Using evolutionary population synthesis we present integrated colours , integrated spectral energy distributions and absorption-line indices defined by the Lick Observatory image dissector scanner ( referred to as Lick/IDS ) system , for an extensive set of instantaneous burst binary stellar populations with and without binary interactions .
The ages of the populations are in the range 1 - 15 Gyr and the metallicities are in the range 0.0001 - 0.03 .
By comparing the results for populations with and without binary interactions we show that the inclusion of binary interactions makes the integrated U - B , B - V , V - R and R - I colours and all Lick/IDS spectral absorption indices ( except for H _ { \beta } ) substantially smaller .
In other words binary evolution makes a population appear bluer .
This effect raises the derived age and metallicity of the population .

We calculate several sets of additional solar-metallicity binary stellar populations to explore the influence of input parameters to the binary evolution algorithm ( the common-envelope ejection efficiency and the stellar wind mass-loss rate ) on the resulting integrated colours .
We also look at the dependence on the choice of distribution functions used to generate the initial binary population .
The results show that variations in the choice of input model parameters and distributions can significantly affect the results .
However , comparing the discrepancies that exist between the colours of various models , we find that the differences are less than those produced between the models with and those without binary interactions .
Therefore it is very necessary to consider binary interactions in order to draw accurate conclusions from evolutionary population synthesis work .