Electrophotographic image forming apparatus having a charge removing means

An image forming apparatus includes a rotatable photosensitive member including a conductive base and an organic photosensitive film located on a surface of the base and containing two charge generation materials which have different light absorbing characteristics from each other and different wavelengths showing a maximum light absorbance from each other; a charging device for charging the organic photosensitive film, the charging device being located in the vicinity of the photosensitive member; a charge removing member including a light source for radiating light toward the organic photosensitive film to uniformize the potential of a surface of the organic photosensitive film, the light having a wavelength in the range between wavelengths corresponding to half of a maximum light absorption of at least one of the two charge generation materials; an exposing device for radiating light corresponding to an image toward the organic photosensitive film charged by the charging device; and a developing device located downstream with respect to the exposing device in a rotation direction of the photosensitive member. The residual carriers in the organic photosensitive film are reduced, and thus the image quality is significantly improved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus using an 
electrophotographic technology, and in particular to an image forming 
apparatus which is provided with a light generation device such as a 
charge removing member for radiating light having a specific wavelength to 
an organic photosensitive film located on a surface of a photosensitive 
drum and forms an image by charging and exposing the photosensitive drum 
to light. 
2. Description of the Related Art 
Conventionally, image forming apparatuses using electrophotographic 
technologies have been actively developed for use as, for example, 
electrostatic copiers or printers. 
Briefly referring to FIG. 3, a conventional image forming apparatus 1 using 
an electrophotographic technology will be described. An image forming 
apparatus 1 includes a rotatable photosensitive drum 3 having a 
photosensitive film 2 located on a surface thereof, a main charger 4 for 
uniformly supplying the photosensitive film 2 with a prescribed level of 
electric charge, an optical device 5 for exposing the photosensitive film 
2 to light and forming an electrostatic latent image on the photosensitive 
film 2, a developing device 6 for developing the electrostatic latent 
image formed on the photosensitive film 2 into a toner image, a transfer 
device 8 for transferring the toner image on the photosensitive film 2 
onto a recording paper sheet 7, a cleaning device 9 provided with a 
cleaning blade for removing the residual toner on the photosensitive film 
2, and a charge removing device 10 for removing the residual charge on the 
photosensitive film 2 and thus setting the surface potential of the 
photosensitive film 2 at a prescribed uniform level. 
In the image forming apparatus 1 having the above-described structure, an 
image is formed in the following manner. 
First, the main charger 4 uniformly supplies the photosensitive film 2 with 
a prescribed level of electric charge. Next, light is radiated to the 
photosensitive film 2 by the optical device 5, thereby forming an 
electrostatic latent image on the photosensitive film 2. Toner is supplied 
to the photosensitive film 2 by the developing device 6, thereby 
developing the electrostatic latent image into a toner image. The toner 
image on the photosensitive film 2 is transferred to the recording paper 
sheet 7 by the transfer device 8. After the transference, the residual 
toner on the photosensitive film 2 is removed by the cleaning device 9. 
Light is radiated on the photosensitive film 2 by the charge removing 
device 10, thereby removing the residual charge on the photosensitive film 
2. Thus, the surface potential of the photosensitive film 2 is uniformly 
set at a prescribed level. Thereafter, the photosensitive film 2 is 
charged again by the main charger 4. Such a process is repeated in 
accordance with the rotation of the photosensitive drum 3. 
The photosensitive film 2 is formed of an inorganic or an organic material. 
Usable inorganic materials include, for example, Se-type materials and 
amorphous Si-type materials. 
Recently, more and more photosensitive films are formed of an organic 
material due to higher safety and easier processibility thereof. Organic 
photosensitive bodies formed of an organic photosensitive material are 
classified into multiple-layer organic photosensitive bodies and 
single-layer organic photosensitive bodies. 
A multiple-layer photosensitive body includes a charge generation layer and 
a charge carrying layer. The charge carrying layer contains a charge 
carrying material. The charge carrying material may be a hole carrying 
material or an electron carrying material. There are various hole carrying 
materials which have a satisfactory carrying ability, whereas no electron 
carrying material having a satisfactory carrying ability has been 
developed. Accordingly, multiple-layer organic photosensitive bodies are 
mostly of a type to be negatively charged. However, when such a type of 
photosensitive body is charged using a charger for performing corona 
discharge, a large amount of ozone is generated. In order to protect the 
human body and the environment, an additional measure to deal with ozone 
is needed. 
