JP2014119729A - Image forming apparatus - Google Patents

Abstract

An image forming apparatus is provided that forms a detection image in a non-transfer area and executes calibration, and prevents a recording material from being stained.
An image forming apparatus includes a plurality of detection images each having a plurality of lines in directions different from a moving direction of a surface of an image carrier between images to be printed formed on the image carrier during printing. , An image forming unit that forms an image while changing image forming conditions relating to density, a detecting unit that detects a plurality of detection images formed on the image carrier and outputs an output signal, and a peak value of an output signal from the detecting unit And a control means for controlling the image forming conditions, and the detection means receives the light emitted from the light emitting means and reflected by the image carrying body. The output signal is a signal corresponding to the difference in the value of the detection signal corresponding to the received light amount including the specularly reflected light components from different positions on the surface of the image carrier and the light receiving means that outputs the detection signal corresponding to the received light amount. An output means for outputting, That.
[Selection] Figure 11

Description

  The present invention relates to calibration in an image forming apparatus such as a color laser printer, a color copying machine, and a color facsimile that mainly employ an electrophotographic process.

  In recent years, an electrophotographic color image forming apparatus is mainly a tandem type in which a photoconductor is provided for each color in order to increase the printing speed. In a tandem type image forming apparatus, for example, a detection image, which is a pattern for detecting color misregistration and density, is formed on an intermediate transfer belt, and reflected light from the detected image is detected by an optical sensor to correct color misregistration and density. That is, calibration is executed. This calibration is executed, for example, when the cartridge including the photoconductor is replaced, after a predetermined number of prints, after a predetermined time has elapsed, or when the temperature / humidity environment changes.

  In an image forming apparatus, when image formation is continuously performed on a plurality of recording materials, the temperature in the apparatus rises, image forming conditions change, and image density and color may change. For this reason, Patent Document 1 discloses a configuration in which calibration is sequentially performed while performing continuous printing. Specifically, the detection image is formed in a region (hereinafter referred to as a non-transfer region) between the trailing edge of the image transferred to the recording material and the leading edge of the image transferred to the next recording material on the intermediate transfer belt. Is to detect. In the configuration of Patent Document 1, it is not necessary to interrupt image formation for executing calibration.

JP 2001-109219 A

  However, in an image forming apparatus that uses an intermediate transfer belt, a cleaning blade or the like that removes the toner image on the intermediate transfer belt is downstream in the rotational movement direction of the intermediate transfer belt from the secondary transfer roller that transfers the toner image to the recording material. Arranged on the side. For this reason, the detected image formed in the non-transfer area adheres to the secondary transfer roller, and therefore, a back smear phenomenon occurs in which the toner of the detected image adheres to the back side of the recording material that passes thereafter. In order to prevent this back stain, the transfer bias output from the secondary transfer roller may be switched to a polarity opposite to that at the time of transfer at the timing when the detected image passes through the secondary transfer roller. However, even if the control for switching the transfer bias to the reverse polarity is performed, if the density of the detected image is high, the detected image still adheres to the secondary transfer roller.

  The present invention provides an image forming apparatus that forms a detection image in a non-transfer area and performs calibration while preventing back-staining on a recording material.

  According to an aspect of the present invention, the image forming apparatus includes a plurality of lines in directions different from the moving direction of the surface of the image carrier between the images to be printed formed on the image carrier during printing. Image forming means for forming a plurality of detection images while changing image forming conditions relating to density, detection means for detecting the plurality of detection images formed on the image carrier and outputting an output signal, and the detection Control means for controlling the image forming condition based on a peak value of an output signal from the means, the detection means is a light emitting means for irradiating light to the image carrier, and the light emitting means emits light. A light receiving unit that receives light reflected by the image carrier and outputs a detection signal corresponding to the amount of light received; and a light reception amount that includes specularly reflected light components from different positions on the surface of the image carrier. Difference in value of the detection signal Characterized in that the response signal and a, and output means for outputting as the output signal.

  It is possible to prevent the recording material from being soiled while forming a detection image in the non-transfer area and performing calibration.

The figure which shows the optical sensor by one Embodiment. The figure which shows the control circuit of the optical sensor by one Embodiment. Explanatory drawing of the signal which the optical sensor by one Embodiment outputs. The figure which shows the signal which the optical sensor by one Embodiment outputs. The figure which shows the characteristic of the amplitude value of the signal which the optical sensor by one Embodiment outputs. Explanatory drawing of the formation method of the pattern image by one Embodiment. The figure which shows the output signal of the optical sensor with respect to the pattern image of FIG.6 (D) and (F). Explanatory drawing of the formation method of the pattern image by one Embodiment. The figure which shows the relationship between the density fluctuation | variation by one Embodiment, and the output signal of an optical sensor. Explanatory drawing of the power determination of a laser beam. Explanatory drawing of the determination of the developing bias by one Embodiment. Explanatory drawing of the formation method of the pattern image by one Embodiment. Explanatory drawing of the pattern image formed with the image data of M dot M space. Explanatory drawing of the determination of the developing bias by one Embodiment. 1 is a diagram illustrating an image forming apparatus according to an embodiment. Explanatory drawing of the process with respect to the output signal of the optical sensor by one Embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings. In the following drawings, the same reference numerals are used for the same components.

<First embodiment>
First, the image forming apparatus 101 according to the present embodiment will be described with reference to FIG. The Y, M, C, and Bk at the end of the reference numerals in FIG. 15 indicate that the colors of the developer (toner) targeted by the corresponding members are yellow, magenta, cyan, and black, respectively. In the following description, when it is not necessary to distinguish between colors, reference numerals without Y, M, C, and Bk at the end are used. The charging unit 2 uniformly charges the photosensitive member 1 that is an image bearing member that is rotationally driven in the direction of the arrow in the figure, and the exposure unit 7 irradiates the photosensitive member 1 with laser light to irradiate the photosensitive member 1. An electrostatic latent image is formed on the surface. The developing unit 3 supplies a toner as a developer to the electrostatic latent image by applying a developing bias, and converts the electrostatic latent image into a toner image that is a visible image. The primary transfer roller 6 transfers the toner image on the photoreceptor 1 to the intermediate transfer belt 8 by the primary transfer bias. The intermediate transfer belt 8 is rotationally driven in the direction of the arrow 81. Each photoconductor 1 transfers the toner image on the intermediate transfer belt 8 so as to form a color image. The cleaning blade 4 removes the toner that is not transferred to the intermediate transfer belt 8 and remains on the photoreceptor 1.

