A back-illuminated CCD baseline drift correction method

Abstract

The application discloses a back-illumination CCD baseline drift correction method, acquires dark noise of multiple horizontal pixels in a CCD to obtain pixel dark noise data; acquires a fitting curved surface function through self-defined function fitting based on the pixel dark noise data; substitutes temperature fed back by a temperature sensor and set integral time into the fitting curved surface function to calculate minimum theoretical dark noise and noise reduction amplitude of the multiple horizontal pixels in the CCD; determines a target baseline, judges whether the minimum theoretical dark noise is greater than the target baseline; and based on the noise reduction amplitude, filters and reduces noise of compensated dark noise to complete correction. The application carries out signal compensation through a signal compensation circuit based on a subtracter designed on hardware and carries out filtering and noise reduction processing on software, so that hardware and software work cooperatively, effectively control the CCD baseline to approach the target baseline, reduce fluctuation amplitude of the CCD baseline, realize CCD baseline drift correction, and increase a dynamic range of a spectrometer detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of back-illuminated CCD, in particular to a back-illuminated CCD baseline drift correction method. BACKGROUND

[0002] Spectral detection technology is widely used in biology, chemistry and polymer materials, etc., wherein back-illuminated CCD is used as a detector by most spectrometers due to its better quantum efficiency, large dynamic range, low dark noise and good response in ultraviolet and near infrared regions.

[0003] Dark current noise, dark current shot noise and other dark noises are generated in the detection process of back-illuminated CCD, and the dark noises increase with the increase of exposure time, i.e. integration time, and temperature rise, resulting in the increase of CCD baseline, i.e. serious baseline drift, and the fluctuation amplitude of the dark noises also increases with the increase of the amount of CCD baseline drift, affecting the dynamic range of the spectrometer detection and the accuracy and reliability of the subsequent spectral analysis algorithm.

[0004] Currently, back-illuminated CCD is collected and controlled by Correlated Double Sampling (CDS), but CDS has no obvious correction effect on the CCD baseline drift caused by the dark noises related to time and temperature; in addition, a refrigeration type CCD is used to control the dark current level and thus control the dark noises, but the cost of the refrigeration type CCD is high; and a refrigerant is added outside the CCD to control the dark current level, and the process of adding the refrigerant causes a sudden temperature drop, resulting in the condensation phenomenon of the photosensitive surface of the CCD, which seriously disperses the detected light of the spectrometer and affects the detection of the CCD.

[0005] Therefore, it is necessary to study how to effectively control the dark noises without using the refrigeration type CCD and various refrigeration operations to avoid serious baseline drift of the CCD. SUMMARY

[0006] To solve the above technical problems, the present application provides a back-illuminated CCD baseline drift correction method, which realizes the correction of the baseline drift of the CCD by designing a signal compensation circuit based on a subtracter on the hardware and performing filtering and noise reduction processing on the software, and increases the dynamic range of the spectrometer detection.

[0007] The technical scheme adopted by the present application is as follows:

[0008] A back-illuminated CCD baseline drift correction method, specifically comprising the following steps:

[0009] S1, collecting the dark noises of a plurality of horizontal pixels inside the CCD to obtain pixel dark noise data;

[0010] S2, fitting a fitting surface function based on the pixel dark noise data through a self-defined function;

[0011] S3, substituting the temperature fed back by the temperature sensor and the set integral time into the fitting surface function to calculate the minimum theoretical dark noise and the noise reduction amplitude of the plurality of horizontal pixels inside the CCD;

[0012] S4, determining a target baseline, judging whether the minimum theoretical dark noise is greater than the target baseline: if yes, performing signal compensation on the CCD; otherwise, normally collecting and then performing filtering and noise reduction processing on the dark noise;

[0013] S5, based on the noise reduction amplitude, performing filtering and noise reduction processing on the compensated dark noise to complete the correction.

