Method and apparatus for multicolor image forming

An apparatus for forming a multicolor image having a color correcting device with an operational processor for correcting and converting an image data composed of a plurality of color data. The apparatus has a device for forming a latent image on an image retainer based on the results of the color correction executed by the color correction device and a plurality of developing devices for developing the latent image thus formed by the device using toner of mutually different colors in order to form the multicolor image by successively forming a plurality of color toner images on the image retainer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a method and an apparatus for forming a 
multicolor image by successively forming different color toner images on 
an image retainer in the fields of electrostatic recording and 
electrophotographic reproducing applications. 
2. Description of the Prior Art: 
Heretofore, multicolor images have been obtained through the 
electrophotographic reproducing method wherein a series of reproducing 
processes, namely, charging, exposing, developing and transferring, is 
repeated on a component color basis and a plurality of different color 
toner images are piled on and transferred to transfer paper. For instance, 
an electrostatic image is formed separately through the processes by blue, 
green and red obtained through a separation filter and developed using 
yellow, magenta, cyan and, if necessary, black toner to form toner images 
which are superposed to form a multicolor image. However, this method of 
forming a multicolored image has disadvantages including the necessity of 
transferring the image to a transfer substance each time development by 
colors is completed, larger equipment and lengthy image-forming time; (2) 
necessity of assuring that the several color images are not out of 
register, i.e., misaligned relative to the repetition of the reproducing 
operation. 
Although there has been devised a method for forming a multicolor image 
while solving the problems above by piling up and developing a plurality 
of toner images on a photosensitive member and completing the transfer 
process at one time, that method still has disadvantages in that the toner 
image obtained in the preceding stage is disturbed or a multicolor image 
lacks color balance because the developer in the following stage is 
blended with the toner contained in the developer in the preceding stage. 
In order to remedy these shortcomings, there has also been disclosed a 
method, for instance, Japanese Patent Laid-Open No. 56-1144452, comprising 
preventing a photosensitive member from contacting a developer layer for 
developing a latent image formed on the photosensitive member, and laying 
an a.c. component on a d.c. bias applied to a developing device to fly the 
toner contained in the developer across a gap. In this method of forming a 
multicolor image, the image is not disturbed because the developer layer 
is prevented from rubbing the toner image formed in and up to the 
preceding stage. Referring to a flowchart of FIG. 1, the principle of the 
image forming method will be described. FIG. 1 illustrates changes in 
potential on the surface of a photosensitive member positively charged. In 
FIG. 1, there are shown an exposed portion PH of a photosensitive member, 
a non-exposed portion DA of the photosensitive member, a rise DUP in the 
potential produced by the positively charged toner T stuck to the exposed 
portion PH in the first development, and a rise CUP in the potential 
produced thereby in the exposed portion PH due to the second development. 
The photosensitive member is uniformly charged by a scorotron charge device 
or the like and provided with a constant surface potential E. The surface 
potential E on the exposed portion PH is reduced to almost zero by first 
image exposure by means of a light source such a laser, cathode raytube or 
LED. At this time, the d.c. component causes a positive bias roughly 
equivalent to the surface potential E in the non-exposed portion to be 
applied to a developing device and the positively charged toner T in the 
developing device is allowed to stick to the exposed portion PH having a 
relatively low potential, so that a first visible image may be formed. The 
potential in the region where the visible image has been formed is 
increased by the DUP because of the positively charged toner T adhering 
thereto and, as the region is charged secondly by the charge device, the 
potential is further raised by the CUP, whereby the initial surface 
potential E is obtained as in the case of the non-exposed portion. 
Subsequently, second image exposure is provided on the surface of the 
photosensitive member where uniform surface potential has been obtained to 
form an electrostatic latent image and a second visible image is obtained 
through the similar developing operation. A multicolor toner image is 
obtained on the photosensitive member by repeating the above processes and 
the image is transferred to recording paper and fixed with heat or under 
pressure to obtain a multicolor image. The toner and charge left on the 
photosensitive member is cleaned in preparation for the formation of the 
following multicolor image. In the above method of forming a multicolor 
image, the second and following charging may be omitted. In case the 
charging is repeated each time without the omission, a charge eliminating 
process may be added before the charging. Moreover, the exposure beam 
source used for each image exposure may be a similar or different one. 
In the above method of forming a multicolor image, for instance, yellow, 
magenta, cyan and black color toner images are often superposed on the 
photosensitive member and the reason for this includes the following: 
Although a black image should be obtained by superposing the three primary 
colors of yellow, magenta and cyan according to the principle of the 
subtractive color process, clear black characters and diagrams can hardly 
be reproduced only by the three primary colors, because toner in actual 
use for the three primary colors has not an ideal adsorptive wavelength 
range and these color toner images are not easily positionally 
synchronized. 
As a result, it is arranged to obtain a fourcolor image which is a more 
faithful reproduction of the document by superposing black as well as 
three primary color toner images as above described. 
In the method of forming a multicolor image, reversal development is also 
used for developing an electrostatic latent image. In reversal 
development, it is only necessary to expose a portion where a toner image 
is formed on the photosensitive member but not to expose the background 
without any gap as is the case with normal development, so that a latent 
image may relatively readily be formed on the photosensitive member with a 
toner image already formed. Moreover, the advantage is that the life of 
the photosensitive member can be prolonged as it is wear-resistant. 
Further, because the second and following charging are effected at the 
same polarity as the toner, electrostatic transfer is implemented without 
trouble. 
As a method of forming a latent image for forming a multicolor image, there 
are those of forming a latent image by directly injecting a charge into an 
image retainer using a multi-stylus electrode and of forming a magnetic 
latent image using magnetic head in addition to that of forming an 
electrostatic latent image by uniformly charging the photosensitive member 
and image exposure. 
Although each of these methods of forming a latent image allows the 
expression of gradation, the problem is that they are not suitable for 
high-speed recording. Moreover, because the gradation thereby expressed 
through such methods is the so-called multistage gradation, a greater 
capacity for image data is required. Accordingly, there has been proposed 
a method for providing image data of gradation in the form of binary 
values, the method comprising converting each picture element to a binary 
value for recording purposes and expressing dummy gradation based on the 
distribution of the binary values to minimize the capacity of image data. 
The density pattern method of FIG. 2 and the dither method of FIG. 3, for 
instance, are used to express the gradation of an image through the above 
method of forming image data of gradation in the binary form. 
The density pattern method shown in FIG. 2 presupposes the conversion of 
one picture element into a plurality of elements. In FIG. 2, there are 
shown a document 1a, each picture element 5a having gradation; a sample 2a 
for extracting a picture element 5a representing the typical density of a 
matrix of the document 1a and processing the value in terms of a 
threshold; a matrix 3a having the threshold density of MxN corresponding 
to the sample; and a pattern 4a provided in a binary form by comparing the 
threshold matrix 3a and the sample 2a. 
