JP2001260412A - Image-forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image-forming apparatus which can maintain an exposure on a photoreceptor to be nearly constant even when a scanning density is changed by a change of a moving speed or a scanning interval of the photoreceptor. SOLUTION: A semiconductor laser 18 of a laser scanner 36Y is turned on based on a light quantity control signal Vrefy corresponding to an image yellow component. An intensity of light beams from the semiconductor laser 18 is detected by a monitor photodiode 38, converted by a current/voltage- converting circuit 40 to a voltage, and outputted to a comparator 42. A detection level for the light intensity is switched when a switch 52 comes in touch with either of variable resistances 50A and 50B by a range switch signal Range which is switched in accordance with the scanning density. The comparator 42 compares the light quantity control signal Vrefy with the detected light intensity and outputs an error signal. The light intensity of light beams from the semiconductor laser 18 is determined by this error signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of printing with a plurality of pixel densities.

[0002]

2. Description of the Related Art Conventionally, there has been proposed a technique for optimizing the amount of light emitted from a light source while switching the pixel density. However, the reason why the appropriate amount of light varies depending on the pixel density is not very clear. In an electrophotographic system, generally, the developed image density largely depends on the surface potential of a photoconductor, and the surface potential of the photoconductor is considered to be a function of exposure energy per unit area on the photoconductor. Therefore, in order to make the image density constant, it is necessary to make the exposure energy per unit area on the photoconductor constant under the condition that the sensitivity and development characteristics of the photoconductor are constant. Here, the exposure energy is represented by light intensity × exposure time.

The pixel density is switched by, for example, (1) changing the main scanning speed while keeping the sub-scanning speed constant, (2) changing the sub-scanning speed while keeping the main scanning period constant, or (3) changing the sub-scanning speed. This is realized by writing an image at a specific period selected from among a plurality of main scanning periods (for example, writing an image every main scanning or writing an image every other time, that is, changing a scanning interval by interlaced scanning). can do.

Comparing these three methods, the method (1) does not change the exposure energy per unit area on the photoconductor, but the methods (2) and (3) use the unit area on the photoconductor. The exposure energy per hit changes. That is, in the method (1), the area irradiated with light per unit time is constant because the sub-scanning speed is constant. Is constant.

On the other hand, in the method (2), the area irradiated with light changes per unit time because the sub-scanning speed changes. Therefore, even if the intensity of the irradiated light is constant, the exposure energy per unit area on the photoconductor changes.

Further, in the method (3), since the ratio of the light irradiation time per unit time changes, even if the irradiation light intensity is constant, the exposure per unit area on the photoreceptor is required. Energy will change.

FIGS. 12 and 13 show when the scanning density is switched.
This shows a state where the light intensity is switched. In each figure, the horizontal axis indicates the sub-scanning position, and the vertical axis indicates the light intensity at the relative exposure amount where the average exposure amount is 1. The broken line is the light intensity of the light beam for each scan,
The solid line indicates the overlapping exposure intensity distribution of each scanning beam.

FIG. 12 is a graph showing a light intensity of 70 μm at a light intensity of 1 / e ^2.
m with a light beam of 600 dpi (dot per inc
h) shows a state in which three lines are written in the main scanning direction. FIG. 13 shows the same beam diameter as that of FIG.
In this state, the light intensity is reduced to half and six lines are written in the main scanning direction. In this case, a line width of about 127 μm is expected, but if the threshold value of the exposure amount in the image forming process is 0.5, it is considered that almost the expected line width can be obtained in any case. In the characteristics of the actual image forming process, the threshold value is not always constant. As shown in FIG. 14, the exposure amount and the photoconductor surface potential show a relatively linear relationship in a certain range, but gradually become saturated. . The photoreceptor surface potential and the image density also show a linear relationship, although the slope differs depending on the method.

The high light intensity distribution portions shown in FIGS. 12 and 13 become saturated with high density in an image portion exposure method generally used in a laser printer, so that ripples hardly appear on images.

