Image forming apparatus and method

Abstract: A development bias calculation and an electrifying bias calculation are executed in this order. In the development bias calculation, a plurality of toner images are formed as first patch images (PI1) while changing the development bias. An optimal development bias, which is necessary to obtain the target density, is determined based on densities of the first patch images (PI1). In the electrifying bias calculation, toner images are formed as second patch images (PI2) while changing the electrifying bias with the development bias fixed to the optimal development bias. An optimal electrifying bias, which is necessary to obtain the target density, is determined based on densities of the second patch images (PI2).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and an image forming method in which a charging bias applied to charging means charges a surface of a photosensitive member, an electrostatic latent image is thereafter formed on the surface of the photosensitive member, and a development bias is thereafter applied to developer means so that a toner visualizes the electrostatic latent image into a toner image. 2. Description of the Related Art This type of an image forming apparatus often sees a change in an image density due to the following factors: fatigue, degradation with age or the like of a photosensitive member and a toner; a change in a temperature, a humidity or the like around the apparatus; and other causes. Noting this, a number of techniques have been proposed which aim at stabilizing an image density through appropriate adjustment of a density control factor such as a charging bias, a development bias, a light exposure dose, etc. For example, the invention described in the Japanese Patent Application Laid-Open Gazette No. 10-239924 requires to properly adjust a charging bias and a development bias in an effort to stabilize an image density. That is, according to this conventional technique, reference patch images are formed on a photosensitive member while changing a charging bias and/or a development bias and an image density of each reference patch is detected. An optimal charging bias and an optimal development bias are thereafter determined based on the detected image densities, and a density of a toner image is accordingly adjusted. However, the conventional technique described above requires to identify a charging bias/development bias characteristic before forming reference patch images, and to set a charging bias and a development bias for creation of reference patch images, such that the characteristic is satisfied. In order to stabilize an image density based on a calculated optimal charging bias and development bias, it is necessary to identify a charging bias/development bias characteristic of each image forming apparatus, which is troublesome. Further, a charging bias/development bias characteristic does not always stay constant but may change with time. If the characteristic changes, it is difficult to accurately calculate an optimal charging bias or an optimal development bias. While appropriate updating of the charging bias/development bias characteristic solves this problem, the updating is bothersome and disadvantageous in terms of maintainability. Meanwhile, other technique for stabilizing an image density is the invention described in Japanese Patent Application Laid-Open Gazette No. 9-50155. According to the described invention, a reference patch image, which is a patch image obtained by outputting groups of three-dot lines for every three dots, is formed on a photosensitive drum, and a sensor reads patch images thus created, whereby a line width is detected. A laser power is controlled based on the detected line width, a light exposure dose is accordingly adjusted so that a desired line width will be obtained, and an ideal line image is obtained. However, a line image is basically a one-dot line which is drawn with one laser beam, and therefore, simply controlling a line width of a multi-dot line as in the conventional technique can not realize a precise adjustment of a line image. JP 08211722 discloses an image forming apparatus comprising means for forming a toner image by an electrophotography process and a toner weight detection means for detecting the image density of a toner image. In a check mode of the prior art device, a test pattern is produced over which the toner concentration varies continuously. Subsequently, the toner weight detection means measures the image density of the test pattern, and an electrification bias and a development bias are adjusted accordingly to produce a predetermined image density. SUMMARY OF THE INVENTION A main object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density at a high accuracy in a simple manner. Other object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density of a line image. In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image based on image densities of a plurality of patch images. According to a first aspect of the present invention, there is provided an image forming apparatus for forming an image which has a predetermined target density, comprising: a photosensitive member; charging means which charges a surface of said photosensitive member; exposing means which forms an electrostatic latent image on the charged surface of said photosensitive member; developing means which visualizes said electrostatic latent image with a toner to form a toner image; transferring means which transfers the toner image from said photosensitive member to a transfer medium; density detecting means which detects an image density of the toner image on said photosensitive member or on said transfer medium as a patch image; and control means which adjusts an image density of said toner image to the target density based on a result of the detection obtained by said density detecting means, characterized in that said patch image is formed by a plurality of one-dot lines which are apart from each other, wherein said density detecting means has a detect area capable of including a plurality of said one-dot lines arranged in said patch image. According to a second aspect of the present invention, there is provided an image forming method in which after charging a surface of a photosensitive member, an electrostatic latent image is formed on the charged surface of said photosensitive member, and developing means visualizes said electrostatic latent image with a toner to form a toner image which has a predetermined target density, characterized in that: after sequentially forming a plurality of toner images, which are each formed by a plurality of one-dot lines which are apart from each other, as patch images while changing a density controlling factor which influences an image density of toner image, densities of said patch images are detected, wherein the density detection is based on detecting a plurality of said one-dot lines, and an optimal density controlling factor which is necessary to obtain said target density is determined based on the densities of said patch images. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a drawing showing a preferred embodiment of an image forming apparatus according to the present invention; Fig. 2 is a block diagram showing an electric structure of the image forming apparatus of Fig. 1; Fig. 3 is a flow chart showing a density adjustment operation in the image forming apparatus of Fig. 1; Fig. 4 is a flow chart showing an operation of development bias calculation of Fig. 3; Fig. 5 is a flow chart showing an operation of the bias calculation of Fig. 4 in a wide range; Figs. 6A through 6D are schematic diagrams showing an operation of the processing of Fig. 5 and an operation of the bias calculation in a narrow range; Fig. 7 is a drawing showing a first patch image; Figs. 8A through 8D are drawings showing an order of forming patch images; Fig. 9 is a flow chart showing an operation of bias calculation (1) of Fig. 4 in the narrow range; Fig. 10 is a flow chart showing an operation of bias calculation (2) of Fig. 4 in the narrow range; Figs. 11 A and 11B are schematic diagrams showing the operation of the processing of Fig. 10; Fig. 12 is a flow chart showing an operation of the charging bias calculation of Fig. 3; Figs. 13A and 13B are schematic diagrams showing the operation of the processing of Fig. 12; Fig. 14 is a drawing showing a second patch image; Figs. 15A and 15B are drawings showing a relationship between the first patch images, a surface potential and a development bias potential; and Figs. 16A and 16B are drawings showing a relationship between the second patch images, a surface potential and a development bias potential. Fig. 17 is a graph showing a light intensity distribution of laser light which is irradiated onto a surface of a photosensitive member; Figs. 18A and 18B are schematic diagrams showing a relationship between one-dot lines and a detect area which a patch sensor detects, with a change in line intervals; Fig. 19 is a view for describing a detect deviation which occurs as positions of the detect area of the patch sensor and one-dot lines change relative to each other; Fig. 20 is a graph showing a change in an output from the patch sensor with a change in line intervals; Fig. 21 is a schematic diagram of other preferred embodiment of a patch image; Fig. 22 is a graph showing attenuation of a surface potential as photosensitive member is exposed at various exposure powers; Fig. 23 is a drawing showing a relationship between a development bias and a contrast potential when the development bias is changed with a charging bias fixed; Fig. 24 is a drawing showing a relationship between a charging bias and a contrast potential when the charging bias is changed with a development bias fixed; Fig. 25 is a drawing showing the relationship between the development bias and the contrast potential; Fig. 26 is a drawing showing variations in the contrast potential and the exposed area potential in accordance with a change in the charging bias; Fig. 27 is a drawing showing a relationship between the development bias and the contrast potential as the charging bias is set according to a first variation; Fig. 28 is a drawing showing a relationship between the charging bias and the development bias in the first variation; Fig. 29 is a drawing showing a relationship between the charging bias and the development bias in a second variation; Fig. 30 is a drawing showing a relationship between