[0014] Further, the pixel dark noise data in the step S1 includes first pixel dark noise data and second pixel dark noise data; wherein the first pixel dark noise data is obtained by collecting the dark noise of the plurality of horizontal pixels inside the CCD at different temperatures through control of the integral time; and the second pixel dark noise data is obtained by collecting the dark noise of the plurality of horizontal pixels inside the CCD at different integral times through control of the temperature.

[0015] Further, the step S2 specifically includes the following steps:

[0016] 1) based on the same horizontal pixel, the relationship between the horizontal pixel dark noise and the integral time is obtained through linear fitting by the least square method, and the specific formula is as follows:

[0017] N p (t)=a p (T)t+b p (T),p=1,2,3...P(1)

[0018] In formula (1), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integral time; T is the temperature; a p , b p are the fitting straight line coefficients corresponding to the horizontal pixel p respectively;

[0019] 2) based on the same horizontal pixel, the relationship between the horizontal pixel dark noise and the temperature is obtained through curve fitting, and the specific formula is as follows:

[0020]

[0021] In formula (2), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integral time; T is the temperature; c p , d pare the fitting curve coefficients corresponding to the horizontal pixel p respectively;

[0022] 3) Based on the same horizontal pixel, the relationship between the horizontal pixel dark noise, integration time and temperature, i.e. the fitting surface function, is obtained by surface fitting, and the specific formula is as follows:

[0023]

[0024] In formula (3), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integration time; T is the temperature; a p , b p , c p , d p are the fitting coefficients corresponding to the horizontal pixel p respectively; f p is the correction amount corresponding to the horizontal pixel dark noise N p .

[0025] Further, the step S4 specifically comprises the following steps:

[0026] 1) Determine the target baseline: according to the standard of commonly used CCD dark noise, when the CCD baseline is controlled at about 2000, the maximum signal-to-noise ratio is as high as 33dB, which indicates that the dark noise level at this time has reached near zero level, and therefore the target baseline is set to 2000;

[0027] 2) Determine whether the minimum theoretical dark noise is greater than the target baseline 2000: if yes, it indicates that the CCD baseline drifts and the baseline is too high, and the CCD is compensated for signal to make the CCD baseline approach the target baseline; otherwise, it indicates that the CCD baseline does not drift, and the dark noise is filtered and denoised after normal acquisition;

[0028] Wherein, the signal compensation of the CCD includes designing a signal compensation circuit and calculating a signal compensation voltage;

[0029] ① The design process of the signal compensation circuit: a signal compensation circuit is arranged between the CCD and the A / D converter, specifically: the pixel dark noise data output by the CCD after one acquisition is sent to the A / D converter through the signal compensation circuit; then the converted data is sent to the FPGA chip through synchronization by the A / D converter; finally, it is sent to the PC through the USB.

[0030] ② The process of calculating the signal compensation voltage:

[0031] The minimum theoretical dark noise in one acquisition is obtained by the fitting surface function, and the compensation dark noise in the signal compensation circuit is obtained based on the target baseline of the CCD dark noise, and the specific calculation formula is as follows:

[0032] ΔN min = Nmin -N Bl (8)

[0033] In formula (8), ΔN min is the compensation dark noise in the signal compensation circuit; N min is the minimum theoretical dark noise; N Bl is the target baseline of the CCD dark noise;

[0034] The A / D converter internal reference voltage is obtained by an A / D converter AD9826 based on 16-bit precision;

[0035] The signal compensation voltage V com is calculated based on the compensation dark noise and the A / D converter internal reference voltage, and the specific calculation formula is as follows:

[0036]

[0037] In formula (9), V com is the signal compensation voltage; ΔN min is the compensation dark noise in the signal compensation circuit; V ref is the A / D converter internal reference voltage, and is 4V.