Thus, it is considered that the method (1) does not change the image density even when the pixel density is switched. However, in order to keep the image density constant in the methods (2) and (3), it is necessary to change the light intensity applied to the photoreceptor to make the exposure energy per unit area constant. Further, in order to keep the exposure energy constant, it is necessary to change the intensity of the light beam to be irradiated.

When a semiconductor laser is used as a light source, the intensity of the light beam can be changed by controlling the amount of current applied. Since the relationship between the amount of applied current and the intensity of the light beam changes greatly depending on the temperature and the like,
In order to stably obtain a predetermined amount of light, generally, a light beam intensity detection circuit that detects the intensity of a light beam emitted from a light source and generates a light beam intensity signal corresponding to the intensity of the light beam; A comparison circuit that compares the signal with the light quantity control signal; and a current control circuit that controls a current value applied to the light source according to the comparison result.

A light beam is emitted from the light source at a light beam intensity corresponding to the light amount control signal. The light quantity control signal is changed according to the exposure potential of the photoconductor, the density of the developed image on the photoconductor, the environmental temperature affecting them, and the number of times the photoconductor is used so that the image density becomes appropriate.

The target value of the light beam intensity control of the light source can be roughly divided into (4) image forming process characteristics such as photoreceptor sensitivity and other image forming conditions, and (5) image forming conditions such as printing speed and scanning density.
Is determined by two conditions.

The image forming process characteristic (4) changes continuously due to variations due to individual components, environmental conditions such as temperature, deterioration due to use, and the like. Generally, in an electrophotographic system, a stable image density or color reproduction is obtained by operating a process control system that detects these conditions and determines a light intensity target value.

On the other hand, the image forming condition of (5) is not usually changed to an arbitrary value in one machine, but is set by switching a plurality of preset conditions, and the printing speed switching and scanning are performed. This is to switch the density. Since the scanning density generally has a standardized scanning density such as 400/600/1200 dpi, it is sufficient to correspond to this.

Also, the printing speed is set discretely from the expected effect. That is, for example, it is considered that the user hardly feels any effect even if the speed is increased to 1.1 times at the time of speed increase, so that a setting of, for example, 2 times speed is required.

[0017]

As described above, the light intensity target value of the light source is largely determined by two different conditions. Therefore, when switching and using image forming conditions such as printing speed and scanning density in the same machine,
There is a need to hold a plurality of data and determine the target value of the light beam intensity control by these calculations, which causes a problem that the control becomes complicated.

Further, as compared with the process characteristics at the time of image formation, light intensity switching depending on image forming conditions needs to be largely and discretely switched. If the target value of the light intensity changes significantly, the substantial setting accuracy of the light intensity changes. For example, when the light intensity control signal is composed of data of 8 bits (0 to 255) and the target value of the light intensity is 100, the light intensity can be set in units of 1% of the target value. In the case of 50, the light intensity can be set only in units of 2% of the target value.

For this reason, it is not possible to accurately adjust the light intensity to the target value, and it is necessary to allow an error in the image density or to increase the number of data bits of the light amount control signal. Crabs need to be compromised.
Further, even if the number of bits of the data of the light amount control signal is unnecessarily increased, the actual setting accuracy is limited by the S / N ratio of the circuit. Therefore, the necessary and sufficient setting accuracy cannot be obtained,
There is a problem that image quality is deteriorated.

The present invention has been made in order to solve the above-mentioned problem. Even when the scanning density is changed by changing the moving speed and the scanning interval of the photoconductor, the exposure amount on the photoconductor is kept substantially constant. It is an object of the present invention to provide an image forming apparatus capable of maintaining the same.

[0021]

According to a first aspect of the present invention, there is provided a light source for emitting a light beam modulated in accordance with image data, and a light source for moving a light beam in a predetermined direction at a predetermined moving speed. And a scanning unit for performing a main scan on the photoconductor in a direction substantially orthogonal to the predetermined direction with the light beam, and changing a scanning density by changing at least one of the moving speed and the main scanning interval. Scanning density changing means, and light source control means for controlling the intensity of a light beam emitted from the light source based on the scanning density so that an exposure amount on the photoconductor corresponding to the image data is substantially constant. And, it is characterized by having.

