Image forming method and apparatus therefor

An image forming method wherein a latent image is formed by one of plurality of latent image forming devices and developed by one of plurality of developing devices. The above steps are repeated to form an image. An image forming apparatus having a first latent image forming device, a first developing device, a second latent image forming device, and a second developing device sequentially arranged in face of an image retainer in this order.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming method and an apparatus 
therefor and, more particularly, to the image forming method and apparatus 
suitable for image formations by the electrophotography. 
2. Description of the Prior Art 
In recent years, there appears an image forming apparatus for making 
full-color copies by using a full-color original image (or document) in 
the image forming by the electrophotography. The multi-color image is very 
favorable not only for reproducing pictures of figures, still lives, 
landscapes and so on but also for diagrams, tables and so on because one 
recorded image can incorporate many pieces of informations. 
From the circumstances described above, there have been proposed a variety 
of methods of and apparatus for forming the multi-color images. 
In one method (as is disclosed in Japanese Patent Laid-Open Nos. 
106743/1977, 144452/1981 and 79261/1983), for example, a plurality of 
latent image forming means and a plurality of developing means are 
arranged around a rotating drum-shaped photosensitive member, visible 
images of different colors are formed and superposed on the drum-shaped 
photosensitive member by repeatedly forming and developing the latent 
images and are transferred altogether to a sheet of recording paper. 
In another method (as is disclosed in Japanese Patent Laid-Open Nos. 
76766/1985 and 95456/1985), one latent image forming means and a plurality 
of developing means are arranged around a rotating drum-shaped 
photosensitive member to form and develop a latent image of one color for 
each rotation of the photosensitive member, and multi-color visible images 
are formed on the photosensitive member as a result of rotations of the 
photosensitive member and are transferred altogether to a sheet of 
recording paper. 
In case the colors to be reproduced are full colors including yellow, 
magenta and cyan colors, and a black color, if necessary, according to the 
former method, the number of the latent image forming means and the 
developing means to be arranged around the photosensitive member is equal 
to the number of the kinds of the above-specified colors so that the 
photosensitive member has an increased diameter to enlarge the size of the 
apparatus. 
According to the latter method, on the contrary, the apparatus can be made 
smaller than that of the former method because of the single latent image 
forming means. However, the photosensitive member has to make the same 
rotations as the number of the kinds of the colors so that the rate of 
forming the multi-color image is dropped. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide the image forming method 
and apparatus which can solve the aforementioned problems accompanying the 
prior art and form an image at a high rate without increasing the size of 
the apparatus. 
According to a feature of the present invention, there is provided an image 
forming method which is characterized in that, when the steps of forming 
and developing latent images by using a plurality of latent image forming 
means and a plurality of developing means for developing the respective 
latent images formed by said latent image forming means are repeated to 
form an image, at least one of said latent image forming means is 
repeatedly used. 
According to a preferable embodiment of the present invention, there is 
provided an image forming apparatus which is characterized: in that first 
latent image forming means, first developing means, second latent image 
forming means and second developing means are sequentially arranged with 
respect to an image retainer; and in that each of said first developing 
means and/or said second developing means is composed of a plurality of 
developing devices. 
Other objects and features of the present invention will be described in 
detail in the following with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the recording apparatus of FIG. 1(a), reference numeral 1 designates a 
drum-shaped image retainer which has a surface layer of a photoconductive 
and photosensitive material such as Se and which is made rotatable in the 
direction of arrow. Numerals 11 and 12 designate chargers for charging the 
surface of the image retainer 1 uniformly. Numerals 21 and 22 designate 
image exposing units for different colors of a color image. Numerals 31 to 
34 designate developing devices which use toners of different colors such 
as yellow, magenta, cyan and black colors as developers. Numerals 13 to 41 
designate a pre-transfer charging device and a pre-transfer exposure lamp 
which are disposed, if necessary, for facilitating either transfer of a 
color image, which is formed of a plurality of color toner images 
superposed on the image retainer 1, to a transfer member P or separation 
of the transfer member P. Numeral 14 designates a transfer device. Numeral 
61 designates a fixing device for fixing the toner images transfered to 
the transfer member P. Numerals 42 and 15 designate a charge eliminating 
lamp and a charge eliminating corona discharger, respectively, one or both 
of which are used in combination. Numeral 51 designates a cleaning device 
which is equipped with a cleaning blade or fur brush for coming into 
contact with the surface of the image retainer 1, from which the color 
image has been transferred, to remove the residual toner from that surface 
and for leaving the surface of the image retainer 1 until it is reached by 
the surface having been subjected to the first development. 
Here, especially in the recording apparatus in which the already charged 
surface of the image retainer 1 is additionally charged, there is 
preferably used a scorotron corona discharger, as shown, which can give a 
stable charge with little influence by the preceding charge. As in this 
recording apparatus using the drum-shaped image retainer 1, on the other 
hand, the image exposing units 21 and 22 may be those which are filtered 
from the slit exposures, as in an electrophotographic copying machine of 
an ordinary monochromatic type. In order to record a clear color image, 
the image exposures may preferably be established by the laser beam 
scanner shown in FIGS. 2(a) and 2(b). 
FIG. 8 shows the operation timing of the main devices of the apparatus 
shown in FIG. 1(a). 
In FIG. 8, EM, EY, EC and EBK show the durations of the latent images under 
writing operation state corresponding to magenta, yellow, cyan and black, 
respectively, and DM, DY, DC and DBK show durations in which the 
developing devices 31, 32, 33 and 34 having magenta, yellow, cyan and 
black toners, respectively, can be operated to develop. 
