JP5564903B2 - Image evaluation method, image evaluation apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to a technique for measuring an electrostatic latent image in electrophotography, in particular, a technique for precisely measuring an electrostatic latent image when a vertical surface emitting laser is used as a light source for optical writing, and an electrophotographic apparatus for improving image quality. .

  In an electrophotographic process applied to a digital copying machine or a laser printer, a vertical surface emitting laser (VCSEL) in which a plurality of light sources are arranged is used for the purpose of achieving high definition of optical writing. It is getting to be. As a result, 600 d. p. i from conventional resolution such as i. p. i, 2400d. p. i, and further 4800d. p. Image formation with a high resolution such as i has become possible. This high resolution realizes a high-definition and high-quality image in paper output.

An example of the above vertical surface emitting laser is shown in FIG. FIG. 29 is a schematic view of a vertical surface emitting laser in which a plurality of light emitting sources are arranged as viewed from the optical axis (perpendicular to the paper surface), and a black circle in the drawing represents a light emitting source of one vertical surface emitting laser. The light emission direction is a direction perpendicular to the paper surface as a parallel light flux. The vertical surface emitting laser is manufactured monolithically on a semiconductor substrate using a semiconductor process, which has an advantage that an array of light emitting sources can be easily formed. The distance between the light emitting sources indicated by reference characters w and h in FIG. 29 is as large as several tens of μm (for example, 30 to 50 μm). is there. The wavelength (λ) of light emission is, for example, 780 nm, and light in the infrared region can be output. In FIG. 29, 30 (5 × 6) light sources are arranged, and these are configured to be capable of independent light emission by electrical control.

  Here, the overshoot which is a feature of the present invention will be described. Overshooting is used as a technique for improving the light emission characteristics of a laser, and can be set and adjusted by inputting / outputting electrical signals. Therefore, it is not limited to a vertical surface emitting laser and is also used for an edge emitting laser.

  Consider that the optical response waveform of an edge-emitting laser or a vertical-surface emitting laser is measured with a light receiver such as a photodiode (PD). In FIG. 30, reference numeral 11 denotes a vertical surface emitting laser, reference numeral 12 denotes an optical system, reference numeral 13 denotes a photodiode, and reference numeral 14 denotes a detector such as an oscilloscope. The optical system 12 uses a scanning optical system described later, but may be a single lens experimentally. In this configuration, the light from the laser light source received by the detector 14 is referred to as an optical response waveform.

  The vertical surface emitting laser 11 is driven by a pulsed input signal indicated by reference numeral 10 in FIG. At this time, the optical response waveform is ideally rectangular as indicated by reference numeral 100 in the figure, but in reality, a delay occurs in the electrical and electronic circuit included in the vertical surface emitting laser 11, and the optical response waveform is rectangular. Instead, the waveform becomes dull as shown by reference numeral 101 in FIG. For this reason, a difference (difference between the waveform 100 and the waveform 101) occurs as shown by reference numeral 102 in the drawing with respect to the desired pulse width (light emission time), and the integrated light quantity is insufficient. Here, the integrated light amount is obtained by integrating the intensity of the light response waveform with the light emission time, the light response waveform is a function f (t), and f (t) is integrated in the range from t0 to t1. That is, the area S. Overshoot is used to make up for this shortage. As indicated by reference numeral 15 in the figure, the start intensity of the input signal is set high to reduce the dullness of the rise of the optical response waveform so that it approaches a rectangle. It is.

  The set value of overshoot needs to be determined appropriately. If the set value is appropriate, an optical response waveform close to a rectangle can be obtained as shown in FIG. However, if the set value is large, an optical response waveform having a large head intensity is obtained as shown in FIG. 31C, and the integrated intensity is also larger than a desired value. If the set value is small, reference numeral 101 in FIG. As shown, the light response waveform has a small integrated intensity.

The above explanation relates to the optical response waveform measured with a photodiode. Find out what optical response waveform is suitable, what is the integrated light intensity, and what is the preferred overshoot setting value. Can do. However, in an actual electrophotographic apparatus such as a digital copying machine or a laser printer, light emitted from a laser light source reaches a photosensitive member through a scanning optical system to form an electrostatic latent image. It will serve as a dimension detector. As described above, the information obtained by the photodiode is one-dimensional, whereas the electrostatic latent image on the photosensitive member is two-dimensional, and information that cannot be obtained by the optical response waveform is obtained. An electrostatic latent image can be said to be an image in the previous stage of an output image (toner image recorded on a recording medium such as paper), and evaluation with an electrostatic latent image is preferable to evaluation with a photodiode. is there.
“Patent Document 1” discloses a technique related to electrostatic latent image measurement using a charged particle beam, and “Patent Document 2” discloses a technique for improving image quality by overshooting in an electrophotographic apparatus. Yes.

The technique disclosed in “Patent Document 1” does not include a technique related to overshoot, and the technique disclosed in “Patent Document 2” obtains a suitable overshoot by an electrostatic latent image and reflects this in an image. The technology to be made is not disclosed.
The present invention relates to an evaluation method for obtaining a suitable value of overshoot from an electrostatic latent image, an evaluation method for obtaining a correlation between the light emission characteristics of a laser light source and the size of the electrostatic latent image, and an image that embodies this evaluation method. An object of the present invention is to provide an evaluation apparatus and an image forming apparatus that forms a high-quality image by reflecting a result obtained by the image evaluation apparatus.

The invention according to claim 1 is an image evaluation method for evaluating an electrostatic latent image formed on a photosensitive member, wherein a plurality of laser light sources, a driving means for emitting laser light from each of the laser light sources, An electrostatic latent image forming unit that scans each laser beam on the photosensitive member to form an electrostatic latent image on the photosensitive member, and an electrostatic latent image measuring unit that measures the electrostatic latent image, One dot, which is the minimum unit of the electrostatic latent image, is formed by a plurality of laser light sources, and the driving means has a function of adjusting the pulse width and overshoot of the laser light source, and the pulse width is set to the one dot. after by the laser light source fraction to form forming a plurality of electrostatic latent image is changed stepwise as the variation of the pulse width determined the size of each of the electrostatic latent image, to further change the overshoot And a laser that forms the one dot with the pulse width And a step of gradually changing the source fraction as the variation of the pulse width by forming a plurality of electrostatic latent images Ru determined the size of each of the electrostatic latent image, the overshoot stepwise The correlation between the pulse width and the size of the electrostatic latent image is obtained by repeating the above steps while changing.

