  We present the design and implementation of a medium-band near-IR filter
tailored for detecting low-mass stars and brown dwarfs from the summit of
Maunakea. The W-band filter is centered at 1.45 micron with a bandpass width of
6%, designed to measure the depth of the H2O water absorption prominent in
objects with spectral types of M6 and later. When combined with standard J- and
H-band photometry, the W-band filter is designed to determine spectral types to
$\approx$1.4 subtypes for late-M and L dwarfs, largely independent of surface
gravity and reddening. This filter's primary application is completing the
census of young substellar objects in star-forming regions, using W-band
selection to greatly reduce contamination by reddened background stars that
impede broad-band imaging surveys. We deployed the filter on the UH 88-inch
telescope to survey $\approx$3 sq. deg. of the NGC 1333, IC 348, and rho
Ophiuchus star-forming regions. Our spectroscopic followup of W-band selected
candidates resulted in the confirmation of 48 ultracool dwarfs with a success
rate of 89%, demonstrating the efficacy of this new filter and selection
method.
