为揭示拉曼奈斯衍射条件下超声驻波成像的物理本质，澄清以往超声驻波成像问题分析中所存在的一些理论上的混淆，提出一种基于傅里叶光学的新的分析方法。通过这种方法分析发现拉曼奈斯衍射条件下激光垂直穿过超声驻波后的光场中含有一系列不同频率的非相干光，分别求出这一系列非相干光单独产生的干涉光强分布，将这一系列非相干光进行强度的线性叠加得到了此光场总强度分布的严格表达式，为目前已有的但尚无严格理论解释的一种声速测量方法提供了理论依据。实验中根据实际条件对此表达式进行了简化并进行了验证。

In order to explore the physics essence of the imaging principle of the ultrasonic standing waves in Raman-Nath diffraction and clarify the confusion of the theory, a new method based on Fourier optics is proposed. The optical field of a laser beam vertically passing through the ultrasonic standing waves in Raman-Nath diffraction is analyzed via this method, which contains a series of incoherent light with different frequencies. The intensity distribution of this field is obtained by linear superposition of incoherent light of different frequencies after calculating the individual interference light intensity distribution. This expression proves the correctness of an existing acoustic velocity measuring method that has not been testified. The simplification of this expression according to actual conditions in the experiment is also accomplished and it is proved experimentally.