Highlight color imaging apparatus

A magnetic brush developer apparatus including a plurality of developer housings each including a plurality of magnetic brush rolls associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically formed images. The physical properties such as conductivity, toner concentration and toner charge level of the CMB developers are such that density fine lines are satisfactorily developed notwithstanding the presence of relatively high cleaning fields.

BACKGROUND OF THE INVENTION 
This invention relates generally to the rendering of latent electrostatic 
visible using multiple colors of dry toner or developer and, more 
particularly, to a developer apparatus including structure for suppressing 
the development of the fringe fields of complementary tri-level images 
while developing acceptable line images, notwithstanding the presence of 
relatively high cleaning fields. 
The invention can be utilized in the art of xerography or in the printing 
arts. In the practice of conventional xerography, it is the general 
procedure to from electrostatic latent images on a xerographic surface by 
first uniformly charging a photoconductive insulating surface or 
photoreceptor. The charge is selectively dissipated in accordance with a 
pattern of activating radiation corresponding to original images. The 
selective dissipation of the charge leaves a latent charge pattern on the 
imaging surface corresponding to the a reas not struck by radiation. 
This charge pattern in made visible by developing it with toner. The toner 
is generally a colored powder which adheres to the charge pattern by 
electrostatic attraction. 
The developed image is then fixed to the imaging surface or is transferred 
to a receiving substrate such as plain paper to which it is fixed by 
suitable fusing techniques. 
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 
issued in the name of Gundlach. The patent to Gundlach teaches the use of 
tri-level xerography as a means to achieve single-pass highlight color 
imaging. As disclosed therein, the charge pattern is developed with toner 
particles of first and second colors. The toner particles of one of the 
colors are positively charged and the toner particles of the other color 
are negatively charged. In one embodiment, the toner particles are 
supplied by a developer which comprises a mixture of triboelectrically 
relatively positive and relatively negative carrier beads. The carrier 
beads support, respectively, the relatively negatively and relatively 
positive toner particles. Such a developer is generally supplied to the 
charge pattern by cascading it a cross the imaging surface supporting the 
charge pattern. In another embodiment, the toner particles are presented 
to the charge pattern by a pair of magnetic brushes. Each brush supplies a 
toner of one color and one charge. In yet another embodiment, the 
development system is biased to about the b ackground voltage. Such 
biasing results in a developed image of improved color sharpness. 
In tri-level xerography, the xerographic contrast on the charge retentive 
surface or photoreceptor is divided three, rather than two, ways as is the 
case in conventional xerography. The photoreceptor is charged, typically 
to 900v. It is exposed imagewise, such that one image corresponding to 
charged image areas (which are subsequently developed by charged area 
development, i.e. CAD) stays at the full photoreceptor potential 
(V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed 
to discharge the photoreceptor to its residual potential, i.e. V.sub.c or 
V.sub.dad (typically 100v) which corresponds to discharged area images 
that are subsequently developed by discharged-area development (DAD). The 
background areas exposed such as to reduce the photoreceptor potential to 
halfway between the V.sub.cad than V.sub.white (about 600v), and the DAD 
developer system is biased about 100v closer to V.sub.dad than V.sub.white 
(about 400v). 
Various techniques have heretofore been employed to develop electrostatic 
images as illustrated by the following disclosures which may be relevant 
to certain aspects of the present invention. 
As disclosed in U.S. Pat. No. 3,457,900, magnetic brushes have been 
designed to give fringe field or solid area development by adjusting the 
conductivity of the carrier. It is also stated therein that they can also 
be made to tone areas of less charge and clean areas of greater charge 
giving what is known in the art as a reverse development. 
As discussed in U.S. Pat. No. 4,397,264 which relates to a conventional 
xerographic image development system, conductive magnetic brush (CMB) 
development and insulating magnetic brush (IMB) development systems suffer 
from limitations in their abilities to meet the full range of copy quality 
requirements. Specifically, insulating magnetic brush development systems 
have difficulty in using one developer roller to develop both lines and 
solid areas. In order to optimize solid area development with an 
insulating developer material, the spacing between the developer roller 
and photoconductive surface must be made quite small. However, low density 
fine line development occurs at a larger spacing to take advantage of the 
accuracy of fringe field development with insulating materials. This 
permits development with high cleaning fields so as to minimize background 
development. 
As further discussed in U.S. Pat. No. 4,397,264 conductive magnetic brush 
development systems inherently fail to faithfully reproduce low density 
lines. Conductive developer materials are not sensitive to fringe fields. 
