Underwater monitoring device and underwater monitoring method

Abstract

Provided are an underwater monitoring device and an underwater monitoring method in which water turbidity, etc., are taken into consideration and with which it is possible to deal with a variety of situations and obtain high measurement accuracy.　An underwater monitoring device according to one aspect of the present invention comprises a light emission member that emits a plurality of laser beams in the visible region, a plurality of light-receiving members that receive the respective reflected beams from the plurality of laser beams in the visible region emitted by the light emission member, and an imaging member that captures an image of a region to the front. An underwater monitoring method according to another aspect of the present invention is characterized in comprising emitting a plurality of visible-region laser beams, receiving reflected beams from each of the visible-region laser beams, and selecting some of the reflected beams from among the plurality of received reflected beams to obtain measurement light, said visible-region laser beams being emitted while being scanned, and image data for a frontward-oriented region being acquired during intermittent periods in the scanning.

Description

Underwater monitoring device and underwater monitoring method

[0001] The present invention relates to an underwater monitoring device and an underwater monitoring method.

[0002] Light detection and ranging (LiDAR) is a technology that uses light to measure distant objects (measurement objects). LiDAR is a technology that irradiates the measurement object with light, measures the light that is reflected by scattering of the irradiated light, and performs various measurements based on the information obtained from the measurement.

[0003] While LiDAR is generally used on land, there is a desire to apply this LiDAR technology underwater as well.

[0004] Technologies relating to underwater LiDAR are described in, for example, Patent Documents 1 to 3 and Non-Patent Document 1 below.

[0005] JP 2011-196955 A JP 2017-228889 A JP 4-103232 A

[0006] https: / / www.vision-systems.com / cameras-accessories / article / 14037190 / 3d-laser-scanner-system-targets-underwater-inspection-applications

[0007] In actual underwater measurements, turbidity varies depending on factors such as various suspended and dissolved substances. The type and amount of these substances vary greatly depending on the measurement location, so it is necessary to respond appropriately to these various conditions. In addition, there is also the issue of measurement accuracy varying because the reflectance of the object being measured varies depending on the wavelength.

[0008] However, none of the techniques described in Patent Documents 1 to 3 and Non-Patent Document 1 take into consideration the turbidity in the water, and there remain problems with the accuracy of the measurement.

[0009] In view of the above, an object of the present invention is to provide an underwater monitoring device and an underwater monitoring method that take into account the turbidity of water and are adaptable to various situations and have high measurement accuracy.

[0010] An underwater monitoring device according to one aspect of the present invention that solves the above problem comprises a light irradiating element that irradiates multiple laser beams in the visible range, multiple light receiving elements that receive the reflected light of each of the multiple laser beams in the visible range irradiated by the light irradiating element, and an imaging element that photographs the area in front.

[0011] In addition, in this respect, although not limited thereto, it is preferable that the plurality of light irradiating members are arranged within an area surrounded by the plurality of light receiving members and the imaging member.

[0012] In addition, in this respect, although not limited thereto, it is preferable that the laser light has a wavelength in the range of 360 nm or more and 830 nm or less.

[0013] In addition, in this respect, although not limited thereto, it is preferable that the light irradiation member includes an optical scanning member.

[0014] In addition, in this respect, although not limited thereto, it is preferable that the imaging member is an RGB camera.

[0015] In addition, in this respect, although not limited thereto, it is preferable to provide a control member that controls the light emitting member, the light receiving member, and the image capturing member.

[0016] In addition, an underwater monitoring method according to another aspect of the present invention irradiates multiple laser lights in the visible range, receives reflected light from each of the laser lights in the visible range, and selects at least a portion of the multiple reflected light lights received as measurement light.

[0017] In addition, in this respect, although not limited thereto, it is preferable to select the reflected light having the highest signal intensity as the selected light.

[0018] In addition, an underwater monitoring method according to another aspect of the present invention irradiates a laser beam in the visible range while scanning, and acquires image data of the area ahead during intermittent periods in the scanning.

[0019] In addition, in this respect, although not limited thereto, the scanning is preferably raster scanning.

[0020] In addition, in this respect, although not limited thereto, it is preferable that the laser light in the visible region is pulsed light.

[0021] As described above, the present invention can provide an underwater monitoring device and an underwater monitoring method that take into account the turbidity in water, can handle various turbidity conditions, and has high measurement accuracy.

