Image forming apparatus

The invention provides a method of forming an image on an imaging surface of a photoreceptor having a light decay characteristic so that the potential of the imaging surface is slowly decayed in an initial period of exposure and the potential of the imaging surface is sharply decayed in a middle period of exposure. The imaging surface is charged to an electric potential, exposed with a light beam, wherein the light beam has the maximum light amount I.sub.0 in the light amount distribution thereof on the imaging surface so as to satisfy following condition: EQU 1.2.times.P.sub.178 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2, in which P.sub.1/2 is a half decay exposure light amount for decaying the potential of the imaging surface from an initial potential value before the exposure to the half of the initial potential value.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrophotographic image forming 
apparatus, and more particularly relates to an image forming apparatus 
which forms an electrostatic latent image on a high .gamma. photoreceptor 
by a modulated light beam which has been modulated according to digital 
image data sent from a computer. 
Recently, in the field of electrophotography in which an electrostatic 
latent image is formed on a photoreceptor and the latent image is 
developed so that a visual image can be obtained, a digital system of 
image forming has been actively investigated by which improvements, 
conversion and editing of images are easily conducted so that image 
forming of high quality is possible. 
An optical scanning system which directly modulates the laser intensity 
utilizing a semiconductor laser, is used for modulation conducted 
according to the signal sent from a computer or a document in this image 
forming apparatus. A dot-shaped exposure is conducted on a photoreceptor, 
which has been charged uniformly beforehand, by the aforementioned optical 
scanning system so that a dot-shaped image can be formed. 
The section of a beam which is illuminated by the aforementioned optical 
scanning system, is circular or oval, and its luminance distribution is 
similar to a normal distribution, both ends of which are spread to the 
right and left. For example, in the case of a semiconductor laser beam, 
the luminance is usually 1-6 mW, and its section on a photoreceptor is a 
very narrow circle or oval of which one or both of the primary and 
subsidiary scanning lengths is 20-100 .mu.m. 
FIG. 10 is a schematic illustration showing the outline of the 
characteristic of a low .gamma. type photoreceptor. 
A low .gamma. type of photoreceptor in which light decay is sharp in the 
beginning of exposure and gentle in the middle of exposure as shown in 
FIG. 2a, has been used as a photoreceptor which is applied to an 
electrophotographic image forming apparatus. 
Concerning the low .gamma. type of photoreceptor, the following have been 
widely known: a mono-layer type such as Se, CdS and the like; and a 
two-layer type composed of an electric charge generating layer and an 
electric charge conveyance layer, the two-layer type being usually used in 
OPC. Light sensitivity of many of the photoreceptors which show the 
aforementioned semiconductor characteristic, is generally low in a low 
electric field, compared with a high electric field, and when electric 
potential is lowered due to an increase in the amount of light, the 
sensitivity is lowered. For that reason, the information concerning the 
surface potential in the low luminance region is important. Specifically, 
the surface potential on the low .gamma. photoreceptor which has been 
formed as an electrostatic latent image, is subsequently detected, and 
exposure is conducted according to the detected surface potential in order 
to control the charging potential. In the way described above, the 
influence can be prevented which is caused by the fluctuation of 
photosensitivity due to the change of environmental factors. Further, the 
charging potential is controlled in order to compensate the fluctuation of 
photosensitivity caused by the deterioration of the photoreceptor. 
If an electrostatic latent image were formed on a low .gamma. photoreceptor 
by beams emitted from the aforementioned optical scanning system, then a 
sharp dot-shaped latent image could not be formed because the sensitivity 
of the aforementioned photoreceptor is generally high in the beginning of 
exposure, so that the fluctuation of the photoreceptor tends to be picked 
up. 
In this case, there are problems as follows. Even when an electrostatic 
latent image formed by the aforementioned beams is preferably developed by 
the method of reversal development, the sharpness of the obtained image is 
low in many cases. Further, recording of high density is difficult. 
In order to take measures to meet the situation described above, the 
inventors have developed an image forming apparatus having a high .gamma. 
type photoreceptor, the light decay characteristic of which is as follows: 
the light decay of the charging potential is not sensitive to a small 
amount of light so that the charging potential is not decayed when the 
photoreceptor receives a small amount of light; and when the photoreceptor 
receives a medium amount of light, the charging potential is sharply 
decayed. 
With a view to solving the conventional problems described above, the first 
object of the present invention is to provide an image forming method 
characterized in that: a sharp latent image is formed without being 
influenced by the change of sensitivity of a photoreceptor caused by the 
fluctuation of environmental factors; and further the middle tone of the 
image can be accurately reproduced by dot exposure. 
By the image forming method to attain the first object of the present 
invention, a latent image is formed on a high .gamma. photoreceptor when a 
modulated beam sent from an optical scanning system is illuminated on the 
photoreceptor and reversal-development is conducted, and the 
aforementioned image forming method is characterized in that: the maximum 
amount of light I.sub.0 in the light amount distribution on the 
aforementioned photoreceptor, and the amount P.sub.1/2 of light of the 
half decay exposure satisfy the following inequality; 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
Further, the present invention is characterized in that: the aforementioned 
modulated beam is pulse-width-modulated. 
The second object of the present invention is to provide an image forming 
apparatus in which an image of stable quality can be formed without being 
influenced by the change of light sensitivity of a high .gamma. type 
photoreceptor, wherein the change of light sensitivity is caused by the 
fluctuation of environmental factors. 
The second object of the present invention can be attained by an image 
forming apparatus in which a high .gamma. type photoreceptor is 
illuminated with a modulated beam sent from an optical scanning system so 
that a latent image is formed and reversal-development is conducted, and 
which comprises: a half decay exposure amount detection means which 
detects the half decay exposure amount P1/2 by which surface potential 
V.sub.0 of the aforementioned photoreceptor can be reduced to 1/2; and a 
light emission amount setting means which sets the amount of light emitted 
by a semiconductor laser to a predetermined value according to the result 
of detection conducted by the aforementioned half decay exposure amount 
detection means. 
The aforementioned light emission amount setting means is characterized in 
that: the amount of light emitted by the semiconductor laser is set to 
1.2-2.5 times that of the half decay exposure amount P.sub.1/2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Before the image forming method to accomplish the first object of the 
present invention is explained, the following will be explained: the 
outline of the light decay characteristic of a high .gamma. type 
photoreceptor; the luminance distribution of modulated beams illuminated 
on the upper surface of the high .gamma. type photoreceptor; and the 
relation between the latent image potential on the upper surface of the 
high .gamma. type photoreceptor and the distribution of the amount of 
exposure. 
First, the relation between the distribution of the amount of exposure and 
the latent image potential on the high .gamma. type photoreceptor will be 
explained as follows. 
FIG. 1 is a graph showing the relation between the latent image potential 
and the amount of exposure on the high .gamma. type photoreceptor of this 
embodiment. The characteristics of photoreceptors made from Se and OPC are 
shown in the drawing for reference. 
In the drawing, the vertical axis indicates the value obtained when latent 
image potential Vl is divided by V.sub.0 so that latent image potential Vl 
can be standardized, and the horizontal axis indicates the value obtained 
when the overall exposure amount I is divided by the half decay exposure 
P.sub.1/2 so that the overall exposure amount I can be standardized. 
The relation between the image quality and the amount of exposure was 
investigated, and the following results were obtained: in the case of a 
high .gamma. photoreceptor, the best image quality was obtained when I was 
1.2-2.5. In the case of a Se photoreceptor, the best image quality was 
obtained when I was set to 3.times.P.sub.1/2 -5.times.P.sub.1/2. In the 
case of an OPC photoreceptor, the best image quality was obtained when I 
was set to 4.times.P.sub.1/2 - 6.times.P.sub.1/2. However, the sharpness 
of the latent image was inferior to that formed by the high .gamma. type 
photoreceptor. 
