Abstract:
High precision image density characteristics are stable maintained over extended periods of time. For this purpose, a first calibration operation is preformed in which a predetermined grayscale pattern is formed on a recording paper and this pattern is read by a reading device to produce a LUT for controlling the laser output in accordance with the image signal (gamma correction). A second calibration operation is performed after the first calibration operation wherein a patch is formed on an image carrier by the laser output controlled by the above LUT, its density is detected by a detector and a correction LUT is generated in accordance with the detected density.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an image processing apparatus for forming an image on a recording medium and its control method. 
     BACKGROUND OF THE INVENTION 
     Conventionally, in order to improve the stability of image quality, an image processing apparatus of this type prints a specific pattern such as a gray scale pattern or the like on a recording medium such as a paper sheet upon completion of a warm-up process upon startup, reads the printed gray scale pattern using an image reader such as a scanner or the like, and feeds back that information to image forming conditions such as γ correction and the like. 
     However, when the image processing apparatus has been used for a long period of time, the fed-back γ correction characteristics alone often fail to obtain an optimal image. 
     For example, in an image processing apparatus using electrophotography, even when image forming conditions such as γ correction and the like are optimally adjusted, the attachment characteristics of the toner with respect to the photosensitive drum charge change over long-term use, and consequently optimal image forming conditions cannot be assured. 
     To solve such problems, a method of making correction using the relationship between the potential data and density has been proposed. However, since the relationship between the potential data and density cannot be determined uniquely, such correction is insufficient. Also, a method of forming a developing patch on, e.g., the surface of a photosensitive drum, converting the output from a photosensor into a density using a density conversion table which is determined in advance, and making γ correction using the converted value is also available. However, since this method detects the density of toner attached onto the surface of the photosensitive drum, the detected density does not always match the final image density. Furthermore, the sensor normally used in this method has insufficient resolution for the attached toner density, and has insufficient performance as an absolute density sensor. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the conventional problems, and has as its object to provide an image processing apparatus which reads a specific pattern such as a gray scale pattern, and can maintain given image density characteristics by feeding back the read information to image forming conditions such as γ correction and the like, even when the apparatus has been used for a long period of time, and its control method. 
     In order to achieve the above object, an image processing apparatus according to the present invention, comprises: 
     forming means for forming a gray scale pattern on an image carrier, and forming a gray scale pattern image by transferring an image corresponding to the gray scale pattern onto a recording sheet; 
     determination means for reading the gray scale pattern image formed on the recording sheet, and determining density correction characteristics of the forming means; 
     holding means for holding the density correction characteristics determined by the determination means; 
     storage means for storing a density of an image formed on the image carrier using the density correction characteristics; and 
     adjustment means for adjusting the density correction characteristics held by the holding means in accordance with a relationship between the density stored in the storage means, and the density of an image formed on the image carrier at a predetermined timing. 
     Also, an image processing apparatus which forms an electrostatic latent image on an image carrier by an image exposure output corresponding to an image signal, develops the electrostatic latent image with toner, and transfers the developed toner image on the image carrier onto a recording medium, comprises: 
     pattern forming means for forming a gray scale pattern based on a predetermined image signal on the recording medium; 
     reading means for reading the gray scale pattern formed by the pattern forming means; 
     first control means for controlling the image exposure output corresponding to the image signal to match the tone of the image signal with the tone of an image recorded on the recording medium by comparing the gray scale pattern read by the reading means with the predetermined image signal; 
     storage means for storing a density value of a toner image formed on the image carrier by the image exposure output controlled by the first control means as a reference density value immediately after the control of the first control means; 
     detection means for detecting a density value of a toner image formed on the image carrier by the image exposure output controlled by the first control means; and 
     second control means for controlling the image exposure output corresponding to the image signal to match the density value detected by the detection means with the reference density value stored in the storage means. 
     The first control means comprises: 
     first table generation means for generating a first table for storing a correspondence between the image signal and image exposure output; and 
     table storage means for storing the first table, 
     the second control means comprises: 
     correction table generation means for generating a correction table for correcting the image signal to match the density value detected by the detection means with the reference density value stored in the storage means; and 
     second table generation means for generating a second table by combining the first table stored in the table storage means with the correction table, and 
     image forming means for forming an image by using the second table. 
     Detection by the detection means and control by the second control means are automatically done at a predetermined timing. 
     A method of controlling an image processing apparatus according to the present invention, comprises: 
     a forming step of forming a gray scale pattern on an image carrier, and forming a gray scale pattern image by transferring an image corresponding to the gray scale pattern onto a recording sheet; 
     a determination step of reading the gray scale pattern image formed on the recording sheet, and determining density correction characteristics of the forming step; 
     a holding step of holding the density correction characteristics determined in the determination step; 
     a storage step of storing a density of an image formed on the image carrier using the density correction characteristics; and 
     an adjustment step of adjusting the density correction characteristics held in the holding step in accordance with a relationship between the density stored in the storage step, and the density of an image formed on the image carrier at a predetermined timing. 
     An method of controlling an image processing apparatus which forms an electrostatic latent image on an image carrier by an image exposure output corresponding to an image signal, develops the electrostatic latent image with toner, and transfers the developed toner image on the image carrier onto a recording medium, comprises: 
     a first control step of controlling the image exposure output corresponding to the image signal to match the tone of the image signal with the tone of an image recorded on the recording medium by reading an image on the recording medium on which a predetermined image is recorded by the image processing apparatus; 
     an storage step of storing a density value of a toner image formed on the image carrier by the image exposure output controlled in the first control step as a reference density value immediately after the first control step; 
     a detection step of inputting the predetermined image signal at a predetermined timing and detecting a density value of a toner image formed on the image carrier at that time; and 
     a second control step of controlling the image exposure output corresponding to the image signal to match the density value detected in the detection step with the reference density value stored in the storage step. 