The dither method shown in FIG. 3(a) is intended to convert a picture 
element into i picture elements A document 1b is divided into density 
matrices on a MxN picture element basis. A sample 2b is subjected to a 
threshold process corresponding to the density matrix of the document 1b, 
the threshold density matrix 3b of MxN corresponds to the sample 2b, and a 
pattern 4b is represented by a binary value obtained by comparing the 
threshold matrix 3b with the sample 2b. 
In the conventional method of expressing gradation, it has been preferred 
to arrange dots in such a manner as to set a space frequency greater. In 
other words, the gradation is, as shown in FIG. 2 or 3(a), expressed by 
the number of dots of predetermined size (dot density). Particularly in 
the dither method, deterioration in resolution has been considered 
minimizable. However, in the aforementioned method of forming a multicolor 
image, gradation is incapable of being satisfactorily expressed because 
the resolving power is reduced as dots are welded together or an image 
looks coarse when the image is formed through the developing, transferring 
and fixing processes. The problem is that, for instance, even the 
resolving power in the order of 16 dots/mm required for the formation of 
an ordinary image cannot be maintained. 
There are two methods of expressing the gradation of a multicolor image: 
(1) different color dots are prevented from overlapping; (2) different 
color dots are allowed to overlap at least partially. In the case of (1), 
the dots are formed in different places within the pattern 4a or 4b as 
shown in FIGS. 2 and 3(a). Accordingly, different color dots are 
distributed separately and two-dimensionally and a dummy mixture of colors 
is formed on recording paper. 
In the case of (2), because different color dots are allowed to exist 
together within the pattern 4a or 4b, the different color dots are at 
least partially overlapped. In the case of (2), though development is 
implemented while the latent potential and the development bias are 
controlled, the formation of a desired latent image is not achieved 
because the overlapped potential is short and the toner dot which has 
already been developed impairs image exposure for the formation of the 
following toner dot. As a result, the tone of the preceding toner dot is 
excessively emphasized, which poses a problem in that the color balance of 
a multicolor image is broken. This constitutes a serious problem when the 
picture element is converted into a binary value to express the color 
balance. Particularly when the method of expressing decentralized 
gradation shown in FIGS. 2 and 3(a) is used, the problem becomes still 
more serious. 
Even when the different color dots are not allowed to overlap in the case 
of (1), the same type of problem occurs because of an unavoidable error in 
positioning which is caused when a latent image of the different color 
toner image is formed and because of the diffusion of the dots. The 
problem is conspicuous when the method of expressing the decentralized 
gradation as in the case of (2) is employed. 
When the aforementioned reversal development is used to form a color toner 
image on a photosensitive member, the following problems are posed: That 
is, as light for exposing an image is barely transmissible through a 
region where the toner has adhered from the development in the preceding 
stage, and the surface potential is thus sufficiently lowered, the toner 
is not allowed to stick to the photosensitive member in the following 
development stage. Even in the case of additive processes, because of 
difficulties in complete positioning and complete development 
corresponding to an electrostatically charged image, the same problem 
occurs. Accordingly, even if it is attempted to develop three primary 
yellow, magenta and cyan colors successively to express various tones, 
there will also be posed problems including the disturbance of the color 
balance and the image near the peripheral edge. Thus, a desired color 
image is not formed. 
Heretofore, a bulb, fluorescent lamp, EL (electroluminescence) or LED 
(light emitting diode) has been used as a light source for providing image 
exposure on a photosensitive member in an apparatus for forming an image 
but the use of a laser as a light source for image exposure is on the 
increase. In other words, the laser beam offers special properties such as 
greater energy per area unit, coherence and higher directivity and, 
because it permits the formation of an image of good quality at high speed 
without noise, much importance has been attached thereto. 
In the apparatus for forming an image using the laser beam, there is used, 
for instance, a He-Ne or He-Cd laser capable of emitting beams whose 
wavelength band ranging from 400 to 600 nm equivalent to the beam 
absorptive wavelength of a photosensitive member for forming an ordinary 
image. 
A laser beam L.sub.3 for image exposure is generated by a laser-beam 
exposure device 1 shown in FIG. 3(b). In FIG. 3(b), a signal 7 from a 
signal source 6 based on, for instance, image data, a facsimile or 
computer is applied to a driver 8 and an optical mudulator 5 such as an 
EOM {Electric Optical Modulator) or AOM (Acoustic Optical Modulator) is 
driven by the driver 8 and the intensity of a laser beam L.sub.1 from a 
light source 2 is modulated. A laser beam L.sub.2 after modulation is 
reflected from the reflecting surface of a polygon 9 rotate at high 
velocity and the laser beam L.sub.3 thus reflected is irradiated on a 
photosensitive member 112 to form an electrostatically charged image. A 
lens 3 is one capable of converging at a diameter (for instance, 50 to 300 
.mu.m) where the laser beam L.sub.1 can be modulated by the modulator 5, 
whereas a lens 4 is a collimate lens for obtaining the parallel laser beam 
L.sub.2 after modulation. A lens 10 is a focusing f.theta. lens and 111 
shows a scanning area by the laser beam L.sub.3. 
A gas laser is used as a beam source for image exposure in an apparatus for 
forming a multicolor image and, as a gas such as helium or neon is used as 
a substance for a conventional laser beam source, the disadvantages 
pointed out include an increase in the size of the beam source means and 
the price thereof. As shown in FIG. 3(b), the driver 8, the beam modulator 
5 and the like are required to modulate the intensity of the laser beam 
L.sub.1 and the laser beam L.sub.3 must be generated without interruption 
while an image is being formed and this makes the amount of the light 
source energy enormous. Particularly in the case of the formation of a 
multicolor image, the amount of image data is large and therefore the 
exposure beam source should be the one which can be operated at high 
velocity and provide excellent tone reproducibility in order to maintain 
the color balance. Accordingly, a laser beam source in demand for an 
apparatus for forming a multicolor image should be not only capable of 
modulating the intensity of light in proportion to image data but also 
compact and less costly. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of the present invention is to provide 
a method for forming a multicolor image with high resolution and an 
excellent color balance when color is reproduced by converting the picture 
element of a document image into a binary value. 
In a method for obtaining a multicolor image by successively forming a 
plurality of toner images having different colors on an image retainer, 
the aforementioned object is attained by using dots constituting the toner 
image and expressing the graduation by the size of the dot. Particularly 
when the dots constituting the plurality of toner images are overlapped, 
the dots should preferably be arranged at mutually different angles. 
