JP2012053331A - Electrostatic latent image measurement apparatus and electrostatic latent image measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrostatic latent image measurement apparatus and an electrostatic latent image measurement method which measure an electrostatic latent image formed on a photoreceptor surface in high accuracy in micron order.SOLUTION: An electrostatic latent image measurement apparatus comprises an electron gun 11, a light source 100, an acousto-optic deflection element 103, a polygon mirror 105, a second electron detector 24 and correction means which corrects the effect of changed drive frequency of the acousto-optic deflection element 103 on diffraction. A scan to a two-dimensional direction is achieved by scanning to a one-dimensional direction with diffraction of the acousto-optic deflection element 103 and scanning to another one-dimensional direction with deflection of the polygon mirror 105.

Description

  The present invention relates to an electrostatic latent image measuring device and an electrostatic latent image measuring method capable of measuring the potential distribution and charge distribution of an electrostatic latent image generated on the surface of a photoreceptor with high resolution on a micron scale. .

  In recent years, there has been an increasing demand for high-speed image formation for multicolor image forming apparatuses, and image forming apparatuses have come to be used for simple printing as on-demand printing systems, and high quality and high accuracy of images are required. ing.

  In an electrophotographic image forming apparatus, the process quality in each process of charging, exposure, development, transfer, and fixing greatly affects the quality of the finally output image. Above all, the state of the electrostatic latent image generated on the photoreceptor by the exposure process is an important factor that directly affects the behavior of the toner particles. Therefore, it is extremely important to realize an image forming apparatus that can obtain a high-quality image by accurately measuring the state of the electrostatic latent image after exposure and accurately evaluating the quality of the electrostatic latent image. Therefore, high-precision measurement on the micron scale is required.

  As a method for calculating the charged potential of the object to be measured on a micron scale, an electrostatic attractive force that occurs as an interaction between the electrostatic latent image and the cantilever when a sensor head such as a cantilever is brought close to a sample having a potential distribution There is a method of measuring an induced current and converting it into a potential distribution.

  However, in order to use this method, it is necessary to bring the sensor head close to the object to be measured. For example, in order to obtain a spatial resolution of 10 μm, the distance between the sensor head and the object to be measured needs to be 10 μm or less.

However, when the sensor head and the object to be measured are brought close to each other, the following major problems occur. It is.
・ Absolute distance measurement is required.
• Measurement takes time, and the state of the latent image changes during that time.
・ Discharge and adsorption occur.
・ The sensor itself disturbs the magnetic field.

  Further, as described in Patent Document 1 and the like, there is a method for measuring an electrostatic latent image using an electron beam, but the sample is limited to an LSI chip or a sample that can store and hold an electrostatic latent image. Has been. That is, a normal photoconductor that causes dark decay cannot be measured. Since a normal dielectric can hold a charge semipermanently, even if measurement is performed over time after forming a charge distribution, the measurement result is not affected. However, in the case of a photoreceptor, since the resistance value is not infinite, the charge cannot be held for a long time, dark decay occurs, and the surface potential decreases with time. The time that the photoconductor can hold the charge is at most several tens of seconds even in the dark room. Therefore, even if an electrostatic latent image is observed in an electron microscope (SEM) after charging and exposure, the electrostatic latent image disappears in the preparation stage.

  Further, as described in Patent Document 2 or 3, since an apparatus for measuring an electrostatic latent image is provided in an apparatus for forming an electrostatic latent image, even a photoconductor sample having dark attenuation may be used. There is a method for measuring an electrostatic latent image. However, in the exposure optical system in this system, scanning with an exposure beam is possible only for one dimension, and does not support two-dimensional scanning. In an actual image forming apparatus, the exposure optical system and the photosensitive drum rotate to perform image formation by scanning the exposure beam two-dimensionally. Therefore, it is necessary to measure the electrostatic latent image in accordance with the actual machine. .

  The present invention relates to an electrostatic latent image measuring apparatus and an electrostatic latent image measuring device that measure an electrostatic latent image generated on the surface of a photoconductor with high accuracy on the order of microns by a two-dimensional exposure optical system using an acousto-optic deflection element. It aims to provide a method.

  (1) The present invention is an electrostatic latent image measuring device for measuring an electrostatic latent image formed on the surface of a photoconductor, wherein the photoconductor is charged with a charged particle beam and charged, and the charged A light source that irradiates and exposes a light beam to the photoconductor, an acousto-optic deflection element that diffracts the light beam from the light source, a deflecting unit that deflects the light beam in a direction perpendicular to the diffraction direction, and a secondary from the charged photoconductor A detecting means for detecting electrons, and a correcting means for correcting the influence of a change in the driving frequency of the acoustooptic deflecting element on the diffraction, the scanning in the one-dimensional direction is diffraction by the acoustooptic deflecting element, and the other one-dimensional The main feature is that it is an electrostatic latent image measuring device that performs scanning in the two-dimensional direction by scanning in the direction by deflection by the deflecting means.

  Although not particularly limited in the present invention, the correcting means corrects a change in diffraction efficiency accompanying a change in driving frequency of the acousto-optic deflecting element by changing a voltage applied to the acousto-optic deflecting element according to the driving frequency. It is preferable to do.

  Although not particularly limited in the present invention, the correcting means changes the amount of light reaching the surface of the photosensitive member accompanying the change in the driving frequency of the acousto-optic deflection element, and changes the amount of light from the light source according to the driving frequency. It is preferable to correct by.

  Further, although not particularly limited in the present invention, it is preferable to further include a light attenuating means for changing the amount of light reaching the surface of the photoreceptor.

  Further, although not particularly limited in the present invention, it is preferable to further include a beam size correcting means for adjusting the diameter of a beam spot formed on the surface of the photosensitive member by a light beam.

  Further, although not particularly limited in the present invention, it is preferable that the beam size correcting means adjust the distance between the light source and the photosensitive member.

  Further, although not particularly limited in the present invention, it is preferable that the beam size correction means adjust the incident angle of the light beam incident on the photosensitive member.

  (2) The present invention relates to an electrostatic latent image measuring method for measuring an electrostatic latent image formed on the surface of a photoreceptor, a charging step of irradiating and charging a charged particle beam to the photoreceptor, and charged photosensitive An exposure process for irradiating a body with a light beam and exposing, a diffraction process for diffracting the light beam from a light source by an acousto-optic deflection element, a deflection process for deflecting the light beam in a direction perpendicular to the diffraction direction, and a charged photoconductor The main feature is that the electrostatic latent image measurement includes a detection step of detecting secondary electrons and a correction step of correcting the influence of a change in driving frequency of the acousto-optic deflection element on diffraction.

