A two-channel spatial phase shifting ESPI (Electronic Speckle Pattern Interferometer) for the deformation measurement of a transient process was developed. The system was composed of two cameras for the simultaneous acquisition of two phase shifted speckle fringe patterns, and several polarization components for spatial phase shifting. The phase shifted speckle fringe patterns were analyzed by three kinds of phase change calculation algorithms. Applications of the system to measurement of the object deformation and the analysis of the fringe patterns are described.
The phase shifting method is based on the recording of three or more speckle interference patterns recorded before and after deformation to calculate the phase distribution. For applying phase shifting method to fast varying process, the phase shifted fringe patterns should be recorded simultaneously. These types of phase shifting method are known as spatial phase shifting. In conventional spatial phase shifting ESPI, at least three frames of speckle interference patterns with mutual phase shift are present simultaneously. Previous multi-camera spatial phase shifting ESPI was composed of three or four cameras for acquiring three or four phase-shifted images simultaneously. However, the optical alignment of three or more cameras is a very difficult and time-consuming process. To overcome these disadvantages, the two-channel spatial phase shifting ESPI using two cameras was developed.
The ESPI could record simultaneously two speckle interference patterns with mutual phase shift using polarization phase shifting. From four frames of the interference patterns, of which 2 frames were recorded before object deformation and 2 frames were recorded after deformation, the phase change due to object deformation could be calculated. In this paper, three kinds of phase change calculation algorithms for two-channel spatial phase shifting ESPI system - 2-buckets phase shifting algorithm, 2-buckets phase shifting with root-mean-square filtering and modified 3-buckets phase shifting using three secondary speckle interference patterns are investigated. Also, the applications of two-channel spatial phase shifting ESPI system to object deformation measurement are described.
From the experimental results, the phase map obtained by the 2-bucket phase shifting method has a lot of speckle noise and low visibility due to speckle decorrelation. A more accurate and clear phase map can be obtained using the least-square-fitting method. The phase map calculated using 3-buckets phase of difference method is also more accurate than that from 2-buckets phase shifting algorithm in two-channel spatial phase shifting ESPI system. In two-channel spatial phase shifting ESPI experiments, the RMS(root-mean-square) phase measurement error using 2-buckets algorithm only is 0.1 wave and the RMS phase error in 2-buckets algorithm with 5x5 root-mean-square filtering and modified 3-buckets algorithm with 7x7 windows are 0.03 waves.
The error analysis in 2-buckets phase shifting and electro-optics holography are also described in this paper. As a result of the error analysis, we found that the phase shifting errors in 2-buckets algorithm and static electro-optic holography are twice as large as those in the conventional 4-bucket phase shifting algorithm, and the RMS phase shifting errors in dynamic electro-optic holography are similar to those in the 4-bucket phase shifting algorithm.
High-speed holographic interferometry was applied to the shockwave and laser-induced plume measurement experiments. In these experiment, three kinds of holographic interferometers were used ; a real-time interferometer with a high-speed framing camera, a real-time interferometer with a streak camera and a double-pulse laser holographic interferometer with a dual-reference-beam module. The three kinds of experimental results were compared with each other and the advantage and disadvantages are discussed.
