CN1635346A - Space modulation type interference spectrum imager on-satellite calibration method - Google Patents

Abstract

An on-board calibration method for a spatially modulated interferometric spectroscopic imager. The method is: a uniform surface light source with a flat spectral curve enters the integrating sphere to obtain a surface light source with higher surface uniformity; the spectral curve of the light source produces an absorption peak; the collimator system produces parallel light with a certain field of view, Become a wide-spectrum calibration light source with characteristic spectral lines, and use the instrument to measure the spectral curve of the calibration light source. The calibration light source is imaged on the slit of the primary image plane by the front mirror of the hyperspectral imager, and then an interference image is generated on the detector array by the interferometer, Fourier lens, and cylindrical mirror, and the calibration light source is obtained by computer restoration. Spectral curve; compare the spectral curve with the pre-obtained spectral curve to obtain the required calibration parameters. Correct the data obtained by the instrument with the above calibration parameters to complete the calibration.

Description

On-board Calibration Method for Spatial Modulation Interferometric Spectral Imager

technical field

The invention relates to an on-board calibration method for a space modulation type interference spectrum imager.

Background technique

Interferometric spectral imaging technology, also known as interferometric imaging spectroscopy technology, was developed in the 1990s and uses remote sensing observation methods. On-board calibration is one of its core key technologies, and it is currently in the research stage internationally.

In engineering practice, due to the nature of materials, the accuracy of installation and other reasons, there will always be a certain gap between the target information obtained by the instrument and the actual target. It is necessary to obtain corrected parameters through calibration to make the results obtained by the instrument as close as possible to the actual target situation. , so calibration is required.

The calibration of the interferometric hyperspectral imager is different from that of the common CCD remote sensing camera. The ordinary CCD remote sensing camera obtains the geometric image of the target, and only needs to perform radiometric calibration to meet the requirements; while the interferometric hyperspectral imager not only obtains the geometric image of the target, but also obtains the spectral information of the target, so it is both To perform radiometric calibration, spectral calibration is also performed.

Because the working principle of the interferometric hyperspectral imager is different from that of the dispersive imaging spectrometer, the detector of the dispersive imaging spectrometer receives the direct spectral information of the target, while the detector of the interferometric imaging spectrometer receives the Fu of the target spectrum. Leaf transformation information. Both have the same purpose, but the method is quite different.

The powerful small satellite launched by the United States in July 2000 is the first practical interferometric hyperspectral imager in the world. At first, the powerful small satellite wanted to use an optical fiber to introduce sunlight into the instrument as a light source for calibration. The data obtained by the instrument is compared with the known solar spectrum, and the corrected parameters are obtained. However, this idea was not put into practice in the end, but was compared and corrected by calibration of the ground radiation field. Due to the lack of understanding of the characteristics of the solar spectrum, there is currently no domestic production of optical fibers suitable for aerospace applications.

Contents of the invention

The invention solves the technical problem that the background technology cannot realize the calibration of the space-borne interferometric hyperspectral imager, that is, the radiometric calibration and the spectral calibration cannot be performed simultaneously.

Technical solution of the present invention is:

An on-board calibration method for a space-modulated interferometric spectroscopic imager, which is special in that the method includes:

1). A uniform surface light source with a flat spectral curve is emitted by the Kohler lighting system or an integrating sphere system light source, and enters the integrating sphere 4 from the entrance 3 to obtain a surface light source with higher surface uniformity.

2). Make the spectral curve of the light source produce an absorption peak. Placed at the outlet 5, dodman-praseodymium glass or other transparent materials with absorption peaks make the spectral curve produce two, three or more absorption peaks.

3). The collimator system produces parallel light with a certain field of view, which becomes a wide-spectrum calibration light source with characteristic spectral lines, and the spectral curve of the calibration light source is measured with standard instruments.

4). The calibration light source is imaged on the slit of the primary image plane through the front mirror of the hyperspectral imager, and then passes through the interferometer, Fourier lens, and cylindrical mirror to generate an interference image on the detector array. After the image is restored by the computer restoration software, the spectral curve of the calibration light source obtained by the hyperspectral imager is obtained.

5). Comparing this spectral curve with the spectral curve we obtained in advance, we can get the calibration parameters we need. The flat broad spectrum can be used for radiometric calibration, and the absorption peak of the light source can be used to determine the spectral resolution of the instrument, as well as the position and relative amplitude of the spectral lines.