  Although not particularly limited in the present invention, the correction step can correct the change in diffraction efficiency associated with the driving frequency of the acousto-optic deflection element by changing the voltage acting on the acousto-optic deflection element according to the driving frequency. preferable.

  Although not particularly limited in the present invention, the correction step corrects a change in the amount of light reaching the surface of the photosensitive member due to the drive frequency of the acousto-optic deflection element by changing the amount of light from the light source according to the drive frequency. It is preferable to do.

  Although not particularly limited in the present invention, it is preferable to change the light amount of the light source within the range of the single mode oscillation region in the step of correcting the light amount from the light source.

  Although not particularly limited in the present invention, in the step of correcting the light amount from the light source, it is preferable to change the light amount of the light source by using an optical attenuating means outside the range of the single mode oscillation region.

  Further, although not particularly limited in the present invention, it is preferable to further include a beam size correcting step for adjusting the diameter of a beam spot formed on the surface of the photosensitive member by a light beam.

  Further, although not particularly limited in the present invention, the beam size correction step is preferably performed by adjusting the distance between the light source and the photosensitive member.

  Although not particularly limited in the present invention, the beam size correction step is preferably performed by adjusting the incident angle of the light beam incident on the photoconductor.

  According to the present invention, an electrostatic latent image generated on the surface of a photoreceptor can be measured with high accuracy on the order of microns. Further, according to the present invention, the change in the amount of light on the sample image surface due to the difference in diffraction efficiency due to the difference in the driving frequency of the acousto-optic deflection element is corrected, and the light amount on the sample image surface is made constant regardless of the driving frequency. Can keep.

It is a model figure which shows the example of the electrostatic latent image measuring device which concerns on this invention. It is a perspective view which shows the exposure optical system of the electrostatic latent image measuring device of FIG. 1 in detail. It is a perspective view which shows the example of the light source of the exposure optical system of FIG. FIG. 3 is a model diagram and a graph showing an example of an acousto-optic deflection element of the exposure optical system in FIG. 2. It is a model figure which shows another example of the electrostatic latent image measuring device which concerns on this invention. It is a top view which shows the example of an electrostatic latent image pattern. It is a model figure which shows the principle of the charge distribution detection by a secondary electron. It is a graph which shows the diffraction efficiency change by the drive frequency of an acousto-optic deflection element. It is a graph which shows the change of the beam diameter by the change of LD drive current. It is a flowchart which shows the Example of the electrostatic latent image measuring method which concerns on this invention. It is a graph which shows the relationship between the voltage applied to an acousto-optic deflection | deviation element, and an intensity | strength modulation voltage. FIG. 12 is a graph showing the state of FIG. 11 after correcting the diffraction efficiency change by the intensity modulation voltage. It is a block diagram which shows the control system of an acousto-optic deflection element. It is a block diagram which shows another control system of an acousto-optic deflection | deviation element. It is a front view shown about a variable ND filter. It is a flowchart which shows another Example of the electrostatic latent image measuring method which concerns on this invention. It is a graph which shows the difference in the depth characteristic by the difference in LD drive current. It is a flowchart which shows another Example of the electrostatic latent image measuring method which concerns on this invention. It is a model figure shown about the beam diameter thickening of the subscanning direction in a sample image surface. It is a model figure which shows the beam diameter change correction | amendment by adjustment of the incident angle of an exposure beam. It is a flowchart which shows another Example of the electrostatic latent image measuring method which concerns on this invention. It is a model figure which shows the incident angle of the exposure beam in a drum-shaped sample. It is a graph which shows the beam diameter change correction | amendment by height adjustment about the sample of FIG. It is a flowchart which shows another Example of the electrostatic latent image measuring method which concerns on this invention.

  Embodiments of an electrostatic latent image measuring device and an electrostatic latent image measuring method according to the present invention will be described below with reference to the drawings.

  First, the configuration of an electrostatic latent image measuring apparatus that measures an electrostatic latent image on a photoreceptor will be described with reference to FIG.

  The electrostatic latent image measurement apparatus 1 includes a charged particle irradiation unit 50, a sample mounting table 60, an exposure optical system 22, a secondary electron detector 24, and an information processing unit 80. Each of these components is connected to a power source (not shown), and the operation is controlled by a control unit including a host computer 90.

  The charged particle irradiation unit 50 includes an electron gun 11 for generating an electron beam as a charged particle beam, an extraction electrode (extractor) 12 that controls the electron beam, an acceleration electrode 13 that controls the energy of the electron beam, An electrostatic lens (condenser lens) 14 for focusing an electron beam generated from an electron gun, a beam blanking electrode (beam blanker) 15 for ON / OFF control of the electron beam, a partition plate 16, and an electron beam A movable diaphragm 17 for controlling the current density of the irradiation current (number of irradiation potentials per unit time), a sting meter 18 for correcting astigmatism of the electron beam that has passed through the beam blanking electrode 15, and a sting meter 18 A scanning lens (deflection electrode) 19 that is a deflection coil for scanning the electron beam that has passed through, and a scanning lens 19 An electrostatic objective lens 20 which again focusing an electron beam which has passed through, a, a beam exit opening 21. A driving power source (not shown) is connected to each lens. Here, the charged particles refer to particles that are affected by an electric field or a magnetic field such as an electron beam or an ion beam. When charging using an ion beam, a liquid metal ion gun or the like is used instead of the electron gun 11.

  Tungsten or Lab 6 is used for the cathode of the electron gun 11. When the photoreceptor 23 is charged by the electron gun 11, scanning using an electron beam is performed in order to uniformly charge the surface of the photoreceptor 23. Instead of scanning and charging the surface of the photoconductor 23, an electron beam may be diffused and charged over the entire surface of the photoconductor 23.

  The sample mounting table 60 is a table on which a plane for mounting the photoconductor 23 is formed. After the photoconductor 23 is mounted on the sample mounting table 60, the inside of the casing of the electrostatic latent image measuring device 1 is evacuated using a vacuum pump (not shown), and the charging characteristics are evaluated.

  As shown in FIG. 2A, the exposure optical system 22 includes an LD (laser diode) 100 having a sensitivity with respect to the photosensitive member, a collimating lens 101, an aperture 102, an acoustooptic deflecting element 103, a cylinder lens 104, a polygon mirror. 105, a scanning lens 106, a synchronization detection mirror 107, a synchronization detection means 108, and the like, and a desired beam diameter and beam profile can be generated on the photoconductor sample. The light source may be a multi-beam light source such as a vertical cavity surface emitting laser (VCSEL) as shown in FIGS. The acousto-optic deflection element 103 can modulate the angle of the laser beam by modulating its drive frequency.

