Image forming device

Abstract

To suppress reproducibility of an image region whose light exposure is low from deteriorating.SOLUTION: An image forming device comprises: an exposure head that exposes a photoreceptor, with a plurality of light-emitting elements arranged along a rotary shaft direction of the photoreceptor; setting means that sets brightness of light exposure of the photoreceptor exposed using the plurality of light-emitting elements; and processing means that generates first image data which are aggregate of bit data for controlling lighting-up or lighting-out of the plurality of light-emitting elements and second image data in which the light-emitting elements that emit light to the first image data are increased or decreased, on the basis of the first image data, which performs processing for outputting the second image data to the exposure head. The processing means generates the second image data by increasing or decreasing the bit data for showing lighting-up of the light-emitting elements in accordance with a ratio between a set value of the brightness of light-exposure set by the setting means and a reference value of brightness of light-exposure, in a first region excluding an edge region in a region where the bit data showing lighting-up of the light-emitting element in the first image data are consecutive.SELECTED DRAWING: Figure 17

Description

[Technical Field]

[0001] The present invention relates to an exposure technique in an image forming apparatus. [Background technology]

[0002] Electrophotographic image forming apparatuses expose a rotating photoconductor to light to form an electrostatic latent image on the photoconductor, and then develop the electrostatic latent image with a developer such as toner to form an image. To expose the photoconductor, the image forming apparatus includes, for example, an exposure device that moves a laser beam spot on the photoconductor in a direction parallel to the rotation axis of the photoconductor. The direction of the rotation axis of the photoconductor (the direction of the rotation axis) is referred to as the main scanning direction. Such an exposure device requires a polygon mirror for moving the laser beam spot on the photoconductor in the main scanning direction, a motor for driving the polygon mirror, and other components. In contrast, Patent Documents 1 and 2 disclose image forming apparatuses that expose a photoconductor with an exposure head equipped with multiple light-emitting elements arranged along the main scanning direction. Using such an exposure head eliminates the need for a polygon mirror, motor, and other components, allowing for a more compact image forming apparatus. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-112856 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-183436 Summary of the Invention [Problem to be solved by the invention]

[0004] The multiple light-emitting elements in the exposure head can be, for example, light-emitting diodes (LEDs) or organic electroluminescent (EL) elements. However, using light-emitting elements with relatively low exposure luminance (exposure intensity), such as LEDs or OLEDs, in the exposure head requires a longer exposure time per pixel. For example, while the exposure time for a single pixel with laser light is within 10 ns, an OLED requires an exposure time of 10 μs or more. If the exposure amount is defined as the integral of the exposure luminance over the exposure time, the potential of the photoconductor after exposure will not be the same even if the exposure amount is the same for different exposure luminances. Specifically, the lower the exposure luminance, the more linear the relationship between the exposure amount and the potential of the photoconductor after exposure. This increased linearity reduces the contrast in low-exposure image areas, such as highlight areas and thin lines, and reduces image reproducibility.

[0005] The present invention provides a technique for suppressing the deterioration of reproducibility in image areas with low exposure. [Means for solving the problem]

[0006] According to one aspect of the present invention, an image forming apparatus comprises a rotationally driven photosensitive member, an exposure head that exposes the photosensitive member using a plurality of light-emitting elements arranged along the rotational axis direction of the photosensitive member, a setting means for setting the exposure brightness of the photosensitive member using the plurality of light-emitting elements, a processing means for generating first image data which is a collection of bit data that controls the turning on or off of the plurality of light-emitting elements, and second image data which increases or decreases the number of light-emitting elements that emit light compared to the first image data based on the first image data, and outputting the second image data to the exposure head, wherein the processing means generates the second image data by increasing or decreasing the bit data indicating the turning on of the light-emitting elements in a first region excluding edge regions within a region where bit data indicating the turning on of the light-emitting elements in the first image data is continuous, in accordance with the ratio between the setting value of the exposure brightness set in the setting means and a reference value of the exposure brightness. [Effects of the Invention]

[0007] According to the present invention, it is possible to suppress the deterioration of reproducibility in image areas with low exposure. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic configuration diagram of an image forming apparatus according to some embodiments. [Figure 2] FIG. 2 is an explanatory diagram illustrating the configuration of an exposure head according to some embodiments. [Figure 3] 5A and 5B are explanatory diagrams illustrating the configuration of a printed circuit board of an exposure head according to some embodiments. [Figure 4] 1A and 1B are diagrams illustrating a light-emitting chip and a light-emitting element array within the light-emitting chip according to some embodiments. [Figure 5] FIG. 1 is a plan view showing a schematic configuration of a light-emitting chip according to some embodiments. [Figure 6] 1 is a cross-sectional view showing a schematic configuration of a light-emitting chip according to some embodiments. [Figure 7] FIG. 10 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. [Figure 8] FIG. 1 is a schematic control configuration diagram of an image forming apparatus according to some embodiments. [Figure 9] FIG. 4 is a circuit diagram showing a control configuration of an exposure head according to some embodiments. [Figure 10] 10 is a signal chart related to accessing a register of a light emitting chip according to some embodiments. [Figure 11] 10 is a signal chart associated with transmitting image data to a light-emitting chip according to some embodiments. [Figure 12] FIG. 2 is a functional block diagram showing a detailed configuration of a light-emitting chip according to some embodiments. [Figure 13] 10A and 10B are diagrams showing examples of the relationship between the line width in input image data and the line width in an output image. [Figure 14] 10 is a flowchart of a process for generating image data to be output to an exposure head according to some embodiments. [Figure 15] FIG. 10 is a diagram showing an example of an edge-preserving filter. [Figure 16]10A and 10B are diagrams showing examples of an LUT and an error diffusion filter. [Figure 17] FIG. 15 is an explanatory diagram of the processing in the flowchart shown in FIG. 14. [Figure 18] 10 is a flowchart of a process for determining an increase amount of exposure brightness according to an embodiment. [Figure 19] 10 is a flowchart of a process for determining an increase amount of exposure brightness according to an embodiment. [Figure 20] 10 is a flowchart of a static elimination process according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0010] First Embodiment FIG. 1 is a diagram illustrating a schematic configuration example of an image forming apparatus 1 according to this embodiment. A reading unit 100 optically reads an original placed on a platen and generates read image data. An image creating unit 103 forms an image on a sheet, for example, based on the read image data generated by the reading unit 100 or based on print image data received from an external device via a network. The image creating unit 103 includes image forming units 101a, 101b, 101c, and 101d. The image forming units 101a, 101b, 101c, and 101d form black, yellow, magenta, and cyan toner images, respectively. The image forming units 101a, 101b, 101c, and 101d have the same configuration and are hereinafter collectively referred to as the image forming unit 101. A photoconductor 102 of the image forming unit 101 is rotated clockwise in the figure during image formation. The charger 107 charges the photoconductor 102. The exposure head 106 exposes the photoconductor 102 to light, thereby forming an electrostatic latent image on the surface of the photoconductor 102. The developer 108 develops the electrostatic latent image on the photoconductor 102 with toner, thereby forming a toner image on the photoconductor 102. The toner image formed on the photoconductor 102 is transferred to a sheet transported on a transfer belt 111. By transferring the toner images of the four photoconductors 102 onto the sheet in an overlapping manner, a color image containing four color components: black, yellow, magenta, and cyan, can be formed.

