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J. Eur. Opt. Society-Rapid Publ. 22, 32( 2026)
Figure 17. SCR in the 8 – 14 μm band.( a) Summer.( b) Winter.
5.2 8 – 14 μm simulation results
Figure 16 shows the simulation results of the infrared optical imaging characteristics of ship targets at different times in the 8 – 14 μm band. Emissivity of the hull, deck and superstructure is 0.90, 0.95, 0.88, respectively. The local SCR in the 8 – 14 μm band is calculated, and the results are shown in Figure 16.
Combining Figures 16 and 17, it can be seen that in summer, the local SCR is the highest at 01:00, and the target detection capability is the strongest, but the target radiation intensity is lower than the background. At 13:00, the target radiation intensity is higher than the background, indicating that the target is detectable. At the other two moments, due to changes in ambient temperature, the background and target radiation characteristics are similar, and the target difficult to detect.
In winter, the local SCR is relatively large at 12:00 and 00:00, and the target detection capability is relatively strong. The critical point of“ thermal crossover” appears around 8:30 and 18:00. In the 8 – 14 μm band, during the critical moment of thermal crossover, the detectability is better in summer than in winter; during non-thermal crossover periods, the opposite is true.
Comparing Figures 14 and 16, it can be seen that the occurrence of the thermal crossover phenomenon in the morning happens earlier in summer than in winter, while the thermal crossover time in the evening occurs later in summer than in winter. This phenomenon is caused by the varying solar radiation in different seasons and differences in the heat absorption characteristics of materials. Comparing Figures 15 and 17, it can be seen that the evolution of ship imaging characteristics at the critical moment of“ thermal crossover” is correlated with both season and time. During non-thermal crossover periods in summer, the detectability of 3 – 5 μm is better than that of 8 – 14 μm, while the opposite is true during non-thermal crossover periods in winter. During the critical moment of thermal crossover in summer, the detectability of 3 – 5 μm is worse than that of 8 – 14 μm, while during the critical moment of thermal crossover in winter, the detectability of 3 – 5 μm and8 – 14 μm is relatively close.
At the thermal crossover moment, the thermal radiation difference between the target and the background is relatively small, and the radiative signal is mainly constrained by atmospheric transmittance and path radiance. In summer, atmospheric humidity and temperature are higher than in winter, causing the target radiative characteristics in the 3 – 5 μm band to be significantly affected by water vapor absorption and atmospheric thermal radiation interference, whereas the 8 – 14 μm band is less influenced by atmospheric radiative transfer, resulting in less pronounced variations in target radiative characteristics. In winter, atmospheric humidity and temperature are low, the difference in atmospheric radiation between the two bands decreases, and the target radiation characteristics tend to converge.
6 Conclusion
To address the limitation of infrared detection systems in detecting surface ships during“ thermal crossover” periods, an optical imaging feature prediction model of ship target, port background, ambient lighting, and atmosphere is established. The imaging characteristics during thermal crossover periods of ship targets in the 3 – 5 μm and 8 – 14 μm bands are simulated and analyzed. The detectability of low-orbit satellites to ship targets under different environmental conditions is analyzed using local SCR. The results show that the radiation difference between the target and the background exhibits a clear stage-wise evolution before and after the thermal crossover. As the background temperature gradually approaches the target temperature, the radiation contrast of the target in infrared imaging decreases significantly, which means that the target ' s detectability weakens over time. Near the moment of thermal crossover, the contrast between the target and the background reaches its lowest value, which will have a significant impact on the detection algorithm and recognition performance. Meanwhile, the sensitivity to thermal crossover varies across different bands, indicating that appropriate band selection is of great significance for target detection. The results will provide data support and theoretical basis for the infrared detection and identification of ship targets in complex marine environments.
Funding
Shaanxi Provincial Natural Science Basic Research Program Project( 2025JC-YBQN-845, 2025JC-YBQN-081); Shaanxi Province Postdoctoral Research Project( 2023BSHEDZZ161).