JP5493364B2 - Image density measuring method, image density measuring apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to an image density measuring method, an image density measuring apparatus, and an image forming apparatus for correcting image density during image formation.

  There are many image products in the market such as printers, copiers, high value-added products such as printers, copiers and communication functions, and commercial printers. Electrophotographic, ink jet, and thermal methods are also used for image forming methods. And so on. As user needs have shifted from monochrome printing to color printing, the demand for color reproduction, such as high-quality photographic prints, has been increasing.

  In color formation of an image, the user sets the color of the image to be output and the output image of the image product based on a color conversion table represented by a lookup table. Conventionally, color sensing and color correction in image products are generally performed by detecting a color patch formed on an intermediate transfer body with an RGB color sensor or the like and correcting the toner amount.

  In recent years, image products with external spectrophotometers and high-accuracy color management are now available on the market. Examples of color evaluation technology and feedback to the image forming process in image products are shown below.

  For example, even when the density variation of an image is large, a patch pattern of one density level for each color is detected, a coarse detection range is limited, and a patch pattern with a detailed density level at the limited level is detected, and a density correction table Is disclosed (see, for example, Patent Document 1).

  In addition, a technique is disclosed in which overcorrection of density is avoided by outputting an image without density correction when the density fluctuation is small by a color sensor (see, for example, Patent Document 2).

  A technology that has multiple visible light sources, a single ultraviolet light source, and light receiving means, and enables color measurement without being affected by fluorescent whitening material contained in paper by removing ultraviolet light components. Is disclosed (for example, see Patent Document 3).

  Further, the output image of the gradation patch pattern is read by a scanner, and the acquired RGB data is converted into XYZ data to obtain colorimetric data. A technique is disclosed in which a conversion curve for converting reflectance into colorimetric data is calculated from reflectance data obtained by a sensor that has read a gradation patch pattern on an intermediate transfer member, and sensor characteristics are corrected (for example, , See Patent Document 4).

  Further, the toner adhesion amount is made uniform between the front and back of the transfer paper, and the difference in gloss when forming double-sided images is reduced. The toner adhesion amount detection means serving as an index for reducing the gloss difference is obtained by acquiring a toner image and deriving the image area ratio and the like. Further, a technique is disclosed in which the gloss control step is executed by controlling the fixing temperature on each surface based on the toner adhesion amount detection values on the front and back surfaces (see, for example, Patent Document 5).

In addition, a technique for deriving spectral characteristics with multiband and specifying the area occupied by each color except black is disclosed (for example, see Patent Document 6).
JP 2006-26936 A JP 2005-321572 A Japanese Patent Laid-Open No. 2005-265752 Japanese Patent Laid-Open No. 2005-43617 JP 2006-3679 A Japanese Patent No. 3935836

  However, with a fixed color conversion table, such as conditions and fluctuations in the usage environment of products, fluctuations in quality of supply products such as toner, deterioration of parts and units that make up the products, and changes over time in mass printing, how accurately the colors are Even if measurement is performed and color correction is performed, correction to the expected color is not performed.

  In addition, in image forming methods such as electrophotography and inkjet, color representation is determined by superimposing halftone images with each color material, so that not only the amount of color material in the thickness direction but also the area ratio in the image area is increased. It is necessary to specify the accuracy.

  Each of the techniques disclosed in Patent Documents 1 to 3 uses a color conversion table, and requires color correction, density correction, and color material correction steps between detection data and an output image. Therefore, it is necessary to appropriately verify and correct the relationship between the correction data for each factor that causes the image to fluctuate. On the other hand, when there is a mismatch between the steps, there is a problem in that an accurate color image cannot be formed.

  Since the technology disclosed in Patent Document 4 is based on the gradation patch pattern of the product itself in order to ensure the accuracy of the sensor, if there are factors that cause image fluctuations such as environmental fluctuations, There is a very high possibility that correction will occur, and there is a risk of providing a product with a permanently low color reproducibility by permanently using the conversion table obtained by this method.

  The technique disclosed in Patent Document 5 is intended to improve glossiness, but is considered to be effective for feedback to image correction as well as color evaluation by using the image area ratio. However, since a fixed correlation between the toner amount and the area ratio is used or a conversion table is referred to, there is a problem that it is not possible to deal with factors that cause the image to fluctuate.

  Although the technique disclosed in Patent Document 6 can detect each color region with high accuracy, there is a problem that it cannot cope with a case where a color material is changed or an image bearing medium has a color. there were.

  In view of the above-described points, it is an object to provide an image density measurement method and an image density measurement apparatus that can correct color variation and color variation in an image, and an image forming apparatus including them.

The image density measurement method includes a step of irradiating a solid image formed on an image bearing medium with white light, the image bearing medium irradiated with the white light, and a one-dimensional image from the solid image on the image bearing medium. A step of branching diffused light in a plurality of directions, and a plurality of one-dimensional light receiving elements through a plurality of filters that transmit light of different wavelengths from each of the branched one-dimensional diffused light from the solid image irradiated with the white light Obtaining a plurality of one-dimensional diffused light images in a row, irradiating an arbitrary halftone image formed on the image bearing medium with white light, the image bearing medium irradiated with the white light, and the a step of branching a one-dimensional diffusion light from any halftone image on the image carrying medium in a plurality of directions, each one-dimensional diffusion light the white light is branched from any halftone image irradiated, different Through a plurality of filters which transmit length of light, the plurality of one-dimensional and a step of obtaining a plurality of one-dimensional diffusion light image by the light receiving element array, the solid image and the halftone image each of the plurality of one-dimensional diffusion light image The spectral characteristics of each pixel of the one-dimensional diffused light image of each of the solid image and the halftone image are estimated, and the halftone image is calculated from the spectral characteristics of each pixel of the estimated one-dimensional diffused light image of the solid image. And a step of deriving the area ratio of the color material existing in the imaging region of each pixel of the one-dimensional diffused light image.

