## 3 Results
### Simulation
#### 3.1.3 Optimization of IMC Method  
During the process of underwater transmission, the backscattering light of particles return the imaging system and introduce noise, especially when imaging long-distance targets in complex media. In the IMC method, the detector and laser are coaxially placed, as the power of illumination increases, so does the intensity of backscattering, which results in laser active imaging systems being unable to detect objects effectively. The detected normalized intensities of the diffuse planar target under AL of 0.42 and 1.42 by IMC model before optimization are shown in Figure 3a,c. It can be seen from the echo timing, in which the black dashed rectangular area was the response of the detector to backscattering, while the red one was the reflected signal of the target. It indicates that with the increase in AL, the influence of the backscattering is enhanced and part of the reflected signals from targets are submerged in the invalid scattering signals, which greatly limits the underwater detection distance.  
With the development of a precise delay in time domain and ultra-fast shutter, the employment of image intensifiers in range-gated technology ensured that the imaging system only received signals in the vicinity of the target and would not be affected by scattering signals on the transmission path [9]. It is an effective method to overcome the backscattering of medium, which can improve the contrast and \(S/\mathrm{N}\) ratio of the detected image. Therefore, referring to the principle of the technology, the echo signals within the gating range were marked effective in the optimized IMC simulation, whereas the photons with channel information, in other words, the backscattered part, was marked as invalid photons. The normalized intensities depicted in Figure 3b,d were obtained by counting the response of the detector to the effective photons exclusively. Comparing the simulation results before and after optimization, the influence of the backscattered photons to the detector was removed, which described the underwater detection process of the range-gated system appropriately.  
The detector was divided into grids in the IMC simulation to separately count the photon energy falling in each bin, and the response curve of the camera was used to establish an illuminance-grayscale lookup table, so the simulation image could be obtained. Original and optimized simulated images of \"Diver\" and \"Double Slit\" with AL equals 0.42 and 1.42 are shown in Figure 4. With the increase in the AL, the photon energy returning to the detector decreased, and obvious background noise appeared in the non-target area with  
Figure 3: Comparison of the normalized intensity before and after optimization of IMC method. (**a,c**) Simulation results before optimization under AL equals 0.42 and 1.42; (**b,d**) simulation results after optimization under AL equals 0.42 and 1.42.  
the loss of image details. The main reason for the phenomena is that the absorption and scattering of the water lead to the attenuation of the laser energy during the transmission process, which, accompanied by the multipath effects, reduces the received photon energy, lowers the contrast and degrades the image quality.  
To further explore the relationship between the detection effect and water quality, and to verify the reliability of the simulation, we built a range-gated imaging system. While obtaining the images corresponding to the simulated situation, imaging experiments in the water with continuously changing ALs were also conducted. Moreover, the influence of the transmission distance on the photon energy returning to the detector were studied.