The present invention presupposing the aforesaid construction features the 
use of a method for expressing gradation according to the size of the dots 
in place of a conventional method for expressing the gradation according 
to the dots being arranged in a decentralized mode. 
Another object of the present invention is to provide an apparatus for 
forming a multicolor image capable of offering a disturbance-free clear 
image with an excellent color balance by processing a plurality of color 
data supplied in the operational processor to form a color image using 
fewer color toners compared with that of the color data. 
In an apparatus for forming a color image by successively forming a 
pluralitv of different color toner images on the image retainer, the 
apparatus comprising means for correcting color image data composed of a 
plurality of different color data, means for forming a latent image on the 
image retainer based on what has been corrected by the means above in 
terms of colors and a plurality of developing means for developing the 
latent image formed by the means above with different color toner, the 
object is attained by using an operational processor for converting the 
image data based on the results obtained from said color correcting means 
for comparing and computing the plurality of color data, the operational 
processor being used to subtract data designating the lowest density among 
the data composed of yellow, magenta and cyan primary colors from each of 
the three primary color data so as to treat the result as a black 
component. 
Still another object of the present invention is to provide an apparatus 
for forming a multicolor image capable of offering a disturbance-free 
clear image with an excellent color balance by correcting and converting 
the tone of image data based on external instructions and further forming 
the color image employing the converted data. The object is attained by an 
apparatus for forming a color image by successively forming a plurality of 
different color toner images on the image retainer, the apparatus 
comprising means for correcting color image data composed of a plurality 
of different color data, means for forming a latent image on the image 
retainer based on what has been corrected by the means above in terms of 
colors and a plurality of developing means for developing the latent image 
formed by the means above with different color toner, the color correcting 
means having an operational processor for converting the tone of the image 
data according to external instructions. 
A further object of the present invention is to provide an apparatus for 
forming a multicolor image using a compact, low-cost, high-speed 
modulation laser beam source therefor. 
The object can be attained by an apparatus for forming a multicolor image 
comprising at least one image exposure means for forming an 
electrostatically charged image by irradiating laser beams on a 
photosensitive member uniformly charged and a plurality of developing 
devices for developing the electrostatically charged image, the laser beam 
being semiconductor laser beam whose intensity has been modulated. 
The present invention features the use of a highly efficient laser beam 
source for controlling current by directly applying the data signal to the 
semiconductor, allowing the modulated laser beam to be oscillated in terms 
of its intensity and forming different color electrostatic latent images 
on a photosensitive member using the laser beam.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, the present invention will be described. 
However, the applications of the present invention are not limited to the 
following embodiments thereof. A method for forming image data will be 
described first. 
In the method of forming a multicolor image acdording to the present 
invention, an output signal of an image pickup element which has scanned a 
multicolored document, a transmission signal from another apparatus such 
as a facsimile or data stored in a memory is utilized as image data. The 
image data is represented by three yellow, magenta and cyan primary color 
data Yi, Mi, Ci and black data BKi. When the multicolor image is formed, 
the image data is sent to an operational processor in a color correcting 
section of FIG. 4 and the operational equation (1), for instance, is used 
to compute desired four color data. 
Equation (1): 
EQU Ym=.alpha..sub.1, Yi-.beta..sub.1 min(Yi, Mi, Ci 
EQU Mm=.alpha..sub.2 Mi-.beta..sub.2 min(Yi, Mi, Ci) 
EQU Cm=.alpha..sub.3 Ci-.beta..sub.3 min(Yi, Mi, Ci) 
EQU BKm=.alpha..sub.4 BKi+.beta..sub.4 min(Yi, Mi, Ci) 
where Ym, Mm, Cm=data after operation; Yi, Mi, Ci=image data input; 
.alpha..sub.1, .alpha..sub.2, .alpha..sub.3, .alpha..sub.4, .beta..sub.1, 
.beta..sub.2, .beta..sub.3, .beta..sub.4 = color correction coefficient 
based on external factors such as developing conditions; and min (Yi, Mi, 
Ci) =tone having the minimum density value among three yellow, magenta and 
cyan primary colors. 
To aid in understanding the equation (1), description will be given as 
follows by taking .alpha..sub.1 .about..alpha..sub.4, .beta..sub.1 
.about..beta..sub.4 being 1 in all cases as an example. Given that the 
tone of the minimum density is cyan (Ci) as shown in FIG. 5, the black 
component is obtained based on the principle of the subtractive color 
process by gathering the quotient obtained by subtracting the density 
equivalent to the cyan from each of the three primary colors. The black 
component is added to the black data BKi to provide black image data shown 
in FIG. 6, whereas what is left after the cyan density or the equivalent 
has been subtracted from each of the three primary colors is treated as 
the image data of each of the three primary colors. It becomes thus 
possible to improve the color balance at the time of development, save 
toner consumption and increase the efficiency of the developing process. 
The four color data Ym, Mm, Cm, BKm corrected by the operational processor 
shown in FIG. 4 are compared with a threshold matrix (described later) 
before being converted into four color data Yo, Mo, Co, BKo in a binary 
form. The data are stored in memories My, Mm, Mc, MBK and given to the 
exposure system according to the instructions from the control means, so 
that an electrostatically charged image can be formed on the 
photosensitive member. The electrostatically charged image should 
preferably be subjected to non-contact development using four kinds of 
developing devices driven by the control means, the developing devices 
containing yellow, magenta, cyan and black toner, respectively. Thus, 
toner images of four colors are superposed and formed on the 
photosensitive member and then transferred to and fixed on transfer paper 
fed in conjunction with the photosensitive member, so that a multicolor 
image is formed thereon. 
As a threshold matrix for use in the present invention, a volute type 
matrix YP, MP, CP or BKP distributed separately by colors as shown in FIG. 
7 is used. The toner image obtained using the threshold matrix is composed 
of centralized dots shown by YD, MD, CD, BKD and the gradation thereof is 
expressed depending on the size of the dot. In addition, because the dot 
of each color is prevented from being overlapped with another, a clear 
multicolor image with an excellent color balance is obtained. 
In addition to the method of expressing the tone separately by colors 
according to the present invention, it is also employed to express the 
gradation by distributing various color dots within the same matrix 
pattern and allowing dots to be overlapped. The problem in that case 
includes the difference in charging between the overlapped portion and 
what is free from overlapping and the impossibility of forming a desired 
latent image because the image exposure is prevented from reaching the 
image retainer as it is blocked by the preceding dot. According to the 
present invention, the aforesaid disadvantages are remedied by providing a 
centralized, instead of decentralized, dot construction even when 
different color dots are superposed and, as shown in FIGS. 8(a) through 
8(d), the occurrence of a moire is prevented by changing the angles of 
different color dots and reducing the overlapping of different color dots, 
so that each tone may visually be emphasized. 
