Abstract:
An image forming apparatus image forming means for forming a toner image using light color toner and dark color toner which have the same hues and which have different densities; detecting means for detecting a density of a toner image for reference which is formed by the image forming means, the reference toner image including a number of portions corresponding to different image density levels; control means for controlling an image forming condition of the image forming means in accordance with an output of the detecting means, wherein a difference between the image density levels corresponding to adjacent ones of the portions in a predetermined image density area including an image density level corresponding to a boundary between an image density area where an image is formed using only the light toner and an image density area where an image is formed using both of the light toner and the dark toner, is smaller than that in a density area other than the predetermined image density area.

Description:
FIELD OF THE INVENTION AND RELATED ART  
       [0001]     The present invention relates to an image forming apparatus which forms an image through an electrophotographic process. In particular, it relates to an image forming apparatus such as a copying machine, a printer, a facsimileing machine, or the like.  
         [0002]     As one of the electrophotographic image forming apparatuses such as a copying machine, a laser beam printer, etc., a full-color image forming apparatus which forms a full-color image by depositing in layers a plurality of monochromatic images different in color, more specifically, yellow (H), magenta (M), cyan (C), and black (Bk) images, has been known.  
         [0003]     For the formation of a high quality image with use of a full-color image forming apparatus such as the above described one, density control is important, which regulates the apparatus in terms of the maximum and intermediary levels of density for monochromatic yellow (Y), magenta (M), cyan (C), and black (Bk) images so that the apparatus will remain consistent in terms of the image density level, regardless of the difference in manufacture tolerance and changes in ambient conditions. Therefore, it is customary to equip a full-color image forming apparatus with a density controlling means for controlling the apparatus in terms of image density.  
         [0004]     There have been proposed various full-color image forming apparatuses equipped with a density detecting means Some of them (for example, one disclosed in Japanese Laid-open Patent Application 2000-231279) are provided with a plurality of image bearing members and a plurality of developing means. Further, at least two of the plurality of developing means are identical in the hue of the developer (toner) therein, but, are different in density (saturation or deepness) of the developer (toner) therein; the developer in one of the two developing means is the same in hue as the developer in the other developing means, but is lower in density than the developer in the other developing means. They employ an image forming method in which each of the plurality of monochromatic images formed to form a single full-color image is formed of a combination of two monochromatic images identical in spectral properties, that is, a monochromatic image formed of the abovementioned developer lower in color density level (which hereinafter will be referred to light color toner), and a monochromatic image formed of the abovementioned developer higher in color density level (which hereinafter will be referred to as deep color toner), using two kinds of lookup tables, that is, a lookup table A for the light color toner, and a lookup table B for the deep color toner, shown in  FIG. 13 .  
         [0005]     According to the lookup tables in  FIG. 13 , the low density areas of the monochromatic image are primarily formed of the light color toner, and the mid density areas of the monochromatic image are formed of the mixture of the light and deep color toners. Further, the high density areas of the monochromatic image are primarily formed of the deep color toner. Therefore, controlling the image forming apparatus with reference to these lookup tables A and B makes it possible to form an image which does not suffer from the problem that the low density areas of an image appear grainy due the low dot density, and also, to reduce the amount of toner which is consumed for the formation of the high density areas of an image. In other words, controlling the image forming apparatus with reference to these lookup tables improves the image forming apparatus in terms of image quality by reducing the graininess level at which the low density areas of an image are formed. It also effective to expand the range in which an image is accurately formed in terms of color reproduction.  
         [0006]     However, the above described image forming method suffers from the following problem. That is, as a large number of images are formed, that is, the image forming apparatus is repeatedly used for a large number of times, changes occur to various conditions under which an image is formed; changes occur to the developing means in terms of development properties, the thickness of the dielectric layer of the photosensitive drum, the transfer efficiency, etc. Changes also occur to the ambient conditions. As these changes occur, the light color toner and the deep color toner change in the γ properties, and the occurrence of this change in the γ property corresponds to the threshold value for the input video signal, below which the light color toner is used, and above which the deep color toner is used. Therefore, the linearity in the relationship between the values of input video signals and the density level of the corresponding areas of the resultant image is lost. As a result, a defective image is formed; for example, an image defective in that the areas of the image, which are intermediary in density, are unnatural in gradation, and an image defective in that it has pseudo-contours.  
       SUMMARY OF THE INVENTION  
       [0007]     The primary object of the present invention is to provide an image forming apparatus capable of an image higher in quality than an image forming apparatus in accordance with the prior art.  
         [0008]     Another object of the present invention is to provide an image forming apparatus superior to an image forming apparatus in accordance with the prior art, in that it is capable of an image superior in the reproduction of the transitional areas of the image, transitional in that the image density changes from the level to be reproduced with the use of the light color toner to the level to be reproduced with the use of the deep color toner.  
