NEO Monitors LaserGas II Open Path (OP) Monitor is a compact, high perfor- mance gas analyser for long distance monitoring in ambient air. Measure- ment lengths up to 1000 m one way are possible. The OP Monitor utilizes a trans- mitter / reflector configuration to mea- sure the average gas concentration along the optical line-of-sight. As option a portable, battery-powered version is available containing a control keypad and a flash memory for data logging.