Optical sensors for detecting obstacles on the guideway of a high speed transportation system were investigated experimentally and analytically. Both approaches are described in this paper. The performance of the optical sensor is dependent upon the extent of ray bending caused by solar heating of the air immediately above the guideway. The effect of ray bending is presented in terms of a relationship between obstacle size and length of the optical path.
Daniel E. Magnus,
"Environmental Limitations of Optical Sensors for Guideway Surveillance," Optical Engineering 11(2), 110254 (1 April 1972). https://doi.org/10.1117/12.7971608