         [0009]     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the-preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, showing-the general structure thereof.  
         [0011]      FIG. 2  is a flowchart which shows the flow of video signals in the image forming apparatus in the first embodiment.  
         [0012]      FIG. 3  is a schematic drawing of an example of a density detecting means in accordance with the present invention.  
         [0013]      FIG. 4  is a graph showing the relationship between the amount of the light color toner on the medium, and the output of the density detecting means, and the relationship between the amount of the deep color toner on the medium, and the output of the density detecting means.  
         [0014]      FIG. 5  is a graph showing the relationship between the values of the input video signals, and the density levels of the images resultant from the input video signals, after the adjustment of the input video signals based on the lookup tables.  
         [0015]      FIG. 6  is a graph showing the effect of the changes in image formation conditions and/or ambient conditions upon the relationship between the value of the input video signals, and the density levels of the images resulting from the input video signals.  
         [0016]      FIG. 7  is a graph showing the relationship between the values of the input video signals generated for the formation of the density level test patches, and the density levels of the images of the test patches resulting from the input video signals for the formation of the density level test patches.  
         [0017]      FIG. 8  is a graph showing the LUT for the light color toner, and the LUT for the deep color toner, in the first embodiment.  
         [0018]      FIG. 9  is a graph showing the relationship between the values of the input video signals generated for the formation of the density level test patches, and the density levels of the images of the test patches resulting from the input video signals for the formation of the density level test patches.  
         [0019]      FIG. 10  is a schematic drawing of the image forming apparatus in the second embodiment, showing the general structure thereof.  
         [0020]      FIG. 11  is a picture of the density level detection test patches in the third embodiment.  
         [0021]      FIG. 12  is a schematic drawing of the image forming apparatus in the fourth embodiment, showing the general structure thereof.  
         [0022]      FIG. 13  is a graph showing the LUT for the light color toner, and the LUT for the deep color toner, for an image forming apparatus in accordance with the prior art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Hereinafter, the image forming apparatuses in accordance with the present invention will be described in detail with reference to the appended drawings.  
       Embodiment 1  
       [0024]     Referring to  FIGS. 1-9 , the first embodiment of the present invention will be described.  
         [0025]     Referring to  FIG. 1 , the image forming apparatus in this embodiment has four processing stations (image forming stations) P (Pa, Pb, Pc, and Pd) as image forming means for forming monochromatic yellow (Y), magenta (M), cyan (C), and black (Bk) images on the four image bearing members, one for one. The four processing stations are aligned straight in the direction in which a recording medium is conveyed. Each processing station has a photosensitive drum  1  ( 1   a,    1   b,    1   c,  and  1   d ), a charging apparatus  2  ( 2   a,    2   b,    2   c,  and  2   d ), an exposing apparatus  3  ( 3   a,    3   b,    3   c,  and  3   d ), a primary developing means  4  ( 4   a,    4   b,    4   c,  and  4   d ), a second developing means  5  ( 5   a,    5   b,    5   c,  and  5   d ), a cleaning apparatus  6  ( 6   a,    6   b,    6   c,  and  6   d ), and a primary transferring means  7  ( 7   a,    7   b,    7   c,  and  7   d ) as a transferring means. The image forming apparatus is provided with an intermediary transfer member  12  as a transferring means for transferring, in coordination with the primary transferring apparatuses  7 , the toner images onto a recording medium p. The intermediary transfer member  12  is stretched between the photosensitive drum  1  and primary transferring apparatus  7 , in each processing station, and is circularly moved in the direction indicated by an arrow mark.  
         [0026]     In this image forming apparatus structured as described above, each of the four image forming stations for forming the four monochromatic toner images, that is, the monochromatic yellow (Y), magenta (M), cyan (C), and black (Bk) toner images, one for one, is provided with two developing means, that is, the first and second developing means  4  and  5 ; two developing means are provided per color. More specifically, the first and second developing means are identical in the color (hue) of the toners therein, but are different in the color density of the toners therein. That is, the first developing means  4  is filled with such developer that is the same in hue, but is lighter in color density (saturation) than the toner in the second developing means  5 .  
         [0027]     In other words, the image forming apparatus in this embodiment has two developing means, that is, the deep color developing means  5  and light color developing means  4 , for each of the four colors, that is, yellow (Y), magenta (M), cyan (C), and black ( 8 k). The deep color developing means  5  and light color developing means  4  are the same in the hue of the toner of the developer they contain, but are different in the color density (saturation) of the toner of the developer they contain; the color of the toner in the second developing means  5  is darker (deeper) than that in the first developing means  4 .  