[0022] 1 is a schematic diagram of an underwater monitoring device 1 according to an embodiment. FIG. 1 is a diagram of the underwater monitoring device 1 according to an embodiment as viewed from the front. FIG. 2 is a schematic diagram of a cross section of the underwater monitoring device 1 according to an embodiment. FIG. 3 is a functional block diagram of the underwater monitoring device 1 according to an embodiment. FIG. 4 is a diagram showing the wavelength dependency of light transmittance in water. FIG. 5 is a diagram showing the wavelength dependency of light transmittance in water. FIG. 6 is a functional block diagram of a control device of the underwater monitoring device according to an embodiment. FIG. 7 is a timing chart showing an example of a waveform of a light emission control signal according to an embodiment. FIG. 8 is a diagram explaining an example of scanning of light irradiated by the underwater monitoring device according to an embodiment. FIG. 9 is a diagram explaining an example of scanning of light irradiated by the underwater monitoring device according to an embodiment. FIG. 10 is a diagram explaining an example of scanning of light irradiated by the underwater monitoring device according to an embodiment.

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention can be embodied in many different forms and is not limited to the specific examples shown in the following embodiments.

[0024] Fig. 1 is a schematic diagram of an underwater monitoring device 1 according to this embodiment (hereinafter referred to as "this device"), Fig. 2 is a diagram showing the arrangement of a light irradiating member 2, a light receiving member 3, and an imaging device 4 when this device 1 is viewed from the front, and Fig. 3 is a schematic diagram showing a cross section of this device 1. Also, Fig. 4 is a functional block diagram of this device 1.

[0025] As described above, the device 1 includes a light irradiation member 2 that irradiates multiple laser beams L in the visible range, multiple light receiving members 3 that receive reflected light R from each of the multiple laser beams L in the visible range irradiated by the light irradiation member 2, and an imaging member 4 that captures an image of the area in front of it. The device 1 also includes a control member 5 that controls the light irradiation member 2, the light receiving members 3, and the imaging member 4. In the device 1, the light irradiation member 2, the light receiving members 3, the imaging device 4, and the control member 5 are housed in a housing 6 and sealed to prevent water from entering. This configuration of the device 1 enables measurement (monitoring) of the surrounding environment, the distance from the measurement object, the color, etc.

[0026] As is clear from the description in this specification, the device 1 is a device used to measure the distance to an object to be measured underwater (hereinafter referred to as the "measurement object") and the reflectance characteristics of the measurement object. The device 1 is not limited to this, and may be in the form of a device fixed to one location underwater for fixed-point observation, or in the form of an attachment device installed on a so-called underwater drone that is equipped with a power member such as a motor and can move freely underwater, or may be in the form of a device combined with other members such as the power member of the underwater drone to form part of it and housed together in a single housing.

[0027] As described above, the present device 1 includes the light irradiation member 2 and is capable of irradiating a plurality of laser beams L in the visible region. The light irradiation member 2 is not limited as long as it has the above-described functions, but is preferably configured to include a light source member 21, a light path adjustment member 22, and a light scanning member 23. Specifically, the light irradiation member 2 of the present device 1 emits light from the light source member 21, then adjusts the light path of the light with the light path adjustment member 22 and causes the light to enter the light scanning member 23, which then changes the direction (optical axis) of the light over time, specifically, enables scanning.

[0028] Furthermore, as described above, the light source member 21 of the present device 1 is a member capable of emitting laser light L in a plurality of visible wavelengths, and is not limited to any particular type as long as it has this function, but is preferably, for example, a laser diode (LD), a fiber laser, etc., but is more preferably a laser diode from the viewpoints of simplicity, space-saving, power-saving, and compactness. Specifically, the use of a laser diode makes it possible to easily emit desired light, and has the advantages of saving space even if laser diodes are installed for each desired wavelength, and of reducing power consumption.

[0029] Furthermore, it is preferable that the wavelengths of the multiple laser beams emitted by the light source member 21 of the present device 1 are all in the visible range. This is because, among a wide range of wavelengths, light in the visible wavelength range is less likely to be absorbed by water. In other words, by using wavelengths in the visible range, it is possible to achieve accurate distance measurement underwater, as they are less likely to be absorbed even in water. Note that wavelengths in the visible range will be described later, but specifically, they are preferably in the range of 360 nm to 830 nm.

