Method, apparatus and software for detection and localization of hidden defects in optically diffuse media
In a method and apparatus, a property of an optically diffuse medium including a first optical absorber having a first concentration and a second optical absorber having a second concentration is determined. A surface area of the medium is imaged at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. A reflectance spectrum of the medium at the surface area at the multiple wavelengths is determined. A derivative of the determined reflectance spectrum around the isosbestic wavelength is determined. From the derivative, a concentration ratio of the first concentration and the second concentration is estimated.
1. An apparatus for determining a property of an optically diffuse medium, the medium comprising a first optical absorber having a first concentration and a second optical absorber having a second concentration, the apparatus comprising:
a light source configured to illuminate a surface area of a volume of the medium;
a filtering device configured to receive reflected light from the surface area of the medium and to transmit filtered light to an optical imaging device, wherein the filtering device is configured to filter multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber;
a two-dimensional, 2D, optical imaging device configured to receive the filtered light from the filtering device;
an image processing component of a data processing device having a processor to load computer instructions enabling the processor to determine a reflectance spectrum of the medium at the surface area at the multiple wavelengths;
a calculating component of a data processing device having the processor to load computer instructions enabling the processor to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and
an estimating component of a data processing device having the processor to load computer instructions enabling the processor to estimate a concentration ratio (ψ) of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around the isosbestic wavelength,
wherein the concentration ratio (ψ) is determined using the partial derivative of a first molar absorption coefficient of the first absorber as a function of the wavelength, and the partial derivative of a second molar absorption coefficient of the second absorber as a function of the wavelength, both taken at a wavelength at which the first molar absorption coefficient and the second molar absorption coefficient are equal.
2. The apparatus according to claim 1 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare the concentration ratio (ψ) to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when the concentration ratio is outside the concentration ratio range.
3. The apparatus according to claim 1 , wherein the filtering device comprises:
a filter support comprising a plurality of different optical filters; and
an actuator to position each one of the optical filters in an optical path from the surface area of the medium to the optical imaging device.
4. The apparatus according to claim 1 , wherein the optical imaging device comprises a CCD camera, an InGaAs camera or a CMOS camera.
5. A non-transitory storage medium storing computer instructions which, when loaded in the processor of the data processing device of claim 1 to carry out the steps of the image processing component, the calculating component and the estimating component.
6. The apparatus according to claim 1 , wherein the concentration ratio (ψ) is determined as:
ψ
=
2
DX
Δλ
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
=
2
DX
Δλ
-
g
f
-
g
where:
f and g are defined by
f
=
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
and
g
=
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
;
λ indicates a wavelength;
λ 0 indicates an isosbestic wavelength;
Δλ indicates a wavelength difference from the isosbestic wavelength;
DX indicates an imaging ratio, defined by subtracting two images taken Δλ apart from an isosbestic wavelength λ 0 and dividing by the sum of the two images;
μ a,m indicates a molar absorption coefficient of absorber m; and
μ a,0 indicates a molar absorption coefficient at isosbestic wavelength.
7. The apparatus according to claim 1 , wherein the derivative of the determined reflectance spectrum around the isosbestic wavelength is a first derivative.
8. The apparatus according to claim 1 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to estimate a concentration ratio of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around different isosbestic wavelengths of the first absorber and the second absorber in different wavelength regions to obtain estimated concentration ratios for each one of the isosbestic wavelengths.
9. The apparatus according to claim 8 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare each one of the estimated concentration ratios to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
10. The apparatus according to claim 8 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth; and
the apparatus further comprising an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect and a depth thereof for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
11. The apparatus according to claim 8 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth and, if small defects are to be located, to select an isosbestic wavelength among different isosbestic wavelengths with a low optical penetration depth.
12. An apparatus for determining a property of an optically diffuse medium, the medium comprising a first optical absorber having a first concentration and a second optical absorber having a second concentration, the apparatus comprising:
a light source configured to illuminate a surface area of a volume of the medium;
a one-dimensional, 1D, or two-dimensional, 2D, hyperspectral optical imaging device configured to receive reflected light from the surface area of the medium;
an image processing component of a data processing device having a processor to load computer instructions enabling the processor to determine the reflectance spectrum of the medium at the surface area at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber;
a calculating component of the data processing device having the processor to load computer instructions enabling the processor to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and
an estimating component of the data processing device having the processor to load computer instructions enabling the processor to estimate a concentration ratio (ψ) of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around the isosbestic wavelength,
wherein the concentration ratio (ψ) is determined using the partial derivative of a first molar absorption coefficient of the first absorber as a function of the wavelength, and the partial derivative of a second molar absorption coefficient of the second absorber as a function of the wavelength, both taken at a wavelength at which the first molar absorption coefficient and the second molar absorption coefficient are equal.
13. The apparatus according to claim 12 , further comprising:
a stage configured to support the medium; and
an actuator configured to drive the stage to move the medium relative to the optical imaging device.
14. The apparatus according to claim 12 , wherein the optical imaging device is a 2D hyperspectral optical imaging device comprising an image sensor having a plurality of pixels, and wherein the image processing component is further configured to:
assign a cluster of pixels to the surface area of the medium;
determine the reflectance spectrum for each pixel of the image sensor; and
determine the reflectance spectrum of the medium at the surface area from the reflectance spectra of the pixels of the cluster.
