Optical wireless communication method for underwater equipment

Abstract

The purpose of the present disclosure is to provide underwater equipment capable of performing control such that, in water, communication light for optical wireless communication is not affected by illumination light, and an image photographed by a camera using the illumination light is not affected by the communication light.　Underwater equipment according to the present invention comprises: an optical transmission / reception unit 13 that can move in water and that performs underwater optical communication with another communication device 20 by means of communication light; an imaging unit 11 that photographs the surroundings; an illumination unit 12 that shines illumination light at an imaging target 30 of the imaging unit; and a control unit 14 that performs time-division control such that a communication time of communication by means of the communication light and a light emission time of the illumination light do not overlap.

Description

Optical wireless communication method for underwater devices

[0001] The present disclosure relates to an optical wireless communication system for spaces requiring illumination.

[0002] In recent years, while aerial drones have been under consideration for social implementation, development of underwater drones aimed at innovation in the underwater field is progressing (see, for example, Non-Patent Document 1). Aerial drones can be controlled wirelessly using radio waves and can also acquire location information using GPS, so their application to, for example, GPS-based delivery systems is progressing. However, the transmission characteristics of various physical media underwater are significantly limited compared to air, and the GPS signals required for control use radio waves, so they cannot reach underwater and location information cannot be acquired. Therefore, current underwater drones are controlled by an onboard operator viewing images from the underwater drone's camera via a cable called a tether.

[0003] Masaki Minami, "Current Status and Issues of Underwater Drone Systems," Measurement and Control 2020, Vol. 59, No. 7, pp. 492-496, Published 2020 / 07 / 17, Online ISSN 1883-8170, Print ISSN 0453-4662

[0004] Such tethered operations have the following issues: (a) multiple personnel are required for operation (pilot and cable operator); (b) there is a risk that the underwater drone itself will become tangled in the cable and become unable to navigate; (c) navigation in narrow spaces is difficult due to the risk of the cable becoming tangled in obstacles; and (d) the navigation range is limited to the length of the tether cable.

[0005] To overcome the aforementioned issues with underwater drones that require tether cables, it is necessary to establish a means of communication between the operator and the underwater drone. Possible communication methods include wireless communication using sound, light, radio waves, etc. Figure 1 summarizes the underwater characteristics and issues of each physical medium: sound, light, and radio waves.

[0006] Underwater communication using light (hereinafter referred to as optical wireless) is an attractive method when considering applications such as transmitting high-definition video from underwater industrial robots, etc. However, since the illumination light used for video capture underwater where sunlight does not reach and the visible light used for optical wireless communication interfere with each other, establishing a method to reduce the mutual influence of the two is a challenge.

[0007] In other words, in order to solve the above-mentioned problems, the present invention aims to provide underwater equipment that can be controlled so that optical wireless communication light is not affected by illumination light underwater, and so that images captured by a camera using illumination light are not affected by communication light.

[0008] In order to achieve the above object, the underwater equipment of the present invention controls the illumination light and communication light to be emitted in a time-division manner, and extracts from the captured images the images captured when the illumination light was being emitted.

[0009] Specifically, the underwater device of the present invention comprises an optical transceiver unit that performs underwater optical communication with other communication devices using communication light; an imaging unit that divides continuously captured images of the surrounding area into frames at predetermined intervals and outputs the frames; a video monitor unit that receives the framed images output by the imaging unit; an illumination unit that illuminates the subject being imaged by the imaging unit with illumination light; and a control unit that performs time-sharing control so that the communication time using the communication light and the emission time of the illumination light do not overlap, and causes the video monitor unit to output the framed images when the illumination light is emitted to the optical transceiver unit.

[0010] Because illumination light and communication light are emitted alternately, it is possible to avoid the influence of illumination light in optical wireless communication. Furthermore, by extracting from the captured images those images captured when illumination light was being emitted, it is possible to obtain images that are not influenced by communication light. Therefore, the present invention can provide an underwater device that can be controlled underwater so that optical wireless communication light is not influenced by illumination light, and so that images captured by a camera using illumination light are not influenced by communication light.

[0011] It is preferable that the control unit of the underwater equipment of the present invention synchronizes the timing at which the photographing unit frames the image with the emission time of the illumination light by comparing the brightness of the framed image when the illumination light is emitted with the brightness of the image before and after the framed image.