In order to achieve low density fine line development with conductive 
developer materials, the cleaning field must be relatively low. This 
produces relatively high background. 
U.S. patent application Ser. No. 913,181 , now U.S. Pat. No. 4,761,668 
filed on Sept. 29, 1986 Parker et al and assigned to the same assignee as 
the instant application which relates to tri-level printing discloses 
apparatus for minimizing the contamination of one dry toner or developer 
by another dry toner or developer used for rendering visible latent 
electrostatic images formed on a charge retentive surface such as a 
photoconductive imaging member. The apparatus causes the otherwise 
contaminating dry toner or developer to be attracted to the charges 
retentive surface in its inter-document and outboard areas. The dry toner 
or developer so attracted is subsequently removed from the imaging member 
at the cleaning station. 
U.S. patent application Ser. No. 78,750, now U.S. Pat. No. 4,761,672 filed 
on July 28, 1987 in the name of Parker et al and assigned to the same 
assignee as the instant application which relates to tri-level printing 
discloses apparatus wherein undesirable transient development conditions 
that occur during start-up and shut-down in a tri-level xerographic system 
when the developer biases are either actuated or de-actuated are obviated 
by using a control strategy that relies on the exposure system to generate 
a spatial voltage ramp on the photoreceptor during machine start-up and 
shut-down. Furthermore, the development systems'bias supplies are 
programmed so that their bias voltages follow the photoreceptor voltage 
ramp at some predetermined offset voltage. This offset is chosen so that 
the cleaning field between any development roll and the photoreceptor is 
always within reasonable limits. As an alternative to synchronizing the 
exposure and developing characteristics, the charging of the photoreceptor 
can be varied in accordance with the change of developer bias voltage. 
U.S. Pat. application Ser. No. 78,743 filed on July 28, 1987 in the name of 
Jerome May and assigned to the same assignee as the instant application 
which relates to tri-level printing discloses apparatus wherein 
undesirable transient development conditions that occur during start-up 
and shut-down in a tri-level xerographic system when the developer biases 
are either actuated or de-actuated are obviated by the provision of 
developer apparatuses having rolls which are adapted to be rotated in a 
predetermined direction for preventing developer contact with the imaging 
surface during periods of start-up and shut-down. rolls of a selected 
developer housing or housings can be rotated in the contact-prevention 
direction to permit use of the tri-level system to be utilized as a single 
color system or for the purpose of agitating developer in only one of the 
housings at a time to insure internal triboelectric equlibrium of the 
developer in that housing. 
U.S. patent application Ser. No. 947,321, now U.S. Pat. No. 4,771,314 filed 
on Dec. 29, 1986 in the name of Parker et al and assigned to the same 
assignee as the instant application which relates to tri-level printing 
discloses printing apparatus for forming toner images in black and at 
least one highlighting color in a single pass of a charge retentive 
imaging surface through the processing areas, including a development 
station, of the printing apparatus. The development station includes a 
pair of developer housings each of which has supported therein a pair of 
magnetic brush development rolls which are electrically biased to provide 
electrostatic development and cleaning fields between the charge retentive 
surface and the developer rolls. The rolls are biased such that the 
development fields between the first rolls in each housing and the charge 
retentive surface are greater than those between the charge retentive 
surface and the second rolls and such that the cleaning fields between the 
second rolls in each housing and the charge retentive surface are greater 
than those between the charge retentive surface and the first rolls. 
U.S. patent application Ser. No. 95,486 filed on Aug. 31, 1987 in the name 
of Delmar Parker and assigned to the same assignee as the instant 
application which relates to tri-level printing discloses a magnetic brush 
developer apparatus comprising a plurality of developer housings each 
including a plurality of magnetic rolls associated therewith. The magnetic 
rolls disposed in a second developer housing are constructed such that the 
radial component of the magnetic force field produces a magnetically free 
development zone intermediate a charge retentive surface and the magnetic 
rolls. The developer is moved through the zone magnetically unconstrained 
and, therefore, subjects the image developed by the first developer 
housing to minimal disturbance. Also the developer is transported from one 
magnetic roll to the next. This apparatus provides an efficient means for 
developing the complementary half of a tri-level latent image while at the 
same time allowing the already developed first half to pass through the 
second housing with minimum image disturbance. 
