  A measurement of the photon polarization in radiative penguin $B$ decays
provides a test of the Standard Model and a probe for New Physics, that can
lead to a deviation from the Standard Model prediction of left-handed photons
in $b\to s \gamma$. We propose a new method to measure the photon polarization
using the baryonic decay $B^- \to \Lambda\bar{p} \gamma$. The $P$-violating
$\Lambda$-hyperon decay allows a measurement of the $\Lambda$ helicity to be
performed, which can be uniquely related to the photon polarization in a
model-independent way. The $B^- \to \Lambda\bar{p} \gamma$ decay was recently
measured to have a large branching fraction providing a possibility to get
meaningful results with the data already available at LHC and B-factory
experiments. An increase of the $B$-meson sample at high luminosity LHC
experiments and Belle II should provide a really stringent test by using this
method already in the near future.
