Color measuring method and device for printed matter

• US 20020135768A1
• Filed: 11/01/2001
• Published: 09/26/2002
• Est. Priority Date: 11/06/2000
• Status: Active Grant

• Current Assignee: Dai Nippon Printing Company Limited
• Original Assignee: Dai Nippon Printing Company Limited
• Inventors: Kudo, Yoshiaki, Sugiyama, Tohru
• Granted Patent: US 6,671,050 B2
• Time in Patent Office: Days
• US Class Current: 356/405
• CPC Class Codes: G01N 21/27 using photo-electric detection G01N21/31 takes precedence; circuits for computing concentration logarithmic circuits G06G7/24; photometric circuits in general G01J
• CPC Class Codes: G01N 21/64 Fluorescence; Phosphorescence

Abstract

The indication of correct color evaluation standards of a printed matter including a paper containing a fluorescent whitening agent, is enabled even in the case where the printed matter is observed under a light source that differs from the light source used for color measurement. A spectral reflectance Pt(λ) of the paper under a light source T and a spectral reflectance Pu(λ) of the paper under a light source U are measured in advance and a differential component, due to the fluorescent whitening agent, is determined as Fp(λ)=Pu(λ)−Pt(λ). For a printed matter of dot percent S, a spectral reflectance Rt(λ, S) under light source T is measured, and the measured values are subject to a correction process of adding an effective component within the differential component Fp(λ) that is in accordance to dot percent S to determine an estimated spectral reflectance Rtu(λ, S) under light source U. Estimation in likewise manner can also be performed for XYZ tristimulus values.

Claims

1. A color measuring method for determining a spectral reflectance, under a predetermined illumination environment, for a printed matter including a paper containing a fluorescent whitening agent, the method being comprised of:

a first step of measuring a spectral reflectance Pt(λ

) of said paper under a first illumination environment and a spectral reflectance Pu(λ

) of said paper under a second illumination environment;

a second step of computing a difference, obtained by subtracting said spectral reflectance Pt(λ

) from said spectral reflectance Pu(λ

), as a differential component Fp(λ

) of said paper due to the fluorescent whitening agent;

a third step of measuring a spectral reflectance Rt(λ

), under said first illumination environment, for a region to be measured of said printed matter; and

a fourth step of performing a correction based on said differential component Fp(λ

) on said spectral reflectance Rt(λ

) to compute an estimated spectral reflectance, under said second illumination environment, for said region to be measured.

2. A color measuring method for printed matter as set forth in claim 1, wherein:

in a process of measurement in the first step and the third step, a result of measurement of a spectral reflectance of a perfect reflecting diffuser is used to perform a correction of eliminating influence of spectral intensity distribution of illumination light that is used.

3. A color measuring method for printed matter as set forth in claim 1, wherein:

in a process of computation in the fourth step, correction based on the differential component Fp(λ

) is not performed on an ink-adhered region within the region to be measured.

4. A color measuring method for printed matter as set forth in claim 3, wherein:

in the third step, a spectral reflectance Rt(λ

, S) under the first illumination environment is measured for a region to be measured, a ratio of an area of an ink-adhered region to an entire area in said region to be measured being S;

in the fourth step, a computation using an equation;

Rtu(λ

, S)=Rt(λ

, S)+Fp(λ

)·

(1−

S)²

 

is performed to determine an estimated spectral reflectance Rtu(λ

, S), under the second illumination environment, for said region to be measured.

5. A color measuring method for printed matter as set forth in claim 1, wherein:

in a process of computation in the fourth step, a correction based on the differential component Fp(λ

) is performed in accordance to an optical transmittance of an ink layer for an ink-adhered region in the region to be measured.

6. A color measuring method for printed matter as set forth in claim 5, wherein:

in the third step, a spectral reflectance Rt(λ

, S) under the first illumination environment is measured for a region to be measured, a ratio of an area of an ink-adhered region to an entire area in said region to be measured being S;

in the fourth step, an excitation coefficient CE and a luminescence coefficient CL are defined, said excitation coefficient CE indicating a proportion of “

total amount of excitation energy supplied to an ink-adhered part”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, said luminescence coefficient CL indicating a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, the excitation coefficient CE and the luminescence coefficient CL of said region to be measured are measured, and computation using an equation;

RRtu(λ

, S)=Rt(λ

, S)+Fp(λ

)·

(1−

S(1−

CE))·

(1−

S(1−

CL))

 

is performed to determine an estimated spectral reflectance RRtu(λ

, S), under the second illumination environment, for said region to be measured.

