JPH0883021A - Image forming device - Google Patents

Abstract

PURPOSE: To improve the developability by changing the intensity of the exposure of an image in association with charge potential so as to make the difference between the charge potential of a photoreceptor drum and a DC bias potential applied to a developing sleeve almost constant. CONSTITUTION: A charger 2 uniformly charges the surface of a photoreceptor drum 1 to a potential VH, and an AC bias voltage VAC generated by an AC voltage power source 70 and a DC bias voltage VDC generated by a variable DC voltage power source 71 are added and the resultant voltage is applied to a developing sleeve 21. Then, the uniformly charged surface of the photoreceptor drum 1 is irradiated with the laser beam of a laser 80 to expose the image. When the developability is changed in this state, the output of the variable DC voltage power source 71 is varied to change the DC bias potential VDC for correcting the developability. At this time, the charge potential VH is changed by the charger 2 so as to made the difference between the charge potential VH of the drum 1 and the DC bias voltage VDC applied to the developing sleeve 21 almost constant and the exposure intensity of the laser 80 is changed by a laser driving part 81 linked with the change.

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 improved in control of charging potential and developing bias potential in a developing portion.

In recent years, a developing device adopted in an image forming apparatus such as a digital color copying machine or a color copying machine of a multi-color image batch transfer system uses a two-component developer composed of toner and carrier as a developer carrier (developing). (Also referred to as a sleeve) to supply a thin layer to narrow the development gap and apply a high electric field bias voltage.

[0003]

2. Description of the Related Art FIG. 5 is a diagram showing a configuration example of a color image forming apparatus (color copying machine) of a multicolor image batch transfer system. In the figure, reference numeral 1 denotes a photosensitive drum which is an image bearing member, and is an OPC photosensitive member coated on the drum, which is electrically grounded and driven and rotated clockwise. Reference numeral 2 denotes a scorotron charger, which uniformly charges the peripheral surface of the photosensitive drum 1 at a potential VH by corona discharge by a grid held at a potential VG and a corona discharge wire. Prior to the charging by the scorotron charger 2, in order to eliminate the history of the photosensitive member up to the previous printing, an exposure device (for example, PCL) 3 using a light emitting diode or the like is used to remove the charge on the peripheral surface of the photosensitive member. Keep it.

After uniformly charging the photoreceptor, the image exposing means 10
Thus, image exposure is performed based on the image signal. The image exposure means 10 uses a laser diode (not shown) as a light emitting source, performs scanning while the optical path is bent by a reflecting mirror 12 via a rotating polygon mirror 11, fθ lens, etc., and rotation of the photosensitive drum 1 (sub scanning). Form an electrostatic latent image. Here, the character portion is exposed to form an inverted latent image in which the character portion has a lower potential VL.

A developing device (developing device) is provided around the periphery of the photosensitive drum 1 with a developer containing toner such as yellow (Y), magenta (M), cyan (C) and black (K) and a carrier. 20 is provided, and first, the development of the first color is carried out by the developing sleeve 21 which contains a magnet and holds the developer and rotates. The developer is composed of a carrier in which ferrite is used as a core and an insulating resin is coated around the core, and a toner whose main material is polyester and pigments corresponding to colors, a charge control agent, silica, titanium oxide, etc. are added. The developer is regulated to a layer thickness (developer) of 100 μm to 600 μm on the developing sleeve 21 by the layer forming means and is conveyed to the developing area.

The gap between the developing sleeve 21 and the photosensitive drum 1 in the developing area is larger than the layer thickness (developer) of 0.2.
The thickness is set to mm to 1.0 mm, and the AC bias having the voltage value VAC and the DC bias having the voltage value VDC are superimposed and applied during this period. Since VDC and VH and toner charge have the same polarity,
The toner that has been given the opportunity to leave the carrier by VAC does not adhere to the VH portion, which has a higher potential than VDC,
It adheres to the VL portion having a lower potential than VDC and is visualized (reversal development).

After the visualization of the first color is completed, the image forming process of the second color is started, the uniform charging is performed again by the scorotron charger 2, and the electrostatic latent image based on the image data of the second color is exposed by the image exposing means. Formed by 10. At this time, the charge removal by the exposure device 3 performed in the image forming process of the first color is not performed because the toner attached to the image portion of the first color scatters due to the rapid decrease in the potential around the image.