[0011] The conveying unit 105 controls the feeding and transport of sheets. Specifically, the conveying unit 105 feeds a sheet from a designated unit among the internal storage units 109a and 109b, the external storage unit 109c, and the manual feed unit 109d to a transport path of the image forming apparatus 1. The fed sheet is transported to the registration rollers 110. The registration rollers 110 transport the sheet onto the transfer belt 111 at an appropriate timing so that the toner images on the photoconductors 102 are transferred to the sheet. As described above, the toner image is transferred to the sheet while the sheet is transported on the transfer belt 111. The fixing unit 104 fixes the toner image to the sheet by applying heat and pressure to the sheet to which the toner image has been transferred. After the toner image is fixed, the sheet is discharged to the outside of the image forming apparatus 1 by the discharge rollers 112. An optical sensor 113 is disposed opposite the transfer belt 111. The optical sensor 113 optically reads the test chart formed on the transfer belt 111 by the image forming unit 101. The results of reading the test chart by the optical sensor 113 are used for correcting positional deviation, density, and the like.

[0012] Although an example has been described here in which a toner image is directly transferred from each photoconductor 102 to a sheet on the transfer belt 111, the toner image may also be indirectly transferred from each photoconductor 102 to a sheet via an intermediate transfer body. Also, although an example has been described here in which a color image is formed using toners of multiple colors, the technology according to the present disclosure is also applicable to an image forming apparatus that forms a monochrome image using toner of a single color.

[0013] 2(A) and 2(B) show the photoconductor 102 and the exposure head 106. The exposure head 106 includes a light-emitting element array 201, a printed circuit board 202 on which the light-emitting element array 201 is mounted, a rod lens array 203, and a housing 204 that holds the rod lens array 203 and the printed circuit board 202. The photoconductor 102 has a cylindrical shape. The exposure head 106 is disposed such that its longitudinal direction is parallel to the direction D1 of the rotation axis of the photoconductor 102 (hereinafter referred to as the axial direction), and the surface on which the rod lens array 203 is attached faces the surface of the photoconductor 102. While the photoconductor 102 rotates in the circumferential direction D2, the light-emitting element array 201 of the exposure head 106 emits light, and the rod lens array 203 focuses the light on the surface of the photoconductor 102.

[0014] 3(A) and 3(B) show an example of the configuration of the printed circuit board 202. Note that Fig. 3(A) shows the surface on which the connector 305 is mounted, and Fig. 3(B) shows the surface on which the light-emitting element array 201 is mounted (the surface opposite to the surface on which the connector 305 is mounted). Fig. 4 is a diagram schematically showing a light-emitting chip 400 and an array of light-emitting elements 602 in the light-emitting chip 400.

[0015] In this embodiment, the light-emitting element array 201 includes a plurality of light-emitting elements arranged two-dimensionally. The light-emitting element array 201 includes, as a whole, N columns of light-emitting elements in the axial direction D1 and M rows of light-emitting elements in the circumferential direction D2 of the photoconductor, where M and N are integers equal to or greater than 2. In the example shown in FIG. 3B, the light-emitting element array 201 is divided into 20 light-emitting chips 400-1 to 400-20, each including a subset of the entire plurality of light-emitting elements, and the light-emitting chips 400-1 to 400-20 are arranged in a staggered pattern along the axial direction D1. The light-emitting chips 400-1 to 400-20 are also collectively referred to as light-emitting chips 400. As shown in FIG. 3B, the area occupied by all of the light-emitting elements of the 20 light-emitting chips in the axial direction D1 is wider than the area occupied by the maximum width W0 of the input image data. Therefore, some light-emitting elements located at both ends in the axial direction D1 may not be used to expose the photoconductor 102 unless image misalignment is detected. Each light-emitting chip 400 on the printed circuit board 202 is connected to the image controller 700 (FIG. 9) via a connector 305. For convenience of explanation, the side with the smaller branch number of the light-emitting chips 400-1 to 400-20 arranged along the axial direction D1 may be referred to as the "left" and the side with the larger branch number as the "right." For example, the light-emitting chip 400-1 is the leftmost light-emitting chip 400, and the light-emitting chip 400-20 is the rightmost light-emitting chip.

[0016] The number J (J=N / 20) of light-emitting elements 602 arranged in each row of one light-emitting chip 400 may be equal to, for example, 748 (J=748). Meanwhile, the number M of light-emitting elements 602 arranged in each column of one light-emitting chip 400 may be equal to, for example, 4 (M=4). That is, in an exemplary embodiment, each light-emitting chip 400 has 748 light-emitting elements 602 in the axial direction D1 and 4 light-emitting elements 602 in the circumferential direction D2, for a total of 2992 (=748×4) light-emitting elements 602. The spacing PC between the center points of adjacent light-emitting elements 602 in the circumferential direction D2 may be, for example, approximately 21.16 μm, which corresponds to a resolution of 1200 dpi. The spacing between the center points of adjacent light-emitting elements 602 in the axial direction D1 may also be approximately 21.16 μm, in which case the 748 light-emitting elements 602 occupy a length of approximately 15.8 mm in the axial direction D1. 4 shows an example in which the light emitting elements 602 are arranged in a grid pattern in each light emitting chip 400. However, the M (M=4) light emitting elements 602 in each row may be arranged in a staircase pattern.