6a, 7a, 8a, 9a show yellow, magenta and cyan threshold matrices, whereas 
6b, 7b, 8b, 9b show the distributions of centralized color dots. 6c, 7c, 
8c,9c represent variations in angles of color dot distributions and, for 
instance, the angle .theta..sub.1 of 6c=90.degree., the angle 
.theta..sub.2 of 7c=45.degree., the angle .theta..sub.3 of 8c=26.6.degree. 
and the angle .theta..sub.4 of 9c=-26.6.degree.. With respect to the angle 
variations, all of the colors may be changed or otherwise the difference 
in color may be between two colors only. Accordingly, even if different 
color toner dots are superposed, a moire will be prevented from occurring 
because the angle at which the dots are arranged and reduction in the 
color balance will be decreased because the tone of each different color 
dot is visually demonstrated. 
(EXAMPLE 1) 
Referring now to the drawings, an embodiment 1 of the present invention 
will be described. 
FIG. 9 is a cross sectional view of an apparatus for forming a multicolor 
image illustrating the embodiment of the present invention. FIG. 10 
illustrates a laser applicable to the apparatus for forming an image of 
FIG. 9. FIG. 11 illustrates a developing device applicable to the 
apparatus for forming an image of FIG. 9. In FIG. 9, a photosensitive drum 
11 is a selenium photosensitive member 120 mm in diameter which is caused 
to rotate at a linear velocity of 120 mm/sec in the direction of an arrow 
and uniformly charged with +600 V by a scorotron charge device 12. The 
uniform charge is subjected to image exposure by image exposure means and 
an electrostatically charged image is formed. 
In other words, data Yi, Mi, Ci, BKi stored in the memory are supplied to 
the operational processor in the color correcting section of FIG. 4 and 
processed by the equation (1), whereby the data Ym, Mm, Cm, BKm subjected 
to the color correction are obtained. The data obtained are compared with 
the threshold matrices of FIGS. 7 or 8 and those Yo, Mo, Co, BKo in the 
form of a binary value are obtained. The data obtained are stored in 
memories My, Mm, Mc, MBK and supplied to the exposure system under 
instructions from the control means. In the present embodiment, a laser 14 
of FIG. 10 is used as an exposure system and an image is exposed by a 
laser beam L from the laser 14, whereby an electrostatic latent image 
corresponding to each color is formed on the photosensitive member 11. 
FIG. 10 shows the details of the laser 14. The helium-neon laser beam 
emitted from a transmission source 33 is supplied to an acoustic optical 
modulator (AOM) 34 through reflecting mirrors 37, 38 and modulated by 
image data in the form of a binary value. The modulated laser beam is 
deflected by a mirror scanner 35 comprising a rotary octahedron, whereby 
image exposure is carried out by scanning the surface of the 
photosensitive member 11 at a constant velocity through a focusing lens 
f-.theta. 36. Numeral 39 designates an inspection device for checking the 
characteristics of the laser beam L. 
Among the electrostatic latent images corresponding to each of the colors, 
what corresponds to yellow is formed by the irradiation of the laser beam 
modulated by the yellow data, which is the one convered into a binary 
value using the YP threshold matrix of FIG. 7. 
The electrostatic latent image corresponding to yellow is developed by a 
first developing device 15, and a first toner image (yellow toner image) 
is formed on the photosensitive member 11. The first toner image is again 
charged with +600 V by the scorotron on the photosensitive member without 
being transferred to recording paper P. Subsequently, the laser beam is 
modulated by magenta converted into a binary value using the threshold 
matrix MP of FIG. 7 and the laser beam thus modulated is radiated onto the 
photosensitive member 11 to form an electrostatic latent image. The 
electrostatic latent image is developed by a second developing device 16 
to form the second toner image (magenta toner image). In the same manner, 
the threshold matrices CP, BKP are successively used and a third as well 
as a fourth developing devices 17, 18 are successively employed to form 
the third toner image (cyan toner image) and the fourth toner image (black 
toner image). These toner images are piled up on the photosensitive member 
11 to form four color toner images. These four color toner images are 
de-electrified by a charge eliminating device 19 and transferred to the 
recording paper P supplied from a paper feeder 20 through the function of 
a transfer electrode 24. In this case, numerals 23, 22 represent a paper 
feeding roller and a guide plate, respectively. The recording paper P 
retaining the toner image transferred is separated from the photosensitive 
member 11 by a separation electrode 25 and carried by a guide 26 and a 
carrier belt 27 to a fixing roller 28 where it is fixed with heat and then 
discharged onto a paper receiver 29. 
On the other hand, upon the completion of the transfer, the photosensitive 
member 11 is de-electrified by a charge eliminating device 31, which is 
not used during the formation of the toner images, whereas the toner left 
on the surface of the photosensitive member 11 is removed by a blade 32 of 
a cleaning device 30, which is also released during the formation of the 
toner images, so that the subsequent formation of a multicolor image may 
be carried out without trouble. 
There are four kinds of developing devices used for the apparatus for 
forming a multicolor image of FIG. 9 and these may be the same or similar 
ones. FIG. 11 is a cross sectional view of the representative first 
developing device 15. A developer D is carried in the direction of an 
arrow G by a magnetic roll 41 having six poles and being driven at a 
velocity of 1,000 r.p.m. in the direction of an arrow F and a sleeve 42 
having a diameter of 30 mm and being rotated at a velocity of 120 mm/sec 
in the direction of an arrow G. The developer D is a two-component 
developer and its thickness is so controlled by a developer layer control 
blade 43 so as to form a developer layer 0.5 mm thick. An agitating screw 
45 for thoroughly agitating the developer D is provided in a developer 
tank 44 and, when the developer D in the developer tank has been consumed, 
a toner supply roller 46 turns to supply the toner T from a toner hopper 
47. 
Moreover, a gap 0.8 mm wide is provided between the sleeve 42 and the 
photosensitive drum 11, wherein a d.c. power supply 48 is installed to 
apply a developing bias to allow reversal developing. An a.c. power supply 
49 is installed in series with the d.c. power supply 48 to oscillate the 
developer D in a developing region E and supply a sufficient amount of the 
developer D to the photosensitive drum 11. R designates a protective 
resistor. The developing bias is provided with a d.c. component of +500 V, 
an a.c. component of 2 KHz and an effective value of 1.5 KV. The toner T 
in the developer carried by the sleeve 42 within the developing device 15 
is supplied with a charge of 20 .mu.c/g until it reaches the developing 
region E. 
