Developing device

A device for developing an electrostatic latent image recorded on a photoconductive layer. The developing device comprises a doner roll for supporting a uniform layer of single-component developing material adjacent to the photoconductive layer. The doner roll is disposed as to create a space gap between the photoconductive layer and the doner roll. The doner roll is made of semiconductive material having a specific resistance ranging from 10.sup.6 to 10.sup.12 .OMEGA.cm. An electrical bias potential is applied across the gap, thereby establishing a field for transferring the developing material from the doner roll to the photoconductive layer.

BACKGROUND OF THE INVENTION 
This invention relates to a device using single-component developing 
material for developing an electrostatic latent image recorded on a 
photoconductive layer. 
In the art of xerography as discussed in U.S. Pat. No. 2,297,991 to 
Carlson, a xerographic plate, which comprises a layer of photoconducting 
and insulating material on a conducting backing, is given a uniform 
electric charge over its surface and is then exposed to the subject matter 
to be reproduced. This exposure results in discharge of the 
photoconductive plate whereby an electrostatic latent image is formed. The 
latent charge pattern is developed or made visible with a charged powder. 
Thereafter, the developed image is transferred to a support member to 
which it is fixed. Controlled development of electrostatic latent image 
can be accomplished by several techniques including cascade, 
magnetic-brush, liquid-dispersion development, etc. Another important 
development technique is called as "transfer development" which is, for 
example, disclosed in U.S. Pat. No. 2,895,847 to Mayo. This development 
process employs a support member such as a "donor" which carries a layer 
of toner particles to be brought into close contact with the electrostatic 
latent image to be developed. 
It is to be noted that the term "transfer development" is generec to 
development techniques where (1) the toner layer is out of contact with 
the photoconductor and the toner particles must traverse an air gap to 
effect development, (2) the toner layer is brought into rolling contact 
with the photoconductor to effect development, and (3) the toner layer is 
brought into contact with the imaged photoconductor and skidded across the 
imaged surface to effect development. Transfer development has also come 
to be known as "touchdown development". 
A serious problem which occurs with transfer type development is fog or 
background development. In order to minimize background development, there 
is proposed, in U.S. Pat. No. 2,289,400 to Moncrieff-Yeates, an out of 
contact transfer development system in which toner particles tranverse an 
air gap between the doner and the xerographic plate to develop the 
electrostatic latent image disposed on the xerographic plate. However, the 
special positioning of the doner and the xerographic plate in relation to 
each other is critical. For example, the length of the air gap or 
development gap must be adjusted at a value less than 0.05 mm and 
preferably less than 0.03 mm. This adjustment involves considerable 
difficulty in maintaining the xerographic plate and the doner within the 
required range of mechanical accuracy. Several attempts have been made to 
overcome the difficulty. For example, in U.S. Pat. Nos. 3,866,574 to 
Hardennrock, 3,890,929 to Walkup, and 3,893,418 to Liebman, a pulse 
generator source is employed for applying pulsed bias potentials to create 
electrical fields across the air gap between the toner carrier member and 
the latent image bearing member. Particularly, the Hardennrock patent 
discloses that optimum line development is effected with a minimum of 
background deposition when the three conditions are established, that is, 
when the air gap length (g) is in the range of 0.05 mm to 0.18 mm, the AC 
electric voltage frequency (f) is in the range of 1.5 kHz to 10 kHz, and 
the pulsed bias potential (V.sub.p-p) is less than 800 volts. 
Furthermore, the conventional transfer type development systems as 
disclosed in the Hardennrock patent utilize the electrostatic forces of 
the latent image to overcome the carrier-toner bond and attract toner 
particles onto the image areas. The toner can transfer from the doner to 
the image areas on the xerographic plate across the air gap when the 
intensity of electrostatic forces associated with the latent image exceeds 
a threshold value which may be referred to as toner transfer threshold 
value. Although the toner bonding forces vary from one toner particle to 
another due to the dispersion of physical and chemical properties of the 
individual toner particles, they are distributed in a narrow range around 
a fixed value. Consequently, development is effected in such binary form 
fashion that toner particles are deposited on the image areas producing 
electrostatic forces exceeding the toner transfer threshold value, while 
no toner particle is deposited on the areas producing electrostatic forces 
less than the threshold value. In other words, the characteristic curve 
representing image density with respect to surface potential has such a 
great gradient (.gamma.) as to cause poor continuous-tone development. In 
addition, the characteristic curve has such a great gradient (.gamma.) as 
to allow only a part of toner particles to traverse the air gap if the 
amplitude of the pulsed bias potential (V.sub.p-p) is less than 800 volts 
even though the toner bonding forces are distributed in a wide range. 
