  We present the design and performance of a polychromatic polarization
modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot tunable
narrow-band imaging spectropolarimer at the Swedish 1-m Solar Telescope (SST).
We discuss the design process in depth, compare two possible modulator designs
through a tolerance analysis, and investigate thermal sensitivity of the
selected design. The trade-offs and procedures described in this paper are
generally applicable in the development of broadband polarization modulators.
The modulator was built and has been operational since 2015. Its measured
performance is close to optimal between 500 and 900~nm, and differences between
the design and as-built modulator are largely understood. We show some example
data, and briefly review scientific work that used data from SST/CRISP and this
modulator.