  According to the disclosed technology, it is possible to provide an image density measuring method and an image density measuring apparatus that can correct color variation and color variation in an image, and an image forming apparatus including the image density measuring method and image density measuring apparatus.

  Hereinafter, embodiments will be described with reference to the drawings.

<First Embodiment>
FIG. 1 is a diagram illustrating an example of an image density measuring apparatus according to the first embodiment. Referring to FIG. 1, the image density measurement apparatus 10 includes a light source 11, a filter 12, a filter replacement unit 13, an image acquisition unit 14, and a calculation unit 15. Reference numeral 19 denotes an image bearing medium.

  The light source 11 has a function of emitting light applied to the image bearing medium 19 through the filter 12. As the light source 11, for example, a xenon lamp, a white LED (Light Emitting Diode) or the like can be used. The xenon lamp can be mounted on a large-scale commercial printing machine, but it is preferable to use a micro light source such as an LED because it is necessary to reduce the overall size of the apparatus for office use.

  The filter 12 has a function of transmitting only light in a predetermined wavelength range, and includes a filter 12a, a filter 12b, and a filter 12c. The filter 12a, the filter 12b, and the filter 12c are arranged on the same plane and have a function of transmitting light in different wavelength ranges. The filter 12a transmits red light, for example, the filter 12b transmits green light, for example, and the filter 12c transmits blue light, for example.

  The filter exchanging means 13 has a function of exchanging the filter 12 by rotating it around a rotation axis parallel to the optical axis direction of the light emitted from the light source 11, for example. The filter replacement unit 13 sequentially drives the filter 12 to a position where light emitted from the light source 11 passes through any one of the filter 12a, the filter 12b, and the filter 12c. That is, it is possible to sequentially irradiate the image bearing medium 19 with light in a plurality of different wavelength ranges. The light source 11, the filter 12, and the filter replacement means 13 may be referred to as light source means. That is, the light source 11, the filter 12, and the filter replacement unit 13 constitute a light source unit that emits a plurality of lights having a predetermined wavelength distribution.

  The image acquisition unit 14 has a function of acquiring a two-dimensional image (two-dimensional diffused light image) from the diffuse reflected light of the light irradiated from the light source 11 through the filter 12 to the image bearing medium 19. The image acquisition unit 14 includes a plurality of pixels. As the image acquisition means 14, for example, an area sensor such as a MOS (Metal Oxide Semiconductor Device), a CMOS (Complimentary Metal Oxide Semiconductor Device), or a CCD (Charge Coupled Device) can be used. The area sensor is a light detection sensor array occupying a certain area.

  The calculation unit 15 has a function of performing calculation based on the two-dimensional image obtained from the image acquisition unit 14. For example, the calculation means 15 can transmit light that passes through each filter from a two-dimensional image that passes through the filter 12a, a two-dimensional image that passes through the filter 12b, and a two-dimensional image that passes through the filter 12c (for example, The reflectance of red light, green light, and blue light is obtained, and converted into numerical values of an arbitrary color system for each pixel of the two-dimensional image. As an example of an arbitrary color system, for example, an XYZ color system can be cited. The XYZ color system is adopted as a CIE (International Commission on Illumination) standard color system, and is a color system based on tristimulus values X, Y, and Z.

  The computing means 15 also calculates the spectral characteristics of each pixel of a two-dimensional image obtained by sequentially irradiating a solid image or a halftone image closely formed on the image bearing medium 19 with a plurality of different wavelength ranges. The area ratio of the color material existing in the imaging region of each pixel of the two-dimensional image of the halftone image is derived from the estimated spectral characteristics of each pixel of the two-dimensional image of the solid image. Details of the function of the computing means 15 will be described later.

  The image bearing medium 19 is a medium that is a target for measuring diffuse reflectance. The image bearing medium 19 is, for example, paper on which no image is formed, paper on which a solid image or a halftone image is formed, or the like. Light emitted from the light source 11 is incident on the image bearing medium 19 at a predetermined angle.

  FIG. 2 is a diagram illustrating another example of the image density measuring apparatus according to the first embodiment. 2, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof may be omitted. Referring to FIG. 2, the image density measuring device 20 is the same as the image density measuring device 10 except that the filter 12 is replaced with a filter 22. Hereinafter, only the filter 22 different from the image density measuring apparatus 10 will be described.

  The filter 22 has a function of transmitting only light in a predetermined wavelength range, and includes six filters: a filter 22a, a filter 22b, a filter 22c, a filter 22d, a filter 22e, and a filter 22f. The filter 22a, the filter 22b, the filter 22c, the filter 22d, the filter 22e, and the filter 22f are arranged on the same plane and have a function of transmitting light in different wavelength ranges. Note that the light source 11, the filter 22, and the filter replacement means 13 may be referred to as light source means. That is, the light source 11, the filter 22, and the filter replacement unit 13 constitute a light source unit that emits a plurality of lights having a predetermined wavelength distribution.

  Hereinafter, details of the image density measuring method and the image density measuring apparatus according to the first embodiment will be described, particularly focusing on the function of the computing means 15.

  FIG. 3 is a diagram illustrating an example of spectral transmittance characteristics of the filters 22a to 22f in the image density measuring apparatus 20. The filters 22a to 22f have a wavelength range for transmission so as to be uniformly dispersed throughout the visible region (approximately 380 nm to 780 nm). Since the amount of reflected light acquired by the image acquisition unit 14 does not have a wavelength distribution, the spectral transmittance characteristics shown in FIG. 3 are obtained by measuring the transmission characteristics of the filters 22a to 22f with a spectrometer and changing the shape of the wavelength distribution. It is obtained by multiplying and specifying according to the acquired light quantity by a method such as approximating a Gaussian distribution.