[0038] Further, the step S5 specifically comprises the following steps:

[0039] The original dark noise N 1p of the signal compensation circuit of a plurality of horizontal pixels is compensated by the value of the compensation dark noise ΔN min , and the new dark noise N1' p generated after noise reduction is generated, and the specific calculation formula is as follows:

[0040] N1' p =N 1p -(N 2p -ΔN min )(10)

[0041] In formula (10), N1' p is the new dark noise generated after noise reduction; N 1p is the original dark noise after passing through the signal compensation circuit; N 2p is the dark noise under the condition of integral time and temperature simulated by the fitting surface function; ΔN min is the compensation dark noise in the signal compensation circuit.

[0042] Compared with the prior art, the present application has the following beneficial effects:

[0043] The present application controls the CCD baseline to approach the target baseline, reduces the fluctuation amplitude of dark noise, realizes the CCD baseline drift correction, increases the dynamic range of the spectrometer detection, and ensures the accuracy and reliability of the subsequent spectral analysis algorithm by designing a signal compensation circuit based on a subtractor on hardware and performing filtering and noise reduction processing on software.

[0044] 2) Without the need of a refrigeration type CCD and various refrigeration operations, the dark noise is controlled, the cost is reduced, and the normal CCD detection is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0045] The present application will be further described below in combination with the drawings and specific embodiments:

[0046] Figure 1 is a flowchart of a back-illumination type CCD baseline drift correction method of the present application;

[0047] Figure 2 is a fitting straight line diagram of the horizontal pixel dark noise and the integration time in the present application;

[0048] Figure 3 is a fitting curve diagram of the horizontal pixel dark noise and the temperature in the present application;

[0049] Figure 4 is a fitting three-dimensional surface diagram of the horizontal pixel dark noise, the integration time and the temperature in the present application;

[0050] Figure 5 is a logic judgment diagram of a back-illumination type CCD baseline drift correction method of the present application;

[0051] Figure 6 is a system framework design diagram of the present application with a signal compensation circuit;

[0052] Figure 7 is a CCD output signal voltage diagram detected by an oscilloscope in the present application;

[0053] Figure 8 is a signal voltage change diagram when the CCD baseline drifts in the present application;

[0054] Figure 9 is a working principle diagram of the signal compensation circuit in the present application;

[0055] Figure 10 is an experimental result diagram using the correction method of the present application. DETAILED DESCRIPTION

[0056] EMBODIMENT

[0057] As Figures 1 to 10As shown, a back-illuminated CCD baseline drift correction method specifically includes the following steps:

[0058] S1, collecting dark noise of multiple horizontal pixels inside the CCD to obtain pixel dark noise data;

[0059] Specifically, the pixel dark noise data in step S1 includes first pixel dark noise data and second pixel dark noise data; wherein the first pixel dark noise data is obtained by controlling the integration time and collecting the dark noise of multiple horizontal pixels inside the CCD at different temperatures; the second pixel dark noise data is obtained by controlling the temperature and collecting the dark noise of multiple horizontal pixels inside the CCD at different integration times;

[0060] In this embodiment, the number of horizontal pixels p is set to 2048.

[0061] S2, obtaining a fitting surface function by fitting a custom function based on pixel dark noise data;

[0062] Specifically, 1) based on the same horizontal pixel, the relationship between the horizontal pixel dark noise and the integration time is obtained by least squares linear fitting, that is, the fitting straight line function. The specific formula is as follows:

[0063] N p (t) = a p (T)t+b p (T),p=1,2,3...P(1)

[0064] In formula (1), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integration time; T is the temperature; a p 、b p are the fitting straight line coefficients corresponding to the horizontal pixel p;

[0065] In this embodiment, Figure 2 As shown, Figure 2 (a), (b), and (c) represent the horizontal pixel dark noise N when the horizontal pixel p is 500, 1000, and 1500, respectively. p The fitting straight line with the integration time t, where the horizontal axis represents the integration time t and the vertical axis represents the horizontal pixel dark noise N p , red, blue and black represent temperatures T of 8℃, 18℃ and 28℃ respectively; Figure 3 It can be seen that the horizontal pixel dark noise N p It is linearly related to the integration time t, and the slope of the fitting line gradually increases with the increase of temperature T; Figure 2 (a), (b), and (c) show that the inherent properties of each horizontal pixel of the CCD make the fitting lines at different horizontal pixel positions have different coefficients;

