JP2007148262A - Image forming method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately evaluate the recording properties of each recording element, and to improve image quality. <P>SOLUTION: A plurality of recording elements are arrayed on a print head 10, and each recording element has different recording properties. The recording quantity of each recording element is expressed by a model formula including the sum of b-th term (b is real number) and the constant term of an input signal value, then, the model formula is calculated. After calculating the constant term based on information read from the image recorded related to two or more kinds of input signal values, the b-th term of the input signal value is calculated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming method and an image forming apparatus for forming an image with a plurality of recording elements.

  In recent years, with the widespread use of digital cameras, it has been highly desired to improve the performance of digital minilab machines as digital output devices, such as the printing capability and image quality. In particular, there is a high demand for large-format printing, and development of an exposure engine using a print head in which a plurality of recording elements suitable for this is arranged in an array is progressing.

In general, the light-emitting recording elements constituting the array-shaped print head have a variation of about 20% to 40% in individual recording characteristics. If the variation correction is insufficient, the variation is recorded as it is when creating a print as unevenness in the density of the image. When reproducing a photograph or the like with continuous gradation, it is necessary to correct the variation to at least 2% or less, and to obtain a higher quality, 1% or less. As a technique relating to this correction, a technique has been proposed in which the density of a correction image is measured, density data corresponding to each recording element is specified, and a correction amount is calculated for each recording element (for example, Patent Document 1, Patent). Reference 2).
JP 2004-299109 A JP 2005-246925 A

  However, there are cases where the tendency differs depending on the density whether the density output from each recording element is higher or lower than the other recording elements. FIG. 22 shows an output result after shading correction in the prior art. In FIG. 22, the horizontal axis indicates the pixel position, and the vertical axis indicates the reflection density. Each pixel corresponds to each recording element. In the example of FIG. 22, the density is uniform at an intermediate density (0.8 to 1.0), but shading unevenness occurs at high density and low density. As described above, even when correction is performed so that the recording amount of each recording element becomes uniform at a certain input signal value, a uniform recording amount may not be obtained with other input signal values.

  Therefore, in order to uniformly correct the recording amount of each recording element, it is necessary to accurately evaluate the recording characteristics for each recording element and make corrections so as to match the target recording characteristics. Further, since the recording amount of the recording element does not become 0 with respect to the input signal value 0 and there may be an offset component, this offset component must also be evaluated.

  For example, in an image forming apparatus using a light-emitting recording element, a method of measuring the exposure amount as an output of the recording element for all input gradations with an optical sensor and correcting the recording characteristics of the recording element based on the measurement result is also conceivable. However, it is not practical to actually measure the exposure amount for all input gradations. In addition, the measurement of the exposure amount requires a special measuring machine that directly measures the exposure amount, and thus manufacturing costs are increased.

  The present invention has been made in view of the above-described problems in the prior art, and provides an image forming method and an image forming apparatus capable of accurately evaluating the recording characteristics of each recording element and improving the image quality. Is an issue.

  In order to solve the above-described problem, the invention according to claim 1 converts the input signal value into the output signal value based on the correction coefficient, and uses the print head in which a plurality of recording elements are arranged in an array. A correction image is recorded on the recording material based on the output signal value, read information is acquired from the correction image by the image reading unit, a correction amount of the recording characteristic of each recording element is calculated based on the read information, In the image forming method of adjusting the correction coefficient based on the correction amount and forming an image using the adjusted correction coefficient, the recording amount of each recording element is a b-order term of the input signal value. (B is a real number) and a model expression including a sum of constant terms, and the model expression is calculated.

  According to a second aspect of the present invention, in the image forming method according to the first aspect, the model formula is calculated based on the read information.

  According to a third aspect of the present invention, in the image forming method according to the first or second aspect, the correction image has a density information acquisition region including a region recorded for two or more types of input signal values. The constant term is calculated based on reading information of the density information acquisition region.

  According to a fourth aspect of the present invention, in the image forming method according to any one of the first to third aspects, the b-th term of the input signal value is calculated after the constant term is calculated. And

  According to a fifth aspect of the present invention, in the image forming method according to any one of the first to fourth aspects, a temporary model formula obtained by removing the constant term from the model formula is calculated, and the temporary model formula is calculated. The second input signal calculated based on the temporary model equation using the ratio of the recording amount at the first input signal value calculated based on the target recording amount at the first input signal value. A virtual recording amount in the second input signal value is calculated from a recording amount in the value, and the constant term is calculated from a difference between the virtual recording amount and a target recording amount in the second input signal value. To do.

  According to a sixth aspect of the present invention, in the image forming method according to any one of the first to fifth aspects, the model formula is calculated using regression analysis.

  According to a seventh aspect of the present invention, in the image forming method according to any one of the first to sixth aspects, the model formula is calculated based on a variation amount of image read information corresponding to a specific input signal value. It is characterized by determining whether to calculate.

  According to an eighth aspect of the present invention, in the image forming method according to any one of the first to seventh aspects, the read information is read information corresponding to a plurality of different densities.

  According to a ninth aspect of the present invention, in the image forming method according to any one of the first to eighth aspects, the print head is a PLZT print head.