On the other hand, use can be made of a two-component developer composed of 
toner and a carrier or one-component developer composed of toner only as 
the developer for use in such a machine. In the case of the two-component 
developer, although the quantity of the toner relative to the carrier must 
be controlled, the advantage is that the charge caused by toner particle 
friction can be controlled. Moreover, because it is unnecessary to mix a 
black magnetic substance with toner particles in the two-component 
developer composed of the magnetic carrier and the non-magnetic toner, 
color toner free from color turbidity can be used because of the magnetic 
substance and a clear color image is advantageously usable. 
The two-component developer used according to the present invention should 
preferably be composed of a magnetic carrier as a carrier and non-magnetic 
toner as toner. 
The composition of the toner is as follows: 
(1) Thermoplastic resin: binder 80.about.90 wt%. 
Example: polystyrene; styrene acryl polymer; polyester; polyvinylbutyral; 
epoxy resin; polyamide resin; polyethylene; and ethylene vinegar vinyl 
copolymer may be added. 
(2) Pigment: coloring agent 0.about.15 wt%. 
Example: black: carbon black; cyan: copper phthalocyanin; sulfonamide 
dielectric dye; yellow: benzidine derivative; magenta: rhodamin B lake, 
carmine 6B, etc. 
(3) Charge control agent: 0.about.5 wt%. 
Plus toner: electron supply dyes of nigglosin series are often used. In 
addition, alcoxylamine, alkylamide, kylate, pigment; 4-grade ammonium 
salt; etc. 
Minus toner: electron receptive organic complex is effective. In addition, 
chlorinated paraffin; chlorinated polyester; polyester with excessive acid 
radical; and chlorinated copper phthalrosianin; etc. 
(4) Fluid agent: 
Example: colloidal silica and hydro silica are mainly used. In addition, 
silicon varnish; metal soap; and non-ion surface active agent. 
(5) Cleaning agent: for use in prevening the filming of toner in the 
photosensitive member. 
Example: fatty acid surface; oxidized silicon having an organic radical on 
the surface; and a surface-active agent of the fluorine series. 
(6) Filler: intended to improve luster on the surface of the image and 
reduce material cost. 
Example: calcium carbonate; cray; talc; pigment; etc. 
Other than the aforementioned materials, a magnetic substance may be 
contained to prevent photographic fog and the scattering of the toner. 
Although 0.1.about.1 .mu.m triiron oxide, r-ferric oxide, chrome dioxide, 
nickel ferrite and iron alloy powder have been proposed as magnetic 
powder, the triiron oxide is now widely used and 5.about.70 wt% thereof is 
mixed with the toner. The resistance of the toner varies with its type and 
the quantity and, in order to obtain sufficient resistance, it is 
preferred to add a less than 55 wt% magnetic substance. Moreover, it is 
also preferred to add a less than 30 wt % magnetic substance to maintain a 
clear color of the color toner. 
As resin suitable for use in fixing toner under pressure, an adhesive resin 
such as wax, polyolefine, ethylene acetic acid vinyl copolymer, 
polyurethane or rubber is selected and used in such a manner that it is 
subjected to plastic deformation with a force of about 20 kg/cm before 
being glued to paper. Capsular toner may also be used. 
Toner can be made using the known method of manufacture and materials. 
With the arrangement according to the present invention, the diameter of 
the toner particle should preferably be about 50 microns in terms of the 
ordinary average weight/particle diameter in connection with resolving 
power to obtain a further preferred image. Although there is no theoretic 
restriction to the toner particle diameter according to the means above, 
it should preferably be about 1.about.30 microns in view of resolving 
power, toner diffusion and carrying. In this embodiment of the present 
invention, each of the four color toners having an average weight/particle 
diameter of 10 .mu.m is used. 
In order to obtain fine points and lines or improve the gradient, the 
magnetic carrier particle should be composed of a magnetic particle and 
resin, for instance, magnetic powder and resin dispersed therein or a 
magnetic particle coated with resin. It should preferably be spherical and 
have an average weight/particle diameter of less than 50 .mu.m and more 
preferably less than 30 .mu.m and more than 5 .mu.m. In the present 
embodiment, carrier particles of all four colors having an average 
weight/particle diameter of 50 .mu.m were used. The average 
weight/particle diameter of the toner and the carrier above was measured 
using a Coulter counter (Coulter Electronics, Inc.). 
To prevent a charge from being readily injected into the carrier particle, 
which tends to impair the formation of a good image, by the bias voltage, 
that is, the carrier from being readily stuck to the surface of the image 
retainer and further prevent the occurrence of the problem including 
insufficiently applied bias voltage, the resistance of the carrier should 
preferably be set at more than 18.sup.8 .OMEGA.cm, more preferably more 
than 10.sup.13 .OMEGA.cm and most preferably more than 10.sup.14 
.OMEGA.cm, whereas the particle diameter mentioned above together with the 
resistance should be employed. In this embodiment, a carrier magnetized at 
50 e.m.u. and of a resin dispersed type having a specific resistance of 
10.sup.14 .OMEGA.cm was used. The specific resistance of the carrier is 
measured by the following method. That is, particles are put into a 
container having a cross section of 0.50 cm.sup.2 and tapped and then a 
load of 1 kg/cm.sup.3 is applied to the packed particles. Subsequently, 
voltage allowing an electric field of 10.sup.2-5 V/cm to be generated is 
applied across the load and the bottom electrode and the value of the 
current flowing then is read and computed as predetermined to obtain the 
intrinsic resistance. The thickness of the carrier particle at this time 
is about 1 mm. 
The method of preparing the carrier thus reduced to particles comprises the 
steps of coating the surface of the magnetic substance with resin using 
the magnetic substance and the thermoplastic resin as described in 
reference with the toner or preparing the particles from the resin and 
fine magnetic substance powder dispersed therein and selecting the 
particles obtained using average particle diameter selecting means. It is 
also preferred to make the carrier spherical to let the toner and the 
carrier be readily agitated and the developer be readily carried and 
moreover prevent the agglutination of the toner particles or the toner and 
carrier particles by improving toner charge controllability. The method of 
preparing the spherical magnetic carrier particle comprises, in the case 
of what is coated with resin, selecting the most sperical one as a 
magnetic substance particle and coating it with resin and, in the case of 
the carrier with a fine magnetic substance particle dispersed therein, 
selecting the most fine magnetic substance particle and making it 
spherical using hot air or water after the formation of a dispersing resin 
particle or directly forming a spherical dispersing resin particle through 
the spray dry method. When a highly gradient four color image was formed 
under the conditions above, it offered not only high resolving power and 
excellent gradation but also clearness with a desired color balance. 
Moreover, clear characters and diagrams were also obtained. When a 
four-color image was also formed with a dot pattern whose different color 
toner dots were overlapped using the threshold matrix of FIG. 8, a clear 
image with a superior color balance was obtained. 
