CN114593894A - Detection distance calculation method for point source target imaging - Google Patents
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Abstract
The invention provides a detection distance calculation method for point source target imaging, which comprises the following steps: s1, converting the target signal and the background signal into a target signal electron number and a background signal electron number respectively; s2, substituting the target signal electron number, the background signal electron number and the characteristic parameters of the photoelectric detector into a signal-to-noise ratio calculation formula; and S3, carrying out inverse operation on the signal-to-noise ratio calculation formula under the premise of knowing the detection signal-to-noise ratio, and obtaining the detection distance. By utilizing the detection distance calculation method for point source target imaging, provided by the invention, the influence of target signals, background signals, an optical system and detector parameters on the detection signal-to-noise ratio is comprehensively considered, and the detection distance from a target to the entrance pupil of the optical system can be accurately calculated by substituting the target signals, the background signals, the optical system and the detector parameters into a signal-to-noise ratio calculation formula.
Description
Technical Field
The invention relates to the technical field of infrared detection distance analysis, in particular to a detection distance calculation method of point source target imaging based on electron number.
Background
The detection capability of the infrared photoelectric system on the point target is closely related to the target radiation characteristic, the background radiation characteristic, the atmospheric condition, the infrared optical system performance, the detector characteristic and other factors. Common foundational RedThe method for calculating the detection capability of the external photoelectric system comprises three methods, namely: 1) based on the transfer function; 2) based on the contrast ratio; 3) based on the signal-to-noise ratio. For the method based on the transfer function, the transfer function of each subsystem is difficult to obtain accurately; contrast-based methods are commonly used in imaging systems for human eye viewing; for point target imaging, a signal-to-noise ratio-based method is generally adopted to establish a detection capability calculation model of an infrared photoelectric system. The physical quantity characterizing the sensitivity of the camera is generally the average detectivity D within the band*Noise equivalent temperature difference NETD, etc. Thus, the general use of D-based*And NETD, such as:
SNR＝|(Lt-Lbg)/Nt|D*(λ)AtAoτa(R)τo/R2(Ad/2tint)1/2 (1)
and D*And NETD are scalable, for a photodetector:
in the formula, D*(λp) For peak detectivity, unit cm Hz1/2·W-1(ii) a NETD is noise equivalent temperature difference of the infrared photoelectric system, and the unit K is; f is the F number of the optical system when testing NETD; tau.otThe transmittance of an optical system is tested when the NETD is tested; c. C2＝1.4388×104μ m · K is a second radiation constant; lambda [ alpha ]pPeak wavelength in μm; t isBFor test background temperature, unit K; a. thedIs the area of a single pixel of the detector in cm2；Δf0The noise equivalent bandwidth of the test system is in Hz; wTBPlanck spectral radiance as background, unit W cm-2。
Prior art uses of photodetectors*Value calculation of the detection Signal-to-noise ratio, but D*The values were measured under laboratory conditions with a background of 20 ° black, a target of 35 ° black, and the target filled with pixels. WhileThe environment is complex in practical use, and the target and the background are measured D*Values are also compared with laboratory measurements D*The situation of the value is different, D of the actual environment measurement*D value to be measured in laboratory*The difference in value causes an error, so that the result of the detection distance calculation does not match the actual result.
Disclosure of Invention
The object of the present invention is to overcome the drawbacks of the prior art, to solve the problem of measuring D under laboratory conditions*Value and actual environmental measurement of D*The method is characterized in that the detection distance calculation method of point source target imaging based on the electron number comprehensively considers the influence of target signals, background signals, an optical system and detector parameters on the detection signal-to-noise ratio and substitutes the target signals, the background signals, the optical system and the detector parameters into a signal-to-noise ratio calculation formula, and under the condition of the known detection signal-to-noise ratio requirement, the detection distance is accurately calculated by utilizing the inverse operation of the signal-to-noise ratio calculation formula.
The invention provides a detection distance calculation method for point source target imaging, which comprises the following steps:
s1, converting the target signal and the background signal into a target signal electron number and a background signal electron number respectively;
s2, substituting the target signal electron number, the background signal electron number and the characteristic parameters of the photoelectric detector into a signal-to-noise ratio calculation formula;
and S3, carrying out inverse operation on the signal-to-noise ratio calculation formula under the premise of knowing the detection signal-to-noise ratio, and obtaining the detection distance.