  According to a second aspect of the present invention, in the image evaluation method according to the first aspect, a suitable overshoot value is obtained from a correlation between the pulse width and the size of the electrostatic latent image.

  According to a third aspect of the present invention, in the image evaluation method according to the first or second aspect, the shape of the electrostatic latent image is a dot, and the size of the electrostatic latent image is the electrostatic latent image forming means. In the main scanning direction.

According to a fourth aspect of the present invention, there is provided an image evaluation method for evaluating an electrostatic latent image formed on a photoconductor, wherein a vertical surface emitting laser having a plurality of light sources and a drive for emitting laser light from each of the light sources. Means, electrostatic latent image forming means for forming an electrostatic latent image on the photosensitive member by scanning each laser beam on the photosensitive member, and electrostatic latent image measuring means for measuring the electrostatic latent image 1 dot, which is the minimum unit of the electrostatic latent image, is formed by a plurality of laser light sources, and the driving means has a function of adjusting the pulse width and overshoot of each light source, after determining the spatial frequency characteristics of gradually changing to form a plurality of electrostatic latent images each electrostatic latent image with a laser light source fraction as the variation of the pulse width for forming the one dot, and further wherein The overshoot is changed and the pulse width is changed to the one dot. A laser light source fraction which forms comprising the step of forming a plurality of electrostatic latent image is varied stepwise Ru calculated spatial frequency characteristic of each of the electrostatic latent image as a variation of the pulse width, the overshoot It is characterized in that a plurality of spatial frequency characteristics are obtained by repeating the above-mentioned process with stepwise change.

  According to a fifth aspect of the present invention, in the image evaluation method according to the fourth aspect, a suitable overshoot value is further obtained from the plurality of spatial frequency characteristics.

  According to a sixth aspect of the present invention, in the image evaluation method according to the fourth or fifth aspect, the spatial frequency characteristics further include an area of a dot-like electrostatic latent image and a diameter when the electrostatic latent image is approximated by a circle. It is characterized by being defined from.

  A seventh aspect of the present invention is an image evaluation apparatus for performing the image evaluation method according to any one of the first to sixth aspects, wherein the charged particle generating means generates charged particles for charging the photoreceptor. A vertical surface emitting laser having a plurality of light sources, driving means for emitting laser light from each light source, and scanning each of the laser light on the photoconductor to form an electrostatic latent image on the photoconductor Electrostatic latent image forming means, electrostatic latent image observing means for observing the electrostatic latent image formed on the photosensitive member, and desired information from the electrostatic latent image observed by the electrostatic latent image observing means. And an information processing means for electronic processing.

Invention according to claim 8, to reflect the image evaluation result obtained by the image evaluation apparatus of an image evaluation method or claim 7, wherein according to any one of claims 1 to 6 for image formation, preferred wherein The image forming apparatus includes a storage unit that stores overshoot .

  According to the present invention, it is possible to provide an image evaluation method for obtaining a correlation between a pulse width and an electrostatic latent image at each overshoot.

It is the schematic which shows the input signal for making a laser light source light-emit. It is the schematic which shows the input signal which added the overshoot. It is the schematic which shows the input signal which changed the pulse width in steps. It is the schematic which shows the input signal which changed the overshoot in steps. It is the schematic which shows the input signal which changed the pulse width and the overshoot in steps. It is the schematic which shows the electrostatic latent image formed when the pulse width and overshoot of an input signal are changed. It is the schematic which shows a dot-shaped electrostatic latent image. It is a diagram showing the relationship between the latent image area and the pulse width by performing measurement while changing the pulse width and overshoot stepwise. It is a diagram showing the relationship between the latent image area and the pulse width by performing measurement while changing the pulse width and overshoot stepwise. It is a diagram showing the relationship between the latent image area and the pulse width by performing measurement while changing the pulse width and overshoot stepwise. It is the schematic which shows the input signal with the same pulse width and different overshoot. It is the schematic which shows the response waveform of the input signal with the same pulse width and different overshoot. It is a diagram which shows the correlation with a pulse width and a main scanning diameter. It is a diagram which shows the correlation with a pulse width and a main scanning diameter. It is the schematic which shows an electrostatic latent image of 1 dot. It is the schematic which shows the input signal which changed the average intensity | strength in steps, making overshoot and pulse width constant. It is the schematic which shows the input signal which changed the average intensity | strength and the overshoot in steps, making overshoot and pulse width constant. It is the schematic which shows the electrostatic latent image formed with the input signal which changed the average intensity | strength and the overshoot in steps, making overshoot and pulse width constant. It is a diagram which shows the relationship between the spatial frequency of an electrostatic latent image, light emission intensity, and an overshoot. It is the schematic which shows the electrostatic latent image formed by changing light emission intensity and overshoot in 3 steps. FIG. 6 is a schematic diagram for explaining the area and spatial frequency of an electrostatic latent image formed by changing the emission intensity in six steps with overshoot as a constant value, and the stability of electrostatic latent image formation. It is a diagram which shows the emitted light intensity measured by changing luminescence intensity in steps. It is a diagram which shows the measurement result which changed the overshoot in 3 steps and changed the light emission intensity every 10%. It is the schematic which shows the scanning optical system used for one Embodiment of this invention. It is the schematic which shows the image evaluation apparatus which employ | adopted one Embodiment of this invention. 1 is a schematic diagram of a main part of an electrophotographic apparatus to which an embodiment of the present invention can be applied. FIG. 3 is a schematic diagram illustrating a layer structure of a photoconductor used in an embodiment of the present invention and a state of charging and light irradiation. It is the schematic which shows the concept at the time of reflecting the result obtained by the image evaluation method and image evaluation apparatus in one Embodiment of this invention in an electrophotographic apparatus. It is the schematic which shows a vertical surface emitting laser. It is the schematic explaining the structure which measures the optical response waveform of a laser with a light receiver. It is the schematic explaining the optimal value of an overshoot.