  FIG. 4 is a diagram showing an example of spectral characteristics of continuous wavelengths of a plurality of sample images. The spectral characteristic shown in FIG. 4 is a continuous wavelength spectral characteristic obtained by measuring a sample image which is a monochromatic image. That is, in FIG. 4, spectral characteristics of continuous wavelengths different from each other are displayed by the number of sample images. FIG. 5 is a diagram illustrating an example of spectral characteristics in six wavelength ranges for a plurality of sample images. The spectral characteristics shown in FIG. 5 are spectral characteristics in six wavelength ranges corresponding to the transmission characteristics of the filters 22a to 22f obtained by measuring the same multiple sample images as in FIG. That is, in FIG. 5, the spectral characteristics in six different wavelength ranges are displayed by the number of sample images.

  An estimation matrix applied to Wiener estimation can be derived from the spectral characteristics shown in FIGS. By multiplying the spectral characteristics of the arbitrary measurement target in the six wavelength ranges by the derived estimation matrix, the spectral characteristics of the continuous wavelength of the arbitrary measurement target can be estimated. Since the method for deriving the estimation matrix is well known, its description is omitted.

  The computing means 15 stores an estimation matrix that is applied in advance to Wiener estimation derived from the spectral characteristics shown in FIGS. 4 and 5. Accordingly, the calculation means 15 estimates the spectral characteristics of the continuous wavelength by multiplying the spectral characteristics obtained from the diffuse reflected light intensities in the six wavelength ranges obtained from the image carrier medium 19 to be measured by the derived estimation matrix. can do. FIG. 6 is a diagram illustrating an example of the spectral characteristics of the continuous wavelength estimated by the computing unit 15. In FIG. 6, the actual measurement value indicated by the solid line is the spectral characteristic of the continuous wavelength of the image bearing medium 19 actually measured by a spectrometer (not shown). The spectral characteristic indicated by the alternate long and short dash line is a spectral characteristic obtained from the diffuse reflected light intensity in the six wavelength ranges obtained from the image bearing medium 19 that is the object of measurement by the computing means 15. The estimated value indicated by a circle is the spectral characteristic of the continuous wavelength estimated by multiplying this spectral characteristic by the estimation matrix. As shown in FIG. 6, the actual measurement value and the estimated value coincide with each other with high accuracy.

  Note that it is also possible to estimate the spectral characteristics of continuous wavelengths by multiplying the diffusely reflected light intensity in the six wavelength ranges obtained from the image bearing medium 19 that is the measurement target by an estimation matrix that is directly derived. However, the method of estimating the spectral characteristics of the continuous wavelength by multiplying the spectral characteristics obtained from the diffuse reflected light intensity in the six wavelength ranges obtained from the image bearing medium 19 that is the measurement target by the derived estimation matrix. Thus, it is possible to estimate the spectral characteristics of the continuous wavelength with higher accuracy.

  The calculating means 15 converts the spectral characteristics of the estimated continuous wavelength into numerical values of an arbitrary color system. In the first embodiment, the XYZ color system is used to convert the values into X, Y, and Z values indicating the tristimulus values of the color. Since the conversion method is well known, its description is omitted.

  The calculating means 15 calculates | requires the area ratio of a color material from X, Y, Z which shows the converted tristimulus value. Here, the area ratio of the color material indicates the ratio of the area occupied by the color material in the image bearing medium 19, and the area ratio of the color material is 100% in the solid image and bright in the halftone image. The area and the dark area are mixed at a predetermined ratio, and the area ratio of the predetermined color material is obtained.

  Here, as a relationship for deriving the color of the image from the area ratio of the color material constituting the image, there is a Neugebauer equation expressed by Equation (Equation 1).

Here, i is eight elements of W, Y, M, C, R, G, B, and BL. Here, W represents paper (image bearing medium 19) itself, Y represents yellow, which is the primary color of the color material, M represents magenta, which is the primary color of the color material, and C represents the primary color of the color material. Indicates cyan. R is red, which is the secondary color of the coloring material Y and the coloring material M, G is green, which is the secondary color of the coloring material M and the coloring material C, and B is a secondary color of the coloring material Y and the coloring material M. Blue is the color, and BL is black. A represents the area ratio of each color material (including W), and X, Y, and Z represent tristimulus values of color.

  Expression (Equation 1) indicates that the color of the image can be uniquely determined by determining the area ratio A of the paper (image bearing medium 19) and each color material. From this, it is considered that if the halftone gradation can be accurately reproduced for each single color material, the color of the superimposed image can be accurately formed. For this purpose, since it is necessary to derive the area ratio of the halftone image formed on the image bearing medium 19, a method for deriving the area ratio of the halftone image will be described below.

  First, the spectral characteristics of the continuous wavelength of the paper that is the image bearing medium 19 and the spectral characteristics of the continuous wavelength of the solid image formed on the paper that is the image bearing medium 19 are acquired. The method for acquiring the spectral characteristics of the continuous wavelength is as described above. Here, the solid image is an image formed by tightly applying any one of the color material Y, the color material M, and the color material C onto the image bearing medium 19. That is, a solid image is formed for each different color material (color material Y, color material M, and color material C).

