Detecting and correcting for low developed mass per unit area

A method of maintaining consistent large solid area development by developing a large area test patch covering the image area of a photorececptor and detecting the lead edge and trail edge density of the test patch using a densitometer to measure reflectance and comparing the lead and trail edge density to the lead and trail edge density of a reference patch. The reference patch is generated after the changing of developer elements such as developer material and photoreceptor and an electrostatic set up performed. Alternatively a density differential between lead and trail edge density of the test patch is detected. Electrostatic parameters such as preset toner concentration control values and decreasing the development field of the test patch are adjusted to maintain constant large solid area development.

BACKGROUND OF THE INVENTION 
The invention relates to optimization of the xerographic process, and more 
particularly, to the automatic compensation for toner concentration drift 
due to developer aging. 
One benchmark in the suitable development of a latent electrostatic image 
on a photoreceptor by toner particles is the correct toner concentration 
in the developer. An incorrect concentration, i.e. too much toner 
concentration, can result in too much background in the developed image. 
That is, the white background of an image becoming gray. On the other 
hand, too little toner concentration can result in deletions or lack of 
toner coverage of the image. Generally, the aging characteristics of 
developer material is that toner concentration decreases over time. As 
toner concentration decreases, solid areas become lighter. 
Under prior art process controls, a relatively small toner control patch is 
developed and sensed to adjust the development process to maintain the 
quality of developed small solid areas. 
Specifically, many machines (both copiers and printers) use optical feed 
back from toner patches to control DMA (developed mass per unit area). The 
toner patch is developed to a partially discharged region of a 
photoreceptor. In toner patch based DMA control systems, the patch voltage 
is held constant. The controller attempts to keep the reflectance of the 
toner patch in range using the toner dispenser as an actuator. When the 
toner patch reflectance is high (the patch is too light) toner is added. 
The assumption is that if toner is added in such a way that the toner 
patch reflectance is kept at its target value then the DMA of the printed 
foreground will be kept at its target. Toner patches are developed to 
partially discharged belt areas because patches developed to fully charged 
areas would be saturated black and their reflectance would have 
insufficient sensitivity to DMA to control toner concentration TC. To 
create a toner patch on a printer a small region of the photoreceptor is 
initially left unexposed (fully charged). A special discharge lamp is then 
used to reduce its surface potential to a target value a fixed number of 
volts above the developer bias. Toner is then developed to the patch and 
its reflectance is read by the optical sensor. As the toner patch gets 
developed toner is deposited on it until the development field is 
sufficiently neutralized. With highly charged toner, less toner will be 
developed to the patch and its reflectance will be below target causing 
toner to be added. With lower charged toner the opposite occurs. 
As developer ages, its charging properties degrade and progressively lower 
toner concentrations are required to keep the toner patch reflectance at 
target. With some developer, the toner concentration gets set sufficiently 
low after as little as 30,000 prints that foreground solids can not be 
properly rendered. In this case sufficient toner is available to keep the 
low density toner patch at target but not enough toner is available to 
render the more demanding foreground solid areas. 
An example of the prior art is, U.S. Pat. No. 4,999,673, assigned to the 
same assignee as the present invention, disclosing the use of a relatively 
small developed half tone image patch to regulate the developer 
parameters. However, these prior art small patch process controls are 
generally inadequate and insensitive to detect large solid area 
development deterioration as discussed above. It would be desirable, 
therefore, to provide a process control technique to detect deterioration 
in large, solid area development. It is also known in the prior art to use 
an electro-optic sensor or any other suitable sensor in the developer 
housing to determine toner concentration. The use of a sensor in the 
housing in addition to the IRD sensor normally used in adjusting 
development, however, adds additional cost and complexity to the system. 
It would also be desirable, therefore, to minimize additional cost and 
complexity in a developer control system that is capable of responding to 
large solid area development deterioration to maintain toner concentration 
and developed mass at a constant level throughout the life of the 
developer. 
It is an object of the present invention therefore to provide a new an 
improved technique to detect deterioration in large, solid area 
development. It is another object of the present invention to minimize 
additional cost and complexity in a developer control system that is 
capable of responding to large solid area development deterioration to 
maintain toner concentration and developed mass at a constant level. Other 
advantages of the present invention will become apparent as the following 
description proceeds, and the features characterizing the invention will 
be pointed out with particularity in the claims annexed to and forming a 
part of this specification. 