[0012] The control unit of the underwater device of the present invention can also control the emission of communication light from the communication partner by notifying the other communication device of the transmission time via the optical transceiver unit based on the communication time and the light emission time.

[0013] The underwater communication system according to the present invention is an underwater communication system comprising the underwater device and the other communication device, wherein the other communication device comprises a communication control unit that determines whether or not it is receiving the communication light from the underwater device and, if it is receiving the communication light, transmits data to the underwater device via the communication light. By determining whether or not there is communication light from the underwater device, the communication partner can stop emitting communication light that affects camera photography.

[0014] The above inventions can be combined as much as possible.

[0015] The present invention can provide underwater equipment that can be controlled underwater so that optical wireless communication light is not affected by illumination light, and so that images captured by a camera using illumination light are not affected by communication light.

[0016] FIG. 1 is a table explaining the characteristics and problems of various wireless communications. FIG. 2 is a diagram explaining an underwater communication system according to the present invention. FIG. 3 is a diagram explaining time-division control of illumination light and communication light performed by a control unit of an underwater device according to the present invention. FIG. 4 is a diagram explaining the underwater communication system according to the present invention. FIG. 5 is a diagram explaining the operation of a communication partner of the underwater communication system according to the present invention. FIG. 6 is a diagram explaining time-division control of illumination light and communication light performed by a control unit of an underwater device according to the present invention. FIG. 7 is a diagram explaining the relationship between the exposure time and the light-emitting time of the imaging unit of an underwater system according to the present invention. FIG. 8 is a diagram explaining the relationship between the exposure time and the light-emitting time of the imaging unit of an underwater system according to the present invention. FIG. 9 is a diagram explaining the relationship between the exposure time and the light-emitting time of the imaging unit of an underwater system according to the present invention. FIG. 10 is a diagram explaining the relationship between the exposure time and the light-emitting time of the imaging unit of an underwater system according to the present invention. FIG. 11 is a diagram explaining the relationship between the exposure time and the light-emitting time of the imaging unit of an underwater system according to the present invention.

[0017] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0018] 2 is a diagram illustrating the underwater communication system of this embodiment. The underwater communication system includes an underwater device 10 and another communication device 20. The underwater device 10 is, for example, an underwater drone. The communication device 20 is, for example, a ship, a submarine, or another underwater drone that communicates with the underwater device 10.

[0019] The underwater device 10 includes an optical transmitter / receiver 13 that performs underwater optical communication with another communication device 20 using communication light, an image capturing unit 11 that captures images of the surroundings, and an illumination unit 12 that illuminates an object 30 to be captured by the image capturing unit with illumination light. The communication device 20 also includes an optical transmitter / receiver 23 that performs underwater optical communication with the underwater device 10.

[0020] The underwater device 10 further includes a control unit 14 that performs time-division control so that the communication time using the communication light and the emission time of the illumination light do not overlap. Fig. 3 is a diagram illustrating the time-division control of the illumination light and the communication light performed by the control unit 14. The control unit 14 controls the illumination unit 12 and the optical transceiver 13 to emit the illumination light and the communication light in a time-division manner.

[0021] Specifically, as shown in FIG. 3 , the emission of illumination light and communication light is controlled so that their emission time t1 and emission time t2 do not overlap (exclusively). Here, the emission time t1 of communication light refers not only to physical emission but also to the communication time using the communication light. That is, when on-off keying is in operation, the off state is the emission time t2 of communication light, even though no light is being output. Furthermore, because there is a possibility that reflected light of communication light from the optical transmitter / receiver 13 may enter the photographing unit 11, and there is a possibility that communication light from the optical transmitter / receiver 23 or its reflected light may enter the photographing unit 11, the emission time of both the optical transmitter / receiver 13 and the optical transmitter / receiver 23 is included in the emission time t2 of communication light.

[0022] Here, it is preferable that the control unit 14 inserts a predetermined guard time tg between the communication time and the light emission time. The guard time tg can further reduce the influence of the illumination light on the optical transceivers 13 and 23 and the influence of the communication light on the image capture unit 11. In addition, it is possible to reduce the power consumption of the illumination light, and extend the operating time of the battery-powered underwater device 10.

[0023] As shown in Figure 4, the control unit 14 controls the emission start time and duration of the optical transceiver unit 13, the emission start time and duration of the illumination unit 12, and the video output of the video monitor unit 16 based on the clock of the clock generation source 15 within the underwater drone, in order to control the emission of illumination light and communication light in a time-division manner.

