The application of liquid crystal techniques to determine heat transfer rates

Different methods using liquid crystals to determine surface temperature are listed. These methods depend on the unique characteristic of cholesteric and chiral nematic liquid crystals to change color with variation of temperature. An explanation of 'color' is given. There is a direct relation between the temperature and the maximum intensity wavelength scattered by the liquid crystal layer. This expression leads directly to the spectral distribution of light and the basic optical properties of liquid crystals which are reviewed. Early methods used the human eye as measurement instrument, while later developed methods went over to cameras taking color pictures and video recordings. It is clear that neither photographic nor video cameras were developed for making accurate heat transfer measurements. They try to reproduce the human color perception as close as possible, and for their application in heat transfer measurement chains basic considerations about their recording technique have to be made. Other color influencing parameters which can deteriorate the measurements are considered. A narrow bandpass filter method is described. The application to a practical case and the difficulties in calculating the heat transfer coefficients are discussed.