  In the exposure optical system 22 according to the present embodiment, the acousto-optic deflection element 103 performs scanning in the one-dimensional direction and the polygon mirror 105 performs scanning in another one-dimensional direction. Scanning is possible.

  The secondary electron detector 24 is a detector such as a scintillator or a photomultiplier tube.

  The information processing unit 80 includes an electron detection unit 81, a signal processing unit 82, a measurement result output unit 83, and an image processing unit 84. The electron detector 81 converts the detection output of the secondary electron detector 24 into an electrical signal corresponding to secondary electrons. The signal processing unit 82 samples the electric signal converted by the electron detection unit 81 at an appropriate sampling time. The measurement result output unit 83 performs various analyzes based on the electrical signal sampled by the signal processing unit 82. The image processing unit 84 converts the analysis result from the measurement result output unit 83 into image data and outputs the image data to a display (not shown) or the like.

  Next, the electroacoustic deflection element 103 described above will be described with reference to FIG. The acousto-optic deflection element 103 is an element that diffracts a traveling laser beam by generating an ultrasonic wave in an optical medium. Since there is no mechanical movable part, high-speed scanning can be realized.

The electroacoustic deflection element 103 is formed by adhering an ultrasonic transducer such as a piezoelectric element to an acoustooptic medium made of a single crystal or glass such as tellurium dioxide (TeO ₂ ) or lead molybdate (PbMo0 ₄ ). By applying an electrical signal to the piezoelectric element from the outside to generate ultrasonic waves and propagating the ultrasonic waves in the acousto-optic medium, it is possible to form a periodic refractive index density in the medium.

Laser light passing through the medium is diffracted by Bragg diffraction, and incident light generates diffracted light of ± 1, 2,. The angle θ between the 0th-order diffracted light and the 1st-order diffracted light is expressed by the following equation where λ is the wavelength of light in the air, fa is the fundamental acoustic wave frequency, and Va is the acoustic wave velocity.
θ = λ · fa / Va

In order to change the deflection angle by Δθ, the fundamental frequency fa is shifted by Δfa. That is, if the acoustic wave frequency modulation is Δfa, Δθ is expressed by the following equation.
Δθ = λ · Δfa / Va

  By driving this element with a voltage controlled oscillator (VCO) and an RF amplifier, the light beam can be scanned in the sub-scanning direction. FIG. 4B shows the relationship between the voltage input to the VCO and the output frequency. As can be seen from FIG. 4B, the light flux can be deflected in a desired direction by inputting an appropriate voltage to the VCO and changing the frequency.

Specifically, when the acoustic wave velocity of TeO ₂ is Va = 650 m / s, fa = 50 MHz, and λ = 655 nm,
θ = 655 × 10 ⁻⁹ × 50 × 10 ⁶ /650=50.38 mrad
It becomes.

By the way, the acousto-optic deflection element 103 has a delay in order to obtain predetermined optical characteristics after giving a modulation signal. This response time (access time) Tres can be expressed by the following equation, where D is the beam size.
Tres = D / Va
Specifically, when D = 5 mm, Tres = 7.7 μs.

For this reason, when the scanning frequency of the changing means by reflection is fv, the response time is
D / Va <1 / fv
What should I do?

That is, when the rotation speed of the polygon mirror 105 is Rm (rpm) and the number of surfaces of the polygon mirror 105 is N, the scanning frequency fv of the polygon scanner is fv = Rm / 60 × N.
And
Va ≧ D × fv
It is necessary to become. For example, in the case of D = 7 mm and fv = 6 kHz, it is necessary to use the acousto-optic deflection element 103 having the characteristics of Va ≧ 42 m / s.

  The configuration of the above-described electrostatic latent image measurement apparatus 1 will be further described with reference to FIG.

  The exposure optical system 22 is preferably disposed outside the vacuum chamber so that vibrations and electromagnetic waves generated when driving the exposure optical system 22 and the polygon mirror 105 do not affect the trajectory of the electron beam. By moving the exposure optical system 22 away from the electron beam trajectory position, the influence of disturbance can be suppressed. It is desirable that the light beam from the exposure optical system 22 enter the vacuum chamber through a transparent incident window.

  In the present embodiment, as shown in FIG. 5, an incident window 31 is provided at which the light beam emitted from an external light source can enter the vacuum chamber 30 at a position of 45 ° with respect to the vertical axis of the vacuum chamber 30. The exposure optical system 22 is arranged outside the vacuum chamber 30. In FIG. 5, the exposure optical system 22 includes a light source 100, a scanning lens 106, a synchronization detection means 107, an acoustooptic deflection element 103, an optical deflector (polygon mirror) 105, and the like. The housing 32 holding the exposure optical system 22 covers the entire exposure optical system 22 and is configured to shield external light (harmful light) incident on the inside of the vacuum chamber 30.

  Since the exposure optical system 22 is arranged away from the vacuum chamber 30, vibrations generated when driving an optical deflector such as the polygon mirror 105 are not directly propagated to the vacuum chamber 30, and the influence thereof can be suppressed. . Further, although not shown in FIG. 5, if a damper is inserted between the structure 33 and the vibration isolation table 34, a higher vibration isolation effect can be obtained.

  A method for measuring an electrostatic latent image by the above-described electrostatic latent image measuring apparatus 1 will be described. First, charging is performed by irradiating the photosensitive member sample 23 with an electron beam. The acceleration voltage | Vacc | is set higher than the acceleration voltage at which the secondary electron emission ratio is 1. Thereby, since the amount of incident electrons exceeds the amount of emitted electrons, electrons are accumulated in the sample, and the photosensitive member sample 23 is charged up. As a result, negative uniform charging occurs in the sample 23. A desired charging potential can be formed by appropriately setting the acceleration voltage and the irradiation time.

  Next, the photosensitive sample 23 is exposed by two-dimensional scanning using the exposure optical system 22. This two-dimensional scanning is realized by diffraction in the sub-scanning direction by the acousto-optic deflection element and deflection in the main scanning direction by the polygon mirror 105.

