Fig. 1. Schematic illustration of the proposed passive imaging through scattering media. ASD, angle-selection device. The inset at the upper-right corner shows that the use of ASD can significantly filter out the light with large incident angles.

Fig. 2. Schematic illustration of the pipeline of the proposed time-domain minimum filtering (TDMF) algorithm. To enhance image quality through multiple measurements, the proposed TDMF algorithm selects minimal pixel values from multiple frames. Note that CLAHE and DCT are used to further enhance the image contrast.

Fig. 3. Sitemap where our outfield experiments were performed. (a) Scene to be imaged and (b) imaging system. The geometric distance between the target and the imager is about 5.9 km.

Fig. 4. Measurement of the equivalent visibility Ve. It is clearly seen that Ve varies with time. The shadow along the solid line of Ve represents the standard deviation of the measurements.

Fig. 5. Experimental demonstration of the effectiveness of ASD: single-shot results. The photo on the left (taken by a cell phone) gives an impression of the visibility of the scene. Raw images taken by the PCO camera at the calibrated effective visibility equal to (a) 2789 m without the use of ASD, (b) 2789 m with the use of ASD, (c) 2428 m with the use of ASD, and (d)–(f) SIR enhanced versions of them, respectively, using the global histogram equalization algorithm.

Fig. 6. Experimental demonstration of the proposed method, i.e., ASD + TDMF, at different visibilities. (a) Raw images taken by the PCO camera with the use of ASD, and the corresponding images enhanced by (b) traditional averaging together with global histogram equalization and (c) the proposed TDMF + CLAHE + DCT method.

Table 1. Pseudocode of the Proposed Algorithm