JP2004338130A - Exposure controller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exposure controller ensuring a uniform quantity of laser light in an exposure scanning region. <P>SOLUTION: The exposure controller comprises a means for detecting the output light power of a semiconductor laser, a means for sampling the output light power detected by the detecting means immediately after starting emission of each dot, and a means for controlling the driving current of the semiconductor laser to a preset reference level when driving of the semiconductor laser is started for each dot and controlling the driving current of the semiconductor laser for that dot based on an output light power sampled by the sampling means immediately after starting emission of that dot and an output light power corresponding to that dot position stored in a storage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to an exposure control device in an image forming apparatus such as a copying machine.
[0002]
[Prior art]
In general, it is known that in a scanning optical system using a semiconductor laser, the exposure power of laser light in a scanning range decreases toward the end of the scanning range due to the cos4 law of the fθ lens.
[0003]
In the case of binary image exposure with a large exposure contrast, such a decrease in light amount is not considered to be a particular problem. However, when dealing with a multi-valued image, particularly a color image, slight exposure unevenness may cause a great loss of image quality. This is particularly noticeable in a system in which the multi-level exposure level is determined by laser power modulation.
[0004]
As a method of compensating for such a defect, a surface treatment for reducing the amount of transmitted light at the center of the fθ lens so that the amount of transmitted light at the center of the fθ lens is relatively equal to the amount of transmitted light at the end of the fθ lens. There is a way to do it. However, since such processing can be performed only with a uniform transmittance distribution, it cannot cope with manufacturing variations of the fθ lens and overall variations of other optical components.
[0005]
Therefore, in order to solve such a problem, it is conceivable to separately measure the transmittance characteristics of each lens or unit in advance, and to perform processing in which the lens transmittance distribution is adjusted in accordance with the characteristics. However, in such a method, the transmittance adjustment processing of the fθ lens must be performed individually, and the cost for the processing may be significantly increased, which is not practical.
[0006]
Further, as disclosed in Japanese Patent Application Laid-Open No. 6-98104, a method of electrically correcting the light amount has been developed. However, this method has a droop characteristic of a semiconductor laser (even when a constant current is applied, It does not take into account the characteristic that the laser power decreases due to heat, etc.), and even if it is possible to adjust the light amount change due to the influence of the optical system itself such as a lens, the light amount change in the scanning area due to the droop characteristic of the semiconductor laser is corrected Can not. In halftone printing or color printing, the effect of a change in light amount due to the droop characteristic of a semiconductor laser on image quality cannot be ignored.
[0007]
[Patent Document] JP-A-6-98104
[Problems to be solved by the invention]
According to the present invention, the exposure can be performed without being influenced by the variation of the absolute light amount between the units due to the optical system components, the light amount unevenness in the exposure scanning area due to the cos 4 power law of the fθ lens, and the light amount change due to the droop characteristic of the semiconductor laser. An object of the present invention is to provide an exposure control device capable of obtaining a uniform laser light amount in a scanning area.
[0009]
[Means for Solving the Problems]
The exposure control device according to claim 1, wherein the laser modulation system is a pulse width modulation system, the exposure scanning region is divided into a plurality of unit regions, and for each unit region, the exposure power in all the unit regions is reduced. A storage device in which a uniform output light power of the semiconductor laser is determined in advance, and a value corresponding to the previously determined output light power of each unit area is stored, and the output light power of the semiconductor laser is detected. Detecting means for sampling, the sampling means for sampling the output light power detected by the detecting means immediately after the start of emission of each dot, and the drive current of the semiconductor laser at the start of driving of the semiconductor laser for each dot is preset. Output light power sampled immediately after the emission of the dot is controlled by the sampling means while controlling to the reference value , Based on the value corresponding to the output optical power corresponding to the dot position stored in the storage device, characterized in that it comprises means for controlling the driving current of the semiconductor laser with respect to the dot.