an exposure power and a surface potential; Fig. 31 is a drawing showing a relationship between the development bias and the contrast potential at the exposure power shown in Fig. 30; Fig. 32 is a drawing showing a relationship between the development bias and the contrast potential as the charging bias is set according to the second variation; Fig. 33 is a drawing showing a relationship between the charging bias and the development bias in a third variation; Fig. 34 is a drawing showing a relationship between an exposure power and a surface potential; Fig. 35 is a drawing showing a relationship between the development bias and the contrast potential at the exposure power shown in Fig. 34; Fig. 36 is a drawing showing a relationship between the development bias and the contrast potential as the charging bias is set according to the third variation; Fig. 37 is a drawing showing the relationship between the development bias and the contrast potential; Fig. 38 is a drawing showing a relationship between the charging bias and the development bias in a fourth variation; Fig. 39 is a drawing showing a relationship between the development bias and the contrast potential as the charging bias is set according to the fourth variation; Fig. 40 is a drawing showing the relationship between the development bias and the contrast potential; Fig. 41 is a drawing showing a relationship between the charging bias and the development bias in a fifth variation; Fig. 42 is a drawing showing a relationship between the development bias and the contrast potential as the charging bias is set according to the fifth variation; and Figs. 43A through 43D are drawings showing an order of forming patch images according to still other preferred embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Overall Structure of Image Forming Apparatus Fig. 1 is a drawing showing a preferred embodiment of an image forming apparatus according to the present invention. Fig. 2 is a block diagram showing an electric structure of the image forming apparatus of Fig. 1. The image forming apparatus is an apparatus which overlaps toner images in four colors of yellow (Y), cyan (C), magenta (M) and black (K) to thereby form a full-color image or uses only a black (K) toner to thereby form a monochrome image. When an image signal is supplied to a main controller 11 of a control unit 1 from an external apparatus such as a host computer, an engine controller 12 controls respective portions of an engine part E in accordance with an instruction from the main controller 11, whereby the image forming apparatus forms an image which corresponds to the image signal on a sheet S. The engine part E is capable of forming a toner image on a photosensitive member 21 of an image carrier unit 2. That is, the image carrier unit 2 comprises the photosensitive member 21 which is rotatable in the direction of an arrow in Fig. 1. Disposed around the photosensitive member 21 and in the rotation direction of the photosensitive member 21 in Fig. 1 are a charging roller 22 which serves as charging means, developers 23Y, 23C, 23M and 23K which serve as developing means, and a cleaning part 24. Applied with a high voltage from a charging bias generation part 121 and in contact with an outer peripheral surface of the photosensitive member 21, the charging roller 22 uniformly charges the outer peripheral surface of the photosensitive member 21. An exposure unit 3 irradiates laser light L toward the outer peripheral surface of the photosensitive member 21 which is charged by the charging roller 22. The exposure unit 3, as shown in Fig. 2, is electrically connected with an image signal switching part 122. In accordance with an image signal which is supplied through the image signal switching part 122, the laser light L scans over the photosensitive member 21 and consequently exposes the photosensitive member 21, whereby an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 21. For example, when the image signal switching part 122 is in conduction with a patch generation module 124, based on an instruction from a CPU 123 of the engine controller 12, a patch image signal outputted from the patch generation module 124 is fed to the exposure unit 3 so that a patch latent image is formed. On the other hand, when the image signal switching part 122 is in conduction with a CPU 111 of the main controller 11, the laser light L scans over and consequently exposes the photosensitive member 21 in accordance with an image signal which is supplied through an interface 112 from an external apparatus such as a host computer, so that an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 21. The electrostatic latent image which is formed in this manner is developed by a developer part 23. In other words, according to the preferred embodiment, disposed as the developer part 23 are the developer 23Y for yellow, the developer 23C for cyan, the developer 23M for magenta and the developer 23K for black which are arranged in this order around the photosensitive member 21. The developers 23Y, 23C, 23M and 23K are each structured so as to freely separate from and come close to the photosensitive member 21. In accordance with an instruction given from the engine controller 12, one of the four developers 23Y, 23C, 23M and 23K selectively contacts the photosensitive member 21. A development bias generation part 125 thereafter applies a high voltage to the photosensitive member 21, and the toner in the selected color moves to the surface of the photosensitive member 21, thereby visualizing the electrostatic latent image on the photosensitive member 21. The voltages supplied to the respective developers may be simply D.C. voltages, or alternatively, A.C. voltages superimposed over D.C. voltages. The toner image developed by the developer part 23 is primarily transferred onto an intermediate transfer belt 41 of a transfer unit 4 in a primary transfer region R1 which is located between the black developer 23K and the cleaning part 24. A structure of the transfer unit 4 will be described in detail later. The cleaning part 24 is disposed at a position further ahead in a circumferential direction (the direction of the arrow in Fig. 1) from the primary transfer region R1, such that a toner remaining on the outer peripheral surface of the photosensitive member 21 after the primary transfer treatment is scraped off. Next, the structure of the transfer unit 4 will be described. According to the preferred embodiment, the transfer unit 4 comprises rollers 42 through 47, the intermediate transfer belt 41 which is spun around the rollers 42 through 47, and a secondary transfer roller 48 which secondarily transfers an intermediate toner image transferred to the intermediate transfer belt 41 onto a sheet S. A transfer bias generation part 126 applies a primary transfer voltage upon the intermediate transfer belt 41. Toner images in the respective colors formed on the photosensitive member 21 are laid one atop the other on the intermediate transfer belt 41 into a color image, while the sheet S is taken out from a cassette 61, a hand-feeding tray 62 or an additional cassette (not shown) by a paper feed part 63 of a paper feed/discharge unit 6 and conveyed to a secondary transfer region R2. The color image is thereafter secondarily transferred onto the sheet S, thereby obtaining a full-color image. Meanwhile, when a monochrome image is to be transferred onto a sheet S, only a black toner image on the photosensitive member 21 is formed on the intermediate transfer belt 41, and transferred onto a sheet conveyed to the secondary transfer region R2 to thereby obtain a monochrome image, as in the case of forming a color image. After secondary transfer treatment, a toner remaining on and sticking to an outer peripheral surface of the intermediate transfer belt 41 is removed by a belt cleaner 49. The belt cleaner 49 is disposed opposite to the roller 46 across the intermediate transfer belt 41, and a cleaner blade contacts the intermediate transfer belt 41 at appropriate timing and scrapes off a toner from the outer peripheral surface of the intermediate transfer belt 41. Further, disposed in the vicinity of the roller 43 is a patch sensor PS which detects a density of a patch image which is formed on the outer peripheral surface of the intermediate transfer belt 41 as described later, and so is a read sensor for synchronization RS which detects a reference position of the intermediate transfer belt 41. Referring to Fig. 1 again, the description on the structure of the engine part E will be continued. The sheet S now seating the toner image transferred by the transfer unit 4 is conveyed by the paper feed part 63 of the paper feed/discharge unit 6 to a fixing unit 5 which is disposed on the downstream side to the secondary transfer region R2 along a predetermined paper feed path (dot-dot-dash line), and the toner image on the conveyed sheet S is fixed on the sheet S. The sheet S is thereafter conveyed to a paper discharge part 64 along the paper feed path 630. The paper discharge part 64 has two paper discharge paths 641a and 641b. The paper discharge path 641a extends from the fixing unit 5 to a standard paper discharge tray, while the paper discharge path 641b extends approximately parallel to the paper discharge path 641a between a paper re-feed part 66 and a multi-bin unit. Three roller pairs 642 through 644 are disposed along the paper discharge paths 641a and 641b, so as to discharge the sheets S toward the standard paper discharge tray or the multi-bin unit and convey the sheets S toward the paper re-feed part 66 for the purpose of forming images on non-printing surfaces of the sheets S. Aiming at conveying a sheet S which was inverted and fed from the paper discharge part 64 as described above to a gate roller pair 637 of the paper feed part 63 along a paper re-feed path 664 (dot-dot-dash line), the paper re-feed part 66 is formed of three paper re-feed roller pairs 661 through 663 which are disposed along the paper re-feed path 664 as shown in Fig. 1. In this manner, the sheet S sent from the paper discharge part 64 is returned to the gate roller pair 637 along the paper re-feed path 664 and a non-printing surface of the sheet S is directed toward the intermediate transfer belt 41 within the paper feed part 63, which makes it possible to secondarily transfer the image onto the non-printing surface. In Fig. 2, denoted at 113 is an image memory which is disposed in the main controller 11 such that the