     The first control step comprises: 
     a first table generation step of generating a first table for storing a correspondence between the image signal and image exposure output; and 
     the table storage step of storing the first table, 
     a second control step comprises: 
     a correction table generation step of generating a correction table for correcting the image signal to match the density value detected in the detection step with the reference density value stored in the storage step; and 
     a second table generation step of generating a second table by combining the first table stored in the table storage step with the correction table, and 
     a method further comprises the image forming step of forming an image using the second table. 
     Furthermore, the detection step and the second control step are automatically executed at a predetermined timing. 
     A computer readable memory according to the present invention is a computer readable memory which stores a control program for an image processing apparatus, which forms an electrostatic latent image on an image carrier by an image exposure output corresponding to an image signal, develops the electrostatic latent image with toner, and transfers the developed toner image on the image carrier onto a recording medium, storing: 
     a forming program for forming a gray scale pattern on the image carrier, and forming a gray scale pattern image by transferring an image corresponding to the gray scale pattern onto the recording sheet; 
     a determination program for reading the gray scale pattern image formed on the recording sheet, and determining density correction characteristics of the forming program; 
     a holding program for holding the density correction characteristics determined by the determination program; 
     a storage program for storing a density of an image formed on the image carrier using the density correction characteristics; and 
     an adjustment program for adjusting the density correction characteristics held in the holding program in accordance with a relationship between the density stored in the storage program, and the density of an image formed on the image carrier at a predetermined timing. 
     A computer readable memory which stores a control program for an image processing apparatus, which forms an electrostatic latent image on an image carrier by an image exposure output corresponding to an image signal, develops the electrostatic latent image with toner, and transfers the developed toner image on the image carrier onto a recording medium, has: 
     a first control program for controlling the image exposure output corresponding to the image signal to match the tone of the image signal with the tone of an image recorded on the recording medium by reading an image on the recording medium on which a predetermined image is recorded by the image processing apparatus; 
     a storage program for storing a density value of a toner image formed on the image carrier by the image exposure output controlled by the first control program as a reference density value immediately after execution of the first control program; 
     a detection program for inputting the predetermined image signal at a predetermined timing and detecting a density value of a toner image formed on the image carrier at that time; and 
     a second control program for controlling the image exposure output corresponding to the image signal to match the density value detected by the detection program with the reference density value stored in the storage program. 
     The first control program includes: 
     a first table generation program for generating a first table for storing the correspondence between the image signal and image exposure output; and 
     a table storage program for storing the first table, 
     the second control program includes: 
     a correction table generation program for generating a correction table for correcting the image signal to match the density value detected by the detection program with the reference density value stored in the storage program; and 
     a second table generation program for generating a second table by combining the first table stored in the table storage program with the correction table. 
     The detection program and the second control program are automatically executed at a predetermined timing. 
     An image processing apparatus comprising: 
     first calibrating means for calibration an image forming apparatus based on an image fixed on a recording medium and generating a look-up table for correcting image data; and 
     second calibrating means for calibrating the image forming apparatus based on an image formed on an image holding medium and generating data for correcting the look-up table. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing of an image processing apparatus according to the first embodiment of the present invention; 
     FIG. 2 is a block diagram showing a reader image processor  108  of the image processing apparatus according to the first embodiment; 
     FIG. 3 is a timing chart showing the timings of the reader image processor  108  of the image processing apparatus according to the first embodiment; 
     FIG. 4 is a control block diagram of the image processing apparatus according to the first embodiment; 
     FIG. 5 is a block diagram showing the image processing apparatus according to the first embodiment; 
     FIG. 6 is a four-quadrant chart showing tone reproduction characteristics; 
     FIG. 7 is a flow chart of a first control system of the image processing apparatus according to the first embodiment; 
     FIGS. 8A to  8 C show the display contents of a display  218  of the image processing apparatus according to the first embodiment; 
     FIGS. 9A to  9 C show the display contents of the display  218  of the image processing apparatus according to the first embodiment; 
     FIGS. 10A to  10 E show the display contents of the display  218  of the image processing apparatus according to the first embodiment; 
     FIG. 11 shows an example of test print  1  of the image processing apparatus according to the first embodiment; 
     FIG. 12 shows an example of test print  2  of the image processing apparatus according to the first embodiment; 
     FIG. 13 shows the layout of test print  1  placed on a platen; 
     FIG. 14 shows the layout of test print  1  placed on the platen; 
     FIG. 15 is a graph showing the relationship between the relative drum surface potential and image density; 
     FIG. 16 is a graph showing the relationship between the absolute humidity and contrast potential; 
     FIG. 17 is a graph showing the relationship between the grid potential and surface potential; 
     FIG. 18 shows reading points for the patch pattern of test print  2 ; 
     FIG. 19 is a graph showing a reading example of test print  2 ; 
     FIG. 20 is a graph showing the density conversion characteristics; 
     FIG. 21 is a block diagram from a photosensor  40  to density conversion; 
     FIG. 22 is a graph showing the spectral characteristics of yellow toner; 
     FIG. 23 is a graph showing the spectral characteristics of magenta toner; 
     FIG. 24 is a graph showing the spectral characteristics of cyan toner; 
     FIG. 25 is a graph showing the spectral characteristics of black toner; 
     FIG. 26 is a graph showing the relationship between the photosensor output and image density; 
     FIGS. 27A and 27B are flow charts of a second control system of the image processing apparatus according to the present invention; 
     FIG. 28 is a graph showing the laser output upon forming a patch; 
     FIG. 29 is a chart showing the patch formation sequence of the second control system; 
     FIG. 30 is a graph showing a change in density detected by a photosensor when identical image signals are input to form patches; 
     FIG. 31 shows graphs used for making correction tables; 
     FIG. 32 is a view showing how to look up a correction table; and 
     FIG. 33 is a schematic view showing the arrangement of an image processing apparatus according to the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be explained in detail hereinafter with reference to the accompanying drawings. Note that the relative layouts of building elements, formulas, numerical values, and the like described in the embodiments of the present invention do not limit the scope of the present invention in any way unless otherwise specified. 
     [First Embodiment] 
     An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
     FIG. 1 is a schematic view showing a full-color image processing apparatus of this embodiment. 