[0024] Meanwhile, there are two control methods for the light emission time of the optical transceiver 23 of the communication device 20: (Method 1) The control unit 14 notifies the other communication device 20 of the transmission time of the other communication device 20 via the optical transceiver 13 based on the communication time and the light emission time. That is, in this method, the control unit 14 acquires information in advance regarding the light emission start time and duration of the optical transceiver 13, the light emission time of the illumination unit 12, and the exposure time of the imaging unit 11, and notifies the communication device 20 of this information, thereby controlling the light emission start time and duration of the optical transceiver 23, i.e., the communication time of the optical transceiver 23.

[0025] (Method 2) FIG. 5 is a flowchart illustrating the operation of the communication control unit 24 in this method. The communication control unit 24 determines whether communication light is being received from the underwater device 10 (step S01). If communication light is being received ("Yes" in step S01), the communication control unit 24 causes the optical transceiver 23 to transmit data to the underwater device 10 via communication light (step S02). After completing data transmission, or if communication light is being received ("No" in step S01), the communication control unit 24 waits until the next communication light is received from the underwater device 10 (step S03). That is, in this method, the communication control unit 24 detects communication light from the optical transceiver 13 and controls the optical transceiver 23 so that only the light emission time of the detected communication light is used as the light emission time (communication time). Here, the control unit 14 may cause the optical transceiver 13 to emit an idle signal to indicate to the communication device 20 the time during which communication is possible, even if no data is being transmitted from the underwater drone 10.

[0026] If the optical transceiver 13 and the optical transceiver 23 are capable of full-duplex communication using wavelength multiplexing or the like, they can emit light (communicate) simultaneously as shown in Fig. 6(A) using both the above-mentioned methods 1 and 2. Also, if the optical transceiver 13 and the optical transceiver 23 are capable of half-duplex communication, they can emit light (communicate) alternately as shown in Fig. 6(B) using the above-mentioned method 1 or the following method 3. (Method 3) A certain period of time after the optical transceiver 13 finishes emitting light is considered to be the communication available time for the optical transceiver 23.

[0027] 11 is a diagram illustrating the operation of the video monitor unit 16. (1) shows the timing of emitting illumination light or communication light, (2) shows the timing of video output from the imaging unit 11, (3) shows an example of video output from the video monitor unit 16 (part 1), (4) shows an example of video output from the video monitor unit 16 (part 2), and (5) shows an example of video output from the video monitor unit 16 (part 3).

[0028] The control unit 14, the illumination unit 12, and the optical transceiver unit 13 can operate synchronously with a clock from a clock generation source 15. Based on the clock, the control unit 14 can cause the illumination unit 12 to start and end illumination light emission L1, cause the optical transceiver unit 13 to start and end communication light emission L2, and extract images from the video monitor unit 16 as follows.

[0029] In accordance with instructions from the control unit 14, the video monitor unit 16 outputs only the video captured during the illumination light emission L1 from among the video received from the imaging unit 11. In other words, while the imaging unit 11 continuously captures and outputs video frames as shown in (2), the video monitor unit 16 can extract video frames of the subject 30 illuminated by the illumination light. The video monitor unit 16 can extract video frames using the following three methods: (3) is an example of reducing the number of frames to one-third; (4) is an example of changing the video information format to one-third; and (5) is an example of supplementing the video captured using communication light with a complementary frame. The complementary frame can be a frame captured during the immediately preceding light emission. This method allows the underwater drone 10 to extract only video frames that are not affected by the communication light and output them to the communication device 20.

[0030] In the underwater drone of this embodiment, the imaging unit 11 does not receive a clock from the clock generation source 15, nor is it controlled by the control unit 14. For this reason, there may be a mismatch between the imaging timing (image frame) of the imaging unit 11 and the time of illumination light emission L1 from the illumination unit 12. For this reason, the control unit 14 synchronizes the image frame and the time of illumination light emission L1 using the method described below.

[0031] The control unit 14 compares the brightness of the framed image when the illumination light is emitted with the brightness of the images before and after the image, thereby synchronizing the timing at which the photographing unit 11 frames the image with the emission time of the illumination light.

[0032] [Method 1] Frequency Adjustment Figure 12 is a diagram illustrating synchronization by frequency adjustment. Before starting communication with the communication device 20, the underwater drone 10 monitors the illuminance (brightness) of the video frame on the video monitor unit 16, thereby confirming the synchronization between video capture and the illumination light and correcting the illumination light cycle.

