Reconstruction Of Refractive Index Fields Research Articles

A new approach for the reconstruction of axisymmetric refractive index fields from background-oriented schlieren measurements

Background-oriented schlieren (BOS) is an optical visualization technique that reconstructs a whole-field flow based on its density gradient. Thanks to the cost-effectiveness of its optical setup and its unlimited field of view, BOS has become an attractive technique for laboratory and large-scale experiments. In BOS, the reconstruction of the refractive index field involves various mathematical calculations, which depend on the flow geometry. In this context, the present study presents a new method for the reconstruction of three-dimensional axisymmetric refractive index fields. The proposed method takes advantage of the bi-sensitivity quality of BOS to gain more accuracy in the reconstructed fields. We demonstrate the method’s precision and robustness against experimental noise throughout a synthetic axisymmetric refraction index field. The application of this new approach is not restricted to the BOS technique but can be extended to several other measurement techniques such as moiré deflectometry, laser speckle photography, and rainbow schlieren.

Tomographic reconstruction of refractive index fields based on laser beam deflection measurement

Refractive index as one of the most important optical properties of materials is the key to utilize refractive index elements. However, it is very difficult to measure or estimate refractive index of media, especially for multi-dimensional inhomogeneous refractive index distribution. Therefore, an inverse scheme is presented for reconstructing refractive index field in rectangular gradient index medium based on ray deflection measurement in this work. A collection of rays from given positions along given directions transit the semitransparent medium and all exit boundary positions are recorded to construct the objective function of inverse problem. Runge–Kutta ray tracing technique is employed to simulate the curved ray path determined by Fermat principle in gradient-index medium. Conjugate gradient method as an inverse algorithm is used to reconstruct refractive index field. A series of simulation experiments are done to show the performance of the inverse strategy we developed. The results indicate that this inverse strategy for reconstructing refractive index field is accurate and effective.

Reconstruction of refractive index fields by using relative fringe data

Tomographic reconstruction techniques used to obtain three-dimensional refractive index fields from interferometric data all assume that the correct fringe number has been assigned. In confined fields or where the imaging beam is smaller than the object field, only relative fringe numbers can be determined from the interferogram because no reference (undisturbed) region is visible. An algorithm that can be applied to iterative reconstruction techniques is described that permits the quantitative reconstruction of three-dimensional fields by using only relative fringe numbers provided that a priori data are supplied. This method is applied to an existing reconstruction technique and is demonstrated by using simulated interferometric data.