  FIG. 7 is a diagram showing an example of spectral characteristics obtained by measuring paper as the image bearing medium 19. FIG. 8 is a diagram showing an example of spectral characteristics obtained by measuring a solid image formed on paper as the image bearing medium 19 and a halftone image having three types of gradations. The tristimulus values converted from the spectral characteristics of the paper shown in FIG. 7 are Xp, Yp, Zp, and the tristimulus values converted from the spectral characteristics of the solid image shown in FIG. 8 are Xm, Ym, Zm, and a solid image. Assuming that the area ratio is 1, a provisional area ratio A is defined, and the tristimulus values of an arbitrary halftone image are Xh, Yh, and Zh, the equation (Equation 2) holds. Here, the overall area ratio is 1, and since there is no other color material, the area ratio of the paper is (1-A).

If the tristimulus values of the halftone image shown in FIG. 8 are Xr, Yr, and Zr, the same relationship as Xh, Yh, and Zh in Equation (2) is established. However, since there is one variable A for the three equations, the area ratio A is not always obtained as one value. Therefore, a method of obtaining the area ratio A as Xh = Xr or a method of obtaining the area ratio A from the average value of A obtained from the respective equations as Xh = Xr, Yh = Yr, Zh = Zr can be considered.

  In addition, since the color is determined by the wavelength distribution of the reflected light and transmitted light from the object, the finally detected color is the intensity distribution for each wavelength of the light source, the reflectance / transmittance distribution for each wavelength of the object, the light receiving means When the image is obtained by simplification of each wavelength, such as the sensitivity for each wavelength, the reflection / transmittance distribution of the optical element used, etc. (usually expressed as a matrix or function, the product of the matrix or function). If the spectral reflectances of paper and color material are Iλ, Pλ, and Tλ, respectively,

It can be expressed as.

  Multiplying the spectral distribution of the paper and the spectral distribution of the colorant for each wavelength results in the color of the image formed on the paper having the color, so the reverse operation is performed, that is, the image formed on the paper having the color If the paper color is removed from the color, the color material color can be separated, and the area ratio can be derived without considering the paper color in the equations (Equation 1) and (Equation 2).

  An example of color separation between paper and an image will be described with reference to FIGS. FIG. 9 is a diagram illustrating an example of spectral characteristics of three different types of paper. In FIG. 9, samples 1 to 3 show three types of paper having different colors. FIG. 10 is a diagram illustrating an example of spectral characteristics of yellow toner images formed on three different colors of paper and plain spectral characteristics of paper. The plain spectral characteristics of the paper are the same as those shown in FIG. Samples 1 to 3 (plain color) show plain spectral characteristics. Samples 1 to 3 (with images) show the spectral characteristics of the yellow toner image. FIG. 11 is a diagram illustrating an example of spectral characteristics of only the toner image from which the influence of the paper color is removed. The spectral characteristics shown in FIG. 11 are spectral characteristics of only the toner image from which the influence of the paper color is obtained by dividing the spectral reflectance of FIG. 10 by the spectral reflectance of FIG. 9 at each wavelength.

  Further, there is a method for obtaining the area ratio A by performing a convergence calculation of A so that the expression (Equation 4) is minimized among the three equations of the expression (Equation 2).

For the convergence calculation of A, the Levenberg-Marquardt method, the quasi-Newton method, or the like can be applied. Since these methods are well known, description thereof will be omitted. In this way, the calculation means 15 can derive the color material area ratio in the image bearing medium 19.

  Actually, halftone images of three color materials (yellow, magenta, cyan) were formed with three different gradations. The area ratio of the color material in each formed halftone image was derived from the spectral characteristics of the color materials (yellow, magenta, cyan) used for forming each of the solid images by the method described above. The results are shown below.

  FIG. 12 is a diagram illustrating an example of a result of deriving the area ratio of the color material in each halftone image from the spectral characteristics for each solid image. In FIG. 12, y spectrum, m spectrum, and c spectrum indicate the area ratios of the color materials derived from the spectral characteristics of the yellow, magenta, and cyan halftone images with respect to the solid images. Further, the actual y measurement, actual m measurement, and actual c measurement indicate the area ratio of the color material actually measured by capturing a halftone image with the image acquisition unit 14.

  As shown in FIG. 12, the area ratio of the color material derived from the spectral characteristics and the area ratio of the actually measured color material coincide with each other with high accuracy. Here, since the area ratio of the color material in the halftone image can be handled as the amount of the color material forming the image, if the area ratio derived from the spectral characteristics for each solid image is obtained, the correction value of the color material The amount can be fed back.

  It is also conceivable to apply the Demicheel relationship.

  In the above method, when the spectral characteristics of the paper are replaced with the spectral characteristics of the intermediate transfer body, the area ratio condition of the halftone image can be measured on the intermediate transfer body. Some comparison is required for comparison with images. For example, on the intermediate transfer body, the area ratio of the halftone image is measured based on the spectral characteristics of the solid image before fixing, and the area ratio of the halftone image is measured based on the spectral characteristics of the solid image on the paper. Measure. Until now, there was a difference in the conditions before and after fixing, so it was not meaningful to compare the colors directly. However, since it is possible to compare by area ratio based on both solid images, Next, it is possible to detect factors of variation in the color of the image, such as the transfer rate of the color material before and after the transfer process and the fix rate of the color material before and after the fixing process.

  The above-described method for deriving the area ratio of the color material is performed for each pixel of the acquired two-dimensional image (two-dimensional diffused light image), and color variation and color variation are determined from the difference in spectral characteristics adjacent to each other in the obtained image. By detecting the area ratio of the color material, directly controlling the amount of the color material, and correcting the lighting time represented by the lighting timing and duty ratio of the light source in the writing optical system, the local color fluctuation and color variation are corrected. It becomes possible.

