Measurements of the degree and angle of polarization of emission from astronomical sources enable physical characterization of the objects and environmental conditions fromwhich the photons originate. However, a lack of suitable instrumentation in the x-ray and gamma-ray regimes has been frustrating astronomers for over three decades. To date, only one unambiguous astrophysical x-ray polarization measurement has been obtained: the Crab Nebula was observed to exhibit 19% linear polarization at 2.6keV.1 Conventional x-ray polarimetry relies on Bragg reflection or Thomson scattering, but instruments operating on the basis of either effect tend to suffer from low sensitivity. The next generation of large x-ray observatories currently under study—such as the International X-ray Observatory2 (IXO) and Astro-H3— have triggered renewed interest in practical x-ray polarization measurements. Device efficiency improvements have been facilitated by the development of novel gaseous detectors that can operate in the x-ray regime.4, 5 We have been pursuing an alternative approach to achieve x-ray polarimetry by developing dichroic filters that work in an analogous way to standard optical Polaroid R © film.6–8 These low-mass devices could add a polarimetry capability to the existing focal-plane microcalorimeter instruments proposed for these new observatories, while avoiding the cost and complexity of developing and operating additional detectors solely dedicated to x-ray polarimetry.6 At visible wavelengths, polarization can be detected using highly ordered materials such as H-sheet Polaroid, a polyvinyl alcohol polymer impregnated with iodine. Polarization of 33.17keV x-rays using ordinary H-sheet material was successfully analyzed for the first time in 1997.9 However, the Figure 1. Energy spectra for an inclusion compound of 1-bromoadamantane in a thiourea host. The red and black curves are for χ = 90 and 0◦, respectively, where χ is the angle between the crystal’s dichroic axis and the polarization direction of the photons. Such measurements were first reported by Chao and collaborators.10