Multiple stellar populations of globular clusters from homogeneous
  Ca--CN photometry. IV. Toward precision populational tagging
  Apparently similar but multifaceted photometric systems are currently being
used to investigate the multiple stellar populations in globular clusters,
without the concrete general agreement on the definition of the multiple
populations. In recent years, an attractive idea of utilization of the widely
used UBI photometry, C_{UBI}, for the populational tagging of the giant stars
in globular clusters has been emerged. We perform a critical analysis of the
cn_{JWL} and the C_{UBI} indices, finding that the populational tagging from
the C_{UBI} index may not be reliable, due to the inherited trait of the
broad-band photometry. As a consequence, the populational number ratios and the
cumulative radial distributions from the C_{UBI} index can be easily in error.
The results for M3, which shows a very strong radial gradient in the
populational number ratio, highlights the strengths of our cn_{JWL} index: both
the HST imaging and the ground-based spectroscopy failed to grasp the correct
picture, that can be easily achieved with our cn_{JWL} index with small
aperture ground-based telescopes, due to the small field of view or crowdedness
in the central part of the cluster.