     An original  101  placed on a platen glass  102  is irradiated with light emitted by a light source  103 , and light reflected by the original  101  forms an image on a CCD sensor  105  via an optical system  104 . The CCD sensor  105  generates red, green, and blue color component signals in units of line sensors using red, green, and blue line sensors, which line up in three arrays. 
     A reading optical system unit including these components scans in the direction of the arrow in FIG. 1 to convert the original into an electrical signal data sequence in units of lines. 
     On the platen glass  102 , a reference white plate  106  and a registration member  107  are placed. The reference white plate  106  is used for determining the white level of the CCD sensor  105  and for performing the shading correction in the scan direction. The registration member  107  register the original to prevent it from being obliquely placed. 
     Image signals obtained by the CCD sensor  105  undergo an image process by a reader image processor  108 , and the processed signals are sent to a printer portion B to undergo further image processing by a printer controller  109 . 
     The image processor  108  will be described below. 
     FIG. 2 is a block diagram showing the flow of image signals in the image processor  108  of a reader portion A according to this embodiment. As shown in FIG. 2, image signals output from the CCD sensor  105  are input to an analog signal processor  201  to undergo gain and offset adjustments. An A/D converter  202  into 8-bit digital image signals R 1 , G 1 , and B 1  then converts the adjusted signals. Subsequently, these signals are input to a shading correction section  203  and undergo known shading correction using signals obtained by reading the reference white plate  106  in units of colors. 
     A clock generator  211  generates clocks in units of pixels. A main scan address counter  212  counts clock pulses from the clock generator  211  and generates a pixel address output for one line. A decoder  213  decodes the main scan address from the main scan address counter  212  and generates CCD drive signals in units of lines such as shift pulses, reset pulses, and the like, a VE signal indicating an effective region in a 1-line read signal from the CCD, and a line sync signal HSYNC. Note that the main scan address counter  212  is cleared in response to the signal HSYNC to start counting of the main scan address for the next line. 
     Since the line sensors of the CCD sensor  105  are spaced predetermined distances from each other, spatial deviations in the sub-scan direction are corrected by a line delay circuit  204  in FIG.  2 . 
     More specifically, the R and G signals are line-delayed with respect to the B signal in the sub-scan direction to be adjusted to the B signal. 
     An input masking section  205  converts a read color space determined by the spectral characteristics of R, G, and B filters of the CCD sensor into an NTSC standard color space, and makes a matrix operation given by:                [               R4           G4                 B4         ]     =       [         a11       a12       a13           a21       a22       a23           a31       a32       a33         ]          [               R3           G3                 B3         ]               (   1   )                                
     A light amount/density converter (LOG converter)  206  comprises a look-up table ROM, and converts luminance signals R 4 , G 4 , and B 4  into density signals C 0 , M 0 , and Y 0 . A line delay memory  207  delays the image signals C 0 , M 0 , and Y 0  by a line delay amount up to determination signals UCR, FILTER, SEN, and the like generated by a black character determination section (not shown) from the signals R 4 , G 4 , and B 4 . 
     A masking &amp; UCR circuit  208  extracts a black signal (Bk) from input three primary color signals Y 1 , M 1 , and C 1 , makes an operation for correcting chromatic blur of recording color agents in the printer portion B, and outputs signals Y 2 , M 2 , C 2  and Bk 2  with a predetermined bit width (8 bits) for each read operation. 
     A γ correction circuit  209  makes density correction to attain ideal tone characteristics of the printer portion B in the reader portion A. On the other hand, a spatial filter processor (output filter)  210  makes an edge emphasis or smoothing process. 
     The processed, frame-sequential image signals M 4 , C 4 , Y 4 , and Bk 4  are sent to the printer controller  109 , and undergo density recording by means of PWM in the printer portion B. 
     Reference numeral  214  denotes a CPU for controlling components inside the reader portion;  215 , a RAM; and  216 , a ROM. Reference numeral  217  denotes a console (control panel) having a display  218 . 
     FIG. 3 shows the timings of respective control signals in the image processor  108  shown in FIG.  2 . Referring to FIG. 3, the signal VSYNC is an image effective period signal in the sub-scan direction, and an image is read (scanned) to form output signals (C), (M), (Y), and (Bk) in turn during periods in which the signal VSYNC is at logic “1”. The signal VE is an image effective period signal in the main scan direction, defines the timings of main scan start positions during periods in which it is at logic “1”, and is mainly used in line count control for line delay. The signal CLOCK is a pixel sync signal, and is used to transfer image data at the timing of its “0”→“1” leading edge. 
     Referring back to FIG. 1, the printer portion B will be explained below. 
     Referring to FIG. 1, a photosensitive drum  4  is uniformly charged by a primary charger  8 . 
     Image data is converted into a laser beam signal via a laser driver included in the printer controller  109  and a laser beam source  110 , and the laser beam signal is reflected by a polygonal mirror  1  and mirror  2 . The uniformly charged surface of the photosensitive drum  4  is then irradiated with that laser beam signal. 
     The photosensitive drum  4  on which a latent image is formed by scanning the laser beam signal rotates in the direction of an arrow shown in FIG.  1 . 
     Then, developers  3  develop images in turn in units of colors. 
     This embodiment uses a two-component system as a developing method, and the color developers  3  are disposed around the photosensitive drum  4  in turn in the order of black (Bk), yellow (Y), cyan (C), and magenta (M) from the upstream side. The developer corresponding to an image signal develops at the developing timing of a latent image region formed on the photosensitive drum. 
     A transfer sheet  6  is wound around the transfer drum  5 , which makes one revolution in the order of M, C, Y, and Bk for a total of four revolutions, thus transferring color toner images onto the transfer sheet  6  to overlap each other. 
     Upon completion of transfer, the transfer sheet  6  is peeled from the transfer drum  5 , and the toner image formed on the sheet  6  is fixed by a pair of fixing rollers  7 , thus completing a full-color image print. 
     A surface potential sensor  12  is located on the upstream side of the developers  3  around the photosensitive drum  4 . 