  The conveyance rollers 14, 15 and 16 convey the recording material in the cassette 13 to the secondary transfer roller 11 along the conveyance path 9. The secondary transfer roller 11 transfers the toner image on the intermediate transfer belt 8 to the recording material by a secondary transfer bias. The toner that is not transferred to the recording material and remains on the intermediate transfer belt 8 is removed by the cleaning blade 21 and collected in a waste toner collecting container 22. The recording material onto which the toner image has been transferred is heated and pressurized in the fixing unit 17 to fix the toner image, and is discharged out of the apparatus by the transport roller 20. The engine control unit 25 includes a microcontroller 26, and performs sequence control of various drive sources (not shown) of the image forming apparatus 101, various controls using sensors, and the like. An optical sensor 27 is provided at a position facing the intermediate transfer belt 8.

  For example, in a tandem type color image forming apparatus, the machine dimensions deviate from the design value due to assembly errors, part tolerances, thermal expansion of parts, etc. during the manufacture of the apparatus, thereby causing a positional deviation for each color. For this reason, a detection image for detecting color misregistration of each color is formed on the intermediate transfer belt 8 or the like, and the reflected light from the formed detection image is detected by the optical sensor 27. Then, based on the detection result, color misregistration is corrected by adjusting the writing position in the main scanning and sub-scanning directions and the image clock for each color. Further, in the image forming apparatus 101, the color, density, and the like of an image output by time-dependent change or continuous printing can change. In order to correct this variation, density control is performed. In density control, a detection image for detecting the density of each color is formed on the intermediate transfer belt 8 and the like, and reflected light from the formed detection image is detected by the optical sensor 27. Then, the maximum density of each color and the halftone gradation characteristics are corrected by feeding back the detection result to process formation conditions such as each voltage condition and laser beam power. The density detection by the optical sensor 27 is generally performed by irradiating a detection image with a light source and detecting the intensity of reflected light with a light receiving element. A signal corresponding to the intensity of the reflected light is processed by the microcontroller 26 and fed back to the process formation conditions. The purpose of the control of the maximum density is to keep the color balance of each color constant, and to prevent the overlaid color image from being overloaded and the fixing failure. On the other hand, halftone gradation control is intended to prevent the output density from deviating from the input image signal due to nonlinear input / output characteristics to prevent a natural image from being formed.

  Hereinafter, the details of the optical sensor 27 of the present embodiment will be described with reference to FIG. FIG. 1A is a perspective view of the optical sensor 27, and FIG. 1B is a view seen from the X direction of FIG. The optical sensor 27 according to the present embodiment includes a light emitting element 272, a light receiving unit 270 including a plurality of light receiving elements 273n, 273p, 274n, and 274p, a control circuit 275, and a light shielding wall 276. ,have. The light emitting element 272 has, for example, an LED chip without a reflector, and irradiates the intermediate transfer belt 8 with a divergent light beam from a point light source without narrowing the irradiation light by an optical member or the like. The light receiving elements of the light receiving unit 270 are arranged in an array along the moving direction 81 of the intermediate transfer belt 8. Each light receiving element is, for example, a photodiode that outputs a current corresponding to the amount of light received. In the present embodiment, the reflected light from the intermediate transfer belt 8 depends on the amount of light received by the light receiving element without passing through an optical member such as a condensing lens for condensing or condensing light. Is converted to a signal. In the following description, the light receiving element 273n and the light receiving element 274n are also referred to as a light receiving part An, and the light receiving elements 273p and 274p are also referred to as a light receiving part Ap. That is, the light receiving unit 270 is configured such that the light receiving unit Ap (first light receiving unit) and the light receiving unit An (second light receiving unit) are alternately arranged. As described above, in this embodiment, one light receiving portion Ap corresponds to one light receiving element 273p, 274p, and one light receiving portion An corresponds to one light receiving element 273n, 274n. The part Ap and one light receiving part An may include a plurality of light receiving elements. In other words, in the present embodiment, the light receiving portions Ap and the light receiving portions An, each including one or more light receiving elements, may be alternately arranged.

  As shown in FIG. 1A, in this embodiment, the widths of the light receiving portions Ap and An in the arrangement direction are equal to Wsns, the pitches of the light receiving portions Ap and the light receiving portions An are equal, and the value Psns is It is twice the width of Ap and the light receiving part An. In the present embodiment, the pitch of the light receiving portions Ap means the distance between the corresponding positions of the adjacent light receiving portions Ap, and does not mean the distance between the adjacent light receiving portions Ap. Similarly, in the present embodiment, the pitch of the light receiving portions An means the distance between the corresponding positions of the adjacent light receiving portions An, and does not mean the distance between the adjacent light receiving portions An. In the present embodiment, the width in the array direction of the light receiving surfaces of each light receiving portion Ap and each light receiving portion An is substantially equal to Wsns, and the light receiving areas of each light receiving portion Ap and each light receiving portion An are equal. .

  The control circuit 275 is electrically connected to the light emitting element 272 and the light receiving unit 270, and constitutes a signal processing circuit described later. The light shielding wall 276 is provided to prevent the light emitted from the light emitting element 272 from entering the light receiving unit 270 without passing through the intermediate transfer belt 8. Note that the surface of the intermediate transfer belt 8 is glossy coated, and the light irradiated from the light emitting element 272 is substantially regularly reflected. On the other hand, in the toner portion of the image formed on the intermediate transfer belt 8, the light emitted from the light emitting element 272 is absorbed or diffusely reflected. For example, when the light source is a red LED, the irradiation light is absorbed by cyan and black toners and diffusely reflected by yellow and magenta toners. For example, when the light source is an infrared LED, the irradiation light is absorbed by the black toner and diffusely reflected by the yellow, magenta, and cyan toners. In FIGS. 1A and 1B, the light irradiated by the light emitting element 272 and regularly reflected on the surface of the intermediate transfer belt 8 is indicated by arrows. In FIG. 1B, the irradiation light is absorbed by the toner or diffusely reflected by the toner, so that the reflected light is weakened in the region where the light receiving portion 270 is formed, that is, the shadow is formed by the toner. Is denoted by reference numeral 279.