  FIG. 1 shows an input signal for causing the laser light source to emit light in pulses. In the figure, the horizontal axis indicates time, and the vertical axis indicates signal intensity, and the pulse width time is several picoseconds to several hundred nanoseconds. The signal intensity on the vertical axis is a set value (arbitrary intensity), which is converted into an injection current value (unit ampere) to the laser light source. The input signal is a rectangular wave (a pulse is a rectangular shape that indicates a short time), but when the emitted light is actually measured with a detector such as a photodiode, the optical response waveform is rectangular. Instead, the rising portion is dull as shown in FIG. The photodiode used for measuring the optical response waveform is preferably one having a high-speed response, and preferably has a time resolution of several picoseconds in order to accurately measure the overshoot response waveform.

  FIG. 2 shows an input signal when overshoot is added. As shown in FIG. 2, the overshoot is to increase the intensity at the start of the pulse. Here, the increased intensity is called the peak intensity and the subsequent intensity is called the average intensity to distinguish them from each other. The value obtained by subtracting is represented by Iov. The peak intensity is generally about several percent to several tens of percent with respect to the average intensity. There is a trade-off relationship between the magnitude of the peak intensity and the lifetime of the laser light source. The larger the peak intensity, the shorter the lifetime of the laser light source. The intensity of overshoot is about several tenths of milliamperes to several milliamperes in terms of current value, and changes linearly with respect to the set value of overshoot. The overshoot width (duration) is a fraction of the pulse width to a few tenths or a few hundredths. For example, when the pulse width is 100 nanoseconds, the overshoot width is several nanoseconds. It is.

  FIG. 3 shows how the pulse width is changed stepwise, and the average intensity at this time is constant. FIG. 4 shows how the overshoot is changed stepwise, and the average intensity and the overshoot width at this time are constant. FIG. 5 shows how both the pulse width and the overshoot are changed stepwise. First, overshoot is determined (overshoot (1)), and the pulse width is changed (W (1) → W (2) → W (3)). Next, the overshoot is changed (overshoot (2)). Similarly, the pulse width is changed from W (1) → W (2) → W (3), and the overshoot is further changed (overshoot (3 )), And the pulse width is changed from W (1) → W (2) → W (3). Here, the pulse width and the overshoot are both changed in three steps, but the change step is not limited to this, and the more the change steps, the more accurate the measurement. This step change in overshoot includes a set value of zero.

  The above-described average intensity, pulse width, and overshoot are preferably configured such that these numerical values are input and executed on a screen of a personal computer connected to the evaluation device, for example. Further, the laser light source may be connected to the control electronic circuit board, and the electronic circuit board may be connected to the personal computer via the interface. The control electronic circuit board has a function of controlling light emission characteristics such as a pulse width, light emission intensity, and an overshoot set value of the laser light source.

  The state of the electrostatic latent image formed on the photoconductor when the pulse width and the overshoot are changed will be described with reference to FIG. This electrostatic latent image is formed by electrostatic latent image forming means which is a scanning optical system described later. In FIG. 6, the electrostatic latent image is formed from left to right (scanning direction), and the formation start position (or time) is Tr and the formation end position (or time) is Tf.

  First, FIG. 6A shows an electrostatic latent image formed with an input signal having a pulse width W (1) and an overshoot OV (1). This electrostatic latent image has a small formation start position (longitudinal size) and a little overshoot (for example, reference numeral 101 in FIG. 30 in the case of an optical response waveform). Next, assuming that the pulse width is W (2), the length of the electrostatic latent image extends from the left to the right by the pulse width as shown in FIG. 6B. At this time, the formation start position (size in the vertical direction) remains small. Further, if the pulse width is W (3), the length of the electrostatic latent image is further extended as shown in FIG. 6C, but the size of the formation start position remains small. The reason why the electrostatic latent image is small at the formation start position is that the amount of light is insufficient as shown by the light response waveform 101.

  Next, the overshoot is changed to OV (2), and an electrostatic latent image is formed with a pulse width W (1). At this time, as shown in FIG. 6D, the size of the electrostatic latent image at the formation start position is larger than that in FIG. 6A, and thereafter the same size. This is because the amount of light is sufficient. When the pulse width is further increased to W (2) and W (3), the length of the electrostatic latent image becomes longer as shown in FIGS. 6 (e) and 6 (f). At this time, the size of the electrostatic latent image at the formation start position does not change. Next, when the overshoot is changed to OV (3) and the pulse width is set to W (1), the formation start position of the electrostatic latent image becomes larger as shown in FIG. This is because the amount of light at the formation start position is large. When the pulse width is further increased to W (2) and W (3), the length of the electrostatic latent image becomes longer as shown in FIGS. 6 (h) and 6 (i). The electrostatic latent image at this time is large at the formation start position and slightly smaller thereafter.

  In the embodiment described above, the electrostatic latent image shown in FIGS. 6D, 6E, and 6F does not change in size in the vertical direction between Tr and Tf, but this is not necessarily preferable. is not. The state in which the size of the electrostatic latent image does not change corresponds to the fact that the integrated intensity of the photoresponse waveform is close to a rectangle.

  FIG. 7 shows a dot-like electrostatic latent image, and there are an area S and a length d as the size of the electrostatic latent image. The length d corresponds to the main scanning direction of the scanning optical system and can also be called the main latent image diameter.

  FIG. 8 shows the relationship between the latent image area S and the pulse width t when measurement is performed by changing the pulse width and overshoot stepwise by the above-described method. Naturally, as the pulse width t increases, the latent image area S increases, and this relationship can be approximated by a straight line in a region where the pulse width t is not too short, and the slope, intercept, correlation, etc. of the straight line can be obtained. At this time, the larger the overshoot, the closer the y-intercept approaches the origin, and a negative value when the overshoot is small. This means that an electrostatic latent image cannot be formed properly if the pulse width t is short and the overshoot is insufficient. That is, the latent image forming ability is high and preferable so that the electrostatic latent image can be formed in the region where the pulse width t is low and the electrostatic latent image forming ability is low.

  In a region where the pulse width t is short, the relationship between the latent image area S and the pulse width t may not be approximated by a straight line as shown in FIG. In this case, approximation is performed with an appropriate curve, and in particular, by obtaining an x-intercept where the latent image area S is zero, the minimum pulse width t required for forming an electrostatic latent image can be obtained.

