Digital holographic interferometry experimental setup to study mass transfer processes

Abstract

For many years, holographic interferometry (HI) has been an optical method widely used to study mass transfer processes in liquids and in transparent gels. During an experiment, changes in the refractive index distribution, and therefore in the concentration distribution, are visualized as an interference fringe pattern. In real-time HI, the first step is to obtain the hologram (an image of the object at a certain time, holographically stored), which is usually recorded on a holographic plate by a photographic method. This hologram represents the object at its reference state. The second step is to obtain an interferogram at different times. The interferogram allows changes in the object to be visualized by combining the current object wave with the reference object wave stored in the hologram. A major drawback of classical HI is not so much the cost of the holographic plates but the difficulty in developing the film. Moreover, all interferograms are obtained by comparing the current state of the object with the only hologram taken. Therefore, the temporal states that are compared cannot be freely chosen. These problems were overcome with digital holographic interferometry (DHI) (Marquadt and Richter). Digital holography (DH) is the digital recording and numerical reconstruction of numerous holograms and it offers the possibility of combining the current object wave with reference waves captured at different times. The use of DH has been possible due to the tremendous development in data processing and in opto-electronics. Digital CCD (Charged Coupled Device) cameras, that digitize the hologram, have continuously increased their resolution in the past years and they are now applicable for optical measurements. Adjustments in the classical HI setup previously used to study mass transfer processes have been made to adapt it to the DHI. The main change has been the use of a CCD camera instead of the holographic plates to record the holograms. Furthermore, a MATLAB program has been developed in order to capture and reconstruct the holograms numerically. In this work it was checked that the new DHI technique worked correctly by comparing the new interferograms with those obtained with optical HI in studies about diffusion and polarized reverse osmosis processes.