由于数字全息采用相干光成像，当再现距离偏离焦点时，再现像的边缘会出现振荡现象，采用传统的清晰度评价函数不能准确实现自动聚焦。通过对再现像进行小波分析可以发现，偏离焦点时的小波变换高频系数的幅值比聚焦时要小得多。针对这一特点，对拉普拉斯算子和小波变换清晰度评价函数进行了改进，将原来利用高频系数之和改为利用聚焦窗口中高频系数的最大幅值为清晰度评价依据。为便于同时观察到数字全息三维空间内的目标，还提出了将不同层面上的再现像进行融合的算法。进行了模拟数字全息自动聚焦及再现像融合实验和用数字全息观察生物标本的实验。实验结果表明，改进后的清晰度评价函数可以准确实现数字全息的自动聚焦；提出的融合算法可以将数字全息再现后得到的一系列再现像融合到一幅图片中。

When the reconstructed image is out of focus, oscillation will appear at the edge of objects, because digital holography is a coherent imaging system. Therefore, it is inaccurate to perform auto-focusing by the traditional evaluation function of definition in digital holography. It can be found through wavelet analysis that the amplitude of high-frequency coefficients out of focus is smaller than that in focus. The evaluation function of definition based on Laplacian operator and wavelet transform is improved. The maximum amplitude of high-frequency coefficients in the focal windows, instead of the sum of high-frequency coefficients, is used to evaluate the definition of the reconstructed image. In order to get a clear image that contains all relevant objects in the three-dimensional (3D) space of digital holography, a fusion algorithm is proposed. The auto-focusing and reconstructed image fusion experiments for simulated digital in-line holography and an ant detection experiment are completed. Experimental results show that the improved evaluation function of definition can accurately achieve auto-focusing for digital holography, and a series of reconstructed image can be fused into a clear image by the proposed fusion algorithm.