EP1170639A1

EP1170639A1 - Blank exposure apparatus and image forming device

Info

Publication number: EP1170639A1
Application number: EP00909643A
Authority: EP
Inventors: Hiroya Copyer Co. Ltd. HIROSE
Original assignee: Copyer Co Ltd; Canon Finetech Inc
Current assignee: Canon Finetech Nisca Inc
Priority date: 1999-03-15
Filing date: 2000-03-15
Publication date: 2002-01-09
Also published as: JP3374906B2; WO2000055693A1; JP2000267403A; EP1170639A4

Abstract

The invention provides a blank exposing device and an image forming apparatus which can prevent occurrence of a black stripe at the front end portion of a recording paper. The blank exposing device, which exposes the surface of a photosensitive object to light and thereby removes electric charge on the surface of it and forms a non-image area on the photosensitive object, comprises a blank exposing lamp formed by arranging a number of light emitting diodes 31, a constant-voltage power supply circuit 35 for applying a constant voltage to each light emitting diode 31, and a switching element 36 for performing an on-off operation of a voltage to be applied to each light emitting diode 31. A control portion 110 controls the switching element 36 to generate a pulse voltage of one or plural pulses and apply the pulse voltage to the blank exposing lamp when changing over the blank exposing lamp from the on state to the off state. Due to this, it is possible to gradually change the quantity of light of the blank exposing lamp when changing over it from the on state to the off state.

Description

TECHNICAL FIELD

The present invention relates to a blank exposing device for removing unnecessary electric charge on a photosensitive object, to be used in such an image forming apparatus as a copying machine, a printer, a facsimile or the like for forming image information on a recording medium by means of an electrophotographic method.

BACKGROUND ART

An image forming apparatus of an electrophotographic method for forming an image on a recording medium using a developer (toner) is widely used as an output apparatus of a computer or a workstation. Such an image forming apparatus makes the surface of a photosensitive object charged uniformly with electricity prior to exposing the photosensitive object to an image light. Hereupon, the photosensitive object turns at a fixed speed. At this time, if the size of an image area on the photosensitive object is smaller than the size of a charged area of the photosensitive object, electric charge remains on a non-image area on the photosensitive object. And this remaining electric charge has toner adhered to it and brings a result that a more quantity of toner than a necessary quantity is consumed. And in order to prevent a recording medium having a toner image transferred onto it from winding round a fixing roller, it is necessary to form a blank space on the front end portion of a recording paper. Due to these, an image forming apparatus is ordinarily provided with a blank exposing device on the periphery of its photosensitive object to remove unnecessary electric charge on the photosensitive object.
This blank exposing device is composed of a light emitting diode array (blank exposing lamp) in which a number of light emitting diodes are arranged. By turning on predetermined light emitting diodes before developing an electrostatic latent image formed on the photosensitive object, it is possible to remove electric charge on an area (non-image area) on the photosensitive object irradiated with light from the said light emitting diodes and thereby prevent unnecessary toner from adhering to the non-image area. Particularly, by using such a blank exposing device, it is possible to perform what is called a frame erasing operation or a trimming operation which adheres toner only to a selected portion of an original but does not adhere toner to the peripheral portion of it, a composite duplication to combine selected portions of plural originals and the like.
By the way, a conventional image forming apparatus has a problem that a black stripe appears on the front end portion of a recording paper (or the front blank space of a recording paper). Figure 10 is a diagram for explaining this black stripe and shows a timing chart when forming an image on a sheet of recording paper. The same figure (a) shows change in electric potential of the surface of a photosensitive object, the same figure (b) shows change in voltage applied to a blank exposing lamp, and the same figure (c) shows change in quantity of light of the blank exposing lamp. And the same figure (d) shows a real image on the recording paper. Here, the respective timing charts of Figures 10(a), 10(b) and 10(c) are drawn correspondingly to the positions on the recording paper of Figure 10(d). The example of Figure 10 forms a solid white image, a solid black image, a halftone image, a solid white image, a halftone image, a solid white image, a solid black image and a solid white image in order from the front end side of the recording paper.
The photosensitive object turns at a fixed speed. The blank exposing lamp is turned on to a non-image area on the photosensitive object and is turned off to an image area, as shown in Figures 10(b) and 10(c). After this, the image area is irradiated with a light carrying image information and thereby the distribution of electric potential as shown in Figure 10(a), namely, an electrostatic latent image is formed. Next, this electrostatic latent image is developed and a toner image is fixed on a recording paper and thereby a recording paper shown in Figure 10(d) is obtained. Seeing this recording paper, it is found that a black stripe appears on the boundary between a blank space of the front end side of the recording paper and a solid white image adjacent to it.

