  The BICEP/Keck experiment (BK) is a series of small-aperture refracting
telescopes observing degree-scale Cosmic Microwave Background (CMB)
polarization from the South Pole in search of a primordial $B$-mode signature.
This $B$-mode signal arises from primordial gravitational waves interacting
with the CMB, and has amplitude parametrized by the tensor-to-scalar ratio $r$.
Since 2016, BICEP3 and the Keck Array have been observing with 4800 total
antenna-coupled transition-edge sensor detectors, with frequency bands spanning
95, 150, 220, and 270 GHz. Here we present the optical performance of these
receivers from 2016 to 2019, including far-field beams measured in situ with an
improved chopped thermal source and instrument spectral response measured with
a field-deployable Fourier Transform Spectrometer. As a pair differencing
experiment, an important systematic that must be controlled is the differential
beam response between the co-located, orthogonally polarized detectors. We
generate per-detector far-field beam maps and the corresponding differential
beam mismatch that is used to estimate the temperature-to-polarization leakage
in our CMB maps and to give feedback on detector and optics fabrication. The
differential beam parameters presented here were estimated using improved
low-level beam map analysis techniques, including efficient removal of
non-Gaussian noise as well as improved spatial masking. These techniques help
minimize systematic uncertainty in the beam analysis, with the goal of
constraining the bias on $r$ induced by temperature-to-polarization leakage to
be subdominant to the statistical uncertainty. This is essential as we progress
to higher detector counts in the next generation of CMB experiments.
