About the approximation of a field of an extended source by a sphere wave in the far illumination region

On the base of a coherent emitting straight segment, the problem of approximate description of the field of an extended source in the far illumination region is considered. It is shown that in regions close to the directions of intensity maximums the source field can be approximated by mean of sphere waves. The sphericity condition in corresponding region is found. The problem of approximation of an extended source field by means of a plane wave is also discusses. As a result of a next approximation of a spherical wave, the condition for the approximation of a field of extended source by means of a plane wave was found.