THE EMS 1302 OPLC Opacity Monitor Is a High Performance Opacity Monitoring System with Double-Pass Transmissometer T he sensor system consists of an optical stack mounting Transmissometer and the transceiver mounted on one side of the stack and a retro reflector mounted on the other. The main light source is electronically modulated and projects a collimated beam of light, which is split into a reference beam and a measurement beam. The measurement beam is projected across the stack to a retroreflector, which reflects it back across the stack to the measurement detector. The output of the measurement and reference detectors are transmitted to the control room mounted monitoring unit where all output signals are analyzed. EMS Model 1302 OPLC ratio-measurement technique provides continuous automatic compensation for variations in light source intensity to ensure prolonged instrument accuracy and stability. Since the Model 1302OPLC is insensitive to the absolute intensity of the light source, it is not affected by light source aging.