Electrophotographic image forming apparatus and method
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an electrophotographic image forming apparatus and method, and more particularly to an image forming apparatus and method of a type controlling an exposure output upon correction of gradations of image data. 
     2. Description of the Related Art 
     In a digital color copier, image signals from an image reader such as a CCD or the like are initially converted to digital image data of, for example, eight bits (256 gradations) by an analog-to-digital converter (A/D converter). Thereafter, in order to enhance the fidelity of half-tone images, an exposure output is controlled upon correction of gradations of the digital image data based upon a gamma-correction table formulated in accordance with non-proportional gradation characteristics called the &#34;gamma-characteristics&#34; between the image data to be reproduced and reproduced image density levels. The gamma-correction table is made in advance based upon a relationship between the image density and the image data and is stored in a ROM or the like. 
     Meanwhile, in a color copier, because color reproduction is performed based upon the subtractive color mixing of toners of cyan, magenta and yellow, any desired color reproduction needs the stability of the image density (the amount of adhesion of toner) of three different colors with respect to a photosensitive member. 
     To this end, an automatic toner density control (ATDC) and an automatic image density control (AIDC) are both performed in the apparatus. More specifically, in the ATDC, the toner density in a developer is detected by a toner density detector means and the amount of toner to be replenished is controlled in accordance with a detection signal thereof, thereby making the toner mixture ratio in the developer substantially constant. On the other hand, the AIDC is performed to adjust the apparatus to changes in the amount of electrostatic charge of the developer or characteristic changes of the photosensitive member caused by environmental changes. In this control, the density of a reference pattern formed on the photosensitive member is initially detected and developing conditions are changed in accordance with a detected density signal, thereby making the amount of adhesion of toner (the toner density) substantially constant. A method of changing a developing bias is widely used with highest reliability as a method of changing the developing conditions. 
     However, when the developing conditions are changed according to the environmental changes or the like, the gradation characteristics also change. Because of this, a single gamma-correction table cannot provide the optimum gradation correction, and therefore, the most appropriate gamma-correction table is selected from among a plurality of gamma-correction tables according to the developing conditions. 
     In this kind of image density control, half-tone images are faithfully reproduced with an appropriate density when the toner mixture ratio is always constant. Even though the toner mixture ratio is made constant by the ATDC, the actual toner mixture ratio always changes according to the area ratio (B/W ratio) between an image portion and a white ground of an original document or copying modes such as, for example, a continuous copying mode or an intermittent copying mode. In other words, even though the toner mixture ratio detected by an ATDC sensor is fed back to a toner replenishing means, it is impossible to immediately replenish, agitate and transport the toner. As a result, the toner mixture ratio changes. 
     Meanwhile, a central value in controlling the toner mixture ratio is occasionally changed. In the image density control by the AIDC, the grid voltage and the developing bias are set low when the humidity is comparatively high. As a result, the 256 gradations of the density must be controlled by a low developing voltage, and changes in surface potential or in the amount of light are followed by wide variations in the density. In contrast, the grid voltage and the developing bias are set high when the humidity is comparatively low. Since these values have respective upper limits due to the pressure resistance of the photosensitive member, saturation of the developing efficiency, or the like, the central value in controlling the toner mixture ratio is occasionally switched according to localities or seasons. 
     When the toner mixture ratio changes, the image density is controlled to be constant by the AIDC. However, since the gamma characteristics change as shown in FIG. 5, desired images cannot always be obtained merely by selecting a gamma-correction table from among a plurality of tables in compliance with the AIDC. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to overcome the above-described disadvantages. 
     It is accordingly an object of the present invention to provide an improved image forming apparatus which is capable of stably forming images, of which gradations are appropriately corrected even though the toner mixture ratio changes. 
     In accomplishing this and other objects, an electrophotographic image forming apparatus according to the present invention comprises a photosensitive member, a charging means for charging the photosensitive member with a given initial potential, an exposure means for forming an electrostatic latent image by applying light to the photosensitive member charged with the initial potential, an exposure control means for controlling the amount of light to be applied to the photosensitive member from the exposure means in compliance with an image density signal indicative of a density of each pixel of an image to be formed, and at least one developing device accommodating a developer consisting of toner and carrier. The electrostatic latent image formed on the photosensitive member is developed by the developer contained in the developing device. 
     The apparatus further comprises a toner density detector means for detecting a toner density in the developer, a memory means for storing a plurality of gradation correction data, and a selector means for selecting one of the plurality of gradation correction data stored in the memory means based upon a toner density detected by the toner density detector means. 
