Color image recording method and apparatus

A color image recording apparatus according to the present invention comprises a light-sensitive drum, a first developing device for developing a first electrostatic latent image on the drum surface as a first toner image, a second developing device for developing a second electrostatic latent image as a second toner image on the drum surface along with the first toner image, and a developing bias voltage source for applying a developing bias voltage to the second developer device comprising a DC component and an AC component of a predetermined period, wherein the AC component has a waveform such that the time for the voltage to change from a value such that an electric field making toner move toward the electrostatic latent image is maximum to a value such that the electric field becomes minimum is set to be at least 1/2 of the predetermined period of the AC component. According to a method of the present invention a first toner is electrostatically attracted to an electrostatic latent image on the surface of a light-sensitive drum, and then a second toner is electrostatically attracted to the electrostatic latent image on the drum surface by applying a periodic AC electric field in which the time during which the electric field develops the electrostatic latent image with the second toner exceeds the time during which the electric field does not develop the electrostatic latent image with the second toner. The developed first and second toner images are then simultaneously transfered to a copy sheet.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a color image recording method and 
apparatus which form a plurality of color images on a electrostatic latent 
image formation member such as a light-sensitive member by the use of an 
electrophotographic recording method and then transfer the plurality of 
color images to a recording medium together, and relates in particular to 
a color image recording method and apparatus providing color images of 
improved quality. 
2. Discussion of the Related Art 
Conventionally many different color image recording methods adopting an 
electrophotographic recording method are known. An example of these 
methods is what is called a superimposing development method, which 
successively superimposes plural images of different colors on a 
electrostatic latent image formation member such as a light-sensitive 
member to form a plurality of images of different colors on it. The 
plurality of color images is transferred together to a recording medium, 
resulting in a color image. This method requires only one electrostatic 
latent image formation member such as a light-sensitive drum, and 
additionally does not need a transfer drum; therefore the recording 
apparatus can be miniaturized, and moreover image formation is exceedingly 
speedy because the plurality of color images can be formed during a single 
rotation of the electrostatic latent image formation member. 
However, the superimposing developing method described above has the 
problem that the toner image deteriorates to a high degree during the 
developing process for the second and subsequent colors because the 
previously developed toner image is damaged as it passes through the 
developing area again, thus leading to a final color image with severe 
deterioration. Another problem occurs in the second developing process 
that part of the toner is rubbed off the toner image formed on the 
electrostatic latent image formation member in the first developing 
process and then contaminates the second developer, which causes a 
reduction of the first toner image density and extreme reduction in the 
life of the second developer. 
To overcome the above-mentioned problems in the superimposing developing 
method, that is, to develop the second and subsequent latent images 
without damaging the toner image previously formed on the electrostatic 
latent image formation member, many techniques have been proposed in 
relation to the electrostatic latent image formation method or the 
developing method for the second and subsequent images. 
Japanese Patent Application Unexamined Publication No. Sho. 55-36889 (1980) 
discloses a two-color image reproduction method successively developing 
two electrostatic latent images formed on the surface of an electrostatic 
latent image formation member by a pair of magnetic brush developing 
devices using developers of different colors wherein the second magnetic 
brush developing device is constructed so that its friction contact force 
is less than that of the first magnetic brush developing device. 
In Japanese Patent Application Unexamined Publication No. Sho. 57-79970 
(1982), a two-color developing apparatus for electrophotography having 
first and second developing devices, forms two electrostatic latent images 
of opposite polarities on an electrostatic latent image formation member, 
and develops them with two different two-component toners of different 
colors. The first and second developing apparatuses have main magnets of 
different magnetic power, and additionally the main magnets are 
constructed so that their shape and the distributions of their magnetic 
flux are different. 
The color image recording apparatus disclosed by Japanese Patent 
Application Unexamined Publication No. Sho. 60-126665 (1985) repeats the 
process of forming an electrostatic latent image on an electrostatic 
latent image formation member in a latent image forming section and 
developing the electrostatic latent image in a developing section a number 
of times, and then transfers the visible image to a recording medium. 
Among the plurality of the developing sections described above, at least 
the second and subsequent developing sections employ a two-component 
developer made by mixing a non-magnetic toner and a magnetic carrier 
having a particle diameter of 50 .mu.m or less. 
The electrophotographic recording apparatus disclosed by Japanese Patent 
Application Examined Publication No. Hei. 2-4903 (1990) comprises a means 
for forming a first latent charge image on a light-sensitive drum member, 
a first developing means for developing the first latent charge image 
formed by the means for forming the first latent charge image on the 
light-sensitive drum member by using a magnetic developer having charged 
toner of a first color, a means for forming a second latent charge image 
on the light-sensitive drum member after development by the first 
developing means, a second developing means for developing the second 
latent charge image formed by the means for forming the second latent 
charge image on the light-sensitive drum member by using a magnetic 
developer having charged toner of a second color, wherein the second 
developing means comprises a fixed magnet roller whose portion facing the 
light-sensitive drum member is between two like magnetic poles, a rotating 
sleeve for carrying developer rotatably installed around the periphery of 
the fixed magnet roller, a means for applying a DC bias potential having 
the same polarity as the latent charge image formed on the light-sensitive 
drum member between the rotating sleeve for carrying developer and the 
light-sensitive drum member, and a means for applying an AC bias potential 
to the magnetic developer, which is attached to the surface of the 
rotating carrying sleeve and carried thereby on the portion facing the 
light-sensitive drum member, and wherein the spacing between the surface 
of the magnetic toner attached to the surface of the rotating carrying 
sleeve and the surface of the light-sensitive drum member facing the 
magnetic developer is maintained to be 0.05-0.5 mm. 
Japanese Patent Application Unexamined Publication No. Hei. 2-77767 
discloses a multi-color electrostatic recording apparatus having an 
electrostatic latent image formation member on which an electrostatic 
latent image is formed, a developer holding member for holding developer 
and rotating in a predetermined direction and performing at least two 
cycles of developing processes by applying a predetermined developing bias 
voltage to the developer holding member for attaching developer held by 
the developer holding member to the electrostatic latent image formed on 
the electrostatic latent image formation member to develop it, wherein 1/2 
of the maximum voltage in one waveform cycle of the developing bias 
voltage applied to the developer holding member is set to be different 
from the average voltage in one waveform cycle of the developing bias 
voltage in the second and subsequent developing processes. 
However, the conventional art described above has some problems. The 
inventions disclosed by Japanese Patent Application Unexamined 
Publications Nos. Sho. 55-36889 (1980), Sho. 57-79970 (1982), and Sho. 
