  The Large Underground Xenon experiment (LUX) searches for dark matter using a
dual-phase xenon detector. LUX uses a custom-developed trigger system for event
selection. In this paper, the trigger efficiency, which is defined as the
probability that an event of interest is selected for offline analysis, is
studied using raw data obtained from both electron recoil (ER) and nuclear
recoil (NR) calibrations. The measured efficiency exceeds 98\% at a pulse area
of 90 detected photons, which is well below the WIMP analysis threshold on the
S2 pulse area. The efficiency also exceeds 98\% at recoil energies of \mbox{0.2
keV} and above for ER, and \mbox{1.3 keV} and above for NR. The measured
trigger efficiency varies between 99\% and 100\% over the fiducial volume of
the detector.