[0030] Furthermore, the present device 1 uses laser light of multiple wavelengths. This is because the wavelengths that are easily transmitted through water (including seawater) vary depending on various conditions (factors), such as the turbidity of the water, and the wavelengths that are easily reflected by the object being measured vary. In other words, by using laser light of multiple wavelengths, it is possible to select an appropriate wavelength from among the wavelengths, enabling highly accurate measurements. Furthermore, the number of wavelengths of the laser light is not limited, but is preferably two or more, preferably three or more, and more preferably four or more. By using three or more wavelengths, it is possible to provide light in each of the red (R), green (G), and blue (B) color regions, which has the advantage of being able to sufficiently distinguish the wavelength regions that are easily transmitted and reflected through water, as described above and below.

[0031] Figures 5 and 6 illustrate how the optimal wavelength varies depending on the situation. Figure 5 shows the transmittance versus wavelength of seawater in a certain port area in Japan in spring. Figure 6 shows the transmittance versus wavelength of seawater in another port area in Japan in summer. As shown in these figures, the maximum transmittance varies depending on various conditions, such as turbidity and the location of measurement. Specifically, in the example of Figure 5, light with a green (G) wavelength has the highest transmittance, while in the example of Figure 6, light with a red (R) wavelength has the highest transmittance. In other words, as shown in these figures, the optimal wavelength varies significantly depending on the environment, so by using laser light with multiple wavelengths, it is possible to select the laser light with the most appropriate wavelength depending on the situation.

[0032] Here, as described above, the preferred wavelength range is the visible range, specifically, the range of 360 nm to 830 nm. Furthermore, as described above, when the present device 1 uses laser light of multiple different wavelengths, it is preferable that the light be in the red (R), green (G), or blue (B) color range. For red (R), the range is preferably 620 nm to 830 nm. For green (G), the range is preferably longer than 500 nm and less than 600 nm. For blue (B), the range is preferably 430 nm to 500 nm. Furthermore, by using these wavelengths, it is possible to obtain the three primary colors of light: red (R), green (G), and blue (B). This not only enables LiDAR to measure the distance to the object being measured, but also potentially allows for the acquisition of various information, as described below.

[0033] The optical path adjusting member 22 of the device 1 is a member for appropriately changing and adjusting the optical path of the laser light emitted by the light source member 21 and guiding it to a desired direction and position. Specific examples of the optical path adjusting member 22 are not limited to, but are preferably configured to include, for example, a lens 221 for focusing or collimating the laser light emitted from the light source member 21, and an optical member (dichroic mirror) 222 for combining the laser light of each color, but are not limited to this.

[0034] The optical scanning member 23 of the device 1 is a member that can change the direction (optical axis) of light incident from the optical path adjusting member 22 over time. The optical scanning member 23 is not limited to, but examples thereof include, but are not limited to, a MEMS mirror and a polygon mirror. The actual scanning movement of the optical scanning member will be described separately below.

[0035] Furthermore, as will be described later, the light irradiation member 2 of the present device 1 is preferably arranged in an area surrounded by a plurality of light receiving members 3, and therefore is preferably made smaller. Specifically, it is preferable to house the light source member 21, the light path adjustment member 22, and the optical scanning member 23 in a cylindrical light irradiation member housing 24. This makes it possible to make the device smaller. In this case, the cross-sectional image of the light irradiation member 2, the light receiving member 3, and the imaging member 4 is as shown in Figure 3 above.

[0036] As described above, the light receiving member 3 of the present device 1 receives the reflected light of each of the multiple laser beams L in the visible range emitted by the light irradiating member 2. According to the present device 1, the light irradiated from the light irradiating member 2 strikes the object to be measured and becomes scattered light, but a portion of this scattered light returns to the side of the present device 1 as reflected light, and by detecting this light with the light receiving element 41, it becomes possible to obtain the necessary information as will be described later.

[0037] As described above, the device 1 is provided with a plurality of light-emitting members 3. The reason why a plurality of light-receiving members 3 are provided is that the light-irradiating member 2 irradiates a plurality of laser beams L in the visible region, and it is necessary to receive light corresponding to each of the plurality of laser beams L in the visible region.

