  Exfoliated graphene monolayers are identified by optical inspection. In order
to improve the monolayer detection, we investigate the angle dependence of the
optical contrast of graphene on a 90nm SiO$_2$/Si substrate. We observe a
significant enhancement of the visibility of graphene by changing the
polarization and the angle of optical incidence. This method can be used to
detect graphene on new substrate designs such as GaAs/AlAs based materials,
which have a much cleaner surface.