SUMMARY OF THE INVENTION 
The present invention is concerned with a method of maintaining consistent 
large solid area development by developing a large area test patch 
covering the image area of a photoreceptor and detecting the lead edge and 
trail edge density of the test patch using a densitometer to measure 
reflectance and comparing the lead and trail edge density to the lead and 
trail edge density of a reference patch. The reference patch is generated 
after the changing of developer elements such as developer material and 
photoreceptor and an electrostatic set up performed. Alternatively a 
density differential between lead and trail edge density of the test patch 
is detected. Electrostatic parameters such as preset toner concentration 
control values and decreasing the development field of the test patch are 
adjusted to maintain constant large solid area development. Communication 
with a remote station regarding adjustment or need for adjustment is also 
contemplated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 schematically depicts an electrophotographic printing machine 
incorporating the features of the present invention therein. It will 
become evident from the following discussion that the present invention 
may be employed in a wide variety of applications such as light lens 
machines, laser printers, and any other suitable digital copier, and is 
not specifically limited in its application to the particular embodiment 
depicted herein. 
Referring to FIG. 1 of the drawings, the electrophotographic printing 
machine employs a photoconductive belt 10. Belt 10 moves in the direction 
of arrow 12 to advance successive portions of the photoconductive surface 
sequentially through the various processing stations disposed about the 
path of movement thereof. Belt 10 is entrained about stripping roller 14, 
tensioning roller 16, and drive roller 18. At charging station A, a corona 
generating device, indicated generally by the reference numeral 20, 
charges the photoconductive belt 10 to a relatively high, substantially 
uniform potential. Corona generating device 20 includes a generally 
U-shaped shield and a charging electrode. A high voltage power supply 22 
is coupled to the shield A change in the output of power supply 22 causes 
corona generating device 20 to vary the charge applied to the 
photoconductive belt 10. Charging station A may be one of the processing 
stations regulated by the control system depicted in FIG. 2. 
Next, the charged portion of the photoconductive surface is advanced 
through imaging station B. At imaging station B, an original document 24 
is positioned face down upon a transparent platen 26. Imaging of a 
document is achieved by lamps 28 which illuminate the document on platen 
26. Light rays reflected from the document are transmitted through lens 
30. Lens 30 focuses the light image of the original document onto the 
charged portion of photoconductive belt 10 to selectively dissipate the 
charge thereon. This records an electrostatic latent image on the 
photoconductive belt which corresponds to the informational areas 
contained within the original document. 
Imaging station B includes a test area generator, indicated generally by 
the reference numeral 32. Test generator 32 comprises a light source 
projecting light rays onto the charged portion of photoconductive belt 10, 
in the interimage region, i.e. between successive electrostatic latent 
images recorded on photoconductive belt 10. A test patch is recorded on 
photoconductive belt 10 typically a one inch square as shown at 88 in FIG. 
3 or any other suitable patch size. The electrostatic latent image and 
test patch are then developed with toner particles at development station 
C. In this way, a toner powder image and a developed test patch is formed 
on photoconductive belt 10. The developed test patch is subsequently 
examined to determine the quality of the toner image being developed on 
the photoconductive belt. 
At development station C, a magnetic brush development system, indicated 
generally by the reference numeral 34, advances a developer material into 
contact with the elctrostatic latent image and test patch recorded on 
photoconductive belt 10. Preferably, magnetic brush development system 34 
includes two magnetic brush developer rollers 36 and 38. These rollers 
each advance the developer material into contact with the latent image and 
test areas. Each developer roller forms a brush comprising carrier 
granules and toner particles. The latent image and test patch attract the 
toner particles from the carrier granules forming a toner powder image on 
the latent image and a developed test patch. As toner particles are 
depleted from the developer material, a toner particle dispenser, 
indicated generally by the reference numeral 40, furnishes additional 
toner particles to housing 42 for subsequent use by developer rollers 36 
and 38, respectively. Toner dispenser 40 includes a container 44 storing a 
supply of toner particles therein. A foam roller 46 disposed in sump 48 
coupled to container 44 dispenses toner particles into an auger 50. Auger 
50 is made from a helical spring mounted in a tube having a plurality of 
apertures therein. Motor 52 rotates the helical spring to advance the 
toner particles through the tube so that toner particles are dispensed 
from the apertures therein. This process station may also be controlled by 
the control system regulating the energization of motor 52. 