[0017] 5 is a plan view showing a schematic configuration of the light-emitting chip 400. The plurality of light-emitting elements 602 of each light-emitting chip 400 are formed on a light-emitting substrate 402, which is, for example, a silicon substrate. A circuit unit 406 for driving the plurality of light-emitting elements 602 is provided on the light-emitting substrate 402. Signal lines for communicating with the image controller 700, power lines for connecting to a power source, and ground lines for connecting to ground are connected to the pads 408-1 to 408-9. The signal lines, power lines, and ground lines may be wires made of, for example, gold.

[0018] FIG. 6 shows a portion of the cross section taken along line AA in FIG. 5. A plurality of lower electrodes 504 are formed on the light-emitting substrate 402. A gap of length d is provided between two adjacent lower electrodes 504. A light-emitting layer 506 is provided on the lower electrodes 504, and an upper electrode 508 is provided on the light-emitting layer 506. The upper electrode 508 is a common electrode for the plurality of lower electrodes 504. When a voltage is applied between the lower electrode 504 and the upper electrode 508, a current flows from the lower electrode 504 to the upper electrode 508, causing the light-emitting layer 506 to emit light. Therefore, one lower electrode 504 and a partial region of the light-emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light-emitting element 602. That is, in this embodiment, the light-emitting substrate 402 includes a plurality of light-emitting elements 602.

[0019] The light-emitting layer 506 may be, for example, an organic EL film. The upper electrode 508 is formed of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light-emitting layer 506. In this embodiment, the entire upper electrode 508 transmits the emission wavelength of the light-emitting layer 506; however, the entire upper electrode 508 does not need to transmit the emission wavelength. Specifically, it is sufficient that a partial region through which light from each light-emitting element 602 passes transmits the emission wavelength. While FIG. 6 shows a single continuous light-emitting layer 506, multiple light-emitting layers 506 each having a width equal to the width W of the lower electrode 504 may be formed on the lower electrode 504. Furthermore, in FIG. 6, the upper electrode 508 is a single common electrode for multiple lower electrodes 504; however, multiple upper electrodes 508 each having a width equal to the width W of the lower electrode 504 may be formed corresponding to each lower electrode 504. Furthermore, among the lower electrodes 504 of each light-emitting chip 400, a first plurality of lower electrodes 504 may be covered with a first light-emitting layer 506, and the second plurality of lower electrodes 504 may be covered with a second light-emitting layer 506. Furthermore, among the lower electrodes 504 of each light-emitting chip 400, a first upper electrode 508 may be commonly formed corresponding to the first plurality of lower electrodes 504, and a second upper electrode 508 may be commonly formed corresponding to the second plurality of lower electrodes 504. Even in such a configuration, one lower electrode 504 and the region of the light-emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light-emitting element 602.

[0020] 4 shows an example in which the light emitting elements 602 are arranged in a grid pattern in each light emitting chip 400, but the M light emitting elements 602 in each row can be arranged in a stepped pattern at a constant pitch. FIG. 7 is an explanatory diagram of multiple exposure using the light emitting elements 602 arranged in a stepped pattern. j_mrepresents the light emitting element 602 in the j-th column from the left in the axial direction D1 and in the m-th row from the top in the circumferential direction. In this example, j is an integer between 0 and 747, and m is an integer between 0 and 3. The circumferential pitch PC of the light emitting elements 602 may be approximately 21.16 μm, as described above. The axial spacing between two adjacent light emitting elements 602 among the M light emitting elements 602 in each column, i.e., the axial pitch P of the light emitting elements 602, is A may be about 5 μm, which corresponds to a resolution of 4800 dpi.

[0021] By arranging the four light-emitting elements 602 in each column in a stepped manner in this way, any two adjacent light-emitting elements 602 occupy areas that partially overlap in the axial direction. While the photoconductor 102 rotates, for example, four light-emitting elements 602 corresponding to each pixel position of input image data with a resolution of 1200 dpi are sequentially illuminated to expose one pixel at 1200 dpi. If the input image data is 2400 dpi, two light-emitting elements 602 are sequentially illuminated to expose one pixel at 2400 dpi. In the following description, the area of the photoconductor 102 exposed by one light-emitting element 602 is referred to as a "dot."

[0022] In the example of FIG. 7, when the pixel value at the left end of the i-th line of the input image data indicates that light is on, the light-emitting element R 0_0 , R 0_1 , R 0_2 , R 0_3 Each of them corresponds to a line L on the surface of the photoreceptor 102. i As a result, the line L i Similarly, when the j-th pixel value from the left of the i-th line of the input image data indicates that light is emitted, the light-emitting element R j_0 , R j_1 , R j_2 , R j_3 Each of them corresponds to a line L on the surface of the photoreceptor 102. iAs a result, the line L i The area corresponding to the j-th pixel from the left of the spot SP j is formed.

[0023] 7, by activating the four light-emitting elements 602 in each row of the light-emitting chip 400 at appropriate times, smooth lines of an electrostatic latent image can be formed on the surface of the photoreceptor 102, each consisting of a series of partially overlapping spots spaced at regular intervals. As a result of these lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is formed.

[0024] Although specific numerical values are used for explanation purposes in this disclosure, these specific numerical values are merely examples, and the present invention is not limited to the specific numerical values used in the embodiments. Specifically, the number of light-emitting chips provided on one printed circuit board is not limited to 20 and can be any number equal to or greater than 1. Furthermore, the light-emitting elements provided on each light-emitting chip 400 are not limited to being arranged in 4 rows and 748 columns, and other numbers of rows and columns are possible. Furthermore, the circumferential pitch and axial pitch of the light-emitting elements are not limited to approximately 21.16 μm and approximately 5 μm, and may be any other values.

[0025] FIG. 8 shows the control configuration of the image forming apparatus. The controller 800 includes a CPU 801 and a memory 802. The CPU 801 has one or more processors. The memory 802 is a collective term for a non-volatile memory device and a volatile memory device, and stores a control program executed by the CPU 801 and control data used by the CPU 801 to control the image forming apparatus. The controller 800 outputs image data to the image controller 700 during image formation. Details of the image controller 700 will be described later using FIG. 9. When performing correction control for misregistration and density, the controller 800 obtains the reading results of a test chart formed on the transfer belt 111 from the optical sensor 113. The temperature and humidity sensor 114 detects the environmental temperature and humidity of the environment in which the image forming apparatus is installed and notifies the controller 800 of the detection results as environmental information.

