JPH0687234A - Image forming method and device therefor - Google Patents

Abstract

PURPOSE:To prevent an occurrence of a void in an image to obtain an ideal image while a cost and an image quality are kept favorable. CONSTITUTION:When an image in a low-coverage part is to be formed, in addition to the modulation of an exposure time of a laser beam, the laser beam is simultaneously modulated in intensity. The intensity of the laser beam at this time is so controlled as to be higher than that at the time of non-intensity modulation. A rate of change of a maximum energy density value of a latent image after an intensity modulation to an input coverage is so controlled as to be continuously changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method and apparatus used in an electrophotographic copying machine, a printer or the like for forming an image using a laser beam emitted from a semiconductor laser, a gas laser or the like. Is.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus,
For example, there is one as shown in FIG. This image forming apparatus irradiates a laser beam LB emitted from the semiconductor laser 100 onto a polygon mirror 102 via a collimator lens 101 and reflects the laser beam LB by the polygon mirror 102 rotating at a high speed. Scanning exposure is performed along the axial direction (main scanning direction) of the photosensitive drum 104. At that time, the image forming apparatus converges the laser beam LB, which is scanned and exposed on the photosensitive drum 104, into a spot shape by the fθ lens 103, makes the moving speed in the main scanning direction constant, and uniformly charges the surface. By scanning and exposing the formed photosensitive drum 104, an electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 104, and an image is formed.

At that time, in order to reproduce the gradation in the above-mentioned image forming apparatus, generally, the exposure time of the laser beam which is scanned and exposed on the photosensitive drum 104 is modulated, so that the laser per pixel is changed. An area gradation method is used in which the exposure area of the beam is changed according to the density.

[0004]

However, the above-mentioned prior art has the following problems. That is, in the image forming apparatus, in order to reproduce the gradation, the area gradation method is used in which the exposure time of the laser beam is modulated to change the exposure area of the laser beam per pixel according to the density. There is. Therefore, when the exposure time of the laser beam is changed according to the density of the image, the energy density of the laser beam has a distribution that is approximated by a Gaussian distribution. However, the maximum value of the energy density of the laser beam with which the photosensitive drum 104 is irradiated also changes as shown in FIG. Therefore,
When the exposure time of the laser beam is changed in order to reproduce the gradation, the maximum value of the energy density of the laser beam also changes accordingly. Not only the exposure time is shortened, but also the maximum value of the energy density of the laser beam is lowered, and the surface potential of the photosensitive drum 104 does not change to a predetermined potential.

In this state, the electrostatic latent image formed as described above is developed by a developing device having a speed difference between the photosensitive drum and the developer carrying member of the developing device, and the developer carrying member has a high coverage. If development is performed while moving from the area to the low coverage area, the following problems occur.

That is, as shown in FIG. 10, the image formed on the photosensitive drum 104 has a high coverage portion (for example, a portion having a density of 100%), which is a high density portion, followed by a low coverage portion ( For example, in the case of continuous images having a density of 20%), as shown by a broken line in FIG. 11, a high coverage portion having a high latent image potential and a low maximum value of the energy density of the laser beam have a low latent image potential. The low low coverage areas will be adjacent. Therefore, if this electrostatic latent image is developed by a developing device having a speed difference between the photosensitive drum and the developer carrying member of the developing device, when the developer carrying member develops the high coverage portion, A large amount of toner is consumed by the development, and the developer remaining on the developer carrier is
Many residual charges of the opposite polarity to the toner are generated. After that, when the low-coverage portion is developed by this developer-carrying member, the developer-carrying member having many residual charges of the opposite polarity to the charging polarity of the toner passes through the low-coverage unit,
A reverse electric field is generated between the developer bearing member and the photosensitive drum, the toner once developed returns to the developer bearing member side, and a blank image as shown in FIG. 15 occurs on the image, and the image quality deteriorates. There was a problem.

Therefore, in order to prevent the occurrence of such image defects, the present applicant has optimized the beam diameter of the laser beam and the resistivity of the developer as disclosed in Japanese Patent Application No. 4-55981. We are proposing to change.