In an attempt to solve the above-described problem, single-layer organic 
photosensitive bodies have been developed. A single-layer organic 
photosensitive body includes a charge carrying medium containing a charge 
generation material diffused therein. By using a charge carrying material 
having a satisfactory hole carrying ability as the charge carrying medium, 
a single-layer photosensitive body of a type to be positively charged can 
be easily formed. 
Compared to a multiple-layer organic photosensitive body, a single-layer 
organic photosensitive body is easier to produce and thus is preferable. 
However, a single-layer photosensitive film formed of a single-layer 
organic photosensitive body retains a generally high level of charge even 
after charge removal. If the photosensitive film is positively charged in 
the state of having residual electrons therein, such residual electrons 
move to a surface of the photosensitive film due to the surface potential 
obtained by charging and thus reduces the surface potential. 
FIG. 4 is a graph illustrating the surface potential and the residual 
potential with respect to repeated rotation of a photosensitive drum 
formed of a conventional organic photosensitive material. In the case that 
an image is formed by the repeated rotation of the photosensitive drum, 
the level of surface potential reduces significantly and the level of 
residual potential increases significantly, causing non-uniformity in the 
image density. Further, residual photocarriers in the organic 
photosensitive film which causes such a high level of residual potential 
undesirably reduces the surface potential obtained by charging. Moreover, 
the organic photosensitive film can be initially charged sufficiently but 
wears out while being used repeatedly. Accordingly, the trap site is 
increased and thus the organic photosensitive film becomes more difficult 
to be charged. 
SUMMARY OF THE INVENTION 
An image forming apparatus according to the present invention includes a 
rotatable photosensitive member including a conductive base and an organic 
photosensitive film located on a surface of the base and containing two 
charge generation materials which have different light absorbing 
characteristics from each other and different wavelengths showing a 
maximum light absorbance from each other; a charging device for charging 
the organic photosensitive film, the charging device being located in the 
vicinity of the photosensitive member; an exposing device for radiating 
light toward the organic photosensitive film charged by the charging 
device; a developing device located downstream with respect to the 
exposing device in a rotation direction of the photosensitive member; and 
a charge removing member including a light source for radiating light 
toward the organic photosensitive film to uniformize the potential of a 
surface of the organic photosensitive film, the light having a wavelength 
in the range between wavelengths corresponding to half of a maximum light 
absorption of at least one of the two charge generation materials. 
In one embodiment of the invention, the image forming apparatus further 
includes a transfer device and a cleaning device. The transfer device is 
located downstream with respect to the exposing device in the rotation 
direction of the photosensitive member. The cleaning device is located 
downstream with respect to the transfer device in the rotation direction 
of the photosensitive member. The charge removing member is located in at 
least one position selected from the group consisting of between the 
cleaning device and the charging device, between the charging device and 
the developing device, between the developing device and the transfer 
device, and between the transfer device and the cleaning device. 
In one embodiment of the invention, the charge removing member is located 
upstream with respect to the charging device in the rotation direction of 
the photosensitive member. 
In one embodiment of the invention, the charge removing member includes a 
single light source which generates light having a wavelength in the range 
which is common to the range between wavelengths corresponding to half of 
the maximum light absorbance of one of the two charge generation materials 
and the range between wavelengths corresponding to half of the maximum 
light absorbance of the other charge generation material. 
In one embodiment of the invention, the charge removing member includes two 
light sources each generating light of a single color having a wavelength 
in the range between wavelengths corresponding to half of the maximum 
absorption of the respective charge generation material. 
In one embodiment of the invention, the photosensitive member has such a 
size that repeated rotation thereof allows transfer of an image on the 
organic photosensitive film onto a single recording medium, and the charge 
removing member radiates light having such an intensity that a residual 
potential on a surface of the photosensitive member left by charge removal 
after the first rotation of the photosensitive member is no greater than 
about 10% of the surface potential thereof obtained by charging and that 
an increase ratio of the residual potential after the final rotation of 
the photosensitive member is no greater than about 30%. 
In one embodiment of the invention, the charge removing member generates 
light having an intensity which is at least about 20 times the intensity 
for required halving of the surface potential obtained by charging. 
Thus, the invention described herein makes possible the advantages of 
providing an image forming apparatus which prevents generation of residual 
carriers in an organic photosensitive film and thus significantly improves 
the image quality. 