[0026] 9 shows the control configuration of the exposure head 106. The image controller 700 is a control circuit that communicates with the printed circuit board 202 via multiple signal lines (wires). The light-emitting control unit 705 terminates the signal lines between the image controller 700 and the printed circuit board 202. The n-th light-emitting chip 400-n (n is an integer from 1 to 20) on the printed circuit board 202 is connected to the light-emitting control unit 705 via a signal line DATAn and a signal line WRITEn. The signal line DATAn is used to transmit image data from the image controller 700 to the light-emitting chip 400-n. The signal line WRITEn is used by the image controller 700 to write control data to a register of the light-emitting chip 400-n.

[0027] One signal line CLK, one signal line SYNC, and one signal line EN are further provided between the light-emitting control unit 705 and each light-emitting chip 400. The signal line CLK is used to transmit a clock signal for transmitting data on the signal lines DATAn and WRITEn. The light-emitting control unit 705 outputs a clock signal generated based on a reference clock signal from the clock generation unit 702 to the signal line CLK. The signals transmitted to the signal lines SYNC and EN will be described later.

[0028] The image data processing unit 703 performs image processing on the image data received from the controller 800 to generate binary bitmap image data for controlling the on / off of light emission of the light-emitting elements 602 of the light-emitting chips 400 on the printed circuit board 202. This image processing may include, for example, raster conversion, tone correction, color conversion, and halftone processing. The image data processing unit 703 performs a thinning process (described later) on the image data after halftone processing (first image data) and transmits the thinned image data (second image data) to the light-emission control unit 705. The register access unit 704 receives control data to be written to the registers in each light-emitting chip 400 from the CPU 701 and transmits it to the light-emission control unit 705. The control data includes a setting value for the light emission luminance (exposure luminance) of each light-emitting element 602 of each light-emitting chip 400. The setting value may be represented, for example, by the value of a drive current to be passed through each light-emitting element 602 to obtain a target exposure luminance, or the value of a drive voltage for passing the drive current.

[0029] FIG. 10 shows the transition of the signal level of each signal line when control data is written to the register of the light-emitting chip 400. An enable signal that is at high level during communication and indicates that communication is in progress is output to the signal line EN. The light-emitting control unit 705 transmits a start bit to the signal line WRITEn in synchronization with the rising edge of the enable signal. Next, the light-emitting control unit 705 transmits a write identification bit indicating a write operation, and then transmits the address of the register to which the control data is to be written (4 bits in this example) and the control data (8 bits in this example). When writing to the register, the light-emitting control unit 705 sets the frequency of the clock signal transmitted to the signal line CLK to, for example, 3 MHz.

[0030] FIG. 11 shows the transition of the signal level of each signal line when image data is transmitted to each light-emitting chip 400. A periodic line synchronization signal indicating the exposure timing of each line on the photoconductor 102 is output to the signal line SYNC. If the peripheral speed of the photoconductor 102 is 200 mm / s and the peripheral resolution is 1200 dpi (approximately 21.16 μm), the line synchronization signal is output at a period of approximately 105.8 μs. The light-emitting control unit 705 transmits image data to the signal lines DATA1 to DATA20 in synchronization with the rising edge of the line synchronization signal. In this embodiment, each light-emitting chip 400 has 2992 light-emitting elements 602, and therefore, image data indicating the light-emitting or non-light-emitting status of each of the total 2992 light-emitting elements 602 must be transmitted to each light-emitting chip 400 within a period of approximately 105.8 μs. Therefore, in this example, as shown in FIG. 11, when transmitting image data, the light-emitting control unit 705 sets the frequency of the clock signal transmitted to the signal line CLK to 30 MHz.

[0031] FIG. 12 is a functional block diagram showing a detailed configuration of one light-emitting chip 400. As also shown in FIG. 5, the light-emitting chip 400 has nine pads 408-1 to 408-9. The pads 408-1 and 408-2 are connected to a power supply voltage VCC via a power supply line. Power is supplied from this power supply voltage VCC to each circuit of the circuit unit 406 of the light-emitting chip 400. The pads 408-3 and 408-4 are connected to ground via a ground line. Each circuit of the circuit unit 406 and the upper electrode 508 are connected to ground via the pads 408-3 and 408-4. The signal line CLK is connected to the transfer unit 1003, the register 1102, and the latch units 1004-001 to 1004-748 via the pad 408-5. The signal lines SYNC and DATAn are connected to the transfer unit 1003 via the pads 408-6 and 408-7. The signal lines EN and WRITEn are connected via pads 408-8 and 408-9 to a register 1102. The register 1102 stores control data including data indicating the setting value of the exposure luminance of the light emitting element 602, for example.

[0032] The transfer unit 1003 receives input image data (second image data) from signal line DATAn, including a series of data values each indicating whether one light-emitting element 602 emits light or not, in synchronization with the clock signal from signal line CLK, starting from the line synchronization signal from signal line SYNC. The transfer unit 1003 performs serial-to-parallel conversion on the series of data values serially received from signal line DATAn in units of M (e.g., M=4) data values. For example, the transfer unit 1003 has four cascade-connected D flip-flops, and parallelizes the data values DATA-1, DATA-2, DATA-3, and DATA-4 input over four clocks, and outputs the parallelized data to latch units 1004-0001 to 1004-748. The transfer unit 1003 also has four D flip-flops for delaying the line synchronization signal, and outputs a first latch signal to latch unit 1004-001 via signal line LAT1 four clocks after the line synchronization signal is input.

[0033] The kth latch unit 1004-k (k is an integer from 1 to 748) holds, in a latch circuit, four data values DATA-1, DATA-2, DATA-3, and DATA-4 input from the transfer unit 1003 simultaneously with the input of the kth latch signal. Except for the last-stage latch unit 1004-748, the kth latch unit 1004-k outputs a (k+1)th latch signal, which is the kth latch signal delayed by four clocks, to the latch unit 1004-(k+1) via the signal line LAT(k+1). The kth latch unit 1004-k continues to output drive signals based on the four data values held in the latch circuit to the current driver 1104 during the signal period of the kth latch signal. For example, there is a four-clock delay between the timing at which the first latch signal is input to the latch unit 1004-1 and the timing at which the second latch signal is input to the latch unit 1004-2. Therefore, the latch unit 1004-1 outputs drive signals based on the first, second, third, and fourth data values to the current driver 1104, while the latch unit 1004-2 outputs drive signals based on the fifth, sixth, seventh, and eighth data values to the current driver 1104. Generally speaking, the latch unit 1004-k outputs drive signals based on the (4k-3), (4k-2), (4k-1), and (4k)th data values to the current driver 1104. Therefore, in the embodiment shown in FIG. 12 , 748 latch units 1004-001 to 1004-748 output 2992 drive signals in approximately parallel to the current driver 1104 for controlling the driving of 2992 (=748×4) light-emitting elements 602. Each drive signal is a binary signal indicating a high level or a low level.

