There is an increasing need for on-site measurements in the manufacturing industry. In several cases, the shape of a work piece needs to be optically measured in high-temperature atmospheres up to 500 °C. Measurement errors caused by a lump of high-temperature air in an optical path were theoretically investigated using a model of a cylindrical lens made of air. Errors predicted by the model qualitatively explained the experimental results well. Quantitatively, the predicted errors were always larger than the experimental ones. The model was found to be useful for estimating the maximum values of the errors, which has important practical uses.