EXAMPLE 2 
FIG. 12 shows an apparatus for forming a threecolor image illustrating 
another embodiment of the present invention. The principal difference 
between this and the above example 1 lies in the fact that a three color, 
namely, yellow, magenta and cyan color, laminated toner image can be 
obtained with one turn of a photosensitive drum 51. In FIG. 12, the 
selenium photosensitive drum is a drum-shaped photosensitive member 200 mm 
in diameter and rotated at a linear velocity of 150 mm/sec in the 
direction of an arrow. A scorotron charge device 52, an exposure device 53 
and a developing device 54 are, as shown in FIG. 12, arranged on the 
surface of the photosensitive member 51 to form a yellow toner image. A 
scorotron charge device 56, an exposure device 57 and developing device 58 
are arranged to form a magenta toner image. Furthermore, a scorotron 
charge device 60, an exposure device 61 and developing device 62 are 
arranged to form a cyan toner image. Accordingly, three-color toner images 
are piled up within a cycle of the photosensitive member 51. In the 
present embodiment, the photosensitive member 51 is charged with +700 V by 
the scorotron charge devices 52, 56, 60. 
Subsequently, the process for three-color image exposure and the formation 
of an equivalent image will be described. The image data stored in 
memories are input to operational processors of color correcting means of 
FIG. 13 and, for instance, desired color data are computed by the 
following equation (II). 
Equation (II): 
EQU Ym=a.sub.1 Yi+.beta..sub.1 Mi+.gamma..sub.1 Ci 
EQU Mm=a.sub.2 Yi+.beta..sub.2 Mi+.gamma..sub.2 Ci 
EQU Cm=a.sub.3 Yi+.beta..sub.3 Mi+.gamma..sub.3 Ci 
where Ym, Mm, Cm =data after computation; Yi, Mi, Ci=image data input; 
a.sub.1, a.sub.2, a.sub.3, .beta..sub.1, .beta..sub.2, .beta..sub.3, 
.gamma..sub.1, .gamma..sub.2, .gamma..sub.3 =, for instance, color 
correcting coefficient of 1, 2, 3, etc. and determined by external factors 
such as the composition of the developer and the voltage applied to the 
developing device. The data Ym, Mm, Cm subjected to color correction are 
converted into a binary value after being compared with the threshold 
matrix of FIG. 7 or 8 as is the case with the first example and set as 
data Yo, Mo, Co. These data are stored in the memories My, Mm, Mc and 
supplied to the exposure system under the instructions from the control 
means. In this example, light emitting diodes (LED) 53 (for yellow), 57 
(for magenta) and 61 (for cyan) are used to expose respective color images 
and form electrostatic latent image on the photosensitive member 51. 
Each of the electrostatic latent images is developed by the corresponding 
developing devices 54 (for yellow), 58 (for magenta) and 62 (for cyan), 
whereby each color toner image is successively superposed on another. 
Although the same types of developing devices 15 as what is shown in FIG. 
11 of the example 1, an effective value of 1.5 KV with a d.c. bias 
component +600 V from a power supply 74 and an a.c. bias component 2 KHz 
from a power supply 73 is applied to the developing device 54 through a 
sleeve 55. 
To the developing device 58, an effective value of 1.2 KV with a d.c. bias 
component +600 V from a power supply 76 and an a.c. bias component 2 KHz 
from a power supply 75 is applied through a sleeve 59, whereas to the 
developing device 62, an effective value of 1.0 KV with a d.c. bias 
component +600 V from a power supply 78 and an a.c. bias component 2 KHz 
from a power supply 77 is applied through a sleeve 63. Each of the color 
toner images is deverloped on a non-contact basis under the above 
conditions. 
Each of the color toner images is transferred onto recording paper P 
through the function of a transfer electrode 66, the paper P being 
supplied from a paper feeder 64 to a paper feeding roll 65. Then the 
recording paper P is separated from the photosensitive member 51 by a 
separating electrode 67, fixed by a hot roll 69 of a fixing device 68 with 
heat and discharged. The charge and the toner left on the photosensitive 
member 51 are de-eliminated and removed by a charge eliminating device 71 
and a blade 72 of a cleaning device 70 in preparation for the following 
formation of an image. 
In this example as in the case of the example 1, a three-color image was 
prepared without superposing different color dots, and with the lamination 
of dots by changing the distribution angle of the dots. It was then proved 
that the image obtained offered high resolving power and clearness with an 
excellent color balance. Moreover, a desired tone was obtained by 
adjusting the parameters .alpha., .beta., .gamma. of FIG. 13. 
Obviously in the method of forming a multicolor image comprising piling a 
plurality of toner images on an image retainer, the resolving power and 
the color balance of the multicolor image can be improved by expressing 
the gradation of the toner image with the difference size of the dot and a 
clearer multicolor image is effectively obtained. 
A further embodiment of the present invention will be described 
subsequently. 
As the developer D in the above examples is caused to be carried without 
contacting the photosensitive member 11, the toner is impelled to the 
latent image by the a.c. bias. In that case, electric force directing the 
toner particles to and from the photosensitive member 11 acts on the 
particles located between the photosensitive member 11 and the developing 
devices (for instance, 15, 16, 17) because of the a.c. current phase 
changing moment by moment. The latter causes the toner on the 
photosensitive member to flow back to the developing devices and may allow 
a different color toner to enter the developing device. 
The following measures can be taken to counter the above problem while 
toner images are superposed. 
(i) Toner having a larger charge quantity is successively used; 
(ii) The amplitude and/or frequency of the a.c. component of the developing 
bias is successively decreased; 
(iii) The developing device being not used is put away from the 
photosensitive member 11; 
(iv) The quantity of toner supplied is gradually increased; 
(v) The potential contrast of the latent image is gradually increased; 
(vi) The gap d between the photosensitive member 11 and the developer layer 
is gradually increased; and 
(vii) The bias (the same polarity as that of the toner) is applied so as to 
prevent different toner from entering the developing device being not 
used. 
Referring to FIG. 14, the functions of the operational processor in another 
apparatus for forming a color image embodying the present invention will 
subsequently be described. The input image data is divided into regions 
having a predetermined size and operational processing is carried out on a 
region basis. In other words, the density data Yi, Mi, Ci, BKi of yellow, 
magenta, cyan and black are processed according to the algorithm of an 
operational process (described later), converted into Yo, Mo, Co, BKo and 
stored in the memories My, Mm, Mc, M.sub.BK. When the whole image data to 
be recorded has been computed and processed, the density data stored in 
the memories are taken out under instructions from the control means and 
the exposure device and the corresponding developing device is driven 
according to the data on a color basis and a color toner image is formed 
on the photosensitive member 11. 