     The exposure control means controls the amount of light based upon gradation correction data selected by the selector means. 
     In another aspect of the present invention, an image forming method comprises the steps of: 
     (a) detecting a toner density in the developer; 
     (b) determining gradation characteristics of the image forming apparatus in compliance with the detected toner density; 
     (c) charging the photosensitive member with a given potential; 
     (d) correcting a digital signal indicative of a density of an image to be formed in compliance with the determined gradation characteristics; 
     (e) converting the corrected digital signal to an analog signal; 
     (f) forming an electrostatic latent image by applying light to the photosensitive member at an amount of light corresponding to the analog signal; and 
     (g) developing the electrostatic latent image by the developing device. 
     According to the present invention, an actual toner density is detected during an image forming operation, and an image formation is performed upon selection of appropriate gradation correction data corresponding to the actual toner density. Accordingly, even if the toner density changes, desired images can be stably obtained, of which gradations are appropriately corrected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and wherein: 
     FIG. 1 is a schematic diagram of an image forming apparatus according to the present invention; 
     FIG. 2 is a flow-chart indicative of a control required for an image forming operation according to a first embodiment of the present invention; 
     FIG. 3 is a graph of a gamma-characteristic curve indicative of a relationship between input levels of image data and the image density; 
     FIG. 4 is a graph of a gamma-correction characteristic curve indicative of a relationship between the input levels of image data and a laser output after gamma-correction; 
     FIG. 5 is a graph of several gamma-characteristic curves with the toner mixture ratio employed as a parameter; 
     FIG. 6 is a graph of several gamma-correction characteristic curves corresponding to respective gamma-correction tables; and 
     FIG. 7 is a flow-chart similar to FIG. 2, according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is schematically shown in FIG. 1 a full-color image forming apparatus embodying the present invention. The image forming apparatus is internally provided with a photosensitive drum 1, a corona charger 2 having an arrangement called the &#34;scorotron&#34; for uniformly charging a photosensitive layer of the photosensitive drum 1, an exposure portion 3 for forming an electrostatic latent image upon exposure of an image formed, a plurality of developing devices 4a-4c for developing the electrostatic latent image using respective colored toners, and a transfer charger 5 for transferring a developed toner image to a transfer material 6. The corona charger 2, the exposure portion 3, the developing devices 4a-4c, and the transfer charger 5 are located around the photosensitive drum 1 in this order in the direction of rotation of the photosensitive drum 1. An AIDC sensor 7 for detecting the amount of toner adhering to the photosensitive drum 1 i.e., the amount of toner adhering to a reference pattern (this pattern is hereinafter referred to as an AIDC pattern) formed on the photosensitive drum 1 for detection of the image density is disposed downstream from the developing devices 4a-4c. The developing devices 4a-4c are provided with respective ATDC sensors 8a-8c mounted thereon for detecting the toner mixture ratio in respective developers contained therein. The developing devices 4a-4c accommodate toner replenishing means 9a-9c and developing sleeves 10a-10c, respectively. 
     The corona charger 2 has a grid 2a, of which the potential is controlled so as to be a predetermined potential corresponding to a surface potential of the photosensitive drum 1 by a grid voltage control circuit 11. The exposure portion 3 receives laser light from a laser diode 12. A developing bias is applied to the developing sleeves 10a-10c of the developing devices 4a-4c by a developing bias control circuit 13. The toner replenishing means 9a-9c are controlled by a toner replenishment control circuit 14 so that the toner mixture ratio in respective developers may be made constant. These grid voltage control circuit 11, developing bias control circuit 13 and toner replenishment control circuit 14 are controlled by a CPU 15. The laser diode 12 is driven by a laser driver 18 on the basis of data obtained upon correction of image data from an image reader 16 such as a CCD or the like, using a gradation control circuit 17. The gradation control circuit 17 is also controlled by the CPU 15. 
     Detection signals of the AIDC sensor 7 and those of the ATDC sensors 8a-8c are converted to digital signals by A/D converters 19a and 19b, respectively, and are inputted into the CPU 15. The CPU 15 is electrically coupled with a ROM 20 in which are stored various control data including a plurality of gamma-correction tables and from which such data are outputted to the CPU 15 as occasion demands. 
     The control of an image forming operation by the CPU 15 will be discussed hereinafter with reference to a flow-chart of FIG. 2. 
     Initialization is performed at step #1 followed by step #2 where various conditions for image formation such as, for example, the required number of copies are inputted. Step #3 determines whether a print switch is data from the ATDC sensors 8a-8c and those from other sensors are inputted at step #4. 