Again, an electrostatic latent image similar to that of the first color is formed on a portion of the photoconductor having a potential of VH all over the peripheral surface of the photoconductor drum without the image of the first color and developed. However, in the portion where the image with the first color image is developed again, the toner adhering to the first color shields light and an electrostatic latent image of potential VM is formed by the electric charge of the toner itself, so that VDC Development is performed according to the potential difference of VM. In the part where the images of the first and second colors overlap,
When the development of the first color is performed by forming an electrostatic latent image of VL, the balance between the first color and the second color is lost, so the exposure amount of the first color may be reduced to an intermediate potential of VH>VM> VL. is there.

An image forming process similar to that of the second color is performed for the third and fourth colors, and a visible image of four colors is formed on the peripheral surface of the photosensitive drum 1. On the other hand, the recording paper P carried out from the paper feeding cassette 15 via the half-moon roller 16 is temporarily stopped, and when the transfer timing is adjusted, the paper feeding roller 17
The paper is fed to the transfer area by the rotation operation of.

In the transfer area, a transfer roller 18 is pressed against the peripheral surface of the photosensitive drum 1 in synchronism with the transfer timing, and the supplied recording paper P is nipped and a multicolor image is transferred at once. To be done.

Then, the recording paper P is discharged at almost the same time by the separating brush 19 which is brought into a pressure contact state, separated by the peripheral surface of the photosensitive drum 1 and conveyed to the fixing device 30, where the heat roller 31 and the fixing roller (pressing roller). ) 32 to heat and pressurize the toner, and then the toner is ejected to the outside of the apparatus through the paper ejection roller 41. The transfer roller 18 and the separation brush 19 are retracted from the peripheral surface of the photosensitive drum 1 after the recording paper P has passed, to prepare for the next toner image formation.

On the other hand, the photosensitive drum 1 from which the recording paper P is separated
Removes and cleans the residual toner by pressing the blade 51 of the cleaning device 50, and again receives the charge removal by the exposure device 3 and the charge by the scorotron charger 2 to start the next image forming process. The blade 51 moves immediately after cleaning the surface of the photoconductor and retracts from the peripheral surface of the photoconductor drum 1.

Note that the image forming apparatus shown in the figure can also be manually fed in addition to being fed by an automatic sheet feeding mechanism. The recording paper P manually fed by the manual feed tray 60 is conveyed by the rotation of the pickup roller 61,
The sheet is fed to the transfer area through the same process as the sheet feeding from the sheet feeding cassette 15 described above.

[0014]

In this type of image forming apparatus, various controls are performed in order to improve developability (density characteristics and gradation characteristics). The developability changes depending on temperature, humidity, the number of recording sheets used, and the like. In order to correct this change in developability, the DC bias potential VDC and the charging potential VH are changed.

FIG. 6 shows the DC bias potential VDC and the charging potential V
It is a figure explaining the relationship of H. The same parts as those in FIG. 5 are designated by the same reference numerals. The photoconductor drum 1 charged to a uniform charging potential VH by the charger 2 rotates clockwise in the figure. Then, image exposure is performed by a laser beam on the surface of the uniformly charged photosensitive drum. The image-exposed portion has an image potential (post-exposure potential) VL. The photoconductor portion subjected to the image exposure reaches the position where the developing device 20 is arranged.

In the developing device 20, the two-component developer 25 is agitated by the agitating screw 26, and the developer is supplied around the developing sleeve 21. The developing sleeve 21 rotates counterclockwise, and the surface of the developing sleeve to which the developer has adhered reaches the developing gap Dsd portion.

An AC bias voltage VAC and a DC bias voltage VDC are applied to the developing sleeve 21 in a superposed manner, and a horizontal magnetic field or a vertical magnetic field is formed in the developing gap. In this state, toner corresponding to the electrostatic latent image adheres to the surface of the photosensitive drum 1. In the drawing, for example, the case of developing one yellow (Y) toner is shown, but the same applies to other M (magenta), C (cyan), and K (black) developing devices.