[0033] 12A illustrates the timing of illumination light emission L1 at time 0 and the video frames captured by the imaging unit 11. At this time 0, the video captured by illumination light emission L1 is only frame K. The illumination light is emitted in a cycle T. The video monitor unit 16 compares the brightness of the video frame (K+nT frame) that the imaging unit 11 should have captured with illumination light emission L1 n cycles later (time nT), and the video frames before and after that (K+nT-1 frame and K+nT+1 frame).

[0034] When the illumination light emission period L1 and the video frame period are synchronized ( FIG. 12B ), the brightness of the K+nT frame is higher than the brightness of the other two frames. Furthermore, when the brightness of two consecutive frames is higher than the frames before and after them ( FIGS. 12C and 12D ), it can be determined that the illumination light emission period L1 and the video frame period are misaligned. Here, when the brightness of the K+nT frame and the K+nT−1 frame is higher than the brightness of the K+nT+1 frame ( FIG. 12C ), the illumination light emission period L1 is faster, so the control unit 14 adjusts the light emission period to be slower. On the other hand, when the brightness of the K+nT frame and the K+nT+1 frame is higher than the brightness of the K+nT−1 frame ( FIG. 12D ), the illumination light emission period L1 is slower, so the control unit 14 adjusts the light emission period to be faster.

[0035] By gradually decreasing the adjustment amount from a large amount, synchronization deviation can be brought into synchronization more quickly. Note that although an example in which the brightness of three video frames is compared is described in Fig. 12, frequency adjustment may be performed by comparing four or more video frames.

[0036] [Method 2] Skew Adjustment Figure 13 is a diagram illustrating synchronization by skew adjustment. Before starting communication with the communication device 20, the control unit 14 of the underwater drone 10 varies the timing of the periodic illumination light emission L1 and monitors the illuminance (brightness) of the video on the video monitor unit 16, thereby checking and correcting the misalignment (skew) between the video capture and the illumination light.

[0037] Skew is confirmed by comparing the brightness of three or more frames of video. When they are synchronized, the brightness of only the K frame will be higher than the brightness of the two frames before and after it (FIG. 13(B)). On the other hand, if the brightness of two consecutive frames is higher than the frames before and after it, it can be determined that there is a skew deviation in the illumination light emission L1. When illumination light emission L1 is early, the brightness of the K-1 frame and the K frame will be higher than the brightness of the K+1 frame (and also the K-2 frame), as shown in FIG. 13(C). Conversely, when illumination light emission L1 is late, the brightness of the K+1 frame and the K frame will be higher than the brightness of the K-1 frame (and also the K+2 frame), as shown in FIG. 13(D).

[0038] If the illumination light emission L1 is to be synchronized with the K frame, in the case of Fig. 13(C), the timing of the illumination light emission L1 is gradually delayed and adjustment is continued until the state of Fig. 13(B) is reached. Similarly, in the case of Fig. 13(D), the timing of the illumination light emission L1 is gradually advanced and adjustment is continued until the state of Fig. 13(B) is reached.

[0039] [Method 3] Adjusting the illumination light emission time Figure 14 is a diagram illustrating synchronization by adjusting the illumination light emission time. Before starting communication with the communication device 20, the control unit 14 of the underwater drone 10 varies the periodic illumination light emission time L1 and monitors the illuminance (brightness) of the image on the image monitor unit 16, thereby confirming and correcting the synchronization between the illumination light emission time L1 and the image frame time.

[0040] The illumination light emission time is confirmed by comparing the brightness of four or more frames of video. For example, as shown in FIG. 14C, if the brightness of three consecutive frames (frames K-1 to K+1) is higher than the brightness of the preceding and following frames (frames K+2 or K-2), it can be determined that the duration of illumination light emission L1 is longer than the captured frame time length. Also, as shown in FIG. 14B, if the brightness of only frame K is higher than the brightness of the preceding and following two frames (frames K-1 and K+1) or three frames (frames K-1, K+1, K+2, or frames K-1, K+1, K-2), this indicates that the duration of illumination light emission L1 is synchronized with the captured frame time length (the duration of illumination light emission L1 is appropriate). However, if the duration of illumination light emission L1 is short, as shown in FIG. 14D, the brightness of frame K is lower than when they are synchronized. Therefore, even if the brightness of only the K frame is higher than the brightness of the other frames, if the brightness of the K frame is lower than the specified value, it can be determined that the duration of illumination light emission L1 is short.