In other words, when a high .gamma. type photoreceptor is adopted to an 
image forming apparatus, it is an important factor to set the amount 
I.sub.0 of exposure of the optical scanning system to 1.2.times.P.sub.1/2 
-2.5.times.P.sub.1/2, which is lower than the amount of exposure in the 
case of an Se or OPC photoreceptor. In the manner described above, the 
characteristic of the high .gamma. type photoreceptor can be fully 
utilized so as to form a latent image. 
Next, the light decay characteristic of the high .gamma. type photoreceptor 
adopted to the present invention will be explained as follows. 
FIG. 3 is a schematic illustration showing the characteristic of the high 
.gamma. type photoreceptor. 
In the drawing, V.sub.1 is charging potential, V.sub.0 is initial potential 
before exposure, L1 is the amount (.mu.J/cm.sup.2) of light of the 
illuminated laser beam which is necessary for initial potential V.sub.0 to 
be decayed to 4/5, and L.sub.2 is the amount (.mu.J/cm.sup.2) of light of 
the illuminated laser beam which is necessary for initial potential 
V.sub.0 to be decayed 1/5. 
The preferable range of L.sub.2 /L.sub.1 is as follows. 
EQU 1.0.ltoreq.L.sub.2 /L.sub.1 .ltoreq.1.5 
In this embodiment, V.sub.1 =1000(V), V.sub.0 =950(V), and L.sub.2 /L.sub.1 
=1.2. The electrical potential of the exposed portion on the photoreceptor 
is 10 V. 
When E.sub.1/2 is defined as the light sensitivity in the position 
corresponding to the middle stage of exposure in which initial electrical 
potential (V.sub.0) is decayed to 1/2 in the light decay curve, and when 
E.sub.9/10 is defined as the light sensitivity in the position 
corresponding to the initial stage in which initial potential (V.sub.0) is 
decayed to 9/10, a photoconductive semiconductor satisfying the following 
relation is selected. 
EQU (E.sub.1/2)/(E.sub.9/10).gtoreq.2 
more preferably; 
EQU (E.sub.1/2)/(E.sub.9/10).gtoreq.5 
In this case, the light sensitivity is defined as the absolute value of 
electrical potential drop with regard to a minute amount of exposure. 
As shown in the light decay curve of the photoreceptor 1 shown in FIG. 3, 
when the amount of light is small, the absolute value of the differential 
coefficient of the electrical potential characteristic curve is small, and 
when the amount of light is increased, the curve is sharply decayed. 
Specifically, as shown in FIG. 3, the light decay curve can be described 
as follows: in the initial stage of exposure, the sensitivity 
characteristic is so bad that the light decay curve is approximately flat; 
and in the middle stage of exposure, which is from L.sub.1 to L.sub.2, the 
sensitivity characteristic becomes very sensitive and the light decay 
curve descends almost linearly to show the ultra high .gamma. 
characteristic. It can be considered that the photoreceptor 1 obtains the 
high .gamma. characteristic having the possibility of the avalanche 
phenomenon under the condition that the photoreceptor 1 is charged to a 
high voltage of +500-+2000 V. In other words, the carrier which has been 
generated on the surface of a photoconductive pigment, is effectively 
trapped on the interface between the pigment and the coating resin, so 
that the light decay is positively restricted. As a result, a very sharp 
avalanche phenomenon occurs in the middle stage of exposure. 
The physical meaning of the appropriate conditions obtained in the 
aforementioned manner, will be studied as follows. 
FIG. 2 is a graph showing an example of luminance distribution of the beam 
which is illuminated on the surface of a photoreceptor by an optical 
scanning system. 
The beam which is illuminated on the photoreceptor 1 for image-formation, 
is distributed in the manner of Gaussian distribution or in a manner which 
is similar to Gaussian distribution. The luminance distribution of the 
beam is on the curve of I=e.sup.-2.times.(x/x0)2. In this Gaussian 
distribution, I=I.sub.0 .times.e.sup.-1/2 in the position where 
x=.+-.x.sub.0 /2. This position is the point a at which the beam luminance 
distribution curve is most sharply changed. 
In the image forming method of the present invention, exposure intensity 
I.sub.0 .times.e.sup.-1/2 in the aforementioned position of x 
=.+-.x.sub.0 /2 where the curve is most sharply changed, is made equal to 
the reduced amount of light P.sub.1/2 which will be described later. 
Namely, it can be considered that a predetermined region including I.sub.0 
=e.sup.1/2 .times.P.sub.1/2 is the appropriate condition. When the 
aforementioned appropriate condition is adopted, latent images can be 
stably formed without being influenced by the change of sensitivity of the 
photoreceptor. Specifically, in some cases, the beam shape does not 
conform to the Gaussian distribution curve or the beam shape could be 
rectangular in the same manner as the pulse-width-modulation. However, the 
curve of the rising and last transition portions can be approximated to 
the Gaussian distribution curve. In the manner described above, 
image-formation can be stably conducted without being influenced by the 
change of sensitivity of the photoreceptor 1 which is caused by 
environmental factors. The diameter of the dot formed by this exposure 
intensity is 1/2 as compared with the case of a conventional Se or OPC 
photoreceptor in which the value is set to x-x.sub.0. Namely, when the 
aforementioned exposure conditions are set, high density recording can be 
conducted by the same optical system. 
Next, the image forming method of the present invention will be explained 
as follows. 
The inventors' eyes were fixed upon the light decay curve of electric 
potential of a photoreceptor in combination with reversal-development. The 
characteristic of the image forming method of the present invention will 
be explained as follows. The image forming method of the present invention 
is provided with a high .gamma. photoreceptor having the light decay 
characteristic such that: the light decay of the charging potential is not 
sensitive with regard to a small amount of light; and in the middle stage 
in which the amount of light exceeds the aforementioned small amount, the 
charging potential is sharply decayed. After the photoreceptor has been 
uniformly charged, an electrostatic latent image is formed on the 
aforementioned photoreceptor under the condition that the maximum amount 
of light I.sub.0 and the half decay amount of exposure P.sub.1/2 satisfy 
the following inequality; 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
where I.sub.0 is the maximum value in the distribution of the light amount 
of the beam, and P1/2 is the amount of exposure light by which the 
electric potential of the aforementioned photoreceptor is decreased to 
1/2. After that, the latent image formed on the photoreceptor is developed 
by the method of reversal-development. 
In the manner described above, image-formation is stably conducted without 
being affected by the fluctuation of environmental factors. In the case 
where the same optical system is utilized, the diameter of the dot formed 
by the aforementioned exposure intensity is approximately 1/2 of the 
diameter of the dot formed by the conventional method in which the 
photoreceptor made from Se or OPC is utilized. In other words, when the 
aforementioned exposure conditions are set, recording of high density can 
be performed by the same optical system. 
In other words, in the case of the aforementioned photoreceptor, the 
carrier which has been generated on the surface of a photoconductive 
pigment, is effectively trapped on the interface between the pigment and 
the coating resin, so that the light decay is positively restricted. As a 
result, a very sharp avalanche phenomenon occurs in the middle stage of 
exposure and an electrical potential drop occurs. In the way described 
above, an electrostatic latent image with high contrast, the electrical 
potential of a non-image portion of which is stable, can be formed, and 
the latent image can be stably developed by the method of 
reversal-development. 