  As described above, according to the first embodiment, a difference in color of an image is detected, and when there is a difference in color, it can be directly fed back to the image device with the amount of color material. Even if the color of the toner itself changes, the solid image is used as a reference, so correction that cannot be handled by a fixed lookup table can be performed, and local color fluctuations and color variations in the image can be performed. It is possible to feed back the color material amount directly from the derived area ratio and correct local color fluctuations and color variations in the image.

  That is, the color of the expected output image against image fluctuation factors such as conditions and fluctuations in the usage environment of the product, quality fluctuations in supply products such as toner, deterioration of parts and units that make up the product, and changes over time in mass printing Image density measuring method and image density measuring apparatus for detecting and feeding back local color fluctuations and color variations in an image based on the area ratio of the color material constituting the image be able to.

<Second Embodiment>
FIG. 13 is a diagram illustrating an example of an image density measuring apparatus according to the second embodiment. 13, the same components as those in FIG. 2 are denoted by the same reference numerals, and the description thereof may be omitted. Referring to FIG. 13, the image density measuring device 30 is the same as the image density measuring device 20 except that a lens 31 is added. Hereinafter, the image density measuring device 30 will be described only with respect to parts different from the image density measuring device 20.

  The lens 31 has a function of condensing the diffuse reflection light of the light irradiated on the image bearing medium 19 on the image acquisition unit 14. Therefore, the image acquisition unit 14 can acquire the total amount of diffuse reflected light of the light irradiated on the image bearing medium 19.

  In the image density measuring device 30, the light emitted from the light source 11 is irradiated to the image bearing medium 19 through the filters 22 a to 22 f that are sequentially exchanged by the filter exchange means 13. The image acquisition unit 14 acquires a multispectral image from the diffuse reflected light of the light irradiated on the image bearing medium 19 and estimates the spectral characteristics of continuous wavelengths at each pixel from the acquired multispectral image. Since the reflected light amount obtained by irradiating the image bearing medium 19 with light in different wavelength ranges corresponding to the filters 22a to 22f is acquired as the total light amount in each wavelength range, the calculation means 15 causes the image The data of each pixel of the multi-spectral image acquired by the acquisition unit 14 is the brightness value of the pixel.

  FIG. 14 is a diagram illustrating an example of spectral transmittance characteristics of the filters 22a to 22f and pixel brightness data. In FIG. 14, a solid line indicates the spectral transmittance characteristics of the filters 22a to 22f that are known or measured by a spectrometer, and a broken line indicates pixel brightness data. By multiplying the spectral transmittance characteristics of the filters 22a to 22f by pixel brightness data, or by converting the spectral transmittance characteristics of the filters 22a to 22f into functions such as Gaussian and adjusting the amplitude according to the light quantity, the reflected light quantity can be reduced. It can be acquired as spectral characteristics of a plurality of bands.

  The spectral characteristic acquired by the calculation means 15 is converted into the spectral characteristic of continuous wavelength by the above-described Wiener estimation, the color difference is measured, and converted into the area ratio, so that the color difference is fed back as the amount of the color material. It becomes possible. Further, by executing the above calculation on all the pixels of the acquired two-dimensional image, a two-dimensional color material area ratio distribution can be obtained, and the correction amount can be determined based on the obtained variation in the area ratio. it can.

  FIG. 15 is a diagram illustrating an example of spectral characteristics of an image when the irradiation angle from the light source is changed with respect to the solid image in the image density measurement device 30. In FIG. 15, the X axis indicates the wavelength and increases toward the right side, the Y axis indicates the spectral reflectance and increases toward the upper side, and the Z axis indicates the irradiation angle from the light source and goes to the back side. It is supposed to be bigger. As shown in FIG. 15, in the image density measuring device 30, it can be seen that the color measurement result varies due to the geometric variation. A method for preventing the color measurement result from fluctuating due to the geometric variation will be described in a third embodiment.

  As described above, according to the second embodiment, when a difference in color of an image is detected and there is a difference in color, the color data is converted into an area ratio, and feedback is directly provided to the imaging device with the amount of color material. It is possible to perform detection that enables robust color correction against color variation factors, and local color variation in the detected image, color variation directly local color material amount variation, It is possible to provide an image density measurement method and an image density measurement apparatus that detect a difference and enable local color correction.

  In addition, it is possible to provide an image density measuring method and an image density measuring apparatus that can detect and correct local color variation and color variation as an area ratio of a mechanical color material offline.

<Third Embodiment>
FIG. 16 is a diagram illustrating an example of an image density measuring apparatus according to the third embodiment. 16, the same components as those in FIGS. 2 and 13 are denoted by the same reference numerals, and the description thereof may be omitted. Referring to FIG. 16, the image density measuring device 40 is the same as the image density measuring device 30 except that a lens 41 is added. Hereinafter, only the parts of the image density measuring device 40 that are different from the image density measuring device 30 will be described.

  The lens 41 is a so-called collimator lens having a function of converting light emitted from the light source 11 into parallel light. In the image density measuring device 40, a lens 41 is provided on the optical axis of the light emitted from the light source 11, and the light emitted from the light source 11 is radiated to the image bearing medium 19 as parallel light. As a result, the image density measuring apparatus 40 can prevent the color measurement result from fluctuating due to the geometric fluctuation as shown in FIG. That is, it is possible to reduce the spectral characteristic fluctuation acquired due to the fluctuation of the light beam angle due to the light divergence state or the focusing state.

  As described above, according to the third embodiment, when a difference in color of an image is detected, and there is a difference in color, the color data is converted into an area ratio, and feedback is directly provided to the imaging device with the amount of color material. It is possible to perform detection that enables robust color correction against color variation factors, and local color variation in the detected image, color variation directly local color material amount variation, It is possible to provide an image density measurement method and an image density measurement apparatus that detect a difference and enable local color correction. Furthermore, it is possible to reduce the spectral characteristic fluctuation obtained due to the fluctuation of the light beam angle.

