Development device

A development device according to the invention has an arrangement wherein a developer carrying member holding a toner thereon and an image bearing member with an electrostatic latent image formed thereon oppose each other across a predetermined gap therebetween, and a power unit applies an alternating voltage to the gap for supplying the toner from the developer carrying member to the image bearing member, the development device satisfying any one the following conditions: ##EQU1## where a (.OMEGA.) denotes a resistance component of an impedance (a+b.multidot.i) of the developer carrying member, -b (.OMEGA.) denotes a capacitative reactance component of the impedance thereof, and f (Hz) denotes a frequency of the alternating voltage.

BACKGROUND OF THE INVENTION 
This application is based on application No. 37269/1998 filed in Japan, the 
contents of which is hereby incorporated by reference. 
1. Field of the Invention 
The present invention relates generally to a development device which is 
used in image forming apparatuses, such as copying machines, printers and 
the like, for developing an electrostatic latent image formed on an image 
bearing member. In particular, the invention relates to a development 
device which is arranged such that a developer carrying member for holding 
a toner thereon and the image bearing member with the electrostatic latent 
image formed thereon oppose each other across a predetermined gap 
therebetween and an alternating voltage is applied to the gap for 
supplying the toner from the developer carrying member to the image 
bearing member, the development device adapted to produce the image which 
does not suffer a significant variations in image density or has a stable 
image density even if the gap between the developer carrying member and 
the image bearing member varies. 
2. Description of the Related Art 
The image forming apparatuses, such as the copying machines and printers, 
have heretofore employed various types of development devices for 
development of the electrostatic latent images formed on the image bearing 
members. The known development devices include those utilizing the two 
component developer comprised of a carrier and a toner, and those 
utilizing the single component developer free from the carrier. 
As the development device of the single component development system, there 
have been known a contact type development device arranged such that the 
developer carrying member comes into contact with the image bearing member 
at a development zone and introduces the developer to the development zone 
for developing the latent image, and a non-contact type development device 
arranged such that the developer carrying member opposes the image bearing 
member across a predetermined gap therebetween and introduces the 
developer to the development zone opposite to the developer carrying 
member, thereby accomplishing the development of the latent image. 
The contact type development device features an excellent reproducibility 
of the electrostatic latent image formed on the image bearing member 
because the latent image is developed by bringing the developer into 
contact with the image bearing member. However, the developer also adheres 
to a non-image area of the image bearing member so that the produced image 
suffers fogging. 
Hence, the prior-art development device is designed to prevent the 
developer from adhering to the non-image area by varying a moving speed of 
the image bearing member from that of the developer carrying member. 
In as much as the contact-type development device has the developer 
carrying member pressed against a surface of the image bearing member at a 
given pressure, the developer carrying member moving at the different 
speed relative to the image bearing member causes abrasion of the surface 
of the image bearing member. Consequently, the production of images with a 
stable density is not ensured. 
In the non-contact type development device with the developer carrying 
member opposing the image bearing member across the predetermined gap 
therebetween, the surface of the image bearing member is not abraded by 
the developer carrying member. However, significant density variations of 
the produced images result from the varied gap, at development zone, 
defined by the developer carrying member and the image bearing member in 
opposing relation. In a case where a minor variation in the gap between 
the image bearing member and the developer carrying member results from 
poor forming precisions of the image bearing member and the developer 
carrying member, for example, the produced images suffer density 
variations. Hence, the images with a stable density cannot be obtained. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a development device 
arranged such that a developer carrying member holding a toner thereon and 
an image bearing member with an electrostatic latent image formed thereon 
define a predetermined gap therebetween, an alternating voltage is applied 
to the gap for supplying the toner from the developer carrying member to 
the image bearing member in non-contacting relation with the developer 
carrying member, the development device adapted to reduce the density 
variations of the produced images despite the varied gap between the 
developer carrying member and the image bearing member thereby ensuring 
the production of favorable images with a constant image density. 