These and other advantages of the present invention will become apparent to 
those skilled in the art upon reading and understanding the following 
detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described by way of illustrative 
examples with reference to the accompanying drawings. The present 
invention is not limited to the following examples. 
According to the present invention, an image forming apparatus includes a 
rotatable photosensitive member including a conductive base and an organic 
photosensitive film located on a surface of the base and containing two 
charge generation materials which have different light absorbing 
characteristics from each other and different wavelengths showing a 
maximum light absorbance from each other. The image forming apparatus 
further includes a charge removing member for generating light having a 
wavelength in the range between wavelengths corresponding to half of the 
maximum light absorbance of at least one of the two charge generation 
materials contained in the organic photosensitive film. A surface 
potential of the organic photosensitive film is uniformized by light 
radiation from the charge removing member. Such radiated light is properly 
absorbed by the organic photosensitive film and thus is prevented from 
reaching a bottom part thereof. Accordingly, generation of carriers caused 
by the light from the charge removing member is prevented. 
For the above reason, the organic photosensitive film is more easily 
charged, and thus the image quality is improved. Furthermore, wearing of 
the organic photosensitive film by light is prevented. 
Since the light generated by the charge removing member is single-color 
light, heat generation by the light is restricted. Accordingly, the inner 
temperature, especially, the surface temperature of the organic 
photosensitive film can be restricted. Thus, characteristics of the 
organic photosensitive film are stabilized, and the aging characteristic 
against repeated use of the image forming apparatus is also stabilized. 
Referring to FIG. 1, an image forming apparatus 11 in one example according 
to the present invention will be described. FIG. 1 is a schematic view of 
the image forming apparatus 11. As is shown in FIG. 1, the image forming 
apparatus 11 includes a rotatable photosensitive drum 13 acting as a 
photosensitive member which includes a drum substrate 30 formed of metal, 
for example, aluminum and a single-layer organic photosensitive film 12 
(hereinafter, referred to simply as a "photosensitive film") located on a 
surface of the drum substrate 30. The photosensitive drum 13 is surrounded 
by a main charger 14 for uniformly supplying the photosensitive film 12 
with a prescribed level of charge, an optical device 15 for generating 
light for exposing the photosensitive film 12 to form an electrostatic 
latent image on the photosensitive film 12, a developing device 16 for 
developing the electrostatic latent image on the photosensitive film 12 
into a toner image, a transfer device 18 for transferring the toner image 
on the photosensitive film 12 onto, for example, a recording paper sheet 
17, a cleaning device 19 for removing the residual toner on the 
photosensitive film 12 after the transference, and a charge removing 
device 20 acting as a charge removing member for removing the residual 
charge on the photosensitive film 12 to uniformize the surface potential 
of the photosensitive film 12. 
The photosensitive film 12 contains two charge generation materials. The 
charge removing member such as the charge removing device 20 includes a 
light source for generating light having a wavelength in the range between 
wavelengths corresponding to half of the maximum light absorbance of at 
least one of the charge generation materials. 
First, the charge removing member which is one feature of the present 
invention will be described. 
Charge removing member 
One or more charge removing members may be provided in the image forming 
apparatus 11. The charge removing member is most preferably located 
upstream with respect to the main charger 14 in a rotation direction of 
the photosensitive drum 13 as the charge removing device 20. The charge 
removing member uniformizes the surface potential of the photosensitive 
film 12 before the photosensitive film 12 is charged. 
The charge removing member includes a removing lamp as a light source. 
Various types of light sources of visible light may be used. For example, 
a halogen lamp, a fluorescent lamp, a cold CRT, a neon lamp for emitting 
light of red, green or other colors, or a light source of single color 
light such as an LED (light emitting diode) for emitting light of red, 
yellow, green or other colors may be used. 
For the charge removing member, a light source which generates light having 
a wavelength in the range between wavelengths corresponding to half of the 
maximum light absorbance of at least one of the charge generation 
materials contained in the photosensitive film 12 is selected. The 
wavelength of the light generated by the charge removing member should be 
determined by the light absorbance characteristic of the photosensitive 
film 12. However, the light absorbance characteristic of the 
photosensitive film 12 generally depends on the charge generation 
materials contained therein. Accordingly, the wavelength of the light 
generated by the charge removing member can be determined by the light 
absorbance characteristic of the charge generation materials. 