  With the above-described configuration, by measuring the electrostatic latent image while changing the pulse width and the overshoot stepwise, the pulse width t and the latent image area can be determined from the correlation between the formed electrostatic latent image and the pulse width. The relationship with S is graphed, and the slope, intercept, correlation coefficient, etc. can be obtained by approximating with a straight line or curve. Thereby, it is possible to provide an image evaluation method for obtaining the correlation between the pulse width and the electrostatic latent image in each overshoot.

  In the graphs shown in FIG. 8 and FIG. 9, it can be seen that a preferable overshoot is OV3 because an electrostatic latent image can be formed even in a region where the pulse width is smaller as the overshoot is larger.

  In addition to the above measurement, it is assumed that the relationship between the pulse width t and the latent image area S as shown in FIG. 10 is obtained, and the desired specification is indicated by a broken line in the figure. In FIG. 10, OV (1) does not satisfy the specifications, but OV (2) and OV (3) satisfy the specifications. Further, OV (2) and OV (3) are close to each other, and it is understood that OV (2) is sufficient for the capability of forming an electrostatic latent image in consideration of, for example, the life of the laser light source. This is because the magnitude of the overshoot is proportional to the current injected into the laser light source, and the life of the laser light source is shortened as a larger current flows. As described above, a suitable overshoot value can be obtained by stepwise changing the pulse width of the laser light source and the overshoot value and obtaining the correlation between the pulse width and the size of the electrostatic latent image. it can. Accordingly, superiority or inferiority can be obtained from the correlation between the pulse width for each overshoot and the electrostatic latent image, and an image evaluation method for determining a suitable overshoot can be provided.

  In FIG. 6, it can be seen that when the dot-like electrostatic latent image has the same pulse width, the larger the overshoot, the larger the electrostatic latent image in the length direction. This is due to the characteristic of the laser light source that the longer the overshoot is set, the longer the light emission delay is eliminated. If input signals having the same pulse width and different overshoot are used as shown in FIG. 11, the response waveform is as shown in FIG. In FIG. 12, t1-t0 corresponds to the pulse width W of the input signal shown in FIG. In FIG. 12, the light emission end positions Tf are aligned and are t1. In contrast, the light emission start position Tr is different for each of OV (1), OV (2), and OV (3), and approaches 0 as the overshoot increases. This is because a delay occurs when the overshoot is insufficient, and the light emission start time is delayed. This delay time is a length of time and is one-dimensional, so it is preferable to use the length of the electrostatic latent image, which is also a one-dimensional quantity, to evaluate such overshoot characteristics, The spread of the pulse width is more preferable because it directly corresponds to the main scanning direction. As the pulse width increases, the length of the electrostatic latent image in the main scanning direction increases.

  When an electrostatic latent image is formed by changing the pulse width and overshoot stepwise, the main scanning diameter is measured, and the correlation between the pulse width and the main scanning diameter is graphed, as shown in FIG. . The main scanning diameter of the electrostatic latent image formed when the pulse width was changed from about 100 nanoseconds to about 400 nanoseconds was about 20 μm to 150 μm. When approximated by a curve as shown in FIG. 13, the x-intercept can be obtained, and it can be seen that an electrostatic latent image can be formed even in a region where the pulse width is short when the overshoot value is large. Further, it may be approximated by a straight line as shown in FIG. As described above, the correlation between the pulse width and the main scanning diameter of the electrostatic latent image is obtained to obtain a suitable value of the overshoot. By using the main scanning diameter which is a value directly corresponding to the pulse width for the size of the electrostatic latent image, a more accurate image evaluation method can be provided.

Here, a specific example of the above-described measurement will be described. A vertical surface emitting laser was used as the light source, and the wavelength was 780 nm. The intensity of the emitted light immediately after emitting the light source was measured with a power meter and found to be 1.4 mW. Under these conditions, a dot-shaped electrostatic latent image was formed using a scanning optical system as shown in FIG. Since the emitted light intensity is reduced to about 1/20 by the scanning optical system, the emitted light intensity reaching the photosensitive member is also this value. As described above, the size of the electrostatic latent image in the main scanning direction increases as the pulse width increases. On the other hand, the size in the sub-scanning direction is substantially constant. When the emitted light was measured by a general-purpose beam profile measuring apparatus (PHOTON manufactured by BeamScan), the size in the sub-scanning direction at the position of the photoconductor was about 60 μm (1 / e ^{2 with} respect to the maximum value of the beam profile). Profile cross-section at).

  FIG. 15A shows an electrostatic latent image of one dot, and this electrostatic latent image is formed from four quarter dots 1, 2, 3, 4 as shown in FIG. ing. One dot formed from each ¼ dot 1, 2, 3, 4 is formed by making four ¼ dots 1, 2, 3, 4 adjacent in the scanning direction in the scanning optical system. When expressing one dot in an electrophotographic apparatus, one dot is usually used. However, when the amount of light from the laser light source is low, four quarter dots may be combined into one dot. Particularly, in the case of a surface emitting laser in which a plurality of light sources are arranged, the amount of light (power) emitted from one light source is small, so that one dot is formed using the light emitted from four light sources. The size of each ¼ dot 1, 2, 3, 4 in the main scanning direction was about 50 μm as a result of measurement by the beam profile measuring apparatus described above. The beam size of each 1/4 dot 1, 2, 3, 4 is not limited to this, and may be 60 μm × 60 μm, 70 μm × 70 μm, 80 μm × 80 μm, or the like when the main scanning direction × sub-scanning direction. Here, if the pulse width of one dot is 100 nanoseconds, the pulse width of each quarter dot 1, 2, 3, 4 is 25 nanoseconds. By using 1 / integer of one dot as a unit of change in pulse width, a more accurate measurement of an electrostatic latent image can be performed. For example, if the total pulse width to be changed is 400 nanoseconds and the unit of change is every 25 nanoseconds, the change stage is 16.