A first development device according to the invention comprises a developer 
carrying member holding a toner thereon and opposing an image bearing 
member with an electrostatic latent image formed thereon across a 
predetermined gap therebetween, and a power unit for applying an 
alternating voltage to the gap between the developer carrying member and 
the image bearing member for supplying the toner from the developer 
carrying member to the image bearing member, the development device 
satisfying relations: 
EQU a.ltoreq.5.times.10.sup.9 /f, and 5.times.10.sup.8 
/f.ltoreq.-b.ltoreq.5.times.10.sup.9 /f (1) 
where a (.OMEGA.) denotes a resistance component of an impedance 
(a+b.multidot.i) of the developer carrying member, -b (.OMEGA.) denotes a 
capacitative reactance component of the impedance thereof, and f (Hz) 
denotes a frequency of the alternating voltage. 
A second development device according to the invention satisfies relations: 
EQU -b.ltoreq.5.times.10.sup.9 /f, and 5.times.10.sup.8 
/f.ltoreq.a.ltoreq.5.times.10.sup.9 /f (2) 
A third development device according to the invention satisfies a relation: 
EQU 5.times.10.sup.8 /f.ltoreq.(a.sup.2 +b.sup.2).sup.1/2 
.ltoreq.5.times.10.sup.9 /f (3) 
If the resistance component "a" and the capacitative reactance component 
"-b" of the impedance (a+b.multidot.i) of the developer carrying member 
and the frequency "f" of the alternating voltage satisfy any one of the 
aforementioned conditions, as in the first to third development devices 
according to the invention, strength variations of the electric field 
applied between the developer carrying member and the image bearing member 
decrease despite the varied the gap between the developer carrying member 
and the image bearing member. This contributes to reduced variations in 
the density of the produced images. In addition, a suitable strength of 
electric field is applied between the developer carrying member and the 
image bearing member so that images sufficient and stable in density are 
produced. 
For defining the aforementioned conditions of the invention, an examination 
was made on how the strength of the electric field applied between the 
developer carrying member and the image bearing member varied in 
association with each .+-.0.05 mm variation of the gap "d" between the 
developer carrying member and the image bearing member. The examination 
was conducted under the following conditions, for example: a relative 
dielectric constant .di-elect cons..sub.P of the image bearing member set 
to 3.0; a thickness of a photoconductive layer of the image bearing member 
set to 20 .mu.m; an area of the development zone set to 1500 mm.sup.2 
where the image bearing member is opposed by the developer carrying 
member; a peak-to-peak value V.sub.PP of 2000 V of the alternating voltage 
applied between the developer carrying member and the image bearing 
member; various frequencies "f" of the alternating voltage applied between 
the developer carrying member and the image bearing member; and various 
resistance components "a" and capacitative reactance components "-b" of 
the developer carrying member. The results are shown in FIGS. 1 to 3. 
FIG. 1 shows the field variations where the gap "d" was 0.20 mm and the 
frequency "f" of the alternating voltage was 2 kHz. FIG. 2 shows the field 
variations where the gap "d" was 0.30 mm and the frequency "f" was 2 kHz. 
FIG. 3 shows the field variations where the gap "d" was 0.20 mm and the 
frequency "f" was 4 kHz. The figures also show how the field strength 
varied in association with each of the various resistance components "a" 
and capacitative reactance components "-b" of the developer carrying 
member. In these figures, hollow circles represent the field strengths 
when the gap "d" was 0.05 mm increased from a set value whereas solid 
circles represent the field strengths when the gap "d" was 0.05 mm 
decreased from the set value. 
In order to ensure that a reduced difference is obtained between a field 
strength in the gap "d" 0.05 mm increased from the set value and a field 
strength in a gap "d" 0.05 mm decreased from the set value and that a 
sufficient amount of toner is supplied from the developer carrying member 
to the image bearing member, there were determined respective ranges of 
the above parameters that achieve the field strength of not less than 
2.times.10.sup.6 V/m. The aforesaid relations 1 to 3 were derived from 
these ranges thus determined. 
It is to be noted here that the development device of the invention may 
employ the developer carrying member which does not include a magnetic 
member. Usable as the developer hereof is a single component developer 
free from the carrier. The single component developer may be a magnetic 
toner containing a magnetic powder or a non-magnetic toner free from the 
magnetic powder. 