DISCLOSURE OF THE INVENTION

The present invention has been performed under such a technical background, and aims at providing a blank exposing device and an image forming apparatus being capable of preventing a black stripe from appearing on the front end portion of a recording paper.
In order to attain the above-mentioned object, a blank exposing device according to the present invention is characterized by comprising a blank exposing lamp for removing electric charge on the surface of a photosensitive object and forming a non-image area on said photosensitive object by exposing said surface of the photosensitive object to light, a light quantity adjusting means for adjusting the quantity of light of said blank exposing lamp, and a control means for controlling said light quantity adjusting means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
And in order to attain the above-mentioned object, a blank exposing device according to the present invention is characterized by comprising a blank exposing lamp for removing electric charge on the surface of a photosensitive object and forming a non-image area on said photosensitive object by exposing said surface of the photosensitive object to light, a constant-voltage power supply circuit for applying a constant voltage to said blank exposing lamp, a switching means for performing an on-off operation of a voltage to be applied to said blank exposing lamp, and a control means for controlling said switching means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
In order to attain the above-mentioned object, the present invention is characterized by an image forming apparatus for forming image information on a recording medium by means of an electrophotographic method, said image forming apparatus comprising a developing device for developing an electrostatic latent image formed on a photosensitive object on the basis of said image information with a mono-component developer, a blank exposing device which has a blank exposing lamp for removing electric charge on the surface of the photosensitive object and forming a non-image area on said photosensitive object by exposing the surface of said photosensitive object to light before performing the development by means of said developing device and a light quantity adjusting means for adjusting the quantity of light of said blank exposing lamp, and a control means for controlling said light quantity adjusting means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
And in order to attain the above-mentioned object, the present invention is characterized by an image forming apparatus for forming image information on a recording medium by means of an electrophotographic method, said image forming apparatus comprising a developing device for developing an electrostatic latent image formed on a photosensitive object on the basis of said image. information with a mono-component developer, a blank exposing device which has a blank exposing lamp for removing electric charge on the surface of the photosensitive object and forming a non-image area on said photosensitive object by exposing the surface of said photosensitive object to light before performing the development by means of said developing device, has a constant-voltage power supply circuit for applying a constant voltage to said blank exposing lamp and has a switching means for performing an on-off operation of a voltage to be applied to said blank exposing lamp, and a control means for controlling said switching means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic composition diagram of a main part of an image forming apparatus being an embodiment of the present invention.
Figure 2 is a schematic composition diagram of a main charger and a blank exposing device of the image forming apparatus shown in Figure 1.
Figure 3 is a schematic perspective view of the main charger of the image forming apparatus shown in Figure 1.
Figure 4 is a schematic perspective view of the blank exposing device of the image forming apparatus shown in Figure 1.
Figure 5 is a schematic circuit diagram for explaining control of turning on/off light emitting diodes in the blank exposing device shown in Figure 4.
Figure 6 is a diagram showing the development characteristics of an image forming apparatus of this embodiment.
Figure 7(a) is a diagram for explaining change in electric potential of an area on a photosensitive object corresponding to the front end portion of a recording paper, and Figure 7(b) is a diagram for explaining change in electric potential of an area on the photosensitive object corresponding to the rear end portion of the recording paper.
Figure 8 is a diagram for explaining control of turning on/off a blank exposing lamp in an area on the photosensitive object corresponding to the front end portion of a recording paper in an image forming apparatus of this embodiment.
Figure 9 is a diagram for explaining other examples of a pulse voltage to be applied to a blank exposing lamp.
Figure 10 is a diagram for explaining a black stripe appearing on the front end portion of a recording paper.