Japanese Patent Publication No. 58-32375 discloses a transfer type 
development method which improves the quality in continuous tone images by 
applying a low-frequency bias voltage to create alternative electric 
fields across the air gap between the toner carrier and the xerographic 
plate. The toner transfers from the toner carrier to the xerographic plate 
during one half cycle of applied voltage, this cycle being termed to toner 
transfer cycle. The toner transfers back to the toner carrier from the 
xerographic plate during the second cycle which is termed to toner 
counter-transfer cycle. The Japanese Publication describes that the 
quality of continuous tone images can be improved to a considerable extent 
by repetitive transfer and counter-transfer cycles when the applied bias 
voltage is at a frequency lower than 1 kHz, while the effect is diminished 
when the biase voltage frequency is higher than 2 kHz. It is considered 
that application of low-frequency bias voltage to create alternative 
electrical fields across the air gap is effective to deposite toner 
particles on image areas in conformity with the latent image pattern with 
high fidelity to its surface potentials in the case where the toner 
bonding forces are distributed in such a narrow range as to effect 
binary-form development. However, the development method disclosed in he 
Japanese Publication is disadvantageous in that (1) the forces produced by 
the electrical fields associated with the image and non-image areas are 
not different on the toner carrier and (2) dot or screen pattern images 
cannot be reproduced with high fidelity since toner particles do not 
transfer along the electrical force lines, resulting in low resolution. 
Therefore, the present invention provides an improved developing device 
which can achieve an excellent reproduction of dot or screen pattern 
images without degrading the quality of reproduction of line and solid 
images. 
SUMMARY OF THE INVENTION 
There is provided, in accordance with the present invention, a device for 
developing an electrostatic latent image recorded on a photoconductive 
layer. The developing device comprises a doner roll for supporting a 
uniform layer of single-component developing material adjacent to the 
photoconductive layer. The doner roll is disposed as to create a space gap 
between the photoconductive layer and the doner roll. The doner roll is 
made of semiconductive material having a specific resistance ranging from 
10.sup.6 to 10.sup.12 .OMEGA.cm. An electrical bias potential is applied 
across the gap, thereby establishing a field for transferring the 
developing material from the doner roll to the photoconductive layer. 
According to the present invention, fringing fields are produced at the 
boundary of an electrostatic latent image in order to reproduce both dot 
or screen pattern images and line images with high fidelity. Since 
substantially no fringing field occurs at the boundary of an electrostatic 
latent image if the air gap between the development electrode and the 
xerographic plate (photoconductive layer) has a minute length (100 to 500 
.mu.m), the development electrode must be separated a substantial distance 
from the xerographic plate. However, separation of the development 
electrode from the photoconductive layer would cause electrical discharge 
between the development electrode and the photoconductive layer and toner 
particles would get a relatively great kinetic energy so that toner 
particles cannot move along the electrical force lines, causing deposition 
of toner particles on the non-image areas. According to the present 
invention, this problem is eliminated by placing an electrical resistive 
layer (doner roll) on the development electrode in such a fashion as to 
increase the electrical length of the space between the development 
electrode and the xerographic plate and at the same time decrease the 
electrical length of the space between the xerographic plate and the toner 
to produce fringing fields at the boundary of the electrostatic latent 
image. It is desirable that the resistive layer placed on the development 
electrode has a specific resistance ranging from 10.sup.6 to 10.sup.12 
.OMEGA.cm. If it is smaller than this range, no fringing field occurs at 
the boundary of the image areas. If it is greater, the contrast of field 
intensities and thus the density of the center portion of the image area 
are too low. 
A high-frequency AC bias potential may be applied across the gap between 
the doner roll and the photoconductive layer in order to facilitate 
transfer of toner particles from the doner roll to the photoconductive 
layer. It is desired that the AC bias potential has a frequency ranging 
from 1 to 10 kHz, preferably from 1 to 3 kHz, and an amplitude ranging 
from 400 to 4500 volts, preferably from 800 to 2500 volts. 