  Hereinafter, the relationship between the pattern image 40 that is a density detection image and the light projected on the light receiving unit 270 will be described. In the present embodiment, the pattern image 40 is a striped toner image in which two lines in the direction orthogonal to the moving direction 81 on the surface of the intermediate transfer belt 80 are formed along the moving direction 81. In the present embodiment, the line is described by taking an optimal solid line as an example, but it may be a dotted line or a broken line having a fine spot shape. The number of lines to be formed is an exemplification, and an arbitrary number of 2 or more can be used. Hereinafter, a portion between lines of the pattern image 40 is referred to as a space. As shown in FIG. 1A, the line width of the pattern image 40 of the present embodiment is Wt40, the space width is Wb40, and the pitch between the lines is Pt40. The line width of the pattern image 40 is, for example, several dots level (600 dpi resolution, 1 dot is 42.3 μm). In the present embodiment, the pitch between lines means the distance between corresponding positions of two adjacent lines, and does not mean the width of the space.

  Since the light emitting element 272 is a type of LED that emits a divergent light beam of a point light source, an image of the pattern image 40 formed on the intermediate transfer belt 8 is projected onto the light receiving unit 270 at a predetermined magnification. In this embodiment, since the intermediate transfer belt 8 and the substrate 271 are arranged in parallel, the incident light from the light emitting element 272 to the intermediate transfer belt 8 and the regular reflection light are equiangular. Furthermore, the light emitting element 272 and the light receiving portion 270 are configured so that the surface height on the substrate 271 is the same. That is, the optical path lengths of incident light (outward path) and regular reflection light (return path) are equal. Therefore, the striped pattern image 40 formed on the intermediate transfer belt 8 is projected onto the light receiving unit 270 as an image having a double size. That is, the line width Wt40, the space width Wb40, and the pitch Pt40 between the lines of the pattern image 40 of the intermediate transfer belt 8 are all twice as large when projected onto the light receiving unit 270. The light and darkness of the pattern image 40 that is twice the size at the position of the light receiving unit 270 is moved by the movement of the intermediate transfer belt 8, and the light receiving unit Ap and the light receiving unit An are along the light and dark moving direction (first direction). Are arranged.

  Next, the control circuit 275 will be described with reference to FIG. The control circuit 275 is connected to the light receiving unit 270 on the substrate 271. Further, inside the control circuit 275, the light receiving elements 273p and 274p constituting the light receiving part Ap are connected, and the light receiving elements 273n and 274n constituting the light receiving part An are connected. The light receiving part Ap is connected to the IV conversion amplifier 281, and the light receiving part An is connected to the IV conversion amplifier 284. When the light receiving unit 270 receives light, the current Iap corresponding to the total light receiving amount of the light receiving unit Ap and the current Ian corresponding to the total light receiving amount of the light receiving unit An are converted into voltages by the IV conversion amplifier, respectively. .

A reference voltage Vref1 generated by dividing the voltage Vcc by the resistors 287 and 288 is input to the non-inverting input terminals of the IV conversion amplifiers 281 and 284 by the voltage follower 289. Therefore, when the light receiving unit 270 is not receiving light, the IV conversion amplifiers 281 and 284 output the reference voltage Vref1, and when receiving light, the IV conversion amplifiers 281 and 284 are respectively voltages expressed by the following equations. S1 and voltage S2 are output.
S1 = Vref1− (R ₂₈₂ × Iap)
S2 = Vref1- ( _R285 * Ian)
Here, R ₂₈₂ is the resistance value of the resistor 282, and R ₂₈₅ is the resistance value of the resistor 285. Capacitors 283 and 286 are provided for phase compensation and noise removal.

  The reference voltage Vref2 is input from the voltage follower 298 to the non-inverting input terminal of the differential amplifier 290 configured with the resistors 291 to 294. The reference voltage Vref2 is generated by dividing the voltage Vcc by the resistors 296 and 297. The differential amplifier 290 performs differential amplification of the reference voltage Vref2 and S1 and S2, and outputs a sensor signal Vsns = Vref2 + S2-S1 which is an output signal of the optical sensor 27 to the terminal 295. Therefore, when the total amount of light received by the light receiving portion Ap is equal to the total amount of light received by the light receiving portion An, the sensor signal becomes the reference voltage Vref2. On the other hand, if the total light receiving amount of the light receiving part Ap is larger than the total light receiving amount of the light receiving part An, the sensor signal becomes larger than the reference voltage Vref2. On the other hand, when the total light receiving amount of the light receiving part Ap is smaller than the total light receiving amount of the light receiving part An, the sensor signal becomes smaller than the reference voltage Vref2. That is, the optical sensor 27 outputs a signal having an amplitude corresponding to a value obtained by subtracting the total light receiving amount of the light receiving unit An from the total light receiving amount of the light receiving unit Ap.

  Next, a lighting circuit of the light emitting element 272 will be described. The control circuit 275 is provided with an operational amplifier 299 and an accompanying circuit for driving the light emitting element 272 at a constant current. The operational amplifier 299 drives the light emitting element 272 by driving the transistor 302 to light it. The current flowing through the light emitting element 272 when the light is on is detected by the resistor 301 and monitored by the inverting input terminal of the operational amplifier 299. On the other hand, a voltage input terminal Trgt for setting the drive current of the light emitting element 272 by the microcontroller 26 is connected to the non-inverting input terminal of the operational amplifier 299. That is, the operational amplifier 299 drives the light emitting element 272 at a constant current so as to have a value set from the terminal Trgt.

  Next, an output signal when the optical sensor 27 detects the pattern image 40 of the intermediate transfer belt 8 will be described. In order to detect the pattern image 40, the microcontroller 26 lights up the light emitting element 272, and the light receiving unit 270 detects the reflected light. The current corresponding to the amount of light received output from the light receiving unit 270 is processed by the control circuit 275 described above and output as a sensor signal. FIG. 3A shows various positional relationships between the light receiving portion 270 of the optical sensor 27 and the brightness of light generated at the position of the light receiving portion 270 by the pattern image 40 formed on the intermediate transfer belt 8. Note that the brightness of the light can actually be generated on the light receiving portion 270, but in FIG. 3A, it is displayed by shifting up and down in order to make the drawing easier to see. FIG. 3B shows the relationship between the output signal of the optical sensor 27 and each state of FIG. Here, the line and space widths Wt40 and Wb40 of the pattern image 40 formed on the intermediate transfer belt 8 are equal to each other and are half the width Wsns of the light receiving part Ap and the light receiving part An.