  The laser light source may be a vertical surface emitting laser or an edge emitting laser. For example, the vertical surface emitting laser has a light emission wavelength (λ) of 780 nm and the number of light sources is 40 (an array of 8 × 5), and the edge surface light emitting laser has a light emission wavelength of 655 nm, but is not limited thereto. The photoreceptor may be used for, for example, a commercially available digital copying apparatus or a laser printer, and has a three-layer structure of UL, CGL, and CTL, where CTL is gallium phthalocyanine and has a thickness of 30 μm.

  Here, FIG. 16 is used to consider a case where the average intensity is changed stepwise while the overshoot and pulse width are constant in the input signal. In FIG. 16, both the overshoot (1) and the pulse width W (1) are constant, and the average intensity as the light emission intensity is changed stepwise as I (1), I (2), I (3). FIG. 17 shows a state where the overshoot is further changed stepwise from the state shown in FIG. In FIG. 17, the emission intensity has a relationship of I (1) <I (2) <I (3), and the overshoot also changes in three stages: overshoot (1), overshoot (2), and overshoot (3). Then, the electrostatic latent image formed on the photoconductor becomes as shown in FIG. As can be seen from FIG. 18, the larger the emission intensity or the larger the overshoot, the larger the electrostatic latent image formed. Note that the changes in the emission intensity and the overshoot are not limited to three steps, and may be other steps.

  When the length (main scanning diameter) of the electrostatic latent image obtained in FIG. 18 is measured and the length is d, the spatial frequency of the electrostatic latent image is 1 / 2d. For example, when the main scanning diameter of the electrostatic latent image is 50 μm (0.05 mm), the spatial frequency is 1 / (2 × 0.05) = 10 (cycle / mm).

  FIG. 19 is a graph showing the relationship between the spatial frequency of the electrostatic latent image shown in FIG. 18, the light emission intensity, and each overshoot. As can be seen from FIG. 19, the larger the emission intensity, the larger the electrostatic latent image, and thus the spatial frequency becomes smaller. Further, by approximating the graph with a curve, a plurality of frequency characteristics such as an approximate curve, its parameters, and a correlation coefficient can be obtained. Thereby, the image evaluation method which calculates | requires light emission intensity and a spatial frequency characteristic in each overshoot can be provided.

  When it is desired to realize a certain spatial frequency F in the graph shown in FIG. 19, it is determined how much light emission intensity is necessary with the curves of the overshoots OV (1), OV (2), and OV (3). It can be seen that the light emission intensity may be the smallest in the overshoot OV (3) (because I1 <I2 <I3). When realizing the same spatial frequency, the smaller the emission intensity, the better, and it can be determined that the overshoot OV (3) is suitable. Thereby, the superiority or inferiority of the characteristics is obtained from the spatial frequency characteristics for each overshoot, and an image evaluation method for determining a suitable overshoot can be provided.

  If the size of the electrostatic latent image does not change regardless of the light emission intensity, the formation of the electrostatic latent image is stable, but actually the size of the electrostatic latent image changes depending on the light emission intensity. Therefore, it can be said that the smaller the rate of change, the more stable the formation of the electrostatic latent image. This depends on the properties of the photoconductor, and the stability of the photoconductor can be estimated from the rate of change.

As shown in FIG. 18, the electrostatic latent image does not have an ideal circular shape, but has a slightly distorted shape. Therefore, it is considered that the radius is approximated by a circular shape. When the area of the electrostatic latent image is S, the radius r is r = (S / n) ^1/2 and the diameter is 2r. This diameter is the equivalent circle diameter.

  FIG. 20 shows an electrostatic latent image formed by changing the light emission intensity and the overshoot in three stages. Here, the emission intensity is the emission intensity (1) <the emission intensity (2) <the emission intensity (3). The formed electrostatic latent images are electrostatic latent image 1, electrostatic latent image 2, and electrostatic latent image 3, respectively, and areas S1, S2, and S3 and equivalent circle diameters d1, d2, and d3 of the electrostatic latent images are set. Ask for each. Here, when the spatial frequency is obtained using the equivalent circle diameter d2, the spatial frequency is 1 / (2 × d2). Next, regarding the electrostatic latent image 2, the stability of the electrostatic latent image formation with respect to the change in the light emission intensity shown above is defined by S3 / S1.

  FIG. 21 shows how an electrostatic latent image is formed and measured with the overshoot set to a certain value and the light emission intensity is changed in six steps to determine the area, spatial frequency, and stability of electrostatic latent image formation. In FIG. 21, the spatial frequency characteristics of the electrostatic latent image are represented by the area of the electrostatic latent image and the equivalent circle diameter.

  It is preferable that the stepwise change in emission intensity is constant. Consider measuring the light emitted from a laser light source with a power meter. When the emission intensity is changed stepwise and the emitted light intensity is measured each time, a linear relationship is obtained within a certain range as shown in FIG. The emission intensity at this time is I (2), a 10% decrease in I (2) is I (1), and a 10% increase in I (2) is I (3).

  FIG. 23 shows the measurement results obtained by changing the overshoot in three steps and changing the emission intensity every 10%. In this example, the light emission intensity was changed to 10 steps and approximated by a straight line. As shown in FIG. 23, it can be seen that the larger the overshoot, the more stable the electrostatic latent image is even at higher spatial frequencies. In other words, it is shown that the gentler the slope of the straight line, the more stable the electrostatic latent image is formed even at a high spatial frequency. For this reason, it is required that a higher overshoot is preferable. Thereby, the spatial frequency characteristic is defined by the area and the spatial frequency of the electrostatic latent image, and this appropriately represents the spatial frequency characteristic of the electrostatic latent image, so that a more accurate image evaluation method can be provided.

  In the above example, a vertical surface emitting laser having a light emission wavelength (λ) of 780 nm was used as the laser light source. When measured with a power meter, the emission intensity was in the range of approximately 10 μW to 70 μW at the position of the photoreceptor. The overshoot OV (3) injects a current of several mA into the laser, the overshoot OV (2) is about half the current amount of OV (3), and the overshoot OV (1) is zero current amount.

