  Context. We study the relation between the known binary fraction and spectral
absorption feature index to judge whether (and potentially which) spectral
absorption feature indices are suitable for determining the binary fraction.
Aims. The determination of the binary fraction is important in studies of
binary star formation, evolutionary population synthesis models, and other
works. The number of binary stars is difficult to determine for nearly all
stellar systems because the individual stars are need to be resolved
photometrically or spectroscopically. By comparison, their integrated spectra
or spectral absorption feature indices are relatively easy to obtain. Results.
We find that the low-resolution (15\,\AA) spectrum is not suitable for this
study and the binary fraction type would affect the results: $f$($q$>0.5) and
$f$(tot)$^{\rm mc}$ exhibit better correlations with the spectral absorption
feature index than $f$(tot)$^{\rm mf}$ and the difference in metallicity would
significantly affect the above relationship. % Finally, to eliminate the
effects of metallicity, age, and dynamical evolution, we only used those GCs
with multiple spectra observed among different regions. % We find that OIII-1,
OIII-2, H$_{\rm \gamma F}$, H$_{\rm \delta F}$, H$_{\rm \gamma A}$, H$_{\rm
\delta A}$, H$_{\rm \beta}$, Ca4455, C$_2$4668, and TiO$_1$ indices have strong
correlations with binary fraction. % The two OIII indices are the most
sensitive to the binary fraction, followed by four Balmer indices -- the two
narrower central bandpass Balmer indices ($\sim$20\AA, F-definition) are more
sensitive than the wider two ($\sim$40\AA, A-definition) and, lastly, the
Ca4455, C$_2$4668, and TiO$_1$ indices.