     The printer portion B also includes a cleaner  9  for cleaning the residual toner on the photosensitive drum  4  upon transfer, and an LED light source  10  (having a main wavelength of around 960 nm) and photodiode  11 , which are used to detect the amount of light reflected by a toner patch pattern formed on the photosensitive drum  4 . 
     FIG. 4 is a block diagram showing the arrangement of the image processing apparatus according to this embodiment. 
     The printer controller  109  is constructed by a CPU  28 , ROM  30 , RAM  32 , test pattern memory  31 , density conversion circuit  42 , and LUT  25 , and can communicate with the reader portion A and a printer engine  100 . 
     The printer controller  109  controls an optical reading device (photosensor)  40 , which includes the LED  10  and photodiode  11 , the primary charger  8 , the laser beam source  110 , the surface potential sensor  12 , and the developers  3 , which are arranged around the photosensitive drum  4 . 
     The printer engine  100  includes an environment sensor  33 , which measures humidity of the air in the apparatus. 
     The surface potential sensor  12  is located on the upstream side of the developers  3 , and controls the grid potential of the primary charger  8  and the developing bias of the developers  3 , as will be described later. 
     FIG. 5 shows an image signal processing circuit for obtaining a halftone image according to this embodiment. 
     A luminance signal of an image is obtained by the CCD sensor  105 , and is converted into a frame-sequential image signal by the reader image processor  108 . The density characteristics of this image signal are converted by the LUT  25  to match with the default γ characteristics of the printer, i.e., so that the density of an original image expressed by the input image matches that of an output image. 
     FIG. 6 is a four-quadrant chart showing tone reproduction. 
     Quadrant I shows the reading characteristics of the reader portion A that converts an original density into a density signal; quadrant II shows the conversion characteristics of the LUT  25  for converting the density signal into a laser output signal; quadrant III shows the recording characteristics of the printer portion B for converting the laser output signal into an output density; and quadrant IV shows the total tone reproduction characteristics of this image processing apparatus, which represent the relationship between the original density and output density. The number of gray levels is 256 since the process is done using 8-bit digital signals. 
     In this image processing apparatus, in order to obtain linear tone characteristics in quadrant IV, nonlinear components of the printer characteristics in quadrant III are corrected by the LUT  25  in quadrant II. 
     Such LUT  25  is generated by the result of arithmetic operations to be described later. 
     Referring back to FIG. 5, the signal converted by the LUT  25  is converted into the signal corresponding to a dot width by a pulse width modulation (PWM) circuit  26 , and the converted signal is sent to the laser driver  27  for controlling the turn on/off of the laser beam. 
     In this embodiment, tone reproduction based on pulse width modulation is used for all the Y, M, C, and K colors. 
     By scanning the laser beam source  110 , a latent image having predetermined tone characteristics is formed on the surface of the photosensitive drum  4 , and a halftone image is reproduced via development, transfer, and fixing processes. 
     (Tone Control of System Including Both Reader/Printer) 
     A first control system that pertains to stabilization of the image reproduction characteristics of a system that includes both the reader portion A and printer portion B will be explained below. 
     Calibration of the printer portion B using the reader portion A will be explained below with reference to the flow chart shown in FIG.  7 . This flow is implemented by the CPU  214  that controls the reader portion A, and the CPU  28  that controls the printer portion B. 
     Upon pressing a mode setting button named “automatic tone correction” and provided on the console  217 , this control starts. In this embodiment, the display  218  comprises a liquid crystal display panel with a pressure sensor (touch panel display) shown in FIGS. 8 to  10 , and the operator can directly make operations on the display  218 . 
     This calibration control will be described below in units of steps in FIG.  7 . 
     {Output Test Print  1 : Step S 51 } 
     In step S 51 , a print start button  81  for test print  1  appears on the display  218  (FIG.  8 A). When the operator presses this button, the printer portion B prints out an image of test print  1  shown in FIG.  11 . 
     At this time, the CPU  214  checks the presence/absence of a paper sheet used to form test print  1 . If no paper sheet is available, an alert display shown in FIG. 8B is made. 
     A standard contrast potential corresponding to an environment is registered as a default value, and this default value is used upon forming test print  1 . 
     The apparatus of this embodiment comprises a plurality of paper cassettes, and a plurality of different paper sizes such as B 4 , A 3 , A 4 , B 5 , and the like are selectable. 
     However, this control is set to use so-called large-size paper sheets, i.e., B 4 , A 3 , 11×17, or LGR sheets to avoid errors resulting from wrong document (portrait, landscape) positions. 
     As shown in FIG. 11, a stripe pattern  61  by halftone densities of four colors Y, M, C, and K is formed as test pattern  1 . 
     The operator visually inspects this pattern  61  to confirm if it is free from any stripe-shaped abnormal image, density non-uniformity, and color non-uniformity. The main scan size of this pattern  61  is set to cover patch patterns  62  and gray scale patterns  71  and  72  for test print  2  (to be described later). 
     If any abnormality is found, test print  1  is printed again, and if abnormality is found again, a message that says “call a service person” is displayed. 
     Note that this pattern  61  may be read by the reader portion A, and whether or not to proceed with the subsequent control may be automatically determined based on density information of that pattern in the scan direction. 
     The patterns  62  are maximum density patches of colors Y, M, C, and Bk. That is, the density signal value is set at  255 . 
     {Read Test Print  1 : Step S 52 } 
     In step S 52 , the operator places this image of test print  1  on the platen glass  102 , as shown in FIG. 13, and presses a read start button  91  shown in FIG.  9 A. 
     At this time, a guidance message for the operator shown in FIG. 9A appears. 
     FIG. 13 is a top view of the platen. An upper left wedge-shaped mark T is an original registration mark of the platen, and the aforementioned message (FIG. 9A) is displayed on the control panel to locate the pattern  61  on the side of the registration mark T and to prevent the wrong side not bearing information from facing down. In this manner, control errors resulting from wrong document positions can be prevented. 
     Upon reading the patterns  62  by the reader portion A, a scan gradually starts from the registration mark T, and a first density gap point A is determined to be the corner of the pattern  61 . The relative coordinate positions of the patch patterns  62  are determined from the coordinate point of the point A, thus reading the density values of the patterns  62 . 
