Based on the polarization splitting principle of the only-phase liquid crystal device, a common path phase-shifting interferometry is built to measure the whole and local phase modulation characteristics of a liquid-crystal spatial light modulator (LC-SLM) with high spatial resolution and high gray level. In this technique, the SLM is not only a device to be measured but also an optical component that is used to splitting the incident linearly polarized light. A specially designed control grayscale image is loaded into the SLM, and then several frames of phase-shifted interferograms are recorded. The wavefront phase can be calculated by using the phase-shifting algorithm. Through changing the loaded images, the phase modulation curve is obtained. The experimental steps and results are presented in the paper, and the uniformity of local phase modulation is also evaluated. The proposed method has stable measuring results and has the advantage of simple principle and convenient operation.
In order to measure the local phase modulation characteristic of a phase-only reflective Liquid crystal spatial light modulator (LCSLM) and accurately evaluate the nonuniformity of phase modulation, a Twyman-Green interferometer is built to simultaneously measure the local phase modulation of multi-region in LCSLM. A series of carrier fringe patterns can be captured, which is modulated by the local control pattern loaded to the LCSLM. The phase distribution from all fringe patterns are extracted respectively with the Fourier transform method. By calculating the variance values of all phase values in each pixel position, a phase variance matrix is obtained. Then the regions of interests (ROIs) from the fringe pattern are extracted with the Otsu segmentation algorithm and the connected region labeling algorithm. At last, the phase modulation of local regions is obtained synchronously with the phase-shift algorithm based on Fourier transform method. The proposed method can greatly improve the measuring efficiency and reduce the environmental impact to great degree.
In the accuracy measurement of phase from interferometers with adjustable fringe contrast, it needs to estimate the contrast of experimental patterns so as to obtain the interference patterns with the maximum contrast. We develop the Fourier-polar transform and combine the directional projection to estimate the global contrast of carrier fringe pattern. The technique is especially used for low-quality fringe pattern such as low contrast and low signal to noise ratio (SNR) that often appear in the interferometric experiment. An illustrative experiment based on the radial shearing interferometer is given. Results generated from this technique are compared with the derived values from theoretical model, and exemplary agreement between both is demonstrated.
A global direction metric method based on Fourier-polar transform is proposed in this paper, which calculates the global fringe direction according to the directional distribution of intensity from the power spectrum of fringe pattern. By introducing polar coordinate transform, the rotation of power spectrum is transformed into translation component, which can make the calculation process simple and fast. Then the original image is projected along the global fringe direction and the mean value of pixel gray is calculated. Also, the fringe pitch can be calculated from the projection curve close to cosine distribution. This detection method of optical fringe parameters uses overall information of the image, and holds good adaptability and robustness to noise and degraded image. Moreover, without any pre-processing operations such as smoothing filter and threshold segmentation is required in this method. It can directly detect two parameters of global fringe direction and fringe spacing, which is of great significance for quantitative analysis of fringe image.
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