In the laser beam scanner of FIG. 2(a), a laser beam emitted from a laser 
121 such as He-Ne laser is turned on and off by an acousto-optical 
modulator 122 and is deflected by a mirror scanner 123, which is composed 
of an octagonal mirror rotated by a drive motor 130, so that it is formed 
through a focusing f-.theta. lens 124 into an image exposure 104 for 
scanning the surface of the image retainer 1 at a constant rate. 
Incidentally, reference numerals 125 and 126 designate mirrors, and 
numeral 127 designates a lens for enlarging the diameter of a beam which 
is incident upon the focusing f-.theta. lens 124 so as to reduce the 
diameter of the beam on the image retainer 1. If the laser beam scanner 
shown in FIG. 2(a) is used for forming the image exposure 104, 
electrostatic images of different colors can be formed with a lag, as will 
be described hereinafter, so that a clear color image can be recorded. On 
the other hand, the laser beam scanner can suitably use the structure 
shown in FIG. 2(b). As shown, a laser beam generated by a semiconductor 
laser 221 is rotationally scanned by a polygon mirror 223, which is 
rotated by a drive motor 230, and has its optical path deflected through 
an f-.theta. lens 224 by a reflecting mirror 237 so that it is projected 
on the surface of the image retainer 1 to form a bright line 239. Numeral 
234 designates an index sensor for detecting the start of the beam scan. 
Numerals 235 and 236 designate cylindrical lenses for correcting the angle 
of deflection. Numerals 238 a, 238b and 238c designate reflecting mirrors 
for forming beam scanning and detecting optical paths. 
When the scan is started, the beam is detected by the index sensor 234 so 
that the modulation of the beam by a first color signal is started by a 
not-shown modulation unit. The beam thus modulated scans the image 
retainer 1 which is uniformly charged in advance by the charging device 11 
or 12. A latent image corresponding to the first color is formed on the 
surface of the drum by the main scan with the laser beam scanner and the 
auxiliary scan resulting from the rotations of the image retainer 1. 
As the case may be, for example, in case the informations of an image to be 
reproduced are arrayed in one page sequentially in the rotating direction 
of the image retainer, only one of the laser beam scanners of the image 
exposing units 21 and 22 may be used. In this case, suitable optical 
systems such as beam splitters may be used and arranged for image 
exposures at two positions of the image retainer. Then, the apparatus may 
possibly have its size reduced and its cost dropped. 
On the other hand, the laser beam 104 should not be limited to the slit 
exposure or the dot exposure of the laser beam, as has been described 
hereinbefore, but can be established by means of an LED, a CRT, a liquid 
crystal or an optical fiber transmitter, for example. In the recording 
apparatus in which the image retainer can take a flat shape as a belt, 
moreover, the image exposure can be a flash exposure. 
On the other hand, the developing devices 31 to 34 having the structure 
shown in FIG. 3 can preferably be used. 
In FIG. 3 reference numeral 131 designates a developing sleeve which is 
made of a non-magnetic material such as aluminum or stainless steel. 
Numeral 132 designates a magnet disposed in the developing sleeve 131 and 
having a plurality of magnetic poles in the circumferential direction. 
Numeral 133 designates a layer thickness regulating blade for regulating 
the thickness of a developer layer which is formed on the developing 
sleeve 131. Numeral 134 designates a scraper blade for removing a 
developer layer after development from the surface of the developing 
sleeve 131. Numeral 135 designates an agitating rotor for agitating the 
developer in a developer reservoir 136. Numeral 137 designates a toner 
hopper. Numeral 138 designates a toner supply roller having toner 
receiving recesses on its surface for supplying toner from the toner 
hopper 137 to the developer reservoir 136. Numeral 139 designates a power 
supply for applying a bias voltage containing a vibratory voltage 
component, as the case may be, to the developing sleeve 131 through a 
protecting resistor 140 to generate an electric field for controlling the 
motions of toner between the developing sleeve 131 and the image retainer 
1. As shown, the developing sleeve 131 and the magnet 132 are rotated in 
the direction of arrows. However, either the developing sleeve 131 or the 
magnet 132 may be fixed, or both the developing sleeve 131 and the magnet 
132 may rotate in the common direction. In case the magnet 132 is fixed, 
it is the current practice to strengthen the magneticism or to arrange two 
identical or different magnetic poles close to each other so that the 
density of magnetic flux of a magnetic pole facing the image retainer 1 
may be higher than that of another magnetic pole. 
In the developing device thus constructed, the magnet 132 has its magnetic 
poles magnetized usually to have a density of magnetic flux of 500 to 
1,500 gauses. By this magnetic force, the developer of the developer 
reservoir 136 is attracted onto the surface of the developing sleeve 131. 
The developer thus attracted has its thickness regulated by the layer 
thickness regulating blade 133 to form a developer layer. This developer 
layer is carried in the same direction as (as shown) or in the opposite 
direction to the rotating direction of the image retainer 1, as indicated 
by the arrow, to develop an electrostatic image of the image retainer 1 in 
a developing region in which the surface of the developing sleeve 131 
faces that of the image retainer 1. The residual developer is scraped off 
from the surface of the developing sleeve 131 by the action of the scraper 
blade 134 until it is returned to the developer reservoir 136. Moreover, 
the developments are preferred to resort to the so-called "non-contact 
development" conditions for at least the second or later developments, 
which are repeated for superposing the color toner images, so that the 
toner having sticked to the image retainer 1 during the preceding 
development may be displaced by a succeeding development. The 
above-specified non-contact development is conducted as follows: the 
developer layer on the developing sleeve 131 is spaced from the image 
retainer 1 while it is supplied with no developing bias; and a DC and AC 
superposed bias is applied to the developing sleeve 131 so that the toner 
may fly under the alternating electric field onto the image retainer 1. 
