Method for Measuring PRNU Characteristics of Area Array Detector of Imaging Spectrometer

Abstract

The present invention discloses a method for measuring the PRNU characteristics of an imaging spectrometer array detector. By constructing an optical model based on a monochromatic LED and an optical pinhole, an ideal correction light source can be obtained through a light field model fitting method of a diffraction spot. The method has a simple structure, reduces the PRNU correction error introduced by the light source, and realizes low-cost, high-precision PRNU testing. By adjusting the spot size and position, pixels in different areas on a single detector can be irradiated with light of different wavelengths. A control group area can be set to realize the study of the relationship between the PRNU of the imaging spectrometer detector and the wavelength, and the relationship between the PRNU and the irradiation time, thereby improving research efficiency and reliability.

Description

Technical Field

[0001] The present invention relates to the technical field of PRNU research of imaging spectrometer detectors, and in particular to a method for measuring the PRNU characteristics of an imaging spectrometer array detector. Background Art

[0002] Hyperspectral imaging technology typically uses scientific-grade detectors to ensure detection efficiency. For example, the PRNU of a scientific-grade CCD can be better than 0.6%, while the uniformity of a typical integrating sphere can only reach 99.5%. This means that the light source introduces 0.5% non-uniformity, resulting in a large PRNU correction error.

[0003] The article "Study on Non-uniformity Correction of Spaceborne Trace Gas Differential Absorption Spectrometer" uses spectral data for column-by-column correction, achieving PRNU correction in different bands. However, this method is based on the rational flat field of the integrating sphere light source, which will introduce additional light source structure. In addition, the number of pixels for single-column correction is insufficient, the data volume is small, and spectral curvature may introduce additional errors.

[0004] The existing patent, "A Method, System, and Device for Correcting PRNU Characteristics in Wavelength Bands of Imaging Spectrometers," employs a two-dimensional Gaussian model for modeling. Using a light field distribution model as the ideal light source for the PRNU correction device, this approach can reduce correction errors caused by light source inhomogeneity and implement PRNU correction in different wavelength bands in a regional manner. However, the current method for obtaining a more ideal Gaussian spot is to use a single longitudinal mode laser. To achieve wide-band selection, a tunable single longitudinal mode continuous laser is required. These lasers are expensive, typically costing over a million dollars, making this method costly. As mentioned above, existing methods are costly and the equipment is bulky and inconvenient to use. Summary of the Invention

[0005] The present invention proposes a method for measuring the PRNU characteristics of an imaging spectrometer array detector, which can solve the problems in the PRNU research of hyperspectral imaging spectrometer detectors, that is, traditional methods cannot obtain an ideal light source, resulting in large detector calibration errors, or complex test systems, high equipment costs, and low efficiency.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for measuring the PRNU characteristics of an imaging spectrometer array detector comprises the following steps:

[0008] S1. Build an optical system and use laser alignment to make λ pq The monochromatic LED, optical pinhole and detector focal plane of the band meet the paraxial approximation conditions;

[0009] S2. Adjust the relative position of the detector and the diffraction hole so that the diffraction spot is located in the detector pixel area Z. pq ;

[0010] S3. Adjust the LED drive current so that the maximum brightness of the light spot in the image reaches 90% of the full scale of the detector under a fixed exposure time;

[0011] S4. Adjust the horizontal distance between the diffraction screen and the focal plane of the detector while maintaining the paraxial approximation condition so that the spot area occupies M×N pixels of the detector;

[0012] S5. Continuously irradiate the detector, the continuous data is t, and t is initialized to 0;

[0013] S6. Imaging the light spot at a fixed exposure time, obtaining L frames of image data, and calculating the average image to obtain the light spot data value U actually obtained by the detector. dn (i, j), i, j are pixel row and column numbers;

[0014] S7. According to the diffraction spot intensity distribution model, i.e., formula (9), combined with the actual spot data value, the spot model value U(i, j) is obtained by fitting;

[0015]

[0016] (mΔx, nΔy) is the discretization function of the diffraction spot, which represents the light intensity value of the pixel in the mth row and nth column on the diffraction spot. The value range of m is 0:M-1, and the value range of n is 0:N-1, indicating a total of M*N pixels. (Δx0, Δy0) represents the discretization of the diffraction screen function, which is divided into the same number as the receiving screen.