The exposure optical system 22 is adjusted so as to form a desired beam diameter and beam profile. The required exposure energy is a factor determined by the characteristics of each photoconductor, but is usually about ² to 10 mJ / m ² . For a photoreceptor sample with low sensitivity, ten or more mJ / m ² may be required. The charging potential and the required exposure energy are preferably set in accordance with the characteristics of the photoconductor and the process conditions of electrostatic latent image measurement.

  In addition, by arbitrarily setting conditions such as beam spot diameter, duty, image frequency, writing density, and image pattern, it is possible to perform latent image measurement under various conditions. As shown in FIG. 6, as an image pattern, (a) 1-dot isolated (1by1), (b) 2by2, (c) 1-dot grid, (d) sub-scan 1-dot line, (e) sub-scan pitch Various patterns such as uneven one dot line can be formed on the photoconductor sample 23.

  Next, the formed electrostatic latent image is measured. The photoreceptor sample 23 is scanned with an electron beam, and the emitted secondary electrons are detected with a scintillator and converted into an electric signal to observe a contrast image. In this case, since the secondary electron detection amount of the charging unit is large and the secondary electron detection amount of the exposure unit is small, a bright and dark contrast image is generated. Since the dark portion can be regarded as a latent image portion by exposure, the light and dark boundary when spot exposure is performed without scanning the beam is the latent image diameter.

  If there is a charge distribution on the surface of the sample 23, an electric field distribution corresponding to the surface charge distribution is formed in the space above the sample 23. For this reason, the secondary electrons generated by the incident electrons are pushed back by this electric field, and the amount reaching the detector 24 may decrease. Accordingly, at the charge leak portion, the exposed portion is black and the non-exposed portion is white, and a contrast image corresponding to the surface charge distribution can be measured.

  FIG. 7A illustrates the potential distribution in the space between the secondary electron detector 24 and the sample 23 in the form of contour lines. The surface of the sample 23 is in a state of being uniformly charged to a negative polarity except for a portion where the potential is attenuated by light attenuation. Since a positive potential is applied to the secondary electron detector 24, the potential increases in the potential contour line group indicated by the solid line as it approaches the secondary electron detector 24 from the surface of the sample 23.

  Accordingly, the secondary electrons el1 and el2 generated at the points Q1 and Q2 in FIG. 7A, which are parts of the sample 23 that are uniformly charged to the negative polarity, are attracted to the positive potential of the secondary electron detector 24. Then, it is displaced as indicated by arrows G 1 and G 2 and is captured by the secondary electron detector 24.

  On the other hand, in FIG. 6A, the point Q3 is a portion where the negative potential is attenuated by light irradiation, and the arrangement of the potential contour lines is indicated by a broken line in the vicinity of the point Q3. In this partial potential distribution, the closer to the point Q3, the higher the potential. In other words, the secondary electron el3 generated in the vicinity of the point Q3 is acted on by the electric force restrained on the sample 23 side as indicated by the arrow G3. For this reason, the secondary electron el3 is captured in a so-called potential hole indicated by a broken potential contour line and cannot move toward the secondary electron detector 24.

  FIG. 7B schematically shows the potential holes. That is, the intensity of the secondary electrons detected by the secondary electron detector 24 (the number of secondary electrons) is such that the portion with the high intensity is “the ground portion of the electrostatic latent image (the portion that is uniformly negatively charged, FIG. 7 (a), which is represented by the points Q1 and Q2), a portion having a low intensity is represented by “an electrostatic latent image portion (a portion irradiated with light, represented by a point Q3 in FIG. 7A). Part) ”.

  Therefore, if the electrical signal obtained by the secondary electron detector 24 is sampled at an appropriate sampling time by the signal processor, the surface potential distribution: V (X, Y) with the sampling time T as a parameter as described above. Can be specified for each “small area corresponding to sampling”, and the signal processing unit configures the surface potential distribution (potential contrast image): V (X, Y) as two-dimensional image data, which is then processed by the information processing unit. When output from 80 and displayed on the display, an electrostatic latent image is obtained as a visible image.

  For example, if the intensity of the secondary electrons to be captured is expressed in terms of brightness, the image portion of the electrostatic latent image is dark and the ground portion has a bright contrast, and is expressed as a light-dark image corresponding to the surface charge distribution ( Output). Of course, if the surface potential distribution is known, the surface charge distribution can also be known.

  According to the above-described configuration, a method of irradiating a sample having a surface charge distribution or a surface potential distribution with a charged particle beam and measuring the state of the sample's charge distribution or potential distribution using a detection signal obtained by the irradiation. In this case, it is possible to grasp the electrostatic characteristics of the photoreceptor by measuring the latent image state when the exposure conditions are changed.

  Finally, the photoconductor sample 23 is neutralized using the LED 25 to prepare for the next measurement. The charged charges generated on the photoreceptor sample can be eliminated by irradiating light.

  By performing the above process, an electrostatic latent image formed under desired conditions can be measured.

  By the way, the acousto-optic deflection element 103 that controls the sub-scanning position of the exposure beam has undesirable characteristics in measuring the latent image. For example, the acousto-optic deflection element 103 has different diffraction efficiencies as shown in FIG. When a two-dimensional pattern latent image is formed and analyzed, it is desirable that the amount of light on the image plane at each diffraction angle is constant. For this reason, a correction mechanism that corrects a change in the amount of light on the sample image plane due to the drive frequency is required.

  Further, it is desirable that the exposure beam incident on the acousto-optic deflection element 103 is single mode oscillation. In the case of multimode oscillation, the light condensing characteristic is remarkably deteriorated. This is because the deflection by the acousto-optic deflection element 103 is a deflection using Bragg diffraction, and if there is oscillation at different wavelengths, diffraction occurs at diffraction angles corresponding to the respective wavelengths.

  Further, the oscillation characteristics of the LD, which is an exposure light source, vary depending on the drive current If. Usually, laser oscillation starts with a drive current of about If = 30 mA, which is the oscillation threshold, but under conditions where the drive current is small, even a single mode oscillation laser performs multimode oscillation including other modes, and the image The amount of light changes on the surface. That is, when the LD driving current is changed, the beam size on the sample image plane at each driving current changes.

  FIG. 9 shows changes in the beam diameter depending on the LD drive current amount. It can be seen that while the beam diameter in the main scanning direction is constant, the beam diameter in the sub-scanning direction, which is the deflection direction of the acousto-optic deflection element, changes significantly.

  In order to reduce the dot diameter, which is one of the problems of the image forming apparatus, it is important to evaluate the influence on the latent image due to the change in the amount of light. However, the characteristics of the acousto-optic deflection element described above are disadvantageous in measuring this effect. Therefore, a mechanism for correcting the change in the beam diameter is required so that the light quantity on the image plane can be changed while keeping the beam size on the sample image plane constant.