  According to a tenth aspect of the present invention, there is provided a print head control unit that converts an input signal value into an output signal value based on a correction coefficient, and a print head in which a plurality of recording elements are arranged in an array, and the output signal value A print head that records a correction image on a recording material based on the image, an image reading unit that acquires read information from the correction image, and a correction amount that calculates a correction amount of a recording characteristic of each recording element based on the read information In the image forming apparatus that includes a calculation unit and a correction information calculation unit that adjusts the correction coefficient based on the correction amount, and that forms an image using the adjusted correction coefficient, the recording of each recording element The quantity is represented by a model expression including a sum of a b-order term (b is a real number) and a constant term of the input signal value, and the correction amount calculation unit calculates the model expression.

  According to an eleventh aspect of the present invention, in the image forming apparatus according to the tenth aspect, the correction amount calculation unit calculates the model formula based on the read information.

  According to a twelfth aspect of the present invention, in the image forming apparatus according to the tenth or eleventh aspect, the correction image has a density information acquisition region including a region recorded for two or more types of input signal values. The correction amount calculation unit calculates the constant term based on reading information of the density information acquisition region.

  According to a thirteenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to twelfth aspects, the correction amount calculating unit calculates the constant term and then calculates the b-th order of the input signal value. A term is calculated.

  According to a fourteenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to thirteenth aspects, the correction amount calculation unit calculates a provisional model formula obtained by removing the constant term from the model formula. Then, using the ratio between the recording amount at the first input signal value calculated based on the temporary model formula and the target recording amount at the first input signal value, the calculation is performed based on the temporary model formula. The virtual recording amount at the second input signal value is calculated from the recorded recording amount at the second input signal value, and the constant term is calculated from the difference between the virtual recording amount and the target recording amount at the second input signal value. Is calculated.

  According to a fifteenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to fourteenth aspects, the correction amount calculation unit calculates the model formula using regression analysis. .

  According to a sixteenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to fifteenth aspects, the correction amount calculating unit adjusts the amount of variation in the read information of the image corresponding to the specific input signal value. Based on this, it is determined whether to calculate the model formula.

  According to a seventeenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to sixteenth aspects, the read information is read information corresponding to a plurality of different densities.

  According to an eighteenth aspect of the present invention, in the image forming apparatus according to any one of the tenth to seventeenth aspects, the print head is a PLZT print head.

  According to the first and tenth aspects of the present invention, by calculating the model formula for each recording element, it is possible to accurately evaluate the recording characteristics of each recording element, and image quality can be achieved even at low and high densities. Can be improved.

  According to the second and eleventh aspects of the present invention, since the model formula is calculated based on the read information of the correction image actually output, it is possible to grasp the variation state of each recording element of the print head. , The accuracy of correction can be improved.

  According to the third and twelfth aspects of the present invention, since the constant term is calculated based on the read information corresponding to a plurality of input signal values, it is difficult to be influenced by noise and can be corrected with high accuracy.

  According to the inventions of claims 4 and 13, since the b-th term of the input signal value is calculated as accurately as possible after calculating the constant term portion that is difficult to actually measure, the accuracy of correction is improved. Can do.

  According to the inventions described in claims 5 and 14, the constant term is calculated from the difference between the virtual recording amount at the second input signal value and the target recording amount at the second input signal value. A term can be calculated.

  According to the sixth and fifteenth aspects of the present invention, it is possible to accurately calculate the model formula even when a small amount of dust or dust is present on the correction image. In addition, it is possible to reduce adverse effects on correction due to measurement variations in the read information.

  According to the seventh and sixteenth aspects of the present invention, when there is a large amount of variation in the read information, there is a possibility that dust or dust may be present on the image, so that inaccurate correction can be prevented.

  According to the eighth and seventeenth aspects of the present invention, since the number of read information data increases, the correction accuracy can be improved. In addition, a plurality of different concentrations include a high concentration (for example, 1.5 to 1.7) and a low concentration (for example, 0.3 to 0.5). High-quality performance can be realized.

  According to the ninth and 18th aspects of the present invention, in a PLZT print head in which the difference in recording characteristics between recording elements is relatively large, the recording characteristics of each recording element can be accurately evaluated and the image quality can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples.

FIG. 1 shows a schematic configuration of an image forming apparatus 1 according to an embodiment of the present invention.
As shown in FIG. 1, the image forming apparatus 1 includes a print head 10, an image control unit 20, and an image reading unit 30.

  The print head 10 forms a latent image by emitting light to the recording material 40 conveyed in the recording material conveyance direction E. A developing device (not shown) is provided at the conveyance destination of the recording material 40.

FIG. 2 is a schematic diagram illustrating the configuration of the print head 10.
As shown in FIG. 2, the print head 10 includes an LED light source 11, a light guide 12, an integrator 13, a PLZT (Plomb Lanthanum Zirconate Titanate) shutter array 14, a SELFOC (registered trademark) lens array 15, and the like. Print head.

  The LED light source 11 is a surface light source having three RBG colors and can be lit independently. The light guide 12 converts the irradiation shape of the surface light source into one dimension. The integrator 13 mixes the light emitted from the light guide 12 and suppresses unevenness in the light amount of the light source.

  The PLZT shutter array 14 is configured by arranging a plurality of recording elements 16 in an array, and has an optical shutter function of transmitting or not transmitting light. The “print head in which a plurality of recording elements are arranged in an array” described in the claims can be any one in which a plurality of recording elements are arranged in one or a plurality of rows at a predetermined interval in order to obtain a desired resolution. Good. As a preferable example of a print head in which a plurality of recording elements are arranged in an array, in addition to the PLZT print head described in this embodiment, an LED light emitting element or a vacuum fluorescent tube is arranged, a liquid crystal shutter array print head, or the like Examples include an optical shutter array, a semiconductor laser arrayed in an array, a thermal head, a light emitting element utilizing an electroluminescence phenomenon such as an organic EL material, and the like.

