Image forming apparatus

Abstract

It suppresses poor fixing of images when they are fixed to the paper. [Solution] The image forming apparatus includes an image data processing unit 703 that generates bitmap data composed of multiple pixels corresponding to image data and performs pixel thinning processing on the bitmap data according to the thinning rate; an image forming unit (101) that forms an image based on the bitmap data that has been thinned by the image data processing unit 703; a transfer member (114) that transfers the image formed by the image forming unit (101) onto paper; a fixing unit (104) that fixes the image on the paper by heating the image on the paper; and an image controller 700 that controls the thinning rate based on information regarding the basis weight of the paper on which the image is to be formed by the image forming unit (101).

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that adjusts the density of an image to be printed.

Background Art

[0002] In a general electrophotographic image forming apparatus, an electrostatic latent image formed by exposing a rotatably driven photoreceptor is developed with toner to form an image. The formed image is transferred onto a sheet of paper and fixed onto the paper by being heated and pressed by a fixing device. The toner is melted by being heated by the fixing device and fixed onto the paper by being pressed. As the exposure device for exposing the photoreceptor, an exposure head using a light-emitting element such as an LED (Light-Emitting Diode) or an organic EL (Electro-Luminescence), or a scanning type laser scanner is used. The light emission pattern of the exposure device is determined based on image data representing the image to be printed.

[0003] Patent Document 1 discloses a technique for suppressing the breakage of thin lines and suppressing the blurring of an image by adjusting the exposure amount of a target pixel according to the information of pixels around the target pixel.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, the fixing device has a limit on the amount of toner that can be fixed onto a sheet of paper. For example, when continuously printing images on thick paper, the heating unit provided in the fixing device loses heat to the thick paper and the toner, so that the temperature at the time of image fixing decreases. As a result, the melting of the toner is not sufficiently performed, the adhesive force between the toner and the paper is lost, and fixing defects such as peeling of the toner after fixing occur.

[0006] Therefore, the inventors considered using the technology described in Patent Document 1 to suppress poor adhesion.

[0007] The present invention aims to suppress poor fixing of images when they are fixed onto paper. [Means for solving the problem]

[0008] The present invention relates to an image forming apparatus that forms an image on paper using toner, and comprises: an image processing means that generates bitmap data composed of a plurality of pixels corresponding to image data and performs pixel thinning processing on the bitmap data according to a thinning rate; an image forming means that forms an image based on the bitmap data on which the thinning processing has been performed by the image processing means; a transfer means that transfers the image formed by the image forming means to paper; a fixing means that fixes the image on the paper by heating the image on the paper; and a control means that controls the thinning rate based on information regarding the basis weight of the paper on which the image is to be formed by the image forming means. [Effects of the Invention]

[0009] According to the present invention, it is possible to suppress poor fixing of images fixed to paper. [Brief explanation of the drawing]

[0010] [Figure 1] A diagram illustrating the configuration of an image forming apparatus. [Figure 2] (a) and (b) are explanatory diagrams of the photoreceptor and exposure head. [Figure 3] (a) and (b) are explanatory diagrams of a printed circuit board. [Figure 4] Diagram illustrating the light-emitting chip. [Figure 5] Plan view of the light-emitting chip. [Figure 6] Cross-sectional view AA in Figure 5. [Figure 7] Image controller configuration diagram. [Figure 8] Timing chart when writing control data. [Figure 9] Timing chart when transmitting printed image data. [Figure 10] Detailed functional configuration diagram of one light-emitting chip. [Figure 11] Explanatory drawing of multi-exposure. [Figure 12] Explanatory drawing of potential decay characteristics. [Figure 13] Graph in which potential decay characteristics are normalized by exposure energy. [Figure 14] Flowchart of edge preservation processing and pixel decimation processing. [Figure 15] Explanatory drawing of edge preservation processing and pixel decimation processing. [Figure 16] (a) to (c) are explanatory drawings of edge preservation processing and pixel decimation processing. [Figure 17] (a) to (c) are explanatory drawings of images obtained by edge preservation processing and pixel decimation processing. [Figure 18] Illustrative diagram of the relationship between paper basis weight information, environmental information, and decimation rate. [Figure 19] Flowchart of edge preservation processing and pixel decimation processing. [Figure 20] (a) and (b) are illustrative diagrams of weighting filters. [Figure 21] Illustrative diagram of a LUT. [Figure 22] (a) to (c) are explanatory drawings of the relationship between printed image data, potential contrast, and toner amount on paper.

Mode for Carrying Out the Invention

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiments, not all of these plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] (Image forming apparatus) Figure 1 is a diagram of the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 1 comprises a reading unit 100, an image forming unit 103, a fixing unit 104, and a transport unit 105. The reading unit 100 optically reads a document placed on the document glass and generates read image data. The image forming unit 103 forms an image on paper based on the read image data generated by the reading unit 100 or print image data acquired from an external device via a network. Such an image forming apparatus 1 can be realized, for example, by a copier, a multifunction printer, an MFP (Multi-Function Peripheral), etc.

[0013] The image forming unit 103 includes a plurality of image forming units 101a, 101b, 101c, and 101d, a transfer belt 111 for transporting paper, and an optical sensor 113. The image forming units 101a, 101b, 101c, and 101d are used to form toner images of black (K), yellow (Y), magenta (M), and cyan (C), respectively. The configurations of the image forming units 101a, 101b, 101c, and 101d are the same, and collectively referred to as the image forming unit 101 below.