The light source emits a light beam modulated according to image data, and is composed of, for example, a semiconductor laser. The photoconductor moves in a predetermined direction at a predetermined moving speed. The photoconductor has a drum shape, for example, and rotates at a predetermined speed in this case. The scanning unit includes, for example, a rotary polygon mirror, an fθ lens, and the like, and performs main scanning on the photoconductor in a direction substantially orthogonal to a predetermined direction by a light beam. That is, for example, a rotating direction (sub-scanning direction) is applied on a rotating photosensitive member by a light beam.
Are scanned in a direction (main scanning direction) orthogonal to.

The scanning density changing means changes the scanning density by changing at least one of the moving speed of the photoconductor and the main scanning interval (interval for writing one line of image). That is, by reducing the moving speed of the photoconductor, the main scanning interval is reduced, and the scanning density can be increased. Also, by increasing the moving speed of the photoconductor,
The main scanning interval is widened and the scanning density can be reduced. Further, for example, by writing an image for one line every two main scans, the scanning density can be reduced to half.

As described above, when changing the scanning density by changing at least one of the moving speed of the photoconductor and the main scanning interval, the exposure amount per unit area of the photoconductor changes.
The image density may not be uniform. That is, when the scanning density is high, the exposure amount per unit area increases, and when the scanning density is low, the exposure amount per unit area decreases.

Therefore, the light quantity control means controls the intensity of the light beam emitted from the light source based on the scanning density so that the exposure amount on the photosensitive member corresponding to the image data becomes substantially constant. For example, as described in claim 3, the intensity of the light beam is reduced as the scanning density increases, and the intensity of the light beam is increased as the scanning density decreases. Thereby, even when the scanning density is changed, the exposure amount per unit area on the photoconductor can be kept substantially constant.

Further, the light source control means compares the detected intensity with the target intensity of the light beam, and compares the detected intensity with the target intensity of the light beam. And a light intensity detection sensitivity switching means for switching the detection sensitivity of the light intensity detection means collectively according to the scanning density.

For example, the light intensity detection sensitivity switching means includes:
When the scanning density is high, that is, when the exposure amount is large, the detection sensitivity of the light intensity detection means is increased, and when the scanning density is low, that is, when the exposure amount is small, the detection sensitivity of the light intensity detection means is reduced. Thus, when the scanning density is high, the control means controls in the direction of decreasing the intensity of the light beam, and when the scanning density is low, the control means controls in the direction of increasing the intensity of the light beam. become. Therefore, the exposure amount on the photoconductor can be kept substantially constant regardless of the scanning density.

According to a fifth aspect of the present invention, there is provided a beam position detecting means for detecting a position of a light beam, and a beam position detecting level switching means for switching a detection level of the beam position detecting means in accordance with a scanning density. And may be further provided.

The beam position detection level switching means switches the detection level of the beam position detection means according to the scanning density. For example, when the scanning density is high, that is, when the light intensity is decreased, the detection level of the beam position detecting means is increased, and when the scanning density is low, that is, when the light intensity is increased, the detection level of the beam position detecting means is increased. Lower. This makes it possible to stably detect the beam position.

According to a second aspect of the present invention, there are provided a plurality of light sources for emitting light beams of respective colors modulated in accordance with image data, a plurality of photosensitive members moving in a predetermined direction at a predetermined moving speed, and A plurality of scanning units for performing main scanning on the photoconductor in a direction substantially orthogonal to the predetermined direction by the light beam; and scanning for changing a scanning density by changing at least one of the moving speed and the main scanning interval. Density changing means, and a plurality of light sources respectively controlling the intensities of light beams emitted from the plurality of light sources such that an exposure amount on the photoconductor corresponding to the image data is substantially constant based on the scanning density. Light source control means.

As described above, in a color image forming apparatus including a plurality of light sources, a plurality of photoconductors, and a plurality of scanning means for each color in order to form a color image, the plurality of light source control means are based on the scanning density. Therefore, even when the scanning density is changed, the exposure amount per unit area on the photoconductor can be kept substantially constant even when the scanning density is changed.

