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Thermochromism is the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses. Thermochromism is one of several types of chromism.
The two basic approaches are based on liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.