     At step #5, AIDC measurements are performed in which a grid voltage V G  and a developing bias V B  are set to respective predetermined reference values for the AIDC pattern and an output of the laser diode 12 is set to a predetermined value required for forming a half-tone AIDC pattern. Upon rotation of the photosensitive drum 1, the corona charger 2 and a developing motor are activated and the developing bias V B  is applied to the developing sleeves 10a-10c of the developing devices 4a-4c. The AIDC pattern is then exposed by the laser diode 12 and is developed by the developing devices 4a-4c. Finally, the image density of the AIDC pattern is detected by the AIDC sensor 7. 
     At step #6, based upon data outputted from the AIDC sensor 7, the most appropriate developing bias V B  and grid voltage V G  are selected using Table 1 below. 
     
                       TABLE 1                                                     
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              Gamma-Correction Table                                      
Level  V.sub.B V.sub.G  6%   7%    8%   9%    10%                         
______________________________________                                    
 0     -100    -200     γ.sub.0,0                                   
                             γ.sub.0,1                              
                                   γ.sub.0,2                        
                                        γ.sub.0,3                   
                                              γ.sub.0,4             
 1     -120    -220     γ.sub.1,0                                   
                             γ.sub.1,1                              
                                   γ.sub.1,2                        
                                        γ.sub.1,3                   
                                              γ.sub.1,4             
 2     -140    -240     γ.sub.2,0                                   
                             γ.sub.2,1                              
                                   γ.sub.2,2                        
                                        γ.sub.2,3                   
                                              γ.sub.2,4             
. . .  . . .   . . .    . . .                                             
                             . . . . . .                                  
                                        . . . . . .                       
. . .  . . .   . . .    . . .                                             
                             . . . . . .                                  
                                        . . . . . .                       
. . .  . . .   . . .    . . .                                             
                             . . . . . .                                  
                                        . . . . . .                       
27     -640    -740     γ.sub.27,0                                  
                             γ.sub.27,1                             
                                   γ.sub.27,2                       
                                        γ.sub.27,3                  
                                              γ.sub.27,4            
28     -660    -760     γ.sub.28,0                                  
                             γ.sub.28,1                             
                                   γ.sub.28,2                       
                                        γ.sub.28,3                  
                                              γ.sub.28,4            
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     In Table 1, the data outputted from the AIDC sensor 7 are classified into density levels from 0 to 28. Set values of the developing bias V B  and the grid voltage V G  are determined as corresponding to respective levels. The density level detected by the AIDC sensor 7 increases with reduction of the AIDC pattern formed on the photosensitive drum 1 in density level. As a result, any image formation is performed with an optimum image density. 
     The selection of gamma-correction tables is performed at step #7. Input levels of image data and the image density are in a non-proportional relationship called the &#34;gamma-characteristics&#34; as shown in FIG. 3. Accordingly, it is necessary to correct the output of the laser diode 12 with respect to the input levels of the image data, as shown in FIG. 4, in order to enhance the fidelity in an image formation of half-tone images. To this end, the correction-characteristics in the output of the laser diode 12 are tabulated. Since the most appropriate gamma-correction table changes according to the developing bias V B  and the grid voltage V G  controlled by the detection levels of the AIDC sensor 7, a large number of gamma-correction tables are formulated, as shown in Table 1, according to the detection levels of the AIDC sensor 7. 
     Table 2 indicates an example of gamma-correction tables .sub.γ0,0 and .sub.γ1,0. 
     
                       TABLE 2                                                     
______________________________________                                    
               Output Level                                               
Input Level      γ.sub.1,0                                          
                         γ.sub.0,0                                  
______________________________________                                    
 0                0       0                                               
 1                13      18                                              
 2                25      38                                              
 3                35      58                                              
 4                46      77                                              
 5                57      93                                              
 6                69     106                                              
 7                83     114                                              
 8                96     120                                              
 9               107     124                                              
 10              116     127                                              
 11              123     130                                              
 12              128     132                                              
. . .            . . .   . . .                                            
. . .            . . .   . . .                                            
246              820     647                                              
247              836     663                                              
248              852     681                                              
249              869     700                                              
250              887     721                                              
251              905     747                                              
252              924     780                                              
253              949     838                                              
254              973     897                                              
255              1023    1023                                             
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     Furthermore, the gamma-characteristics change according to the toner density in a developer, as shown in FIG. 5, wherein the image density in a medium density portion becomes deep and that in a high density portion saturates. Accordingly, it is necessary to select one of the gamma-correction tables in compliance with the toner mixture ratio detected by the ATDC sensors 8a-8c. 