[0032]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a multicolor image forming apparatus 10 to which the present invention is applied. As shown in FIG. 1, the multicolor image forming apparatus 10 includes an image control unit 12. The image control unit 12 converts the input image data into Y (yellow)
Component, M (magenta) component, C (cyan) component, and K (black) component image data 14Y, 14M, 14
C, 14K to generate laser scanning devices 16Y, 16M,
16C and 16K.

The laser scanning devices 16Y, 16M, 16C,
Each of 16K has the same configuration, and includes a semiconductor laser 18, a collimator lens 20, a polygon mirror 22, an fθ lens 24, a mirror 25, and a scanning beam detection sensor 26, as shown in FIG. Hereinafter, the laser scanning device 16Y will be described as a representative.

The laser scanning device 16Y includes a semiconductor laser 18
It has. The semiconductor laser 18 outputs the image data 14
The laser beam 28 modulated according to Y, 14M, 14C, and 14K is emitted.

A collimator lens 20 and a polygon mirror 22 are provided on the emission side of the laser beam 28 of the semiconductor laser 18.
Is arranged. The laser beam 28 emitted from the semiconductor laser 18 is collimated by the collimator lens 20, and then enters the reflecting surface of the polygon mirror 22 rotating at a predetermined speed in the direction of arrow A in FIG.

Laser beam 28 of polygon mirror 22
The fθ lens 24 is disposed on the exit side of the lens. The laser beam 28 deflected along a predetermined direction by being reflected by the reflection surface of the polygon mirror 22
Photoconductor 3 charged by a charger (not shown) via
Irradiated at 0Y (see also FIG. 1).

The laser beam 28 applied to the photoreceptor 30Y scans the main surface of the photoreceptor 30Y along the direction parallel to the axis of the photoreceptor 30Y as the polygon mirror 22 rotates (main scanning). Then, an electrostatic latent image of a Y image is formed on the photoconductor 30Y. In the sub-scanning, the photoconductor 30Y is moved in FIG.
By rotating in the direction of arrow C.
In the same manner as above, an electrostatic latent image of the M component, the C component, and the K component is formed on each of the photoconductors 30M to 30K.

The laser beam 2 of the fθ lens 24
8, a mirror 25 is disposed at a position corresponding to the end on the scanning start side in the entire scanning range of the laser beam 28, and the laser beam 28 reflected by the mirror 25 detects the scanning beam. The light enters the sensor 26. A signal output from the scanning beam detection sensor 26 is used as a main scanning write start signal (SOS signal) to start writing an image in each main scan (modulation of the laser beam 28 emitted from the semiconductor laser 18 is started). Timing).

The electrostatic latent images of the respective color components thus formed on the photoconductors 30Y to 30K are developed with the respective color toners by a developing device (not shown). Then, the toner images formed on the photoconductors 30Y to 30K are sequentially transferred onto the intermediate transfer body 32 conveyed in the direction of arrow B in FIG. By adjusting the scanning timing of the laser beam 28 on the photoconductors 30Y to 30K in advance, the toner images are accurately superimposed and transferred onto the intermediate transfer body 32.

The toner image transferred to the intermediate transfer body 32 is transferred onto a recording medium 34 such as paper. The toner image transferred onto the recording medium 34 is fused and fixed by a fixing device (not shown). As shown in FIG. 1, the image forming apparatus 10 includes a density detection sensor 35, which can detect the density of the toner image of each color transferred onto the intermediate transfer body 32 by the density detection sensor 35. it can.

The laser scanning devices 16Y, 16M, 1
6C and 16K are light amount control circuits 36 as shown in FIG.
Y, 36M, 36C and 36K, respectively. The image control unit 12 determines the light intensity of the laser beam based on the density of the toner image of each color detected by the density detection sensor 35 so that the density of each color becomes an appropriate density, and determines the determined light intensity. Light amount control signals Vrefy, Vrefm, Vrefc, Vrefk for instructing are output to light amount control circuits 36Y, 36M, 36C, 36K, respectively.

Light intensity control circuits 36Y, 36M, 36C, 3
6K have the same configuration, the configuration of the light amount control circuit 36Y will be described below.
Description of 36C and 36K is omitted.

