AN UPPER LIMIT ON THE THERMAL DEFOCUSING OF A LIGHT BEAM.
NAVAL RESEARCH LAB WASHINGTON D C
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The authors discuss the defocussing of a light beam due to the heating of the atmosphere caused by the beam itself. Under certain assumptions, the calculation of the intensities a long distance from the source far-field pattern is very simple, and does not involve the tracing of individual rays through the intermediate region. Of the assumptions made, that requiring the absence of wind and convection is physically the most severe one all assumptions tend to increase the size of the pattern calculated, so that the result is an upper limit to the real situation. A general expression for the far-field pattern is obtained, which is easily evaluable in terms of any assmued initial power profile. However, the gross features, and particularly the size, of the far-field pattern is found to be only slightly dependent on the details of the profile. For a given power density, the size of the far-field pattern is independent of the original beam radius. Author