         [0028]     When it is said that ordinary two toners, which primarily are the mixture of resin and coloring component (pigment), are the same in hue, but different in density (saturation), it usually means that the two toners are practically the same in the spectral characteristics of the coloring ingredient (pigment), but are different in the amount of the coloring component. When one is called “light color toner” of the two toners which are the same in color (hue), the light color toner is the one which is lower in color density (saturation).  
         [0029]     The image forming apparatus in this embodiment uses two toners different in color density in order to form a monochromatic toner image of a given color, and the two toners different in color density used for forming a single monochromatic toner image may sometimes be referred to as “dense (dark) toner” and “light toner”.  
         [0030]     When two toners are said to be the same in hue, it means that the two toners are the same in the spectral characteristics of the coloring component (pigment) as described above. In the following description of the present invention, however, it means that the two toners are the same in terms of the ordinary concept of color. For example, two toners may be said to be the same in hue in that both are of magenta, cyan, yellow, or black color.  
         [0031]     In the following description of the present invention, when one of the two toners of the same hue is referred to as light color toner, it means that when the amount by which this toner is deposited on recording medium is 0.5 mg/cm 2 , the portion of the recording medium covered with this toner is no more than 1.0 in optical density after the image fixation, whereas the portion of the recording medium covered with the other toner, or the deep color toner (toner more saturated in color) is no less than 1.0 in optical density.  
         [0032]     In this embodiment, the amount of the pigment in the deep color toner is adjusted so that when the amount by which the deep color toner is deposited on recording medium is 0.5 mg/cm 2 , the optical density of the recording medium covered with this toner is 1.6, whereas that for the light color toner is 0.8. These two toners different in color density (saturation) are used in various ratios to reproduce a desired level (gradation level) of color density.  
         [0033]     In terms of the direction, indicated by an arrow mark, in which the photosensitive drum  1  is rotated, the first developing means  4  is located on the upstream of the second developing means  5 .  
         [0034]     The normal image forming steps carried out to form an image, by the image forming apparatus structured as described above are as follows:  
         [0035]     In each of the plurality of process stations P, a toner image, which is different in color from the toner images in the other processing stations P, is formed on the photosensitive drum  1  through the electrophotographic process (comprising: charging, exposing, and developing steps).  
         [0036]     First, in each processing station P, the charging step, in which the peripheral surface of the photosensitive drum  1  is uniformly charged by the charging apparatus  2  as a charging means, is carried out.  
         [0037]     Meanwhile, in each processing station P, image formation data are read by an image reading portion  20 , are processed by a controlling means  15  as the controller for controlling the image forming operation, and are transmitted to a laser driver  3  ( 3   a,    3   b,    3   c,  and  3   d ), which is a part of the exposing apparatus as a latent image forming means for forming a latent image on the photosensitive drum  1 .  
         [0038]     In this embodiment, an original is read twice by the original reading portion  20 , for each processing station. More specifically, when the original is read for the first time, the obtained image formation data are processed by the controlling means  15  into the video signals for the first developing means  4 , whereas when the original is read for the second time, the obtained image formation data are processed by the controlling means  15  into the video signals for the second developing means  5 . The flowchart which shows the essential steps of this process of outputting the video signals is given in  FIG. 2   
         [0039]     First, regarding the first reading of an original for the formation of a latent image (exposure of photosensitive drum  1 ), the original placed on the original reading portion  20  is scanned (si), and the optical data obtained from the original are converted (s 1 ) by a CCD  14  into electrical signals, which are converted (s 3 ) by an A/D conversion apparatus into digital signals. The thus obtained digital signals are processed (s 4 ) by the image formation data processing block, and the R, G, and B signals are converted (s 5 ) in color into CMYK signals. Then, the CMYK signals are subjected to the γ correction step (s 6 ), and are converted (s 7 ) into the video signals for the light color toner, in accordance with the lookup table (which hereinafter will be referred to as “LUT”). Then, the video signals for the light color toner are digitized (s 8 ). The thus obtained digital image formation data are stored (s 9 ), are converted (s 10 ) into analog signals, are transferred to the laser driver  3 , and are used (s 11 ) for image formation. The LUT for the light color toner, which is used in the above described step s 7 , is represented by the line indicated by a referential letter A, in  FIG. 8 .  
         [0040]     The electrostatic latent image formed through the exposure in the above described step (s 11 ) is developed by the first developing means  4  which uses the light color toner. Then, the toner image formed by the first developing means  4  is transferred (primary transfer) onto the intermediary transfer belt  12  by the primary transferring apparatus  7  as a transferring means.  