[0014] The image forming unit 101 comprises a photoreceptor 102, a charger 107, an exposure head 106, and a developer 108. The photoreceptor 102 is a drum-shaped image carrier having a photosensitive layer on its surface. The photoreceptor 102 is rotated clockwise around the drum axis in the direction shown in Figure 1. The charger 107 uniformly charges the surface of the rotating photoreceptor 102 to a predetermined potential with a predetermined polarity. The exposure head 106 is an exposure device that exposes the surface of the uniformly charged photoreceptor 102 to form an electrostatic latent image on the surface of the photoreceptor 102. As will be described in detail later, the exposure head 106 in this embodiment has a configuration in which a plurality of light-emitting elements are arranged in a planar manner.

[0015] The developer unit 108 develops the electrostatic latent image formed on the photoreceptor 102 with a developer (e.g., toner) to form a toner image on the surface of the photoreceptor 102. Transfer members 114 are provided at positions opposite each photoreceptor 102 via a transfer belt 111. The toner images formed on the surface of the photoreceptor 102 are sequentially transferred to the paper being transported on the transfer belt 111 by applying a transfer voltage to the transfer members 114. The toner images of the four photoreceptors 102 are superimposed and transferred to the paper. As a result, a color image containing four color components—black, yellow, magenta, and cyan—is formed on the paper. The optical sensor 113 optically reads the adjustment image transferred to the paper.

[0016] The transport unit 105 controls the feeding of paper. Paper can be fed from the internal storage units 109a and 109b, the external storage unit 109c, and the manual feed unit 109d. The transport unit 105 feeds paper into the transport path from one of the internal storage units 109a and 109b, the external storage unit 109c, or the manual feed unit 109d according to the instructions. A registration roller 110 is provided in the transport path. The fed paper is transported to the registration roller 110.

[0017] The registration roller 110 corrects the skew of the paper and transports the paper onto the transfer belt 111 at the appropriate timing so that the toner images of each photoreceptor 102 are transferred to predetermined positions on the paper. As described above, the toner images are sequentially superimposed and transferred onto the paper as it is transported on the transfer belt 111. The paper on which the toner images have been transferred is transported to the fuser unit 104. The fuser unit 104 fixes the toner images onto the paper by heating and pressurizing the paper on which the toner images have been transferred. After the toner images have been fixed, the paper is discharged to the outside of the image forming apparatus 1 by the discharge roller 112.

[0018] Inside the image forming apparatus 1, there is an image controller 700 that performs various image processing on read image data and print image data, such as color space conversion, filtering, scaling, resolution conversion, and quantization. The image controller 700 performs image processing on the read image data and print image data to generate print image data for the image forming unit 103 to perform image formation. The image controller 700 includes an image processing module that performs the various image processing described above.

[0019] The image controller 700 corrects the image formation conditions based on the reading results of the adjustment image from the optical sensor 113. Details of the image controller 700 will be described later.

[0020] Although this description explains a configuration in which toner images are directly transferred from each photoreceptor 102 to the paper on the transfer belt 111, the toner images may also be transferred indirectly from each photoreceptor 102 to the paper via an intermediate transfer body. Furthermore, although this description explains an example of forming a color image using multiple toner colors, the technology according to this embodiment is also applicable to an image forming apparatus that forms a monochrome image using a single toner color.

[0021] (Exposure head) Figure 2 is an explanatory diagram of the photoreceptor 102 and the exposure head 106. Figure 2(a) is a perspective view of the photoreceptor 102 and the exposure head 106. Figure 2(b) is an explanatory diagram of the exposure position. The exposure head 106 includes a light-emitting element array 201 having a plurality of light-emitting elements, 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.

[0022] As described above, the photoreceptor 102 is drum-shaped. The exposure head 106 is positioned so that its longitudinal direction (arrangement direction of the light-emitting elements) is parallel to the drum axis direction D1 of the photoreceptor 102, and the mounting surface of the rod lens array 203 faces the surface of the photoreceptor 102. While the photoreceptor 102 rotates in the circumferential direction D2, the light-emitting element array 201 (light-emitting elements) of the exposure head 106 emits light, and the rod lens array 203 focuses this light onto the surface of the photoreceptor 102. The surface of the photoreceptor 102 is uniformly charged by the charger 107, and the potential at the position where the light is focused is displaced. The position where the potential is displaced becomes an electrostatic latent image.

[0023] The light-emitting element array 201 consists of multiple light-emitting elements arranged in a planar configuration. Examples of light-emitting elements include LEDs and organic ELs. The drum axis direction D1 is the primary scanning direction, and the circumferential direction D2 is a secondary scanning direction intersecting the primary scanning direction.

[0024] Figure 3 is an explanatory diagram of the printed circuit board 202. The connector 305 and the light-emitting element array 201 are mounted on different sides of the printed circuit board 202. Figure 3(a) shows the side of the printed circuit board 202 on which the connector 305 is mounted. Figure 3(b) shows the side of the printed circuit board 202 on which the light-emitting element array 201 is mounted. The light-emitting element array 201 comprises a plurality of light-emitting chips 400, each having multiple light-emitting elements. In this embodiment, there are 20 light-emitting chips 400 (light-emitting chips 400-1 to 400-20). The light-emitting chips 400-1 to 400-20 are arranged in a staggered pattern in the main scanning direction.

[0025] As shown in Figure 3(b), the area occupied by all the light-emitting elements of the 20 light-emitting chips 400-1 to 400-20 in the main scanning direction is wider than the area occupied by the maximum width W0 of the image shown in the print image data. Therefore, some light-emitting elements located at both ends of the main scanning direction may not be used to expose the photoreceptor 102 unless an image misalignment is detected. Each light-emitting chip 400 on the printed circuit board 202 is connected to the image controller 700 via a connector 305.