In the developing method according to this proposal, a magnetic brush of a developer in which an insulating toner and magnetic particles are mixed is formed on the surface of a developer carrier containing a magnetic pole whose position is fixed,
This magnetic brush is brought into contact with or close to an electrostatic latent image holding member having an electrostatic latent image formed on its surface by uniformly charging and irradiating a laser beam based on a digital signal, and at the same time, developing the electrostatic latent image bearing member and the developing member. In a developing method of applying a bias voltage containing an AC component between the insulating toner and the agent carrier to make the insulating toner adhere to the electrostatic latent image to be visualized, the insulating toner has an average particle diameter of 8 μm or less. Use one
The frequency f (Hz) of the AC component of the bias voltage is in the range of 4000 ≦ f, and the magnetic particles have an electrical resistivity ρ (Ω · c).
m) is configured to use a laser beam having a diameter of r (μm) within the range of r ≦ −12 × log (ρ) +196 (r ≦ 100).

However, in this case, if the change of the maximum value of the energy density due to the change of the exposure time of the laser beam is made small by making the beam diameter of the laser beam small, the beam diameter of the laser beam is narrowed down. This requires a special optical system, which causes a new problem that the cost is significantly increased. In addition, by lowering the resistivity of the developer, the residual charge of the developer is released when the developer carrier develops the high coverage portion, and a reverse electric field is prevented from occurring when the low coverage portion is subsequently developed. In such a case, since the resistance of the developer is low, there is a new problem that deterioration of gradation and leakage of developing bias occur. Therefore, there is a new problem that there is a limit in optimizing the beam diameter of the laser beam and the resistivity of the developer while preventing the occurrence of a blank image while achieving other image quality and cost.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its purpose is to prevent occurrence of a blank image while achieving cost and image quality at the same time, and to obtain a good image. An object of the present invention is to provide an image forming method and a device therefor.

[0011]

In order to solve the above problems, the present invention scans and exposes an electrostatic latent image bearing member with a laser beam modulated according to an image signal to form an image. In the image forming method capable of reproducing the gradation by modulating the exposure time of the laser beam scanning-exposed on the electrostatic latent image carrier according to the coverage of the image density, the image of the low coverage portion In order to form the laser beam, the intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam so that the intensity of the laser beam becomes larger than that during no intensity modulation, and the maximum energy density of the latent image after intensity modulation. It is configured to control the rate of change of the value with respect to the input coverage so as to continuously change.

Further, the image forming apparatus according to the present invention scans and exposes the laser beam modulated according to the image signal on the electrostatic latent image carrier to form an image, and also the electrostatic latent image carrier. In the image forming apparatus capable of reproducing the gradation by modulating the exposure time of the laser beam scanning-exposed above according to the coverage of the image density,
When forming an image of a low coverage area, the intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam so that the intensity of the laser beam becomes larger than that during no intensity modulation and The control unit is configured to control so that the rate of change of the maximum energy density value of the image with respect to the input coverage continuously changes.

When a laser beam modulated according to the above image signal is scanned and exposed on the electrostatic latent image carrier to form an image, for example, a line screen is used.

[0014]

According to the present invention, when the image of the low coverage portion is formed, the intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam, and the intensity of the laser beam becomes larger than that in the non-intensity modulation. And is configured to control so that the rate of change of the maximum energy density value of the latent image after intensity modulation with respect to the input coverage changes continuously, so that an image in which the low coverage area is continuous with the high coverage area is formed. Is developed by a developing device in which there is a difference in speed between the electrostatic latent image carrier and the developer carrier of the developing device, and the developer carrier is developed while moving from the high coverage area to the low coverage area. However, the image of the low-coverage area is not only modulated by the exposure time modulation of the laser beam but also by the intensity modulation of the laser beam at the same time. Because There is set to be greater than the time of no intensity modulation, the latent image potential of the low coverage section,
It is possible to prevent deterioration in image quality due to reduction in the reverse electric field generated on the developer carrying member after developing the high coverage portion and development in the high coverage portion, resulting in reduction in concentration in the low coverage portion.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 2 shows a laser scanning type image forming apparatus which is an embodiment of the image forming apparatus according to the present invention.

