Abstract: In measuring a hot rolling train wheel, optic rays are curved due to the non-uniform distribution of surrounding air temperature, leading to significant errors.Air temperature distribution is analyzed with a finite element method.Linear distribution of refractive index is calculated for certain measurement positions. Reasonable positions are chosen according to the amount and direction of curly light rays so that influence of the ray curvature is avoid.

Key words: temperature distribution, thermal optic error, air refractive index, finite element