[0026] For the sake of clarity, in the following explanation, the side with the smaller sub-number of the light-emitting chips 400-1 to 400-20, which are aligned in the main scanning direction, will be referred to as "left," and the side with the larger sub-number as "right." For example, light-emitting chip 400-1 is the leftmost light-emitting chip 400, and light-emitting chip 400-20 is the rightmost light-emitting chip. Figure 4 illustrates the two rightmost light-emitting chips 400-n and 400-n+1.

[0027] Figure 4 is an explanatory diagram of the light-emitting chip 400. The light-emitting element array 201 of this embodiment includes multiple light-emitting elements arranged in N columns in the main scanning direction and M rows in the sub-scanning direction. M and N are integers of 2 or more. The number of light-emitting elements 602 arranged in each row (main scanning direction) of one light-emitting chip 400, J (J=N / 20), is, for example, 748 (J=748). The number of light-emitting elements 602 arranged in each column (sub-scanning direction) of one light-emitting chip 400, M, is, for example, 4 (M=4). In other words, in this example, the light-emitting chip 400 has a total of 2992 (=748×4) light-emitting elements 602: 748 in the main scanning direction (drum axis direction D1) and 4 in the sub-scanning direction (circumferential direction D2).

[0028] The distance PC between the center points of adjacent light-emitting elements 602 in the sub-scanning direction is approximately 21.16 μm when the resolution is 1200 dpi. Similarly, the distance between the center points of adjacent light-emitting elements 602 in the main scanning direction is also approximately 21.16 μm. In this case, the 748 light-emitting elements 602 have a length of approximately 15.8 mm in the main scanning direction.

[0029] Figure 4 shows an example where the light-emitting elements 602 of each light-emitting chip 400 are arranged in a perfectly grid pattern for convenience. However, in reality, the M (M=4) light-emitting elements 602 in each row are arranged in a stepped pattern. This point will be discussed later.

[0030] Figure 5 is a plan view of the light-emitting chip 400. Multiple light-emitting elements 602 are formed on a light-emitting substrate 402, which is, for example, a silicon substrate. The light-emitting substrate 402 also has a circuit section 406 for driving the multiple light-emitting elements 602 mounted on it. The light-emitting substrate 402 is provided with pads 408-1 to 408-9 to which signal lines for communication with the image controller 700, power lines for connection to a power supply, and ground lines for grounding are connected. The signal lines, power lines, and ground lines are, for example, wires made of gold.

[0031] Figure 6 is a cross-sectional view of AA in Figure 5. Multiple 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 single common electrode for the multiple lower electrodes 504.

[0032] A potential difference is generated between the lower electrode 504 and the upper electrode 508, causing a current to flow from the lower electrode 504 to the upper electrode 508, which in turn causes the light-emitting layer 506 to emit light. Therefore, one lower electrode 504 and a portion of the light-emitting layer 506 and upper electrode 508 corresponding to that lower electrode 504 constitute one light-emitting element 602. In this way, multiple light-emitting elements 602 are formed on the light-emitting substrate 402.

[0033] For example, an organic EL film is used for the light-emitting layer 506. The upper electrode 508 is made of a transparent electrode such as indium tin oxide (ITO) so as to transmit a predetermined wavelength (emission wavelength) of light emitted from the light-emitting layer 506. In this embodiment, the entire upper electrode 508 transmits the emission wavelength of the light-emitting layer 506, but it is not necessary for the entire upper electrode 508 to transmit the emission wavelength. Specifically, it is sufficient for only a portion of the region through which light from each light-emitting element 602 passes to transmit the emission wavelength.

[0034] In this embodiment, the light-emitting layer 506 is formed as a single continuous light-emitting layer 506, but multiple light-emitting layers 506, each having a width equivalent to the width W of the lower electrode 504, may be formed correspondingly on the lower electrode 504. Furthermore, a first group of lower electrodes 504 of each light-emitting chip 400 may be covered by a first light-emitting layer 506, and a second group of lower electrodes 504 may be covered by a second light-emitting layer 506. Also, a first upper electrode 508 may be commonly formed corresponding to a first group of lower electrodes 504 of each light-emitting chip 400, and a second upper electrode 508 may be commonly formed corresponding to a second group of lower electrodes 504. Even in these configurations, one lower electrode 504 and the regions of the light-emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light-emitting element 602.

[0035] (Image controller 700) Figure 7 is a diagram showing the configuration of an image controller 700 that controls the lighting and extinguishing of the light-emitting chip 400. The image controller 700 can communicate with the printed circuit board 202 via multiple signal lines (wires). The image controller 700 includes a CPU (Central Processing Unit) 701, a clock generation unit 702, an image data processing unit 703, a register access unit 704, and a light emission control unit 705.

[0036] The light emission control unit 705, together with the exposure head 106, constitutes the exposure apparatus. The light emission control unit 705 terminates the signal lines to the printed circuit board 202. The nth light emission chip 400-n on the printed circuit board 202 is connected to the light emission control unit 705 by the signal lines DATAn and WRITEn. The signal line DATAn is a signal line for transmitting print image data from the image controller 700 to the light emission chip 400-n. The signal line WRITEn is a signal line for the image controller 700 to write control data to the registers of the light emission chip 400-n.