Further, in the present invention, the aforementioned modulated beam is 
made by the method of pulse width modulation, so that an electrostatic 
latent image with high contrast, the electrical potential of the non-image 
portion of which is stable, can be formed and reversaly developed stably. 
Referring now to FIG. 9, the outline of the structure of the image forming 
apparatus 100 of this embodiment will be explained as follows. 
FIG. 9 is a sectional view showing the outline of the structure of the 
image forming apparatus of this embodiment. 
In the color image forming apparatus 100, a photoreceptor is uniformly 
charged, then shading correction, gradation correction and masking 
correction are conducted on the image density signal sent from a computer 
or a scanner. A dot-shaped light is obtained by pulse-width modulation in 
accordance with a modulation signal which has been obtained by comparing 
an analog image density signal obtained by D/A-converting the 
aforementioned digital image density signal, with a reference signal, 
wherein the aforementioned obtained modulation signal is binarized. Then, 
a dot-shaped electrostatic latent image is formed by the aforementioned 
dot-shaped light. The latent image is reversaly developed by toner so that 
a dot-shaped toner image is formed. The above-described exposure and 
development processes are repeatedly conducted so that color toner images 
can be formed on a photoreceptor 1, and the above-described color toner 
images are transferred, separated and fixed so that a final color image is 
obtained. 
The image forming apparatus 100 comprises: the drum-shaped photoreceptor 1 
(which will be called a photoreceptor, hereinafter), a scorotron charger 2 
which gives a uniform electric charge on the aforementioned photoreceptor 
1, an optical scanning system 30, developing units 4A, 4B, 4C, 4D in which 
toners of yellow, magenta, cyan and black are loaded, a pre-transfer 
charger 61, a scorotron transfer unit 62, a separator 63, a fixing roller 
64, a cleaning unit 70, and a discharger 74. 
The structure of each portion of the image forming apparatus of this 
embodiment will be explained as follows. 
FIG. 4 is a sectional view showing an example of the specific structure of 
a high .gamma. photoreceptor. 
The main structure of the embodiment will be explained as follows. 
As shown in FIG. 4, the photoreceptor 1 comprises a conductive support 1A, 
a middle layer 1B and a photosensitive layer 1C. The thickness of the 
photosensitive layer 1C is 5-100.mu., and preferably 10-100.mu.. The 
photosensitive layer 1C is composed in such a manner that: the drum-like 
conductive support 1A made from aluminum is utilized, the diameter of 
which is 150 mm; the intermediate layer 1B made of ethylene-vinyl acetate 
copolymer, the thickness of which is 0.1 .mu.m, is formed on the 
aforementioned support 1A; and the photoconductive layer 1C, the layer 
thickness of which is 35 .mu.m, is formed on the aforementioned 
intermediate layer 1B. 
A drum made of aluminum, steel, copper or the like, the diameter of which 
is 150 mm, is used as the conductive support 1A. The following may be used 
as the conductive support 1A: a belt-like support made of paper or plastic 
on which a metal layer is laminated or vapor-deposited; or a metallic belt 
such as a nickel belt made by the method of electroforming. The 
intermediate layer 1B is preferably provided with the following properties 
so that the intermediate layer 1B can withstand a high potential of 
.+-.500-.+-.2000 V, for example, in the case of positive charging, the 
migration of electrons from the conductive support 1C is prevented; and 
the intermediate layer 1B has a hole mobility so that an excellent light 
decay characteristic can be obtained due to an avalanche phenomenon. 
Therefore, a positive charging type of electric charge conveyance 
material, which is described in the specification of Japanese Patent 
Publication Open to Inspection No. 44662/1988 proposed by the applicant, 
is preferably added into the intermediate layer 1B by not more than 10 
weight %. 
For example, the following resins, which are applied to a photosensitive 
layer for use in electrophotography, may be used for the intermediate 
layer 1B. 
(1) Vinyl polymer such as polyvinyl alcohol (poval), polyvinyl methylether, 
and polyvinyl ethylether 
(2) Nitrogen containing vinyl polymer such as polyvinyl amine, poly-N-vinyl 
imidazole, polyvinyl pyridine (polyvinyl pyridinium salt), polyvinyl 
pyrrolidone, and vinylpyrrolidonevinyl acetate copolymer 
(3) Polyether polymer-such as polyethylene oxide, polyethylene glycol, and 
polypropylene glycol 
(4) Acrylic acid polymer such as polyacrylic acid and its salt, polyacrylic 
amide, and poly-.beta.-hydroxyethyl acrylate 
(5) Methacrylate polymer such as polymethacrylate and its salt, 
polymethacrylate amide, and polyhydroxypropyl methaacrylate 
(6) Ether cellulose polymer such as methyl cellulose, ethyl cellulose, 
carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl 
methylcellulose 
(7) Polyethyleneimine polymer such as polyethyleneimine 
(8) Polyamino acid such as polyalanine, polyserine, poly-L-glutamic acid, 
poly(hydroxyethyl)-L-glutamine, poly-.delta.-carboxymethyl-L-cystein, 
polyproline, lysine-tyrosine copolymer, glutamic acid-lysine-alanine 
copolymer, silkfibroin, and casein 
(9) Starch and its derivatives such as starchacetate, hydroxyne 
ethylstarch, starchacetate, hydroxyethylstarch, aminestarch, and 
phosphatestarch 
(10) Polymer soluble in a mixed solvent of water and alcohol such as 
soluble nylon which is polyamide and methoxymethylnylon (8 type nylon) 
The photosensitive layer 1C is formed in such a manner that: electric 
conveyance material is not essentially contained in the photosensitive 
layer 1C; phthalocyanine particulates, the diameter of which is 0.1-1 
.mu.m, as photoconductive pigment, an antioxidant, and a binder resin are 
mixed and dispersed, in a solvent of binder resin, so that a coating 
slurry can be prepared; the aforementioned coating slurry is coated on the 
intermediate layer; the coated intermediate layer is dried; and heat 
treatment is conducted, if necessary. 
When both the photoconductive material and electric charge conveyance 
material are contained, a photoconductive material including a 
photoconductive pigment and an electric charge conveyance material, the 
weight of which is not more that 1/5, preferably 1/100-1/10 of the 
aforementioned photoconductive pigment, and an antioxidant are dispersed 
in a binder resin so that a photosensitive layer is formed. 
In this embodiment, color toner images are superimposed on the 
photoreceptor, so that a photoreceptor, the spectral sensitivity of which 
is on the long wavelength side, is needed so that the beam sent from the 
optical scanning system can not be shielded by the color toner images. 
FIG. 5 is a block diagram showing the outline of the structure of the 
optical scanning system adopted to the image forming apparatus of this 
embodiment, and FIG. 6 is a block diagram showing the outline of the 
structure of the modulation circuit adopted to this embodiment. 
In the optical scanning system 30, a semiconductor laser 31 is oscillated 
by a modulated signal which has been obtained by pulse-modulating of an 
image density signal sent from a page memory (which is not shown in the 
drawing). The laser beam is deflected by a polygonal mirror 36 which is 
rotated at a predetermined speed. The deflected beam is refracted by an 
f.theta. lens 37 and cylindrical lenses 35a, 35b so that a minute 
dot-shaped beam can scan the surface of the photoreceptor 1 which has been 
uniformly charged. 
The optical scanning system 30 comprises: the semiconductor laser 31 which 
is used as a coherent light source; a collimator lens 32 and a prism 33 
which are used as an optical modulation system; the polygonal mirror 36 
and the f.theta. lens 37 which are used as an optical deflection system; 
the cylindrical lenses 35a, 35b which are used as an optical tilt 
correction system, wherein the tilt is caused by the polygonal mirror 36; 
and reflection mirrors 38a, 38b. 