  In addition, it is possible to provide an image density measuring method and an image density measuring apparatus that can detect and correct local color variation and color variation as an area ratio of a mechanical color material offline.

<Fourth embodiment>
FIG. 17 is a diagram illustrating an example of an image density measuring apparatus according to the fourth embodiment. Referring to FIG. 17, the image density measuring device 50 includes a light source 51, an imaging element 52, a branching optical system 53, a filter 54a, a filter 54b, a filter 54c, an image acquisition unit 55a, and an image acquisition. Means 55b and image acquisition means 55c are provided. Reference numeral 59 denotes an image bearing medium. In FIG. 17, the calculation means and the like are omitted. The longitudinal direction of the one-dimensional shape of each optical element is a direction perpendicular to the paper surface.

  The light source 51 has a function of emitting light applied to the image bearing medium 19. As the light source 51, for example, a one-dimensional light source such as a white LED (Light Emitting Diode) array can be used. The imaging element 52 has a function of collecting one-dimensional diffuse reflected light from an image on the image bearing medium 59 and forming an image on the image acquisition units 55a to 55c. As the imaging element 52, for example, a SELFOC lens or the like can be used.

  The branching optical system 53 has a function of branching the one-dimensional diffuse reflected light that has passed through the imaging element 52 in a plurality of directions. As the branching optical system 53, for example, a combination of prisms or the like can be used. The filters 54a to 54c have a function of transmitting light in different wavelength ranges. The image acquisition means 55a to 55c have a function of receiving one-dimensional diffuse reflected light branched in a plurality of directions by the branching optical system 53 and acquiring a one-dimensional image. As the image acquisition means 55a to 55c, for example, a one-dimensional light receiving element such as a PD (Photo detector) array or a line sensor can be used.

  In the image density measuring device 50, light emitted from the light source 11 (shown by a broken line in FIG. 17) irradiates the image bearing medium 59 or the image on the image bearing medium 59 at a predetermined angle, and the image bearing medium 59 or the image bearing medium. One-dimensional diffuse reflected light is generated from the image on the medium 59. The one-dimensional diffuse reflected light enters the branching optical system 53 via the imaging element 52. The one-dimensional diffuse reflected light incident on the branching optical system 53 branches in three directions by the branching optical system 53 and forms images on the image acquisition units 55a to 55c via the filters 54a to 54c.

  For example, in an image product such as a printer or a copying machine, the image bearing medium 59 and the image on the image bearing medium 59 are normally moved, such as rotated or conveyed. The optical system of the image density measuring device 50 shown in FIG. 17 receives a plurality of one-dimensional diffuse reflected light in a state where the image carrying medium 59 and the image on the image carrying medium 59 are moving in the direction of the arrow in FIG. A two-dimensional diffuse reflection image can be acquired from these images.

  First to third embodiments for each pixel in a one-dimensional image (one-dimensional multispectral image) acquired by the image acquisition means 55a to 55c or a two-dimensional image (two-dimensional multispectral image) that is a combination thereof. By using the image density measuring method shown in Fig. 1, it is possible to detect the color variation and color variation of the image by the area ratio of the mechanical color material image.

  Also, the branching optical system shown in FIG. 17 can correspond to six branches, the filter can correspond to six wavelength ranges, and six image acquisition means can be installed. By doing in this way, a 6-band one-dimensional multiband image is acquired, and a two-dimensional multiband image can be acquired by scanning accompanying the movement of the image carrying medium and the image on the image carrying medium.

  As described above, according to the fourth embodiment, even if an image bearing medium represented by paper or an intermediate transfer belt or an image on the image bearing medium is rotated or conveyed in the image product, Without stopping, the entire diffusely reflected light image is acquired by using the image carrying medium and the movement of the image on the image carrying medium, and the spectral characteristics are derived to detect the difference in the color of the image. If there is a difference in color, the color data can be converted into an area ratio and fed back directly to the imaging device with the amount of color material, and detection that enables robust color correction against color variation factors can be performed. And an image density measurement method and an image density capable of directly detecting local color fluctuations and color variations in the detected image as local color material quantity fluctuations and differences, and enabling local color correction. A measuring device can be provided.

  Also, an image density measurement method and an image density capable of detecting and correcting local color fluctuations and color variations as the area ratio of the mechanical color material in the same state as the operation state in the image product offline. A measuring device can be provided.

<Fifth embodiment>
FIG. 18 is a diagram illustrating an example of an electrophotographic image forming apparatus according to the fifth embodiment. Referring to FIG. 18, the image forming apparatus 60 includes a paper feed cassette 61a, a paper feed cassette 61b, a paper feed cassette 61c, a transport roller 62, a calculation unit 63, a writing optical system 64, and a photosensitive member 65. An intermediate transfer member 66, a fixing roller 67, and a paper discharge roller 68.
Reference numeral 69 denotes paper as an image bearing medium.

  In the image forming apparatus 60, the paper 69 conveyed by the guides and conveyance rollers 62 (not shown) from the paper feeding cassettes 61 a to 61 c is subjected to color system conversion and image processing by the calculation means 63, and is processed by the writing optical system 64. The photosensitive member 65 is exposed, developed with a coloring material. The developed image is transferred onto the intermediate transfer member 66 and then onto the paper 69 from the intermediate transfer member 66. The image transferred onto the paper 69 is fixed by the fixing roller 67, and the paper 69 on which the image has been formed is discharged by the paper discharge roller 68.