         [0041]     Then, the original is scanned for the second time (s 12 ). In order to form a toner image of the deep color toner after forming a toner image of the light color toner, it is necessary to read (scan) the original again, due to requirement related to the memory. The image formation signals obtained by the second scanning of the original are processed through steps (s 12 -s 17 ) similar to the steps through which the image formation signals obtained by the first scanning of the original are processed, up to the correction step. Thereafter, the signals are converted (s 18 ) into the signals for the deep color toner, in accordance with the LUT for the deep color toner, and then, are digitized (s 19 ). The thus obtained digital image formation data are stored (s 20 ), are converted (s 21 ) into analog signals, are transferred to laser driver  3 , and are used to drive (s 22 ) the laser driver  3  to form an image of the deep color toner. The LUT to be used in the step (s 18 ) to obtain the signals for the deep color toner is represented by the line indicated by a letter B, in  FIG. 8 .  
         [0042]     As the above described step s 22 , or the latent image formation step, is carried out, an electrostatic latent image is formed, through the exposure, on the uniformly charged peripheral surface of the photosensitive drum  1 . Next, this electrostatic latent image is developed through the developing step carried out by the second developing means  5  which uses the deep color toner, yielding a toner image formed of the deep color toner The thus obtained toner image is transferred (primary transfer) by the primary transferring apparatus  7 , onto the intermediary transfer belt  12 , onto which the toner image formed of the light color toner has been transferred. As a result, a toner image formed of the deep color toner and light color toner is yielded on the intermediary transfer belt  12 .  
         [0043]     In other words, through the video signal processing steps shown in  FIG. 2 , the original is sorted into the areas which are to be reproduced with the use of only the light color toner, the areas which are to be reproduced with the use of both the light and deep color toners, and the areas which are to be reproduced with the use of only the deep color toner, and then, whether only one of the developing means  4  and  5  is to be used, or which developing means is to be used if only one of the developing means  4  and  5  is to be used, is determined based on the results of the sorting.  
         [0044]     As for the transferring means, the intermediary transfer member  12  is circularly moved by the suspensive rollers  12   a  and  12   b  at the same speed as the rotational velocity of each of the plurality of photosensitive drums  1 , through the contact area (nip) between the primary transferring apparatus  7  and photosensitive drum  1 , in each processing station P (Pa, Pb, Pc, and Pd), with its outwardly facing surface, in terms of the loop which the intermediary transfer member  12  forms, kept in contact with the peripheral surface of the photosensitive drum  1 . Thus, as the intermediary transfer member  12  is moved sequentially through the plurality of primary transfer stations, the toner image formed on the peripheral surface of the photosensitive drum  1 , of the two toner images formed in layers on the peripheral surface of the photosensitive drum  1 , of the two toners different in color density, in each processing station P (Pa, Pb, Pc, and Pd) is transferred in layers onto the intermediary transfer member  12 , yielding a single multicolor image, which is conveyed, while remaining on the intermediary transfer member  12 , to the secondary transfer station  11 , by the circularly movement of the intermediary transfer member  12 .  
         [0045]     The multicolor image formed on the intermediary transfer member  12 , of the plurality of monochromatic toner images formed of the two toners different in color density, in the plurality of processing stations P, one for one, is transferred (secondary transfer) in the secondary transfer station  11 , onto the recording medium p delivered to the secondary transfer station  11  from the sheet feeder cassette  13 , and then, is fixed to the recording medium p by the fixing apparatus  9 . Thereafter, the recording medium p is discharged as a final product (copy) from the image forming apparatus.  
         [0046]     In other words, according to the flowchart given in  FIG. 2 , the image formation signals are processed, in the step s 7 , in accordance with the LUT for the light color toner, so that the areas of the image, which are low in color density, are primarily developed with the light color toner. As a result, the latent image is developed so that a monochromatic image, the low color density areas of which are lower in the color density of each dot, will be yielded In other words, the flowchart makes it possible to minimize the shortcoming of a digital image that a digital image appears grainy. Further, another set of image formation signals are processed, in step p 18 , in accordance with the LUT for the deep color toner. In other words, according to the flowchart in  FIG. 2 , two monochromatic images different in color density are formed per color component (into which optical image of original is separated), through two sets of image formation steps, that is, the image formation signal processing step, latent image forming step, and developing step, and are transferred in layers onto the intermediary transfer belt  12 , through the primary transfer step, yielding thereby a single monochromatic image formed of two monochromatic images formed of the deep and light color toners, one for one, which are the same in hue and different in color density.  
         [0047]     Described next will be the control to be carried out to form a satisfactory image, regardless of the changes in the apparatus conditions attributable to the usage and the changes in the ambient conditions, through an image forming process such as the one described above, in which two developing means different in the color density of the toners they use are used per color component. In this embodiment, the image forming process is controlled by revising the LUTs used for processing the video signals.  