Since the charges on conventional single-component developing material are 
distributed in a relatively narrow range, there is a clear toner transfer 
threshold value, causing development effected in binary form or on-off 
fashion when the developing material is used for out of contact transfer 
development. According to the present invention, the toner particle 
charges are distributed in such a wide range as to achieve an excellent 
continuous tone development. In this case, the desirable toner charge 
distribution has a variance of .+-.15 .mu.C/g.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to the description of the preferred embodiments of the present 
invention, the serious problems which occur with the transfer type 
development method disclosed in Japanese Patent Publication No. 58-32375 
will be described with reference to FIGS. 1 and 2 for a better 
understanding of the present invention. 
One problem with the prior art out of contact transfer development method 
is in that the electrical forces cannot be resolved on the xerographic 
plate so that the image and non-image areas have the same potential when 
the electrostatic latent image has a high spatial frequency if the gap 
between the xerographic plate and toner carrier has a length greater than 
0.1 mm. In other words, narrow line images or dot pattern images collapse. 
The image collapse problem will be described in connection with a value M 
which is used to indicate the degree of collapse and is given as: 
##EQU1## 
where .DELTA.D is the difference in image density between the image and 
non-image areas. 
It can be seen from FIG. 1, which shows the value M in relation to the 
spatial frequency, that the resolution of an electrosttic latent image 
formed on the xerographic plate is still high with a spatial frequency of 
5 lines/mm, while the resolution thereof is rather low for a spatial 
frequency of 6 lines/mm. It was found from microphotographs that image 
collapse results in a reduction of the value M. As shown in FIG. 2, 
collapse occurs for a dot or screen pattern image to produce a deviation 
between the original and reproduced images with a spatial frequency of 65 
lines/mm. As a result, the image resulting from development of a dot or 
screen pattern image having a great number of lines is dark over its whole 
area and is unclear with low contrast. In order to overcome this problem, 
the inventors conducted experiments using the development method disclosed 
in Japanese Patent Publication No. 58-32375. The result is that the 
quality of the continuous tone images are improved to a considerable 
extent and the images are reproduced with higher fidelity to the surface 
potential on the xerographic plate, while this advantageous effect is 
obtained only for a spatial frequency higher than 65 lines/mm. 
The reason for this is that collapse occurs on dot or screen pattern images 
because the electrical fields produced by the electrostatic latent image 
has poor fidelity to the latent image so that the force of the electrical 
fields associated with the image and non-image areas are the same on the 
photoconductor, that is, there is no contrast in the electrical fields 
rather than because the development is effected in binary form with a 
great gradient (.gamma.). 
When the toner carrier does not have a proper resistivity and thickness, 
for example, when it is a normally used metal sleeve, no contour 
electrical field occurs in association with the image periphery at a 
position near the xerographic plate. Consequently, toner particles 
transfer towards the image and non-image areas without clear distinction 
and get kinetic energy in the development gap to fly away from the 
electrical force lines so that a part of toner particles are deposited on 
the non-image areas. 
The principles of the present invention will be described with reference to 
FIGS. 3 to 9. Referring to FIG. 3, which is a schematic view showing the 
contour of the electrical field in the region of an electrostatic latent 
image formed on a xerographic plate. 
The xerographic plate comprises a photosensitive insulating layer 10 placed 
on a conductive substrate 15. Arranged in spaced relation to the 
xerographic plate is a toner carrier 12 of a resistance material. A 
development electrodes 14 is placed in contact with the toner carrier 12. 
An alternative voltage source 18 is connected to apply a high-frequency AC 
bias voltage between the development electrode 14 and the conductive 
substrate 15. 
Various controlled parameters are set to control the electrical fields 
produced by the latent image on the xerographic plate so as to produce 
fringing fields at the boundary of the latent image, permitting high 
quality reproduction of dot or screen pattern images and minute 
reproduction of line images with high fidelity. These controlled 
parameters include the resistance, thickness and dielectric constant of 
the toner carrier 12, and the distance between the photoconductive 
insulating layer 10 and the toner carrier 12. 