BEST MODE FOR CARRYING OUT THE INVENTION

A best mode for carrying out the invention related to the present application is described with reference to the drawings in the following. Figure 1 is a schematic composition diagram of a main part of an image forming apparatus being an embodiment of the present invention, Figure 2 is a schematic composition diagram of a main charger and a blank exposing device of the image forming apparatus, Figure 3 is a schematic perspective view of the main charger of the image forming apparatus, Figure 4 is a schematic perspective view of the blank exposing device of the image forming apparatus, and Figure 5 is a schematic circuit diagram for explaining control of turning on/off light emitting diodes in the blank exposing device.
As shown in Figures 1 and 2, an image forming apparatus of this embodiment is provided with a photosensitive object 10, a main charger 20, a blank exposing device 30, a developing device 40, a transfer charger 50, a separation charger 60, a fixing device 70, a cleaning device 80, a pre-exposure device 90 and a control portion (see Figure 5).
The photosensitive object 10 turns at a predetermined circumferential speed (process speed) in the clockwise direction as shown by an arrow A. The main charger 20, which charges uniformly the surface of the photosensitive object 10 with electricity by means of corona discharge, comprises a wire 21 which is made from tungsten in general, a shield 22, a grid 23 and a power supply 24, as shown in Figures 2 and 3. A high voltage is applied to the wire 21 by the power supply 24. The grid 23 has a number of openings 23a formed by applying an etching process to a plate member of about 0.1 mm in thickness, as shown in Figure 3. By this, it is possible to make the photosensitive object 10 uniformly charged with electricity. And the blank exposing device 30 removes electric charge on the surface of the photosensitive object 10 corresponding to a non-image area and prevents unnecessary developer (toner) from adhering to the photosensitive object 10.
An original lighting lamp not illustrated irradiates an original with a scanning light and reads image information of the original. And an irradiation light L carrying the image information is led through an unshown optical system to the photosensitive object 10, and thereby an electrostatic latent image is formed on the photosensitive object 10.
The developing device 40 supplies toner onto the photosensitive object 10 by rotation of a sleeve 41 in the direction of an arrow B. Due to this, an electrostatic latent image formed on the photosensitive object 10 is visualized. Hereupon, this embodiment uses a mono-component toner as its toner. On the one hand, a transfer paper being a recording medium is fed by a pair of resist rollers 121 and 122 along the direction of an arrow C in such timing that the front end of an image on the photosensitive object 10 and the front end of the transfer paper coincide with each other. And the toner on the surface of the photosensitive object 10 is transferred onto the transfer paper by the transfer charger 50.
The separation charger 60 removes electric charge from the reverse face of the transfer paper having a toner image transferred onto it, and separates the transfer paper static-electrically from the photosensitive object 10. The static-electrically separated transfer paper is carried in the direction of an arrow D and is fed to the fixing device 70. The fixing device 70 fixes the toner image on the transfer paper by means of the heat and pressure of a pair of fixing rollers 71 and 72. Due to this, the toner image is semipermanently recorded. After this, the transfer paper is discharged from the image forming apparatus main body to the outside.
On the other hand, untransferred toner slightly remaining on the photosensitive object 10 is recovered from the photosensitive object 10 by the cleaning device 80.
And the residual electric potential being traces of an electrostatic latent image on the photosensitive object 10 is eliminated by the pre-exposure device 90. And the control portion 110 supervises control of each portion of the image forming apparatus.
Next, the blank exposing device 30 is described in detail. The blank exposing device 30 performs the following two main roles. That is to say, these are a role of removing electric charge on a non-image area on the photosensitive object 10 corresponding to blank spaces at the front end and the rear end of a recording paper by irradiating the whole area in the axial direction of the photosensitive object 10, and a role of removing electric charge on a non-image area on the photosensitive object 10 corresponding to, for example, the peripheral area of the recording paper by irradiating a partial area in the axial direction of the photosensitive object 10. As shown in Figures 2, 4 and 5, such a blank exposing device 30 has a light emitting diode array (blank exposing lamp) formed by arranging a number of light emitting diodes 31, a substrate 32 mounted with the light emitting diode array, a housing 33, a partition wall 34, a constant-voltage power supply circuit 35 for applying a constant voltage to the respective light emitting diodes 31 and a switching element 36 of a transistor or the like for performing an on-off operation of a voltage to be applied to the respective light emitting diodes 31.