  The state 0 is a state in which the shadow due to the line of the pattern image 40 of the intermediate transfer belt 8 does not reach the detection area of the optical sensor 27, and the regular reflection light is incident on all the light receiving parts Ap and the light receiving parts An. Therefore, the output of the optical sensor 27 at this time is the analog reference voltage Vref2 indicated by “0” in FIG.

  State 1 is a state in which a shadow of one line of the pattern image 40 covers one of the light receiving parts An. In this state, since the regular reflection light to one of the light receiving parts An is blocked, the current Ian is almost halved and the output S2 becomes larger than the output S1. Accordingly, the output of the optical sensor 27 is higher than the analog reference voltage Vref2 indicated by “1” in FIG.

  State 2 is a state in which the movement of the intermediate transfer belt 8 has progressed, and a shadow of one of the lines of the pattern image 40 covers one of the light receiving parts Ap. In this state, since the regular reflection light to one of the light receiving parts Ap is blocked, the current Iap is halved and the output S1 becomes larger than the output S2. Therefore, the output of the optical sensor 27 has a potential lower than the analog reference voltage Vref2 indicated by “2” in FIG.

  State 3 is a state in which the intermediate transfer belt 8 is further moved and shadows due to lines of the pattern image 40 cover all the light receiving portions An. In this state, since the regular reflection light to all the light receiving parts An is blocked, the current Ian is reduced to almost 0, and the output S2 becomes the maximum value. Therefore, the output of the optical sensor 27 becomes maximum as shown by “3” in FIG. In addition, when the color of the toner of the pattern image 40 is a color that diffuses the irradiation light, the pattern reflection is performed so that the diffuse reflection light from the line of the pattern image 40 is uniformly diffused to all the light receiving elements. Since the pitch of the image 40 is formed, the increase / decrease amounts of the currents Iap and Ian due to the diffused light are equal. Accordingly, the diffuse reflected light is canceled by the differential processing in the control circuit 275, and the output signal of the optical sensor 27 is not affected by the diffuse reflected light.

  State 4 is a state in which the intermediate transfer belt 8 is further moved, and the shadows due to the lines of the pattern image 40 cover all the light receiving parts Ap. In this state, since the regular reflection light to all the light receiving parts Ap is blocked, the current Iap is reduced to almost 0, and the output S1 becomes the maximum value. Accordingly, the output of the optical sensor 27 is minimized as indicated by “4” in FIG.

  The state 5 is a state in which the intermediate transfer belt 8 is further moved and the shadow of the line of the pattern image 40 covers one light receiving part Ap. In this state, the specularly reflected light to one of the light receiving portions Ap is blocked, so that the output of the optical sensor 27 is lower than the analog reference voltage Vref2 indicated by “5” in FIG.

  State 6 is a state in which the intermediate transfer belt 8 further moves and the shadow of the line of the pattern image 40 is outside the light receiving unit 270. Like the state 0, the voltage output from the optical sensor 27 is an analog reference. The voltage becomes Vref2.

  As described above, when the brightness of the light projected from the pattern image 40 passes through the light receiving unit 270, the optical sensor 27 outputs a sensor signal that vibrates around the reference voltage Vref2. The optical sensor 27 outputs a signal having an amplitude corresponding to a value obtained by subtracting the total amount of light received by the light receiving unit An from the amount of light transmitted and received by the light receiving unit Ap. Is maximized when the duty ratio is 50%, that is, when the widths of the bright and dark portions are the same. FIG. 5 shows the relationship between the light / dark ratio of light and the maximum value of the amplitude of the output signal of the optical sensor 27.

  In FIG. 3, the pattern image 40 has two lines, but the number of lines in the pattern image 40 can be any number. FIG. 4 shows an output signal of the optical sensor 27 when the pattern image 40 having five lines is formed. By setting the number of pattern images to five, compared to the output signal shown in FIG. 3B, the number of occurrences of the maximum amplitude and the minimum amplitude is increased by three times corresponding to the increased number of lines. I understand that.

  Next, the pattern image 40 and the arrangement pitch of the light receiving parts Ap and An in the optical sensor 27 will be described. The arrangement pitch of the light receiving portions Ap and An of the optical sensor 27 shown in FIG. 1, that is, Psns in FIG. 1 can be manufactured with various values. On the other hand, the line pitch of the pattern image 40 is limited depending on the resolution of the image forming apparatus. For example, if the resolution of the image forming apparatus is 600 dpi, the size of one dot is about 42.3 μm. Therefore, in the sub-scanning direction, it is basically possible to form a stripe pattern with toner only in units of an integral multiple. On the other hand, the stripe pattern formed on the intermediate transfer belt 8 and the regular reflection image projected onto the light receiving unit 270 using the optical sensor 27 have a ratio of 1: 2 in this embodiment. That is, it is optimal that the arrangement pitch (Psns) of the light receiving portions An and Ap of the optical sensor 27 is twice as large as one dot size of the image forming apparatus, and the light receiving portions An and Ap of the present embodiment are arranged at this arrangement pitch. It is formed with.

  Next, a method for forming the above-described striped pattern image 40 will be described. In the present embodiment, in order to form a striped pattern with toner having a line and space width of K dots (K is an integer), the line is M dots and the space is N dots (M <N: M and N Is an integer) image data. In the present embodiment, there is a relationship of M + N = 2K.

  FIG. 6 is an explanatory diagram of a method of forming a pattern image 40 in which a 2-dot line and a 2-dot space are repeated. FIG. 6A shows a pattern image 40 that is a formation target. Since the line and space are 2 dots, the line pitch of the pattern image 40 is 4 dots (about 169.3 μm). FIG. 6B is a diagram in which a partition line in units of one dot is added to the diagram of FIG. In the image forming apparatus, an image is formed by providing each pixel data for each dot. Both the line width and the space width are equal to 2 dots, and in the image data, the line to space ratio is 50 to 50, that is, the duty ratio is 50%.

  However, the duty ratio of the developed pattern image 40 does not become 50%. FIG. 6C shows a developed one-dot toner image. As shown in FIG. 6C, the toner image that is actually developed does not become an ideal area of one dot square, but has a substantially circular shape larger than the ideal size of 42.3 μm square. For example, in FIG. 6C, the diameter is about 85 μm. A pattern image 40 formed when the dot diameter of the toner image is 85 μm shown in FIG. 6C is shown in FIG. As shown in FIG. 6D, the development regions of the adjacent toner image dots overlap each other, but the width of the adjacent two dot toner images in the sub-scanning direction, that is, the line width LD2 is It becomes larger than the target TW. Therefore, the space width LS2 is smaller than the target TW. That is, the duty ratio of the pattern image 40 to be formed is not 50%.

