
as that of the faces of another detector.
U.S. Pat. No. 4,605,956 to Cok discloses an electronic color camera having
a single-chip solid state color image sensor, includes a color dependent
birefringent spatial filter that deflects red and blue light from portions
of an image sampled by the neighboring green sensitive image sensing
elements onto red and blue sensitive image sensing elements. Signal
processing electronics produces interpolated red and blue signal values by
forming red and blue hue component values at the red and blue sampling
locations, interpolating the hue component values, and producing the
interpolated red and blue values, and green signal values at the
interpolation locations. As a result, color fringes at monochrome edges
are completely eliminated, and are substantially reduced at colored edges.
In accordance with one aspect of the present invention, there is provided
an apparatus for sensing and subsampling luminance and chrominance of
features on an object being scanned in a fast scan direction and a slow
scan direction. The apparatus includes an array of n subsampling sensor
sets spanning a fast scan direction width of n times m pixels, each of the
sensor sets including, aligned in a slow scan direction column, a set of m
pixel-sized green sensors aligned in a fast scan direction line for
determining pixel-by-pixel green color and luminance, a red sensor
extending m pixels wide in the fast scan direction and m pixels long in
the slow scan direction for detecting red color and a blue sensor
extending m pixels wide in the fast scan direction and m pixels long in
the slow scan direction for detecting blue color. The apparatus also
includes a main processor for determining a high resolution luminance
according to a luminance output of the green sensors whereby the luminance
outputs are transformed into Y tristimulus values and thereafter to high
resolution L* values corresponding to the features of the object and a low
resolution subsampled chrominance output according to the red and blue
sensor color determinations, whereby the luminance and chrominance outputs
are transformed into X, Y and Z tristimulus values and thereafter to color
corrected L*, a* and b* values corresponding to the features of the
object.
In accordance with another aspect of the present invention, there is
provided an apparatus for sensing and subsampling luminance and
chrominance of features on an object being scanned in a fast scan
direction and a slow scan direction by an array having green, red and blue
sensors. The apparatus includes a main processor for determining a high
resolution luminance according to a luminance output of the green sensors
and a low resolution subsampled chrominance output according to a summed
green sensor output of n green sensors, a summed red sensor output of n
red sensors and a summed blue sensor output of n blue sensors, whereby the
luminance and chrominance outputs are transformed by the main processor
into X, Y and Z tristimulus values and thereafter to L*, a* and b* values
corresponding to the features of the object.
In accordance with another aspect of the present invention, there is
provided an electrophotographic printing machine having an apparatus for
sensing and subsampling luminance and chrominance of a sheet having
multicolored indicia thereon, and means responsive to L*, a* and b* values
for reproducing a copy of the sheet being scanned in a fast scan direction
and a slow scan direction. The apparatus includes an array of n
subsampling sensor sets spanning a fast scan direction width of n times m
pixels, each of the sensor sets including, aligned in a slow scan
direction column, a set of m pixel-sized green sensors aligned in a fast
scan direction line for determining pixel-by-pixel green color and
luminance, a red sensor extending m pixels wide in the fast scan direction
and m pixels long in the slow scan direction for detecting red color and a
blue sensor extending m pixels wide in the fast scan direction and m
pixels long in the slow scan direction for detecting blue color. The
apparatus also includes a main processor for determining a high resolution
luminance according to a luminance output of the green sensors whereby the
luminance outputs are transformed into Y tristimulus values and thereafter
to high resolution L* values corresponding to the multicolored indicia on
the sheet and a low resolution subsampled chrominance output according to
the red sensor and the blue sensor color determinations, whereby the
luminance and chrominance outputs are transformed into X, Y and Z
tristimulus values and thereafter to color corrected L* a* and b* values
corresponding to the multicolored indicia on the sheet.
Further aspects and advantages of the present invention will become
apparent from the following description of the various embodiments and
characteristic features of the present invention.