60-126665 (1985) are magnetic brush developing methods with a low friction 
contact force; the developing efficiency or developing capability is 
lowered in the second developing process, and therefore a sufficient image 
density cannot be obtained for the second and subsequent colors. 
The non-contact developing method disclosed by Japanese Patent Application 
Examined Publication No. Hei. 2-903 is constructed to maintain the spacing 
of 0.05-0.5 mm between the surface of the magnetic developer attached to 
the peripheral surface of the rotating carrying sleeve for the magnetic 
developer and the surface of the light-sensitive drum member facing the 
magnetic developer. For this reason, it is difficult to make the pile 
height of the magnetic brush uniform to a tolerance of 50 .mu.m or less 
when the spacing is set to be minimum, and besides the low pile density 
causes a reduction of developing efficiency, thus resulting in 
insufficient and excessively uneven image density. 
Another non-contact developing method in Japanese Patent Application 
Unexamined Publication No. Hei. 2-77767 (1990) provides an alternating 
electric field, and by arranging that one half of the peak voltage in a 
cycle of the bias waveform is different from the average voltage, makes it 
easier to transfer the developer to the electrostatic latent image 
formation member, and thus improves development efficiency. In this 
method, although the developing efficiency is improved, the second toner 
adheres to the first image area, (referred to as color mixing,) because an 
electric field strong enough to develop an image is also present in the 
first image area which has been already developed. Moreover, in the case 
where the first and second toner are charged with different polarities, an 
electric field in the direction effective to develop the image with the 
second toner removes the first toner from the surface of the electrostatic 
latent image formation member, which aggravates the reduction of the 
density of the first toner image and deterioration of the second developer 
because it is contaminated by the first toner. 
Further, as one of the other methods related to a non-contact developing 
method, the developing apparatus disclosed by Japanese Patent Application 
Unexamined Publication No. Sho. 60-176069 (1985) is also known though it 
is not necessarily employed for a color image recording method. This 
invention is characterized in that a horizontal magnetic field is applied 
to the developer in a developing area, developer is held on a developer 
holding member without contacting an electrostatic latent image formation 
member and that an oscillating electric field is used for development. 
This method requires a strong oscillating electric field to obtain 
sufficient developing density, and accordingly, if the method is adopted 
for the superimposing developing method, the problem occurs that first 
toner image is removed by the effect of the oscillating electric field, 
thus leading to reduction of density of the first toner image and 
contamination of the second developer with the first toner. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above circumstances and 
has as an object to overcome the above-mentioned problems. 
A further object of the present invention is to provide a color image 
recording method and apparatus preventing deterioration of image quality 
and image density of a first image. 
Another object of the present invention is to provide a color image 
recording method and apparatus avoiding color mixing and contamination of 
a second developing device by a first toner. 
Another object of the present invention is to provide a color image 
recording method and apparatus generating an image with sufficient image 
density in development for the second and subsequent colors. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and attained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the object and in accordance with the purpose of the invention, 
as embodied and broadly described herein, a color image recording 
apparatus of this invention comprises an electrostatic latent image 
formation member, a first toner image forming means for forming a first 
toner image electrostatically on the electrostatic latent image formation 
member, a second toner image forming means for forming a second toner 
image electrostatically on the electrostatic latent image formation member 
on which the first toner image has been previously formed, and a 
developing bias voltage applying means for applying a developing bias 
voltage to the second toner image forming means, wherein the developing 
bias voltage consists of a DC component and an AC component of a 
predetermined period and the AC component has a waveform such that the 
time for the voltage to change from a value such that an electric field 
making toner move toward the electrostatic latent image is maximum to a 
value such that the electric field becomes minimum is set to be at least 
1/2 of the predetermined period of the AC component. 
A color image recording method of this invention comprises the steps of 
attaching a first toner electrostatically to an electrostatic latent image 
formation member, attaching a second toner electrostatically to the 
electrostatic latent image formation member by applying a periodic AC 
electric field in which the time in which the electric field develops an 
electrostatic latent image with toner exceeds the time in which the 
electric field does not develop the electrostatic latent image with toner, 
and transferring the first and second toner held on the electrostatic 
latent image formation member simultaneously to a recording medium. 
In the color image recording method according to the present invention, the 
AC developing bias voltage for the second and subsequent developing 
processes is determined such that the time from a maximum voltage of the 
electric field developing an electrostatic latent image on an 
electrostatic latent image formation member with toner to a minimum 
voltage of the electric field developing the electrostatic latent image on 
an electrostatic latent image formation member with toner is at least 1/2 
of one period of the AC component; therefore the time for the acceleration 
of the toner being transferred to the electrostatic latent image formation 
member to develop the electrostatic latent image change from maximum to 
minimum is increased. As a result, toner developing the electrostatic 
latent image can be easily transferred to the electrostatic latent image 
formation member, which improves developing efficiency in the second and 
subsequent developing processes and provides a sufficient developing 
density even though the AC component of the developing bias voltage is 
small. Since the waveform of the developing bias voltage is selected such 
that an average of the maximum and minimum voltage in one cycle is equal 
to the average voltage value for one cycle, the AC voltage can be set to 
be low to the degree not to affect or remove toner already attached to the 
electrostatic latent image formation member, thus avoiding disturbance of 
the first toner image, reduction of the image density or shortening of the 
life of the second developer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of a color image recording apparatus according to the 
present invention will be described in detail based on the drawings. 
First Embodiment 
FIG. 1 shows the construction of a first embodiment of a color image 
recording apparatus employing a color image recording method according to 
the present invention. 
In the figure, 1 is a light-sensitive drum employed as an electrostatic 
latent image formation member, which is made by forming a thin 
light-sensitive layer 1b on a cylindrical member 1a of conductive 
material. As the light-sensitive layer 1b, may be used, for example, a 
negatively charged organic light-sensitive member, referred to as OPC. The 
outside diameter of the light-sensitive drum 1 is 100 mm and its surface 
linear speed, in other words the process speed is 160 mm/s for instance. 
Around the light-sensitive drum 1, are installed in order in the direction 
of rotation of the light-sensitive drum 1 a first electrical charger 2a, a 
first exposing means 3a, a first developing device 4a, a second electrical 
charger 2b, a second exposing means 3b, a second developing device 4b, a 
pre-transfer corotron 5, a transfer corotron 6, a separating corotron 7, a 
cleaner 8 and an erase lamp 9. 
Any exposing means capable of providing exposure corresponding to image 
information, such as a laser writing device, an LED array or a liquid 
crystal light valve consisting of a uniform light source and a liquid 
crystal shutter may be adopted as the first exposing means 3a and the 
second exposing means 3b described above according to the purpose. These 
first exposing means 3a and second exposing means 3b may serve for either 
image portion exposure or background exposure, selected as the need 
demands. 