According to the development device of the invention, the electron 
conductive material is preferably used for imparting the impedance to the 
developer carrying member. 
The use of the electron conductive material for imparting the impedance to 
the developer carrying member is effective to reduce the variations in the 
electric field applied between the image bearing member and the developer 
carrying member when the environmental conditions around the development 
device, such as temperature, humidity and the like, vary. This further 
ensures the production of images with a more stable density. 
According to the development device of the invention, the developer 
carrying member is arranged such that at least a resilient layer is formed 
on a conductive roller. Preferably, the resilient layer has a thickness of 
not more than 2 mm. 
By using the developer carrying member wherein at least the resilient layer 
is formed on the conductive roller, a regulating member for regulating a 
quantity of toner held by the developer carrying member is prevented from 
pulverizing the toner particles. With the thickness of 2 mm or less, the 
resilient layer in the developer carrying member suffers smaller 
variations in the thickness thereof under the varying environmental 
conditions including temperature, humidity and the like. Thus, the 
production of images with the stable density is ensured. 
According to a development device of the invention, it is preferred that 
the aforesaid alternating voltage as well as a direct voltage are applied 
between the developer carrying member and the image bearing member while 
the following conditions are satisfied: 
EQU 1.5.ltoreq.V.sub.PP (kV).ltoreq.2.0, and .vertline.V.sub.o -V.sub.i 
.vertline./2.ltoreq..vertline.V.sub.DC .vertline. (4) 
where V.sub.PP (kV) denotes a peak-to-peak value of the alternating 
voltage, V.sub.DC (V) denotes the direct voltage, V.sub.o (V) denotes a 
potential of a non-image area of the image bearing member, and V.sub.i (V) 
denotes a potential of an image area thereof. 
If the alternating voltage together with the direct voltage are applied 
between the developer carrying member and the image bearing member while 
the peak-to-peak value V.sub.PP and the direct voltage V.sub.DC satisfy 
the aforesaid conditions 4, an occurrence of leakage between the developer 
carrying member and the image bearing member is prevented. This leads to 
the prevention of appearance of black spots in a background portion of the 
produced image and also to the reduced field variations caused by the 
varied gap at the development zone. Accordingly, the production of images 
with the stable density is ensured. 
According to a development device of the invention, it is preferred that 
the frequency f (Hz) of the alternating voltage applied between the 
developer carrying member and the image bearing member satisfies a 
condition of 1000&lt;f&lt;5000 and that the alternating voltage acts for a 
shorter period of time to effect an electric field in a toner leading 
direction toward the image bearing member than to effect the electric 
field in a toner leading direction toward the developer carrying member. 
If the alternating voltage has the frequency limited within the aforesaid 
range and effects the electric field in the toner leading direction toward 
the image bearing member in a manner to allow a shorter duration thereof 
than that of the field in the toner leading direction toward the developer 
carrying member, the production of images with the suitable density is 
ensured despite the increased absolute value of the peak-to-peak value 
V.sub.PP of the alternating voltage or of the direct voltage V.sub.DC. 
These and other objects, advantages and features of the invention will 
become apparent from the following description thereof taken in 
conjunction with the accompanying drawings which illustrate specific 
embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, a development device according to preferred embodiments of the 
invention will hereinbelow be described. Based on various examples of the 
invention, a specific explanation will be made on that the development 
device satisfying any one of the conditions of the invention provides 
images which are reduced in density variations and have a sufficient 
density despite a varied gap between the developer carrying member and the 
image bearing member. 
EXAMPLES 1 to 10 
As shown in FIG. 4, these examples each employed a developer carrying 
member 10 including a conductive roller 11 formed of a metal and a 
resilient layer 12 formed of any one of various materials and laid in a 
thickness of 1 mm on the conductive roller. 
In Examples 1 to 3, the resilient layer 12 was formed of an electron 
conductive material including a silicone rubber to which carbon black was 
added in different proportions for varying a resistance component and a 
capacitative reactance component of the layer. In Examples 4 to 6, the 
resilient layer was formed of an electron conductive material including an 
ethylene-propylene-diene-methylene rubber (EPDM rubber) to which carbon 
black was added in different proportions for varying the resistance and 
capacitative reactance components of the layer. In Examples 7 and 8, the 
resilient layer was formed of an ion conductive material varied in the 
resistance and the capacitative reactance components. In Examples 9 and 
10, the resilient layer was formed of an epichlorohydrin rubber, a kind of 
ion conductive material, which was varied in the resistance and 
capacitative reactance components. 