As shown in FIG. 3, the light quantity control circuit 36Y includes a semiconductor laser 18, a monitor photodiode 38 integrated with the semiconductor laser 18, and a current / voltage conversion circuit 4
0, comparator 42, LPF (low-pass filter) 44, voltage / current conversion (V / I) circuit 46, and SW (switch)
48.

The monitor photodiode 38 receives light emitted from the rear surface of the semiconductor laser 18 and outputs the light to the current-voltage conversion circuit 40 as a photocurrent. The photocurrent output from the monitor photodiode 38 is converted into a voltage and converted into a voltage by the current-voltage conversion circuit 40, and is output to the comparator 42 as a light intensity detection signal. Since the rear emission light of the semiconductor laser 18 is proportional to the light intensity of the front emission light contributing to image formation, the light intensity detection signal is proportional to the front emission light of the semiconductor laser 18.

The light emitted from the front surface of the semiconductor laser 18 is attenuated on the scanning optical system up to the photosensitive member 30Y. Since the amount of attenuation varies among individual units under the influence of the aperture ratio, transmittance, and reflectance of the scanning optical system, the current-voltage conversion circuit 40 has an amplification factor adjusting function. The relationship between the light intensity on the photoreceptor 30Y and the output voltage of the light intensity detection signal is set to a predetermined relationship.

That is, as shown in FIG. 4, the current-voltage conversion circuit 40 is composed of two variable resistors 50A and 50B and a switch 52, and a range switching signal Ra
Whether the switch 52 contacts the variable resistor 50A or the variable resistor 50B is determined according to the nge. For example, when the scanning density is lowered by changing the rotation speed of the photoconductor 30Y or the scanning interval (for example, 600 dpi) and the range switching signal Range is at a high level, the switch 52 contacts the variable resistor 50A and the range 1 Is selected and the scanning density is increased by changing the rotation speed of the photoconductor 30Y or changing the scanning interval (for example, 120
0 dpi) and the range switching signal Range is at a low level, the switch 52 comes into contact with the variable resistor 50B and the range 2 is selected.

As a result, the current path is switched, and the voltage level of the light intensity detection signal, that is, the voltage input to the comparator 42 changes. Note that the range switching signal Ran
ge is switched at the same time as switching of the rotation speed of the photoconductor 30Y or switching of the scanning interval, that is, switching of the scanning density. When the rotation speed of the photoconductor 30Y is switched, the movement speed of the intermediate transfer body 32 and the recording medium 34 is also switched. Also, the range switching signal Rang
As shown in FIG. 3, e is simultaneously output to the respective light quantity control circuits, so that the range, that is, the amplification factor is simultaneously switched.

The light amount control signal Vrefy output from the image control unit 12 is input to the comparator 42. Thus, the comparator 42 compares the light amount control signal Vrefy with the light detection signal, and outputs the error signal to the low-pass filter 4.
4 is output. At this time, when the light quantity detection signal is larger than the light quantity control signal Vrefy, the error signal becomes a negative voltage, and when the light quantity detection signal is smaller than the light quantity control signal Vrefy, the error signal becomes a positive voltage.

The resistance values of the variable resistors 50A and 50B are determined by the light intensity on the photosensitive member 30Y and the light amount control signal Vref.
The relationship between y and the control voltage is set in advance in an assembling process or the like so as to have a predetermined relationship, for example, a relationship as shown in FIG. That is, when the scanning density is increased by changing the rotation speed of the photoconductor 30Y or the scanning interval (for example, 1200 dpi), the exposure amount on the photoconductor 30Y becomes relatively large. The resistance value of the variable resistor 50A is set to a value that increases the sensitivity of the light intensity detection so that is relatively small. This allows
Since the voltage level of the light intensity detection signal is higher than in the normal case, the error signal increases in the negative direction, the light intensity is weakened, and the exposure amount on the photoconductor 30Y becomes relatively small.