         [0048]     To describe in more detail, the above described image forming apparatus is reset so that the image formation conditions, such as the conditions under which the photosensitive drums  1  are charged and exposed by the image forming means, the conditions under which a latent image is developed, and the conditions under which a toner image is transferred, are set to the defaults. Then, the data for generating the video signals for forming density level detection test patches, which are stored in the ROM or the like, are read by the means for forming the electrostatic latent images for density level detection test, that is, a density level detection test patch forming means, for example, the controller (controlling means)  15 , and a desired image density level is inputted. Then, the electrostatic latent image for density level detection test, which reflects the inputted image density level is formed, and is developed by the developing means to be used for developing the latent image in accordance with the intended image. As a result, the image of the density level detection test patch (images to be used for devising LUT) is formed, and is transferred (primary transfer) onto the intermediary transfer medium  12 . Then, the color density level of the toner image of the density level detection test patch on the intermediary transfer member  12  is detected by the density detecting means (density sensor)  21 , which is positioned upstream of the second transfer station  11 , in terms of the moving direction of the intermediary transfer member  12 , so that it faces the intermediary transfer belt  12 . The thus obtained density level of the image of the density level detection test patch is used as the output density level.  
         [0049]     Then, based on the relationship between the inputted color density level, and the outputted color density level detected by the color density sensor  21 , the controller  15  as a controlling means adjusts the image formation conditions, as will be described below, in order to yield a satisfactory image. More specifically, the gradation reference, which in this embodiment is the LUT, set in the video signal processing portion of the controller  15 , is revised so that a satisfactory (vivid) image, in terms of gradation, is always formed regardless of the gradational variations.  
         [0050]     Referring to  FIG. 3 , the density sensor  21  in this embodiment comprises a light emitting element  23 , a light receiving element  24  such as a photo-diode, Cds, or the like, and a holder  22  to which the light emitting element  23  and light receiving element  24  are attached. The beam of light from the light emitting element  23  is projected onto the image T of the density detection patch (which hereinafter will be referred to patch image T) on the belt  12 , and is partially received by the light receiving element  24  after being deflected (diffused) by the patch image T, in order to measure the density level of the patch image T. Generally, light reflected by a given surface includes the portion literally reflected by the surface and the portion diffused by the surface. In this embodiment, a density sensor of the diffuse light type is used as the density sensor  21 , and the incident angle θ and reflection angle φ are set to 15° and 45°, respectively. The outputs of the density sensor  21  when the light color toner was used, and the outputs of the density sensor  21  when the deep color toner was used, are given in  FIG. 4 .  
         [0051]     The controlling means  15  automatically revises the gradation setting, in real time, by changing the values set in the lookup table stored in the γ correcting portion of the video signal processing portion, based on, for example, a LUT revision table, in response to the image density level of the patch image T detected by the density sensor  21 .  
         [0052]     Further, the controlling means  15  stabilizes the image forming apparatus in terms of image quality, by sequentially revising the image formation conditions, that is, the conditions under which the photosensitive drums  1  are charged, the conditions under which the photosensitive drums  1  are exposed, the conditions under which images are transferred, etc., which are set in the video signal processing portion. In other words, the controlling means  15  stabilizes the image forming apparatus in terms of image quality by revising the image formation conditions. Since the image forming apparatus is controlled in image density, based on the LUT revised through the above described steps, the relationship between the input video signals and the density of the image resultant from the inputted video signals becomes linear, as shown in  FIG. 5 , making it possible to yield an image satisfactory in terms of density level reproduction. Referring to  FIG. 5 , incidentally, the input video signals means the video signals resulting from the reading of the original by the original reading apparatus  20 , and the output image density level means the density level of the image resulting from the input video signals.  
         [0053]     As described above, in this embodiment, the image formation operation is controlled by the controlling means  15  in accordance with the LUT. Therefore, a satisfactory image can be formed.  
         [0054]     However, as a large number of copies are made, that is, the image forming operation is repeated a large number of times, and/or the ambient conditions of the image forming apparatus change, the image formation conditions, such as the developmental properties of the developing means  4  and  5 , the thickness of the dielectric layer of the photosensitive drum  1 , the transfer efficiency or the like in the secondary transfer station  11 , change. As a result, the light and deep color toners change in the γ property, making nonlinear the relationship between input image density level and output image density level, roughly at the density level (which hereinafter will be referred to as mid image density level) where the light color toner and deep color toner begin to be used in mixture, as shown in  FIG. 6 . Therefore, it becomes unlikely for an image satisfactory in terms of color density reproduction to be formed. Instead, an unsatisfactory image, for example, an image unnatural in gradation across the areas where color density is in the mid range, an image suffering from pseudo-contours, or the like, is likely to be yielded.  
