The OPSIS AR500 Analyser is the central unit of an OPSIS ambient air quality monitoring system. It receives light from one or more light paths via a fibre optic cable and provides data for presentation through OPSIS software. The analyser has a high-performance spectrometer. Here, received light is converted into digital signals and analysed by the built-in computer. The computer software includes the spectrographic ‘fingerprints’ of a number of user-specified compounds. Using Beer Lambert’s Law, it detects and measures these compounds through the depletions they cause in the spectrum of received light.The system refines results by calculating margins of error for each value and by noting the proportion of transmitted light received by the analyser. This allows both data and system verification. OPSIS is a fully modular system which may be updated or expanded at any time. An OPSIS analyser includes interfaces for a multiplexer (for multiple light paths) and for a data logger (when integrating external signals from e.g. meteorological sensors) as standard. Furthermore, it is prepared for the addition of automatic calibration equipment. Communication ports are available for data transfer via modem, LAN or Internet, using optional communication devices. Updating an OPSIS system to monitor additional gases often only involves a simple software upgrade.