[0037] Between the light emission control unit 705 and each light-emitting chip 400, there are further signal lines: CLK, SYNC, and EN. Signal line CLK transmits a clock signal for data transmission via signal lines DATAn and WRITEn. The clock generation unit 702 generates a reference clock signal and transmits it to each part of the image controller 700. The light emission control unit 705 transmits the clock signal it has generated based on the reference clock signal obtained from the clock generation unit 702 to each light-emitting chip 400 via signal line CLK.

[0038] The CPU 701 controls the overall operation of the image forming apparatus 1. The image data processing unit 703 performs predetermined image processing on read image data and print image data acquired from the reading unit 100 or an external device. By performing image processing, the image data processing unit 703 generates binary bitmap image data (print image data) for controlling the emission of light-emitting elements 602 of the light-emitting chip 400 on the printed circuit board 202.

[0039] The image processing performed by the image data processing unit 703 includes, for example, raster conversion, gradation correction, color conversion, and halftone processing (dithering). The image data processing unit 703 transmits the generated binary image data (print 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 emission chip 400 from the CPU 701 and transmits it to the light emission control unit 705.

[0040] The image forming apparatus 1 has an exposure head 106 in each of its image forming sections 101a to 101d. In other words, in this embodiment, four printed circuit boards 202 are provided. The image controller 700 is connected to these four printed circuit boards 202 and controls the lighting of multiple light-emitting elements 602 mounted on each of the four printed circuit boards 202. To this end, the image controller 700 generates and transmits four print image data corresponding to each of the four exposure heads 106. The four print image data are image data for yellow image formation, magenta image formation, cyan image formation, and black image formation.

[0041] Figure 8 is a timing chart for writing control data to the registers of each light-emitting chip 400. Figure 8 shows the signal level transitions of each signal line when writing control data to the registers of the light-emitting chip 400. An enable signal is transmitted to the signal line EN, which is high during communication to indicate that communication is in progress. 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. Subsequently, the light-emitting control unit 705 transmits a write identification bit to the signal line WRITEn to indicate that it is a write operation, and then transmits the address of the register to which the control data will be written (4 bits in this case) and the control data (8 bits in this case). When writing to a register, the light-emitting control unit 705 sets the frequency of the clock signal transmitted on the signal line CLK to 3 [MHz], for example.

[0042] Figure 9 is a timing chart for transmitting print image data to each light-emitting chip 400, illustrating the transition of the signal levels of each signal line. A periodic line synchronization signal indicating the exposure timing of each line in the photoreceptor 102 is transmitted to the SYNC signal line. Assuming a peripheral speed of 200 [mm / s] and a circumferential resolution of 1200 [dpi] (approximately 21.16 [μm]) of the photoreceptor 102, the line synchronization signal is output with a period of approximately 105.8 [μs].

[0043] The light emission control unit 705 transmits print image data via signal lines DATA1 to DATA20 in synchronization with the rising edge of the line synchronization signal. Since each light emission chip 400 in this embodiment has 2992 light-emitting elements 602, it is necessary to transmit print image data to each light emission chip 400 to control the illumination (lighting) of each of the 2992 light-emitting elements 602 within a period of approximately 105.8 [μs]. Therefore, in this embodiment, as shown in Figure 9, when transmitting print image data, the light emission control unit 705 sets the frequency of the clock signal transmitted via signal line CLK to 30 [MHz].

[0044] Figure 10 is a detailed functional configuration diagram of one light-emitting chip 400 (the nth light-emitting chip 400-n). The circuit section 406 includes a register 1102, a transfer section 1103, latch sections 1004-001 to 1004-748, and a current drive section 1104.

[0045] As explained in Figure 5, the light-emitting chip 400 has nine pads 408-1 to 408-9. Pads 408-1 and 408-2 are supplied with a power supply voltage VCC via a power supply line. Each part of the circuit section 406 of the light-emitting chip 400 is supplied with a power supply voltage VCC via pads 408-1 and 408-2. Pads 408-3 and 408-4 are grounded via a ground line. Each part of the circuit section 406 and the upper electrode 508 are grounded via pads 408-3 and 408-4.

[0046] Pad 408-5 is connected to the signal line CLK. The signal line CLK is connected to the transfer unit 1103, register 1102, and latch units 1004-001 to 1004-748 via pad 408-5. Pad 408-6 is connected to the signal line SYNC. Pad 408-7 is connected to the signal line DATAn. The signal lines SYNC and DATAn are connected to the transfer unit 1103 via pads 408-6 and 408-7. Pad 408-8 is connected to the signal line EN. Pad 408-9 is connected to the signal line WRITEn. The signal lines EN and WRITEn are connected to register 1102 via pads 408-8 and 408-9. Register 1102 stores control data, for example, indicating the light emission intensity of the light-emitting element 602.

[0047] The transfer unit 1103 uses a line synchronization signal obtained from the signal line SYNC as a starting point and synchronizes with a clock signal obtained from the signal line CLK to obtain print image data from the signal line DATAn, which includes a series of pixel values ​​indicating whether each of the light-emitting elements 602 is on or off. The transfer unit 1103 performs serial-to-parallel conversion on the series of pixel values ​​obtained serially from the signal line DATAn in units of M (for example, M=4).

[0048] For example, the transfer unit 1103 has four cascaded D flip-flops. The transfer unit 1103 parallelizes the pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input over a period of four clock cycles and transmits them sequentially to the latch units 1004-001 to 1004-748. The transfer unit 1103 also has four more D flip-flops for delaying the line synchronization signal. The transfer unit 1103 outputs a first latch signal to the latch unit 1004-001 via the signal line LAT1 at a timing four clock cycles after the line synchronization signal is input. The first latch signal is, for example, a signal obtained by delaying the line synchronization signal by four clock cycles.

