  Non-Hermitian systems have attracted considerable attention for their broad
impacts on various physical platforms and peculiar applications. In
non-Hermitian systems, both eigenvalues and eigenstates simultaneously coalesce
at exceptional points (EPs). As one of the remarkable features of EPs, the
field chirality is commonly considered perfect, which is utilized as an
intriguing feature to control wave propagation and regarded as a criterion of
EP. However, in this work, we discover an imperfect chirality of eigenmodes at
the EPs in an optical whispering gallery mode (WGM) microcavity perturbed by
two strong nanoscatterers. This counterintuitive phenomenon originates from a
strong frequency-dependence of the scattering between the counterpropagating
waves at an "effective scatterer", which could be explained by a
first-principle-based model considering a dynamic multiple-scattering process
of the azimuthally propagating modes. We find that the generally imperfect
chirality at the EP tends to be globally perfect with the decrease of the
scattering effect induced by the nanoscatterers. Furthermore, the chirality
also becomes locally perfect with the decrease of the relative azimuthal angle
between the two strong nanoscatterers. This work provides a new understanding
of the general properties of chirality at EPs. It will benefit the potential
applications enabled by the chirality features of non-Hermitian systems at EPs.