[0017] S8. According to the PRNU factor calculation formula (10), the data value of the spot M'×N' area is obtained

[0018] Select the M' row and N' column pixels in the central spot for calculation, and the model value U(i, j) is different from the actual data value U dn The deviation of (i,j) is the detector PRNU factor The value of:

[0019]

[0020] Adjust the irradiation time t, set t = t + 2h, that is, increase the irradiation time by 2 hours, repeat steps S5 to S8 until the time reaches the set total time t end ;

[0021] Plot the detector in band λ pq The relationship between the PRNU factor and time under light irradiation, curve;

[0022] Repeat steps S1 to S10 to obtain other wavelengths curve;

[0023] Through the above steps, you can get the The curve can be used to determine the attenuation change law of the PRNU characteristics of the detector under light irradiation of different bands as the irradiation time increases.

[0024] It can be seen from the above technical solution that the method for measuring the PRNU characteristics of the imaging spectrometer array detector of the present invention, the present invention establishes an optical model based on a monochromatic LED and an optical pinhole, and can obtain an ideal correction light source through the light field model fitting method of the diffraction spot. The structure is simple, the PRNU correction error introduced by the light source is reduced, and low-cost, high-precision PRNU testing is achieved; by adjusting the spot size and position, different bands of light radiation can be performed on pixels in different areas on a single detector, and a control group area can be set to realize the study of the relationship between the PRNU and the band of the imaging spectrometer detector and the relationship between the PRNU and the irradiation time, thereby improving research efficiency and reliability.

[0025] Specifically, the present invention uses simple yet high-precision optical components to construct a simple optical system consisting of a monochromatic LED point light source combined with an optical pinhole. This system then extracts the light intensity distribution model of the diffraction spot, which serves as the ideal PRNU (Precision Numerical Unit) light source for the imaging spectrometer detector. This reduces imaging errors introduced by complex optical systems and provides more reliable results. The spot wavelength, energy, and size are controllable, enabling long-term monochromatic light irradiation testing of the detector area. This allows for multi-band illumination radiation with a single detector, and allows for control groups to be set up to study the relationship between the detector PRNU and the illumination wavelength, as well as the relationship between PRNU and irradiation time. Consequently, the present invention offers low cost, a reliable solution, and improved efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 1 is a schematic diagram of the structure of a test system according to an embodiment of the present invention;

[0027] Figure 2 This is an optical principle diagram of an LED diffraction spot according to an embodiment of the present invention;

[0028] Figure 3 Schematic diagram of the LED diffraction optical physics model according to an embodiment of the present invention;

[0029] Figure 4 2. This is a schematic diagram of the spot distribution for measuring the PRNU characteristics of the detector according to an embodiment of the present invention;

[0030] Figure 5 Flowchart of the research method for PRNU characteristics of area array detectors. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0032] like Figure 1 As shown, the PRNU characteristic measurement method of the imaging spectrometer array detector described in this embodiment, wherein the system is constructed as follows Figure 1 The PRNU research and testing system shown in the figure consists of an adjustable LED driver, an optical pinhole with a diameter of 25 to 50 μm, an imaging spectrometer detector, a three-dimensional displacement platform, and a control computer. The control computer controls the adjustable LED driver to drive a monochromatic UV LED with controllable brightness. After the LED passes through the optical pinhole, diffraction occurs, and the imaging spectrometer detector captures the diffraction spot. The three-dimensional displacement platform uses the computer to set the detector's relative position and distance from the diffraction pinhole.

[0033] The diffraction spot follows the physical law of optical diffraction and can be described as follows: Figure 2 The optical principle diagram shown is used to calculate the light field distribution expression of the diffraction spot on the imaging focal plane of the detector, that is, the light spot intensity distribution model.

[0034] Figure 2 The monochromatic LED is a circular light-emitting surface source with a diameter of 3.5 mm, the diffraction screen is an optical pinhole with a diameter of 20 to 50 μm, and the receiving screen is the imaging focal plane of the detector of the imaging spectrometer; the distance from the light source to the diffraction screen is 3500 mm, and the distance from the diffraction screen to the receiving screen is 25 to 30 mm. A laser collimator and optical adjustment method are used to set the planes where the three components are located to be parallel, and the center of the LED and the optical pinhole are in the same horizontal position in space, so that the optical system meets the paraxial approximation condition.

[0035] Based on this, an optical physics model is established as follows Figure 3 The calculation process of the light field model expression is as follows:

[0036] The center of the diffraction screen is the origin of the XYZ space coordinate system, the normal vector of the plane where the diffraction screen is located is the Z axis, and d is the distance from the diffraction screen to the focal plane of the detector, as shown in Figure 3 , U0(x0,y0,0) is the complex amplitude of the light wave at point (x0,y0,0) on the diffraction screen, U(x,y,d) is the complex amplitude of point (x,y,d) on the focal plane of the detector, and θ is the angle between the vector from point (x0,y0,0) to point (x,y,d) and the normal vector n.