     During reading, a message shown in FIG. 9B is displayed. On the other hand, when test print  1  is set in a wrong direction or at a wrong position, and cannot be read, a message shown in FIG. 9C is displayed. When the operator re-places the sheet and presses the read key  92 , test pattern  1  is read again. 
     The read R, G, and B values are converted into optical densities by:                                  M   =       -     k   m       ×   log                 10        (   G255   )                   C   =       -     k   c       ×   log                 10        (   R255   )                         Y   =       -     k   y       ×   log                 10        (   B255   )                         Bk   =       -     k   bk       ×   log                 10        (   G255   )               }           (   2   )                                
     To obtain the same values as those obtained by a commercially available densitometer, the values are adjusted using correction coefficients k m , k c , k y , and k bk . 
     Also, another LUT may be used to convert RGB luminance information into MCYBk density information. 
     {Compute Contrast Potential: Step S 53 } 
     A method of correcting the maximum density based on the obtained density information will be explained below. 
     FIG. 15 shows the relationship between the relative drum surface potential and the image density obtained by the aforementioned arithmetic operations. 
     A contrast potential is defined as the difference between the developing bias potential and the surface potential of the photosensitive drum upon irradiating a laser beam signal corresponding to a maximum level of image data after the primary charger charges the drum surface. 
     Assume that when the contrast potential is set for value A, the obtained maximum density is D A . In a density range around maximum density D A , the image density linearly normally corresponds to the relative drum surface potential, as indicated by solid curve L. 
     In the two-component developing system, the toner density in the developer may often vary and become low. That is, in the density range around the maximum density, nonlinear characteristics may be obtained, as indicated by broken curve N. 
     Hence, when the target value of the final maximum density is 1.6, the controlled variable is determined by setting the target value to be 1.7 in consideration of a margin of 0.1. 
     Contrast potential B for obtaining the maximum image density value=1.7 is given by; 
       B =( A+Ka )×1.7/( D   A )  (3) 
     where Ka is a correction coefficient, whose value is preferably optimized depending on the type of developing method. 
     The contrast potential must be frequently changed in correspondence with the environment. 
     Therefore, a correction coefficient Vcont.rate (=B/A) is saved in a backed-up RAM. Then a change in environment (humidity) is detected at 30 min intervals on the basis of the output from the aforementioned environment sensor  33  that monitors the humidity in the apparatus. By using the relation shown in FIG. 16, the value A is determined based on the detection result, and A×Vcont.rate is calculated to obtain contrast potential B. 
     A method of obtaining the grid potential and developing bias potential from the contrast potential will be briefly explained below. 
     FIG. 17 shows the relationship between the grid potential and the surface potential of the photosensitive drum. 
     Surface potential V L  when the laser beam is set at the lowest level, and surface potential V H  when the laser beam is set at the highest level while the grid potential is set at −200 V are measured by a surface potential sensor  12 . 
     Likewise, V L  and V H  when the grid potential is set at −400 V are measured. 
     The relationship between the grid potential and surface potential can be obtained by interpolation and extrapolation of those data for −200 V and −400 V. 
     The control for obtaining the potential data is called potential measurement control. 
     Developing bias V DC  is set by subtracting V bg  (set at 100 V in this case) from V L , for avoiding toner from becoming attached to an image. 
     Contrast potential Vcont is the differential voltage between developing bias V DC  and V H , and the maximum density increases with increasing Vcont, as described above. 
     The grid potential and developing bias potential (V) required for setting contrast potential B given by equation (3) can be computed based on the relationship shown in FIG.  17 . 
     In step S 53  in FIG. 7, the contrast potential is computed so that the maximum density is set 0.1 higher than the final target value, and the grid potential and developing bias potential are set to obtain the computed contrast potential. 
     {Compare Contrast Potential: Step S 54 } 
     It is checked in step S 54  if the computed contrast potential falls within a control range. If the computed contrast potential deviates from the control range, it is determined that the developer or the like is abnormal, and an error flag is set to indicate the corresponding color developer to be checked so that a service person can check it in a predetermined service mode. 
     {Correct Contrast Potential: Step S 55 } 
     In step S 55 , contrast potential B is corrected to a limit value of the control range to proceed with the control. 
     In this manner, the CPU  28  sets the grid potential and bias potential to obtain contrast potential B computed in step S 53 . 
     FIG. 20 shows the density conversion characteristics. With the maximum density control of this embodiment, which sets the maximum density to be higher than the final target value, the printer characteristics in quadrant III are converted, as indicated by solid curve J. 
     If such control is not done, printer characteristics that cannot reach 1.6, as indicated by broken curve H, may be set. In case of the characteristics indicated by broken curve H, since the LUT  25  does not have correction for increasing the maximum density independently of its setups, it is impossible to reproduce density between density D H  and the maximum density value=1.6. 
     When a target value is slightly higher than the set maximum density, as indicated by solid curve J, the density reproduction range can be reliably assured by the total tone characteristics in quadrant IV. 
     (Test Print  2 ) 
     {Output Test Print  2 : Step S 56 } 
     Subsequently, a print start button 150 of an image of test print  2  appears on the control panel, as shown in FIG.  10 A. When the operator presses this button, an image of test print  2  shown in FIG. 12 is printed out (S 56 ). During the print process, a message shown in FIG. 10B is displayed. 
     Test print  2  consists of 4 (columns)×16 (rows) gradation patches for a total of 64 gray levels, which are selected from 256 gray levels by assigning more laser output levels to a lower density range than to a higher density range. In this manner, the tone characteristics of, especially, a highlight portion can be satisfactorily adjusted. 
     Referring to FIG. 12, reference numeral  71  denotes a patch at a resolution of 200 lpi (lines/inch); and  72 , a patch at a resolution of 400 lpi (lines per inch). Images of these resolutions can be formed by preparing a plurality of different periods of triangular waves, which are used in comparison with image data to be processed in the PWM circuit  26 . 