FIG. 4 shows three different reproduction curves of reproduction density 
vs. original density for different specific resistances. The fidelity of 
reproduction of a dot or screen pattern image to an original image of 175 
lines per inch was tested with a toner carrier having a thickness (l) of 1 
mm and a dielectric constant (.epsilon.) of 20, that is, a dielectric 
thickness (l/.epsilon.) of 5.times.10.sup.-5 m. The reproduction fidelity 
will be at a maximum with no image collapse when the gradient of the 
reproduction curve is 1. It can be seen that for a specified resistance 
less than 10.sup.6 .OMEGA.cm, the reproduction curve curves at a high 
value of original density, as indicated by the solid curve of FIG. 4. This 
represents occurrence of collapse on the image areas, resulting in a 
so-called "dark image". For a specified resistance of 10.sup.7 .OMEGA.cm, 
the reproduction curve gets close to a line, as indicated by the broken 
line of FIG. 4. When the specific resistance is greater than 10.sup.8 
.OMEGA.cm, a linear relationship is established between the reproduction 
density (Dout) and the original density (Din), as indicated by the 
one-dotted line of FIG. 4. When the gradient of the reproduction curve is 
substantially 1, the dot or screen pattern image was reproduced with high 
fidelity and high resolution. 
If the toner carrier layer has an excessively great thickness, the fringing 
fields at the boundary of the electrostatic latent image will be 
intensified to such an extent as to degrade the uniformity of development 
of the solid black areas. FIG. 5 illustrates the results of a series of 
solid area development uniformity tests, where the solid curve relates to 
a toner carrier thickness (l) of 8 mm (or toner carrier dielectric length 
(l/.epsilon.) of 4.0.times.10.sup.-4 m), the one-dotted curve relates to a 
toner carrier thickness of 5 mm (or toner carrier dielectric length of 
2..times.10.sup.-4 m), and the two-dotted curve relates to a toner carrier 
thickness less than 3 mm (or toner carrier dielectric length less than 
1.5.times.10.sup.-4 m). In FIG. 5, the point C indicates a limit above 
which the uniformity of development of solid black areas is permissible. 
It can be seen from these test results that, for a toner carrier thickness 
less than 3 mm, uniform development of solid black areas can be achieved 
when the toner carrier specific resistance is in the range of 10.sup.6 to 
10.sup.12 .OMEGA.cm. For a toner carrier thickness of 5 mm, solid black 
areas can be developed with permissible uniformity when the specific 
resistance of the toner carrier layer is less than 10.sup.10 .OMEGA.cm. 
With the toner carrier thickness of 8 mm, solid black areas can be 
developed with permissible uniformity when the toner carrier specific 
resistance is less than 10.sup.8 .OMEGA.cm. Various tests show that both 
high quality development of dot or screen pattern images and uniform 
development of solid black areas can be achieved when the specific 
resistance (.rho.) of the toner carrier layer is in the range of 10.sup.6 
to 10.sup.12 .OMEGA.cm and when the dielectric length of the toner carrier 
layer is less than 4.0.times.10.sup.-4 m. 
FIG. 6 illustrates the results of toner deposition tests for different 
development bias voltage sources connected to the development electrode. 
Line (a) relates to a bias voltage of a 300 volt DC voltage superposed on 
a 2000 volt AC voltage having a frequency of 3 kHz, line (b) relates to a 
bias voltage of a 300 volt DC voltage superposed on a 2000 volt AC voltage 
having a requency of 2 kHz, and line (c) relates to a bias voltage of a 
300 volt DC voltage superimposed on a 2000 volt AC voltage having a 
frequency of 1 kHz. Line (d) relates to a bias voltage of a 300 volt DC 
voltage. Application of a 300 volt DC voltage is effective to prevent 
toner deposition on the non-image areas. In these tests, the development 
gap length was 150.mu., the toner carrier specific resistance (.rho.) was 
10.sup. .OMEGA.cm, the toner carrier thickness (l) was 1 mm, the toner 
carrier dielectric constant (.epsilon.) was 20, and the xerographic plate 
background potential was 250 volts. With a 300 volt DC bias voltage 
applied to the development electrode, substantially no toner could 
traverse the development gap, as shown by line (d). Lines (a), (b) and (c) 
indicate that the amount of toner particles deposited on the image areas 
is in a linear relationship to the xerographic plate surface potential, 
that is, the electrostatic latent image can be developed with high 
fidelity, when the bias voltage comprises a 300 volt DC voltage superposed 
on a 2000 volt AC voltage having a frequency ranging from 1 kHz to 3 kHz. 
As can be seen from FIG. 6, the gradient (.gamma.) of the toner deposition 
lines is dependent upon the frequency of the AC voltage component of the 
bias voltage applied to the development electrode. High quality 
development was achieved for an AC bias voltage frequency higher than 1 
kHz, although the toner cannot move in response to the bias voltage 
application when the AC bias voltage frequency is higher than 10 kHz. It 
is therefore considered that the upper limit of the AC bias voltage 
frequency is 10 kHz. 