The first developing device 4a uses a magnetic brush developing method and 
uses for example, a two-component developer comprising red toner and 
ferrite particle carrier having an average particle diameter of 100 .mu.m. 
On the other hand, the second developing device 4b adopts a non-contact 
developing method and uses for example, a two-component developer 
comprising black toner and carrier made by mixing magnetic particles such 
as magnetite into a synthetic resin having average particle diameter of 40 
.mu.m. 
The developing device used in this embodiment is illustrated with FIG. 2. 
The second developing device 4b comprises a rotatable non-magnetic 
cylindrical sleeve 11 as a developer holding member and a magnetic roller 
12 arranged inside the sleeve 11. The magnetic roller 12 is fixed so that 
an approximate midpoint between magnetic poles 14 and 15 faces the 
light-sensitive drum 1. The thickness of developer attached to the 
non-magnetic cylindrical sleeve 11 for transfer is maintained constant by 
a thickness regulating member 13, and then developer is carried to a 
developing area facing the light-sensitive drum 1 by rotation of the 
non-magnetic cylindrical sleeve 11. In this embodiment, the spacing 
between the light-sensitive drum 1 and the non-magnetic cylindrical sleeve 
11 is set to be 500 .mu.m, and the thickness of the developer in the 
portion facing the light-sensitive drum is set to be 350 .mu.m. The 
outside diameter of the non-magnetic cylindrical sleeve 11 is 25 mm. The 
magnetic pole 14 is at 70.degree. to the magnetic pole 15. 
A predetermined developing bias voltage, which comprises an AC voltage 
superimposed on a DC voltage, is applied to the non-magnetic cylindrical 
sleeve 11 by a developing bias voltage power supply 20, as will be 
described later. The DC component of the developing bias voltage is set to 
be, for example, -500 V to prevent fogging and color mixing into the first 
image. 
The light-sensitive drum 1 is driven by a driving means (not shown in the 
figure) to rotate in the direction shown by an arrow. As shown in FIG. 
3(a), the surface of the light-sensitive drum 1 is first uniformly charged 
at a predetermined voltage, for example -600 V, by the first electrical 
charger 2a. After that an image to be developed by the first color on the 
surface of the light-sensitive drum 1 is formed by the first exposing 
means 3a, whereby an electrostatic latent image corresponding to an image 
of the first color is formed as shown in FIG. 3(b). The exposure 
corresponding to the first color image may be carried out by exposing the 
image portion, for example, resulting in a reduction of the negative 
potential of the image portion to -100 V. As FIG. 3(c) shows, the 
electrostatic latent image corresponding to the first color image formed 
on the surface of the light-sensitive drum 1 is developed with toner of a 
first color, namely, red toner by the first developing device 4a, thus 
providing a toner image. At that time a DC voltage of, for example -450 V 
is applied to a developing sleeve of the first developing device 4a as the 
developing bias voltage. 
The surface of the light-sensitive drum 1 is then uniformly charged at a 
predetermined voltage, for example -650 V, by the second electrical 
charger 2b as shown in FIG. 3(d), and the potential of the image portion 
of the first image rises to -600 V. After that an image to be developed by 
the second color on the surface of the light-sensitive drum 1 is formed by 
the second exposing means 3b, whereby an electrostatic latent image 
corresponding to an image of the second color is formed as shown in FIG. 
3(e). The exposure corresponding to the second color image may also be 
carried out by exposing the image portion, for example, resulting in a 
reduction of the negative potential of the image portion to -100 V. As 
FIG. 3(f) shows, the electrostatic latent image corresponding to the 
second color image formed on the surface of the light-sensitive drum 1 is 
developed with toner of a second color, namely, black toner by the second 
developing device 4b, thus providing a toner image. At that time a 
predetermined developing bias voltage is applied to the non-magnetic 
cylindrical sleeve 11 of the second developing device 4b by the power 
supply 20 for developing bias voltage, as will be described later. 
The toner image formed in red and black on the surface of the 
light-sensitive drum 1 is charged by the pre-transfer corotron 5 as 
required, whereby the first-color toner and second-color toner are made to 
have the same polarities when they are different polarities, and the 
transfer characteristics are improved by an increase of the charge amount. 
The first-color toner and the second-color toner are charged by the 
transfer corotron 6 and transferred to the recording medium 10. The 
separating corotron 7 charges the recording medium 10 so as to be 
separated from the surface of the light-sensitive drum 1. The recording 
medium 10 having been separated from the light-sensitive drum 1 is carried 
to a fuser (not shown in the figure) and toner images of two colors are 
fused on the surface of the recording medium 10, thus completing the color 
image recording. 
After transferring toner image and separating the recording medium 10, 
residual toner on the surface of the light-sensitive drum 1 is removed by 
the cleaner 8, and then residual charge is exposed to the erase lamp 9 to 
be eliminated so that the light-sensitive drum 1 is able to prepare for 
the next process of color image recording. 
In this embodiment, the developing bias voltage in at least the second and 
subsequent developing processes is an AC voltage superimposed on a DC 
voltage. The AC voltage waveform is such that the time from the maximum 
value of the electric field developing the electrostatic latent image (on 
an electrostatic latent image formation member) with toner to the minimum 
value of the electric field developing the electrostatic latent image (on 
an electrostatic latent image formation member) with toner is at least 1/2 
of one cycle of the AC component. 
FIG. 4 shows a circuit diagram of a developing bias voltage power supply 
for applying developing bias voltage to the second developing device. 
In the figure, 20 refers to the developing bias voltage power supply, which 
comprises an AC voltage generator 21 and a DC voltage generator 22 for 
outputting AC voltage superimposed on a DC voltage. 
As shown in FIG. 5, the AC voltage generator 21 is constructed to output AC 
voltage having different waveform depending on whether the gradient dv/dt 
of the AC voltage is positive or negative. That is, when the gradient 
dv/dt is positive, the AC voltage generator 21 generates an AC voltage 
V.sub.AC whose value V.sub.AC1 is expressed by the following equation: 
EQU V.sub.AC1 =(V.sub.PP /2).times.sin(.omega..sub.1 t) 
and on the other hand, when dv/dt is negative, generates an AC voltage 
V.sub.AC whose value V.sub.AC2 is expressed by the following equation: 
EQU V.sub.AC2 =(V.sub.PP /2).times.sin(.omega..sub.2 t) 
wherein .omega..sub.2 is set to be larger than .omega..sub.1 as will be 
explained later. The relation between .omega..sub.1 and .omega..sub.2 
varies depending on the polarity of the toner. 