  As shown in FIG. 3, in the PLZT shutter array 14, the recording elements 16 are arranged in a staggered manner in two rows in a direction perpendicular to the recording material conveyance direction E, and one line image is formed in the main scanning direction F in two rows. To do. Although FIG. 3 shows an example of a staggered arrangement, it may be arranged in a straight line. In FIG. 2, the recording elements 16 are arranged in an array in the depth direction of the drawing. The polarizers 17 and 18 shown in FIG. 2 are arranged in a crossed Nicols relationship. When a predetermined voltage is applied to each recording element 16, the polarization direction of incident light changes and the light is transmitted through the output-side polarizer 18. The light to be controlled can be controlled. The recording amount (exposure amount) of the recording element 16 is controlled by a pulse width modulation (PWM) method. The recording amount means an output amount when the print head 10 performs recording on the recording material 40. For example, when the print head 10 is a photographic photosensitive material, the recording amount corresponds to an exposure amount. If it is a system, it corresponds to the ink discharge amount.

  FIG. 4 shows how the recording amount varies with time of the recording element 16. The horizontal axis represents time (input signal value), and the vertical axis represents the recording amount. A graph G1 shown in FIG. 4 is an ideal light emission waveform, and a graph G2 is an actual light emission waveform of the recording element 16. The offset component in the graph G2 in FIG. 4 corresponds to light (leakage light) emitted even when the shutter is closed, and depends on the performance of the polarizers 17 and 18, the performance of the shutter itself, and the like. Further, the graph G2 has a more gradual rise and fall than the graph G1, and has a nonlinear response characteristic.

  The SELFOC lens array 15 is obtained by integrating a plurality of optically equivalent SELFOC lenses in parallel with each other, and forms an image of the light transmitted through the PLZT shutter array 14 on the recording material 40.

  The image control unit 20 performs various image processing and correction processing on the image data to be output in the image forming apparatus 1 and outputs the image data to the print head 10. The detailed configuration of the image control unit 20 will be described later.

  The image reading unit 30 includes a light source, a CCD (Charge Coupled Device), an A / D converter (all not shown), and the like. The image reading unit 30 irradiates a document placed on a document table (not shown) with light from a light source, and converts the reflected light into an electrical signal (analog signal) by a CCD, so that each of the three RGB color components is converted. Read information is acquired. The acquired read information is converted into digital data by an A / D converter and sent to the image control unit 20.

  The read information means information indicating optical density (also simply referred to as density) read by an arbitrary image reading apparatus or density measuring means, or numerical information calculated based on the optical density. In addition, the optical density itself may be used, but it may be a reflectance, a transmittance, a light absorptivity, etc., or a function value corresponding to these one-to-one, such as a logarithmic value, etc. It may be a statistic. The density is preferable as read information because the linearity is relatively high and the feedback of the correction result is more accurate. In addition, since the reading information corresponding to the density can obtain the same effect as the density, when the image is read by an image reading device such as a flatbed scanner, the signal value acquired by the image reading device It may be a function value that has a one-to-one correspondence with this signal value, such as a logarithmic value, or a relative value thereof.

  As the recording material 40, color photographic printing paper such as a silver halide photosensitive material is used. The recording material 40 has a layer structure in which a coloring layer that develops each color forms a layer structure, and a layer that develops cyan by exposure to red light (R), and green light (G) that is exposed. A layer that develops color on magenta and a layer that develops yellow when exposed to blue light (B) are provided.

FIG. 5 is a block diagram illustrating a functional configuration of the image forming apparatus 1.
As shown in FIG. 5, the image control unit 20 includes an image processing unit 21, a media adjustment LUT (Look Up Table) 22, a correction information recording unit 23, a print head control unit 24, a correction amount calculation unit 25, and a correction information calculation. Part 26 is provided.

  The image processing unit 21 converts various types of digital image data such as image data of a digital camera and image data of a negative film read image into a common image input signal value.

  The media adjustment LUT 22 is an LUT for eliminating a difference in output that differs for each media type, and converts an image input signal value into an input signal value x. The input signal value is converted appropriately by various LUTs to eliminate the difference depending on the media type for the image data of the digital camera and the image data of the image read from the negative film (or image input signal value). Image data.

  The correction information recording unit 23 includes an input signal section information table 231 as data for making the recording amount uniform for each recording element n (n: recording element number corresponding to each recording element) and for each RGB color. A correction coefficient information table 232 is recorded.

  The input signal section information table 231 is a table in which the input signal value x and the input signal section k are associated with each other. The input signal value x is divided into a plurality of sections, and a number is assigned to each section. FIG. 6A is a graph showing the number of the input signal section k with respect to the input signal value x, and FIG. 6B is an example of the input signal section information table 231. In FIG. 6B, the input signal section k for the input signal value x is associated with each RGB. Note that the number of intervals into which the input signal value x is divided and the interval of one interval may be different for each RGB.