On the other hand, when the scanning density is lowered by changing the rotation speed of the photosensitive member 30Y or changing the scanning interval (for example, 6
In 00 dpi), since the exposure amount on the photoconductor 30Y is relatively small, the variable resistor 50B is set to a value such that the light intensity is increased so that the exposure amount is relatively large and the light intensity detection sensitivity is reduced. Set the resistance value of. As a result, the voltage level of the light intensity detection signal becomes lower than in the normal case, so that the error signal increases in the positive direction, the light intensity is increased, and the exposure amount on the photoconductor 30Y becomes relatively large. In this case, the resistance of the variable resistor 50A is set to be smaller than the resistance of the variable resistor 50B.

As described above, the sensitivity of the light intensity detection is increased when the scanning density is high, and the sensitivity of the light intensity detection is decreased when the scanning density is low. Even when the scanning density is changed by changing the scanning interval, the exposure amount on the photoconductor 30Y can be kept substantially constant.

The error signal is integrated by the low-pass filter 44, converted into a current by the voltage-current conversion circuit 46, and output to the semiconductor laser 18 via the SW 48.

Thus, the light detection signal corresponding to the intensity of the light emitted from the semiconductor laser 18 and the light amount control signal Vre
The current supplied to the semiconductor laser 18 is controlled so that fy coincides with fy.

If the variable range of the light intensity of the laser beam 28 is widened, it is necessary to widen the image writing timing by the beam detection circuit for detecting the scanning beam. If the light intensity of the laser beam 28 is too high, unnecessary light reflected from the periphery of the beam detection sensor 26 may be detected, causing a shift in image synchronization.
For this reason, it is necessary to design the beam detection circuit so as not to react to incident light having a level lower than the original light intensity of the laser beam 28. When the light intensity of the laser beam 28 is low, May not be reached.

Therefore, the image forming apparatus 10 is provided with a beam detecting circuit 54 as shown in FIG. 6 which can switch the sensitivity according to the light intensity of the laser beam 28.

As shown in FIG. 6, the beam detection circuit 54 includes a beam detection sensor 26 composed of two photodiodes 26A and 26B. When the photodiodes 26A and 26B detect the laser beam 28, output signals a and b as shown in FIG. The voltage levels of the output signals a and b indicate the light intensity of the light input to the photodiodes 26A and 26B. The comparator 56 compares the voltage levels of the output signals a and b of the photodiodes 26A and 26B, and outputs a differential signal c as shown in FIG. That is, the differential signal c becomes a high level when the voltage level of the output signal a is higher than the voltage level of the output signal b, as shown in FIGS. Is lower than the voltage level of the output signal b. Then, the position where the differential signal c is at the high level is defined as a division position X between the photodiode 26A and the photodiode 26B, and is used as a reference for the passing position of the laser beam 28.

The output signals a and b from the photodiodes 26A and 26B are output from the current-voltage conversion resistors 60A and 60B.
The signal is input to the comparator 62 as an output signal d as shown in FIG. Further, the output signal d is input to the comparator 66 as an output signal d 'as shown in FIG.

The comparators 62 and 66 have the reference voltage V
ref is input. Thus, the comparator 62 compares the voltage level of the output signal d with the reference voltage Vref,
When the voltage level of the output signal d is higher than the reference voltage Vref, a high level is output, and when the voltage level of the output signal d is lower than the reference voltage Vref, a low level is output.

Similarly, the comparator 66 compares the voltage level of the output signal d 'with the reference voltage Vref, and outputs a high level when the voltage level of the output signal d' is higher than the reference voltage Vref. When the voltage level of the signal d 'is lower than the reference voltage Vref, a low level is output.

The output signals of the comparators 62 and 66 are
It is input to the T circuit 68. Also, the SELECT circuit 68
Is also supplied with a range switching signal Range. S
In the ELECT circuit 68, the range switching signal Rang
e, that is, when the range 1 is selected (when the light intensity is increased), the output of the comparator 66 to which the output signal d ′ is input is selected, and when the range 2 is selected (the light intensity is selected). When the output signal d is input, the output of the comparator 62 to which the output signal d is input is selected and output to the AND circuit 58 as an output signal e as shown in FIG. That is, the gain is decreased when the light intensity is increased, and the gain is increased when the light intensity is decreased. In this way, FIG.
The level of the unnecessary light Y to be eliminated as shown in FIG.