image memory stores image data supplied from an external apparatus such as a host computer through the interface 112, denoted at 127 is a RAM which temporarily stores control data for controlling the engine part E, a calculation result obtained by the CPU 123, etc., and denoted at 128 is a ROM which stores a calculation program which is executed by the CPU 123. B. Density Adjustment by Image Forming Apparatus Now, a description will be given on how the image forming apparatus having such a structure as described above adjusts a density of an image. Fig. 3 is a flow chart showing a density adjustment operation in the image forming apparatus of Fig. 1. In the image forming apparatus, as shown in Fig. 3, it is determined at a step S1 whether the density adjustment operation should be executed to thereby update a charging bias and a development bias. For example, the image forming apparatus may start setting the biases when the image forming apparatus becomes ready to form an image after a main power source of the image forming apparatus is turned on. Alternatively, the image forming apparatus may set the biases every few hours while a timer (not shown) disposed in the image forming apparatus measures hours of continuous use. When it is determined YES at the step S1 and setting of the biases is accordingly started, steps S2 and S3 are executed to calculate an optimal development bias, and the calculated bias is set as the development bias (step S4). Following this, a step S5 is executed to calculate an optimal charging bias, and the calculated bias is set as the charging bias (step S6). The charging bias and the development bias are optimized in this manner. In the following, a detailed description will be given on an operation of each one of the development bias calculation (step S3) and the charging bias calculation (step S5). B-1. Development Bias Calculation Fig. 4 is a flow chart showing an operation of the development bias calculation shown in Fig. 3. In the development bias calculation (step S3), the CPU 123 determines whether this is the first calculation or the second or subsequent calculation after the main power source of the image forming apparatus is turned on (step S301). When the current calculation is the first one, after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S311), an immediately subsequent step S312 is executed. In other words, a plurality of patch images are formed while gradually changing the development bias at relatively long intervals within a relatively wide range, thereby tentatively identifying a development bias which is necessary to obtain an optimal image density based on densities of the respective patch images. Now, an operation of this processing will be described in detail with reference to Figs. 5 and 6A through 6D. Fig. 5 is a flow chart showing an operation of the bias calculation of Fig. 4 within a wide range. Figs. 6A through 6D are schematic diagrams showing an operation of the processing of Fig. 5 and an operation of the bias calculation within narrow range which will be described later. During this calculation, a color in which patch images are to be generated is set as the first color, e.g., yellow (step S312a). With the charging bias set to a default value which is set in advance at the step S2, the development bias is set to four different values which are apart at relatively long intervals (first intervals) within the wide range (step S312b). For instance, in this preferred embodiment, the wide range is the entirety of a programmable range (Vb01 - Vb10) of development bias which can be supplied to the developer part 23 from the development bias generation part 125, and four points Vb01, Vb04, Vb07 and Vb10 within the wide range (Vb01 - Vb10) are set as development biases. In this manner, according to this preferred embodiment, the first intervals W1 are: W1 = Vb10 - Vb07 = Vb07 - Vb04 = Vb04 - Vb01 Four yellow solid images (Fig. 7) are sequentially formed on the photosensitive member 21 with this bias setup, and the solid images are transferred onto the outer peripheral surface of the intermediate transfer belt 41 as shown in Fig. 8A to thereby form first patch images PI1 (step S312c). The first patch images PI1 are solid images in this preferred embodiment. The reason of this will be described in detail later. At a subsequent step S312d, whether patch images are formed in all of patch generation colors is determined. While a result of the judgement stays NO, the next color is set as a patch generation color (step S312e) and the steps S312b and S312c are repeated. This adds further first patch images PI1 on the outer peripheral surface of the intermediate transfer belt 41, in the order of cyan (C), magenta (M) and black (K), as shown in Figs. 8B through 8D. On the contrary, when it is determined YES at the step S312d, image densities of the sixteen (= 4 types × 4 colors) patch images PI1 are measured on the basis of a signal outputted from the patch sensor PS (step S312f). While the image densities of the patch images PI1 are measured at once after forming the patch images PI1 in all patch generation colors in this preferred embodiment, the image densities of the patch images PI1 may be measured sequentially color by color every time the patch images PI1 in one patch generation color are formed. This applies to the later bias calculation (Figs. 9, 10 and 12) as well. Following this, a development bias corresponding to a target density is calculated at a step S312g, and the calculated bias is stored temporarily in the RAM 127 as an interim bias. When a measurement result (image density) matches with the target density, a development bias corresponding to this image density may be used as the interim bias. When the two density values fail to match, as shown in Fig. 6B, it is possible to calculate an interim bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Vb04) and data D (Vb07) which are on the both sides of the target density. Once the interim bias is determined in this manner, the bias calculation (1) in the narrow range shown in Fig. 4 is executed. Fig. 9 is a flow chart showing an operation of the bias calculation (1) of Fig. 4 in the narrow range. During this calculation, a color in which patch images are to be generated is set as the first color, e.g., yellow (step S313a), as in the earlier calculation (step S312). With the charging bias set to the default value which is set in advance at the step S2, the development bias is set to four different values which are apart at narrower intervals (second intervals) than the first intervals W1 within a narrow range which includes the interim bias (step S313b). For instance, in this preferred embodiment, the narrow range is approximately 1/3 of the programmable rang (Vb01 - Vb10) of development bias. When the interim bias is between development biases Vb05 and Vb06 as shown in Fig. 6B, four points Vb04, Vb05, Vb06 and Vb07 are set as development biases (Fig. 6C). In this manner, according to this preferred embodiment, the second intervals W2 are: W2 = Vb07 - Vb06 = Vb06 - Vb05 = Vb05 - Vb04 Four yellow solid images (Fig. 7) are sequentially formed on the photosensitive member 21 with this bias setup, and the solid images are transferred onto the outer peripheral surface of the intermediate transfer belt 41 as shown in Fig. 8A to thereby form first patch images PI1 (step S313c). As in the earlier calculation (step S312), the next color is set as a patch generation color (step S313e) and the steps S313b and S313c are repeated until it is determined at a step S313d that patch images are formed in all of patch generation colors. As a result, first patch images PI1 are further formed on the outer peripheral surface of the intermediate transfer belt 41, in the order of cyan (C), magenta (M) and black (K). Once sixteen (= 4 types × 4 colors) patch images PI1 are formed on the intermediate transfer belt 41 in this manner, image densities of the respective patch images PI1 are measured on the basis of a signal outputted from the patch sensor PS (step S313f). Following this, at a step S313g, a development bias corresponding to a target density is calculated. When a measurement result (image density) matches with the target density, a development bias corresponding to this image density may be used as an optimal development bias. When the two density values fail to match, as shown in Fig. 6D, it is possible to calculate an optimal development bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Vb05) and data D (Vb06) which are on the both sides of the target density. The RAM 127 stores the optimal development bias which is calculated in this manner (step S302 in Fig. 4), and reads it out as the development bias during calculation of the charging bias which will be described later or while an image is formed in a normal manner. Thus, the preferred embodiment described above carries out a two-stage development bias calculation. In the first stage, patch images PI1 are formed at the first intervals W1 in the wide range to calculate a development bias, which is necessary to obtain an image having a target density, as an interim development bias. In the second stage, patch images PI1 are formed at the narrower intervals (i.e., the second intervals) W2 in the narrow range which includes the interim bias to calculate a development bias which is necessary to achieve the target density. Finally, the calculated bias is set as an optimal development bias. This realizes the following effects. For example, upon turning on of the main power source of the image forming apparatus, it is totally impossible to predict variations in characteristics of the photosensitive member and the toners, humidity and temperatures around the apparatus, etc. Hence, it is necessary to form patch images after setting a development bias such that the programmable range (Vb01 - Vb10) of development biases is entirely covered and to determine an optimal development bias. Therefore, the optimal development bias can be obtained by the following approach: The approach requires to divide the programmable range (Vb01 - Vb10) of development biases into a plurality of narrow ranges and to execute similar processing to the bias calculation (1) described above in each one of the narrow ranges. However, this comparative approach has a problem that the number of steps to be executed increases in proportion to the number of the divided ranges and calculation of an optimal development bias therefore takes time. Conversely, if the