The housing 33 is provided with a partition wall 34 for regulating the width of exposure in order to make it possible to perform exposure so as to clearly define the boundary between an image area and a non-image area, namely, between a charged area and a non-charged area along the axial direction of the photosensitive object 10. This partition wall 34 has a number of openings 34a formed in it. The formation of the openings 34a regulates the width of exposure of the individual light emitting diodes 31 onto the surface of the photosensitive object 10. By turning on predetermined light emitting diodes 31, electric charge on the surface of the photosensitive object 10 within the width of exposure of them is removed. The timing of turning on/off each light emitting diode 31 is controlled by the control portion 110.
And the control portion 110 controls the switching element 36 so that the quantity of light of each light emitting diode 31 gradually changes when changing over each light emitting diode 31 from the on state to the off state. For example, the control portion 110 sends an on/off signal to the switching element 36, changes a voltage outputted from the constant-voltage power supply circuit 35 into a pulse voltage and applies it to the light emitting diodes 31. The control of turning on/off the blank exposing lamp by means of such a pulse voltage is described in detail later.
Next, the development characteristics of an image forming apparatus of this embodiment is described. Figure 6 shows the development characteristics of the image forming apparatus. Here, the ordinate represents the density of an image and the abscissa represents the electric potential of the surface of a photosensitive object 10 (hereinafter, referred to as a photosensitive object potential also). And the density of an image is represented by a Macbeth reflection density.
In the image forming apparatus of this embodiment, a development method using a mono-component toner is adopted and a photosensitive object potential when an ordinary image is formed is between 150 V and 700 V. For example, in case of forming such an image of 1.2 or more in image density as a solid black image, the photosensitive object potential is set so as to be about 700 V. And in case of forming such a halftone image as an image of about 0.4 in image density, the photosensitive object potential is set so as to be about 400 V, and in case of forming a solid white image being a white ground portion, the photosensitive object potential is set so as to be about 150 V.
Generally, in an image forming apparatus using an electrophotographic method, a phenomenon that toner is slightly put on a white ground portion of a recording paper, namely, what is called a fogging phenomenon occurs. This is a phenomenon caused by a fact that a photosensitive object potential is within the range of regular fog from 150 V to 300 V. On the other hand, the photosensitive object potential being lower than 150 V comes into the range of reversal fog. At this time, the toner of reverse polarity existing in the developing device 40, namely, what is called the reversal toner is developed and the toner results in being adhered to an area on the photosensitive object 10 to be a non-image area in image information.
By the way, the inventors have found that a black stripe appearing on the front end portion of a recording paper (or the front blank space portion of a recording paper) is caused by a sharp change in photosensitive object potential which occurs when the blank exposing device 30 removes electric charge on the surface of the photosensitive object 10. This fact is described in detail. Figure 7(a) is a diagram for explaining change in electric potential of an area on a photosensitive object 10 corresponding to the front end portion of a recording paper, and Figure 7(b) is a diagram for explaining change in electric potential of an area on the photosensitive object 10 corresponding to the rear end portion of a recording paper. Here, in Figures 7(a) and 7(b), the ordinate represents electric potential of the surface of the photosensitive object 10 (a photosensitive object potential) and the abscissa represents time (or position along the direction of rotation of the photosensitive object 10). And the photosensitive object potential is an electric potential obtained after exposure by an irradiation light L.