FIG. 7 illustrates that peak-to-peak voltage (V.sub.p-p) of the AC bias 
voltage, which is required to overcome the carrier-toner bond and deposite 
toner particles on the xerographic plate, in connection with the carrier 
thickness (l) plus the development gap length (g). In the tests, the toner 
carrier specific resistance (.rho.) was 10.sup.10 .OMEGA.cm, the toner 
carrier dielectric constant (.epsilon.) was 20, the xerographic plate 
background potential was 250 volts, and the applied AC bias voltage 
frequency was 2 kHz. It can be seen from FIG. 7 that the AC bias voltage 
is required to have a peak-to-peak voltage (V.sub.p-p) greater than 400 
volts when the toner carrier thickness (l) is 20 .mu.m and the development 
gap length is 80 .mu.m, a peak-to-peak value greater than 1000 volts when 
the sum of the toner carrier thickness and the development gap length is 1 
mm, and a peak-to-peak value greater than 3000 volts when the sum of the 
toner carrier thickness and the development gap length is 3 mm. Although 
the required peak-to-peak value (V.sub.p-p) is also dependent upon the 
toner carrier specific resistance (.rho. ), the toner carrier dielectric 
constant (.epsilon.) and the AC bias voltage frequency (f). It may be said 
that toner particles can traverse the development gap if the AC bias 
voltage peak-to-peak value (V.sub.p-p) is in the range of 400 to 4500 
volts, preferably in the range of 800 to 2500 volts. 
The quality of reproduction of continuous tone images is improved by 
distributing the quantity of electrical charges on the toner in a wider 
range. FIG. 8A illustrates the relationship between the xerographic plate 
surface potential and the force bonding toner particles on the toner 
carrier, and FIG. 8B illustrates the relationship between the xerographic 
plate surface potential and the amount of toner deposited on the 
xerographic plate. The problem which occurs with the out of contact 
transfer type development as disclosed disclosed in U.S. Pat. No. 
3,866,574 is that development is effected in an on-off fashion with a 
great gradient (.gamma.) of the characteristic curve representing image 
density with respect to surface potential. This problem will be described 
with reference to FIG. 8. Assuming now that the charge on the toner is Q1 
and the xerographic plate surface potential is V, the intensity of the 
electrostatic forces acting on the toner is in direct proportion to the 
product Q1.times.V of the toner charge Q1 and the surface potential V. On 
the other hand, the electrical force bonding the toner on the toner 
carrier (development resistance) is in direct proportion to the square of 
the toner charge Q1. Toner particles are deposited on the xerographic 
plate at points having a potential greater than a threshold value Vc at 
which the electrostatic force acting on the toner overcomes the electrical 
force bonding toner particles on the toner carrier. As a result, 
development is effected in an on-off fashion with a great gradient 
(.gamma.). That is, assuming that, in FIG. 8A, F1 is the force bonding the 
toner having a charge Q1 on the toner carrier, toner particles, which have 
a charge Q1, will traverse the development gap when the xerographic plate 
surface potential is greater than a threshold value Vc1, whereas toner 
particles, which have a charge Q2 greater than the charge Q1, will 
traverse the development gap when the xerographic plate surface potential 
is greater than a threshold value Vc2 greater than Vc1. The charges Q on 
conventional one-component developer particles are distributed in a 
relatively narrow range, resulting in development effected in on-off 
fashion with a great gradient. Japanese Patent Publication No. 58-32375 
discloses a method which can improve the on-off type development, that is, 
the quality of halftone reproduction by applying a low-frequency 
alternating voltage to repeat two cycles of operation. During one cycle, 
the toner transfers from the toner carrier to the xerographic plate. 
During the second cycle, the toner is transferred back from the 
xerographic plate to the toner carrier. On the other hand, since the 
present invention, which adjusts the intensity of the development 
electrical fields in accordance with developer carrier resistance, 
developer carrier thickness, developer carrier dielectric constant, and 
development gap length, requires a high-frequency alternating bias 
voltage, it is impossible to improve the continuous tone development in 
the conventional manner. In the present invention, therefore, the charges 
on toner particles are distributed in a proper wide range so that the 
development threshold potential values Vc can be distributed in a proper 
wide range to improve the conventional development effected in an on-off 
fashion. In FIG. 9, curve (a) relates to the case where the toner particle 
charges are distributed with a variance of .+-.3 .mu.C/g around an average 
charge Q, that is, the gradient (.gamma.) is great, and curve (b) relates 
to the case where the toner charges are distributed with a variance of 
.+-.15 .mu.C/g and exhibits excellent continuous tone development. 