The AC voltage generator 21 described above can generate an AC voltage of a 
predetermined frequency, in which the rise time T.sub.1 from the minimum 
voltage V.sub.min to a maximum voltage V.sub.max and the fall time T.sub.2 
from the maximum voltage V.sub.max to the minimum voltage V.sub.min are 
different. Moreover, the AC voltage generator 21 can vary the ratio of the 
time for the voltage to change from the value such that the electric field 
developing the electrostatic latent image (on the electrostatic latent 
image formation member) is maximum to the value such that the electric 
field developing an electrostatic latent image (on the electrostatic 
latent image formation member) is minimum to one cycle of the AC component 
T, which is referred to as the fall-time factor D.sub.f, where D.sub.f 
=T.sub.1 /(T.sub.1 +T.sub.2). Correspondingly the rise-time factor D.sub.r 
is given by D.sub.r =T.sub.2 /(T.sub.1 +T.sub.2). The period T of the AC 
voltage V.sub.AC is constant as follows: 
EQU T=T.sub.1 +T.sub.2 =1/f=1/1500 s 
where the frequency f is 1500 Hz, but the amplitude V.sub.PP of the AC 
voltage V.sub.AC can be changed. 
The rise-time factor D.sub.r and fall-time factor D.sub.f are collectively 
referred to as the active-time factor D.sub.a. The reason for both the 
rise-time factor D.sub.r and fall-time factor D.sub.f corresponding to the 
active-time factor D.sub.a is that the direction of the electric field 
acting on the toner depends on the polarities of the developing bias 
voltage and the toner. As FIG. 7 shows, to conduct reverse development, 
that is, when the polarities of the developing bias voltage and toner are 
both negative, the voltage at which the electric field developing the 
electrostatic latent image with toner becomes maximum is V.sub.min in FIG. 
7 and the voltage at which the electric field developing the electrostatic 
latent image with toner becomes minimum is V.sub.max, and therefore the 
active-time factor in this case is T.sub.A /T.sub.B. 
On the other hand, when the polarity of the developing bias voltage is 
negative and the polarity of toner is positive, that is, to conduct normal 
development, the voltage at which the electric field developing the 
electrostatic latent image with toner becomes maximum is V.sub.max and the 
voltage at which the electric field developing the electrostatic latent 
image with toner becomes minimum is V.sub.min as shown in FIG. 10: 
therefore the active-time factor is given by T.sub.A /T.sub.B. 
The DC voltage generator 22 consists of a DC power supply 23 which is 
well-known and provides a variable output voltage V.sub.DC. The output 
voltage V.sub.DC is input to a capacitor C2, which acts as a bypass 
capacitor for the AC output current to be superimposed on the DC voltage. 
The above-described output voltage V.sub.DC can be controlled by an 
external V.sub.DC control signal. 
The construction of the AC voltage generator 21 will now be described in 
more detail. The AC voltage generator 21 has a waveform signal generating 
means 24 which determines the waveform of the output AC voltage. The 
waveform signal generating means 24 has a first binary counter 25. The 
first binary counter 25, a pulse oscillator 26 connected thereto, and 
peripheral gate circuits constitute a circuit for changing the count-up 
timing of the address output to a PROM 30 from a second binary counter 29, 
as will be described later, depending on whether the most significant bit 
of the count value of the second binary counter 29 is zero or one. 
The pulse oscillator 26 is connected to the first binary counter 25 through 
NAND circuits 27 and 28 and generates a clock pulse signal CLK of a 
predetermined frequency, for example 2.88 MHz. Through the NAND circuits 
27 and 28, the clock pulse signal CLK generated by the pulse oscillator 26 
is input to a CLKDWN terminal and a CLKUP terminal. Active-time ratio 
control data is input to the first binary counter 25 described above and 
can be selected appropriately by an active-time ratio control data setting 
means (not shown in the figure). 
When the most significant bit of the output of the second binary counter 29 
is zero, a signal which is input to the CLKDWN terminal of the first 
binary counter 25 inputs through the NOT and NAND circuits 27, and the 
first binary counter 25 decrements the active-time factor control data by 
one on every input of a clock pulse from the pulse oscillator 26. The 
first binary counter 25 then outputs a clock pulse signal from a B.O 
(borrow out) terminal each time a count-down series is finished. For that 
reason, the frequency of the clock pulse signal output from the first 
binary counter 25 is (the frequency of the pulse oscillator 
26).times.(active-time factor control data). On the other hand, when the 
most significant bit of the output of the second binary counter 29 is one, 
a signal is input to the CLKUP terminal of the first binary counter 25 
through NAND circuit 28, and the first binary counter 25 increments the 
active-time factor control data by one on every input of a clock pulse 
from the pulse oscillator 26, and further outputs the clock pulse signal 
from a C.O (carry out) terminal each time a count-up series is finished. 
For that reason, the frequency of the clock pulse signal output from the 
first binary counter 25 is (the frequency of the pulse oscillator 
26).times.(15-active-time factor control data). 
The clock pulse signal output from the first binary counter 25 is then 
input to the second binary counter 29. The second binary counter 29 counts 
based on the clock pulse signal output from the first binary counter 25 
and outputs a signal corresponding to a count value from output terminals 
Q(.sub.1A...Q.sub.4B). At the same time, the second binary counter 29 also 
outputs the most significant bit of the count value from the relevant 
output terminal Q to the first binary counter 25. 
The counting of the second binary counter 29 is conducted based on the 
clock pulse signal output from the first binary counter 25. As described 
above, the clock pulse signal has different output frequencies as the most 
significant bit of the output of the binary counter 29 is zero or one; 
therefore, the frequency of count values output from the second binary 
counter 29 depends on whether the most significant bit of the output of 
the binary counter 29 is zero or one. 
The signal output from the second binary counter 29 is input to the PROM 
30. As shown in FIG. 6, data values corresponding to sine wave are stored 
in the PROM 30 with addresses in the range 0d (00h) to 255d (7Fh). The 
PROM 30 outputs sine waveform data in accordance with the address signal 
input from the second binary counter 29; therefore the PROM 30 outputs a 
waveform similar to the sine wave shown in FIG. 6 when the signals are 
input from the second binary counter 29 with a predetermined period. 
As described above, the signal output from the second binary counter 29 has 
different frequencies based on the active-time factor control data 
depending on whether the most significant bit of the count value of the 
second binary counter 29 is zero or one. As a result, the PROM 30 outputs 
data which corresponds to waveform of sine wave and has different 
frequencies in accordance with whether the gradient of the sine wave dv/dt 
is positive (rise portion) or negative (fall portion). 
The output data from the PROM 30 described above is then input to a 
digital-analog converter 31 which outputs an analog current from output 
terminal OUT1 corresponding to the output data of PROM 30 shown in FIG. 6. 
