  The vast majority of binaries containing a compact object and a regular star
spend most of their time in a quiescent state where no strong interactions
occur between components. Detection of these binaries is extremely challenging
and only few candidates have been detected through optical spectroscopy.
Self-lensing represents a new means of detecting compact objects in binaries,
where gravitational lensing of the light from the visible component by the
compact object produces periodic optical flares. Here we show that current and
planned large-area optical surveys can detect a significant number ($\sim
100$-$10,000$s) of these self-lensing binaries and provide insights into the
properties of the compact lenses. We show that many of the predicted population
of observable self-lensing binaries will be observed with multiple self-lensing
flares; this both improves the chances of detection and also immediately
distinguishes them from chance-alignment micro-lensing events. Through
self-lensing we can investigate long - but previously hidden - stages of binary
evolution and consequently provide new constraints on evolutionary models which
impact on the number and nature of double compact object mergers.