On the other hand, curve (c) relates to toner charge distribution with a 
variance of .+-.20 .mu.C/g and illustrates that the minimum or threshold 
value of the xerographic plate surface potential at which toner particles 
are deposited on the xerographic plate is negative, causing fog or 
background development. The fog problem occurs due to toner particles 
charged in the opposite polarity. Test results show that the fog or 
background development problem occurs when the toner particles charged in 
the positive polarity are distributed with a variance of .+-.10 .mu.C/g or 
more. It is desired that toner particles charged in the opposite polarity 
are distributed with a variance of .+-.15 .mu.C/g. 
The basic structure of the developing device of the present invention 
comprises a hopper for containing a single-component toner, a toner 
carrier mounted on a shaft for rotation near an electrostatic latent image 
bearing member, the toner carrier being made of a semiconductive material 
having a specific resistance ranging from 10.sup.6 to 10.sup.12 .OMEGA.cm, 
a magnet roller secured within the toner carrier, the magnet roller having 
a plurality of magnetic polarities, a toner metering means for metering 
the amount of toner deposited on the toner carrier, and an AC power source 
electrically connected to the toner carrier. In a preferred embodiment, 
the toner carrier has a thickness ranging from 0.5 to 5 mm, preferably 
from 1 to 2 mm. The toner carrier is made of phenolic plastic having a 
specific resistance ranging from 10.sup.6 to 10.sup.12 .OMEGA.cm. The 
surface of the toner carrier is polished longitudinally to a predetermined 
roughness for carrying toner particles thereon. The toner metering member 
is positioned just above the toner carrier for metering the amount of 
toner on the toner carrier. The toner metering member may be a 
non-magnetic leaf spring having a resilient member secured thereon by 
thermocompression bonding, the resilient member having a thickness ranging 
from 0.1 to 3 mm, preferably from 0.5 to 1.5 mm and a hardness ranging 
from 30.degree. to 70.degree. and preferably from 40.degree. to 
60.degree.. The resilient member is made of rubber, silicone rubber or the 
like. The resilient member is in contact with the semiconductive roller at 
a position corresponding to the magnetic pole of the magnet roller under a 
line pressure of 50 to 200 g/cm. 
The following Examples further specifically define the surprisingly 
advantageous developing device of this invention. The parts and 
percentages are by weight unless otherwise indicated. 
The Examples below are intended to illustrate various preferred embodiments 
of the improved developing device of this invention. 
EXAMPLE 1 
Referring to FIG. 10, which is a schematic cross-sectional view of the 
developing device according to the present invention, a drum as a 
photosensitive surface 1 thereon bearing an electrostatic latent image. 
The drum may be rotated in a clockwise direction for predetermined 
processes to thereby produce an electrostatic latent image thereon, and 
then reaches the developing station. These processes may be accomplished 
in any suitable manner as well known in the art. For example, the 
photosensitive surface 1 is subject to an overall uniform distribution of 
electrical charges and then exposed to an optical image. The 
photosensitive surface 1 is shown as having an electrostatic latent image 
2 carried thereon, the latent image corresponding to the dot or screen 
pattern of an original document. The initial surface potential as -900 
volts and the background potential was -150 volts. 
The developing station comprises a toner reservoir or hopper 3, a magnet 
roll 5 fixed to unshown opposite side plates, a semiconductor sleeve 
(toner carrier) 16 rotatably mounted in surrounding relation about the 
periphery of the magnetic roll surface, and a toner metering device 17. 