[0034] The current driver 1104 has 2992 light-emitting drive circuits corresponding to the 2992 light-emitting elements 602, each of which includes a partial region of the light-emitting layer 506. Each light-emitting drive circuit applies a drive voltage corresponding to the exposure brightness indicated by the control data in the register 1102 to the light-emitting layer 506 of the corresponding light-emitting element 602 while the corresponding drive signal indicates a high level, which means that light is on. This causes a drive current to flow through the light-emitting layer 506, causing the light-emitting element 602 to emit light. Note that the control data may indicate one individual exposure intensity for each light-emitting element 602, one exposure intensity for each group of light-emitting elements 602, or one exposure intensity common to all light-emitting elements 602.

[0035] For example, when using light-emitting elements with low exposure luminance (exposure intensity), such as the exposure head 106 in this embodiment, the exposure time per pixel must be longer, e.g., 10 μs or longer, to ensure the required exposure dose, compared to when using laser light. In such cases, the linearity of the relationship between the exposure dose and the potential of the photoconductor 102 after exposure increases. This linearity reduces the contrast in image regions with low exposure, such as highlight areas and thin lines, and image reproducibility decreases. As an example, FIG. 13 shows the relationship between the line width indicated by the input image data and the line width of the output image (the formed image) when organic EL elements are used as light-emitting elements. The horizontal axis of FIG. 13 represents the line width in dots at 600 dpi. Note that a dot corresponds to the area of the photoconductor 102 exposed by one light-emitting element 602. Because of the 600 dpi resolution, the dot spacing is approximately 42.3 μm. The vertical axis of FIG. 13 represents the line width of the output image in μm. As shown in FIG. 13, if the line width indicated by the input image data is less than 4 dots (approximately 170 μm), the line width of the output image will be narrower than the line width indicated by the input image data.

[0036] In this embodiment, the set value of the exposure luminance is increased from a reference value to improve the reproducibility of highlight regions and thin lines. The reference value of the exposure luminance is a luminance value for setting the maximum density as the target density. In the following description, the exposure luminance set to the reference value is also referred to as the reference luminance. When the exposure luminance is increased from the reference luminance to improve the reproducibility of highlight regions and thin lines, the density of regions other than highlight regions and lines that are not thin lines increases above the target density. Therefore, in this embodiment, for regions where the density will increase above the target density, some of the multiple light-emitting elements 602 that expose those regions are changed from emitting (on) to not emitting (off) light, thereby controlling the density of these regions to approach the target density.

[0037] 14 is a flowchart of the process performed by the image forming apparatus. In S10, the controller 800 determines the amount of increase in the exposure luminance from the reference luminance. The amount of increase in the exposure luminance may be, for example, a predetermined value. In this case, the amount of increase in the exposure luminance is stored in advance in the memory 802. The amount of increase in the exposure luminance may also be determined based on the state of the photoconductor 102.

[0038] For example, the thickness of the photoreceptor 102 may decrease as the surface of the photoreceptor 102 is scraped during image formation. As the thickness of the photoreceptor 102 decreases, the distance between the surface of the photoreceptor 102 and the substrate set at ground potential decreases, increasing the electric field strength of the electrostatic latent image. Therefore, as the thickness of the photoreceptor 102 decreases, the reproducibility of highlights increases. Therefore, the thickness of the photoreceptor 102 can be estimated, and the increase in exposure luminance can be determined based on the estimated thickness of the photoreceptor 102. Specifically, the thinner the estimated thickness of the photoreceptor 102, the smaller the increase in exposure luminance can be. In this case, determination information indicating the relationship between the thickness and the increase in exposure luminance is stored in advance in the memory 802. The controller 800 can estimate the thickness of the photoreceptor 102 based on the cumulative number of sheets on which images have been formed using the photoreceptor 102 and the value of the discharge current when the charger 107 charges the photoreceptor 102. The controller 800 notifies the image controller 700 of the determined increase in exposure luminance.

[0039] In S11, image controller 700 selects an edge-preserving filter and LUT to be applied based on the increase in exposure luminance. FIGS. 15(A), 15(B), and 15(C) show edge-preserving filters used when the increase in exposure luminance is 20%, 15%, and 10%, respectively. The values included in the edge-preserving filters correspond to "dots" on photoconductor 102. Note that the shaded dots in FIGS. 15(A) to 15(C) indicate dots of interest. For simplicity, the numerical values of the edge-preserving filters in FIGS. 15(A) to 15(C) indicate the x portion of (x / 1024). That is, although the value of the dot of interest in FIGS. 15(A) to 15(C) is "512," this means that the actual value is "512 / 1024." The dotted line, dashed line, and solid line in Fig. 16(A) show the relationship between the input value and the output value indicated by the LUT used when the increase in exposure luminance is 20%, 15%, and 10%, respectively. Note that Fig. 16(A) shows the output value when the input value is 191 or more, but when the input value is less than 191, the output value is the same as the input value.

[0040] Returning to FIG. 14, the image data processing unit 703 of the image controller 700 performs thinning processing shown in S12 to S15 on the dithered image data. In the following description, the dithered image data will be referred to as first image data. The first image data is a collection of binary bit data for controlling the emission (on) or non-emission (off) of the corresponding light-emitting element 602. For example, the first image data indicates a value of "1" when the light-emitting element 602 is to be emitted, and a value of "0" when the light-emitting element 602 is not to be emitted. As described above, the area of the photoconductor 102 to which the light-emitting element 602 irradiates light will be referred to as a "dot." In this case, the first image data is data indicating whether each dot on the photoconductor 102 is to be exposed. In the following description, a dot indicated by the first image data to be exposed will be referred to as an exposed dot, and a dot indicated by the first image data not to be exposed will be referred to as a non-exposed dot. Therefore, the first image data is data indicating whether to form an exposed dot on the photoconductor 102 or to form a non-exposed dot by not forming an exposed dot.