The following is a brief description of each drawing used to describe the
present invention, and thus, are being presented for illustrative purposes
only and should not be limited of the scope of the present invention,
wherein:
FIG. 1 is a block diagram showing one embodiment of an image processor of
the present invention;
FIG. 2 is an elevational view showing an embodiment of a subsampling sensor
array in accordance with the present invention;
FIG. 3 is an elevational view showing a prior art sensor array;
FIG. 4 is a block diagram showing another embodiment of an image processor
of the present invention;
FIG. 5 is a block diagram showing a dual-slope A/D converter that may be
employed in the present invention;
FIG. 6 is graphical representation of luminance values versus Y tristimulus
values;
FIG. 7 is graphical representation of dual slope A/D transfer
characteristics;
FIG. 8 is graphical representation of the A/D range (slope) of the
luminance component of the color space conversion;
FIG. 9 is graphical representation of the A/D range (slope) conversion of
the X, Y and Z tristimulus components; and
FIG. 10 is a schematic elevational view showing an exemplary
electrophotographic printing )machine incorporating features of the
present invention therein.
While the present invention will hereinafter be described in connection
with preferred embodiments thereof, it will be understood that it is not
intended to limit the invention to these embodiments. On the contrary, it
is intended to cover all alternatives, modifications and equivalents, as
may be included within the spirit and scope of the invention as defined by
the appended claims.
For a general understanding of the features of the present invention,
reference is made to the drawings. FIG. 10 is a schematic elevational view
showing an electrophotographic printing machine which may incorporate
features of the present invention therein. It will become evident from the
following discussion that the subsampling apparatus of the present
invention is equally Well suited for use in a wide variety of color
scanners coupled with printing systems, image memory storage systems and
other devices, and therefore are not limited in application to the
particular systems shown herein. While the present invention will
hereinafter be described in connection with preferred embodiments, it will
be understood that it is not intended to limit the invention to a
particular embodiment. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included within the
spirit and scope of the invention as defined by the appended claims.
To begin by way of general explanation, FIG. 10 is a schematic elevational
view showing an electrophotographic printing machine which may incorporate
features of the present invention therein, it will become evident from the
following discussion that the present invention is equally well suited for
use in a wide variety of copying and printing systems, and is not
necessarily limited in its application to the particular system shown
herein. As shown in FIG. 10, during operation of the printing system, a
multiple color original document 38 is positioned on a raster input
scanner (RIS), indicated generally by the reference numeral 10. The RIS
contains document illumination lamps, Optics, a mechanical scanning drive,
and a charge coupled device (CCD array) or full width subsampling scanning
sensor array 11, such as shown and described in greater detail conjunction
with FIGS. 2 and 3 herein. Sensor array 11 of the RIS captures the entire
image from original document 38 and converts it to a series of raster scan
lines and moreover measures a set of primary color densities, i.e. red,
green and blue densities, at each point of the original document. Sensor
array 11 transmits chrominance data as electrical signals to an image
processing system (IPS), indicated generally by the reference numeral 12.
IPS 12 converts the set of red, green and blue density signals to a set of
colorimetric coordinates, as more fully described in association with
FIGS. 1 through 9 herein.
IPS 12 also contains data control electronics which prepare and manage the
image data flow to a raster output scanner (ROS), indicated generally by
the reference numeral 16. A user interface (UI), indicated generally by
the reference numeral 14, is in communication with IPS 12. UI 14 enables
an operator to control the various operator adjustable functions. The
operator actuates the appropriate keys of UI 14 to adjust the parameters
of the copy. UI 14 may be a touch screen, or any other suitable control
panel, providing an operator interface with the system. The output signal
from UI 14 is transmitted to IPS 12. The IPS then transmits signals
corresponding to the desired image to ROS 16, which creates the output
copy image. ROS 16 includes a laser with rotating polygon mirror blocks.
Preferably, a nine facet polygon is used. The ROS illuminates, via mirror
37, the charged portion of a photoconductive belt 20 of a printer or
marking engine, indicated generally by the reference numeral 18, at a rate
of about 400 pixels per inch, to achieve a set of subtractive primary
latent images. The ROS will expose the photoconductive belt to record
three or four latent images which correspond to the signals transmitted
from IPS 12. One latent image is developed with cyan developer material.
Another latent image is developed with magenta developer material and the
third latent image is developed with yellow developer material. A black
latent image may be developed in lieu of or in addition to other (colored)
latent images. These developed images are transferred to a copy sheet in
superimposed registration with one another to form a multicolored image on
the copy sheet. This multicolored image is then fused to the copy sheet
forming a color copy.