The AND circuit 58 performs an AND operation on the output signal c and the output signal e and outputs an output signal f as shown in FIG. 7E, ie, a beam detection signal. Thus, by taking the AND of the output signal c and the output signal e, unnecessary light Y is eliminated, and a stable beam detection signal can be obtained.

As shown in FIG. 8, the output signal a and the output signal b biased by a predetermined amount by the bias current source 70 are amplified by amplifiers 72 and 74, respectively (FIG. 9).
(See (A)) may be compared by the comparator 76 and used as the output signal f. In this case, the gains of the amplifiers 70 and 72 are switched by the range switching signal Range. That is,
If range 1 is selected, amplifiers 70 and 72
And if range 2 is selected,
The gain of the amplifiers 70 and 72 is increased. As a result, FIG.
As shown in (B), the output signal f is a stable beam detection signal from which unnecessary light is eliminated.

Next, a case where the scanning density is changed by switching the scanning interval will be described. FIG. 10 shows a signal output circuit 80 for outputting a line signal and a page signal for determining the timing of image writing in the image control unit 12.

As shown in FIG. 10, the main scanning counter 82 of the signal output circuit 80 includes the beam detection circuit 54 described above.
The beam detection signal output from is input. Further, a scanning interval switching signal is also input to the main scanning counter 82. This scanning interval switching signal switches at the same timing as the range switching signal Range. For example, when the scanning density is low (for example, 600 dpi), the scanning interval switching signal becomes high level, and when the scanning density is high (for example, 1200).
In the case of dpi), the level is low.

When the scanning interval switching signal is at a low level, that is, when the scanning density is high, the main scanning counter 82 outputs a line signal every time a beam detection signal as shown in FIG. When the scanning interval switching signal is at a high level, that is, when the scanning density is low, FIG.
As shown in (B) and (C), a line signal is output every time the beam detection signal is input twice. That is, a line signal is output every two main scans.

The image controller 12 receives this line signal and outputs one line of image data (Video Data) to each laser scanning device as shown in FIG. 11D. Thereby, each laser scanning device turns on and off the semiconductor laser 18 according to the image data as shown in FIG. 11A (IMAGE part in the figure). That is, when the scanning density is high, one line of image data is written each time main scanning is performed. When the scanning density is low, one line of image data is written every two main scans. Thereby, the scanning interval, that is, the scanning density can be changed. Note that FIG.
The next image data (Video Data) is output to the image control unit 12 by outputting the data request signal (DATA Req) output after writing the image as shown in (E).

The line signal is also output to the magnitude comparing section 84. The magnitude comparing section 84 counts the number of times the line signal is output. If the count value of the line signal output count is compared with a predetermined page leading edge position count value 86 and matches, the page signal (Page Sync) is set to a high level, and compared with the page trailing edge position count value 88 and matched. The page signal (Page Sync) is set to low level. That is, a page signal is output until image formation for one page is completed.

As described above, the light intensity, the light intensity detection sensitivity, the resistance value of the variable resistor, and the beam position detection sensitivity for making the exposure amount constant when the scanning density is changed by changing the photoconductor speed. Is shown in Table 1 below. Table 2 below shows the relationship among the light intensity, the light intensity detection sensitivity, the resistance value of the variable resistor, and the beam position detection sensitivity for making the exposure amount constant when the scanning density is changed by switching the scanning interval. .

[0070]

[Table 1]

[0071]

[Table 2]

As described above, the image forming apparatus 10
In the case where the scanning density is switched by switching the rotation speed of the photoconductor or the scanning interval, the sensitivity of light intensity detection is increased when the scanning density is high, and the sensitivity of light intensity detection is decreased when the scanning density is low. , The exposure amount on the photoconductor can be kept almost constant regardless of the switching of the scanning density. Therefore, the same image density and color reproduction can be obtained even when the scanning density is changed. Further, since the sensitivity of the light intensity detection is adjusted by the variable resistor, the apparatus is not complicated, and good image quality can be obtained at low cost. Further, since the sensitivity of beam detection can be switched according to the light intensity of the laser beam, stable beam detection can be performed even when the light intensity is changed.