6). Use the above calibration parameters to correct the data obtained by the instrument to complete the calibration.

The above-mentioned collimating lens focal length and relative aperture are identical with imaging spectrometer front mirror; Then image on the detector through the interferometer, Fourier lens, and cylindrical mirror; collimator 6——change the light source into parallel light; thus, the calibration light source generated by the entire calibration system becomes a beam with characteristic spectral lines wide-spectrum light source. The wide-spectrum light source can be used for radiometric calibration, and the characteristic spectral lines of the light source can be analyzed through spectral restoration to determine the change of the spectral line position and achieve the purpose of relative calibration on the star; , and finally the light is refracted by the mirror 7 and introduced into the optical system of the instrument for calibration.

The above-mentioned calibration refers to the process of determining the accurate value output by the remote sensor, and the main method is to measure the response of the remote sensor to a target with known radiation characteristics. For the interferometric hyperspectral imager, the calibration content mainly includes two aspects: spectral calibration and radiometric calibration, and radiometric calibration includes relative radiometric calibration and absolute radiometric calibration. Calibration is used to determine the relative radiation response of each spectral band of the instrument and the relative radiation response of different units of the detector. Absolute radiation calibration is used to determine the output signal of the instrument so that it can reflect the absolute radiation intensity of the input signal.

The spectral calibration is to determine the position, frequency and maximum optical path difference of the interferogram zero optical path difference, so as to determine the center wavelength and half-wave width of each spectral segment. It is very important to ensure the spectral resolution of the instrument, and at the same time it is the most important criterion for the normal working state of the interferometer.

The radiometric calibration: Usually, the relative radiometric calibration is mainly performed on the star.

The calibration light source with known spectral intensity distribution B(σ) enters the hyperspectral imager, and the interference intensity distribution of the pixel is obtained as:

I(x)=∫K(σ)B(σ)cos(2πxσ)dσ+IN

IN is the zero-input response of the pixel, x is the optical path difference, and σ is the wavenumber. K(σ) is the spectral response function of the system.

After filtering and phase correction, the above interferogram is subjected to inverse Fourier transform to obtain the restored spectral intensity distribution B’(σ), then there is

B'(σ)=K(i,j)×B(σ)

In K(i, j), i and j represent the number of pixels in the spatial direction and the number of spectral segments, respectively. K(i, j) is the spectral correction value of the i-th detector unit, (j=1, 2...N, N is the number of spectral segments), that is, the spectral response function.

Using the same method, the correction value can be obtained for each column (i=2...M) unit, and finally a complete two-dimensional correction coefficient matrix K(i, j) can be obtained to provide spectral radiance correction data and complete the spectral radiance degree calibration.

The present invention has the following advantages:

1. Realized the calibration of the spaceborne interferometric hyperspectral imager;

2. Radiometric calibration and spectral calibration can be completed at the same time;

3. Calibration accuracy can reach: relative accuracy is 5%, absolute accuracy is 10%;

4. Materials and instruments do not need to be specially made;

5. The purpose of calibration can be achieved by using existing materials and instruments.

Description of drawings

The accompanying drawing is a structural schematic diagram of the on-board calibration optical system of the present invention.

Description of reference numerals: 1-light source, 2-condenser, 3-inlet, 4-integrating sphere, 5-exit, 6-collimating mirror system, 7-reflector.

Detailed ways

The spectrum curve of the light source of the calibration system of the present invention can be measured in advance. The spectrum of the tungsten-halogen lamp is relatively flat. After passing through the integrating sphere system, there will be some sharp absorption peaks on the spectral line. The number and position of the absorption peaks are related to the material of the integrating sphere. The flat part of the spectrum is used for radiometric calibration, and the position of the absorption peak and the full width at half maximum are used for spectral calibration.

Referring to accompanying drawing, the present invention is made up of Cora lighting system , integrating sphere system and collimating mirror system .

1. A uniform surface light source with a flat spectral curve is emitted by the Kohler lighting system or an integrating sphere system light source, and enters the integrating sphere 4 from the entrance 3 to obtain a surface light source with higher surface uniformity.

