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The HSI ( Hue , Saturation, Intensity) model describes color in terms of how it is perceived by the human eye. It is what an artist refers to as pigment; it is what we think of as color - yellow, orange, cyan and magenta are examples of different hues. An artist usually starts with a highly saturated (pure), and intense (bright) pigment. Some white is then added to reduce its saturation and some black to reduce its intensity. Red and Pink are two different saturations of the same hue, namely Red. The main advantages of HSI model is that, it is useful while comparing two colors, or for changing a color from one to another. For example, changing a value from Cyan to Magenta is more easily accomplished in an HSI model; only the H value needs to be changed. Making the same change in an RGB view is less intuitive; since we must know the correct amounts of Red, Green and Blue needed to create Magenta. In short, the RGB model is suited for image color generation, whereas the HSI model is suited for image color description. Owing to these inherent advantages, hue is best suited to establish a unique relation between hue and temperature in the pertinent LCT experiments. Therefore, though the colors of the thermochromic liquid crystals (TLCs) are observed by a data acquisition system that senses RGB, the R-, G-, and B-data are not used to calibrate the TLCs. Instead. the RGB color space is first transformed to the HSI color space. Invariably, temperature turns out to be a monotonic function of the hue. Thus, a calibration curve is immediately obtainable. The calibration data is mostly insensitive to image intensity and external illumination.