[0066] 2) Based on the same horizontal pixel, the horizontal pixel dark noise N is obtained by curve fitting p The relationship with temperature T is the fitting curve function, and the specific formula is as follows:

[0067]

[0068] In formula (2), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integration time; T is the temperature; c p d p are the fitting curve coefficients corresponding to the horizontal pixel p;

[0069] In this embodiment, Figure 3 As shown, Figure 3 (a), (b), and (c) represent the horizontal pixel dark noise N when the horizontal pixel p is 500, 1000, and 1500, respectively. p The fitting curve of temperature T, where the horizontal axis represents temperature T and the vertical axis represents the horizontal pixel dark noise N p , red, blue and black represent the integration time t of 10s, 30s and 60s respectively; Figure 4 It can be seen that the horizontal pixel dark noise N p It has an exponential relationship with the temperature T, and the curvature of the fitting curve gradually increases with the increase of the integration time t; Figure 3 The inherent properties of each horizontal pixel of the CCD in (a), (b), and (c) result in different coefficients for the fitting curves at different horizontal pixel positions.

[0070] 3) Based on the same horizontal pixel, the relationship between horizontal pixel dark noise, integration time and temperature is obtained through surface fitting, that is, the fitting surface function. The specific formula is as follows:

[0071]

[0072] In formula (3), N p is the horizontal pixel dark noise; p is the horizontal pixel; P is the horizontal pixel value; t is the integration time; T is the temperature; a p 、b p 、c p d p are the fitting coefficients corresponding to the horizontal pixel p; f p is the corresponding horizontal pixel dark noise N p The correction constant of

[0073] In this embodiment, Figure 4The fitting three-dimensional surface graph of horizontal pixel dark noise, integration time and temperature is shown. The specific fitting process is: taking horizontal pixel p = 500 as an example, first, the fitting linear function is obtained in the interval of t = 8 ms-60 s using formula (1), then the three-dimensional matrix is obtained by arranging the fitting linear function in the range of T = 0 ℃-28 ℃, and finally the three-dimensional schematic diagram of the fitting surface function is obtained by surface fitting.

[0074] In addition, in the process of fitting, the coefficient of determination R 2 (0≤R 2 ≤1) is used to evaluate the fitting effect of the fitting linear function, the fitting curve function and the fitting surface function in turn. The larger and the closer to 1 the coefficient of determination R 2 is, the better the fitting effect is. In this example, through Matlab calculation, Figures 2-4 the coefficients of determination of the fitting linear function, the fitting curve function and the fitting surface function based on the same horizontal pixel all reach 0.99≤R 2 ≤1, which shows that the fitting linear function, the fitting curve function and the fitting surface function conform to the true situation.

[0075] S3, the temperature feedback by the temperature sensor and the set integration time are substituted into the fitting surface function, and the minimum theoretical dark noise and the noise reduction amplitude of multiple horizontal pixels inside the CCD are calculated;

[0076] S4, the target baseline is determined, and it is judged whether the minimum theoretical dark noise is greater than the target baseline: if yes, the signal compensation is performed on the CCD; otherwise, the dark noise is filtered and reduced after normal acquisition;

[0077] Specifically, 1) determining the target baseline: according to the standard of commonly used CCD dark noise, when the CCD baseline is controlled at about 2000, the maximum signal-to-noise ratio is as high as 33 dB, which shows that the dark noise level at this time has reached near zero level, so the target baseline is set to 2000;

[0078] 2) as shown in the figure, it is judged whether the minimum theoretical dark noise is greater than the target baseline 2000: if yes, it indicates that the CCD baseline has drifted and the baseline is too high, and the signal compensation is performed on the CCD to make the CCD baseline approach the target baseline; otherwise, it indicates that the CCD baseline has not drifted, and the dark noise is filtered and reduced after normal acquisition; Figure 5