FIG. 7 is a diagram showing the relationship between the photoconductor potential and the developing bias potential. The AC bias voltage VAC swings up and down by Va with respect to the DC bias potential VDC. Therefore, the potential of the developing sleeve 21 fluctuates within the range shown by the thick solid line in the figure. Here, the amplitude Va of the AC voltage VAC is 1.4 kV and the DC bias potential VDC is -600.
Assuming V, the potential of the developing sleeve 21 changes from −2000V to + 800V. On the other hand, when the charging potential VH of the photoconductor drum 1 is −750 V and the post-exposure potential VL is −50 V, the maximum value of the potential difference applied to the developing gap Dsd is the post-exposure potential VL portion, Vmax1 is 1.95 kV, and the charge potential VH portion. Then Vmax2 becomes 1.55 kV. That is, such a high electric field is applied to the development gap Dsd.

FIG. 8 is a diagram showing the relationship between the post-exposure potential VL and the exposure amount EL. The vertical axis represents the potential after exposure and the horizontal axis represents the exposure amount. In order to perform the above-mentioned reversal development, the charging potential VH
Is set higher than the DC bias potential VDC.
In FIG. 6A, f is the potential VL characteristic after exposure. As shown, when the exposure amount is increased, the post-exposure potential VL decreases. Here, the DC bias potential VDC and the post-exposure potential V
The potential difference VG between L is called a DC gap and is a parameter that affects the developability. If the DC gap is increased, the developability is generally improved, and if the DC gap is decreased, the developability is deteriorated.

The developability changes due to changes in temperature, humidity, the number of recording sheets used, changes over time, and the like. The fact that the developability changes while the DC gap remains the same generally means that the developability deteriorates. Therefore, when the developability deteriorates,
It is necessary to correct this DC gap accordingly. In order to correct the DC gap, a method of raising and lowering the DC bias potential VDC can be considered. In this case, when raising the DC bias potential VDC, it is necessary to also raise the charging potential VH. The reason is as follows.

When the charging potential VH is left as it is and only the DC bias potential VDC is raised, finally, VDC is reached.
Will exceed VH. As a result, the relationship between VH and VDC is reversed and ground fog occurs. On the contrary, if only the DC bias potential VDC is lowered, carriers are developed by the electric field between VH and VDC. Therefore, normally, the charging potential VH is also changed in association with the DC bias potential VDC.

When the charging potential VH is raised together with the DC bias potential VDC to improve the developing property, the post-exposure potential VL also rises as shown by f ', as shown in FIG.
The C gap is not sufficiently widened and the effect of raising the DC bias potential VDC is diminished, and the developability cannot be improved so much (f in the figure is the original VL characteristic). For this reason, white spots occur in the low-density portion, a delicate gradation image cannot be stably reproduced, and in the case of the multicolor image superposition method, the balance between the first and second colors is There was a problem that it collapsed and a change in chromaticity occurred.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of reliably improving the developability.

[0024]

SUMMARY OF THE INVENTION According to the present invention for solving the above-mentioned problems, a predetermined developing gap is formed with respect to an image carrier, and a bias voltage is applied to a developer carrier facing the image carrier for development. In the image forming apparatus, when the charging potential of the image carrier is VH and the DC bias potential applied to the developer carrier is VDC, when the charging potential VH is changed so that VH-VDC becomes substantially constant, It is characterized in that an image exposure driving means for changing the image exposure intensity in association with the change of the charging potential VH is provided.

In this case, the image exposure driving means changes the charging potential VH so that VH-VDC becomes substantially constant according to the output of the temperature sensor, the humidity sensor and the number of used recording sheets in the use environment. In addition, it is preferable to change the image exposure intensity in conjunction with the change in charging potential VH in order to improve the developability.

Further, when the image exposure is performed with a laser beam, the image exposure driving means is interlocked with the change of the charging potential VH when changing the charging potential VH so that VH-VDC is substantially constant. It is preferable to control the output of the laser in order to improve the developability.

Further, when the image exposure is performed with a laser beam, the image exposure driving means is interlocked with the change of the charging potential VH when changing the charging potential VH so that VH-VDC is substantially constant. It is preferable to control the pulse width for driving the laser in order to improve the developability.

