DOAS Products for Gas Analysis

A wide range of products are combined into customised systems

Differential Optical Absorption Spectroscopy (DOAS) is the core technology in the OPSIS product portfolio, where a combination of light transmitters, light receivers, optical fibres, and analysers enable the measurement of gaseous pollutants in ambient air.


Each monitoring system is built around an analyser. Its task is to measure the absorption of certain wavelengths of light coming from the monitoring path, and use this to calculate the levels of various compounds, such as NO, NH3, NO2, SO2, O3, benzene, toluene, styrene, and formaldehyde.

The AR500 is the most common type of analyser. It operates in the ultraviolet wavelength range and the technique is then called UV-DOAS. When monitoring in the infrared range, an AR550 analyser based on FTIR-DOAS technology is often used. In some cases, IR measurements can also be made with an analyser that is essentially of the UV-DOAS type. It is then called AR520.

Each individual analyser is adapted to monitor the levels of the specific substances required. Two analysers of the same type can therefore have completely different monitoring capabilities.


The DOAS technology is based on the emission of white (broadband) light in a narrow beam from a xenon lamp (emitter) and its capture by a receiver. Along the beam of light, which can be several hundred metres long, different types of pollutants absorb different wavelengths of light. It is this absorption that is measured in the analyser. The open beam of light is also called the monitoring path because that is where the monitored concentrations of air pollutants are found.

OPSIS offers several different products to form the monitoring path. The ER110, ER120N, and ER150 consist of separate emitter and receiver units. The ER120 combines the emitter and receiver in the same unit. A reflector (RR090) ensures that the emitted light is sent back to the ER120 unit. The choice of ER model depends on the length of the measurement path and the conditions at the installation site.


OPSIS emitters and receivers are placed outdoors and can withstand very harsh environments. The analysers are more sensitive and must be placed in an indoor climate. To transmit the light from the receiver to the analyser, special types of optical fibres called OF60 and OF100 are used. The former is used with AR500 and AR520 analysers, the latter with AR550. The maximum length of the fibres depends on the wavelengths to be transmitted, but they can in some cases be up to 100 m.


A basic system for monitoring gas concentrations in ambient air consists of an analyser, an emitter/receiver set or a combined emitter/receiver and reflector, and an optical fibre cable that transmits the light to the analyser. The system can be configured to measure a wide range of gaseous compounds. A single analyser can monitor concentrations along several different monitoring paths. All in all, this provides a complete monitoring system that is both versatile and cost-effective. Read more on the page on turnkey solutions.

It is straightforward to supplement an existing system with the monitoring of additional substances and on more monitoring paths.


The OPSIS DOAS systems have been tested and approved by several international test institutes and national authorities, including the U.S. EPA and the German TÜV.


On our blog page you can read more about our air quality measurement products and what they are used for, for example:

Inside the Gas Analyser - the Spectrometer 

Selecting the Right Air Quality Monitor 

DOAS Explained 

How Does a Gas Analyser Work? 


MCERTS Certificate Benzene

MCERTS Certificate SO2, NO2, O3

Swedish EPA SO2, NO2, O3

TÜV Certificate SO2, NO2, O3

U.S. EPA Certificate SO2, NO2, O3, PM2.5, PM10

CCEP Certificate SO2, NO2, O3


OPSIS Open-path DOAS

OPSIS DOAS in Swansea, by Enviro Technology

Benzene Monitoring

Fence-line Monitoring of Fugitive Emissions

Contact Anders A OPSIS 720X480pxl

I'm happy to answer questions about our air quality monitoring products!