  The correction coefficient information table 232 is a table for recording correction coefficients α (k, n) and β (k, n) for each recording element n, for each input signal section k, and for each RGB. The correction coefficient is a coefficient for adjusting the recording amount of each recording element 16 so that each recording element 16 of the print head 10 can record on the recording material 40 with a uniform recording amount. As will be described in the present embodiment, it is preferable that each of the recording elements n has a correction coefficient for each of a plurality of different densities (the original recording amount and input signal value). FIG. 7 shows an example of the correction coefficient information table 232. The initial value of the correction coefficient α (k, n) is set to 1, and the initial value of the correction coefficient β (k, n) is set to 0.

The print head controller 24 converts the input signal value x into the output signal value x ′ based on the correction coefficients α and β as shown in the following conversion equation (1), and outputs the output signal value x ′ to the print head 10. The output signal value refers to data transferred to the print head 10 for recording on the recording material 40.

  Processing in the correction information recording unit 23 and the print head control unit 24 will be described with reference to FIG. When correcting the input signal value x for the recording element n, first, the input signal interval k corresponding to the input signal value x is obtained based on the input signal interval information table 231. Next, based on the correction coefficient information table 232, correction coefficients α (k, n) and β (k, n) corresponding to the recording element n and the input signal interval k are obtained. Next, the output signal value x ′ of the printing element n is calculated by multiplying the correction coefficient α (k, n) and the input signal value x, and further adding the correction coefficient β (k, n), and printing. Output to the head 10. FIG. 9 shows conversion from the input signal value x to the output signal value x ′ based on the correction coefficients α (k, n) and β (k, n) for each recording element n and for each input signal section k. In the calculation, it is set to 0 when the output signal value x ′ becomes a negative value.

  The print head 10 records the correction image 41 on the recording material 40 with the recording amount (exposure amount) z based on the output signal value x ′. FIG. 10 shows an example of the correction image 41. As illustrated in FIG. 10, the correction image 41 includes a density information acquisition region 411, a recording element position information acquisition region 412, and a recording direction information acquisition region 413.

  The density information acquisition region 411 preferably has two or more types of regions for acquiring density information corresponding to a plurality of different input signal values x. The density information acquisition region 411 preferably includes a plurality of different densities. Here, the different density means that there is a density difference with respect to the recording material conveyance direction, and the difference in the recording element arrangement direction is treated separately as density unevenness. Further, it is preferable that there is a difference of about 0.1 or more as different concentrations, and it is preferable that the difference in density between the maximum density region and the minimum density region is 1.0 or more. By recording using different input signal values, density information acquisition regions having different densities may be formed. In the density information acquisition area 411, the RGB exposure colors (basic colors) are recorded at the same location, and the cyan color component (R exposure), the magenta color component (G exposure), and the yellow color component, which are pigments of the basic colors, are recorded. An image in which (B exposure) is colored, that is, a so-called gray image is preferable.

  The density information acquisition area 411 preferably includes the density of the linear portion of the characteristic curve of the photosensitive material used as the recording material 40. FIG. 11 shows a characteristic curve of the photosensitive material. The horizontal axis represents the logarithm of the recording amount, and the vertical axis represents the image density with respect to the recording amount. The straight line portion of the characteristic curve is a portion where the change in density (the slope of the graph) with respect to the change in logarithm of the recording amount is constant as shown in the H part of FIG. In the linear portion of the characteristic curve, since the difference in density with respect to the change in recording amount is large, the accuracy of correction is improved.

  The recording element position information acquisition area 412 is an area for specifying the position of each recording element 16 and is an image recorded at a predetermined interval in the recording element array direction.

  The recording direction information acquisition area 413 is a part that reads information for determining the recording element array direction and the recording material conveyance direction. For example, by recording the recording direction information acquisition area 413 in different colors (cyan and magenta, etc.) at different positions on the correction image 41, the upper recording direction information acquisition area 413 in FIG. 10 is cyan and the lower recording direction. If the information acquisition area 413 is magenta, it is determined that the elements are arranged in ascending order from the recording element number 1 from left to right in the horizontal direction. Conversely, the upper recording direction information acquisition area 413 is magenta and the lower recording direction information acquisition area. If 413 is cyan, it is determined that the printing elements are arranged in ascending order from the recording element number 1 from the right to the left.

  The correction amount calculation unit 25 illustrated in FIG. 5 is based on the read information acquired from the correction image 41 by the image reading unit 30, that is, the relationship between the input signal value x and the recording amount z of the recording element 16, that is, the recording element. Sixteen recording characteristics are evaluated, and correction amounts Δs and Δγ for making the characteristics uniform are calculated. Here, the correction amount means a correction amount for adjusting the recording amount of each recording element 16 so that each recording element 16 of the print head 10 can record on the recording material 40 with a uniform recording amount. Such adjustment is preferably performed for each recording element 16.

  The correction information calculation unit 26 adjusts the correction coefficients α and β based on the correction amounts Δs and Δγ calculated by the correction amount calculation unit 25. Specifically, the correction information calculation unit 26 calculates new correction coefficients α and β from the current correction coefficients α and β based on the correction amounts Δs and Δγ, and records them in the correction information recording unit 23. Calculation of new correction coefficients α and β from the current correction coefficients α and β based on the correction amounts Δs and Δγ is called correction coefficient adjustment.

Next, the operation of the image forming apparatus 1 will be described.
FIG. 12 is a flowchart showing recording characteristic correction processing.

  First, the image data of the correction image is converted into an image input signal value by the image processing unit 21, and the image input signal value is converted into an input signal value x by the media adjustment LUT 22 (step S1).