[0038] The light receiving member 3 of the present device 1 is not limited as long as it has the above-mentioned function, but is preferably configured to be able to output an electrical signal based on the received light, and specific examples thereof include a photoelectric conversion element 31, and optical elements such as a bandpass filter 32 and a lens 33 arranged in front of the photoelectric conversion element 31. Here, the photoelectric conversion element 31 is not limited to, but examples thereof include a phototransistor, a photodiode, a phototube, a photomultiplier tube, etc. This allows the received laser light to be output to the control device 5 as an electrical signal.

[0039] The imaging device 4 of the device 1 is a device that can receive light input from the outside and create an image. The use of the imaging device 4 has the advantage that information about the surroundings of the device 1 can be obtained as an image together with color information.

[0040] More specifically, the imaging device 4 is preferably a so-called camera device, and more specifically, it is preferably equipped with a light receiving element 41, an optical element such as a lens 42 provided in front of the light receiving element 41, and a camera control device 43 connected to the light receiving element 41.

[0041] The light-receiving element 41 of the imaging device 4 is an element that receives light and outputs an electrical signal based on the received light. This electrical signal is then input to the camera control device 43, which creates image data based on the electrical signal. From this perspective, the light-receiving element 41 can be, but is not limited to, a photoelectric conversion element such as a CCD or CMOS. In this case, the camera device is preferably a so-called RGB camera, in which a light-receiving element 41 is provided for each of the three primary colors of light (red (R), green (G), and blue (B)). The use of an RGB camera has the advantage of being able to acquire color information underwater to obtain images, and furthermore, by combining this with distance information, a wider variety of information can be obtained.

[0042] Furthermore, the camera control device 43 receives the input of the electrical signal output from the light receiving element 41 as described above and creates image data. Image data refers to data containing image information, and this image data is output to the outside of the camera control device 43 and displayed as an image on a display device or the like. A specific example of the camera control device 43 can be, but is not limited to, a printed circuit board or the like in which resistors, capacitors, IC chips, etc. are arranged on a plastic substrate on which metal wiring is formed, and these are connected by the metal wiring to form an electronic circuit that receives the electrical signal from the light receiving element and creates image data.

[0043] In the present device 1, the light irradiation member 2 is preferably disposed within an area surrounded by the plurality of light receiving members 3 and the imaging member 4. By arranging the light irradiation member 2 within an area surrounded by the light receiving members 3, the distance between the light irradiation member 2 and each of the plurality of light receiving members 3 can be made as uniform as possible when the laser light travels back and forth. Furthermore, the optical system that emits the laser light, i.e., the light source member 21 and the light path adjustment member 22, which require more precise handling, can be arranged in a position offset from the plurality of light receiving members 3 within the housing 6, which has the effect of facilitating precision adjustment work and reducing the external size of the housing. In the present device 1, the number of light receiving members 3 is preferably the same as the number of wavelengths emitted by the light irradiation member 2, i.e., the number of light source members 21. Preferably, there are three or more light receiving members 3 corresponding to the red (R), green (G), and blue (B) regions, respectively. However, there may be four or more light receiving members 3, and more preferably five or more light receiving members 3, as long as the installation area is secured.

[0044] The control member 5 of the device 1 is electrically connected to the light irradiating member 2, the light receiving member 3, and the imaging device 4, and serves to control these members. This enables various processes to be performed, and the distance to the object to be measured can be measured with high accuracy.

[0045] 7 is a diagram showing the functional blocks of the control member 5 of the device 1. As shown in this figure, the control member 5 of the device 1 is configured to include a light emission control signal generation unit 51, a drive signal generation unit 52, a time-of-flight measurement unit 53, an image data control unit 54, and a main control unit 55. In the example shown in this figure, the device 1 is connected to an external computer or the like by wire or wirelessly, and receives commands from the operator of the device 1 to perform desired processing.

[0046] The control member 5 of the device 1 is not limited as long as it has the above-mentioned functions and can be realized in various configurations, but for example, it may be a printed circuit board or the like in which resistors, capacitors, IC chips, etc. are arranged on a plastic substrate on which metal wiring is formed, and these are connected by the metal wiring to form an electronic circuit that receives electrical signals from the light-receiving elements and creates image data, similar to the camera control device 43. Furthermore, this printed circuit board is preferably not only configured with a so-called gate array that performs predetermined processing, but also with a programmable so-called FPGA (Field-Programmable Gate Array).

