  Contextuality is considered as an intrinsic signature of non-classicality,
and a crucial resource for achieving unique advantages of quantum information
processing. However, recently there have been debates on whether classical
fields may also demonstrate contextuality. Here we experimentally configure a
contextuality test for optical fields, adopting various definitions of
measurement events, and analyse how the definitions affect the emergence of
non-classical correlations. The heralded single photon state, a typical
non-classical light field, manifests contextuality in our setup, while
contextuality for classical coherent fields strongly depends on the specific
definition of measurement events which is equivalent to filtering the
non-classical component of the input state. Our results highlight the
importance of definition of measurement events to demonstrate contextuality,
and link the contextual correlations to non-classicality defined by
quasi-probabilities in phase space.