         [0055]     In this embodiment, therefore, the image density levels to be inputted for forming the images of the density level detection test patches for revising the LUT are selected so that the detection of the density levels of the patch images, the density levels of which are at, or in the adjacencies of, the mid image density level, is prioritized.  
         [0056]     In this embodiment, as the values for the video signals to be inputted to form the patch images for determining the relationship between the input signal level and the output density level,  16 ,  48 ,  80 ,  112 ,  120 ,  128 ,  136 ,  144 ,  176 ,  208 , and  240  are selected from among the  255  values (that is,  256  gradation levels) used to indicate the density level of an image of a solid color.  FIG. 7 , in which the abovementioned values for the video signals inputted for patch formation, and the corresponding density levels of the patch images, are plotted, shows the relationship between the input signals and output signals in terms of the image density. As will be evident from this graph, the values for the input video signal are selected so that the interval between the adjacent two values is smaller, near  128 ; in other words, the detection of the density level is concentrated to the values near  128 .  
         [0057]     More specifically, the abovementioned values are selected in consideration of the following facts (problems). That is, not only is it difficult to confirm whether or not the relationship between the input video signal and the density level of the resultant image is linear in the areas of the image, where the density is in the mid range, but also, if the larger the interval between the adjacent two density levels selected for the density level detection test patches, the more unclear the changes in the γ property, whereas the narrower the interval, the greater the number of the patch images to be formed to detect the relationship between the input video signals and the density level of the resultant image, and therefore, the longer the down time, or the time spent to detect the relationship, and also, the greater the toner consumption, and therefore, the higher the image formation cost.  
         [0058]     More specifically, the number by which the patch images, which are formed of the mixture of the light and deep color toners, and the image density levels of which are in the adjacencies of the borderline density level between the density level range in which patch images are formed of the light color toner alone, and the density level range in which patch images are formed of the mixture of the light and deep color toners, are formed, and the number by which the patch images, which are formed of the mixture of the light and deep color toners, and the image density levels of which are in the adjacencies of the border line density level between the density level range in which patch images are formed of the mixture of the light and deep color toners, and the density level range in which patch images are formed of the deep color toner alone, are formed, are made greater than the number by which the patch images which are formed with the use of the deep color toner alone, and the density levels of which are in the mid to high portion of the density level range in which patch images are formed of the deep color toner alone, are formed, and the number by which the patch images, which are formed of the light color toner alone, and the density levels of which are in the low to mid portion of the density level range in which patch images are formed of the light color toner alone, are formed.  
         [0059]     As described above, it is desired that the patch images, the image density levels of which fall within the adjacencies of the mid density level at which the toner used for forming a monochromatic image is switched from the light color toner to the mixture of the light and deep color toners, are formed by a greater number than the patch images, the image density levels of which do not fall within the abovementioned range, and their actual density levels are detected.  
         [0060]     Referring to  FIG. 8 , in which in order to make it easier to understand the abovementioned mid density levels, the overall range of the values for the input video signals are divided into an image density range R 1  in which only the light color toner is used, an image density range R 2  in which the mixture of the light and deep color toners are used, and an image density range R 3  in which only the deep color toner is used, the abovementioned mid density level means the borderline between the image density ranges R 1  and R 2 .  
         [0061]     In other words, the patch images, the theoretical density levels of which fall within the adjacencies of the borderline between the image density ranges R 1  and R 2 , are formed by a larger number than the patch images, the theoretical density levels of which do not fall therein, and their actual density levels are detected by the density sensor  21  to more precisely determine the relationship between the input density and output density. Therefore, it is possible to keep linear the relationship between the input density level and output density level. In other words, it is possible to satisfactorily control the image density.  
         [0062]     Referring to  FIG. 9 , regarding one of the characteristic features of this embodiment of the present invention, the image density can be even more satisfactorily controlled by forming, by a greater number, the patch images, the theoretical density levels of which fall within the adjacencies of the borderline between the image density ranges R 2  and R 3 , in addition to the patch images, the theoretical density levels of which fall within the adjacencies of the borderline between the image density ranges R 1  and R 2 , than the patch images, the theoretical density levels of which do not fall therein.  