The hopper 3 has a supply of single-component magnetic developer 4 
comprising toner particles. The toner is comprised of about 55% by weight 
of magnetic powder, about 22.5% by weight of dimethylamide methyl 
methacryulate (main binder), and about 22.5% by weight of a mixture of 
styrene butadiene and polyethylene wax. The magnetic roll 5 is magnetized 
to have a plurality of magnet segments N and S in such a way that 
respective adjacent magnet segments are of opposite polarity. The 
semiconductive sleeve 16, which is made of phenolic plastic having a 
specific resistance of 10.sup.10 .OMEGA.cm and a specific inductive 
capacity .epsilon.=20, has a cylindrical form with a thickness of about 
1.2 mm. The peripheral surface of the sleeve 16 is polished to a roughness 
Rz=10 .mu.m. The toner metering device 17 comprises a leaf spring 171 made 
of non-magnetic stainless steel and a resilient member 172 secured on the 
leaf spring 171 by thermocompression bonding. The leaf spring 171 is 
secured at its one end to the hopper at such an angle that the leaf spring 
172 can urge the resilient member 172 in contact with the semiconductor 
sleeve 16. The contact pressure is about 150 g/cm. The leaf spring 171 has 
a thickness of about 0.1 mm. The resilient member 172 is made of silicone 
rubber and it has a thickness of about 1 mm. The toner metering device 17 
forms a uniform toner layer on the semiconductive sleeve 16. The reference 
numeral 10 designates a bias voltage source which comprises an AC voltage 
source 8 connected in series with a DC voltage source 9 for applying an AC 
voltage superposed on a DC voltage to the semiconductive sleeve 16. 
The magnetic roll 5 creates fields between respective adjacent magnet 
segments to attract toner particles on the semiconductor sleeve 16 in the 
hopper 3. The toner bristles on the semiconductor sleeve 16 at positions 
corresponding to the magnetic segments of the magnet roll 5. Rotation of 
the semiconductive sleeve 16 permits the toner particles to be conveyed 
through the toner metering device 17. The toner metering device 17 has a 
resilient member 172 which engages in pressure contact with the 
semiconductive sleeve 16 to meter the toner is such a way as to form a 
uniform toner layer on the semiconductive sleeve 16 and also to 
triboelectrically charge the toner particles. When the toner reaches the 
development area A in which the semiconductive sleeve 16 faces, with a 
development gap, to the photosensitive surface 1, the toner bristles again 
and comes close to the photosensitive surface 1 to permit toner particles 
to be transferred into contact with the photosensitive surface 1 where the 
greater electrostatic attraction of the latent image will overcome the 
attraction between the toner and the semiconductive sleeve 16, causing 
toner to be stripped off the semiconductive sleeve 16 and 
electrostatically bonded to the charged image to effect development 
thereof. The amount of toner particles forming the uniform toner layer was 
about 2.0 mg/cm.sup.2. 
The developing device was placed in a xerographic machine in such a manner 
as to provide a 300 .mu.m gap between the semiconductive sleeve 16 and the 
photosensitive surface 1. The toner on the semiconductive sleeve was out 
of contact with the photosensitive surface 1. A bias voltage was applied 
from the bias voltage source 10 to the semiconductive sleeve 16. The 
frequency of the AC voltage applied from the AC voltage source 8 was about 
2.4 kHz and the peak-to-peak voltage (V.sub.p-p) thereof was about 2400 
volts. The DC voltage applied from the DC voltage source 9 was about -250 
volts. A very clean reproduction of the dot or screen pattern image was 
achieved from an original document. 
Various dot pattern image reproduction tests have been performed for 
different semiconductive sleeve materials under the above conditions. It 
was found that an excellent image reproduction can be achieved when the 
specific resistance of the semiconductive sleeve material is in the range 
of 10.sup.6 to 10.sup.12 .OMEGA.cm. 
EXAMPLE 2 
Using the same developing device as described in connection with the first 
Example, various dot pattern image reproduction tests have been performed 
for different development gap lengths (g) and different AC voltage 
peak-to-peak values (V.sub.p-p). It was found that the dot pattern image 
reproduction quality charges with sharp contrast on the opposite sides of 
each of two peak-to-peak voltage threshold lines each of which is 
represented as a linear function of development gap length (g). 
FIG. 11 illustrates the results of the dot pattern image reproduction 
tests. In FIG. 11, marks x indicates the points at which the peak-to-peak 
values (V.sub.p-p) are plotted against a development gap length (g) and 
they indicates the conditions resulting in poor dot pattern image 
reproduction. Marks o indicates the points at which the peak-to-peak value 
(V.sub.p-p) are plotted against a development gap length (g) and they 
indicate the conditions resulting in an excellent dot pattern image 
reproduction. The upper line is represented as V.sub.p-p =10 g+300 and the 
lower line is represented as 6 g+200. It is, therefore, apparent that an 
excellent dot pattern image reproduction can be achieved by setting the 
peak-to-peak value (V.sub.p-p) in the range of 6 g+200 to 10 g+300 if the 
development gap is set at a fixed value (g) for any of reasons. This 
facilitates the design of developing devices. 