[0041] In S12, the image data processing unit 703 converts the first image data into 8 bits. For example, the image data processing unit 703 converts a data value indicating an exposed dot (e.g., a value "1") to 255, and leaves a data value indicating a non-exposed dot (e.g., a value "0") as 0. In S13, the image data processing unit 703 applies the edge-preserving filter selected in S11 to the 8-bit converted first image data. In S14, the image data processing unit 703 converts the data values of the first image data after filtering with the edge-preserving filter based on the LUT selected in S11. As is clear from FIG. 16(A), by using the LUT, data values equal to or greater than the threshold are converted to the maximum value (255), and data values less than the threshold are left unchanged. The threshold value differs depending on the LUT used.

[0042] In S15, the image data processing unit 703 performs binarization processing using an error diffusion filter shown in FIG. 16B on the image data converted using the LUT. As will be described in detail later, the dots targeted by the error diffusion filter are those dots indicated as exposure dots by the first image data whose values have not been converted by the LUT. In the following description, the exposure dots targeted by the error diffusion filter are referred to as processing target dots. Through the binarization processing using the error diffusion filter, some of the processing target dots are converted from exposure dots to non-exposure dots. The first image data subjected to the thinning processing shown in S12 to S15 of FIG. 14 is output to the exposure head 106 as second image data. Although not shown in FIG. 14, the CPU 701 of the image controller 700 sets a setting value indicating an exposure luminance obtained by increasing the reference luminance by the increase amount determined in S10 in the register 1102 of each light-emitting chip 400. The exposure head 106 exposes the photoconductor 102 based on the second image data using the exposure luminance obtained by increasing the reference luminance by the increase amount determined in S10.

[0043] FIG. 17 is a diagram visually expressing the thinning process in S12 to S15 in FIG. 14. Note that FIG. 17 is a schematic diagram for facilitating understanding of the thinning process in S12 to S15 in FIG. 14, and does not show actual processing using the edge-preserving filter shown in FIG. 15 or the LUT and error diffusion filter shown in FIG. 16. FIG. 17(A) shows exposed dots in the first image data. According to FIG. 17(A), all of the 13 dot x 13 dot area are exposed dots. Note that, although not shown, all dots outside the 13 dot x 13 dot area are non-exposed dots.

[0044] In S12, the image data processing unit 703 sets all data values of the 13 x 13 dots shown in Fig. 17(A) to 255, sets the data values of the other dots to 0, and applies the edge-preserving filter in S13. Fig. 17(B) shows the data values after the edge-preserving filter is applied. In Fig. 17(B), the lower the density of the shading, the smaller the data value. The edge-preserving filter causes the data values of the dots to become smaller as they move toward the inside of the 13 dot x 13 dot area.

[0045] Figure 17(C) shows the state after the data values of the first image data to which the edge-preserving filter shown in Figure 17(B) has been applied have been converted using the LUT. As shown in Figure 16(A), the LUT converts data values equal to or greater than the threshold to the maximum value (255) and leaves other dots unchanged. In Figure 17(C), the data values of the four surrounding dots in a 13-dot x 13-dot area are converted to 255, while the 5 x 5 dots within that area have the same values as in Figure 17(B). As mentioned above, the 5 x 5 = 25 dots within Figure 17(C) are the dots to be processed.

[0046] Figure 17(D) shows the state after binarization processing using an error diffusion filter is performed on the first image data converted using the LUT shown in Figure 17(C). In Figure 17(D), the white dots indicate dots that have been converted to non-exposed dots out of the 25 dots to be processed. Figure 17(D) shows that 3 dots out of the 25 dots to be processed have been converted to non-exposed dots.

[0047] The edge-preserving filter and LUT used when the increase in exposure luminance is X% are designed so that approximately X% of the dots to be processed become non-exposed dots. Furthermore, the edge-preserving filter and LUT are designed so that the edge region of Y dots surrounding an area of continuous exposed dots is not converted into a non-exposed dot. For example, in the example of Figure 17, Y=4, and the edge region of 4 dots surrounding it is not converted into a non-exposed dot.

[0048] In the case of a small highlight area where exposure dots are continuous, all of the exposure dots are included in the edge area of the surrounding Y dots. In other words, all of the exposure dots in the highlight area are not processed dots. This improves the reproducibility of the highlight area. The same applies to thin lines that are thinner than a specified width. On the other hand, in the case of an area that is different from the highlight area or a line that is equal to or wider than a specified width, of the exposure dots excluding the edge area of the surrounding Y dots, the number of exposure dots corresponding to the increase in exposure luminance (X%) is converted into non-exposed dots. As a result, for areas that are different from the highlight area or lines that are equal to or wider than a specified width, the increase in density caused by increasing the exposure luminance by X% from the reference luminance can be suppressed, and the target density can be approached.

[0049] In this way, the greater the increase in exposure luminance relative to the reference luminance, the greater the number of conversion dots converted from exposed dots to non-exposed dots. More specifically, the greater the ratio of the set value (exposure luminance) to the reference value (reference luminance), the greater the number of conversion dots relative to the number of target dots. This configuration improves the reproducibility of highlight areas and the like, while also bringing the density of other areas closer to the target density. Note that the target dots are the area (first area) excluding the edge area within the exposure area, which is the area of the photoconductor 102 where the first image data indicates that exposure dots will be continuous. The width Y of the edge area is determined in advance and is reflected in the design of the edge-preserving filter and LUT.

[0050] In this embodiment, the dots to be converted from exposed dots to non-exposed dots are determined by binarization processing using an error diffusion filter, but it is also possible to perform this processing using dot-dispersed binarization processing such as the blue noise mask method or the FM screen method.

[0051] As described above, in the first region, excluding the edge region, of the region in the first image data where bit data indicating the lighting of the light-emitting element 602 is continuous, the second image data is generated by increasing or decreasing the bit data indicating the lighting of the light-emitting element based on the ratio of the set value to the reference value. This configuration improves the reproducibility of highlight regions and the like, and makes it possible to bring the density of other regions closer to the target density.

