  Frequency Selective Bolometers (FSBs) are a new type of detector for
millimeter and sub-millimeter wavelengths that are transparent to all but a
narrow range of frequencies as set by characteristics of the absorber itself.
Therefore, stacks of FSBs tuned to different frequencies provide a low-loss
compact method for utilizing a large fraction of the light collected by a
telescope. Tests of prototype FSBs, described here, indicate that the
absorption spectra are well predicted by models, that peak absolute absorption
efficiencies of order 50% are attainable, and that their out-of-band
transmission is high.