     In this embodiment, as shown in Table 1, a large number of gamma-correction tables are arrayed in the form of a matrix in accordance with detection levels of the AIDC sensor 7 and the toner mixture ratio and are stored in the ROM 20. From among these gamma-correction tables, the most appropriate one is selected on the basis of detection data of the AIDC sensor 7 and those of the ATDC sensors 8a-8c. 
     FIG. 6 indicates an example of the correction characteristics of gamma-correction tables .sub.γ0,0, .sub.γ0,1, .sub.γ0,2, .sub.γ0,3, .sub.γ0,4, when the detection level of the AIDC sensor 7 is &#34;0&#34;. 
     At step #8, an image forming operation is performed in accordance with image forming conditions set based upon detection values of the ATDC sensors 8a-8c, the CPU 15 gives the toner replenishment control circuit 14 instructions for replenishing toner by driving the toner replenishing means 9a-9c so that the toner mixture ratio in the developing devices 4a-4c may be a predetermined value. 
     Thereafter, step #9 determines whether the image forming operation is terminated. Unless the image forming operation is terminated, the procedure returns to step #6 to further proceed with an image forming operation in another color. In contrast, if the image forming operation is terminated at step #9, the procedure returns to step #2. 
     It is to be noted that the flow-chart of FIG. 2 may be replaced by a flow-chart shown in FIG. 7, which indicates a second embodiment of the present invention. 
     Steps from #11 to #13 in the flow-chart of FIG. 7 are the same as steps from #1 to #3 in the flow-chart of FIG. 2. In the flow-chart of FIG. 7, however, if a print switch is turned on at step #13, AIDC measurements and selection of the most appropriate developing bias V B  and grid voltage V G  are performed at step #14 and #15, respectively, prior to input of data from the ATDC sensors 8a-8c and other sensors. 
     In the second embodiment shown by the flow-chart FIG. 7, detection values of the ATDC sensors 8a-8c are repeatedly inputted into. the CPU 15 at step #16 until an image forming operation is terminated. At subsequent step #17, the gamma-correction tables are selected based upon such detection values. Accordingly, even if the toner density is lowered during a large number of successive image forming operations, the gamma-correction tables are appropriately changed, thereby enabling the optimum half-tone image reproduction at all times. 
     In the above-described embodiments, since the developing bias V B  and the grid voltage V G  are set on the basis of the detection signals of the AIDC sensor 7, the image formation is stably performed with an optimum image density. Furthermore, even if the toner density changes due to continuous image forming operations, the gradation characteristics can be maintained unchanged. The reason for this is that an output of the laser diode 12 is controlled in accordance with the most appropriate gamma-correction table selected from among a large number of gamma-correction tables on the basis of the detection signals of the AIDC sensor 7 and the ATDC sensors 8a-8c. As a result, half-tone images can be stably obtained. 
     In the above-described embodiments, the description is made with respect to an image forming apparatus of the intensity modulation type in which the laser driver 18 controls the intensity of light emitted from the laser diode 12 by changing an electric current supplied thereto on the basis of image data corrected by the gradation control circuit 17. However, the present invention is also applicable to an image forming apparatus of the pulse-width modulation type in which the time period during which light is emitted from the laser diode 12 is controlled by changing the pulse-width of an electric current supplied to the laser diode 12 in compliance with image data. 
     Furthermore, in the above-described embodiments, the intensity of laser light from the laser diode 12 is controlled in compliance with input levels of image data and the gamma-correction tables are changed in compliance with detection values of the ATDC sensors 8a-8c in the toner density of developers. However, in an apparatus for performing gradation representation in the image density by the use of an area gradation method such as, for example, a dither method, an optimum dither matrix may be selected from among a plurality of threshold patterns (dither matrixes) stored in a ROM in compliance with detection values of the ATDC sensors 8a-8c. In this case, the density of half-tone images can be controlled in compliance with changes in the toner density by controlling an output of the laser diode 12 using the selected dither matrix. 
     It is to be noted that the present invention is also applicable to a laser printer, a facsimile machine, or the like. 
     As is clear from the above, according to the present invention, a gradation correction table appropriate for a toner mixture ratio actually detected during an image forming operation is selectively determined. Accordingly, even if the toner mixture ratio changes, gradations of images are properly corrected, thus ensuring stable image formations. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein.