Device and method for absolute calibration of spectro-imagers

Abstract

The invention relates to a spectrometric calibration device (1), comprising an intensity-calibrated broadband light source (2) and a cosine-corrected transmission diffuser (3) coupled to said light source. The invention also relates to the use of this device for the absolute calibration of both micro and macro spectro-imaging systems. Figure 1

Description

Title of the invention: Device and method for absolute calibration of spectro-imagers. Technical field

[0001] The field of the invention is that of the calibration of spectro-imagers, such as hyperspectral imagers, particularly those used to perform luminescence imaging measurements (for example, to characterize photovoltaic cells or study semiconductors such as light-emitting diodes) or fluorescence imaging measurements (in the field of biology, for example). Prior art

[0002] Despite significant advances in the field of photovoltaic materials, new materials are struggling to be economically competitive with silicon-based materials. This is mainly due to a lack of understanding and control of the non-uniformity of the active layers, thus hindering the optimization of optoelectronic properties. In order to bring the next generations of solar cells to market, researchers must be able to study the spatial variation of their materials' properties on a larger scale.

[0003] To meet this need, hyperspectral imaging provides electroluminescence (EL) and photoluminescence (PL) maps which allow for the rapid characterization of structural and physical properties of a photovoltaic material.

[0004] The principle of hyperspectral measurement is as follows: the light spectrum of the object to be tested (light response to a luminous flux or to an electric current in the case of a solar cell) passes through the imager so that its image can be analyzed. An image is composed of the sum of point sources (at least one point source) coming from different positions of the object as seen by the imager's lens. Each point source produces a collimated polychromatic beam that arrives at a specific angle of incidence on a spectral filtering system such as a volume Bragg grating.

[0005] A volume Bragg grating is a diffraction grating that has a periodic modulation of the refractive index through the volume of a photosensitive material (for example, photothermo-refractive glass). This volume acts as a transparent window for all wavelengths except for a narrow spectral band of a few nanometers, which is diffracted in another direction, at an angle dependent on the wavelength. Only this diffracted beam reaches a vertical line of the imager detector, which detects a monochromatic image. One-dimensional. The adjacent wavelength is projected onto the adjacent line; there is a different wavelength on each line. The wavelength selected on each line is adjusted by changing the angle of incidence between the polychromatic source beam and the grating. A scan takes place.

[0006] Once all wavelengths have been scanned, an image reconstruction process is used which provides a "hyperspectral cube", i.e. one image per wavelength and one spectrum per pixel.

[0007] It should be noted that there are alternatives to the spectral filtering system constituted by the volume Bragg network, for example tunable bandpass filters.

[0008] Implementing an absolute calibration of a hyperspectral imaging system makes it possible to determine the absolute number of photons emitted by each point on the surface of a sample at each wavelength. This allows researchers to obtain quantitative information instead of data expressed in arbitrary units and thus better understand the optical properties of the sample.

[0009] An absolute calibration method, mentioned for example in Delamarre, A., Lombez, L., & Guillemoles, JF (2012). Characterization of solar cells using electroluminescence and photoluminescence hyperspectral images. Journal of Photonics for Energy, 2 (1), 027004-027004, comprises four steps:

[0010] 1) The first step consists of performing a calibration using a line lamp. This involves of a classic procedure which consists of imaging lines of known emission wavelength to deduce the spectral dispersion of the imaging system and correct any spectral aberrations.

[0011] 2) The second step consists of performing a relative spectral calibration of the transmittance of the imaging system over the entire field of view. To do this, an extended source, spatially homogeneous over the area to be calibrated, with a known spectrum and Lambertian emission geometry (uniform luminance at each emitted wavelength) is imaged; for this, a device called an integrating sphere coupled to a halogen lamp is used.

[0012] An integrating sphere is a spherical cavity covered with a perfect Lambertian reflector, having at least one orifice for injecting light from a lamp and at least one orifice for emitting Lambertian radiation. An integrating sphere with a diameter of 10 cm is conventionally used to generate a Lambertian source with a diameter of a millimeter.

