  We investigate high resolution imaging polarimetry of HD 169142 taken in the
R' and I' bands with the SPHERE/ZIMPOL instrument for an accurate quantitative
measurement of the radiation scattered by the circumstellar disk. We observe a
strong dependence of the disk polarimetry on the atmospheric turbulences, which
strongly impact the AO performance. With our non-coronagraphic data we can
analyze the polarimetric signal of the disk simultaneously with the strongly
variable stellar PSF, correct for the convolution effects to determine the
intrinsic polarization of the disk with high precision. We also extract the
disk intensity signal and derive the fractional polarization. We compare the
scattered flux from the inner and outer disk rings with the corresponding
thermal dust emissions measured in the IR and estimate the ratio between
scattered and absorbed radiation. We obtain ratios between the integrated disk
polarization flux and total system flux of 0.43% for the R' band and 0.55% for
the I' band. This indicates a reddish color for the light reflection by the
dust. The inner disk ring contributes about 75% to the total disk flux. The
obtained fractional polarization for the bright inner disk ring is 23.6% for
the I' band and similar for the R' band. The ratio between scattered disk flux
and star flux is about 2.3%. This is much smaller than the derived IR excess of
17.6% for the disk components observed in scattered light. This indicates that
only a small fraction of the radiation illuminating the disk is scattered; most
is absorbed and reemitted in the IR. We conclude that accurate, quantitative
measurements of the scattered light from circumstellar disks are possible with
ground based high contrast AO systems, if the PSF convolution effects are
properly taken into account, and this provides important new constraints on the
properties of the scattering dust.
