The spatial field correlations within a Bessel-correlated spherical source with large size parameter (radius much greater than the wavelength) are investigated. Exact expressions for the field spectral density, in terms of a partial-wave series, as well as for the magnitude of the complex degree of spectral coherence (correlating the field at an arbitrary internal point with that at the center of the sphere), are obtained. Asymptotic approximations for a large size parameter are found by applying complex angular-momentum techniques. The results are discussed and plotted for a wide range of values of the ratio between the source spatial coherence length and the wavelength. The asymptotic approximations are found to be quite accurate. The field ranges from extreme incoherence to full spatial coherence. The latter is attained only when the source’s coherence length becomes much larger than its Fresnel radius. Results previously obtained in the limit of an unbounded source are verified. Features arising from finite size and boundary effects are discussed.
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