FIG. 3 shows the state in which the development is conducted under the 
non-contact development conditions. 
Moreover, the developing devices 31 to 34 may preferably use the so-called 
"two-component developer", which is composed of a mixture of a 
non-magnetic toner and a magnetic carrier and which need not contain a 
black or brown magnetic material in its toner but can provide a clear 
color toner and can control the charge of the toner easily. Specifically, 
the magnetic carrier is prepared by dispersing and containing fine 
particles of a ferromagnetic or paramagnetic material such as 
ferrosoferric oxide, .gamma.-ferric oxide, chromium dioxide, manganese 
oxide, ferrite or manganese-copper alloy in a resin such as styrene, 
vinyl, ethylene, rosin-modified, acrylic, polyamide, epoxy or polyester 
resin. Otherwise, the magnetic carrier is prepared by covering the 
surfaces of the particles of those magnetic materials with the 
aforementioned resins. The magnetic carrier may preferably be an 
insulating carrier having a resistivity of 10.sup.8 .OMEGA.cm or higher, 
more preferably 10.sup.13 .OMEGA.cm or higher. If this resistivity were 
low, there would arise problems that charges are injected into the carrier 
particles, if the bias voltage is applied to the developing sleeve 131, to 
make the carrier particles liable to stick to the surface of the image 
retainer 1, and that the bias voltage cannot be applied sufficiently. 
Especially if the carriers stick to the image retainer 1, the color tone 
of a color image is adversely affected. 
Incidentally, the resistivity takes a value obtained by tapping particles 
in a container having a sectional area of 0.50 cm.sup.2, by applying a 
load of 1 kg/cm.sup.2 to the tapped particles, and by reading a current 
value when a voltage for establishing an electric field of 1,000 V/cm is 
applied between the load and the bottom electrode. 
On the other hand, the carrier having an average particle diameter smaller 
than 5 .mu.m will have an excessively weak magneticism whereas the carrier 
having an average particle diameter larger than 50 .mu.m will not improve 
the image but is liable to cause the breakdown or discharge so that it 
will not allow application of high voltage. From these tendencies, it can 
be concluded that the average particle diameter of the carrier be 
preferably within an range of 5 .mu.m to 40 .mu.m. A fluidizing agent such 
as hydrophobic silica is suitably applied as an additive, if necessary. 
The toner is preferably prepared by adding a variety of pigments and, if 
necessary, a charge controller to a resin to have an average particle 
diameter of 1 to 20 .mu.m and an average quantity of charge of 3 to 300 
.mu.c/g, more preferably 10 to 100 .mu.c/g. The toner becomes reluctant to 
leave the carrier, if its average particle diameter is smaller than 1 
.mu.m, and liable to deteriorate the resolution of an image if its average 
particle diameter exceeds 20 .mu.m. 
If the developer made of the mixture of the insulating carrier and the 
toner described above is used, the bias voltage to be applied to the 
developing sleeve 131 of FIG. 3 can be so set without any danger of 
leakage that the toner can sufficiently stick to an electrostatic image 
without any fogging. Incidentally, in order to effectively control the 
developing movement of the toner by the application of such bias voltage, 
the toner may contain such a magnetic material within a range retaining 
the color clearness as is used in the magnetic carrier. 
The structure of the developing device and the composition of the developer 
thus far described are preferably used in the method of the present 
invention. However, the present invention should not be limited thereto 
but can use the developing devices and the developers, as are disclosed in 
Japanese Patent Laid-Open Nos. 30537/1975, 18656 to 18659/1980, 
144452/1981, and 116553 and 116554/1983. More preferably, the non-contact 
development conditions with the two-component developers may be used, as 
are disclosed in the specifications of Japanese Patent Applications Nos. 
57446/1983, 96900 to 96903/1983, 97973/1983, 192710 and 192711/1985, 
14537/1985, 14539/1985, and 176069/1985, all of which are assigned to the 
present Applicant. Of these, the developing device disclosed in Japanese 
Patent Laid-Open No. 176069/1985 is preferred because the development is 
conducted in a portion having a thin developer layer between magnetic 
poles with the magnet being fixed in the developing sleeve so that the 
developing gap can be narrowed to establish a sufficiently strong 
developing electric field thereby to provide a high developing 
performance. The presence of the irrotational magnet is also advantageous 
for the image forming apparatus which is equipped with a plurality of 
developing devices. 
Incidentally, each image exposure has to be conducted in an accurately 
registered position on the image retainer. The positions of these image 
exposures can be easily and accurately determined by the ordinary position 
detection and image exposure timing control using a photosensor, by which 
one (or several, if necessary, not shown) registration index marker 
(although not shown) disposed in a predetermined position of the image 
retainer is detected for each rotation of the image retainer, so that the 
image obtained has no color deviation. 
In the aforementioned recording method, as has been described hereinbefore, 
the toner image formed on the image retainer 1 is transferred directly to 
the transfer member P from the image retainer 1 by the transfer device 14 
without any use of a transfer drum so that the apparatus can be 
small-sized without any color deviation. 
