  A stable light source obtained using sum-frequency generation (SFG) is
developed for high-resolution spectroscopy at 578 nm. Hyperfine transitions of
molecular iodine are observed by using the SFG light source with saturation
spectroscopy. The light source is frequency stabilized to the observed
hyperfine transition and achieves a stability of 2*10-12 for a 1-s averaging
time. The absolute frequency of the light source stabilized on the a1 component
of the R(37)16-1 transition is determined as 518304551833 (2) kHz. This
transition serves as a frequency reference for the 1S0 - 3P0 optical clock
transition in neutral ytterbium (Yb).