In FIG. 1, reference numeral 1 denotes a semiconductor laser, to which a laser modulator 6 is supplied with an excitation current pulse-modulated in accordance with an image signal for each pixel. The laser beam LB emitted from the semiconductor laser 1 is collimator lens 2
It is irradiated onto the polygon mirror 3 via. The collimator lens 2 is for condensing and collimating the laser beam LB emitted from the semiconductor laser 1 so as to be diffused.

The laser beam LB applied to the polygon mirror 3 rotates at a high speed.
Is reflected by and is scanned and exposed along the axial direction (main scanning direction) of the photosensitive drum 4. At this time, the laser beam LB is converged by the fθ lens 5, the moving speed in the main scanning direction is constant, and the surface of the photoconductor drum 4 is uniformly exposed by scanning exposure.
An electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 4. The electrostatic latent image formed on the photoconductor drum 4 is developed by the developing device 7 with toner that is a color pigment to be visualized. Then, the toner image formed on the photoconductor drum 4 is transferred onto a transfer sheet and is also fixed onto the transfer sheet by heat and pressure by a fixing device to form an image. .

At this time, the laser beam LB used in this embodiment has, for example, a wavelength of 780 nm, a beam diameter of 64 μm, and an image screen of 40 per inch.
Regarding the potentials of the zero line screen and the photosensitive drum 4, the potential of the non-image portion is −650 V, and the potential of the entire exposed portion V _low is −20.
It is set to 0 respectively.

The developing device 7 for developing the electrostatic latent image formed on the photosensitive drum 4 is, as shown in FIG. 3, contained in a housing 9 for containing a two-component developer 8 composed of toner and carrier. A cylindrical developer carrier 10 that adsorbs the developer 8 on the surface and conveys the developer 8 by rotating the developer carrier 10.
A developer regulating member 11 that regulates the amount of the developer 8 attached to the surface of the developer carrier 10, and a screw auger that rotates to stir and convey the developer 8 and supply the developer to the developer carrier 10. 12 and 12.

The developer carrying member 10 contains a magnet roll 13 fixed so as not to rotate, and a sleeve 14 rotatably supported around the magnet roll 13. The sleeve 14 is arranged so as to face the photoconductor drum 4 at a distance of 0.5 mm, and a portion where the surfaces thereof are close to each other is a developing area. Further, the magnet roll 13 disposed inside has a plurality of magnetic poles, and a magnetic brush of magnetic particles is formed on the surface of the sleeve by the magnetic field formed between the adjacent magnetic poles. It can be transported. The rotation speed of the sleeve 14 is set so that the peripheral speed ratio between the photosensitive drum 4 and the sleeve 14 is about 1: 2. That is,
In the developing device 7, there is a difference in speed between the photosensitive drum 4 and the developer carrying member 10 of the developing device 7.
Moves at a speed faster than that around the photosensitive drum 4.

A DC superimposing AC voltage is applied as a developing bias voltage from the DC power supply and the AC power supply to the developer carrying member 10, and with respect to the surface exposure image portion voltage of -200 V of the photosensitive drum 4. The development potential contrast (difference between the DC component voltage and the overall exposure potential of the photosensitive drum 4) is about 300V, that is, the DC component is -500V.

Further, the AC component of the developing bias voltage has an amplitude value of 1.5 KV in peak-to-peak value and a frequency of 6 KHz, and its waveform is set to be rectangular.

By the way, in this embodiment, when the laser beam LB emitted from the semiconductor laser 1 is irradiated onto the photosensitive drum 4 to form an electrostatic latent image, gradation can be reproduced. In addition to controlling the area of the laser beam, the intensity modulation is performed at the same time according to the coverage value to prevent the occurrence of a blank image due to an excessive potential difference between the high coverage area and the low coverage area. Has been done.

That is, as shown in FIG. 4, the output characteristic of the semiconductor laser 1 is such that the optical output hardly increases up to the threshold current I _th even if the excitation current I increases, but the excitation current I is equal to the threshold current I _{th. When} it becomes _th or more, the optical output P sharply rises and linearly increases up to the maximum value I _max of the excitation current I.

