## 6 Experimental Results and Discussion
### Performance Evaluation of AISRT in the Different Mediums  
As mentioned before, signal attenuation and blocking are very common when the signals are transmitted in the medium. As a kind of wireless waves, visible light also encounters signal attenuation in the medium, especially in liquids such as water. The scattering and absorption effects of photons in water are particularly obvious. This effect would even affect image transmission quality. In order to further investigate the transmission features in the different mediums, a series of important parameters such as MSE, MAE, PSNR, and SSIM were measured and analyzed. These four parameters were calculated with MATLAB based on grayscale images which converted from RGB images. The RGB images were actually transmitted through this AISRT system. In this test, we collected the RGB images at four receiving nodes: 10 cm distance from the transmitter, the first elbow, the second elbow, and the final receiving node outside the pipe. For each node, three image frames as experiment samples have been captured. As demonstrated in Figures 6 and 7, three landmarks were labeled on the pipe wall [41; 42; 43]. In order to observe easily and truly reveal the deterioration of the image during the transmission, these three regular graphical landmarks 1, 2, and 3 were utilized to indicate three common situations, including corrosion, leakage, and blockage, respectively. Figure 5: Waveform analysis of AISRT (**a**) waveform of first relay node, (**b**) waveform of second relay node, and (**c**) waveform of final receiver. Figure 6: Analog image transmission with visible light in the empty pipe. As depicted in Table 1, landmarks 1 and 2 had the same real size \(\mathrm{L}\times\mathrm{W}=6.5\;\mathrm{cm}\times 2\;\mathrm{cm}\) and same radian of \(3\pi/4\) from the camera vision in the pipe. These two landmarks were located on the pipe wall where is 10 cm from the end of the pipe and 15 cm from the camera. Landmark 3 had the size of radius=1.5 cm and height =1.2 cm. The location is almost 15 cm from the end of the pipe and 10 cm from the camera. From Figures 6 and 7, we could observe that whether in the water or an empty pipe, the loss of image color is the most obvious. From 10 cm distance, the color has been lost. Also, the loss of image quality became extremely huger with the increase of communication distance. This loss could be mainly reflected in the three aspects: the noise, the structure, and the brightness of the image. The two typical parameters MSE and MAE could both reveal the error between the original image and the final received image. In particular, MAE could better reflect the actual situation of image abnormal error value, while MSE was more sensitive to abnormal values. In Figure 8a,b, with the increase of transmission distance in the water pipe, the noise of signal and pixel error became much larger. Therefore MSE increased significantly from 1911.3 to 22,400 and MAE rose from 30.726 to 141.25. The increase rate of MSE at the second elbow became higher in Figure 8a, which could be also observed from the image of Figure 7d to Figure 7e. The noise of the transmission channel would also have an impact on image sharpness and landmark recognition. PSNR was another important standard for measuring image quality. As mentioned before, the higher the value of PSNR, the higher the image quality. Within a distance of 10 cm between transmitter and receiver, PSNR could reach 16.528 and 15.318 in the empty and water pipe respectively. It appeared that the image signals were able to maintain good transmission condition whether in empty pipe or in water pipe within a short distance. However, as shown in Figure 9a, due to the scattering and absorption effects of water on the optical signal, PSNR of water pipe met a serious decline from 15.318 to 4.6284. The noise could also affect the recognition of target landmarks.