A V.sub.AC control signal is input to the digital-analog converter 31 and 
a current output is linearly modulated by the V.sub.AC control signal, 
which makes it possible to linearly control the amplitude of the output 
current from the digital-analog converter 31 using the V.sub.AC control 
signal. 
The output current from the digital-analog converter 31 is further input to 
an I/V converter 32 to undergo current-to-voltage conversion. The output 
from the I/V converter 32 is output to a buffer amplifier 33 through a 
capacitor C. At that time, the I/V converter 32 can control the gain by a 
variable resistance R1 so that the required AC output voltage is obtained 
using the V.sub.AC control signal. 
An AC voltage is output from the buffer amplifier 33 corresponding to the 
waveform signal output from the waveform signal generating means 24 such 
as described above, and increased by a transformer T, and then output from 
an output terminal V.sub.OUT to the second developing device 4b as the 
developing bias voltage superimposed on a DC voltage. 
If the voltage V.sub.AC is 1500 V and the output of the buffer amplifier 33 
is 15 V, the transformer T has a transformer ratio of 100. The buffer 
amplifier 33 is a power buffer whose voltage gain is 1 and its output of 
AC voltage V.sub.AC is the output of the voltage waveform from the 
waveform signal generating means 24 multiplied by the transformer ratio of 
the transformer T. 
As described above, the active-time factor (D.sub.a =T.sub.1 /(T.sub.1 
+T.sub.2)) of the AC voltage V.sub.AC output from the AC voltage generator 
21 can be varied in accordance with the active-time factor control data. 
That is, as shown in FIG. 5, the time T.sub.2 for the voltage to change 
from the value such that the electric field developing the electrostatic 
latent image on the electrostatic latent image formation member is maximum 
to the value such that the electric field developing an electrostatic 
latent image on the electrostatic latent image formation member is minimum 
is expressed by: 
T.sub.2 =(the period of the pulse oscillator 26).times.(15-active-time 
factor control data).times.128 
and the remainder T.sub.1 is expressed by: 
T.sub.1 =(the period of the pulse oscillator 26).times.active-time factor 
control data.times.128. 
The number 128 is half of the number of data values for one period stored 
in the PROM 30, and accordingly, one period of the AC voltage T is: 
T=T.sub.1 +T.sub.2 =(the period of the pulse oscillator 
26).times.15.times.128, 
and the active-time factor is given by: 
D.sub.a =active-time factor control data/15. 
When the frequency of the pulse oscillator 26 is 2.88 MHz, the output is 
1500 Hz. As shown in the table below, the active-time factor can be 
controlled in accordance with the equation D.sub.a =active-time factor 
control data/15. 
TABLE 1 
______________________________________ 
Active-time factor 
control data D.sub.a 
______________________________________ 
2d 0.133 
3d 0.200 
4d 0.267 
5d 0.333 
6d 0.400 
7d 0.467 
8d 0.533 
9d 0.600 
10d 0.667 
11d 0.733 
12d 0.800 
13d 0.867 
______________________________________ 
The AC voltage generator of this embodiment has been explained based on the 
construction shown in FIG. 4, but it is clear that the active-time factor 
of the AC voltage V.sub.AC can be controlled with higher resolution by 
altering the first binary counter 26 and its peripheral circuits. Though 
the waveform signal generating means 24 is installed inside the power 
supply 20 in FIG. 4, the waveform signal can equally be generated by a CPU 
and digital-analog converter in the controller and the power supply 20 may 
be constituted without the buffer amplifier 33 and the DC voltage 
generator 22. Moreover, analog circuits can also be used to generate the 
waveform signal, though the waveform signal generating means 24 consists 
of digital circuits in this embodiment. The analog circuits can be 
substituted for the digital circuits. 
The color image recording apparatus of this embodiment having the 
above-described construction carries out development by the second 
developing device as follows. 
After formation of the first toner image, a second electrostatic latent 
image is formed on the surface of the light-sensitive drum and then 
developed by the second developing device 4b, that is, the second 
electrostatic latent image formed on the light-sensitive drum 1 is 
developed by toner contained in developer attached to the surface of the 
non-magnetic cylindrical sleeve 11 as shown in FIG. 2. At that time, an AC 
voltage superimposed on a predetermined DC voltage is applied as a 
developing bias voltage having waveform shown in FIG. 7 by the developing 
bias voltage power supply 20. Negatively charged toner contained in the 
developer is therefore ejected toward the light-sensitive drum 1 from the 
non-magnetic cylindrical sleeve 11 by the electric field formed by the AC 
component of the developing bias voltage. The AC voltage is applied so 
that the time from a point that the voltage makes the electric field 
developing the electrostatic latent image by toner maximum to a point that 
the voltage makes the electric field developing the electrostatic latent 
image by toner minimum is 1/2 or more of one period of the AC component. 
Consequently, the electric field E generated by the above-described AC 
voltage applies a force F expressed as follows to toner to be ejected 
toward the light-sensitive drum 1 from the non-magnetic cylindrical sleeve 
11. 
EQU F=qE 
If the spacing between the light-sensitive drum 1 and the non-magnetic 
cylindrical sleeve 11 is d, E is expressed by the following equation: 
EQU E=.DELTA.V/d=(V.sub.BIAS -V.sub.IMAGE)/d 
and then the motion of the toner mentioned above can be described by an 
equation of motion as follows: 
EQU F=ma=qE. 
That is to say, the force F=qE is applied to toner, and the time for the 
acceleration effective to move the toner in the direction which develops 
the second electrostatic latent image on the light-sensitive drum 1 to 
change from the maximum to minimum is increased. For this reason, the 
distance moved by the toner in a certain time is increased as the ratio of 
the time for the voltage T.sub.2 to change from the value such that the 
electric field carrying out development is maximum to the value such that 
the electric field carrying out development is minimum to one period T of 
the AC component, namely, the active-time factor increases as shown in 
FIG. 8. Toner is ejected toward the light-sensitive drum 1 easily, which 
leads to effective development of the second electrostatic latent image 
and prevents reduction of the second toner density. 
The waveform of the developing bias voltage is set to be such that an 
average voltage value of the maximum and minimum voltage is equal to the 
average voltage during one cycle; therefore, the second toner does not 
easily attach to the first toner image already developed, thus avoiding 
turbulence of the first image, reduction of the image density, and color 
mixing, and preventing the first toner from entering the second developing 
device. 