programmable range is divided into a smaller number of narrow ranges, although the problem described earlier is solved, bias intervals within each divided range become wider than the second bias intervals W2. This creates another problem that an accuracy of calculating an optimal development bias drops down and an image density therefore cannot be accurately adjusted to the target density. In contrast, according to the above embodiment, a development bias is tentatively calculated through the bias calculation processing (step S312) in the wide range, and the development bias is changed at the narrower intervals (i.e., the second intervals) W2 in the narrow range in the vicinity of the interim bias, so that an optimal development bias is finally calculated. Hence, it is possible to more accurately calculate an optimal development bias in a shorter period of time than in the comparative approach above. By the way, while an optimal electrifying bias and an optimal development bias change due to fatigue, degradation with age or the like of a photosensitive member, a toner, etc., the changes possess a continuity to a certain extent. Hence, where an image density is repeatedly adjusted, it is possible to predict an optimal development bias based on an image density which is measured immediately previously (e.g., the step S313f, and steps S322f and S510 which will described later). Noting this, in the bias calculation (step S3) according to this preferred embodiment, when the current calculation is determined to be the second or subsequent calculation after the main power source of the image forming apparatus is turned on, that is, when it is determined at the step S301 in Fig. 4 to follow the SECOND OR SUBSEQUENT path, after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S321), an immediately subsequent step S322 is executed. In other words, bias calculation (2) within the narrow range is executed to thereby calculate an optimal development bias without calculating an interim bias. Now, an operation of this processing will be described in detail with reference to Fig. 10. Fig. 10 is a flow chart showing an operation of the bias calculation (2) of Fig. 4 within the narrow range. Figs. 11A and 11B are schematic diagrams showing the operation of the processing shown in Fig. 10. This calculation processing is largely different from the bias calculation (1) within the narrow range described earlier in regard to the following. During the calculation (1) shown in Fig. 9, the charging bias set to the default value, and four different types of development biases are set based on an interim bias (step S313b). Meanwhile, during the bias calculation (2), the charging bias is the optimal charging bias which is calculated through immediately preceding measurement and stored in the RAM 127, and four different types of development biases are set within the narrow range based on the optimal development bias which is stored in the RAM 127 (step S322b). The bias calculation (2) is structured otherwise the same as the bias calculation (1), and therefore, a redundant description will be simply omitted. In this manner, during the second or subsequent density adjustment, the four different types of development biases are set. The four biases are apart at the second intervals within the narrow range using the development bias which is calculated immediately previously (preceding optimal development bias) without calculating an interim bias, the patch images are formed in the respective colors, and the optimal development bias is calculated. Hence, it is possible to calculate an optimal development bias in a further shorter time. The engine controller 12 writes the optimal development bias which is calculated in this manner over the preceding optimal development bias which is already stored in the RAM 127, thereby updating the optimal development bias (step S302 in Fig. 4). The sequence thereafter returns to Fig. 3 which requires to read the optimal development bias from the RAM 127 and set the retrieved optimal development bias as the development bias. An optimal charging bias is thereafter calculated (step S5) and set as the charging bias (step S6). B-2. Optimal Charging Bias Calculation Fig. 12 is a flow chart showing an operation of the charging bias calculation of Fig. 3. Figs. 13A and 13B are schematic diagrams showing the operation of the processing shown in Fig. 12. During the charging bias calculation (step S5), after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S501), a color in which second patch images are to be generated is set as the first color, e.g., yellow at a step S502. As in the development bias calculation, the CPU 123 determines whether the current charging bias calculation is first such calculation or the second or subsequent calculation after the main power source of the image forming apparatus is turned on (step S503). When the current calculation is determined to be the first one, a step S504 is executed. When the current calculation is determined to be the second or subsequent calculation, a step S505 is executed. At the step S504, the charging bias is set to four different values. The four biases are apart at relatively narrow intervals (third intervals) within the narrow range which includes the default value. Meanwhile, at the step S505, the charging bias is set to four different values which are apart at relatively narrow intervals (third intervals) within the narrow range which includes a preceding optimal charging bias. In this manner, unlike the development bias calculation, the charging bias calculation executes only narrow-range calculation without calculating within the wide range. In this preferred embodiment, the narrow range is approximately 1/3 of a programmable range (Va01 - Va10) of charging bias. When the default value or an immediately preceding optimal charging bias is between charging biases Va05 and Vb06 as shown in Fig. 13A, four points Va04, Va05, Va06 and Va07 are set as charging biases. That is, according to this preferred embodiment, the third intervals W3 are: W3 = Va07 - Va06 = Va06 - Va05 = Va05 - Va04 Once four types of charging biases are set up for the yellow color in this manner, respective yellow halftone images (See Fig. 14) are sequentially formed on the photosensitive member 21 and transferred onto the outer peripheral surface of the intermediate transfer belt 41, whereby second patch images PI2 are formed (Fig. 8A: step S506). The charging bias is increased stepwise because when a charging bias is to be changed stepwise, increasing the charging bias achieves a superior response of the power source as compared to decreasing the charging bias. In the preferred embodiment above, the second patch images PI2 are halftone images which are defined by a plurality of one-dot lines which are arranged parallel to each other but apart from each other at the intervals of five lines (n = 5). The reason of this will be described in detail later together with the reason why the first patch images are solid images. At a subsequent step S507, whether the second patch images are formed in all of patch generation colors is judged. While a result of the judgement stays NO, the next color is set as a patch generation color (step S508) and the steps S503 through S507 are repeated. This adds further second patch images PI2 on the outer peripheral surface of the intermediate transfer belt 41, in the order of cyan (C), magenta (M) and black (K), as shown in Figs. 8B through 8D. On the contrary, when it is determined YES at the step S507, image densities of the sixteen (= 4 types × 4 colors) patch images PI2 are measured on the basis of a signal outputted from the patch sensor PS (step S509). Following this, a charging bias corresponding to a target density is calculated (step S510), and the calculated charging bias is stored in the RAM 127 as an optimal charging bias (step S511). When a measurement result (image density) matches with the target density, a charging bias corresponding to this image density may be used as an optimal charging bias. When the two density values fail to match, as shown in Fig. 13B, it is possible to calculate an optimal charging bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Va05) and data D (Va06) which are on the both sides of the target density. Once the optimal charging bias is determined in this manner, the optimal charging bias calculated as described above is read from the RAM 127 and set as the charging bias, in addition to the optimal development bias already set as the development bias. When an image is formed with this setup, the resultant image has the target density. In other words, the image density is stable. As described above, according to this preferred embodiment, it is possible to calculate an optimal charging bias and an optimal development bias without using a charging bias/development bias characteristic which is essential in the conventional technique to adjust an image density. Hence, it is possible to adjust an image density to a target density and accordingly stabilize the image density in a simple manner. Further, even despite a change with time in a charging bias/development bias characteristic, this preferred embodiment allows to accurately calculate an optimal charging bias and an optimal development bias without an influence of the change. Further, as described above, since calculation of an optimal development bias is achieved in the two stages of bias calculation in the wide range (step S312) and bias calculation in the narrow range (step S313), it is possible to calculate the optimal development bias at a high accuracy in a short period of time. Further, this preferred embodiment makes it possible to calculate an optimal charging bias and an optimal development bias, adjust an image density to a target density, and stabilize the image density. According to this preferred embodiment, in particular, each patch image PI2 is formed by a plurality of one-dot lines which are arranged apart from each other. Since an image density of each such patch image PI2 is detected and an image density of a toner image is adjusted to a target density based on the detected image densities of the patch images PI2, it is possible to stabilize an image density of not only a line image which is formed by a P-dot (P ≥ 2) line but of a line image which is formed by a one-dot line, and hence, to stably form a fine image with an appropriate image density. Further, with respect to calculation of an optimal charging