System and method for compensating light source drift at different wavelengths with a single reference channel in a light measuring device
In accordance with a broad aspect of the present invention the system and method described are directed to correcting with a single reference channel thermally dependent spectrum shifts of a light source used with a spectrophotometer. In one embodiment of the system and method described, a light measuring device uses the measured light intensity values to generate a correction coefficient for use in correcting measured light from an object under analysis.
1. A computer implemented method to compensate temperature induced light source drift in a spectrophotometer using a light measuring device having a processor, a memory, a light source configured to emit broad band light, a sample sensor configured to output a sample signal in response to light being incident upon the sensor, a reference sensor configured to output a reference signal in response to light being incident upon the reference sensor, and a calibration measurement application stored in the memory and executable by the processor, the method comprising the steps of:
emitting a plurality of flashes of light from the light source such that the light is incident on a reference target and a calibration target,
measuring, for each of the plurality of light flashes, the intensity of each wavelength of the light reflected off a calibration target using the sample sensor, and the intensity of the light reflected off a reference target using the reference sensor;
transforming the measured values into a compensation coefficient value based on the relationship between the sample sensor measurement and the reference sensor measurement at each wavelength;
obtaining a sample measurement of the intensity at different wavelengths of light reflected off of a sample under analysis with the sample sensor and a reference measurement of the intensity of light reflected off of the reference target;
generating a corrected sample measurement according to a compensation algorithm that incorporates the compensation coefficient; and
outputting the corrected sample signal.
2. The method of claim 1 , wherein the compensation algorithm is:
I
c
(
λ
)
=
I
(
λ
)
+
k
(
λ
)
*
[
I
(
λ
)
I
W
(
λ
)
]
*
(
T
1
-
T
0
)
where I(λ) is the measured sample signal at wavelength λ when the object is obtained, k(λ) is the transformed coefficient value at wavelength λ, T 1 is the measured reference channel signal when the object is measured, T 0 is a measured reference signal value obtained during the measuring step, I W (λ) is a measured sample signal value obtained during the measuring step and where I c (λ) is the corrected sample signal.
3. The method of claim 2 , wherein I W (λ) and T 0 are derived from the measurements obtained in the measuring step.
4. The method of claim 2 , wherein I W (λ) and T 0 are selected from among the values obtained in the measuring step.
5. The method of claim 2 , further comprising the step of:
generating a reflectance R(λ) value of the object based on I C (λ), according to the following:
R (λ)= I C (λ) R W (λ)/ I W (λ).
6. The method of claim 1 , wherein the transforming step includes performing a linear regression on the sample and reference signals at each wavelength to generate the compensation coefficient.
7. The method of claim 1 , where the light source is an LED.
8. The method of claim 1 , where the sample sensor is a spectrophotometer.
9. The method of claim 1 , where the light source is subject to thermally dependent spectrum shift.
10. The method of claim 1 , wherein the reference sensor is selected from a broadband sensor, a narrowband sensor, a single band sensor, or a multiband sensor.
11. The method of claim 1 , wherein the reference target is part of the inside surface of an integrating sphere.
12. A system for compensating thermally dependent light source drift in a spectrophotometer, the system comprising:
a. a spectrophotometer having a processor, a memory, a light source configured to emit broad-band light, a sample sensor configured to output a sample signal in response to light being incident upon the sensor, a reference sensor configured to output a reference signal in response to light being incident upon the reference sensor,
b. the processor of the spectrophotometer configured by code executing therein to implement the steps of:
emitting a plurality of flashes of light from the light source such that the light is incident on a reference target and a calibration target,
measuring for each of the plurality of light flashes, the intensity at each wavelength of the light reflected off a calibration target using the sample sensor and the intensity of the light reflected off a reference target using the reference sensor;
transforming the measured values into a compensation coefficient based on the relationship between the signal channel measurement and the reference channel measurement at each wavelength;
obtaining a sample measurement of the intensity at different wavelengths of light reflected off of a sample under analysis with the sample sensor and a reference measurement of the intensity of light reflected off of the reference target;
generating a corrected sample signal according to:
I
c
(
λ
)
=
I
(
λ
)
+
k
(
λ
)
*
[
I
(
λ
)
I
W
(
λ
)
]
*
(
T
1
-
T
0
)
where I(λ) is the measured sample signal at wavelength λ when the object is measured, k(λ) is the generated coefficient at wavelength λ, T 1 is the measured reference channel signal when the object is measured, T 0 is a reference signal value obtained from the measuring step, I W (λ) is a measured sample signal value obtained from the measuring step and where I c (λ) is the corrected sample signal; and
outputting the corrected sample signal.
13. The method of claim 12 , wherein I W (λ) and T 0 are the average values of the measurements obtained in the measuring step.
14. The method of claim 13 , further comprising the step of:
generating a reflectance R(λ) value of the object based on I C (λ), according to the following:
R (λ)= I C (λ) R W (λ)/ I W (λ).
15. The method of claim 12 , wherein I W (λ) and T 0 are selected from among the values obtained in the measuring step.
16. The method of claim 12 , wherein the transforming step includes performing a linear regression on the sample and reference signals at each wavelength to generate the compensation coefficient.
17. The method of claim 12 , where the light source is an LED.
18. The method of claim 12 , wherein in the reference target is an integrating sphere.
19. A system for compensating thermally dependent light source drift in a spectrophotometer, the system comprising:
a. a spectrophotometer having a processor, a memory, a light source configured to emit broad-band light, a sample sensor configured to output a sample signal in response to light being incident upon the sensor, a reference sensor configured to output a reference signal in response to light being incident upon the reference sensor,
b. the processor of the spectrophotometer configured by code executing therein to implement the steps of:
accessing a calibration coefficient from the memory;
obtaining a sample measurement of the intensity at different wavelengths of light reflected off of a sample under analysis with the sample sensor and a reference measurement of the intensity of light reflected off of the reference target;
generating a corrected sample signal according to:
I
c
(
λ
)
=
I
(
λ
)
+
k
(
λ
)
*
[
I
(
λ
)
I
W
(
λ
)
]
*
(
T
1
-
T
0
)
where I(λ) is the measured sample signal at wavelength λ when the object is measured, k(λ) is the generated coefficient at wavelength λ, T 1 is the measured reference channel signal when the object is measured, T 0 is a reference signal value obtained from the measuring step, I W (λ) is a measured sample signal value obtained from the measuring step and where I c (λ) is the corrected sample signal; and
outputting the corrected sample signal.
20. The system for compensating thermally dependent light source drift in a spectrophotometer of claim 19 , wherein the calibration coefficient value stored in the memory is determined according to the steps of:
emitting a plurality of flashes of the light from the light source such that the light is incident on a reference target and a calibration target,
measuring for each of the plurality of light flashes the intensity each wavelength of light reflected off a calibration target using the sample sensor the intensity of the light reflected off a reference target using the reference sensor;
transforming the measured values into a compensation coefficient based on the relationship between the signal channel measurement and the reference channel measurement at each wavelength.