[0049] The k-th latch unit 1004-k (where k is an integer from 1 to 748) latches the four pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input from the transfer unit 1103 simultaneously with the input of the k-th latch signal. Except for the last stage latch unit 1004-748, the k-th latch unit 1004-k delays the k-th latch signal by 4 clock cycles and outputs the (k+1) latch signal to the latch unit 1004-(k+1) via the signal line LAT(k+1). The k-th latch unit 1004-k continues to output a drive signal based on the four latched pixel values ​​to the current drive unit 1104 for the duration of the signal cycle of the k-th latch signal.

[0050] For example, there is a delay of 4 clock cycles between the timing when the first latch signal is input to latch unit 1004-001 and the timing when the second latch signal is input to latch unit 1004-002. Therefore, latch unit 1004-001 outputs a drive signal based on the 1st to 4th pixel values ​​to the current drive unit 1104, while latch unit 1004-002 outputs a drive signal based on the 5th to 8th pixel values ​​to the current drive unit 1104.

[0051] Generally speaking, the latch unit 1004-k outputs a drive signal based on the (4k-3) to (4k)th pixel value to the current drive unit 1104. Therefore, in Figure 10, 748 latch units 1004-001 to 1004-748 output 2992 drive signals to the current drive unit 1104 in roughly parallel order to control the driving of 2992 (=748 × 4) light-emitting elements 602. Each drive signal is a binary signal indicating high / low level.

[0052] The current drive unit 1104 has 2992 light-emitting drive circuits, each corresponding to one of the 2992 light-emitting elements 602, including a portion of the light-emitting layer 506. Each light-emitting drive circuit applies a drive voltage corresponding to the light-emitting intensity indicated by the control data in the register 1102 to the light-emitting layer 506 of the corresponding light-emitting element 602 while the drive signal is high, meaning the light-emitting element 602 is turned on (lit). This causes current to flow through the light-emitting layer 506, causing the light-emitting element 602 to emit light. The control data may indicate one individual light-emitting intensity for each light-emitting element 602, one light-emitting intensity for each group of light-emitting elements 602, or one light-emitting intensity common to all light-emitting elements 602.

[0053] (Multiple exposure control) Figure 4 shows an example where the light-emitting elements 602 of each light-emitting chip 400 are arranged in a grid pattern. In reality, however, the M light-emitting elements 602 in each row are arranged in a stepped pattern at a constant pitch. Figure 11 is an explanatory diagram of multiple exposure using light-emitting elements 602 arranged in a stepped pattern. Figure 11 partially illustrates the arrangement of the light-emitting elements 602 of light-emitting chip 400-1 when M=4.

[0054] Rj_m(j={0,1,…,747}, m={0,1,2,3}) represents the light-emitting element 602 located in the j-th column from the left in the main scanning direction and in the m-th row from the top in the sub-scanning direction. The pitch PC of the light-emitting element in the sub-scanning direction is determined by the size of the light-emitting element 602, and is approximately 21.16 [μm] as shown above. The spacing between two adjacent light-emitting elements in each column of M light-emitting elements, i.e., the pitch PA of the light-emitting element 602 in the main scanning direction, is approximately 5 [μm] for a resolution of 4800 [dpi].

[0055] In this way, the four light-emitting elements 602 in each row are arranged in a stepped pattern, so that any two adjacent light-emitting elements 602 have a range that partially overlaps in the main scanning direction. As the photoreceptor 102 rotates, the four light-emitting elements 602 in the row corresponding to each pixel position in the print image data emit light sequentially, forming a spot on the surface of the photoreceptor 102 corresponding to each pixel position.

[0056] For example, four light-emitting elements 602, each having 4800 dpi position information for a row corresponding to each pixel position in print image data with a resolution of 1200 dpi, emit light sequentially as the photoreceptor 102 rotates. This forms a 1200 dpi spot on the surface of the photoreceptor 102 corresponding to each pixel position in the print image data. If the print image data has a resolution of 2400 dpi, two light-emitting elements 602, each having 4800 dpi position information for a row corresponding to each pixel position in the print image data, overlap. This forms a 2400 dpi spot on the surface of the photoreceptor 102 corresponding to each pixel position.

[0057] In the example shown in Figure 11, when the print image data instructs the leftmost point of the i-th line to be turned on, the light-emitting elements R0_0, R0_1, R0_2, and R0_3 sequentially emit light at timings corresponding to the line Li on the surface of the photoreceptor 102. As a result, the spot area at the leftmost point of line Li is multiple-exposed, forming spot SP0. Similarly, when the print image data instructs the j-th point from the left of the i-th line to be turned on, the light-emitting elements Rj_0, Rj_1, Rj_2, and Rj_3 sequentially emit light at timings corresponding to the line Li on the surface of the photoreceptor 102. As a result, the j-th spot area from the left of line Li is multiple-exposed, forming the corresponding spot SPj.

[0058] Thus, in this embodiment, the light-emitting elements in two adjacent rows in the main scanning direction occupy a range that partially overlaps in the main scanning direction. Similarly, among two adjacent light-emitting chips 400 in the main scanning direction, the light-emitting elements in the rightmost column of the left light-emitting chip 400 and the light-emitting elements in the leftmost column of the right light-emitting chip 400 also occupy a range that partially overlaps in the main scanning direction (see Figure 3(b)).