As shown in FIG. 5, each of the developer carrying members of Examples 1 to 
10 was disposed in contact with an electrode roller 20. An AC voltage 
source 30 and a resistance 40 were connected across the developer carrying 
member and the electrode roller, between which an AC voltage was applied 
by the AC voltage source 30. Measurement was taken on waveforms of the AC 
voltage thus applied and of a voltage across the resistance 40. As seen in 
FIG. 6, the waveform of the applied AC voltage, represented by a solid 
line, had a different peak value and phase from those of the waveform, 
represented by a broken line, of the voltage across the resistance 40. 
As to each of the aforesaid developer carrying members 10, there were 
determined a peak value V.sub.P of the AC voltage applied by the AC 
voltage source 30, a peak value V.sub.R of the voltage across the 
resistance 40, and a phase shift .phi. between the waveform of the applied 
AC voltage and that of the voltage across the resistance 40. On the other 
hand, a resistance component "a" and a capacitative reactance component 
"-b" of each developer carrying member 10 were determined by using a 
resistance value R of the resistance 40 in the following equations: 
EQU a=(V.sub.P .multidot.cos .phi./V.sub.R -1).multidot.R 
EQU -b=V.sub.P .multidot.R sin .phi./V.sub.R 
The results are shown in the following Table 1. 
Next, each of the developer carrying members of Examples 1 to 10 was spaced 
from the electrode roller 20 by a predetermined distance, as shown in FIG. 
7. 
As to each of the developer carrying members 10 of Examples 1 to 10, 
variations in the peak value V.sub.R of the voltage across the resistance 
40 were examined in association with various peak values V.sub.P of the AC 
voltage under the following conditions: the gap "d" between each developer 
carrying member and the electrode roller 20 set to 0.20 mm and 0.30 mm, 
respectively; and the frequency "d" from the AC voltage source 30 set to 2 
kHz. FIGS. 8 to 17 show the results. In these figures, the solid circles 
.circle-solid. represent the peak values when the gap "d" between the 
developer carrying member 10 and the electrode roller 20 was 0.20 mm 
whereas the hollow circles .largecircle. represent the peak values when 
the gap "d" therebetween was 0.30 mm. 
It is to be noted that in order to ensure that the aforesaid developer 
carrying member 10 provides a sufficient image density, the voltage across 
the resistance 40 must be not less than a predetermined level. 
Furthermore, as the voltage V.sub.R across the resistance 40 associated 
with the gap "d" of 0.20 mm and the voltage V.sub.R associated with the 
gap "d" of 0.30 mm present a smaller difference therebetween, reduced are 
the image density variations due to the varied development gap. 
As to each of the developer carrying members 10 of Examples 4, 5 and 8, the 
variations in the peak value V.sub.R of the voltage across the resistance 
40 were examined in association with the various peak values V.sub.P of 
the AC voltage under the following conditions: the frequency "f" of the AC 
voltage from the AC voltage source 30 set to 2 kHz and 4 kHz, 
respectively; the gap "d" between the developer carrying member 10 and the 
electrode roller 20 set to 0.20 mm and 0.30 mm, respectively. The results 
are shown in FIGS. 18a-b to 20a-b. FIGS. 18a to 20a each show the peak 
values V.sub.R where the AC voltage had the frequency "f" of 2 kHz whereas 
FIGS. 18b to 20b each show the peak values V.sub.R where the AC voltage 
had the frequency "f" of 4 kHz. In these figures, the solid circles 
.circle-solid. represent the peak values V.sub.R when the gap d between 
the developer carrying member 10 and the electrode roller 20 was 0.20 mm 
whereas the hollow circles .largecircle. represent the peak values V.sub.R 
when the gap "d" was 0.30 mm. 