[0041] The three synchronization methods have been described above. If the control unit 14 finds that the synchronization of the illumination light is not accurate enough, it may stop communication with the communication device 20 (stop the emission of communication light L2), monitor the emission of illumination light L1, and periodically provide a monitoring time for fine-tuning the synchronization.

[0042] The relationship between the exposure time of the image capture unit and the light emission time of the illumination unit is described below. (Relationship 1) FIG. 7 is a diagram illustrating Relationship 1. As a premise, the illumination light emission L1 and the communication light emission L2 are emitted exclusively, as described with reference to FIGS. 3 to 5 . The control unit 14 controls the exposure Exp time of the image capture unit 11 so that it is outside the communication light emission L2 time. In other words, the exposure Exp time and the illumination light emission L1 time do not necessarily need to coincide. If the illumination light emission timing and the exposure timing cannot be controlled accurately, it is sufficient to control the exposure Exp time so that it is outside the communication light emission L2 time, as shown in FIG. 7 . FIG. 7(A): Case where the exposure Exp time is longer than the illumination light emission L1 time. FIG. 7(B): Case where the illumination light emission L1 time is longer than the exposure Exp time. FIG. 7(C): ​​Case where the illumination light emission start timing is later than the exposure start timing and the illumination light emission end timing is later than the exposure end timing. FIG. 7D: A case in which the exposure start timing is later than the illumination light emission start timing, and the exposure end timing is later than the illumination light emission end timing.

[0043] (Relationship 2) Figure 8 is a diagram illustrating Relationship 2. Relationship 2 is the case where the exposure of the imaging unit 11 is a rolling shutter. In this relationship, the illumination light emission L1 and the communication light emission L2 are also assumed to be emitted exclusively as explained in Figures 3 to 5. Then, the time when the exposure of all the lines overlaps (T2 ｋ ~T3 ｋ The control unit 14 adjusts the exposure time Exp so that the illumination light emission L1 starts at T2 ｋ After that, the end time of the illumination light emission L1 is T3 ｋ and the start time of communication light emission L2 is T4 ｋ After that, the end time of communication light emission L2 is T1 ｋ＋１ The control unit 14 controls the imaging unit 11, the illumination unit 12, and the light transmitting / receiving unit 13 so that the exposure time of all the lines is earlier than the exposure time of the light receiving unit 13. By performing control as shown in FIG.

[0044] (Relationship 3) FIG. 9 is a diagram illustrating Relationship 3. Relationship 3 is a relationship when the exposure of the photographing unit 11 is a rolling shutter, and further, the communication light of either or both of the optical transceiver 13 and the optical transceiver 23 does not affect the photographing unit 11 during the exposure Exp. In this relationship, the control unit 14 controls the illumination unit 12 for the time of illumination light emission L1 and the optical transceiver 13 for the time of communication light emission L2-1, as described in FIG. 8. On the other hand, the communication control unit 24 controls the communication light emission L2-2 of the optical transceiver 23, which outputs communication light that does not affect the photographing unit 11, after the end time of illumination light emission L1 (time T3 in FIG. 9). ｋ 9 ) and before the start time of the next illumination light emission L1 (time T2 ｋ＋１ 9, the control unit 14 and the communication control unit 24 perform control as shown in FIG. 9, thereby extending the available communication time and improving the throughput.

[0045] (Relationship 4) Fig. 10 is a diagram illustrating Relationship 4. Relationship 4 is a relationship when the exposure of the imaging unit 11 is a rolling shutter and the exposure Exp time of each line is short. For example, the exposure start time T2 of line n is short. ｋ is the exposure end time T3 of line 1 ｋ In this case, the control unit 14 sets the start time of the illumination light emission L1 to the exposure start time T1 of the first line 1 to be exposed. ｋ Before the end of illumination light emission L1, the exposure end time T4 of line n is the last exposure. ｋ 10, the control unit 14 controls the exposure time Exp of all lines to be constant even if the exposure time Exp is short.

[0046] In the case of Relationship 4, a method for synchronizing the video frame and the illumination light when the image capturing unit 11 uses a rolling shutter will be described with reference to Fig. 15. Fig. 15 is a diagram illustrating the relationship between the video frame and the illumination light when the image capturing unit 11 uses a rolling shutter.