With continued reference to FIG. 10, printer or marking engine 18 is an
electrophotographic printing machine. Photoconductive belt 20 of marking
engine 18 is preferably made from a photoconductive material. The
photoconductive belt moves in the direction of arrow 22 to advance
successive portions of the photoconductive surface sequentially through
the various processing stations disposed about the path of movement
thereof. Photoconductive belt 20 is entrained about rollers 24 and 26,
tensioning roller 28, and drive roller 30. Drive roller 30 is rotated by a
motor 32 coupled thereto by suitable means such as a belt drive. As roller
30 rotates, it advances belt 20 in the direction of arrow 22.
Initially, a portion of photoconductive belt 20 passes through a charging
station, indicated generally by the reference numeral 33. At charging
station 33, a corona generating device 34 charges photoconductive belt 20
to a relatively high, substantially uniform potential.
Next, the charged photoconductive surface is rotated to an exposure
station, indicated generally by the reference numeral 35. Exposure station
35 receives a modulated light beam corresponding to information derived by
RIS 10 having multicolored original document 38 positioned thereat. The
modulated light beam impinges on the surface of photoconductive belt 20.
The beam illuminates the charged portion of the photoconductive belt to
form an electrostatic latent image. The photoconductive belt is exposed
three or four times to record three or four latent images thereon.
After the electrostatic latent images have been recorded on photoconductive
belt 20, the belt advances such latent images to a development station,
indicated generally by the reference numeral 39. The development station
includes four individual developer units indicated by reference numerals
40, 42, 44 and 46. The developer units are of a type generally referred to
in the art as "magnetic brush development units." Typically, a magnetic
brush development system employs a magnetizable developer material
including magnetic carrier granules having toner particles adhering
triboelectrically thereto. The developer material is continually brought
through a directional flux field to form a brush of developer material.
The developer material is constantly moving so as to continually provide
the brush With fresh developer material. Development is achieved by
bringing the brush of developer material into contact with the
photoconductive surface. Developer units 40, 42, and 44, respectively,
apply toner particles of a specific color which corresponds to the
complement of the specific color separated electrostatic latent image
recorded on the photoconductive surface.
The color of each of the toner particles is adapted to absorb light within
a preselected spectral region of the electromagnetic wave spectrum. For
example, an electrostatic latent image formed by discharging the portions
of charge on the photoconductive belt corresponding to the green regions
of the original document will record the red and blue portions as areas of
relatively high charge density on photoconductive belt 20, while the green
areas will be reduced to a voltage level ineffective for development. The
charged areas are then made visible by having developer unit 40 apply
green absorbing (magenta) toner particles onto the electrostatic latent
image recorded on photoconductive belt 20. Similarly, a blue separation is
developed by developer unit 42 with blue absorbing (yellow) toner
particles, while the red separation is developed by developer unit 44 with
red absorbing (cyan) toner particles. Developer unit 46 contains black
toner particles and may be used to develop the electrostatic latent image
formed from a black and white original document. Each of the developer
units is moved into and out of an operative position. In the operative
position, the magnetic brush is substantially adjacent the photoconductive
belt, while in the nonoperative position, the magnetic brush is Spaced
therefrom. During development of each electrostatic latent image, only one
developer unit is in the operative position, the remaining developer units
are in the nonoperative position.
After development, the toner image is moved to a transfer station,
indicated generally by the reference numeral 65. Transfer station 65
includes a transfer zone, generally indicated by reference numeral 64. In
transfer zone 64, the toner image is transferred to a sheet of support
material, such as plain paper amongst others. At transfer station 65, a
sheet transport apparatus, indicated generally by the reference numeral
48, moves the sheet into contact with photoconductive belt 20. Sheet
transport 48 has a pair of spaced belts 54 entrained about a pair of
substantially cylindrical rollers 50 and 52. A sheet gripper 84 (not shown
in FIG. 6) extends between belts 54 and moves in unison therewith. A sheet
25 is advanced from a stack of sheets 56 disposed on a tray. A friction
retard feeder 58 advances the uppermost sheet from stack 56 onto a
pre-transfer transport 60. Transport | | |