  Next, the correction coefficients α (k, n) and β (k, n) recorded in the correction information recording unit 23 are recorded by the print head control unit 24 for each recording element n, for each input signal section k, and for each RGB. The input signal value x is converted into the output signal value x ′ based on the above, and the correction image 41 is recorded based on the output signal value x ′ by the print head 10, developed by the developing device, and output (step S2). ).

  Next, the correction image 41 is measured by the image reading unit 30, and reading information is acquired for each of RGB (step S3). When the correction image 41 is read by the image reading unit 30, it is preferable to fix the correction image 41 using a pressing member. The pressing member preferably has a substantially uniform concentration, such as black, and is preferably made of a soft material such as rubber or sponge that can be easily bent. Moreover, it is preferable that the material is low in chargeability and hardly adheres to dust. By using the pressing member, the floating of the correction image 41 due to curling or the like is reduced, and the accuracy of the edge determination of the image is improved.

  Here, the inclination of the correction image 41 is determined based on the reading information acquired by the image reading unit 30, and the reading information of the correction image 41 is rotated according to the inclination. Specifically, markers serving as reference for inclination determination are recorded at a plurality of different positions in the correction image 41, and the inclination of the correction image 41 is determined based on the marker position. Since the inclination is determined using the marker, the rotation process can be easily performed. Further, by performing the rotation process, the operability when the correction image 41 is set in the image reading unit 30 is improved, and the correction accuracy is improved.

  Next, the variation amount of the read information corresponding to the specific input signal value is compared with a predetermined threshold value (step S4). For example, individual reading information corresponding to a specific input signal value may be compared with a threshold value, or an average value may be calculated from individual reading information corresponding to a specific input signal value, and the average value and each reading value may be calculated. Differences from information may be calculated, and each difference may be compared with a threshold value. If the variation amount of the read information is large, dust or dust may be present in the correction image 41. Therefore, if the variation amount of the read information is larger than the threshold (step S4; YES), the correction calculation is stopped. (Step S5), the process proceeds to Step S11. Furthermore, in order to more accurately determine whether to cancel correction calculation, the size of variation may be determined by appropriately combining a plurality of threshold values, or a plurality of threshold values may be provided for each RGB color.

  In step S4, when the variation amount of the read information is equal to or less than the threshold value (step S4; NO), the correction amount calculation unit 25 uses the model formula indicating the recording characteristics of the recording element n for each recording element n, that is, A relational expression between the input signal value x and the recording amount z is calculated (step S6).

ここで、ｘは入力信号値、ｚは記録量、ａ(ｎ)は記録量に対する入力信号値の重み係数、ｂ(ｎ)は記録特性の非線形性を表すパラメータ、ｃ(ｎ)は記録量のオフセット成分（定数項）である。 The recording characteristic model formula calculation process will be described with reference to FIG. The recording characteristic model formula calculation process is a process of calculating characteristic parameters a (n), b (n), and c (n) in the model formula indicating the recording characteristics of each recording element n shown in the following formula (2) ( a (n), b (n), c (n) are real numbers).

Here, x is an input signal value, z is a recording amount, a (n) is a weighting factor of the input signal value with respect to the recording amount, b (n) is a parameter indicating nonlinearity of recording characteristics, and c (n) is a recording amount. Offset component (constant term).

As shown in FIG. 13, first, the density y (x, n) is calculated from the read information corresponding to each recording element n and each input signal value x (step S21). Then, the average density y _ave (x) in the recording element array direction of the density y (x, n) at each input signal value x is calculated (step S22). For example, as shown in FIG. 14, the average of the concentrations corresponding to the input signal value x ₁ ..., Y (x ₁ , n−1), y (x ₁ , n), y (x ₁ , n + 1),. The average density is y _ave (x ₁ ).

Next, as shown in FIG. 15, the average density y _ave (x) is plotted against the logarithmic input signal value log (x). Then, the measured values are interpolated, and the measured values are extrapolated outside the range, and the average recording characteristic log (x) −y _ave (x) of the recording element 16 is obtained. In this average recording characteristic log (x) −y _ave (x), y _ave (x) is redefined as y and log (x) is defined as log (z), and z is an index of the recording amount. In this way, a density y-log recording amount log (z) table is created (step S23).

  Next, based on the relationship of density y−logarithmic recording amount log (z), the density y (x, n) output based on the input signal value x in the recording element n becomes the logarithmic recording amount log (z (x, x, n)) (step S24). Then, for each recording element n, the input signal value x corresponding to the same density and the recording amount z are associated, and a log (x) -log (z (x, n)) table is created (step S25). ).

Next, a target recording characteristic z = z ₀ (x) is calculated (step S26). FIG. 16 shows an example when the target recording characteristic is z ₀ (x) = x.

Next, the reference input signal value x ₀ is calculated (step S27). The reference input signal value x ₀ is an input signal value necessary for the recorded density to be a predetermined density (reference density). The reference density is preferably set to a density at which unevenness is conspicuous, that is, a density that is most sensitive to human eyes (for example, a density of 0.8 to 1.0). Specifically, the average value of the input signal value x, which records a corresponding concentration stage reference density of the correction image 41 is calculated as the reference input signal value x _0. Reference input signal value x ₀ is calculated for each RGB color. It should be noted that the type of recording material 40, variations in lot, the sensitivity is changed by the variation of the development conditions, changes the reference input signal value x ₀ for obtaining a reference density by the state. Therefore, the media adjustment LUT 22 is adjusted in advance so that a reference density is obtained when a predetermined reference image input signal value is input to the media adjustment LUT 22.