Hereinafter, a method for selecting the charge removing member will be 
described. FIG. 2 is a graph illustrating the light absorbance 
characteristics L1 and L2 of the two charge generation materials contained 
in the photosensitive film 12. In the case where the wavelengths .lambda.1 
and .lambda.2 corresponding to the maximum light absorbances of the light 
absorbance characteristics represented by L1 and L2 are sufficiently close 
to each other, the charge removing member can be a single light source. In 
the case where the charge removing member is a single light source, the 
wavelength of the light generated by the charge removing member is 
preferably in the range between wavelengths corresponding to half of the 
maximum light absorbance of at least one of the two charge generation 
materials. More preferably, the peak wavelength of the light generated by 
the charge removing member is in the range between wavelengths 
corresponding to half of the maximum light absorbance of either of the two 
charge generation materials. Most preferably, the peak wavelength of the 
light generated by the charge removing member is in the range which is 
common to the range between wavelengths corresponding to half of the 
maximum light absorbance of one of the two charge generation materials and 
such a range of the other charge generation material. Light having such a 
peak wavelength is preferable in terms of charge removing efficiency and 
stability against repeated use. "Peak wavelength" is the wavelength 
corresponding to the maximum intensity in a wavelength spectrum of the 
light radiated by the charge removing member. The charge removing member 
preferably generates single-color light. In the case where one charge 
removing member includes a plurality of light sources, such light sources 
need to be selected in consideration of the above-described points. 
In the case where the wavelengths .lambda.1 and .lambda.2, corresponding to 
the maximum light absorbances of the light absorbance characteristics L1 
and L2, are far from each other, one charge removing member includes a 
plurality of light sources. Preferably, such a plurality of light sources 
generate single-color light of different colors, each having a wavelength 
in the range between wavelengths corresponding to half of the maximum 
light absorbance of the respective charge generation material. More 
preferably, the peak wavelength of single-color light generated by each 
light source is in the range between wavelengths corresponding to half of 
the maximum light absorbance of the respective charge generation material. 
The light generated by the charge removing member has a wavelength spectrum 
of a certain range. Among components of the light generated by the charge 
removing member, a component having a wavelength which is in the range 
between wavelengths corresponding to half of the intensity at the peak 
wavelength is preferably radiated in consideration of the intensity of 
light and the like. 
As is described above, one or more charge removing members may be provided 
in the image forming apparatus 11. Most preferably, the charge removing 
member is located upstream with respect to the main charger 14 in the 
rotation direction of the photosensitive drum 13 as the charge removing 
member 20. Another charge removing member may be a blank lamp 26 located 
between the main charger 14 and the developing device 16 for radiating 
light to a part of the photosensitive film 12 to perform masking, 
trimming, or other processing. Still another charge removing member may be 
a pre-transference removing device 27 located between the developing 
device 16 and the transfer device 18 for removing the charge on the 
photosensitive film 12 before transference. Still another charge removing 
member may be a pre-cleaning removing device 28 located between the 
transfer device 18 and the cleaning device 19 for removing the charge on 
the photosensitive film 12 before cleaning is performed by the cleaning 
device 19. 
The charge removing members 20, 26, 27 and 28 radiate single-color light or 
light from two single-color light sources to the photosensitive film 12. 
Since such radiated light is excellently absorbed into the photosensitive 
film 12 and thus is prevented from reaching a bottom part of the 
photosensitive film 12, generation of carriers at the bottom part of the 
photosensitive film 12 by the light radiated by any of the charge removing 
members is avoided. Accordingly, the photosensitive film 12 is more easily 
charged, thus significantly enhancing the image quality. Furthermore, 
wearing of the photosensitive film 12 caused by the light is restricted. 
Since the charge removing members generate single-color light, heat 
generation by the light is restricted, thus restricting an increase in the 
inner temperature of the image forming apparatus 11. 
Especially, an increase in the surface temperature of the photosensitive 
film 12 is restricted. Due to such restriction, the characteristics of the 
photosensitive film 12 are stabilized, and the aging characteristic of the 
image forming apparatus 11 after repeated use is stabilized. 
Before charging the photosensitive film 12, the charge on the 
photosensitive film 12 is removed to reduce the surface potential of the 
photosensitive film 12 to, for example, 100 V or less. In order to realize 
such a level of the surface potential, the charge removing member may 
preferably generate light of about 5 lux.multidot.sec or more, preferably 
about 10 lux.multidot.sec or more, depending on the type of the 
photosensitive film 12. The illuminance of the light used for charge 
removal by each charge removing member is preferably about 200 
lux.multidot.sec or less. If any charge removing member generates light of 
more than about 200 lux.multidot.sec., the image quality possibly 
deteriorates due to wearing of the photosensitive film 12. 