Therefore, in this embodiment, as shown in FIG. 6E, the pattern image 40 is formed by image data in which a line is 1 dot and a space is 3 dots. A pattern image 40 formed from the image data of FIG. 6E is shown in FIG. The diameter of the 1-dot toner image is 85 μm as shown in FIG. As shown in FIG. 6F, the toner line width LD1 is 85 μm, which is substantially equal to the target TW (2 dots). Therefore, the space width LS3 is also substantially equal to the target TW. That is, by forming the pattern image 40 using the image data of the 1-dot line and the 3-dot space, it is possible to form the pattern image 40 having a duty ratio of approximately 50%. That is, when the diameter of a 1-dot toner image is 85 μm as in this example, the actual toner image is thicker by about 1 dot than the 1-dot size of 600 dpi (about 42.3 μm), and the actual space portion Is narrowed by about 1 dot. Therefore, in this example, if the line is M dots and the space is N dots and N = M + 2, the ratio of the developed toner image line to space can be about 50:50. If the diameter of the toner image is about 100 μm, and if N = M + 3, the ratio of the line and space of the developed toner image can be about 50:50. This can be expressed as an equation:
N = M + (A−1) × 2
It becomes. Here, A is the ratio of the diameter of one dot actually formed to the calculated diameter of one dot of the toner image. Since N is an integer, the value obtained by the above formula is obtained by rounding up or down the decimal point. When the toner image diameter is 85 μm and 100 μm, N = M + (2-1) × 2 = M + 2
N = M + (2.5-1) × 2 = M + 3
It becomes.

  As already described, when the duty ratio is not 50%, the peak value of the output signal of the optical sensor 27 decreases as shown in FIG. FIG. 7 shows sensor signals output by the optical sensor 27 for the pattern image of FIG. 6D and the pattern image shown in FIG.

  Next, a method for detecting the density fluctuation by forming the pattern image 40 in the non-transfer area during printing will be described. Note that the image forming apparatus 101 performs conventionally known density control when the printing process is not performed. The following processing is performed for each color, but only one color will be described below. First, the image forming apparatus 101 determines the exposure intensity, that is, the power of the laser beam that is a reference for forming a pattern image with a duty ratio of approximately 50% in a state after normal density control. Set it.

  Below, the determination of the power of the reference laser beam will be described. In this example, as already described, in order to form a pattern image 40 having both a line width and a space width of 2 dots, image data forming a 1-dot line and a 3-dot space is used. As shown in FIG. 10A, the microcontroller 26 forms a striped pattern image 50 in which the power of the laser beam is changed in stages on the intermediate transfer belt 8. Note that 80h, FFh, and the like in FIG. 10A are laser beam powers that are PWM-controlled with 256 gradations. Then, the pattern image 50 is detected by the optical sensor 27. As shown in FIG. 10B, the peak value of the output signal of the optical sensor 27 becomes maximum when the duty ratio is 50%. Therefore, the microcontroller 26 determines the position of the pattern image 50 at which the peak value of the output signal of the optical sensor 27 is maximum, and determines the power of the laser beam when the determined position is formed. The determined power of the laser beam is such that the line duty ratio is 50%. The microcontroller 26 uses the laser beam power thus obtained and the developing bias of the developing unit 3 when the pattern image 50 is formed in subsequent image formation.

  At this time, the image forming apparatus 101 operates at a position indicated by a black circle in the graph of FIG. 9E showing the relationship between the developing bias and the amplitude value of the output signal of the optical sensor 27. Further, a pattern image with a duty ratio of 50% shown in FIG. 9B is formed by this developing bias. Note that FIG. 9B displays a pattern image and a state in which the pattern image is enlarged at a micro level.

  When density variation occurs from the state shown in FIG. 9E, the relationship between the output peak value of the optical sensor 27 and the developing bias shifts as shown by the black circles in FIGS. 9D and 9F. To do. Therefore, if the developing bias is used as it is, the duty ratio of the pattern image is not 50% as shown in FIGS. 9A and 9C. 9A is a pattern image for the state of FIG. 9D, and FIG. 9C is a pattern image for the state of FIG. 9F. 9A and 9C respectively show a pattern image and a state in which the pattern image is enlarged on a micro level. In this embodiment, density correction is performed during the printing process by controlling the developing bias so that the state shown in FIG.

  When performing density correction during printing, the image forming apparatus 101 according to the present embodiment forms a pattern image shown in FIG. 11A in a non-transfer area of the intermediate transfer belt 8, that is, in an area between images to be printed. 51 is formed. In FIG. 11A, seven pattern images 51 including two lines in the non-transfer area are formed at positions A1 to A7. The number of pattern images 51 formed in one non-transfer area may be another number, or the pattern images 51 may be formed in a plurality of non-transfer areas. That is, for example, the four pattern images 51 of A1 to A4 can be formed in one non-transfer area, and the three pattern images 51 of A5 to A7 can be formed in one non-transfer area. Furthermore, for example, seven pattern images 51 of A1 to A7 can be formed in different non-transfer areas. In the present embodiment, the development bias at the time of forming each pattern image 51 is changed stepwise. In FIG. 11A, the developing bias when forming the pattern image 51 at the position A1 is the smallest, and thereafter the developing bias is increased in order to form the pattern image 51 at the position A7. The development bias is maximized.

  The peak value of the output signal when the optical sensor 27 detects the pattern image 51 is maximum when the duty ratio is 50%. Therefore, the peak value of the output signal of the optical sensor 27 when each pattern image 51 is detected is obtained, and the development bias when the pattern image 51 having the maximum peak value is formed is determined by the microcontroller 26 after density correction. Is set as the development bias. For example, when the relationship between the developing bias and the output of the optical sensor 27 shown in FIG. 11B is obtained, density correction is performed by setting the developing bias when the pattern image 51 at the position A4 is formed. Done. Thereafter, when the relationship between the developing bias when the pattern image 51 is formed and the output of the optical sensor 27 again becomes as shown in FIG. 11C, the developing bias is set to the developing bias used at the position A3. Thus, density correction is performed.