ここで、ｘは入力信号値、ｚは記録量、ａ’(ｎ)は記録量に対する入力信号値の重み係数、ｂ’(ｎ)は記録特性の非線形性を表すパラメータである。 Next, as shown in FIG. 16, the relationship of log (x) −log (z (x, n)) associated in step S25 is the temporary value obtained by removing the constant term c (n) from equation (2). A ′ (n) and b ′ (n) are obtained by regression analysis as expressed by the model formula (the following formula (3)) (a ′ (n) and b ′ (n) are real numbers). The recording amount z (x, n) is obtained as a function of the input signal value x and the recording element n. Then, the recording amount z (x ₀ , n) at the reference input signal value x ₀ of each recording element n is calculated (step S28).

Here, x is an input signal value, z is a recording amount, a ′ (n) is a weighting factor of the input signal value with respect to the recording amount, and b ′ (n) is a parameter representing nonlinearity of the recording characteristics.

Next, as shown in the following formula (4), and recording the amount of z (x _0, n) in the reference input signal value x ₀ for each recording element n calculated in step S28, the target in the reference input signal value x ₀ From the recording amount z ₀ (x ₀ ), the correction amount d (n) for the reference input signal value x ₀ of each recording element n is calculated (step S29).

Next, as shown in FIG. 17, the input signal value x _L (x _L <x ₀ ) of the recording element n is based on log (x) −log (z (x, n)) associated in step S25. recording the amount of z (x _L in), n) is calculated, the recording amount z (x _L, and n), based on the correction amount d (n), as shown in the following formula (5), the input signal The virtual recording amount z ′ (x _L , n) at the value x _L is calculated.

Then, as shown in the following formula (6), from the difference between the virtual recording amount z ′ (x _L , n) and the target recording amount z ₀ (x _L ) at the input signal value x _L , A constant term c (n), which is an offset component of the recording amount z, is calculated (step S30).

Next, for each recording element n, assuming that the data set of log (x) and log (z−c (n)) conforms to the following equation (7) obtained by modifying the model equation (2), the least square method ( By regression analysis, parameters a (n) and b (n) indicating recording characteristics are calculated (step S31).

Next, returning to FIG. 12, the input signal interval k (x) is calculated (step S7). With reference to FIG. 18, a method of calculating the input signal interval k (x) will be described. As shown in FIG. 18, the measured input signal values are {x ₁ , x ₂ ,..., X _t } (x ₁ <x ₂ <... <x _t ), and the input signal values x ₁ to x _t are equally spaced. , (P− ₁ (= x ₁ ), p ₂ ,..., P _u (= x _t )}). {P ₁ , p ₂ ,..., P _u } obtained by equally dividing the input signal value in this measurement range, the minimum input signal value 0, the maximum input signal value x _max , and the reference input signal value x ₀ are rearranged in ascending order. .
0 <p ₁ <... <p _j <x ₀ <p _{j + 1} ... <p _u <x _max
Then, the input signal interval k (x) is re-divided using these values as boundary values of the intervals.

  When the type of the recording material 40 to be output changes, the media adjustment LUT 22 is changed so that the same output density can be obtained even with different recording materials 40. Therefore, the image input signal value (image data of the correction image) is fixed, but the input signal value x output from the media adjustment LUT 22 is changed. The input signal interval k (x) is appropriately changed depending on the range of the input signal value x used at the time of correction.

FIG. 19 shows the recording amount z = z (x, n) and the target recording characteristic z = z ₀ (x) of the recording element n. The recording amount z = z (x, n) of the recording element n in FIG. 19 is an equation (2) including c (n) calculated in step S30 and a (n) and b (n) calculated in step S31. ). The minimum input signal value of the input signal section k (x) is x _k0 , the maximum input signal value of the input signal section k (x) is x _k1, and in each input signal section k (x), according to the following equation (8): Correction amounts Δs (k, n) and Δγ (k, n) are calculated (step S8).

  Then, the calculated correction amounts Δs (k, n), Δγ (k, n) and the input signal interval k (x) are recorded (step S9). The input signal section k (x) is recorded in the input signal section information table 231.

Next, the correction information calculation unit 26 performs correction coefficient update processing (step S10).
The correction coefficient update process will be described with reference to FIG.

  As shown in FIG. 20, first, it is determined whether or not the input signal interval k (x) has been changed (step S41). If the input signal section k (x) has not been changed (step S41; NO), the current correction coefficients α and β are read from the correction information recording unit 23 (step S42).

On the other hand, when the input signal interval k (x) is changed (step S41; YES), correction coefficients α and β based on the new input signal interval k (x) are calculated (step S43). As shown in FIG. 21, the case where the original input signal sections k−1 and k are changed to a new input signal section k ′ will be described as an example. The correction coefficients in the original input signal section k−1 are α (k−1, n) and β (k−1, n), and the correction coefficients in the original input signal section k are α (k, n) and β (k , N), p ₁ , p ₂ , and p ₃ are the boundary values of the input signal value x in the original input signal interval k−1, k, and q _{1 is} the boundary value of the input signal value x in the new input signal interval k ′. , Q ₂ , output signal values x ′ ₁ , x ′ _{2 at} q ₁ , q ₂ are obtained by the following equation (9).