The blank exposing lamp is changed over from the on state to the off state on an area on the photosensitive object 10 corresponding to the front end portion of a recording paper. A portion irradiated with light from the blank exposing lamp becomes a blank space portion, and a portion not irradiated becomes an image area. At this time, the photosensitive object potential is sharply changed at the boundary between the blank space portion and the image area adjacent to it, and at this time a transient falling and rising phenomenon occurs and potential edges E₁ and E₂ are formed. And when the photosensitive object 10 further turns, its image area is irradiated with an irradiation light L carrying image information. For example, it is assumed that this irradiation light L contains image information being a solid white image.
In this case, as shown in Figure 7(a), the photosensitive object potential V_SL of a blank space portion is about 50 V and the photosensitive object potential V_L of a solid white image area is about 150 V. And the photosensitive object potential at the blank space side is decreased by the potential edge E₁ from V_SL (about 50 V) to about 0 V near the boundary between the blank space portion and the solid white image area, and on the other hand the photosensitive object potential at the solid white image area side is increased by the potential edge E₂ from V_L (about 150 V) to about 200 V. As seen from Figure 6, therefore, a portion in which the potential edge E₁ is formed has an electric potential at which a reversal fogging phenomenon appears conspicuously. That is to say, it has been found that a black stripe appears at the front end portion of a recording paper due to this reversal fogging phenomenon. On the other hand, as seen from Figure 6, the potential of a portion in which the potential edge E₂ is formed is in the range of regular fog, and it is thought that such a potential of this degree has little influence upon the image density. That is to say, the potential edge E₂ little contributes to occurrence of a black stripe at the front end portion of a recording paper.
And the reason why no black stripe appears at the rear end portion of a recording paper (or the rear blank space portion of a recording paper) is as follows. First, a change in quantity of light of the blank exposing lamp participates in this. That is to say, as shown in Figure 10(c), when the blank exposing lamp is changed over from the on state to the off state the quantity of light of the blank exposing lamp sharply falls, but when it is changed over from the off state to the on state a certain time is required until a necessary quantity of light is reached and therefore the quantity of light of the blank exposing lamp rises gradually. That is to say, the blank exposing lamp has a predetermined delay time when it is turned on. Further, as shown in Figure 2, according to the positional relation between the main charger 20 and the blank exposing device 30, electric charge on the photosensitive object 10 is removed by the blank exposing device 30 from immediately after the photosensitive object 10 has passed the charging area X of the main charger 20. When the blank exposing lamp is changed over from the on state to the off state, since the photosensitive object 10 has already obtained a sufficient quantity of light from the blank exposing lamp in the exposure area Y of the blank exposing device 30, the electric potential of the surface of the photosensitive object 10 is sharply changed at the boundary between the areas X and Y. On the other hand, when the blank exposing lamp is changed over from the off state to the on state, the photosensitive object 10 cannot obtain a sufficient quantity of light from the blank exposing lamp at the downstream side of the area Y, and a portion in which electric charge is not completely removed appears on the photosensitive object 10. Moreover, at this time, there is also the influence of a delay time required for turning on the blank exposing lamp, as mentioned above. Due to this, as shown in Figure 7(b), the electric potential of an area on the photosensitive object 10 corresponding to the rear end portion of a recording paper changes gradually. In short, it is thought that since no potential edge is formed, no black stripe appears at the rear end portion of a recording paper.
Next, the control of turning on/off the blank exposing lamp in an area on the photosensitive object 10 corresponding to the front end portion of a recording paper in an image forming apparatus of this embodiment is described. Figure 8(a) is a diagram showing an ideal change in photosensitive object potential, Figure 8(b) is a diagram showing an ideal change in quantity of light of the blank exposing lamp, Figure 8(c) is a diagram for explaining a pulse voltage to be applied to the blank exposing lamp, and Figure 8(d) is a diagram showing a concrete example of a pulse voltage to be applied to the blank exposing lamp.
As easily understood from the above description, in order to suppress occurrence of a black stripe at the front end portion of a recording paper (or the front blank space portion of a recording paper), it is enough to gradually change the quantity of light of the blank exposing lamp as shown in Figure 8(b) when the blank exposing lamp is changed over from the on state to the off state. In other words, as shown in Figure 8(a), it is enough to suppress formation of a potential edge by gradually changing the photosensitive object potential.