[0013] A correction factor is calculated for each pixel and each wavelength which makes it possible to match the spectral output of the system to the known spectrum of the source.

[0014] 3) The third step consists of performing an absolute power calibration of a few pixels. For this, a monochromatic light source of limited surface area and A laser beam with a known power output and an emission cone smaller than that of the imaging lens (i.e., a laser beam exiting a fiber) is imaged. Since the power is known, an absolute calibration of the system is obtained at the wavelength of the monochromatic source, from which it is possible to extrapolate to the entire spectral range using the relative calibration of the second step.

[0015] 4) The fourth step consists of carrying out a spatial calibration using a target. The goal is to calculate the spatial correspondence between the image of the target and a defined area of ​​the sample; that is, to determine the correspondence between a pixel of the camera and a unit of length. This correspondence is necessary for calculating the detected light intensity in absolute units.

[0016] This absolute calibration method has drawbacks. First, the time required to perform the absolute calibration is significant, on the order of one hour, and the numerous steps involved introduce measurement errors and lead to many calculations. Second, the power calibration is unstable and therefore exhibits poor reproducibility. Similarly, the fourth step introduces a significant error / uncertainty into the calibration process. Furthermore, in addition to being expensive, the integrating sphere is bulky (at least 10 cm in diameter), which may sometimes require disassembling and reassembling the sample holder. Finally, this method cannot perform absolute calibration when the field of view becomes large (on the order of a centimeter), as the integrating sphere would then need to be more than one meter in diameter. Description of the invention

[0017] The invention aims to enable absolute calibration of a spectro-imaging system by overcoming one or both of the aforementioned drawbacks. To this end, the invention proposes a spectrometric calibration device comprising a broadband light source calibrated in intensity and a cosine-corrected transmission diffuser coupled to said light source.

[0018] Some preferred but not limiting aspects of this device are as follows:

[0019] - the diffuser is coupled to the light source via a fiber optics;

[0020] - it further includes a ray lamp suitable for being coupled to the diffuser;

[0021] - the light source is further stabilized in intensity;

[0022] - the diffuser includes a diffusing glass of the bubble glass type.

[0023] The invention also relates to a method for the absolute calibration of a spectro-imaging system using such a device. This method comprises:

[0024] - obtaining at least one cube of spectral data acquired by a camera of the spectro-imaging system with the diffuser of said device positioned in a field of view of the camera;

[0025] - the determination of a calibration cube from at least one data cube spectral obtained and an emission spectrum of the light source of said device, the calibration cube comprising, for each of a plurality of wavelengths, a correction parameter per pixel imaged by the camera.

[0026] In one possible embodiment, said obtaining comprises obtaining a plurality of spectral data cubes, each for one of a plurality of diffuser locations in the camera's field of view, and said determination comprises: • the interpolation of the spectral data cubes obtained across the entire field of view so as to obtain a field-of-view cube that includes, for each wavelength of said plurality of wavelengths, an intensity value per pixel imaged by the camera; and • the comparison of the cube of the field of view to the emission spectrum of the light source.

[0027] The invention also relates to a data processing unit, comprising a processor configured to implement the process as defined above, and to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to implement the process. Brief description of the drawings

[0028] Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0029] - [Fig. 1] is a diagram of a device according to the invention for calibration spectrometric;

[0030] - [Fig.2A] and [Fig.2B] represent measurements, respectively at 600 nm and 1300 nm, of the intensity identifier of the light scattered by a device according to the invention;

[0031] - Figure 3 represents, from left to right, an emission spectrum of the source calibrated luminous, a spectrum measured by a hyperspectral imaging system and the response of the hyperspectral imaging system, or device function;

[0032] - [Fig. 4] represents different possible locations of the diffuser for a wide-field calibration;

[0033] - [Fig. 5] represents a correction surface determined by interpolation of spectral measurements carried out for each of the different locations of [Fig.4].

[0034] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0035] With reference to [Fig. 1], the invention provides a spectrometric calibration device 1, for example for the absolute calibration of a spectro-imaging system such as a multispectral or hyperspectral imaging system. This device comprises a light source 2 and a diffuser 3 coupled to the light source.