Referring to FIGS. 15(a), 15(b) and FIGS. 16(a), 16 (b), the algorithm of 
the operational process will subsequently be described. FIGS. 15(a), 15(b) 
are histograms illustrating the sum total of color density levels by 
colors within a region thus divided. Taking the data of FIG. 15(a) as an 
example, FIG. 15(a) is converted to FIG. 15(b) utilizing that black is 
obtained when three primary colors of yellow, magenta and cyan having 
equal density levels are blended. That is, the portion Yi displaying a 
minimum density value in the input data is subtracted from the data Yi, 
Mi, Ci and replaced with black. This is expressed by the following 
equation. 
EQU Yo=Yi-min(Yi, Mi, Ci) 
EQU Mo=Mi-min(Yi, Mi, Ci) 
EQU Co=Ci-min(Yi, Mi, Ci) 
EQU Bo=Bi+min(Yi, Mi, Ci) 
FIGS. 16(a), 16(b) illustrate the color density data of each color allotted 
to each picture element within a region (composed of 4.times.4 picture 
elements) thus divided in FIGS. 15(a), 16(b). In FIG. 16(a), the input 
data are directly allotted, whereas in FIG. 16(b) the data converted by 
the aforementioned equation are allotted. A comparison of FIGS. 16(a), 16 
(b) allows the conversion of a fairly large number of data out of the 
three primary color data into black, so that the quantity of toner stuck 
because of development may be decreased. As a result, the quantity of the 
toner consumed may effectively be saved in the first place. Secondly, the 
problem which occurs when reversal development is repeated on the same 
photosensitive member in that the toner images are not readily superposed 
can be solved because the density of the toner stuck to the photosensitive 
member 11 is reduced and color reproduction is not seriously obstructed. 
Accordingly, the quantity of the toner consumed is decreased and a color 
image offering an excellent color balance can be obtained. 
In the operational process based on the algorithm, any input data may be 
used, provided that it contains color data. In the case of a television 
image for instance wherein it is scanned by an electron beam according to 
the signal transmitted and the luminance of three primary colors, blue, 
green and red is indicated through the additive processes, they are 
converted into the density levels of three primary colors, yellow, magenta 
and cyan through the subtractive color process by taking the difference 
between each level and the saturated quantity of the three primary colors. 
The analog output signals of Y, M, C of the pickup element may directly be 
used as input data for operational processing and moreover it is possible 
to use the analog signal converted into a digital form or, if necessary, 
what is supplied with additional different data. The aforesaid color data 
may be more than three-color or multicolor data. 
FIG. 17 shows the operating timing of each image forming device in the 
apparatus for forming a color image according to the present invention. 
The horizontal axis represents each image forming cycle time (second), 
whereas the vertical axis represents the operation of each device. In FIG. 
17, a bias of +500 V is applied to the developing device without operating 
during four developing processes to prevent different color toner from 
entering the device and a bias of -300 V is applied thereto immediately 
before and after development to prevent the toner from flying out. The 
magnetic roll and the sleeve are so controlled as to rotate only at the 
time development. When a four-color image was formed under such 
conditions, a clear color image free from distortion with a good color 
balance was obtained. 
As set forth above, the quantity of the toner consumed for development can 
be saved by carrying out the operational process to convert the three 
primary color density common to one another in the input data as color 
correcting means for the apparatus for forming a color image according to 
the present invention. The shortcoming attributed to the formation of a 
color image by superposing multicolor toner images on the photosensitive 
member can be remedied, whereas a clear image free from disturbance with a 
good color balance can effectively be obtained. 
Referring to still another embodiment of the present invention as shown in 
FIGS. 18(a).about.18(c), 19(a).about.19(c), the algorithm of the 
operational process will be described. FIGS. 18(a).about.18 (c) show 
historams of the sum of color density levels by colors within a region 
thus divided. Taking the input data shown in FIG. 18(a) as an example, 
FIG. 18(a) is converted into FIG. 18(b) utilizing black obtained when 
three primary yellow, magenta and cyan colors having an equal density 
level. That is, the portion Ci displaying a minimum density out of the 
input data is subtracted from the data Yi, Mi, Ci and the rest is replaced 
with the black component. The image data thus processed contributes to the 
reduction of the toner quantity stuck to the photosensitive member 11 and 
the advantage is that the quantity of the toner consumed can be saved to a 
greater extent. However, there is still posed a problem in that, because 
the following toner image is not readily superposed on the preceding one 
when toner images are successively piled on the photosensitive member by 
reversal development, the tone of the preceding toner image is 
over-emphasized. 
In this example, the operational process of the image data is not limited 
to the replacement of the density level to solve the above problem and the 
parameter for the operational process is so arranged as to be externally 
determined. The parameter may be determined by, for instance, detecting 
the composition of the developer, the bias voltage to the developing 
device and the order of each color toner development and feeding back the 
results obtained. Furthermore, it may be determined by the operator's 
manual operation based on the data determined experimentally. 
The method of operationally processing the aforementioned image data is 
expressed by the following equation: 
EQU Bo=.alpha..sub.2 Bi+.beta..sub.2 min(Yi, Mi, Ci) 
EQU Yo=.alpha..sub.2 Yi+.beta..sub.2 min(Yi, Mi, Ci) 
EQU Mo=.alpha..sub.3 Mi+.beta..sub.3 min(Yi, Mi, Ci) 
EQU Co=.alpha..sub.4 Ci+.beta..sub.4 min(Yi, Mi, Ci) 
where Bi, Yi, Mi, Ci=each color input data showing the density of black, 
yellow, magenta, cyan being input to the operational processor; Bo, Yo, 
Mo, Co=each color data indicating the density of black, yellow, magenta, 
cyan converted by the operational processor; min (Yi, Mi, Ci)=data 
designating a minimum density among three primary color data Yi, Mi, Ci; 
and .alpha..sub.1, .alpha..sub.2, .alpha..sub.3, .alpha..sub.4, 
.beta..sub.1, .beta..sub.2, .beta..sub.3, .beta..sub.4 =parameters 
externally instructed. The histogram of each color density level of input 
data Yi, Mi, Ci, Bi is shown in FIG. 18(a), whereas the histogram of each 
color density level of the data Yo, Mo, Co, Bo obtained by computing the 
parameters .alpha..sub.1 through .alpha..sub.4 and .beta..sub.1 through 
.beta..sub.4, assuming that they are all one and the toner is developed in 
the order of yellow, magenta, cyan and black, is shown in FIG. 18(b). The 
histogram when the parameter .beta..sub.2 is set at 1.5, is shown in FIG. 
18(c). (However, Yo' indicates the data converted to yellow when 
.beta..sub.2 is set at 1.5.) 