The semiconductor laser 31 is made from GaAlAs. Its maximum output is 5 mW, 
its optical efficiency 25%, and its divergent angle is 
8.degree.-16.degree. in the direction parallel with the composition 
surface and 20.degree.-36.degree. in the direction perpendicular to the 
composition surface. Since color toner images are superimposed on the 
surface of the photoreceptor 1 in sequence, it is preferable to utilize 
light, the wavelength of which is so long that the absorption by colored 
toner is small, for exposure. In this case, the wavelength of the beam is 
800 nm. 
The collimator lens 32 is used so as to effectively adjust the beam 
diameter. Its numerical aperture N.A is 0.33 and the transmission factor 
is not less than 97%. The collimator lens 32 is used so as to improve 
spherical aberration and spot size condition. 
The transmission factor of the prism 33 is not less than 80%, and the 
diameter of the beam sent from the semiconductor laser 31 is compressed to 
1/3 by the prism 33. 
The optical deflection system is used so as to converge the beam (light 
flux) and used so as to reduce the Petzval's summation and astigmatism in 
order to make the scanning surface flat. 
The polygonal mirror 36 is provided with 8 polygonal surfaces, and when it 
is rotated at a revolution speed of 16535.4 rpm, the surface of the 
photoreceptor 1 can be scanned by the beam. It should be understood that 
not only a polygonal mirror but also other units can be used for the 
aforementioned purpose as far as they function in the same way as the 
polygonal mirror. 
The f.theta. lens 37 decreases the Petzval's summation and astigmatism so 
as to eliminate the curvature of field. In the manner described above, the 
scanning surface is made flat. 
As an optical correction system, the cylindrical lenses 35a, 35b are 
provided before and after the polygonal mirror 36 in order to decrease the 
unevenness of scanning lines which is caused by the tilt of the polygonal 
mirror 36. In this way, the tilt angle of the polygonal mirror becomes 120 
sec P--P, and the correction coefficient of tilt angle becomes not less 
than 1/20. The cylindrical lens 35b is used to form an image on the 
surface of the photoreceptor 1 by the beam. The spread of the dot-shaped 
beam is 20.5.+-.5 .mu.m in the primary scanning direction, and 
82.5.+-.12.5 .mu.m in the subsidiary scanning direction. On the other 
hand, it was possible to set the recording density of both primary and 
subsidiary scanning to 800 dpi. Pulse-width-modulation is utilized in the 
primary scanning. Namely, according to the present invention, it has 
become possible to conduct a recording of high density by setting an 
appropriate exposure on a high .gamma. photoreceptor. 
Further, a modulation circuit 200 is provided in the control circuit of the 
optical scanning system 30. An index sensor 39 and an index detection 
circuit 39a are provided as a synchronizing system. A polygonal driver 360 
is provided as a deflection system. 
The beam sent from the optical deflection system is incident upon the index 
sensor 39 through a reflection mirror 38b by the action of the 
synchronizing system. The index sensor 39 is induced by the beam and 
outputs an electric current. The current is current/voltage A/V-converted 
by the index detection circuit 39a and outputted as an index signal. The 
surface position of the polygonal mirror 36 which is rotated at a 
predetermined speed, is detected by this index signal, and optical 
scanning is conducted by a modulated digital image density signal, which 
will be described later, according to the raster scanning system, wherein 
the period of the scanning is that of the primary scanning direction. The 
scanning frequency is 2204.72 Hz, the effective printing width is not less 
than 297 mm, and the effective exposure width is not less than 306 mm. 
A modulation circuit 200 is provided for the purpose of outputting a 
pulse-width-modulation signal which has been binarized after comparing a 
reference wave with an analog density signal which has been obtained by 
D/A-converting a digital image density signal of a predetermined bit, for 
example, 8 bits. As shown in FIG. 6, the modulation circuit 200 comprises 
a reference wave signal generating circuit 210, a buffer circuit 220, a 
comparator 240, and a D/A-converter 230. The modulation signal outputted 
from the modulation circuit 200 is utilized as a drive signal of an LD 
drive circuit 31a. 
In a reference wave signal generating circuit 210, triangular waves are 
generated by an integrator composed of a variable resistor 211 and a 
condenser 212. The aforementioned triangular wave is inputted into a base 
terminal of a transistor 221 through a condenser 213 and a protective 
resistor 215. The reference wave signal generating circuit 210 is provided 
with two variable resistors. In other words, the variable resistor 211 is 
provided for adjusting the amplitude of the triangular wave. A variable 
resistor 214 is provided for adjusting the bias or the offset of the 
triangular wave. The triangular wave is inputted into a positive input 
terminal of the comparator 240 through the buffer circuit 220. In the 
comparator 240, a comparison is made between the reference wave which has 
passed through the buffer circuit 220 as described above and the analog 
density signal obtained by D/A-converting of the digital image density 
signal of a predetermined bit, for example, of 8 bits, using the 
D/A-converter 230. Then, the compared signal is binarized. After that, the 
signal is outputted from the output signal of the comparator 240 through 
an amplifier 241 in the form of a pulse-width-modulation signal 
synchronized with an image clock DCK. The exposure intensity is made 
variable by this amplifier 241. 
A semiconductor laser 31 is oscillated by an LD drive circuit 31a according 
to a modulation signal sent from the modulation circuit 200. The LD drive 
circuit 31a drives in such a manner that: a signal corresponding to the 
light amount of the beam sent from the semiconductor laser 31 is fed back 
so that the amount of light can become constant. 
FIG. 8 is a sectional view showing the developing unit which is applied to 
the image forming apparatus of the embodiment. 
Developing units 4A, 4B, 4C, 4D have the same structure as illustrated in 
FIG. 9, wherein the colors of developers loaded in the developing units 
are different, so that the structure of the developing unit 40 will be 
explained as follows since it is typical. 
The developing device 40 is provided with: a sleeve 43 including a magnetic 
roller 44 having N and S poles which is rotated inside a developing tank 
made of a lower casing 42 and an upper casing 41; a scraper 45 made of an 
elastic plate, which is mounted on a stationary member 46 fixed to an 
upper casing 41, and which comes into contact with a sleeve 43 with 
pressure; the first and second screw-shaped stirring members 47, 48; and a 
sleeve cleaning roller 49. The first stirring member 47 conveys the 
developer toward the viewer's side, and the second stirring member 48 
conveys the developer to the far side with regard to the viewer. A wall 42 
is installed between the stirring members 47, 48 so that the developer can 
not be accumulated in the tank. Instead of the scraper 45, a thin layer 
forming means composed of a magnetic plate or a magnetic rod may be 
installed. 
The sleeve cleaning roller 49 is rotated in the direction of an arrow so 
that the developer which has passed through the developing region and in 
which the toner component has been consumed, can be scraped off from the 
sleeve 43. Therefore, the developer conveyed into the developing region 
can be replaced with a new one, so that the developing conditions are 
stabilized. 
In order to prevent the occurrence of fogging, the sleeve 43 is connected 
with a development bias circuit 80 which impresses a voltage having a DC 
current bias component, through a protective resistance (not illustrated 
in the drawing). 
In the case described above, a two-component type of developer D is used 
which is characterized in that: the particle size of the toner is 1-20 
.mu.m; and silica particulates processed by amine compounds or silica 
particulates to which other additives are added, are used as the electric 
charge controlling agent. Small sized carrier particles are advantageous 
from the viewpoint of resolving power and gradation reproducibility. For 
example, when a small carrier, the particle size of which is 5-50 .mu.m, 
is used, a uniform height of magnetic brush can be formed. 