By the way, ordinarily the blank exposing device 30 using light emitting diodes 31 can easily obtain a necessary quantity of light and moreover adopts an inexpensive constant-voltage power supply circuit 35. Therefore, it is easily thought to gradually change the quantity of light of the blank exposing lamp by providing with a constant-current circuit in place of the constant-voltage power supply circuit 35, or by providing with a light adjusting device separately. However, these methods bring a result of high cost.
As shown in Figure 5, therefore, the blank exposing device 30 of this embodiment is provided with the switching element 36 for performing an on-off operation of a voltage to be applied to the blank exposing lamp, and the control portion 110 controls the switching element 36 to generate a pulse voltage of one or plural pulses as shown in Figure 8(c) and apply the pulse voltage to the blank exposing lamp when changing over the blank exposing lamp from the on state to the off state. Due to this, the quantity of light of the blank exposing lamp substantially falls gradually as shown in Figure 8(b) and therefore the photosensitive object potential also changes gradually as shown in Figure 8(a), and thus it is possible to suppress formation of a sharp potential edge.
An image forming apparatus of this embodiment actually applies a pulse voltage shown in Figure 8(d) to the blank exposing lamp. This pulse voltage is such an irregular pulse voltage that it starts at the on state, is kept in the off state for 6 milliseconds, in the on state for 4 milliseconds, in the off state for 4 milliseconds, in the on state for 2 milliseconds and then comes completely into the off state. The inventors have set such a pulse condition, aiming at reducing the quantity of light of the blank exposing lamp by 10 to 90 % in 80 to 100 milliseconds when changing over the blank exposing lamp from the on state to the off state. Hereupon, the quantity of light of a conventional blank exposing lamp has been reduced by 10 to 90 % in 20 or less milliseconds. By using such a pulse voltage shown in Figure 8(d), it has been possible to realize a sufficiently slow change in photosensitive object potential and completely suppress occurrence of a black stripe.
Generally, when determining such a pulse condition that a slow change in photosensitive object potential can be obtained, it is necessary to consider the circumferential speed (process speed) or sensitivity of the photosensitive object 10, the arrangement or composition of the main charger 20 and the blank exposing device 30, and the like. Due to this, there are various proper pulse conditions according to these respective conditions. Figure 9 shows other examples of a pulse voltage to be applied to the blank exposing lamp.
For example, as shown in Figure 9(a), a pulse voltage having pulse intervals T₁, T₂ and T₃ being made shorter in stages can be used. And as shown in Figure 9(b), a pulse voltage having a fixed duty factor "t₁/(t₁ + t₂)" can be used. Here, time t₁ of the off state and time t₂ of the on state are different from each other (t₂ < t₁). If the longer time t₁ of the off state and the shorter time t₂ of the on state are set taking a delay time required for turning on the blank exposing lamp into account, it is possible to more properly control the change in quantity of light of the blank exposing lamp. And the duty factor may be changed in stages.
Further, a pulse voltage shown in Figure 9(c) is more increased in number of pulses than the pulse voltage shown in Figure 9(a).
An image forming apparatus of this embodiment is provided with a switching element for performing an on-off operation of a constant-voltage power supply circuit and makes a control portion control the switching element to generate a pulse voltage of one or plural pulses and apply the pulse voltage to the blank exposing lamp when changing over the blank exposing lamp from the on state to the off state, and thereby can make the quantity of light of the blank exposing lamp fall gradually. Thanks to this, since the photosensitive object potential also gradually changes, it is possible to suppress formation of a sharp potential edge and suppress occurrence of a black stripe at the front end portion of a recording paper.
Particularly, since this image forming apparatus uses a constant-voltage power supply circuit as it is without using an expensive constant-current power supply circuit, it has also a merit that it is not so high in cost and is comparatively simple in control.
The present invention is not limited to the above-mentioned embodiment but can be modified variously within the scope of its gist.
For example, in the above-mentioned embodiment, a constant-current power supply circuit may be used in place of the constant-voltage power supply circuit. And it is acceptable also to use a light adjusting device and change the quantity of light of the blank exposing lamp.