FIGS. 19(a).about.19(c ) correspond to each of FIGS. 18(a).about.18(c). In 
FIG. 19(a), the input data of FIG. 18(a) are directly allotted, whereas in 
FIG. 19(b), 19 (c) the data converted by the operational processor are 
allotted. A comparison of FIGS. 19(a).about.19(c) shows that the quantity 
of toner adhering to the photosensitive member 11 in the case of data 
conversion shown in FIGS. 19(b), 19(c) is by far smaller than that in FIG. 
19(a). Particularly in FIG. 19(c), the yellow toner image precedingly 
develop is suppressed and the color balance is seen to have been 
optimized. 
In the operational processing by the algorithm, any input data may be used 
as far as it contains color data. For instance, the luminance levels of 
blue, green and red are defined as three primary colors through the 
additive processes when a television image is transmitted but, if the 
difference between the saturated quantities at the color levels is 
obtained, they are converted to the density levels of three primary 
colors, yellow, magenta and cyan through the subtractive color process. 
Moreover, the analog output signals of yellow, magenta and cyan of a 
pickup element may directly be used as the input data of the operational 
processing. It is also possible to digitalize the analog data and, if 
necessary, add different data to provide the input data. 
The color data in the apparatus for forming a color image as above 
described is applicable to more than three to four colors. 
FIG. 17 shows the operating timing of each image forming device in the 
apparatus for forming a color image in this example. 
As set forth above, according to this embodiment, the shortcoming when 
multicolor toner images are piled up to form a color image can be remedied 
by subjecting the color data to the operational process and conversion 
according to instructions based on external data as means for correcting 
colors in the apparatus for forming a color image, whereas a clear image 
free from disturbance with a good color balance can effectively be 
obtained. 
An additional embodiment of the present invention will subsequently be 
described. As the semiconductor laser beam source, a semiconductor device 
120 having a double heterostructure of gallium, aluminum or arsenic series 
shown in FIG. 20 is used. In FIG. 20, there is shown such a semiconductor 
device comprising an upper electrode 121, a lower electrode 125 to which a 
data signal subjected to color correction is applied, an upper clad layer 
122 composed of P-Alz.Ga(1-y).As, an active layer 123 composed of 
n-Alx.Ga(1-x).As, a lower clad layer 124 composed of n-Aly.Ga(1-y).As, a 
substrate 125 composed of n-Ga.As and a laser beam L oscillating out of 
the active layer 123. However, X, Y, Z takes either value of 0 or 1. 
Although the oscillating wavelength of such a semiconductor laser beam is 
relatively as long as 700 to 830 nm, as a photosensitive member suitable 
for the semiconductor laser beam, the use of a functional separated type 
photosensitive member composed of a charge generating layer (CGL) and a 
charge transport layer (CTL) is preferred. In other words, a Se evaporated 
layer containing 10.about.40 wt% tellurium, a resin dispersed layer 
containing a photoconductive bisazo pigment or triceazo pigment, a 
hydrogenized nitrigenized amorphous silicon evaporated layer, a resin 
dispersed layer containing vanadilphtharosianin pigment and the like is 
used as the CGL. As the CTL, for instance, polyvinylcarbazole, a 
polyallylalkan amino compound, an oxisadiazole derivative, sellenium 
evaporated layer, etc. are used. 
FIG. 21 shows the cross section of the apparatus for forming a multicolor 
image in this embodiment. FIG. 21 shows the cross section of a laser beam 
exposure device. FIG. 22 shows current modulation characteristics. 
FIG. 21, a drum photosensitive member 130 having a diameter of 120 mm 
comprises a CGL with sellenium 1 .mu.m thick being evaporated on a drum 
aluminum substrate, the sellenium containing 35 wt% tellurium, and a CTL 
formed thereon with sellenium 20 .mu.m. The drum photosensitive member 130 
is turned at a peripheral velocity of 120 mm/sec in the direction of an 
arrow. A laser beam Lk is generated by a laser beam exposure device 132. 
The construction of the exposure device 132 is shown in FIG. 22. A beam 
source 150 has a double heterostructure (DH) of gallium. arsenic-gallium. 
aluminum. arsenic as shown in FIG. 20 and generates a beam of 750 nm at 15 
mw. 
As shown in FIG. 23, the output characteristics of the laser beam source 
150 are such that the output P (mw) slightly increases as current i (mA) 
increases and, when the current i reaches a predetermined level, it 
produces laser oscillation and sharply increases its output. When a pulse 
current (II) is charged, a sharp laser oscillating output like (III) is 
obtained. The laser beam from the beam source 150 of FIG. 22 is changed 
into a parallel beam by a collimate lens 151 through reflecting mirrors 
154, 155 and reflected by a polygon 152 and then irradiated on the 
photosensitive member 130 through a f.theta. lens 153. 
In FIG. 21, after the photosensitive member 130 is charged beforehand with 
+600 V by a charge device 12, it is subjected to image exposure by a laser 
beam Lky modulate by yellow data to form an electrostatically charged 
image, which is developed by a developing device 15, whereby a first toner 
image (yellow toner image) is formed. A developer D stored in the 
developing device 15 is a two-component developer comprising a mixture of 
the toner and a carrier at 1:9 weight ratio. The toner contains as 
coloring agent, a benzidine derivative and as a charge controlling agent, 
a nigrosin dye, of an average weight/particle diameter of 10 .mu.m, and a 
specific resistance of more than 10.sup.14 .OMEGA.cm, with triiron 
dispersed in resin. The carrier has an average weight/particle diameter of 
30 .mu.m, magnetization at 50 emu/g and a specific resistance of more than 
10.sup.14 .OMEGA.cm. 
Above average weight/particle diameter is measured by Courter counter (made 
by Courter Electronics, Inc.). 
In the apparatus for forming a multicolor image according to the present 
embodiment, after the image data from a multicolor document is processed 
by the operational processor as shown in FIG. 14, the data is sent to the 
laser beam source 150 of FIG. 22. According to the signal of the corrected 
image data, the current i (mA) of FIG. 23 for driving the beam source 150 
is controlled and the intensity P (mw) of the laser beam oscillated from 
the beam source 150 is modulated. When the laser beam Lk thus modulated in 
terms of its intensity is irradiated on the uniformly charged 
photosensitive merber 130, because the charge quantity diffused according 
to the intensity of the laser beam Lk varies, an electrostatiacally 
charged image with gradation corresponding to the data signal is formed on 
the photosensitive member. Consequently, a multicolor toner image having 
gradation is obtained by developing electrostatically charged image using 
a different color toner. 
As set forth above, by using a semiconductor laser as the light source of 
the image exposure device for the apparatus for forming a multicolor image 
in the present embodiment, the quantity of the power consumption of the 
light source can be reduced and at the same time the exposure device is 
made compact and less costly, whereas a multicolor image with gradation 
and a superior color balance can effectively be obtained.