In the case where one charge removing member is located upstream with 
respect to the main charger 14 in the rotation direction of the 
photosensitive drum 13 as the charge removing device 20, the intensity of 
charge removing light is selected so that the residual potential left by 
charge removal after the first rotation of the photosensitive drum 13 is 
about 10% or less of the surface potential obtained by charging and that 
the increase ratio of the residual potential obtained by charge removal 
after the final rotation of the photosensitive drum 13 is about 30% or 
less. The increase ratio of the residual potential is represented with 
respect to the residual potential left by charge removal after the first 
rotation. If the residual potential after the first rotation is about 10% 
or more of the surface potential obtained by charging, the surface 
potential obtained after the second or later rotations is reduced, which 
is undesirable. If the increase ratio after the final rotation is more 
than about 30%, the surface potential for a single recording medium is 
reduced, thereby causing non-uniformity in the image density. The 
intensity of the light generated by the charge removing device 20 is 
preferably at least 20 times the intensity of light required for reducing 
the surface potential to half. If such intensity is less than 20 times, 
the intensity of the charge removing light is not sufficient to reduce the 
residual charge. 
Main Charger 
Returning to FIG. 1, the main charger 14 includes a discharge wire 21 for 
performing corona discharge, a shielding case 22 surrounding the discharge 
wire 21 and having an opening opposed to the photosensitive drum 13, and a 
metal grid 23 located at the opening of the shielding case 22. The 
discharge wire 21 is connected to a power source 25 for supplying the 
discharge wire 21 with a necessary amount of current for the corona 
discharge. The shielding case 22 is grounded. 
As the main charger 14, a scorotron charger is preferably used. In the case 
when a scorotron charger is used, the surface potential of the 
photosensitive drum 13 at a charging position reaches and is maintained at 
a prescribed maximum limit for the following reason. 
A current Icc from the power source 25 flowing to the discharge wire 21 is 
branched into a discharge current Isc flowing to the shielding case 22, a 
discharge current Igc flowing to the grid 23, and a discharge current Ipc 
flowing to the photosensitive drum 13. In order to allow the discharge 
current from the discharge wire 21 to reach the surface of the 
photosensitive film 12 through the grid 23, the surface potential of 
photosensitive film 12 needs to be lower than the potential of the grid 
23. 
When the discharge current Ipc is supplied to the charging position of the 
photosensitive film 12 by discharge performed by the discharge wire 21, 
the surface potential of the photosensitive film 12 gradually rises. When 
the surface potential of the photosensitive film 12 becomes substantially 
equal to the potential of the grid 23, no discharge occurs thereafter 
between the grid 23 and the photosensitive film 12. Accordingly, the 
current Icc supplied to the discharge wire 21 is branched only to the 
discharge currents Isc and Igc. Accordingly, the surface potential of the 
photosensitive film 12 is generally determined by the potential of the 
grid 23 and is maintained in the vicinity of the potential of the grid 23. 
Generally, it is preferable to charge the photosensitive film 12 by the 
main charger 14 so that the surface potential of the photosensitive film 
12 is in the range between about 500 V and about 1,000 V, preferably in 
the range between about 700 V and about 850 V and that 
.DELTA.SP/.DELTA.Icc.ltoreq.0.5 V/.mu.A. In order to perform such 
charging, it is preferable to apply a high voltage of about 4 to about 7 
kV to the discharge wire 21 of the main charger 14 when performing corona 
discharge. 
Optical device, developing device and transfer device 
In the image forming apparatus 11 according to the present invention, the 
optical device 15 is used for exposing the electrostatic latent image to 
form a toner image. An optical device 15 includes an optical system 
including a lens, a reflecting mirror and the like, and a laser 
oscillator, or the like may be used. 
The developing device 16 is provided with a developing roller 16a for 
supplying the surface of the photosensitive film 12 with a mono-component 
or a two-component toner which is charged. 
As the transfer device 18, a corona charger similar to the one used as the 
main charger 14 or a contact charger may be used. 
Organic photosensitive film 
In the image forming apparatus 11 according to the present invention, the 
photosensitive film 12 can be of various types. In a preferable 
embodiment, the photosensitive film 12 is a single-layer organic 
photosensitive film of a type to be positively charged, which is formed by 
diffusing a charge generation material in a charge carrying medium. 