  In FIG. 11, seven pattern images 51 are formed while changing the developing bias. Here, by reducing the value of the developing bias to be used, the density control time is shortened. However, in this case, the maximum value of the peak value of the output signal of the optical sensor 27 may not be directly detected. In this case, as will be described below with reference to FIG. 14, it is possible to determine the developing bias that maximizes the peak value of the output signal of the optical sensor 27 by linear interpolation.

  FIG. 14A shows a pattern image 52 formed when density correction is performed during the printing process. In FIG. 14A, four pattern images 52 including five lines are formed in the non-transfer area. Also in the present embodiment, the development bias at the time of forming the pattern image 52 from the positions B1 to B4 is changed stepwise. In FIG. 14A, it is assumed that the development bias at position B1 is the smallest, and thereafter the development bias is increased in order, and the development bias at position B4 is the largest.

  As shown in FIG. 14B, the peak value of the output signal of the optical sensor 27 is larger when the pattern image 52 at the position B2 is detected than when the pattern image 52 at the position B1 is detected. Yes. Further, the peak value of the output signal of the optical sensor 27 is larger when the pattern image 52 at the position B3 is detected than when the pattern image 52 at the position B4 is detected. Therefore, it can be seen that the peak value of the output signal of the optical sensor 27 becomes maximum at the developing bias between the developing bias formed at the position B2 and the developing bias formed at the position B3.

  Here, the developing bias and the peak value of the output signal of the optical sensor 27 are proportional to each other in the increasing region and the decreasing region. Therefore, as shown in FIG. 14B, linear interpolation is performed between the development bias and the peak value of the output signal of the optical sensor 27 when the pattern image 52 is formed at the positions B1 and B2. Similarly, linear interpolation is performed between the developing bias when the pattern image 52 is formed at the positions B3 and B4 and the peak value of the output signal of the optical sensor 27. And those intersection B0 is calculated | required. The development bias corresponding to the obtained intersection B0 is the development bias that maximizes the peak value of the output signal of the optical sensor 27.

  In the pattern image 52, there are five lines. However, if there are five lines, the peak value of the output of the optical sensor 27 continues multiple times as shown in FIG. This is because the noise component of the signal can be reduced by averaging. In the above-described embodiment, as the image forming condition related to the density, the power of the laser beam is controlled before printing, and the developing bias is controlled during printing. All of these control the development contrast, which is the difference between the potential at the position where the electrostatic latent image on the photoreceptor 1 is formed and the development bias. Therefore, an arbitrary value for changing the development contrast can be controlled before printing or during printing.

  Further, in the present embodiment, density control is performed with reference to the line width at which the duty ratio is 50%. Therefore, the maximum value of the peak value of the output signal of the optical sensor 27 is set as the target value, and the development contrast is controlled so that the peak value of the output signal of the optical sensor 27 becomes the target value. However, the line width at a predetermined duty ratio can be set as the target value. In this case, the development contrast is controlled using the peak value of the output signal of the optical sensor 27 corresponding to the target line width as the target value. In FIG. 5, when the target duty ratio is not 50%, there are two duty ratios corresponding to the target value, but the control unit 25 determines that the duty ratio of the pattern image detected by the optical sensor 27 is It can be determined by the position with respect to the maximum value. In the present embodiment, the regular reflection image of the pattern image is doubled at the position of the light receiving unit 270. However, if the pitch of the line of the regular reflection image generated at the position of the light receiving unit 270 by the pattern image is equal to the pitch of the light receiving unit An and the pitch of the light receiving unit Ap, it may be an arbitrary multiple.

  In the above-described embodiment, the line of the detection image 40 has been described as being in a direction orthogonal to the moving direction of the intermediate transfer belt 8. For example, the line is drawn obliquely with respect to the orthogonal direction. Also good. That is, the detected image 40 may be an image in which the toner amount (developer amount) regularly changes in the moving direction of the intermediate transfer belt 8 and includes a line in a direction different from the moving direction of the detected image 40. be able to.

  With the above configuration, a calibration can be executed by forming a detection image in the non-transfer area. The pattern images 40, 51, and 52 used in this embodiment are striped patterns having a very small width, and can prevent the back stain on the recording material.

<Second embodiment>
In the first embodiment, the diameter of a one-dot toner image is 85 μm, which is almost equal to the target line width. However, the diameter of a 1-dot toner image varies from product to product due to various factors of the image forming apparatus and is not the same. In the present embodiment, a case where the diameter of a one-dot toner image is larger than the formation target line width will be described. In the following description, it is assumed that the diameter of a one-dot toner image is 100 μm. Since other parts are the same as those in the first embodiment, the description thereof is omitted.

  FIGS. 8A and 8B are the same as FIGS. 6A and 6B, respectively. FIG. 8C is a one-dot toner image in this embodiment, and the diameter thereof is about 100 μm. FIG. 8D is a toner image formed with 2-dot line and space image data. As shown in FIG. 8D, the line width LD2 is thicker than the target line width Tw, and the space width LS2 is narrower than the target space width Tw. FIG. 6E shows image data of a 1-dot line and a 3-dot space, and FIG. 6F shows a toner image formed from the image data of FIG. As shown in FIG. 6C, since the diameter of a 1-dot toner image is 100 μm, the formed line width LD1 is also 100 μm, and thus becomes thicker than the target line width Tw = 84.6 μm. That is, the duty ratio is not 50%. Therefore, in this embodiment, as shown in FIG. 8G, the duty ratio is adjusted to 50% by reducing the density of the pixel data of each pixel forming the line. Specifically, the laser scanning time of each pixel can be adjusted in multiple stages by PWM control, and the scanning time of each pixel is defined so that the diameter of a one-dot toner image is approximately 85 μm. FIG. 8H is a toner image formed by the image data of FIG. As shown in FIG. 8G, by reducing the density of the pixel corresponding to the line, shortening the scanning time, and setting the diameter of the toner image to about 85 μm, the line width targeted for the LD 11 is 84.6 μm. Can be made approximately equal.

  As described above, the method of setting the toner image line to space ratio to 50:50 by adjusting the exposure intensity of the laser beam in the case where the diameter of the one-dot toner image is 100 μm has been described. However, the exposure intensity of the laser beam may be further finely adjusted to 50:50 in a state where the M dot and the (M + 3) space are about 50:50.

<Third embodiment>
In the first embodiment and the second embodiment, a pattern image having a duty ratio of approximately 50% is formed using image data for forming a line of 1 dot width. However, the image data may be image data for forming a line having a plurality of dot widths. In the present embodiment, a case where image data of a line having a width of 2 dots is used will be described. Since other parts are the same as those in the first embodiment, the description thereof is omitted.