Preferably, in step S1, the calculation formula of the target signal electron number is:
in the formula (1), StThe electron number of the target signal is shown, and eta is the quantum efficiency of the photoelectric detector; h is the Planck constant; v is the frequency of light, c/lambda, c is the speed of light, lambda is the wavelength of light; t is tintIs the integration time; phi is a unit oftA radiant flux at the target;
in the formula (2), AtIs the effective radiation area of the target; l is a radical of an alcoholtIs the radiance of the target; tau isaIs the average atmospheric transmittance between the target and the device; tau isoIs the transmittance of the optical system; d is the entrance pupil diameter of the optical system; r is a detection distance; n is the pixel number occupied by the imaging light spot of the target on the target surface of the photoelectric detector;
the formula for calculating the electron number of the background signal is as follows:
in the formula (3), SbBackground signal electron count; l isbIs the sky background radiance; a. thedThe area of a single pixel of the photoelectric detector; f is the relative aperture number of the optical system.
Preferably, in step S2, the snr calculation formula is:
in the formula (4), SNR is the detection signal-to-noise ratio; sigmaRRead noise for the photodetector; deIs the dark current of the photodetector.
Preferably, in step S3, the calculation formula of the detection distance R is:
preferably, step S1 specifically includes the following steps: s110, respectively converting the target signal and the background signal into a target signal photon number and a background signal photon number; and S120, converting the target signal photon number and the background signal photon number into a target signal electron number and a background signal electron number respectively through the quantum efficiency of the photoelectric detector.
Preferably, when the target is circular, the calculation formula of the number of pixels N occupied by the imaging light spot of the target on the target surface of the photodetector is as follows:
Ω＝Ad/f2 (7)
ΩIMG＝π(θIMG/2)2 (8)
θTGT＝r/R (10)
wherein omegaIMGImaging a corresponding solid angle for the target; omega is a solid angle corresponding to a single pixel of the detector; a. thedThe area of a single pixel of the detector; f is the system focal length; thetaIMGThe average opening angle of the target on the image plane; thetaTGTThe geometric opening angle of the target on the image plane; r is the target diameter; lambda is the central wavelength of the system working waveband; r is target observation slope distance; r is0Is the atmospheric coherence length; d is the primary mirror diameter.
Preferably, when the target is square, the calculation formula of the number of pixels N occupied by the imaging light spot of the target on the target surface of the photodetector is as follows:
θTGT1＝a/R (13)
θTGT2＝b/R (14)
wherein omegaIMGImaging a corresponding solid angle for the target; omega is a solid angle corresponding to a single pixel of the detector; a and b are respectively the side length of the target; a. thedThe area of a single pixel of the detector; f is the system focal length; theta.theta.IMGThe average opening angle of the target on the image plane is taken; thetaTGT1And thetaTGT2Respectively the geometric opening angles of the target on the image plane; lambda is the central wavelength of the system working waveband; r is target observation slope distance; r is0Is the atmospheric coherence length; d is the primary mirror diameter.
By utilizing the detection distance calculation method for point source target imaging provided by the invention, the influence of characteristic parameters of a target, a background, an optical system and a detector on a detection signal-to-noise ratio can be comprehensively considered, the influence factors of the target and the background signals received by a detector pixel are fully explained, the detection distance is accurately calculated, and references are provided for system parameter optimization and detector type selection, for example:
(1) the target signal is in direct proportion to the entrance pupil area of the infrared photoelectric system, while the background signal is in inverse proportion to the relative aperture number of the optical system, so that the relative aperture number of the optical system is increased while the aperture of the telescope is increased as much as possible to achieve higher detection capability on the basis of ensuring that the target is still detected as a point source on an image;
(2) the saturation integration time of the detector under different observation scenes and target characteristics can be determined according to the full-trap charge number of the photoelectric detector, a target signal, a background signal and characteristic parameters of the photoelectric detector, and a decision basis can be provided for task execution;
(3) for daytime detection, a large full-well charge photodetector may guarantee a longer integration time, thereby extending the detection distance.
Drawings
FIG. 1 is a first flow diagram of a method of detection range calculation for point source target imaging according to one embodiment of the invention;
FIG. 2 is a second flow chart diagram of a method of detection range calculation for point source target imaging according to one embodiment of the invention;
FIG. 3 is a schematic view of an observation of the Infrared Standard Star HD156283 according to one embodiment of the invention.
