  Information about the three-dimensional structure of solar magnetic fields is
encoded in the polarized spectra of solar radiation by a host of physical
processes. To extract this information, solar spectra must be obtained in a
variety of magnetically sensitive spectral lines at high spatial, spectral, and
temporal resolution with high precision. The need to observe many different
spectral lines drives the development of Stokes polarimeters with a high degree
of wavelength diversity. We present a new paradigm for the design of
polarization modulators that operate over a wide wavelength range with near
optimal polarimetric efficiency and are directly applicable to the next
generation of multi-line Stokes polarimeters. These modulators are not
achromatic in the usual sense because their polarimetric properties vary with
wavelength, but they do so in an optimal way. Thus we refer to these modulators
as polychromatic. We present here the theory behind polychromatic modulators,
illustrate the concept with design examples, and present the performance
properties of a prototype polychromatic modulator.