The development bias circuit 80 is provided with: an AC current power 
source which supplies an AC bias in order to oscillate the toner between 
the sleeve 43 and the photoreceptor 1 in the developing region in which 
the toner conveyed by the sleeve is electrostatically transferred onto the 
photoreceptor 1; and a high voltage DC current power source which supplies 
a DC current bias. In this example, V.sub.DC =800 V, V.sub.AC =700 V, and 
the frequency is 3 KHz. As described above, the development bias circuit 
80 generates an oscillating electric field between the sleeve 43 and the 
photoreceptor 1, so that the particles of the developer are oscillated in 
the space between the sleeve 43 and the photoreceptor 1. Accordingly, a 
toner image can be formed on the photoreceptor 1 under the condition that 
developer D does not come into contact with the photoreceptor, so that the 
toner image formed previously is not damaged. 
In the case of non-contact development, the developer does not come into 
contact with the latent image, so that it is difficult to develop a fine 
latent image. However, when a sharp latent image is formed by a high 
.gamma. type photoreceptor, a fine portion of the latent image can be 
developed accurately. 
For that reason, the embodiment in which the high .gamma. type 
photoreceptor is utilized is effective not only in the case of contact 
development but also in the case of non-contact development. 
Referring to FIG. 7(a)-(f), the image forming process in the image forming 
apparatus 100 of the embodiment will be explained as follows. 
FIG. 7(a)-(f) are time charts which explain the operation of the image 
forming apparatus of this embodiment, wherein the image-formation is 
conducted according to the pulse-width-modulation. 
In the drawing, (a) shows a clock DCK. In the drawing, (b) shows an analog 
density signal obtained by D/A-conversion after color or gradation 
correction. The signal shown by a dotted line in (c) is an analog density 
signal indicating the density of an image which has been D/A-converted. 
The signal shown by a solid line is a reference wave signal. In the 
drawing (d) shows a pulse-width-modulated signal sent from the modulation 
circuit 200. The density signal corresponding to a recording pixel is 
synchronized with the reference signal, and a pulse-width-modulation 
signal corresponding to the image density is generated. In the drawing, 
(e) shows an exposure dot distribution on the photoreceptor 1. Namely, the 
exposure dot distribution is originally rectangular. However, in this 
case, the exposure dot distribution is spread due to MTF of the optical 
system. In this exposure dot distribution, the position of the amount 
P.sub.1/2 of half decay exposure is indicated by a broken line, and the 
portion above the broken line is formed as a latent image due to the 
characteristic of a high .gamma. type of photoreceptor. In the drawing, 
(f) shows that a latent image is obtained which is composed of large and 
small dots corresponding to the density signal. This shows a sharp 
dot-shaped toner image in which no blur has occurred. When the diameter of 
the dot-shaped toner image is changed, a toner image, the gradation 
property of which has been improved, can be obtained. 
The image forming process by the image forming apparatus 100 will be 
explained as follows. 
First, the photoreceptor 1 is uniformly charged by a scorotron charger 2, 
and an electrostatic latent image corresponding to yellow is formed when 
the photoreceptor 1 is illuminated with a laser beam which has been 
modulated by yellow data (digital density data). The aforementioned 
electrostatic latent image corresponding to yellow, is developed by the 
first developing unit 4A, and the first dot-shaped toner image (yellow 
toner image) which is very sharp, is formed on the photoreceptor 1. This 
first toner image is not transferred onto recording paper P, and the 
photoreceptor 1 is charged again by the scorotron charger 2. 
Next, the laser beams are optically modulated by magenta data (digital 
density data), and the photoreceptor 1 is illuminated by the modulated 
laser beams so that an electrostatic latent image can be formed. This 
electrostatic latent image is developed by the second developing unit 4b 
and the second toner image (a magenta toner image) is formed. In the same 
manner described before, discharging, charging and illumination of laser 
beams are conducted, then the toner images are developed by the third 
developing unit 4C in order that the third toner image (a cyan toner 
image) is formed. In this way, a three-color toner image in which toner 
images are superimposed, is formed on the photoreceptor 1. Finally, the 
fourth toner image (a black toner image) is formed so that a four-color 
toner images are formed on the photoreceptor 1. 
According to the image forming apparatus 100 of the present invention, the 
photoreceptor has an excellent high .gamma. characteristic. Due to the 
high .gamma. characteristic, a latent image can be stably formed when the 
processes of charging and exposure are repeated a plurality of times so 
that toner images can be superimposed. In other words, even when a toner 
image is illuminated by a laser beam according to a digital signal, a 
sharp dot-shaped electrostatic latent image in which a fringe is 
eliminated, can be formed. As a result, a highly sharp toner image can be 
obtained. 
After the photoreceptor 1 has been charged by the charger 61 (this process 
may be omitted), this four-color toner images are transferred by a 
transfer unit 62 onto recording paper P. 
Recording paper P which conveys transferred toner images, is separated from 
the photoreceptor 1 by a separation electric pole 63, conveyed by a guide 
and a conveyance belt into a fixing unit 64, fixed by the method of heat 
fixing, and delivered onto a delivery tray. 
The photoreceptor 1 which has finished transferring the toner image is 
prepared for the next multi-color image forming in such a manner that: the 
residual toner on the surface is removed by a blade, a fur brush or a 
magnetic brush of a cleaning unit 70 which has been released during toner 
image forming; and the photoreceptor is discharged by a discharger 74 
composed of a corona discharger or a lamp. The lamp and the corona 
discharger 74 may be provided upstream of the cleaning means. 
In the aforementioned apparatus, the appropriate conditions were 
investigated in such a manner that: the maximum amount I.sub.0 of light in 
the laser beam optical distribution was changed with regard to the amount 
P.sub.1/2 of half decay exposure. 
__________________________________________________________________________ 
I.sub.o /P.sub.1/2 
0.4 
0.6 
0.8 
1.0 
1.2 
1.4 
1.6 
1.8 
2.0 
2.2 
2.4 
2.6 
2.8 
3.0 
__________________________________________________________________________ 
Monocolor Image 
x x x x .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.DELTA. 
.DELTA. 
.DELTA. 
MultiColor Image 
x x x x x .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
.DELTA. 
.DELTA. 
.DELTA. 
__________________________________________________________________________ 
Mark .largecircle. shows that the image quality is good, mark .DELTA. shows 
that the image quality is a little inferior, and mark X shows that the 
image quality is bad. 
In the above table, Mono-color image indicates the case in which mono-color 
images were formed by the aforementioned color image forming apparatus. 
Multicolor image indicates the case in which color images were formed by 
the aforementioned color image apparatus. 
In the case of mono-color images, the appropriate condition was as follows. 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
In the case of multicolor images, the appropriate condition was as follows. 
EQU 1.4.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
In this embodiment, color image was formed in such a manner that image 
exposure was conducted on the toner image which had already been formed on 
the photoreceptor. Therefore, when the amount of exposure light was small, 
the light was intercepted by the toner on the surface of the 
photoreceptor, so that the latent image was not formed completely, which 
can be considered to be the reason why the condition had been changed. 
By the image forming method of this embodiment, a latent image is formed on 
a high .gamma. photoreceptor 1 when a modulated beam sent from an optical 
scanning system 30 is illuminated on the photoreceptor 1 and 
reversal-development is conducted, and the aforementioned image forming 
method is characterized in that: the maximum amount of light I.sub.0 in 
the beam distribution on the aforementioned photoreceptor 1, and the 
amount P.sub.1/2 of light of the half decay exposure satisfy the 
following inequality. 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
In the manner described above, the reproduction of half tone can be 
accurately performed by dot-exposure. 
