  We theoretically analyze and experimentally measure the extrinsic angular
momentum contribution of topologically structured darkness found within
fractional vortex beams, and show that this structured darkness can be
explained by evanescent waves at phase discontinuities in the generating optic.
We also demonstrate the first direct measurement of the intrinsic orbital
angular momentum of light with both intrinsic and extrinsic angular momentum,
and explain why the total orbital angular momenta of fractional vortices do not
match the winding number of their generating phases.