     Note that the image processing apparatus of this embodiment forms a halftone image at a resolution of 200 lpi, and a line image such as a character or the like at a resolution of 400 lpi. Patterns at identical gray levels are output at these two different resolutions. When the tone characteristics vary considerably due to the resolution difference, different gray levels are preferably set in correspondence with the resolutions. 
     Also, test print  2  is generated by a pattern generator  29  without operating the LUT  25 . 
     {Read Test Print  2 : Step S 57 } 
     FIG. 14 is a top view of the printout of test print  2  placed on the platen glass  102 . An upper left wedge-shaped mark T is an original registration mark, and a message (FIG. 10C) is displayed on the control panel to locate a Bk pattern on the side of the registration mark T and to prevent the wrong side from facing down. In this manner, control errors resulting from wrong document positions can be prevented. 
     Upon reading the patterns by the reader portion A, a scan gradually starts from the registration mark T. When a first density gap point B is obtained, the relative coordinate positions of respective color patches are determined based on the coordinate point of the point B, thus reading the individual patches (S 57 ). 
     As shown in FIG. 18, 16 reading points (x) are set per patch ( 73  in FIG.  12 ), and the obtained signals are averaged. The number of points is preferably optimized depending on the reader and image processing apparatus. 
     {Generate and Set LUT  25 : Step S 58 } 
     FIG. 19 plots, as the output density, density values, which are obtained by converting R, G, and B signals obtained by averaging the values at 16 points in units of patches into density values by the aforementioned conversion method into optical densities, along the left ordinate, and plots the laser output level along the abscissa. 
     Furthermore, as indicated by the right ordinate, the base density of a paper sheet (0.06 in this embodiment) is normalized to level 0, and the maximum density (1.60) of the image processing apparatus of this embodiment to level 255. 
     When obtained data indicates a peculiarly high density like point C or low density like point D, such data is highly likely to be obtained due to contamination on the platen glass  102  or errors on the test pattern. For this reason, to preserve continuity of a data sequence, the slope of the curve is corrected by a limiter. More specifically, when the slope assumes 3 or more, it is fixed at 3; when it assumes a minus value, the same density level as the previous level is set. 
     The contents of the LUT  25  can be easily generated by replacing the coordinate axes, i.e., the density level shown in FIG. 19 by the input level (the density signal axis in FIG.  6 ), and the laser output level by the output level (the laser output signal axis in FIG.  6 ), as described above. The values of density levels, which do not correspond to any patch are computed by interpolation. 
     At this time, a constraint condition is set so that zero output level is obtained in response to zero input level. 
     In step S 58 , the conversion contents generated by the aforementioned process are set in the LUT  25 . 
     In this manner, the contrast potential control and generation of the γ conversion table by the first control system using the reader are completed. During the aforementioned processes, a message shown in FIG. 10D is displayed, and upon completion of the process, a message shown in FIG. 10E is displayed. 
     The control of the first control system has been explained. In this tone control, since the laser output is controlled so that an input image signal can correspond to a finally recorded image on a paper sheet as a transfer medium, very accurate control can be realized, and an output image with high tone accuracy can be obtained. However, since the transfer medium must be read, such control cannot be done so frequently. Hence, a second control system to be described below is executed a plurality of number of times between the execution timings of the first control system so as to stabilize image reproduction characteristics over a long period of time. 
     (Second Control System for Long-term Stabilization) 
     The second control system, which is executed to stabilize image reproduction characteristics obtained by the first control system over a long period of time, will be explained below. 
     FIG. 21 shows a processing circuit for processing a signal from the photosensor  40 , which comprises the LED  10  and photodiode  11  that oppose the photosensitive drum  4 . Near infrared light, which is reflected by the photosensitive drum  4  and enters the photosensor  40 , is converted thereby into an electrical signal, which is converted from an output voltage ranging from 0 to 5 V into a digital signal ranging from levels 0 to 255 by an A/D converter  41 . The digital signal is converted into a density value by the density conversion circuit  42 . 
     Note that the toners used in this embodiment are yellow, magenta, and cyan color toners, and are formed by dispersing respective color agents using a styrene-based copolymer resin as a binder. 
     The spectral characteristics of yellow, magenta, and cyan toners have 80% or higher reflectance of near infrared light (960 nm), as shown in FIGS. 22 to  24  in the order listed. Upon forming these color toner images, the two-component developing method advantageous in color purity and transmittance is used. 
     On the other hand, in this embodiment, black toner has around 10% reflectance of near infrared light (960 nm), as shown in FIG. 25, since carbon black is used as a color agent to obtain pure black, although the identical two-component developing method is used. 
     The photosensitive drum  4  is an OPC drum which has around 40% reflectance of near infrared light, and an amorphous silicon-based drum or the like may be used if its reflectance is same as that of the OPC drum. 
     FIG. 26 shows the relationship between the output from the photosensor  40  and the output image density when the density on the photosensitive drum  4  is changed stepwise by dot area modulation. 
     Assume that the output from the sensor  9  while no toner becomes attached to the photosensitive drum  4  is set at 2.5 V, i.e., level 128. 
     As can be seen from FIG. 26, the output from the photosensor  40  increases with increasing area coverage and image density of yellow, magenta, and cyan color toners. 
     On the other hand, the output from the photosensor  40  decreases with increasing area coverage and image density of black toner. 
     By preparing a table for converting sensor output signals dedicated to the individual colors into toner image densities on the photosensitive drum using the aforementioned characteristics, the toner image densities of the individual colors can be obtained with high precision. Since a change in toner image density corresponds to the final image density on a paper sheet, the second control system estimates a change in characteristics of the apparatus from a change in toner image density upon inputting identical image signals, and corrects the characteristics so that the output image density linearly corresponds to the image signal. 
     FIG. 27A is a flow chart showing the second control for setting a reference density value. This control is implemented by the CPU  28 . 
     After confirming that the LUT of the first control (automatic tone correction) is set (S 271 ), developed patches obtained by forming and developing patch patterns in units of colors (Y, M, C, Bk) on the photosensitive drum (S 272 ) are detected by the photosensor  40  (S 273 ). 