[0036] A spectro-imager is defined here as a device that measures the light spectrum of one or more spatially defined areas of a light-emitting object. When multiple areas are observed, several scenarios are possible: either the detector comprises several pixels (hyperspectral imager using a camera), or a single area moves by scanning the object (confocal imager equipped with a spectrometer such as a confocal microscope). Ultimately, the spectro-imager provides a one- or two-dimensional optical image with spectral resolution of the object.

[0037] The light source 2 is a broadband source calibrated for intensity. By broadband source, it is understood that the light source 2 generates a range of wavelengths that covers a large portion of the electromagnetic spectrum, for example from 250 nm to 1500 nm. By source calibrated for intensity, it is understood that the emission spectrum of the light source 2 is known. This emission spectrum may have been measured using a spectrogonometer.

[0038] The light source 2 is preferably intensity-stabilized to ensure the accuracy and reproducibility of the absolute calibration with respect to the calibration obtained from a calibrated spectrogram or the prior method described above. This stable operation can be ensured by current control of the power supply to the source 2. The stability of the light source is, for example, guaranteed to + / - 5% over 200 hours.

[0039] Alternatively, the power of the light source 2 can be measured during calibration and then relatively before each use.

[0040] The light source 2 can be a halogen lamp, a xenon lamp or a deuterium lamp.

[0041] The diffuser 3 is a cosine-corrected transmission diffuser that ensures the Lambertianity (or orthotropy) of the scattered light over the entire spectral range of interest and over an angle greater than the aperture angles of the objectives used by the hyperspectral imaging system. In this regard, measurements of the intensity identifier of the scattered light at 600 nm and 1300 nm, respectively, are shown in Figures 2A and 2B. These measurements demonstrate near-perfect Lambertianity at several wavelengths.

[0042] The diffuser 3 may include a diffusing glass of the bubble glass type ('milk glass' in English). Such a bubble glass may be a diffuser made from a pure and opaque synthetic silica glass, such as the diffusil® model from the company Opsira.

[0043] The light source 2 and the diffuser 3 can be coupled via an optical fiber 4, for example a fiber from a bundle of optical fibers.

[0044] The luminance of light from the diffuser 3 on the central area (typically 1mm at the center), reasonably homogeneous (verified by imaging) is measured for example using a calibrated spectro-goniometer or a calibrated hyperspectral imager with the prior method described above.

[0045] Device 1 according to the invention makes it possible to perform absolute calibration of a spectro-imaging system such as a hyperspectral imaging system without having to resort to the second, third and fourth steps of the prior method described above. This device is also more compact compared to the prior assembly comprising an integrating sphere coupled to a halogen lamp.

[0046] In a possible embodiment shown in [Fig. 1], the device 1 may further include a line lamp 5 suitable for being coupled to the diffuser, for example via an optical fiber 6. This optical fiber 6 may form a second path of a fiber bundle also accommodating the fiber 4 coupling the light source 2 and the diffuser 3. This line lamp can be used to carry out the first step of the prior method described above.

[0047] The invention is not limited to the device described above and also extends to the use of this device for calibrating a spectro-imaging system, such as a multispectral or hyperspectral spectro-imaging system. A hyperspectral spectro-imaging system may, in particular, utilize a Bragg volume as previously mentioned or a tunable bandpass filter.

[0048] This method is implemented following the positioning of the diffuser 3 of the calibration device 1 according to the invention within the field of view of a camera of the spectro-imaging system and the acquisition by the camera of at least one spectral data cube. A spectral data cube is here a stack of monochromatic images of the diffuser, with one spectrum per pixel imaged by the camera. The cube thus has two spatial dimensions (x and y) and one spectral dimension (z), the face of the cube being a function of the spatial coordinates and the depth being a function of the wavelength.

[0049] The method comprises obtaining at least one spectral data cube acquired by the camera and determining a calibration cube from the at least one spectral data cube obtained and an emission spectrum of the light source 2 of the calibration device 1 according to the invention. The calibration cube comprises, for each of a plurality of wavelengths, a correction parameter per pixel imaged by the camera.