[0052] Second Embodiment Next, the second embodiment will be described, focusing on differences from the first embodiment. In the first embodiment, the exposure luminance was increased by X% from the reference luminance, and approximately X% of the dots to be processed were converted to non-exposed dots. The reference luminance of the light-emitting element 602 is the luminance required to achieve the maximum density as its target density. The maximum density depends on the latent image contrast Vc, which is the difference between the charging potential Vd of the photoconductor 102 by the charger 107 and the exposure potential VL of the photoconductor 102 exposed by the exposure head 106. In other words, the reference luminance is the luminance required to achieve the latent image contrast Vc as its target value. Here, the target value of the latent image contrast Vc required to achieve the maximum density as its target density varies depending on the installation environment of the image forming apparatus. Generally, the target value of the latent image contrast Vc required to achieve the maximum density as its target density increases as the relative humidity RH inside the image forming apparatus 1 increases. To increase the value (absolute value) of the latent image contrast Vc, the reference luminance must increase. Therefore, the reference luminance must increase as the relative humidity RH inside the image forming apparatus 1 increases. That is, the value of the reference luminance changes depending on the relative humidity RH inside the image forming apparatus 1.

[0053] Here, when the thinning process, which increases the exposure luminance by X% from the reference luminance and converts approximately X% of the dots to non-exposed dots, is implemented in an integrated circuit such as an ASIC, the range of exposure luminance may be limited depending on the value of X. As an example, if the increase in exposure luminance (= the amount of thinning of the dots to be processed) X is 10%, the light-emitting element 602 can emit light within a range from the first luminance value to the fourth luminance value, if it is 15%, the light-emitting element 602 can emit light within a range from the second luminance value to the fifth luminance value, and if it is 20%, the light-emitting element 602 can emit light within a range from the third luminance value to the sixth luminance value. Note that here, the luminance increases in the order of the first luminance value to the sixth luminance value.

[0054] In such a case, if the exposure luminance obtained by increasing the luminance value (reference luminance) by 20% to achieve the required latent image contrast Vc would be lower than the third luminance value depending on the relative humidity RH, the increase amount X cannot be 20%. In other words, the exposure luminance obtained by increasing the luminance value (reference luminance) by 20% to achieve the required latent image contrast Vc must be within the range from the third luminance value to the sixth luminance value. Similarly, if the exposure luminance obtained by increasing the luminance value (reference luminance) by 10% to achieve the required latent image contrast Vc could be higher than the fourth luminance value depending on the relative humidity RH, the increase amount X cannot be 10%. In other words, the exposure luminance obtained by increasing the luminance value (reference luminance) by 10% to achieve the required latent image contrast Vc must be within the range from the first luminance value to the fourth luminance value.

[0055] Therefore, in this embodiment, the required latent image contrast Vc is determined based on the environmental information, and the increase amount X of the exposure luminance is determined based on the determined latent image contrast Vc.

[0056] FIG. 18 is a flowchart of the process for determining the exposure luminance increase amount X according to this embodiment. The process in FIG. 18 corresponds to the process performed in S10 of FIG. 14 in the first embodiment. In S20, the controller 800 acquires information about the relative humidity RH inside the image forming apparatus from the temperature and humidity sensor 114. In S21, the controller 800 determines the target value of the latent image contrast Vc based on the relative humidity RH. The controller 800 is pre-stored with a formula for calculating the target value of the latent image contrast Vc from the relative humidity RH, and the controller 800 can calculate the target value of the latent image contrast Vc from the relative humidity RH using this formula. Instead of the formula, the controller 800 can also be configured to pre-store information indicating the correspondence between the relative humidity RH and the target value of the latent image contrast Vc. In this case, the controller 800 calculates the target value of the latent image contrast Vc from the relative humidity RH based on this information.

[0057] In S22, the controller 800 determines the increase amount X of the exposure luminance based on the target value of the latent image contrast Vc calculated in S21. Note that information indicating the correspondence between the target value of the latent image contrast Vc and the increase amount X of the exposure luminance is set in advance in the controller 800, and the controller 800 determines the increase amount X based on this information.

[0058] In addition, the controller 800 can also determine the value of the reference brightness, i.e., the reference value, based on the target value of the latent image contrast Vc, and determine the increase amount so that the value of the exposure brightness obtained by increasing the determined reference value by the increase amount is within the range of exposure brightness possible for that increase amount.

[0059] By determining the increase amount X of exposure brightness in this manner, the reproducibility of highlight areas and thin lines less than a predetermined width can be maintained, and the density of areas other than highlight areas and lines with a width equal to or greater than a predetermined width can be brought closer to the target density.

[0060] Third Embodiment Next, the third embodiment will be described, focusing on the differences from the second embodiment. In the second embodiment, the required latent image contrast Vc was determined based on environmental information, and the increase amount X of exposure luminance was determined based on the determined latent image contrast Vc. In this embodiment, the increase amount X of exposure luminance is determined according to the detection result of the patch image detected by the optical sensor 113. FIG. 19 is a flowchart of the process of determining the increase amount X of exposure luminance according to this embodiment. Note that the process in FIG. 19 corresponds to the process performed in S10 of FIG. 14 in the first embodiment.

[0061] In S30, the controller 800 determines, based on an environmental table (not shown), the charging potential Vd of the photoconductor 102, the developing potential Vdc output by the developer 108, and the thinning amount X of the target dots, based on environmental information (temperature and humidity) acquired from the temperature and humidity sensor 114. Next, in S31, the controller 800 forms multiple patch images on the transfer belt 111 using multiple different exposure luminances. Note that at this time, the values determined in S30 are used for the charging potential Vd, the developing potential Vd, and the thinning amount X of the target dots. In S32, the controller 800 determines the density of each of the multiple patch images based on the detection results of the multiple patch images by the optical sensor 113. Then, in S32, the controller 800 determines the exposure luminance that will make the maximum density its target value, based on the detected densities of the multiple patch images and a target maximum density. Note that, since the patch image has been subjected to thinning processing with the thinning amount X, the exposure luminance determined here is not the reference luminance but the exposure luminance obtained by increasing the reference luminance by the increase amount X.