[0028]

As can be seen from the characteristic f1 of FIG. 8A, the post-exposure potential VL decreases as the exposure amount increases. Focusing on this characteristic, it is only necessary to increase the DC bias potential VDC as shown in FIG. 8B and increase the exposure amount when the post-exposure potential characteristic rises as indicated by f ′. Therefore, when the DC bias potential VDC is increased, control may be performed so as to increase the exposure amount in conjunction with this. As a result, the characteristic f ′ can be returned to the characteristic f, and the developability can be improved.

Therefore, when changing the charging potential VH so that VH-VDC becomes substantially constant, an image exposure driving means is provided which changes the image exposure intensity in association with the change in the charging potential VH. Thereby, the DC gap can be changed and the developability can be improved.

In this case, the image exposure driving means changes the charging potential VH so that VH-VDC becomes substantially constant in accordance with the outputs of the temperature sensor, the humidity sensor and the number of used recording sheets in the use environment. Further, if the image exposure intensity is also changed in association with the change of the charging potential VH, the developability can be improved in response to the change of the developing environment.

When the charging potential VH is changed so that VH-VDC is substantially constant, the DC gap can be improved by controlling the laser output in conjunction with the change in the charging potential VH. The developability can be improved.

Further, when the charging potential VH is changed so that VH-VDC is substantially constant, the DC gap is changed by controlling the pulse width for driving the laser in conjunction with the change of the charging potential VH. And the developability can be improved.

[0033]

1 is a block diagram showing the configuration of an embodiment of the present invention. The same parts as those in FIG. 6 are designated by the same reference numerals. In the figure, 1 is a photosensitive drum as an image carrier,
Reference numeral 2 is a charger for uniformly charging the surface of the photoconductor drum with a potential VH. Reference numeral 20 denotes a developing device which is arranged so as to face the photosensitive drum 1. The developing device 20 includes a developing sleeve 21 as a developer carrying member, a multi-pole permanent magnet 23 disposed inside the developing sleeve 21, a two-component developer 25 including a carrier and toner, and a stirring screw 2 for stirring the two-component developer 25.
6, a supply roller 27 and a case 28 containing these.

Reference numeral 70 is an AC voltage source, and 71 is a variable DC voltage source whose output can be varied. The AC voltage source 70 generates an AC bias voltage VAC, and the variable DC voltage source 71 generates a DC bias voltage VDC. These AC bias voltage V
The AC and the DC bias voltage VDC are added and applied to the developing sleeve 21. Reference numeral 80 is a laser for image exposure, 81 is a laser drive unit that drives the laser according to an image signal, and 82 is a mirror that reflects the laser beam and makes it enter the surface of the photosensitive drum. The operation of the apparatus configured as described above will be described below.

Uniform potential (charging potential) VH by the charger 2.
The photosensitive drum 1 charged to the right rotates in the clockwise direction in the figure. Then, the output of the laser 80 driven by the laser driving unit 81 is reflected by the mirror 8 on the surface of the uniformly charged photosensitive drum.
The laser beam is reflected by the laser beam 2 and the image exposure is performed. The image-exposed portion has the post-exposure potential VL. The photoconductor portion subjected to the image exposure reaches the position where the developing device 20 is arranged.

In the developing device 20, the two-component developer 25 is agitated by the agitating screw 26, and the developer is supplied around the developing sleeve 21 by the supply roller 27. The developing sleeve 21 rotates counterclockwise, and the surface of the developing sleeve to which the developer has adhered reaches the developing gap Dsd portion.

The aforementioned AC bias voltage VAC and DC bias voltage VDC are applied to the developing sleeve 21, and a horizontal magnetic field or a vertical magnetic field is formed in the developing gap by the multi-pole permanent magnet 23. When the developability changes in this state, the output of the variable DC voltage source 71 is changed to change the DC bias potential VDC in order to correct the developability. At this time, the charging potential VH is also changed by the charger 2 so that VH-VDC becomes substantially constant. At this time, the laser drive unit 81 drives the laser 80 to control the exposure amount to be changed in conjunction with each other, whereby development with improved developability can be performed.