When the aforementioned modulated beam is made by the method of 
pulse-width-modulation in the image forming method of this embodiment, an 
electrostatic latent image of high contrast, the electric potential of the 
non-image portion of which is stable, can be formed and it can be stably 
reversal-developed. In the manner described above, the reproduction of 
half tone can be accurately performed by dot-exposure. 
Other exposure means such as an LED or a liquid crystal shutter can be also 
applied to the present invention. In the aforementioned cases, the shape 
of a dot intensity distribution is similar to that of the Gaussian 
distribution, so that it is preferable to set the value of P.sub.1/2 in 
the region where exposure intensity is sharply decreased. In other words, 
the exposure condition which is the same as that of the present invention 
is preferable to the high .gamma. photoreceptor. 
According to the present invention, a latent image is formed on a high 
.gamma. photoreceptor when a modulated beam sent from an optical scanning 
system is illuminated on the photoreceptor and reversal-development is 
conducted, and the aforementioned image forming method is characterized in 
that: the maximum amount of light I.sub.0 in the beam distribution on the 
aforementioned photoreceptor, and the amount P.sub.1/2 of light of the 
half decay exposure satisfy the following inequality. 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
Accordingly, it is possible to provide an image forming method by which the 
reproduction of half tone can be accurately performed without being 
affected by the fluctuation of sensitivity of the photoreceptor caused by 
environmental factors. 
When the aforementioned modulated beam is made by the method of 
pulse-width-modulation in the image forming method of the present 
invention, an electrostatic latent image of high contrast, the electric 
potential of the non-image portion of which is stable, can be formed and 
it can be stably reversaly developed. In the manner described above, the 
image forming method can be provided in which the reproduction of half 
tone can be accurately performed by dot-exposure. 
Referring now to FIG. 11 and FIG. 12, the outline of structure of an image 
forming apparatus 100 to accomplish the second object of the present 
invention will be explained as follows. 
FIG. 11 is a block diagram showing the outline of structure of the image 
forming apparatus of this embodiment. 
By the color image forming apparatus 100, a color image can be obtained in 
the following manner. After a high .gamma. photoreceptor 1 is uniformly 
charged, a dot-shaped electrostatic latent image is formed by a dot-shaped 
light which has been pulse-modulated or intensity-modulated according to a 
modulation signal obtained by modulating a digital image density signal 
sent from a page memory. The formed latent image is reversaly developed so 
that a dot-shaped toner image is formed. The aforementioned exposure and 
developing processes are repeated so that color toner images can be formed 
on the surface of the photoreceptor 1, and the color toner images is 
transferred, separated and fixed so that a final color image is obtained. 
The image forming apparatus 100 comprises: the drum-shaped photoreceptor 1 
which is rotated in the direction of an arrow; a scorotron charger 2 which 
gives a uniform electric charge on the photoreceptor 1; an optical 
scanning system 3; developing units 4-7 in which toners of yellow, 
magenta, cyan and black are loaded; a scorotron transfer unit 8; a 
separator 9; a cleaning unit 10; and a discharger 11. Electric power is 
supplied by a high voltage power unit 50 to the developing units 4-6, the 
scorotron charger 2, the scorotron separator 8, and the separator 9. 
The image forming apparatus 100 of this embodiment is provided with: a half 
decay exposure light amount detecting means to detect half decay exposure 
light amount P.sub.1/2 by which surface potential V.sub.0 of the high 
.gamma. photoreceptor 1 can be reduced to 1/2; and a light emitting amount 
setting means which sets the amount of light emitted by semiconductor 
laser LD to a predetermined value according to the detection result sent 
from the aforementioned half decay exposure light amount detecting means. 
The half decay exposure light amount detecting means is composed of 
electric potential probe P, an electrometer 510, and a microprocessor 500 
(which will be called MPU, hereinafter). As shown in FIG. 11 and FIG. 12, 
electric potential probe P is placed in the position close to the surface 
of the photoreceptor 1 between the scorotron charger 2 and the developing 
unit 4. After the surface of the photoreceptor has been illuminated 
according to the reference pattern data, the surface potential on the high 
.gamma. photoreceptor 1 is detected by electric potential probe P. In this 
embodiment, electric potential probe P is provided for the purpose of 
detecting 1/2.times.V.sub.0 of the high .gamma. photoreceptor. The detail 
of electric potential probe P will be described later. Electric potential 
probe P is connected with the electrometer 510, and the detection signal 
of the aforementioned electric potential probe P is inputted into the 
electrometer 510 and a digital electric potential signal indicating the 
voltage is outputted from the electrometer 510 into MPU 500. The light 
emitting amount setting means is composed of MPU 500 and a variable DC 
power source 440. The detail will be described later. 
MPU 500 detects the surface potential of the high .gamma. photoreceptor 1 
and controls the output voltage of the high voltage power unit 50 so that 
the surface potential of the photoreceptor 1 can be a predetermined value. 
FIG. 12 is a plan view showing the structure of the essential portion of 
the image forming apparatus of this embodiment. 
The optical scanning system 3 is operated as follows: semiconductor laser 
LD is oscillated according to the modulation signal obtained by 
pulse-width-modulating or intensity-modulating the image density data 
which has been read out from the page memory 200 (which is shown in FIG. 
13); the obtained laser beam is deflected by a polygonal mirror 33 which 
is rotated at a predetermined speed; and scanning is conducted through an 
f.theta. lens 34 and cylindrical lenses 32, 35 on the surface of the 
photoreceptor 1 which has been uniformly charged. Electric potential probe 
P is placed approximately in the middle portion on the photoreceptor 1 
corresponding to the image forming region. 
The optical scanning system 3 comprises: the semiconductor laser LD which 
is used as a coherent light source; a collimator lens 31 which is used as 
an optical focusing system; the polygonal mirror 33 and the f.theta. lens 
35 which are used as an optical deflection system; the cylindrical lenses 
32, 35 which are used as an optical tilt correction system, wherein the 
tilt is caused by the polygonal mirror 33; and a reflection mirror 36. 
The semiconductor laser LD is made from GaAlAs. Its maximum output is 5 mW, 
its optical efficiency 25%, and its divergent angle is 
8.degree.-16.degree. in the direction in parallel with the composition 
surface and 20.degree.-36.degree. in the direction perpendicular to the 
composition surface. Since color toner images are superimposed on the 
surface of the photoreceptor 1 in sequence, it is preferable to utilize 
the light, the wavelength of which is so long that the absorption by 
colored toner is small, for exposure. In this case, the wavelength of the 
beam is 800 nm. 
The collimator lens 31 is used so as to effectively adjust the beam 
diameter. Its numerical aperture N.A is 0.33 and the transmission factor 
is not less than 97%. The collimator lens 32 is used so as to improve 
spherical aberration and spot size condition. 
The optical deflection system is used so as to converge the beam (light 
flux) and used so as to reduce the Petzval's summation and astigmatism in 
order to make the scanning surface flat. 
The polygonal mirror 33 is provided with 8 polygonal surfaces, and when it 
is rotated at a revolution speed of 16535.4 rpm, the surface of the 
photoreceptor 1 can be scanned by the beam. It should be understood that 
not only a polygonal mirror but also other units can be used for the 
aforementioned purpose as far as they function in the same way as the 
polygonal mirror. 