The methods of the present invention shown in FIGS. 4 to 7 can be 
implemented by the recording apparatus thus far described. Incidentally, 
all of FIGS. 4 to 7 show the stage before which a second development has 
been conducted. 
FIG. 4 shows an embodiment of the present invention, in which an 
electrostatic image is formed by the electrostatic image forming method 
for forming an image exposed portion in the background and the 
electrostatic image in the unexposed portion and in which the development 
is effected as a result that the toner for charging in an opposite 
polarity sticks to the electrostatic image. According to the recording 
apparatus of FIG. 1(a), more specifically, the surface of the image 
retainer 1 in its initial stage, which has its charge eliminated by the 
charge eliminating devices 15 and 42 and cleaned by the cleaning device 51 
to have a zero potential, is uniformly subjected for its first rotation to 
a first charging operation by the charging device 11. The surface thus 
charged is subjected to a first image exposure to the image exposing unit 
21, such that the potential in the portion other than the electrostatic 
image is substantially at zero. The potential thus obtained effects the 
first development with the electrostatic image substantially equal to the 
potential of the first charging operation either of the developing devices 
31 and 32, which uses the developer of the color toner corresponding to 
the first image exposure, so that the toner T charged in the opposite 
polarity sticks. 
A second charging operation is uniformly conducted by the charging device 
12. A second image exposure for reducing the potential in the portion 
other than the electrostatic image substantially to zero is conducted 
again with the image exposing unit 22. The electrostatic image thus 
obtained is subjected to a second development with a toner T' by either of 
the remaining developing devices 33 and 34, which uses the developer of 
the corresponding color. 
Next, for second rotation, third and fourth electrostatic image formations 
and developments are repeated with the pre-transfer charging device and 
exposing lamp 13 and 41, the transfer device 14, the charge eliminating 
devices 15 and 42 and the cleaning device 51 being inoperative. When the 
fourth development is conducted to form a color image having the color 
toner images superposed, the pre-transfer charging device 13 and the 
pre-transfer exposing lamp 41 are operated until the color image passes. 
Then, the color image is transferred by the transfer device 14 to the 
transfer member P which is being fed in synchronism with the rotation of 
the image retainer 1. The color image thus transferred is fixed on the 
transfer member P by the fixing device 61. The surface of the image 
retainer 1 bearing the transferred color image has its charges eliminated 
by the charge eliminating devices 15 and 42 and is cleaned by the cleaning 
device 51 to restore its initial state. Thus, the one color image 
recording cycle is completed in the embodiment of the present invention. 
In other words, the charging operations for the individual formations of 
the electrostatic images are conducted twice by the charging devices 11 
and 12, and the image exposures are also conducted twice by the two 
exposing devices which are made by the laser beam scanner of FIG. 2, for 
example. As a result, the recording apparatus can be made in a small size 
and at a low cost and can record at a high speed. 
In this embodiment of FIG. 4, the developments are conducted by the 
developing method using the toners for charging the electrostatic images 
in opposite polarities. As a result, the developed densities of the 
individual colors can be easily increased to record a clear color image 
easily. Incidentally, in order to avoid the color mixture, the DC biases 
in the developments may be set to be sequentially the higher at the later 
steps. The charged potentials may accordingly be set to become 
sequentially the higher. 
FIGS. 5 to 7 show embodiments of reversal development according to the 
present invention, in which an electrostatic image is formed by the method 
of forming an image exposed portion into the electrostatic image at a 
lower potential than the background and in which the developments are 
carried out such that toners for charging the electrostatic images at the 
same polarity as that of the background stick to the electrostatic images. 
In the embodiment of FIG. 5 using the recording apparatus of FIG. 1(a), the 
surface of the image retainer 1 in the same initial state as that of FIG. 
4 is uniformly charged for a first rotation by the charging device 11. 
This charged surface is subjected to a first image exposure to have a 
substantially zero potential in the electrostatic image by projecting the 
image exposure to the image exposing unit 21 with the laser beam scanner 
of FIG. 2. The electrostatic image thus obtained is subjected to a first 
development by that of the developing devices 31 and 32, which uses the 
developer (which has its toner for charging in the same polarity as that 
of the charging operation of the image retainer 1, as is different from 
the embodiment of FIG. 4) containing the color toner corresponding to the 
image exposure. For forming subsequent latent image formations, a second 
image exposure is conducted by projecting the image exposure to the image 
exposing unit 22 in a position displaced from the position of the 
preceding projection with the laser beam scanner. The resultant 
electrostatic image having a substantially zero potential is developed by 
either the remaining developing devices 33 or 34 using the developer 
containing the corresponding color toner. For a second rotation, third and 
fourth electrostatic image formations and developments are repeated. Then, 
the one color image recording cycle is completed like FIG. 4. 
Incidentally, in this embodiment, the electrostatic image having the 
substantially zero potential will not take a potential substantially equal 
to that of the background, as shown, even if it is developed to carry the 
toner T for charging it in the same polarity as that of the image retainer 
1. As a result, during the development for applying the toner T' of 
different color to the electrostatic image formed later, the toner T' will 
frequently stick to the previous toner T of the electrostatic image 
portion despite of no previous exposure, i.e., not write. Since, however, 
the laser beam scanner which can be formed as a unit is used for 
generating the image exposure. The laser beam scanner can be arranged 
around the image retainer 1, the projection position of each image 
exposure can be displaced very simply, so that no recharging for forming 
the latent image for the second and following colors is necessary because 
the charge for the latent image of the first color can be used as it is. 
