Measurements of Concentration differences between Liquid Mixtures using Digital Holographic Interferometry

Abstract

We present an alternative method to detect and measure the concentration changes in liquid solutions. The method uses Digital Holographic Interferometry (DHI) and is based on measuring refractive index variations. The first hologram is recorded when a wavefront from light comes across an ordinary cylindrical glass container filled with a liquid solution. The second hologram is recorded after slight changing the liquid's concentration. Differences in phase obtained from the correlation of the first hologram with the second one provide information about the refractive index variation, which is directly related to the changes in physical properties related to the concentration. The method can be used − with high sensitivity, accuracy, and speed − either to detect adulterations or to measure a slight change of concentration in the order of 0.001 moles which is equivalent to a difference of 0.003 g of sodium chloride in solutions. The method also enables to measure and calculate the phase difference among each pixel of two samples. This makes it possible to generate a global measurement of the phase difference of the entire sensed region.

Digital Holographic Interferometry without Phase Unwrapping by a Convolutional Neural Network for Concentration Measurements in Liquid Samples

“…These holograms were not used before in the training process. It should be noted that the errors in Table 6 are smaller than the values obtained with the conventional DHI method [5]. However, in Table 7 the errors are slightly higher; however, the precision can be improved if the image dataset is increased.…”

“…The optical system for concentration measurements was based on DHI. The principles and mathematical equations of DHI are well known in the literature [5,6]. In DHI, phase difference maps were obtained from the correlation between two holograms.…”

“…where is the new phase that indicates a change in the optical path length, i.e., = + Δ . Therefore, the concentration difference between two liquid samples can be described as [5]:…”

“…The refractive index difference is related to the change of concentration (CON) and the temperature (T) between liquid samples. In the case of aqueous salt mixtures, the liquid samples have a linear relationship between the refractive index and concentration (CON), which is considered to be constant at 1.71 × 10 −3 at a temperature of 20 • C. Therefore, the concentration difference between two liquid samples can be described as [5]:…”

“…There are many methods and tools for the estimation of concentrations in liquid samples; however, most of them are invasive and destructive [2][3][4]. A technique that is able to perform measurements of concentration differences with high accuracy, in a non-invasive and non-destructive way, is digital holographic interferometry (DHI) [5].…”