The f.theta. lens 34 decreases the Petzval's summation and astigmatism so 
as to eliminate the curvature of field. In the manner described above, the 
scanning surface is made flat. 
As an optical correction system, the cylindrical lenses 32, 35 are provided 
before and after the polygonal mirror 33 in order to decrease the 
unevenness of scanning lines which is caused by the tilt of the polygonal 
mirror 33. In this way, the tilt angle of the polygonal mirror becomes 120 
sec P--P, and the correction coefficient of tilt angle becomes not less 
than 1/20. The cylindrical lens 35 is used to form an image on the surface 
of the photoreceptor 1 by the beam. The spread of the dot-shaped beam is 
20.5.+-.5 .mu.m in the primary scanning direction and 82.5.+-.12.5 .mu.m 
in the subsidiary scanning direction at the maximum exposure intensity of 
1/l.sup.2. On the other hand, it was possible to set the recording density 
of both primary and subsidiary scanning to 800 dpi. Pulse-width-modulation 
is utilized in the primary scanning. Namely, according to the present 
invention, it has become possible to conduct a recording of high density 
by setting an appropriate exposure on a high .gamma. photoreceptor. 
FIG. 13 is a block diagram showing the control circuit of the optical 
scanning circuit of this embodiment. 
The control circuit of the optical scanning system 30 is used for forming a 
dot-shaped electrostatic latent image by a dot-shaped light which has been 
obtained as follows: a comparison is made between the reference wave 
signal and the analog image density signal obtained by D/A-converting the 
digital image density signal sent from the page memory 200, and the signal 
is binarized or differentially amplified so that a modulated signal is 
obtained; and pulse-width-modulation or intensity-modulation is conducted 
in accordance with the obtained modulated signal so that a modulated 
dot-shaped light can be obtained. The optical scanning system 30 is 
provided with the aforementioned half decay exposure light amount 
detecting means and light emitting amount setting means, so that the 
electric current which flows in semiconductor laser LD is changed in 
accordance with the fluctuation of light sensitivity of the photoreceptor 
1, and the amount of emitted light is always controlled in the range of 
1.2 to 3.0 times of half decay exposure light amount P.sub.1/2. 
The control circuit of the optical scanning system comprises: electric 
potential probe P and an electrometer 550 which compose the half decay 
exposure light amount detecting means; an MPU 500 which composes the light 
emitting amount setting means; a page memory 200; a reading-out circuit 
250; a modulation circuit 300; an LD drive circuit 400; an MPU 500; a high 
voltage electric power unit 50; an index sensor 37; an index detection 
circuit (not illustrated in the drawing); and a polygonal driver (not 
illustrated in the drawing) which is used as the deflection system. 
The beam sent from the optical deflection system is incident upon the index 
sensor 37 through a reflection mirror 36. The index sensor 37 is induced 
by the beam and outputs an electric current. The aforementioned electric 
current is Current/Voltage-converted (A/V-converted) by the index 
detection circuit and the obtained signal is outputted as an index signal. 
The surface position of the polygonal mirror 36, which is rotated at a 
predetermined speed, is detected by this index signal, and optical 
scanning is conducted by the raster scanning system in accordance with the 
modulated digital image density signal which will be described later, 
wherein the period of scanning is the same as that of primary scanning. 
The scanning frequency is 2204.72 Hz, the effective printing width is not 
less than 297 mm, and the effective exposure width is not less than 306 
mm. 
The page memory 200 stores the digital image density signal (which is 
called the image density data) by the unit of one page, which has been 
obtained by correcting the image density signal sent from the computer or 
the scanner by the method of shading-correction and gradation correction. 
In this case, the unit of one page is defined as the amount of data 
corresponding to one image formed on the surface of the high .gamma. 
photoreceptor 1. In the case of this embodiment, the reference pattern 
data, the amount of which is one scanning line of data or several scanning 
lines of data, is attached to the head of the one page of data, wherein 
the reference pattern data is defined as the image density data by which 
light is continuously emitted from the semiconductor laser LD. 
The reading-out circuit 250 reads out the image density data by the unit of 
one scanning line according to the aforementioned index signal, wherein 
the reading-out operation is synchronized with the pixel clock. 
The modulation circuit 300 outputs the intensity-modulation signal obtained 
by differentially amplifying the binarized pulse-width-modulation signal 
or analog density signal which has been obtained as follows: a comparison 
is made between the reference signal and the analog density signal 
obtained by D/A-converting the image density data of 8-bit, for example, 
wherein the image density data is inputted synchronously with the pixel 
clock, and the obtained signal is binarized so that the 
pulse-width-modulation signal or analog density signal is obtained. 
The LD drive circuit 400 makes the semiconductor laser LD oscillate in 
accordance with the modulation signal sent from the modulation circuit 
300. The LD drive circuit 400 comprises: a photo-coupler type of 
semiconductor layer 410 composed of the semiconductor laser LD and 
photo-diode PD; an amplifying circuit 420 which amplifies the modulation 
signal at a predetermined amplification factor; a differential amplifier 
430; and a D/A-converting circuit 440. In the manner described above, a 
feedback loop is formed, so that the amplification factor by the 
amplifying circuit 420 is stabilized and the electric current which flows 
in the semiconductor laser LD can be controlled to be a predetermined 
value, and at the same time the target value of the electric current which 
flows the semiconductor laser LD can be determined in accordance with the 
output voltage of the D/A-converting circuit 440. 
In the case of the photo-coupler type of semiconductor laser 410, a 
photo-diode FD generates an electric current in accordance with the amount 
of light emitted by the semiconductor laser LD. Voltage according to the 
aforementioned electric current is generated by resistor R, and this 
voltage is inputted into the positive terminal of the differential 
amplifier 430. The electric potential difference between the voltage 
inputted into the positive terminal and the reference voltage inputted 
into the negative terminal of the differential amplifier 430 is fed back 
to the amplifying circuit 430, so that the amplification factor can be 
stabilized. The DC voltage impressed upon the negative terminal of the 
differential amplifier 430 corresponds to the target value of the electric 
current which flows in the semiconductor laser LD. This DC voltage is 
supplied from the D/A-converting circuit 440. In the manner described 
above, the LD drive circuit 400 feeds back the signal corresponding to the 
amount of light of the beam sent from the semiconductor LD so that the 
amount of light emitted by the semiconductor laser LD can become constant. 
MPU 500 composes: a half decay exposure light amount detection means which 
detects half decay exposure light amount P.sub.1/2 ; and an emitting light 
amount setting means which sets the amount of emitting light to 1.2-3.0 
times of half decay exposure amount P.sub.1/2. MPU 500 is also used as a 
charging voltage adjusting means which adjusts the output of the high 
voltage power unit 50 to predetermined voltage V0 according to the output 
signal of the electrometer 510. 
The output voltage of a variable DC power source 440 can be adjusted 
according to the control signal sent from MPU 500. In the manner described 
above, the electric current which flows the semiconductor laser LD is 
changed so that the amount of light emitted by the laser can be adjusted 
to 1.2-2.5 times of half decay exposure light amount P.sub.1/2. 
The operation is explained as follows which is conducted when the amount of 
light of the beam illuminated on the surface of the high .gamma. 
photoreceptor 1 is set, wherein the beam is sent from the optical scanning 
system 3. 
FIG. 14 is a flow chart which shows the operation of the control circuit of 
the optical scanning system of this embodiment. FIG. 15(a) and FIG. 15(b) 
are graphs which shows the relation between the amount of exposure light 
of the beam illuminated by the optical scanning system 3 and the electric 
potential on the surface of the high .gamma. photoreceptor, in which the 
intensity of laser beam is changed with respect to the exposed position on 
the surface of photoreceptor so that the exposure light amount is changed 
with respect to the exposed position on the surface of photoreceptor. 
