天光背景下混浊大气中成像质量的分析方法

调制传递函数(MTF)定量描述混浊介质对图像质量的影响,它是混浊介质的固有光学特性,利用等效原理可以获得MTF从低频到高频的完整特征.在实际应用中,混浊介质中的图像质量不仅取决于介质的MTF,还与图像的背景辐射密切相关.本文从混浊大气中图像退化机理出发,理论分析了空间频域中天光背景下图像质量的退化过程.参考等效原理,提出了一种考虑天光背景的表观MTF,得到了表观MTF与介质MTF和天光背景的定量关系,从而得到了一种分析背景辐射下混浊介质中图像质量的有效便捷方法.针对图像质量优化方法,从空间频域的角度提出了一种评价原则.

Image quality analysis method under background radiation in turbid atmosphere

Image quality is seriously degraded when propagating through the turbid atmosphere. It is practical to characterize the degradation process in terms of modulation transfer function (MTF). The MTF can describe the effect of the turbid medium on imaging quantitatively in spatial frequency domain, including attenuation and multiple scattering. It is inherent property of the turbid medium. The whole spatial frequency characteristic of the turbid atmosphere MTF can be acquired through the equivalence principle, i, e., the equivalence between the MTF of a turbid medium and the transmitted radiance from the medium under isotropic diffuse illumination. In practice, the image quality is not only affected by the turbid medium MTF but also related tightly to the background radiation. The influence of scattered background radiation on imaging was almost not considered in the past when dealing with the imaging problem in the turbid atmosphere. In this paper, this issue is considered in detail. The analysis results demonstrate that the scattered background radiation increases the zero frequency component of image in spatial frequency domain. As a result, it degrades the image contrast seriously in spatial domain. Based on the optical model of image degradation in the atmosphere, the theoretical analysis is carried out to study the image quality degradation process in spatial frequency domain. The formalized MTF is proposed, which considers the effects of attenuation, multiple scattering and scattered background radiation by the turbid medium on image quality. The quantitative relation among the formalized MTF, turbid medium MTF and background radiation is confirmed. Image blur simulations show that the results from the formalized MTF are more consistent with actual scenes than results only from turbid medium MTF. Thus, the formalized MTF can describe the image degradation process through atmosphere comprehensively. The image restoration results indicate that the formalized MTF method performs better than dark channel prior method. In order to evaluate different image restoration methods effectively in spatial frequency domain, spectrum area (A_S) is proposed. The A_S is the area of middle-high frequency information of the region of interest in restored image. So A_S can represent the scene details in the restored image. The higher the A_S, the better the image quality is, which is demonstrated in this paper. In conclusion, the formalized MTF provides a more effective method for image quality analysis and assessment. Additionally, it also supplies a new standpoint for researching atmospheric degradation mechanism and correction method for imaging in turbid atmosphere. Then, A_S can be an effective reference for correction to the method evaluation.