According to the results, if the frequency "f" of the AC voltage from the 
AC voltage source 30 is increased, the varied gap "d" between the 
developer carrying member 10 and the electrode roller 20 causes smaller 
variations in the peak value V.sub.R of the voltage across the resistance 
40. Thus, the influence of the varied gap Ado is reduced. However, the 
peak values V.sub.R of the voltage across the resistance 40 decrease so 
that the produced images have low image densities. 
Next, as shown in FIG. 21, an arrangement was made such that each of the 
developer carrying members of Examples 1 to 10 opposed an image bearing 
member 50 across a predetermined gap "d" therebetween while the AC voltage 
source 30 and a DC voltage source 60 were connected across the developer 
carrying member 10 and the image bearing member 50. In this arrangement, 
an AC voltage from the AC voltage source 30 as well as a suitable DC 
voltage from the DC voltage source 60 were applied to the gap d for 
effecting a reversal development process under the following conditions: a 
peak-to-peak value V.sub.PP of the AC voltage set to 2 kHz; and the 
frequency "f" thereof set to 2 kHz and 4 kHz, respectively. Evaluation was 
made on the stability of the electric field produced between each 
developer carrying member 10 and the image bearing member 50 and on the 
density of the produced images. The results are shown in the following 
Table 1. 
As to the field stability, the field strength variations associated with 
the gap "d" of 0.20 mm between each developer carrying member 10 and the 
image bearing member 50 and those associated with the gap "d" of 0.30 mm 
were examined. The results were compared with the variations in strength 
of the electric field produced between a developer carrying member 10, 
formed of the conductive roller, and the image bearing member. According 
to Table 1, a developer carrying member presenting the field strength 
variations of not more than 90% was considered to be .largecircle. whereas 
any other developer carrying member was considered to be X. As to the 
image density, a developer carrying member providing a sufficient image 
density was considered to be .largecircle., that providing a substantially 
acceptable image density was considered to be .DELTA., that failing to 
provide the sufficient image density was considered to be X. 
TABLE 1 
______________________________________ 
Field Image 
a -b Stability Density 
Example 
(.OMEGA.) 
(.OMEGA.) 
2 kHz 4 kHz 2 kHz 4 kHz 
______________________________________ 
1 2 .times. 10.sup.4 
1 .times. 10.sup.4 
x x .smallcircle. 
.smallcircle. 
2 5 .times. 10.sup.5 
4 .times. 10.sup.5 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
3 1 .times. 10.sup.6 
5 .times. 10.sup.6 
.smallcircle. 
.smallcircle. 
x x 
4 4 .times. 10.sup.4 
2 .times. 10.sup.5 
x .smallcircle. 
.smallcircle. 
.smallcircle. 
5 1 .times. 10.sup.5 
1 .times. 10.sup.6 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
6 2 .times. 10.sup.5 
4 .times. 10.sup.6 
.smallcircle. 
.smallcircle. 
x x 
7 4 .times. 10.sup.5 
1 .times. 10.sup.5 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
8 1 .times. 10.sup.6 
5 .times. 10.sup.5 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.DELTA. 
9 3 .times. 10.sup.5 
2 .times. 10.sup.5 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
10 3 .times. 10.sup.6 
5 .times. 10.sup.5 
.smallcircle. 
.smallcircle. 
x x 
______________________________________ 
Where the AC voltage having the frequency "f" of 2 kHz was applied by the 
AC voltage source 30, the developer carrying members 10 of Examples 2, 5 
and 7 to 9 satisfied of the aforementioned conditions 1 to 3 of the 
invention. Where the AC voltage having the frequency of 4 kHz was applied 
by the AC voltage source 30, the developer members 10 of Examples 2, 4, 5, 
and 7 to 9 satisfied any one of the aforementioned conditions 1 to 3 of 
the invention. 
If any one of the aforementioned conditions 1 to 3 of the invention was 
satisfied, the electric field applied between the developer carrying 
member 10 and the image bearing member 50 was reduced in strength 
variations despite the varied gap "d" therebetween. Accordingly, images 
stable and sufficient in image density were produced. 