  FIG. 12A shows image data with a line width of 2 dots and a space width of 3 dots, and FIG. 12B shows a pattern image formed with the image data of FIG. The diameter of a 1-dot toner image is 85 μm. As shown in FIG. 12B, LD2 is thicker than 105.75 μm, which is equal to the target line width Tw, which is 2.5 dots in this example. Therefore, in this embodiment, as shown in FIG. 12C, image data in which the scanning time of the pixels constituting the line is shortened to reduce the density is used. FIG. 12D shows the diameter of the toner image formed by the image data of the pixels constituting the line in FIG. With the image data shown in FIG. 12C, the diameter of the one-dot toner image is 63.5 μm as shown in FIG. FIG. 12E is a pattern image formed with the image data of FIG. In FIG. 12E, the line width LD21 and the space width LS31 are both about 105.8 μm, which is substantially equal to the target width TW. As described above, a pattern image having a duty ratio of approximately 50% can be formed even if the line width of the image data is 2 dots or more.

  For image data of M dot (M is a natural number) line and M dot space, it is difficult to set the duty ratio of the pattern image to 50% even if the power of the laser beam is controlled by the microcontroller 26. The reason will be described below. FIG. 13A shows image data of a 3-dot wide line and a 3-dot wide space. As already described, in the image data of FIG. 13A, the line width of the pattern image to be formed is thick, so that the image data in which the density of the pixels forming the line is lowered as shown in FIG. 13B. Is used. At this time, in order to make the duty ratio approximately 50%, the dot diameter of the toner image must be reduced to about 42.3 μm as shown in FIG. FIG. 13D is a pattern image formed with the image data of FIG. As shown in FIG. 13D, since the dot diameter of the toner image becomes too small, a gap is generated between the dots of the toner image. Due to this gap, the amplitude of the output signal of the optical sensor 27 decreases. Conversely, if the pixel density is adjusted so that the dot diameter does not cause a gap, the line width becomes too thick.

  As described above, the image data for forming a pattern image with a duty ratio of 50% needs to have a line width smaller than the space width.

  The line width of the pattern image only needs to be able to detect minute fluctuations in the line and space widths, and may be larger than the several dot width used in the above embodiments. However, if the line and space widths are too thick, the dynamic range with respect to minute fluctuations in the width is reduced, so for example, up to about 10 dots is a reasonable range.

<Other embodiments>
In the above-described embodiment, the differential processing of the signal indicating the temporal change in the amount of light received by each of the light receiving part Ap and the light receiving part An is performed. The light receiving part Ap and the light receiving part An are arranged along the moving direction of the intermediate transfer belt 8. Therefore, the temporal changes in the amounts of light received by the light receiving part Ap and the light receiving part An are shifted from each other by a time determined by the distance between the light receiving part Ap and the light receiving part An and the moving speed of the intermediate transfer belt 8. Accordingly, the differential processing of the signals indicating the temporal changes in the received light amounts of the light receiving unit Ap and the light receiving unit An is performed by, for example, bifurcating the signal corresponding to the received light amount output by one light receiving unit, It can also be realized by performing differential processing while shifting by a predetermined time. The time shifted at this time is determined by the distance between the light receiving portion Ap and the light receiving portion An and the moving speed of the intermediate transfer belt 8. That is, in each of the above embodiments, a differential process is performed between the total amount of light received by one or more light receivers Ap and the total amount of light received by one or more light receivers An. This is equivalent to performing differential processing of a signal indicating the amount of received light with a sum of one or more first time positions and a sum of one or more second time positions. For example, when a pattern image of a plurality of lines is moved, the amount of light received by the light receiving unit vibrates at a period corresponding to the plurality of lines. Therefore, for example, the first time positions are set so that the periods are in phase with each other, and the second time positions are also set so that the periods are in phase with each other. This corresponds to, for example, setting the time interval between the plurality of first time positions as the time during which the intermediate transfer belt 8 moves by the line pitch. The same applies to the time intervals between the plurality of second time positions. On the other hand, the first time position and the second time position are set so as to be in opposite phases, for example, so that the phases of this period are different.

  Moreover, in the said embodiment, the relationship between the width | variety of the arrangement direction of the light-receiving surface of the light-receiving part Ap and the light-receiving part An, and the width | variety of a line was demonstrated. Here, the light receiving portions Ap and An receive the reflected light from a certain region at the same time on the light receiving surface, which is equivalent to obtaining the average value of the reflected light received simultaneously. Therefore, increasing the width of the light receiving surface in the arrangement direction is equivalent to, for example, obtaining a moving average with respect to a signal indicating a temporal change in the amount of light received output from the light receiving part Ap and the light receiving part An. Here, in the form of performing differential processing at different time positions of signals output from one light receiving unit, a memory is required to branch and shift the signals. Therefore, by using this memory, two sections of the first section and the second section are set in the signal output from one light receiving unit, the moving average value of the first section, and the second section The differential processing of the moving average value can be easily performed. Thereby, the section according to the width of the line can be easily set without changing the width of the light receiving surface. At this time, the time interval between the first section and the second section corresponds to the distance between the light receiving section Ap and the light receiving section An in the above embodiment, and the section lengths of the first section and the second section are as follows: This corresponds to the width of the light receiving surface in the arrangement direction.

  FIG. 16 is a configuration diagram for performing the above-described processing on a signal output from one light receiving unit. In the present embodiment in which differential processing of signals output from one light receiving unit is performed at different times, for example, the optical sensor 27 simply outputs a light detection signal corresponding to the amount of light received by the light receiving unit to the engine control unit 25. To do. Note that the sampling unit 31, the moving average processing units 32 and 33, and the differential processing unit 34 in FIG. 16 are provided in the engine control unit 25, for example. However, the sampling unit 31, the moving average processing units 32 and 33, and the differential processing unit 34 may be provided in the control circuit 275, for example. The light detection signals are sampled by the sampling unit 31 and output to the moving average processing units 32 and 33, respectively. The moving average processing units 32 and 33 obtain a moving average value of a section having a predetermined length, and output the obtained moving average values to the differential processing unit 34, respectively. As described above, the period between the section corresponding to the moving average value output by the moving average processing unit 32 and the section corresponding to the moving average value output by the moving average processing unit 33 at the same time is, for example, The phase of the light detection signal is set to be different. The differential processing unit 34 performs differential processing of the moving average value from the moving average processing units 32 and 33. With this configuration, the differential processing unit 34 outputs, for example, an output signal similar to the output signal of the differential amplifier 290 in FIG.