In the second developing device 4b, the magnetic roller 12 is fixed inside 
the rotatable non-magnetic cylindrical sleeve 11 so that an approximate 
midpoint between magnetic poles 14 and 15 faces the light-sensitive drum 
1. A magnetic brush formed between the magnetic poles 14 and 15 in the 
magnetic roller 12 is approximately parallel to the surface of the 
light-sensitive drum 1, and therefore, a uniform magnetic brush, not a 
non-uniform magnetic brush, acts on the light-sensitive drum 1, which 
forms a toner image of predetermined density efficiently and precisely. 
Second Embodiment 
Now a second embodiment of the present invention will be described, in 
which the ratio of time for the AC component of the developing bias 
voltage to change from a value such that the electric field developing the 
electrostatic latent image is maximum to a value such that the electric 
field becomes minimum to one period of AC component is adjustable. FIG. 11 
is the same as FIG. 1 except for the exposing means 3 and developing 
device 4. 
The developing bias voltage power supply 20 is the same as the one shown in 
FIG. 4. As shown in FIG. 12, a toner density detection means 40 consisting 
of a piezoelectric element or the like is installed inside the housing of 
the developing device 4. The output from the toner density detection means 
40 is input to a CPU 41 which determines a active-time factor by referring 
to a lookup table previously stored in a ROM 42 so that the predetermined 
image density may be obtained. The active-time factor determined by the 
CPU 41 is input to an active-time factor control means 43. The active-time 
factor control means 43 inputs a digital active-time factor control data 
value as shown in Table 1 to the developing bias voltage power supply 20. 
By using the developing device 4 in the color image recording apparatus of 
FIG. 11, the image density can be adjusted without fogging or development 
of carrier particles when the toner density varies as a result of toner 
shortage. The developing density may be equally detected instead of the 
toner density. The active-time factor mentioned above can be changed 
manually in accordance with the image density. 
First experiment 
Using the color image recording apparatus shown in FIG. 1, a color image 
was experimentally recorded using image exposure in the first and second 
exposure and reverse development in the first and second development. The 
toner was negatively charged both in the first developer and in the second 
developer. 
Now the image formation process in this first experiment will be explained 
based on FIG. 3. 
As shown in FIG. 3(a), the surface of an OPC light-sensitive drum 1 is 
uniformly charged to -600 V by a first electrical charger 2a. Image 
exposure is carried out by using a laser beam to form a negative latent 
image having an exposed portion potential of -100 V (FIG. 3(b)). The 
negative latent image is reverse developed by a first developing device 3a 
with a DC component of the developing bias voltage of -450 V (FIG. 3(c)). 
Second charging is carried out by the second electrical charger 2b as 
shown in FIG. 3(d). After that, the potentials of the first image portion 
and the first background (non-image) portion are -600 V and -650 V, 
respectively. A negative latent image with an exposed portion potential of 
-100 V is formed by a laser beam (FIG. 3(e)), and then reverse developed 
by a second developing device 3b (FIG. 3(f)). 
The second developing device 3b used in this experiment will now be 
illustrated based on FIG. 2. 
In this experiment, the spacing between the light-sensitive drum 1 and the 
non-magnetic cylindrical sleeve 11 is set to be 500 .mu.m and thickness of 
the developer layer where it faces the light-sensitive drum 1 is set to be 
350 .mu.m. The outside diameter of the non-magnetic cylindrical sleeve 11 
is 25 mm. The magnetic pole 14 is at 70.degree. to the magnetic pole 15. 
A developing bias voltage, which comprises an AC voltage superimposed on a 
DC voltage, is applied to the nonmagnetic cylindrical sleeve 11 by the 
developing bias voltage power supply 20. The DC component of the 
developing bias voltage is set to be -500 V to prevent fogging and color 
mixing in the first image. 
Here, a frequency of 1500 Hz is employed for the AC component of the second 
developing bias voltage and the voltage and active-time factor are varied 
to investigate the relation between the active-time factor and reduction 
of the first and second developing densities, and deterioration of the 
first toner image. The waveform of the developing bias voltage is shown in 
FIG. 7. Here, the AC voltage V.sub.P-P refers to .vertline.V.sub.max 
-V.sub.min .vertline.. The active-time factor is, as described above, the 
ratio of the time (T.sub.A) for the voltage to vary from the value such 
that the electric field developing the electrostatic latent image on the 
light-sensitive drum 1 is maximum to the value such that the electric 
field developing the electrostatic latent image on the light-sensitive 
drum 1 is minimum to the period of the AC component T.sub.B. 
The result of the above-described experiment is shown in Table 2 below. To 
evaluate the result of the experiment, and in particular to evaluate the 
image density, a solid image was measured using a reflection densitometer 
(trade name: "X-RITE 310"). For the second image density, a density of at 
least 1.3 is sufficient both for solid blocks and line images, and 
therefore an image density of 1.3 or more is indicated as "Good" and less 
than 1.3 is indicated as "Poor" in the table. The reduction of the first 
image density is measured by the difference obtained by subtracting the 
density of a two-color image after second development from the density of 
a one-color image: "Good" indicates that there is no difference and "Bad" 
shows that there is a difference, that is, that reduction of the first 
image density occurs. Deterioration of the first image is measured by the 
degree of its occurrence, where "Good" shows that no deterioration occurs, 
"Acceptable" shows that some deterioration occurs but is acceptable from a 
practical viewpoint (the rate of thickening of a linear image is not more 
than .+-.10%), and "Bad" indicates that image deterioration is 
unacceptable (the rate of thickening of a linear image is in excess of 
.+-.10%). 
TABLE 2 
______________________________________ 
Active- Deterioration 
Reduction of 
V.sub.P--P 
time of the the first Second image 
(kV) factor first image 
image density 
density 
______________________________________ 
0.75 0.25 Good 0.0 Good 1.20 Poor 
0.40 Good 0.0 Good 1.25 Poor 
0.50 Good 0.0 Good 1.30 Good 
0.60 Good 0.0 Good 1.32 Good 
0.75 Good 0.0 Good 1.35 Good 
1.00 0.25 Good 0.0 Good 1.25 Poor 
0.40 Good 0.0 Good 1.28 Poor 
0.50 Good 0.0 Good 1.35 Good 
0.60 Good 0.0 Good 1.38 Good 
0.75 Good 0.0 Good 1.40 Good 
1.50 0.25 Acceptable 0.0 Good 1.36 Good 
0.40 Acceptable 0.0 Good 1.39 Good 
0.50 Acceptable 0.0 Good 1.40 Good 
0.60 Acceptable 0.0 Good 1.40 Good 
0.75 Acceptable 0.0 Good 1.40 Good 
2.00 0.25 Bad 0.3 Bad 1.40 Good 
0.40 Bad 0.3 Bad 1.40 Good 
0.50 Bad 0.3 Bad 1.40 Good 
0.60 Bad 0.3 Bad 1.40 Good 
0.75 Bad 0.3 Bad 1.40 Good 
______________________________________ 
As will be seen from Table 2, by setting the active-time factor to be 0.5 
or more for the same voltage value V.sub.PP, the efficiency of development 
is improved. In other words, by making the time T.sub.2 for the voltage to 
change from a value such that the electric field developing the 
electrostatic latent image on the light-sensitive drum 1 is maximum to a 
value such that the electric field developing the electrostatic latent 
image on the light-sensitive drum 1 is minimum is 1/2 or more of one 
period of the AC component T, the efficiency of development is improved. 
