  The non-separability between the spatial and polarisation Degrees of Freedom
(DoFs) of complex vector light fields has drawn significant attention in recent
time. Key to this are its remarkable similarities with quantum entanglement,
with quantum-like effects observed at the classical level. Crucially, this
parallelism enables the use of quantum tools to quantify the coupling between
the spatial and polarisation DoFs, usually implemented with
polarisation-dependent spatial light modulators, which requires the splitting
of the vector mode into two orthogonal polarisation components. Here we put
forward a novel approach that relies on the use of Digital Micromirror Devices
(DMDs) for fast, cheap and robust measurement, while the
polarisation-independent nature of DMDs enables a reduction in the number of
required measurements by 25\%. We tested our approach experimentally on
cylindrical vector modes with arbitrary degrees of non-separability, of great
relevance in a wide variety of applications. Our technique provides a reliable
way to measure in real time the purity of vector modes, paving the way to novel
applications where the degree of non-separability can be used as an optical
sensor.