Further, as shown in FIG. 5, the laser beam LB emitted from the semiconductor laser 1 has a distribution in which its energy density can be approximated by a Gaussian distribution centering on the maximum energy density value.

Further, in order to be able to reproduce the gradation in the above-mentioned image forming apparatus, basically, by modulating the exposure time of the laser beam LB scanning-exposed on the photoconductor drum 5, the per-pixel An area gradation method is used in which the exposed area of the laser beam LB is changed according to the density.

The semiconductor laser 1 for emitting the laser beam is supplied from the laser modulation means 6 with an excitation current which is pulse-modulated according to the image signal of each pixel. The time modulation of the pulse wave of the excitation current I is performed corresponding to the input coverage having the width of one pixel. Ie 1
Assuming that the pixel width is L, the laser beam scanning speed is S, and the input coverage is C, the laser lighting time T is T =
It is time-modulated so as to be L · C / S.

On the other hand, the intensity modulation of the pulse wave of the excitation current I supplied to the semiconductor laser 1 is performed according to the following procedure.

That is, assuming that the intensity distribution of the laser beam LB emitted from the semiconductor laser 1 is a Gaussian distribution, the exposure energy distribution when the laser beam LB is time-modulated as described above is the main distribution on the photosensitive drum 4. In the central cross section parallel to the scanning direction, it is expressed by the following formula 1.

[0031]

[Equation 1]

Here, x is a position in the main scanning direction, a is a beam diameter, and m is an intensity modulation coefficient.

The change in the maximum energy density value with respect to the coverage is expressed by the following equation (2).

[0034]

[Equation 2]

The intensity modulation coefficient m is represented by the relative value of the intensity modulation rate, where the beam intensity that gives the potential V _low of the entire surface exposed portion is 1.

The diameter a of the laser beam is defined as follows. That is, as shown in FIG. 5, the energy density of the laser beam decreases with distance from the center of the beam in the radial direction, but the energy density is 1 / e ² ≈the maximum value (value of the center x of the beam = 0). 13.5
The diameter at the position of%, that is, the value of a shown in FIG.

In this embodiment, the intensity modulation coefficient m _max at the time of maximum modulation is set to 1.365. This value is determined so that the maximum output of the laser beam and the gradation reproducibility are good. FIG. 6 shows changes with respect to the coverage of dE / dC at various intensity modulation coefficients m.

Further, in this embodiment, the intensity modulation coefficient m for each coverage is the difference in the maximum energy density value of the latent image between no intensity modulation and intensity modulation in the vicinity of 20% of the coverage where the blank image defect is most noticeable. As shown in FIG. 7, it is set to move on a straight line connecting the input coverage 5%, the maximum modulation intensity coefficient m at point A and the coverage 50%, and the intensity modulation coefficient at point B in FIG.

That is, the intensity modulation coefficient m for each coverage is determined by the following equation (3).

[0040]

[Equation 3]

FIG. 8 is a graph showing the value of the maximum energy difference of the latent image between the non-intensity modulation and the intensity modulation with respect to the coverage.

However, the intensity modulation start and end coverage can be freely set depending on the situation of the blank image defect.

FIG. 9 is a block diagram showing the control circuit of the image forming apparatus.

In the figure, reference numeral 20 denotes an image signal, and this image signal 20 is transmitted through an amplifier 21 to an A / D converter.
It is input to the converter 22. One of the image signals 20 converted into a digital signal by the A / D converter 22 is input to a modulation intensity calculation circuit 23, a predetermined modulation intensity is calculated, and an AND circuit is calculated via a D / A converter 24. 26 is output. On the other hand, the other part of the image signal 20 converted into a digital signal by the A / D converter 22 is input to a pulse width modulator 25, where a predetermined pulse width modulation is performed, and the image signal 20 is passed through a D / A converter 24. It is output to the AND circuit 26. Then, an image signal is input from the AND circuit 26 to the semiconductor laser 1 via the amplifier 27.