bias, since the charging bias calculation is executed with an optimal development bias calculated through immediately preceding calculation set as a development bias, it is possible to accurately calculate an optimal charging bias. Further, during the second or following calculation of a development bias and a charging bias, since the biases are calculated based on immediately preceding results of image density measurements (i.e., an optimal charging bias and an optimal development bias), it is possible to accurately calculate an optimal charging bias and an optimal development bias in a short period of time. C. Patch Images By the way, the following is the reason why solid images are used as the first patch images for calculation of a development bias, while for calculation of a charging bias, used as the second patch images are halftone images in which a plurality of one-dot lines are arranged parallel to each other but apart from each other at intervals of n lines. As an electrostatic latent image LI1 of a solid image (first patch image) PI1 (See Fig. 7) is formed on the surface of the photosensitive member 21 which is charged uniformly at a surface potential V0, a surface potential corresponding to the electrostatic latent image LI1 largely drops down to a potential (exposed area potential) Von as shown in Figs. 15A and 15B, whereby a well potential is developed. Now, even if the charging bias is increased to raise the surface potential of the photosensitive member 21 from the potential V0 up to a potential V0', the exposed area potential will not depart largely from the potential Von. Hence, a toner density is determined only in accordance with the development bias Vb despite any small change in the charging bias. Meanwhile, a halftone image (second patch image) PI2 (See Fig. 14) contains one-dot lines DL formed at predetermined intervals. As an electrostatic latent image LI2 of the halftone image is formed on the surface of the photosensitive member 21 which is charged uniformly at a surface potential V0, surface potentials corresponding to the positions of the lines largely drop down to the potential (exposed area potential) Von, as shown in Figs. 16A and 16B. As a result, a comb-shaped well potential is developed. If the charging bias is increased in a similar manner to described above to raise the surface potential of the photosensitive member 21 from the potential V0 up to the potential V0', the exposed area potential corresponding to each line changes greatly from the potential Von to a potential Von'. Hence, as the charging bias changes, a toner density corresponding to the development bias Vb changes with the change in the charging bias. A relationship between such bias setup (the optimal development bias and the optimal charging bias) and a toner density will be described in detail in "D. Setting of Charging Bias in Development Bias Calculation" below. From the above, it is found that use of a solid image reduces the influence of the charging bias over the toner density, and therefore, it is possible to adjust an image density of the solid image by means of adjustment of the development bias. In short, when the development bias calculation is executed using solid images as the first patch images as in the preferred embodiment above, it is possible to accurately calculate an optimal development bias regardless of the value of the charging bias. Further, to form an image in a stable manner, adjustment at a maximum gradation (maximum density) alone is not sufficient. Density adjustment of a line image is necessary as well. However, when halftone images of line images are used, as shown in Figs. 16A and 16B, the set development bias and the set charging bias strongly influence an eventual image. To deal with this, the preferred embodiment above requires to calculate an optimal development bias first. While changing the charging bias with the development bias set to the optimal development bias, the second patch images of halftone images are formed. As a result, therefore, the optimal charging bias needed to obtain an image density, which meets the target density, is calculated. In addition, a line image (second patch image PI2) is formed by a halftone image which is obtained by arranging a plurality of one-dot lines parallel to each other but apart from each other at intervals of n lines, for the following reason. That is, although one approach to adjust an image density of a one-dot line is to form the second patch image PI2 as a single one-dot line and detect a density of the one-dot line with the patch sensor PS, since an image density of a one-dot line is extremely low, it is difficult to detect an image density of a one-dot line with the patch sensor PS. Noting this, the present invention requires to form a patch image with a plurality of one-dot lines to solve this problem. Where a patch image is formed by a plurality of one-dot lines, the issue is how to arrange the one-dot lines for the following reason. Laser light L irradiated toward the photosensitive member 21 from the exposure unit 3 has a light intensity distribution of a Gaussian type as that shown in Fig. 17, for example. In a normal apparatus design, in most cases, a design spot diameter is set which is needed to attain a design resolution. An apparatus is designed such that a spot diameter approximately at 50 % of a maximum light intensity matches a design resolution. However, an effective exposure spot diameter corresponding to 1/e 2 which is effective as an exposure power is larger than the design spot diameter. Hence, when a line interval between adjacent one-dot lines DL is narrow, a toner adheres between the lines. In other words, if the line interval n between the adjacent one-dot lines DL (Fig. 16 A) is one line, adjacent effective exposure spots partially overlap with each other, a surface potential at the overlap position changes, and a toner adheres. Because of this, it is necessary that a line interval between adjacent one-dot lines DL is at least two lines or more. Conversely, the following problem occurs if the line intervals are too wide. That is, a sensitivity of the patch sensor PS to detect an image density is closely related with the number of one-dot lines DL which are contained in a detect area of the patch sensor PS. Where a density change of each one-dot line DL is X and the number of lines covered by the detect area is m, an image density change Δ detected by the patch sensor PS is: Δ = m • X Thus, the larger the number of lines contained in the detect area is, the higher the detect sensitivity is. For instance, as shown in Fig. 18A, with line intervals of n1, when the number of lines contained in the detect area IR of the patch sensor PS is five, an image density change Δa is: Δa = 5 • X On the other hand, as shown in Fig. 18B, with line intervals n2 (> n1), the number of lines contained in the detect area IR of the patch sensor PS decreases to four, and therefore, an image density change Δb is: Δb = 4 • X thereby decreasing the detect sensitivity. While results of various experiments have identified that it is necessary to improve the detect sensitivity of the patch sensor PS approximately one digit in order to ensure sufficient density adjustment, the number of lines contained in the detect area IR must be set to ten or larger for that purpose. Now, where the size of the detect area IR is Ø (mm) and the design resolution of the apparatus, namely, the number of dots contained in a unit length (1 mm) is R, if the line intervals are n, the number of lines m within the detect area IR is: m = Ø • R/(1+ n) For the number of lines m to be ten or larger, the following must be satisfied: Ø • R/(1+n) 10 Modifying the inequality, n (Ø • R-10)/10 Thus, if the line intervals n are set so as to satisfy the inequality (1) above, it is possible to detect image densities of the patch images PI2 at an excellent detect sensitivity. While where the patch sensor PS is to read image densities, repeated reading while changing a read position aims at improving the detect accuracy. If images to be detected are patch images in which one-dot lines are arranged parallel to each other but apart from each other at predetermined intervals, due to positional differences between the detect area of the patch sensor PS and the patch images relative to each other, the number of one-dot lines contained in the detect area differs maximum one line. When the detect area IR of the patch sensor PS and the patch image PI2 are positioned relative to each other as shown in Fig. 19A, for example, the number of one-dot lines DL contained in the detect area IR is five, whereas the relative positions are as shown in Fig. 19B, the number of the lines is six. Hence, even though the patch sensor PS reads the same patch image PI2, the patch sensor PS detects different image densities in the two different situations, and the detect deviation between the two different situations is: Detect deviation (%) = (1 / m) × 100 where m denotes the number of the lines contained in the detect area IR. Thus, the larger the number of the lines m contained in the detect area IR becomes, the smaller the detect deviation becomes. This makes it possible to improve the accuracy of measurement. For highly accurate control of densities, it is necessary to suppress the detect deviation to 5 % or smaller, and therefore, it is desirable to set the number of the lines m to twenty or larger. In short, the inequality below must be satisfied: Ø • R/(1 + n) ≥ 20 Modifying the inequality, n (Ø • R - 20)/20 Thus, if the line intervals n are set so as to satisfy the inequality (2) above, it is possible to suppress the detect deviation and detect image densities of the patch images PI2 at an even better detect accuracy. An actual example as described below was tried to verify the condition above regarding the line intervals. In the actual example, patch images were created while changing the line intervals n under the following conditions and voltages detected by the patch sensor PS were measured, thereby obtaining a graph as that shown in Fig. 20: Design resolution R: 23.6 lines / mm (600 DPI); and Size of detect area IR of patch sensor PS Ø: 8 mm The result in the graph well matches with the condition described above regarding the line intervals. That is, while it is necessary to set the line intervals n to two or larger in order to avoid a mutual influence between adjacent one-dot lines, as clearly seen in Fig. 20, if the line intervals n are set to 1, it