Detailed Description
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same reference numerals are used for the same blocks. In the case of the same reference numerals, their names and functions are also the same. Therefore, detailed description thereof will not be repeated.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not to be construed as limiting the invention.
The invention aims to solve the problem of D measured under laboratory conditions*Value and actual environmental measurement of D*The method is dependent on the characteristic parameters (quantum efficiency, dark current and read noise) of the photoelectric detector, and comprehensively considers the influence of the characteristic parameters of the target, the background, the optical system and the photoelectric detector on the detection signal-to-noise ratio, so that the calculation result of the detection distance is more accurate.
The overall thought of the invention is as follows: the method comprises the steps of respectively converting a target signal and a background signal into photon numbers when a photoelectric detection system observes a target, converting the photon numbers into electron numbers by combining the quantum efficiency of a photoelectric detector, and substituting the electron numbers into a signal-to-noise ratio calculation formula to calculate the detection signal-to-noise ratio. And under the premise of knowing the detection signal-to-noise ratio, carrying out inverse operation on the signal-to-noise ratio calculation formula to calculate the detection distance.
It should be noted that the calculation method of the present invention is applicable to any wavelength band of photoelectric detection system, such as infrared photoelectric detection system, ultraviolet photoelectric detection system, etc., the photoelectric detection system includes two components, namely an optical system and a detector, the optical system is used for imaging the target, and the detector is used for performing photoelectric conversion on the target image.
The following will describe the method for calculating the detection distance for imaging the point source target according to the present invention in detail.
Fig. 1 and 2 respectively show two flows of a detection distance calculation method for point source target imaging according to an embodiment of the present invention.
As shown in fig. 1 and fig. 2, a method for calculating a detection distance of point source target imaging according to an embodiment of the present invention includes the following steps:
and S1, converting the target signal and the background signal into a target signal electron number and a background signal electron number respectively.
Converting the target signal and the background signal into the target signal electron number and the background signal electron number according to the following two steps:
and S110, respectively converting the target signal and the background signal into a target signal photon number and a background signal photon number.
The formula for converting the target signal into the number of target signal photons is as follows:
in the formula (1), tintIs the integration time; h is Planck constant, h is 6.626176 × 10-34W·s2(ii) a V is the frequency of light, c is the speed of light, c is 2.997925 × 108m·s-1λ is the wavelength of light; phi is atFor the radiation flux, S, of the object on a single photo-detector pixelMFor the target at integration time tintThe number of target photons generated.
In the formula (2), AtIs the effective radiation area of the target; l istThe target radiance can be calculated by a Planck formula according to the target temperature and the surface emissivity; tau isaTargeted to the equipmentAverage atmospheric transmittance, τoThe transmittance of the optical system is shown as D, the entrance pupil diameter of the optical system is shown as R, the detection distance, namely the distance from the target to the entrance pupil of the optical system is shown as R, and the number of pixels occupied by the imaging light spots of the target on the target surface of the photoelectric detector is shown as N.
When the number of pixels occupied by imaging light spots of a target on a target surface of a photoelectric detector is N, for a circular target, the diameter of the target is R, the central wavelength of a system working waveband is lambda, the observation slant distance of the target is R, and the atmospheric coherence length is R0And D is the diameter of the primary mirror, the average field angle of the target on the image plane is as follows:
wherein, thetaTGTR/R, then:
in the formula, omegaIMGImaging a corresponding solid angle for the target; omega is a solid angle corresponding to a single pixel of the detector; a. thedThe area of a single pixel of the detector; f is the system focal length; thetaIMGThe average opening angle of the target on the image plane; thetaTGTIs the geometric opening angle of the object on the image plane. The target 80% of the energy is generally considered to be centered within the K pixels calculated above.
For a square object, let the side lengths of the object be a and b, θTGT1＝a/R，θTGT2b/R, then:
wherein, thetaTGT1And thetaTGT2Respectively the geometric opening angle of the object in the image plane.
Atmospheric disturbance 2 "corresponds to a coherence length of:
corresponding atmospheric coherence length:
r0＝r0(@550nm)×(λ/550nm)6/5 (7)
according to the formula r0＝r0|α＝90°(sinα)3/5And calculating to obtain the atmospheric coherence length of the alpha-degree observation elevation angle.