Next, a test was conducted by using each of the developer carrying members 
10 of Examples 2, 5 and 7 to 9 in the device shown in FIG. 7. In the test, 
the AC voltage at the frequency "f" of 2 kHz was applied by the AC voltage 
source 30 to the gap "d" between each developer carrying member 10 and the 
electrode roller 20, the gap "d" set to 0.20 mm and 0.30 mm. The test 
examined the variations in the peak value V.sub.R of the voltage across 
the resistance 40 in association with the various peak values V.sub.P of 
the AC voltage under high temperature/high humidity conditions of 
30.degree. C. in temperature and 85% in humidity and low temperature/low 
humidity conditions of 10.degree. C. in temperature and 15% in humidity. 
The results are shown in FIGS. 22 to 26, wherein the solid circles 
.circle-solid. represent the peak values V.sub.R associated with the gap 
"d" of 0.20 mm under the high temperature/high humidity conditions whereas 
the hollow circles .circle-solid. represent the peak values V.sub.R 
associated with the gap "d" of 0.30 mm under the high temperature/high 
humidity conditions. On the other hand, the solid triangles 
.tangle-solidup. represent the peak values V.sub.R associated with the gap 
"d" of 0.20 mm under the low temperature/low humidity conditions whereas 
the hollow triangles .tangle-solidup. represent the peak values V.sub.R 
associated with the gap"d " of 0.30 mm under the low temperature/low 
humidity conditions. 
According to the results, the developer carrying members 10 of Examples 2 
and 5 achieved smaller variations in the peak value V.sub.R of the voltage 
across the resistance 40 at the varied temperatures and humidities, as 
compared with the developer carrying members 10 of Examples 7 to 9. The 
developer carrying members 10 of Examples 2 and 5 included the resilient 
layer 12 formed of the electron conductive material containing silicone 
rubber or EPDM rubber with carbon black added thereto, whereas those of 
Examples 7 to 9 included the resilient layer 12 formed of the ion 
conductive material containing urethane rubber or epichlorohydrin rubber. 
Thus, the developer carrying member 10 wherein the resilient layer 12 is 
formed of the electron conductive material ensures that the images with 
stable image density are produced under the varied temperatures, 
humidities and the like. 
In each of the developer carrying members 10 of Examples 1 to 10, the 
thickness of the resilient layer 12 was changed to examine a variation 
thereof under the aforementioned high temperature/high humidity conditions 
and low temperature/low humidity conditions. With increase in the 
thickness thereof, the resilient layers 12 suffered greater thickness 
variations due to the environmental changes. This also resulted in the 
variation of the gap "d" between the developer carrying member 10 and the 
image bearing member 50 and hence, the produced images were varied in 
image density. 
Where the resilient layer 12 in each of the above developer carrying 
members 10 had a thickness of not more than 2 mm, the resilient layer 12 
presented the thickness variations of not more than 50 .mu.m in the 
environmental changes between the aforementioned high temperature/high 
humidity conditions and low temperature/low humidity conditions. Thus, the 
image density variations due to the environmental changes were decreased. 
EXAMPLE 11 
Example 11 used a developer carrying member 10 having a resistance 
component "a" of 5.times.10.sup.5 .OMEGA. and a capacitative reactance 
component "-b" of 4.times.10.sup.5 .OMEGA.. As shown in FIG. 21, an 
arrangement was made such that the developer carrying member 10 opposed 
the image bearing member 50 across a predetermined gap "d" therebetween 
while the AC voltage source 30 and the DC voltage source 60 were connected 
across the developer carrying member 10 and the image bearing member 50. 
In this arrangement, the reversal development process was performed by 
applying, between the developer carrying member 10 and the image bearing 
member 50, an AC voltage from the AC voltage source 30 and a DC voltage 
V.sub.DC from the DC voltage source 60, thereby producing a halftone image 
with a target density of about 0.6. The AC voltage had a rectangular 
waveform, a frequency "f" of 2 kHz and a period ratio (duty ratio) of 50%, 
the period during which the voltage effected an electric field in a toner 
leading direction toward the image bearing member 50. 