U.S. patent application Ser. No. 31,627 filed on March 3, 1987, and 
abandoned and filed as continuation application Ser. No. 220,408 on June 
28, 1988 in the name of Parker et al and assigned to the same assignee as 
the instant application which relates to tri-level printing discloses an 
electronic printer employing tri-level xerography to superimpose two 
images with perfect registration during the single pass of a charge 
retentive member past the processing stations of the printer. One part of 
the composite image is formed using Magnetic Ink Character Recognition 
(MICR) toner, while the other part of the image is printed with less 
expensive black, or color toner. For example, the magnetically readable 
information on a check is printed with MICR toner and the rest of the 
check in color or in black toner that is not magnetically readable. 
In tri-level xerography, the images comprise charged area images and 
discharged area images. Such images are commonly referred to as charged 
area development (CAD) images and discharged area development images, 
respectively. In a typical configuration where the charge retentive 
surface is uniformly charged negative, the CAD image is developed using a 
charged area development (CAD) system including a positive black toner 
with subsequent development of the discharged area using a discharged area 
development (DAD) system including a negative colored toner. When a CAD 
image or a background (V.sub.white) region moves past the DAD housing a 
reverse development or cleaning field is established between the image and 
the developer rolls of that housing. The magnitude of the field is 
determined by the difference between the voltage level of the CAD image 
after development which is approximately equal to the CAD bias voltage 
V.sub.bb (FIG. 1b), or the background, V.sub.white and the bias voltage on 
the discharged area development (DAD) system which is V.sub.cb. The field 
thus established tends to cause the negative toner to migrate away from 
the photoreceptor towards the developer rolls. Thus, when a fine line 
moves trough the DAD developer housing, particularly with its smallest 
dimension travelling in the process direction, the development field 
generated by the fine line doesn't have time to attract enough toner that 
has drifted away from the photoreceptor surface into the developer back to 
the charge retentive surface to adequately develop the DAD fine line 
image. Because of the toner's inertia, it takes it a finite time for the 
toner to move in response to a rapidly changing development field, and in 
the case of a fine line, perpendicular to the process direction, there may 
not be sufficient time if the toner has migrated too far into the 
developer. Thus, line images may be improperly developed. This phenomenon 
is known as a developer history effect, which in this case is manifest as 
an underdeveloped fine line. 
The shortcomings of CMB and IMB development systems discussed above with 
respect to conventional xerography have, heretofore, been present in 
highlight color xerography as well. In fact, the problem of not being to 
able to develop low density fine lines with CMB developer in the presence 
of relatively high cleaning fields has resulted in the wide use of IMB 
developer. However, in a tri-level highlight color system the use of IMB 
developer has been found to be unacceptable. Its use results in the 
development of fringe fields in a color different from the rest of the 
image. Thus, for example, in a system that uses black and red developers, 
the black images would have a red border around them while red images 
would have a black border around them. 
The development of such fringe fields is caused by the reverse development 
or cleaning fields established between the developer biases and a 
complementary image (either developed or latent) on the charge retentive 
surface. The colored around the black image results from the field 
established due to the difference (V.sub.bb -V.sub.cb, see FIG. 1b) 
between the developer biases as the black images passes through the red 
developer housing while the black border around the red image results from 
the field established due to the difference (V.sub.bb -V.sub.c) between 
the bias on the black developer housing and the voltage level of the red 
latent image on the charge retentive surface as that image passes through 
the black developer housing. 
Since the use of IMB developer has been found to be unacceptable in a 
tri-level highlight system for the reason noted, and since CM developer as 
disclosed in the prior art cannot develop line images in the presence of 
relatively high cleaning fields it would appear that tri-level highlight 
color imaging in a single pass is not viable. 
BRIEF SUMMARY OF THE INVENTION 
We have discovered that source of the failure of CMB developers in 
developing line images is attributable to certain properties of 
conventional developer materials as well as other aspects of the basic 
xerographic process. Thus, the development of optically (as opposed to 
electronically) formed images in ahighlight color system using CMB 
developer yields images that are not totally acceptable. Optical image 
formation systems typically possess a large degree of flare 
(non-image-forming light) which makes it difficult to develop fine lines 
using CMB developer, particularly when the smaller dimension of the image 
moves in the process direction. This, we found, is because the conductive 
developer, in the presence of large cleaning fields, moves toward the 
developer system, i.e. away from the imaging surface. Therefore, there 
isn't sufficient time for the toner to travel back to the imaging surface 
to adequately develop the line images. Even when electronically formed 
images have been used in highlight color systems, the resulting developed 
images were still not optimally formed. We discovered that the effects of 
adverse cleaning fields could be obviated by modifying the toner 
concentrations of conventional xerographic developers, which are too small 
(i.e. 1.0% by weight or less), and by modifying the charge levels of 
conventional xerographic developers, which are too high (i.e. 25 to 30 
microcoulombs/gram), and further by spacing the developer rolls from the 
charge retentive surface at a distance in the order of 0.040 to 0.120 
inch. 