In FIG. 21, a straight line passing through two points (q ₁ , x ′ ₁ ) and (q ₂ , x ′ ₂ ) is a conversion formula in a new input signal section k ′. Therefore, the correction coefficients α (k ′, n) and β (k ′, n) in the new input signal section k ′ are obtained by the following equation (10).

Next, a new correction coefficient α (k, n), based on correction amounts Δs (k, n) and Δγ (k, n) for each input signal section k (x) according to the following equation (11). β (k, n) is calculated (step S44) and recorded in the correction coefficient information table 232 of the correction information recording unit 23 (step S45).

In the equation (11), α (k, n) and β (k, n) on the left side are new correction coefficients, and α (k, n) and β (k, n) on the right side are new correction coefficients. It is a correction coefficient before being applied. Further, x _k0 is the minimum input signal value in the input signal section k (x), and x _k1 is the maximum input signal value in the input signal section k (x).

  Next, returning to FIG. 12, when the correction is repeated without completing the correction (step S11; NO), the process returns to step S1.

  In step S11, when the correction is finished (step S11; YES), the recording characteristic correction process is finished. After the recording characteristic correction process, image formation is performed using the correction coefficients α (k, n) and β (k, n) adjusted by the recording characteristic correction process.

In the course of repeating the recording characteristic correction process, if the correction coefficients α and β diverge (do not converge), the correction coefficient initialization process is performed. In the initialization process of the correction coefficient, first, the correction coefficients α and β for each input signal section k are discarded, and all input signal values x are set based on the reference input signal value x ₀ and its output signal value x ′ _0. On the other hand, the same correction coefficient α = x ′ ₀ / x ₀ , β = 0 is set. As a result, only the recording amount at the reference input signal value x ₀ can be made uniform, and the amount of work is reduced compared to the case where the recording characteristic correction process is performed again from the beginning.

  As described above, according to the image forming apparatus 1, by calculating the model formula for each recording element 16, it is possible to accurately evaluate the recording characteristics of each recording element 16, and low density or high density. The image quality can also be improved. In addition, since the model equation includes a constant term c (n) corresponding to leakage light, it is more effective when the amount of leakage light increases due to an increase in recording speed.

  Further, since the model formula is calculated based on the read information corresponding to the plurality of densities of the correction image 41 that is actually output, it is possible to grasp the variation state of each recording element 16 of the print head 10 and correct the correction. Accuracy can be improved. Here, in order to improve the image quality even in the high density region and the low density region, a high density (for example, 1.5 to 1.7) and a low density (for example, 0.3 to 1.7) among a plurality of different densities. 0.5) is preferably included. In addition, since the constant term c (n) is calculated based on the read information corresponding to a plurality of input signal values, it is difficult to be affected by noise and correction can be performed with high accuracy. In addition, the apparatus can be miniaturized as compared with a method in which the recording amount (exposure amount) of the recording element is measured by an optical sensor or the like to evaluate the recording characteristics.

  In addition, since the model formula is calculated using regression analysis, the model formula can be accurately calculated even when a slight amount of dust or dust is present on the correction image. In addition, it is possible to reduce adverse effects on correction due to measurement variations in the read information.

  Further, since it is determined whether to calculate the model formula (whether the correction calculation is continued or stopped) based on the variation amount of the reading information of the image corresponding to the specific input signal value, the variation amount of the reading information If the image is large, that is, if there is a possibility that large dust or dust may be present on the image, inaccurate correction can be prevented.

The image reading device used in the present invention is preferably a device having a line-shaped CCD and reading an image as the CCD on the line scans, and includes various scanners such as a flatbed scanner and a drum scanner. .
Further, when acquiring the reading information of the correction image using the image reading apparatus, it is preferable to read the correction image at a higher resolution than the resolution for recording on the recording material using the arrayed print head. .

  In addition, the recording characteristic correction process may be performed on the entire print head area, or in order to shorten the correction time, only a part to be used or only a part where the density unevenness is conspicuous is applied to a part of the print head. You may do it for.

  The description in the above embodiment is an example of the image forming apparatus according to the present invention, and the present invention is not limited to this. The detailed configuration and detailed operation of each part of the image forming apparatus can be changed as appropriate without departing from the spirit of the present invention.

  For example, in the above embodiment, the recording amount z of the recording element n is represented by the sum of the b-order term and the constant term of the input signal value x as shown in Expression (2). A plurality of b-order terms of the signal value x may be included.