7. A color measuring method for printed matter as set forth in claim 6, wherein:

a spectral transmittance R(λ

) of the ink-adhered part, a spectral transmittance P(λ

) of the non-ink-adhered part, an excitation spectrum PE(λ

) of the paper, and a luminescence spectrum PL(λ

) of the paper are measured; and

the excitation coefficient CE and the luminescence coefficient CL of said ink-adhered part are determined using the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

8. A color measuring method for printed matter as set forth in claim 5, wherein:

in the third step, a spectral reflectance Rt(λ

, S₁, S₂, S₁₂) under the first illumination environment is measured for a region to be measured, ratios of areas of a first region, a second region, a third region and a fourth region with respect to an entire area in said region to be measured being S₁, S₂, S₁₂, S_p, respectively, only a first ink being adhered in said first region, only a second ink being adhered in said second region, both the first ink and the second ink being adhered in said third region, and neither ink being adhered in said fourth region;

in the fourth step, an excitation coefficient CE and a luminescence coefficient CL are defined, said excitation coefficient CE indicating a proportion of “

total amount of excitation energy supplied to an ink-adhered part”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, said luminescence coefficient CL indicating a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, excitation coefficients CE₁, CE₂, and CE₁₂ and luminescence coefficients CL₁, CL₂, and CL₁₂ of said first region, said second region, and said third region, respectively, are measured, and computation using an equation;

RRtu(λ

, S₁, S₂, S₁₂)=Rt(λ

, S₁, S₂, S₁₂)+Fp(λ

)·

(S_p+S₁·

CE₁+S₂·

CE₂+S₁₂·

CE₁₂)·

(S_p+S₁·

CL₁+S₂·

CL₂+S₁₂·

CL₁₂)

 

is performed to determine an estimated spectral reflectance RRtu (λ

, S₁, S₂, S₁₂), under the second illumination environment, for said region to be measured.

9. A color measuring method for printed matter as set forth in claim 8, wherein:

a spectral transmittance R(λ

) of the ink-adhered part, a spectral transmittance P (λ

) of the non-ink-adhered part, an excitation spectrum PE(λ

) of the paper, and a luminescence spectrum PL(λ

) of the paper are measured; and

the excitation coefficient CE and the luminescence coefficient CL of said ink-adhered part are determined using the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

10. A color measuring method for printed matter as set forth in claim 5, wherein:

in the third step, a spectral reflectance under the first illumination environment is measured for a region to be measured, on which printing using a plurality of inks is performed and a total of n kinds of ink-adhered regions are formed by mixing of a first type of region in which only an ink of a single color is adhered and a second type of region in which a plurality of inks are adhered in an overlapping manner, a ratio of an area of an i-th region with respect to an entire area in said region to be measured being Si;

in the fourth step, an excitation coefficient CE and a luminescence coefficient CL are defined, said excitation coefficient CE indicating a proportion of “

total amount of excitation energy supplied to an ink-adheredpart”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, said luminescence coefficient CL indicating a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, and the excitation coefficient CE and the luminescence coefficient CL of each of said n kinds of ink-adhered regions are measured; and

in a process of computation in the fourth step, a correction based on the differential component Fp(λ

) is performed on said n kinds of ink-adhered regions in said region to be measured in accordance to the excitation coefficients CE and the luminescence coefficients CL determined for the respective n kinds of ink-adhered regions.

11. A color measuring method for printed matter as set forth in claim 10, wherein:

a spectral transmittance R(λ

) of the ink-adhered part, a spectral transmittance P (λ

) of the non-ink-adhered part, an excitation spectrum PE(λ

) of the paper, and a luminescence spectrum PL(λ

) of the paper are measured; and

the excitation coefficient CE and the luminescence coefficient CL of said ink-adhered part are determined using the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

13. A color measuring method for printed matter as set forth in claim 12, wherein:

in a process of measurement in the first step and the third step, XYZ tristimulus values are determined by computation using spectral reflectances obtained by measurement.

14. A color measuring method for printed matter as set forth in claim 12, wherein:

in a process of computation in the fourth step, corrections based on the differential components Fp(X), Fp(Y), and Fp(Z) are not performed on an ink-adhered region within the region to be measured.

15. A color measuring method for printed matter as set forth in claim 14, wherein:

in the third step, XYZ tristimulus values Rt(X, S), Rt(Y, S), and Rt(Z, S) under the first illumination environment is measured for a region to be measured, a ratio of an area of an ink-adhered region to an entire area in said region to be measured being S;

in the fourth step, computations using the following equations;

Rtu(X, S)=Rt(X, S)+Fp(X)·

(1−

S)²Rtu(Y, S)=Rt(Y, S)+Fp(Y)·

(1−

S)²Rtu(Z, S)=Rt(Z, S)+Fp(Z)·

(1−

S)²

 

are performed to determine estimated XYZ tristimulus values Rtu(X, S), Rtu(Y, S), and Rtu(Z, S), under the second illumination environment, for said region to be measured.

