We present a novel strategy to uncover the Galactic population of quiescent black holes ( BHs ) .
This is based on a new concept , the photometric mass function ( PMF ) , which opens up the possibility of an efficient identification of dynamical BHs in large fields-of-view .
This exploits the width of the disc H \alpha emission line , combined with orbital period information .
We here show that H \alpha widths can be recovered using a combination of customized H \alpha filters .
By setting a width cut-off at 2200 km s ^ { -1 } we are able to cleanly remove other Galactic populations of H \alpha emitters , including \sim 99.9 % of cataclysmic variables ( CVs ) .
Only short period ( P _ { orb } < 2.1 h ) eclipsing CVs and AGNs will contaminate the sample but these can be easily flagged through photometric variability and , in the latter case , also mid-IR colours .
We also describe the strategy of a deep ( r=22 ) Galactic plane survey based on the concept of PMFs : HAWKs , the HAlpha-Width Kilo-deg Survey .
We estimate that \sim 800 deg ^ { 2 } are required to unveil \sim 50 new dynamical BHs , a three-fold improvement over the known population .
For comparison , a century would be needed to produce an enlarged sample of 50 dynamical BHs from X-ray transients at the current discovery rate .