Infrared propagation at low altitudes is determined by extinction that is due to molecules and aerosol particles and ray bending by refraction, three effects that control the mean value of the signal. Interference causes the signal to fluctuate, or scintillate, about the mean value. We discuss the design, calibration, and limitations of a field instrument for measuring optical propagation inside the midwave and long-wave infrared atmospheric windows. The instrument, which is accurate to ?10%, has been used to investigate aerosol, refractive, and scintillation phenomena in the marine boundary layer.