  Geometrical chirality is a property of objects that describes
three-dimensional mirror-symmetry violation and therefore it requires a
non-vanishing spatial extent. In contrary, optical chirality describes only the
local handedness of electromagnetic fields and neglects the spatial geometrical
structure of optical beams. In this manuscript, we put forward the physical
significance of geometrical chirality of spatial structure of optical beams,
which we term "Kelvin's chirality". Further, we report on an experiment
revealing the coupling of Kelvin's chirality to optical chirality upon
transmission of a focused beam through a planar medium. Our work emphasizes the
importance of Kelvin's chirality in all light-matter interaction experiments
involving structured light beams with spatially inhomogeneous phase and
polarization distributions.
