The principle of operation is based on the temperature dependence of the bandgap of GaAs. The GaAs crystal fixed on the tip of the fibre will be transparent at a wavelength above 850 nm. The position of the band edge is temperature-dependent and is shifted about 0.4 nm/K. The light is directed via the optical fibre to the crystal, where it is absorbed and partially reflected into the fibre. A miniature spectrometer provides a spectrum with the position of the band edge, from which the temperature is calculated.