  A revised metallicity calibration of the Stroemgren (b-y),m_1 diagram, based
on a sample of globular cluster and field red giant stars, is presented. This
new calibration has been used to determine Stroemgren metallicities
([Fe/H]_phot) for more than 1400 red giants in omega Centauri. Most of the
stars belong to a metal poor ([Fe/H]_phot = -1.7 dex), old population. A second
more metal rich ([Fe/H]_phot = -0.9 dex) population is found to be about 3 Gyr
younger and more centrally concentrated than the old one. The CN-rich stars do
not seem to follow the radial distribution of the metal rich population.
