We derive a general law for the M<sup>2</sup> factor of any beam generated by a partially coherent Schell-model source. The fourth power of <i>M</i> differs from its minimum value, attained in the coherent limit, by a term proportional to the second derivative of the modulus of the spectral degree of coherence evaluated for zero argument. Examples are given for cases of practical interest.
© 1999 Optical Society of America
M. Santarsiero, F. Gori, R. Borghi, G. Cincotti, and P. Vahimaa, "Spreading properties of beams radiated bypartially coherent Schell-model sources," J. Opt. Soc. Am. A 16, 106-112 (1999)