  The third-generation South Pole Telescope camera is designed to measure the
cosmic microwave background across three frequency bands (95, 150 and 220 GHz)
with ~16,000 transition-edge sensor (TES) bolometers. Each multichroic pixel on
a detector wafer has a broadband sinuous antenna that couples power to six
TESs, one for each of the three observing bands and both polarization
directions, via lumped element filters. Ten detector wafers populate the focal
plane, which is coupled to the sky via a large-aperture optical system. Here we
present the frequency band characterization with Fourier transform
spectroscopy, measurements of optical time constants, beam properties, and
optical and polarization efficiencies of the focal plane. The detectors have
frequency bands consistent with our simulations, and have high average optical
efficiency which is 86%, 77% and 66% for the 95, 150 and 220 GHz detectors. The
time constants of the detectors are mostly between 0.5 ms and 5 ms. The beam is
round with the correct size, and the polarization efficiency is more than 90%
for most of the bolometers