[0059] The pitch PA of the light-emitting elements 602 in the main scanning direction is constant (approximately 5 [μm]) across the entirety of the 20 light-emitting chips 400. The four light-emitting elements in each row of these light-emitting chips 400 emit light sequentially at appropriate timings, forming a smooth electrostatic latent image line on the surface of the photoreceptor 102, consisting of a series of spots that partially overlap each other at regular intervals. As such lines are continuously formed in the sub-scanning direction, a two-dimensional electrostatic latent image is generated on the surface of the photoreceptor 102.

[0060] The specific numerical values ​​used in the above description are all examples, and the values ​​are not limited to those used in this embodiment. For example, the number of light-emitting chips 400 mounted on one printed circuit board 202 is not limited to 20, but can be 1 or more. The number of light-emitting elements 602 contained in each light-emitting chip 400 is also not limited to 2992. In this embodiment, one light-emitting chip 400 is composed of 4 sets of 748 light-emitting elements 602 arranged along the main scanning direction, but the number of sets can be 1 or more. The light-emitting elements 602 are arranged at pitches of approximately 21.16 [μm] and approximately 5 [μm], corresponding to the resolutions of the main scanning direction and sub-scanning direction, but the spacing between the light-emitting elements 602 can be set according to the resolution and number. The number and spacing of the light-emitting elements 602 on the exposure head 106 can be determined by the resolution and image size of the image formed by the image forming apparatus 1.

[0061] (Potential decay characteristics of the photoreceptor) Figure 12 is an explanatory diagram of the potential decay characteristic (EV curve) that shows the relationship between the exposure amount (image formation conditions) to the photoreceptor 102 and the surface potential of the photoreceptor 102. In an image forming apparatus using an analog exposure method, the reciprocity law of the photoreceptor holds true, but in a digital exposure method using pulse exposure such as a semiconductor laser, the photoreceptor is in a state of reciprocity failure. Reciprocity failure means that the reciprocity law, which states that if the product of light intensity and exposure time is constant, the amount of reacting substance will also be the same, does not hold true, and it is a characteristic in which the combined factors of exposure amount and the photoreceptor affect the formation of an electrostatic latent image.

[0062] In the digital exposure method of this embodiment, an organic EL is used as the light source. Organic EL has the characteristic of having a lower light emission amount compared to semiconductor lasers. For this reason, the exposure time for one pixel is longer with organic EL compared to semiconductor lasers. Specifically, while the exposure time for semiconductor lasers is 10 nanoseconds, it is 10 microseconds or more with organic EL. Therefore, organic EL emits light for a longer period of time with a lower light intensity than in the analog exposure method, thereby securing the amount of light necessary to attenuate the potential of the photoreceptor.

[0063] The dashed line in Figure 12 represents EV curve A when using long exposure in this embodiment. The solid line in Figure 12 represents EV curve B when using conventional pulse exposure. In this embodiment, the exposure time for one pixel is more than 1000 times the exposure time in the case of conventional pulse exposure. Comparing EV curve A for long exposure where the reciprocity rule is nearly true with EV curve B for pulse exposure where the reciprocity rule is not true, it is found that when the same amount of light (energy) is applied, the potential can be attenuated more effectively in the case of long exposure where the reciprocity rule is nearly true.

[0064] Figure 13 is a graph of the potential decay characteristics normalized by the exposure energy. Here, the surface potential (VD) of the photoreceptor 102 after charging is set to -600[V], and the surface potential (VL) of the portion of the photoreceptor 102 exposed to achieve the set solid image density is set to -200[V], and the potential decay characteristics are normalized by the exposure energy. As shown in Figure 13, EV curve A in the case of long exposure has more linear characteristics than EV curve B in the case of pulse exposure. When the normalized potential decay characteristics are like EV curve A in Figure 13, it becomes difficult to reproduce highlight areas and ultrafine lines, which is a characteristic of electrophotographic methods, and the image quality deteriorates.

[0065] (Edge preservation and pixel downsampling) To prevent such a degradation in image quality, a decimation process that limits the exposure of a predetermined number of pixels from each pixel in the entire image will be described. Pixel decimation is performed using error diffusion processing. However, if error diffusion processing is performed as is, pixels will be randomly decimated. When pixels at the edges of dots or lines are decimated by error diffusion processing, variations in the image shape occur, resulting in a decrease in the reproducibility improvement effect and a decrease in granularity. Therefore, in this embodiment, pixel decimation processing is performed along with edge preservation processing to suppress variations in the image shape and maintain the reproducibility improvement effect and granularity. In edge preservation processing, pixels at the edges are excluded from the pixels to be decimated so that the edges at the boundaries of the image are not decimated. Figure 14 is a flowchart of the edge preservation processing and pixel decimation processing.

[0066] The image controller 700, using the image data processing unit 703, performs gradation correction and dithering on the read image data or print image data acquired from the reading unit 100 or an external device, and generates 1-bit intermediate data (bitmap data). In the process shown in Figure 14, the intermediate data represents an image with a resolution of 2400 dpi.

[0067] The image controller 700 converts the intermediate data to 8 bits (S1). This allows each pixel value in the image to take values ​​from 0 to 255. Using the 8-bit intermediate data, the image controller 700 performs a weighted filter process for each pixel, referencing surrounding pixels to increase the pixel value of the image edges (S2). The image controller 700 uses a LUT (Look Up Table) process to change the pixel values ​​of pixels whose pixel values ​​in the intermediate data after the weighted filter process are above a threshold to a constant value, in this case "255" (S3). The image controller 700 then thins out pixels whose pixel values ​​are not the constant value (not "255") by error diffusion processing according to the amount of light increase (S4).