Briefly, the present invention uses a magnetic brush developer apparatus 
comprising a plurality of developer housings each including a plurality of 
magnetic rolls associated therewith. Conductive magnetic brush (CMB) 
developer is provided in each of the developer housings. The CMB developer 
is used to develop electronically formed images. The developer 
conductivity, as measured in a Gutman conductivity cell, is in the range 
of 10-.sup.9 to 10-.sup.13 (ohm-cm)-.sup.1. The toner concentration of the 
developer is 2.0 to 3.0% by weight and the charge level is less than 20 
microcoulombs/gram. Additionally, the developer rolls are spaced from the 
charge retentive surface a distance in the order of 0.040 to 0.120 inch.

DETAILED DESCRIPTION OF THE PREFERRED 
EMBODIMENT OF THE INVENTION 
For a better understanding of the concept tri-level imaging, a description 
thereof will now be made with reference to FIGS. 1a and 1b. FIG. 1a 
illustrates the tri-level electrostatic latent image in more detail. Here 
V.sub.o is the initial charge level, V.sub.ddp the dark discharge 
potential (unexposed), V.sub.w the white discharge level and V.sub.c the 
photoreceptor residual potential (full exposure). 
Color discrimination in the development of the electrostatic latent image 
is achieved by passing the photoreceptor through two developer housings in 
tandem which housings are electrically biased to voltages which are offset 
from the background voltage V.sub.w, the direction of offset depending on 
the polarity or sign of toner in the housing. One housing (for the sake of 
illustration, the first) contains developer with black toner having 
triboelectric properties such that the toner is driven to the most highly 
charged (V.sub.ddp) areas of the latent image by the electric field 
between the photoreceptor and the development rolls biased at V.sub.bb (V 
black bias) as shown in FIG. 1b. Conversely, the triboelectric charge on 
the colored toner in the second housing is chosen so that the toner is 
urged towards parts of the latent image at residual potential, V.sub.c by 
the electric field existing between the photoreceptor and the development 
rolls in the second housing at bias voltage V.sub.cb (V color bias). 
As shown in FIG. 2, a printing machine incorporating our invention may 
utilize a charge retentive member in the form of a photoconductive belt 10 
consisting of a photoconductive surface and an electrically conductive 
substrate and mounted for movement past a charging station A, an exposure 
station B, developer stations C, transfer station D and cleaning station 
F. Belt 10 moves in the direction of arrow 16 to advance successive 
portions thereof sequentially through the various processing stations 
disposed about the path of movement thereof. Belt 10 is entrained about a 
plurality of rollers 18, 20 and 22, the former of which can be used as a 
drive roller and the latter of which can be used to provide suitable 
tensioning of the photoreceptor belt 10. Motor 23 rotates roller 18 to 
advance belt 10 in the direction of arrow 16. Roller 18 is coupled to 
motor 23 by suitable means such as a belt drive. 
As can be seen by further reference to FIG. 2, initially successive 
portions of belt 10 pass through charging station A. At charging station 
A, a corona discharge device such as a scorotton, corotron or dicorotron 
indicated generally by the reference numeral 24, charges the belt 10 to a 
selectively high uniform positive or negative potential, V.sub.o. 
Preferably charging is negative. Any suitable control, well known in the 
art, may be employed for controlling the corona discharge device 24. 
Next, the charged portions of the photoreceptor surface are advanced 
through exposure station B. At exposure station B, the uniformly charged 
photoreceptor or charge retentive surface 10 is exposed to a laser based 
input and/or output scanning device 25 which causes the charged retentive 
surface to be discharged in accordance with the output from the scanning 
device. Preferably the scanning device is a three level laser Raster 
Output Scanner (ROS). The ROS output is set via a programmable power 
supply 26 which driven by means of a controller 27 via a digital to analog 
converter 28. Alternatively, the ROS could be replaced by a conventional 
xerographic exposure device. 
The photoreceptor, which is initially charged to a voltage V.sub.o, 
undergoes dark decay to a level V.sub.ddp. When exposed at the exposure 
station B it is discharged to V.sub.w imagewise in the background (white) 
image areas and to V.sub.c which is near zero or ground potential in the 
highlight (i.e. color other than black) color parts of the image. See FIG. 
