Spatially incoherent imaging systems are characterized by a linear-in-intensity relationship between the object and the image distributions. While strict spatial incoherence is theoretically not achievable, a particular imaging system may be made effectively linear in intensity by a choice of the appropriate illumination source location and size. The requirement for source size for effectively incoherent illumination of a two-dimensional object is well known. I extend the arguments for choosing the source size in a two-dimensional imaging system to develop necessary conditions for the source size for effectively spatially incoherent illumination of a three-dimensional object. While the conditions are necessary, they are not sufficient, since coherence in the direction of the optical axis is not addressed.
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