[0047] The light emission control signal generating section 51 in the control member 5 generates a light emission control signal in response to an external command signal and outputs this to the light irradiating member 2, and the light irradiating member 2 emits light based on this.

[0048] As is clear from the above description, the light emission control signal generated by the control member 5 is a signal that is input to the light irradiation member 2, more specifically, the light source member 21. The light source member 21 can emit a predetermined amount of light based on this light emission control signal. It is preferable to generate as many light emission control signals as the number of light source members 21, more specifically, the number of light source members 21 that are to emit light. Figure 8 shows an example of the waveform of the light emission control signal generated by the control member 5. In this example, a pulse-shaped light emission control signal is input to each of three light sources of red (R), blue (G), and green (B).

[0049] Furthermore, the waveform of the light emission control signal is not limited as long as it can emit light from the light source member 21 and achieve the effects of the present device 1, but it is preferable that it be a pulsed waveform that occurs at regular intervals, as shown in this figure. This makes it possible to improve the peak value of the laser light, which has the effect of enabling accurate measurements in scenes with high turbidity.

[0050] Furthermore, when a plurality of light source members 21 are provided, it is preferable that the light emission control signal has a waveform formed so that light is emitted at different times. By slightly shifting the times, the angles of the laser light emitted into space vary slightly, which has the advantage of making it possible to irradiate a large number of measurement points.

[0051] Furthermore, the drive signal generating unit 52 in the control device 5 of the present device 1 generates a drive signal, which is a signal for controlling the optical scanning member 23 in the light irradiation member 2. The optical scanning member 23 is driven by the drive signal generating unit 52, making it possible to emit laser light generated by the light source member 21 in a desired direction. The waveform of the drive signal can be adjusted as appropriate depending on the type of optical scanning member 23 used, etc.

[0052] The control member 5 also includes a time-of-flight measurement unit 53. The time-of-flight measurement unit 53 calculates time-of-flight data and distance data based on the electrical signal received from the light-receiving member 3. Theoretically, the difference between the time data when the light-irradiating member 2 irradiates light and the time data when the light-receiving member 3 receives light can be calculated as time-of-flight data. In other words, "time-of-flight data" refers to data containing information about the time of flight (time of flight) of light. Knowing this time-of-flight data allows multiplying it by light speed data containing information about the speed of light to measure the round-trip flight distance data of the laser light (half of which is the distance between the device 1 and the object being measured). In other words, "distance data" refers to data containing information about the distance between the device 1 and the object being measured, and includes information about the value obtained by halving the product of the time-of-flight data and the light speed data. The device 1 is intended for use in a medium, more specifically, underwater. For more accurate measurements, light speed data in a medium, more specifically, underwater, is preferable. The acquired distance data is output to the main control unit 55.

[0053] The control device 5 also includes an image data control unit 54. The image data control unit 54 is connected to the imaging device 4 and receives image data from the imaging device 4 (acquires image data), controlling the timing of this image acquisition. More specifically, the device 1 not only has a so-called LiDAR function that measures the distance to the measurement target using the light-irradiating member 2 and the light-receiving member 3, but also allows for direct image acquisition by simultaneously installing an imaging device. However, when attempting to acquire an image using the imaging device, if light from the light-irradiating member 2 is reflected back to the device 1, the reflected light will be recognized as a bright spot. In other words, this bright spot may cause noise in the image. Therefore, by providing the image data control unit 54, this noise can be removed. An example of this is shown in the same diagram (Figure 8) as the light emission control signal.

[0054] As shown in this diagram, the image data control unit 54 issues a trigger signal at regular intervals in synchronization with the time period when the LiDAR is not measuring. After issuing this trigger signal, the image data control unit 54 acquires image data for a certain period of time. This has the advantage of being able to obtain an image of the surroundings excluding the period when bright spots are occurring, as described above. The acquired image data is then output to the main control unit 55.

[0055] The main control unit 55 is connected to the light emission control signal generation unit 51, the drive signal generation unit 52, the time-of-flight measurement unit 53, and the image data control unit 54, and performs overall control, i.e., main control. The main control unit 55 serves as a relay point that records various data, such as distance data input from the time-of-flight measurement unit 53 and image data input from the image data control unit 54, and outputs the data to an externally connected computer or the like.