[0037] According to the knowledge of diffraction optics, the complex amplitude of the light wave on the detector can be expressed by formula 1:

[0038]

[0039] Since θ is very small here, the tilt factor K(θ) can be set to 1, the denominator r in the integral term is r = d, and k in the formula is the wave vector, that is, 2π / λ. Figure 3 As shown, r can be expressed by formula 2:

[0040]

[0041] Formula 2 is expressed in Taylor series form:

[0042]

[0043] The third term in the brackets of formula 3 is very small and can be ignored, so we can also use:

[0044]

[0045] Therefore, Formula 1 can be written as:

[0046]

[0047] After expansion:

[0048]

[0049] Combined with the diffraction screen size in this design, the radius is 12.5 to 25 μm, and d is 25 to 30 mm, that is, Therefore, in order to simplify the calculation, the exponential part of the integral term in formula (6) is simplified to obtain:

[0050]

[0051] Combined with the definition of Fourier transform, Formula 7 can be written as:

[0052]

[0053] Since the actual calculation needs to convert it into digital form and perform discretization processing on it, we get:

[0054]

[0055] (mΔx, nΔy) is the discretization function of the diffraction spot, which represents the light intensity value of the pixel in the mth row and nth column on the diffraction spot. The value range of m is 0:M-1, and the value range of n is 0:N-1, indicating a total of M*N pixels; (mΔx0, nΔy0) represents the discretization of the diffraction screen function, which divides it into the same number as the receiving screen.

[0056] According to formula (9), combined with the construction parameters of the test system, the intensity distribution model of the diffraction spot obtained by the detector focal plane can be calculated. The M' row and N' column pixels in the central spot are selected for calculation. The model value U(i, j) is the same as the actual data value U dn The deviation of (i,j) is the detector PRNU factor The value of:

[0057]

[0058] like Figure 4 The figure shows the spot distribution during the PRNU characteristic study of the detector. The number of pixels in the focal plane of the detector is H rows and W columns, and the irradiation area of the diffraction spot is M rows and N columns. To ensure data reliability, M and N ≥ 200. Pixels in the M' row and N' column of the spot are selected for PRNU characteristic study. To ensure test reliability, the values of M' and N' cannot be too large, because PRNU noise is a type of noise directly related to light intensity. That is, the greater the light intensity, the greater the proportion of PRNU noise in the total noise. Other noises are unrelated to light intensity, such as power supply noise and circuit reset noise. Therefore, when the light intensity is weak, the impact of such noise on the test results is also greater. Here, the values of M' and N' are selected based on 0.6 times the grayscale value of the two light intensity points in the center of the spot. Usually, M' and N' ≥ 100.

[0059] The distribution of diffraction spots on the focal plane of the detector can be used to study the PRNU variation characteristics of the detector under monochromatic light irradiation. A long-term irradiation experiment is conducted on a fixed pixel area of the detector using a light spot of a specific wavelength, and by comparing and analyzing the results with the non-irradiated pixel area, the PRNU variation characteristics of the detector pixel under the irradiation of the monochromatic light spot are studied. Figure 4 As shown, according to the monochromatic light band (λ ij ) number of pixels on the focal plane of the detector is divided into J×K experimental areas (Z ij ), each experimental area includes a monochromatic spot irradiation area and a reference group pixel area, which contains λ 11 -λ JK A monochromatic spot.

[0060] In summary, after the test system is built, the flow chart of the PRNU characteristics research method of the array detector is as follows: Figure 5 shown.

[0061] The specific steps are as follows:

[0062] 1) Build an optical system and use laser alignment and other methods to make λ pq The monochromatic LED, optical pinhole and detector focal plane of the band meet the paraxial approximation conditions;

[0063] 2) Adjust the relative position of the detector and the diffraction hole (optical pinhole) so that the diffraction spot is located in the detector pixel area Z pq ;

[0064] 3) Adjust the LED drive current so that the maximum brightness of the light spot in the image reaches 90% of the detector's full scale at a fixed exposure time (which can be, but is not limited to, 500mS);

[0065] 4) Adjust the horizontal distance between the diffraction screen and the focal plane of the detector while maintaining the paraxial approximation condition so that the spot area occupies M×N detector pixels, where M and N ≥ 200;

[0066] 5) Continuously irradiate the detector, the continuous data is t, and t=0 is initialized;

[0067] 6) The light spot is imaged at a fixed exposure time, and L (L ≥ 500) image data are obtained and the average image is obtained to obtain the light spot data value U actually obtained by the detector. dn (i, j), i, j are pixel row and column numbers;

