讨论了基于天空背景下红外图像空间噪声组成，分析了各种空间噪声的形成原因，给出了红外图像噪声类型及其对图像质量的不同影响。分析结果认为红外图像空间噪声主要为高斯噪声和椒盐噪声。综合考虑均值法和中值法在处理噪声类型上的不同，提出了一种基于简单阈值的改进型中值法。对于不同的阈值，其算子有所不同。而M×N处理区域内每一点算子的权值取决于区域灰度中值，该点灰度越接近中值权值越大。处理后的图像背景更平滑，信噪比和改进的峰值信噪比提高了1．2 dB以上，表明改进型中值法优于传统滤波方法。

The components of the space noise in an infrared image with the sky background were discussed. The reasons for forming all types of noises were analysed. The noise types of the IR image were classified and their effects on the image were investigated. The analysis indicates that the space nosie in the IR image mainly represents the Gauss noise and spiced-salt noise. An improved median filtering based on a simple threshold is introduced by considering the difference in the noise disposal of the averaging method and the median filtering. For the different thresholds,the operators are different too. The weight selection of every operator in the M×N area depends on the gray median in the area. The more the gray value is close to the median,the more the weight of operator is. The background of the image becomes smoother after the preprocessing. The SNR and improved PSNR increase by 1.2 dB. It indicates that the improved median filtering works better than the traditional ones.