  In other words, it can be said that the above embodiment takes the difference in the amount of reflected light including the regular reflected light components from different positions on the surface of the intermediate transfer belt 8 before and after the pattern image. For example, this is apparent from the fact that the regular reflection light received by the light receiving part Ap and the light receiving part An at the same time is reflected at different positions on the surface of the intermediate transfer belt 8 before and after the pattern image. Also, in the form of performing differential processing at different time positions of signals output by one light receiving unit, the difference in the amount of reflected light including specularly reflected light components from different positions on the surface of the pattern image and the surrounding intermediate transfer belt 8 is also obtained. It is what you take. For example, it is assumed that differential processing is performed between a first time position of the light detection signal and a second time position after the first time position. In addition, the position on the pattern image that is the reflection position of the regular reflection light to the light receiving unit in the first time is defined as the first position, and the reflection position of the regular reflection light to the light reception unit in the second time is the second position. Position. In this case, the distance between the first position and the second position is equal to a value obtained by multiplying the moving speed of the surface of the intermediate transfer belt 8 by the difference between the first time and the second time. Therefore, performing the differential processing between the first time position and the second time position means that the total amount of received light when the light receiving unit receives specularly reflected light from the first position and the light receiving unit is second. This is because it corresponds to performing differential processing with the total amount of light received when the regular reflection light is received from the position.

  As described above, when the light emitting element irradiates the intermediate transfer belt 8 with the divergent light beam, a certain wide range of the intermediate transfer belt 8 is illuminated by the light emitting element. Therefore, the diffusely reflected light by the line of the pattern image received by the light receiving element becomes substantially constant while the pattern image passes through this irradiation region. Therefore, the diffuse reflection light can be removed or suppressed and only the specular reflection light component can be extracted by the difference in the light reception amounts of the plurality of light receiving elements or the difference in the light reception amounts at different time positions of one light receiving element. With this configuration, it is possible to control the density while suppressing the influence of diffuse reflected light.

  The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (15)

During printing, a plurality of detected images each having a plurality of lines in directions different from the moving direction of the surface of the image carrier between the images to be printed formed on the image carrier, and the image forming conditions related to the density are changed. Image forming means for forming
Detection means for detecting the plurality of detection images formed on the image carrier and outputting an output signal;
Control means for controlling the image forming conditions based on the peak value of the output signal from the detection means;
With
The detection means includes
A light emitting means for irradiating the image carrier with light;
A light receiving means for receiving the light emitted from the light emitting means and reflected by the image carrier and outputting a detection signal corresponding to the amount of received light;
An output means for outputting, as the output signal, a signal corresponding to a difference in value of the detection signal corresponding to the amount of received light including specularly reflected light components from different positions on the surface of the image carrier;
An image forming apparatus comprising:   In order to form the detection image, the control means has a width in the moving direction of the surface of the image carrier of the line of M dots (M is an integer), and a width between the lines is N dots (N is an integer) The image forming apparatus according to claim 1, wherein M <N.   When the ratio of the diameter of one dot actually formed to the calculated diameter of one dot of the toner image is A, the N dots are rounded up to the decimal point of the value obtained by M + (A−1) × 2. The image forming apparatus according to claim 2, wherein the image forming apparatus is an integer obtained by rounding down.   The image forming apparatus according to claim 2, wherein a scan time of each pixel is defined in the image data. The light receiving means includes a plurality of first light receiving portions and a plurality of second light receiving portions arranged alternately,
The output means outputs, as the output signal, a signal corresponding to a difference between a total received light amount of the plurality of first light receiving units and a total received light amount of the plurality of second light receiving units. Item 5. The image forming apparatus according to any one of Items 1 to 4.   The pitch of the plurality of shadows by the plurality of lines of the detection image generated at the position where the light receiving unit is provided when the light emitting unit irradiates the detection image with light is the pitch of the plurality of first light receiving units. The image forming apparatus according to claim 5, wherein the pitch is equal to a pitch of the plurality of second light receiving units. When the light emitting means irradiates the detection image with light, the shadow due to the line of the detection image generated at the position where the light receiving means is provided moves in the first direction by the movement of the surface of the image carrier,
The image forming apparatus according to claim 5, wherein widths of the light receiving surfaces of the first light receiving unit and the light receiving surfaces of the second light receiving unit in the first direction are equal to each other.   The output means includes a difference between a sum of values of a plurality of first time positions of the detection signal and a sum of values of a plurality of second time positions separated from the first time position by a predetermined period. 5. The image forming apparatus according to claim 1, wherein a signal corresponding to the output is output as the output signal. 6.   The value of the first time position is an average value of the first interval of the detection signal, and the value of the second time position is a second value separated from the first interval by the predetermined period. The image forming apparatus according to claim 8, wherein the image forming apparatus is an average value of the intervals.   The time interval between the plurality of first time positions and the time interval between the plurality of second time positions correspond to a time during which the image carrier moves by the pitch of the plurality of lines. The image forming apparatus according to 8 or 9.   The control means detects the plurality of detection images with the detection means, obtains a relationship between the image forming condition and the peak value of the output signal from the detection means, and determines the peak of the output signal from the detection means. The image forming apparatus according to claim 1, wherein the image forming condition is controlled so that the value becomes a target value.   The image forming apparatus according to claim 11, wherein the target value is a maximum value of a peak value of an output signal from the detection unit.   The control means forms an image having a plurality of lines in a direction different from the moving direction of the surface of the image carrier on the image carrier while printing is not being performed, while changing the image formation conditions regarding density. The image forming condition according to claim 12, wherein the image forming condition is set so that the image is detected by the detecting unit, and a peak value of an output signal of the detecting unit becomes the target value. apparatus. The image forming unit includes a photosensitive member, a charging unit that charges the photosensitive member, an exposure unit that exposes the photosensitive member to form an electrostatic latent image, and a developing bias to apply the electrostatic latent image. And developing means for developing with a developer,
The image forming condition is a development contrast that is a difference between the development bias and a potential at a position exposed by the exposure unit of the photoreceptor.
The image forming apparatus according to claim 13.   The control means controls the development contrast by changing the exposure intensity of the exposure means while printing is not being performed, and controls the development contrast by changing the development bias during printing. The image forming apparatus according to claim 14.

Images