In a device for measuring the complete polarization state of light over a spectral bandwidth, an optical input signal (41) with wavelengths of light within a spectral band is incident on two or more diffraction gratings (42, 44, 46, 48), or incident from at least two directions on one or more diffraction gratings (72, 74), and the intensity is measured as a function of wavelength for at least four of the diffraction spectra produced by the grating(s). The polarization state of light is then calculated as a function of wavelength over the spectral bandwidth from the intensity measurements.