Data processing: measuring – calibrating – or testing – Calibration or correction system
Reexamination Certificate
1998-12-04
2001-08-07
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
C356S073000, C356S407000, C356S408000, C356S425000, C250S559040
Reexamination Certificate
active
06272440
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for determining the color and/or composition of a material.
The invention further relates to an apparatus for determining color and/or composition of a material.
Color is a property of an object which depends on the object, conditions of illumination, and the observer. In general, the light reflected or transmitted by a non-self-luminous object depends on the nature of the light simultaneously incident on the object, and the geometrical relation of the light source and object. The perceived color of the reflected or transmitted light depends additionally on the visual receptivity of the observer, and the geometrical relation of the observer to said light.
The apparent reflectance of a non-self-luminous object in a particular geometrical relation to the light source and observer is defined to be the ratio of the spectral power in each wavelength band of the reflected light to the spectral power of the same wavelength band of the incident light:
R
(
λ
)
=
reflected
(
λ
)
incident
(
λ
)
(
1
)
Similarly, the apparent transmittance of a non-self-luminous object in a particular relation to the light source and observer is defined to be the ratio of the spectral power in each wavelength band of the transmitted light to the spectral power of the same wavelength band of the incident light:
T
(
λ
)
=
transmitted
(
λ
)
incident
(
λ
)
(
2
)
Absorbance is often used instead of transmittance, being the ratio of spectral power in each wavelength band of the absorbed light to the spectral power of the same wavelength band of the incident light. Thus, it is the complement of transmittance:
A
(
λ
)
=
absorbed
(
λ
)
incident
(
λ
)
=
1
-
transmitted
(
λ
)
incident
(
λ
)
=
1
-
T
(
λ
)
(
3
)
An alternative definition of absorbance is the logarithm of the absorbance as defined in (3). Absorbance and transmittance are interchangeable by trivial modification of any expression in which one or the other appears. In this specification, where either absorbance or transmittance is used, it is to be understood in each case that the equivalent formulation using the other is tacitly implied and within the scope of the specification. Similarly, while this specification expresses reflectance, transmittance, and other quantities as functions of wavelengths, equivalent expressions as functions of frequency or wave number are also in common use. These quantities can be easily converted between the different formulations. Thus, wherever a quantity is expressed as a function of wavelength, it is to be understood in each case that the equivalent formulations using functions of frequency or wave number are tacitly implied and within the scope of the specification.
Clearly, reflectance and transmittance as defined in (1) and (2) have meaning only for wavelength bands in which the incident light has sufficient power to be detectable. Accordingly, rich light sources, having significant amounts of energy at all humanly visible wavelengths, are normally used for measuring them.
Since the perceived color of an object depends on so many factors, standardization of definitions is most important for each of the variables. Standards authorities, such as the CIE (Commission Internationale d'Éclairage), have specified generally accepted standard illuminants having particular spectral power distributions, and color measurement devices usually contain means for approximating one or two such illuminants. Such means is often a rich physical light source with specific optical filters. The C, D55, D65, and D75 sources are frequently encountered, but others such as A, D60, F2, etc. may also be found in industrial applications.
Similarly, since human observers may match color samples differently depending on the size of the color samples, standard spectral observers have been defined for 2 degree and 10 degree fields of view.
Since human vision reduces many wavelength bands in a light spectrum into a three dimensional signal in the retina, color is conventionally expressed as colorimetric quantities having three values. Colorimetric systems in common use include for example CIE Tristimulus; CIE Chromaticity, Lightness; CIE L*a*b*; Hunter L,a,b; Hue Angle, Saturation Value and Dominant wavelength, Excitation purity, Lightness.
Under given conditions of illumination and geometry, CIE tristimulus values may be calculated for the standard spectral observers using formulae which are defined by the CIE. These tristimulus values provide a base from which the other colorimetric quantities can be calculated using formulae defined by the pertinent standards authorities. Such formulae are occasionally revised, as the state of the art is improved. Some auxiliary colorimetric quantities are also of importance in appearance specifications. These are also derived from the tristimulus values, with definitions provided by the CIE and other standards authorities. They include for example tint; whiteness index; yellowness index and blue reflectance.
The tristimulus values are calculated from the apparent reflectance or transmittance of an object, using the spectral power distribution of the illuminant for which the object's color appearance is to be evaluated. Conventionally, tristimulus values are defined as integrals but are normally evaluated as finite approximations:
X
=
k
∫
380
780
R
(
λ
)
S
(
λ
)
x
_
(
λ
)
ⅆ
λ
=
k
∑
j
=
1
N
R
j
S
j
x
_
j
δλ
(4a)
Y
=
k
∫
380
780
R
(
λ
)
S
(
λ
)
y
_
(
λ
)
ⅆ
λ
=
k
∑
j
=
1
N
R
j
S
j
y
_
j
δλ
(4b)
Z
=
k
∫
380
780
R
(
λ
)
S
(
λ
)
z
_
(
λ
)
ⅆ
λ
=
k
∑
j
=
1
N
R
j
S
j
z
_
j
δλ
(4c)
where k is a normalization factor, S is the spectral power distribution of the target illuminant, and x, y, z, are the standard observer functions, tabulated at uniform wavelength intervals. In the case that the reflectance data is abridged or truncated, or measured at non-standard wavelength intervals, there are various recommended techniques for interpolation, extrapolation or resampling. Similar equations to (4a-4c) and corresponding methods are used in calculating tristimulus from transmittance spectra. Note that the spectral power distribution of the illuminant used in evaluations (4a-4c) need not be the same as the spectral power distribution of the source used to illuminate the sample during measurement of reflectance or transmittance. It is assumed that the reflectance and transmittance do not depend on the light source.
Each industry tends to have a preferred colorimetric system, although there may be regional differences in such preference. For example, Hunter L,a,b is used widely in the papermaking industry in the U.S.A., but rarely elsewhere, as CIE L*a*b is preferred in the papermaking industry in most other regions, and is also used in the U.S.A. The CIE L*a*b values are defined (1976) for photopic conditions as follows:
L
*
=
116
(
Y
/
Y
n
)
1
/
3
-
16
(5a)
a
*
=
500
[
(
X
/
X
n
)
1
/
3
-
(
Y
/
Y
n
)
1
/
3
]
(5b)
b
*
=
200
[
(
Y
/
Y
n
)
1
/
3
-
(
Z
/
Z
n
)
1
/
3
]
(5c)
where X
n
, Y
n
, and Z
n
are the tristimulus values for the illuminant. Photopic conditions exist when the ratios X/X
n
, Y/Y
n
, and Z/Z
n
all exceed 0.008856; otherwise either mesopic or scotopic conditions exist, and the equations used differ from (5a), (5b) and (5c), as described in ASTM test method E308-90, for example. These and other issues of colorimetry are well known per se, and are not further discussed. Measurement of color and evaluation of colorimetric quantities in photopic, mesopic, and scotopic conditions are contemplated
Shakespeare John
Shakespeare Tarja
Hoff Marc S.
Ladas & Parry
Metso Paper Automation Inc.
Vo Hien
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