  The mathematical notion of a spectral singularity admits a physical
interpretation as a zero-width resonance. It finds an optical realization as a
certain type of lasing effect that occurs at the threshold gain. We explore
spectral singularities of a complex spherical barrier potential and study their
realization as transverse spherical electromagnetic waves emitted by a gain
medium with a spherical geometry. In particular, for a typical dye laser
material, we obtain a lower bound on the size of the gain medium for the
occurence of this kind of spectral singularities.
