  We show that the photoluminescence intensity of single-walled carbon
nanotubes is much stronger in tubes with large chiral angles - armchair tubes -
because exciton resonances make the luminescence of zigzag tubes intrinsically
weak. This exciton-exciton resonance depends on the electronic structure of the
tubes and is found more often in nanotubes of the +1 family. Armchair tubes do
not necessarily grow preferentially with present growth techniques; they just
have stronger luminescence. Our analysis allows to normalize photoluminescence
intensities and find the abundance of nanotube chiralities in macroscopic
samples.