A densitometer 54, positioned adjacent the photoconductive belt between 
developer station C and transfer station D, generates electrical signals 
proportional to the developed test patch reflectance. These signals are 
conveyed to a control system and suitably processed for regulating the 
processing stations of the printing machine. Further details of the 
control system are shown in FIG. 2. Preferably, densitometer 54 is an 
infrared densitometer, energized at 15 volts DC and about 50 milliamps. 
The surface of the infrared densitometer is about 7 millimeters from the 
surface of photoconductive belt 10. Densitometer 54 includes a 
semiconductor light emitting diode typically having a 940 nanometer peak 
output wavelength with a 60 nanometer one-half power bandwidth. The power 
output is approximately 45 milliwatts. A photodiode receives the light 
rays reflected from the developed half tone test patch and converts the 
measured light ray input to an electrical output signal. The infrared 
densitometer is also used to periodically measure the light rays reflected 
from the bare photoconductive surface, i.e. without developed toner 
particles, to provide a reference level for calculation of a suitable 
signal ratio. After development the toner powder image is advanced to 
transfer station D. 
At transfer station D, a copy sheet 56 is moved into contact with the toner 
powder image. The copy sheet is advanced to transfer station D by a sheet 
feeding apparatus 60. Preferably, sheet feeding apparatus 60 includes a 
feed roll 62 contacting the uppermost sheet of a stack 64 of sheets. Feed 
rolls 62 rotate so as to advance the uppermost sheet from stack 64 into 
chute. Chute guides the advancing sheet from stack 64 into contact with 
the photoconductive belt in a timed sequence so that the toner powder 
image developed thereon contacts the advancing sheet at transfer station 
D. At transfer station D, a corona generating device 58 sprays ions onto 
the backside of sheet 56. This attracts the toner powder image from 
photoconductive belt 10 to copy sheet 56. After transfer, the copy sheet 
is separated form belt 10 and a conveyor advances the copy sheet, in the 
direction of arrow 66, to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 68 which permanently affixes the transferred toner 
powder image to the copy sheet. Preferably, fuser assembly 68 includes a 
heated fuser roller 70 and a pressure roller 72 with the powder image on 
the copy sheet contacting fuser roller 70. In this manner, the toner 
powder image is permanently affixed to sheet 56. After fusing, chute 74 
guides the advancing sheet 56 to catch tray 76 for subsequent removal from 
the printing machine by the operator. 
After the copy sheet is separated from photoconductive belt 10, the 
residual toner particles and the toner particles adhering to the test 
patch are cleaned from photoconductive belt 10. These particles are 
removed from photoconductive belt 10 at cleaning station F. Cleaning 
station F includes a rotatably mounted fiberous brush 78 in contact with 
photoconductive belt 10. The particles are cleaned from photoconductive 
belt 10 by the rotation of brush 78. Subsequent to cleaning, a discharge 
lamp (not shown) floods photoconductive belt 10 with light to dissipate 
any residual electrostatic charge remaining thereon prior to the charging 
thereof for the next successive imaging cycle. 
As illustrated in FIG. 2, infrared densitometer 54 detects the density of 
the developed test patch and produces an electrical output signal 
indicative thereof. The electrical signal produced by the infrared 
densitometer is proportional to the reflected light intensity which is 
related to the change in density. 
In addition, an electrical output signal is periodically generated by 
infrared densitometer 54 corresponding to the bare or undeveloped 
photoconductive surface. These signals are conveyed to controller 80 
through suitable conversion circuitry 82. Controller 80 forms the ratio of 
the developed test patch signal/bare photoconductive surface signal and 
generates electrical error signals proportional thereto. The error signal 
is transmitted to logic interface 84 which processes the error signal so 
that it controls the respective processing station 86. For example, if the 
charging station is the processing station being controlled, the logic 
interface transmits the error signal in the appropriate form to the high 
voltage power supply to regulate charging of the photoconductive surface. 
When toner concentration is being controlled, motor 52 (FIG. 1) is 
energized causing toner dispenser 40 to discharge toner particles into 
developer housing 42. This increases the concentration of toner particles 
in the developer mixture. During operation of the electrophotographic 
printing machine, any of the selected processing stations can be 
simultaneously controlled by the control loop depicted in FIG. 2. For 
example, in addition to controlling charging and toner concentration, the 
electrical bias applied to the developer roller may also be regulated. By 
regulating a plurality of processing stations, larger variations from the 
nominal conditions and faster returns to the nominal conditions are 
possible. Thus, the various printing machine processing stations have 
wider latitude. 
