  For applications such as airport border control, biometric technologies that
can process many capture subjects quickly, efficiently, with weak supervision,
and with minimal discomfort are desirable. Facial recognition is particularly
appealing because it is minimally invasive yet offers relatively good
recognition performance. Unfortunately, the combination of weak supervision and
minimal invasiveness makes even highly accurate facial recognition systems
susceptible to spoofing via presentation attacks. Thus, there is great demand
for an effective and low cost system capable of rejecting such attacks.To this
end we introduce PARAPH -- a novel hardware extension that exploits different
measurements of light polarization to yield an image space in which
presentation media are readily discernible from Bona Fide facial
characteristics. The PARAPH system is inexpensive with an added cost of less
than 10 US dollars. The system makes two polarization measurements in rapid
succession, allowing them to be approximately pixel-aligned, with a frame rate
limited by the camera, not the system. There are no moving parts above the
molecular level, due to the efficient use of twisted nematic liquid crystals.
We present evaluation images using three presentation attack media next to an
actual face -- high quality photos on glossy and matte paper and a video of the
face on an LCD. In each case, the actual face in the image generated by PARAPH
is structurally discernible from the presentations, which appear either as
noise (print attacks) or saturated images (replay attacks).