  In the image forming process, when the image density measuring apparatus according to the first to fourth embodiments is installed in the A part in FIG. 18, images are formed in the sheet feeding cassettes 61a to 61c. The color of the paper 69 before being detected is detected. The detected color of the paper 69 is applied to derive tristimulus values from the spectral characteristics of the paper 69 in the above-described equations (Equation 1) to (Equation 4). When color variation or color variation is detected on the paper 69, it is possible to guarantee image quality such as warning or rejection as defective paper. It is also possible to calculate the distribution of the color material in accordance with the color variation of the paper 69.

  Even when the image density measuring apparatus according to the first to fourth embodiments is installed in the B part in FIG. 18, the same effects as when the image density measuring apparatus is installed in the A part are obtained. However, since the sheet 69 being transported is targeted, synchronization of detection timing is required. Further, there is a high possibility that the posture will change when the paper 69 is transported, and it is necessary to take measures such as increasing the tension of the paper 69.

  When the image density measuring apparatus according to the first embodiment to the fourth embodiment is installed in the part C in FIG. 18, the tristimulus values of the intermediate transfer body 66 are determined from the spectral characteristics of the intermediate transfer body 66. Thus, an inspection pattern such as a color patch composed of a solid image and / or a halftone image is formed on the intermediate transfer member 66, and the area ratio of the formed inspection pattern can be detected based on the solid image.

  Further, the transfer rate or the remaining transfer amount can be estimated by the toner amount represented by the area ratio from the area ratio of the color patch where the halftone image is formed before and after the transfer to the paper 69. In addition, when color variation and color variation in the image are detected, if the same color variation and color variation are continuously detected, it is determined that the variation is due to the characteristic variation of the intermediate transfer body 66, and writing optical By locally controlling the exposure pattern and the amount of light from the light source in the system 64, local image quality degradation can be eliminated.

  When the image density measuring device according to the first to fourth embodiments is installed in the D part in FIG. 18, this is related to the case where the image density measuring apparatus is installed in the C part. The image transfer amount can be detected.

  When the image density measuring apparatus according to the first to fourth embodiments is installed in the E portion in FIG. 18, the spectral characteristics of the paper 69 itself and the image on the paper 69 after fixing are displayed. Spectral characteristics can be acquired, and the area ratio can be estimated from each tristimulus value and fed back as a correction value when color fluctuations such as the image to be output and the output result, or changes over time during continuous output are detected. It becomes.

  Since local color variation and color variation can be detected by detection at any of the positions A to E, toner supply, writing optical system control, offline photoconductors and intermediate transfer belts, etc. It can be developed to grasp characteristics.

  In addition, since color correction is possible without referring to a lookup table, it is possible to perform robust correction against any variation factors. It can also be used to correct the conversion parameter of the lookup table, for example, when the spectral characteristics of the output result are different.

  As described above, according to the fifth embodiment, the image density measuring apparatus according to the first to fourth embodiments is installed at a predetermined position of the image forming apparatus, so that an intermediate transfer member, paper, etc. The color on the image bearing medium can be detected, the difference from the expected image can be derived by the area ratio of the color material constituting the image, and fed back to the output image.

  In addition, it is possible to perform robust color correction for each color variation factor, provide a high-quality image expected by the user, and write optical system for local color variation and color variation in the detected image By locally controlling the exposure density and the like, it is possible to provide a uniformly high-quality image that can be corrected.

  As described above, according to the first to fifth embodiments, the area ratio of the entire image and the area is derived directly from the image bearing medium represented by the intermediate transfer member and paper and the color evaluation data of the image. It is possible to provide an image density measuring method and an image density measuring apparatus capable of correcting color variation and color dispersion in the image forming apparatus, and an image forming apparatus including them. Unlike the method of collectively detecting the spectral characteristics of a predetermined area, it is possible to cope with color fluctuations and local density fluctuations in an image.

  The preferred embodiment has been described in detail above. However, the present invention is not limited to the above-described embodiment, and various modifications and substitutions may be made to the above-described embodiment without departing from the scope described in the claims. Can be added.

  For example, the image density measurement method, the image density measurement apparatus, and the image forming apparatus according to each embodiment are preferably applied to sensing and feedback for image density correction at the time of image formation such as an electrophotographic method and an inkjet method. However, it can also be applied to image formation such as a color proofer. Moreover, it can also be applied to image design, color material development, etc. as an offline image evaluation apparatus.

  In the first to third embodiments, the example using the filter replacement means is shown. However, without using the filter replacement means, a plurality of light sources are installed, and the filters of the respective colors are fixed to the respective light sources. Images can also be acquired by sequentially irradiating from the other direction at the same angle.

It is a figure which shows the example of the image density measuring device which concerns on 1st Embodiment. It is a figure which shows the other example of the image density measuring apparatus which concerns on 1st Embodiment. It is a figure which shows the example of the spectral transmittance characteristic of the filters 22a-22f in the image density measurement apparatus 20. FIG. It is a figure which shows the example of the spectral characteristic of the continuous wavelength of a some sample image. It is a figure which shows the example of the spectral characteristics in six wavelength ranges with respect to a several sample image. It is a figure which shows the example of the spectral characteristic of the continuous wavelength estimated by the calculating means. It is a figure which shows the example of the spectral characteristic obtained by measuring the paper which is the image carrier medium 19. FIG. It is a figure which shows the example of the spectral characteristic obtained by measuring the solid image formed on the paper which is the image carrier medium 19, and the halftone image which has three types of gradations. It is a figure which shows the example of the spectral characteristic of three types of paper from which a color differs. It is a figure which shows the example of the spectral characteristic of the yellow toner image formed on the paper in which three types of colors differ, and the plain spectral characteristic of paper. FIG. 6 is a diagram illustrating an example of spectral characteristics of only a toner image from which the influence of paper color is removed. It is a figure which shows the example of the result derived | led-out from the spectral characteristic with respect to each solid image for the area ratio of the color material in each halftone image. It is a figure which shows the example of the image density measuring apparatus which concerns on 2nd Embodiment. It is a figure which shows the example of the spectral transmittance characteristic of filters 22a-22f, and the brightness data of a pixel. It is a figure which shows the example of the spectral characteristic of an image at the time of changing the irradiation angle from a light source with respect to a solid image in the image density measurement apparatus. It is a figure which shows the example of the image density measuring device which concerns on 3rd Embodiment. It is a figure which shows the example of the image density measuring device which concerns on 4th Embodiment. It is a figure which shows the example of the image forming apparatus of the electrophotographic system which concerns on 5th Embodiment.