Referring now to FIG. 3, there is shown test patch 88 recorded in the 
interimage region of photoconductive belt 10. At the development station, 
the test patch is developed and infrared densitometer 54 (FIG. 2) detects 
the density of the developed test patch and generates an electrical 
signal. It has been discovered in accordance with the present invention 
that by the periodic development of a test patch illustrated at 94, in the 
image area 90 of the photoreceptor, sufficient data can be acquired to not 
only sense the large solid area deterioration previously undetectable, but 
appropriate adjustments can be made as described below. In particular, 
initially a reference patch similar to test patch 94 as shown is developed 
and sensor 54 detects the lead edge and trail edge density of the 
reference patch. 
The reference patch is generated after the changing of developer elements 
such as developer material and photoreceptor and an electrostatic set up 
performed. Reference data for the lead edge and trail edge density of the 
reference patch is stored, for example in memory 80A in FIG. 2, for later 
comparison with the lead and trail edge density of the test patches 94. 
The use of the reference data compensates for irregularities and 
differences in machine components and systems. 
With reference to FIG. 4, after the corona generating device 20 charges the 
photoconductive belt 10 to a relatively high substantially uniform 
potential, a document is illuminated by lamps 28. Light rays reflected 
from the document focus the light image of the original document onto the 
charged portion of the photoconductive belt to selectively dissipate the 
charge. The dark areas or image areas of the document reflect less light 
and therefore dissipate the charge on the photoconductor less than the 
white or background portions of the document which reflect a large 
proportion of the light to significantly dissipate the charge on the 
photoconductive belt. As a result, for example, in one particular 
embodiment, the charge on the photoconductive belt 10, representing the 
white or background areas is dissipated to a minus 100 volts and the 
portion on the photoconductive belt representing the black portions of the 
document are dissipated to a minus 580 volts as illustrated by 102 and 104 
respectively. 
With a given bias on the developer rolls of a minus 210 volts as 
illustrated at 106, this results in a field of 370 volts, referred to as 
the development or image field illustrated at 108 between the image 
portions of the document and a field of 110 volts illustrated at 109 
between the developer roll and the photoreceptor belt for the white or 
background areas, referred to as the background field. Due to degradation 
of component parts of the Xerographic system such as the photoconductive 
belt and the developer system, various voltages such as the image voltage 
(-580), bias voltage (-210V), and background voltage (-100V) are subject 
to change which in turn alter the development field 108 and background 
field 109. 
It has been discovered that the density differential from the lead edge to 
the trail edge of a relatively large solid areas is measurable as the 
toner concentration decreases. Under normal process controls, the toner 
control patch is too small or insufficiently dense to be sensitive to this 
differential. In accordance with the present invention, with reference to 
FIG. 3, the density of the lead edge 94 of the large solid area developed 
patch 92, is determined as well as the density of the trail edge 96. By 
comparing the lead and trail edge signals to reference lead and trail edge 
signals, predetermined electrostatic parameters can be adjusted to 
compensate for large solid area toner concentration decreases. 
Alternatively, only a density differential between lead and trail edge 
density of the test patch is detected and the test patch differential used 
to compensate for large solid area toner concentration decreases. 
In a preferred embodiment, at predetermined intervals, a dead cycle is 
initiated and the normal toner dispense suspended. This dead cycle for 
example, can be initiated at power on or after the production of a given 
number of copies or based upon time use of the machine. The patch 
generator 32 that normally provides the relatively small test patch 88 in 
the photoreceptor space between images 90, is used to project a large test 
patch 92 in the image area 90. Preferably, the control patch is one pitch 
long or the normal image cycle. This is the time period equivalent to the 
area of the photoreceptor to project and develop a single image. 
The patch generator discharges the voltage at the lead and trail edge of 
the image to a lower level as is well known in order that the developed 
image will be in the active sense region of the infrared densitometer. The 
middle of the image remains at a nominal voltage level to simulate a long 
solid area. Once the image has been generated by the patch generator 32, 
and developed, the electrostatic volt meter 33 as illustrated in FIG. 1, 
measures the background voltage on the photoreceptor. That is, after the 
projection of the image, the potential on the photoreceptor belt 
corresponding to the white or background areas of the document is 
measured. This is the background voltage 102 as illustrated in FIG. 4. The 
image of the patch 92 is developed and the lead and trail edge densities 
are measured by the densitometer. 
