Range gating imaging simulation method based on Monte Carlo

Abstract

The invention provides a Monte Carlo-based range gating imaging simulation method, which belongs to the field of scattering medium imaging, and in the simulation process, a delay distance S0 is used for replacing the delay time T of a detector in a range gating system, and a distance range S1 is used for replacing the gate opening time, namely the gate width # imgabs0 #, of the detector in the range gating system. Calculating transmission characteristics of photons in the scattering medium by using a Monte Carlo method, wherein the transmission characteristics comprise scattering and absorption of light by the scattering medium; photons emitted from the scattering medium are screened through a range gating method, the number and spatial distribution of the photons reaching the detector are obtained through statistics, and the number and spatial distribution are range gating images. The method provides a theoretical basis for the research of obtaining target information through the scattering medium and the research of feature extraction, target recognition and the like of scattering medium imaging, and has important scientific research significance and value.

Description

Technical Field

[0001] The invention relates to the technical field of scattering medium imaging, in particular to a Monte Carlo-based range-gated imaging simulation method. Background Art

[0002] At present, underwater imaging detection technology mainly includes two ways: acoustic detection and photoelectric detection. Acoustic detection technology has a long detection distance, but the spatial resolution of imaging is poor; underwater photoelectric imaging technology has high resolution and can quickly and intuitively transmit two-dimensional or even three-dimensional information of targets and scenes. However, due to the rapid attenuation of light along the transmission path in water, underwater photoelectric imaging requires auxiliary lighting, and because underwater photoelectric imaging equipment often has the same direction of illumination and imaging system, the backscattering of illumination light becomes an important factor affecting the working distance of underwater photoelectric imaging system. Therefore, reducing backscattering and improving image contrast have become the focus of underwater photoelectric imaging technology research. The range gating technology can suppress backscattering and significantly increase its imaging distance, making underwater laser range gating imaging technology one of the most effective photoelectric imaging technologies for underwater carrier platforms.

[0003] Some scientific research and technology institutions have conducted effective research and development on underwater range-gated optoelectronic imaging systems, such as the LUCIE series of products from Canada's DRDC Valcartier (National Defense Research Institute), which can work 200 meters underwater when mounted on an ROV to detect and monitor ports and deep seas. This product has developed three generations (2006-2009). Since 2006, Beijing Institute of Technology has conducted a lot of research on underwater range-gated imaging technology, including the use of a 5ns pulse width range-gated ICCD imaging system and a DPLNd:YAG high-power pulse laser to develop an underwater range-gated imaging experimental system, which has achieved direct detection of targets 40 meters away underwater, and has made a lot of improvements to handheld or desktop real-time underwater range-gated imaging instruments. Other units that have carried out related work include the 1411 Institute of Electronics and the 205 Institute of Ordnance. The working environment of underwater range-gated imaging systems is mostly the ocean, and the ocean environment itself is very complex. In addition, the design and manufacturing of each link of the underwater range-gated imaging system are not ideal. These factors together lead to the degradation of the quality of the acquired images. Summary of the invention

[0004] In view of this, an object of the present invention is to provide a Monte Carlo-based range-gated imaging simulation method to achieve active light illumination reflective optical imaging simulation.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A Monte Carlo-based distance-gated imaging simulation method, including; The simulation process is as follows: Photons emitted by a pulsed laser propagate in a scattering medium. The target object is located within the scattering medium. When photons reach the target object, diffuse reflection occurs, and then the photons are transmitted back in the scattering medium and finally reach a detector on the same side as the light source; Record the position and distance of each photon movement, and accumulate to obtain the total path S of the photons; Let the laser pulse emission time be 0, the detector gate is opened at time T, and closed at T + moment; Only when the photons reach the detector and the total path S is greater than the delay distance S0 and less than the sum of the delay distance S0 and the distance range S1, they are collected by the detector; Repeat the above process, and count the number of photons reaching the detector and the spatial distribution, which is the distance-gated image with the delay time T and the gate width ; Whether the photons irradiate the target object is determined by judging whether the position of the photons is within the range of the target object; The light intensity of the photons after diffuse reflection is equal to the light intensity of the incident photons multiplied by the reflectivity of the target object at that position, and the reflection angle is randomly selected within the range of 0 - 180 degrees; Use the delay distance S0 to replace the delay time T of the detector in the distance-gated system and use the distance range S1 to replace the gate opening time of the detector in the distance-gated system, that is, the gate width .