  Therefore, by using a mechanism that corrects and adjusts the change in the light amount and the change in the beam diameter, which will be described later, the change in the light amount due to the change in the diffraction efficiency and the change in the beam diameter due to the change in the drive current of the LD The influence on measurement can be ignored, and a suitable measurement environment can be realized.

  A method for measuring a two-dimensional latent image pattern by two-dimensional exposure will be described with reference to FIG. Here, N represents the total number of scanning lines required for the two-dimensional exposure pattern to be exposed, and I represents the number of the line on which scanning is performed.

  First, in step S1, the LD drive current is determined according to the sample image plane light quantity to be measured.

  Next, in step S2, the change in the beam diameter due to the change in the LD drive current is corrected using a beam diameter change correction mechanism described later, and the beam size is determined.

  Next, in step S3, the photosensitive member sample is irradiated with an electron beam to uniformly generate charged charges on the photosensitive member sample.

  Next, in step S4, the sub-scanning position of the exposure beam corresponding to the I-th line is determined according to the exposure pattern, and the drive frequency of the acousto-optic deflection element corresponding to that position is determined.

  Next, in step S5, a change in diffraction efficiency due to the drive frequency of the acousto-optic deflection element is corrected using an image plane light quantity correction mechanism described later.

  Next, in step S6, the acoustooptic deflector is driven based on the conditions determined in the previous steps, and scanning and exposure are performed for one line of the two-dimensional exposure pattern. The line scanned at this time is defined as the I-th line in the flowchart of FIG.

  Next, in step S7, it is determined whether or not the scanning line I has reached the total number N of lines to be exposed. If I has not reached N, the process returns to step S4, and scanning and exposure are performed by repeating S4 to S7 again for the next scanning line (I + 1). In this way, by repeating S4 to S7 until the total number of lines N is reached, a two-dimensional latent image is formed.

  If I has reached N in step S7, the process proceeds to step S8. In step S8, electrostatic latent image measurement is performed by irradiating the specimen on which the latent image pattern is formed with an electron beam and detecting secondary electrons.

  After the electrostatic latent image is measured in step S8, the photoconductor sample is neutralized using the LED in step S9, and the next measurement is prepared.

  A method for correcting a change in diffraction efficiency in order to correct a change in the amount of light reaching the surface of the photosensitive member accompanying a change in the drive frequency of the acousto-optic deflector used in the above-described two-dimensional latent image pattern measurement method will be described. . Here, a method will be described in which the intensity modulation voltage acting on the acousto-optic deflection element is changed for each drive frequency, the diffraction efficiency at each diffraction angle is made constant, and the change in light quantity is corrected.

  The intensity modulation voltage is a voltage applied to the high frequency amplifier. The ultrasonic intensity acting on the acousto-optic deflection element is modulated through the ultrasonic transducer.

The diffracted light intensity of the acoustooptic deflector is as follows: diffracted light intensity is I ₁ , incident light intensity is I ₀ , incident light wavelength is λ ₀ , element performance index is M ₂ , acoustic beam width is L, and acoustic beam height is H Assuming that the ultrasonic power is P, the relationship of Equation 1 and Equation 2 is established.

  Thus, the diffracted light intensity of the acousto-optic deflection element increases and decreases as the ultrasonic intensity increases and decreases. Therefore, the diffraction efficiency can be changed by changing the intensity modulation voltage. By utilizing this characteristic and appropriately setting the intensity modulation voltage for each diffraction angle, the diffraction efficiency at each ultrasonic intensity can be made constant.

  FIG. 11 shows corrected intensity modulation voltage characteristics for correcting the difference in diffraction efficiency shown in FIG. By acquiring the correction intensity modulation voltage under a plurality of points in advance under the use conditions of the image forming apparatus and specifying the intensity modulation voltage corresponding to each diffraction angle at the time of scanning using the correction intensity modulation voltage characteristic obtained by the approximate expression, As shown in FIG. 12, the diffraction efficiency within the scanning range can be kept constant.

  FIG. 13 shows the configuration of the control system 2 for the acousto-optic deflection element. The latent image pattern information formed from the host computer 90 is sent to the control board 85 that controls the exposure conditions such as the exposure optical system and the mechanical shutter. The control board 85 sends the respective information to the acousto-optic deflection element controller 86 in order to designate the drive frequency and intensity modulation voltage of the acousto-optic deflection element 103 according to the latent image pattern information. The acousto-optic deflection element control unit 86 receives information on the correction intensity modulation voltage corresponding to the drive frequency from the correction intensity modulation voltage memory 87 storing the correction intensity modulation voltage characteristic obtained in advance, and based on the information. The acousto-optic deflection element 103 is driven.

  In this way, the control system 2 functions as a means for correcting a change in the amount of light reaching the surface of the photosensitive member due to a change in the drive frequency of the acousto-optic deflection element. As a result, the change in the amount of light due to the change in diffraction efficiency can be ignored, and the latent image measurement of the two-dimensional pattern can be performed under suitable conditions.

  In order to correct the change in the amount of light reaching the surface of the photoconductor due to the change in the drive frequency of the acousto-optic deflection element used in the above-described two-dimensional latent image pattern measurement method, the light source light amount is set for each drive frequency. It may be corrected. Hereinafter, a method for correcting a light amount change by correcting the light source light amount for each drive frequency will be described.

  As shown in FIG. 8, since the diffraction efficiency of the acousto-optic deflection element differs for each driving frequency, the amount of light on the sample image surface also differs for each driving frequency. In order to correct this, there is a method in which the light quantity of the light source is changed so that the light quantity of the image plane at each driving frequency is constant with respect to the light quantity of the image plane at a certain driving frequency.

  Similar to the intensity modulation voltage characteristic described above, the correction light quantity under the use conditions of the image forming apparatus is obtained in advance at a plurality of points, and the light quantity corresponding to each diffraction angle is designated at the time of scanning using the correction light quantity characteristic obtained by the approximate expression. As a result, the amount of light on the sample image plane can be kept constant regardless of the diffraction angle. The configuration of the control system 2 of the acousto-optic deflection element 103 at this time is shown in FIG. When an LD is used as the light source, the beam size changes depending on the drive current. Therefore, the light amount change region as the correction light amount is a light amount change region that contributes little to the beam size change on the sample image plane due to multimode oscillation. Is desirable. If a further change in the amount of light is required, it is possible to correct the amount of light on the image plane while maintaining the target beam size by using a beam size adjustment mechanism described later corresponding to the change in the beam size. .

