Open-path tuneable diode laser absorption spectroscopy applications

Open-path tuneable diode laser absorption spectroscopy applications 

In the early days of tuneable diode laser spectroscopy, as is true for many modem-day applications, the experimental set-up of a TDLAS system was comprised of a gas cell as its centrepiece. This sampled the analyte across a closed volume allowing for the control of the internal and external parameters (e.g. ambient temperature, gas pressure in the cell, etc.). Then, flow-type systems were introduced later, in which the overall configuration was modified so that the gas could be continuously exchanged in a flow through the cell, normally assisted by a pump at the exit of the cell, as shown in Section 28.2. 

However, there is no principal reason why a cell configuration (i.e. a closed path) is required at all. It 

Open-path TDLAS can be realized in three common implementations (i) the laser beam traverses the measurement gas volume and the absorption is recorded by a detector placed opposite to the laser source (ii) the laser transmitter and the detector receiver are mounted adjacent to each other, with the laser beam being reflected back into the detector by a retro-reflector at the far side of the gas volume and (iii) the laser and detector are mounted again side by side, but a reflecting mirror is not used and instead the (diffuse) reflections from ambient targets beyond the gas volume are recorded. Note that multipass arrangements are normally difficult to implement, and in the latter case impossible. Below, we discuss some representative examples for each of the three cases. 

However, aging and bad maintenance of a car may nullify these efforts. It is quite clear that a small percentage of vehicles, known as gross polluters , make a large contribution to motor vehicle pollution. The consensus is that CO contribution increases exponentially with vehicle age, and that from this age-dependence one derives that 50 per cent of CO emission comes from only 10 per cent of vehicles. 

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