[0062] In S33, the controller 800 determines whether the exposure luminance determined in S32 is equal to or greater than the lower limit and equal to or less than the upper limit of the exposure luminance when the increase amount is X. If the exposure luminance is equal to or greater than the lower limit and equal to or less than the upper limit, the controller 800 sets the thinning amount X as the increase amount of the exposure luminance, determines the exposure luminance determined in S32 as the increased exposure luminance, and ends the processing of FIG. 19. On the other hand, if the exposure luminance determined in S32 is not within the range from the lower limit to the upper limit for the thinning amount X, the controller 800 adjusts the value of X by a predetermined amount and repeats the processing from S31. Note that if the exposure luminance is smaller than the lower limit, the controller 800 increases the thinning amount X. If the exposure luminance is greater than the upper limit, the controller 800 decreases the thinning amount X.

[0063] By determining the increase amount X of exposure brightness in this manner, the reproducibility of highlight areas and thin lines less than a predetermined width can be maintained, and the density of areas other than highlight areas and lines with a width equal to or greater than a predetermined width can be brought closer to the target density.

[0064] In this embodiment, the density of a patch image formed on the photoconductor 102 is measured by transferring the patch image to the transfer belt 111, which is another member. However, a configuration in which the density of a patch image formed on the photoconductor 102 is measured may also be used.

[0065] <Fourth embodiment> Next, the fourth embodiment will be described, focusing on the differences from the first to third embodiments. At the end of image formation, the voltage applied to the charger 107 is stopped, and then the exposure head 106 removes the residual potential on the surface of the photoconductor 102. Repeated removal of this charge reduces the luminous efficiency of the light-emitting elements 602, which may affect the life of the light-emitting elements 602. For this reason, in this embodiment, the exposure luminance of the exposure head 106 in the removal of charge after image formation is set lower than the exposure luminance during image formation.

[0066] FIG. 20 is a flowchart of this embodiment. The processing of FIG. 20 starts after image formation is completed. In S40, the controller 800 stops applying voltage to the charger 107. This causes the charger 107 to stop outputting the charging voltage. In S41, the controller 800 sets the exposure luminance during static elimination. The exposure luminance during static elimination is lower than the exposure luminance used in the immediately preceding image formation, that is, a luminance obtained by increasing the reference luminance by X%. In S42, the controller 800 eliminates static from the photoconductor 102.

[0067] As described above, by setting the exposure luminance during static elimination lower than the exposure luminance during image formation, it is possible to suppress the effect on the life of the exposure head 106 and perform static elimination efficiently.

[0068] <Other embodiments> In the first to fourth embodiments, the exposure luminance is increased from the reference luminance. However, the configurations of the first to fourth embodiments can also be applied when it is necessary to decrease the exposure luminance from the reference luminance. Therefore, the increase amount in the first to fourth embodiments can be interpreted as a fluctuation amount. In this case, the second image data is generated by increasing or decreasing the bit data indicating the illumination of the light-emitting element 602 based on the ratio of the set value to the reference value in the first region, excluding the edge region, of the region where the bit data indicating the illumination of the light-emitting element 602 in the first image data is continuous.

[0069] [Other embodiments] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0070] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0071] 102: photosensitive member, 106: exposure head, 701: CPU, 703: image data processing unit

Claims

1. a photosensitive member that is driven to rotate; an exposure head that exposes the photosensitive member to light using a plurality of light-emitting elements arranged along the rotation axis direction of the photosensitive member; a setting unit for setting the exposure luminance of the photosensitive member by the plurality of light-emitting elements; a processing means for generating first image data, which is a set of bit data for controlling the lighting or extinguishing of the plurality of light-emitting elements, and second image data, which increases or decreases the number of light-emitting elements that emit light based on the first image data, and outputting the second image data to the exposure head; Equipped with An image forming apparatus, wherein the processing means generates the second image data by increasing or decreasing the bit data indicating the lighting of the light-emitting element in a first region, excluding edge regions, within an area in which bit data indicating the lighting of the light-emitting element in the first image data is continuous, in accordance with the ratio between the setting value of the exposure brightness set by the setting means and the reference value of the exposure brightness.

2. 2. The image forming apparatus according to claim 1, wherein the number of bit data in the first area that is increased or decreased to indicate lighting of the light-emitting element in the first area is based on a ratio of the set value to the reference value.

3. 2. The image forming apparatus according to claim 1, wherein the processing means performs filtering on the first image data using an edge-preserving filter, and determines the first region based on the filtered first image data.

4. a discharge unit that discharges the photosensitive member by exposing the photosensitive member with the exposure head after the image formation based on the second image data is completed; 2. The image forming apparatus according to claim 1, wherein the setting unit sets the exposure luminance setting value when discharging the photosensitive member to a value lower than the setting value when forming an image based on the second image data.

5. 2. The image forming apparatus according to claim 1, wherein the reference value is a value of the exposure luminance for making the maximum density of the image formed by the image forming apparatus a target density.

6. The image forming apparatus according to claim 1 , wherein the difference between the set value and the reference value is a predetermined value.

7. The image forming apparatus according to claim 1 , further comprising a determining unit that determines the setting value.

8. 8. The image forming apparatus according to claim 7, wherein said determining unit estimates a film thickness of said photosensitive member, and determines said set value based on the estimated film thickness of said photosensitive member.

9. 9. The image forming apparatus according to claim 8, wherein the setting unit lowers the set value as the thickness of the photosensitive member decreases.

10. 8. The image forming apparatus according to claim 7, wherein said determining means determines said set value based on a relative humidity inside said image forming apparatus.

11. 11. The image forming apparatus according to claim 10, wherein the determining unit determines a target value of the latent image contrast based on the relative humidity, and determines the set value based on the determined target value.

12. the range of the exposure luminance is limited according to the amount of variation of the set value from the reference value, The image forming apparatus according to claim 11, wherein the determining means determines the reference value based on the target value, and determines the amount of variation so that the value of the exposure luminance obtained by varying the determined reference value by the amount of variation falls within the range of the exposure luminance for the amount of variation, thereby determining the setting value.

13. 8. The image forming apparatus according to claim 7, wherein the determining unit detects the density of a patch image formed on the photosensitive member or a patch image formed on the photosensitive member and transferred to another member, and determines the setting value based on the detected density of the patch image.

14. the range of the exposure luminance is limited according to the amount of variation of the set value from the reference value, The image forming apparatus according to claim 13, wherein the determining means determines the setting value obtained by varying the reference value by the variation amount based on the density of the detected patch image, and determines the setting value so that the variation amount at the determined setting value is within the range of the exposure luminance at the variation amount.

Images