is not possible to distinguish from solid images. On the contrary, it is desirable to set the line intervals n such that the inequality (1) above is satisfied in order to obtain a sufficient detect sensitivity. Therefore, in the actual example, it is desirable to set the line intervals n to seventeen or smaller, i.e., satisfy the following: n (8 × 23.6 - 10) / 10 = 17.88 (lines) In this respect, as clearly seen in Fig. 20, if the line intervals n are 18 or larger, it is not possible to distinguish from a blank image, and hence, it is difficult to accurately detect image densities. Further, it is desirable to satisfy the inequality (2) described above for highly accurate detection with a suppressed detect deviation. Therefore, in the actual example, it is desirable to set the line intervals n to eight or smaller, i.e., satisfy the following: n (8 × 23.6 - 20) / 20 = 8.44 (lines) Thus, it is most desirable to set the line intervals n to five in the actual example. In addition, although the patch images PI2 are images which are obtained by arranging a plurality of one-dot lines DL parallel to each other but apart from each other at the predetermined intervals n in the preferred embodiment above, as shown in Fig. 21, for instance, perpendicular lattice images PI2' may be used which are obtained by arranging a plurality of one-dot lines DL in the configuration of a lattice. In this case, the detect area IR of the patch sensor PS covers more lines, and hence, the detect sensitivity is better and a larger improvement is made to the accuracy as compared to where the patch images PI2 are formed by one-dot lines which are arranged parallel to each other (See Fig. 14). Moreover, it is possible to widen the line intervals n, owing to the increased number of lines. Widening the line intervals particularly in the sub-scanning direction reduces an influence by an uneven density in the drive direction, which in turn allows to control while detecting more stable images. Of course, a lattice structure of patch images is not limited to a perpendicular lattice, but may be various types of lattices in which case as well a similar effect is obtained. D. Setting of Charging Bias in Development Bias Calculation By the way, when second patch images are formed while changing a charging bias, an exposed area potential (bright part potential) Von of a latent image sometimes largely changes as the charging bias changes. Fig. 22 is a graph showing attenuation of a surface potential as a photosensitive member is exposed at various exposure powers, in which curves C(Va-1), C(Va-2), C(Va-3) and C(Va-4) express attenuation of a surface potential caused by charging at charging biases Va-1 through Va-4 which are different from each other. In Fig. 22, "EXPOSURE POWER" denotes a dose of exposure applied upon a photosensitive member 21 per unit area from the exposure unit 3. As clearly shown in Fig. 22, a surface potential in a surface area of the exposed photosensitive member 21, namely, the exposed area potential changes in accordance with the charging bias and the exposure power supplied to the exposed photosensitive member 21 from the exposure unit 3. The exposed area potential is approximately the same between the attenuation curves regardless of a value of the charging bias when the exposure power is relatively large. On the other hand, the exposed area potential is different in accordance with the charging bias when the exposure power is relatively small. Such a tendency is as already described with reference to Figs. 15A, 15B, 16A and 16B. Hence, when the exposure power is set relatively high, even if the charging bias set during the development bias calculation is largely deviated from the optimal charging bias, a contrast potential (= development bias - surface potential) during the development bias calculation matches with a contrast potential after setting of the optimal charging bias. Therefore, it is possible to stably form an image at a target density by means of the optimal development bias and the optimal charging bias which are calculated according to the preferred embodiment above. Conversely, when the exposure power is set relatively small, since the surface potential differs depending on the charging bias, it is sometimes impossible to stably form an image at a target density even despite setting the optimal development bias and the optimal charging bias which are calculated according to the preferred embodiment above. This is because when the charging bias set during the development bias calculation is largely deviated from the optimal charging bias, the contrast potential (= development bias - surface potential) during the development bias calculation becomes different from the contrast potential after setting of the optimal charging bias. With the contrast potential varied in such a manner, it is difficult to stabilize an image density. Noting this, in a preferred embodiment described below, the charging bias is changed in accordance with a change in the development bias during the development bias calculation processing, to thereby solve the problem above which occurs when the exposure power is relatively small. First, a relationship between the development bias Vb and the contrast potential will be described before describing how the charging bias is specifically changed. During the development bias calculation processing, as shown in Fig. 23 for instance, if the charging bias is fixed at a bias Va-2 and latent images of first patch images are formed by exposing light at an exposure power P1, the exposed area potential of the latent images become a potential Von1. As the development bias Vb is changed in this condition, a contrast potential Vcon1 changes in accordance with the change in the development bias Vb, thereby changing densities of the first patch images. Hence, during the development bias calculation according to the preferred embodiment described above, a plurality of first patch images is formed while changing only the development bias Vb and the optimal development bias is thereafter determined. On the other hand, during the charging bias calculation processing, as shown in Fig. 24 for example, the charging bias is set to various levels while fixing the development bias to the optimal development bias Vb, and latent images of second patch images are formed by exposing light at an exposure power P2. The exposed area potential of the latent images becomes largely different between the different charging bias levels. Since second patch images are halftone images as those shown in Fig. 16A. Hence, even though the latent images are formed with an exposure beam having the exposure power P1, an effective exposure power for exposure with an isolated beam is smaller than the exposure power P1. As a result, the lowest potential level of a comb-shaped well potential is not as low as the lowest potential level that is observed during solid exposure. Noting a macro surface potential of halftone latent images, this is the same as solid exposure at the exposure power P2 that is smaller than the exposure power P1. Therefore, considering that the latent images of the second patch images are images solidly exposed at the exposure power P2, the exposed area potential of these latent images becomes largely different depending on the charging bias. For instance, the exposed area potential becomes a potential Von2-2 to generate the contrast potential Vcon2-2 when the charging bias has the level Va-2, whereas when the charging bias has the level Va-3, the exposed area potential becomes a potential Von2-3 to generate the contrast potential Vcon2-3. In this manner, the contrast potential Vcon2 changes as the charging bias Va changes, and a density of the second patch image accordingly changes. For this reason, the charging bias calculation according to the preferred embodiment described above requires to form a plurality of second patch images while changing only the charging bias Va in order to determine an optimal charging bias. If the optimal charging bias resulting from such charging bias calculation processing is different from the charging bias set during the development bias calculation (i.e., the charging bias Va-2 in Fig. 23), the contrast potential Vcon1 determined through the development bias calculation is changed. Hence, despite application of the optimal development bias, an image density may deviate from a target density. The possibility of this is high particularly when the exposure power drops. Fig. 25 shows a relationship between the development bias Vb and the contrast potential that is identified based on the optimal attenuation curves C(Va-a) and C(Va-b). In Fig. 25, the horizontal axis denotes the development bias Vb while the vertical axis denotes the contrast potential. Further, straight lines L(P1, Va-a), L(P1, Va-b), L(P2, Va-a) and L(P2, Va-b) respectively denote contrast potentials Vcon1-a, Vcon1-b, Vcon2-a and Vcon2-b which are shown in Fig. 26. When first patch images are formed with the charging bias Va-a, changing the development bias Vb causes proportional change in the contrast potential Vcon1-a as denoted at the straight line L(P1, Va-a) shown in Fig. 25. Meanwhile, when first patch images are formed with the charging bias Va-b, changing the development bias Vb causes proportional change in the contrast potential Vcon1-b as denoted at the straight line L(P1, Va-b) shown in Fig. 25. When second patch images are formed with the charging bias Va-a, changing the development bias Vb causes proportional change in the contrast potential Vcon2-a as denoted at the straight line L(P2, Va-a) shown in Fig. 25. Further, when second patch images are formed with the charging bias Va-b, changing the development bias Vb causes proportional change in the contrast potential Vcon2-b as denoted at the straight line L(P2, Va-b) shown in Fig. 25. A development bias/contrast potential characteristic is determined based on the optimal attenuation curves in this manner. In Fig. 25, a target contrast potential Vcon01 corresponds to the target density during the development bias calculation processing and a target contrast potential Vcon02 corresponds to the target density during the charging bias calculation processing. In order to even more accurately adjust a density, it is necessary to set the optimal development bias Vb and the optimal charging bias Va such that these two contrast potentials Vcon01 and Vcon02 are simultaneously satisfied. According to this embodiment, during the development