  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.
