We have developed an open-path hand-held gas detector incorporating a distributed feedback InGaAs laser diode at 1.65µm. Incorporated into a handheld transceiver unit, the emitted laser beam is backscattered from nearby surfaces, collected and focussed onto an amplified InGaAs detector using a 150mm diameter plastic Fresnel lens. At ranges of 4-5metres, a typical backscattered signal is 10's of nanowatts of laser light. Applying 2nd derivative wavelength modulation spectroscopy gives a sensitivity to methane of better than 10ppm over a one metre path length. A number of demonstration units have been fabricated and successfully evaluated by end users.

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We have developed a quantitative model of the performance of laser pointer style gas leak detectors, which are based on remote detection of backscattered radiation. The model incorporates instrumental noise limits, the reflectivity of the target background surface and a mathematical description of gas leak dispersion in constant wind speed and turbulence conditions. We have investigated optimum instrument performance and limits of detection in simulated leak detection situations. We predict that the optimum height for instruments is at eye level or above, giving an operating range of 10 m or more for most background surfaces, in wind speeds of up to 2.5 ms(-1). For ground based leak sources, we find laser pointer measurements are dominated by gas concentrations over a short distance close to the target surface, making their readings intuitive to end users in most cases. This finding is consistent with the results of field trials.