  We have developed a suite of novel infrared-blocking filters made by
embedding scattering particles in a polymer aerogel substrate. Our developments
allow us to tune the spectral performance of the filters based on both the
composition of the base aerogel material and the properties of the scattering
particles. Our filters are targeted for use in a variety of applications, from
ground-based CMB experiments to planetary science probes. We summarize the
formulations we have fabricated and tested to date, including several polyimide
base aerogel formulations incorporating a range of size distributions of
diamond scattering particles. We also describe the spectral characterization
techniques used to measure the filters' optical properties, including the
development of a mm-wave Fourier transform spectrometer testbed.