2. Make the spectral curve of the light source produce an absorption peak. Placed at the outlet 5, dodman-praseodymium glass or other transparent materials with absorption peaks make the spectral curve produce two, three or more absorption peaks.

3. The collimator system produces parallel light with a certain field of view, which becomes a wide-spectrum calibration light source with characteristic spectral lines, and the spectral curve of the calibration light source is measured with standard instruments.

4. The calibration light source is imaged on the slit of the primary image plane through the front mirror of the hyperspectral imager, and then passes through the interferometer, Fourier lens, and cylindrical mirror to generate an interference image on the detector array. After the image is restored by the computer restoration software, the spectral curve of the calibration light source obtained by the hyperspectral imager is obtained.

5. Comparing this spectral curve with the spectral curve we obtained in advance, we can get the calibration parameters we need. The flat broad spectrum can be used for radiometric calibration, and the absorption peak of the light source can be used to determine the spectral resolution of the instrument, as well as the position and relative amplitude of the spectral lines.

6. Use the above calibration parameters to correct the data obtained by the instrument to complete the calibration.

The focal length and relative aperture of the collimating mirror are the same as those of the front mirror of the imaging spectrometer.

The collimating mirror 6 and the front mirror of the hyperspectral imager converge the light emitted by the integrating sphere 4 on the slit of the optical system of the hyperspectral imager, and then pass through the interferometer, Fourier lens, and cylindrical mirror on the detector imaging. Collimating mirror 6——to change the light source into parallel light.

In this way, the calibration light source generated by the entire calibration system becomes a wide-spectrum light source with characteristic spectral lines. The wide-spectrum light source can be used for radiometric calibration, and the characteristic spectral lines of the light source can be analyzed through spectral restoration to determine the changes in the position of the spectral lines and achieve the purpose of relative calibration on the star. After passing through the collimating mirror system 6, the light is finally refracted by the reflector 7 and introduced into the optical system of the instrument for calibration. Lambert Plate 8,

In engineering practice, the signal collected by the instrument will change due to the nature of the material, the accuracy of processing and assembly, and the difference between the space environment and the ground environment. Therefore, it is necessary to obtain corrected parameters through calibration.

Supplementary note:

Calibration refers to the process of determining the accurate output value of the remote sensor. The main method is to measure the response of the remote sensor to a target with known radiation characteristics. For the interferometric hyperspectral imager, the calibration content mainly includes two aspects: spectral calibration and radiometric calibration, and radiometric calibration includes relative radiometric calibration and absolute radiometric calibration. Calibration is used to determine the relative radiation response of each spectral band of the instrument and the relative radiation response of different units of the detector. Absolute radiation calibration is used to determine the output signal of the instrument so that it can reflect the absolute radiation intensity of the input signal.

Spectral calibration: It is to determine the position, frequency and maximum optical path difference of the interferogram, so as to determine the center wavelength and half-wave width of each spectral segment. It is very important to ensure the spectral resolution of the instrument, and at the same time it is the most important criterion for the normal working state of the interferometer.

Radiometric calibration: Usually, the relative radiometric calibration is mainly performed on the star.

The calibration light source with known spectral intensity distribution B(σ) enters the hyperspectral imager, and the interference intensity distribution of the pixel is obtained as:

I(x)=∫K(σ)B(σ)cos(2πxσ)dσ+IN

IN is the zero-input response of the pixel, x is the optical path difference, and σ is the wavenumber. K(σ) is the spectral response function of the system.

After filtering and phase correction, the above interferogram is subjected to inverse Fourier transform to obtain the restored spectral intensity distribution B’(σ), then there is

B'(σ)=K(i,j)×B(σ)

In K(i, j), i and j represent the number of pixels in the spatial direction and the number of spectral segments, respectively. K(i, j) is the spectral correction value of the i-th detector unit, (j=1, 2...N, N is the number of spectral segments), that is, the spectral response function.

Using the same method, the correction value can be obtained for each column (i=2...M) unit, and finally a complete two-dimensional correction coefficient matrix K(i, j) can be obtained to provide spectral radiance correction data and complete the spectral radiance degree calibration.

Claims (3)

1. Calibrating method on 1 one kinds of spatial modulation type interference spectrum imager stars, it is characterized in that: this method comprises:

   1). by Cola illuminator or integration sphere light source systemLight source sends the smooth uniform area light source of the curve of spectrum, enters integrating sphere 4 by inlet 3, obtains the higher area source of surface uniformity.