[0079] Among them, the signal compensation of the CCD includes designing a signal compensation circuit and calculating a signal compensation voltage;

[0080] ① the design process of the signal compensation circuit: as shown in the figure, Figure 6 ​As shown, a signal compensation circuit is arranged between the CCD and the A / D converter, specifically, the pixel dark noise data output by the CCD after one collection is sent to the A / D converter through the signal compensation circuit; then the converted data is sent to the FPGA on-chip ram through synchronization; finally, it is sent to the PC through the USB2.0.

[0081] The working principle of the signal compensation circuit is as follows: Figure 9 As shown, the reset signal is compensated based on the voltage variation relationship of the horizontal pixel signal and the reset signal, specifically as follows: 1) when the CCD analog signal, i.e. the CCD dark noise signal, enters the reset signal stable stage, the switch S1 is connected to the D / A converter, and the switch S2 is connected to the resistor R4, the system constitutes a subtractor circuit, and the instruction data calculated by the fitting function in the library is sent to the D / A converter through the I 2 The D / A converter outputs a signal compensation voltage controlled by the C serial bus, and the reset signal is compensated by the subtractor differential circuit composed of the operational amplifier; 2) after the reset signal collection is completed, the CCD analog signal enters the horizontal pixel signal output stage, and the switches S1 and S2 are disconnected, and the system forms a follow-up circuit for effective signal collection: if the minimum theoretical dark noise calculated by the fitting surface function is less than the target baseline, the current collection does not perform signal compensation, and the switches S1 and S2 remain disconnected to form a follow-up circuit.

[0082] The principle of the signal compensation voltage is as follows: Figure 7 As shown, it represents the CCD output signal voltage detected by the oscilloscope, wherein the label ① represents the reset buffer stage, the label ② represents the reset signal stable stage, and the label ③ represents the horizontal pixel signal output stage.

[0083] In this embodiment, the reset signal and the horizontal pixel signal voltage are collected by the correlated double sampling method, and the reset signal voltage is eliminated by subtracting the horizontal pixel signal voltage from the reset signal voltage to complete signal extraction. Assuming that the dark noise is 0, the calculation formula of the CCD output signal voltage after the correlated double sampling is as follows:

[0084] V out = V res -V signal (4)

[0085] In formula (4), V out is the CCD output signal voltage after the correlated double sampling; V res is the reset signal voltage; and V signal is the horizontal pixel signal voltage.

[0086] In this embodiment, long integration time will affect the noise level of the effective signal. As the integration time increases, the charge accumulation inside the CCD will also increase, and the temperature rise will also speed up the rate of charge accumulation. These factors will cause the horizontal pixel signal voltage to decrease, i.e. V signal The numerical value decreases, and the reset signal voltage V res does not change with the integration time, resulting in an increase in the CCD output signal voltage, which is manifested in the graphical interface as an increase in the CCD baseline and an increase in the noise floating level.

[0087] As Figure 8 shown, it represents a schematic diagram of the change of signal voltage when the CCD baseline drifts, where V s ' ignal Bl The specific formula is as follows:

[0088] V n = V Bl -V s ' ignal (5)

[0089] In formula (5), V n is the charge accumulation voltage under the condition of CCD baseline drift, i.e. the voltage difference between V Bl and V s ' ignal V Bl is the horizontal pixel signal voltage corresponding to the target baseline N Bl ; and V s ' ignal is the horizontal pixel signal voltage when the CCD baseline drifts.

[0090] When the integration time and temperature change cause charge accumulation, the CCD reset signal voltage V res does not change, and at this time the CCD output signal voltage V out is the output signal voltage after the baseline drift, which is specifically as follows:

[0091] V out = V res -(V Bl -V n ) (6)

[0092] In formula (6), V out is the CCD output signal voltage after correlated double sampling; V res is the reset signal voltage; V Bl is the horizontal pixel signal voltage corresponding to the target baseline N Bl ; and V n is the charge accumulation voltage under the condition of CCD baseline drift.

