Light source wavelength correction
View Patent ↗A wavelength correction function provides corrected reflectance values from actual reflectance values taken in a reflectance-base instrument. The correction is provided as a function of measured reflectance values and a predefined set of high resolution reflectance values established for the reflectance-based instrument implementing the wavelength correction function.
1. A method of correcting one or more reflectance values when a center wavelength of one or more light sources used to generate corresponding light signals is different from a specified center wavelength for the one or more light sources, the method comprising the steps of:
A. defining, for each of the one or more light sources, a reference spectral distribution {L*} that is characteristic of the one or more light sources and composed of reference light intensity values over a set of reference wavelengths;
B. defining, for each of the one or more light sources, a spectral distribution {L} comprising actual light intensity values over the set of wavelengths;
C. determining the actual reflectance R of a set of reflected signals;
D. defining a set of detector sensitivity data {D} corresponding to the set of detectors receiving the set of reflected signals;
E. determining high resolution reflectance values {r};
F. determining a correction factor as a function of {L}, {L*}, {r} and {D}; and
G. applying the correction factor to R to determine R*.
2. The method of claim 1 , wherein determining the correction factor in step F is valid up to a range of at least about ±8 nanometers around the specified center wavelength.
3. The method of claim 1 , wherein the one or more light sources comprise LEDs.
4. The method of claim 1 , wherein at least one of the one or more light sources is an infrared light source and determining {r} in step E comprises measuring reflectance values R IR in the infrared range and determining r IR as a constant representing an average of R IR , where each value in {r} equals the value of (R/R IR )·r IR at a corresponding wavelength.
5. The method of claim 4 , wherein the values of {r} are determined at discrete wavelength intervals.
6. The method of claim 1 , wherein the one or more light sources and set of detectors comprise part of a reflectometer.
7. The method of claim 1 , wherein determining a correction factor as a function of {L}, {L*}, {r} and {D} comprises determining a correction factor
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8. A center wavelength correction system configured to correct one or more reflectance values when a center wavelength of one or more light sources used to generate corresponding light signals is different from a specified center wavelength for the one or more light sources, the system comprising:
A. a spectral distribution module configured to determine, for each of the one or more light sources, a spectral distribution {L} comprising actual light intensity values over the set of wavelengths;
B. a reflectance module configure to determine actual reflectance R from a set of reflected signals;
C. at least one storage device comprising:
1) for each of the one or more light sources, a reference spectral distribution {L* } that is characteristic of the one or more light sources and composed of reference light intensity values over a set of reference wavelengths;
2) high resolution reflectance values {r}; and
3) detector sensitivity data {D} corresponding to the set of detectors receiving the set of reflected signals;
D. a correction function module configured to determine a correction factor at a given wavelength as a function of {L}, {L*}, {r} and {D} and to apply the correction factor to R to determine R*.
9. The system of claim 8 , wherein the correction function module is configured to determine the correction factor within a range of at least about ±8 nanometers around the specified center wavelength.
10. The system of claim 8 , wherein the one or more light sources comprise LEDs.
11. The system of claim 8 , wherein at least one of the one or more light sources is an infrared light source and the correction function is configured to determine {r} as a function of measured reflectance values R IR an the infrared range and a constant r IR that represents an average of R IR , where each value in {r} equals the value of (R/R IR )r IR at a corresponding wavelength.
12. The system of claim 11 , wherein the values of {r} are determined at discrete wavelength intervals.
13. The system of claim 8 , wherein the one or more light sources and set of detectors comprise part of a reflectometer.
14. The system of claim 8 , wherein the correction function module is configured to determine a correction factor
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15. A wavelength correction means for correcting one or more reflectance values when a center wavelength of one or more light sources used to generate corresponding light signals is different from a specified center wavelength for the one or more light sources, the system comprising:
A. a spectral distribution means for determining, for each of the one or more light sources, a spectral distribution {L} comprising actual light intensity values over the set of wavelengths;
B. a reflectance means for determining actual reflectance R from a set of reflected signals;
C. at least one storage means far storing:
1) for each of the one or more light sources, a reference spectral distribution {L} that is characteristic of the one or more light sources and composed of reference light intensity values over a set of reference wavelengths;
2) high resolution reflectance values {r}; and
3) detector sensitivity data {D} corresponding to the set of detectors receiving the set of reflected signals;
D. a correction function means for determining a correction factor at a given wavelength as a function of {L}, {L*}, {r} and {D} and to apply the correction factor to R to determine R*.
16. The means of claim 15 , wherein the correction function means includes means for determining the correction factor within a range of at least about ±8 nanometers around the specified center wavelength.
17. The means of claim 15 , wherein the one or more light sources comprise LEDs.
18. The system of claim 15 , wherein at least one of the one or more light sources is an infrared light source and the correction function means includes means for determining {r} as a function of measured reflectance values R IR in the infrared range and a constant r IR that represents an average of R IR , where each value in {r} equals the value of (R/R IR )r IR at a corresponding wavelength.
19. The system of claim 18 , wherein the correction function means includes means for determining values of {r} at discrete wavelength intervals.
20. The system of claim 15 , wherein wavelength correction means comprises a portion of a reflectometer means.
21. The wavelength correction means of claim 15 , wherein the correction function means is configured to determine a correction factor
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22. A reflectometer comprising:
A. a set of light sources;
B. a set of detectors;
C. a reflectance assembly configured to direct light signals from the set of light sources onto a test product and to direct light signals reflected from the test product onto the set of detectors;
D. at least one storage device configured to store a reference spectral distribution {L*}, a set of high resolution reflectance values {r}, a set of detector sensitivity data {D} corresponding to the set of detectors, a measured spectral distribution {L} corresponding tote set of light sources, and a set of measured reflectance values R; and
E. a correction function module for determining a correction factor at a given wavelength as a function of {L}, {L*}, {r} and {D} and to apply the correction factor to R to determine R*.
23. The reflectometer of claim 22 , wherein the set of light sources comprises a set of LEDs.
24. The reflectometer of claim 22 , wherein the correction function module is configured to determine a correction factor
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25. A wavelength correction module, in a reflectance-based system comprising a set of light sources, a set of detectors, and a reflectance assembly configured to direct light signals from the set of light sources onto a test product and to direct light signals reflected from the test product onto the set of detectors, the wavelength correction module comprising:
A. at least one storage device configured to store a reference spectral distribution {L*}, a set of high resolution reflectance values {r}, a set of detector sensitivity data {D} corresponding to the set of detectors, a measured spectral distribution {L} corresponding to the set of light sources, and a set of measured reflectance values R; and
B. a correction function module for determining a correction factor at a given wavelengths a function of {L}, {L*}, {r} and {D} and to apply the correction factor to R to determine R*.
26. The wavelength correction module of claim 25 , wherein the set of light sources comprises a set of LEDs.
27. The wavelength correction module of claim 25 , wherein the correction function module is configured to determine a correction factor
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