A reciprocity inequality is derived, involving the effective size of a plan
ar, secondary, Gaussian Schell-model source and the effective angular sprea
d of the beam that the source generates. The analysis is shown to imply tha
t a fully spatially coherent source of that class (which generates the lowe
st-order Hermite-Gaussian laser mode) has certain minimal properties. (C) 2
000 Optical Society of America OCIS code: 030.1640.