  Intensity, wavevector, phase, and polarization are the most important
parameters of any light beam. Understanding the wavevector distribution has
emerged as a very important problem in recent days, especially at nanoscale. It
provides unique information about the light-matter interaction. Back focal
plane or Fourier plane imaging and spectroscopy techniques help to measure
wavevector distribution not only from single molecules and single
nanostructures but also from metasurfaces and metamaterials. This review
provides a birds-eye view on the technique of back focal imaging and
spectroscopy, different methodologies used in developing the technique and
applications including angular emission patterns of fluorescence and Raman
signals from molecules, elastic scattering etc. We first discuss on the
information one can obtain at the back focal plane of the objective lens
according to both imaging and spectroscopy viewpoints and then discuss the
possible configurations utilized to project back focal plane of the objective
lens onto the imaging camera or to the spectroscope. We also discuss the
possible sources of error in such measurements and possible ways to overcome it
and then elucidate the possible applications.