Any charge generation material which is generally used by those of ordinary 
skill in the art may be used. An organic photoconductive pigment is 
especially preferable. Preferably, the charge generation material is a 
photoconductive organic pigment such as a phthalocyanine-type pigment, a 
perylene-type pigment, a quinacridone-type pigment, a pyranetron-type 
pigment, a bisazo-type pigment, or a trisazo-type pigment. In the image 
forming apparatus 11, two or more such photoconductive pigments are used 
in combination. 
The charge carrier medium may be formed by diffusing a charge carrying 
material in a bonding resin. 
As the charge carrying material, a hole carrying material or an electron 
carrying material which is generally used by those of ordinary skill in 
the art may be used. 
As the hole carrying material, a phenylenediamine-type compound, for 
example, N,N,N',N'-tetrakis(3-methylphenyl)-m-phenylenediamine, 
poly-N-vinylcarbazole, phenanthrene, N-ethylcarbazole, 
2,5-diphenyl-1,3,4-oxyadiazole, 
2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, 
bis-diethylaminophenyl-1,3,6-oxadiazole, 
4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane, 
2,4,5-triaminophenylimidazole, 
2,5-bis(4-diethylaminophenyl)-1,3,4-triazole, 
1-phenyl-3-(4-diethylaminostyril)-5-(4-diethylaminophenyl)-2-pyrazoline, 
or p-diethylaminobenzaldehyde(diphenylhydrazone) may be used. Such 
compounds may be used independently or in combination of two or more. 
As the electron carrying material, phenoquinone, for example, 
3,5,3',5'-tetraphenyldiphenoquinone, 2-nitro-9-fluorenone, 
2,7-dinitro-9-fluorenone, 2,4,7-trinitro-9-fluorenone, 
2,4,5,7-tetranitro-9-fluorenone, 2-nitrobenzothiophene, 
2,4,8-trinitrothioxantone, dinitroanthracene, dinitroacridine, or 
dinitroantoquinone may be used. Such materials may be used independently 
or in combination of two or more. 
As the bonding resin, for example, a styrene-type polymer, a 
styrene-butadiene copolymer, a styrene-acrylonitrile copolymer, a 
styrene-maleic acid copolymer, an acryl-type polymer, a styrene-acryl-type 
copolymer, a styrene-vinyl acetate copolymer, a poly(vinyl chloride), a 
vinyl chloride-vinyl acetate copolymer, polyester, an alkyd resin, 
polyamide, polyurethane, an epoxy resin, polycarbonate, polyallylate, 
polysulfone, a diallylphthalate resin, a silicone resin, a ketone resin, a 
polyvinylbutylale resin, a polyether resin, a phenol resin; a photocurable 
resin such as epoxy acrylate or urethane acrylate; or other copolymers may 
be used. A photoconductive polymer such as poly-N-vinylcarbazole may also 
be used. 
The amount of the charge generation material contained in the 
photosensitive film 12 is preferably about 0.1 to about 50 parts, more 
preferably about 0.5 to about 30 parts with respect to 100 parts of the 
bonding resin. The amount of the charge carrying material contained in the 
photosensitive film 12 is preferably about 20 to about 500 parts, more 
preferably about 30 to about 200 parts with respect to 100 parts of the 
bonding resin. The photosensitive film 12 preferably has a thickness of 
about 10 to about 40 .mu.m, more preferably about 22 to about 32 .mu.m in 
order to obtain a sufficiently high surface potential, a high durability 
against repeated image forming, and high sensitivity. 
The drum substrate 30 may be formed of any conductive material, preferably 
conductive metal. In general, the drum substrate 30 is formed of a plain 
aluminum tube or an aluminum tube with an alumetized surface. The drum 
substrate 30 may also be formed of a conductive resin, a conductive film 
or the like. The substrate may be provided in the form of a sheet, a belt, 
a drum or the like. 
The photosensitive film 12 is formed in the following manner. 
The bonding resin is dissolved in a solvent, and the charge generation 
material and, if necessary, the charge carrying material are diffused in 
the dissolved bonding resin to prepare a composition. The composition is 
applied to the surface of the drum substrate 30 and dried. As the solvent, 
for example, an amide-type solvent such as N,N-dimethylformamide or 
N-N-dimethylacetoamide; a cyclic ether such as tetrahydrofuran or dioxan; 
dimethylsulfoxide; an aromatic solvent such as benzene, toluene, or 
xylene; ketone such as methylethylketone; N-methyl-2-pyrrolidone; or 
phenol such as phenol or cresol may be used. 