[0068] Figures 15 and 16 are explanatory diagrams for such edge preservation and pixel decimation processes. Figure 15 shows the transition of pixel values ​​in the intermediate data after each process in Figure 14. Figure 16 shows the weighting filter (Figure 16(a)), LUT (Figure 16(b)), and error diffusion filter (Figure 16(c)).

[0069] The image controller 700 performs the S2 weighted filter processing using the weighted filter shown in Figure 16(a). The image controller 700 performs the S3 LUT processing using the LUT shown in Figure 16(b). In the LUT processing, the pixel values ​​of pixels whose pixel values ​​in the intermediate data after weighted filtering are greater than or equal to a threshold are changed to "255". Here, the threshold is set to "220". The pixel values ​​of pixels whose pixel values ​​are "220" or greater are changed to "255". The image controller 700 performs the S4 error diffusion processing using the error diffusion filter shown in Figure 16(c). Here, in order to increase the potential contrast of the highlights, the light intensity increase is set to 20%, and the number of pixels to be downsampled correspondingly is set to 20%. The potential contrast is adjusted by controlling the exposure amount. In other words, exposure amount and potential contrast are examples of image formation conditions.

[0070] Through the above process, the exposure to pixels excluding those at the edges of the image can be limited by a pixel decimation rate. Pixels at the edges of the image are excluded from the pixel decimation process when their pixel value exceeds a threshold due to a weighted filter. This makes it possible to reduce the potential contrast beyond the range of exposure variation. As a result, it becomes possible to adjust the amount of toner to adjust the image density and prevent fixing failures.

[0071] Figure 17 is an explanatory diagram of the image obtained by such edge preservation and pixel decimation processes. Figure 17(a) shows a line pattern, Figure 17(b) shows a dither pattern, and Figure 17(c) shows a solid image, illustrating the exposure patterns (exposure positions) for each. In the image, black and shaded areas are exposed pixels (exposed areas), and white areas are unexposed pixels (unexposed areas).

[0072] As shown in the line pattern and dither pattern, pixels in images with very thin lines or highlights are not downsampled. In other words, pixel downsampled is not performed unless the image is larger than a predetermined size. Also, as is clear from the exposure patterns of the line pattern, dither pattern, and solid image, the pixel values ​​at the edges of the image are set to "255" due to the weighting filter processing in S2 and the LUT processing in S3. As a result, the pixel downsampled processing by the diffusion error processing in S4 is not performed on the edges, and the pixel values ​​at the edges are preserved.

[0073] The image controller 700 generates print image data from the intermediate data after the downsampling process and controls the exposure of the exposure head 106 according to the print image data. As a result, the image forming unit 101 forms an electrostatic latent image on the photoreceptor 102. The process shown in Figure 14 is performed for each of the image data for yellow, magenta, cyan, and black. In this way, the image controller 700 performs 20% downsampling of pixels from the intermediate data (bitmap data) after dithering, while preserving edges at a resolution of 2400 [dpi] per pixel, which is higher than the resolution of the dithering process. This reduces the potential contrast and thus the amount of toner consumed without compromising image quality. In this embodiment, the downsampling process was performed by error diffusion binarization, but binarization is not limited to the error diffusion method. Binarization can also be performed by dot dispersion type binarization methods such as the blue noise mask method and the FM screen method.

[0074] (Adjustment of edge preservation and pixel downsampling processes) Edge preservation and pixel downsampling can be modified according to paper weight information and environmental information. If the potential contrast is to be lowered below the potential contrast achievable solely by adjusting the light intensity of the exposure head 106, pixel downsampling is performed. Furthermore, the downsampling rate is not limited to the aforementioned 20%, but can be increased or decreased as appropriate according to the required amount of toner, i.e., the desired potential contrast level.

[0075] One case where reducing toner usage is desirable is when forming images on thick paper, which is prone to fixing failures, as mentioned above. Fixing failures are even more likely when the image forming apparatus 1 is used in a low-temperature environment. Figure 18 is an example diagram showing the relationship between conditions such as paper weight and environmental information and the pixel thinning rate. When printing on plain paper, the pixel thinning rate is 0%. When printing on thick paper with a high basis weight, the heat of the fuser unit 104 is easily lost, so the pixel thinning rate is set to 20%. Here, plain paper refers to, for example, paper with a basis weight of 90 g / m². 2 ] to 128 [g / m³ 2This is a type of paper, and thick paper is, for example, paper with a basis weight of 129 g / m². 2 ] to 350 [g / m³ 2 This is the paper type. Furthermore, if the operating environment of the image forming apparatus 1 is a low-temperature environment, the heat of the fixing unit 104 during continuous printing will not recover easily, so the pixel decimation rate is set to 40%. However, the present invention does not limit the decimation rate to the above values. For example, the decimation rate for plain paper may be 10%, and the decimation rate for thick paper may be 25%. Furthermore, for example, the decimation rate for thick paper in a low-temperature environment may be 50%.

[0076] Although not illustrated in Figure 18, the amount of pixels thinned according to the thinning rate is also determined by the surface properties of the paper. For example, when printing on paper with deep surface irregularities (e.g., embossed paper), heat is lost from the fuser unit 104, and heat is not easily transferred to the toner that has entered the recesses on the surface of the paper, so the pixel thinning rate is set to 20% or more. To maintain productivity, it is necessary to reduce the amount of toner, but if it is not possible to reduce the amount of toner, it is also possible to reduce productivity without changing the thinning rate and print while ensuring sufficient time for the heat of the fuser unit 104 to recover. In this case, the user can set the thinning rate. In addition, the thinning rate or amount of thinning can also be limited in toner-saving mode, which is used to reduce toner usage in addition to preventing fixing failures. It is also possible to determine the pixel thinning rate according to the number of pages to be printed continuously. For example, when printing an image on a large number of sheets of paper, the temperature of the fuser unit 104 tends to decrease as the number of sheets increases. For this reason, the more pages to be printed continuously, the higher the pixel thinning rate is set.

