Calibration of a Sheet of Thermosensitive Liquid Crystals Viewed Non-Orthogonally

A thin adhesive sheet of encapsulated thermochromic liquid crystals (TLCs) is attached to a submerged plate, impinged by an ionic jet in an electrohydrodynamics experiment. In order to measure the local temperature and obtain the heat transfer coefficients on the plate, a proper calibration of the liquid crystals is needed. A colour can be expressed as a weighted combination of three attributes: hue, saturation and intensity. In TLCs, hue is a function of temperature in a narrow range. However, TLCs colour also depends on the angle under which a point on the sheet is viewed, as well as on the lighting conditions. Due to the particular specifications and inherent constraints of our experimental apparatus, the calibration procedure becomes fairly complicated. In fact, the digital camera for the acquisition of the colour pattern has to be positioned on the side of the sheet. Besides, the TLCs are non-uniformly lighted, including the presence of shadows. The relationship between the hue field and the temperature field on the plate is then obtained by means of a filtering and interpolation technique, as described in the paper. The overall accuracy of the developed technique is ±0.1 K.