Generally, in reproduction machines an electrostatic set up is performed as 
part of a periodic maintenance. The periodic maintenance could include the 
replacement of the developer unit or developer materials, changing of the 
photoreceptor, or any other necessary maintenance procedures. As part of 
the maintenance, the system is set for a suitable quality product. At this 
time, in accordance with the present invention, an elongated reference 
patch in the image area, such as illustrated at 94 in FIG. 3, is developed 
and the developed reference patch is monitored by the densitometer 54. 
Electrical signals proportional to the developed reference patch are 
generated at the leading edge 96 and the trailing edge 92 of the reference 
patch. Representations of these signals at the leading edge and trailing 
edge of the reference patch are stored at any suitable memory such as 
memory 80A of controller 80 shown in FIG. 2. These signals are stored and 
used as a lead edge and trail edge reference against the signals produced 
by test patches periodically developed during the operation of the 
machine. 
For any given machine, lead and trail edge signals from developed test 
patches are compared with the reference lead and trail edge signals stored 
representations stored in memory at suitable intervals. The interval may 
vary from machine to machine, for example, at the completion of a set 
number of copies. For example, in high volume machines the interval could 
be after the reproduction of 500,000 copies or in much lower volume 
machines as often as after the completion of 20,000 copies. At any rate, 
at the proper time a long test patch is generated and developed in the 
image area of the photoreceptor and the lead and trail edges of the test 
patch are sensed by desitometor 54. 
The lead edge of the sensed test patch is then compared to the lead edge 
reference signal stored in memory. If the ratio is approximately 1.0, 
there is no substantial change or difference between the recently measured 
lead edge of a test patch and the reference lead edge stored in memory. It 
should be noted that the range or value to determine the substantial or 
significant difference between measured values is a design choice and can 
vary dependent upon design considerations and machine environment. If the 
ratio is approximately 1.0, then the trail edge of the recently measured 
test patch is compared with the trail edge reference signal stored in 
memory. If the ratio of the trail edge signals is approximately 1.0, then 
there is no significant change in the developability in the system. 
However, if the ratio of trail edge signals significantly differs from 
1.0, then a suitable adjustment is made in the development system. 
Likewise, if the ratio of the leading edge signals differs significantly 
from 1.0, a suitable change will be made to the development system. It 
should also be noted that the change to the development system based upon 
lead edge or trail edge difference is also a design choice. It should be 
further noted that in the alternative, changes to the development system 
can be made based upon the test patch lead to trail edge differential 
rather than comparing to reference signals. 
In general, a significant deviation or variation of measured test patch 
signals from the reference patch signals means there is not enough toner 
in the toner sump. This requires an adjustment to increase toner 
concentration. Any suitable method can be used to increase toner 
concentration. For example, depending upon the relationship of the test 
lead and trail edges with the reference lead and trail edges, a control 
value can be changed. That is, a digital or bit value stored in memory 
used as a reference for toner concentration can be changed to indicate a 
new level of toner concentration. Alternatively, there could be a decrease 
in the development field of the test patch. 
It should be noted that all sample solid areas, preferably, are imaged on 
dead cycle frames and would thus not be printed on paper. Upon the 
detection of a solids rendering problem as indicated by a comparison of 
the lead and trail edge signals, the xerographic set up or system can be 
adjusted to compensate or correct for the problem. Another option is 
simply to declare a fault in systems which are not able to automatically 
internally correct the fault. Another alternative is to transmit 
representations of the sensed lead and trail edged signals via modems 110 
to a remote service station illustrated at 112 in FIG. 2. At remote 
service station 112, automatic corrections could be determined and 
suitable data on instructions transmitted from remote service station 112 
to modems 110 to controller 80 to provide suitable corrective action. 
The remote service station 112 could include an expert system 114 as 
illustrated in FIG. 5, including a Knowledge Base 202 having a set of 
rules embodying an expert's knowledge about the operation, diagnosis, and 
correction of the machine, an Inference Engine 204 to efficiently apply 
the rules of the Knowledge Base 202 to solve machine problems, an Operator 
Interface 206 to communicate between the operator and the Expert System, 
and Rule Editor 208 to assist in modifying the Knowledge Base 202. In 
operation, the Inference Engine 204 applies the Knowledge Base 202 rules 
to solve machine problems, compares the rules to data entered by the user 
about the problem, tracks the status of the hypothesis being tested and 
hypotheses that have been confirmed or rejected, asks questions to obtain 
needed data, states conclusions to the user, and even explains the chain 
of reasoning used to reach a conclusion. The function of the Operator 
Interface is to provide dialogue 210, that is ask questions, request data, 
and state conclusions in a natural language and translate the operator 
input into computer language. 
