  The efficient certification of nonclassical effects of light forms the basis
for applications in optical quantum technologies. We derive general correlation
conditions for the verification of nonclassical light based on multiplexed
detection. The obtained nonclassicality criteria are valid for
imperfectly-balanced multiplexing scenarios with on-off detectors and do not
require any knowledge about the detector system. In this sense they are fully
independent of the detector system. In our experiment, we study light emitted
by clusters of single-photon emitters, whose photon number may exceed the
number of detection channels. Even under such conditions, our criteria certify
nonclassicality with high statistical significance.