The present invention has a remarkable advantage when a single-layer 
organic photosensitive body of a type to be positively charged is used. 
Such a type of photosensitive body is advantageous in generating very 
little ozone when charged. In the case that such a type of photosensitive 
body is used, a perylene-type pigment, an azo-type pigment or a 
combination of the two is preferably used as the charge generation 
material. As the charge carrying material, a diphenoquinone derivative 
such as 2,6-dimethyl-2',6-ditert-butyl-diphenoquinone, a diamine-type 
compound such as 
3,3'-dimethyl-N,N,N',N'-tetrakis-4-methylphenyl(1,1'-biphenyl)-4,4'-diamin 
e, a fluorene-type compound, or a hydrazone-type compound is preferably 
used. 
In this example, an electrostatic copier is used as the image forming 
apparatus. Needless to say, the present invention is applicable to any 
image forming apparatus for forming an image using an electrophotographic 
technology. 
EXAMPLE 
Preparation of the photosensitive film 
Materials having the following compositions were mixed by a ball mill and 
diffused for 2 hours to prepare a photosensitive liquid used for a 
single-layer organic photosensitive film 12. 
__________________________________________________________________________ 
Bisazo pigment represented by formula I (maximum light absorbance at 550 
nm; range between wavelengths 5 parts 
corresponding to half of the maximum light absorbance: 490 to 585 nm) 
Perylene pigment represented by formula II (maximum light absorbance at 
550 nm; range between wavelengths 5 parts 
corresponding to half of the maximum light absorbance: 450 to 570 nm) 
3,3'-dimethyl-N,N,N',N'-tetrakis-4-methylphenyl(1,1'-biphenyl)-4,4'-diamin 
e (hole carrying material) 100 
parts 
3,3'-dimethyl-5,5'-ditert-butyl-4,4'-diphenoquinone 50 parts 
Polycarbonate resin 150 
parts 
Dichloromethane 800 
parts 
__________________________________________________________________________ 
Formula I 
##STR1## 
Formula II 
##STR2## 
An aluminum cylinder having an outer diameter of 30 mm was immersed in 
the photosensitive liquid and dried at a temperature of 110.degree. C. 
for 30 minutes, thereby forming an organic photosensitive film having a 
thickness of 30 .mu.m on an outer surface of the aluminum cylinder. In 
this manner, the single-layer electrophotographic photosensitive drum 13 
of a type to be positively charged was obtained. Evaluation of the image 
For the following experiments, an apparatus improved from DC-2556 produced 
by Mira Industrial Co., Ltd. equipped with the image forming apparatus 11 
shown in FIG. 3 was used. The improved apparatus was obtained by replacing 
the developing section of DC-2556 with a surface potential sensor. An 
aging test was performed using the following charge removing members, 
namely, charge removing lamps. In the aging test, each section of the 
image forming apparatus 11 was operated without performing developing or 
using any recording medium. The circumferential speed of the 
photosensitive drum 13 was set at 300 mm/sec., and the initial surface 
potential of an area corresponding to the developing section was 800 V. 
The intensity of light generated by the charge removing lamp was selected 
so that the surface potential after charge removal would be 50 V. 
Example 1 
A single charge removing lamp for generating light having a peak wavelength 
in the range which is common to the range between wavelengths 
corresponding to half of the maximum light absorbance of one of the two 
charge generation materials and such a range of the other charge 
generation material was used. Specifically, a green LED for emitting light 
having a wavelength of 565 nm (the range between wavelengths corresponding 
to the intensity at the peak wavelength: 555 to 580 nm) was used. 
Example 2 
A charge removing lamp for generating light having a peak wavelength 
exceeding the range between wavelengths corresponding to half of the 
maximum light absorbance of one of the two charge generation materials and 
a charge removing lamp for emitting light having a peak wavelength within 
the range between wavelengths corresponding to half of the maximum light 
absorbance of the other charge generation materials were used in 
combination. Specifically, a yellow LED having a wavelength of 580 nm (the 
range between wavelengths corresponding to the intensity at the peak 
wavelength: 520 to 600 nm) was used as the former, and a green LED for 
emitting light having a wavelength of 565 nm (the range between 
wavelengths corresponding to the intensity at the peak wavelength: 555 to 
580 nm) was used as the latter. 
