Method for color measurement
View Patent ↗The present disclosure is directed to a method for producing color spectral data of an article illuminated with lights emitted from one or more light emitting units, such as light emitting diodes (LEDs). The method is useful for obtaining accurate color spectral data of the article and for matching the color data produced from another color measuring instrument. The method is particularly useful for obtaining color spectral data using light emitting diodes as light source for color measurement. This disclosure is also directed to a color measuring system using the method disclosed therein.
1. A method for producing color spectral data of an article, said method comprising the steps of:
a) measuring individual specimen reflection intensity data S n of lights emitted from a number n light unit of a first set of one or more light emitting units and reflected by said article using one or more detectors, said S n is measured over a range of wavelength λ and under a voltage U n applied to said number n light unit;
b) obtaining individual characterized spectral power distribution data SPDC n,λ,t of said number n light unit, said SPDC n,λ,t characterizes the number n light unit over the range of wavelength λ, at one or more known temperatures t and under one or more individual voltages V n,t applied to said number n light unit;
c) generating individual raw spectral power distribution data SPDR n,λ of said number n light unit based on said SPDC n,λ,t and the U n ;
d) generating individual normalized spectral power distribution data SPDN n,λ of the number n light unit by normalizing said SPDR n,λ ;
e) obtaining an individual white normalization factor W n of the number n light unit;
f) repeating the steps of a) through e) for k light units of said first set of one or more light emitting units; and
g) producing a high resolution spectrum H λ for said k light units by a process comprising the steps of:
g1) generating a COG n of the number n light unit using the formula (i):
COG
n
=
∑
λ
=
380
760
λ
×
SPDN
n
,
λ
∑
λ
=
380
760
SPDN
n
,
λ
;
(
i
)
g2) generating a specimen reflectance value L n unit using the formula (ii):
L n =S n ×W n (ii);
g3) generating a spectral optimization seed value T λ by interpolating said COG n and said L n ;
g4) optionally, generating an initial medium resolution spectrum M λ based on the T λ ;
g5) generating an initial high resolution spectrum H λ by interpolating from the spectral optimization seed value T λ , or optionally from the initial medium resolution spectrum M λ ;
g6) calculating an estimated low resolution reflectance value E n at each COG n using the formula (iii):
E
n
=
∑
λ
=
380
760
H
λ
×
SPDN
n
,
λ
;
(
iii
)
g7) calculating an error function Δ using the formula (iv):
Δ
=
∑
n
=
1
k
(
E
n
-
L
n
)
2
;
(
iv
)
g8) adjusting the spectral optimization seed value T λ to a subsequent T λ , or optionally adjusting the initial medium resolution spectrum M λ to a subsequent M λ , and repeating steps g5) through g7) using the subsequent T λ to replace the spectral optimization seed value T λ or using the subsequent M λ to replace the initial medium resolution spectrum M λ , if the error function Δ in step g7) is not converged; and
g9) producing said high resolution spectrum H λ based on the spectral optimization seed value T λ or the subsequent T λ , or optionally, based on said initial medium resolution spectrum M λ or said subsequent M λ , when the error function Δ is converged;
wherein:
k is the number of the light emitting units measured;
n is an index of the light emitting units and is in a range of from 1 to k;
λ is a wavelength of lights in a range of from 380 nm to 760 nm.
2. The method of claim 1 further comprising the step of: g10) producing a color spectral data F j from the formula (v) based on said high resolution spectrum H λ and a slit function B j,λ ,
F
j
=
∑
λ
=
380
760
H
λ
×
B
j
,
λ
∑
λ
=
380
760
B
j
,
λ
;
(
v
)
wherein:
said color spectral data F j match a target color spectral data of said article produced from a target color measuring instrument having the slit function B j,λ and a second set of one or more light emitting units.
3. A method of claim 1 further comprising the steps of producing a color spectral data F j from the formula (v) based on said high resolution spectrum H λ using an artificial neural network trained with H λ values and corresponding F j values.
4. The method of claim 2 or 3 further comprising the step of generating color space data values based on said color spectral data F j , said color space data values are selected from XYZ values, L,a,b values, L*,a*,b* values, L,C,h values, or a combination thereof.
5. The method of claim 2 or 3 further comprising the step of outputting said color spectral data F j to an output device.
6. The method of claim 1 , wherein said individual white normalization factor W n of the number n light unit is generated by the steps of:
e1) obtaining individual reference reflection intensity data S n ′ of lights emitted from said number n light unit and reflected by a white reference standard having a reflection factor WR n ; and
e2) generating the W n using the formula (vi):
W
n
=
WR
n
S
n
′
.
(
vi
)
7. The method of claim 6 , wherein said individual reference reflection intensity data S n ′ is measured at one or more detection angles in a range of from 15° to 115° from the aspecular reflection.
8. The method of claim 6 , wherein said individual reference reflection intensity data S n ′ is measured at one or more detection angles selected from 15°, 25°, 45°, 75°, 110°, or a combination thereof, from the aspecular reflection.
9. The method of claim 1 , wherein said individual normalized spectral power distribution data SPDN n,λ of the number n light unit is generated by normalizing said SPDR n,λ to an area-under-the curve equal to a predetermined value.
