Open path measurements are made feasible by use of the Variable-Focus Retro-reflector , described in a separate topic.

Other systems have used cube-corner arrays to turn a collimated beam of infrared radiation back on itself. This creates the serious problem of having to use a beam splitter to separate the returned beam from the outgoing beam. The beam-splitter will waste 75% of the energy, thus reducing the measurement sensitivity by a factor of four. Another problem is that cube corner arrays and beam splitters are very expensive. The variable-focus retro-reflector is inexpensive and efficient. If you are working indoors, you can adjust the focus to fit the size of your room. If you are working out-of-doors, you can fit the focus to the length of a fence-line. It is also feasible to use auxiliary plane mirrors to set up multiple-passing in the open path, as illustrated in the figure here.

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