  Similarly to the acousto-optic deflection element control system 2 shown in FIG. 13, the latent image pattern information formed from the host computer 90 is sent to the control board 85 that controls the exposure conditions such as the exposure optical system and the mechanical shutter. The control board 85 sends the respective information to the acousto-optic deflection element controller 86 in order to designate the drive frequency and intensity modulation voltage of the acousto-optic deflection element 103 according to the latent image pattern information. The acousto-optic deflection element control unit 86 receives information on the correction light quantity corresponding to the drive frequency from the correction light quantity memory 88 storing the correction light quantity characteristic obtained in advance, and the acousto-optic deflection element 103 based on the information. Drive.

  By using the above configuration, it is possible to correct a change in light amount due to a change in diffraction efficiency. In addition, to realize a suitable environment for measuring electrostatic latent images without being affected by changes in deflection characteristics such as thermal characteristics in the acousto-optic deflection element, which is a concern when changing the correction intensity voltage Can do.

  Next, a method for correcting a change in the beam size on the sample image plane at each drive current accompanying a change in the LD drive current will be described.

  As an adjustment mechanism for correcting the beam size change due to the LD drive current, there is a method of changing the LD light source light amount only in a region where the contribution to the beam size change on the sample image plane due to multimode oscillation is small. Also, in this method, when changing the amount of light outside the above range, the LD driving current is not changed, and the light amount of the image plane is changed by arranging an optical attenuation mechanism such as an attenuation filter in the exposure optical system. The amount of image plane light is changed while keeping the beam size constant. By using this method, the oscillation characteristics of the LD do not change, and only the amount of light reaching the image plane can be changed.

  The position where the light attenuation mechanism is disposed may be any position in the exposure optical system. For example, when an attenuation filter (ND filter) is used as the light attenuation mechanism, a method of replacing one having a different transmittance for each measurement may be used, but a variable ND filter may be used as shown in FIG.

  The variable ND filter shown in FIG. 15A is a circular ND filter whose optical density is continuously changed in the circumferential direction, and the amount of transmitted light can be continuously changed by rotating the filter. Moreover, you may use the rectangular ND filter which changed the optical density continuously to the linear direction shown in FIG.15 (b). The transmittance can be changed arbitrarily by electrically controlling the laser light transmission position with a stepping motor or the like. By using this configuration, it is possible to easily control the exposure amount for each light source, omit the troublesome work associated with the selection and replacement of the ND filter for each measurement, and easily control arbitrary transmittance.

  A method for measuring the latent image at each light amount by repeatedly changing the image surface light amount using this adjustment mechanism will be described with reference to the flowchart of FIG.

  First, in step S11, the LD drive current is determined according to the sample image plane light quantity to be measured.

  Next, in step S12, the transmittance of the attenuation filter is determined.

  Next, in step S13, the photosensitive member sample is irradiated with an electron beam to generate a charged charge on the photosensitive member sample.

  Next, in step S14, two-dimensional exposure is performed. This step S14 includes steps S4 to S7 of the flowchart shown in FIG.

  Next, in step S15, electrostatic latent image measurement is performed by irradiating the sample on which the latent image pattern is formed with an electron beam and detecting secondary electrons.

  Next, in step S16, the photoreceptor sample is neutralized using an LED.

  Next, in step S17, it is determined whether to measure the latent image characteristics with different light amounts.

  If it is determined in step S17 that the measurement is continued, the process proceeds to step S18, and the transmittance of the attenuation filter is changed according to the sample image plane light quantity to be measured. And it returns to step S13 again.

  By measuring the latent image based on the above configuration and method, the influence of the change in the light amount due to the change in the diffraction efficiency and the change in the beam size due to the change in the LD drive current can be ignored. A suitable environment can be realized.

  A mechanism for changing the distance between the sample image plane and the light source may be used as an adjustment mechanism for varying the image plane light quantity while keeping the beam diameter constant.

  The change in the beam diameter in the sub-scanning direction shown in FIG. 9 is represented by the depth characteristic as shown in the graph of FIG. Compared to the beam diameter when the current amount If = 30 mA, the beam diameter when If = 40 mA becomes smaller and the depth characteristic becomes narrower. At this time, the beam waist position does not change at any current amount. By utilizing this difference in depth characteristics, the beam diameter at the lower limit drive current within the measurement range is set as the target beam diameter. In addition, when the LD drive current is outside the measurement range, the beam diameter change is corrected so that the target beam diameter is obtained. This makes it possible to achieve a target beam diameter regardless of the LD drive current.

For example, when the beam diameter at If = 30 mA is used as a reference and the diameter is kept constant even at If = 40 mA, the image plane position is set to the beam waist position up to the position where the beam diameter is the same as shown in FIG. Just make fun of it. That is, the change in the beam diameter due to the change in the LD drive current amount can be corrected by the distance adjustment mechanism that adjusts the distance between the sample image plane and the light source. However, the condition must be within the movable range of the adjustment mechanism so that the beam diameter in the main scanning direction does not change. As this distance adjustment mechanism, a mechanism is used in which an exposure optical system installed outside the vacuum unit is mounted on a microstage and the exposure optical system is moved manually or automatically within the movable distance.

  A method for measuring the latent image characteristic by the change in the amount of light on the image plane when the distance adjusting mechanism is used will be described with reference to the flowchart of FIG.

  First, in step S21, a drive current to be a reference sub-scanning beam diameter is determined. Here, it is desirable to select the lower limit drive current within the measurement range in order to correct the beam diameter change by intentionally increasing the beam diameter at different drive currents using the depth characteristics of the beam diameter.

  Next, in step S22, the position to be used as the sample image plane is selected within the depth characteristic, and the beam diameter on the reference sample image plane is determined. At this time, the image plane spot position is desirably a beam waist position where the beam characteristics are estimated to be stable.

  Next, in step S23, the photosensitive member sample is irradiated with an electron beam to generate a charged charge on the photosensitive member sample.

  Next, in step S24, two-dimensional exposure is performed. This step S24 includes steps S4 to S7 in the flowchart shown in FIG.

  Next, in step S25, the sample on which the latent image pattern is formed is irradiated with an electron beam, and secondary electrons are detected to perform electrostatic latent image measurement.

  Next, in step S26, the charge of the photoreceptor sample is removed using the LED.

