  We present RingFinder, a tool for finding galaxy-scale strong gravitational
lenses in multiband imaging data. By construction, the method is sensitive to
configurations involving a massive foreground early-type galaxy and a faint,
background, blue source. RingFinder detects the presence of blue residuals
embedded in an otherwise smooth red light distribution by difference imaging in
two bands. The method is automated for efficient application to current and
future surveys, having originally been designed for the 150-deg2 Canada France
Hawaii Telescope Legacy Survey (CFHTLS). We describe each of the steps of
RingFinder. We then carry out extensive simulations to assess completeness and
purity. For sources with magnification mu>4, RingFinder reaches 42% (resp. 25%)
completeness and 29% (resp. 86%) purity before (resp. after) visual inspection.
The completeness of RingFinder is substantially improved in the particular
range of Einstein radii 0.8 < REin < 2. and lensed images brighter than g =
22.5, where it can be as high as 70%. RingFinder does not introduce any
significant bias in the source or deflector population. We conclude by
presenting the final catalog of RingFinder CFHTLS galaxy-scale strong lens
candidates. Additional information obtained with Hubble Space Telescope and
Keck Adaptive Optics high resolution imaging, and with Keck and Very Large
Telescope spectroscopy, is used to assess the validity of our classification,
and measure the redshift of the foreground and the background objects. From an
initial sample of 640,000 early type galaxies, RingFinder returns 2500
candidates, which we further reduce by visual inspection to 330 candidates. We
confirm 33 new gravitational lenses from the main sample of candidates, plus an
additional 16 systems taken from earlier versions of RingFinder. First
applications are presented in the SL2S galaxy-scale Lens Sample paper series.