The flare angle of the light spot caused by the atmospheric dispersion effect is as follows:
1.22λ/r0(@ center wavelength λ)&Actual observation elevation angle α);
the spot angle caused by diffraction effects is: 1.22X lambda/D.
The formula for converting the background signal into the number of background signal photons is:
in the formula (8), LbIs the sky background radiance; a. thedThe area of a single pixel of the photoelectric detector; f is the relative aperture number of the optical system, S'bAt an integration time tintThe number of background signal photons generated.
And S120, converting the target signal photon number and the background signal photon number into a target signal electron number and a background signal electron number respectively through the quantum efficiency of the photoelectric detector.
The formula for converting the number of target signal photons into the number of target signal electrons is as follows:
St＝η·SM (9)
in the formula (9), η is the quantum efficiency of the photodetector, the quantum efficiency characterizes the photoelectric conversion efficiency of the photodetector, and StIs the target signal electron number.
The formula for converting the number of background signal photons into the number of background signal electrons is as follows:
Sb＝η·S′b (10)
and S2, substituting the target signal electron number, the background signal electron number and the characteristic parameters of the photoelectric detector into a signal-to-noise ratio calculation formula.
The characteristic parameters of the photodetector include readout noise and dark current of the photodetector.
The signal-to-noise ratio calculation formula is as follows:
in the formula (11), SNR is the detection signal-to-noise ratio, σRRead noise for the photodetector; deIs the dark current of the photodetector.
And S3, carrying out inverse operation on the signal-to-noise ratio calculation formula under the premise of knowing the detection signal-to-noise ratio, and obtaining the detection distance.
The detection distance is the distance from the object to the entrance pupil of the optical system.
When the detection signal-to-noise ratio SNR is a known quantity, the detection distance R is reversely derived by performing an inverse operation on equation (11) and calculating all other parameters in equation (11) as known quantities.
The calculation formula of the detection distance R is:
in one embodiment of the present invention, the total charge number of the target pixel can be obtained from equation (12):
for daytime detection, a large well-filled charge number photodetector may guarantee a longer integration time, thereby extending the detection distance.
Under the condition that the full-trap charge number S of the photoelectric detector is a known quantity, the saturation integration time value can be reversely deduced according to the formula (13), the saturation integration time of the detector under different observation scenes and target characteristics can be determined, and a decision basis can be provided for task execution.
In order to verify the accuracy of the detection distance calculation method for point source target imaging, provided by the invention, a medium wave infrared telescope system with the caliber of 680mm is used for verification, and system parameters are shown in the following table:
because the energy of a flying target or a space target received by the infrared photoelectric system is difficult to accurately estimate, the received energy comprises the reflection of the target on sunlight, the reflection of the target on earth heat radiation and the reflection of the target on the sunlight reflected by the earth besides the self heat radiation of the target; in addition, accurate parameters such as target temperature, surface emissivity and flight attitude are difficult to know, so that the accuracy of a detection distance formula is verified by using an infrared standard star with known radiation quantity.
When the detection distance is calculated, the known quantity of the target is the target area and the target radiation brightness, the target detection distance when the detection signal-to-noise ratio SNR is 5 is obtained, and the infrared standard star knows the radiation illuminance outside the atmosphere, and for the radiation illuminance E, because:
in the formula (14), ItIs the target radiation intensity. Equation (2) becomes:
therefore, the detection signal-to-noise ratio can be calculated according to the known infrared standard star irradiance value, and then the detection signal-to-noise ratio is compared with the detection signal-to-noise ratio of the shot image, so that the accuracy of the method is verified.
As shown in fig. 3, the infrared standard star HD156283 photographed on 19/4/2019 is 1.72 noise, 7966 background mean, 16.47 average SNR, 31.94 peak SNR, 10ms integration time, 40 ° (corresponding to the atmospheric transmittance 0.5145) observation angle, and 4.1 × 10 theoretical irradiance value of HD156283 in the range of 3.7 μm to 4.8 μm-15W/cm2. Presuming that the background radiance is about 1.7W/m according to the image digital value and the integration time2And/sr, from which a theoretical signal-to-noise ratio of 20 is calculated. The value of D of the photodetector is 6 x 1011cm·Hz1/2W-1The theoretical signal-to-noise ratio is calculated to be 22 according to the formula (1) in the background technology, and the theory is consistent with the actual measurement result, so that the calculation result of the detection distance calculation method for point source target imaging provided by the invention is proved to be accurate.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.