[0056] Here, the underwater distance measurement method (underwater monitoring method (hereinafter referred to as "this method")) realized by this device 1 will be described.

[0057] First, this method includes the steps of (S1-1) irradiating a plurality of laser beams in the visible range, (S1-2) receiving reflected light from each of the laser beams in the visible range, and (S1-3) selecting the reflected light with the strongest signal intensity from the plurality of received reflected light beams as the measurement light.

[0058] First, this method includes a step (S1-1) of irradiating a plurality of laser beams in the visible region. As is clear from the above description, this step is performed by the light irradiation member 2. In this step, as described above, it is preferable to emit a plurality of laser beams with different wavelengths at different times, but as long as the light receiving member 3 can sufficiently separate the light beams of the plurality of wavelengths, it is also possible to irradiate a plurality of laser beams with different wavelengths simultaneously.

[0059] Furthermore, in this method, multiple laser beams in the visible range are irradiated. However, as mentioned above, the wavelength range that is easily transmitted varies depending on the underwater conditions, such as turbidity. Therefore, to maximize accuracy, it is preferable to start measurement by using the laser beam with the wavelength that has the highest reflected signal intensity ratio among the multiple laser beams with different wavelengths (if the irradiation intensity is the same, it is preferable to use the laser beam with the highest reflected signal intensity). This allows for the most accurate measurement. Furthermore, since the wavelength selection may vary depending on the sunlight entering the water, it is preferable to irradiate the water with laser beams of multiple different wavelengths again after a certain period of time has passed and compare the intensities. Of course, it is also preferable to irradiate multiple wavelengths of light simultaneously or with a time lag during measurement.

[0060] Although there are no limitations on the selection of this wavelength, it is preferable to select the wavelength that has the highest signal strength in the following formula.

[0061] In the above formula, S is the signal strength, k is a proportionality constant, and R λ is the reflectance of the object to be measured at wavelength λ, T λ indicates the transmittance at wavelength λ for the forward and backward paths, and Ps indicates the source power of the light source member. In this formula, it can be considered that the transmittance in air is approximately 100% (T=1). On the other hand, the unknowns are k and R λ , Ps, so by measuring the signal strength while changing Ps at least three times, the reflectance of the object being measured in air can be determined. Then, by performing a similar measurement underwater, the transmittance of seawater at wavelength λ can be determined. Then, by substituting the seawater transmittance at each wavelength as T into the above equation in advance, the reflectance of the object being measured in the sea can be determined. Then, by using S, which has the largest value, accuracy can be improved.

[0062] Even more advantageously, because LiDAR can measure distance, it can also measure transmittance per unit distance, as shown on the vertical axis in Figures 5 and 6. This makes it possible to determine the reflectance of an object at any distance, as long as the object is within a measurable range.

[0063] Furthermore, in underwater monitoring devices such as this one that are equipped with both LiDAR and a camera, knowing the distance from the LiDAR to the object being measured can be used to improve the image quality of the camera.

[0064] Underwater camera images appear bluish because light in the red wavelength band is absorbed, causing the red component to attenuate with increasing distance. Therefore, in conventional underwater camera-only systems, the camera itself does not have the ability to measure distance. Therefore, to restore the original color, complex image processing techniques have been used to first estimate the distance to the subject and then adjust the color of the photograph (see, for example, Reference 1 (https: / / openaccess.thecvf.com / content_CVPR_2019 / papers / Akkaynak_Sea-Thru_A_Method_for_Removing_Water_From_Underwater_Images_CVPR_2019_paper.pdf)). However, this method suffers from a decrease in the accuracy of color restoration and requires a huge amount of computational resources for the subsequent calculation process (long processing times on a high-performance computer).If LiDAR and a camera are installed simultaneously, as in this method, the distance values ​​obtained by LiDAR can be used to restore the color of the camera, making it possible to complete the task accurately and in a short time.

[0065] Furthermore, the light of a preferred wavelength to be selected can be adjusted appropriately depending on the purpose of monitoring, and the following can be considered, for example.

[0066] The purpose of monitoring may be to emphasize structural measurement of the object being measured, i.e., to obtain highly accurate and precise 3D data of the object being measured (Case A), or to measure the reflectance characteristics of the object being measured, i.e., to identify or classify the material of the object by measuring the reflectance characteristics of the object in a manner similar to multispectral measurement (Case B).