  In the above example, the change in overshoot has been described as three stages, but the change is not limited to three stages. The upper limit of the overshoot is determined by the current value that can be injected into the laser light source. The upper limit is a current value that does not destroy the laser light source, and the lower limit value is zero. How much width is provided between the upper limit value and the lower limit value depends on the control of the circuit and is, for example, a value of 4 bits. It is preferable that the light emission intensity can be set by, for example, a personal computer connected to the evaluation apparatus. The laser light source is preferably connected to an electronic circuit board for control, and the electronic circuit board is preferably connected to a personal computer via an interface.

  FIG. 24 shows an example of a scanning optical system. The scanning optical system includes a light source 131, a collimating lens 132, a cylindrical lens 133, a first mirror 134, a polygon mirror 135 as an electrostatic latent image forming unit, a first mirror, and a first mirror 134. A scanning lens 136, a second scanning lens 137, a second mirror 138, and the like are included. The polygon mirror 135 is rotationally driven at a high speed of around 10,000 rpm to several tens of thousands rpm around a rotation shaft 1351 by a driving means (not shown), and scans incident light. The scanning direction by the polygon mirror 135 is the main scanning direction, which corresponds to the longitudinal direction of the photoreceptor 139. The photosensitive member 139 is rotationally driven around a rotational shaft 1391 by a driving unit (not shown), and this rotational direction becomes the sub-scanning direction. An electrostatic latent image 1392 is formed on the photoreceptor 139 by light irradiation from the polygon mirror 135.

  In the configuration described above, the photoconductor 139 may not be included in the scanning optical system. An opening (aperture) (not shown) is provided between the light source 131 and the collimating lens 132, and the emission intensity of the emitted light emitted from the light source 131 is measured at this opening position, and the probe of the optical power meter is located at this position. Is arranged. The light emission intensity on the photoconductor 139 is measured at the position where the electrostatic latent image 1392 is formed. Examples of the light source 131 include a vertical surface emitting laser (VCSEL) having a wavelength (λ) of 780 nm, an edge emitting laser having a wavelength (λ) of 655 nm, and the wavelength is not limited thereto.

  FIG. 25 shows an image evaluation apparatus of the present invention. In the figure, an image evaluation apparatus 141 includes an electron gun 142, a condenser lens 143, a scanning lens 144, an objective lens 145, a light irradiation optical system 146 as an electrostatic latent image forming unit, a sample (photosensitive member) 147, a sample stage 1450, An electronic detector 1430, an interface 148, a vacuum pump 149, an electronic computer (computer and display) 1410 as information processing means, and the like are included. In the figure, reference numeral 1440 is an exaggerated and enlarged image of an electrostatic latent image of an arbitrary pattern formed by light irradiation on a charged photoconductor, and reference numeral 1420 is input data and observed electrostatic latent image data on a computer. -Each image processing is shown.

  In the above configuration, the means for generating charged particles for charging the photoconductor 147 as the sample is the electron gun 142 and the lenses 143, 144, and 145, and the means for irradiating the photoconductor 147 with light is a light irradiation optical system. 146, and means for observing the electrostatic latent image 1440 having an arbitrary pattern formed on the photoreceptor 147 are an electron gun 142, lenses 143, 144, 145, and an electron detector 1430 as electrostatic latent image measuring means. is there. The electron gun 142 may be used in, for example, a scanning electron microscope (SEM), and may be any one such as a hot filament or field emission.

Considering that the electron gun 142 is used to charge the photoconductor 147 with electrons, the electron gun 142 and the photoconductor 147 need to be placed in a vacuum. This is not a configuration unique to the present apparatus, but is similar to the SEM. For this reason, a vacuum pump 149 is disposed to evacuate the entire apparatus. In the above configuration, only one vacuum pump 149 is used for convenience, but a rotary pump, a turbo molecular pump, an ion sputtering pump, or the like may be used in combination depending on the degree of vacuum to be achieved. The charged particle is generally an electron having a negative polarity, but may be an ion such as an ion gun used in FIB (Focused Ion Beam). The ions used here are ion species such as Ga ⁺ . The condenser lens 143, the scanning lens 144, and the objective lens 145 are all magnetic field lenses or electrostatic lenses. The charged particle beam is scanned two-dimensionally on the photosensitive member 147 by the scanning lens 144.

  When charging the photoconductor 147, electrons are preferably used. When electrons are used, the charged potential on the surface of the photoconductor 147 is mainly influenced by the acceleration voltage of the electrons and the irradiation time. The higher the electron acceleration voltage, the higher the charged potential on the surface of the photoconductor 147. In the present invention, it is necessary to make the conditions equivalent to those of electrophotography. The conditions are an acceleration voltage (Vacc) of about 1.0 to several kV and an irradiation time of about several tens of seconds to several tens of minutes. When the charging potential is measured using, for example, a general-purpose surface potential meter, a value of about −600 to −1000 V is obtained, and the charging potential is equivalent to that of an actual copying apparatus or printer. Further, instead of changing the acceleration voltage, the charging potential may be adjusted by applying a bias voltage to the photoconductor 147 or the like.

  A means for forming an electrostatic latent image 1440 having an arbitrary pattern by irradiating light on the photoconductor 147 is a light irradiating optical system 146, specifically the scanning optical system shown in FIG. In the configuration shown in FIG. 25, the light irradiation optical system 146 is arranged inside the apparatus, but may be arranged outside the apparatus (in the atmosphere). In this case, in order to maintain the vacuum inside the apparatus, the image evaluation apparatus 141 is arranged by sealing a transparent window such as glass having high transmittance so that the light from the light irradiation optical system 146 is transferred to the photoreceptor 147. You can guide it to.

  The scanning optical system shown in FIG. 24 is provided with a vertical surface emitting laser or an edge emitting laser as the light source 131. These laser light sources are equipped with an electronic electric circuit for controlling light emission characteristics, and the control is performed by an electronic computer 1410 via an interface 148. The pulse width, light emission intensity, and overshoot set value described above are performed while viewing the screen of the electronic computer 1410. In FIG. 25, the sample photoconductor 147 is a flat plate (or a sheet), but may be a cylinder (or a drum like the photoconductor 139 shown in FIG. 24). The sample stage 1450 is configured to be three-dimensionally movable in the x direction, the y direction, and the z direction.

