Calibration of a sheet of thermosensitive liquid crystals viewed non-orthogonally W. Grassi, D. Testi * , D. Della Vista, G. Torelli LOTHAR (LOw gravity and THermal Advanced Research laboratory), Department of Energetics ‘‘L. Poggi’’, University of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy Received 4 May 2006; received in revised form 23 October 2006; accepted 26 October 2006 Available online 12 November 2006 Abstract 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 shad- ows. 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid crystal thermography; Image processing; Image filtering; Calibration; Hue–temperature curve 1. Introduction The evaluation of convective heat transfer coeffi- cients in an experiment of complex geometry or fluid dynamic flow, in which high velocity and tem- perature gradients are present, needs temperature measurement techniques capable of mapping the entire heat transfer surface. Traditional sensors such as thermocouples or resistance thermometers, besides monitoring only a discrete number of sites on the surface, might disturb the physical phenom- enon owing to the presence of the instrument probes. A simple, low cost, reliable and almost non-intrusive technique for surface temperature measurements is the optical method of liquid crystal thermography. Thermochromic liquid crystals (TLCs) have the valuable property of exhibiting a selective reflec- tance of non-coherent light as a function of their temperature. The internal structure of the crystals changes with temperature over a known, reproduc- 0263-2241/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2006.10.020 * Corresponding author. Tel.: +39 050 2217 109; fax: +39 050 2217 150. E-mail addresses: w.grassi@ing.unipi.it (W. Grassi), d.tes- ti@ing.unipi.it (D. Testi). Measurement 40 (2007) 898–903 www.elsevier.com/locate/measurement