If the active-time factor is set to be as described above, a sufficient 
image density can be obtained when the voltage is at least 0.75 kV. When 
the voltage V.sub.PP is 2 kV, the electric field to remove toner from the 
surface of the light-sensitive drum 1 grows in force, and therefore 
reduction of the first image density or deterioration of the first image 
occurs. If the active-time factor is set to be as is mentioned above and 
the voltage value V.sub.PP is set to be from 0.75 kV to 2 kV, a sufficient 
developing density can be obtained without reducing the first image 
density or causing deterioration of the first image. 
Second experiment 
Color image recording was carried out using the color image recording 
apparatus shown in FIG. 1, adopting image portion exposure for the first 
exposure, background (non-image) portion exposure for the second exposure, 
reverse development for the first development and normal development for 
the second development. The toner contained in the first developer was 
negatively charged, and the toner contained in the second developer was 
positively charged. 
Now the image formation process is explained based on FIGS. 9(a)-9(e). 
The surface of the OPC light-sensitive drum 1 is uniformly charged to -600 
V by the first electrical charger 2a (shown in FIG. 9(a)). The image 
portion exposure is carried out by using a laser beam to form a negative 
latent image having an exposed portion potential of -100 V (FIG. 9(b)). 
The negative latent image is reverse developed with a developing bias 
voltage of -450 V by the first developing device 3a (shown in FIG. 9(c)). 
Next, a positive latent image having an exposed portion potential of -130 
V was formed by the laser beam (shown in FIG. 9(d)). After the second 
exposure, the potential of the first image portion is -100 V, and the 
potential of the background (non-image) portion is -560 V. Here, the 
normal development is carried out by the second developing device 3b 
(shown in FIG. 9(e)). In this experiment, a second electrical charger 2b 
does not carry out charging, and therefore, the second electrical charger 
2 b may be omitted. 
The second developing device 4b used in this experiment is the same as that 
of the first experiment. The developing bias voltage has an AC voltage 
superimposed on a DC voltage. The AC component of the developing bias 
voltage is set to be -230 V to prevent fogging and color mixing with the 
first image. 
Here, the relation between different values of the voltage value and the 
active-time factor and reduction of the second developing density, 
reduction of the first image density and deterioration of the first image 
is examined by adopting a frequency of 1500 Hz for the AC component of the 
second developing bias voltage. The waveform of developing bias voltage is 
shown in FIG. 10, wherein V.sub.P-P refers to .vertline.V.sub.max 
-V.sub.min .vertline.. The active-time factor refers to the ratio of the 
time T.sub.A to change from the voltage V.sub.max at which the electric 
field developing the electrostatic latent image on the light-sensitive 
drum 1 is maximum to the voltage V.sub.min at which the electric field 
developing the electrostatic latent image on the light-sensitive drum 1 is 
minimum to one period of the AC component of the developing bias voltage 
T.sub.B. The evaluation was carried out in the same way as in the first 
experiment. The result is shown in Table 3 below. 
TABLE 3 
______________________________________ 
Active- Deterioration 
Reduction of 
V.sub.P--P 
time of the first 
the first Second image 
(kV) factor image image density 
density 
______________________________________ 
0.75 0.25 Good 0.0 Good 1.23 Poor 
0.40 Good 0.0 Good 1.27 Poor 
0.50 Good 0.0 Good 1.31 Good 
0.60 Good 0.0 Good 1.35 Good 
0.75 Good 0.0 Good 1.38 Good 
1.00 0.25 Good 0.0 Good 1.25 Poor 
0.40 Good 0.0 Good 1.28 Poor 
0.50 Good 0.0 Good 1.33 Good 
0.60 Good 0.0 Good 1.37 Good 
0.75 Good 0.0 Good 1.40 Good 
1.50 0.25 Acceptable 0.0 Good 1.37 Good 
0.40 Acceptable 0.0 Good 1.40 Good 
0.50 Acceptable 0.0 Good 1.40 Good 
0.60 Acceptable 0.0 Good 1.40 Good 
0.75 Acceptable 0.0 Good 1.40 Good 
2.00 0.25 Bad 0.15 Bad 1.40 Good 
0.40 Bad 0.15 Bad 1.40 Good 
0.50 Bad 0.15 Bad 1.40 Good 
0.60 Bad 0.15 Bad 1.40 Good 
0.75 Bad 0.15 Bad 1.40 Good 
______________________________________ 
As will be seen from Table 3, by setting the active-time factor to be 0.5 
or more for the same voltage value V.sub.PP, PG,38 the developing 
efficiency is improved. In other words, by making the time T.sub.2 for the 
voltage to change from a value such that the electric field developing the 
electrostatic latent image on the light-sensitive drum 1 is maximum to a 
value such that the electric field developing the electrostatic latent 
image on the light-sensitive drum 1 is minimum is 1/2 or more of one 
period of the AC component T, the efficiency of development is improved. 
If the active-time factor is set to be as described above, a sufficient 
image density (1.3 or more) can be obtained when the voltage is 0.75 kV or 
more. When the voltage V.sub.PP is 2 kV, the electric field to remove 
toner from the surface of the light-sensitive drum 1 grows in force, and 
therefore reducing of the first image density or turbulence of the first 
image occur. If the duty factor is set to be as is mentioned above and the 
voltage value V.sub.PP is set to be within 0.75 kV to 2 kV, sufficient 
(satisfactory) developing density can be obtained without reducing the 
first image density and causing turbulence of the first image. 