         [0063]     The reason not only are the patch images, the image density levels of which fall within the adjacencies of the intermediary density level, that is, the borderline between the image density ranges R 1  and R 2 , that is, the borderline between the image density range in which only the light color toner is used, and the image density range in which the light color toner is used in combination with the deep color toner, but also, the patch images, the image density levels of which fall within the adjacencies of the intermediary density level, that is, the borderline between the image density ranges R 2  and R 3 , are formed by a greater number than the patch images, the density levels of which do not fall in the adjacencies of the borderline between the image density ranges R 1  and R 2 , and the adjacencies of the image density range R 2  and R 3 , is that the effects of the changes which occur to the developing means through the usage, upon the γ property, and the effects of the changes in the ambient conditions, upon the γ property, are larger when the image density level of the portion of the image being formed is at or in the adjacencies of these borderlines.  
         [0064]     As will be evident from the above description of this embodiment, in the case of the image forming apparatus in this embodiment, in which each of the plurality of monochromatic images, different in color, formed to form a single multicolor image, is formed of two toners, that is, light and deep color toners, which are the same in hue, but, are different in color density, and the image density is controlled by revising the LUT in response to the output of the density sensor which detects the image density levels of the images of the density level detection test patches, the formation of the patch images, the image density levels of which fall in the adjacencies of the image density level at which the toner used for the formation of the monochromatic image is switched from the light color toner to the mixture of the light and deep color toners, is prioritized, and the density levels of the resultant patch images are detected by the density sensor. Therefore, even if the processing conditions of the image forming apparatus change due to the formation of a large number of images (copies), and/or the ambient conditions change, the relationship between the video signals and the density level of the image resulting from the video signals remains linear, making it possible to always form a color image of high quality.  
       Embodiment 2  
       [0065]     Next, referring to  FIG. 10 , the second embodiment of the present invention will be described.  
         [0066]     In this embodiment, the image formation stations Pb and Pc for forming the magenta (M) and cyan (C) images are provided with both the first and second developing means  4  and  5  in the above described first embodiment, and the image formation stations Pa and Pd for forming the yellow (Y) and black (Bk) images are provided with only the second developing means  5 , that is, the developing means which uses the deep color toner.  
         [0067]     Yellow (Y) color is higher in brightness. Therefore, the graininess of the yellow areas of an image is difficult to visually detect, even if the areas are low in density. Thus, the effect of the usage of the light yellow toner is insignificant.  
         [0068]     As for black (Bk) color, it is rare that photographic image or the like images, which require high quality, have black areas which are low in density. Further, a letter or the like image usually is solid. Therefore, effect of the usage of the light black toner is insignificant.  
         [0069]     In this embodiment, the process of forming a magenta (M) image and the process of forming a cyan (C) image are controlled in the manner similar to that in the first embodiment. As a result, the relationship between the input video signal and the density level of the resultant image can be kept linear, making it possible to yield a color image of high quality, in terms of the density of the magenta and cyan color areas of the image, regardless of the changes in the ambient conditions, even after the developing apparatuses change in properties through the usage.  
         [0070]     Moreover, the component count of the developing means is smaller than that in the first embodiment, and also, the memory capacity necessary for the LUT can be reduced. Therefore, it is possible to provide an image forming apparatus, which is smaller, lower in cost, and simpler to control.  
       Embodiment 3  
       [0071]     Next, referring to  FIG. 11 , the third embodiment of the present invention will be described. The general structure of the image forming apparatus in this embodiment is the same as that of the image forming apparatus in the first embodiment, and therefore, the same referential numbers and symbols as those used for the designation of the components, means, etc., of the image forming apparatus in the first embodiment are used to designate the corresponding component, means, etc., of this image forming apparatus.  
         [0072]     In this embodiment, the density of the patch image formed to control the image forming apparatus in terms of image density is detected by the density sensor  21  positioned next to the intermediary transfer member  12 , facing the intermediary transfer member  12 . In this embodiment, however, the density level of the test patch image is detected by the original reading portion  20  after the test patch image is transferred onto the recording medium p, and the control is carried out in response to the thus detected image density level of the test patch image.  
         [0073]     Referring to  FIG. 11 , a test pattern print  30  contains four rows of color patches, that is, the row of the eleven yellow color patches, row of the eleven magenta color patches, row of the eleven cyan color patches, and row of the eleven black color patches. The eleven color patches in each row of color patches are different in density level (gradation level). Out of the 256 levels of density (gradation level), which this image forming apparatus is enabled to reproduce, the mid density value and the values close thereto are primarily selected as the values for the density levels for the density level detection test patches, and the images of the density level detection test patches, the density levels of which fall in the low density range, or high density range, are formed by a substantially smaller number than the number by which the images of the density level detection test patches, the density levels of which fall on or within the adjacencies of the mid density value.  
         [0074]     Thus, the images of the density detection test patches are not formed by an excessive number. Therefore, it is possible to control the image forming apparatus in terms of the density level at which the toner used for the formation of a monochromatic image is switched from the light color toner to the mixture of the light and deep color toners, while reducing the toner consumption and the time required for forming the test prints.  