An essential element of the Expert System is the dialogue feature 209 to 
enable the Expert System to proceed with analysis upon receipt of 
additional data from an operator or tech rep. The Expert System includes 
memory with a profile of expected machine performance and parameters 
portion, a current switch and sensor information portion, and a table of 
historical machine performance and utilization events. The system monitors 
status conditions and initiates external communication relative to the 
status conditions of the machine. This procedure includes the steps of 
monitoring the predetermined status conditions relative to the operation 
of the machine, recognizing the deviation of the machine operation from 
said predetermined status conditions, recognizing the inability of the 
machine to automatically respond to the deviation to self correct, and, 
determining the need for external response to provide additional 
information for evaluation for further analysis. It should be noted that 
the use of the remote station and expert system is merely an alternative 
corrective procedure. For further details, reference is made to U.S. Pat. 
No. 5,138,377 and 5,057,866 incorporated herein. 
As stated above, the development field (the voltage of the image on the 
photoreceptor less the developer bias voltage) and toner patch development 
field (the voltage of the toner patch less the voltage of the developer 
bias) could be lowered by a common factor to correct the developer to 
maintain consistent solid area development. Lowering the toner patch 
development field would cause the control system to raise the toner 
concentration, eliminating the toner supply problem and thus the solids 
rendering problem. The increase in toner concentration increases 
developability. Lowering the development field prevents the developed mass 
per unit area from becoming excessive as a result of the developability 
increase. 
With reference to FIG. 6, block 132 represents the completion of an 
electrostatic set up for a machine. The electronic set up, generally 
follows the changing of key components in the machine such as new 
developer material for the replacement of the photoreceptor. Upon making 
these type of changes, the tech rep adjusts the machine to a given quality 
level setting key parameters such as charging voltage and developer bias. 
After completion of the electrostatic set up, a reference patch is 
developed in the image area of the photoreceptor. The reference patch is a 
generally elongated patch that will be the standard or reference for large 
area solid development of the machine. The developed reference patch is 
sensed and signals representing the lead edge and trail edge of the 
elongated patch are stored in memory as illustrated at 136. 
Block 138 illustrates the development of periodic test patches to be 
compared to the reference patch. A typical test patch is illustrated at 94 
in FIG. 3 which is also similar to the test patch used to obtain the 
reference readings shown at block 136. At block 140, the lead edge and 
trail edges 96 and 92 of the test patch are sensed. As discussed above, 
the routine or periodic development and sensing of a test patch varies 
with the type machine as well as the machine environment. 
It should be understood that once the lead and trail edge measurements for 
the test patch have been obtained, the scope of the present invention 
covers any suitable method to compare the sample test readings with the 
reference readings and to make appropriate corrections to the developer 
system. In one embodiment, as illustrated at decision block 142, the ratio 
of the reference lead edge signal to the test patch lead edge signal is 
determined. If the ratio is not substantially one, that is, there is a 
significant or substantial difference between the lead edge signal of the 
reference patch to the lead edge signal from the test patch, then a 
developer adjustment is made as shown at block 144. The adjustment can be 
any suitable adjustment such as changing a quality bit or indicator stored 
in memory or to decrease the development field for the development of the 
test patch or to proportionally adjust the patch development and solid 
development fields. It should be understood that any suitable adjustment 
is contemplated for a particular development system and related 
electrostatic fields that compensates for the developer system aging. If, 
on the other hand, the ratio of the lead edge signals of the test and 
reference patches are substantially equal, then there is a determination 
of the ratio of the trail edge signals from the test patch and reference 
patch as shown at block 146. If there is a significant difference, then a 
developer adjustment is made as shown at block 148. The adjustment made as 
indicated at block 148 will be any suitable adjustment that will 
compensate for the unequal relationship of the trail edge signals of the 
reference and test patches. If there is not a significant difference in 
both the lead and trail edge signals between the test patch and the 
reference patch, then as shown at block 150 no adjustment will be made. 
While there has been illustrated and described what is at present 
considered to be a preferred embodiment of the present invention, it will 
be appreciated that numerous changes and modifications are likely to occur 
to those skilled in the art, and it is intended to cover in the appended 
claims all those changes and modifications which fall within the true 
spirit and scope of the present invention.