     Note that the laser output of each color patch uses a density signal (image signal) of level 96. 
     Hence, the output signal is determined based on the LUT  25  generated by the first control. For example, in case of the LUT made by using a graph shown in FIG. 28, a laser output signal of level 120 is obtained in response to an input signal of level 96. Since LUT are set in units of colors, the laser output signals are set in units of colors. 
     The laser output signals are fixed until the LUT are updated by the first control, but are not output values based on LUT determined by correction control (to be described later). 
     The sequence for forming patches on the photosensitive drum  4  is executed as shown in FIG.  29 . 
     In this embodiment, in order to efficiently obtain accurate density data within a short period of time, identical color patches are formed and measured at 180° opposing angular positions on the photosensitive drum to compensate for any decentering of the photosensitive drum, are sampled a plurality of number of times, and the sampling results are averaged. 
     By forming patches of another color to sandwich those patches, data for two colors are obtained per round. 
     In this manner, data for four colors are obtained in two rounds, and density values are obtained using the density conversion table  42   a  shown in FIG.  21 . FIG. 26 shows an example of a graph from which this density conversion table  42   a  is made. 
     The density value obtained by this density conversion table cannot be used as absolute density. This is because the photosensor does not have high resolution unlike the CCD used in the reader, and patches are not final images fixed on a paper sheet. However, a change amount in that density value can be deemed to correspond to that of the final image density. Hence, a density value obtained by the second control immediately after the first control, i.e., a toner density on the photosensitive drum obtained upon inputting an image signal of level 96 is determined to be a reference density value, a change in tone density value on the photosensitive drum from the reference density value upon executing the second control at a predetermined timing is checked, and a correction table is generated based on that change amount. Then, the generated correction table is combined with the LUT  25  obtained by the first control, and γ correction is made using the combined table. 
     In other words, since the first control guarantees an output density corresponding to an image signal in the LUT  25  immediately after that control, patches are formed using the laser output based on the LUT  25  immediately after the first control, the density values of those patches are stored, and the sensor is calibrated for any degradation of the photosensitive drum or the like using the stored density values as guaranteed reference density values. That is, a change in density value of each patch is checked using the stored value as the reference value, and the LUT  25  is corrected so that the patch density matches the reference value. In this manner, by executing the second control for making correction with reference to the LUT  25  at a predetermined timing, a change in image density characteristics over long-term use can be accurately coped with. 
     Such control will be described in more detail below. 
     As described above, the second control for acquiring a reference density is executed immediately after the first control so as to obtain a reference density value. Then, the LUT obtained by the first control is corrected on the basis of the difference between the reference density value, and the density value detected by the second control for correction, which is executed at a later, at the desired time. 
     In the image processing apparatus of this embodiment, the second control for correction is executed when the main switch of the image processing apparatus is turned on, or a predetermined period of time after the main switch is turned on, or in accordance with the outputs from temperature and humidity sensors (not shown) for detecting environmental variations (FIG.  27 B). 
     The sequence and output signal of this control are the same as those in the conditions upon acquiring the reference density. 
     When the main switch is turned on, developed patches are formed on the photosensitive drum using the LUT  25  obtained by the first control system as in step S 272  above (S 275 ). In this case, respective color patches are formed using laser outputs (obtained using the LUT  25 ) corresponding to an image signal of level 96. 
     Then, the developed patch densities are read by a patch sensor (photosensor) (S 276 ). The measured patch densities are compared with the reference patch density to compute their differences. 
     FIG. 30 is a graph for explaining a change amount of the density detected by the photosensor when patches are formed based on identical input image signals. 
     More specifically, in FIG. 30, when the reference density value is at position A, and a correction density detected when the main switch is turned on is B, the difference between the density values plotted along the ordinate corresponds to a change amount from the reference density. 
     Referring to FIG. 31, a graph a has correction characteristics which consider basic characteristics of the image processing apparatus of this embodiment, and change in the direction of an arrow in correspondence with the density change amount. In this embodiment, the correction characteristics have a peak at an image signal of level 96, and the output signal at that time is set at level 48. A correction value (0 to 48: ordinate) corresponding to an input image signal (abscissa) is obtained using the graph a, and the actual correction amount of an image signal (input signal) is obtained by computing: 
     
       
         (correction value (0 to 48))×[−density change amount)/correction characteristic peak value (48)] 
       
     
     The above formula is computed for all 256 levels of image signals, and the computation results are added to a linear graph b (input signal=output signal), thus preparing a graph c. 
     For example, when the input image signal has level 48 and the density change amount is “10”, a value along the ordinate when the value along the abscissa of the graph a is “48” is read. Assuming that the read value is “40”, it is substituted in the above formula to obtain 40×−10/48=−8.3. 
     Hence, the value in graph c is 48−8.3=39.7=about 40. A correction table which generates an output “40” in response to an input “48” is combined with the LUT  25  to prepare a single table. The correction table can be arbitrary set depending on the specifications of the apparatus. 
     As shown in FIG. 32, a new table that makes the output data (OUT) of the LUT  25  generated by the first control correspond to the input address (IN) of the correction table replaces the LUT  25  generated by the first control, and an image is formed in practice. That is, in case of the correction table and LUT  25  shown in FIG. 32, according to the new table, an output of level  14  is obtained in response to an input of level 5, and an output of level 20 in response to an input of level 8. 
     The LUT  25  generated by the first control is saved in another storage area, and every time the second control for correction is repeated to generate the correction table, the LUT  25  is read out and is combined with the correction table, thus maintaining the initial characteristics. 
     Normally, the main switch of this image processing apparatus is kept off during the night, and is turned on in the morning. Hence, the second control system is normally initiated once per day. 
     By contrast, the first control system cannot be frequently executed since it requires manual operations. 
     Thus, upon installing the image processing apparatus, a service person executes the first control system and if no problem is found in an image, the second control system automatically maintains the characteristics, at that time, for a short term. When the characteristics have gradually changed over a long term, the first control system executes calibration. In this manner, the roles of the two control systems can be distributed, and as a consequence, given tone characteristics can be maintained until the service life of the image processing apparatus expires. 