Further, it is not necessary to detect the image density, calculate the appropriate light intensity, and hold the light intensity control value individually for each image density, and a value at one image density can be applied in common. .

[0074]

As described above, according to the present invention,
Even when the scanning density is changed by changing at least one of the moving speed of the photoconductor and the main scanning interval, the exposure amount per unit area on the photoconductor can be kept substantially constant, and the beam position can be detected stably. Has the effect of being able to

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus.

FIG. 2 is a schematic configuration diagram of a laser scanning device.

FIG. 3 is a circuit diagram of a light quantity control circuit.

FIG. 4 is a detailed diagram of a light amount control circuit.

FIG. 5 is a diagram showing a relationship between a voltage level of a light quantity control signal and an emitted light intensity.

FIG. 6 is a circuit diagram of a beam detection circuit.

FIG. 7 is a waveform chart showing waveforms at various parts of the beam detection circuit.

FIG. 8 is a circuit diagram showing another example of the beam detection circuit.

FIG. 9 is a waveform chart showing waveforms at various parts of the beam detection circuit of FIG. 8;

FIG. 10 is a schematic configuration diagram of a signal output circuit.

FIG. 11 is a timing chart showing the output timing of each signal.

FIG. 12 is a diagram illustrating a relationship between a sub-scanning position and a relative exposure amount.

FIG. 13 is a diagram illustrating a relationship between a sub-scanning position and a relative exposure amount.

FIG. 14 is a diagram illustrating a relationship between an exposure amount and a surface potential of a photoconductor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Image control part 14Y, 14M, 14C, 14K Light amount control signal 16Y, 16M, 16C, 16K Laser scanning device 30Y, 30M, 30C, 30K Photoconductor 36Y, 36M, 36C, 36K Light amount control device 40 Current voltage Conversion circuit 54 Beam detection circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 1/113 H04N 1/04 104A 9A001 1/23 103 F term (Reference) 2C362 AA21 AA22 AA53 AA54 AA63 AA64 AA68 BA69 BB32.

Claims (5)

[Claims] A light source that emits a light beam modulated in accordance with image data; a photoconductor that moves at a predetermined moving speed in a predetermined direction; Scanning means for performing main scanning in a direction to be scanned, scanning density changing means for changing a scanning density by changing at least one of the moving speed and the main scanning interval, and corresponding to the image data based on the scanning density. An image forming apparatus comprising: a light source control unit that controls an intensity of a light beam emitted from the light source so that an exposure amount on the photoconductor is substantially constant. 2. A plurality of light sources for emitting light beams of respective colors modulated in accordance with image data; a plurality of photoconductors moving at a predetermined moving speed in a predetermined direction; A plurality of scanning means for performing main scanning in a direction substantially orthogonal to the predetermined direction by a light beam; a scanning density changing means for changing a scanning density by changing at least one of the moving speed and the main scanning interval; A plurality of light source control units each controlling an intensity of a light beam emitted from the plurality of light sources such that an exposure amount on the photoconductor corresponding to the image data is substantially constant based on a scanning density. Image forming apparatus provided. 3. The light source control unit according to claim 1, wherein the intensity of the light beam is reduced as the scanning density increases, and the intensity of the light beam is increased as the scanning density decreases. Item 3. The image forming apparatus according to Item 2. 4. The light source control unit compares a detected intensity with a target intensity of the light beam, and compares the detected intensity with a target intensity of the light beam. 4. The control device according to claim 1, further comprising: a control unit configured to control an intensity; and a light intensity detection sensitivity switching unit that collectively switches a detection sensitivity of the light intensity detection unit according to the scanning density. The image forming apparatus according to claim 1. 5. A beam position detecting means for detecting a position of the light beam, and a beam position detection level switching means for switching a detection level of the beam position detecting means according to the scanning density. 5. The method according to claim 1, wherein:
The image forming apparatus according to claim 1.