bias calculation processing, as shown in Fig. 27, the development bias Vb is varied in its programmable range while at the same time changing the charging bias from the level Va-a to the level Va-b. As the charging biases Va-a and Va-b are set so that the two target contrast potentials Vcon01 and Vcon02 are simultaneously satisfied with approximately the same development bias Vb0, the optimal development bias Vb and the optimal charging bias Va are set at a high accuracy. Now, as variations of the charging bias during the development bias calculation processing, five variations will be described. In each one of the five variations below, the charging bias increases as the development bias increases. (1) First variation : Fig. 28 Fig. 28 is a drawing showing a first variation of the development bias and the charging bias during the development bias calculation processing. In the first variation, a quantity of change ΔVa (= Va-b - Va-a) in the charging bias is set equal to a quantity of change ΔVb in the development bias, and the charging bias Va is set to a value which is expressed as below: Va = Vb + C where C is a constant that is determined in accordance with a structure, operations and the like of an image forming apparatus. (2) Second variation : Fig. 34 Fig. 34 is a drawing showing a second variation of the development bias and the charging bias during the development bias calculation processing. In the second variation, a quantity of change Va (= Va-b - Va-a) in the charging bias is set smaller than a quantity of change ΔVb in the development bias. Such setup is suitable to a situation where, as shown in Fig. 30, the exposure power P1 during the development bias calculation processing is relatively high thereby accompanying a small change in the exposed area potential Von1 with a change in the charging bias, whereas the exposure power P2 during the charging bias calculation processing is relatively low thereby accompanying a large change in the potential Von2 with a change in the charging bias. The reason of this will now be described with reference to Figs. 30 through 32. Where an attenuation characteristic is as shown in Fig. 30, the straight line L(P2, Va-a) and the straight line L(P2, Va-b) shown in Fig. 31 are apart relatively far from each other. Because of this, even when the charging bias is changed from the level Va-a to the level Va-b, the contrast potential Vcon2 shows only a small change, thereby making it impossible sometimes to calculate appropriate values which are necessary to obtain the target contrast potential Vcon02. To deal with this, the second variation requires to set a charging bias change ΔVa smaller than a quantity of change ΔVb in the development bias Vb. Hence, the straight line L(P2, Va-b) shifts closer to the straight line L(P2, Va-a) as shown in Fig. 32, accompanying a large change in the contrast potential Vcon2. As a result, it is possible to reliably calculate appropriate values (the optimal development bias and the optimal charging bias) which are necessary to obtain the target contrast potential Vcon02. (3) Third variation : Fig. 33 Fig. 33 is a drawing showing a third variation of the development bias and the charging bias during the development bias calculation processing. In the third variation, a quantity of change ΔVa (= Va-b - Va-a) in the charging bias is set larger than a quantity of change Vb in the development bias. Such setup is suitable to a situation where, as shown in Fig. 34, the exposure power P1 during the development bias calculation processing is relatively high thereby accompanying a small change in the exposed area potential Von1 with a change in the charging bias, and the exposure power P2 during the charging bias calculation processing is also relatively high thereby accompanying a small change in the potential Von2 with a change in the charging bias. The reason of this will now be described with reference to Figs. 34 through 36. Where an attenuation characteristic is as shown in Fig. 34, the straight line L(P2, Va-a) and the straight line L(P2, Va-b) shown in Fig. 35 are apart relatively close to each other. In this condition, even when the charging bias is changed from the level Va-a to the level Va-b, the exposed area potentials Von2-a, Von2-b of second patch images shows only a small change, which arrives at virtually one optimal solution (the optimal charging bias). Because of this, as shown in Fig. 35, the target contrast potential Vcon01 of first patch images and the target contrast potential Vcon02 of second patch images sometimes become inconsistent to each other. In short, a deviation ΔVb0 is sometimes created between the optimal development bias Vb0 of first patch images and the optimal development bias of second patch images. To deal with this, the third variation requires to set the charging bias change ΔVa larger than a quantity of change ΔVb in the development bias Vb (Fig. 33). Hence, the straight line L(P2, Va-b) is far from the straight line L(P2, Va-a) as shown in Fig. 36, thereby expanding a range of an optimal solution. This ensures consistency between the target contrast potential Vcon01 of first patch images and the target contrast potential Vcon02 of second patch images. (4) Fourth variation : Fig. 38 It is desirable to set the charging bias in accordance with a change in the development bias such that a development bias Vb01 satisfying the target contrast potential Vcon01 and a development bias Vb02 satisfying the target contrast potential Vcon02 become approximately equal to each other, as described above. However, depending on a process of forming images, as described earlier, it is difficult in some cases to match the development biases Vb01 and Vb02 with a linear change in the charging bias. For example, when the charging bias is changed according to the first variation (Fig. 28), the development bias Vb02 sometimes becomes smaller than the development bias Vb01 as shown in Fig. 37 to thereby create a deviation ΔVb0 to the development bias. When this occurs, the charging bias may be changed logarithmically as shown in Fig. 38, which moves the development bias Vb02 which satisfies the target contrast potential Vcon02 closer to the development bias Vb01 which satisfies the target contrast potential Vcon01 so that the two development biases Vb01 and Vb02 approximately match with each other (Fig. 39). (5) Fifth variation : Fig. 41 When the charging bias is changed according to the first variation (Fig. 28), the development bias Vb02 sometimes becomes larger than the development bias Vb01 as shown in Fig. 40, creating a deviation ΔVb0 to the development bias. When this occurs, the charging bias may be changed exponentially as shown in Fig. 41, which moves the development bias Vb02 which satisfies the target contrast potential Vcon02 closer to the development bias Vb01 which satisfies the target contrast potential Vcon01 so that the two development biases Vb01 and Vb02 approximately match with each other (Fig. 42). E. Other The present invention is not limited to the preferred embodiment above, but can be modified in various manners other than those described above without departing from the scope of the present invention as claimed. For example, although the foregoing requires to use the charging roller 22 as the charging means, a charging brush may be used. The present invention is also applicable to an image forming apparatus in which non-contact charging means charges the photosensitive member 21, instead of an image forming apparatus utilizing such contact charging in which a conductive member, such as a charging roller and a charging brush, touches a surface of a photosensitive member 21 for charging. Further, while the patch images PI1 are formed as clusters in each color as shown in Figs. 8A through 8D in the preferred embodiment described above, the patch images PI1 may be formed in each color in turn as shown in Fig. 43A through 43D. More specifically, first, yellow patch images PI1(Y) are formed on the intermediately transfer belt 41 at relatively wide intervals. Next, cyan patch images PI1(C) are formed one by one, starting at a position which is shifted by one patch image and a blank between the adjacent-patch images in the sub scanning direction (the right-hand side in Figs. 43A through 43D) as viewed from the yellow patch images PI1(Y). Following this, magenta patch images PI1(M) and black patch images PI1(K) are formed in a similar manner. Where the respective patch images are thus formed at relatively wide intervals, it is possible to ensure a stabilization time for switching of the biases, and hence, to form the respective patch images at the set biases without fail. Although the description immediately above is related to first patch images, the same directly applies to second patch images as well. Further, while the preferred embodiment above is related to an image forming apparatus which is capable of forming a color image using toners in four colors, an application of the present invention is not limited to this. The present invention is naturally applicable to an image forming apparatus which forms only a monochrome image as well. In addition, although the image forming apparatus according to the preferred embodiment above is a printer for forming an image supplied from an external apparatus such as a host computer through the interface 112 on a sheet such as a copying paper, a transfer paper, a form and a transparent sheet for an over-head projector, the present invention is applicable to image forming apparatuses of the electrophotographic method in general such as a copier machine and a facsimile machine. Further, in the preferred embodiment above, toner images on the photosensitive member 21 are transferred onto the intermediate transfer belt 41, image densities of patch images formed by said toner images are detected, and an optimal development bias and an optimal charging bias are thereafter calculated based on the detected image densities. However, the present invention is also applicable to an image forming apparatus in which a toner image is transferred onto other transfer medium except for the intermediate transfer belt 41, to thereby form a patch image. The other transfer medium includes a transfer drum, a transfer belt, a transfer sheet, an intermediate transfer drum, an intermediate transfer sheet, a reflection-type recording sheet, a transmission memory sheet, etc. Further, instead of forming a patch image on a transfer medium, a patch sensor may