   2). make the curve of spectrum of this light source produce absorption peak.Place didymium glass or other has the absorption peak transparent material and makes its curve of spectrum produce two, three or more absorption peak at outlet 5 places.

   3). produce directional light through calibration lens system, promptly with certain visual angleBecome the wide range scaling light source that has characteristic spectral line, record the curve of spectrum of this scaling light source with reference instrument.

   4). scaling light source is imaged on image planes slit through the preset lens of hyperspectral imager, and then lists the generation interference image through interferometer, fourier transform lens, cylindrical mirror at detector array.This image after computing machine recovery software rejuvenation, obtains the curve of spectrum of the scaling light source of hyperspectral imager acquisition,

   5). with this curve of spectrum and the comparison of the curve of spectrum that we obtain in advance, just can obtain the calibration parameter of our needs.Smooth wide range can be used for the radiancy calibration, and the absorption peak of light source can be used for determining the spectral resolution of instrument, and the position of spectral line and relative amplitude.

   6). with above-mentioned calibration parameter the data that instrument obtains are revised, finished calibration.
2. Calibrating method on the 2 spatial modulation type interference spectrum imager stars according to claim 1, it is characterized in that: described collimating mirror focal length is identical with the imaging spectrometer preset lens with relative aperture; Collimating mirror 6 with the preset lens of hyperspectral imager with the optical convergence of integrating sphere 4 outgoing on the slit of hyperspectral imager optical system, then by interferometer, Fourier lens, cylindrical mirror imaging on detector; Collimating mirror 6---light source is become directional light; Thus, the scaling light source of whole scaling system generation just becomes the wide spectrum light source that has characteristic spectral line.Wide spectrum light source can be as the radiancy calibration, and the characteristic spectral line of light source carries out the spectral characteristic analysis by spectrum recovering, can determine the situation of change of spectrum line position, reaches the purpose of relative calibration on the star; Again through calibration lens system 6, last light is through catoptron 7 light path of turning back, and the optical system that is introduced into instrument is calibrated.
3. Calibrating method on the 3 spatial modulation type interference spectrum imager stars according to claim 1 and 2, it is characterized in that: described calibration is meant determines that remote sensor exports the process of accurate numerical value, and main means are to measure the response of remote sensor to a known radiation characteristic target.Concerning interference type super spectrum imager, the calibration content comprises that mainly spectral calibration and radiancy calibrate two aspects, wherein the radiancy calibration comprises that again relative radiancy is calibrated and the absolute radiation degree is calibrated two kinds, the radiancy calibration is used for determining the relative radiometric response of each spectral coverage of instrument and the relative radiometric response of detector different units relatively, absolute radiometric calibration is used for determining the instrument output signal, enables to react the absolute radiation intensity of input signal.

   Described spectral calibration: be exactly position, frequency and the maximum optical path difference of determining the interferogram zero optical path difference, thereby determine the centre wavelength and the half band-width of each spectral coverage.It is very important to the spectral resolution that guarantees instrument, and whether the duty to interferometer normally is topmost criterion simultaneously.

   Described radiancy calibration: mainly carry out relative radiancy calibration usually on the star.

   The scaling light source of known spectra intensity distributions B (σ) enters hyperspectral imager, and the interference strength that obtains pixel is distributed as:

   I(x)＝∫K(σ)B(σ)cos(2π×σ)dσ+IN

   IN is the zero input response of pixel, and x is an optical path difference, and σ is a wave number.K (σ) is the spectral response functions of system.

   Above-mentioned interference figure carries out inversefouriertransform after filtering, phase place correction, obtain recovered light spectral intensity distribution B ' (σ), then has

   B’(σ)＝K(i，j)×B(σ)

   K (i, j) in, i and j be representation space direction pixel number and spectral coverage number respectively.(i j) is exactly the spectrum modified value of i row detector unit, (j=1,2 to K ... N, N are the spectral coverage number), i.e. spectral response functions.

   Use the same method, can be listed as (i=2 each ... M) modified value is obtained in the unit, and (i j), provides spectral radiant emittance correction data, finishes the calibration of spectral radiant emittance to obtain a complete two-dimentional correction factor matrix K at last.

Images