Example 3 
A single charge removing lamp for generating light having a peak wavelength 
exceeding both the ranges between wavelengths corresponding to half of the 
maximum light absorbance of the two charge generation materials. 
Specifically, a yellow LED for emitting light having a wavelength of 580 nm 
(the range between wavelengths corresponding to the intensity at the peak 
wavelength: 520 to 600 nm) was used. 
Comparative example 
In the comparative example, the range between wavelengths corresponding to 
half of the intensity at the peak wavelength of the light generated by a 
single charge removing lamp does not overlap the range between wavelengths 
corresponding to half of the maximum light absorbance of either of the two 
charge generation materials. Specifically, a red LED for emitting light 
having a wavelength of 660 nm (the range between wavelengths corresponding 
to the intensity at the peak wavelength: 628 to 675 nm) was used. 
Results 
The dark potential (the surface potential of the photosensitive film 12 
obtained when the photosensitive film 12 is not exposed to light; SP0) and 
the bright potential (the surface potential obtained by exposing the 
photosensitive film 12 at the prescribed illuminance of 3.5 
lux.multidot.sec; V3.5) which were obtained after the first, 1,000th, and 
3,000th rotation of the photosensitive drum 13 were measured and are shown 
in Table 1. The bright potential after the 1,000th and 3,000th rotation 
were obtained by the same intensity of light as the bright potential after 
the first rotation. 
TABLE 1 
______________________________________ 
1st 1,000th 3,000th 
SPO V3.5 SPO V3.5 SPO V3.5 
______________________________________ 
Ex 1 800 250 800 245 795 250 
Ex 2 800 250 790 250 785 255 
Ex 3 800 250 785 255 780 255 
Comparative 
800 250 750 230 700 200 
example 
______________________________________ 
It is appreciated from Table 1 that the aging characteristic against 
repeated use is most stable in Example 1 using a charge removing lamp for 
emitting light having a peak wavelength in the range which is common to 
the range between wavelengths corresponding to half of the maximum light 
absorbance of one of the two charge generation materials (a perylene 
pigment and a bisazo pigment) and such a range of the other charge 
generation material. 
In Example 2, due to the charge removing lamp for generating light having a 
peak wavelength in the range between wavelengths corresponding to half of 
the maximum light absorbance of the one of the two charge generation 
materials, the aging characteristic is stable against repeated use. 
Compared with Example 1, however, the dark potential is slightly lower and 
the bright potential is slightly higher. 
As is appreciated from the results in Example 3, a charge removing effect 
can still be achieved if the range between wavelengths corresponding to 
half of the intensity at the peak wavelength of the light generated by the 
single light source overlaps the range between wavelengths corresponding 
to half of the maximum light absorbance of one of the two charge 
generation materials even though the peak wavelength of such light exceeds 
the above-described two ranges. 
As is appreciated from the results in the comparative example, in the case 
where the range between wavelengths corresponding to half of the intensity 
at the peak wavelength of the light generated by the single charge 
removing lamp does not overlap the range between wavelengths corresponding 
to half of the maximum light absorbance of either of the two charge 
generation materials, the aging characteristic is not sufficiently stable. 
As is understood from the above description, it is effective to select a 
charge removing lamp for generating light having a wavelength in the range 
between wavelengths corresponding to half of at least one of the two 
charge generation materials. 
According to the present invention, the charge removing member radiates 
light having a prescribed wavelength toward an organic photosensitive film 
of a photosensitive member. The light thus radiated is excellently 
absorbed by the photosensitive film and prevented from reaching the deep 
part of the photosensitive film. Thus, generation of carriers in the deep 
part of the photosensitive film by the light generated by the charge 
removing member is prevented. Therefore, the amount of residual carriers 
which exist on the photosensitive film when the photosensitive film is 
charged by the main charger is reduced, and thus the photosensitive film 
is more easily charged. As a result, the image quality is significantly 
improved. Furthermore, wearing of the photosensitive film by light is 
alleviated. Since the light generated by the charge removing member is 
preferably a single-color light, heat generation by the light is 
restricted. Accordingly, the inner temperature of the image forming 
apparatus, especially, the surface potential of the photosensitive film is 
restricted. Due to such restriction of heat, the characteristics of the 
photosensitive film are stabilized. The aging characteristic against 
repeated use of the image forming apparatus is also stabilized. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.