With the above structure, the image forming apparatus according to this embodiment modulates the laser beam as follows. That is, in order to form an image in the image forming apparatus, as shown in FIG.
Is supplied with a pulsed excitation current from the laser modulator 6 according to the image signal of each pixel, and the laser beam LB emitted from the semiconductor laser 1 according to the excitation current is collimated by the collimator lens 2 and the polygon. Scanning exposure is performed along the axial direction (main scanning direction) of the photosensitive drum 4 via the mirror 3 and the fθ lens 5. Then, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 4, and the image is formed.

At this time, since the excitation current supplied to the semiconductor laser 1 can reproduce the gradation, the exposure time T of the laser beam LB scanning-exposed on the photosensitive drum 5 is shown in FIG. As shown, the pulse width modulator 25 modulates the input coverage having a width of one pixel so that T = L · C / S as described above.

By the way, in this embodiment, when reproducing the gradation, in addition to the above-mentioned area gradation, that is, the modulation of the exposure time, the exposure intensity of the laser beam LB is changed to the input coverage (5 to 50% area only). 9), as shown in FIG. 9, the intensity modulation calculation circuit 23 performs modulation according to the equation (1) as described above.

Therefore, even if the maximum value of the energy density of the laser beam becomes lower as the exposure time of the laser beam LB becomes shorter in the region where the image density is low and the input coverage is low, the laser beam corresponding to the input coverage is obtained. The exposure intensity of the LB is modulated, and the region of low input coverage is modulated so that the exposure intensity of the laser beam LB is higher than usual as shown in FIG.

As a result, in the image area on the photosensitive drum 4, as shown by the solid line in FIG. 1, the potential of the image changes according to the input coverage, and in the low input coverage area, the conventional one shown by the broken line in the figure. The change in the electric potential of the image area is larger than that of the technique.

Therefore, as shown in FIG. 10, the image formed on the photosensitive drum 104 has a low coverage portion (next to the image of the high coverage portion (for example, a portion having a density of 100%) which is a high density portion. For example, in the case of continuous images with a density of 20%), as shown by the solid line in FIG. 11, a high coverage portion with a high latent image potential and a low maximum energy density of the laser beam and a latent image potential The low low coverage areas will be adjacent. However, in the low input coverage area, the change in the potential of the image area becomes larger than that in the conventional technique shown by the broken line in FIG. Therefore, when this electrostatic latent image is developed by the developing device 7 in which the speeds of the photosensitive drum 4 and the developer carrying member 10 of the developing device 7 are different, when the developer carrying member 10 develops the high coverage portion, Developer carrier 1
The toner on 0 is consumed in a large amount by the development, and a large amount of residual charge having a polarity opposite to that of the toner is generated in the developer remaining on the developer carrier 10. After that, when the low coverage area is developed by the developer carrying body 10, the developer carrying body 10 in which a large amount of residual charge having a polarity opposite to the charging polarity of the toner exists passes through the low coverage area.

However, in the low input coverage area, the change in the potential of the image area is large and the potential of the image area is close to the potential of the image area in the high input coverage area. Even when the developer carrying member 10 in which the presence of the developer passes through the low coverage portion, the reverse electric field generated between the developer carrying member 10 and the photosensitive drum 4 is small. Therefore, the toner once developed on the photoconductor drum 4 is unlikely to return to the developer carrier 10 side due to the reverse electric field generated between the developer carrier 10 and the photoconductor drum 4.
It is possible to prevent a blank image from being generated on the image and prevent deterioration of the image quality.

As described above, when the image of the low coverage portion is formed, the intensity of the laser beam LB is simultaneously modulated in addition to the exposure time modulation of the laser beam LB. It is configured to control so that the rate of change of the maximum energy density value of the latent image after intensity modulation with respect to the input coverage continuously changes so as to be large. Of the photosensitive drum 4
Even when the development is performed by the developing device 7 in which the speed of the developer carrying member 10 of the developing device 7 is different, and the developing agent carrying member 10 is developed while moving from the high coverage portion to the low coverage portion, The image of the low coverage portion is set so that the intensity of the laser beam LB is simultaneously modulated in addition to the exposure time modulation of the laser beam LB, and the intensity of the laser beam LB is set to be larger than that in the non-intensity modulation. The latent image potential of the coverage portion is close to that of the high coverage portion, and the developer carrying member 1 after developing the high coverage portion 1
It is possible to prevent the reverse electric field generated above 0 from becoming small and the concentration of the low coverage portion from lowering and the image quality from lowering.

