  We investigate the impact of instrumental systematic errors on the potential
of cosmic microwave background polarization experiments targeting primordial
B-modes. To do so, we introduce spin-weighted Muller matrix-valued fields
describing the linear response of the imperfect optical system and receiver,
and give a careful discussion of the behaviour of the induced systematic
effects under rotation of the instrument. We give the correspondence between
the matrix components and known optical and receiver imperfections, and compare
the likely performance of pseudo-correlation receivers and those that modulate
the polarization with a half-wave plate. The latter is shown to have the
significant advantage of not coupling the total intensity into polarization for
perfect optics, but potential effects like optical distortions that may be
introduced by the quasi-optical wave plate warrant further investigation. A
fast method for tolerancing time-invariant systematic effects is presented,
which propagates errors through to power spectra and cosmological parameters.
The method extends previous studies to an arbitrary scan strategy, and
eliminates the need for time-consuming Monte-Carlo simulations in the early
phases of instrument and survey design. We illustrate the method with both
simple parametrized forms for the systematics and with beams based on
physical-optics simulations. Example results are given in the context of
next-generation experiments targeting tensor-to-scalar ratios r ~ 0.01.
