  The Wavelength-shifting Optical Module (WOM) is a novel photosensor concept
for the instrumentation of large detector volumes with single-photon
sensitivity. The key objective is to improve the signal-to-noise ratio which is
achieved by decoupling the photosensitive area of a sensor from the cathode
area of its photomultiplier tube (PMT).
  The WOM consists of a transparent tube with two PMTs attached to its ends.
The tube is coated with wavelength-shifting paint absorbing ultra-violet
photons with nearly $100\,\%$ efficiency. Depending on the environment, e.g.
air (ice), up to $73\,\%$ $(41\,\%)$ of the subsequently emitted optical
photons can be captured by total internal reflection and propagate towards the
PMTs where they are recorded.
  The optical properties of the paint, the geometry of the tube and the
coupling of the tube to the PMTs have been optimized for maximal sensitivity
based on theoretical derivations and experimental evaluations. Prototypes were
built to demonstrate the technique and to develop a reproducible construction
process.
  Important measurable characteristics of the WOM are the wavelength dependent
effective area, the transit time spread of detected photons and the
signal-to-noise ratio. The WOM outperforms bare PMTs especially with respect to
the low signal-to-noise ratio with an increase of a factor up to 8.9 in air
(5.2 in ice). Since the gain in sensitivity is mostly in the UV-regime, the WOM
is an ideal sensor for Cherenkov and scintillation detectors.
