Digital holographic imaging of refractive index distributions for defect detection

Any change in physical quantities like density and temperature, affect the refractive index distribution of materials. So, any discontinuity in the translucent materials can be imaged or detected by mapping its spatiotemporally evolving refractive index under thermal stressing. With the help of digital holography, both amplitude and phase of the object under investigation can be obtained and so it emerges as one of the most resourceful tools for high contrast imaging of transparent and semi-transparent materials. The reconstructed phase using digital holography, provides information regarding the refractive index distribution existing in such objects. By mapping the refractive index distributions across the object which is subjected to thermal stress, thermal conductivity of objects can be studied. Moreover, the fact that a defect or any form of optical inhomogeneity will show discontinuity in the refractive index distributions on change in temperature, can be used to detect defects. The wavefront passing through such a region of non-uniformity in refractive index will carry the information about it as a spatio-temporally varying phase and thus will lead to defect detection in phase objects. Here we describe our efforts in the development of a single beam lens less Fourier transform digital holographic interferometric technique for the imaging spatio-temporally evolving refractive index distributions and its application in defect detection.