Third experiment 
An experiment using different active-time factors in image formation was 
carried out using the color image recording apparatus shown in FIG. 11 
under the following conditions: 
Exposure: image portion exposure 
Development: non-contact, two-component, reverse development 
Developer: negatively charged black toner (average particle diameter 10 
.mu.m), positively charged carrier with magnetic particles dispersed in it 
(average particle diameter 45 .mu.m, density 2.2 g/cm.sup.3) 
Toner concentration: 3%, 9%, 12% and 15% 
Process speed: 200 mm/sec 
Process of image formation: as shown in FIG. 14 
DC component of developing bias voltage: -500 V 
AC component of developing bias voltage: composite sine wave with frequency 
of 1500 Hz and V.sub.P-P of 1.5 kV 
The results of this experiment are shown in Table 4. The density of the 
second image was evaluated in the same way as in the first and second 
experiments. With regard to fogging, "None" indicates that no fogging 
occurred, "Acceptable" indicates that some fogging occurred but was 
acceptable from a practical viewpoint, and "Bad" indicates that fogging 
occurred and the active-time factor is inadequate. To evaluate the carrier 
particle attachment, the area ratio of carrier particles in the background 
portion was measured by an image resolution apparatus in comparison with 
an image in which the area ratio of line images to the background portion 
is 1:1 and the lines are formed at a spacing of 2 lines per mm. In Table 
4, "Good" indicates that the area ratio is less than 1.0, and "Poor" 
indicates that it is 1.0 or more. 
TABLE 4 
__________________________________________________________________________ 
Toner 
concen- 
Duty 
Image Occurrence 
Carrier Overall 
tration(% 
factor 
density of fogging 
area ratio 
evaluation 
__________________________________________________________________________ 
3 0.25 
1.11 
Poor 
None 0.49 
Good 
Poor 
0.40 
1.18 
Poor 
None 0.43 
Good 
Poor 
0.50 
1.24 
Poor 
None 0.24 
Good 
Poor 
0.60 
1.31 
Good 
None 0.17 
Good 
Good 
0.75 
1.35 
Good 
None 0.12 
Good 
Good 
9 0.25 
1.21 
Poor 
None 0.71 
Good 
Poor 
0.40 
1.27 
Poor 
None 0.62 
Good 
Poor 
0.50 
1.34 
Good 
None 0.44 
Good 
Good 
0.60 
1.38 
Good 
None 0.37 
Good 
Good 
0.75 
1.40 
Good 
None 0.25 
Good 
Good 
12 0.25 
1.36 
Good 
None 0.89 
Good 
Good 
0.40 
1.39 
Good 
None 0.77 
Good 
Good 
0.50 
1.40 
Good 
None 0.59 
Good 
Good 
0.60 
1.40 
Good 
Acceptable 
0.45 
Good 
Good 
0.75 
1.40 
Good 
Acceptable 
0.29 
Good 
Good 
15 0.25 
1.40 
Good 
None 0.93 
Good 
Good 
0.40 
1.40 
Good 
Acceptable 
0.87 
Good 
Good 
0.50 
1.40 
Good 
Acceptable 
0.72 
Good 
Good 
0.60 
1.40 
Good 
Bad 0.51 
Good 
Poor 
0.75 
1.40 
Good 
Bad 0.32 
Good 
Poor 
__________________________________________________________________________ 
As will be seen from Table 4, when the duty factor is high, the image 
density increases and fogging increases. On the other hand, the rate of 
attachment of carrier particles is reduced, and therefore the image 
density can be adjusted while preventing the occurrence of fogging and the 
attachment of carrier particles. 
Fourth experiment 
A fourth experiment was carried out in the same way as the third experiment 
except that three types of toner, with charge densities of -3 .mu.C/g, -8 
.mu.C/g, and -12 .mu.C/g were used and the toner concentration was 
constant at 9%. The results of the experiment are shown in Table 5. 
TABLE 5 
______________________________________ 
Over- 
Occur- all 
Charge 
Duty Image rence of 
Carrier evalu- 
density 
factor density fogging 
area ratio 
ation 
______________________________________ 
-3 0.25 1.11 Poor None 0.49 Good Poor 
.mu.C/g 
0.40 1.18 Poor None 0.43 Good Poor 
0.50 1.24 Poor None 0.24 Good Poor 
0.60 1.31 Good None 0.17 Good Good 
0.75 1.35 Good None 0.12 Good Good 
-8 0.25 1.21 Poor None 0.71 Good Poor 
.mu.C/g 
0.40 1.27 Poor None 0.62 Good Poor 
0.50 1.34 Good None 0.44 Good Good 
0.60 1.38 Good None 0.37 Good Good 
0.75 1.40 Good None 0.25 Good Good 
-12 0.25 1.36 Good None 0.89 Good Good 
.mu.C/g 
0.40 1.39 Good None 0.77 Good Good 
0.50 1.40 Good None 0.59 Good Good 
0.60 1.40 Good Accept- 
0.45 Good Good 
able 
0.75 1.40 Good Accept- 
0.29 Good Good 
able 
______________________________________ 
As will be seen from Table 5, when the charge density of the toner is 
small, the image density becomes low and on the other hand, if the charge 
amount of toner is large, the carrier particles tend to attach to the 
recording medium. By controlling the active-time factor in accordance with 
the charge density of the toner, the image density can be adjusted while 
preventing fogging and carrier attachment. 
The above-described control of the active-time factor may be carried out in 
combination with control of the DC component of the developing bias 
voltage, control of the charging potential or control of the exposure 
amount. A contact developing method or a one-component developing method 
may equally serve as the developing method. 
A composite sine wave is adopted as the waveform of the AC component of the 
second developing bias voltage in the above-described embodiment, but a 
triangular pulse wave or other waveform may also provide the same effect. 
The above-described embodiment may be applied to a non-contact developing 
method using a two-component developer or a contact developing method for 
the second development. Moreover, they can also use a contact magnetic 
brush developing method with a low contact friction force using a 
single-component magnetic toner or a two-component developer. 
Furthermore, though the approximate midpoint between the opposite magnetic 
poles adjacent to each other in the developer holding member faces the 
electrostatic latent image formation member in the embodiment described 
above, it is possible to make the approximate midpoint between similar 
magnetic poles adjacent to each other face the electrostatic latent image 
formation member. The above-described embodiment is also adaptable to a 
developing method in which a magnetic pole in the developer holding member 
approximately faces the electrostatic latent image formation member. 
A light-sensitive member is employed as the latent image formation member 
in the embodiment described above. However, a dielectric member may be 
used as the latent image formation member to form the electrostatic latent 
image by an electrical discharge recording member such as used in an 
electrostatic printer or by an ion current control head as disclosed in 
Japanese Patent Application Unexamined Publication No. Sho. 59-190854 
(1984), and the like. 
Additionally, the above-described embodiments relate to recording 
apparatuses for two-color reproduction, but the electrostatic latent image 
formation process is not limited to these embodiments and may also be used 
in a recording apparatus for three or more colors. 
The foregoing description of preferred embodiments of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and modifications and variations are possible in light of the 
above teachings or may be acquired from practice of the invention. The 
embodiment was chosen and described in order to explain the principles of 
the invention and its practical application to enable one skilled in the 
art to utilize the invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. It is 
intended that the scope of the invention be defined by the claims appended 
hereto, and their equivalents.