  The optical properties of (001)-oriented NbP single crystals have been
studied in a wide spectral range from 6 meV to 3 eV from room temperature down
to 10 K. The itinerant carriers lead to a Drude-like contribution to the
optical response; we can further identify two pronounced phonon modes and
interband transitions starting already at rather low frequencies. By comparing
our experimental findings to the calculated interband optical conductivity, we
can assign the features observed in the measured conductivity to certain
interband transitions. In particular, we find that transitions between the
electronic bands spilt by spin-orbit coupling dominate the interband
conductivity of NbP below 100 meV. At low temperatures, the momentum-relaxing
scattering rate of the itinerant carriers in NbP is very small, leading to
macroscopic characteristic length scales of the momentum relaxation of
approximately 0.5 $\mu$m.