[0067] On the other hand, the device can operate in two ways: using only one wavelength per unit time (Case 1) or using three wavelengths simultaneously (Case 2). In other words, four possible combinations are possible: Cases A and B and Cases 1 and 2. The algorithm for using the laser based on these combinations is shown in Table 1. Ideally, if a device is equipped with three wavelengths, it would be desirable to be able to measure using all three wavelengths at all times. However, for example, there are cases where (1) simultaneous distance calculations using all three wavelengths per unit time are not possible due to limitations in the processing power of the downstream distance calculation circuitry, or (2) when mounted on an underwater drone (such as the aforementioned ROV or AUV), low power consumption is desirable to extend the duration of a single dive measurement. Therefore, Cases A and B can be considered to be Cases 1 and 2, respectively, resulting in a total of four use cases. These are summarized in Table 1.

[0068] In Table 1, for example, measuring 3D data of a structure when the turbidity is high corresponds to Case A. Therefore, if there are no constraints on the device's downstream processing capacity or power consumption, 3D data can be obtained using three wavelengths simultaneously. However, if there are constraints, as shown in Table 1, a preliminary survey is first conducted, i.e., a survey is conducted by emitting a laser for each wavelength, and the signal strength S shown in Equation 1 is calculated for each wavelength to determine the strongest one. Then, actual measurement work is performed using the laser of that wavelength. After an appropriate time has passed, the above preliminary survey is conducted again, and if the wavelength with the highest intensity is found to be different, actual measurement work is then performed using the wavelength with the strongest intensity. This makes it possible to obtain useful 3D data even in highly turbid waters.

[0069] In this method, it is preferable to irradiate a laser beam in the visible range while scanning. Because the cross-sectional area of ​​the emitted laser beam is a minute point area, even if the laser beam is irradiated on the object to be measured, only the distance to the point on the object to be measured can be measured. Therefore, by irradiating the laser beam while scanning, it is possible to measure distances over a wide area by determining the distances at multiple points, even though they are only points. This scanning can be achieved by the optical scanning member 23 described above.

[0070] The optical scanning method is not particularly limited, but examples include raster scanning and rotary scanning. However, when considering a typical case where the light receiving element 41 is a photoelectric conversion element such as a CCD or CMOS, raster scanning is preferred because it is simpler and corresponds to the cells arranged in a grid pattern in the photoelectric conversion element. Figure 9 shows an example of raster scanning. In raster scanning, the scanning starts from one corner (the upper left corner in the figure, the intersection of the left vertical line and the upper horizontal line) and gradually moves down to the other vertical line (the right vertical line) (moving from the upper horizontal line to the lower horizontal line) (this is called a "line"), then turns back to the other vertical line (the left vertical line), turns back again, and repeats until it reaches the other corner (the lower right corner in the figure, the intersection of the right vertical line and the lower horizontal line). This makes it possible to scan a certain area without omissions. After reaching the other corner (bottom right in the figure), it returns in a straight line (in a non-emitting state) to one corner (top left in the figure), and begins scanning again for the next frame (i.e., the period from one corner to the other corner and back to the one corner is one frame).

[0071] Incidentally, when irradiating a plurality of laser beams with different wavelengths, if the plurality of laser beams are irradiated simultaneously, they will be irradiated at the same position during scanning, but if the laser beams are emitted at different times, various innovations are possible. For example, as shown in Fig. 10, laser beams with different wavelengths may be emitted for each frame, or as shown in Fig. 11, the wavelength of light emitted may be different for each line, such that one line has one wavelength, the next line has another wavelength, and the next line has yet another wavelength, or as shown in Fig. 12, laser beams with one wavelength may be emitted with one pulse, laser beams with another wavelength may be emitted with the next pulse, and laser beams with yet another wavelength may be emitted with the next pulse.

[0072] In addition, this method preferably includes a step of acquiring image data of the forward region during intermittent periods in the scan (S1-1-2). As described above, it is possible to measure the distance to the object by irradiating laser light from the light-irradiating member 2 and receiving the reflected light with the light-receiving device 3. However, there is a problem in that this reflected light enters the imaging device 4 and becomes a bright spot, making the image of the forward region difficult to see. Therefore, this step has the advantage of eliminating noise caused by the reflected light by acquiring image data with the imaging device 4 during intermittent periods (non-emitting periods when the laser returns linearly from the other corner to one corner). An example of this is shown in FIG. 8.

