Time-Series Surveys and Pulsating Stars: The Near-Infrared Perspective
  The purpose of this review is to discuss the advantages and problems of
near-infrared surveys in observing pulsating stars in the Milky Way. One of the
advantages of near-infrared surveys, when compared to optical counterparts, is
that the interstellar extinction is significantly smaller. As we see in this
review, a significant volume of the Galactic disk can be reached by infrared
surveys but not by optical ones. Towards highly obscured regions in the
Galactic mid-plane, however, the interstellar extinction causes serious
problems even with near-infrared data in understanding the observational
results. After a review on previous and current near-infrared surveys, we
discuss the effects of the interstellar extinction in optical (including {\it
Gaia}) to near-infrared broad bands based on a simple calculation using
synthetic spectral energy distribution. We then review the recent results on
classical Cepheids towards the Galactic center and the bulge, as a case study,
to see the impact of the uncertainty in the extinction law. The extinction law,
i.e. the wavelength dependency of the extinction, is not fully characterized,
and its uncertainty makes it hard to make the correction. Its characterization
is an urgent task in order to exploit the outcomes of ongoing large-scale
surveys of pulsating stars, e.g. for drawing a map of pulsating stars across
the Galactic disk.