16. A color measuring method for printed matter as set forth in claim 12, wherein:

in a process of computation in the fourth step, corrections based on the differential components Fp(X), Fp(Y), and Fp(Z) are performed in accordance to an optical transmittance of an ink layer for an ink-adhered region in the region to be measured.

17. A color measuring method for printed matter as set forth in claim 16, wherein:

in the third step, XYZ tristimulus values Rt(X, S) , Rt(Y, S), and Rt(Z, S) under the first illumination environment is measured for a region to be measured, a ratio of an area of an ink-adhered region to an entire area in said region to be measured being S;

in the fourth step, an excitation coefficient CE and a luminescence coefficient CL are defined, said excitation coefficient CE indicating a proportion of “

total amount of excitation energy supplied to an ink-adhered part”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, said luminescence coefficient CL indicating a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, the excitation coefficient CE and the luminescence coefficient CL of said region to be measured are measured, and computations using the following equations;

RRtu(X, S)=Rt(X, S)+Fp(X)·

(1−

S(1−

CE))·

(1−

S(1−

CL))RRtu(Y, S)=Rt(Y, S)+Fp(Y)·

(1−

S(1−

CE))·

(1−

S(1−

CL))RRtu(Z, S)=Rt(Z, S)+Fp(Z)·

(1−

S(1−

CE))·

(1−

S(1−

CL))

 

are performed to determine estimated XYZ tristimulus values RRtu(X, S), RRtu (Y, S), and RRtu (Z, S), under the second illumination environment, for said region to be measured.

18. A color measuring method for printed matter as set forth in claim 17, wherein:

a spectral transmittance R(λ

) of the ink-adhered part, a spectral transmittance P(λ

) of the non-ink-adhered part, an excitation spectrum PE(λ

) of the paper, and a luminescence spectrum PL(λ

) of the paper are measured; and

the excitation coefficient CE and the luminescence coefficient CL of said ink-adhered part are determined using the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

19. A color measuring method for printed matter as set forth in claim 16, wherein:

in the third step, XYZ tristimulus values under the first illumination environment are measured for a region to be measured, on which printing using a plurality of inks is performed and a total of n kinds of ink-adhered regions are formed by mixing of a first region in which only an ink of a single color is adhered and a second region in which a plurality of inks are adhered in an overlapping manner, a ratio of an area of an i-th region with respect to an entire area in said region to be measured being Si;

in the fourth step, an excitation coefficient CE and a luminescence coefficient CL are defined, said excitation coefficient CE indicating a proportion of “

total amount of excitation energy supplied to an ink-adhered part”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, said luminescence coefficient CL indicating a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, and the excitation coefficient CE and the luminescence coefficient CL of each of said n kinds of ink-adhered regions are measured; and

in a process of computation in the fourth step, corrections based on the differential components Fp(X), Fp(Y) , and Fp(Z) are performed on said n kinds of ink-adhered regions in said region to be measured in accordance to the excitation coefficients CE and the luminescence coefficients CL determined for the respective n kinds of ink-adhered regions.

20. A color measuring method for printed matter as set forth in claim 19, wherein:

a spectral transmittance R(λ

) of the ink-adhered part, a spectral transmittance P(λ

) of the non-ink-adhered part, an excitation spectrum PE(λ

) of the paper, and a luminescence spectrum PL(λ

) of the paper are measured; and

the excitation coefficient CE and the luminescence coefficient CL of said ink-adhered part are determined using the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

12. A color measuring method for determining XYZ tristimulus values, under a predetermined illumination environment, for a printed matter including a paper containing a fluorescent whitening agent, the method being comprised of:

a first step of measuring XYZ tristimulus values Pt(X), Pt(Y), and Pt(Z) of said paper under a first illumination environment and XYZ tristimulus values Pu(X), Pu(Y), and Pu(Z) of said paper under a second illumination environment;

a second step of computing differences, obtained by subtracting said XYZ tristimulus values Pt(X), Pt(Y), and Pt(Z) from said XYZ tristimulus values Pu(X), Pu(Y), and Pu(Z), respectively, as differential components Fp(X), Fp(Y), and Fp(Z) of said paper due to the fluorescent whitening agent;

a third step of measuring XYZ tristimulus values Rt(X), Rt(Y), and Rt(Z), under said first illumination environment, for a region to be measured of said printed matter; and

a fourth step of performing corrections based on said differential components Fp(X), Fp(Y), and Fp(Z) on said XYZ tristimulus values Rt(X), Rt(Y), and Rt(Z), respectively, to compute estimated XYZ tristimulus values, under said second illumination environment, for said region to be measured.