[0006] In a preferred embodiment, during the simulation process, by setting the delay distance S0, S0 = C × T, where C is the speed of light, to replace the delay time T of the detector in the distance-gated system.

[0007] In a preferred embodiment, during the simulation process, use the distance range S1 to replace the gate opening time of the detector in the distance-gated system, that is, the gate width , S1 = C × .

[0008] In a preferred embodiment, in the distance-gated system, let the laser pulse emission time be 0, the detector gate is opened at time T, and closed at T + moment; During the simulation process, based on Monte Carlo simulation, record the total path S of the photons during the entire propagation process; If S < S0, it can be known that the photons reach the detector before the gate is opened, and at this time the photons are not counted; If S0 < S < (S0 + S1), it can be known that the photons reach the detector during the gate opening, and at this time the photons are counted; If (S0 + S1) < S, it can be known that the photons reach the detector after the gate is closed, and at this time the photons are not counted.

[0009] Compared with the prior art, the present invention has the following beneficial effects: The method of the present invention provides a Monte Carlo-based distance-gated optical imaging simulation method for simulating the imaging results of a distance-gated system in a scattering medium. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a flow chart of the method implementation of the embodiment of the present invention.

[0011] Figure 2 It is a schematic diagram of a Monte Carlo-based range-gated imaging simulation model in an embodiment of the present invention. DETAILED DESCRIPTION

[0012] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0013] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0014] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application; as used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0015] refer to Figure 1 , 2 , a Monte Carlo-based range-gated imaging simulation method, Figure 1 The implementation steps of the Monte Carlo-based range-gated imaging simulation method provided in this embodiment are shown, including: 1) Delay distance S0 replaces delay time T for simulation.

[0016] 2) Distance range S1 instead of door width simulation.

[0017] 3) Transmission characteristics simulation.

[0018] 4) Receiving reflected photons simulation Among them, the delay distance S0 replaces the delay time T simulation step, In the simulation process, the delay distance S0 is used to replace the delay time T of the detector in the range-gated system. In the range-gated imaging system, to image the target at a distance L, the detector needs to be opened after the delay time of the pulse laser from emission to the target position and then reflected from the target to the detector T. The speed of light is C, and L=CT / 2. In the simulation process, the delay distance S0 is set, S0=CT, to replace the delay time T of the detector in the range-gated system. At this time, S0 is exactly the total distance S that the pulse laser in the range-gated system travels from emission to target position and then reflected from the target to the detector without scattering; if the pulse laser does not reach the target, it scatters with the scattering medium in advance and returns to the detector. The total distance S of this part of the laser will be less than S0, which is non-target light for the detector imaging, which will affect the imaging quality of the system. In the simulation process, the total distance S of the pulse laser is judged. If the total distance S<S0, it can be known that the pulse laser is not the target light at this time and is not received by the detector.

[0019] Among them, the distance range S1 replaces the door width Simulation steps: In the simulation process, the distance range S1 is used to replace the gate opening time of the detector in the distance gating system, that is, the gate width. In the range-gated imaging system, the imaging depth of field By door width On the other hand, the range resolution of the range-gated imaging system is determined by the laser pulse width and the gate width. Determine the laser pulse width and gate width The smaller it is, the less backscattered light the detector receives, and the higher the signal-to-noise ratio of its imaging. During the simulation, the distance range S1 is set, S1= , to replace the gate width in the range gating system Assume that the laser pulse emission time is 0, the detector gate is opened at time T, and at T+ At this time, (S0+S1) is exactly the total distance S that the pulse laser in the range gating system has traveled from the start of emission to the closing of the detector gating gate; if the pulse laser reaches the target but is scattered by the scattering medium and returns to the detector, the total distance S that this part of the laser has traveled will be greater than (S0+S1). By judging the total distance S that the pulse laser has traveled, if the total distance S>(S0+S1), it can be known that the pulse laser is scattered light at this time and is not received by the detector.