  Next, in step S27, it is determined whether to measure the latent image characteristics with different light amounts. If the measurement is not continued, the series of steps ends. When the measurement is continued, the process proceeds to step S28.

  In step S28, the LD drive current is changed according to the sample image plane light quantity to be measured.

  Next, in step S29, a moving distance having a beam diameter that matches the reference beam diameter is derived based on the previously measured depth characteristics for each drive current.

  Next, in step S30, the exposure optical system is moved based on the derived movement distance. And it returns to step S23 again.

  By performing latent image measurement based on the above configuration and process, it is possible to ignore the effects of changes in light quantity due to changes in diffraction efficiency and changes in beam size due to changes in LD drive current. A suitable environment can be realized. Further, a mechanism for mounting a light attenuation mechanism such as a neutral density filter in the exposure optical system, a control mechanism for changing the transmittance, and the like can be omitted, and the system configuration can be simplified. .

  Further, a mechanism for adjusting the incident angle of the exposure beam with respect to the sample may be used as an adjustment mechanism for changing the image plane light quantity while keeping the beam size constant. Hereinafter, the incident angle adjusting mechanism will be described.

  In the electrostatic latent image measuring apparatus of the present invention, as shown in FIG. 5, the exposure beam from the exposure optical system installed outside has an inclination in the sub-scanning direction with respect to the sample, and has a predetermined incident angle. Reach the sample. Therefore, as shown in FIG. 19, the beam diameter in the sub-scanning direction on the sample image plane is 1 / cos θ times larger than the beam diameter of the exposure light source itself. The beam diameter in the main scanning direction does not change.

  By utilizing this characteristic, it is possible to correct the beam diameter change due to the amount of LD drive current by changing the incident angle of the exposure beam with respect to the sample with a predetermined incident angle as a reference. The beam diameter serving as a reference at this time is preferably at the drive current that can minimize the beam size on the image plane, that is, at the upper limit drive current within the measurement range. In addition, the reference incident angle that is the incident angle of the exposure beam with respect to the sample at this time is θ = 45 °. In addition, regarding the reference incident angle of the exposure beam, a different angle may be used depending on the configuration of the electrostatic latent image measurement apparatus.

For example, when the beam diameter of If = 40 mA is used as a reference and the diameter is kept constant even when the drive current amount is If = 40 mA or less, as shown in FIG. 20, in order to correct the beam diameter increase accompanying the decrease in the amount of current. The incident angle should be reduced from θ ₁ to θ ₂ . As a specific example, when the beam diameter of If = 30 mA is 10% larger than the beam diameter of If = 40 mA, if the reference incident angle θ ₁ = 45 °, the incident angle is set to about θ ₂ = 39 °. If changed, the change in the beam diameter due to the change in the LD drive current amount can be corrected. It is preferable to perform correction by measuring the beam diameter change amount in advance and obtaining the incident angle change amount for correcting the beam diameter change amount.

This incident angle adjustment mechanism can be realized by mounting an exposure optical system installed outside the vacuum section or a sample stage on an inclination stage and controlling the inclination manually or automatically within the movable range. A method for measuring the latent image characteristics by the change in the amount of light on the image plane when this adjustment mechanism is used will be described with reference to the flowchart of FIG.

  First, in step S31, a driving current having a reference sub-scanning beam diameter is determined. It is desirable that the reference sub-scanning beam diameter is that at the upper limit driving current within the measurement range in which the beam size on the image plane can be minimized.

  Next, in step S32, which position is set as the sample image plane in the depth characteristic is selected, and the beam diameter on the reference sample image plane is determined. At this time, the image plane spot position is desirably a beam waist position where beam characteristics are considered to be stable.

  Next, in step S33, the photosensitive member sample is irradiated with an electron beam to generate a charged charge on the photosensitive member sample.

  Next, in step S34, two-dimensional exposure is performed. This step S34 includes steps S4 to S7 of the flowchart shown in FIG.

  Next, in step S35, the electrostatic latent image measurement is performed by irradiating the sample on which the latent image pattern is formed with an electron beam and detecting secondary electrons.

  Next, in step S36, the charge of the photoreceptor sample is removed using the LED.

  Next, in step S37, it is determined whether to measure the latent image characteristics with different light amounts. When the latent image characteristics are not measured with different amounts of light, the entire process ends. On the other hand, when continuing the measurement, the process proceeds to S38.

  In step S38, the LD drive current is changed according to the sample image plane light quantity to be measured.

  Next, in step S39, an incident angle change amount for correcting the beam diameter change amount for each drive current measured in advance is derived.

  Next, in step S40, the inclination of the exposure optical system or the sample stage is changed based on the derived incident angle change amount. And it returns to step S33 again.

  By performing latent image measurement based on the above configuration and process, it is possible to ignore the effects of changes in light quantity due to changes in diffraction efficiency and changes in beam size due to changes in LD drive current. A suitable environment can be realized. Further, a mechanism for mounting a light attenuation mechanism such as a neutral density filter in the exposure optical system, a control mechanism for changing the transmittance, and the like can be omitted, and the system configuration can be simplified. .

  Further, as a mechanism for changing the incident angle of the exposure beam with respect to the sample, a mechanism for changing the sample position may be used.

  For example, when using a sample with curvature such as a slice of a drum-shaped photoconductor as a sample for measuring a latent image, the incident angle of the exposure beam to the photoconductor sample can be changed by changing the height position of the photoconductor sample. It is possible to change. The incident angle at this time is obtained from the angle formed by the exposure beam incident on the drum-shaped sample and the normal to the surface of the drum-shaped sample at the point where the exposure beam is incident as shown in FIG. When the beam diameter of If = 40 mA is used as a reference and when it is desired to keep the diameter constant even with a drive current amount of If = 40 mA or less, as shown in FIG. 23, in order to correct the beam diameter increase accompanying a decrease in the amount of current. The sample height may be changed to reduce the incident angle.

For example, when the beam diameter at If = 30 mA is 10% larger than the beam diameter at If = 40 mA, if the reference incident angle θ ₁ = 45 °, the incident angle is θ2 = What is necessary is just to change to 39 degrees. When the beam is incident on the central axis of the cylindrical photoconductor with φ = 30 mm, the height of the sample may be moved by 3.3 mm. As the adjustment mechanism, a microstage or the like that can be moved in the height direction is used as the sample stage, and a mechanism that moves the height position manually or automatically within the movable range may be used. Further, the same effect can be obtained by fixing the sample height and providing a mechanism for moving the sample in the Y-axis direction in the figure and moving the sample by a predetermined distance.