     Automatic tone correction as the first control means is executed, and a developed patch is read and is set to be a reference density of a patch sensor as the second control for acquiring a reference density on the basis of the LUT generated by the first control. 
     Then, the LUT generated by the automatic tone correction is corrected in accordance with the change amount between the reference density and a patch density value of the second control for correction executed after the second control for acquiring the reference density, thus maintaining the image density characteristics obtained by the automatic tone correction for a long period of time. Not only upon installing said image processing apparatus but also upon servicing it, a service person can execute the first control system. Further, by the user&#39;s operation, the first control system may be executed. 
     In this embodiment, the correction characteristics shown in graph a in FIG. 31 are set at values that can cope with both plus and minus density change amounts. Alternatively, independent correction characteristics respectively corresponding to the plus and minus sides can be used to optimize the control. 
     Furthermore, a plurality of correction characteristics may be prepared, and an optimal correction LUT may be selected in correspondence with the change amount, thus obtaining the same effect as above. 
     In this embodiment, an image is formed by a laser beam. Also, the present invention can be applied to an image forming apparatus using an exposure device such as an LED other than the laser. 
     In this embodiment, a reflection sensor is used. If the photosensitive drum is made of a material with high transparency, a transmission sensor may be used. 
     [Second Embodiment] 
     This embodiment will exemplify an image processing apparatus using an intermediate transfer medium. The second control in this embodiment provides a photosensor above the intermediate transfer medium to detect the developed patch density. 
     FIG. 33 shows a printer portion of the image processing apparatus of this embodiment. A rotary developer is used, and yellow, magenta, and cyan developing cartridges  3 Y,  3 M,  3 C are housed in a rotary. Each developing cartridge moves to the developing position to develop a latent image at a desired timing. A black cartridge  3 Bk is fixed in position, and the time required for rotating the rotary developer can be reduced when an image is printed using only black. 
     A toner image formed on the photosensitive drum  4  in accordance with each color image information is transferred in turn onto the intermediate transfer medium  331 . In case of a full-color image, after four-color tone images are transferred onto the intermediate transfer medium, they are simultaneously transferred onto a recording medium fed from a paper feed unit. The recording medium is exhausted outside the apparatus via a fixing process by a fixing device, thus obtaining a full-color print. 
     The charger for the photosensitive drum in the image processing apparatus of this embodiment uses a contact charging method. The applied voltages are an AC bias (constant current) for obtaining uniform charging, and a DC bias (constant voltage) for determining a charging potential. As is known, the contact charging method in which AC bias+DC bias are superposed considerably deteriorates the photosensitive drum, and especially shaves the surface layer. When a corona charger in the first embodiment is used, the shaving amount is around 1 μm after 100,000 rotations, while the contact charging method in which AC bias+DC bias are superposed in this embodiment shaves around 12 μm after 100,000 rotations. 
     Therefore, it is not preferable to form patches on the photosensitive drum in terms of long-term stabilization since variation factors upon reading patches increase. 
     On the other hand, the intermediate transfer medium suffers less deterioration factors compared to the photosensitive drum, and the tone characteristics can be more stabilized. 
     Hence, the sensor for the second control of the image processing apparatus of this embodiment is provided above the intermediate transfer medium. 
     In this embodiment as well, automatic tone correction as the first control means is executed, patches are formed on the intermediate transfer medium on the basis of the LUT generated by the first control, and developed patched are read as the second control for acquiring a reference density. Then, the LUT generated by the automatic tone correction is corrected in correspondence with the change amount between the obtained reference density of the patch sensor, and the patch density value of the second control for correction executed after that for acquiring the reference density, thus maintaining the image density characteristics obtained by the automatic tone correction for a long period of time. 
     In this embodiment, shaving of the photosensitive drum is exemplified as its change factor, but the present invention can be applied to various other change factors such as deterioration due to a discharge product, scratches formed in the cleaning process, and the like. 
     In this embodiment, patches are read on the intermediate transfer medium. Also, the present invention can be applied if an arrangement for reading developed patches is provided to another portion, e.g., a transfer belt for conveying a recording medium, or the like. 
     In this embodiment, a reflection sensor is used. If the intermediate transfer medium, transfer belt, or the like is made of a material with high transparency, a transmission sensor may be used. 
     In this embodiment, an image is formed by a laser beam. Also, the present invention can be applied to an image forming apparatus using an exposure device such as an LED other than the laser. 
     [Other Embodiments] 
     Note that the present invention may be applied to either a system constituted by a plurality of devices (e.g., a host computer, an interface device, a reader, a printer, and the like), or an apparatus consisting of a single equipment (e.g., a copying machine, a facsimile apparatus, or the like). 
     For example, when the present invention is applied to a printer, the LUT  25  may be generated using a scanner only when the first control system is executed. 
     The objects of the present invention are also achieved by supplying a storage medium, which records a program code of a software program that can implement the functions of the above mentioned embodiments to the system or apparatus, and reading out and executing the program code stored in the storage medium by a computer (or a CPU or MPU) of the system or apparatus. In this case, the program code itself read out from the storage medium implements the functions of the above mentioned embodiments, and the storage medium, which stores the program code constitutes the present invention. The functions of the above mentioned embodiments may be implemented not only by executing the readout program code by the computer but also by some or all of actual processing operations executed by an OS (operating system) running on the computer on the basis of an instruction of the program code. 
     Furthermore, the functions of the above mentioned embodiments may be implemented by some or all of actual processing operations executed by a CPU or the like arranged in a function extension board or a function extension unit, which is inserted in or connected to the computer, after the program code read out from the storage medium is written in a memory of the extension board or unit. 
     When the present invention is applied to the storage medium, the storage medium stores the program codes corresponding to the aforementioned flow charts (shown in FIG.  27 A and/or FIG.  27 B). 
     According to the present invention, an image processing apparatus which can stably maintain high precision image density characteristics for a long period of time, and its control method can be provided. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.