[0073] As described above, the device 1 can provide an underwater monitoring device and an underwater monitoring method that take into account the turbidity in the water, can handle various turbidity conditions, and has high measurement accuracy.

[0074] In particular, as described above, the present device 1 may perform measurements by irradiating laser light of multiple different wavelengths, or by employing a single light beam with the highest signal intensity (signal intensity ratio) of the returning light. Each method has its own advantages. A detailed explanation follows. (A) Measurement at a Single Wavelength Performing measurements at a single wavelength offers the advantage of ensuring a larger number of measurement points and improving measurement accuracy compared to measurements at multiple wavelengths. In particular, employing light beams with the highest signal intensity (signal intensity ratio) of the returning reflected light further improves measurement accuracy. In particular, distance measurement calculations are limited by the capabilities of the control device and external computers. Therefore, using a single wavelength has the advantage of reducing the computational load. (B) Measurement at Multiple Wavelengths Even when measuring at a single wavelength, pulse-shaped signals always have intervals (blank periods). Therefore, if laser light of other wavelengths can be irradiated during these intervals, the time resolution can be further increased without wasting these intervals and without reducing the time resolution of the single wavelength. This is shown in Figure 8 above. This allows for more accurate measurements.

[0075] Furthermore, the device 1, by combining the light-irradiating member 2 and the light-receiving member 3, can obtain three-dimensional information with depth from the distance information between the device 1 and the object being measured. On the other hand, the imaging device 4 can obtain a planar image with color information. In other words, by combining these, color image information with depth can be obtained. While the device 1 can obtain color information by using laser light of multiple different wavelengths as described above, this is merely based on the reflected light intensity of the irradiated light. Therefore, there is an advantage in that more detailed information can be obtained by overlaying and correlating the information on the light intensity of multiple different reflected lights with the color information obtained from the imaging device 4. Furthermore, by using a high-resolution camera, advanced recognition functions can be added by combining it with AI (artificial intelligence) technology. Furthermore, when acquiring the training data (teacher data) required for AI, the 3D measurement function obtained from LiDAR makes it possible to acquire more comprehensive data.

[0076] The present invention has industrial applicability as an underwater monitoring device and an underwater monitoring method.

[0077] 1. Underwater monitoring device 2. Light irradiation member 21. Light source member 22. Light path adjustment member 221. Lens 222. Optical member (dichroic mirror) 23. Light scanning member 3. Light receiving member 31. Photoelectric conversion element 32. Band-pass filter 33. Lens 4. Imaging device 41. Light receiving element 42. Lens 43. Camera control device 5. Control device 51. Light emission control signal creation unit 52. Drive signal generation unit 53. Time-of-flight measurement unit 54. Image data control unit 55. Main control unit

Claims

1. An underwater monitoring device comprising: a light-irradiating member that irradiates multiple laser beams in the visible range; multiple light-receiving members that receive the reflected light of each of the multiple laser beams in the visible range irradiated by the light-irradiating member; and an imaging member that images the area in front.

2. An underwater monitoring device according to claim 1, wherein said plurality of light irradiating members are arranged within an area surrounded by said plurality of light receiving members and said imaging member.

3. An underwater monitoring device as described in claim 1, wherein the laser light is in the range of 360 nm to 830 nm.

4. The underwater monitoring device of claim 1, wherein said light irradiating member comprises a light scanning member.

5. The underwater monitoring device according to claim 1, wherein said imaging member is an RGB camera.

6. An underwater monitoring device according to claim 1, further comprising a control member for controlling said light irradiating member, said light receiving member and said imaging member.

7. An underwater monitoring method comprising: irradiating a plurality of laser beams in the visible region; receiving reflected light from each of the laser beams in the visible region; and selecting at least a portion of the plurality of reflected light beams to be used as measurement light.

8. An underwater monitoring method according to claim 7, further comprising the step of selecting the reflected light having the highest signal strength as the selected light.

9. An underwater monitoring method comprising irradiating a visible laser beam while scanning, and acquiring image data of the area ahead during intermittent periods in the scanning.

10. The underwater monitoring method according to claim 9, wherein said scanning is a raster scan.

11. The underwater monitoring method according to claim 9, wherein the laser light in the visible region is a pulsed emission light.

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