  A method for measuring the latent image characteristics due to the change in the amount of light on the image plane when this adjustment mechanism is used will be described with reference to the flowchart of FIG.

  First, in step S41, a drive current having a reference sub-scanning beam diameter is determined.

  Next, in step S42, the position to be used as the sample image plane in the depth characteristic is selected, and the beam diameter on the reference sample image plane is determined. At this time, the image plane spot position is desirably a beam waist position where beam characteristics are considered to be stable.

  Next, in step S43, the photosensitive member sample is irradiated with an electron beam to generate a charged charge on the photosensitive member sample.

  Next, in step S44, two-dimensional exposure is performed. This step S44 includes steps S4 to S7 of the flowchart shown in FIG.

  Next, in step S45, the electrostatic latent image is measured by irradiating the sample on which the latent image pattern is formed with an electron beam and detecting secondary electrons.

  Next, in step S46, the charge of the photoreceptor sample is removed using the LED.

  Next, in step S47, a determination is made as to whether to measure latent image characteristics with different light amounts. If the measurement is not continued, the entire process is completed. When the measurement is continued, the process proceeds to step S48.

  In step S48, the LD drive current is changed according to the sample image plane light quantity to be measured.

  Next, in step S49, in order to correct the change in the beam diameter due to the change in the driving current measured in advance, the amount of movement of the sample height is derived.

  Next, in step S50, the height of the sample is changed based on the derived movement amount. And it returns to step S43 again.

  By performing latent image measurement based on the above configuration and process, it is possible to ignore the effects of changes in light intensity due to changes in diffraction efficiency and changes in beam size due to changes in LD drive current. It is possible to realize a suitable environment for performing the above. In addition, the mechanism for mounting an optical attenuation mechanism such as a neutral density filter in the exposure optical system and its transmittance have been changed without being affected by changes in the optical characteristics due to changes in the spot position of the image plane within the depth characteristics. The control mechanism for doing so can be omitted, and the system configuration can be simplified.

DESCRIPTION OF SYMBOLS 1 Electrostatic latent image measuring device 50 Charged particle irradiation part 11 Electron gun 12 Extraction electrode (extractor)
13 Accelerating electrode 14 Electrostatic lens (condenser lens)
15 Beam blanking electrode (beam blanker)
16 Partition plate 17 Movable diaphragm 18 Sting meter 19 Scan lens (deflection electrode)
DESCRIPTION OF SYMBOLS 20 Electrostatic objective lens 21 Beam emission opening part 60 Sample mounting base 22 Exposure optical system 24 Secondary electron detector 80 Information processing part 81 Electron detection part 82 Signal processing part 83 Measurement result output part 84 Image processing part 90 Host computer

Japanese Patent Laid-Open No. 03-049143 JP 2003-295696 A JP 2008-233376 A

Claims (15)

An electrostatic latent image measuring device that measures an electrostatic latent image formed on the surface of a photoreceptor,
Charging means for irradiating and charging the photosensitive member with a charged particle beam;
A light source for irradiating the charged photosensitive member with a light beam for exposure;
An acousto-optic deflection element that diffracts a light beam from the light source;
Deflecting means for deflecting the light beam in a direction perpendicular to the diffraction direction;
Detecting means for detecting secondary electrons from the charged photoreceptor;
Correction means for correcting the influence of the change in the driving frequency of the acousto-optic deflection element on diffraction;
With
An electrostatic latent image measuring device that scans in a two-dimensional direction by scanning in the one-dimensional direction by diffraction by the acousto-optic deflection element, and scanning in another one-dimensional direction by deflection by the deflecting means.   The correction means corrects a change in diffraction efficiency associated with a change in driving frequency of the acoustooptic deflecting element by changing a voltage applied to the acoustooptic deflecting element according to the driving frequency. Electrostatic latent image measuring device.   2. The correction unit corrects a change in the amount of light reaching the surface of the photosensitive member accompanying a change in the drive frequency of the acousto-optic deflection element by changing the amount of light from the light source according to the drive frequency. Electrostatic latent image measuring device.   The electrostatic latent image measuring device according to claim 1, further comprising a light attenuating unit that changes the amount of light reaching the surface of the photosensitive member.   The electrostatic latent image measuring device according to claim 1, further comprising a beam size correcting unit that adjusts a diameter of a beam spot formed on the surface of the photosensitive member by the light beam.   The electrostatic latent image measuring device according to claim 5, wherein the beam size correcting unit adjusts a distance between the light source and the photosensitive member.   The electrostatic latent image measuring device according to claim 5, wherein the beam size correcting unit adjusts an incident angle of the light beam incident on the photosensitive member. An electrostatic latent image measuring method for measuring an electrostatic latent image formed on a surface of a photoreceptor,
A charging step of irradiating the photosensitive member with a charged particle beam for charging;
An exposure step of irradiating and exposing the charged photoreceptor with a light beam;
A diffraction step of diffracting the light beam from the light source by an acousto-optic deflection element;
A deflection step of deflecting the light beam in a direction perpendicular to the diffraction direction;
A detection step of detecting secondary electrons from the charged photoreceptor;
A correction step of correcting the influence of the change in the driving frequency of the acousto-optic deflection element on the diffraction;
An electrostatic latent image measuring method comprising:   The electrostatic correction according to claim 8, wherein the correcting step corrects a change in diffraction efficiency associated with a driving frequency of the acoustooptic deflecting element by changing a voltage acting on the acoustooptic deflecting element according to the driving frequency. Latent image measurement method.   The static correction according to claim 8, wherein the correcting step corrects a change in the amount of light reaching the surface of the photosensitive member according to the driving frequency of the acousto-optic deflection element by changing the amount of light from the light source according to the driving frequency. Electrostatic image measurement method.   The electrostatic latent image measurement method according to claim 10, wherein in the step of correcting the amount of light from the light source, the amount of light of the light source is changed within a single mode oscillation region.   The electrostatic latent image measuring method according to claim 10, wherein in the step of correcting the light amount from the light source, the light amount of the light source is changed using a light attenuating means outside the range of the single mode oscillation region.   The electrostatic latent image measuring method according to claim 8, further comprising a beam size correction step of adjusting a diameter of a beam spot formed on the surface of the photosensitive member by the light beam.   The electrostatic latent image measurement method according to claim 13, wherein the beam size correction step is performed by adjusting a distance between the light source and the photoconductor.   The electrostatic latent image measuring method according to claim 13, wherein the beam size correcting step is performed by adjusting an incident angle of the light beam incident on the photosensitive member.

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