When marine targets on the sea surface are detected or continuously monitored using mid-infrared sensors, the results are negatively impacted by the radiance due to sun glint, which can cause the sensors to be saturated and target signals to be obscured. Polarization detection technology has been relied upon in the past to solve this problem owing to the polarization properties of sun glint. The traditional single-polarizer system for suppressing sun glint depends on the geometric position of the sun and the sensor. The system can only be successfully applied when the polarization degree of sun glint is obvious or when the sun glint is not strong. In certain situations, the residual sun-glint radiance is greater than the sensor-saturation response. In contrast, the dual-polarizer system, in which the polarizer has a fixed direction, achieves good contrast enhancement; the scenario applicability, however, is still limited. Furthermore, the polarization directional reflectance tends to vary across different observation scenes, which restricts the scope of the aforementioned methods. Therefore, to solve the original problem, in this study, based on the analysis of the mid-infrared polarization properties of sun glint on the sea surface, a mid-infrared switchable dual-polarizer detection system is designed. For different detection scenes, two work modes, both combined with an image-processing algorithm, are selected to realize the extraction of ship targets covered in sun glint.
In this study, to deal with a variety of sun-glint scenes, a mid-infrared switchable dual-polarizer detection system (Fig. 2) that can simultaneously suppress the s- and p-polarized components of sun glint is proposed (Fig. 3). Polarizer 1 was placed in front of the sensor and polarizer 2 on the rotatable support. A single-polarizer was used when the rotatable support was empty. We established whether one or two polarizers should be used based on the type and amount of sun glint, and accordingly named the scenarios as single-polarizer mode and dual-polarizer mode, respectively. The amount of sun-glint radiance can be calculated theoretically using Cox and Munk’s "slope" probability model and Fresnel’s law. The polarized type of sun-glint radiance is related to the sea surface conditions of the sensor points, the observation geometric conditions, and the atmospheric transmittance of the path. The degree of polarization of sun glint is always positive in the mid-infrared wavelength band and it reaches a maximum of 100% as the observation angle approaches the Brewster angle. At this stage, the sun glint is dominated by the s-polarized component, which can be suppressed through the single-polarizer mode. However, a p-polarized component exists in sun glint in most viewing cases, and the intensities of the p- and s-polarized components are nearly equal to the approximate horizontal observation angle. At this stage, a dual-polarizer was utilized to suppress the residual p-polarized component. Because the working mode of single/dual-polarizers can switch flexibly, the proposed polarization detection system can significantly reduce sun glint and enhance the contrast of target images for a variety of sun-glint scenes.
Two groups of sun-glint scenes in "the lake in Zizhuyuan" and "the off shore in Changdao, Yantai city" are selected for experiments, so as to clarify the scene adaptability under different polarization working modes.
The image obtained through the single-polarizer mode can effectively highlight the obscured target (a big boat with size 2.8 m×1.5 m and a small boat with size 1.2 m×0.8 m); a similar contrast enhancement occurs in the dual-polarizer mode (Fig. 6). For the small-boat target, the relative contrast in the single-polarizer mode is 0.61, but clutter bright spots appear around the target owing to the effect of wind and waves in the dual-polarizer mode, and the relative contrast drops to 0.18 (Table 2). Therefore, for long-distance near Brewster angle detection of small targets, the single-polarizer mode should be adopted. The dual-polarizer mode is adopted to further suppress the p-polarized component for a short distance with an approximate horizontal observation angle to improve the relative contrast of the target (Fig. 8). Compared with the single-polarizer mode, the relative contrast of the target is increased from 0.15 to 0.68 (Table 3). For the best angle of the two polarizers, the sun-glint radiance can be calculated according to MODTRAN and Cox and Munk’s models; the theoretical calculation can subsequently be carried out by comparing the contrast function.
Based on the polarization properties of sun glint on the sea surface, a switchable mid-infrared detection system is proposed for a variety of detection scenes. Two polarization working modes are formulated to suppress the sun glint by calculating the polarized type and amount of sun-glint radiance. The single-polarizer mode is utilized when the s-polarized component dominates, whereas the dual-polarizer mode is used when the residual p-polarized component is greater than the sensor-saturation response after single-polarizer suppression. The suppressed image is then processed to realize effective ship target detection in sun glint. Therefore, the proposed mid-infrared dual-polarization detection system can significantly enhance the contrast of target images and effectively improve the detection ability for various sun-glint scenes.