Various dot pattern image reproduction tests have also been performed for 
different AC voltage frequencies ranging from 1.0 to 3.0 kHz. It was found 
that the quality of reproduction of dot pattern images is independent of 
the frequency of the AC voltage applied from the bias voltage source 10 to 
the semiconductive sleeve 16 in this frequency range. 
EXAMPLE 3 
Referring to FIG. 12, which is a schematic cross-sectional view of a 
modified form of developing device of the present invention. Like 
reference numerals have been applied to the components which are similar 
to those of FIG. 10. The developing device is substantially the same as 
that described in connection with the first Example except that the 
structure of the toner metering device. In this example, the toner 
metering device comprises a magnetic trimmer 27 made of ferromagnetic 
material. The magnetic trimmer 27 is formed at its tip end with a slant 
surface to provide a sharp edge 271 extending in parallel with the magnet 
segments of the magnetic roll 5. The magnetic trimmer 27 is secured at the 
other end thereof to the hopper 3 in such a fashion that the trimmer edge 
271 faces to the semiconductive sleeve 16 with a uniform gap. The trimmer 
edge 271 is magnetized. The length of the uniform gap between the magnetic 
trimmer 27 and the semiconductive sleeve 16 is 0.6 mm. Since there are 
produced uniform force lines in the uniform gap, the amount of toner 
passing the gap to the development area A remains constant. 
Using this developing device placed in a xerographic machine, various dot 
pattern image reproduction tests have been performed in the same manner as 
described in connection with the first Example. During these tests, an 
excellent reproduction of dot pattern images was achieved. 
Although the present invention has been described in connection with the 
use of magnetic toner, it is to be noted that the use of the magnet roll 5 
fixed within the semiconductor sleeve 16 permits selective use of magnetic 
toner for reproduction of black images and non-magnetic toner, which has a 
high degree of transparency, for reproduction of bright color images. 
EXAMPLE 4 
Using red-color non-magnetic toner in the same developing devices as 
described in connection with the first and third Examples except that the 
peak-to-peak voltage (V.sub.p-p) of the AC voltage applied from the AC 
voltage source 8 is about 2500 volts and the DC voltage applied from the 
DC voltage source 9 is about -350 volts, various dot pattern image 
reproduction tests have been performed. During these tests, an excellent 
reproduction of dot pattern images was achieved. 
In Examples 1 to 4, the charges Q on toner particles were measured. It was 
found that the toner charges Q are distributed in a wide range with a 
variance ranging from -5 .mu.C/g to 25 .mu.C/g. As shown in FIG. 13, the 
present invention can achieve an excellent reproduction of dot or screen 
pattern images with high fidelity. 
It is, therefore, apparent that there has been provided in accordance with 
the present invention, a developing device which can develop dot or screen 
pattern images with high reproductivity and high fidelity without 
degrading the quality of line and solid images. The present invention 
achieves an excellent reproduction of dot or screen pattern images by 
making the sleeve or toner carrier out of semiconductive material. It is 
to be noted that the sleeve may comprise a base member made of 
non-magnetic conductive material, the base member being coated with 
semiconductive material having a specific resistance ranging from 10.sup.6 
to 10.sup.12 .OMEGA.cm. Test results show that the semiconductive coating 
layer should have a thickness equal to or greater than 500 .mu.m to have a 
similar effect. In addition, a high-frequency bias voltage may be applied 
to achieve a high-quality dot pattern image reproduction. The bias voltage 
comprises a high-frequency AC voltage component superposed on a DC voltage 
component. The AC voltage component has a peak-to-peak value determined as 
a function of the length of the gap between the sleeve and the latent 
image carrier. 
While the developing device of the present invention has been described 
above for use in conjunction with copying machines, nevertheless the 
developing device can be used for a variety of applications. For example, 
the developing device of the present invention can be used with a printer, 
in which case, the developing device develops electrostatic latent images 
formed on a dielectric member. While the present invention has been 
described in conjunction with specific embodiments thereof, it is evident 
that many alternatives, modifications and variations will be apparent to 
those skilled in the art. Accordingly, it is intended to embrace all 
alternatives, modifications and variations that fall within the scope of 
the appended claims.