The liability for the electrostatic images of different colors to be 
superposed can be reduced by setting the DC biases at sequentially lower 
absolute values for the individual developments. Thus, it is possible to 
form a color image, especially a multi-color image having an excellent 
clearness. 
An embodiment of FIG. 6 is an improvement over that of FIG. 5, in which an 
additional electrostatic image cannot be positively formed on the 
preceding electrostatic image and in which the toner of different color 
may possibly be caused, although very little, to stick to the previously 
developed electrostatic image portion by a later development. In the 
embodiment of FIG. 6, more specifically, the process from the initial step 
to the first development is common to that of the embodiment of FIG. 5. 
However, the subsequent steps are different from those of the embodiment 
of FIG. 5. Specifically, a second charging operation is uniformly 
conducted by the charging device 12. This charged surface is subjected to 
a second image exposure and a second development. Likewise, third and 
fourth electrostatic image formations and developments are subsequently 
repeated. Thus, in the embodiment of FIG. 6, as like as the embodiment 
shown in FIG. 4, in which after the preceding development the surface of 
the image retainer 1 is again charged uniformly for the succeeding 
electrostatic image formations and developments, an electrostatic image 
can be formed on the portion bearing the preceding electrostatic image. 
Even in case, moreover, the portion of the succeeding electrostatic image 
is displaced from that of the preceding one, there can be attained an 
effect that the toner of different color hardly sticks to the portion of 
the image to which the preceding toner has sticked. 
FIG. 7 presents an embodiment for preventing the succeeding toner of 
different color from sticking to the portion to which the preceding toner 
has sticked. This embodiment is the same in the process up to the first 
development as that of the embodiments of FIGS. 5 and 6. After the first 
development, however, the surface of the image retainer 1 is uniformly 
exposed by the use of an exposing lamp 71, as shown in FIG. 1(b). Then, a 
second charging operation is conducted by the charging device 12. 
Alternatively, the second charging operation is uniformly conducted in 
advance by the charging device 12, and a weak but uniform exposure is then 
conducted by the exposing lamp 71 shown by phantom line. Then, a second 
image exposure and a second development are conducted. Subsequently, third 
and fourth electrostatic image formations and developments are likewise 
repeated. Here, if a uniform exposure is conducted after the development, 
the portion developed to bear the toner does not have its charged 
eliminated but is maintained at a high potential whereas the remaining 
portion is dropped to a substantially zero potential. By the second 
charging operation, the surface of the image retainer 1 can be charged 
such that the potential at the portion bearing the toner is made slightly 
higher than that at the remaining portion to be formed with the 
electrostatic image. After the development, moreover, if the second 
charging operation is conducted in advance to uniformly charge the surface 
of the image retainer 1 and then the uniform and weak exposure is 
conducted, the charged state of the surface of the image retainer 1 is 
similar to that in case the uniform exposure is conducted in advance. 
Therefore, when a succeeding electrostatic image having its position 
displaced is to be developed, a toner of different color is effectively 
prevented from sticking to the portion bearing the preceding toner because 
the latter portion is at a higher potential. 
In any of the embodiments thus far described, the developing devices 31 to 
34 may preferably use the developer of the mixture of the toner and the 
insulating carrier, and the developments may also preferably conducted 
under the non-contact development conditions. This prevents the mixture of 
the toners of different colors, as has been described hereinbefore. 
Moreover, application of the bias voltage suitable for the toner control 
to the developing sleeve 131 of the developing device is facilitated so 
that a color image having a high developing density and an excellent 
clearness can be recorded even in the electrostatic image forming and 
developing methods, in which the image exposing device such as the laser 
beam scanner can be advantageously used, as in the embodiments of FIGS. 5 
and 7. 
As can be understood from FIGS. 4 to 7, the charging device and the image 
exposing device constitute together the electrostatic image (i.e., latent 
image) forming means. 
Next, the embodiments of FIGS. 4 to 7 will be described more specifically 
as the following examples 1 to 3: 
EXAMPLE 1 
(Embodiment of FIG. 4) 
The recording apparatus shown in FIG. 1(a) was used. The image retainer 1 
had an OPC (i.e., Organic Photoconductive) surface layer and a 
circumferential velocity of 90 mm/sec. The surface of this image retainer 
1 was charged to -600 V by the charging device 11 using the scorotron 
corona discharger, and this charged surface was subjected to a first image 
exposure with blue image information by the image exposing unit 21. As a 
result, the image retainer 1 was formed with an electrostatic image in 
which the background potential of the exposed portion was at -50 V whereas 
the potential of the unexposed portion was -600 V. This electrostatic 
image was subjected to a first development by the developing device 31 
shown in FIG. 3. 
The developing device 31 used the developer which was composed of: a 
carrier containing 50 wt % of magnetite dispersed in a resin and having an 
average particle diameter of 20 .mu.m, a magneticism of 30 em.mu./g and a 
resistivity of 10.sup.14 .OMEGA.cm or higher; and a non-magnetic toner 
prepared by adding 10 wt parts of benzidine derivative as a yellow pigment 
and another charge controlling agent to a styrene-acrylic resin and having 
an average particle diameter of 10 .mu.m. The using condition was that the 
ratio of the toner to the carrier was 25 wt %. Moreover, the developing 
device 31 resorted to the non-contact development conditions, in which: 
the developing sleeve 131 had an external diameter of 30 mm and the number 
of revolutions of 100 r.p.m.; the magnet 32 had the density of magnetic 
flux of 1,000 gauses on the developing sleeve of its N and S magnetic 
poles and the number of revolutions of 1,000 r.p.m.; the developer layer 
in the developing region had a thickness of 0.7 mm; the gap between the 
developing sleeve 131 and the image retainer 1 was 0.8 mm; and the 
developing sleeve 131 was supplied with the superposed voltage of a DC 
voltage of -100 V and an AC voltage of 3 kHz and 1,000 V (at an effective 
value). 