[0093] ​In addition, the reset signal voltage V res is needed before the CCD is correlated double-sampled

[0094] V out res com Bl n (7)

[0095] In formula (7), V out is the CCD output signal voltage after correlated double-sampling; V res is the reset signal voltage; V com is the signal compensation voltage; V Bl is the target baseline N Bl corresponding to the horizontal pixel signal voltage; and V n is the charge accumulation voltage in the case of CCD baseline drift.

[0096] When V com = V n , the floating of V out can be reduced, thereby correcting the CCD dark noise baseline drift phenomenon.

[0097] 2. Process of calculating the signal compensation voltage:

[0098] The minimum theoretical dark noise in one collection is obtained by fitting the surface function, and the compensation dark noise in the signal compensation circuit is obtained based on the target baseline of the CCD dark noise. The specific calculation formula is as follows:

[0099] ΔN min = N min -N Bl (8)

[0100] In formula (8), ΔN min is the compensation dark noise in the signal compensation circuit; N min is the minimum theoretical dark noise; and N Bl is the target baseline of the CCD dark noise.

[0101] The A / D converter internal reference voltage is obtained based on the 16-bit precision A / D converter AD9826;

[0102] The signal compensation voltage V com is calculated based on the compensation dark noise and the A / D converter internal reference voltage. The specific calculation formula is as follows:

[0103]

[0104] In formula (9), V com ​​​​V is a signal compensation voltage; ΔN min V is a compensation dark noise in the signal compensation circuit; ref V is an internal reference voltage of the A / D converter, taking 4V;

[0105] In this embodiment, the minimum theoretical dark noise N min is taken as a standard to obtain the compensation dark noise ΔN min , which can effectively avoid the problem of weakening the effective signal caused by the excessive compensation dark noise; for different types of CCD, the collection and fitting operation under different integration time and temperature is carried out before installation, and the multiple fitting functions obtained by fitting are built in the library in the spectrometer. When the spectrometer works, the operation instruction containing the integration time is input, and the temperature is fed back in real time through the temperature sensor, and then the corresponding fitting function in the library is called to obtain the signal compensation voltage.

[0106] S5, based on the noise reduction amplitude, the dark noise after compensation is filtered and denoised to complete the correction;

[0107] Specifically, the original dark noise N 1p of the signal compensation circuit is compensated by the value of the compensation dark noise ΔN min , and the new dark noise N1' p generated after denoising is generated, and the specific calculation formula is as follows:

[0108] N1' p = N 1p -(N 2p -ΔN min )(10)

[0109] In formula (10), N1' p is the new dark noise generated after denoising; N 1p is the original dark noise after the signal compensation circuit; N 2p is the dark noise under the conditions of integration time and temperature simulated by the fitting surface function; ΔN min is the compensation dark noise in the signal compensation circuit;

[0110] In this embodiment, through the signal compensation circuit, the dark noise value of 2048 horizontal pixels in the CCD has been compensated by ΔN min , so that the baseline drift range of the CCD is controlled to a certain extent, but the fluctuation amplitude of the CCD dark noise is still large at this time, and at this time, the fluctuation amplitude of the dark noise needs to be reduced through the denoising means; then 2048 fitting surface functions are used to simulate the dark noise N 2p (p = 1, 2, 3... P) under the conditions of integration time t and temperature T, and it is compared with the original dark noise N 1pThe subtraction operation is performed on the original signals of the CCD using the correction method of the present application to perform signal compensation and filtering and noise reduction processing, and then the effect of the CCD baseline drift correction is analyzed, as shown in