[0050] Figure 3 shows on the left the emission spectrum I(X) of the calibrated light source 2 (or irradiance), and in the center a spectrum S(X) per unit time measured by a hyperspectral imaging system at one pixel of the image of the diffuser 3 of the calibration device according to the invention. The raw spectrum measured per unit time S(X), shown in the center of Figure 3, depends on the response of the hyperspectral imaging system according to the equation S(X) = I(X) * R(X), where R(X) corresponds to the response of the hyperspectral imaging system, or device function. Since I(X) is known, it is possible to derive R(X) according to nz . SÜ). On the right of [Fig. 3], R(X) represents " U ) - a slice of the calibration cube according to the spectral dimension, associated with a pixel of the image of diffuser 3.

[0051] Under the reasonable assumption of the mechanical and optical stability of the imaging system, a subsequent new measurement from a single point in the field of view allows, through relative calibration, the acquisition of a new calibration dataset. Such operation is well suited to a modification of the imaging system, such as the addition of a filter.

[0052] This calibration method can be adapted to perform absolute calibration over a large area, for example, on the order of 15 x 15 cm, and to enable the creation of macroscopic maps known as wide-field maps. To do this, several spectral data cubes are acquired by moving the diffuser throughout the field of view. Thus, obtaining at least one spectral data cube involves acquiring a plurality of spectral data cubes, each for one of a plurality of diffuser locations within the camera's field of view. Figure 4 illustrates, as an example, different locations AY that can be used successively for acquiring a spectral data cube.

[0053] In this case, the determination of the calibration cube includes the interpolation (for example, a biharmonic spline interpolation coupled with an extrapolation by natural neighbors) of the spectral data cubes obtained over the entire field of view so as to obtain a field of view cube which includes, for each wavelength of said plurality of wavelengths, an intensity value per pixel imaged by the camera.

[0054] Figure 5 shows the result of such an interpolation, for a given wavelength (in this case 1100 nm), across the entire field of view of the measurements taken for the different locations and represented by circles. The cube of the field of view thus contains a correction surface over the entire field of view of the type shown in Figure 5, and this for each of the wavelengths. The determination of the calibration cube continues with the comparison of the field of view cube to the emission spectrum of the light source in accordance with the previously mentioned equation.

[0055] The invention also extends to a data processing unit comprising a processor configured to implement the method described above and to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to implement this method.

Claims

Demands

1. Spectrometric calibration device (1), comprising an intensity-calibrated broadband light source (2) and a cosine-corrected transmission diffuser (3) coupled to said light source.

2. Device according to claim 1, wherein the diffuser (3) is coupled to the light source (2) via an optical fiber (4).

3. Device according to any one of claims 1 and 2, further comprising a line lamp (5) suitable for being coupled to the diffuser (3).

4. Device according to any one of claims 1 to 3, wherein the light source (2) is further stabilized in intensity.

5. Device according to any one of claims 1 to 3, wherein the diffuser (3) comprises a diffusing glass of the bubble glass type.

6. Method for absolute calibration of a spectro-imaging system by means of a device (1) according to any one of claims 1 to 5, comprising: - obtaining at least one spectral data cube acquired by a camera of the spectro-imaging system with the diffuser (3) of said device positioned in a field of view of the camera; - determining a calibration cube from the at least one spectral data cube obtained and an emission spectrum of the light source of said device, the calibration cube comprising, for each of a plurality of wavelengths, a correction parameter per pixel imaged by the camera.

7. A method according to claim 6, wherein: - said obtaining comprises obtaining a plurality of spectral data cubes, each for one of a plurality of locations (AY) of the diffuser in the camera's field of view; and - said determining comprises: interpolating the obtained spectral data cubes to the entire field of view so as to obtain a field of view cube which comprises, for each wavelength of said plurality of wavelengths, an intensity value per pixel imaged by the camera; and comparing the field of view cube to the emission spectrum of the light source. 10

8. Data processing unit, comprising a processor configured to implement the method according to one of claims 6 and 7

9. / . Product computer program comprising instructions which, when the program is executed by a computer, cause the computer to implement the method according to one of claims 6 and 7.

Images