1 is a schematic configuration diagram of an image forming apparatus 1 in an embodiment of the present invention. 2 is a schematic diagram illustrating a configuration of a print head 10. FIG. 3 is a diagram illustrating an arrangement state of recording elements 16. FIG. FIG. 6 is a diagram illustrating a change in recording amount with respect to time of a recording element. 2 is a block diagram illustrating a functional configuration of the image forming apparatus 1. FIG. (A) is a graph representing the number of the input signal section k with respect to the input signal value x. (B) is an example of the input signal section information table 231. 4 is an example of a correction coefficient information table 232; FIG. 5 is a diagram for explaining processing in a correction information recording unit and a print head control unit. It is a figure which shows the conversion from the input signal value x based on the correction coefficients (alpha) and (beta) to the output signal value x '. It is a figure which shows the example of the image 41 for a correction | amendment. It is a figure which shows the characteristic curve of a photosensitive material. It is a flowchart which shows a recording characteristic correction process. It is a flowchart which shows a recording characteristic model formula calculation process. It is a figure for _{demonstrating} the calculation method of average density _yave (x). It is a figure for demonstrating matching with the input signal value x and the recording amount z. It is a diagram for explaining a method of calculating a correction amount d (n) in the reference input signal value x _0. It is a figure for demonstrating the calculation method of the constant term c (n). It is a figure for demonstrating the calculation method of the input signal area k (x). It is a figure for demonstrating the calculation method of correction amount (DELTA) s and (DELTA) gamma. It is a flowchart which shows the update process of a correction coefficient. It is a figure for demonstrating the calculation method of the correction coefficients (alpha) and (beta) based on the new input signal area k (x). It is a figure which shows the output result after the shading correction in a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Print head 11 LED light source 14 PLZT shutter array 15 Selfoc lens array 16 Recording element 17, 18 Polarizer 20 Image control part 23 Correction information recording part 24 Print head control part 25 Correction amount calculation part 26 Correction information calculation Part 30 image reading part 40 recording material 41 image for correction

Claims (18)

An input signal value is converted into an output signal value based on a correction coefficient, and a correction image is recorded on a recording material based on the output signal value using a print head in which a plurality of recording elements are arranged in an array. Reading information is acquired from the correction image by a reading unit, a correction amount of a recording characteristic of each recording element is calculated based on the reading information, the correction coefficient is adjusted based on the correction amount, and the adjustment is performed. In an image forming method of forming an image using the corrected correction coefficient,
The recording amount of each recording element is represented by a model formula including the sum of the b-th term (b is a real number) of the input signal value and a constant term,
An image forming method characterized by calculating the model formula. The image forming method according to claim 1,
The image forming method, wherein the model formula is calculated based on the read information. The image forming method according to claim 1 or 2,
The correction image has a density information acquisition region including a region recorded for two or more types of input signal values,
The image forming method, wherein the constant term is calculated based on reading information of the density information acquisition region. In the image forming method according to any one of claims 1 to 3,
An image forming method, wherein after calculating the constant term, a b-order term of the input signal value is calculated. In the image forming method according to any one of claims 1 to 4,
Calculate a temporary model expression obtained by removing the constant term from the model expression,
Using the ratio between the recording amount at the first input signal value calculated based on the temporary model formula and the target recording amount at the first input signal value, the calculation was performed based on the temporary model formula. Calculating a virtual recording amount at the second input signal value from a recording amount at the second input signal value;
An image forming method, wherein the constant term is calculated from a difference between the virtual recording amount and a target recording amount in the second input signal value. In the image forming method according to any one of claims 1 to 5,
An image forming method, wherein the model formula is calculated using regression analysis. In the image forming method according to any one of claims 1 to 6,
An image forming method comprising: determining whether to calculate the model formula based on a variation amount of read information of an image corresponding to a specific input signal value. In the image forming method according to any one of claims 1 to 7,
The image forming method, wherein the read information is read information corresponding to a plurality of different densities. In the image forming method according to any one of claims 1 to 8,
The image forming method, wherein the print head is a PLZT print head. A print head control unit that converts an input signal value into an output signal value based on a correction coefficient, and a print head in which a plurality of recording elements are arranged in an array, and a correction image is applied to the recording material based on the output signal value A print head for recording, an image reading unit for acquiring read information from the correction image, a correction amount calculating unit for calculating a correction amount of a recording characteristic of each recording element based on the read information, and the correction amount A correction information calculation unit that adjusts the correction coefficient, and an image forming apparatus that forms an image using the adjusted correction coefficient,
The recording amount of each recording element is represented by a model formula including the sum of the b-th term (b is a real number) of the input signal value and a constant term,
The image forming apparatus, wherein the correction amount calculation unit calculates the model formula. The image forming apparatus according to claim 10.
The image forming apparatus, wherein the correction amount calculation unit calculates the model formula based on the read information. The image forming apparatus according to claim 10 or 11,
The correction image has a density information acquisition region including a region recorded for two or more types of input signal values,
The image forming apparatus, wherein the correction amount calculation unit calculates the constant term based on reading information of the density information acquisition region. The image forming apparatus according to any one of claims 10 to 12,
The image forming apparatus, wherein the correction amount calculation unit calculates a b-order term of the input signal value after calculating the constant term. The image forming apparatus according to any one of claims 10 to 13,
The correction amount calculation unit calculates a provisional model expression obtained by removing the constant term from the model expression, and records the first input signal value calculated based on the provisional model expression and the first amount. A virtual recording amount at the second input signal value is calculated from a recording amount at the second input signal value calculated based on the temporary model formula using a ratio of the input signal value to the target recording amount, An image forming apparatus, wherein the constant term is calculated from a difference between the virtual recording amount and a target recording amount in the second input signal value. The image forming apparatus according to any one of claims 10 to 14,
The image forming apparatus, wherein the correction amount calculation unit calculates the model formula using regression analysis. The image forming apparatus according to any one of claims 10 to 15,
The image forming apparatus, wherein the correction amount calculation unit determines whether to calculate the model formula based on a variation amount of read information of an image corresponding to a specific input signal value. The image forming apparatus according to claim 10,
The image forming apparatus, wherein the read information is read information corresponding to a plurality of different densities. The image forming apparatus according to any one of claims 10 to 17,
The image forming apparatus, wherein the print head is a PLZT print head.

Images