The above embodiments of the present invention should not be construed as limiting the scope of the present invention. Any other corresponding changes and modifications made according to the technical idea of the present invention should be included in the protection scope of the claims of the present invention.
Claims (7)
1. A detection distance calculation method for point source target imaging is characterized by comprising the following steps:
s1, converting the target signal and the background signal into a target signal electron number and a background signal electron number respectively;
s2, substituting the target signal electron number, the background signal electron number and the characteristic parameters of the photoelectric detector into a signal-to-noise ratio calculation formula;
s3, carrying out inverse operation on the signal-to-noise ratio calculation formula under the premise of knowing the detection signal-to-noise ratio, and obtaining the target detection distance under the detection signal-to-noise ratio.
2. The method for calculating the detection distance of point source target imaging according to claim 1, wherein in step S1, the calculation formula of the target signal electron number is:
in the formula (1), StThe electron number of the target signal is shown, and eta is the quantum efficiency of the photoelectric detector; h is the Planck constant; v is the frequency of light, c/lambda, c is the speed of light, lambda is the wavelength of light; t is tintIs the integration time; phi is atA radiant flux at the target;
in the formula (2), AtIs the effective radiation area of the target; l istIs the radiance of the target; tau isaIs the average atmospheric transmittance between the target and the device; tau isoIs the transmittance of the optical system; d is the entrance pupil diameter of the optical system; r is a detection distance;n is the pixel number occupied by the imaging light spot of the target on the target surface of the photoelectric detector;
the calculation formula of the background signal electron number is as follows:
in the formula (3), SbBackground signal electron count; l isbIs the sky background radiance; a. thedThe area of a single pixel of the photoelectric detector; f is the relative aperture number of the optical system.
3. The method for calculating the detection distance for imaging a point source target according to claim 2, wherein in step S2, the signal-to-noise ratio calculation formula is:
in the formula (4), SNR is the detection signal-to-noise ratio; sigmaRIs the readout noise of the photodetector, and has the unit of e-; deThe dark current of the photodetector is expressed in the unit of e-/s/pixel.
4. The method for calculating the detection distance for point source target imaging according to claim 3, wherein in step S3, the calculation formula of the detection distance R is:
5. the method for calculating the detection distance of point source target imaging according to claim 1, wherein the step S1 specifically comprises the steps of:
s110, respectively converting the target signal and the background signal into a target signal photon number and a background signal photon number;
and S120, converting the target signal photon number and the background signal photon number into a target signal electron number and a background signal electron number respectively through the quantum efficiency of the photoelectric detector.
6. The method for calculating the detection distance of point source target imaging according to claim 2, wherein when the target is circular, the calculation formula of the number of pixels N occupied by the imaging light spot of the target on the target surface of the photoelectric detector is as follows:
Ω＝Ad/f2 (7)
ΩIMG＝π(θIMG/2)2 (8)
θTGT＝r/R (10)
wherein omegaIMGImaging a corresponding solid angle for the target; omega is a solid angle corresponding to a single pixel of the detector; a. thedThe area of a single pixel of the detector; f is the system focal length; thetaIMGThe average opening angle of the target on the image plane; thetaTGTThe geometric opening angle of the target on the image plane; r is the target diameter; lambda is the central wavelength of the system working waveband; r is target observation slope distance; r is0Is the atmospheric coherence length; d is the primary mirror diameter.
7. The method for calculating the detection distance of point source target imaging according to claim 2, wherein when the target is square, the calculation formula of the number of pixels N occupied by the imaging light spot of the target on the target surface of the photoelectric detector is as follows:
θTGT1＝a/R (13)
θTGT2＝b/R (14)
wherein omegaIMGImaging a corresponding solid angle for the target; omega is a solid angle corresponding to a single pixel of the detector; a and b are respectively the side length of the target; a. thedThe area of a single pixel of the detector; f is the system focal length; thetaIMGThe average opening angle of the target on the image plane; thetaTGT1And thetaTGT2Respectively the geometric opening angles of the target on the image plane; lambda is the central wavelength of the system working waveband; r is target observation slope distance; r is0Is the atmospheric coherence length; d is the primary mirror diameter.
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