  Here, an outline of a general electrophotographic process will be described. The electrophotographic process is performed by charging, exposing, developing, transferring, cleaning, and fixing processes. In the development, a charger is used in a dark place to charge the surface of the photoconductor as an optical semiconductor. In the exposure, the charged photosensitive member is irradiated with light, and the charge of the portion irradiated with the light is removed, thereby forming an electrostatic latent image. Currently, negatives are the mainstream in digital equipment. In the development, toner, which is fine particles charged to a polarity opposite to that of the electrostatic latent image, is electrostatically attached to the electrostatic latent image. In the transfer, a recording paper as a recording medium is superimposed on the developed toner image, a charge disposed on the back side of the recording paper is applied to the recording paper with a charge opposite to the charged polarity of the toner, and the electrostatic force is applied to the toner. Transfer to recording paper. In the cleaning, residual toner remaining on the photosensitive member without being transferred is removed using a cleaner such as a blade or a magnetic brush. For fixing, the recording paper is sent to a heat roller fixing machine, and the toner adhering to the recording paper is fixed by heat and pressure. FIG. 26 shows an example of an electrophotographic apparatus that performs the above-described steps of charging, exposure, development, transfer, cleaning, and fixing.

  In the electrophotographic apparatus shown in FIG. 26, reference numeral 151 denotes a photoconductor (OPC), reference numeral 152 denotes a charging charger, reference numeral 153 denotes a light source for exposure, reference numeral 154 denotes toner, reference numeral 155 denotes recording paper, and reference numeral 156 denotes Reference numeral 157 denotes a transfer charger, reference numeral 157 denotes a fixing roller, reference numeral 158 denotes a cleaning blade, and reference numeral 159 denotes a static elimination light source. Corresponding to the above-described electrophotographic process, the photosensitive member 151 is charged by the charging charger 152 and then exposed by the exposure light source 153, and the toner 154 is electrostatically attached to the formed electrostatic latent image. Development is performed. After development, the toner image on the photoconductor 151 is transferred to the recording paper 155 by the transfer charger 156, and transfer is performed. The residual toner on the photoconductor 151 is removed by the cleaning blade 158, and cleaning is performed. The recording paper 155 on which the toner image is transferred is sent to the fixing roller 157 for fixing. In the fixing step, the toner is mainly fixed on a paper such as a recording paper 155, but may be fixed to a polymer material such as an overhead projector (OHP) sheet.

  The development process described above is classified into several methods, and the developer and toner used are classified according to the method. First of all, there are a dry method and a wet method. At present, the dry method is the mainstream because it is superior in speed. As the developer used in the dry process, there are a one-component developer having only a toner and a two-component developer having a toner and a carrier. The toner used for the one-component developer and the two-component developer is a non-magnetic toner. There is a magnetic toner. Most of the toner is composed of a polymer and may contain a colorant or the like, and its size is several μm to several tens of μm. The carrier is made of a metal-based material such as iron powder, and its size is several tens to several hundreds of μm.

  The photoconductor is a photoconductive material, and an organic electrophotographic photoconductor (OPC) is mainly used because it is low in cost and has good processability. Generally, OPC has a multilayer structure, and an intermediate layer is provided on a conductive support (conductive layer), and a charge generation layer (CGL) and a charge transfer layer (CTL) are stacked. In this configuration (forward layer configuration), the negative charging method is used, and in the configuration in which the stacking order of CGL and CTL is reversed, the positive charging method is used. There is also a single-layer photoreceptor using a material in which charge generation and charge transport are mixed. The intermediate layer is provided to prevent charge leakage and the like. FIG. 27 schematically shows the above-described layer configuration, charging and light irradiation.

  FIG. 27 shows a normal layer structure. In the figure, reference numeral 161 denotes a conductive layer, reference numeral 162 denotes a charge generation layer, reference numeral 163 denotes a charge transport layer, reference numeral 164 denotes irradiation light, and reference numeral 165 denotes hole charge transport. Reference numeral 166 denotes an electron, and reference numeral 167 denotes a hole. When light is applied to the OPC whose surface is charged, light is absorbed in the CGL and positive and negative charges are generated. As for the positive and negative charges, one charge is injected into the CTL and the other charge is injected into the conductive support by the electric field on the surface. The charge injected into the CTL passes through the CTL and reaches the surface of the photoreceptor, and cancels out the charge existing on the surface. Then, the irradiation light is scanned two-dimensionally to form a character or image pattern. The above-described charge generation or movement occurs according to this pattern, and the surface charge is canceled. This is an electrostatic latent image. In general, the thickness of CGL is about sub-μm, and the thickness of CTL is about several tens of μm. The electrostatic latent image is a character or an image, but the minimum unit is a dot.

  The principle of forming an electrostatic latent image on the photoreceptor is as described above. In the present invention, a configuration using electrons is suitable as a means for observing the electrostatic latent image. For this, the electron gun 142 shown in FIG. 25 can be used, but the current value needs to be reduced. When the current value at the time is several nA, the current value at the time of observation is set to several pA and one thousandth. This is to prevent the electrostatic latent image from disappearing due to electron irradiation, and the electrostatic latent image disappears faster as the electron beam having a higher current value is irradiated.

  The electron detector 1430 shown in FIG. 24 is a secondary electron detector. The observation electrons reach the photoconductor 147 and emit secondary electrons from the photoconductor 147. The principle that an electrostatic latent image can be measured is that secondary electrons are not emitted so much from a place where an electrostatic latent image is formed, and a large amount of secondary electrons are emitted from a place where an electrostatic latent image is not formed. By being released. The secondary electron detector 1430 includes a phosphor that emits light when irradiated with electrons, a light guide, and a photoelectron multiplier that doubles the emitted light. The secondary electron detector 1430 is a scanning type detector, and follows the scanning of the electron beam on the photoreceptor 147.

  Each component shown in FIG. 24 is controlled by the electronic computer 1410 via the interface 148. Detailed control of each component is performed by software in the electronic computer 1410. If the electrostatic latent image 1440 is formed in FIG. 24, the observed electrostatic latent image 1440 is output on the display of the electronic computer 1410, and various data and image processing are performed and output as a processing status 1420. The The electronic computer 1410 performs the above-described data processing, calculation, etc., and obtains a value to be evaluated. This data processing and calculation are not special, and can be performed using general-purpose data and image processing software. Thereby, the correlation between the pulse width and the size of the electrostatic latent image, the frequency characteristics of the light emission intensity and the size of the electrostatic latent image, and the preferred overshoot value obtained from these are determined.