[0010]
According to a second aspect of the present invention, in the exposure control device in which the laser modulation system is an intensity modulation system, the exposure scanning region is divided into a plurality of unit regions, and the exposure power in all the unit regions is uniform for each unit region. A storage device in which the output light power of the semiconductor laser is determined in advance and a value corresponding to the previously determined output light power of each unit area is stored, and the output light power of the semiconductor laser is detected. Detecting means for sampling the output light power detected by the detecting means immediately after the start of light emission of each dot, and the driving current of the semiconductor laser for each dot at the start of driving of the semiconductor laser for each dot. In addition to controlling the current value according to the gradation of the data, sampling is performed immediately after the emission of the dot by the sampling means. Driving the semiconductor laser for the dot based on the coupled output light power, a value corresponding to the output light power corresponding to the dot position stored in the storage device, and the gradation of input image data for the dot; It is characterized by having a means for controlling the current.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to an exposure control device of a copying machine will be described with reference to the drawings.
[0012]
FIG. 1 shows the configuration of the exposure control device.
[0013]
The exposure control device includes a semiconductor laser drive circuit 1, a semiconductor laser 2, a target value storage unit 3, and a dot-unit sample hold circuit 4. The semiconductor laser 2 includes a laser light emitting unit 2a and a built-in PIN photodiode 2b for detecting the power of light output from the laser light emitting unit 2a.
[0014]
During the manufacture of the exposure unit, the fluctuation characteristics of the amount of light transmitted through the lens and other optical components in the unit (exposure power on the photoreceptor surface) in the exposure scanning area are measured. At the time of this measurement, control (for example, feedback control) is performed so that the output light power of the semiconductor laser 2 becomes uniform in the exposure scanning area. FIG. 2 schematically shows the measured exposure power on the photoreceptor surface with respect to the image height (exposure scanning width).
[0015]
Then, based on the measurement result shown in FIG. 2, the output light power of the semiconductor laser 2 with respect to the image height (exposure scanning width) so that the exposure power on the photoconductor surface at each dot position in the exposure scanning area becomes uniform. Is calculated in advance. FIG. 3 schematically shows the output light power of the semiconductor laser 2 with respect to the calculated image height (exposure scanning width).
[0016]
The target value storage unit 3 stores the output light of the semiconductor laser 2 with respect to the image height (each dot position in the exposure scanning area) such that the exposure power on the photoconductor surface at each dot position in the exposure scanning area becomes uniform. A value corresponding to the power (target value) is stored.
[0017]
First, the operation of the semiconductor laser drive circuit 1 when the laser modulation method is the pulse width modulation method will be described.
[0018]
The semiconductor laser drive circuit 1 outputs a drive current having a pulse width according to the input image data. At this time, the drive current value is controlled as follows. First, when a drive current having a pulse width corresponding to input image data is output, the drive current value is set to a predetermined reference current value when the pulse rises.
[0019]
The dot-unit sample-and-hold circuit 4 samples a detection signal (a value corresponding to the actual output light power) of the photodiode 2b immediately after the start of light emission of each light-emitting dot, and sends the signal to the semiconductor laser drive circuit 1. The semiconductor laser drive circuit 1 compares the output light power detected by the dot unit sample hold circuit 4 with a target value corresponding to the dot position corresponding to the input image data stored in the target value storage unit 3, The drive current is controlled within the dot so that the output light power of the semiconductor laser 2 matches the target value. Such an operation is repeated for each dot.
[0020]
In the above-described embodiment, the target value is stored in the target value storage unit 3 in units of one dot in the exposure scanning direction. However, as shown in FIG. And the target value may be stored for each of the divided areas.
[0021]
Next, the operation of the semiconductor laser drive circuit 1 when the laser modulation method is the intensity modulation method will be described.
[0022]
In this case, as shown in FIG. 5, a correction value when the output light power of the semiconductor laser 2 for the image height 0 in FIG. 3 is set to 0 is calculated. Then, the correction value for the image height (each dot position in the exposure scanning area) is stored in the target value storage unit 3.
