1. Field of the Invention
The present invention relates to color processing which takes a fluorescent material contained in a medium into consideration.
2. Description of the Related Art
Color matching processing used to match reproduction colors in an image input device, image display device, and image output device associates the reproduction colors among the respective devices in consideration of color reproduction characteristics of a matching target device.
The sequence for obtaining color reproduction characteristics of an image output device will be described below with reference to
X=k∫R(λ)S(λ)
Y=k∫R(λ)S(λ)
Z=k∫R(λ)S(λ)
where k=100/∫S(λ)
an integral range ranges from 380 to 780 nm.
That is, when the image output device prints color patches 12 of a large number of colors on the medium 11, and obtains colorimetric values (for example, tristimulus values XYZ 27) of the respective color patches, thus obtaining the relationship between signal values (for example, RGB values) 21 input to the image output device upon printing the color patches 12 and the colorimetric values. This correspondence relationship represents the color reproduction characteristics of the image output device.
However, when a material that produces fluorescence (for example, a fluorescent material such as a fluorescent whitener) is used as that of a medium (for example, a print sheet) used in image formation, the spectral radiance R(λ) measured by the aforementioned method is often different from that under the viewing illuminant. Note that the fluorescent material emits light in a wavelength range (fluorescence wavelength range) different from an excitation wavelength range contained in illumination light. In general, a fluorescence wavelength is longer than an excitation wavelength.
Graphs of
As shown in
The spectral radiance R(λ) of a sample containing a fluorescent material, an excitation wavelength which falls within an ultraviolet range, will be described below with reference to
As a method of obtaining tristimulus values from a sample printed on a medium containing a fluorescent material, a method using a bi-spectral radiance factor is available. The bi-spectral radiance factor is a bivariable function F(μ, λ) that represents a spectral radiance of a sample with respect to incident light of a wavelength μ, and can express a fluorescence amount radiated in a wavelength range different from that of irradiated light. Using the bi-spectral radiance factor, CIEXYZ values that consider fluorescence can be calculated using:
X=k∫λ{∫μF(μ,λ)S(μ)dμ·
Y=k∫λ{∫μF(μ,λ)S(μ)dμ·
Z=k∫λ{∫μF(μ,λ)S(μ)dμ·
where k=100/∫λS(λ)y(λ)dλ,
an integral range of ∫λ ranges from 380 to 780 nm, and
an integral range of ∫μ ranges from 300 to 780 nm.
However, measurement of the bi-spectral radiance factor requires a measuring device such as a bi-spectral fluorescence colorimeter which includes spectroscopes on both the illumination and light-receiving sides. In addition, the bi-spectral radiance factors for incident light have to be measured per sample in increments of 10 nm within the range from 300 nm to 780 nm, and the measurement requires an immense amount of time. In other words, it is not practical to prepare a large number of color charts obtained by printing a large number of color patches on a large number of media, and to measure bi-spectral radiance factors for respective color patches.
In case of a sample printed on a medium containing a fluorescent material, only excitation light according to a spectral transmittance of a color material reaches the medium on a portion where the color material covers a surface of the medium. Therefore, fluorescence radiated from the medium depends not only on the excitation light amount and the fluorescence characteristics of the medium but also on the spectral reflectance of the color material and a factor at which the color material covers the surface of the medium (to be referred to as a covering factor hereinafter). Japanese Patent Laid-Open No. 2006-292510 (patent literature 1) describes an invention that estimates bi-spectral radiance factors of a sample based on bi-spectral radiance factors of a medium, a spectral transmittance of a color material, and a covering factor of the color material.
According to the invention of patent literature 1, as an estimation reference of the covering factor, a reference printed surface on which color materials alone and combinations of a plurality of color materials are printed at a covering ratio of 100%, and the spectral radiances of the reference printed surface are measured. Next, the spectral radiances of samples are measured, and the covering ratios of the color materials on the samples are estimated based on the spectral radiances of the reference printed surface. Then, the bi-spectral radiance factors of the samples are estimated from the estimated covering ratios, bi-spectral radiance factors of a medium, which are measured in advance, and spectral transmittances of the respective color materials.
