  Diffuse interstellar bands are usually observed in spectra of hot stars,
where interstellar lines are rarely blended with stellar ones. The need for hot
stars is a strong limitation in the number of sightlines we can observe and the
distribution of sightlines in the Galaxy, as hot stars are rare and
concentrated in the Galactic plane. We are introducing a new method, where
interstellar lines can be observed in spectra of cool stars in large
spectroscopic surveys. The method is completely automated and does not require
prior knowledge of the stellar parameters. If known, the stellar parameters
only reduce the computational time and are not involved in the extraction of
the interstellar spectrum. The main step in extracting interstellar lines is a
construction of the stellar spectrum, which is in our method done by finding
other observed spectra that lack interstellar features and are otherwise very
similar to the spectrum in question. Such spectra are then combined into a
single stellar spectrum template, which matches the stellar component in an
observed spectrum. We demonstrate the performance of this new method on a
sample of 482,430 spectra observed in RAVE survey. However, many spectra have
to be combined (48 on average) in order to achieve a S/N ratio high enough to
measure the DIB's profile, hence limiting the spatial information about the
ISM. Only one strong interstellar line is included in the RAVE spectral range,
a diffuse interstellar band at 8620 \AA. We compare its equivalent width with
extinction maps and with Bayesian reddening, calculated for individual stars,
and provide a linear relation between the equivalent width and reddening.
Separately from the introduced method, we calculate equivalent widths of the
diffuse interstellar band in spectra of hot stars with known extinction and
compare all three linear relations with each other and with relations from the
literature.