In order to confirm the effect of the present invention, the present inventors prototyped an image forming apparatus as shown in FIG. 2 and changed the resistivity of the magnetic particles of the developing device 7 to form an image. The following shows the result of comparison between the conventional image forming apparatus that only performs exposure time modulation and the blank image defect.

The developer used in this case is a mixture of toner having an average particle diameter of 7 μm and magnetic particles (ferrite carrier) having an average particle diameter of 50 μm, and the toner concentration is 7%. Set.

FIG. 12 shows the resistivity of the magnetic particles of the developer as 10.
⁹ shows the results of observing the conditions under which white spot image defects occur at ¹¹ , 10 ¹³ , and 10 ¹⁵ Ω · m.

In this figure, the state of occurrence of white spots was evaluated by dividing it into the following three grades. Grade 1: No white spots Grade 2: White spots occur but is within the allowable range Grade 3: White spots occur and is outside the allowable range

As is apparent from FIG. 12, according to the image forming apparatus of the present embodiment, the allowable range of the resistivity of the magnetic particles in which the blank image defect occurs can be expanded.

[0058]

The present invention has the above-described structure and operation, and provides an image forming method and apparatus capable of obtaining a good image while preventing the occurrence of a blank image while achieving cost and image quality at the same time. Can be provided.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between latent image potential and coverage in an embodiment of the image forming method according to the present invention.

FIG. 2 is a configuration perspective view showing a main part of the image forming apparatus.

FIG. 3 is a configuration diagram showing a developing device.

FIG. 4 is a graph showing the relationship between the optical output of a semiconductor laser and the excitation current.

FIG. 5 is a graph showing the energy density of the optical output of a semiconductor laser.

FIG. 6 is a graph showing the rate of change of the maximum energy density value of the latent image with respect to the input coverage.

FIG. 7 is a graph showing a change rate of a maximum energy density value of a latent image with respect to an input coverage.

FIG. 8 is a graph showing the relationship between the coverage and the difference in the maximum energy value of the latent image between non-intensity modulation and intensity modulation.

FIG. 9 is a block diagram showing a control circuit.

FIG. 10 is a schematic diagram showing an image formation state.

FIG. 11 is a graph showing a latent image potential.

FIG. 12 is a graph showing the relationship between the resistivity of magnetic particles and the blank grade.

FIG. 13 is a configuration perspective view showing a main part of a conventional image forming apparatus.

FIG. 14 is a graph showing exposure energy.

FIG. 15 is a schematic diagram showing a state in which a blank image is generated.

[Explanation of symbols]

1 semiconductor laser, LB laser beam, 4 photosensitive drum, 6 laser modulation means.

Claims (2)

[Claims] 1. A laser beam which is modulated according to an image signal is scanned and exposed on an electrostatic latent image carrier to form an image, and a laser beam which is scanned and exposed on the electrostatic latent image carrier is formed. By modulating the exposure time according to the coverage of the image density, in the image forming method capable of reproducing the gradation, when forming the image of the low coverage portion,
The intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam so that the intensity of the laser beam becomes larger than that during non-intensity modulation, and the change in the maximum energy density value of the latent image after intensity modulation with respect to the input coverage. An image forming method characterized in that the rate is controlled so as to continuously change. 2. A laser beam which is modulated according to an image signal is scanned and exposed on the electrostatic latent image carrier to form an image, and a laser beam which is scanned and exposed on the electrostatic latent image carrier is formed. In the image forming apparatus capable of reproducing the gradation by modulating the exposure time according to the coverage of the image density, when forming the image of the low coverage portion,
The intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam so that the intensity of the laser beam becomes larger than that during non-intensity modulation, and the change in the maximum energy density value of the latent image after intensity modulation with respect to the input coverage. An image forming apparatus, comprising: a control unit that controls the rate so as to continuously change.