Although 
is the best tracer to derive quantitative
SFRs in galaxies, beyond 
redshifts into
the near infrared and for many applications it becomes more
practical to measure the emission lines in the blue.
Since the feature can be readily observed from
the ground to redshifts of at least unity, it provides a powerful
means of studying the systematics of galactic
star formation to cosmological redshifts and lookback times.
In Appendix A we discuss how the SFR can be measured
using this emission line as SFR tracer.