[0077] Figure 19 is a flowchart of the edge preservation and pixel decimation processes with adjustable decimation rates. Figure 20 is an example of a weighted filter. Figure 21 is an example of a LUT. The processes S1 to S4 are the same as in Figure 14, so their explanation is omitted.

[0078] The image controller 700 acquires information regarding the basis weight of the paper to be printed, environmental information (e.g., temperature or humidity) detected by environmental sensors installed in the image forming apparatus 1, and information regarding the print job (S5). In S5, the image controller 700 determines the pixel decimation rate and selects the weighting filter and LUT to be used (S6).

[0079] The decimation rate is determined by preparing a table containing information such as that shown in Figure 18, and selecting a pixel decimation rate from this table that matches the information obtained in processing S5. Processing S6 selects a weighting filter from either Figure 20(a) or (b) and a LUT shown in Figure 21, depending on the pixel decimation rate. Figure 20(a) is the weighting filter selected when the pixel decimation rate is 20%, and Figure 20(b) is the weighting filter selected when the pixel decimation rate is 40%. The solid line in Figure 21 shows the LUT selected when the pixel decimation rate is 20%, and the dashed line in Figure 21 shows the LUT selected when the pixel decimation rate is 40%. Processing S2 is performed using the weighting filter selected in S6. Processing S3 is performed using the LUT selected in S6. The LUT determines the threshold value that is compared with the pixel value in processing S3.

[0080] Figure 22 is an explanatory diagram illustrating the relationship between print image data, potential contrast, and toner amount on paper when the pixel decimation rate is changed to 0% (no decimation), 20%, and 40%. When printing a solid image, the image with a pixel decimation rate of 20% (Figure 22(a)) has a higher image density than the image with a pixel decimation rate of 40% (Figure 22(b)). As shown in Figure 22(c), the higher the pixel decimation rate, the lower the potential contrast and the less toner is used on the paper. In this way, by setting the desired toner amount and performing edge preservation and pixel decimation processing according to the printing requirements, it is possible to print images without compromising image quality.

[0081] As described above, the image forming apparatus 1 of this embodiment preserves pixels in the edge portions of the image by performing weighted filtering on intermediate data, and performs pixel decimation according to the pixel decimation rate. The pixel decimation rate is determined based on paper weight information and environmental information. This makes it possible to reduce the potential contrast to the required level. By adjusting the pixel decimation rate, it is possible to set a wide range of potential contrasts, i.e., toner amounts, without changing the combination of the exposure head 106 and the photoreceptor 102. As a result, it is possible to reduce the amount of toner on the paper and appropriately suppress fixing failures.

Claims

1. An image forming apparatus that uses toner to form an image on paper, Image processing means that generates bitmap data composed of multiple pixels corresponding to image data, and performs pixel decimation processing on the bitmap data according to the decimation rate, Image forming means that forms an image based on the bitmap data that has undergone the downsampling process by the image processing means, A transfer means for transferring the image formed by the image forming means onto paper, Fixing means for fixing the image on the paper by heating the image on the paper, The image forming means is characterized by having a control means for controlling the thinning rate based on information regarding the basis weight of the paper on which the image is to be formed. Image forming apparatus.

2. The control means is If the paper on which the image is to be formed by the image forming means is plain paper, the thinning ratio is determined to a first value. If the paper on which the image is to be formed by the image forming means is thick paper with a larger basis weight than ordinary paper, the thinning ratio is determined to a second value in which the number of pixels thinned is greater than the first value. The image forming apparatus according to claim 1.

3. The control means is characterized by controlling the thinning rate based on the information relating to the basis weight of the paper on which the image is to be formed by the image forming means and environmental information detected by a sensor provided in the image forming apparatus. The image forming apparatus according to claim 1.

4. The image forming means comprises a photoreceptor, a charger for charging the photoreceptor, an exposure apparatus for exposing the photoreceptor charged by the charger to form an electrostatic latent image, and a developer for developing the electrostatic latent image using toner. The exposure apparatus is characterized in that the exposure of the photoreceptor is controlled based on the bitmap data that has undergone the downsampling process by the image processing means. The image forming apparatus according to claim 1.

5. The image processing means is characterized by performing a filter process to increase the pixel value of each pixel in the bitmap data, designating pixels with a pixel value equal to or greater than a threshold among the filtered pixels as edge pixels, and performing the decimation process on the pixels of the bitmap data excluding the edge portions. The image forming apparatus according to claim 1.

6. The image processing means is characterized by selecting a filter to be used for the filtering process according to the decimation rate. The image forming apparatus according to claim 5.

7. The image processing means is characterized by changing the pixel values ​​of pixels having pixel values ​​above the threshold to a constant value, and performing the downsampling process on pixels other than those with a constant value. The image forming apparatus according to claim 5.

8. The image processing means is characterized by determining the threshold according to the decimation rate. The image forming apparatus according to claim 5.

9. The control means is characterized by determining the thinning rate based on the number of sheets to be printed consecutively. The image forming apparatus according to claim 1.

Images