[0111] The original signals of the CCD are compensated and filtered and noise reduced using the correction method of the present application, and then the effect of the CCD baseline drift correction is analyzed, as shown in Figure 10 The changes of the CCD baseline before and after the correction under different integration times and temperatures are shown. First, the test is performed on three groups of different integration times at room temperature T = 28℃, as shown in Figure 10 (a), (b) and (c); second, the test is performed on three groups of different temperatures at t = 60s, as shown in (d), (e) and (f), and the experimental results are more significant. Figure 10

[0112] To verify the effect of the correction method of the present application, the average value AVG and the root mean square effective value RMSE of the CCD dark noise are used to evaluate the effect of the CCD baseline correction, wherein the AVG and the RMSE of the CCD dark noise output data reflect the numerical size of the CCD baseline and the noise volatility. In order to realize the stability control and the near-zero baseline level control of the CCD baseline drift, the AVG and the RMSE of the CCD dark noise output data are smaller, the better. In the present embodiment, the integration time of the CCD is between 8ms and 60s, and the temperature is within 0℃-28℃, so it is necessary to ensure that the CCD baseline is stable and as close to zero as possible within the integration time and temperature range. The specific results are shown in Table 1.

[0113] Table 1 Comparison results of the CCD output signal voltage before and after the correction

[0114]

[0115] As shown in Table 1, when t = 60s and T = 28℃, the CCD baseline is about 29500, indicating that the CCD baseline drift is extremely serious. After the method of the present embodiment is used, the CCD baseline is controlled at about the target baseline 2000, and the dark noise baseline correction percentage is as high as 93.41%. Within the integration time range of 8ms-60s and the temperature range of 0℃-28℃, the CCD baseline is stable at the target baseline 2000, which meets the experimental requirements and also meets the dark noise baseline stability requirements of most high-performance spectrometers.

[0116] ​In addition, the fluctuation range of the dark noise of the CCD is also well controlled after the noise reduction. When the maximum integration time t is 60s at T=28℃, the RSME is only about 570.0, and the fluctuation range of the dark noise of the CCD is small. For the spectrometer with the maximum dynamic range of the CCD being 65535, the maximum signal-to-noise ratio can still be as high as 28db when the integration time is 60s.

[0117] The present application controls the CCD baseline to approach the target baseline, reduces the fluctuation range of the CCD baseline, realizes the CCD baseline drift correction and near-zero baseline level control, increases the dynamic range of the spectrometer detection, and guarantees the accuracy and reliability of the subsequent spectral analysis algorithm by designing the signal compensation circuit based on the subtracter on the hardware to compensate the analog signal output by the CCD and filtering and reducing the noise of the digital signal collected by the A / D converter on the software, and by the cooperation of the hardware and the software.

[0118] The above-described embodiments are merely preferred embodiments of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the principles and the essence of the present application shall fall within the protection scope of the present application as defined by the claims.

Claims

1. A back-illuminated CCD baseline drift correction method, characterized in that: The specific steps include: S1, collecting dark noise of multiple horizontal pixels inside the CCD to obtain pixel dark noise data; S2, obtaining a fitting surface function by fitting a custom function based on pixel dark noise data; S3, substituting the temperature feedback from the temperature sensor and the set integration time into the fitting surface function to calculate the minimum theoretical dark noise and the noise reduction amplitude of multiple horizontal pixels inside the CCD; S4. Determine the target baseline and judge whether the minimum theoretical dark noise is greater than the target baseline: if so, perform signal compensation on the CCD; Otherwise, the dark noise is filtered and denoised after normal acquisition; Specifically, 1) Determine the target baseline: According to the commonly used CCD dark noise standard, when the CCD baseline is controlled at around 2000, the maximum signal-to-noise ratio is as high as 33dB, indicating that the dark noise level at this time is already close to zero. Therefore, the target baseline is set to 2000; 2) Determine whether the minimum theoretical dark noise is greater than the target baseline of 2000. If so, it indicates that the CCD baseline has drifted and is too high. Perform signal compensation on the CCD to bring the CCD baseline closer to the target baseline. Otherwise, it indicates that the CCD baseline has not drifted, and the dark noise is filtered and reduced after normal acquisition; Among them, signal compensation for CCD includes designing signal compensation circuit and calculating signal compensation voltage; ① Design process of signal compensation circuit: A signal compensation circuit is set up between the CCD and the A / D converter. Specifically, the pixel dark noise data generated by the CCD after completing one acquisition is sent to the A / D converter through the signal compensation circuit; then the A / D converter sends the converted data to the FPGA chip through synchronization; finally, it is sent to the PC via USB. ②The process of calculating signal compensation voltage: The minimum theoretical dark noise in one acquisition is obtained by fitting the surface function, and the compensation dark noise in the signal compensation circuit is obtained based on the target baseline of the CCD dark noise. The internal reference voltage of the A / D converter is obtained based on the 16-bit precision A / D converter AD9826; The signal compensation voltage is calculated based on the compensation dark noise and the internal reference voltage of the A / D converter; S5. Perform filtering and noise reduction processing on the compensated dark noise based on the noise reduction amplitude to complete correction.