10, 20, 30, 40, 50 Image density measuring device 11, 51 Light source 12, 12a, 12b, 12c, 22, 22a, 22b, 22c, 22d, 22e, 22f, 54a, 54b, 54c Filter 13 Filter replacement means 14 , 55a, 55b, 55c Image acquisition means 15, 63 Calculation means 19, 59, 69 Image carrier medium 31, 41 Lens 52 Imaging element 53 Branch optical system 60 Image forming apparatus 61a, 61b, 61c Paper feed cassette 62 Conveying roller 64 Write optical system 65 Photoconductor 66 Intermediate transfer member 67 Fixing roller 68 Paper discharge roller

Claims (4)

Irradiating the solid image formed on the image bearing medium with white light;
Branching the image bearing medium irradiated with the white light and a one-dimensional diffused light from a solid image on the image bearing medium in a plurality of directions;
Each one-dimensional diffused light branched from the solid image irradiated with the white light is acquired as a plurality of one-dimensional diffused light images by a plurality of one-dimensional light receiving element arrays through a plurality of filters that transmit light of different wavelengths. And a process of
Irradiating an arbitrary halftone image formed on the image bearing medium with white light;
Branching the image bearing medium irradiated with the white light and one-dimensional diffused light from an arbitrary halftone image on the image bearing medium in a plurality of directions;
Each one-dimensional diffused light branched from an arbitrary halftone image irradiated with the white light is passed through a plurality of filters that transmit light of different wavelengths, and a plurality of one-dimensional diffusions are performed by the plurality of one- dimensional light receiving element arrays. Acquiring as a light image ;
Estimating spectral characteristics of each pixel of the one-dimensional diffused light image of each of the solid image and the halftone image from a plurality of one-dimensional diffused light images of the solid image and the halftone image;
Deriving the area ratio of the color material existing in the imaging region of each pixel of the one-dimensional diffused light image of the halftone image from the estimated spectral characteristic of each pixel of the one-dimensional diffused light image of the solid image; A method for measuring image density. Irradiating the solid image formed on the image bearing medium with white light;
Branching the image bearing medium irradiated with the white light and a one-dimensional diffused light from a solid image on the image bearing medium in a plurality of directions;
Each one-dimensional diffused light branched from the solid image irradiated with the white light is acquired as a plurality of one-dimensional diffused light images by a plurality of one-dimensional light receiving element arrays through a plurality of filters that transmit light of different wavelengths. And a process of
Irradiating an arbitrary halftone image formed on the image bearing medium with white light;
Branching the image bearing medium irradiated with the white light and one-dimensional diffused light from an arbitrary halftone image on the image bearing medium in a plurality of directions;
Each one-dimensional diffused light branched from an arbitrary halftone image irradiated with the white light is passed through a plurality of filters that transmit light of different wavelengths, and a plurality of one-dimensional diffusions are performed by the plurality of one- dimensional light receiving element arrays. Acquiring as a light image ;
Obtaining a two-dimensional diffused light image of each of the solid image and the halftone image from a plurality of one-dimensional diffused light images of each of the solid image and the halftone image ;
A step of estimating the spectral characteristic of each pixel of the solid image and from said halftone image each of the two-dimensional diffuse optical image of each of the solid image and the halftone image two-dimensional spreading light image,
Deriving the area ratio of the color material existing in the imaging region of each pixel of the two-dimensional diffused light image of the halftone image from the estimated spectral characteristics of each pixel of the two-dimensional diffused light image of the solid image; A method for measuring image density. A light source means for uniformly irradiating the image on the image bearing medium moving within the product and white light on the image bearing medium in a one-dimensional direction perpendicular to the moving direction;
Optical path branching means for branching the one-dimensional diffused light from the image bearing medium irradiated with the white light and an image on the image bearing medium in a plurality of directions;
A plurality of filters having different wavelength characteristics that transmit each one-dimensional diffused light branched by the optical path branching means;
Image acquisition means for receiving one-dimensional diffused light transmitted through the plurality of filters and acquiring it as a one-dimensional diffused light image ;
Imaging means for forming an image on the image acquisition means with one-dimensional diffused light from the image bearing medium and an image;
Spectral characteristics of an arbitrary halftone image are detected by detecting spectral characteristics of the image-bearing medium and the image in an imaging region of each pixel of the one-dimensional diffused light image or a two-dimensional diffused light image including a plurality of the one-dimensional diffused light images. And an image density measuring device having an arithmetic means for deriving an area ratio of the color material from the image density. Comprising an image density measurement apparatus 請 Motomeko 3 wherein derives the area ratio derived from the spectral characteristics of the image of the expected output, the difference in area ratio derived from the spectral characteristic of the output result of the image, the image An image forming apparatus that corrects a color correction value of an imaging region of each pixel of a two-dimensional diffused light image acquired by a density measuring device based on an area ratio of the image.