1a. 
At development station C, a magnetic brush development system, indicated 
generally by the reference numeral 30 advances developer materials into 
contact with the electrostatic latent images. The development system 30 
comprises first and second developer housings 32 and 34. Preferably, each 
magnetic brush development housing includes a pair of magnetic brush 
developer rollers. Thus, the housing 32 contains a pair of rollers 35, 36 
while the housing 34 contains a pair of magnetic brush rollers 37, 38. 
Each pair of rollers advances its respective developer material into 
contact with the latent image. Appropriate developer biasing is 
accomplished via power supplies 41 and 43 electrically connected to 
respective developer housings 32 and 34. 
Color discrimination in the development of the electrostatic latent image 
is achieved by passing the photoreceptor past the two developer housings 
32 and 34 in a single pass with the magnetic brush rolls 35, 36, 37 and 38 
electrically biased to voltages which are offset from the background 
voltage V.sub.w, the direction of offset depending on the polarity of 
toner in the housing. One housing e.g. 32 (for the sake of illustration, 
the first) contains developer with black toner 40 having triboelectric 
properties such that the toner is driven to the most highly charged 
(V.sub.ddp) areas of the latent image by the electrostatic field 
(development field) between the photoreceptor and the development rolls 
biased at V.sub.bb as shown in FIG. 1b. Conversely, the triboelectric 
charge on colored toner 42 in the second housing is chosen so that the 
toner is urged towards parts of the latent image at residual potential, 
V.sub.c by the electrostatic field (development field) existing between 
the photoreceptor and the development rolls in the second housing at bias 
voltages V.sub.cb. 
In an operative embodiment of the invention, good quality images including 
line images were produced using developers 40 and 42 which comprise 
cnductive magnetic brush (CMB) developer material with a conductivity in 
the range of 10-.sup.13 (ohm-cm)-.sup.1. These developers comprise an 
insulative toner and a conductive carrier, the conductivity of the carrier 
being in the order of 10-.sup.9 to 10-.sup.10 (ohm-cm)-.sup.1. The toner 
concentration of the developers 40 and 42 is in the order of 2.0to 3.0% by 
the weight and the charge level is less than 20 microcoulombs/gram. The 
developer rolls spaced from the charge retentive surface in the order of 
0.040 to 0.120 inch. 
In tri-level xerography, the entire photoreceptor voltage difference 
(V.sub.ddp -V.sub.c, as shown in FIG. 1a) is shared equally between the 
charged area development (CAD) and the discharged area development (DAD). 
This corresponds to approximately 800 volts (if a realistic photoreceptor 
value for V.sub.ddp of 900volts and a residual discharge voltage of 100 
volts are assumed). Allowing an additional 100 volts for the cleaning 
fields (V.sub.bb -V.sub.white and V.sub.white -V.sub.cb) in each 
development housing means an actual development contrast voltage for CAD 
of approximately 300 volts and an approximately equal amount for DAD. In 
the foregoing case the 300 volts of contrast voltage is provided by 
electrically biasing the first developer housing to a voltage level of 
approximately 600 volts and the second developer housing to a voltage 
level of 400 volts. 
A sheet of support material 58 is moved into contact with the toner image 
at transfer station D. The sheet of support material is advanced to 
transffer station D by conventional sheet feeding apparatus, not shown. 
Preferably, sheet feeding apparatus includes a feed roll contacting the 
uppermost sheet of a stack of copy sheets. Feed rolls rotate so as to 
advance the uppermost sheet from the stack into a chute which directs the 
advancing sheet of support material into contact with photoconductive 
surface of belt 10 in a timed sequence so that the toner powder image 
developed thereon contacts the advancing sheet of support material at 
transfer station D. 
Because the composite image developed on the photoreceptor consists of both 
positive and negative toner, a pre-transfer corona discharge member 56 is 
provided to condition the toner for effective transfer to a substrate 
using corona discharge. 
Transfer station D includes a corona generating device 60 which sprays ions 
of a suitable polarity onto the backside of sheet 58. This attracts the 
charged toner powder images from the belt 10 to sheet 58. After transfer, 
the sheet continues to move, in the direction of arrow 62, onto a conveyor 
(not shown) which advances the sheet to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 64, which permanently affixes the transferred powder 
image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser 
roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 
and backup roller 68 with the toner powder image contacting fuser roller 
66. In this manner, the toner powder image is permanently affixed to sheet 
58. After fusing, a chute, not shown, guides the advancing sheet 58 to a 
catch tray, also not shown, for subsequent removal from the printing 
machine by the operator. 
