We report a B -mode power spectrum measurement from the cosmic microwave background (CMB) polarization anisotropy observations made using the SPTpol instrument on the South Pole Telescope. This work uses 500 deg2 of SPTpol data, a five-fold increase over the last SPTpol B -mode release. As a result, the bandpower uncertainties have been reduced by more than a factor of two, and the measurement extends to lower multipoles: 52 of SPTpol data, a five-fold increase over the last SPTpol B -mode release. As a result, the bandpower uncertainties have been reduced by more than a factor of two, and the measurement extends to lower multipoles: 52 of SPTpol data, a five-fold increase over the last SPTpol B -mode release. As a result, the bandpower uncertainties have been reduced by more than a factor of two, and the measurement extends to lower multipoles: 52 of SPTpol data, a five-fold increase over the last SPTpol B -mode release. As a result, the bandpower uncertainties have been reduced by more than a factor of two, and the measurement extends to lower multipoles: 52 -71 , corresponding to a 18.1 σ detection of power. With a prior on the galactic dust from Planck, WMAP and BICEP2/Keck observations, the SPTpol B -mode data can be used to set an upper limit on the tensor-to-scalar ratio, r , corresponding to a 18.1 σ detection of power. With a prior on the galactic dust from Planck, WMAP and BICEP2/Keck observations, the SPTpol B -mode data can be used to set an upper limit on the tensor-to-scalar ratio, r lens, the data prefer Alens=1.17 ±0.13 . These data are currently the most precise measurements of B -mode power at ℓ>320 .
