  Single-star stellar population (ssSSP) models are usually used for stellar
population studies. However, more than 50% of stars are in binaries and evolve
differently from single stars. This suggests that the effects of binary
interactions should be considered when modeling the stellar populations of
galaxies and star clusters. Via a rapid spectral stellar population synthesis
(RPS) model, we give detailed studies of the effects of binary interactions on
the Lick indices and colours of stellar populations, and on the determination
of the stellar ages and metallicities of populations.
  The results show that binary interactions make stellar populations less
luminous, bluer, with larger age-sensitive Lick index (Hbeta) and less
metallicity-sensitive indices (e.g., Mgb, Fe5270 and Fe5335) compared to
ssSSPs. It also shows that when ssSSP models are used to determine the ages and
metallicities of stellar populations, lower ages or metallicities will be
obtained, taking two line indices (Hbeta and [MgFe]) and two colours (e.g., u-R
and R-K), respectively. Some relations for linking the stellar-population
parameters obtained by ssSSPs to those obtained by binary-star stellar
populations (bsSSPs) are presented in the work. This can help us to get some
absolute values for stellar-population parameters and is useful for absolute
studies. However, it is found that the relative luminosity-weighted stellar
ages and metallicities obtained via ssSSPs and bsSSPs are similar. This
suggests that ssSSPs can be used for most stellar population studies, except in
some special cases.
