Assessing the effective penetration depth of mid-wave infrared radiation in water for fluid dynamic measurements

The infrared optical properties of most fluids are known to be strongly wavelength-dependent. Since the conventional infrared detectors are sensitive to ranges of wavelengths, of practical interest are usually the integral fluid properties within a certain spectral band. Here, we propose a simple yet reliable way to measure (in-situ) the effective thickness of the infrared emitting (sub-millimetre) fluid layer. Using the proposed approach, we estimate the maximum penetration depth of mid-wave (3.7–4.8 µm) infrared radiation in room-temperature still water confined between two infrared-transparent walls. The results are of particular importance for the applications of infrared thermal imaging to studies of boundary layer and near-wall aqueous flows.