[0047] In the case of relationship 4, even if the illumination light emission L1 is synchronized with the exposure timing of the imaging unit 11, the brightness of the previous and next frames varies depending on the position of the line. For example, focusing on frame K-1, frames of higher-numbered lines become brighter due to the timing of illumination light emission L1. Specifically, in frame K-1, the overlapping time with illumination light emission L1 from line 1 to line n is longer, so the brightness of frame K-1 becomes brighter from line 1 to line n. Similarly, focusing on frame K+1, frames of lower-numbered lines become brighter due to the timing of illumination light emission L1. Specifically, the overlapping time with illumination light emission L1 from line 1 to line n is shorter in frame K+1, so the brightness of frame K+1 becomes darker from line 1 to line n. In other words, the overlapping time with illumination light emission L1 from line n to line 1 is longer in frame K+1, so the brightness of frame K+1 becomes brighter from line n to line 1.

[0048] Therefore, the control unit 14 compares the brightness of the K-1th and K+1st frames for each line with the brightness of the corresponding lines before and after that. Specifically, if the number of lines is three, the control unit 14 compares the brightness of the K-1th frame of line 1 with the K+1th frame of line 3, or the brightness of the K+1st frame of line 1 with the K-1st frame of line 3. By comparing the brightness, it is possible to calculate the correction amount (the correction amount for the start time and duration of the illumination light emission L1) for synchronizing the illumination light emission L1 with the video frame.

[0049] Alternatively, the video frame and the illumination light may be synchronized as shown in Fig. 16. In the synchronization method of Fig. 16, the control unit 14 can adjust the correction amount (the correction amount of the start time and the correction amount of the duration of the illumination light emission L1) for synchronizing the illumination light emission L1 with the video frame so that the brightness of the consecutive K frames corresponding to L1 does not change. A specific example will be described. L1 （ｊ） The image captured by the light of K （ｊ） When adjusting the start time of L1, the K （ｊ） Frame brightness and K （ｊ－１）No change in the brightness of the frame and K-1 of line 1 （ｊ） The adjustment can be made when the brightness of the frame is low enough. When adjusting the end time of L1, （ｊ） Frame brightness and K （ｊ－１） There is no change in the brightness of the frame and the K-1 （ｊ） Once the brightness of the frame is low enough, it can be adjusted.

[0050] The control unit 14 uses the correction amount to modify the start time and duration of the illumination light emission L1 from the illumination unit 12. The control unit 14 also transmits the correction amount to the communication device 20 via the optical transceiver 13 and the optical transceiver 23, and the communication control unit 24 adjusts the period, skew, and length of the communication light emission L2 from the optical transceiver 23 based on the correction amount so that the illumination light emission L1 and the communication light emission L2 do not overlap.

[0051] 10: Underwater device (underwater drone) 11: Photography unit 12: Lighting unit 13: Optical transmitter / receiver unit 14: Control unit 15: Clock generator 16: Video monitor unit 20: Communication device 23: Optical transmitter / receiver unit 24: Communication control unit

Claims

1. An underwater device capable of moving underwater, comprising: an optical transceiver unit that performs underwater optical communication with other communication devices using communication light; an imaging unit that divides continuously captured images of the surrounding area into frames at predetermined intervals and outputs the frames; a video monitor unit that receives the framed images output by the imaging unit; an illumination unit that illuminates the subject being photographed by the imaging unit with illumination light; and a control unit that performs time-sharing control so that the communication time using the communication light and the illumination light emission time do not overlap, and that causes the video monitor unit to output the framed images when the illumination light is emitted to the optical transceiver unit.

2. The underwater equipment described in claim 1, characterized in that the control unit synchronizes the timing at which the photographing unit frames the image with the emission time of the illumination light by comparing the brightness of the framed image when the illumination light is emitted with the brightness of the image before and after the framed image.

3. The underwater device according to claim 1, characterized in that the control unit notifies the other communication device of the transmission time via the optical transceiver unit based on the communication time and the light emission time.

4. An underwater communication system comprising an underwater device according to any one of claims 1 to 3 and the other communication device, wherein the other communication device is provided with a communication control unit that determines whether or not the communication light from the underwater device is being received, and if the communication light is being received, transmits data to the underwater device via the communication light.

Images