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J. Eur. Opt. Society-Rapid Publ. 22, 32( 2026)
Figure 15. SCR in the 3 – 5 μm band.( a) Summer.( b) Winter.
L a ðkÞ ¼
Z r
0
L k ðr 0 Þexp s 0 ðr 0 Þ�s 0 ðrÞ dr 0 ð21Þ
where r is the distance between the target and the observation point; L( r’) is the spectral radiance of the path point; τ’ is the atmospheric path optical depth.
Meanwhile, aerosol particles and molecules in the atmosphere have absorption and scattering effects on the upward radiation of the target and background, resulting in the attenuation of the target and background radiation. The atmospheric attenuation capacity of a target varies under different meteorological conditions. Atmospheric spectral transmittance is generally used to describe the degree of atmospheric attenuation. Once the scene geometry is determined, atmospheric path radiation and atmospheric transmittance can be calculated using MODTRAN. The atmospheric parameters are configured as follows: atmospheric condition is mid-latitude summer, aerosol model is naval ocean, weather conditions are cloudless and rainless, and visibility is 23 km. The calculation results are presented in Figure 12.
Meanwhile, the atmospheric conditions are chosen to be mid-latitude winter to calculate the atmospheric radiation transfer characteristics under winter conditions. The calculation results are shown in Figure 13.
5 Results
5.1 3 – 5μm simulation results
The simulation parameters are selected as follows: focal length is 1.06 m, optical system transmittance is 0.7, aperture is 0.53 m, pixel size is 40 lm, and orbital altitude is 795 km, emissivity of the hull, deck and superstructure is 0.85, 0.90, 0.78, respectively. In target detection research, the signal-to-clutter ratio( SCR) has significant advantages as an evaluation metric. The SCR [ 18 ] can quantitatively characterize the distinguishability of a target’ s radiated relative to background clutter, reflecting the target’ s detectability under complex background conditions.
SCR ¼ I trg � I bg
ð22Þ r b
where SCR is the local signal-to-clutter ratio; Ī trg is the average radiation luminance of the target; Ī bg is the mean background value within an area twice the size of the target; σ b is the background variance within an area twice the size of the target. Figure 14 shows the simulation results of the infrared imaging characteristics of ship targets at different times in 3 – 5 μm band. Figure 15 shows the SCR in the 3 – 5 μm band.
As can be seen from Figures 14 and 15, the radiation values of the target and background change dynamically throughout the day. Due to seasonal differences, the target radiation value generally higher in summer than in winter. The difference in radiation between the target and background is greatest at 13:00 and 01:00 in summer, resulting in higher SCR values, making the target most easily detectable at these times. Due to the lack of solar radiation at night, the target radiation is generally lower than the background radiation, appearing as a dark target at 6:00. As solar radiation gradually increases after sunrise, the target temperature rises rapidly. During the period from 6:00 to 6:30, the difference between the target and the background radiation gradually decreases, and at 6:30, the radiation of the target and the background reaches a state of approximately equality, which is the zero-crossing moment of thermal equilibrium. At this time, the contrast between the target and the background is extremely low, and the target is almost submerged by background clutter, which is a typical thermal crossover stage. Subsequently, between 6:30 and 7:00, the target radiation continues to increase and gradually exceeds the background. At 7:00, the radiation contrast between the target and the background reverses, and the target changed from dark to bright. Between 19:00 and 20:00, the radiation difference between the target and the background undergoes another reversal of the contrast relationship. At 19:30, the target is in a typical thermal crossover state, where the radiation intensity of the target and the background are approximately equal and the contrast is significantly reduced. After that, the target radiation further decreases and falls below the background radiation, and the target changes from a bright target to a dark target.
In winter, the local SCR is large at 12:00 and 00:00, and the target is more detectable. However, the target radiation is larger than the background at 12:00, and the opposite is true at 00:00. The critical points for“ thermal crossover” occur around 8:30 and 18:30. Therefore, it can be seen that in the 3 – 5 μm band, at the critical moment of thermal crossover, the detectability is better in winter than in summer; while in non-thermal crossover periods, the opposite is true.