[0023]
When the semiconductor laser drive circuit 1 outputs a drive current corresponding to the input image data, the drive current value is set to a reference current value having a magnitude corresponding to the gradation of the input image data when the drive current rises. A reference current value having a magnitude corresponding to the gradation of the input image data is set in advance.
[0024]
The dot-unit sample-and-hold circuit 4 samples a detection signal (a value corresponding to the actual output light power) of the photodiode 2b immediately after the start of light emission of each light-emitting dot, and sends the signal to the semiconductor laser drive circuit 1. The semiconductor laser driving circuit 1 converts the output light power detected by the dot unit sample hold circuit 4 and a correction value corresponding to the dot position corresponding to the input image data stored in the target value storage section 3 into the correction value of the image data. The drive current is controlled within the dot so that the output light power of the semiconductor laser 2 is compared with a value (target value) obtained by adding the output light power corresponding to the gray scale. Such an operation is repeated for each dot.
[0025]
In the above embodiment, the correction value is stored in the target value storage unit 3 for each dot in the exposure scanning direction. However, the exposure scanning area is divided into a plurality of areas of a plurality of dots, and The correction value may be stored for each divided area.
[0026]
【The invention's effect】
According to the present invention, the absolute amount of light due to the optical system components varies from unit to unit, the amount of light in the exposure scanning area due to the cos4 power law of the fθ lens, and the change in the amount of light due to the droop characteristic of the semiconductor laser. As a result, a uniform amount of laser light can be obtained in the exposure scanning area.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an exposure control device.
FIG. 2 is a graph showing measured values of exposure power on a photoconductor surface with respect to an image height (exposure scanning width).
FIG. 3 is a graph showing output light power of a semiconductor laser 2 with respect to a calculated image height (exposure scanning width).
4 is a graph showing an example in which a target value is calculated for each divided area of the exposure scanning area. FIG. 5 is a graph showing a case where the output light power of the semiconductor laser 2 with respect to an image height of 0 in FIG. It is a graph which shows a correction value.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 semiconductor laser drive circuit 2 semiconductor laser 2 a laser emitting unit 2 b built-in PIN photodiode 3 target value storage unit 4 dot unit sample hold circuit

Claims (2)

In an exposure control device in which the laser modulation method is a pulse width modulation method,
The exposure scanning area is divided into a plurality of unit areas, and for each unit area, the output light power of the semiconductor laser is determined in advance so that the exposure power in all the unit areas is uniform. A storage device in which a value corresponding to the output light power for each area is stored,
Detecting means for detecting the output light power of the semiconductor laser,
Sampling means for sampling the output light power detected by the detecting means immediately after the start of emission of each dot, and controlling the drive current of the semiconductor laser to a preset reference value at the start of driving the semiconductor laser for each dot, Based on the output light power sampled by the sampling means immediately after the start of emission of the dot and a value corresponding to the output light power corresponding to the dot position stored in the storage device. Means for controlling the drive current,
An exposure control device comprising: In an exposure control device in which the laser modulation method is an intensity modulation method,
The exposure scanning area is divided into a plurality of unit areas, and for each unit area, the output light power of the semiconductor laser is determined in advance so that the exposure power in all the unit areas is uniform. A storage device in which a value corresponding to the output light power for each area is stored,
Detecting means for detecting the output light power of the semiconductor laser,
Sampling means for sampling the output light power detected by the detection means immediately after the start of emission of each dot; and, at the start of driving the semiconductor laser for each dot, the drive current of the semiconductor laser to the gradation of the input image data for that dot. While controlling the current value in accordance with the output light power sampled by the sampling means immediately after the start of emission of the dot, a value corresponding to the output light power corresponding to the dot position stored in the storage device, and Means for controlling the drive current of the semiconductor laser for the dot based on the gradation of the input image data for the dot,
An exposure control device comprising:

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