Instrumentation to detect the presence of, or to measure the concentration of, a gas or pollutant in a gaseous environment. With the use of spectroscopic techniques and energy beams having suitable wavelengths, the presence and concentrations of these can be detected and measured, not only in flowing streams such as in process streams or exhaust stacks, but above a ground area, around its perimeter, or across the road. A calibration means including an internal audit cell and a retrodirective reflector is also shown.