         [0075]     As for the image density levels of the eleven test patches in each of the four rows of test patches, the density level of the test patch, which is deepest in density, is represented by a value of 255, and the values of the density levels of the eleven test patches for each color are  16 ,  48 ,  80 ,  112 ,  120 ,  128 ,  136 ,  144 ,  176 ,  208 , and  240 , as they were in the first embodiment. The video signals for forming the images of these eleven test patches, the density levels of which have the above listed values, one for one, are generated with the use of the test patch generating means.  
         [0076]     After the formation of the groups of patch images, the groups of patch images on the test print  30  are read by the original reading portion  20 .  
         [0077]     In order to accurately detect the density level of the images of the test patches, the density level of each test patch image was detected at 16 points of the test patch, and the obtained signals are averaged. The value obtained by averaging the  16  values obtained by detecting the density level of each test patch image at 16 different points of the test patch image, RGB signals are converted by the optical density converting method into the density values for Y, M, C, and Sk, and the LOT is revised in response to the thus obtained density values for Y, M, C, and Bk; a new LUT is set up.  
         [0078]     By carrying out the above described image density control, it was possible to maintain linearity in the relationship between the input video signals and the density level of the reproduced image, in spite of the changes in the processing conditions which occurred through an operation for forming a large number of copies, repetition of the image forming operations, and/or changes in the ambient conditions. As a result, it was possible to continuously form images of high quality.  
         [0079]     Further, the test patch images tested for image density control in this embodiment are the test patch images which had been transferred onto the recording mediums p, and had been fixed to the recording mediums p by being put through the fixing device  9 . They are virtually the same in terms of image density level as that of the image to be formed for actual usage. Thus, the image density control in this embodiment is more accurate than that in the first embodiment.  
         [0080]     Referring to  FIGS. 1 and 10 , in the first to third embodiments, the density sensor  21  was positioned so that it faced the intermediary transfer member  12 , which was a transfer belt for a multilayer direct image transfer method. Obviously, however, the density sensor  21  may be positioned so that it faces the peripheral surface of the photosensitive drum  1 . Placing the density sensor  21  so that it faces the peripheral surface of the photosensitive drum  1  is just as effective as placing the density sensor  21  so that it faces the intermediary transfer member  12 .  
       Embodiment 4  
       [0081]     Next, referring to  FIG. 12 , the fourth embodiment of the present invention will be described.  
         [0082]     This embodiment is an example of the application of the present invention to an image forming apparatus employing the multilayer direct image transferring method. In this embodiment, a plurality of image formation stations Pa-Pd, similar in structure as those shown in  FIG. 1 , are disposed along the transfer belt  12 . The recording medium p from a cassette  13  is borne on the surface of the transfer belt  12 , and is conveyed by the transfer belt  12  through the image formation stations Pa-Pd, in which it remains pinched between the transfer roller  7  as a transferring means, and the photosensitive drum  1 , so that the a plurality of monochromatic toner images are transferred in layers directly onto the recording medium p. After the direct transfer, the recording medium p is conveyed through the fixing device  9 , in which the plurality of monochromatic toner images on the recording medium p are fixed. Thereafter, the recording medium p is discharged frog the image forming apparatus. Obviously, a plurality of image formation stations Pa-Pd, similar to those shown in  FIG. 10 , may be disposed along the transfer belt  12 .  
         [0083]     In this embodiment, the images of the density level test patches are formed on the portion of the transfer belt  12  other than where the recording medium p is borne, or on the recording medium p borne on the transfer belt  12 , and then, the test patch images are test for density level by the density sensor  21 . The image control in this embodiment is the same as those in the above described first to third embodiments.  
         [0084]     According to the above described first to fourth embodiments, it is possible to keep linear the relationship between the input video signals and the density levels of the resultant images, even if the condition of an image forming apparatus changes because of the formation of a large number of images, and/or the changes in the ambient conditions. Therefore, it is possible to always form images of high quality.  
         [0085]     Incidentally, in the above, the first to fourth embodiments were described with reference to an image forming apparatus of an inline type However, the number of the photosensitive drum  1  does not need to be limited to the number in these embodiments. For example, a plurality of developing means may be disposed in the adjacencies of the peripheral surface of a single photosensitive drum.  
         [0086]     Further, the measurements, materials, and shapes of the structural components of the image forming apparatus, and the positional relationship among them, in the first to fourth embodiments of the present invention, are not intended to limit the scope of the present invention, unless specifically noted.  
         [0087]     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.  
         [0088]     This application claims priority from Japanese Patent Application No. 433950/2003 filed Dec. 26, 2003, which is hereby incorporated by reference.