Passive scintillometer: theory and field measurements

Abstract

We propose design of a low-cost passive scintillometer that measures the strength of optical turbulence by analyzing scintillation in the image of a straight edge between the two areas of uniform, but distinct brightness. Two theoretical approaches to scintillations description are developed that are based on the phase screen and rigorous path integral propagation imaging models. Asymptotic analysis of both models leads to four distinctive imaging situations. We propose two uniform approximations that cover the most promising conclusion for the passive scintillometer applications. Both the strength of scintillation and the width of the area where scintillations exist can be used to estimate the turbulence strength. We present the results of the proof-of-concept experiment where images of the specifically made target were taken by a consumer grade camera equipped with a telephoto lens. We describe the image processing that separates the target characteristics from the turbulence scintillations. The spatial profiles of the image variances at three spectral bands and for several apertures were calculated. As was expected, scintillations are concentrated around the sharp edge and are absent at the uniformly bright parts of the target. Observed edge scintillation variances are within the theoretical limits, and provide reasonable estimates for the turbulence structure constant across the three spectral bands. However, the width of the scintillation area and dependence on the aperture are inconsistent with the thin lens imaging theory for larger apertures. We attribute this to the Additional bench tests on bar targets revealed that the camera and lens resolution are well below the nominal thin lens diffraction limit, and a different optical setup should be used in order to exploit the spatial distribution and aperture dependence of fluctuations.