The invention of patent literature 1 prepares for the spectral radiances of the reference printed surface, the bi-spectral radiance factors of the medium, and the spectral transmittances of the respective color materials in advance, so as to estimate the bi-spectral radiance factors of the samples. Furthermore, in order to estimate the covering ratios, total spectral radiances of the samples have to be measured under a measurement illuminant. Therefore, according to the invention of patent literature 1, measurements can be simplified compared to a case in which bi-spectral radiance factors of a large number of color patches are measured. However, various kinds of information have to be prepared for estimation, and the invention of patent literature 1 is not practical.
In one aspect, a color processing method comprising the steps of: obtaining spectral reflectances of color patches formed on a medium; inputting bi-spectral radiance factors of the medium; and computing bi-spectral radiance factors of the color patches based on the spectral reflectances of the color patches and the bi-spectral radiance factors of the medium.
According to the aspect, bi-spectral radiance factors of samples can be easily obtained.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Color processing according to embodiments of the present invention will be described in detail hereinafter with reference to the drawings.
As described above, when a medium contains a fluorescent material, bi-spectral radiance factors of samples have to be used so as to accurately obtain colorimetric values of the samples under an arbitrary viewing environment. However, measurements of the bi-spectral radiance factors require a measuring device such as a bi-spectral fluorescence colorimeter and an immense amount of measurement time. Hence, in this embodiment, from bi-spectral radiance factors of a medium, bi-spectral radiance factors, which are caused by the medium and color materials, on samples on which color materials are superposed, are computed. Note that color patches formed by printing color materials on the medium will be referred to as samples hereinafter.
[Arrangement of Apparatus]
The arrangement of a color processing apparatus 100 of this embodiment will be described below with reference to the block diagram shown in
The CPU 101 loads a color processing program which implements color processing of this embodiment, and data from the HDD 103 or storage device 108 onto the RAM, and executes the color processing program according to a user instruction via a user interface, which is displayed on the monitor 106. Then, the CPU 101 stores the color processing result in the HDD 103 or storage device 108.
Color Processing Unit
The functional arrangement of a color processing unit 201 implemented by the color processing program will be described below with reference to the block diagram shown in
A media characteristic input unit 202 inputs bi-spectral radiance factors of a medium from a storage unit 210, which corresponds to the HDD 103 or storage device 108, in accordance with a user instruction input from the operation unit 107. A viewing illuminant information input unit 203 inputs spectral radiances of a viewing illuminant from the storage unit 210 in accordance with a user instruction input from the operation unit 107. A sample information input unit 204 inputs spectral radiances of samples by controlling the measuring device 109 or inputs them from the storage unit 210 in accordance with a user instruction input from the operation unit 107. The computing unit 205 computes bi-spectral radiance factors of the samples based on the bi-spectral radiance factors of the medium and the spectral radiances of the samples. A calculation unit 206 calculates colorimetric values (for example, CIE tristimulus values XYZ) of the samples based on the computed bi-spectral radiance factors of the samples and the spectral radiances of the viewing illuminant.
[Color Processing]
The processing of the color processing unit 201 will be described below with reference to the flowchart shown in
An example of the UI of the color processing unit 201 will be described below with reference to
The color processing unit 201 controls the viewing illuminant information input unit 203 to input the spectral radiances of the viewing illuminant designated by the user from the storage unit 210 (S12).
An example of the data structure of spectral radiance data will be described below with reference to
The color processing unit 201 controls the media characteristic input unit 202 to input the bi-spectral radiance factors of the medium designated by the user from the storage unit 210 (S13).
