Twin Background Subtraction Technique for liquid film thickness and temperature measurements

The novel technique is based on non-invasive optical, infrared measurement, named Twin Background Subtraction (TBS). The technique allows the simultaneous measurement of the temperature and of the thickness of a liquid thin film thickness within an infrared transparent ducts or walls. The maximum thickness measurable depends on the attenuation coefficient of the fluid for the particular IR wavelengths investigated. A mid-wave IR camera obtains two signals coming from two different backgrounds and attenuated by the liquid film positioned in in-between. Two simplified equations give the signals S1 and S2 recorded by the MWIR camera (3.6-5.1µm) as a function of the film temperature TF and each background temperature T1 and T2. The solution of the system of equations gives us the film temperature TF and the film transmissivity τF that, together with the Beer-Lambert law correlating the liquid transmissivity with its equivalent linear attenuation coefficient, is used to estimate the film thickness.