  The image evaluation apparatus 141 described above may have apparatus restrictions. For example, the scanning range of the electron beam cannot be widened, or the size of the sample (photosensitive member 147) is limited. With respect to the electron beam search range, it is possible to scan a range (several mm squares) to some extent when sacrificing resolution on the order of several nanometers compared to a general-purpose SEM. However, many photoconductors are several centimeters in diameter and several tens of centimeters in the longitudinal direction, and it is difficult to scan the entire surface of the photoconductor at once, and partial scanning is repeated by moving the photoconductor. . In addition, when a photoconductor of this size is used as a sample, the sample stage becomes large, and a vacuum chamber needs to have a large capacity. Therefore, it is necessary to devise measures such as cutting out and measuring the photoconductor. In this case, scanning may be performed within a very narrow range.

  FIG. 28 shows a concept when the results obtained by the above-described image evaluation method and image evaluation apparatus are reflected in an electrophotographic apparatus such as a digital copying apparatus or a laser printer. First, the laser light source, scanning optical system, photoconductor, etc. to be used in the electrophotographic apparatus are determined as initial conditions, and the above-described evaluation (the overshoot is changed stepwise by changing the overshoot stepwise) using an actual machine or an experimental machine equipped with these. And the correlation between the size of the electrostatic latent image and the relationship between the light emission intensity and the frequency characteristic). Then, a determination is made as to whether or not this result has achieved a desired image specification. If it has been achieved, a suitable overshoot is determined, and if not, the initial conditions and device specifications are reviewed. The determined overshoot is stored in the storage means of the actual machine, and this overshoot is used at the time of image formation. The storage means gives this overshoot as an initial setting value to an electric / electronic circuit or an electronic computer attached to the apparatus and its software, and the electrophotographic apparatus forms a high-resolution and high-quality image based on this overshoot.

131 Laser light source
135 Electrostatic latent image forming means (polygon mirror)
139, 147, 151 Photoconductor 141 Image evaluation device 146 Electrostatic latent image forming means (light irradiation optical system)
153 Laser light source (light source for exposure)
1392, 1440 Electrostatic latent image 1410 Information processing means (electronic computer)
1430 Electrostatic latent image measuring means (electronic detector)

JP 2003-295696 A JP 2001-96794 A

Claims (8)

An image evaluation method for evaluating an electrostatic latent image formed on a photoreceptor,
Electrostatic latent image formation for forming a plurality of laser light sources, driving means for emitting laser light from each of the laser light sources, and scanning each of the laser light on the photoconductor to form an electrostatic latent image on the photoconductor Means and electrostatic latent image measuring means for measuring the electrostatic latent image,
One dot, which is the minimum unit of the electrostatic latent image, is formed by a plurality of laser light sources, and the driving means has a function of adjusting the pulse width and overshoot of the laser light source, and the pulse width is set to the one dot. after by the laser light source fraction to form forming a plurality of electrostatic latent image is changed stepwise as the variation of the pulse width determined the size of each of the electrostatic latent image, to further change the overshoot At the same time, the pulse width is changed stepwise by setting the amount of change in the pulse width to 1 / the number of laser light sources forming one dot, and a plurality of electrostatic latent images are formed to determine the size of each electrostatic latent image. that step has an image evaluation method characterized by using the relationship between the size of the electrostatic latent image with the pulse width by the stepwise changing overshoot repeating the process. The image evaluation method according to claim 1,
An image evaluation method characterized in that a suitable overshoot value is obtained from a correlation between the pulse width and the size of the electrostatic latent image. The image evaluation method according to claim 1 or 2,
An image evaluation method, wherein the electrostatic latent image has a dot shape, and the size of the electrostatic latent image is a length in a main scanning direction of the electrostatic latent image forming unit. An image evaluation method for evaluating an electrostatic latent image formed on a photoreceptor,
A vertical surface emitting laser having a plurality of light sources, driving means for emitting laser light from each of the light sources, and a static image for forming an electrostatic latent image on the photosensitive member by scanning each of the laser light on the photosensitive member. Using an electrostatic latent image forming means and an electrostatic latent image measuring means for measuring the electrostatic latent image,
One dot, which is the minimum unit of the electrostatic latent image, is formed by a plurality of laser light sources, and the driving means has a function of adjusting the pulse width and overshoot of each light source, and the pulse width is set to the one dot. Change the overshoot after forming a plurality of electrostatic latent images by stepwise changing the number of laser light sources to be formed as the amount of change in pulse width, and obtaining the spatial frequency characteristics of each electrostatic latent image. And changing the pulse width stepwise with a change amount of the pulse width of the number of laser light sources forming one dot to form a plurality of electrostatic latent images, and the spatial frequency characteristics of each electrostatic latent image have Ru seek step, image evaluation method characterized by determining a plurality of spatial frequency characteristics by repeating the process by gradually changing the overshoot. The image evaluation method according to claim 4,
An image evaluation method characterized in that a suitable overshoot value is obtained from the plurality of spatial frequency characteristics. The image evaluation method according to claim 4 or 5,
The image evaluation method according to claim 1, wherein the spatial frequency characteristic is determined from an area of a dot-like electrostatic latent image and a diameter when the electrostatic latent image is approximated by a circle. An image evaluation apparatus for performing the image evaluation method according to claim 1,
Charged particle generating means for generating charged particles for charging the photosensitive member, a vertical surface emitting laser having a plurality of light sources, driving means for emitting laser light from each of the light sources, and each of the laser light to the photosensitive member An electrostatic latent image forming unit that scans above and forms an electrostatic latent image on the photoconductor, an electrostatic latent image observation unit that observes the electrostatic latent image formed on the photoconductor, and the static An image evaluation apparatus comprising: information processing means for electronically processing desired information from the electrostatic latent image observed by the electrostatic latent image observation means. Storage means for reflecting the image evaluation result obtained by the image evaluation method according to any one of claims 1 to 6 or the image evaluation apparatus according to claim 7 in image formation and storing the preferred overshoot. An image forming apparatus comprising:

Images