be disposed so as to detect a density of a patch image which is formed on a photosensitive member. In this case, the patch sensor detects image densities of patch images on the photosensitive member and an optimal development bias and an optimal charging bias are calculated based on the detected image densities. Further, in the preferred embodiment above, the RAM 127 of the engine controller 12 stores an optimal development bias and an optimal charging bias. Hence, when the main power source of the image forming apparatus is turned off, the contents stored in the RAM 127 disappear. When the main power source is turned on once again, the image forming apparatus recognizes the current development bias calculation and the current charging bias calculation as "the first" calculation and executes processing in accordance with this recognition. Instead of this, a nonvolatile memory such as an EEPROM may be used to store an optimal development bias and an optimal charging bias which are calculated in sequence, so that as the main power source is turned on once again, the processing for "the second or subsequent" calculation is executed during the development bias calculation and the charging bias calculation. Further, although the optimal development bias is determined in the two-stage calculation during the development bias calculation processing after it is judged that it is the "FIRST TIME" in the preferred embodiment described above, the optimal development bias may be calculated only through the bias calculation processing in the wide range (step S312) alone. Further, the narrow range is defined as approximately 1/3 of the programmable range (Vb01 - Vb10) of development bias in the preferred embodiment above. Although the width of the narrow range is not limited to this, if the width of the narrow range is wide, the use of the narrow range becomes less meaningful and degrades the accuracy of calculation of an optimal development bias. For this reason, it is necessary to set the narrow range as approximately 1/2 of or narrower than the programmable range for development bias. This also applies to the narrow range for charging biases as well. Further, although the four types of biases are set in the wide and the narrow ranges in the preferred embodiment described above, the number of bias values (the number of patch images) in the range is not limited to this but may be optional to the extent that more than one types of bias values are used. Alternatively, the number of bias values may be different between the wide range and the narrow range such that the number of patch images is different between the wide range and the narrow range. Further, while the first patch images are each a solid image whose area ration is 100% in the preferred embodiment above, an image whose area ratio is approximately 80% or more may be used instead of using a solid image. Even when such an image is used as the first patch images, a similar effect to that promised when solid images are used is obtained. The term "area ratio" refers to a ratio of dots to the area of a patch image as a whole. Further, although the preferred embodiment above requires to change a charging bias which is supplied to the charging roller 22 as a density controlling factor to sequentially form patch images PI2, PI2', other density controlling factor may be used, i.e., patch images of more than one one-dot lines may be formed while changing a development bias, an exposure dose, etc. In such a modification as well, as densities of the patch images are detected and an optimal value which is needed to achieve a target density is determined based on the detected image densities, it is possible to stabilize an image density of a line image. Further, in the preferred embodiment above, after executing the development bias calculation (step S3), the charging bias calculation (step S5) is further executed, in order to calculate an optimal development bias and an optimal charging bias. However, the manner in which an optimal development bias and an optimal charging bias are calculated is not limited to this. For example, a plurality of patch images may be formed while changing the development bias and the charging bias at the same time, so that an optimal development bias and an optimal charging bias are calculated based on image densities of the patch images and density adjustment is executed. In this case, memory means such as a RAM and a ROM stores the development bias and the charging bias for every density adjustment and the memory means reads out the most recent development bias and the most recent charging bias in preparation for the next density adjustment. The plurality of patch images are formed while changing the development bias and the charging bias at the same time based on the most recent development bias and the most recent charging bias. This realizes a similar effect to that according to the preferred embodiment above. Still further, the present invention is applicable to where calculation of an optimal development bias is executed first and an optimal charging bias is thereafter calculated followed by density adjustment, in which case as well it is possible to achieve a similar effect to that described above. Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as other embodiments of the present invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments.

Claims: An image forming apparatus for forming an image which has a predetermined target density, comprising: a photosensitive member (21); charging means which charges a surface of said photosensitive member (21); exposing means (3) which forms an electrostatic latent image on the charged surface of said photosensitive member (21); developing means (23, 23Y, 23M, 23C, 23K) which visualizes said electrostatic latent image with a toner to form a toner image; transferring means (4) which transfers the toner image from said photosensitive member (21) to a transfer medium (41, S); density detecting means (PS) which detects an image density of the toner image on said photosensitive member (21) or on said transfer medium (41, S) as a patch image (PI1,PI2); and control means (1, 12) which adjusts an image density of said toner image to the target density based on a result of the detection obtained by said density detecting means (PS), characterized in that : said patch image (PI2) is formed by a plurality of one-dot lines which are apart from each other, wherein said density detecting means (PS) has a detect area capable of including a plurality of said one-dot lines arranged in said patch image (PI2). The image forming apparatus according to claim 1, wherein said control means (1, 12) is arranged to form said plurality of patch images (PI1, PI2) one after another while changing said charging bias stepwise, and is arranged to determine an optimal charging bias, which is necessary to obtain said target density, based on densities of said patch images (PI2) detected by said density detecting means (PS). The image forming apparatus according to claim 1 or 2, wherein said control means (1, 12) is arranged to form said plurality of patch images (PI2) while increasing said charging bias stepwise. The image forming apparatus according to claim 1, 2 or 3, wherein said patch images (PI2) are lattice images, each of said lattice images consisting of said plurality of one-dot lines which are arranged in the shape of a lattice. The image forming apparatus according to claim 1, 2 or 3, wherein said patch images (PI2) are perpendicular lattice images, each of said perpendicular lattice images consisting of said plurality of one-dot lines which are arranged perpendicular to each other in the shape of a lattice. The image forming apparatus according to any of claims 1 to 5, wherein said charging means comprises a conductor (22) upon which said charging bias is applied, and said charging means is arranged to charge the surface of said photosensitive member (21) as said conductor (22) touches the surface of said photosensitive member (21). The image forming apparatus according to any of claims 1 to 6, wherein said plurality of one-dot lines are approximately parallel to each other, and adjacent two of said one-dot lines are apart from each other at an interval of n-lines, the line interval n being two or more. The image forming apparatus according to any of claims 1 to 7, wherein the line interval n between adjacent two of said one-dot lines is an integer which further satisfies: n ≤ (Ø • R - 10)/10 where Ø denotes a size of a detect area of said density detecting means (PS) and R denotes a resolution of said image forming apparatus. The image forming apparatus according to any of claims 1 to 7, wherein the line interval n between adjacent two of said one-dot lines is an integer which further satisfies: n ≤ (Ø • R - 20)/20 where Ø denotes a size of a detect area of said density detecting means (PS) and R denotes a resolution of said image forming apparatus. An image forming method in which after charging a surface of a photosensitive member (21), an electrostatic latent image is formed on the charged surface of said photosensitive member (21), and developing means (23, 23Y, 23M, 23C, 23K) visualizes said electrostatic latent image with a toner to form a toner image which has a predetermined target density, characterized in that : after sequentially forming a plurality of toner images, which are each formed by a plurality of one-dot lines which are apart from each other, as patch images (PI2) while changing a density controlling factor which influences an image density of toner image, densities of said patch images (PI2) are detected, wherein the density detection is based on detecting a plurality of said one-dot lines, and an optimal density controlling factor which is necessary to obtain said target density is determined based on the densities of said patch images (PI2). The image forming method according to claim 10, wherein after sequentially forming said plurality of toner images as said patch images (PI2) while changing a charging bias which is applied to said charging means as said density controlling factor, the densities of said patch images (PI2) are detected, and an optimal charging bias, which is necessary to obtain said target density, is determined based on the densities of said patch images (PI2).

Inventors: HAMA TAKASHI, NAKAZATO HIROSHI, HAMA, TAKASHI, NAKAZATO, HIROSHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 2000
Main classification: G03G15
IPC codes: G03G15