  The analysis of semi-inclusive deep inelastic electron scattering off
polarized $^3$He at finite momentum transfers, aimed at the extraction of the
quark transverse-momentum distributions in the neutron, requires the use of a
distorted spin-dependent spectral function for $^3$He, which takes care of the
final state interaction effects. This quantity is introduced in the
non-relativistic case, and its generalization in a Poincar\'e covariant
framework, in plane wave impulse approximation for the moment being, is
outlined. Studying the light-front spin-dependent spectral function for a J=1/2
system, such as the nucleon, it is found that, within the light-front dynamics
with a fixed number of constituents and in the valence approximation, only
three of the six leading twist T-even transverse-momentum distributions are
independent.
