  We measure the spatial extent of individual localized photonic states in a
slow-light photonic crystal waveguide. The size of the states is measured by
perturbing each state individually through a local electromagnetic interaction
with a near-field probe. We find localized states which are not observed in
transmission and show that these states are shorter than the waveguide. We also
directly obtain near-field measurements of the participation ratio, from which
the size of the states can be derived, in quantitative agreement with the size
measured with the perturbation method.