Since the intensity of laser beam has a predetermined relation with the 
driving electric current of the LD driving circuit, the exposure amount 
for each of the exposed position on the surface of photoreceptor is 
detected by the measurement of the driving electric current for each of 
the exposed position. Therefore, when the surface potential is measured 
for each of the exposed position and a half decay position on which half 
decay took place is detected, a half decay light amount P.sub.1/2 is 
obtained by the driving electric current used for the half decay position. 
When the main switch has been turned on, the image forming apparatus 100 is 
initialized in such a manner that: the rotating high .gamma. photoreceptor 
is charged by the scorotron charger 2 and discharged by the discharger 11. 
While the aforementioned operation is conducted, MPU 500 detects charging 
potential V.sub.0 of the image forming region on the high .gamma. 
photoreceptor 1 through electric potential probe P and the electrometer 
550, and adjusts the output of the high voltage power unit 50 so that 
charging voltage V.sub.0 can be a predetermined value (S1). 
When the index signal is inputted into the reading-out circuit 250, the 
index circuit 250 reads out a reference pattern data from the page memory 
200 synchronously with the pixel clock and sends the data to the 
modulation circuit 300. The modulation circuit 300 sends the binarized 
pulse-width-modulation signal to the LD drive circuit 400, wherein the 
pulse-width-modualtion signal is obtained by comparing the reference 
signal and the analog reference pattern signal which has been obtained by 
D/A-converting the reference pattern data of 8 bits for example, which 
reference pattern data is inputted synchronously with the pixel clock. A 
black solid pattern composed of a predetermined region is used as this 
reference pattern. According to this analog reference pattern signal, the 
electric current which has been amplified by the amplifying circuit 420 at 
a predetermined amplification factor, continuously flows in the 
semiconductor laser LD. At this moment, MPU 500 changes stepwise the 
output voltage of the D/A-converting circuit 440 synchronously with the 
pixel clock. In the manner described above, the output current of the 
differential amplifier 430 can be changed stepwise, and the amount of 
light emitted by the semiconductor laser LD can be changed stepwise. The 
image region on the high .gamma. photoreceptor 1 is illuminated with a 
beam, the exposure amount of which is changed stepwise as illustrated in 
FIG. 15(a) . In the manner described above, a latent image pattern 
corresponding to the exposure intensity which changes as illustrated in 
FIG. 15(b), can be formed in the image forming region of the high .gamma. 
photoreceptor 1 (S2). 
MPU 500 measures the surface potential of the reference pattern through 
electric potential probe P and electrometer 510 (S3). MPU500 detects the 
position according to the outputted value where the surface potential is 
1/2.times.V.sub.0 and determines P.sub.1/2 (S4). MPU 500 sets a value of 
predetermined times of P.sub.1/2 as the output value of the D/A 
converting circuit 440 (S5). In the way described above, the output 
current from the amplifying circuit 420 can be determined. As explained 
above, in the present invention, the luminance of the beam illuminated on 
the high .gamma. photoreceptor 1 by the optical scanning system 3 can be 
set to a predetermined value with regard to half decay exposure amount 
P.sub.1/2, wherein the predetermined value is 1.2-2.5 times of P.sub.1/2. 
In the aforementioned case, detection was conducted in such a manner that 
the value of 1/2 V.sub.0 can be detected. However, the position where the 
absolute value of the differential value of the output may be determined 
to be P.sub.1/2. 
In this example, the operation to determine the amount of light is carried 
out before a series of image forming processes such as charging, exposure 
and development are conducted, and the aforementioned operation is carried 
out at each image forming process. 
The present invention is not limited to the aforementioned manner. The 
operation to determine the amount of light may be carried out at each time 
when a predetermined number of image formation have been performed. It may 
be carried out when it is predicted from a statistic viewpoint that the 
fluctuation of light sensitivity of the photoreceptor will occur. 
As described above, in this embodiment, an image of stable quality could 
been formed without being affected by the fluctuation of light sensitivity 
of the high .gamma. photoreceptor 1 caused by the change of environmental 
factors, by the image forming apparatus 100 in which image formation is 
conducted as follows. A latent image is formed by illuminating the high 
.gamma. photoreceptor 1 with a modulated beam sent from the optical 
scanning system 3, wherein the photoreceptor 1 has a light decay 
characteristic which is characterized in that: when the amount of light is 
small, the light decay of charging potential V.sub.0 is not so sensitive 
that the potential is hardly decayed; and when the amount of light exceeds 
the aforementioned small amount region, the charging potential is sharply 
decayed. After the latent image has been formed, reversal development is 
conducted. The aforementioned image forming apparatus 100 comprises: 
electric potential probe P and the electrometer 510 which are used as a 
half decay exposure light amount detecting means which detects half decay 
exposure light amount P.sub.1/2 by which surface potential V.sub.0 of the 
aforementioned photoreceptor 1 is decreased to 1/2; MPU 500 and the 
D/A-converting circuit 440 which are used as an emitting light amount 
setting means which sets the amount of light emitted by semiconductor 
laser LD to a predetermined value according to the result of detection 
conducted by the aforementioned electrometer 510. 
The aforementioned MPU 500 and D/A-converting circuit 440 which are used as 
an emitting light amount setting means, make the maximum light amount 
I.sub.0 and the half decay exposure amount P.sub.1/2 to satisfy the 
following inequality so that a latent image is formed on the 
aforementioned photoreceptor and the latent image is reversaly developed 
so that a stable image can be formed without being affected by the change 
of sensitivity of the photoreceptor caused by the fluctuation of 
environmental factors: 
EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2 
where I.sub.0 is the maximum light amount in the distribution of the amount 
of light of the beam illuminated on the high .gamma. photoreceptor 1 which 
has been uniformly charged, and P.sub.1/2 is the half decay exposure 
amount which reduces the electric potential of the aforementioned 
photoreceptor 1. It is shown that a high density recording can be carried 
out by the dot diameter formed by this exposure intensity using the same 
optical scanning system even in the case of conventional Se and OPC, when 
the exposure conditions are set to the aforementioned ones. 
In this embodiment, an electrometer is used to detect P.sub.1/2. However, 
the present invention is not limited to the electrometer, and it is 
possible to use a reference toner image. For example, in FIG. 11, a 
reference toner image is developed by the black toner developing unit 7a, 
and the output is detected by reflection density sensor S. Then, the 
position where the reflection density becomes a specific density or the 
position where the absolute value of differentiation becomes maximum, is 
defined as P.sub.1/2. 
The image forming apparatus of the present invention in which a latent 
image is formed by illuminating on the surface of a high .gamma. 
photoreceptor with a modulated beam sent from an optical scanning system 
and the formed latent image is reversaly developed, comprises: a half 
decay exposure light amount detecting means which detects half decay light 
amount P.sub.1/2 to reduce surface potential V.sub.0 of the 
aforementioned photoreceptor to 1/2; and an emitting light amount setting 
means which sets the amount of light of the laser beam emitted by a 
semiconductor, to a predetermined value according to the results of the 
detection conducted by the aforementioned half decay exposure amount 
detection means. Therefore, the present invention can provide an image 
forming apparatus which can form an image of stable quality without being 
affected by the change of light sensitivity of a high .gamma. 
photoreceptor which is caused by the fluctuation of environmental factors. 
The aforementioned emitting light amount setting means is characterized in 
that: the light amount is set to 1.2-3.0 times of half decay exposure 
light amount P.sub.1/2.