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Long path systems 1. Active systems
The basic idea in optical design of long path system was to use multipass optical cell. Figure schematically illustrates the spatial arrangement of the system's mirrors, their mobility, and their mounting. The dashed line designates the position of images on the field mirror surface. The numbers on the images indicate the succession of their creation during the adjustment of the system for thirty passes (for additional info see Appl. Opt., Vol.30(1), 51). The cell* contained 5 convex mirrors all having radius of 500 mm. It was possible to obtain optical path as long as 16 m at a distance between opposite mirrors of 500 mm. In our experiments we used the simple image configuration providing optical path of 2m length. Light from uncoooled LED with l=4.7 µm passed through the LiF lens with f=8 mm, D=9 mm and input slit. The output light was focused by another LiF lens onto PbSe detector with 2.6 x 2.6 mm sensitive area mounted on 2 stage thermoelectric cooler. SNR of the monitor at current pulses of 5A(t=30 ms, f=500 Hz) and averaging of 500 pulses was one decade. Detector temperature was -40°C. Three mirror design has been recently used in open path ethanol detector utilizing 3.4 um diode optopairs. Utilization of 5 cm wide concave mirrors allows measurements in the 40-100 m long path.
2. Passive systems Immersion lens PDs are capable detecting radiation from distant objects whose temperature differs from background. For instance it was possible to trace hydrocarbons (Lanbda 3.4 um) in the out coming smog at the soot factory (see photo on the left). Such systems can be based on a lens or a two mirror optical scheme presented in the right figure.
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