In open path spectroscopy, tunable diode lasers (TDLs) are designed to focus on single absorption wavelengths specific to a compound of concern in the gaseous form. They are capable of achieving low detection limits and are virtually interferent-free. Open path TDLs are used in atmospheric pollutant studies, fenceline monitoring, process line/tank leak detection, industrial gas-purity applications, and monitoring and control of combustion processes