FIG. 2 is a diagram showing the experimental results according to the present invention. Condition 1 is that the DC bias potential VDC is -600V, the charging potential VH is -750V, the exposure amount by the laser beam is 200 μW expressed by the optical power on the photosensitive drum surface, the post-exposure potential VL is -200V, and the DC gap is 400V. is there. This condition is used as a reference condition (initial condition). Condition 2
Has a direct current bias potential VDC of -800V, a charging potential VH of -950V, an exposure amount of a laser beam of 280 .mu.W expressed by optical power on the surface of the photosensitive drum, a post-exposure potential VL of -200V, and a DC gap of 600V. Condition 2 is a case where the DC gap is widened to improve the developability. Condition 3 is DC bias potential VDC -450V, charging potential VH
Is -600 V, the exposure amount by the laser beam is 150 μW expressed by the optical power on the photosensitive drum surface, and the potential VL after exposure.
Is -200V and the DC gap is 250V. Condition 3
Shows the case where the DC gap is narrowed to lower the developability. Condition 4 is a DC bias potential VDC of -800V and a charging potential V
H is -920 V, the exposure amount by the laser beam is 250 μW expressed by the optical power on the surface of the photoconductor drum, and the post-exposure potential V
L is -280V and DC gap is 520V. Although the exposure amount is represented by the output of the laser, when the laser is started by the pulse width modulation method, if the duty of the drive pulse in condition 1 is 70%, condition 2 is 100.
%, Condition 3 is 50%, and condition 4 is 85%.

FIG. 3 is a diagram showing the relationship between the post-exposure potential VL and the exposure amount EL according to the present invention. The vertical axis shows the potential VL after exposure.
The horizontal axis represents the exposure amount EL. Here, the exposure amount EL is
This is an expression when a gradation of a plurality of bits is given to one dot, and this exposure amount EL is not necessarily the same as the corrected exposure amount EL of the present invention. In the figure, f1 is the condition 1,
The characteristics of conditions 2 and 3, f2 is the characteristics of condition 4, and f3 is the characteristics of only raising the DC bias potential VDC.

When the DC bias potential VDC is changed,
When the charging potential VH is changed accordingly, the post-exposure potential VL changes with the same exposure amount. In particular, V of the halftone potential (corresponding post-exposure potential in which the gray scale on the horizontal axis is near the middle)
The change is large at L (characteristic f3).

Condition 1 is an initial condition and condition 2 is a case where the developability is improved. In order to improve the developing property, the DC bias potential VDC and the charging potential VH are increased in conjunction with each other. However, since the potential VL after exposure also rises as it is, the exposure amount is increased from the previous 200 μW to 280 μW and D
The C gap is increased from 400V to 600V. Thereby, the image quality can be kept constant.

Condition 1 is an initial condition and condition 3 is a case where the developability is lowered. In order to lower the developability, the DC bias potential VDC and the charging potential VH are lowered in conjunction with each other. However, since the post-exposure potential VL is lowered in this state, the exposure amount is reduced from 200 μW until then to 150 μW.
The C gap is lowered from 400V to 250V. Thereby, the image quality can be kept constant. F1 in FIG. 3 indicates that the image quality of Condition 1 to Condition 3 is constant. Under these conditions 1 to 3, the post-exposure potential VL is constant at -200V.

The characteristic f2 in FIG. 3 is the characteristic of condition 4.
The characteristic is intermediate between the characteristics f1 and f3. The potential VL after the halftone exposure is intentionally changed so that the rate of change of the DC gap becomes constant with respect to the change of the exposure amount. By performing such control, the gradation characteristic can be held constant.

FIG. 4 is a diagram showing the relationship between the DC gap and the exposure amount EL according to the present invention. The vertical axis represents the DC gap, and the horizontal axis represents the exposure amount EL. f1 is the characteristic of condition 1, f2 is the characteristic of condition 2, f3 is the characteristic of condition 3, and f4 is the characteristic of condition 4. These characteristics indicate that the DC gap increases as the exposure amount increases.

Among these characteristics, f4 is the ratio of the DC gap b of the condition 1 in the low exposure amount region to the DC gap in the low exposure amount region of the condition 4 to the DC gap B of the condition 1 in the high exposure amount region and the condition. 4 is the same as the ratio with the DC gap A of 4. That is, b / a = B /
It indicates that the exposure amount is controlled so that A is satisfied.