21. A color measuring device for determining a spectral reflectance, under a predetermined illumination environment, of a printed matter including a paper containing a fluorescent whitening agent, the color measuring device being comprised of:

a spectral reflectance measuring device, which measures a spectral reflectance of an object to be measured under a first illumination environment;

a storage unit, which stores a differential component Fp(λ

), obtained by subtracting a spectral reflectance Pt(λ

) of a specific paper under said first illumination environment from a spectral reflectance Pu(λ

) of said paper under a second illumination environment; and

a computational processing unit, which performs a correction based on said differential component Fp(λ

) on a spectral reflectance Rt(λ

), which is measured by said spectral reflectance measuring device for a region to be measured in the printed matter including said specific paper, to compute an estimated spectral reflectance, under said second illumination environment, for said region to be measured.

22. A color measuring device for printed matter as set forth in claim 21, wherein:

the computational processing unit has a function of inputting an area ratio S of an ink-adhered region with respect to an entire area within the region to be measured and performs computation using the following equation on a spectral reflectance Rt(λ

, S), which has been measured for said region to be measured by the spectral reflectance measuring device;

Rtu(λ

, S)=Rt(λ

, S)+Fp(λ

)·

(1−

S)²

 

so as to determine an estimated spectral reflectance Rtu(λ

, S), under the second illumination environment, of said region to be measured.

23. A color measuring device for printed matter as set forth in claim 22, wherein:

a dot percent measuring device, which measures an area ratio S of an ink-adhered region with respect to an entire area within the region to be measured, is furthermore equipped.

24. A color measuring device for printed matter as set forth in claim 21, wherein:

the computational processing unit has a function of inputting, with regard to the region to be measured, an area ratio S of an ink-adhered region with respect to an entire area, an excitation coefficient CE, which indicates a proportion of “

total amount of excitation energy supplied to the ink-adhered part”

per unit area with respect to “

total amount of excitation energy supplied to a non-ink-adhered part”

per unit area in the case where said parts are illuminated under the same conditions from an exterior, and a luminescence coefficient CL, which indicates a proportion of “

total amount of luminescence energy emitted from the ink-adhered part and observed”

per unit area with respect to “

total amount of luminescence energy emitted from the non-ink-adhered part and observed”

per unit area in the case where emission of fluorescence from each of said parts occurs under the same conditions inside the paper, and performs computation using the following equation on a spectral reflectance Rt(λ

, S), which has been measured for said region to be measured by said spectral reflectance measuring device;

RRtu(λ

, S)=Rt(λ

, S)+Fp(λ

)·

(1−

S(1−

CE))·

(1−

S(1−

CL))

 

so as to determine an estimated spectral reflectance RRtu(λ

, S), under the second illumination environment, of said region to be measured.

25. A color measuring device for printed matter as set forth in claim 24, furthermore equipped with:

a dot percent measuring device, which measures an area ratio S of an ink-adhered region with respect to an entire area within the region to be measured;

a transmittance measuring device, which measures a spectral transmittance R(λ

) of an ink-adhered part in the region to be measured and a spectral transmittance P(λ

) of a non-ink-adhered part in the region to be measured; and

a coefficient computation unit, which computes an excitation coefficient CE and a luminescence coefficient CL of said ink-adhered part, using said spectral transmittance R(λ

) and said spectral transmittance P(λ

) which have been measured by said transmittance measuring device, and an excitation spectrum PE(λ

) of the paper and a luminescence spectrum PL(λ

) of the paper in accordance to the following equations;

CE=∫

R(λ

)·

PE(λ

)dλ

/∫

P(λ

)·

PE(λ

)dλ

CL=∫

R(λ

)·

PL(λ

)dλ

/∫

P(λ

)·

PL(λ

)dλ

.

26. A color measuring device for printed matter as set forth in claim 25, wherein:

a spectrofluorometer, which measures an excitation spectrum PE(λ

) of the paper and a luminescence spectrum PL(λ

) of the paper, is furthermore equipped and the coefficient computation unit uses the excitation spectrum PE(λ

) of the paper and the luminescence spectrum PL(λ

) of the paper measured by said spectrofluorometer in performing computation for determining the excitation coefficient CE and the luminescence coefficient CL.

27. A color measuring device for printed matter as set forth in claim 21, wherein:

the computational processing unit has a function of computing XYZ tristimulus values based on a predetermined spectral reflectance and computes estimated XYZ tristimulus values, under the second illumination environment, for the region to be measured.

28. A color measuring device for printed matter as set forth in claim 27, wherein:

the storage unit stores the differential component Fp(λ

) in a form of XYZ tristimulus values; and

the computational processing unit performs computation using said differential component stored in the form of XYZ tristimulus values.

29. A program, which makes a computer function as the computational processing unit in a color measuring device for printed matter as set forth in claim 21, or a computer-readable medium that stores said program.