[0020] Among them, the transmission characteristics simulation steps are: First, according to the characteristics of the pulsed laser light source, the wavelength of the incident photon is set to 532 nm, the polarization state is linear polarization, and the position, scattering angle and azimuth parameters of the incident photon are randomly generated according to the spatial intensity distribution, emission angle and other properties of the pulsed laser light source using a random sampling method. The statistical characteristics of a large number of photons meet the intensity distribution, emission angle and other properties of the pulsed laser light source. The photon enters the scattering medium and scatters with the scattering particles. The scattering phase function is calculated using the Mie scattering theory, and the probability distribution of the scattering angle is determined based on this. Through the random sampling method, the position and distance information of the photon at each step in the transmission process is obtained and recorded. When the photon irradiates the target object, that is, when the photon position is within the range of the target object, reflection occurs, and the intensity of the photon is equal to the intensity of the photon before reflection multiplied by the reflectivity of the target object at that point. The photon reflection process is regarded as diffuse reflection, and the reflection direction during reflection is randomly generated in the range of 0~180° using the rejection sampling method. The reflected photon is transmitted again in the scattering medium until the photon leaves the scattering medium. During the transmission characteristics simulation process, the position and distance of each photon movement are recorded, and the total distance S of the photon is accumulated to obtain.

[0021] Among them, the step of receiving the reflected photon simulation is: It is based on the distance gating principle to screen the photons entering the detector so that the detector only collects photons within the gate width, thereby reducing the impact of backscattered light in active light illumination reflective imaging and improving imaging quality.

Claims

1. A Monte Carlo-based range-gated imaging simulation method, characterized in that: include; The simulation process is as follows: the photons emitted by the pulsed laser propagate in the scattering medium, the target object is located in the scattering medium, and when the photons reach the target object, diffuse reflection occurs. Then the photons are transmitted in the scattering medium in the opposite direction, and finally reach the detector on the same side as the light source. The position and distance of each photon movement are recorded, and the total distance S of the photons is accumulated. Assume that the laser pulse emission time is 0, the detector gate is opened at time T, and at T+ The detector is closed at all times; only when the photons reach the detector and the total distance S is greater than the delay distance S0 and less than the sum of the delay distance S0 and the distance range S1, can they be collected by the detector; repeat the above process, and count the number of photons reaching the detector and their spatial distribution, which is the delay time T, the gate width The distance-gated image is obtained by measuring whether the photon is irradiated on the target object or not. The light intensity of the photon after diffuse reflection is equal to the light intensity of the incident photon multiplied by the reflectivity of the target object at that position. The reflection angle is randomly selected within the range of 0-180 degrees. The delay distance S0 is used to replace the delay time T of the detector in the distance-gated system, and the distance range S1 is used to replace the gate opening time, i.e., gate width, of the detector in the distance-gated system. .

2. The Monte Carlo-based range-gated imaging simulation method according to claim 1, characterized in that: During the simulation, the delay distance S0 is set, S0=C×T, where C is the speed of light, to replace the delay time T of the detector in the range gating system.

3. The Monte Carlo-based range-gated imaging simulation method according to claim 1, characterized in that: In the simulation process, the distance range S1 is used to replace the gate opening time of the detector in the distance gating system, that is, the gate width. , S1=C× .

4. The Monte Carlo-based range-gated imaging simulation method according to claim 1, characterized in that: In a range-gating system, let the laser pulse emission time be 0, and the detector gating gate opens at time T and closes at time T + ; During the simulation process, based on Monte Carlo simulation, record the total path S of photons during the entire propagation process; If S < S0, it can be known that photons reach the detector before the gating gate opens, and at this time the photons are not counted; If S0 < S < (S0 + S1), it can be known that photons reach the detector during the opening of the gating gate, and at this time the photons are counted; If (S0 + S1) < S, it can be known that photons reach the detector after the gating gate closes, and at this time the photons are not counted.

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