This book gives insight of the retro reflective fiber optic sensors and its use in different in industrial applications. It provides complete description of the generalized mathematical models developed using ray tracing method and its analysis in terms of performance parameters of the sensor. Mathematical models are developed with the purpose of designing sensor probes for innovative industrial applications. Systematic method of designing, fabrication and testing of different sensors for identified applications is described in detail. In addition, configuration details and experimental test reports for indigenously designed sensors are also presented in the book.