[0068] 7) The spot model value U(i, j) is obtained by fitting the diffraction spot intensity distribution model (Formula 9) combined with the actual spot data value;

[0069] 8) According to the PRNU factor calculation formula, formula 10, according to the spot M'×N'(', N'≥100)

[0070] The data value of the area is obtained

[0071] 9) Adjust the irradiation time t, set t = t + 2h, that is, increase the irradiation time by 2 hours, repeat steps 5 to 8 until the total time reaches the set time t end The total duration can be set according to the situation. Generally, t end >

[0072] 200ms;

[0073] 10) Draw the detector in band λ pq The relationship between the PRNU factor and time under light irradiation,

[0074] curve;

[0075] 11) Repeat steps 1 to 10 to obtain other wavelengths curve.

[0076] Through the above steps, you can get the The curve can be used to study the attenuation change law of the PRNU characteristics of the detector under light irradiation of different bands with irradiation time, and realize the study of the PRNU characteristics of the imaging spectrometer array detector.

[0077] In summary, the embodiment of the present invention is built through a system platform, that is, a simple optical design of a monochromatic LED point light source combined with an optical pinhole is built through simple but high-precision optical components; then the light intensity distribution model of the diffraction spot is obtained as the PRNU ideal light source of the imaging spectrometer detector, reducing the imaging error caused by the complex optical system. The diffraction spot is used as the ideal light source for PRNU testing, which reduces the imaging error caused by the complex optical system and makes the result more reliable; at the same time, the light spot band, energy, and size are controllable, so that long-term monochromatic light irradiation testing of the detector area can be realized, that is, multi-band light radiation is completed through a single detector, and a control group can be set to realize the relationship between the detector PRNU and the light band and the relationship between PRNU and irradiation time.

[0078] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for measuring the PRNU characteristics of an imaging spectrometer array detector, characterized in that: The following steps are included: S1. Build an optical system and use laser alignment to make λ pq The monochromatic LED, optical pinhole and detector focal plane of the band meet the paraxial approximation conditions; S2. Adjust the relative position of the detector and the diffraction hole so that the diffraction spot is located in the detector pixel area Z. pq ; S3, adjust the LED drive current so that the maximum brightness of the light spot in the image reaches 90% of the full scale of the detector under a fixed exposure time; S4. Adjust the horizontal distance between the diffraction screen and the focal plane of the detector while maintaining the paraxial approximation condition so that the spot area occupies M×N pixels of the detector; S5. Continuously irradiate the detector, the continuous data is t, and t is initialized to 0; S6. Imaging the light spot at a fixed exposure time, obtaining L frames of image data, and calculating the average image to obtain the light spot data value U actually obtained by the detector. dn (i, j), i, j are pixel row and column numbers; S7. According to the diffraction spot intensity distribution model, that is, formula (9), combined with the actual spot data value, the spot model value U(i, j) is obtained by fitting; （9） It is the discretization function of the diffraction spot, which represents the light intensity value of the pixel in the mth row and nth column on the diffraction spot. The value range of m is 0:M-1, and the value range of n is 0:N-1, indicating a total of M*N pixels. represents the discretization of the diffraction screen function, dividing it into the same number as the receiving screen; S8. According to the PRNU factor calculation formula (10), the data value of the spot M'×N' area is obtained. (λ pq , t); Select the M' row and N' column pixels in the central spot for calculation, the model value and actually get the data value The deviation is the detector PRNU factor The value of: ； S9, adjust the irradiation time t, set t=t+2h, that is, increase the irradiation time by 2 hours, repeat steps S5~S8 until the time reaches the set total time t end ; S10, plot the detector in band λ pq The relationship between the PRNU factor and time under light irradiation, (λ pq , t) -t curve; Repeat steps S1 to S10 to obtain other wavelengths (λ pq , t) -t curve; Obtain the multi-band (λ pq , t)-t curve, the attenuation change law of the PRNU characteristics of the detector under light irradiation of different bands with irradiation time can be determined.

2. The method for measuring the PRNU characteristics of an imaging spectrometer array detector according to claim 1, wherein: M≥200, N≥200.

3. The method for measuring the PRNU characteristics of an imaging spectrometer array detector according to claim 1, wherein: described ≥100, ≥100.

4. The method for measuring the PRNU characteristics of an imaging spectrometer array detector according to claim 1, wherein: t end >200ms。 5. The method for measuring the PRNU characteristics of an imaging spectrometer array detector according to claim 1, wherein: L≥500。

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