The reversal development process was performed under the following 
conditions: a potential V.sub.o of -800 V at the non-image area of the 
image bearing member 50; a potential V.sub.i of -50 V at an image area 
thereof; the gap"d" between the developer carrying member 10 and the image 
bearing member 50 set to 0.1 mm, 0.2 mm and 0.3 mm, respectively; the 
peak-to-peak value V.sub.PP of the AC voltage from the AC voltage source 
30 set to 1.0 kV, 1.5 kV and 2.0 kV, respectively; and the DC voltage 
V.sub.DC from the DC voltage source 60 set to -100 V, -250 V and -400 V, 
respectively. Variations in the image density of the produced images were 
examined. The results are shown in FIG. 27 in which an occurrence of 
leakage means a local insulation breakdown produced between the non-image 
area of the image bearing member 50 and the developer carrying member 10. 
According to the results, the increased peak-to-peak value V.sub.PP of the 
AC voltage applied by the Ac voltage source 30 resulted in the increased 
density of the produced images and in the decreased density variations due 
to the varied gap "d" between the developer carrying member 10 and the 
image bearing member 50. Unfortunately, however, the leakage tended to 
occur between the non-image area of the image bearing member 50 and the 
developer carrying member 10, producing the local insulation breakdown 
therebetween. Consequently, spot-like toner adhesion to the non-image area 
resulted. 
On the other hand, the increased absolute value of the DC voltage V.sub.DC 
applied by the DC voltage source 60 resulted in the increased density of 
the produced images and in the decreased leakage produced between the 
non-image area of the image bearing member 50 and the developer carrying 
member 10. 
Hence, in order to decrease the image density variations caused by the 
varied gap "d" between the developer carrying member 10 and the image 
bearing member 50 and to suppress the occurrence of leakage between the 
non-image area of the image bearing member 50 and the developer carrying 
member 10, it was preferred to increase the peak-to-peak value V.sub.PP of 
the AC voltage from the AC voltage source 30 and the absolute value of the 
DC voltage V.sub.DC from the DC voltage source 60. 
However, the increased peak-to-peak value V.sub.PP of the AC voltage from 
the AC voltage source 30 in combination with the increased DC voltage 
V.sub.DC from the DC voltage source 60 resulted in a great increase in the 
density of the produced images. When the peak-to-peak value V.sub.PP of 
the AC voltage was at 2.0 kV and the absolute value of the DC voltage 
V.sub.DC was at -400 V, the produced image suffered an excessive increase 
in image density. 
Next, the AC voltage applied by the AC voltage source 30 was varied in its 
duty ratio to 50%, 35% and 25% for adjustment of the density of the 
produced images. 
The reversal development process was performed under the following 
conditions: the gap "d" between the developer carrying member 10 and the 
image bearing member 50 set to 0.1 mm, 0.2 mm and 0.3 mm, respectively; 
the peak-to-peak value V.sub.PP of the AC voltage from the AC voltage 
source 30 set to 1.5 kV and 2.0 kV, respectively; and the DC voltage 
V.sub.DC from the DC voltage source 60 set to -400 V, -500 V and -600 V, 
respectively. The densities of the produced images were measured to 
determine the density variations. FIG. 28 shows the densities of the 
images produced with the DC voltage V.sub.DC set to -400 V; FIG. 29 shows 
the densities of the images produced with the DC voltage V.sub.DC set to 
-500 V; and FIG. 30 shows the densities of the images produced with the DC 
voltage V.sub.DC set to -600 V. 
According to the results, where the DC voltage Vc had the low absolute 
value of -400 V, the decreased duty ratio of the AC voltage increased the 
image density variations due to the varied gap "d" between the developer 
carrying member 10 and the image bearing member 50. Where, on the other 
hand, the DC voltage V.sub.DC was increased in the absolute value thereof, 
the image density variations due to the varied gap "d" were reduced 
despite the decreased duty ratio of the AC voltage. In addition, the 
occurrence of leakage between the non-image area of the image bearing 
member 50 and the developer carrying member 10 was suppressed. 
