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We present RingFinder, a tool for finding galaxy-scale strong gravitationallenses in multiband imaging data. By construction, the method is sensitive toconfigurations involving a massive foreground early-type galaxy and a faint,background, blue source. RingFinder detects the presence of blue residualsembedded in an otherwise smooth red light distribution by difference imaging intwo bands. The method is automated for efficient application to current andfuture surveys, having originally been designed for the 150-deg2 Canada FranceHawaii Telescope Legacy Survey (CFHTLS). We describe each of the steps ofRingFinder. We then carry out extensive simulations to assess completeness andpurity. 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 particularrange 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 anysignificant bias in the source or deflector population. We conclude bypresenting the final catalog of RingFinder CFHTLS galaxy-scale strong lenscandidates. Additional information obtained with Hubble Space Telescope andKeck Adaptive Optics high resolution imaging, and with Keck and Very LargeTelescope spectroscopy, is used to assess the validity of our classification,and measure the redshift of the foreground and the background objects. From aninitial sample of 640,000 early type galaxies, RingFinder returns 2500candidates, which we further reduce by visual inspection to 330 candidates. Weconfirm 33 new gravitational lenses from the main sample of candidates, plus anadditional 16 systems taken from earlier versions of RingFinder. Firstapplications are presented in the SL2S galaxy-scale Lens Sample paper series.

RingFinder: automated detection of galaxy-scale gravitational lenses in ground-based multi-filter imaging data