Optics: measuring and testing – By shade or color – Trichromatic examination
Reexamination Certificate
2001-09-07
2004-07-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By shade or color
Trichromatic examination
C356S405000, C382S167000
Reexamination Certificate
active
06760108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a correction based on media white points in case of creating data used in a color matching process.
2. Related Background Art
FIG. 1
is a conceptional view showing a general color matching process executed among different devices. Input data being R (red), G (green) and B (blue) data is transformed to X, Y and Z data on a color space not depending on the device, by an input profile. Since colors other than colors in a color reproduction range (gamut) of an output device can not be represented by the output device, color gamut mapping is executed to the input data to obtain data on the color space not depending on the device such that all of the colors can be included in the color reproduction range of the output device. After executing the color gamut mapping, the input data is transformed from the data on the color space not depending on the device to C (cyan), M (magenta), Y (yellow) and K (black) data on a color space depending on the output device.
In the color matching process, a reference white point and environment light are fixed. For example, in profiles regulated by ICC (International Color Consortium), PCS (Profile Connection Space) for connecting profiles has XYZ values (i.e., X, Y and Z values) and Lab values under the reference of D
50
. Therefore, correct color reproduction for input originals and print outputs can be guaranteed in case of performing observation under a light source of which characteristic is defined by the reference of D
50
, and the correct color reproduction can not be guaranteed under a light source of other characteristics.
In case of observing the same sample (e.g., an image) under different light sources, the XYZ values for the sample to be observed are naturally differed in their values. In order to make a prediction of the XYZ values under the different light sources, there are transformation systems such as a prediction expression and the like according to (1) ratio transformation, (2) Von Kries transformation, (3) color perception model, and the like.
The ratio transformation is such a method of performing a ratio transformation of W
2
/W
1
in order to transform XYZ values under a reference white point W
1
to XYZ values under a reference white point W
2
. When this method is adopted to a Lab uniform color space, Lab values under the W
1
coincide with Lab values under the W
2
. For example, when XYZ values for a sample under the W
1
(Xw
1
, Yw
1
, Zw
1
) are assumed as XYZ values (X
1
, Y
1
, Z
1
), and XYZ values for a sample under the W
2
(Xw
2
, Yw
2
, Zw
2
) are assumed as XYZ values (X
2
, Y
2
, Z
2
), following relationship is obtained according to the ratio transformation.
X
2
=(
Xw
2
/
Xw
1
)·
X
1
Y
2
=(
Yw
2
/
Yw
1
)·
Y
1
Z
2
=(
Zw
2
/
Zw
1
)·
Z
1
(1)
The Von Kries transformation is such a method of performing a ratio transformation of W
2
′/W
1
′ on a human's color perception space PQR in order to transform the XYZ values under the W
1
to the XYZ values under the W
2
. When this method is adopted to the Lab uniform color space, the Lab values under the W
1
are not coincided with the Lab values under the W
2
. For example, when the XYZ values for the sample under the W
1
(Xw
1
, Yw
1
, Zw
1
) are assumed as the XYZ values (X
1
, Y
1
, Z
1
), and the XYZ values for the sample under the W
2
(Xw
2
, Yw
2
, Zw
2
) are assumed as the XYZ values (X
2
, Y
2
, Z
2
), following relationship is obtained according to the Von Kries transformation.
[
X2
Y2
Z2
]
=
[
M
-
1
]
[
P2
/
P1
0
0
0
Q2
/
Q1
0
0
0
R2
/
R1
]
[
M
]
[
X1
Y1
Z1
]
(
2
)
where,
[
P1
Q1
R1
]
=
[
M
]
[
Xw1
Yw1
Zw1
]
[
P2
Q2
R2
]
=
[
M
]
[
Xw2
Yw2
Zw2
]
[
M
]
=
[
0.40024
0.70760
-
0.08081
-
0.22630
1.16532
0.04570
0
0
0.91822
]
[
M
-
1
]
=
[
1.85995
-
1.12939
0.21990
0.36119
0.63881
0
0
0
1.08906
]
The prediction expression according to the color perception model is a method of performing transformation by utilizing, for example, a human's color perception space QMH (or JCH) such as a space related in the CIECAM97s in order to transform XYZ values under an observation condition VC
1
(including W
1
) to XYZ values under an observation condition VC
2
(including W
2
). Where, symbol Q in the abbreviation QMH denotes brightness, symbol M denotes colorfulness and symbol H denotes huequadrature or hueangle. Symbol J in the abbreviation JCH denotes lightness, symbol C denotes chroma and symbol H denotes huequadrature or hueangle.
For example, when the XYZ values for the sample under the W
1
(Xw
1
, Yw
1
, Zw
1
) are assumed as the XYZ values (X
1
, Y
1
, Z
1
), and the XYZ values for the sample under the W
2
(Xw
2
, Yw
2
, Zw
2
) are assumed as the XYZ values (X
2
, Y
2
, Z
2
), the following transformation is performed according to the color perception model.
That is, (
X
1
,
Y
1
,
Z
1
)→[CIECAM97s forward transformation]→(
Q, M, H
) or (
J, C, H
)→[CIECAM97s inverse transformation]→(
X
2
,
Y
2
,
Z
2
) (3)
However, the above description depends on a case that the sample is represented on ideal media (white point on medium corresponds to complete reflection and black point on medium corresponds to complete absorption), while the media actually used have different conditions.
For example, since colors displayed on a monitor are represented by light source colors, a white point on medium for color data of R=G=B=225 can be considered as relative brightness of Y=100%, however, a black point on medium for color data of R=G=B=0 does not correspond to relative brightness of Y=0%. Since color of printed matter corresponds to color of the object (or objective color), whiteness in paper represented by C=M=Y=K=0% has a certain reflection ratio &ggr;, therefore, the relative brightness of Y=100% can not be obtained. Even color represented by C=M=Y=K=100% being the most dark color is not resulted in the relative brightness of Y=0%.
Since the whiteness in paper of the printed matter does not have characteristic of complete reflection, it does not coincide with the white point (e.g., D
50
) of light source to be irradiated. Similarly, black (C=M=Y=K=100%) available in a device does not exist on a gray axis of the light source (chroma identical to that of the light source) to be irradiated.
Hereinafter, a white point (color corresponding to R=G=B=255 in an RGB device, and corresponding to C=M=Y=K=0% in a CMYK device) represented on medium of each device is defined as “media white point”. A black point (color corresponding to R=G=B=0 in the RGB device, and corresponding to C=M=Y=K=100% in the CMYK device) represented on medium of each device is defined as “media black point”.
As in the above description, since “white point of light source”, “media white point”, “black point of light source” and “media black point” are different, in case of performing comparison of images on different media under the same light source, there sometimes occurs a case that impressions of those images are different from each other. For example, in a case where white points on media are different from each other such as white points on a blank (white) paper and newspaper (or a recycled paper), when those media are laterally arranged for the comparison, it is obviously recognized by a person that the whiteness in paper are different from each other. However, in case of individually performing the comparison for each of media, even if a media white point is considerably different from other media white points, like the newspaper, the person perceives the whiteness on a paper as “white”. This phenomenon occurs due to a fact that human's visual perception is adapted to the color of white. And, if t
Font Frank G.
Punnoose Roy M.
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