10. The method of claim 1 , wherein said individual specimen reflection intensity data S n is measured at one or more detection angles in a range of from 15° to 115° from the aspecular reflection.
11. The method of claim 1 , wherein said individual specimen reflection intensity data S n is measured at one or more detection angles selected from 15°, 25°, 45°, 75°, 110°, or a combination thereof, from the aspecular reflection.
12. The method of claim 1 further comprising the step of generating color space data values based on said high resolution spectrum H λ , said color space data values are selected from XYZ values, L,a,b values, L*,a*,b* values, L,C,h values, or a combination thereof.
13. A process for matching a color an article, said process comprising the steps of producing color spectral data of said article according to the method of any one of the claims 1 - 2 , 3 and 6 - 11 , 12 .
14. A process for producing one or more matching formulations to match a color an article, said process comprising the steps of producing color spectral data of said article according to the method of any one of the claims 1 - 2 , 3 and 6 - 11 , 12 .
15. A color measuring system for producing color spectral data of an article, said color measuring system comprising:
one or more light emitting units;
one or more light detectors;
an output device;
one or more computing devices communicating among the light emitting units, the light detectors, the output device and each of the computing devices; and
a non-transitory computing program product comprising computer executable codes, when in operation, causing said one or more computing devices of said color measuring system to perform a computing process comprising the steps of:
a) receiving from the light detectors individual specimen reflection intensity data S n of lights emitted from a number n light unit of a first set of one or more light emitting units and reflected by said article, said S n is measured over a range of wavelength λ and under a voltage U n applied to said number n light unit;
b) receiving individual characterized spectral power distribution data SPDC n,λ,t of said number n light unit, said SPDC n,λ,t characterizes the number n light unit over the range of wavelength λ, at one or more known temperatures t and under one or more individual voltages V n,t applied to said number n light unit;
c) generating individual raw spectral power distribution data SPDR n,λ of said number n light unit based on said SPDC n,λ,t and the U n ;
d) generating individual normalized spectral power distribution data SPDN n,λ of the number n light unit by normalizing said SPDR n,λ ;
e) receiving an individual white normalization factor W n of the number n light unit;
f) repeating the steps of a) through e) for k light units of said first set of one or more light emitting units; and
g) producing a high resolution spectrum H λ for said k light units by a process comprising the steps of:
g1) generating a COG n of the number n light unit using the formula (i):
COG
n
=
∑
λ
=
380
760
λ
×
SPDN
n
,
λ
∑
λ
=
380
760
SPDN
n
,
λ
;
(
i
)
g2) generating a specimen reflectance value L n unit using the formula (ii):
L n =S n ×W n (ii);
g3) generating a spectral optimization seed value T λ by interpolating said COG n and said L n ;
g4) optionally, generating an initial medium resolution spectrum M λ based on the T λ ;
g5) generating an initial high resolution spectrum H λ by interpolating from the spectral optimization seed value T λ , or optionally from the initial medium resolution spectrum M λ ;
g6) calculating an estimated low resolution reflectance value E n at each COG n using the formula (iii):
E
n
=
∑
λ
=
380
760
H
λ
×
SPDN
n
,
λ
;
(
iii
)
g7) calculating an error function Δ using the formula (iv):
Δ
=
∑
n
=
1
k
(
E
n
-
L
n
)
2
;
(
iv
)
g8) adjusting the spectral optimization seed value T λ to a subsequent T λ , or optionally adjusting the initial medium resolution spectrum M λ to a subsequent M λ , and repeating steps g5) through g7) using the subsequent T λ to replace the spectral optimization seed value T λ or using the subsequent M λ to replace the initial medium resolution spectrum M λ , if the error function Δ in step g7) is not converged; and
g9) producing said high resolution spectrum H λ , based on the spectral optimization seed value T λ or the subsequent T λ , or optionally, based on said initial medium resolution spectrum M λ or said subsequent M λ , when the error function Δ is converged;
wherein:
k is the number of the light emitting units measured;
n is an index of the light emitting units and is in a range of from 1 to k;
λ is a wavelength of lights in a range of from 380 nm to 760 nm.
16. The color measuring system of claim 15 , wherein said one or more light emitting units are light emitting diodes (LEDs).
17. The color measuring system of claim 15 , wherein said computer program product further comprises computer executable codes, when in operation, causing said one or more computing devices to perform a computing process of:
g10) producing a color spectral data F j from the formula (v) based on said high resolution spectrum H λ and a slit function B j,λ ,
F
j
=
∑
λ
=
380
760
H
λ
×
B
j
,
λ
∑
λ
=
380
760
B
j
,
λ
;
(
v
)
wherein:
said color spectral data F j match a target color spectral data of said article produced from a target color measuring instrument having the slit function B j,λ and a second set of one or more light emitting units.
18. The color measuring system of claim 17 , wherein said first set and said second set of one or more light emitting units are the same or different.
19. The color measuring system of claim 15 , wherein said computer program product further comprises computer executable codes, when in operation, causing said computing devices to perform a computing process of:
generating said individual white normalization factor W n of the number n light unit by the steps of:
e1) receiving individual reference reflection intensity data S n ′ of lights emitted from said number n light unit and reflected by a white reference standard having a reflection factor WR n ; and
e2) generating the W n using the formula (vi):
W
n
=
WR
n
S
n
′
.
(
vi
)