The Statistical and Correlation Properties of Light Scattered by a Random Phase Screen
We report measurements of the statistical and correlation properties of laser light scattered by a random phase screen produced in the laboratory by turbulent mixing of the convective hot air flow above an electric heater and the surrounding cooler air. A phase screen is a localized region of space containing refractive index variations which cause random fluctuations in the phase of electromagnetic radiation propagating through it. On further free propagation of the emergent radiation, intensity fluctuations develop through two distinct mechanisms. Firstly, the individual refractive index inhomogeneities act as lenses which bend the light “rays” and produce a random focussing of the radiation. Secondly, further from the screen, the radiation scattered by different, independent inhomogeneities can overlap to form fringes and more complex interference patterns (e.g. speckle). Phase screens have been of interest for many years because of their role in various natural phenomena such as the fading of radio signals reflected from the ionosphere , the scintillation of distant radio sources due to the solar wind  and the twinkling of starlight caused by atmospheric fluctuations . Nevertheless our measurements appear to be the first which cover a wide range of screen-detector distance, thereby providing focussing curves such as Figure 2.
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