INDUSTRIAL APPLICABILITY

As described above, a blank exposing device of the present invention is provided with a switching means for performing an on-off operation of a constant-voltage power supply circuit and makes a control means control the switching means to generate, for example, a pulse voltage of one or plural pulses and apply the pulse voltage to the blank exposing lamp when changing over the blank exposing lamp from the on state to the off state, and thereby can make the quantity of light of the blank exposing lamp fall gradually. Due to this, since the photosensitive object potential also gradually changes, it is possible to suppress formation of a sharp potential edge and suppress occurrence of a black stripe at the front end portion of a recording paper. Therefore, a blank exposing device according to the present invention can be used in an image forming apparatus for forming image information on a recording medium by means of an electrophotographic method.
And since an image forming apparatus of the present invention can suppress occurrence of a black stripe at the front end portion of a recording paper in the same way by being provided with the above-mentioned blank exposing device, the image forming apparatus can be used in a copying machine, a printer, a facsimile and the like for forming image information on a recording medium by means of an electrophotographic method.

Claims

A blank exposing device comprising;

a blank exposing lamp for removing electric charge on the surface of a photosensitive object and forming a non-image area on said photosensitive object by exposing said surface of the photosensitive object to light,

a light quantity adjusting means for adjusting the quantity of light of said blank exposing lamp, and

a control means for controlling said light quantity adjusting means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
A blank exposing device comprising;

a blank exposing lamp for removing electric charge on the surface of a photosensitive object and forming a non-image area on said photosensitive object by exposing said surface of the photosensitive object to light,

a constant-voltage power supply circuit for applying a constant voltage to said blank exposing lamp,

a switching means for performing an on-off operation of a voltage to be applied to said blank exposing lamp, and

a control means for controlling said switching means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
A blank exposing device according to claim 2,
wherein said control means controls said switching means to generate a pulse voltage of one or plural pulses from said constant-voltage power supply circuit when changing over said blank exposing lamp from the on state to the off state.
A blank exposing device according to claim 3,
wherein said pulse voltage of plural pulses is obtained by changing the pulse intervals in stages.
A blank exposing device according to claim 3,
wherein said pulse voltage of plural pulses has a fixed duty factor.
A blank exposing device according to claim 3,
wherein said pulse voltage of plural pulses is obtained by changing the duty factor in stages.
A blank exposing device according to claim 2, 3, 4, 5 or 6, wherein said blank exposing lamp is composed of a plurality of light emitting diodes.
An image forming apparatus for forming image information on a recording medium by means of an electrophotographic method, comprising;

a developing device for developing an electrostatic latent image formed on a photosensitive object on the basis of said image information with a mono-component developer,

a blank exposing device which has a blank exposing lamp for removing electric charge on the surface of the photosensitive object and forming a non-image area on said photosensitive object by exposing the surface of said photosensitive object to light before performing the development by means of said developing device and a light quantity adjusting means for adjusting the quantity of light of said blank exposing lamp, and

a control means for controlling said light quantity adjusting means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
An image forming apparatus for forming image information on a recording medium by means of an electrophotographic method, comprising;

a developing device for developing an electrostatic latent image formed on a photosensitive object on the basis of said image information with a mono-component developer,

a blank exposing device which has a blank exposing lamp for removing electric charge on the surface of the photosensitive object and forming a non-image area on said photosensitive object by exposing the surface of said photosensitive object to light before performing the development by means of said developing device, a constant-voltage power supply circuit for applying a constant voltage to said blank exposing lamp and a switching means for performing an on-off operation of a voltage to be applied to said blank exposing lamp, and

a control means for controlling said switching means so that the quantity of light of said blank exposing lamp gradually changes when changing over said blank exposing lamp from the on state to the off state.
An image forming apparatus according to claim 9,
wherein said control means controls said switching means to generate a pulse voltage of one or plural pulses from said constant-voltage power supply circuit when changing over said blank exposing lamp from the on state to the off state.