Highlighted Articles from Optica Publishing Group Journals
Spotlight Summary by Johann Toudert
Simultaneous measurement of liquid-film thickness and solute concentration of aqueous solutions of two urea derivatives using NIR absorption
Determining simultaneously the thickness and composition of liquid films by non-invasive optical measurements is needed for applications in various fields. In this featured article, Marc Lubnow and coauthors report an approach enabling such a determination for aqueous films containing urea and dimethylurea, which are of interest for cosmetics, exhaust gas treatment, or health monitoring. First, as a calibration, the near-infrared transmittance of films with a broad range of known thicknesses and compositions is measured at, at least, four wavelengths where water, urea, and dimethylurea selectively absorb light. For each film, data is processed using Beer-Lambert’s law to determine the thickness and the extinction coefficient at each wavelength. It is shown that these coefficients vary linearly with the urea and dimethylurea concentrations. Then, the near-infrared transmittance of an unknown film is measured. By using the same data processing method and the previously determined linear relation, the unknown thickness and concentrations are obtained. Using an FTIR setup, this approach enables a 10% relative accuracy for thicknesses and concentrations up to 1 mm and 40 wt%, respectively. A similar accuracy is expected when using a filtered broadband light source instead of an FTIR, thus opening the way to accurate measurements with portable and cost-effective setups.