While the electrostatic image was being developed by the developing device 
31, the remaining similar developing devices 32 to 34 shown in FIG. 3 were 
kept away from their developing states. This could be achieved by 
isolating the developing sleeve 131 from the power supply 139 into a 
floating state or by positively applying a DC bias voltage in the same 
polarity as that of the charged image retainer 1, i.e., in the opposite 
polarity to that of the charged toner to the developing sleeve 131. Since 
the developing devices 32 to 34 are used for the developing operations 
under the non-contact conditions as like as the developing device 31, the 
developer layer on the developing sleeve 131 need not be troublesomely 
eliminated. The developing device 33 used the developer having the 
composition, in which the toner of the developer of the developing device 
31 was replaced by the toner containing polytungstorate as the magenta 
pigment in place of the yellow pigment. The developing device 32 used the 
developer having the composition, in which the toner is replaced by the 
toner containing copper phthalocyanine as the cyan pigment. The developing 
device 34 used the developer having the composition, in which the toner 
was replaced by the toner containing carbon black as the black pigment. 
Naturally, these color toners may contain other pigments, dyes, and the 
orders of the colors to be developed and the developing devices can be 
suitably selected. 
The surface of the image retainer 1 having been subjected to the first 
development was recharged to -650 V by the action of the charging device 
12. The charged surface was subjected to a second image exposure with 
green image information by the image exposing unit 22 and then to a second 
development with the magenta toner by the developing device 33 under the 
non-contact development conditions in which the superposed voltage of a DC 
voltage of -150 V and an AC voltage of 1,000 V was applied to the 
developing sleeve 131. Next, for a second rotation, a charging step, an 
image exposure to red image information by the image exposing unit 21 and 
a third development with the cyan toner by the developing device 32; and a 
charging step, an image exposure to black image information by the image 
exposing unit 22 and a fourth development with the black toner by the 
developing device 34 were repeated. Incidentally, in the second and later 
development, the amplitudes, frequencies, time selected conversions of the 
DC bias and AC components of the voltage to be applied to the developing 
sleeve 131 were suitably changed in conformity to the changes in the 
surface potential, developing characteristics and color reproducibility of 
the image retainer 1. Especially, the sequential increase in the absolute 
value of the DC bias as well as the charging potential was effective to 
prevent the color mixture of the toners. 
When the fourth development was conducted to form the four-color images on 
the image retainer 1, they were prepared by the pre-transfer charging 
device 13 and the pre-transfer exposing lamp 41 and were transferred to 
the transfer member P by the transfer device 14 until they are fixed by 
the fixing device 61. The suitable exposure by the pre-transfer exposing 
lamp 41 is effective for making the transfer member P liable to be 
separated from the image retainer 1. The image retainer 1 thus having the 
color images transferred thereto had its charges eliminated by the charge 
eliminating devices 15 and 42 and further its residual toners removed from 
its surface by the cleaning blade or sponge roller of the cleaning device 
51. Thus, the one cycle process for recording the color image was 
completely finished when the surface bearing the color image passed over 
the cleaning device 51. 
The color image thus recorded was clear with its individual colors 
exhibiting sufficient densities. However, the toner mixture was slightly 
found in the portion where the color toners sticked densely. 
EXAMPLE 2 
(Embodiment of FIG. 5) 
The recording apparatus shown in FIG. 1(a) was used. The image retainer 1 
had a surface layer of a photosensitive material of Se and a 
circumferential velocity of 180 mm/sec. The surface of this image retainer 
1 was charged to +800 V by the charging device 11 using the scorotron 
corona discharging device, and the charged surface was subjected to a 
first image expsoure with a density of 16 dots/mm by the image exposing 
unit of FIG. 2 using the He-Ne laser. As a result, an electrostatic image 
having a potential of +50 V in the exposed portion and a background 
potential of +800 V was formed on the image retainer 1. This electrostatic 
image was subjected to a first development by the developing device 31 
shown in FIG. 3. 
Incidentally, the development conditions by the developing device 31 were 
the same as those of the Example 1 except that the developer had the 
carrier of an average particle diameter of 30 .mu.m and a ratio of 20 wt % 
of the toner to the carrier, and that a superposed voltage of a DC voltage 
of +600 V and an AC voltage of 1.5 kHz and 700 V (in an effective value) 
was applied to the developing sleeve 131. Moreover, the conditions of the 
remaining developing devices 32 to 34 were the same as those of the 
Example 1 except the bias voltage. In this Example 2, however, the bias 
voltage for holding the developing device, which does not participate in 
the development, in its non-developing state has an opposite polarity to 
that of the charge of the toner and the charge of the image retainer 1. 
The surface of the image retainer 1 having been subjected to a first 
development was subjected to a second image exposure by the image exposing 
unit 22 without the action of the charging device 12 and with not change 
in the density but displacement of the dot positions. The surface thus 
exposed was then subjected to a second development with the magenta toner 
by the developing device 33. For a second rotation, a third development 
with the cyan toner by the developing device 32 and a fourth development 
with the black toner by the developing device 34 were repeated. 