In the present invention, the laser drive unit 8 is used for controlling the exposure amount.
1 shows a case in which the output control of the laser 80 is performed by 1 (see FIG. 1). As the form of the output control, there are used a method of controlling the output itself of the laser 80 and a method of controlling the pulse width (PWM method) with the output level of the laser 80 unchanged. The effect is the same in both cases.

The change in the developing property appears in accordance with the temperature change, the humidity change, the number of recording sheets used, and the change over time in the developing environment. Therefore, a temperature sensor, a humidity sensor, and a counter for detecting the temperature change, the humidity change, and the number of used recording sheets are provided, and the laser drive unit 81 receives the outputs of these sensors and the counter to recognize the change in the developability. It is preferable to control the output of the laser 80 and change the exposure amount according to the result.

[0048]

As described above in detail, according to the present invention, a predetermined developing gap is formed on the image carrier,
In an image forming apparatus configured to apply a bias voltage to a developer carrier facing each other for development, a charging potential of the image carrier is VH and a DC bias potential applied to the developer carrier is VDC.
Then, when the charging potential VH is changed so that VH-VDC becomes substantially constant, the developing property is improved by providing an image exposure driving unit that changes the image exposure intensity in conjunction with the change of the charging potential VH. It can surely be improved.

In this case, the image exposure driving means changes the charging potential VH so that VH-VDC becomes substantially constant in accordance with the outputs of the temperature sensor, the humidity sensor and the number of used recording sheets in the developing environment. By changing the image exposure intensity in conjunction with the change in the charging potential VH,
The developability can be surely improved in response to changes in the development conditions.

Further, when the image exposure is performed with a laser beam, the image exposure driving means is interlocked with the change of the charging potential VH when changing the charging potential VH so that VH-VDC is substantially constant. By controlling the output of the laser, the developability can be surely improved.

Further, when the image exposure is performed with a laser beam, the image exposure driving means is interlocked with the change of the charging potential VH when changing the charging potential VH so that VH-VDC is substantially constant. By controlling the pulse width for driving the laser with the laser, the developability can be surely improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an experimental result according to the present invention.

FIG. 3 is a diagram showing a relationship between a post-exposure potential and an exposure amount according to the present invention.

FIG. 4 is a diagram showing a relationship between a DC gap and an exposure amount according to the present invention.

FIG. 5 is a diagram showing a configuration example of a multi-color image batch transfer type color image forming apparatus.

FIG. 6 is a diagram illustrating a relationship between a DC bias potential VDC and a charging potential VH.

FIG. 7 is a diagram showing a relationship between a photoconductor potential and a developing bias potential.

FIG. 8 is a diagram showing a relationship between a post-exposure potential and an exposure amount E.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor drum 2 Charging device 20 Developing device 21 Developing sleeve 23 Multipolar permanent magnet 25 Two-component developer 26 Stirring screw 27 Supply roller 70 AC voltage source 71 DC voltage source 80 Laser 81 Laser drive unit 82 Mirror

Claims (4)

[Claims] 1. An image forming apparatus having a structure in which a predetermined developing gap is formed with respect to an image carrier, and a developing device is developed by applying a bias voltage to a developer carrier facing the image carrier. When the DC bias potential applied to the developer carrying member is VDC, and when the charging potential VH is changed so that VH-VDC is substantially constant, the image exposure intensity also changes in conjunction with the change of the charging potential VH. An image forming apparatus comprising: an image exposure driving unit for controlling the image exposure. 2. The image exposure driving means, when changing the charging potential VH so that VH-VDC becomes substantially constant in accordance with the output of the temperature sensor, the humidity sensor and the number of used recording sheets in the developing environment, The image forming apparatus according to claim 1, wherein the image exposure intensity is also changed in association with the change of the charging potential VH. 3. When the image exposure is performed with a laser beam, the image exposure driving means changes the charging potential VH so that VH-VDC is substantially constant, and the charging potential VH is changed.
3. The image forming apparatus according to claim 1, wherein the output of the laser is controlled in association with the change of the. 4. When the image exposure is performed with a laser beam, the image exposure driving means changes the charging potential VH so that VH-VDC is substantially constant, and the charging potential VH is changed.
3. The image forming apparatus according to claim 1, wherein the pulse width for driving the laser is controlled in association with the change in the pulse width.