2. The method for correcting baseline drift of a back-illuminated CCD according to claim 1, wherein: The pixel dark noise data in step S1 includes first pixel dark noise data and second pixel dark noise data; wherein the first pixel dark noise data is obtained by controlling the integration time and collecting the dark noise of multiple horizontal pixels inside the CCD at different temperatures; the second pixel dark noise data is obtained by controlling the temperature and collecting the dark noise of multiple horizontal pixels inside the CCD at different integration times.

3. The method for correcting baseline drift of a back-illuminated CCD according to claim 1, wherein: The step S2 specifically includes the following steps: 1) Based on the same horizontal pixel, the relationship between horizontal pixel dark noise and integration time is obtained by least squares linear fitting. The specific formula is as follows: （1） In formula (1), is the horizontal pixel dark noise; is the horizontal pixel; is the horizontal pixel value; is the integration time; is temperature; 、 The corresponding horizontal pixels The fitting straight line coefficient of 2) Based on the same horizontal pixel, the relationship between horizontal pixel dark noise and temperature is obtained through curve fitting. The specific formula is as follows: （2） In formula (2), is the horizontal pixel dark noise; is the horizontal pixel; is the horizontal pixel value; is the integration time; is temperature; 、 The corresponding horizontal pixels The fitting curve coefficient of 3) Based on the same horizontal pixel, the relationship between horizontal pixel dark noise, integration time and temperature is obtained through surface fitting, which is the fitting surface function. The specific formula is as follows: （3） In formula (3), is the horizontal pixel dark noise; is the horizontal pixel; is the horizontal pixel value; is the integration time; is temperature; 、 、 、 The corresponding horizontal pixels The fitting coefficient of is the corresponding horizontal pixel dark noise The corrected constant.

4. The method for correcting baseline drift of a back-illuminated CCD according to claim 1, wherein: The specific calculation formula for the compensated dark noise in the signal compensation circuit in step S4 is as follows: （8） In formula (8), Compensating for dark noise in the signal compensation circuit; is the minimum theoretical dark noise; is the target baseline of CCD dark noise; The specific calculation formula for the signal compensation voltage is as follows: （9） In formula (9), Compensate voltage for the signal; Compensating for dark noise in the signal compensation circuit; The internal reference voltage of the A / D converter is 4V.

5. The method for correcting baseline drift of a back-illuminated CCD according to claim 1, wherein: The step S5 specifically includes the following steps: Original dark noise of signal compensation circuit Multiple horizontal pixels are compensated to compensate for dark noise The value of the new dark noise generated after denoising , the specific calculation formula is as follows: （10） In formula (10), is the new dark noise generated after denoising; is the original dark noise after the signal compensation circuit; The dark noise under the conditions of integration time and temperature is simulated by fitting surface function; Compensates for dark noise in the signal compensation circuit.

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