Incidentally, in and after the second developments, the amplitudes, 
frequencies, time selected conversions of the DC bias and AC components of 
the voltage to be applied to the developing sleeve 131 were suitably 
changed in conformity to the changes in the surface potential, developing 
characteristics and color reproducibility of the image retainer 1. 
Especially, in this Example, the sequential reduction in the DC biases for 
each step is effective for preventing the color mixture of the toners. 
When the fourth development was conducted to form four-color images on the 
image retainer 1, they were transferred to and fixed on the transfer 
member P, like the Example 1, and the one color image recording cycle was 
then completed by eliminating the charges of and cleaning the image 
retainer 1. 
The color image thus recorded was as clear as that of the Example 1. 
EXAMPLE 3 
(Embodiment of FIG. 6) 
The color image recording was conducted by using the same apparatus as that 
of the Example 2 under the same conditions as those of the Example 2 
except that the voltage to be applied to the developing sleeve 131 of the 
developing device was the superposed voltage of a DC voltage of +600 V and 
an AC voltage of 1,000 Hz and 500 V (in an effective value), and that the 
surface potential of the image retainer 1 was subsequently recharged to 
+900 V by the charging device 12. 
The color image recorded had less color mixture of the toners in the 
portion, where the individual color toners densely sticked, to provide a 
clearer image than that of the Example 2. 
Incidentally, according to this Example, as has been touched hereinbefore, 
the portion of the preceding image exposure and the portion of the 
succeeding image exposure can be superposed. In this case, the order of 
the colors to be developed exerts considerable influence on the clearness 
of the color image to make it necessary to determine the color order 
carefully. 
EXAMPLE 4 
(Embodiment of FIG. 7) 
The recording apparatus used had the exposing lamp 71 (as indicated by the 
phantom line) between the charging device 12 and the image exposing unit 
22, as shown in FIG. 1(b). The color image recording was conducted under 
the same conditions as those of the Example 2 except that the voltage to 
be applied to the developing sleeve 131 of the developing device was the 
superposed voltage of a DC voltage of +450 V and an AC voltage of 2 kHz 
and 500 V (in an effective value), and that before each of second and 
later image exposures the surface of the image retainer 1 was charged to 
have a potential +600 V by the charging device 11 or 12 and subjected to a 
uniform and weak exposure to drop its potential to +500 V by the exposing 
lamp 71 (as indicated by the phantom line). 
The color image thus recorded was remarkably clear because no color mixture 
of the toners was present even in the portion where the individual color 
toners sticked closely to each other. 
In this Example, too, the portion of the preceding image exposure and the 
portion of the succeeding image exposure can be superposed like the 
Example 3. 
According to this Example, by using two sets of apparatus for forming the 
electrostatic images by four repetitions, the recording apparatus can be 
constructed in a small size and at a low cost, and the recording speed is 
relatively, and further the synchronized control of the individual image 
exposures can be done easily and accurately because toner images of plural 
colors are formed on the image retainer 1 and transferred at a time. 
Moreover, each development can be conducted either by the method of 
applying the toner to be charged in the opposite polarity to an 
electrostatic image which can have its density controlled relatively 
easily or by the method of applying the toner to be charged in the same 
polarity to an electrostatic image which can use the image exposing unit 
as the image exposing device. According to either method, furthermore, the 
development can be conducted under the non-contact development conditions 
to record a color image having a sufficient developing density and an 
excellent clearness. 
In addition to the Examples described above, the present invention can be 
modified in various manners. 
In case any color image need not be formed, i.e., a monochromatic image is 
to be formed, for example, the latent image can be formed by any 
combination of the charging device 11 or 12 and the image exposing unit 21 
or 22. In case the photosensitive layer of the image retainer 1 has a 
large dark decay, the latent image may preferably be formed by the 
combination of the charging device 12 and the image exposing unit 22 and 
developed by the developing device 34. This is because the short time 
intervals (or distances) between the individual charging, image exposing 
and developing steps can be utilized. The combinations may be selected on 
the basis of the same concept in case a mono-color image is to be formed 
of toner of another color such as the yellow, magenta or cyan color. 
In the dichromatic or trichromatic case, any combination can naturally be 
selected in accordance with the necessity or performance. 
Further, it is possible to superpose the image informations of 2n (wherein 
n is the rotation number of the drum) on the image retainer and to 
transfer them on the transfer paper, if not only the information from the 
document of one sheet is written by the image exposing unit on the image 
retainer but also the informations of the document of plural sheets or the 
different image informations from the external input signals are written 
on the image retainer, and developed by some developing devices. 
By suitably selecting the combination of the image exposures and the 
developing devices, a variety of color images can also be formed. 
By feeding toners of the same color but different lightnesses (e.g., black 
and grey toners) to the individual developing devices, it is possible to 
form a white and black (or mono-color) image having a gradation (for 
reproducing a delicate density difference). 
In addition, the image composition can be conducted on the image retainer 1 
by performing the image exposures of different image informations coming 
from the image exposing units 21 and 22. 