An example of the data structure of bi-spectral radiance factor data will be described below with reference to
An overview of a measurement of a bi-spectral radiance factor will be described below with reference to
The color processing unit 201 controls the sample information input unit 204 to input spectral reflectances of a sample according to a user instruction (S14).
An example of the data structure of spectral reflectance data will be described below with reference to
An overview of a measurement of a spectral reflectance of a sample will be described below with reference to
Subsequently, the sample 18 is irradiated with radiated light of the illuminant 13 without going through the filter 17, and radiated light from the sample 18 is received by the light-receiving device 15 via the spectroscope 14, thereby measuring a spectral radiance in the excitation wavelength range. That is, in order to measure a spectral reflectance in the excitation wavelength range, for example, UV cutoff is skipped. Then, the measured spectral radiance is divided by that of the illuminant 13, thus obtaining a spectral reflectance of the sample 18 in the excitation wavelength range.
The general measurement sequence of a spectral reflectance of a sample has been described. The measurement sequence of a spectral reflectance of a sample is summarized as follows. (i) Using an illuminant which radiates light in the visible range, in other words, an illuminant which does not radiate any light whose wavelength is shorter than the visible range and falls outside the visible range or which has a sufficiently low intensity of radiated light outside the visible range, a spectral reflectance of the sample 18 is measured in the visible range. (ii) Using an illuminant which radiates light outside the visible range (an illuminant which may or may not radiate light in the visible range), a spectral reflectance of the sample 18 is measured in a range outside the visible range.
Next, the color processing unit 201 controls the computing unit 205 to compute the bi-spectral radiance factors of the sample based on the bi-spectral radiance factors of the medium and the spectral reflectances of the sample (S15). Furthermore, the color processing unit 201 controls the calculation unit 206 to calculate colorimetric values based on the bi-spectral radiance factors of the sample and the spectral radiances of the viewing illuminant using equations (2) (S16). Moreover, the color processing unit 201 stores the calculated colorimetric values in a file having the file name designated by the user and saves the file in the storage unit 210 (S17). Note that the computing unit 205 and calculation unit 206 repeat computation and calculation processes in correspondence with the number of samples (color patches), thus calculating colorimetric values when color patches printed on the medium designated by the user are viewed under the viewing illuminant designated by the user.
Computing Unit
A spectral reflectance of a medium 31 and that of a sample will be described below with reference to the conceptual views shown in
On the other hand,
In other words, a spectral reflectance R0(λ) of a sample is considered as a result obtained by multiplying the spectral reflectance Fw(λ) of the medium 31 by a square of an attenuation of light by the color material 32. That is, the spectral reflectance R0(λ) of the sample and the spectral reflectance Fw(λ) of the medium 31 have the following relationship:
R0(λ)=T(λ)2Fw(λ)
T(λ)=√{R0(λ)/Fw(λ)} (3)
where T(λ) is a spectral attenuation (a reciprocal of a spectral transmittance) of the color material.
The processing (S15) of the computing unit 205 will be described below with reference to the flowchart shown in
The computing unit 205 inputs bi-spectral radiance factors Fw(μ, λ) of the medium (S51), and inputs the spectral reflectances R0(λ) of the sample (S52). In the bi-spectral radiance factor data shown in
T(λ)=√{R0(λ)/Fw(λ,λ)} (4)
Next, the computing unit 205 calculates each bi-spectral radiance factor Fe(μ, λ) of the sample (S54) using:
Fe(μ,λ)=Fw(μ,λ)T(μ)T(λ) (5)
where
μ is a wavelength of incident light,
λ is a wavelength of reflected light,
T(μ) is an attenuation at the wavelength μ, and
T(λ) is an attenuation at the wavelength λ.