15. The apparatus according to claim 12 , wherein the concentration ratio (ψ) is determined as:
ψ
=
2
DX
Δλ
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
=
2
DX
Δλ
-
g
f
-
g
where:
f and g are defined by
f
=
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
and
g
=
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
;
λ indicates a wavelength;
λ 0 indicates an isosbestic wavelength;
Δλ indicates a wavelength difference from the isosbestic wavelength;
DX indicates an imaging ratio, defined by subtracting two images taken Δλ apart from an isosbestic wavelength λ 0 and dividing by the sum of the two images;
μ a,m indicates a molar absorption coefficient of absorber m; and
μ a,0 indicates a molar absorption coefficient at isosbestic wavelength.
16. The apparatus according to claim 12 , wherein the derivative of the determined reflectance spectrum around the isosbestic wavelength is a first derivative.
17. The apparatus according to claim 12 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare the concentration ratio (ψ) to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when the concentration ratio is outside the concentration ratio range.
18. The apparatus according to claim 12 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to estimate a concentration ratio of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around different isosbestic wavelengths of the first absorber and the second absorber in different wavelength regions to obtain estimated concentration ratios for each one of the isosbestic wavelengths.
19. The apparatus according to claim 18 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare each one of the estimated concentration ratios to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
20. The apparatus according to claim 18 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth; and
the apparatus further comprising an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect and a depth thereof for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
21. The apparatus according to claim 18 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth and, if small defects are to be located, to select an isosbestic wavelength among different isosbestic wavelengths with a low optical penetration depth.
22. A non-transitory storage medium storing computer instructions which, when loaded in the processor of the data processing device of claim 12 to carry out the steps of the image processing component, the calculating component and the estimating component.
23. An apparatus for determining a property of an optically diffuse medium, the medium comprising a first optical absorber having a first concentration and a second optical absorber having a second concentration, the apparatus comprising:
a light source configured to illuminate a surface area of a volume of the medium;
a spectrograph configured to receive reflected light from the surface area of the medium;
a light processing component of a data processing device having a processor to load computer instructions enabling the processor to determine the reflectance spectrum of the medium at the surface area at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber;
a calculating component of the data processing device having the processor to load computer instructions enabling the processor to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and
an estimating component of the data processing device having the processor to load computer instructions enabling the processor to estimate a concentration ratio (ψ) of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around the isosbestic wavelength,
wherein the concentration ratio (ψ) is determined using the partial derivative of a first molar absorption coefficient of the first absorber as a function of the wavelength, and the partial derivative of a second molar absorption coefficient of the second absorber as a function of the wavelength, both taken at a wavelength at which the first molar absorption coefficient and the second molar absorption coefficient are equal.
24. The apparatus according to claim 23 , further comprising:
a stage configured to support the medium; and
an actuator configured to drive the stage to move the medium relative to the optical imaging device.
25. The apparatus according to claim 23 , further comprising:
a scanning device configured to transmit reflected light from different surface areas of the medium to the spectrograph.
26. The apparatus according to claim 23 , wherein the concentration ratio (ψ) is determined as:
ψ
=
2
DX
Δ
λ
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
-
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
=
2
DX
Δλ
-
g
f
-
g
where:
f and g are defined by
f
=
∂
μ
a
,
1
∂
λ
λ
0
μ
a
,
0
and
g
=
∂
μ
a
,
2
∂
λ
λ
0
μ
a
,
0
;
λ indicates a wavelength;
λ 0 indicates an isosbestic wavelength;
Δλ indicates a wavelength difference from the isosbestic wavelength;
DX indicates an imaging ratio, defined by subtracting two images taken Δλ apart from an isosbestic wavelength λ 0 and dividing by the sum of the two images;
μ a,m indicates a molar absorption coefficient of absorber m; and
μ a,0 indicates a molar absorption coefficient at isosbestic wavelength.
27. The apparatus according to claim 23 , wherein the derivative of the determined reflectance spectrum around the isosbestic wavelength is a first derivative.
28. The apparatus according to claim 23 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare the concentration ratio (ψ) to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when the concentration ratio is outside the concentration ratio range.
29. The apparatus according to claim 23 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to estimate a concentration ratio of the first concentration and the second concentration from the derivative of the determined reflectance spectrum around different isosbestic wavelengths of the first absorber and the second absorber in different wavelength regions to obtain estimated concentration ratios for each one of the isosbestic wavelengths.
30. The apparatus according to claim 29 , further comprising:
a comparator of the data processing device having the processor to load computer instructions enabling the processor to compare each one of the estimated concentration ratios to a predetermined concentration ratio range; and
an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
31. The apparatus according to claim 29 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth; and
the apparatus further comprising an output component of the data processing device having the processor to load computer instructions enabling the processor to indicate a defect and a depth thereof for the surface area when at least one of the concentration ratios is outside the concentration ratio range.
32. The apparatus according to claim 29 , wherein the estimating component of the data processing device further has the processor to load computer instructions enabling the processor to associate each isosbestic wavelength with a respective optical penetration depth and, if small defects are to be located, to select an isosbestic wavelength among different isosbestic wavelengths with a low optical penetration depth.
33. A non-transitory storage medium storing computer instructions which, when loaded in the processor of the data processing device of claim 23 to carry out the steps of the image processing component, the calculating component and the estimating component.