As has been touched hereinbefore, furthermore, the present invention should 
be limited neither to the recording apparatus having the drum-shaped image 
retainer nor to the transfer of a color image to the transfer member. In 
short, the present invention can also be applied to a modification, in 
which the image forming member is one to be applied to a base such as 
electrofax paper so that a color image formed on the member is not 
transferred but fixed. In this modification, the pre-transfer charging 
device, the pre-transfer exposing lamp, the transfer device, the cleaning 
device and so on can be dispensed with. Of these, the pre-transfer 
charging device, the pre-transfer exposing lamp and the charge eliminating 
device can also be omitted in the case of transfer. And, this transfer 
itself may be a pressure one or through an intermediate transfer member. 
It is also natural that the fixing should not be limited to that using 
heat rollers. 
Other embodiments of the apparatus of the present invention different from 
the embodiment shown in FIG. 1(a) will be explained with reference to 
FIGS. 9 and 10. 
In the embodiment shown in FIG. 9, two image exposing units and three 
developing devices. Different color toners of yellow, magenta and cyan may 
be used as the color toners of developers for the three developing 
devices. However, in this embodiment, red toner is used as the color toner 
of developer for the developing device 35, blue toner is used as the color 
toner of developer for the developing device 36, and black toner is used 
as the color toner of developer for the developing device 37. The process 
of image formation using this apparatus will be explained hereunder. 
First rotation of image retainer 1 
1. The image retainer 1 of the photosensitive member (O.P.C.) is charged 
uniformly to -600 V by the first charging device 11 (scorotron). 
2. A red color image information for forming a latent image corresponding 
to red color is written by the first image exposing unit (infrared ray 
laser) 21. Thus, the potential on the image exposed portion is reduced to 
-20 V. 
3. A reverse development is carried out by the developing device 35 having 
red color toner. 
4. The photosensitive member on which the red color toner image exists is 
charged uniformly again to -600 V by the second charging device 
(scorotoron) 12. 
5. A blue color image information for forming a latent image corresponding 
to blue color is written by the second image exposing unit (infrared ray 
laser) 22. Thus, the potential on the image exposed portion is reduced to 
-20 V. 
6. A reverse development is carried our by the developing device 36 having 
blue toner. 
Second rotation of image retainer 1 
7. The photosensitive member on which the red color toner image and the 
blue color toner image exist is charged uniformly further to -600 V by the 
second charging device 12. The first charging device 11 may be used 
instead of the second charging device 12. 
8. An information corresponding to black color is written by the second 
image exposing unit 22. The first image exposing unit 21 may be used 
instead of the second image exposing unit 22. 
9. A reverse development is carried out by the developing device 37 having 
black toner. 
The remaining processes of the image information are the same with that 
explained in FIG. 1. 
It may be possible to form the toner images by using the image exposing 
units 21 and 22 suitably combined with the developing devices 35, 36 and 
37. It is preferable to use the non-contact development. 
In the embodiment shown in FIG. 10, three image exposing units and four 
developing devices are used. The process of image formation using this 
apparatus will be explained hereunder. 
First rotation of image retainer 1 
1. The image retainer 1 of the photosensitive member (O.P.C.) is charged 
uniformly to -600 V by the first charging device 11 (scorotron). 
2. A yellow color image information for forming a latent image 
corresponding to yellow color is written by the first image exposing unit 
(infrared ray laser) 21. Thus, the potential on the image exposed portion 
is reduced to -20 V. 
3. A reverse development is carried out by the developing device 31 having 
yellow color toner. 
4. The photosensitive member on which the red color toner image exists is 
charged uniformly again to -600 V by the second charging device 
(scorotron) 12. 
5. A magenta color image information for forming a latent image 
corresponding to magenta color is written by the second image exposing 
unit (infrared ray laser) 22. Thus, the potential on the image exposed 
portion is reduced to -20 V. 
6. A reverse development is carried out by the developing device 32 having 
magenta toner. 
7. The photosensitive member on which the yellow color toner image and the 
magenta color toner image exist is charged uniformly further to -600 V by 
the third charging device 16. The first charging device 11 may be used 
instead of the second charging device 12. 
8. An information corresponding to cyan color is written by the third image 
exposing unit (infrared ray laser) 23. Thus, the potential on the image 
exposed portion is reduced to -20 V. 
9. A reverse development is carried out by the developing device 33 having 
cyan toner. 
Second rotation of image retainer 1 
10. The photosensitive member on which the yellow color toner image, cyan 
color toner image and the magenta color toner image exist is charged 
uniformly further to -600 V by the third charging device 16. The charging 
device 11 or 12 may be used instead of the third charging device 16. 
11. An information corresponding to black color is written by the third 
image exposing unit 23. The image exposing unit 21 or 22 may be used 
instead of the third image exposing unit 23. 
12. A reverse development is carried out by the developing device 34 having 
black toner. 
The remaining processes of the image formation are the same with that 
explained in FIG. 1. 
As has been described hereinbefore, the present invention can enjoy the 
following effects: 
(1) Since there are used a plurality of latent image forming means at least 
one of which is used repeatedly, latent images of the number corresponding 
to that of this repetition can be formed only by said at least one latent 
image forming means. As a result, the number of the latent image forming 
means can be reduced to make a small-sized image forming apparatus. 
(2) At least one of the plural latent image forming means is used 
repeatedly, and developing means are used for developing the respective 
latent images formed by those latent image forming means. As a result, the 
number of movements of the image retainer (or the number of rotations in 
case it has a drum shape) for retaining the aforementioned latent images 
and the visible images formed by developing the latent images can be made 
as many as the number of the latent image forming means. As a result, the 
image can be formed at a high speed such that the number of movements of 
the image retainer is smaller than that of the developing means.