Unfortunately, with increase in the absolute value of the DC voltage 
V.sub.DC, the leakage was more likely to occur between the image area of 
the image bearing member 50 and the developer carrying member 10. Where 
the DC voltage V.sub.DC was at -600 V, the occurrence of leakage was 
observed between the image area of the image bearing member 50 and the 
developer carrying member 10 when the AC voltage with the peak-to-peak 
value V.sub.PP of 1.5 kV was applied to the gap "d" of 0.21 mm between the 
developer carrying member 10 and the image bearing member 50 and when the 
AC voltage with the peak-to-peak value V.sub.PP of 2.0 kV was applied to 
the gap "d" of 0.25 mm. 
Where the DC voltage was at -500 V, on the other hand, substantially the 
same gap "d" was associated with the onset of leakages occurring between 
the image area of the image bearing member 50 and the developer carrying 
member 10 and between the non-image area thereof and the developer 
carrying member 10. 
In order to ensure that the image density variations due to the varied gap 
"d" do are decreased and that images with a suitable density are produced, 
it was preferred to limit the peak-to-peak value V.sub.PP of the AC 
voltage from the AC voltage source 30 within the range of between 1.5 kV 
and 2.0 kV and to decrease the ratio (duty ratio) of the period during 
which the AC voltage effected the electric field in the toner leading 
direction toward the image bearing member 50. Incidentally, the DC voltage 
V.sub.DC from the DC voltage source 60 may be set to a suitable value for 
adjustment of the density of the produced images and suppression of the 
leakage, with consideration given to the potential V.sub.o of the 
non-image area and that V.sub.i of the image area of the image bearing 
member 50. 
Next, a test was conducted on the developer carrying member 10 of Example 
11 having the resistance component "a" of 5.times.10.sup.5 .OMEGA. and the 
capacitative reactance component "-b" of 4.times.10.sup.5 .OMEGA. and a 
developer carrying member 10 of Comparative Example 1 having a resistance 
component "a" of 2.times.10.sup.4 .OMEGA. and a capacitative reactance 
component "-b" of 1.times.10.sup.4 .OMEGA.. 
The reversal development process was performed by applying between each 
developer carrying member 10 and the image bearing member 50 an AC voltage 
from the AC voltage source 30 and a DC voltage V.sub.DC from the DC 
voltage source 60. The image density variations were examined under the 
following conditions: 2 kHz in the frequency "f" of the AC voltage; 1.7 kV 
in the peak-to-peak value V.sub.PP of the AC voltage; 30% in the duty 
ratio of the AC voltage; -500 V in the DC voltage V.sub.DC ; and the gap 
"d" between the developer carrying member 10 and the image bearing member 
50 set to 0.1 mm, 0.2 mm and 0.3 mm, respectively. FIG. 31a shows the 
densities of the images produced by using the developer carrying member 10 
of Example 11 whereas FIG. 31b shows the densities of the images produced 
by using the developer carrying member 10 of Comparative Example 1. 
It is to be noted that any one of the aforementioned conditions of the 
invention was satisfied by the use of the developer carrying member 10 of 
Example 11 but none of the conditions was satisfied by the use of the 
developer carrying member 10 of Comparative Example 1. 
The following fact was found from a comparison between the images produced 
by the use of the developer carrying member 10 of Example 11 and those 
produced by the use of the developer carrying member 10 of Comparative 
Example 1. The developer carrying member 10 of Example 11 is effective to 
reduce the image density variations due to the varied gap "d " between the 
developer carrying member 10 and the image bearing member 50 and to 
suppress the leakage produced between the image bearing member 50 and the 
developer carrying member 10. 
In the above test, the AC voltage from the AC voltage source 30 had the 
frequency "f" of 2 kHz. Where the frequency "f" was not more than 1 kHz, 
the produced image tended to suffer fogs in a non-image portion thereof. 
Where the frequency "f" was not less than 5 kHz, the produced image 
suffered a poor density. 
Hence, it was found that the AC voltage from the AC voltage source 30 
preferably has a frequency "f" in the range of between 1 kHz and 5 kHz. 
Although the present invention has been fully described by way of examples 
hereof, it is to be noted that various changes and modifications will be 
apparent to those skilled in the art. Therefore, unless otherwise such 
changes and modifications depart from the scope of the present invention, 
they should be construed as being included therein.