The computation concept of the bi-spectral radiance factor Fe(μ, λ) of the sample will be described below with reference to
When the bi-spectral radiance factors of the medium and the spectral radiances of color patches printed on the medium are measured in this way, the bi-spectral radiance factors of the color patches can be computed. Therefore, using the bi-spectral radiance factors of the color patches, colorimetric values of color patches printed on various media under various viewing illuminants can be accurately calculated. Note that various media include those which do not contain any florescent material, those containing fluorescent materials which have excitation wavelengths within a range outside the visible range and produce fluorescence in the visible range, and those containing fluorescent materials which have excitation wavelengths within the visible range and produce fluorescence in the visible range. Also, various illuminants include those which emit light rays of excitation wavelengths of florescent materials contained in media, and those which do not emit light rays of excitation wavelengths of florescent materials contained in media.
Color processing according to the second embodiment of the present invention will be described hereinafter. Note that the same reference numerals in the second embodiment denote the same components as in the first embodiment, and a detailed description thereof will not be repeated.
Functions of the color processing unit 201 shown in
A measurement unit 402 measures, for example, spectral radiances of an illuminant and spectral reflectances of samples. A computing unit 403 inputs the spectral radiances of the illuminant and the spectral reflectances of the samples measured by the measurement unit 402, calculates bi-spectral radiance factors of the samples, and computes colorimetric values of the samples under a viewing illuminant. A storage unit 404 stores measured values of the measurement unit 402, various data required for computations of the computing unit 403, and computation results. That is, the measurement unit 402 corresponds to the measuring device 109 shown in
Note that the bi-spectral radiance factors of a medium are saved in the storage unit 404. Data stored in the storage unit 404 can be read out by an external computer or information processing apparatus via an interface (not shown; for example, a USB interface).
The first and second embodiments have exemplified the case in which colorimetric values (XYZ values) of samples corresponding to a viewing illuminant are output. However, the present invention is not limited to such specific output values. For example, the bi-spectral radiance factors of samples may be output, or CIELab values on a uniform color space or values (Jab values) on a color appearance space such as CIECAM02 may be output. A profile may be created using the colorimetric values of the computation results, and the created profile may be output. The profile describes color reproduction characteristics of a device used by a color management system. For example, a general profile for an RGB printer stores an LUT which represents the correspondence relationship between RGB values and XYZ values (or Lab values or Jab values) of color patches of 93=729 colors obtained by respectively dividing RGB value ranges in 9 steps.
Also, the present invention is applicable to a measuring device incorporated in an image input device, image display device, or image output device. For example, when the present invention is applied to a measuring device incorporated in a printer, the present invention can be used in calibration of color reproduction characteristics of that printer, and creation and updating of a profile of that printer.
In the example of the above description, the media characteristic input unit 202 inputs the bi-spectral radiance factors of a medium from the storage unit 210. Alternatively, the media characteristic input unit 202 may control the measuring device 109 to measure the bi-spectral radiance factors of the medium and may input the bi-spectral radiance factors of the measurement results. Likewise, in the example of the above description, the viewing illuminant information input unit 203 inputs the spectral radiances of a viewing illuminant from the storage unit 210. Alternatively, the viewing illuminant information input unit 203 may control the measuring device 109 to measure the spectral radiances of the viewing illuminant, and may input the spectral radiances of the measurement results.
Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (for example, computer-readable medium).
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2010-186539, filed Aug. 23, 2010, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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2010-186539 | Aug 2010 | JP | national |
Number | Name | Date | Kind |
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5636015 | Imura et al. | Jun 1997 | A |
6278521 | Jablonski et al. | Aug 2001 | B1 |
20060227319 | Imura | Oct 2006 | A1 |
Number | Date | Country |
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2006-292510 | Oct 2006 | JP |
Entry |
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Mahnax Mohammadi, “Developing and Imaging Bi-Spectrometer for Fluorescent Materials” Feb. 23, 2009. http://www.cis.rit.edu/DocumentLibrary/admin/uploads/CIS000111.pdf. |
Number | Date | Country | |
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20120044497 A1 | Feb 2012 | US |