After the sheet of support material is separated from photoconductive 
surface of belt 10, the residual toner particles carried by the non-image 
areas on the photoconductive surface are removed therefrom. These 
particles are removed at cleaning station F. 
Subsequent to cleaning, a discharge lamp (not shown) floods the 
photoconductive surface with light to dissipate any residual electrostatic 
charge remaining prior to the charging thereof for the successive imaging 
cycle. 
The magnetic brush rolls 35 and 36 may comprise any conventional structures 
known in the art that provides a magnetic field that forms the developer 
material in the housing 32 into a brush-like configuration in the 
development zone between the rolls 35 and 36 and the charge retentive 
surface. This arrangement effects development of one of the two tri-level 
images contained on the charge retentive surface in a well known manner. 
The magnetic brush rolls 37 and 38 on the other hand are constructed such 
that development of the other of the two tri-level image is accomplished 
with minimal disturbance of the first image. To this end, the magnetic 
rolls 37 and 38 comprise magnetic force fields as depicted in FIGS. 3a and 
3b, respectively. As shown therein, the radial force profiles of these two 
rolls are such as to cause developer to be picked up from the developer 
housing 32 and conveyed to the top of the roll 37 where the developer 
becomes magnetically unconstrained. The developer is moved through the 
development zone in a magnetically unconstrained manner until it is 
attracted to the roll 38 due to the radial magnetic forces of that roll. 
Magnetic poles are designated N (north) or S (south). 
As illustrated in the drawings, the magnetic fields are plotted around the 
central axis of a two-roll magnetic brush development system such as the 
one comprising rolls 37, 38. For a multiple rolls development system 
comprising more than two rolls, roll 38 is replicated. The rolls are 
driven synchronously in this example, although it is also possible to have 
independent drive mechanisms for each roller. 
FIG. 3 depicts the radial components, respectively, of rolls 37 and 38. As 
illustrated in the drawing, the magnetic fields are plotted around the 
central axis of a two-roll magnetic brush development system such as the 
one comprising rolls 37, 38. For a multiple roll development system 
comprising more than two rolls, roll 38 is replicated. 
The development system additionally consists of a sump, or reservoir, of 
magnetic developer material, and optionally a mixing system, paddle wheel, 
or other apparatus to maintain the developing properties of the material 
in the sump. The developer rolls are rotating non-magnetic cylinders or 
shells having roughened or longitudinally corrugated surfaces to urge the 
developer along by frictional forces around fixed internal magnets. The 
shells are driven synchronously in this example; it is also possible to 
have independent drive mechanisms for each roller. 
During the development process of the system, the direction of rotation of 
the shell around either fixed magnet is counterclockwise. However, the 
system can also be configured to develop in the clockwise direction with 
no compromise in performance, depending on the desired properties of the 
development system with respect to the direction of the photoreceptor 
(i.e., against-mode or with-mode development). 
In the case described, the photoreceptor is located above the development 
rolls. The developer materials are transported from left to right from the 
sump to roll 37, to Roll 38, back to the sump. 
A broad radial pole 80 of roll 37 (FIG. 3) positioned at 6 o'clock serves 
to lift magnetic developer material from a donor roll in the sump or 
housing 32. The combination of tangential and radial fields starting with 
pole 84 transport the developer material along the surface of the 
developer roll until about the 11 o'clock position of roll 37. At that 
point, the developer becomes magnetically unconstrained due to the lack of 
poles or strong poles in this area to constrain the developer in a 
brush-like configuration. 
The developer is moved magnetically unconstrained through the part of the 
development zone delineated by the roll 37 and the charge retentive 
surface until the developer comes under the influence of a strong radial 
south pole 86 of the magnetic 38. Movement through the aforementioned zone 
is effected through the cooperation of the charge retentive surface and 
the developer shell. The pole 86 serves to effect transition of the 
developer from the roll 37 to the roll 38 without magnetically 
constraining the developer so as to cause scavenging of the first image as 
it passes the second developer housing. As will be observed, the poles 
following the pole 86 in the clockwise direction are progressively weaker 
so that the development zone delineated by the roll 38 and the charge 
retentive surface. 
In view of the foregoing description it should now be apparent that there 
has been provided an image forming method and apparatus which forms 
tri-level images that are fringe-free and possess a high fidelity (i.e. a 
faithful reproduction of the original image) even when optically formed.