  We have investigated the effect of imperfect circular polarization on the
angle-resolved photoemission spectroscopy signal, using graphene as a
prototypical system that can be understood within tight-binding formalism. We
found that perfect left- and right-circularly polarized lights give the same
photoelectron intensity distribution around a constant energy contour of the
graphene $\pi$ band. On the other hand, upon breaking the purity of the
polarization, photoelectron intensity starts to show circular dichroism, which
is enhanced with further increasing the imperfection. Our results predict the
existence of an additional factor for the circular dichroism observed in the
photoemission signal from graphene and hence suggest the importance of
experimental conditions to understand circular dichroism observed via
photoemission spectroscopy.
