Index of refraction measurements in liquid substances of full field using holographic interferometry

Abstract

We present a novel method based on Digital Holographic Interferometry to detect slight physical variations of refractive index with high sensitivity in liquid substances. The technique is grounded in the measurement of a phase difference between two reconstructed wavefields. The optical system was tested using a series of sodium chloride (NaCl) solutions to detect a variation in its physical property such as concentration. A first hologram records a wavefront coming from the light scattered by a common cylindrical glass container filled with certain NaCl solution. Later, a second hologram is recorded when the solution mentioned above slightly changes its concentration. The difference between the phase maps obtained from the correlation from the two holograms will provide information about a refractive index variation, which is directly related to a concentration change. The achieved results have proven to be more accurate and faster to get than with other techniques. The process requires just a few special optical elements and is able to measure the three-dimensional distribution of the refractive index of a sample. This method can be extended to identify adulteration in liquids, measure the variation in refractive index in gaseous flames, apart from analyzing and visualizing the mechanical properties of a liquid sample.